Patent Publication Number: US-6033672-A

Title: Method of stimulating an immune response to caprine arthritis-encephalitis virus (CAEV) in humans through the administration of CAEV immunogens

Description:
This work was supported, in part, by grant 5-UO1-HD32632 awarded by the National Institutes of Health. The United States Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to the fields of immunology and medicine and more specifically to methods for providing immunoprotection against HIV-1 infection. 
     2. Background Information 
     The incidence of acquired immunodeficiency syndrome (AIDS) has reached an epidemic level, particularly in third world countries in Africa, Asia and the Caribbean. Despite the expenditure of billions of dollars for research to discover drugs to treat or cure the disease, however, only modest progress has been made in identifying drugs that can delay the progress of the disease. 
     The causative agent of AIDS is the human immunodeficiency virus (HIV-1). HIV-1 infects a particular cell type of the immune system, the T cell. Upon entering a T cell, the HIV-1 genomic DNA is incorporated into the T cell genome, where it directs synthesis of viral proteins. New copies of the HIV-1 virus then are released from the infected T cell and further infect additional T cells. Ultimately, the infected T cells die and the HIV-1 infected individual&#39;s immune system becomes depleted and cannot ward off subsequent infections. As a result, victims of AIDS typically die from infections that normally would cause, at worst, a mild illness in a healthy individual. 
     Initially, treatments for AIDS were limited to methods of treating the infections that resulted from the depleted immune response. Later, drugs such as AZT and ddI, which are known as nucleoside analogs, were identified that could interfere with replication of the HIV-1 DNA. These drugs, particularly in combination, can prolong the life of AIDS patients. More recently, a class of drugs called protease inhibitors have been reported to be even more effective in inhibiting HIV-1 replication. It is hoped that the protease inhibitors, perhaps in combination with the nucleoside analogs, will further prolong the life of AIDS patients and even result in cures. 
     While the use of drugs as described above provides a means to kill the virus in HIV-1 infected individuals, such drugs are useful only after a person has become infected; they have no enhancing effect on the immune system. Clearly, a more preferable approach to stifling the AIDS epidemic would be to prevent HIV-1 infection in the first place. Vaccines, which stimulate a person&#39;s immune response against the vaccinating agent, are the logical choice for preventing HIV-1 infection. For example, vaccines have been used to prevent or reduce the severity of various viral diseases, including polio, measles, smallpox and influenza. In addition, a vaccine can stimulate the immune system in individuals already infected with a virus. 
     Numerous approaches have been made to develop a vaccine that would increase a person&#39;s resistance to infection with the AIDS virus. However, while various types of HIV-1 vaccines have been designed and have been tested in clinical trials, each suffers from a serious limitation. For example, vaccines composed of portions of an HIV-1 protein or using a killed HIV-1 virus have been produced. However, HIV-1 does not have a stable structure, but changes its identity rapidly as it reproduces. As a result, an immune response that is stimulated against a particular HIV-1 protein or strain of HIV-1 is ineffective against a changed form of the virus. Furthermore, portions of the HIV-1 surface proteins that potentially would be most effective in a vaccine, in fact stimulate the production of antibodies that facilitate HIV-1 infection. 
     Attenuated vaccines, which consist of live but reproductively defective viruses, also have been proposed. However, there is justified concern for injecting such an HIV-1 virus into an individual, particularly an otherwise healthy person. Thus, a need exists for a vaccine that provides a broad based immune response against HIV-1 but does not carry the attendant risks and limitations associated with the use of HIV-1 as the vaccinating agent. The present invention satisfies this need and provides additional advantages. 
     SUMMARY OF THE INVENTION 
     The present invention provides a vaccine comprising a caprine arthritis-encephalitis virus (CAEV) immunogen and a pharmaceutically acceptable carrier. A vaccine of the invention is useful, for example, to stimulate an immunoprotective response against CAEV or against human immunodeficiency virus-1 (HIV-1) in a human individual and to vaccinate a human, a goat or other mammal against CAEV infection. 
     The invention also provides a method of stimulating an immune response in an individual against CAEV or HIV-1 by administering a CAEV immunogen to the individual. Such a method is useful, for example, to increase the resistance to HIV-1 infection or to CAEV infection of an individual not previously exposed to HIV-1 or to CAEV, respectively, or to reduce the severity of a pathology caused by HIV-1 in an HIV-1 infected individual or of CAEV in a CAEV infected individual. In addition, the invention provides a method of stimulating an immune response in vitro by contacting a lymphocyte with a CAEV immunogen. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a vaccine comprising a caprine arthritis-encephalitis virus (CAEV) immunogen and a pharmaceutically acceptable carrier. As disclosed herein, a vaccine of the invention is useful to stimulate an immunoprotective response against HIV-1 in a human individual. In addition, a vaccine of the invention is useful to vaccinate an individual such as a human, a goat or other mammal against CAEV infection. 
     As used herein, the term &#34;CAEV immunogen&#34; means a CAEV viral particle, which can be a live, attenuated or killed CAEV, or an immunogenic portion of a CAEV viral particle, which can be a portion of a CAEV viral particle or a peptide fragment of a CAEV protein such as a peptide fragment of the gp135 envelope glycoprotein (see Table 1; env). A CAEV immunogen is characterized in that it can stimulate an immune response, either in vivo or ex vivo. In particular, a CAEV immunogen can stimulate an immune response against CAEV when administered to a human, a goat or other mammal and can stimulate an immune response that is cross-reactive against HIV-1 when administered to a human. The cross-reactive immune response against HIV-1 that is generated by CAEV infection is reminiscent of the cross-reactive immune response against smallpox virus that is generated by cowpox infection. 
     
                       TABLE 1                                                     
______________________________________                                    
CAEV IMMUNOGENS                                                           
______________________________________                                    
env                                                                       
        26     ERKREGFTAG    (SEQ ID NO: 1)                               
        48     SHHGNDSRRR    (SEQ ID NO: 2)                               
        54     SRRRRRKS      (SEQ ID NO: 3)                               
        514    RKETGTLGG     (SEQ ID NO: 4)                               
        620    RKKRELSHKRKKR (SEQ ID NO: 5)                               
pol                                                                       
        13     RMQRKERHK     (SEQ ID NO: 6)                               
        37     VRSSYGITSA    (SEQ ID NO: 7)                               
        83     GRIKLQGIGG    (SEQ ID NO: 8)                               
        307    QEILEDWIQQ    (SEQ ID NO: 9)                               
        1033   KRINNKYNKNS   (SEQ ID NO: 10)                              
gag                                                                       
        123    DGLLEQEEKK    (SEQ ID NO: 11)                              
        141    SVFPIVVQAA    (SEQ ID NO: 12)                              
        306    AIDAEPTV      (SEQ ID NO: 13)                              
______________________________________                                    
 
    
     env, pol and gag refer to the envelope, polymerase and nucleocapsid core protein coding regions, respectively, of CAEV. Numbers preceding each sequence indicates the amino acid position in each CAEV protein that corresponds to the first amino acid shown for each peptide. 
     A peptide fragment of the 70K protein, which is present in the U1 ribonucleoprotein complex, that is homologous to an amino acid sequence present in HIV-1 can act as a surrogate immunogen, which stimulates an immune response that cross-reacts with HIV-1 (Douvas and Takehana, AIDS Res. Hum. Retrovir. 10:253-262 (1994), which is incorporated herein by reference). As disclosed herein, various peptide fragments of CAEV proteins are homologous to 70K and HIV-1 sequences and, therefore, can be useful as CAEV immunogens for stimulating an immune response in an individual that cross-reacts with HIV-1. 
