The invention relates to the generation and the use of flavivirus pseudo- particles containing native functional prM and E envelope glycoproteins assembled onto retroviral core particles. These particles are highly infectious and constitute a valid model of flavivirus virion whose handling does not require category 3 or 4 laboratories.

Flaviviruses are small, icosahedral enveloped viruses that constitute a genus within the family Flaviviridae, which also includes the genera pestivirus and hepacivirus (human hepatitis C viruses). Several flaviviruses are important mosquito-or tick-borne human pathogens, such as the yellow fever, dengue, Japanese encephalitis, West Nile and tick-borne encephalitis (TBE) viruses.

Flavivirus structural proteins and non structural proteins are expressed from a single polyprotein precursor and individually released in their respective cell compartments upon cleavage by cellular and viral protease. By analogy with other members of the Flaviviridae, flavivirus genomic organization suggests a virus consisting of a nucleocapsid comprising a viral genome and core protein (C) coated by a lipid envelop containing the two envelope giycoproteins prM and E. The E protein is the major constituent of the virion surface and has the dual function of binding cell receptors and mediating lowpH-triggered membrane fusion in the endosome. Virus assembly takes place in the endoplasmic reticulum and first leads to the generation of fusion-incompetent, immature virions in which the E protein forms a stable heterodimeric complex with precursor of M (prM). Immature virions are transported through the cellular endocytotic pathway and, shortly before their release, prM is cleaved by furin or a related protease in the trans-Golgi network to generate fusion-competent mature infectious virions.

West Nile virus (WNV), a single-stranded positive-polarity RNA virus, is the etiologic agent of West Nile encephalitis. WNV is maintained in a natural cycle between mosquitoes and birds but also infects humans, horses, and other animals. It is endemic in parts of Africa, Europe, the Middle East, and

Asia (Hubalek and Halouzka, 1999), and outbreaks in the United States over the past 3 years indicate that it has established its presence in the Western Hemisphere (Lanciotti et al., 1999). Humans develop a febrile illness, with a subset of cases progressing to a meningitis or encephalitis syndrome (Hubalek and Halouzka, 1999). Currently, no specific therapy or vaccine has been approved for human use.

The molecular basis of WNV infection in humans and other animals is not clearly established. Although prior rodent models of WNV infection have shown evidence of viral replication in serum and the central nervous system (CNS), many of the mechanistic questions about pathogenesis and the immune response remain unanswered. For example, a peripheral site of replication has not been defined and the mechanism of spread to the CNS remains unclear.

Additionally, a target cell for infection in the CNS has not been definitively identified, and whether tissue injury is related directly to viral infection or to the immune response remains unknown. Finally, although susceptibility to severe WNV infection correlates with an impaired immune system (Hubalek and Halouzka, 1999), the mechanisms by which the innate and adaptive immune responses limit disease have not been established. Experiments with related and unrelated RNA viruses have suggested that antibody may have an important protective role against WNV infection. Passive administration of monoclonal antibodies (MAbs) limits the encephalitis caused by some flaviviruses (Saint Louis encephalitis, Japanese encephalitis, and yellow fever viruses) and nonflaviviruses (Sindbis, murine hepatitis, and lymphocytic choriomeningitis viruses). However, for many of these viruses the mechanism by which antibody attenuates CNS infection has not been clearly demonstrated.

The in vitro properties of MAbs, including isotype, avidity, and neutralization, do not necessarily correlate with protection, as antibodies may limit viral infection through different mechanisms at several stages of pathogenesis. Antibody may decrease viral load in the CNS by limiting hematogenous spread through direct neutralization, complement activation, or increased viral uptake by phagocytic cells. Alternatively, antibodies may act directly in the CNS by preventing replication and spread in neurons.

The invention describes the formation and the use of infectious flavivirus pseudo-particles harboring unmodified prM and E glycoproteins.

Definitions The terms"vecto","cloning vector"and"expression vector'mean the vehicle by which a DNA or RNA sequence (e. g. a foreign gene) can be introduced into a host cell, so as to transform the host and promote expression (e. g. transcription and translation) of the introduced sequence. Vectors typically comprise the DNA of a transmissible agent, into which foreign DNA is inserted.

A common way to insert one segment of DNA into another segment of DNA involves the use of enzymes called restriction enzymes that cleave DNA at specific sites (specific groups of nucleotides) called restriction sites. Generally, foreign DNA is inserted at one or more restriction sites of the vector DNA, and then is carried by the vector into a host cell along with the transmissible vector DNA. A segment or sequence of DNA having inserted or added DNA, such as an expression vector, can also be called a"DNA construcf'. A common type of vector is a"plasmid", which generally is a self-contained molecule of double- stranded DNA, usually of bacterial origin, that can readily accept additional (foreign) DNA and which can readily be introduced into a suitable host cell. A plasmid vector often contains coding DNA and promoter DNA and has one or more restriction sites suitable for inserting foreign DNA. Coding DNA is a DNA sequence that encodes a particular amino acid sequence for a particular protein or enzyme. Promoter DNA is a DNA sequence which initiates, regulates, or otherwise mediates or controls the expression of the coding DNA.

Promoter DNA and coding DNA may be from the same gene or from different genes, and may be from the same or different organisms. A large number of vectors, including plasmid and fungal vectors, have been described for replication and/or expression in a variety of eukaryotic and prokaryotic hosts.

A"coding sequence"or a sequence"encoding"an expression product, such as a RNA, polypeptide, protein, or enzyme, is a nucleotide sequence that, when expressed, results in the production of that RNA, polypeptide, protein, or

enzyme, i. e. , the nucleotide sequence encodes an amino acid sequence for that polypeptide, protein or enzyme.

The term"transfection"means the introduction of a foreign nucleic acid (DNA, cDNA or RNA) into a cell so that the host cell will express the introduced gene or sequence to produce a desired substance, typically a protein coded by the introduced gene or sequence. The introduced gene may include regulatory or control sequences, such as start, stop, promoter, signal, secretion, or other sequences used by a cell's genetic machinery. A host cell that receives and expresses introduced DNA or RNA has been"transformed".

The term"host celrl means any cell of any organism that is selected, modified, transformed, grown, or used or manipulated in any way, for the production of a substance by the cell, for example the expression by the cell of a gene, a DNA sequence, a protein, a virion. In the context of the invention, the host cell is a mammalian cell. In particular, the host cell may be selected from the group consisting of mononuclear macrophages, monocytes or dendritic cells, endothelial cells, fibroblasts, neurone/neuroblastoma cells, hepatocyte/hepatoma cells. Suitable host cells include 293T human embryo kidney cells (ATCC CRL-1573), Huh-7 human hepatocellular carcinoma (Nakabayashi et a/$2), Hela human epithelial adenocarcinoma (CCL-2), PS, LLMK2, SW13, Neuro-2a mouse neuroblastoma (CCL-131), HepG2 human hepatocellular carcinoma (HB-8065), TE671 human rhabdomyosarcoma (CRL-8805), Vero African green monkey kidney (CCL-81), and BHK-21 golden hamster kidney (CCL-10).

As used herein, the term"permissive celP'is meant for a cell that is permissive for flavivirus infection. Examples of permissive cells include the above-mentioned mammalian mononuclear macrophages, monocytes or dendritic cells, endothelial cells, fibroblasts, neurone/neuroblastoma cells, hepatocyte/hepatoma cells.

