Expression and Diagnostic Use of pENV-3 Encoded Peptides Which Are Immunologically Reactive With Antibodies to LAV

1. Technical Field

The present invention relates in general to the expression of viral proteins through the use of recombinant DNA technology, and more specifically, to the expression of proteins which are immunologically reactive with antibodies to lymphadenopathy-associated virus (LAV).

2. Background Art

Acquired immune deficiency syndrome (AIDS) is a transmissible deficiency of cellular immunity characterized by opportunistic infections and certain rare malignancies. The dominant risk groups for AIDS include homosexually active males, intravenous drug abusers, recipients of transfusions and blood products, and the heterosexual partners and children of high-risk individuals, suggesting the involvement of an infectious agent transmitted through initmate contact or blood products.

Recent evidence indicates that the infectious agent responsible for disease transmission is a novel lymphotropic retrovirus, known as lymphadenopathy-associated virus (Barre-Sinoussi et al., Science 220: 868 (1983)). Similar viruses have been reported by other scientific groups (Popovic et al., Science 224: 497 (1984); Levy et al., Science 225: 840 (1984); Vilmer et al., Lancet 1: 753 (1983)) and designated human T-cell lymphotropic virus type III (HTLV-III), AIDS-associated retrovirus (ARV), or immune deficiency-associated virus (IDAV). Still more recent data indicates that LAV, HTLV-III, ARV, and IDAV share several important characteristics, including substantial nucleotide homology (Wain-Hobson et al., Cell 40: 9 (1985); Muesing et al., Nature 313: 450 (1985); Sanchez-Pescador et al., Science 227: 484 (1985)), and should be considered isolates of the same virus, although there is a likelihood that strain-to-strain variations among the viral isolates will exist. In addition to exhibiting substantial nucleotide homology, the isolates are similar with respect to morphology, cytopathology, requirements for optimum reverse transcriptase activity, and at least some antigenic properties (Levy, supra; Schupbach et al., Science 224: 503 (1984)).

As noted above, the virus is known to be transmissable through blood products (blood, blood serum, blood plasma, and fractions thereof), making it important to screen the blood products to determine if the donor has been exposed to the virus. This can be done in any of several ways, including enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies to LAV and related viruses. Individuals whose blood contains antibodies to LAV are said to be "seropositive." Blood from seropositive donors may be eliminated from the blood supply upon detection, thereby helping to prevent the spread of the disease.

The immune response of individuals exposed to LAV is variable. Antibodies can be produced to any of several viral proteins, including p13, p18, p25, p36, gp43, p55, gp65, gp110, etc. (Schupbach et al., New Engl. J. Med. 312: 265 (1985)). It is also likely that not all individuals will make antibodies to the same proteins or to the same epitope on a given protein.

The detection of seropositive individuals, as currently practiced, has several inherent problems. Foremost among these problems is the need to isolate antigen from whole viruses for the immunological assays. This isolation requires the manipulation of large volumes of live, potentially infectious virus, and as such poses a significant safety hazard. In addition, there are concerns relating to the yield, purity, and reproducibility of virus from one preparation to another. This may result in an unacceptable number of false positives and/or negatives. Consequently, there is a need in the art for alternative methods of producing viral antigens which are useful in blood screening assays and which further provide other related advantages.

Briefly stated, the present invention discloses a peptide which is immunologically reactive with antibodies to LAV, the peptide comprising the amino acid sequence of FIG. 5, starting with isoleucine, number 1, and ending with threonine, number 173. A recombinant plasmid capable of replication in bacterial host cells is also disclosed. The plasmid includes procaryotic transciptional and translational signals for expression, followed by a DNA sequence coding for a peptide comprising the amino acid sequence described above. In a preferred embodiment, signals are chosen from an operon, such as the trp operon, which is inducible and/or suppressible. Bacterial cells, such as E. coli, which have been transformed with the recombinant plasmid described above, are also disclosed.

Another aspect of the invention discloses a method for preparing proteins which are immunologically reactive with antibodies to LAV. The method comprises introducing a recombinant plasmid capable of replication into bacterial host cells. The plasmid includes procaryotic transciptional and translational signals for expression, followed by a DNA sequence coding for a peptide comprising the amino acid sequence of FIG. 5, starting with isoleucine, number 1, and ending with threonine, number 173, said peptide being immunologically reactive with antibodies to LAV. Subsequent to the introduction of the plasmid, the bacterial host is grown in an appropriate medium. Expression of the protein may then be induced and the protein product of the sequence isolated from the bacterial host. The protein product may be purified subsequent to isolation, for example, by gel permeation chromatography.

