CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 62/433,210, filed on December 12, 2016, all of which is expressly incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] The analysis of biological samples, including the identification, characterization, and re-engineering of proteins, nucleic acids, carbohydrates, and other important biomolecules, has benefited greatly from the scaling up of sample numbers and the scaling down of sample sizes. For example, the two-dimensional microarrays of biological materials, such as DNA microarrays, have enabled the development of high-throughput screening methods involving multiplexed approaches for processing samples and detecting results.

[0003] The above approaches have, in some cases, benefited from their combination with optical sensing technology to identify specimens of interest using fluorescent or other corresponding specific and sensitive labeling approaches.

[0004] While such techniques provide analytical information about a particular sample, for example the presence and potentially the amount of a particular biomolecule in a solution or the sequence of a particular nucleic acid or polypeptide, they typically do not allow for the recovery of a biological sample identified by the assay without inactivating or otherwise damaging the sample of interest.

[0005] There is therefore a continuing need to develop improved microscale screening and analysis methods and systems with high throughput capabilities, and particularly methods and systems that enable recovery of samples identified in the screening and analysis. SUMMARY OF THE INVENTION

[0006] The present disclosure addresses these and other needs by providing in one aspect methods of screening a population of variant proteins comprising the steps of:

providing a microcapillary array comprising a plurality of microcapillaries, each microcapillary comprising a variant protein, an immobilized target molecule, and a reporter element, wherein the variant protein associates with the immobilized target molecule with a particular affinity; and

measuring a signal from at least one reporter element that indicates association of at least one variant protein with at least one immobilized target molecule to identify at least one microcapillary of interest.

[0007] In some embodiments, the methods further comprise the step of isolating the contents of the microcapillary of interest.

[0008] In another aspect are provided systems for screening a population of variant proteins comprising:

an array comprising a plurality of microcapillaries, each microcapillary comprising a variant protein, an immobilized target molecule, and a reporter element, wherein the variant protein associates with the immobilized target molecule with a particular affinity.

[0009] In some embodiments, the systems further comprise a microscope.

[0010] In some embodiments, the systems further comprise an optical source and a detector.

[0011] In some embodiments, the systems further comprise an extraction device.

[0012] In some embodiments, the systems further comprise a two-stage sample recovery element.

[0013] In some embodiments, the present invention provides a method of screening a population of variant proteins comprising the steps of:

providing a microcapillary array comprising a plurality of microcapillaries, each microcapillary comprising a variant protein, an immobilized target molecule, and a reporter element, wherein the variant protein associates with the immobilized target molecule with a particular affinity; and

measuring a signal from at least one reporter element that indicates association of at least one variant protein with at least one immobilized target molecule to identify at least one microcapillary of interest.

[0014] In some embodiments, the variant protein is expressed by an expression system.

[0015] In some embodiments, the expression system is a cell-free expression system.

[0016] In some embodiments, the expression system is a cellular expression system.

[0017] In some embodiments, the cellular expression system is an animal system, a fungal system, a bacterial system, an insect system, or a plant system.

[0018] In some embodiments, the cellular expression system is a yeast system.

[0019] In some embodiments, the variant protein is a soluble protein.

[0020] In some embodiments, the target molecule is a target protein or polypeptide, a target nucleic acid, a target carbohydrate, or a combination of each.

[0021] In some embodiments, the target molecule is immobilized on a surface.

[0022] In some embodiments, the surface is a surface of a cell.

[0023] In some embodiments, the target molecule is a native protein.

[0024] In some embodiments, the surface is a surface of a bead.

[0025] In some embodiments, the surface is a surface of a microcapillary wall.

[0026] In some embodiments, the surface is a surface configured to settle in the microcapillary by gravitational sedimentation.

[0027] In some embodiments, the reporter element is a labeled antibody or other binding molecule.

[0028] In some embodiments, the labeled antibody or other binding molecule is a fluorescently-labeled antibody or other binding molecule.

[0029] In some embodiments, the labeled antibody is a primary or a secondary antibody.

[0030] In some embodiments, the labeled antibody or other binding molecule is an enzyme-linked antibody or other binding molecule.

[0031] In some embodiments, the reporter element is activated within a cell, and the target molecule is immobilized on a surface of the cell. [0032] In some embodiments, the reporter element comprises a green fluorescent protein or variant.

[0033] In some embodiments, the signal is a fluorescent signal, an absorbance signal, a bright-field signal, or a dark-field signal.

[0034] In some embodiments, each microcapillary in the microcapillary array comprises 0 to 5 variant proteins from the population of variant proteins.

[0035] In some embodiments, the microcapillary array comprises at least 100,000, at least 300,000, at least 1,000,000, at least 3,000,000, or at least 10,000,000 microcapillaries.

[0036] In some embodiments, each microcapillary further comprises an agent to improve viability of the cellular expression system.

[0037] In some embodiments, the agent is methylcellulose, dextran pluronic F-

68, polyethylene glycol, or polyvinyl alcohol.

[0038] In some embodiments, the agent is a growth medium.

[0039] In some embodiments, the signal is measured by an optical detector.

[0040] In some embodiments, the signal is measured by a microscope.

[0041] In some embodiments, the further comprises the step of isolating the contents of the microcapillary of interest.

[0042] In some embodiments, the contents of the microcapillary of interest are isolated by pulsing the microcapillary of interest with a laser.

[0043] In some embodiments, the laser is a diode-pumped Q-switched laser.

[0044] In some embodiments, the laser is directed at the water-glass interface between the microcapillary wall and the sample contained in the microcapillary.

[0045] In some embodiments, the contents of the microcapillary of interest are isolated using a two-stage sample recovery element.

[0046] In some embodiments, the microcapillary does not comprise a microparticle capable of inhibiting the transmission of electromagnetic radiation, a magnetic microparticle, a magnetic bead, or an electromagnetic radiation absorbent material.

[0047] The present invention also provides a system for screening a population of variant proteins comprising:

an array comprising a plurality of microcapillaries, each microcapillary comprising a variant protein, an immobilized target molecule, and a reporter element, wherein the variant protein associates with the immobilized target molecule with a particular affinity.

[0048] In some embodiments, the variant protein is expressed by an expression system.

[0049] In some embodiments, the expression system is a cell-free expression system.

[0050] In some embodiments, the expression system is a cellular expression system.

[0051] In some embodiments, the cellular expression system is an animal system, a fungal system, a bacterial system, an insect system, or a plant system.

[0052] In some embodiments, the cellular expression system is a yeast system.

[0053] In some embodiments, the variant protein is a soluble protein.

[0054] In some embodiments, the target molecule is a target protein or polypeptide, a target nucleic acid, a target carbohydrate, or a combination of each.

[0055] In some embodiments, the target molecule is immobilized on a surface.

[0056] In some embodiments, the surface is a surface of a cell.

[0057] In some embodiments, the target molecule is a native protein.

[0058] In some embodiments, the surface is a surface of a bead.

[0059] In some embodiments, the surface is a surface of a microcapillary wall.

[0060] In some embodiments, the surface is a surface configured to settle in the microcapillary by gravitational sedimentation.

[0061] In some embodiments, the reporter element is a labeled antibody or other binding molecule.

[0062] In some embodiments, the labeled antibody or other binding molecule is a fluorescently-labeled antibody or other binding molecule.

[0063] In some embodiments, the labeled antibody is a primary or a secondary antibody.

[0064] In some embodiments, the labeled antibody or other binding molecule is an enzyme-linked antibody or other binding molecule.

[0065] In some embodiments, the reporter element is activated within a cell, and the target molecule is immobilized on a surface of the cell. [0066] In some embodiments, the reporter element comprises a green fluorescent protein or variant.

[0067] In some embodiments, the signal is a fluorescent signal, an absorbance signal, a bright-field signal, or a dark-field signal.

[0068] In some embodiments, each microcapillary in the microcapillary array comprises 0 to 5 variant proteins from the population of variant proteins.

[0069] In some embodiments, the microcapillary array comprises at least 100,000, at least 300,000, at least 1,000,000, at least 3,000,000, or at least 10,000,000 microcapillaries.

[0070] In some embodiments, each microcapillary further comprises an agent to improve viability of the cellular expression system.

[0071] In some embodiments, the agent is methylcellulose, dextran pluronic F-

68, polyethylene glycol, or polyvinyl alcohol.

[0072] In some embodiments, the agent is a growth medium.

[0073] In some embodiments, the system further comprises an optical source and a detector.

[0074] In some embodiments, the system further comprises a microscope.

[0075] In some embodiments, n the system further comprises an extraction device.

[0076] In some embodiments, the extraction device comprises a diode-pumped Q-switched laser.

[0077] In some embodiments, the system further comprises a two-stage sample recovery element.