"Flavivirus"is a genus of the Flaviviridae virus family which also includes pestivirus and hepacivirus. Flavivirus genome encodes a single polyprotein NH2-C-prM-E-NS1-NS2a-NS2b-NS3-NS4a-NS4b-NS5-COOH that is processed co and post-translationally into both structural (N-terminal nucleocapsid protein termed"Core" (C), and glycoproteins prM and E) and

non-structural (NS) proteins. In the context of the invention, flavivirus is of any member, genotype, strain, subtype and variants thereof.

The term"varianf'refers to the homologous polynucleotide sequences and corresponding amino acid sequences found in the different flavivirus strains owing to flavivirus hypervariability.

Tick borne viruses such as tick-borne encephalitis virus or Louping ill virus, and Mosquito-borne viruses such as Dengue Virus (DEN), Yellow Fever Virus (YFV), West Nile Virus (WNV), or japanese encephalitis virus, are examples of flaviviruses.

Preferably, said flavivirus is selected from the group consisting of West Nile Virus (WNV), Dengue Virus (DEN) or Yellow Fever Virus (YFV). Preferably said flavivirus is a West Nile Virus (WNV), still preferably the Israelian strain ISR98-G001). Preferably said flavivirus is a Dengue Virus (DEN), still preferably from the DEN-1 (Fga/89) or from the DEN-2, DEN-3 and DEN-4 strains. Preferably said flavivirus is a Yellow Fever Virus (YFV), still preferably from the 17D-204-Pasteur strain.

The term"flavivirus-like particles"or"flavivirus pseudo-patticles"as used herein refers to non naturally occurring viral particles that comprise an envelope protein of fiavivirus. The flavivirus pseudo-particles of the invention are infectious for a target cell. The particles of the invention more particularly comprise retroviral core proteins. Such particles may be readily produced by one skilled in genetic engineering techniques. One can for instance refer to EP 1 201 750 that describes production of synthetic retroviral particles expressing an antigen for modulating an immune response.

In the context of the invention, the term"infectious"is used to describe the capacity of the particles of the invention to complete the initial steps of viral cycle that lead to cell entry. However, upon interaction with the host cell, flavivirus-like particles may or may not produce progeny viruses.

The term"an envelope protein of flavivirus"denotes the native prM or E glycoprotein of flavivirus, or a mutant thereof.

By an"prM glycoprotein"or"prM proteins meant an envelope prM protein from any genotype, subtype and variant of flavivirus strains.

By an"E glycoprotein"or"E proteins meant an envelope E protein from any genotype subtype and variant of flavivirus strains.

Preferably, prM and E glycoproteins are derived from a same flavivirus strain.

The term"mutanf'or"mutation"is meant for alteration of the DNA sequence that result in a modification of the amino acid sequence of native prM or E proteins. Such a modification can be for instance the substitution and/or deletion of one or more amino acids. Mutants notably include fragments of native prM and E proteins. Variants are particular examples of naturally occurring mutants. Mutants are more particularly contemplated as useful for identifying the structural elements of prM and/or E proteins necessary for maintaining cell infectivity or for increasing prM and/or E antigenicity for vaccination purposes.

As used herein, the term"flavivirus core"denotes a native core protein of the various flavivirus strains, a fragment thereof, or a variant thereof. The flavivirus core provides a signal peptide for the prM or optionally E linked thereto that allows protein translocation to the endoplasmic reticulum. West Nile virus core signal peptide corresponds to the last 22 residues of the carboxy-terminus of WNV core (QKKRGGKTGIAVMIGLIASVGA, SEQ ID No1).

Dengue virus core signal peptide corresponds to the last 20 residues of the carboxy-terminus of DEN-1 core (MNRRKRSVTMLLMLLPTVLA, SEQ ID No2).

Yellow fever virus core signal peptide corresponds to the last 20 residues of the carboxy-terminus of YFV core (RSHDVLTVQFLILGMLLMTG, SEQ ID No 3).

According to an embodiment, the core protein is thus a N-terminally truncated form of flavivirus core (AC) that comprises the flavivirus core signal peptide.

Preferably AC comprises the last 20, preferably the last 22, residues of the carboxy-terminus of flavivirus core. In particular, flavivirus core may consist in the last 60 residues of the carboxy-terminus of flavivirus core.

Upon completion of its addressing function, the core protein is processed by a cellular protease and thereby cleaved from the polyprotein prME. Accordingly, the flavivirus core protein is not found in the pseudo- particles according to the invention.

In the context of the invention, the terms"native"or"unmodified'are indifferently used to describe a wild-type, full-length protein.

The term"polyprotein"as used herein is used to describe a protein construct made up of individual proteins that are joined together in a sequence whereby they retain their original relevant biological activities.

The term"a polyprotein comprising a flavivirus core protein linked to flavivirus prM protein andlor flavivirus E protein"or"a polyprotein comprising successively a flavivirus core protein, and a flavivirus prM protein andlor a flavivirus E protein"includes the CprME, CEprM, CprM, CE, ACprME, ACEprM, ACprM, and ACE polyproteins."CprME"denotes a polyprotein comprising successively a flavivirus core protein, a flavivirus prM protein and a flavivirus E protein."CEprM'denotes a polyprotein comprising successively a flavivirus core protein, a flavivirus E protein and a flavivirus prM protein."CprM'denotes a polyprotein comprising a flavivirus core protein linked to a filavivirus prM protein."CE"denotes a polyprotein comprising a flavivirus core protein linked to a flavivirus E protein."ACprME'denotes a polyprotein comprising successively a carboxy terminus of flavivirus core protein, and flavivirus prM and flavivirus E proteins."ACEprM"denotes a polyprotein comprising successively a carboxy terminus of flavivirus core protein, and fiavivirus E and fiavivirus prM proteins.

"ACprM"denotes a polyprotein comprising a carboxy terminus of flavivirus core protein linked to a flavivirus prM protein."ACF'denotes a polyprotein comprising a carboxy terminus of flavivirus core protein linked to a flavivirus E protein. ACprME, as well as ACE, have been built by inserting a stop codon at the end of E, whereas ACEprM and ACprM have been built by inserting a stop codon at the end of prM.

By"retrovirus"is meant a virus whose genome consists of a RNA molecule and that comprises a reverse-transcriptase, ie. a member of the Retroviridae family. Retroviruses are divided into Oncovirus, Lentivirus and Spumavirus. Preferably said retrovirus is an oncovirus, e. g. MLV, ALV, RSV, or MPMV, a lentivirus, e. g. HIV-1, HIV-2, SIV, EIAV, or CAEV, or a spumavirus such as HFV. Genomes of these retroviruses are readily available in databanks.

In the context of the invention"a nucleic sequence comprising a packaging competent retrovirus-derived genome"is intended for a sequence that comprises the retroviral nucleic acid sequences known as"cis-acting" sequences. These include the Long Terminal Repeats (LTRs) for the control of transcription and integration, the psi sequence necessary for encapsidation, and the Primer Binding site (PBS) and polypurine track (PPT) sequences necessary for reverse transcription of the retroviral genome. Advantageously, said nucleic acid sequence comprising a packaging competent retrovirus- derived genome further comprises a transgene.

Said retroviral genome may be replication-defective or replication- competent, in the absence of any trans-complementing function. A replication- competent genome would further comprise the gag, pol, and env retroviral genes. In a replication-defective genome, the viral genes gag, pol, and env are deleted. However, assembly of viral pseudo-particles may be achieved by providing another vector that comprises gag, pol and env but that is defective for the"cis"sequences. Their expression allows the encapsidation of the transgene, excluding the genes necessary for the multiplication of the viral genome and for the formation of complete viral particles.