A further aspect of the invention discloses a method for determining the presence of antibodies to LAV in a biological fluid. In one embodiment, the method comprises incubating the biological fluid with a protein produced by bacterial cells transformed with a recombinant plasmid as described above, thereby forming a reaction mixture, and subsequently analyzing the reaction mixture to determine the presence of the antibodies. In a preferred embodiment, the step of analyzing the reaction mixture comprises contacting the reaction mixture with a labeled specific binding partner for the antibody. In another embodiment, the presence of antibodies to LAV is determined by carrying out a competition between a sample suspected of containing antibody and a labeled monoclonal antibody for binding to an immobilized recombinant protein. The reaction mixture is subsequently analyzed to determine the amount of labeled antibody bound to the solid phase.

Yet another aspect of the invention discloses a method for determining the presence of LAV antigen in a biological fluid, comprising incubating the biological fluid, either sequentially or simultaneously, with a labeled protein produced by bacterial cells transformed with a recombinant plasmid as described above, and an antibody to the protein such that specific binding occurs. Subsequently, the reaction mixture formed during the incubation is analyzed to determine the amount of label associated with the antibody.

A method for producing antibodies to LAV comprising immunizing an animal with a protein produced by bacterial cells transformed with a recombinant plasmid as described above is also disclosed.

An additional aspect of the present invention discloses a method for determining the presence of antibodies to LAV in a biological fluid, comprising conjugating latex beads to a protein produced by bacterial cells transformed with a recombinant plasmid capable of replication in bacterial host cells, the plasmid including procaryotic transcriptional and translational signals for expression. The signals are followed by a DNA sequence coding for a peptide comprising the amino acid sequence of FIG. 5, starting with isoleucine, number 1, and ending with threonine, number 173, the peptide being immunologically reactive with antobodies to LAV. Subsequently, the biological fluid is incubated with the bead/protein conjugate, thereby forming a reaction mixture. The reaction mixture is then analyzed to determine the presence of the antibodies.

The proteins produced within the present invention, when used with a suitable carrier or diluent, form an immunologically effective vaccine composition. By administering to an individual an immunogenically effective amount of a protein encoded by a DNA sequence comprising the amino acid sequence of FIG. 5, starting with isoleucine, number 1, and ending with threonine, number 173, attached to a physiologically acceptable carrier, infection caused by the virus responsible for AIDS can be prevented.

Other aspects of the invention will become evident upon reference to the following detailed description and attached drawings.

FIG. 1 illustrates the construction of mpSW19 from .lambda.J19.

FIG. 2 illustrates the trp E expression vector pJH 11, including the polylinker sequences.

FIG. 3 illustrates the origin of the LAV insert in pENV-3.

FIG. 4 illustrates the construction of pENV-3 from pJH 11 and mpSW19.

FIG. 5 illustrates the amino acid sequence of ENV-3.

FIG. 6 illustrates the nucleotide sequence of the env region and its protein product.

Prior to setting forth the invention, it may be helpful to an understanding thereof to set forth definitions of certain terms to be used hereinafter.

Lymphadenopathy-Associated Virus (LAV):

A human T-lymphotropic retrovirus. For purposes of the present invention, a virus is considered to be the same as or equivalent to LAV if it substantially fulfills the following criteria:

(a) the virus is tropic for T-lymphocytes, especially T-helper cells (CD4.sup.+, according to the international nomenclature defined in Bernard et al., eds., Leucocyte Typing, New York: Springer Verlag (1984));

(b) the virus is cytopathic for infected CD4.sup.+ cells (rather than transforming, as are HTLV-I and II);

(c) the virus encodes an RNA-dependent DNA polymerase (reverse transcriptase) which is Mg.sup.2+ -dependent (optimum concentration 5 mM, optimum pH 7.8), not inhibitable by actinomycin D and can employ oligo (dT).sub.12-18 as a primer for reverse transcription from its 3' LTR;

(d) the virus bands in a sucrose gradient at a density of approximately 1.16;

(e) the virus can be labeled with [.sup.3 H] uridine;

(f) the virus is distinct by immunological and nucleotide sequence criteria from members of the HTLV-I/II family of viruses (by this criterion HTLV-III is not to be considered a member of the HTLV-I/II family);

(g) the virus is substantially cross-reactive immunologically with the proteins encoded by the gag and env regions of LAV; and

(h) the virus shares substantial nucleotide homology (75-100%) and amino acid seqence homology (75-100%) with LAV.