[0078] In some embodiments, the microcapillary does not comprise a microparticle capable of inhibiting the transmission of electromagnetic radiation, a magnetic microparticle, a magnetic bead, or an electromagnetic radiation absorbent material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0079] FIG. 1 A- FIG. 1C schematically illustrate the steps of an exemplary microcapillary screening assay. The illustration on the left in each panel is a cross- sectional view from the side of a single microcapillary. The illustration on the right in each panel is a bottom view of a subsection of the array of

microcapillaries. The shading in each case is intended to illustrate an

electromagnetic signal, such as fluorescence.

[0080] FIG. 2A-FIG. 2C show the bottom view of a subsection of a

microcapillary array illustrating hybridoma screening against mammalian cells, where the cells are imaged using either bright-field (FIG. 2A), LiveGreen (FIG.

2B), or a fluorescent anti-mouse secondary antibody (FIG. 2C).

[0081] FIG. 3 shows images of a microcapillary containing both an A431 target cell and a hybridoma cell over the course of a 4 hour incubation.

[0082] FIG. 4A - FIG. 4B show images of a subsection of a microcapillary array highlighting expressing and non-expressing yeast cells against mammalian cells, where the cells are imaged using either bright-field (FIG. 4A) or a fluorescent antibody (FIG. 4B).

[0083] FIG. 5A- FIG. 5G illustrate the growth of an immortalized human cell in a microcapillary array over the course of 6 days.

[0084] FIG. 6A- FIG. 6E are different views of a microscope system designed to carry out the screening methods of the instant disclosure.

[0085] FIG. 7 shows an exemplary embodiment of cells (including mammalian or yeast) expression and binding to mammalian cells using the present invention. Each one of the four panels represents one microcapillary microcavity over time.

[0086] FIG. 8A - FIG. 8B shows an exemplary embodiment of cells (including mammalian or yeast) expression and binding to 2 or more mammalian cell types using the present invention. 8A) Each one of the four panels represents one microcapillary microcavity over time. 8B) Provides potential readouts from the exemplary assay embodiment.

[0087] FIG. 9 shows exemplary fluorescence and bright-field data generated by the exemplary assay described in Example 6.

[0088] FIG. 10 shows an exemplary embodiment of cells (including mammalian or yeast) expression and binding to immobilized targets on a solid support (such as a bead) using the present invention. Each one of the four panels represents one microcapillary microcavity over time. [0089] FIG. 11 shows an exemplary embodiment of cells (including mammalian or yeast) expression and functional reporter response using the present invention. Each one of the four panels represents one microcapillary microcavity over time. Reporters can include any detectable reporter, including for example GFP, YFP, and/or RFP, as well as any fluorophores described herein or known in the art.

[0090] FIG. 12 provides examples of IgGl, IgG2, IgG3, and IgG4 sequences.

[0091] FIG. 13A - FIG. 13B. Data provides A) image of beads from Example 7 titration experiment. B) Graph showing the optimum range of signal was around 1 :500-1 :5000 (lower range of manufacturer recommended ranges).

DETAILED DESCRIPTION OF THE INVENTION

[0092] Microcapillary arrays have recently been employed in approaches for high-throughput analysis and protein engineering with large numbers of biological samples, for example in an approach that has been termed "microcapillary single- cell analysis and laser extraction" or '^SCALE". See Chen et al. (2016) Nature Chem. Biol. 12:76-81; DOI: 10.1038/NCHEMBIO.1978. This approach relies on the spatial segregation of single cells within a microcapillary array, and thus enables repeated imaging, cell growth, and protein expression of the separate samples within each microcapillary of the microcapillary array. Accordingly, the technique enables massively parallel, quantitative biochemical and biophysical measurements on millions or multi-millions of samples within a microcapillary array, for example, in the analysis of millions or multi-millions of protein variants expressed from yeast, bacteria, or other suitable cells distributed throughout the array. Advantageously, the approach has allowed the simultaneous time-resolved kinetic analysis of the multiplexed samples, as well as the sorting of those cells based on targeted phenotypic features.

[0093] The development of μSCALE methods and apparatus for the quantitative biochemical and biophysical analysis of populations of biological variants has also been reported in U.S. Patent Application Publication No. 2016/0244749 Al, which is incorporated by reference herein in its entirety. Extraction of the contents of a desired microcapillary according to the μSCALE approach requires, however, the inclusion of a radiation-absorbing material in each sample and the directing of electromagnetic radiation from a pulsed laser into this material, thus adding complexity to the extraction methods. In addition, earlier methods of screening of biological variants in arrays of microcavities relied on the addition of

microparticles to the arrayed samples to partially or completely inhibit the transmission of electromagnetic radiation into and out of the sample in order to minimize signal emitted from microcavities lacking a desired binding activity. See U.S. Patent Application Publication No. U.S. 2014/0011690 Al. In some aspects of the instant disclosure, the screening methods do not rely on these additional sample components or manipulations, thus simplifying and improving the efficiency of the screening techniques.

[0094] In specific applications of these approaches, and as will be disclosed in more detail herein, the target molecule can be immobilized on a surface, such as the surface of a particle (e.g., a magnetic particle), a cell, or a microcapillary wall. The interaction between a variant protein and a target molecule in these approaches can then be measured by several methods, including methods utilizing detectable antibodies and methods of measuring detectable signals generated within the target cells. It will be understood that such methods can be used in high-throughput screens to discover protein variants that bind to target molecules, for example a target molecule on a cell or other surface.

Methods of Screening

[0095] Accordingly, in some aspects, the instant disclosure provides methods of screening a population of variant proteins comprising the steps of:

providing a microcapillary array comprising a plurality of microcapillaries, each microcapillary comprising a variant protein, an immobilized target molecule, and a reporter element, wherein the variant protein associates with the immobilized target molecule with a particular affinity; and

measuring a signal from at least one reporter element that indicates association of at least one variant protein with at least one immobilized target molecule to identify at least one microcapillary of interest.

[0096] In these methods, the microcapillary arrays preferably comprise a plurality of longitudinally fused capillaries, for example fused silica capillaries, although any other suitable material may be utilized in the arrays. See, e.g. , PCT International Patent Publication Nos. WO2012/007537 and WO2014/008056, the disclosures of which are incorporated by reference herein in their entireties. Such arrays can be fabricated, for example, by bundling millions or billions of silica capillaries and fusing them together through a thermal process, although other suitable methods of fabrication may also be employed. The fusing process may comprise, for example, the steps of i) heating a capillary single draw glass that is drawn under tension into a single clad fiber; ii) creating a capillary multi draw single capillary from the single draw glass by bundling, heating, and drawing; iii) creating a capillary multi-multi draw multi capillary from the multi draw single capillary by additional bundling, heating, and drawing; iv) creating a block assembly of drawn glass from the multi-multi draw multi capillary by stacking in a pressing block; v) creating a block pressing block from the block assembly by treating with heat and pressure; and vi) creating a block forming block by cutting the block pressing block at a precise length (e.g., 1 mm).

[0097] In some embodiments, the fabrication method further comprises slicing the silica capillaries, thereby forming very high-density glass microcapillary arrays. In some embodiments, the microcapillary arrays may be cut to

approximately 1 millimeter in height, but even shorter microcapillary arrays are contemplated, including arrays of 10 μιτι in height or even shorter. In some embodiments, even longer microcapillary arrays are contemplated, including arrays of 10 mm or even longer.

[0098] Such processes form very high-density microcapillary arrays that are suitable for use in the present methods. In an exemplary array, each microcapillary has an approximate 5 μιτι diameter and approximately 66% open space (i.e. , representing the lumen of each microcapillary). In some arrays, the proportion of the array that is open ranges between about 50% and about 90%, for example about 60 to 75%, such as a microcapillary array provided by Hamamatsu that has an open area of about 67%. In one particular example, a 10x10 cm array having 5 μιτι diameter microcapillaries and approximately 66% open space has about 330 million total microcapillaries.

[0099] In various embodiments, the internal diameter of each microcapillary in the array ranges from between approximately 1 μιτι and 500 μιτι. In some arrays, each microcapillary can have an internal diameter in the range between approximately 1 μιη and 300 μηι; optionally between approximately 1 μιη and 100 μιη; further optionally between approximately 1 μιη and 75 μηι; still further optionally between approximately 1 μιη and 50 μηι; and still further optionally between approximately 5 μηι and 50 μηι.

[0100] In some microcapillary arrays, the open area of the array comprises up to 90% of the open area (OA), so that, when the pore diameter varies between 1 μηι and 500 μιτι, the number of microcapillaries per cm of the array varies between approximately 460 and over 1 1 million. In some microcapillary arrays, the open area of the array comprises about 67% of the open area, so that, when the pore size varies between 1 μηι and 500 μιτι, the number of microcapillaries per square cm of the array varies between approximately 340 and over 800,000.