     Examples of peptide fragments of CAEV that are homologous to amino acid sequences present in 70K and HIV-1 are provided in the Table 1. Other such CAEV peptides useful as a CAEV immunogen can be identified by searching the CAEV DNA or amino acid sequence to identify CAEV peptides that are homologous to the immunologically homologous sequences of 70K and HIV-1 (see Douvas and Takehana, supra, 1994). Furthermore, methods for determining which CAEV peptides that are homologous to 70K and to HIV-1 peptides also are useful as CAEV immunogens are disclosed herein, including methods such as ELISA or western blot or viral neutralization assays (Example I), or otherwise known in the art (see, for example, Harlow and Lane, Antibodies: A laboratory manual (Cold Spring Harbor Laboratory Press 1988), which is incorporated herein by reference). 
     As used herein, the term &#34;vaccine&#34; means a composition containing an immunogen which, upon administration to an individual, which generally is a mammal such as a human or a goat, stimulates an immune response in the individual. In particular, a vaccine of the invention contains a CAEV immunogen, which can be administered, for example, to a goat, wherein an immune response against CAEV is stimulated, or can be administered to a human, wherein an immune response against CAEV that is cross-reactive against HIV-1 is stimulated. Thus, a CAEV immunogen is useful as a surrogate immunogen for stimulating an immune response against HIV-1. 
     It is recognized that a vaccine of the invention can be administered to an individual that is not infected with HIV-1, in which case the vaccine stimulates an immune response that can protect the individual from infection due to subsequent exposure to HIV-1, or can be administered to an HIV-1 infected individual, in which case the vaccine can stimulate an immune response so as to counter the immunopathology of HIV-1 (see, for example, Cease and Berzofsky, Ann. Rev. Immunol. 12:923-989 (1994)). Thus, a vaccine of the invention can be useful to increase an individual&#39;s resistance to HIV-1 infection or to reduce the severity of a pathology due to HIV-1 in an HIV-1 infected individual. 
     Prior to the present disclosure, it was not known that CAEV can infect humans. As disclosed herein, however, CAEV infection of human may be prevalent, particularly in populations where consumption of raw goat milk is common or where individuals in the population are otherwise exposed to goats (see Example I.C.). While it has not yet been determined whether CAEV infection causes a pathology such as arthritis or encephalitis in humans as it does in goats, if such a CAEV induced pathology exists, then it will be important to vaccinate persons involved in the goat farming industry against CAEV infection. A vaccine of the invention can be useful for this purpose. In addition, the diagnostic ELISA assay disclosed herein can be useful to identify CAEV infected individuals and can be particularly useful, for example, to screen blood supplies to identify CAEV infected blood, thereby preventing the spread of CAEV infection through administration of CAEV contaminated blood. 
     Since it has now been determined that CAEV infects humans, it is recognized that there is a likelihood that CAEV also can infect other mammals, including, for example, cows, horses, sheep, dogs, cats or other mammals. Thus, an ELISA of the invention can be useful for diagnosing CAEV infection in any such mammal. Furthermore, if it is determined that CAEV infects other such mammals, a vaccine of the invention can be useful for preventing or limiting the spread of CAEV infection in those mammals. 
     A vaccine of the invention also can be particularly useful for stimulating an immune response in a goat so as to increase the goat&#39;s resistance to CAEV infection. Such a vaccine can have a beneficial effect on the goat farming industry by preventing the spread of CAEV, which can cause arthritis, encephalitis and related pathologies in infected animals. 
     As disclosed herein, exposure of a human individual to CAEV also can stimulate an immune response to CAEV that is cross-reactive with HIV-1 (Example I.E.). CAEV is a retrovirus, subtype lentivirus, that is related to human immunodeficiency virus-1 (HIV-1). CAEV infects goats and causes arthritis and abnormalities of the immune system of infected animals (see, for example, Banks et al., Arthrit. Rheum. 30:1046-1053 (1987); Crawford et al., Science 207:997-999 (1980)). 
     Visna maedi virus (VMV) is another lentivirus that is closely related to CAEV and HIV-1 and causes a disease in sheep similar to that caused by CAEV in goats. Thus, VMV or an immunogenic fragment of VMV can be substituted for a CAEV immunogen or, if desired, can be used in combination with a CAEV immunogen in a vaccine of the invention. A vaccine containing a VMV immunogen is useful for stimulating an immune response in a human that cross-reacts against HIV-1. 
     CAEV causes a persistent infection in goats and is associated with three disease syndromes, including arthritis, which occurs in 20-30% of infected animals; leukoencephalitis, which occurs in young animals; and sporadic neurologic disease, which occurs in adult goats. CAEV infection is found world wide and was identified as the cause of arthritis and encephalitis in goats in 1980. In 1985 and 1986, the relationship between the nucleotide and amino acid sequences of CAEV and HIV-1 (then called HTLV-III) was described. CAEV and HIV-1 are closely related phylogenetically and share a high degree of homology, including, for example, between their RNA dependent DNA polymerases (pol) and between gp120/41 in HIV-1 and gp135/38 in CAEV (see, for example, Gonda et al., Proc. Natl. Acad. Sci. USA 83:4007-4111 (1986); Gonda et al., Retroviridae 3:83-109 (1994); Garry et al., Retroviridae 4:491-603 (1995); each of which is incorporated herein by reference). 
     CAEV is transmitted among goats through infected milk, particularly colostrum, and infection is spread by the agricultural practice of pooling colostrum to feed young animals. CAEV multiplies in cells of the monocyte/macrophage lineage and in fibroblast cell lines, but does not infect T cells. Macrophages expressing CAEV are distributed in the synovia, lungs, central nervous system, lymph nodes, spleen, gastro-intestinal tract and mammary glands of infected goats. 
     Prior to the present disclosure, CAEV was not known to infect humans. However, the detection of seroconversion by ELISA and western blot analyses using human blood samples (Example I.C.) and the detection of proviral DNA, corresponding to CAEV gag and pol genes, by polymerase chain reaction (PCR) analysis in genomic DNA obtained from an MCTD patient (Example I.D.) provides demonstrative evidence that CAEV infects humans. The incorporation of viral DNA sequences into host cell DNA is a hallmark of infection by the lentiviruses, including CAEV and HIV-1. 
     Based on the results disclosed herein, CAEV may infect as many as half the population of Mexico and Central America, particularly those who consume raw goat milk, which is common in these areas, or are otherwise exposed to goats (see Example I.C.). Although there is no evidence that CAEV causes a pathologic condition in humans, it is important, nevertheless, to determine the scope of CAEV infection in the human population in order to identify whether, for example, a pathology of unknown etiology correlates with CAEV infection. The ELISA assay provided herein allows for routine screening for CAEV infection in humans and provides the additional advantage of being useful for screening goats or other mammals to identify the presence of CAEV infection. 
     PCR analysis also was performed on genomic DNA obtained from three groups of goats from different geographical regions and from a CAEV infected human (see Example I.D.). The PCR analysis of human and goat genomic DNA identified at least two potential strains of CAEV, which differ from the DNA sequence of a reference CAEV strain (reference CAEV DNA sequence is available as GenBank Accession No. M33677, which is incorporated herein by reference). Remarkably, the immune responses generated in two of the three groups of CAEV infected goats and in several CAEV infected human individuals were cross-reactive with HIV-1, indicating that either of these strains can be used as a source of a CAEV immunogen for a vaccine of the invention. 