Immunologically Reactive:

An antigen and an antibody are said to be "immunologically reactive" if they are capable of binding specifically to each other, typically with an affinity of at least 10.sup.6 M.sup.-1, more often at least 10.sup.8 M.sup.-1.

Transformed or Transformation:

The process of stably and heritably altering the genotype of a recipient cell or microorganism by the introduction of DNA.

Lymphadenopathy-associated virus (LAV) can be isolated from patients with AIDS or lymphadenopathy syndrome. The lymph nodes of such patients are typically biopsied and placed in culture medium supplemented as necessary to support growth. A mitogen such as interleukin-2 (IL-2) or phytohemagglutinin (PHA) can be included; anti-serum to human interferon can also be included. Reverse transcriptase activity typically appears about day 15 of culture, indicating the presence of virus. The virus can be concentrated from the culture supernatant using a non-ionic detergent, followed by banding in a sucrose gradient. These and other methods of purification are well known in the art and are described, for example, in Montelaro et al., J. Virol. 42: 1029 (1982).

LAV can be propagated in any of a number of ways. It can be cultured in T-lymphocytes derived from umbilical cord or peripheral blood or from bone marrow. Alternatively, it can be propagated in immortalized T-cells or B-cells; see, for example, Popovic et al., Science 224: 497 (1984), and Montagnier et al., Science 225: 63 (1984). Growth of the virus is usually monitored by the presence of reverse transcriptase activity.

A genomic clone of LAV can be prepared by any of several methods well known in the art, including but not limited to those described by Hahn et al., Nature 312: 166 (1984); Alizon et al., Nature 312: 757 (1984); Luciw et al., Nature 312: 760 (1984); and Muesing et al., Nature 313: 450 (1985).

Briefly, in one of these methods (Alizon et al.) DNA is isolated from LAV-infected T-cells of a healthy donor, partially digested with a restriction endonuclease such as Hind III, and the resultant digest fractionated electrophoretically. Fragments which correspond in size to the size of the entire LAV genome (approximately 9.2 Kb) are eluted from the gel, precipitated, resuspended, and ligated into the arms of an appropriately restricted vector. The ligation mix is packaged into bacteriophage particles. Bacteria are transformed with the bacteriophage, and the clones are screened in situ for LAV inserts using a suitable probe (such as a cDNA made from LAV-RNA). From such a clone, the desired region of LAV can be subcloned into a bacterial plasmid vector, such as pUC 18. Further subcloning may be desirable to remove unwanted sequences and to add additional restriction sites (in the form of a polylinker) at either end for the purpose of facilitating cloning into an expression vector.

The LAV sequences can then be subcloned into an inducible expression vector. A variety of expression vectors are known in the art and include .lambda.gt 11:Tn5 (Hall et al., Nature 311: 379 (1984)); trp E (Paul et al., Eur. J. Cell. Biol. 31: 171 (1983)); pINIII (Masui et al., Biotechnology, January 1984, p. 81).

The resultant proteins can be partially purified and used for a variety of purposes, including utilization as immunogens and antigens in immunoassays. For use as immunogens, the proteins can be injected into an animal, such as a mouse, rabbit, goat, etc., either in buffered solution or in adjuvant. Alternatively, the proteins can be purified by polyacrylamide gel electrophoresis and the bands of interest excised from the gel, triturated, and resuspended in buffer for injection into the host animal. Polyclonal or monoclonal antibodies can be prepared. For use as antigens in immunoassays, the proteins can be employed in labeled or unlabeled form. When the proteins are labeled, the lables can include radioisotopes, fluorophores, enzymes, luminescers, or particles. These and other labels are well known in the art and are described, for example, in the following U.S. Pat. Nos.: 3,766,162; 3,791,932; 3,817,837; 3,996,345; and 4,233,402.

Assays employing the recombinant proteins of the instant invention can be heterogeneous (i.e., requiring a separation step) or homogeneous. If the assay is heterogeneous, a variety of separation means can be employed, including centrifugation, filtration, chromatography, or magnetism.

One preferred assay for the screening of blood products or other physiological fluids for the presence of antibodies is by an ELISA. Typically, antigen is adsorbed to the surface of a microtiter well. Residual protein-binding sites on the surface are then blocked with an appropriate agent, such as bovine serum albumin (BSA), heat-inactivated normal goat serum (NGS), or BLOTTO, (a buffered solution of nonfat dry milk which also contains a preservative, salts, and an antifoaming agent). The well is then incubated with a sample suspected of containing specific antibody. The sample can be applied neat, or, more often, it can be diluted, usually in a buffered solution which contains a small amount (0.1-5.0% by weight) of protein, such as BSA, NGS, or BLOTTO. After incubating for a sufficient length of time to allow specific binding to occur, the well is washed to remove unbound protein and then incubated with a labeled, anti-human immunoglobulin antibody (.alpha.HuIg). The label can be chosen from a variety of enzymes, including horseradish peroxidase (HRP), .beta.-galactosidase, alkaline phosphatase, and glucose oxidase. Sufficient time is allowed for specific binding to occur, then the well is again washed to remove unbound conjugate, and the substrate for the enzyme is added. Color is allowed to develop and the optical density of the contents of the well is determined visually or instrumentally.

