This invention relates to biochemical assays, and more particularly to assays for antibodies to double-stranded nucleic acids. Polymeric chains of deoxyribonucleic acid (DN A), being integral parts of the nuclear material of biological cells in double-stranded form (dsDNA), do not ordinarily stimulate the immune system to form antibodies. At least one disease, systemic lupus erythematosus, is however characterized by apparent abnormalities of the immune system that causes antibodies to be formed to dsDNA with fatal results. Antibodies to dsDNA occur almost exclusively in systemic lupus erythematosus and assay for such antibodies is one of the laboratory tests used in the diagnosis and determination of the prognosis of the disease. Heretofore, measurement of anti-dsDNA antibody has typically been accomplished by using a radioactive immunoassay in which a complex formed between the antibody and antigen is labelled with a radioactive material, and a measurement of the radioactivity of the resulting product is measured as with a Geiger counter, photochemically or the like. The radioactive materials used in such assays may pose a significant danger, and where they have limited half-lives, frequent calibration is necessary. Alternatively one can use an enzyme- linked immunoabsorbent assay employing an enzyme linked to a molecule specific to an antibody bound to an antigen on a solid phase, e.g. peroxidase phosphate. A chromogenic substrate is added to generate a colored end product in the presence of the enzyme portion of the ligand. While such an assay is stable, it is far less sensitive than radioimmunoassays. Unfortunately also, rheumatoid factors tend to react with the Fc portion of the human IgG and thus may seriously interfere with the assay. The known Crithidia assay does assay for true dsDNA but, being at best semi- quantitative, cannot follow the activity of the disease. A principal object of the present invention is therefore to provide an assay for anti- dsDNA antibody, which assay is stable, yet highly sensitive, and uses no materials that pose a significant health threat to the assayer. Yet other objects of the present invention are to provide such an assay that, being linearly quantitative, has particular utility in clinical medicine; and to provide such an assay that measures only IgG antibody that relates to the disease activity.

Generally these and other objects of the present invention are realized by an assay that incorporates a capture system for double stranded IgG antibody and thereby allows measurement of the antibody only. The assay captures the anti-dsDNA portion of IgG in a human serum specimen by the Fc part of a molecule using solid phase immobilized F(ab')2 fragment of anti-human IgG specific for Fc, the captured IgG being then incubated with a synthetic dsDNA tagged with a moiety from which a signal proportional to the quantity o said synthetic dsDNA can be elicited. Upon eliciting a signal from the moiety, one may quantify the amount of antibody to dsDNA. More specifically, a preferred embodiment of the present invention involves immobilizing F(ab')2 fragment of anti-human IgG antibody, such as goat anti-human IgG antibody, specific for the Fc part of a molecule, on an inert substrate or immunoadsorbent. The present invention employs F(ab')2 to avoid rheumatoid factors that would otherwise interfere with the assay because the latter tend to react with the Fc portion of the IgG. A dilution of a human patient serum specimen believed to contain an anti-dsDNA portion o IgG, is then permitted to contact the coated substrate under conditions and for a period o time sufficient for a proportion of the patient IgG to become bound by reaction with th animal anti-IgG antibody. Synthetic double-stranded DNA is labelled, for example, with digoxigenin inasmuch as the use of the latter must result in dsDNA antibody that does no have a single-stranded or denatured component. The digoxigenin is added to the serum, again under conditions and for a time sufficient to bind to any anti-DNA antibodies whic may be present in the serum. The extent to which the labelled dsDNA is bound to the soli phase immunoadsorbent is determined by adding an anti-digoxigenin antibody coupled t alkaline phosphatase. The amount of alkaline phosphatase present is measured by the furthe addition of a chemiluminescent substrate and the resulting luminescence due to reaction o the alkaline phosphatase with the substrate, if any, is measured with a luminometer . The us of chemiluminescence as a tag or label provides sufficient sensitivity to provide excellen quantitative determination. The levels of luminescence obtained are compared to standar levels provided by the similar reaction of standard quantities of anti-dsDNA antibodies an expressed as U/mL, thereby relating the amount of bound labelled synthetic double-strande DNA detected to a predetermined quantitative relationship between the amount of labelle double-stranded DNA and the amount of animal anti-human IgG antibody to determine th amount of human IgG in the serum.