     CAEV infected goats have been used as an animal model for human rheumatoid arthritis, which is the most well known and prevalent syndrome in a cluster of arthritic/autoimmune disorders, the systemic rheumatic disorders. The systemic rheumatic disorders are about three times more common in women than men. 
     Mixed connective tissue disease (MCTD) is another systemic rheumatic disorder. MCTD is characterized by autoantibodies to the U1 snRNP splicing complex. MCTD is considered to be an overlap syndrome, in that it embraces clinical and serologic features common to three other systemic rheumatic disorders--systemic lupus erythematosus (SLE), scleroderma and polymyositis. The clinical profile of MCTD includes arthritis, lymphadenopathy, vasculitis, myositis, Sjogren&#39;s syndrome (lymphocytic infiltration of salivary and lacrimal glands) and immune dysregulation. 
     The clinical profile manifest in MCTD individuals also occurs in HIV-1 infected individuals. However, these clinical manifestations have a more severe course and are more refractory to therapy in HIV-1 infected persons. For example, Sjogren&#39;s syndrome in MCTD individuals is characterized by localized lymphocytic infiltration, whereas widespread glandular infiltration, referred to as diffuse infiltrative lymphocytosis syndrome, occurs in the corresponding HIV-1 associated syndrome. 
     Severe necrotizing vasculitis accompanied by neuropathy, inflammatory polymyopathy unrelated to drug therapy and other forms of vascular and neuromuscular disease associated with rheumatic disorders also occur in HIV-1 infected individuals. Necrotizing lymphadenopathy, which results in massive destruction of lymph node structure, occurs in MCTD, SLE and HIV-1 infection. Two classical rheumatic disorders, psoriatic arthritis and Reiter&#39;s syndrome, are the earliest manifestations of HIV-1 infection and are more refractory to chemotherapeutic agents when associated with HIV-1 infection. 
     The clinical similarities of HIV-1 infection and the systemic rheumatic disorders such as MCTD suggest that HIV-1 can be a precipitant of rheumatic pathology or that a phylogenetically related, but more clinically benign lentivirus is an etiologic factor in rheumatic disorders. Serologically, MCTD is characterized by the presence of anti-RNP (ribonucleoprotein) antibodies specific for the U1 snRNP particle, which is positioned at 5&#39;-end of introns during nuclear RNA splicing. Anti-RNP antibodies also are present in some SLE and scleroderma patients. The U1 snRNP particle is composed of a core of U1 RNA and a cluster of RNA binding proteins, including the 70K, A and C proteins. Anti-RNP antibodies inhibit RNA splicing by binding U1 snRNP. 
     Although the humoral anti-RNP autoantibody response is pathognomonic in MCTD, the 70K protein, for example, contains both T cell and B cell epitopes and the humoral and cellular autoimmune manifestations are interdependent. T cell clones responsive to 70K epitopes have been isolated and include CD8+ T cells having cytotoxic T lymphocyte activity and CD4+ T cells having helper T cell activity. Some of the 70K T cell epitopes are homologous to T cell epitopes of HIV-1 (see Douvas and Takehana, supra, 1994). As disclosed herein, CAEV proteins, including CAEV gp135, also contain epitopes that are homologous to 70K and HIV-1 T cell epitopes (see Table 1, supra). Thus, a CAEV immunogen comprising such an epitope can induce a broad based cross-reactive immune response against HIV-1. 
     In view of the clinical similarities of HIV-1 infection and MCTD and the identification of CAEV infection in MCTD patients and in otherwise healthy individuals, it was important to determine whether individuals infected with CAEV develop an immune response that cross-reacts with HIV-1. The level of immunity to the HIV-1 protein, gp120, in blood obtained from CAEV infected individuals was examined by ELISA and by western blot analysis (see Example I.E.). In addition to exploring the relationship between CAEV infection and the development of an immune response to HIV-1, the relationship between CAEV infection and MCTD was investigated. Since some MCTD patients from Mexico or Central America have antibodies that react to HIV-1 gp120 and inhibit HIV-1 infectivity in vitro, it remained to be established whether CAEV infected persons that do not have MCTD also develop immunity to HIV-1. 
     As disclosed herein, CAEV infection in humans, whether suffering from MCTD or otherwise healthy, stimulated a humoral immune response that cross-reacted with the HIV-1 gp120 or p24 viral antigen (see Example I.E.). Significantly, none of the humans examined was infected by HIV-1 as determined using clinically certified assays. These results indicate that immunization of an individual with a CAEV immunogen can generate cross-reactive immunity to HIV-1, thereby increasing the resistance to HIV-1 infection in an uninfected individual or reducing the severity of a pathology due to HIV-1 in an HIV-1 infected individual. Furthermore, the presence of a systemic rheumatic disorder such as MCTD in a CAEV infected patient substantially increased the level of HIV-1 cross-reactivity (Example I.E.). Thus, administration of a CAEV immunogen and an autoantigen such as a 70K immunogen, which generates a cross-reactive immune response against HIV-1 (Douvas and Takehana, supra, 1994), can be particularly effective in stimulating an immune response that cross-reacts with HIV-1 in a human individual. 
     A CAEV immunogen can be immunogenic by itself or can be attached to a carrier molecule such as bovine serum albumen or an inert carrier such that the CAEV immunogen-carrier complex can stimulate an immune response (see, for example, Harlow and Lane, supra, 1988). For example, where a vaccine contains the entire CAEV virus, the virus is immunogenic and can stimulate an immune response in a vaccinated individual. CAEV is readily available from the American Type Culture Collection (ATCC VR-905). 
     A vaccine of the invention also can contain a peptide portion of a CAEV protein such as gp135 (see Table 1, supra). It is recognized that each of the peptides exemplified in the Table 1 or otherwise useful in the invention may not be immunogenic by itself. However, it is well known that such haptens can be attached to a carrier molecule so as to be rendered immunogenic (see, for example, Harlow and Lane, supra, 1988). An immune response can be stimulated in vivo or ex vivo. For example, immune cells, including T cells and B cells, can be obtained from a subject and placed in a tissue culture medium. The cells then can be contacted with a CAEV immunogen, which can stimulate the immune cells and induce a T helper cell and cytotoxic T cell immune response. 
     A vaccine of the invention contains, in addition to a CAEV immunogen, a pharmaceutically acceptable carrier. Pharmaceutically acceptable carriers are well known in the art and include aqueous solutions such as physiologically buffered saline or other solvents or vehicles such as glycols, glycerol, oils such as olive oil or injectable organic esters. If desired, a vaccine of the invention can contain a pharmaceutically acceptable carrier that is an adjuvant. Adjuvants such as alum or Freund&#39;s complete or incomplete adjuvant, or other proprietary adjuvants are known in the art and commercially available (Ribi Immunochem Research, Inc.; Hamilton Mont.). The addition of an adjuvant generally decreases the amount of a CAEV immunogen required to stimulate an immune response. 
     A pharmaceutically acceptable carrier also can contain a physiologically acceptable compound that acts, for example, to stabilize the CAEV immunogen or increase its absorption. Physiologically acceptable compounds include, for example, carbohydrates, such as glucose, sucrose or dextrans; antioxidants, such as ascorbic acid or glutathione; chelating agents; low molecular weight proteins; or other stabilizers or excipients. One skilled in the art would know that the choice of a pharmaceutically acceptable carrier, including a physiologically acceptable compound, depends, for example, on the route of administration of the vaccine and on the particular physico-chemical characteristics of the CAEV immunogen. Various routes for administering a vaccine to stimulate an immune response are known in the art and include, for example, intravenous, intradermal and subcutaneous injection, oral administration and transdermal administration. 