As used herein, the term"transgene"designates the gene that is expressed in the target cell upon infection by the particles of the invention.

Examples of transgenes include a gene encoding a molecule of therapeutic interest, a marker gene, a gene coding for an immune modulator, an antigen, or a suicide gene.

A"marker gene"denotes a gene whose expression is detectable. For instance marker gene expression can generate a detectable signal, such as a fluorescence emission, a chromogenic reaction, or confer a growth advantage to the cells wherein it is expressed (antibiotic resistance genes).

An"immune modulatoK'refers to the product of a gene that modifies the activity of the immune system of a subject in vivo. Examples of immune modulators include cytokines, (e. g. interleukins, interferons, or haematopoietic colony stimulating factors), chemokines, and the like. Expression of an immune modulator by transformed cells may change the cellular environment and alter

differentiation of immune cells and thus modify the type and the strength of immune response elicited against a given antigen.

An"antigen"refers to a molecule, such as a peptide, a polypeptide or a protein, against which an immune response is sought. Said antigen may be for instance a tumor, a bacterial, a pathogenic, a proteic, or a viral antigen.

A"suicide gene"is meant for a gene whose expression in cells induces programmed-cell death (apoptosis) such as the conditional Herpes Simplex virus type I thymidine kinase gene.

The"core protein from a retrovirus"refers to proteins encoded by the gag and pol genes. The gag gene encodes a polyprotein which is further processed by the retroviral protease into structural proteins that comprise the core. The pol gene encodes the retroviral protease, reverse-transcriptase, and integrase. Expression of the gag gene alone is sufficient to induce the release of core particles ; yet such particles are not infectious as a result of absence of the pol gene-encoded retroviral replication enzymes. Therefore, the"core protein from a retrovirus"may denote proteins encoded by the gag and pol genes or by the gag gene alone.

A"pharmaceutically acceptable carrier"refers to any vehicle wherein the vaccine composition according to the invention may be formulated. It includes a saline solution such as phosphate buffer saline. In general, a diluent or carrier is selected on the basis of the mode and route of administration, and standard pharmaceutical practice.

In the context of the present application,"vaccination"is intended for prophylactic or therapeutical vaccination."Therapeutical vaccination"is meant for vaccination of a patient with flavivirus infection.

According to the invention, the term"subjecf'or"patient"is meant for any mammal, human or animal, likely to be infected with flaviviruses.

Production of flavivirus particles The inventors have generated infectious pseudo-particles that contain functional, and more particularly unmodified, flavivirus glycoproteins assembled onto retroviral core particles. Flavivirus prME are expressed from a polyprotein containing the core (C) protein or a fragment thereof, in particular the carboxy-

terminus of the C protein, which served as signal peptide for prM or E, and the prM and/or E glycoproteins.

The invention thus provides a method for producing flavivirus (FV)-like particles ex vivo comprising the steps of: - providing a first nucleic acid sequence comprising a packaging competent retrovirus-derived genome; - providing a second nucleic acid sequence comprising a cDNA encoding the core proteins from said retrovirus; - providing a third nucleic acid sequence comprising a cDNA encoding a polyprotein comprising successively a flavivirus core protein, and a flavivirus prM protein and/or a flavivirus E protein; - transfecting host cells with said nucleic acid sequences and maintaining the transfected cells in culture for sufficient time to allow expression of the cDNAs to produce structural proteins from flavivirus and retrovirus ; and allowing the structural proteins to form virus-like particles.

Said cDNA encoding the core proteins from said retrovirus may contain the gag and pol genes, or alternatively the gag gene alone.

The invention further provides a method for producing flavivirus (FV)-like particles il1 vivo, which method comprises the steps of : -providing a first nucleic acid sequence comprising a packaging competent retrovirus-derived genome; - providing a second nucleic acid sequence comprising a cDNA encoding the core proteins from said retrovirus; - providing a third nucleic acid sequence comprising a cDNA encoding a polyprotein comprising successively a flavivirus core protein, and a flavivirus prM protein and/or a flavivirus E protein; - transfecting cells of a subject in vivo with said nucleic acid sequences, to allow expression of the cDNAs to produce structural proteins from flaviviruses and retrovirus; and to allow the structural proteins to form virus-like particles.

Another aspect of the invention is the use of three nucleic acid sequences for the preparation of a medicament useful as a vaccine against a flavivirus, wherein the nucleic acid sequences are:

- a first nucleic acid sequence comprising a packaging competent retroviral-derived genome; - a second nucleic acid sequence comprising a cDNA encoding core proteins from said retrovirus; - a third nucleic acid sequence comprising a cDNA encoding a polyprotein comprising successively a flavivirus core protein, and a flavivirus prM protein and/or a flavivirus E protein; and, when transferred into cells of a subject, the nucleic acid sequences allow the production of structural proteins from flaviviruses and retrovirus, wherein the structural proteins form virus-like particles that are immunogenic.

For the purpose of transfection, said first, second and third nucleic acid sequences may be carried on a same vector, or on two or three separated vectors.

In particular, plasmoviruses, adenoretroviruses, replicating pseudo- viruses, and replicating virus vectors derived from measle virus, are examples of vectors suitable for carrying the above-mentioned sequences. A plasmovirus vaccine consists in such a plasmid DNA preparation, that allow expression of flavivirus pseudo-particles after administration in an patient in order to elicit a immune response against said flavivirus. Administration of such a plasmovirus vaccine being achieved for preventive vaccination into people at risk for flavivirus-induced disease or for therapeutic vaccination into flavivirus-infected patients. Adenoretroviruses consist in an alternative way to provide the above- mentioned nucleic acid sequences encoding flavivirus pseudo-particles. In this case, it is possible to design three independent adenoretroviruses, i. e. recombinant adenoviruses, that encode the three nucleic acid sequences mentioned above (retroviral core and genome and flavivirus glycoproteins), or, alternatively, it is also possible to design a single adenoretrovirus, derived from "guttless"recombinant adenoviruses, that contains the different nucleic acid sequences. Such adenoretroviruses can be administered to patient as for plasmoviruses, in order to elicit an anti-flavivirus immune response. Replicating pseudo-retroviruses are another alternative possibility to express all the above- mentioned nucleic acid sequences encoding the flavivirus pseudo-particles.

Such structures are in fact flavivirus-pseudo-particles whose genome is

engineered to allow, following infection, its propagation into cells of an inoculated patient, thereby inducing the production of further replicating flavivirus pseudo-particles. In this case the genome of a retrovirus is modified so as to express the flavivirus E and prM glycoproteins in place of the retroviral Env gene (encoding the retroviral glycoproteins). The genes encoding the retroviral core proteins are left unchanged. Furthermore an additional gene, encoding a marker gene or an immunomodulator, for example, can be expressed from this genome. Measle virus (MV) vectors are derived from vaccinal strains of MV and harbour a replication-competent MV genome containg an additional gene encoding an immunogen. Replication of such recombinant MV elicits a strong immune response against this immunogen (Combredet et al., 2003) as shown, for example, in mice inoculated with MV vectors expressing HIV-1 antigens that induce strong neutralizing antibody and cellular immune response to HIV (Lorin et al., 2004). In the context of this invention, MV-derived vectors harbour, as additional gene, a construct that express the flavivirus prME protein as well as the retroviral core proteins encoded by the gag and pol gene, or, alternatively, the gag gene only.