Pure double-stranded DNA molecules are provided for use in the anti-dsDNA assay. The synthesis of the DNA from a single-stranded DNA M 13 phage molecule is initiated with random primers and carried out with dATP, dGTP, dCTP, dTTP and digoxigenin-labelled deoxyuridine-triphosphate in the presence of sequenase enzyme, as will be described hereinafter in detail. Not all antibodies present in individuals have the same affinity or avidity for the dsDNA antigen as does the standard antibody preparation. Differences in avidity between the standard and the individual patient samples results in non-linear dose response curves. Because of this non-linearity, a two-fold dilution of a sample may not result in a U/mL value that is one-half of the undiluted sample result. The binding activity, however, of a patient sample will be reproducible and consistent at a given dilution. For this reason, all samples with u/mL over a 150 U standard are diluted as little as possible for reanalysis, and some patients may need to always be analyzed at a set dilution so that sequential results over time can be compared . Molecular biology techniques are used to generate pure, double-stranded DNA molecules for use in the anti-dsDNA assay. The DNA is typically synthesized from single-stranded DNA M13 circular phage molecules (0.2 μg/μL, USB #70704). This DNA synthesis is initiated with random primers and carried out with dATP, dGTP, dCTP, dTTP and digoxigenin-labeled deoxyuridine-triphosphate (duTP) (Boeringer Mannheim #1093088), in the presence of sequenase enzyme, a DNA polymerase (USB #707775). Digoxigenin is a known steroid that occurs naturally in digitalis plants. All pipet tips used for this procedure must be autoclaved and should not be touched without gloved hands. An example of the DNA labeling/synthesis procedure is as follows: An M13 sequencing forward primer (24 Mer), 20μM (New England Biolabs #1224), is annealed to the M 13 DNA by pipetting 30 μL of 20μM forward primer and 120 μL of 5X sequenase buffer (USB #70765) into a Perkin Elmer reaction tube. A mixture is formed by pipetting 5 μL of the annealed M 13 DNA and 10 μL of the single-stranded DNA M 13 circular phage molecules (0.2 μg/μL) into a reaction tube where they are mixed, spun in a centrifuge for about five seconds or less, and placed into a Perkin-Elmer DNA Thermal Cycler machine. The mixture is heated in the machine to 65°C over two minutes, such temperature held for five minutes, and then allowed to cool slowly to about 4°C over fifteen minutes. When cooled, the tube is removed from the machine and placed immediately on ice. While the

primer is annealing, a labeling cocktail mixture is prepared by adding 5.50 μL each of 10μM adenine stock solution, guanine stock solution, and cytosine stock solution, 3.575 μL of lOμM thymidine stock solution, 19.250 μL of 1 μM digoxigenin labeled duTP stock solution, 27.50 μL of 0.1M dithiothreitol, (USB (#70726), and 15.675 μL of distilled water, to a total of 82.50μL. To an empty reaction tube, 56 μL of the enzyme dilution buffer is added. The sequenase enzyme is removed from the freezer where it should be kept, 8 μL thereof is added to the enzyme dilution buffer, and 55 μL of this mixture is added to the labelling cocktail mixture. 5 μL of this prepared labeling cocktail/sequenase solution is added to the iced tube containing M13 DNA with annealed primer, mixed well and pulse spun. The tube is placed into a "floater" in a 37°C water bath, incubated for 45 minutes, removed and placed on ice and immediately 2 μL 0.5M EDTA is added to stop any further reaction. A kit for labeling reactions of 10ng-3μg DNA with digoxigenin-labeled deoxyuridine-triphosphate is commercially available from Boehringer-Mannheim Chemicals, Indianapolis, Indiana, together with instructions for carrying out the procedure. Known agarose gel electrophoresis and modified Southern blot procedures are preferably used to check that the molecular weight of the newly prepare DNA and the efficiency of the digoxigenin labeling are correct. Ultraviolet spectrophotometry is also preferably employed to adjust the concentration of the DNA to lots previously prepared and to ensure its purity. The present invention preferably involves the preparation of an appropriate solid phase support for anti-human IgG antibody. First an 0.05 M Tris buffer, ph 9.5 is made b weighing out 5.789 gm Tris base (Sigma Chemical Co., St. Louis. Mo., T-1503) and 0.334 gm Tris acid (Sigma, T-3253). These materials are added to a 1.0 L graduated cylinder, Q.S. to 1.0 L with distilled water and the pH checked to ensure that it is at 9.5. This buffe is stored at 4°C where it will remain stable for about three months. Preferably the day befor the assay is to be performed, a 2.5 uG/mL solution of the capture antibody, preferably a animal anti-human IgG antibody, such as goat anti-human IgG (Fc specific Jackson #109-006 008 supplied at 1.1 mg/mL), is prepared by adding 25 μL of the stock F(ab)'2 anti-huma IgG (1.1 mg/mL to 11 mL of the 0.05 M Tris coating buffer (1:500 dilution). Using Titerteck 8 channel pipet, 100 μL of the anti-IgG/coating buffer is added to each well of 96-well black, microfluor, microtiter plate. The plate is covered with a plastic plate seale