     If desired, a vaccine of the invention also can contain a 70K immunogen, which is a surrogate immunogen that stimulates an immune response that cross-reacts against HIV-1 (Douvas and Takehana, supra, 1994). The addition of a 70K immunogen to the vaccine is advantageous because it is an autoimmune protein that represents an enrichment of sequences that can stimulate early shared clones, which are anti-70K latent T cells and B cells that can be rapidly activated to react with HIV-1 or can produce cross-reactive anti-gp120/41 antibodies that can neutralize HIV-1. The addition of a 70K immunogen also can be advantageous in that it can provide co-amplification or synergistic amplification of shared clones with gp120/41. In addition, immunogenic fragments of 70K that do not contain amino acid sequences that stimulate antibodies that mediate the deleterious effects associated with HIV-1 infection can be selected (Douvas and Takehana, supra, 1994). 
     A CAEV immunogen can be produced by various methods, including, for example, by recombinant DNA methods or by methods of chemical synthesis. For example, if a CAEV protein or a peptide fragment thereof is desired, the protein or peptide fragment can be produced by cloning the appropriate CAEV coding sequence into an expression vector such as a baculovirus vector and the protein or peptide can be expressed in and isolated from the appropriate host cell. The nucleic acid sequence of CAEV is available as GenBank Accession No. M33677 (see, also, Saltarelli et al., Virology 179:347-364 (1990); Knowles et al., J. Virol. 65:5744-5750 (1991), each of which is incorporated by reference). In addition, it can be desirable to produce the recombinant protein or peptide in a mammalian cell, which can perform a desired post-translational modification. Appropriate expression vectors and host cells are well known in the art and commercially available, and the skilled artisan would know how to select an appropriate vector/host cell system based on a particular need. 
     Where the CAEV immunogen selected is a peptide immunogen such as the CAEV immunogens exemplified in the Table 1, the peptide can be synthesized using well known method of chemical synthesis, including, for example, automated solid phase methods. Chemical synthesis of a CAEV immunogen can be particularly desirable because the method allows for the introduction of amino acid analogs into the peptide. For example, it can be desirable to synthesize a CAEV immunogen containing one or a few (D)-amino acid substitutions for corresponding naturally occurring (L)-amino acids. The incorporation of a (D)-amino acid can confer desirable properties on the CAEV immunogen, including, for example, increasing the stability of the peptide, which can be particularly useful for preparing a vaccine that is to be distributed to regions of the world where refrigeration is not always available or dependable. In addition, a CAEV peptide can be synthesized and, if desired, can include an additional cysteine residue, which can facilitate specific attachment of the peptide to a matrix or to a carrier molecule such as keyhole limpet hemocyanin using appropriate oxidizing conditions. 
     The present invention also provides a method of stimulating an individual&#39;s immune response against CAEV or stimulating an immune response that is cross-reactive against HIV-1 by administering a therapeutically effective amount of a CAEV immunogen to the subject. If desired, VMV or an immunogenic fragment of VMV can be substituted for the CAEV immunogen. In addition, it can be advantageous to administer a 70K immunogen to the individual, either in combination with the CAEV or VMV immunogen or as a separate treatment. 
     Although reference is made herein to the administration of a CAEV immunogen to an individual, or to vaccination of an individual, it is recognized that an immune response against HIV-1 can be stimulated in vivo or ex vivo. Each of these methods is encompassed within the present invention. For example, it can be desirable to stimulate an immune response against HIV-1 ex vivo where the individual to be treated has AIDS. In this case, lymphocytes can be removed from the individual and immunized in culture. At the same time, the lymphocyte population can be expanded. The stimulated, expanded immune cells then can be reinfused into the individual, thereby providing a therapeutic advantage to the individual. Furthermore, even if a method of the invention such as the ex vivo immunization and reinfusion method is not be curative, such a treatment can be palliative and, therefore, can increase the quality of life of an individual suffering from AIDS. 
     As disclosed herein, vaccination of an individual with a CAEV immunogen stimulates an immune response that is cross-reactive against HIV-1. vaccination against HIV-1 using a CAEV immunogen, either alone or in combination with a 70K immunogen, provides significant advantages over the use, for example, of a killed or attenuated form of HIV-1 or an HIV-1 protein antigen as a vaccine in that CAEV is not known to be a human pathogen and does not infect T cells. In addition, CAEV infection, particularly in individuals suffering from MCTD, provides a broad based immunologic resistance to HIV-1, including a B cell response and a T cell response against various strains of HIV-1. For example, in the population of Hispanic individuals examined in the study disclosed in Example I, greater than 60% of the individuals were infected with CAEV (see Example I.D.). In addition, 22% of the CAEV infected, but otherwise healthy, individuals demonstrated cross-reactivity to HIV-1 and 55% of the CAEV infected, MCTD patients demonstrated cross-reactivity to HIV-1 (Example I.E.). Controlled methods of immunization with a CAEV immunogen, alone or in combination with a 70K immunogen, can increase the frequency of individuals demonstrating cross-reactive immunity to HIV-1 (see Example II). 
     In order to stimulate an immune response against HIV-1, a therapeutically effective amount of a CAEV immunogen is administered to an individual. As used herein, the term &#34;therapeutically effective amount&#34; means an amount of an immunogen required to stimulate an immune response. The amount of an immunogen such as a CAEV immunogen that constitutes a therapeutically effective amount will vary, depending, for example, on whether stimulation of the immune response is in vivo or ex vivo; on whether the administration is a first administration or a booster administration; whether an adjuvant is administered with the immunogen; and, when administered in vivo, on the route of administration. In general, a therapeutically effective amount of a CAEV immunogen is about 50 μg to about 50 mg, preferably about 500 μg to about 5 mg. For example, where a CAEV virus is administered, a therapeutically effective amount of the particulate CAEV immunogen can be about 100 μg to about 2 mg, whereas, where a soluble CAEV immunogen is administered, a therapeutically effective amount can be about 1 mg to about 5 mg. Methods for determining a therapeutically effective amount of an immunogen are routine and well known in art (see Example II; see, also, Powell and Newman, Vaccine Design: The subunit and adjuvant approach (Plenum Publ. Corp.; 1994), which is incorporated herein by reference). 
     Methods for vaccinating an individual so as to stimulate an immune response also are well known (Harlow and Lane, supra, 1988). For example, the immunogen can be administered intradermally, intramuscularly or intravenously. In addition, it can be advantageous to administer one or more booster immunizations. The need to administer a booster immunization and the timing of such booster immunizations can be determined experimentally by measuring, for example, the presence of anti-HIV-1 antibodies in a vaccinated individual&#39;s serum, using the methods disclosed herein. 
     A method of the invention can be useful for increasing the resistance to HIV-1 infection of an individual not previously exposed to HIV-1. Similarly, a method of the invention can be useful for increasing the resistance to CAEV infection of a human or goat or other susceptible mammal not previously exposed to CAEV. As used herein, the term &#34;increasing the resistance,,&#34; when used in reference to HIV-1 or CAEV infection, means that the likelihood of infection by the virus is reduced due to the stimulation of a immune response against the virus and consequent generation of memory immune cells, which can be rapidly mobilized against and sequester subsequent viral infection. In the case of CAEV, resistance is increased due to vaccination with a CAEV or a VMV immunogen, respectively, whereas, in the case of HIV-1, resistance is increased to vaccination with a CAEV or a VMV immunogen, which stimulates an immune response that cross-reacts with HIV-1. 