For convenience, reagents for an ELISA may be provided in the form of kits. These kits can include microtiter plates to which viral proteins made by recombinant techniques have been pre-adsorbed, various diluents and buffers, labeled conjugates for the detection of specifically bound antibodies, and other signal-generating reagents, such as enzyme substrates, cofactors, and chromogens.

Due to the fact that a significant number of AIDS patients are highly immunosuppressed and have progressively lost the ability to make high-titer antibodies, the present invention utilizes a portion of the envelope (env) region of the LAV genome which codes for a protein which is immunologically reactive with antibodies to LAV. The presence of antibodies to the envelope glycoprotein of LAV is a better indicator of LAV infection that is the presence of antibodies to other viral proteins, because the antibody titers to the envelope glycoprotein are thought to persist during the later stages of the disease, while antibody to other proteins, such as the core proteins, may decline to less than detectable levels.

Another application of the recombinant proteins of this invention is use as human vaccines. The recombinant proteins can be extensively purified and formulated in a convenient manner, generally in concentrations of 1 .mu.g to 20 mg per kg of host. Physiologically acceptable carriers, such as sterile water, saline, buffered saline, etc., can be employed. Adjuvants, such as aluminum hydroxide, can also be employed. Adjuvants, such as aluminum hydroxide, can also be employed. The vaccine can be administered by intravenous, subcutaneous, intramuscular, or peritoneal injection. One injection can be sufficient, but more often, multiple injections at weekly to monthly intervals are preferred.

Alternatively, vaccinia virus recombinants can be constructed which express regions of the LAV genome. For example, the constructs of this invention can be inserted into a plasmid such as pMM34 (Mackett, et al., Science 227: 433, 1985) and vaccinia virus hybrids containing the resultant chimeric plasmic, formed by homologous recombination. Immunization with such recombinant virus vaccines has been shown to be effective in eliciting protective immunity in animals to hepatitis B virus and vesicular stomatitis virus (Smith et al., Nature 311: 578, 1984).

The use of a recombinant protein vaccine in this manner eliminates the need to compose vaccines from inactivated preparations or avirulent strains of pathogenic microorganisms.

The genetic organization of LAV, 5'LTR-gag-pol-Q-env-F-3'-LTR, is unique among retroviruses. In all other known replication-competent retroviruses, the pol and env genes overlap; in LAV, they are separated by an open reading frame Q (Wain-Hobson et al., Cell 40: 9 (1985)).

The env open reading frame has a possible initiation site (methionine codon) at the eighth triplet and hence is expected to code for a protein of molecular weight 97 kd. This is consistent with the apparent molecular weight (110 kd) of the LAV glycoprotein on denaturing polyacrylamide gels. There are 32 potential N-glycosylation sites in the env gene (Asn-x-Ser/Thr). There are also three hydrophobic regions, characteristic of retroviral envelope proteins, corresponding to a signal peptide (encoded by nucleotides 5815-5850), a second region (7315-7350), and a transmembrane segment (7831-7896). The second region is preceded by an Arg/Lys-rich region which is thought to represent a proteolytic processing site (Wain-Hobson, supra).

As noted above, antibodies to the LAV glycoprotein (the env gene product) appear early in the course of virus infection and persist in the later stages of disease as well (Kitchen et al., Nature 312: 367 (1984)). Antibodies to the glycoprotein are, therefore, a particularly good indicator of prior exposure to LAV, making the glycoprotein, or portions thereof, particularly useful in blood screening assays.

A particularly preferred portion of the env region of the LAV genome is ENV-3. As shown in FIGS. 3 and 4, ENV-3 spans the nucleotide sequence from bp 7178 to bp 7698. Referring now to FIG. 5, included within the region are the following sub-regions (overlined in FIG. 5): (1) a hydrophobic peptide encoded by bp 7315 through 7350; (2) a dodecapeptide containing putative proteolytic progressing sites for cleavage of the envelope precursor protein to gp 65 and gp 43; (3) a highly conserved region around two cysteine residues; (4) a conserved sequence at the 3' (carboxy) terminus; and (5) a conserved sequence at the 5' (amino) terminus.