and incubated for 6 hours at room temperature. The hybridization procedure to carry out the assay of the present invention is described in a document entitled, Genius™, Nonradioactive DNA Labeling and Detection Kit, published by Boehringer-Mannheim, September, 1988, the contents of which are incorporated herein by reference. Specifically, a blocking buffer is prepared by adding 250 mg bovine serum albumin (Sigma A-2153) to 25 mL of the 0.05 M Tris buffer and mixing gently to ensure complete solubilization of the bovine serum albumin in the buffer. The coating solution is now flicked out of the microtiter plate into a sink, the plate is inverted and the tops of the wells blotted with paper towels to dry thoroughly each well. Then, 200 μL of blocking buffer is added to each well, the plate is covered with a plastic plate sealer and incubated at 4°C overnight. Preparatory to performing the assay of the present invention, fresh solutions of washing and sample dilution buffers are prepared by as follows. A 0.01M phosphate buffer solution -10X (hereinafter PBS) is prepared by weighing out the following: 85.0 g. NaCl (Sigma, S-9625) 11.5 g. Na ₂ HPO ₄ (Baker Chemical Co., Phillipsburg, N.J. 3828-5 anhydrous) 2.0 g. KC1 (Sigma, P-4504) 2.0 g. KH ₂ PO ₄ (Sigma, P-5379) 1.0 g. Thimerosal (Sigma, T-5125) The foregoing materials are placed in a 1.0L graduated cylinder, distilled water is added to Q.S. to 1.0L, and the solution mixed. This stock solution of phosphate buffer solution (PBS) should be at pH 6.8. A PBS/TWEEN buffer (0.01M PBS/0.05 % polyoxyethylene-sorbitan monolaureate, (hereinafter Tween)) buffer is prepared by mixing 100 mL of 10X PBS solution with 900 m of distilled water. 0.5 mL of Tween (Sigma, P-1379) is added and mixed gently to avoid foaming. A PBS wash buffer (0.01M PBS) is prepared by mixing 45 mL of stock 10X PBS solution with 450 mL of distilled water. A sample dilution buffer (0.01M PBS/ 0.05 % Tween/ 0.02 % bovine serum albumi (hereinafter BSA)) is prepared by mixing 5.0 mL with 45 mL of distilled water and the

adding 50 μL of Tween and 10 mg BSA (Sigma, A-2135). Standards and controls are prepared as follows from a stock serum sample taken fro a patient with a known high concentration of DNA antibodies: A "high" standard is prepared from by dilution in diluting buffer to a standardize IgG concentration of 825 mg/mL, as by mixing an appropriate amounts of μL of the standar or stock serum in the diluting buffer. A "medium" standard, prepared from the "high" standard to be 60% of the valu of the latter, is made by mixing 600 μL of high standard with 400 μL of diluting buffer. Dilutions are prepared for all controls and standards by adding 10 μL of serum o each to 490 μL of sample dilution buffer (BSA) in respective test tubes. For patients fro a previous run which proved to be strongly positive above the highest standard value, tw dilutions are made, one being a mixture of 50 μL aliquots of normal Lit serum and th patient serum, the other being a mixture of 150 μL normal Lit serum and 50 μL of th patient serum. To two 490 μL aliquots of the sample dilution buffer, 10 μL are added fro each of these dilutions to provide 1:50 dilutions. Similarly, 1:50 dilutions are prepared fo each patient sample to be assayed. The incubated plate, prepared as hereinbefore described is placed on a platewashe and the blocking buffer is aspirated from it. The aspirated plate is then washed wit PBS/Tween buffer a total of six times. The plate is then removed from the washer, blotte with paper towels so that the wells are completely free of buffer. For each standard, contro and patient sample, 100 μL of each is added to respective wells in the plate, and 100 μL o the sample dilution buffer is added to duplicate blank wells. The plate is then covered wit a plate sealer and incubated in a humid chamber at 37°C for 1 1/2 hours. Just prior to th end of the incubation period a 1 :500 dilution is made of the dsDNA-digoxigenin solution b adding 24 μL of stock DNA solution to 12 mL of diluting buffer (0.1 μL/mL) At the en of the incubation period, the diluted sample solution are aspirated and the plate is washe with PBS/Tween buffer. The plate is then blotted and tapped to dry completely. With a channel pipet, 100 μL aliquots of the diluted dsDNA-digoxigenin solution ar added to each well of the plate which is then covered with a plate sealer and again incubate in a humid chamber at 37°C for 1 1/2 hours. At the end of this incubation time, the dilute dsDNA-digoxigenin solution is aspirated from the plate and the plate washed wit PBS/Tween buffer. The plate is then blotted and tapped to dry completely. Just before th