     In addition, a method of the invention can be useful in an individual, including a human or a mammal such as a goat, that currently is infected with a virus and is suffering a pathology due to the infection. In particular, a method of administering a CAEV immunogen to a human in order to stimulate an immune response in an HIV-1 infected individual that is cross-reactive against HIV-1 can be useful for decreasing the immunopathology of HIV-1. Specifically, the stimulation of such a response in an HIV-1 infected individual can prevent the further spread of HIV-1 infection in the individual, thereby reducing the depletion of T cells in the individual. Since many of the pathologies observed in an HIV-1 infected individual are due to the depleted immune system, a treatment that reduces depletion of the immune system can provide a therapeutic advantage. 
     Based on the studies disclosed herein, a significant population of persons, particularly in Mexico and Central America, may be infected with CAEV. Thus, it is important that a simple and inexpensive assay for diagnosing an individual infected with CAEV be available. Accordingly, the invention further provides method of diagnosing a CAEV infection in a sample suspected of being infected with CAEV. For example, the invention provides an enzyme linked immunoadsorption assay (ELISA) useful for diagnosing the presence of a CAEV infection in an individual suspected of having a CAEV infection, by obtaining a sample such as a blood sample from the individual, contacting the sample with a CAEV antigen and detecting binding of an anti-CAEV antibody present in the sample to the CAEV antigen, wherein such a detection is diagnostic of CAEV infection. Thus, methods and reagents for performing a diagnostic assay of the invention are disclosed herein or otherwise known in the art (see, for example, Litt, U.S. Pat. No. 4,092,408, issued May 30, 1978, which is incorporated herein by reference). 
     A method of the invention is exemplified by the ELISA (Example I.C.), in which CAEV gp135 was used as the CAEV antigen. However, a CAEV antigen useful in a method of the invention can be selected from essentially any antigenic fragment of CAEV or can be the entire CAEV viral particle. For purposes of the describing a method of the invention, the term &#34;CAEV antigen&#34; is intended to be synonymous with the term &#34;CAEV immunogen&#34; as defined above. 
     A competition ELISA can be particularly useful for diagnosing the presence of a CAEV infection in a sample. A competition ELISA is performed similarly to a standard ELISA, except that one or more monoclonal antibodies, each specific for a CAEV epitope such as the epitopes defined by the CAEV immunogens shown in Table 1, is added to the reaction to compete with an anti-CAEV antibody present in the sample. The use of a competition ELISA can produce enhanced sensitivity for detecting the presence of an anti-CAEV antibody in a sample. Methods for performing a competition ELISA such as the threshold ligand receptor assay of Buechler et al. (U.S. Pat. No. 5,089,391, issued Feb. 18, 1992, which is incorporated herein by reference) are well known in the art. 
     Monoclonal antibodies specific for particular CAEV epitopes can be prepared using well known methods (see, for example, Harlow and Lane, supra, 1988). For example, a mouse can be immunized with a CAEV immunogen (see, for example, Table 1), then spleen cells are collected from a mouse having a high titer of antibody for the particular CAEV immunogen. Methods for identifying an anti-CAEV immunogen antibody having an appropriate specificity and affinity and, therefore, useful in the invention are disclosed herein or otherwise known in the art and include, for example, enzyme-linked immunoadsorption assays, radioimmunoassays and precipitin assays (see Example I; see, also, Harlow and Lane, supra, 1988, chap. 14). The mouse spleen cells can be fused to an appropriate myeloma cell line such as SP/02 or P3x653.Ag8 myeloma cells to produce hybridoma cells. Cloned hybridoma cell lines can be screened using a labelled CAEV immunogen to identify clones that secrete monoclonal antibodies specific for the CAEV immunogen. Hybridomas that express antibodies having a desirable specificity and affinity can be isolated and utilized as a continuous source of monoclonal antibodies useful in the competition ELISA. 
     As used herein, the term &#34;sample,&#34; when used in reference to a diagnostic method of the invention, means a tissue specimen or a fluid specimen such as a blood, which can be whole blood, plasma or serum, or a urine specimen, which is obtained from an individual such as a human, goat or other mammal to be tested for CAEV infection. Methods of obtaining such a sample, including an appropriate tissue sample, are well known and routine in the art. 
     A method of the invention can diagnose CAEV infection in an individual due, in part, to the presence of anti-CAEV antibodies in a sample obtained from the individual. A sample obtained from an individual suspected of being infected with CAEV is contacted with a CAEV antigen under suitable conditions, which allow the CAEV antigen to bind with an anti-CAEV antibody, if present, in the sample. Thus, a method of the invention can be an ELISA, which is exemplified herein, or can be a radioimmunoassay, western blot or other such assay based on the detection of a specific antibody-antigen interaction (see, for example, Harlow and Lane, supra, 1988; see, also, Gribnau et al., U.S. Pat. No. 4,373,932, issued Feb. 15, 1983, which is incorporated herein by reference). 
     Methods for detecting an antigen-antibody interaction such as the interaction of a CAEV antigen with an anti-CAEV antibody present in a sample are well known in the art and include, for example, the use of a detectably labelled antigen or the use of a detectably labelled second antibody, which is an antibody that specifically binds a particular class of antibody such as IgG, IgA, IgM, IgA, IgD or IgE from a particular mammalian species (see, for example, Greene and David, U.S. Pat. No. 4,376,110, issued Aug. 1983, which is incorporated herein by reference). For example, if a sample is a blood serum or blood plasma sample from a human individual, a second antibody can be a goat, anti-human IgG (see Example I.C.). Such second antibodies can be prepared using well known methods or can be purchased from a commercial source. Methods for detectably labeling a CAEV antigen or an antibody also are well known and routine in the art. 
     CAEV gp135 was selected as the antigen in the exemplified ELISA because the presence of anti-gp135 antibodies in antiserum collected from a human individual, besides being indicative of infection of the individual with CAEV, also indicates that the individual can have antibodies that are cross-reactive with HIV-1. In fact, 22% of the human individuals identified as CAEV infected by the CAEV ELISA also had antibodies cross-reactive to HIV-1 as determined HIV-1 ELISA and western blot assays (see Example I.E.). In addition, 55% of the MCTD individuals that were infected with CAEV as determined using the CAEV ELISA also had antibodies cross-reactive to HIV-1. 
     An ELISA of the invention also can be used to screen mammals other than humans, including, for example, a herd of goats or other mammals susceptible to CAEV infection, to identify CAEV infected individuals. The availability of a simple diagnostic ELISA for identifying CAEV infected goats can be a boon, for example, to the goat farming industry because CAEV infected animals can be identified and removed from the herd well before the clinical signs of CAEV infection occur and before the disease is spread to uninfected goats. 
     A method of the invention such as an ELISA is particularly convenient if it is available in the form of a kit. Such an ELISA kit, for example, can contain a CAEV antigen such as gp135 attached to a solid support such as a plastic support. For example, the kit can contain a microtiter plate such as a 96 well plate containing a CAEV antigen attached to some or all of the wells. Other solid supports useful in an ELISA assay are known in the art. Use, for example, of a 96 well microtiter plate in an ELISA provides the additional advantage that, upon selection of an appropriate detectable label such as a radioactive, fluorescent or luminescent label, the assay can be automated, thereby allowing rapid through put of a large number of samples, including, if desired, appropriate standard or control samples. 