In some embodiments, the pore size is 1 μιτι, 5 μιτι, 10 μηι 50 μιτι, 100 μιτι, 250 μηι 350 or 500 μηι. In some embodiments, the pore size is between 5 μηι and 500 μιη. In some embodiments, the pore size is between 10 μηι and 450 μηι. In some embodiments, the pore size is between 50 μηι and 500 μηι. In some embodiments, the pore size is between 100 μηι and 500 μηι. In some embodiments, the pore size is between 250 μηι and 500 μηι. In some embodiments, the pore size is between 350 μηι and 500 μηι. In some embodiments, the pore size is between 100 μηι and 450 μηι. In some embodiments, the pore size is between 250 μηι and 450 μηι. In some embodiments, the number of microcapillaries per square cm of the array is approximately 400; 500; 1000; 2,000; 3,000; 4,000; 5,000; 6,000; 7,000; 8,000; 9,000; 10,000; 20,000; 50,000, 100,000; 200,000; 300,000; 400,000; 500,000; 600, 000; 700,000; or 800,000. In some embodiments, the number of microcapillaries per square cm of the array varies between approximately 500 and 800,000. In some embodiments, the number of microcapillaries per square cm of the array varies between approximately 1000 and 700,000. In some embodiments, the number of microcapillaries per square cm of the array varies between

approximately 2000 and 600,000. In some embodiments, the number of microcapillaries per square cm of the array varies between approximately 10,000 and 800,000. In some embodiments, the number of microcapillaries per square cm of the array varies between approximately 10,000 and 700,000. In some embodiments, the number of microcapillaries per square cm of the array varies between approximately 50,000 and 800,000. In some embodiments, the number of microcapillaries per square cm of the array varies between approximately 50,000 and 700,000. In some embodiments, the number of microcapillaries per square cm of the array varies between approximately 100,000 and 700,000. In some embodiments, the number of microcapillaries per square cm of the array varies between approximately 100,000 and 600,000. In some embodiments, the number of microcapillaries per square cm of the array varies between approximately 100,000 and 500,000. In some embodiments, the number of microcapillaries per square cm of the array varies between approximately 500,000 and 800,000.

[0101] In one particular embodiment, a microcapillary array can be

manufactured by bonding billions of silica capillaries and then fusing them together through a thermal process. After that slices (0.5 mm or more) are cut out to form a very high aspect ratio glass microcapillary array. Arrays are also commercially available, such as from Hamamatsu Photonics K. K. (Japan), Incom, Inc. (Massachusetts), Photonis Technologies, S.A.S. (France) Inc., and others. In some embodiments, the microcapillaries of the array are closed at one end with a solid substrate attached to the array.

[0102] The microcapillary arrays of the instant screening methods can comprise any number of microcapillaries within the array. In some embodiments, the microcapillary array comprises at least 100,000, at least 300,000, at least

1,000,000, at least 3,000,000, at least 10,000,000, or even more microcapillaries. In some embodiments, the array comprises at least 100,000, at least 200,000, at least 300,000, at least 400,000, at least 500,000, at least 600,000, at least 700,000, at least 800,000, at least 1,000,000, at least 1,500,000, at least 2,000,000, at least 2,500,000, or at least 3,000,000 or more microcapillaries. The number of microcapillaries within an array is preferably chosen in view of the size of the variant protein library to be screened.

[0103] As described above, each capillary in the microcapillary arrays used in the instant screening methods comprises a variant protein, an immobilized target molecule, and a reporter element, where the variant protein is one of the population of variant proteins that is being subjected to the screening method. The population of variant proteins can be any population of proteins that can be suitably distributed within a microcapillary array. Ideally, the population of variant proteins is distributed in the microcapillary array so that each microcapillary comprises a small number of different variant proteins, preferably just a single different variant protein per microcapillary. Importantly, the population of variant proteins is chosen in combination with the immobilized target molecule, such that at least some of the proteins in the population can associate with the immobilized target molecule with a particular affinity, such that the association is detectable by measuring a signal from a reporter element.

[0104] The term "protein", as used herein, refers both to full-length proteins or polypeptide sequences and to fragments thereof. Such fragments may include fragments that retain a functional activity, such as, for example, a binding activity. The terms "protein" and "polypeptide" are used interchangeably throughout the disclosure and include chains of amino acids covalently linked through peptide bonds, where each amino acid in the polypeptide may be referred to as an "amino acid residue". Use of the terms "protein" or "polypeptide" should not be considered limited to any particular length of polypeptide, e.g. , any particular number of amino acid residues. The subject proteins may include proteins having non-peptidic modifications, such as post-translational modifications, including glycosylation, acetylation, phosphorylation, sulfation, or the like, or other chemical modifications, such as alkylation, acetylation, esterification, PEGylation, or the like. Additional modifications, such as the inclusion of non-natural amino acids within a polypeptide sequence or non-peptide bonds between amino acid residues should also be considered within the scope of the definition of the term "protein" or "polypeptide".

[0105] The population of variant proteins is preferably a population of proteins having minor variations, for example a population of proteins where each protein has a slightly different amino acid sequence. The screening assays can, therefore, identify variant protein sequences having desirable properties. Because the screens can be performed in such large numbers at microscopic scale, huge numbers of variant proteins can be assayed in relatively short times. [0106] Variant proteins and/or variant polypeptides can include but are not limited to secreted proteins. In some embodiments, the secreted proteins are from a recombinant protein and/or polypeptide library. In some embodiments, the secreted proteins are from a recombinant protein and/or polypeptide library. In some embodiments, the secreted proteins are from a recombinant protein and/or polypeptide library from a mammalian cell line. In some embodiments, the recombinant protein and/or polypeptide library comprises full length mammalian antibodies. In some embodiments, the recombinant protein and/or polypeptide library comprises full length mammalian antibodies, including IgGl, IgG2, and IgG4 antibodies and variants thereof. In some embodiments, the recombinant protein and/or polypeptide library comprises full length human antibodies. In some embodiments, the recombinant protein and/or polypeptide library comprises full length human antibodies, including IgGl, IgG2, and IgG4 antibodies. In some embodiments, the recombinant protein and/or polypeptide library comprises full length mouse antibodies. In some embodiments, the recombinant protein and/or polypeptide library comprises full length mouse antibodies, including IgGl, IgG2, and IgG4 antibodies. In some embodiments, the recombinant protein and/or polypeptide library comprises full length rat antibodies. In some embodiments, the recombinant protein and/or polypeptide library comprises full length rat antibodies, including IgGl, IgG2, and IgG4 antibodies. In some embodiments, the recombinant protein and/or polypeptide library comprises antibody fragments (Fab). In some embodiments, the recombinant protein and/or polypeptide library comprises single chain variable fragments (scFv). In some embodiments, the recombinant protein and/or polypeptide library comprises natural protein ligands. In some embodiments, the recombinant protein and/or polypeptide library comprises natural protein ligands to a defined target protein and/or polypeptide.

[0107] In some embodiments, each microcapillary in the microcapillary array comprises 0 to 5 different variant proteins from the population of variant proteins. In specific embodiments, each microcapillary in the microcapillary array comprises 0 to 4, 0 to 3, 0 to 2, or even 0 to 1 different variant proteins from the population of variant proteins. It should be understood that the different variant proteins in the population of variant proteins differ in their molecular structure, whether the difference is in their amino acid sequence or in some other chemical modification of the protein.

[0108] It should be understood that each microcapillary will typically comprise many multiple copies of the same variant protein, depending on the source and expression level of the particular variant protein (see below). In some

embodiments, each microcapillary will comprise thousands, tens of thousands, hundreds of thousands, millions, billions, or even more molecules of a particular variant protein, depending on how the variant protein is delivered to or expressed within the microcapillary. In some embodiments, the variant protein can bind to one, two, three, or four or more target molecules. In some embodiments, the variant protein can bind to one target molecule. In some embodiments, the variant protein can bind to two target molecules. In some embodiments, the variant protein can bind to three target molecules. In some embodiments, the variant protein can bind to four target molecules. In some embodiments, the variant protein can bind to more than four target molecules. In some embodiments, this assay can alternatively be used to screen for antibodies that bind both a mouse and human (or other combination of animals) variant of a target protein, i.e. finding "cross-reactive" antibodies. For example, the presence of stained cells and the presence of stained beads within a microcapillary indicates the presence of an antibody which binds to the "target protein" (for example, a mouse target) and which also binds to the "target protein analog" (for example, a human target), in order to identify antibodies which bind to both a mouse and human target. For example, the presence of stained cells and the presence of stained beads within a microcapillary indicates the presence of an antibody which binds to the "target protein" (for example, a cynomolgus target) and which also binds to the "target protein analog" (for example, a human target), in order to identify antibodies which bind to both a cynomolgus and human target.