end of the incubation period, a 1 :500 dilution is made of the stock anti-digoxigenin solutio in sample diluent, and 100 μL aliquots of this dilution are added to each well. Again th plate is covered with a plate sealer and again incubated for a third time in a humid chamber at 37°C for 1 1/2 hours. At the end of this third incubation, the diluted stock anti- digoxigenin solution is aspirated from the plate, the plate washed with PBS/Tween buffer and the latter replace then with PBS without Tween. The wash lines are primed to ensur complete removal of the PBS/Tween buffer, the plate washed with PBS without Tween, blotted and tapped to dry completely. During the third incubation period at least 12 mL of the chemiluminescence alkalin phosphatase substrate, such as Lumiphos 530, is removed from refrigeration, allowed t come to room temperature, and 100 μL aliquots thereof added to each dried well after th incubation period. Detection of the digoxigenin-labeled probe is preferably accomplished with an antibody-enzyme conjugate such as anti-digoxigenin-alkaline phosphatase. The latter may be visualized by known enzyme-linked color reactions but in the present invention, is preferably accomplished by chemiluminescent detection. To this end, the support carryin the hybridized probe and bound antibody conjugate, is reacted with a know chemiluminescent substrate for alkaline phosphatase, such as a 1,2-dioxetane commerciall available in a solution known as Lumi-Phos™ 530 from Boehringer Mannheim. The reactio protocol can be carried out as described in a publication entitled Lumi-Phos™ 530 availabl from Lumigen, Inc., a division of Boehringer Mannheim Chemicals. In order to detect th chemiluminescence, it is preferred to use a luminometer such as an Amerlite chemiluminescent detector available from Eastman Kodak Co., Rochester, New York. Immediately following the addition of the chemiluminescent substrate, the plate i placed into a luminometer and run according to the instructions for that particular device, providing readings for each well. The plate is left in the machine and at the end of a 4 minute incubation period, the machine instructed to reread the wells, providing a second se of data. To determine if the reaction is at an endpoint, the initial and final readings of th "medium" and "high" standards are compared and should be within 5 units for each sample Preferably, the machine will provide curve fitting statistics and the mean value for each se of replicate wells for all standards, controls and patients. Normal individuals with have 1 U/mL of dsDNA antibody. A run is acceptable on the Amerlite detector if all of the following criteria are met

(1) the curve fit factor is < 10.0; the light index is > 10.0; and the % difference for each point in the standard curve provided by the luminometer is <5.0, (2) for the controls, the "medium" and "high" controls at the beginning and end of the batch of samples are each within 5 units; replicate wells should have CVs less than 10%; and the mean values of all controls must be within their accepted duality control limits, and (3) for the patient samples, replicate wells should have CVs less than 10%; and values above the highest standard must be repeated on a subsequent run to obtain a value within the standard curve. For patients that are run in dilutions of normal serum, the values may be calculated as follows: For the 1:2 volume dilution:. Reported value in U/mL = A x (B +C)/B where A = U/mL from the standard curve ; B = IgG level in the patient serum; and C = IgG level in the normal control serum. For the 1:4 volume dilution: Reported value = A + (B+3Q/B For all patient samples above the top standard, the next run should be repeated with additional dilutions . It will be appreciated that readings above the top standard are diagnostic of a diseased condition characterized by antibodies formed to dsDNA, most probably systemic lupus erythematosus. It is suggested that as such disease progresses, the extent to which the assay increases beyond the normal U/ml of dsDNA antibody is indicative of the activity of the disease. Since certain changes may be made in the above method without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted in an illustrative and not in a limiting sense.