     A kit such as an ELISA kit also can contain, if desired, a standard reagent such as a predetermined amount of an anti-gp135 antibody. Such reagents can provide a means to readily determine whether a sample such as a blood sample or a blood serum sample obtained from an individual contains an amount of circulating anti-CAEV antibody. Accordingly, a kit of the invention can contain, for example, an appropriate buffer, which provides suitable conditions for binding of an anti-CAEV antibody present in the sample to a CAEV antigen attached to the solid support. By providing such reagents in the form of a kit, the ELISA assay can be standardized such that results of different tests performed in different places at different times can be compared with each other. It is recognized that, for a selected CAEV antigen, a population of serum samples obtained from uninfected individuals (controls) should be analyzed in order to determine a base line level of reactivity to the particular CAEV antigen. 
     A method of the invention such as the exemplified CAEV ELISA can be useful to detect the presence of circulating anti-CAEV antibodies or to follow the development of an immune response. For example, an individual can be vaccinated with a vaccine of the invention and the development of the immune response can be followed using the ELISA of the invention. Such a method can be useful for determining whether a booster immunization is required and, if necessary, the optimal time for administering the booster. In addition, a CAEV assay such as the exemplified ELISA can be useful for screening a sample of blood provided by a blood donor in order to identify and remove CAEV contaminated blood from a blood bank. 
    
    
     The following examples are intended to illustrate but not limit the present invention. 
     EXAMPLE I 
     Characterization of HIV-1 Cross-Reactivity in CAEV Infected Individuals 
     This example describes the methods used to demonstrate that CAEV infected individuals generate an immune response that is cross-reactive with HIV-1 gp120. 
     A. Human subjects: 
     More than 50 human subjects were involved in the studies disclosed herein. The subjects were categorized based on their disease status (MCTD or normal) and their place of origin (Mexico/Central America or the United States) and were grouped as follows: 
     Group A. MCTD--Hispanic born females, clinically diagnosed with MCTD. 
     Group B. MCTD--U.S. born black or Caucasian females, clinically diagnosed with MCTD. 
     Group C. MCTD--Guadalajara female residents, clinically diagnosed with MCTD. 
     Group D. Normal (non-MCTD)--Hispanic born females. 
     Group E. Normal--non-Hispanic males and females, including two workers in a veterinary service that treats infected goats. 
     Group F. Normal--4 female and 4 male residents of a village located 30 km from Guadalajara, including the owner of a herd of 150 goats and 5 members of his family. 
     Group G. Scleroderma patients, all having anti-Scl-70/type 1 antibodies. 
     B. Blood Samples and Reagents: 
     Human and goat (see Section I.F., below) blood samples were collected and the serum component was used for immunologic testing. The antigens used in the ELISA and western blot assays to measure levels of serum antibodies included crude CAEV and affinity purified gp135 and recombinant HIV-1 gp120 and HIV-1 p24. Recombinant HIV-1 gp120 and p24 were purchased from Intracel (Shepreth UK). 
     Crude CAEV is prepared as described by Klevjer-Anderson et al., (Virology 110:113-119 (1981), which is incorporated herein by reference). Essentially, CAEV is grown in primary fetal goat synovial membrane (SM) cells grown in DMEM with a bicarbonate-HEPES buffer supplemented with 2 mM glutamine, 100 mg/ml streptomycin and 100 units/ml penicillin and contain in g 10% FBS. Medium of CAEV infected SM cultures is collected and clarified by centrifugation at 600×g. 
     Aliquots of the crude CAEV preparation can be stored frozen at  -  70° C. The crude CAEV preparation can be used to isolate purified CAEV or can be used to prepare purified CAEV proteins. Purified CAEV proteins such as purified CAEV gp135 also can be prepared by cloning a nucleic acid molecule encoding gp135 (GenBank Accession NO. M33677; see, also, Saltarelli et al., supra, 1990; Knowles et al., supra, 1991) into an expression vector using routine methods of recombinant DNA technology and purifying the gp135 as described below. 
     CAEV gp135 is affinity purified from the crude CAEV preparation using an anti-gp135 antibody conjugated to a chromatography matrix. Essentially, the crude CAEV preparation is adjusted to 0.1% SDS to lyse the CAEV viral particles, then applied to an anti-gp135 antibody affinity chromatography column or recombinantly produced gp135 is applied to the affinity column. 
     Anti-gp135 antibody is isolated from CAEV infected goat serum or from serum obtained from goats immunized with a peptide fragment of CAEV gp135. If a CAEV infected goat is used as the source of serum, a goat having a high titer of anti-gp135 antibodies is used as the source of serum. Such serum can be identified by using western blot analysis to screen serum samples obtained from several different goats. Anti-gp135 antibodies are isolated from the serum by ammonium sulfate precipitation, followed by column chromatography to obtain the IgG fraction. The anti-gp135 antibodies are attached to a chromatography matrix such as cyanogen bromide activated Sepharose™. Methods for isolating IgG antibodies and attaching such antibodies to a matrix are well known and routine in the art (see, for example, Harlow and Lane, supra, 1988). 
     The crude CAEV preparation or recombinantly produced gp135 in PBS is loaded onto the anti-gp135 antibody affinity column and gp135 is allowed to bind to the antibodies overnight at 4° C. Following binding, the column is washed with PBS, then bound gp135 is eluted with 10 mM HCl, 0.15 M NaCl. Purity of the gp135 antigen is determined by SDS-PAGE and silver staining. 
     C. CAEV ELISA and Western Blot Assays: 
     For ELISA assays, crude CAEV or affinity purified gp135 antigen was diluted 1:20 with phosphate buffered saline (PBS; final concentration gp135 approx. 1 μg/ml) and 50 μl was added to each well of a 96 well plate (Corning). The plate was covered and incubated overnight at 4° C., then antigen was removed and the wells were washed 3× for 1 min each with 90 μl washing solution (1 mg BSA/ml PBS). 100 μl blocking solution (10 mg BSA/ml PBS) was added to each well and plates were incubated 60 min at room temperature (RT), then the blocking solution was removed and the wells were washed 3× for 1 min each with washing solution. 
     Fifty μl of diluted serum sample (1:100 in washing solution) was added to each well and plates were incubated 60 min at RT. Blank (control) wells contained washing solution, alone. Following incubation, wells were washed as above, then 50 μl diluted second antibody was added and incubated as above. Where the serum sample to be analyzed was from humans, the second antibody was goat anti-human IgG conjugated to horse radish peroxidase (HRP; Zymed Laboratories, Inc.; South San Francisco Calif.; cat. # 62-8420) diluted 1:1000 in PBS. Where the serum sample to be analyzed was from goats, the second antibody was rabbit anti-goat IgG-HRP (Jackson ImmunoResearch Labs, Inc.; West Grove Pa.; cat. # 305035003, lot # 28671) diluted 1:3000 in PBS. Following incubation, wells were washed 2× with washing solution, then 2× with PBS. 