Replication of this MV-based vector can induce the sustained secretion of fiavivirus pseudo-particles that, in turn, can elicit a vigourous immune response against fiavivirus proteins.

An example of flavivirus prME and retroviral expression constructs is shown in figures 1A and 1B. Preferably, said flavivirus is selected from the group consisting of West Nile Virus, Dengue Virus and Yellow Fever Virus.

According to a specific embodiment, said packaging competent retroviral genome and core proteins are derived from a retrovirus selected from the group consisting of MLV, ALV, RSV, MPMV, HIV-1, HIV-2, SIV, EIAV, CAEV, and HFV.

Advantageously, the packaging competent retroviral genome further comprises a marker gene or an immune modulator.

In the method of the invention, said polyprotein may comprise a flavivirus core protein linked to a flavivirus prM protein, or a flavivirus core protein linked to a flavivirus E protein, or successively a flavivirus core protein,

a flavivirus prM protein and a flavivirus E protein, or successively a flavivirus core protein, a flavivirus E protein and a flavivirus prM protein.

According to an embodiment, prM and/or E are native proteins.

According to another embodiment, prM and/or E glycoproteins are mutated to obtain particles that are useful for characterizing the glycoprotein determinants for flavivirus infectivity.

Preferably, said prM and E glycoproteins are both derived from a same flavivirus strain.

According to another embodiment said flavivirus core protein is a carboxy terminus form (AC) of flavivirus core protein that comprises the core protein signal peptide. In particular, said flavivirus core protein may comprise the last 20, preferably the last 22, amino acids of the carboxy-terminus of flavivirus core.

In accordance with the present invention there may be employed conventional molecular biology, microbiology, and recombinant DNA techniques within the skill of the art. Such techniques are explained fully in the literature. See, e. g., Sambrook et al., 1989 ; DNA Cloning : A Practical Approach, Volumes I and 11 (D. N. Glover ed. 1985); Oligonucleotide Synthesis (M. J. Gait ed. 1984) ; Nucleic Acid Hybridization [B. D. Hames U ; S. J. Higgins eds. (1985)] ; Transcription and Translation [B. D. Hames & S. J. Higgins, eds.

(1984)] ; Animal Cell Culture [R. I. Freshney, ed. (1986)] ; Immobilized Cells and Enzymes [IRL Press, (1986)] ; B. Perbal, A Practical Guide To Molecular Cloning (1984); F. M. Ausubel et al., 1994.

In particular, the vectors of the invention may be introduced into the target cell by means of any technique known for the delivery of nucleic acids to the nucleus of cells, either in culture, ex vivo, or in vivo.

Introduction of the nucleic acid sequences may be performed by any standard method well known by one skilled in the art, e. g. transfection, electroporation, microinjection, transduction, cell fusion, DEAE dextran, calcium phosphate precipitation, or use of a gene gun (see for instance Wu et al., 1992 ; Wu et al, 1988).

The donor nucleic acid targeting system can also be introduced by lipofection. In certain embodiments, the use of liposomes and/or nanoparticles

is contemplated for the introduction of the donor nucleic acid targeting system into host cells. Nanocapsules can generally entrap compounds in a stable and reproducible way. Ultrafine particles (sized around 0.1 pm) that can be designed using biodegradable polyalkyl-cyanoacrylate polymers are contemplated for use in the present invention, and such particles may be are easily made.

Liposomes are formed from phospholipids that are dispersed in an aqueous medium and spontaneously form multilamellar concentric bilayer vesicles (also termed multilamellar vesicles (MLVs) ). MLVs generally have diameters of from 25 nm to 4 um. Sonication of MLVs results in the formation of small unilamellar vesicles (SUVs) with diameters in the range of 200 to 500 A, containing an aqueous solution in the core. The use of cationic lipids may promote encapsulation of negatively charged nucleic acids, and also promote fusion with negatively charged cell membranes (Feigner et al., 1989).

In vivo targeted gene delivery is described in international patent publication WO 95/28 494. Alternatively, the vector can be introduced in vivo by lipofection, using liposomes or nanoparticles as above described. It is also possible to introduce the vector in vivo using techniques that are similar to the techniques that are employed in vitro (e.g. transfection, electroporation...).

Transformed cells The invention further relates to a transformed host cell that contains : - a first nucleic acid sequence comprising a packaging competent retrovirus-derived genome; - a second nucleic acid sequence comprising a cDNA encoding the core proteins from said retrovirus; and - a third nucleic acid sequence comprising a cDNA encoding a polyprotein comprising successively a flavivirus core protein, and a flavivirus prM protein and/or a flavivirus E protein.

The core proteins from said retrovirus may be encoded either by the gag and pol genes or by the gag gene alone.

Such a transformed host cell is obtainable as described in a method above.

In another aspect, the invention relates to the use of a transformed host cell as defined above, for the identification of molecules capable of interfering with flavivirus entry in cells. The invention provides in particular a method of ex vivo screening or identification of molecules capable of interfering with flavivirus entry in cells comprising comparison of the level of transformed host cell fusion to a target host cell, in the presence or the absence of a candidate molecule.

Said method preferably comprises the steps consisting of: - co-culturing a transformed host cell with a target host cell, in the absence or presence of a candidate molecule, under conditions that allow syncytia formation, i. e. cell-cell fusion, and flavivirus-like particle entry in target host cell in the absence of any candidate molecule ; - assessing syncytia formation in the absence and in the presence of said candidate molecule ; - comparing syncytia formation measured in presence of said candidate molecule with syncytia formation measured in absence of any candidate molecule ; - identifying as a molecule capable of interfering with flavivirus entry the candidate molecule for which syncytia formation, as measured in the presence of said molecule, is decreased as compared to syncytia formation measured in the absence of any candidate molecule.

Contacting a transformed host cell with a target host cell, and a candidate molecule can be carried out by contacting simultaneously said transformed host cell, target host cell and candidate molecule. Otherwise, two of these three elements can be contacted under conditions sufficient to allow their interaction before addition of the third missing element.

Preferably said target host cell is not transformed, Le. said target host cell does not contains at least one of the first, second, and third nucleic acid sequence as defined above.

Syncytia formation can be readily assessed by one skilled in the art.

Briefly, the coculture is submitted to a acidic pH drop by incubation for 5 min at pH-5 and incubated in a nomal medium for an additional 12 hrs. Cultures are then stained by adding the May-Grunwald and Giemsa solutions (MERCK) according to the manufacturer recommendations. Cells containing two or more

nuclei can be defined as syncytia. A fusion index is then defined as the percentage of (N-S) /T where N is the number of nuclei in the syncytia, S is the number of syncytia and T is the total number of nuclei counted.

Flavivirus-like particles In the method described above no structural modifications of the prME glycoproteins are required for their correct assembly on retroviral cores. The method of the invention thus makes it possible to generate high titre infectious flavivirus pseudo-particles with functional prME glycoproteins. As demonstrated herein, these particles constitute a valid model of flavivirus virion as regards to early steps of viral infection cycle.

The invention further relates to an infectious flavivirus-like particle, comprising the core proteins from a retrovirus, and prM and/or E flavivirus glycoprotein (s). Such a particle is obtainable by a method as described above.

According to an embodiment, the infectious particle of the invention may comprise native flavivirus prM protein, native flavivirus E protein, or native flavivirus prM protein and native flavivirus E protein. Preferably said prM and E glycoproteins are both derived from a same flavivirus strain. According to another embodiment, priv ! and/or E glycoproteins are mutated.