Data obtained using synthetic peptides which correspond to the different sub-regions of ENV-3 indicate that the most important regions are those encompassing or in close proximity to the cysteine residues and those encompassing or in close proximity to the proteolytic processing region. The synthesis and screening of certain of these peptides are described in co-pending U.S. patent application Ser. No. 721,237, filed Apr. 8, 1985 by Watanabe, the text of which is hereby incorporated by reference.

Additionally, ENV-3 is almost totally devoid of potential glycosylation sites; hence the antigenicity of the recombinant protein can be reasonably expected to mimic that of the native protein. The entire sequence of the env gene and its protein product is shown in FIG. 6, in which the potential glycosylation sites are overlined. Referring to FIG. 6, it can be seen that a large stretch of ENV-3 is devoid of such sites, and that in comparison to the remainder of the gene, ENV-3 as a whole is strikingly underglycosylated.

In the following example, an LAV genomic clone designated .lambda.J19 was subcloned into the bacterial plasmid vector, pUC 18. The resultant subclone, designated pBT-1 (assigned ATCC Accession Number 53069), was further subcloned to yield pRS-3, which contained predominantly env, F, and LTR region sequences. The env and part of the F sequences were further subcloned into M13mp18, and then regions of the env sequence were transferred into the trp E inducible expression vector. The env DNA was inserted in-frame downstream of the trp E gene, resulting in the expression of a trp E-env fusion protein when E. coli were tranformed with this construct. The resultant protein was partially purified and characterized by its reactivity in ELISA with sera from known seropositive and known seronegative individuals.

The following example is offered by way of illustration, and not by way of limitation.

Any of several bacterial expression systems can be used to express foreign proteins. The trp E system was chosen for the expression LAV env sequences because it contains a strong inducible promoter and because its expression can also be suppressed so that foreign (and potentially toxic) protein does not accumulate within the bacteria for long periods of time.

Since there is no rapid and convenient way to screen or select for bacteria transformed with vector containing a foreign insert (as opposed to bacteria transformed with vector alone), it is convenient to subclone the desired region of a large piece of DNA (such as phage DNA) into a vector containing a selection system for inserts before transferring it to the trp E expression vector. This facilitates the screening of transformed bacteria for ones containing the desired insert.

The approach of applicants was, therefore, to first subclone in two steps most of the env region of the LAV genome into a transfer vector, M13mp18 (FIG. 1). Then, the Bam HI-Hind III fragment of this subclone was ligated into trp expression vector pJH 11. DNA from the resulting subclone was further modified to produce the desired product, pENV-3.

1. Subcloning LAV genome

a. Preparation of phage DNA

The entire LAV genome was obtained from the Pasteur Institut in the form of .lambda.phage particles containing a 9.2 Kb genomic DNA insert in the Hind III site of phage .lambda.L47.1. This clone is referred to as .lambda.J19 and is described in Wain-Hobson et al., Cell 40: 9 (1985). .lambda.J19 phage particles were transfected into the Q359 strain of E. coli K-12 (the genotype of Q359 is hsdRk.sup.-, hsdMk.sup.+, supF, .phi.80, P2) according to the procedure of Maniatis et al., Molecular Cloning: A Laboratory Manual, New York: Cold Spring Harbor Laboratory, 1982, at p. 64. A single plaque was picked and the phage amplified by the plate lysate method (Maniatis, supra, at p. 65). After a nine-hour incubation at 37.degree. C., the plates (100 mm diameter) containing confluent plaques were overlaid with 5 ml of 100 mM NaCl/20 mM MgSO.sub.4 /50 mM Tris, pH 7.5. After incubating for twelve hours at 4.degree. C., the liquid was collected and extracted two times with an equal volume of chloroform.

To 10 ml of the resultant aqueous phase containing phage particles was added 2 ml 0.25M EDTA/2.5% SDS/0.5M Tris, pH 9, and the suspension was incubated at 70.degree. C. for fifteen minutes to disrupt the phage. 2.5 ml 8M potassium acetate was added, and the solution was incubated on ice for fifteen minutes, then centrifuged for ten minutes at 12,000.times.g at 4.degree. C. to pellet protein. The supernatant was transferred to a 50 ml polypropylene centrifuge tube and extracted with an equal volume of phenol (pH 8, equilibrated with 1M Tris, pH 8) at 20.degree. C. The aqueous phase was then extracted with an equal volume of chloroform:isoamylalcohol (24:1) at 20.degree. C. To the aqueous phase was then added 2.5 volumes of 95% ethanol to precipitate the DNA. After centrifugation, the DNA pellet was dried and resuspended in 10 mM Tris HCl, pH 7.4/1 mM EDTA.