     Seventy-five μl color development solution (10 mg O-phenyldiamine (Sigma) in 25 ml ELISA buffer; 50 μl 30% H 2  O 2 , prepared fresh; ELISA buffer is 500 ml 0.1 M citric acid, 500 ml 0.2 M Na 2  HPO 4 , adjusted to pH 5.0) was added to each well and incubated 10 min at RT in the dark. Color development was terminated by adding 25 μl 6N H 2  SO 4 , then absorption was measured at OD 490 . 
     For CAEV western blot analysis, 0.1-0.5 μg aliquots of gp135 in SDS sample buffer (Laemmli, Nature 227:680-685 (1970), which is incorporated herein by reference), were separated by 10% SDS-PAGE, then transferred electrophoretically onto a nitrocellulose membrane. The membrane was incubated for 1 hr in 50 ml blocking buffer (1% BSA in WBS; WBS is 9 g NaCl, 1.21 g Tris-base, 0.25 ml NP-40 made up to 1 liter, pH 7.4), then washed 3× with washing buffer (0.1% BSA in WBS). The membrane was cut into strips, representing the lanes on the gel. 
     Serum samples were diluted 1:100 (20 μl serum/2 ml washing buffer; 40 μl/2 ml if plasma used), then added to an incubation tray containing a nitrocellulose strip and shaken for 60 min at RT. Following incubation, the strips were washed 3× with washing buffer, then 2 ml second antibody (as above, but diluted with washing buffer) was added and incubation continued for 60 min at RT. Strips were washed 3× with washing buffer and the enzyme reaction was initiated by adding 2 ml color development solution (10 mg 3,3-diaminobenzidine tetrahydrochloride (Sigma Chemical Co.; St. Louis Mo.; cat. # D 5905), 1 ml 0.05 M Tris, pH 7.5, 39 ml WBS, 80 μl H 2  O 2 , prepared fresh). Incubation was continued from 10 min at RT with gentle shaking, then color development was terminated by transferring the membrane to distilled water. 
     High reactivity to extracts of CAEV and to purified gp135 was observed in 62% of the serum samples obtained from Hispanic born MCTD patients (Table 2), including those from LAC/USC (group A) and those from Guadalajara, Mexico (group C). Serum samples from the disease control (group G) and from non-Hispanic healthy individuals, excluding the two veterinarians, were weakly reactive or non-reactive. The human serum samples that reacted to CAEV on western blots contained antibodies to the same polypeptides as those reacting with serum samples obtained from CAEV infected goats. 
     The reactivity to CAEV of serum samples from the normal individuals correlated with a history of drinking raw goat milk or other exposure to goats. The highest reactivities were present in the owner of the Guadalajara goat herd and in one of the veterinarians, who, in addition to treating CAEV infected goats, also reported regularly consuming raw goat milk. The second veterinarian, who also treated CAEV infected goats, also had positive reactivity. 
     In summary, 22 of 39 MCTD patients and 11 of 23 non-MCTD individuals had positive reactivity for CAEV infection, including 20 of 33 Hispanic MCTD patients (61%) and 9 of 14 Hispanic non-MCTD individuals (64%; see Table 2). These results indicate that over 60% of the Hispanic individuals examined in this study were infected with CAEV. In the CAEV reactive individuals, the reactivity of serum obtained from Hispanic MCTD patients was higher than that of the non-MCTD Hispanic individuals. 
     
                       TABLE 2                                                     
______________________________________                                    
The relationship between CAEV gp135 and HIV-1 gp120 reactivity            
in MCTD patients and normals                                              
positive gp135    negative gp135                                          
______________________________________                                    
1. MCTD               3. MCTD                                             
n = 22; hispanic = 20 n = 17; hispanic = 13                               
total gp120(+)                                                            
             ##STR1## total gp120(+)                                      
                                     ##STR2##                             
hispanic gp120(+)                                                         
             ##STR3## hispanic gp120(+)                                   
                                     ##STR4##                             
2. NL                 4. NL                                               
n = 11; hispanic = 9  n = 9; hispanic = 5                                 
total gp120(+)                                                            
             ##STR5## total gp120(+)                                      
                                     ##STR6##                             
hispanic gp120(+)                                                         
             ##STR7## hispanic gp120(+)                                   
                                     ##STR8##                             
______________________________________                                    
Summary 1: CAEV(+)                                                        
Total Hispanic  29/47 = 62%                                               
MCTD - hispanic 20/33 = 61%                                               
NL - hispanic    9/14 = 64%                                               
Total Nonhispanic                                                         
                 2/15 = 13%                                               
MCTD - nonhispanic                                                        
                 2/6 = 33%                                                
NL - nonhispanic                                                          
                 2/6 = 33%                                                
Summary 2: CAEV(+)/gp120(+)                                               
hispanic MCTD   12/22 = 55%                                               
hispanic NL      2/9 = 22%                                                
CAEV(-)/gp120(+)                                                          
hispanic MCTD and NL                                                      
                 0/20 = 0%                                                
non-hispanic MCTD and NL                                                  
                 0/11 = 0%                                                
 
    
     D. PCR analysis: 
     PCR analysis for identification of CAEV gag and pol genes was performed using genomic DNA isolated from peripheral blood mononuclear cells (PBMC). PBMC were obtained by Ficoll density gradient centrifugation of whole citrated or heparinized blood collected from selected human individuals and from WA, UC Davis and Gua goats (see below). 
     CAEV-specific genomic DNA sequences encoding regions of the gag protein were amplified in two stages. The first stage produced amplification products representing nucleotides 1057 to 1553 of CAEV. Primers were 5&#39;-GCAGTTGGCATATTATGCTACTAC-3&#39; (SEQ ID NO: 14; CAEV nucleotides 1057-1080) and 5&#39;-CTTGTTGTACTCTTTGTCCTAGTG-3&#39; (SEQ ID NO: 15; nucleotides 1530-1553). The second stage produced amplification products within the product of the first amplification product, from nucleotides 1329 to 1501. The primers for the second stage were 5&#39;-GAGCAGTAAGACATATGGCGGCAC-3&#39; (SEQ ID NO: 16; nucleotides 1329-1351) and 5&#39;-TGATGCATTTGTATATAAGATAGTGTTAGCTT-3&#39; (SEQ ID NO: 17; nucleotides 1471-1510). 
     DNA encoding the CAEV pol also was examined by PCR analysis. Primers for the first stage of amplification were 5&#39;-GGATTTGAACTACACCCGCAG-3&#39; (SEQ ID NO: 18; CAEV nucleotides 2845-2865) and 5&#39;-CCTGTTGATACTATGAACCCTAGAC-3&#39; (SEQ ID NO: 19; nucleotides 3404-3518); primers for the second stage were 5&#39;-AAGAACCTAAGCATCCCGCAAC-3&#39; (SEQ ID NO: 20; nucleotides 3223-3244) and 5&#39;-GTGATGTTCCCTAATTGCAATTCTAGTC-3&#39; (SEQ ID NO: 21; nucleotides 3341-3368). 
     Amplifications were performed using an annealing temperature of 52° C. and an elongation temperature of 72° C. and were allowed to proceed for 38 cycles. Taq polymerase or pfu Taq polymerase (Promega Corp.; Madison Wis.) was used as recommended by manufacturer. One μl of the reaction mixture from the first stage of amplification was used for the second stage of amplification, which was performed under identical conditions. The products following the second stage of amplification were separated by agarose electrophoresis and collected by eluting the appropriate band. The amplified DNA samples then were cloned into a TA3 pCR™ vector (Invitrogen; La Jolla Calif.) and sequenced using a Sequenase™ Version 2.0 DNA sequencing kit (U.S. Biochemical Corp.; Cleveland Ohio) as recommended by the manufacturer. The cloned DNA sequences were compared to a reference CAEV DNA sequence (GenBank Accession No. M33677). 