Said retrovirus may be selected from the group consisting of ALV, ALV, RSV, MPMV, HIV-1, HI\J-2, Sl\/, EIAV, CAEV, and HFV.

Advantageously, said infectious particles further carry a transgene. For instance said transgene may be a marker gene which make it possible to follow-up cell infection by the infectious particles of the invention and can find application for instance in the identification of a cell receptor involved in flavivirus entry. Said transgene can also be a gene encoding a molecule of therapeutic interest and/or a suicide gene. Accordingly, the particles of the invention that specifically target mammalian mononuclear macrophages, monocytes or dendritic cells, endothelial cells, fibroblasts, neurone/neuroblastoma cells, hepatocyte/hepatoma cells comprise a useful vector for gene transfer and/or gene therapy.

Use of the infectious flavivirus-like particles of the invention High infectivity of these particles makes it possible for the investigation of the role of flavivirus prM and E glycoproteins and their potential receptors in cell entry, flavivirus host-range and neutralisation by antibodies from flavivirus patient sera.

The invention therefore concerns the use of a flavivirus-like infectious particle as described above, for ex vivo identification of a cell receptor for flavivirus prM and/or E glycoprotein.

According to an embodiment, the invention provides a method for ex vivo identification of a receptor for flavivirus prM and/or E glycoprotein comprising detection of the binding of said particle to a cell receptor. More specifically, the method may comprise the steps consisting of: - contacting a cell susceptible to flavivirus infection with an infectious flavivirus-like particle of the invention, under conditions sufficient to allow specific binding of said particle to a receptor expressed at the surface of said cell ; -detecting binding of said particle to a receptor; and - identifying said receptor.

A cell susceptible to flavivirus infection may preferably be selected from the group consisting of mammalian mononuclear macrophages, monocytes or dendritic cells, endothelial cells, fibroblasts, neurone/neuroblastoma cells, hepatocyte/hepatoma cells.

Detection of particle binding to a receptor can be achieved according to classical procedures well-known by one skilled in the art. For instance, this could involve radioactive, enzyme or fluorescent labelling of the particles of the invention, and subsequent detection with an appropriate method. A number of fluorescent materials are known and can be utilized as labels. These include, for example, fluorescein, rhodamine, auramine, Texas Red. Enzyme labels consist in conjugation of an enzyme to a molecule of interest, e. g. a polypeptide, and can be detected by any of colorimetric, spectrophotometric, or fluorospectrophotometric techniques. Flow cytometry analysis (FACS) together with labelled antibodies directed against prM or E proteins harboured by the pseudo-particles of the invention is also appropriate.

According to another embodiment, the invention provides a method for ex vivo identifying a cell receptor for flavivirus comprising the step consisting of: - transfecting a cell which is not permissive for flavivirus infection with a nucleic acid sequence encoding a protein likely to be a flavivirus receptor; - contacting said transformed cell with a flavivirus-like particle of the invention; - determining whether said transformed cell has become permissive or not for flavivirus infection; and - identifying as a cell receptor for flavivirus said protein expressed by the transformed cell that has become permissive.

Determination of whether the transformed cell has become permissive for flavivirus infection can be readily achieved using the flavivirus-like particles of the invention. In particular, where said particles carry a marker gene, such as GFP, permissivity (ive. the capacity of cells to be infected with flaviviruses, and in particu) ar with ftavivirus-iike partides) can be assessed by FACS analysis of the transformed cells. Where the marker gene is an antibiotic resistance gene, identification of cells infected by flavivirus-like particle is readily achieved through exposure to said antibiotic.

Where one does not suspect a given protein to be a receptor for flavivirus entry in cells, the above method can advantageously be adapted for the screening and the identification of a cell receptor for flaviviruses. In particular, an expression cDNA library can be prepared, for instance from a cDNA library obtained by reverse-transcription of cellular mRNAs from a cell permissive for flavivirus infection. Expression of such a cDNA library would be driven by a constitutive promoter whose nucleic acid sequence has been fused to the cDNA library in suitable vectors. Such a library would contain a vector encoding a cell receptor for flaviviruses. Non permissive cells can then be transfected with this expression library and further screened for the identification of a cell receptor for flaviviruses.

To this end, the invention proposes a method for ex vivo identifying a cell receptor for flavivirus comprising the step consisting of: - providing an expression cDNA library obtained from a cell permissive for flavivirus infection;

- transfecting cells that are not permissive for flavivirus infection with said expression cDNA library; - contacting said transformed cells with flavivirus-like particles of the invention; - identifying and isolating those transformed cells that have become permissive for flavivirus infection; - isolating the expression vector transfected in cells that have become permissive; and - identifying as a receptor for flavivirus the proteins encoded by the cDNA sequence of said isolated expression vectors.

Determination of whether the transformed cell has become permissive for flavivirus infection can be readily achieved using the flavivirus-like particles of the invention. In particular, where said particles carry a marker gene, such as GFP, permissivity (i. e. the capacity of cells to be infected with flaviviruses, and in particular with flavivirus-like particles) can be assessed by FACS analysis of the transformed cells. Where the marker gene is an antibiotic resistance gene, identification of cells infected by flavivirus-like particle is readily achieved through exposure to said antibiotic.

Advantageously, the expression cDNA library is expressed from retroviral vectors that comprises glycoproteins that allow infection of the flavivirus non permissive cells. Such glycoproteins can be the VSV-G glycoprotein derived from vesicular stomatitis virus (VSV) whose receptor is expressed in most cell types ex vivo. Such viral particles can be assembled using a packaging competent retrovirus-derived genome that comprises the expression cDNA library, and optionally a marker gene. According to this embodiment the method for isolating the expression vector expressed in cells that have become permissive to infection by the flavivirus-like particles of the invention is greatly facilitated. Indeed this latter embodiment is particularly advantageous in that the process of cell infection with retroviral vectors has greater efficacy, as compared to cell transfection. Furthermore, cell infection leads to stable integration of viral genome in the cellular genome. Accordingly, transgenes, i. e. cDNA and marker gene that are carried by the pseudo-particles of the invention, are found to be stably expressed by infected cells. This is in

contrast with classical vectors used for transfection that do not integrate into cellular genome and for which expression may be transient.

In another aspect, the invention relates to the use of an infectious particle as defined above, for the identification of molecules capable of interfering with flavivirus entry in cells. In particular, herein is provided a method of ex vivo screening or identification of molecules capable of interfering with flavivirus entry in cells comprising comparison of the level of cell infection by the particles of the invention in the presence or the absence of a candidate molecule. Said method preferably comprises the steps consisting of: - contacting a cell susceptible to flavivirus infection with an infectious flavivirus-like particle, in the absence or presence of a candidate molecule, under conditions that allow cell infection with flavivirus-like particle in the absence of any candidate molecule ; - assessing cell infectivity in the absence and in the presence of said candidate molecule ; -comparing cell infectivity measured in presence of said candidate molecule with cell infectivity measured in absence of any candidate molecule ; - identifying as a molecule capable of interfering with flavivirus entry the candidate molecule for which cell infectivity, as measured in the presence of said molecule, is decreased as compared to cell infectivity measured in the absence of any candidate molecule.

Contacting a cell susceptible to flavivirus infection with an infectious flavivirus-like particle, and a candidate molecule can be carried out by contacting simultaneously said cell, flavivirus-like particle and candidate molecule. Otherwise, two of these three elements can be contacted under conditions sufficient to allow their interaction before addition of the third missing element.