b. Subcloning the env region

Approximately 12 .mu.g of .lambda.J19 DNA prepared in A.1.a above was digested to completion with the restriction enzyme Sst I (Bethesda Research Labs, Bethesda, MD), which only cuts in the LTR regions of this isolate of LAV. The digest mixture was electrophoresed at 1 V/cm through 0.9% agarose in 0.089M Tris-borate/0.089M boric acid/1 mM EDTA. The position of the 9.1 Kb fragment was determined relative to molecular weight standards after staining with ethidium bromide. This band was electroeluted into NA45 paper (Schleicher and Schuell, Keene, NH). The DNA was recovered from the paper according to instructions provided by the manufacturer.

The 9.1 Kb Sst I fragment was ligated into the Sst I digested vector pUC 18, at a ratio of 10 insert molecules:1 vecor molecule. E. coli strain HB101 was transformed with the ligation mix by the CaCl.sub.2 procedure of Maniatis, et al. (supra) and plated onto LB plus ampicillin (200 .mu.g/ml) agar plates.

Single colonies were picked and diluted into 3 ml LB plus ampicillin medium and grown overnight at 37.degree. C. with constant shaking. Plasmid DNA was prepared by the alkaline lysis method (Maniatis et al., supra, at p. 368). One colony was selected which contained the 9.1 Kb Sst I insert in an orientation such that the Eco RI site in the polylinker was closest to the 5' end of the LAV genome, as determined by restriction analysis of the plasmid DNA. This subclone was designated pBT-1 (FIG. 1).

Plasmid pBT-1 was then digested with Eco RI, and the vector, still containing the 3' end of the LAV genome, was religated. This served to remove the 5' Eco RI fragments from the plasmid. HB101 cells were transformed with the ligation mixture, and a colony containing the insert pRS-3 was identified by restriction analysis of the purified plasmid DNA.

The sequence between the Kpn I sites at bp 5889 and bp 8572 [numbering according to Wain-Hobson et al., Cell 40: 9 (1985)], containing most of the env region, was then transferred from pRS-3 to the M13mp18 vector. This was done to further remove extraneous (non-env) sequences and also to place the env sequences in a vector which utilizes .beta.-galactosidase rather than ampicillin as the selectable/screenable marker. This was advantageous because the trp E expression vector uses ampicillin resistance as its selectable marker; transfer of an insert from one vector (pUC 18) encoding the ampicillin resistance factor to another (trp expression vector pJH 11) would make screening for the desired insert plus vector more difficult. M13 phage were screened by usingthe chromogenic substrate 5-bromo-4-chloro-3-indolyl-.beta.-galactoside (Sigma Chemical Co.) to score for inactivation of .beta.-galactosidase due to insertion of env sequence. Those phage containing inserts were screened for the orientation of the insert relative to restriction sites in the polylinker region. DNA was isolated from recombinant mpSW19, which has the DNA insert in the correct orientation.

2. Insertion of the env sequence into trp vectors

The expression vectors contained the E. coli trp operon promoter, operator, and trp E gene inserted into pBR322 (FIG. 2). The trp E gene was truncated at its 5'-most Bgl II site by the insertion of a polylinker sequence (Konopka et al., J. Virol. 51: 223 (1984)). Based on the reading frame of the restriction sites within the polylinker, one of the three possible trp vectors, pJH 11 (FIG. 2), was chosen to express the portion ENV-3 (FIG. 3). The insertion of ENV-3 into pJH11 results in the production of a fusion protein with trp E sequences at the amino-terminal end followed by ENV-3 sequences (Spindler et al., J. Virol. 49: 132 (1984)).

pENV-3 (assigned ATCC Accession Number 53072) was constructed by ligation of the env region between mpSW19's Bam H1 site (in the polylinker) and Hind III site (bp 7698) into Bam H1 and Hind III restricted pJH 11 (FIG. 4). E. coli HB101 were transformed with the ligation reaction, and the ampicillin-resistant colonies were screened by minilysates. DNA from an appropriate colony was isolated, digested with Bam H1 and Bg1 II (New England Biolabs), and religated. This DNA was used to transform E. coli HB101. The resultant ampicillin-resistant bacteria were screened by restriction analysis for recombinant plasmids which had deleted the Bam H1 to Bg1 II fragment (FIG. 4). Deletion of the fragment brought the env sequences between the Bg1 II site (bp 7178) and the Hind III site (bp 7698) into the correct reading frame.