[0109] The population of variant proteins is typically generated using a genetic library in a biological expression system, for example in an in vitro {i.e. , cell-free) expression system or in an in vivo or cellular expression system. Exemplary cellular expression systems include, for example, animal systems (e.g., mammalian systems), fungal systems (e.g. , yeast systems), bacterial systems, insect systems, or plant systems. In specific embodiments, the expression system is a mammalian system or a yeast system. The expression system, whether cellular or cell-free, typically comprises a library of genetic material encoding the population of variant proteins. Cellular expression systems offer the advantage that cells with a desirable phenotype, for example cells that express a particular variant protein of interest, such as a variant protein capable of associating with an immobilized target molecule with high affinity, can be grown and multiplied, thus facilitating and simplifying the identification and characterization of the proteins of interest expressed by the cells. In some embodiments, the biological expression system comprises a mammalian cell line. In some embodiments, the mammalian cell line is selected from the group consisting of CHO-K1, CHO-S, HEK293T, and/or any derivatives of these cell types. In some embodiments, the mammalian cell line is CHO-K1. In some embodiments, the mammalian cell line is CHO-S. In some embodiments, the mammalian cell line is HEK293T. In some embodiments, the mammalian cell line is selected from the group consisting of human, mouse, and/or rat hybridoma cell lines. In some embodiments, the mammalian cell line is a human hybridoma cell line. In some embodiments, the mammalian cell line is a mouse hybridoma cell line. In some embodiments, the mammalian cell line is a rat hybridoma cell line.

[0110] Genetic libraries encoding large populations of variant proteins are well known in the art of bioengineering. Such libraries are often utilized in systems relying on the process of directed evolution to identify proteins with advantageous properties, such as high-affinity binding to target molecules, stability, high expression, or particular spectroscopic, e.g., fluorescence, or enzymatic activities. Often the libraries include genetic fusions with sequences from the host expression system, for example fragments of proteins directing subcellular localization, where the expressed population of variant fusion proteins are directed by the targeting fragment to a particular location of the cell or virus particle for purposes of activity screening of the variant protein population. Large numbers of variant proteins (e.g. , 10 ⁶ variants, 10 ^s variants, 10 ¹⁰ variants, 10 ¹² variants, or even more variants) can be generated using routine bioengineering techniques, as is well known in the art. Such libraries can include any of the variant proteins described herein, including antibodies, antibody fragments, single chain variable fragments, or natural protein ligands.

[0111] Accordingly, in some embodiments, the variant proteins are soluble proteins, for example soluble proteins that are secreted by a cellular expression system. Exemplary soluble variant proteins include antibodies and antibody fragments, alternative protein scaffolds, such as disulfide-bonded peptide scaffolds, extracellular domains of cell-surface receptor proteins, receptor ligands, such as, for example, G-protein coupled receptor ligands, other peptide hormones, lectins, and the like. Advantageously, the variant proteins screened for binding activity in the instant methods do not need to be covalently attached to the cell or virus that expresses them in order to be identified following a screening assay, since a variant protein with a desired binding activity and the cell that expressed it remain co- localized within the same microcapillary throughout the assay. Isolation of the contents of the desired microcapillary, followed by propagation of the cell or virus clone responsible for expression of the desired variant protein, thereby enables the identification and characterization of that protein. Unlike screening assays where a variant protein of interest is displayed by fusion of the protein to a molecule on the surface of a cell or virus particle, the variant proteins identified in the instant screening methods need not be altered in any way following their identification. The observed activities of the variant proteins in the screens are thus more likely to represent the actual activities of those proteins in their subsequent applications.

[0112] In other embodiments, however, it may be desirable for the variant proteins to be membrane-associated proteins, for example proteins remaining associated with the surface of a cell or a viral particle in an expression system. Screening of cell-associated variant proteins may be desirable where the variant protein and its target molecule mediate interactions between two cells within a biological tissue. The ability to screen against cell-associated variant proteins may also be desirable in screening for interactions with traditionally "non-druggable" protein targets, such as, for example, G-protein coupled receptors or ion channels.

[0113] In addition to a variant protein, each microcapillary in the microcapillary arrays of the instant screening methods also comprises an immobilized target molecule. The immobilized target molecule serves as the potential binding partner for the variant protein of the screening assay. Unlike the population of variant proteins, where each microcapillary ideally contains a variant protein of slightly different sequence, the immobilized target molecules ideally have the same molecular structure in each microcapillary of the array. In some embodiments, there is no binding or other interaction between the variant protein and another agent or molecule (e.g., the target molecule) prior to the addition of the variant protein to the microcapillary. In some embodiments, the interaction between the variant protein and the target molecule occurs within the microcapillary and/or microcavity.

[0114] In some embodiments, the target molecule is a target protein or polypeptide, a target nucleic acid, a target carbohydrate, a target lipid, or a combination of two or more of these target molecules. For example, in some embodiments the target molecule can be a lipid-modified or glycosylated protein. In some embodiments, the target molecule is immobilized on a surface. In more specific embodiments, the target molecule is immobilized on the surface of a cell, such as a target cell, the surface of a bead, the surface of a microcapillary wall, or another suitable surface. In other more specific embodiments, the target molecule is a native protein, for example a native protein immobilized on the surface of a cell. In still other more specific embodiments, the target molecule is immobilized on a surface configured to settle in the microcapillary by gravitational

sedimentation. In some embodiments, one, two, three, or four, or more target molecules are employed, in order to identify variants that bind to one, two, three, or four, or more target molecules. In some embodiments, the target molecules are contained separately in separate and different microcapillaries. In some embodiments, the target molecules are contained separately in separate and different microcapillaries within a single array. In some embodiments, the target molecules are contained separately in separate and different microcapillaries within one or more arrays. In some embodiments, the target molecules are contained together in a single microcapillary. In some embodiments, the target molecules are contained together in a single microcapillary within a single array. In some embodiments, the one, two, three, or four, or more target molecules to which the variant binds are derivatives or variants of an original target molecule, including chemical modifications, secondary post-translational modifications, or sequence identity variants (including, for example, variants with 70%, 75%, 80%, 85%, 90%, 95%, or 99% sequence identity to an original nucleic acid or amino acid target sequence).

[0115] As previously noted, in the methods of the instant disclosure, the variant protein associates with the immobilized target molecule with a particular affinity within a microcapillary. Importantly, such affinities should be sufficiently strong for variant proteins of interest that the association can be measured by a signal from a reporter element. Binding affinities are typically assessed by a dissociation constant as is well understood by those of ordinary skill in the art, where the lower the dissociation constant, the higher the affinity. In some embodiments, the association between the variant protein of interest and the immobilized target molecule displays a dissociation constant in the millimolar to micromolar range. In specific embodiments, the association displays a dissociation constant from micromolar to high nanomolar (i.e., 10 ^"6 M to 10 ^"8 M). In more specific embodiments, the association displays a dissociation constant from lower nanomolar to high picomolar (i.e. , 10 ^"8 M to 10 ^"10 M). In even more specific embodiments, the association displays a dissociation constant in the picomolar range (i.e. , 10 ^"10 M to 10 ^"12 M), or even lower. In some embodiments, a first cell expresses and secretes the variant protein or polypeptide and a second cell comprises the target, such that the first cells binds to the second cell. In some embodiments, the second cell expresses the target. In some embodiments, the second cell is labeled with the target. In some embodiments, the first cell binds to the second cell in the microcapillary. In some embodiments, the first cell binds to the second cell in the microcapillary and/or microcavity.

[0116] In addition to a variant protein and an immobilized target molecule, each microcapillary in the microcapillary array of the instant screening methods also comprises a reporter element. Importantly, the reporter element provides a measureable signal indicative of the association of a variant protein with an immobilized target molecule and thus serves to identify a microcapillary containing variant proteins of interest. [0117] In some embodiments, the reporter element is a labeled antibody or other molecule capable of binding to each variant protein in the population of variant proteins. More specifically, the reporter element is a fluorescently-labeled antibody or other binding molecule.

[0118] In some embodiments, the labeled antibody is a labeled primary antibody or a labeled secondary antibody. For purposes of this disclosure, a primary antibody is typically considered to be an antibody that binds directly to an antigen of interest, whereas a secondary antibody is typically considered to be an antibody that binds to a constant region on a primary antibody for purposes of labeling the primary antibody. Accordingly, secondary antibodies are frequently labeled with fluorophores or other detectable labels or are labeled with enzymes that are capable of generating detectable signals. They are generally specific for a primary antibody from a different species. For example, a goat or other animal species may be used to generate secondary antibodies against a mouse, chicken, rabbit, or nearly any primary antibody other than an antibody from that animal species, as is understood by those of ordinary skill in the art. In specific embodiments, the labeled antibody is a fluorescent antibody or an enzyme-linked antibody.