Cell infectivity can be readily assessed by one skilled in the art. One can take advantage of the embodiment wherein the infectious flavivirus-like particle carries a detectable marker gene to detect cell infection. In a preferred embodiment, the marker gene is a fluorescent marker gene, such as GFP, and the infection is detected by means of fluorescence measurement, for instance by flow cytometry analysis of cells contacted with said infectious particles.

A cell suitable to be used in the method of identification of molecules interfering with flavivirus cell entry may be any permissive cell for flavivirus infection, as described above.

Such molecules capable of interfering with flavivirus entry in cells may constitute new antiviral drugs.

The infectious particles of the invention are further useful for diagnosis of flavivirus infection and follow-up of flavivirus infection, for instance to assess efficacy of a therapy in a patient.

The invention thus concerns the use of an infectious flavivirus-like particle for the in vitro detection of antibodies directed against flavivirus in a biological sample from a subject susceptible to be infected with flaviviruses.

Said biological sample may be a biological fluid, such as blood or serum, or a tissue biopsy. In a specific embodiment, said antibodies are directed against prix and/or E flavivirus glycoproteins.

Accordingly, the invention provides a method of in vitro diagnosis of a flavivirus infection in a patient comprising detecting immune complexes formed by interaction of anti-flavivirus antibodies likely to be present in a biological sample of the patient, with flavivirus-like particle of the invention. Said method may in particular comprise the steps consisting of : - contacting a biological sample with an infectious flavivirus-like particle of the invention under conditions sufficient to allow formation of complexes by binding of said infectious particle to antibodies directed against flavivirus present in the biological sample ; - detecting said complexes, which presence is indicative of a flavivirus infection.

The presence of antibodies reactive with flavivirus-like particles can be detected using standard electrophoretic and immunodiagnostic techniques, including immunoassays such as competition, direct reaction, or sandwich type assays. Such assays include, but are not limited to, Western blots ; agglutination tests; enzyme-labeled and mediated immunoassays, such as ELISAs ; biotin/avidin type assays; radioimmunoassays; immunoelectrophoresis ; immunoprecipitation, etc. The reactions generally

include revealing labels such as fluorescent, chemiluminescent, radioactive, enzymatic labels or dye molecules, or other methods for detecting the formation of a complex between the flavivirus-like particle and the antibody or antibodies reacted therewith.

In another embodiment, said method of in vitro diagnosis of a flavivirus infection in a patient comprises detecting an inhibitory effect of anti-flavivirus antibodies likely to be present in a biological sample of the patient, on the infection of a permissive cell by a flavivirus-like particle of the invention. Said method may in particular comprise the steps consisting of: -contacting a cell permissive for flavivirus infection with a flavivirus-like particle and a biological sample ; - comparing cell infectivity measured in presence of said biological sample with cell infectivity measured in absence of said biological sample ; - detecting the inhibition of flavivirus-like particle infection of a permissive cell as a decrease in cell infectivity measured in presence of said biological sample compared with cell infectivity measured in absence of said biological sample, said inhibition being indicative of a flavivirus infection.

This embodiment is advantageous in that the method relies on the detection of the specific antibodies that are neutralizing for cell infection, that is those patient's antibodies that are effective against viraemia.

In a further embodiment of this invention, commercial diagnostic kits may be useful to carry out the above diagnosis methods, by detecting the presence or absence of immune complexes formed by flavivirus particles and antibodies directed against flavivirus in a biological sample from a subject susceptible to be infected with flaviviruses, or by detecting an inhibition of flavivirus-like particle infection of a permissive cell by anti-flavivirus neutralizing antibodies likely to be present in a biological sample of the patient. Such kits may comprise at least a flavivirus-like particle of the present invention. Where the method involves detection of immune complexes, the kits may further comprise appropriate means of detection of said immune complexes.

Preferably the kit of the invention further comprises directions, and protocols, depending upon the method selected, e. g. ,"competitive","sandwich", and the

like. The kits may also contain peripheral reagents such as buffers, stabilizers, etc..

In another aspect of the invention, the infectious flavivirus-like particles may be used for vaccination purposes.

According to an embodiment, the invention thus proposes a method of vaccination, notably against flavivirus infection, that comprises administration of a flavivirus-like particle to a subject in need thereof. The invention also relates to a vaccine composition comprising a flavivirus-like particle and a pharmaceutical acceptable carrier. The invention further provides an immunogenic composition comprising in a pharmaceutical acceptable carrier, a flavivirus-like particle disclosed herein.

The vaccine and immunogenic compositions of the invention may be drawn to confer immunity, or elicit an immune response against flaviviruses.

However, where the flavivirus-like particles of the invention further carry an additional gene encoding another antigen, different from flavivirus antigens, the invention provides a recombinant viral vaccine useful to raise an immune response against said antigen. Actually, the use of pseudo-particles described herein makes it possible to improve the elicited immune response through combining several presentations and processing pathways of an antigen. For instance, a vaccine composition of the invention, when administered, results in the flavivirus-like particles infecting cells of the host. The transgene encoding the antigen is then integrated in the cellular genome, and subsequently expressed by the cell, such that there is both a cellular and a humoral immune response elicited by the vaccine composition.

Advantageously, the flavivirus-like particles may further carry a transgene encoding an immune modulator, which allows for enhancement of the raised immune reaction.

The vaccination or immunogenic composition of the present invention may additionally contain an adjuvant. A number of adjuvants are known to those skilled in the art. Examples of suitable adjuvants include, for example, include aluminum hydroxide; Saponin; detergents such as Tween 80; animal, mineral or vegetable oils, Corynebacterium or Propionibacterium-derived

adjuvants; Mycobacterium bovis (Bacillus Calmette and Guerinn, or BCG); cytokines; acrylic acid polymers such as carbomer; EMA; or combinations thereof.

The route of administration is any conventional route used in the vaccine field. As general guidance, a vaccine composition of the invention is administered via a mucosal surface, e. g. , an ocular, intranasal, pulmonary, oral, intestinal, rectal, vaginal, and urinary tract surface; or via a parenteral route, e. g. , by an intravenous, subcutaneous, intraperitoneal, intradermal, intraepidermal, or intramuscular route. The choice of administration route depends on the formulation that is selected.

In still another embodiment the particles of the invention may be used as vectors for gene transfer and/or gene therapy. Gene therapy is defined as the introduction of genetic material into a cell in order to either change its phenotype or genotype. Owing to their tropism for either primary or cancerous permissive cells, the flavivirus-like particles described herein comprise an efficient gene delivery system amenable to scale up for reproducibly producing large titers of infectious, replication-defective flavivirus-like particles.

Accordingly, the invention relates to a method for v/vo or/n ro transferring a transgene of interest in a cell, which method comprises infecting a cell with a flavivirus-like particle of the invention, wherein the particle carries a transgene of interest.

The invention further relates to the use of a flavivirus-like particle of the invention, that carries a transgene of interest, for the preparation of a medicament for the prevention or treatment of a disease in a patient, wherein the flavivirus-like particle allows the transfer of the transgene of interest into a cell of the patient, and encodes a product that has a prophylactic or therapeutic effect against the disease.

Preferably, the targeted cell is selected from the group consisting of mammalian mononuclear macrophages, monocytes or dendritic cells, endothelial cells, fibroblasts, neurone/neuroblastoma cells, hepatocyte/hepatoma cells.

The invention will be further understood in view of the following examples and the annexed figures.