1. Transformation of E. coli with the trp-env construct

The recombinant trp-env expression plasmid was transferred from E. coli HB101 into E. coli C600 because the latter is a potentially better host for protein production. Transfer involved transformation of CaCl.sub.2 -shocked C600 with supercoiled DNA from minilysates of HB101. Bacteria were plated in the presence of ampicillin and tryptophan as described (Konopka et al., J. Virol. 51: 223 (1984)). Drug-resistant colonies were screened by minilysates to confirm the presence of the appropriate plasmid.

2. Expression of trp-env protein

Growth and induction of E. coli C600 transformed by the trp expression vector were as described (Spindler et al., J. Virol. 49: 132 (1984); Konopka et al., J. Virol. 51: 223 (1984)). Briefly, minimal medium containing tryptophan (20 .mu.g/ml) and ampicillin (100 .mu.g/ml) was inoculated with transformed bacteria from glycerol stocks. Cultures were grown with aeration at 37.degree. C. overnight. The overnight cultures were then inoculated at 1:100 into fresh minimal medium containing ampicillin (100 .mu.g/ml) but no tryptophan. These cultures were grown with aeration for 2-3 hours (up to early log phase) at 37.degree. C. The inducer, 3-.beta.-indoleacrylic acid (Sigma), was added to a final concentration of 20 .mu.g/ml from freshly made stocks at 20 mg/ml in 95% ethanol.

Induced cultures were grown at 37.degree. C. with aeration for 4 to 5 hours and then pelleted and, optionally, frozen. Protein yields from pENV-3 were between 10-30 mg/liter.

Fusion proteins were partially purified from cell pellets as described (Konopka et al., J. Virol. 51: 223 (1984)). Briefly, bacteria were resuspended in 100 ml of 50 mM Tris, pH 7.5/0.5 mM EDTA/150 mM NaCl (TNE) per liter of induced culture. Lysozyme (Sigma) was added to a final concentration of 1 mg/ml. After fifteen minutes at 0.degree. C., NP40 was added to the mixture to a final concentration of 0.2% for ten minutes at 0.degree. C. 1-2 mg of DNase (Sigma) was then added with 150 ml of DNase buffer (1.5M NaCl/12 mM MgCl.sub.2). Reaction mixtures were incubated for one hour at 0.degree. C. with frequent stirring. Insoluble proteins were then pelleted by centrifugation for fifteen minutes at 8000.times.g at 0.degree. C. Pellets were washed two times in TNE and then analyzed for the presence of insoluble proteins by denaturing polyacrylamide gel electrophoresis. Proteins were visualized by staining with Coomassie Brilliant Blue R.

The pellet containing the insoluble material from one liter of bacteria was solubilized in 6 mls of 6M guanidinium chloride/0.1M Tris HCl, pH 7.8. To this solution was added approximately 60 mg of solid dithiothreitol (DTT), and the reduction was allowed to proceed for thirty minutes at 37.degree. C.

The solubilized reduced protein was subjected to gel permeation chromatography on a 9.4 mm.times.25 cm GF-250 Zorbax column (Du Pont, Wilmington, DE). A 100 .mu.l aliquot of the protein solution was injected onto the column and eluted with 6M guanidinium chloride at a flow rate of 0.5 ml/minute. The optical density at 280 nm was monitored, and fractions were collected at 30-second intervals. Fraction number 32, i.e., the fraction eluting between 7.75 and 8.00 mls, was used for the subsequent assays. Note: Inclusion of DTT will result in formation of insoluble metal sulfides when inferior grades of guanidinium chloride are used.

The trp-pENV3 fusion protein has also been purified as follows:

Fusion protein was partially purified from cell pellets as an insoluble pellet (described above). The pellet containing the insoluble material from four liters of bacteria was solubilized in 4 mls of 6M guanidinium chloride/0.1M Tris HCl, pH 7.8. To this solution was added approximately 100 mg of solid dithiothreitol (DTT), and the reduction was allowed to proceed for one hour at 37.degree. C.

The solubilized, reduced protein was subjected to gel-permeation chromatography on a 2.6.times.87 cm column of Fractogel TSK HW-50(S) (MCB Manufacturing Chemists, Gibbstown, NJ). The 4 mls of reduced protein were pumped onto the column and eluted with 2.times.10.sup.-4 M DTT/2.times.10.sup.-3 M ethylenediaminetetraacetic acid (EDTA)/6M guanidinium chloride (American Research Products Co., South Euclid, OH). The column was run at 0.5 mls/minute, the optical density at 280 nm was monitored, and fractions were collected at ten-minute intervals. Fractions 34 and 35, the two fractions from the center of the UV absorbing peak, were used in subsequent assays.