[0119] In some of the method embodiments, for example in the screening methods illustrated in FIGs. 1A-1C, the variant protein mediates the association of a reporter element with a target molecule, in this example, a target molecule on the surface of a target cell. As shown in FIG. IB, where the variant protein (here designated as a "secreted protein") has sufficient affinity for its target molecule on the target cell that the variant proteins associate with the target cell under the conditions of the microcapillary solution. The reporter element (here designated as "fluorescent detection antibodies") binds to the variant protein, ideally at an epitope that does not affect the affinity of the variant protein for the target molecule, as shown in FIG. 1C.

[0120] As would be understood by those of ordinary skill in the art, when a soluble reporter element, such as a fluorescent antibody, is used in the instant screening methods, the signal emitted by any excess reporter element remaining free in solution (i.e. , either not bound to a variant protein or bound to a variant protein that is not bound to a target molecule) within the microcapillary should not be so high that it overwhelms the signal of reporter elements associated with a target molecule via a variant protein (see, e.g. , the unassociated fluorescent detection antibodies illustrated in FIG. 1C). Such background signals can be minimized, however, by limiting the concentration of labeled antibody or other reporter element within the microcapillary solution. In addition, where signals from the screening methods are measured using a fluorescent microscope, configuring the microscope to image a relatively narrow depth of field bracketing the location of the target molecules (e.g. , the bottom of the microcapillaries when target cells have settled there by gravitational sedimentation) can minimize the background signal from reporter elements not associated with the target molecule.

[0121] In other embodiments, the reporter element is an intracellular reporter element that generates a detectable signal in connection with a binding event, such as, for example, the association of a variant protein with an immobilized target molecule, for example, a receptor or other target molecule on the surface of the cell. In these embodiments, the reporter element may comprise an entire cellular pathway, such as, for example, an intracellular signaling pathway. Such a pathway should include, or be engineered to include, a detectable signal as the downstream readout of the pathway. In contrast to the assays illustrated in FIGs. 1 A-IC, where the detectable signal is bound to the outer surface of the target cell, the detectable signal in these embodiments would typically be generated inside the target cell.

[0122] Many intracellular signaling pathways have been developed for use in high throughput screening assays, in particular in drug discovery screens, and can be adapted for use in the instant assays. See, e.g. , Michelini et al. (2010) Anal. Bioanal. Chem. 398:227-38. In particular, any cellular assay where a binding event with a target molecule on the surface of a cell results in the generation of a measurable signal, in particular a fluorescent signal, can be used as a reporter element in the instant assays. Preferably, the cells can be engineered to express a target molecule of interest on their surface, so that the binding of a particular variant protein to the target molecule and the consequent activation of the intracellular signaling pathway result in the production of a detectable signal from the reporter element, thus enabling the identification of the microcapillary as a positive hit. The expression of a green fluorescent protein (GFP), or any of a wide variety of variant fluorescent proteins, is often used as a readout in such cellular assays and can serve as the reporter element endpoint in the instant methods. Alternatively, the signaling readout can be provided by luciferase or other related enzymes that produce bioluminescent signals, as is well understood by those of ordinary skill in the art. See, e.g. , Kelkar et al. (2012) Curr. Opin. Pharmacol. 12:592-600. Reporter elements can also include RFP (red fluorescent protein) as well as YFP (yellow fluorescent protein), and variants thereof. Other well-known enzymatic reporters from bacterial and plant systems include β-galactosidase, chloramphenicol acetyltransferase, β-glucuronidase (GUS), and the like, which can be adapted for use in the instant screening assays with suitable colorogenic substrates. Transcriptional reporters using firefly luciferase and GFP have been used extensively to study the function and regulation of transcription factors. They can likewise be adapted for use in the instant screening assays. Exemplary intracellular signaling systems are available commercially, for example the Cignal™ Reporter Assay kits from Qiagen (see, e.g. ,

www.sabiosciences.com/reporterassays.php), which are available with either luciferase or GFP readouts. Such systems can be suitably re-engineered for use in the instant screening methods.

[0123] It should be understood that a variant protein expression system, in particular where the expression system is a cellular expression system, can be combined with the immobilized target molecule and the reporter element (or suitable components, such as cellular components, responsible for generating the immobilized target molecule and/or reporter element) prior to the expression of the variant proteins and/or prior to delivery of an assay mixture into the array of microcapillaries. Such approaches advantageously allow for flexibility and control in the timing of interactions between the components compared to prior art microcapillary screening systems, where all of the components of the screening assays are typically mixed and loaded into the microcapillaries in static form. In contrast, the instant methods enable some or all of the components of a binding assay to be generated in situ within the microcapillaries, either by allowing for the growth of cellular components, the expression of genetic components, or both. In some embodiments, cell culture media and/or growth agents are employed in order to maintain the health of the cells during the assay process. In some embodiments, components are included that facilitate the metabolic health of the cells.

[0124] It should also be understood that the concentrations of each component of the screening assay within a microcapillary, including the concentration of the variant protein, the concentration of the immobilized target molecule, and the concentration of the reporter element, can be modulated as desired in an assay in order to achieve an optimal outcome. In particular, it may be desirable to modulate the concentration of variant protein and/or immobilized target molecule to achieve the desired level of association between these components. The level of association will also depend on the particular affinity between these components, wherein a higher affinity results in a higher level of association for a given concentration of the components, and a lower affinity results in a lower level of association of the components for a given concentration. Concentration of the reporter element may likewise be modulated in order to achieve optimum levels of signal output, as would be understood by those of ordinary skill in the art. In some embodiments, the reporter element employed includes a secondary antibody, including those commercially available. In some embodiments the dilution range is 1 :200-1 :2000. In some embodiments the dilution range is 1 :300-1 :2000. In some embodiments the dilution range is 1 :300-1 : 1500. In some embodiments the dilution range is 1 :400-1 : 1500. In some embodiments the dilution range is 1 :500- 1 : 1500. In some embodiments the dilution range is 1 :200-1 : 1000. In some embodiments the dilution range is 1 :500-l : 1000. In some embodiments the dilution range is 1 : 1000-1 :2000. In some embodiments the dilution range is 1 : 1500-1 :2000. In some embodiments, the dilution is 1 :200, 1 :300, 1 :400, 1 :500, 1 :600, 1 :700, 1 :800, 1 :900, 1 : 1000, 1 : 1500, or 1 :2000. In some embodiments, the fluorophore can include but is not limited to AlexaFluor 3, AlexaFluor 5,

AlexaFluor 350, AlexaFluor 405, AlexaFluor 430, AlexaFluor 488, AlexaFluor 500, AlexaFluor 514, AlexaFluor 532, AlexaFluor 546, AlexaFluor 555,

AlexaFluor 568, AlexaFluor 594, AlexaFluor 610, AlexaFluor 633, AlexaFluor 647, AlexaFluor 660, AlexaFluor 680, AlexaFluor 700, and AlexaFluor 750 (Molecular Probes AlexaFluor dyes, available from Life Technologies, Inc.

(USA)). In some embodiments, the fluorophore can include but is not limited to Cy dyes, including Cy2, Cy3, Cy3B, Cy3.5, Cy5, Cy5.5 and Cy7 (available from GE Life Sciences or Lumiprobes). In some embodiments the fluorophore can include but is not limited to DyLight 350, DyLight 405, DyLight 488, DyLight 550, DyLight 594, DyLight 633, DyLight 650, DyLight 680, DyLight 750 and DyLight 800 (available from Thermo Scientific (USA)). In some embodiments, the fluorophore can include but is not limited to a FluoProbes 390, FluoProbes 488, FluoProbes 532, FluoProbes 547H, FluoProbes 594, FluoProbes 647H, FluoProbes 682, FluoProbes 752 and FluoProbes 782, AMCA, DEAC (7- Diethylaminocoumarin-3-carboxylic acid); 7-Hydroxy-4-methylcoumarin-3; 7- Hydroxycoumarin-3; MCA (7-Methoxycoumarin-4-acetic acid); 7- Methoxycoumarin-3; AMF (4'-(Aminomethyl)fluorescein); 5-DTAF (5-(4,6- Dichlorotriazinyl)aminofluorescein); 6-DTAF (6-(4,6-