LEGEND TO THE FIGURES : Figures 1A and 1 B show flavivirus prME and retroviral expression constructs. (A): A cDNA derived from a flavivirus polyprotein gene was used to express the prM and E glycoproteins and the carboxy-terminus of the C protein, which provides the signal peptide for prM (SP prM). The position of the stop codon (star) inserted in the expression constructs to terminate translation of the proteins is shown. The transmembrane domain (TMD) of prM provides the signal peptide (SP E) for the E glycoprotein. NTR, non-translated region;. (B) The expression constructs encoding the different components required to assemble infectious pseudo-particles are shown. The filled boxes represent the viral genes and the marker gene (GFP) transferred to the infected cells. The open boxes show the cis-acting sequences. LTR, long terminal repeat ; CMV, human cytomegalovirus immediate-early promoter; PBS, primer binding site ;, packaging sequence; PPT, poly-purine track; polyA, polyadenylation site.

Vector particles were produced by co-transfection of plasmids harboring the packaging functions, the transfer vector and the viral glycoproteins (GP) into 293T cells. The viral GPs were the West Nile Virus, Dengue virus and Yellow fever virus prM and E glycoproteins, expressed a ACprME polyprotein (Fig.

6A), the VSV-G or the RD114 glycoproteins. The supernatants of transfected cells were collected during transient expression, concentrated by ultracentrifugation, and used for target cell transduction.

Figure 2 depicts the results of infectivity of FVpp generated without prM and E glycoproteins (no prME), without retroviral core proteins (no core), with MLV-G2A assembly-defective core proteins (G2A core mutant), with prM and E glycoproteins expressed expressed from the same vector in cis (core prME).

FVpp were treated with 25 uM AZT (3'-azido-3'-deoxythymidine ; Sigma-Aldrich, France) (core prME + AZT) or with 1/50 diluted sera from animals immunized with prM-E immunogens (core prME + N-serum) before and during infection of target cells. Infectious titres, expressed as TU/ml, were determined on Vero

target cells and are displayed as mean standard deviations of up to four experiments.

EXAMPLES: EXAMPLE 1 : Generation of flavivirus pseudo-particles (FVpp) Flavivirus pseudo-particles (FVpp) were generated by assembling full- length, unmodified prM and E glycoproteins onto retroviral core proteins derived from murine leukemia virus (MLV) (Figure 1).

Construction of expression vectors encoding the viral components, ie., prM, E, or prME glycoproteins and viral core proteins Plasmids expressing wild type prME polyproteins were constructed by standard methods (Sambrook et al., 1989).

Briefly, the phCMV-AC prME-WNV expression vector encoding the prM and E glycoproteins from West Nile Virus was generated by inserting a blunt- ended fragment encoding the last 22 residues of flavivirus core (C) and all of prM and E proteins into the BamHI digested and Klenow blunted vector phCMV-G (Negre et al., 2000).

The sequence of the ACprME open reading frame of West Nile Virus is shown in SEQ ID No 4, whereas the aminoacid sequence if its ACprME polyprotein is shown in SEQ ID No 5.

The sequence of the ACprME open reading frame of Dengue Virus is shown in SEQ ID No 6, whereas the aminoacid sequence if its ACprME polyprotein is shown in SEQ ID No 7.

The sequence of the ACprME open reading frame of Yellow Fever Virus is shown in SEQ ID No 8, whereas the aminoacid sequence if its ACprME polyprotein is shown in SEQ ID No 9.

Expression vectors for prME glycoproteins of Yellow Fever Virus were constructed by similar strategies.

Flavivirus prME glycoproteins were therefore expressed from a polyprotein containing the carboxy-terminus of the core (AC) protein, which

served as signal peptide for the prM glycoproteins whereas the transmembrane domain of prM served as signal peptide for E glycoproteins.

Generation of flavivirus pseudo-particles Retroviruses were chosen as platforms for assembly of FVpp because their cores can incorporate a variety of different cellular and viral glycoproteins and because they can easily package and integrate genetic markers into host cell DNA.

FVpp were produced by transfecting 293T human embryo kidney cells (ATCC CRL-1573) with three expression vectors encoding a ACprME polyprotein, the MLV core proteins, encoded by the gag and pol gene or, alternatively, by the gag gene alone, and a packaging-competent MLV-derived genome harbouring the GFP (green fluorescent protein) marker gene. This construct contains the GFP marker gene, whose expression is driven by the CMV (cyto-megalovirus) immediate early promoter. Both CMV and GFP nucleic acid sequences where inserted in a retroviral vector derived from MLV in which the gag, pol and env viral gene where removed and in which the retroviral cis- acting elements that control vector genome packaging, reverse transcription and integration were retained.

Control pseudo-parl : icles were generat@d with the V8V-G glycoprotein (Negre et a/., 2000), the RD114 virus envelope glycoprotein (Sandrin et aL, 2002), and/or with assembly-defective MLV core proteins (MLV-G2A) (Swanstrom et al., 1997).

Briefly, expression constructs were transfected into 2. 5x106 293T cells seeded the day before in 10 cm plates using a calcium-phosphate transfection protocol (Clontech, France) according to the manufacturer's recommendations.

The medium (8 ml/plate) was replaced 16 hrs after transfection. Supernatants containing the pseudo-particles were harvested 24 hrs later, filtered through 0.45 pm-pore-sized membranes and processed as described before (Nègre et a/., 2000).

Immunoblot analysis of structural components of the pseudo-particles Lysates of transfected cells and of pseudo-particles pelleted through 20% sucrose-cushions were immunobloted. Expression of prM and E glycoproteins from West Nile virus or Dengue Virus or Yellow Fever virus and of MLV core proteins was revealed with monoclonal antibodies against prM and E (gift from P Despres, Institut Pasteur, Paris) and with an anti-capsid (MLV CA) antiserum (ViroMed Biosafety Laboratories, USA), as described previously (Sandrin et a/., 2002). VSV-G expressed in control pseudo-particles was detected with the monoclonal antibody P5D4 (Sigma-Aldrich, France).

Analysis of immunoblots of transfected cells showed that the structural components of the pseudo-particles were readily detected at the expected molecular weights ; i. e. 30 kDa for prM, 60 kDa for E, 60 kDa for VSV-G and 70 kDa for the RD114 glycoprotein. MLV core proteins were detected as a Gag protein precusor of 65 kDa that was partially processed by the MLV protease into mature core components. No difference in this expression pattern were noticed, whether the retroviral core proteins were encoded by the gag and pol genes or by the gag gene alone, except that the gag proteins expressed when using the gag gene alone were not processed, as a result of absence of the viral protease encoded by the pol gene.

The prM and E glycoproteins were readily detected in the pellets of purified virions generated with the wild-type MLV core particles but not with the viral assembly-deficient MLV-G2A mutant.

Comparison of the relative levels of VSV-G or flavivirus glycoproteins in producer cell lysates versus viral pellets suggests efficient incorporation of prM and E into viral particles. Likewise, could prM and E glycoproteins efficiently assemble on retroviral core proteins derived from HIV-1.

Altogether, these results indicate that transient over-expression of prM and E in 293T cells leads to specific and efficient incorporation of flavivirus glycoproteins into pseudo-particles generated with retroviral cores. Since flaviviruses bud from the ER, this implies that FVpp form by budding into the ER lumen or, alternatively, that a fraction of prME to reach the cell surface where MLV budding normally occurs (Swanstrom and Wills, 1997).