1. Analysis by Western blots

Aliquots from the insoluble protein preparations expressed by pENV-3 were solubilized in 2% sodium dodecylsulphate/100 mM Tris, pH 6.8/20% glycerol/1.5M .beta.-mercaptoethanol and electrophoresed on denaturing polyacrylamide gels. proteins were electrotransferred onto nitrocellulose (BA85, L Schleicher and Schuell, Keene, NH) and the filters blocked with 5% bovine serum albumin (Sigma). Filters were then probed with E. coli-adsorbed human sera pooled from AIDS patients. The filters were developed with HRP-conjugated goat .alpha.HuIg. The pool was reactive with the trp-pENV-3 fusion proteins but not with trp E protein alone.

2. Analysis by ELISA

Insoluble protein preparations expressed by pENV-3 were solubilized in 3M guanidinium chloride. Insoluble material was removed by centrifugation at 15,000.times.g for five minutes. Solubilized protein concentrations were determined by the method of Bradford (Analytical Biochemistry 72: 248 (1976)). The proteins were diluted in 0.05M carbonate/bicarbonate buffer (pH 9.6) to a final concentration of 1-2 .mu.g/ml. Fifty .mu.l aliquots were loaded per microtiter well and incubated at 4.degree. C. overnight. Plates were then blocked with BLOTTO (5% [w/v] nonfat dry milk/0.01% thimerosol/0.01% antifoam A in0.01M sodium phosphate, pH 7.2/0.15M sodium chloride) for one hour at 37.degree. C. Pooled seropositive sera, sera from male homosexuals or LAS patients, and sera from healthy heterosexuals were diluted 1:100 with a 1:1 mixture of BLOTTO and PBS (0.01M sodium phosphate, pH 7.3/0.15M NaCl), and 50 .mu.l of diluted serum was added per well for one hour at 37.degree. C. The sera were removed, and the plates were washed three times in wash buffer (0.15M NaCl/0.05% [w/v] Tween 20) before adding 100 .mu.l of the goat anti-human IgG/horseradish peroxidase conjugate (diluted 1:10,000 in 50 mM NaCitrate/0.05% Tween 20/1% heat-inactivated normal goat serum; obtained from Antibodies, Inc., Davis, CA) for one hour at 37.degree. C. The conjugate was removed and the plates washed three times with 0.15M NaCl/0.05% (w/v) Tween 20. The ELISA assay was developed by adding 100 .mu.l/well of substrate solution (10 mg o-phenylenediamine in 50 ml 0.05M sodium citrate, pH 7.0) for thirty minutes at room temperature. Reactions were stopped with 100 .mu.l/well of 3N H.sub.2 SO.sub.4 and the optical density at 490 nm determined by an automated ELISA reader. Protein produced by pENV-3 was found to be highly and consistently reactive with known seropositive sera.

Further screening of the pENV-3 encoded protein was carried out using material from the guanidinium chloride-solubilized pellets described above against a panel of thirty-one human serum samples. The panel included eight sera from healthy heterosexuals (defined as negative in a whole virus ELISA), two serum pools from AIDS patients, and twenty-one sera from individuals diagnosed as LAS. Sera from LAS individuals were confirmed as seropositive in a whole virus ELISA. Twelve were further confirmed by Western blot analysis. The results are shown in Table 1. These results demonstrate that sera from AIDS or LAS patients were more reactive with pENV-3 encoded protein than were sera from normal individuals. This delineation indicates that the trp-env fusion proteins can be used to screen for the presence of sera reactive with the AIDS causative virus, LAV.

3. Fluorescence slide test for detection of serum antibody to LAV

Soluble protein produced as described above is conjugated to latex beads, and the protein/bead preparation is ethanol fixed onto microscope slides. An aliquot of patient serum is incubated with the protein/beads on a slide. The slides are washed, and FITC-labeled anti-human immunoglobulin in Evans blue counterstain is added. The slides are washed, and mounting medium and coverslip applied to each.

Alternatively, the protein/bead preparation is placed in test tubes for incubation with patient serum. The tubes are centrifuged and washed, and FITC-labeled anti-human immunoglobulin in Evans blue counterstain is added. The tubes are centrifuged and the supernatant aspirated. An aliquot of the beads is placed on a microscope slide and ethanol fixed, and coverslips are mounted.

All slides are examined by fluorescence microscopy. If test serum is antibody positive, beads appear as fluorescent green spheres; if test serum is antibody negative, beads appear as red spheres.

From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.