Dichlorotriazinyl)aminofluorescein); 6-FAM (6-Carboxyfluorescein), 5(6)-FAM cadaverine; 5-FAM cadaverine; 5(6)-FAM ethylenediamme; 5-FAM

ethylenediamme; 5-FITC (FITC Isomer I; fluorescein-5-isothiocyanate); 5-FITC cadaverin; Fluorescein-5-maleimide; 5-IAF (5-Iodoacetamidofluorescein); 6- JOE (6-Carboxy-4',5'-dichloro-2',7'-dimethoxyfluorescein); 5-CRl 10 (5- Carboxyrhodamine 110); 6-CRl 10 (6-Carboxyrhodamine 110); 5-CR6G (5- Carboxyrhodamine 6G); 6-CR6G (6-Carboxyrhodamine 6G); 5(6)- Carboxyrhodamine 6G cadaverine; 5(6)-Caroxyrhodamine 6G ethylenediamme; 5- ROX (5-Carboxy-X-rhodamine); 6-ROX (6-Carboxy-X-rhodamine); 5-TAMRA (5-Carboxytetramethylrhodamine); 6-TAMRA (6-Carboxytetramethylrhodamine); 5-TAMRA cadaverine; 6-TAMRA cadaverine; 5-TAMRA ethylenediamme; 6- TAMRA ethylenediamme; 5-TMR C6 maleimide; 6-TMR C6 maleimide; TR C2 maleimide; TR cadaverine; 5-TRITC; G isomer (Tetramethylrhodamine-5- isothiocyanate); 6-TRITC; R isomer (Tetramethylrhodamine-6-isothiocyanate); Dansyl cadaverine (5-Dimethylaminonaphthalene-l-(N-(5- aminopentyl))sulfonamide); EDANS C2 maleimide; fluorescamine; NBD; and pyrromethene and derivatives thereof. In some embodiments, the reporter element used can be a donkey anti-goat IgG secondary antibody labeled with AlexaFluor 633. [0125] In some embodiments, each microcapillary in the microcapillary arrays of the instant screening methods further comprises an agent or agents to improve viability of the cellular expression system. Specifically, the agent or agents is included to prevent cell damage during the step of isolating the contents of the microcapillary of interest, for example by a laser pulse (see below). In preferred embodiments, the agent is methylcellulose (for example at 0.001 to 10 wt %), dextran (for example at 0.5 to 10 wt %), pluronic F-68 (for example at 0.01 to 10 wt %), polyethylene glycol ("PEG") (for example at 0.01 to 10 wt %), polyvinyl alcohol ("PVA") (for example at 0.01 to 10 wt %), or the like. Alternatively, or in addition, each microcapillary in the microcapillary arrays of the instant screening methods can further comprise a growth additive, such as, for example, 50% conditioned growth media, 25% standard growth media, or 25% serum. In some embodiments, the conditioned growth media is conditioned for 24 hours. In some embodiments, the added agent is insulin, transferrin, ethanolamine, selenium, an insulin-like growth factor, or a combination of these agents or any of the agents recited above.

[0126] The screening methods of the instant disclosure preferably include the further step of measuring a signal from at least one reporter element that indicates association of at least one variant protein with at least one immobilized target molecule to identify at least one microcapillary of interest. In some embodiments, the signal measured is a fluorescent signal, an absorbance signal, a bright-field signal, a dark-field signal, a phase contrast signal, or the like. Accordingly, the measuring step can be performed by an appropriate detector device, for example a device capable of detecting electromagnetic radiation or any other suitable signal. In specific embodiments, the measuring step is performed by a microscope, such as a fluorescence microscope or any other microscope configured to detect the above- mentioned signals.

[0127] It should be understood that in preferred embodiments, the

microcapillaries utilized in the instant screening methods do not comprise microparticles capable of inhibiting the transmission of electromagnetic radiation. In other words, the microcapillaries are preferably fully transparent to

electromagnetic radiation incident on the microcapillary array, in particular along the longitudinal axes of the microcapillaries. In other preferred embodiments, the microcapillaries of the instant screening methods do not comprise magnetic microparticles or beads. In still other preferred embodiments, the microcapillaries of the instant screening methods do not comprise microparticles capable of inhibiting the transmission of electromagnetic radiation, magnetic microparticles, or magnetic beads.

[0128] In other preferred embodiments, the microcapillaries utilized in the instant screening methods do not comprise an electromagnetic radiation absorbent material. It should be understood, however, that the component of a reporter element responsible generating a measurable signal in the screening method, for example the fluorophore on a fluorescent antibody, should not be considered an electromagnetic radiation absorbent material for purposes of this aspect of the invention.

[0129] In some embodiments, the instant screening methods further comprise the step of isolating the contents of the microcapillary of interest. In specific embodiments, the contents of the microcapillary of interest are isolated by pulsing the microcapillary of interest with a laser. More specifically, the laser can be a diode laser or a diode-pumped Q-switched Nd:YLF laser. In some embodiments, the laser can be directed at the water-glass interface between the microcapillary wall and the sample contained in the microcapillary. Without intending to be bound by theory, it is believed that firing a UV laser at this interface can break the meniscus/water surface tension that normally holds a sample in the microcapillary, thus allowing the sample to fall out of the array via the force of gravity. In other embodiments, the contents of the microcapillary of interest are isolated by laser- triggered vapor force expansion. In some embodiments, the contents of the microcapillary are isolated by breaking the glass of the microcapillary itself.

[0130] In some embodiments, the microcapillary screening methods of the instant invention allow for screening reactions and/or interactions (including binding interactions) that occur between the variant protein and the target molecule within minutes of the addition of the components to the microcapillary. In some embodiments, the reactions and/or interactions between the variant protein and the target molecule occur and/or are detectable within about 1 minute to about 10 minutes. In some embodiments, the reactions and/or interactions between the variant protein and the target molecule occur and/or are detectable within about 1 hour to about 6 hours. In some embodiments, the reactions and/or interactions occur within about 1 hour, about 2 hours, about 4 hours, about 6, about 10 hours, about 12 hours, about 16 hours, about 24 hours, about 36 hours, or about 48 hours, hours minute to about 10 minutes. In some embodiments, the reactions and/or interactions between the variant protein and the target molecule occur and/or are detectable within a period of time such that the cells within the microcapillary are alive and healthy. In some embodiments, the reactions and/or interactions between the variant protein and the target molecule occur and/or are detectable within a period of time such that the cells within the microcapillary are viable. In some embodiments, the cells can be grown after removal from the microcapillary and/or microcavity. In some embodiments, the cells are viable after removal from the microcapillary and/or microcavity. In some embodiments, the reactions and/or interactions the between the variant protein and the target molecule occur within the microcapillary.

Systems for Screening

[0131] According to another aspect of the invention are provided systems for screening a population of variant proteins comprising:

an array comprising a plurality of microcapillaries, each microcapillary comprising a variant protein, an immobilized target molecule, and a reporter element, wherein the variant protein associates with the immobilized target molecule with a particular affinity. The components of these screening devices are described in detail above and any of those can be incorporated into a system for screening.

[0132] In some embodiments, the screening systems further comprise an optical source and a detector. The optical source and detector are chosen according to the particular reporter element used in the screening system. For example, where the reporter element generates a fluorescent signal, the optical source provides excitation light of an appropriate wavelength to excite the fluorescent probe. Likewise, the detector is chosen to be sensitive to the wavelength of light emitted by the fluorescent probe. The optical source and the detector may, for example, be components of a microscope, such as a fluorescent microscope, or they may be separate devices, as would be understood by those of ordinary skill in the art.

[0133] In some embodiments, the screening system further comprises an extraction device, for example a diode laser, a diode-pumped Q-switched laser, or other appropriate component for isolating the contents of a microcapillary of interest.

[0134] An exemplary microscope for screening populations of variant proteins according to the instant methods is illustrated in the drawings of FIGs. 6A-6E. FIG. 6A shows a perspective view from above the microscope, illustrating both the array -holding stage and the sample-recovery stage. FIG. 6B shows a front view of the device. FIG. 6C shows a view from the right side. A magnified view of the right side of the device is provided in FIG. 6D, which illustrates in detail the relationship between the array -holding stage and the sample-recovery stage. FIG. 6E provides an exploded view of various components of a multi-stage sample- recovery microscope suitable for use in the instant screening systems.

[0135] It will be readily apparent to one of ordinary skill in the relevant arts that other suitable modifications and adaptations to the methods and applications described herein can be made without departing from the scope of the invention or any embodiment thereof. Having now described the present invention in detail, the same will be more clearly understood by reference to the following Examples, which are included herewith for purposes of illustration only and are not intended to be limiting of the invention. In some embodiments, any aspects disclosed under the methods section above are also readily applicable to the systems of the instant invention. The systems of the invention can be employed with any of the methods described herein.

EXEMPLARY EMBODIMENTS:

[0136] The present application provides a method of screening a population of variant proteins comprising the steps of:

providing a microcapillary array comprising a plurality of microcapillaries, each microcapillary comprising a variant protein, an immobilized target molecule, and a reporter element, wherein the variant protein associates with the immobilized target molecule with a particular affinity; and

measuring a signal from at least one reporter element that indicates association of at least one variant protein with at least one immobilized target molecule to identify at least one microcapillary of interest.