EXAMPLE 2: FVpp cell line infectivity Infectivity of FVpp generated with West Nile virus or Dengue virus was assessed on the following target cell lines : Hela human epithelial adenocarcinoma (CCL-2), TE671 human rhabdomyosarcoma (ATCC CRL- 8805), and VERO African green monkey kidney (CCL-81).

Target cells (seeded the day before 8x104 cells/well in 12-well plates) were incubated for 3 hrs with dilutions of supernatants from producer cells containing the FVpp, then washed and cultured until expression of the GFP marker gene harboured by the virions was assessed by FACS analysis 72 hrs later (Sandrin et a/., 2002). Since the FVpp were generated with replication- defective viral components, this procedure allowed evaluation of the specific infectivity of the pseudo-particles after a one-round infection process.

Infectious titres of up to 5x104 TU/ml were detected for the FVpp generated with West Nile virus on Vero African green monkey cells (Figure 2).

Upon concentration of the producer cell supernatants by ultra-centrifugation (Sandrin et a/., 2002), infectious titres of 106 TU/ml, on average, could be readily obtained.

Incubation of FVpp and target cells with AZT, a reverse-transcriptase inhibitor that prevents conversion of the retroviral RNA genome of the FVpp as integration-competent proviral DNA, inhibited transduction (Figure 2).

Moreover, long-term expression of GFP could be demonstrated after serial passaging of the infected cells for more than one month.

These results indicated that infection of the target cells led to integration in host cell DNA and stable expression of the GFP marker gene transduced by the FVpp. Additionally, no infection could be obtained with viral particles lacking both prM and E glycoproteins, or lacking core proteins, or, alternatively, when using the MLV-G2A assembly-defective core proteins (Figure 2).

Similar results were obtained with FVpp habouring West Nile virus prM and E glycoproteins on Hela and TE671 cells, as well as with FVpp generated with Dengue virus glycoproteins on Vero, Hela, and TE671 cells.

Finally, the infectivity of FVpp harbouring West Nile virus prM and E glycoproteins could be neutralized by sera from mice immunized with

inactivated West Nile virus (Fig. 2), as well as with West Nile virus vaccine candidates or with purified antibodies against West Nile virus glycoproteins.

Altogether, these results demonstrated that FVpp harbouring the prM and E glycoproteins and retroviral core proteins were infectious, leading to retroviral-mediated integration of their vector genome. These data indicate that FVpp closely mimic the early events of infection by wild-type flavivirus and can be used as a useful bio-safe infection assay to detect neutralizing compounds such as antibodies.

Example 3: Host-range of pseudo-particles derived from West-Nile virus The host range of pseudo-particles harbouring the prM-E glycoproteins from West Nile virus (WNVpp), from Dengue Virus (DENpp) or from Yellow Fever virus (YFVpp) was investigated by measuring their infectivity on a set of vertebrate target cells (Table 1). A wide range of cell types were found permissive to infection by WNVpp, including cells from rodent and primate species.

Table 1. Permissivity of target cell types to infection with pseudo-particles. WNVpp DENpp YFVpp RD114pp VSV-Gpp -L-SIGN + L-SIGN-L-SIGN + L-SIGN-L-SIGN + L-SIGN-L-SIGN + L-SIGN-L-SIGN + L-SIGN HuH7 1, 78E+03 8, 67E+03 2, 52E+02 5, 01 E+03 9, 14E+02 3, 03E+03 4, 52E+07 1, 70E+07 4, 84E+07 4, 34E+07 Hela 1, 60E+02 5, 17E+03 ND 3, 04E+02 ND 1, 50E+02 2, 20E+06 9, 87E+05 6, 03E+07 1, 71E+07 TE671 3, 56E+03 NT ND NT NT NT NT NT 5, 56E+07 NT 293T 4, OOE+02 NT ND NT NT NT NT NT 1, 33E+07 NT HT1080 1, 37E+03 NT ND NT NT NT NT NT 3, 05E+07 NT U118 1, 83E+03 NT ND D FIT NT NT NT NT 1, 68E+07 NT U87 1, 47E+03 NT ND NT NT NT NT NT 7, 49E+06 NT PLC 1, 39E+03 NT ND NT NT NT NT NT 1, 61E+07 NT Hep3B 1, 71 E+03 NT 1, 32E+02 NT NT NT NT NT 9, 87E+06 NT SW13 2, 42E+02 NT ND NT 1, 54E+02 NT 2, 64E+07 NT NT NT GM209 4, 70E+02 NT 6, 48E+01 NT NT NT NT NT 6, 48E+06 NT A431 1, 67E+03 NT 7, 20E+01 NT NT NT NT NT 5, 03E+07 NT A549 NT NT NT NT 1, 20E+01 NT 8, 90E+06 NT NT NT sMAGI 2, 81 E+03 6, 32E+03 1, 48E+02 3, 42E+02 1, 35E+02 6, 39E+02 ND ND 3, 75E+07 NT VERO 4, 51 E+02 1, 53E+03 ND 1, 53E+02 4, 68E+01 1, 02E+02 5, 50E+07 1, OOE+07 6, 61 E+07 5, 73E+07 HOS 1, 05E+03 3, 49E+02 ND 3, 60E+00 ND 4, 32E+01 5, 21 E+07 3, 07E+06 4, 27E+07 NT XC 4, 94E+02 NT 1, 30E+01 NT NT NT NT NT 2, 94E+07 NT RAW 3, 60E+01 NT ND NT NT NT NT NT 1, 19E+07 NT CHO 7, 80E+01 NT ND NT NT NT NT NT 2, 33E+07 NT NIH3T3 4, 16E+01 NT 3, 20E+00 NT 9, 60E+00 NT ND NT NT NT L-ND NT ND NT NT NT NT NT 1, 82E+07 NT L929 ND NT ND NT ND NT ND NT NT NT

Ectopic expression of type C lectins, such as L-SIGN in VERO, Huh-7 or HeLa target cells, was found to stimulate the infectivity of the pseudo-particles by up to 30 fold for WNVpp in HeLa cells (Table 1). In contrast, the control pseudo-particles obtained with the VSV-G or with the RD114 glycoproteins were not sensitive to expression of type C lectins (Table 1). To analyse further the role of L-SIGN in the increase in infectivity of WNVpp, the interaction between the virion and the cell surface was investigated. First it was found that upon expression of L-SIGN, the binding of the WNVpp to target cell surface was increased by up to 10 fold. Second, removal of the N-glycosylation at position amino-acid 153 of the WNV E glycoprotein resulted in WNVpp that splayed significant decrease in sensivity to the type C lectins, as was revealed by both reduced binding to target cell surface and infectivity. This indicated that C type lectins form one of the receptor components for flaviviruses and probably stimulate infection by capturing the virions to the cell surface.

In conclusion, the invention provides a tool that allows precise investigation of viral assembly of prME glycoproteins (processing, maturation) and their role in cell entry of flaviviruses. No structural modifications of the pr ! v) E gtycoproteins were required for their correct assembly on retroviral and lentiviral cores and to generate high titre infectious FVpp with functional prME glycoproteins. The insertion of a marker gene into the FVpp allowed precise and rapid determination of the infectivity of these pseudo-particles by flow- cytometry.

The development of these functional, infectious flavivirus pseudo- particles make it now possible to investigate early events of flavivirus infection, such as the identification of novel flavivirus receptor (s) or co-receptor (s), to carry out diagnostic assays for the detection of neutralising antibodies in seroconverted patients, and to develop efficient inhibitors to flavivirus infection, such as immunological inhibitors, vaccines and neutralizing antibodies, and pharmacological inhibitors.

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