[0137] In some embodiments, the variant protein is expressed by an expression system.

[0138] In some embodiments, the expression system is a cell-free expression system.

[0139] In some embodiments, the expression system is a cellular expression system.

[0140] In some embodiments, the cellular expression system is an animal system, a fungal system, a bacterial system, an insect system, or a plant system.

[0141] In some embodiments, the cellular expression system is a yeast system.

[0142] In some embodiments, the variant protein is a soluble protein.

[0143] In some embodiments, the target molecule is a target protein or polypeptide, a target nucleic acid, a target carbohydrate, or a combination of each.

[0144] In some embodiments, the target molecule is immobilized on a surface.

[0145] In some embodiments, the surface is a surface of a cell.

[0146] In some embodiments, the target molecule is a native protein.

[0147] In some embodiments, the surface is a surface of a bead.

[0148] In some embodiments, the surface is a surface of a microcapillary wall.

[0149] In some embodiments, the surface is a surface configured to settle in the microcapillary by gravitational sedimentation.

[0150] In some embodiments, the reporter element is a labeled antibody or other binding molecule.

[0151] In some embodiments, the labeled antibody or other binding molecule is a fluorescently-labeled antibody or other binding molecule.

[0152] In some embodiments, the labeled antibody is a primary or a secondary antibody.

[0153] In some embodiments, the labeled antibody or other binding molecule is an enzyme-linked antibody or other binding molecule. [0154] In some embodiments, the reporter element is activated within a cell, and the target molecule is immobilized on a surface of the cell.

[0155] In some embodiments, the reporter element comprises a green fluorescent protein or variant.

[0156] In some embodiments, the signal is a fluorescent signal, an absorbance signal, a bright-field signal, or a dark-field signal.

[0157] In some embodiments, each microcapillary in the microcapillary array comprises 0 to 5 variant proteins from the population of variant proteins.

[0158] In some embodiments, microcapillary array comprises at least 100,000, at least 300,000, at least 1,000,000, at least 3,000,000, or at least 10,000,000 microcapillaries.

[0159] In some embodiments, each microcapillary further comprises an agent to improve viability of the cellular expression system.

[0160] In some embodiments, the agent is methylcellulose, dextran pluronic F-

68, polyethylene glycol, or polyvinyl alcohol.

[0161] In some embodiments, the agent is a growth medium.

[0162] In some embodiments, the signal is measured by an optical detector.

[0163] In some embodiments, the signal is measured by a microscope.

[0164] In some embodiments, the method further comprises the step of isolating the contents of the microcapillary of interest.

[0165] In some embodiments, the contents of the microcapillary of interest are isolated by pulsing the microcapillary of interest with a laser.

EXAMPLES

Example 1. Screening for a Secreted EGFR-binding Protein

[0166] FIG. 1 A-FIG. 1C illustrate an exemplary screening method for a soluble protein capable of associating with a cell-surface protein (e.g., the epidermal growth factor receptor ("EGFR")) as the immobilized target molecule, in this case an immobilized target protein. FIG. 1 A (left panel) shows the target cell, which expresses EGFR on its surface. Also shown is a "library expressing cell", which expresses a population of variant proteins, and a number "fluorescent detection antibodies" in the microcapillary solution. A bottom view of the microcapillary array is illustrated in the right panel.

[0167] Components of each microcapillary according to this screening assay:

1. Cells secreting the variant protein of interest (the "library expressing cell").

The variant protein of interest is preferably a member of a population of variant proteins, i.e., a protein library.

2. Target protein immobilized on a surface of a "target cell". In this example, the target protein is a native, cell-surface receptor (i.e. , EGFR).

Alternatively, however, the target protein could be immobilized on another surface, such as a bead surface or a surface of the microcapillary itself.

3. Reporter element

a. In this example, the reporter element corresponds to a fluorescently- labeled antibody specific for the secreted protein (i.e. , the

"fluorescent detection antibodies"). The antibody specifically localizes to an epitope on the secreted protein but ideally does not interfere with the binding of the secreted protein to the target protein on the target cell.

b. Alternatively, the reporter element can be a signaling pathway

within the cells that express the target protein. If a secreted variant protein binds the target protein on the cell surface and activates the signaling pathway within the target cell, the binding interaction will generate a fluorescent signal within the cell (not shown).

4. Reaction buffer:

a. Can be media for the library-expressing cells or for the target cells (for example, such media could be hybridoma medium: such as one commercially available, including for example from ThermoFIshcer, see, the World Wide Web at

thermofisher.com/order/catalog/product/11279023; CD Hybridoma Medium).

b. Can be a mammalian imaging solution (for example, such an

imaging solution can be optically clear, physiological solution buffered with HEPES at pH 7.4). Illustration of method:

[0168] Step 1 : Add all components into microcapillary (see FIG. 1A).

[0169] Step 2: A specific "secreted protein" is expressed by the library- expressing cell into the microcapillary. Secreted protein variants capable of binding to the target protein are localized to the target cell surface as shown (see FIG. IB).

[0170] Step 3: Fluorescent detection antibodies associated with the bound secreted protein variants are observed in association with target cells in specific microcapillaries (see FIG. 1C).

Detailed description and sample data:

[0171] To demonstrate this method, a yeast vector library expressing a protein designed to bind to EGFR on human cancer cells was created. In this library, some yeast variants were capable of expressing the protein, while other variants were not able to express the protein. Yeast cells, cancer cells, and a fluorescent antibody against the expressed protein were added to the microcapillary. After 18 hours, the microcapillary array was imaged. Further details and results of the screen are provided in Example 3 below.

Example 2. Hybridoma Screening Against Mammalian Cells

General background

[0172] Current methods to screen binding interactions between proteins or other target molecules typically rely on the use of "display" methods, e.g., phage display, bacterial display, yeast display, mammalian display, or virus display. In the display methods, a library of genes encoding protein variants is expressed at the surface of the cell or phage. The protein variants are incubated with a soluble version of the target molecule in order to identify protein variants capable of binding to the target. The library can be screened by panning or by fluorescence- activated cell sorting ("FACS"). Such assays have two primary limitations: 1) the engineered protein is typically tethered to the display platform; and 2) it is usually advantageous for a soluble form of the target molecule to exist. Therefore, it can be difficult to develop reliable assays for variant proteins that bind to many target molecules, in particular membrane proteins, such as G-protein coupled receptors and other such receptors. Hybridoma screening against mammalian cells

[0173] To identify antibody variants with specific binding to a target molecule, hybridomas (which secrete antibody variants) were added to a cancer cell line that expressed high levels of EGFR as the target molecule. Labeled antibodies specific for the secreted antibodies were then added.

Materials;

[0174] Cells:

[0175] Mouse hybridoma

[0176] A431 target cells (human cancer cell line expressing high levels of EGFR)

Detection antibodies:

[0177] Anti-mouse secondary antibody labeled with Alexa488 (a fluorophore)

Media for cell culture:

[0178] DMEM-10% fetal bovine serum

[0179] DMEM-10% horse serum

[0180] Cell line growth and preparation. Mouse hybridoma cells were cultured in complete media (Dulbecco's Modified Eagle's Medium with 10% horse serum). The hybridoma cells were washed twice with PBSA and suspended in complete media at 600 cells /uL. The A431 cells were cultured in complete media

(Dulbecco's Modified Eagle's Medium with 10% fetal bovine serum). The A431 cells were washed twice with PBSA and stained with a LiveGreen fluorescent signal. The A431 cells were then suspended in the complete media containing hybridoma at a final concentration of 1800 cells /uL.

[0181] Assay setup. Following mixing of the two cell types, detection antibodies were added to the reaction mixture: 1 : 100 dilution of secondary (anti-mouse Alexa488). This reaction mixture was then loaded into an ethanol-sterilized, corona-treated microcapillary array (40 μιτι diameter, 1 mm thick). A 2 mm slab of 1% weight/volume agarose was placed on the array to help prevent evaporation. After each hour, the sample was imaged under fluorescence and bright-field microscopy. Sample data:

[0182] FIGs. 2A-2C show images of a subsection of the microcapillary array showing either all of the cells (FIG. 2A, bright-field signal), A431 target cells (FIG. 2B, LiveGreen signal), or cells labeled with the fluorescent anti-mouse secondary antibody (FIG. 2C, Ab-a555 signal). Microcapillaries containing hybridoma cells that express antibodies specific for EGFR are indicated with two arrows in each image.

[0183] FIG. 3 shows images of a microcapillary containing both an A431 target cell and a hybridoma cell over the course of a 4 hour incubation, where the antibody binding signal to the A431 target cells increased during the time course of the assay as mouse antibodies specific to EGFR are produced (middle column). LiveGreen staining of the A431 target cells declined over the same time period (right column).

Example 3. Yeast Library Screening Against Mammalian Cells

[0184] To determine the best secretion yeast plasmid vectors, a yeast vector library expressing scaffold proteins designed to bind to EGFR on a cancer cell surface was created. This library contained yeast cells with various soluble expression levels of a scaffold protein. Using the described assay, the variant expression library was screened to recover the plasmid vectors with high expression of the desired scaffold protein. In this experiment, the secreted scaffold has a c-Myc tag, which can be labeled with fluorescently-labeled antibodies.

Materials;

[0185] Cells:

[0186] Yeast secretion library of scaffold proteins

A431 cells (human cancer cell line expressing high levels of EGFR)

[0187] Detection antibodies:

[0188] Chicken anti-c-Myc

[0189] Anti-chicken secondary antibody labeled with Alexa488

[0190] Media for cell culture:

[0191] DMEM-10% FBS

[0192] SD-CAA minimal yeast media [0193] Reaction buffer:

[0194] SD-CAA minimal yeast media

Methods;

[0195] Cell line growth and preparation. The yeast library was grown in SD- CAA minimal yeast media (20 g dextrose; 6.7 g Difco yeast nitrogen base; 5 g Bacto casamino acids; 5.4 g Na2HPC>4; 8.56 g NaH ₂ P0 ₄ »H ₂ 0; dissolved in deionized H ₂ 0 to a volume of 1 liter). After growth, the yeast cells were washed twice with PBSA (phosphate-buffered saline + 1 mg/ml BSA) and suspended in SD-CAA at a final concentration of 2,400 cells/uL.

[0196] The A431 cells were cultured in complete media (Dulbecco's Modified Eagle's Medium with 10% fetal bovine serum). The A431 cells were washed twice with PBSA and suspended in the SD-CAA containing yeast cells at a final concentration of 600 cells /uL.

[0197] Assay setup. Following mixing of the two cell types, two antibodies were added to the reaction mixture: 1 :250 dilution of an unlabeled primary antibody (chicken anti-c-Myc) and 1 :200 dilution of a labeled secondary antibody (anti- chicken Alexa488). This reaction mixture was then loaded into an ethanol- sterilized, corona-treated microcapillary array (40 μιτι diameter, 1 mm thick). A 2 mm slab of 1% weight/volume agarose was placed on the array to help prevent evaporation. After 18 hours of growth, the sample was imaged under fluorescence and bright-field microscopy.

[0198] Microcapillary array extraction. A Triton UV laser was used to extract the contents of desired capillaries. The laser operates for 18 ± 2 ms (n = 5 measurements), delivering a train of pulses at 2.5 kHz with a total energy of approximately 100 μΐ The microcapillary contents were extracted onto a glass coverslip, which was then placed in yeast growth media (liquid medium or agar plates) to propagate the extracted cells.

Sample data;

[0199] FIGs. 4A and 4B show images of a subsection of the microcapillary array that identifies microcapillaries with expressing and non-expressing cells using bright-field imaging (FIG. 4A) and fluorescence imaging (FIG. 4B). Example 4. Growth of Cultured Human Cells in a Microcapillary Array

[0200] FIGs. 5A-5G demonstrate the growth of K562 cells (a human

immortalized myelogenous leukemia cell-line) in growth media over the course of 6 days within an array of microcapillaries. A bright-field image of the same section of the array was taken every 24 hours. FIG. 5 A: Day 0; FIG. 5B: Day 1; FIG. 5C: Day 2; FIG. 5D: Day 3; FIG. 5E: Day 4; FIG. 5F: Day 5; and FIG. 5G: Day 6. A 40 μιη scale bar is shown in each image.

Example 5. Hybridoma Screening Against Mammalian Reporter Cells

[0201] To identify antibody variants that activate specific signaling pathways, hybridomas secreting different antibody variants were added into a microcapillary array with a reporter cell. For example, the reporter cell can be from Qiagen (see http://www.sabiosciences.com/reporter_assay _product/HTML/CCS-013L.html). If a protein variant binds the reporter cell and activates the signaling pathway, the reporter cell expresses a fluorescent protein. The signal fluorescence of activated cells is observed in microcapillaries that contain desirable protein variants and used to isolate the contents of those microcapillaries.

Example 6. Sample Data for Multiple Target Binding

Study goal:

[0202] Identify hybridoma secreting antibody that would specifically bind target protein, but not bind to a similarly structured protein. See, for example, FIG. 9. This method allowed for and will continue to allow for screening of antibody libraries.

Materials:

[0203] A hybridoma library.

[0204] The target protein would be displayed on a cancer cell type. See, for example, FIG. 9.

[0205] The target protein analog is immobilized on beads. See, for example,

FIG. 9.

Protocol:

1. Cultured cells that displayed target protein A on surface. 2. Labeled protein Dynabeads Biotin Binder with target protein B following manufacturing directions.

3. Cultured rat hybridoma library.

4. Diluted cells, beads, and hybridoma so that there was an average of 1 hybridoma per microcapillary, ~2 cells of target protein A cells, and -10 beads covered of target protein B.

5. Added anti-rat secondary antibody (reporter element) to prepared cell sample.

6. Loaded sample into array.

7. Incubated 1 hours before imaging.

8. Recovered cells that bind cells with target protein A but not beads with target protein B.

Sample result:

[0206] Identified microcapillaries that have properly stained cells but no stained beads. The presence of stained cells and the absence of stained beads within a microcapillary indicated the presence of an antibody which bound to the target protein but did not bind to the target protein analog.

[0207] In some embodiments, the assay tested in this example can alternatively be adapted used to screen for antibodies that bind both a mouse and human (or other combination of animals) variant of a target protein, i.e. finding "cross- reactive" antibodies. For example, the presence of stained cells and the presence of stained beads within a microcapillary indicates the presence of an antibody which binds to the "target protein" (for example, a mouse target) and which also binds to the "target protein analog" (for example, a human target), in order to identify antibodies which bind to both a mouse and human target. For example, the presence of stained cells and the presence of stained beads within a microcapillary indicates the presence of an antibody which binds to the "target protein" (for example, a cynomolgus target) and which also binds to the "target protein analog" (for example, a human target), in order to identify antibodies which bind to both a cynomolgus and human target. Example 7. Titration of reporter element for optimum signal output

[0208] A frequent reporter element used is a fluorescently labeled secondary antibody. In this assay format, the secondary antibody was added in the beginning of the assay, and over time this antibody bound to the secreted antibody (the variant protein), which was bound to the target protein.

[0209] A key consideration was the amount of secondary antibody used. If there was too much secondary antibody, the background noise would be too high. If too little secondary antibody was used, the signal would be too low. In this experiment, beads were labeled with the primary antibody and then titrated with various levels of the secondary antibody to determine the optimal signal to noise ratio.

[0210] Reporter element used was Donkey anti-Goat IgG Secondary Antibody labeled with Alexa Fluor 633.

[0211] Manufacturer recommended range of usage: 1 :200-l :2000 dilution range.

[0212] Dilution series tested was 1 : 100, 1 :200, 1 :500, 1 : 1000, 1 :2000, 1 :5000. Image of beads from the experiment is shown in FIG. 13.

[0213] All patents, patent publications, and other published references mentioned herein are hereby incorporated by reference in their entireties as if each had been individually and specifically incorporated by reference herein.

[0214] While specific examples have been provided, the above description is illustrative and not restrictive. Any one or more of the features of the previously described embodiments can be combined in any manner with one or more features of any other embodiments in the present invention. Furthermore, many variations of the invention will become apparent to those skilled in the art upon review of the specification. The scope of the invention should, therefore, be determined by reference to the appended claims, along with their full scope of equivalents.

[0215] The examples set forth above are provided to give those of ordinary skill in the art a complete disclosure and description of how to make and use the embodiments of the compositions, systems and methods of the invention, and are not intended to limit the scope of what the inventors regard as their invention. Modifications of the above-described modes for carrying out the invention that are obvious to persons of skill in the art are intended to be within the scope of the following claims. All patents and publications mentioned in the specification are indicative of the levels of skill of those skilled in the art to which the invention pertains. All references cited in this disclosure are incorporated by reference to the same extent as if each reference had been incorporated by reference in its entirety individually.

[0216] All headings and section designations are used for clarity and reference purposes only and are not to be considered limiting in any way. For example, those of skill in the art will appreciate the usefulness of combining various aspects from different headings and sections as appropriate according to the spirit and scope of the invention described herein.

[0217] All references cited herein are hereby incorporated by reference herein in their entireties and for all purposes to the same extent as if each individual publication or patent or patent application was specifically and individually indicated to be incorporated by reference in its entirety for all purposes.

[0218] Many modifications and variations of this application can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. The specific embodiments and examples described herein are offered by way of example only, and the application is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which the claims are entitled.