     PCR analysis revealed that CAEV proviral DNA was present in genomic DNA of individuals that were CAEV positive as determined by ELISA or western blot analysis. Specifically, DNA encoding CAEV gag and pol sequences was amplified from genomic DNA of an MCTD patient (group A) and from the CAEV infected UC Davis goat. Comparison with the reference CAEV gag sequence demonstrated that the gag sequence in the MCTD patient diverged from the reference sequence by 8.2% and that the gag sequence in the UC Davis goat diverged by 8.8%. In addition, comparison with the reference CAEV pol sequence demonstrated that the CAEV pol sequence in the MCTD patient diverged by 7.6% and the CAEV po1 sequence in the UC Davis goat diverged by 7.6%. 
     The relatively high degree of similarity between the CAEV sequences present in the human genomic DNA samples and the reference goat CAEV sequence provides definitive evidence of CAEV infection in humans. The average divergence of about 8% between the sequences indicates that the human CAEV infection can be due to a distinctive, human-adapted CAEV strain. Additional DNA sequencing can clarify whether a human-adapted CAEV strain is responsible for human CAEV infection. 
     E. HIV-1 ELISA and Western Blot Assays: 
     Certified HIV-1 ELISA and western blot diagnostic kits were purchased from Organon Teknika (Cambridge UK) and the assays were performed as recommended by the supplier. Based on the clinical criteria using the certified HIV-1 ELISA and western blot assays, all of the human serum samples were negative for HIV-1, except for known HIV-1 positive sera and antibodies used as positive controls. 
     ELISA and western blot assays were performed using the individual recombinant HIV-1 antigens, gp120 and p24, and HRP-conjugated rabbit anti-goat and goat anti-human second antibodies (Zymed Laboratories Inc.) as previously described (Douvas and Takehana, supra, 1994; Crow et al., Cell. Immunol. 121:99-112 (1989), which is incorporated herein by reference). Subsets of sera that reacted with CAEV gp135 also reacted with HIV-1 gp120, including a total of 12 MCTD sera reacted with HIV-1 gp120 (see Table 2, supra). 
     As discussed in Section I.C., above, 22 of 39 MCTD patients and 11 of 23 non-MCTD individuals were positive for CAEV infection, including 20 of 33 Hispanic MCTD patients (61%) and 9 of 14 Hispanic non-MCTD individuals (64%; Table 2). The results of the HIV-1 assays further indicated that 12 of the 22 CAEV positive Hispanic MCTD patients (55%) and 2 of the 9 CAEV positive Hispanic non-MCTD individuals (22%) also showed positive reactivity to HIV-1 gp120 (see Table 2). In contrast, none of the CAEV negative Hispanic MCTD or non-MCTD individuals was positive for HIV-1 gp120. 
     These results indicate that CAEV infection of otherwise healthy individuals occurs without the concomitant development of clinical disease, including without developing symptoms of MCTD. However, exposure to CAEV results in the development of an immune response to CAEV that generalizes to HIV-1 in a subset of infected individuals. In addition, in CAEV positive MCTD patients, the reactivity to CAEV is about 1.6 times stronger than in CAEV positive non-MCTD individuals and the frequency of cross-reactivity to HIV-1 gp120 is more frequent. These results demonstrate that cross-reactivity to HIV-1 develops as a result of infection with CAEV and that the cross-reactive immune response is increased in an individual with an autoimmune disease. 
     F. CAEV Infected Goats: 
     Three groups of goats were studied as follows: 
     Group I. WA--uninfected goats and experimentally infected (CAEV) goats maintained at Washington State University, Pullman Wash. 
     Group II. UC Davis--a naturally infected (CAEV) goat maintained at the University of California, Davis Calif. 
     Group III. Gua--20 naturally infected (CAEV) goats belonging to a goat herder in Guadalajara Mexico (see Group F, above). 
     Serum samples collected from the various CAEV infected or uninfected goats were examined. The Gua goats (Group III) lacked reactivity to CAEV gp135 but about 50% were reactive against CAEV gp38 and gag. In addition, the Gua goats, but not the WA goats showed strong reactivity to HIV-1 gp120 and p24. The UC Davis goat was reactive against both gp135, gp120 and p24. 
     These results demonstrate that differences exist between the immunologic reactivities of the WA and Gua goats to CAEV and to HIV-1 antigens. These differences may derive from different viral strains or may be due to the different routes of infection. For example, the WA goats were infected intravenously, whereas the Gua goats and the UC Davis goat were naturally infected via ingestion of infected milk. 
     G. HIV-1 Viral Neutralization Assay: 
     Virus neutralization assays are performed in 48 well tissue culture plates using PHA-activated normal donor PBMC maintained in growth medium at a concentration of 1×10 6  cells/ml as targets for infection. Various dilutions of HIVIG™, heat-inactivated (56° C., 30 min) MCTD sera or purified IgG are incubated with 10 or 50 TCID 50  (tissue culture infecting dose-50) of virus for 30 min at 37° C. 
     Negative controls for the MCTD sera and for HIVIG™ include heat inactivated serum from individuals not infected by HIV-1 and not affected by a systemic rheumatic disorder, and IVIG™, respectively. HIVIG™, which is a neutralizing IgG preparation from pooled serum from 9 seropositive donors in early stages of HIV-1 infection, and IVIG™, which is IgG from normal donors, are obtained from North American Biologicals Inc. (Miami Fla.). TCID 50  is determined using routine methods of viral titering. 
     Following incubation of the serum or IgG and the virus, serum/IgG-virus mixtures are added to the target cells in triplicate. After an overnight incubation at 37° C., the plates are centrifuged at 1000 rpm for 5 min and the medium is completely replaced. This washing step is repeated 3× to ensure complete removal of the serum/IgG-virus inoculum. On day 4 of culture, the medium is changed and on day 7 the supernatant is harvested for quantitation of p24 core antigen. Percent inhibition is determined by comparing the mean p24 core antigen concentration in each serum/IgG treated tissue culture well with that of the specific negative control. 
     In parallel, a control is prepared to measure any non-specific reduction in the amount of p24 core antigen measured due to a serum or HIVIG™ not being completely removed by the washing steps. At the time that the serum/IgG-virus mixtures are added to the target cells, each serum, IVIG™ or HIVIG™ also is added to target cells without preincubation with virus. These wells are treated in parallel with the experimental cultures. After 7 days of culture, the supernatant from each well is mixed 1:1 with that of the negative control and the p24 antigen concentration is quantitated. A measurement that is less than 50% the value of the negative control is considered non-specific inhibition of p24 antigen, which can be due to the presence of residual p24 antibodies. 
     EXAMPLE II 
     Use of a CAEV Vaccine to Vaccinate an Individual 
     This example describes a method of administering a CAEV vaccine to an individual in order to stimulate an immune response against CAEV. 
     CAEV virus is prepared by inoculation of human fetal synovial cell (see Example I.B.; see also, Klevjer-Anderson et al., supra, 1981). Cell supernatants are harvested and CAEV virions are pelleted by centrifugation at 150,000×g at 4° C. to obtain a sterile suspension of approximately 2×10 8  pfu/ml, containing 50% glycerol as a diluent (see Graham et al., J. Infect. Dis. 166:244-252 (1992), which is incorporated herein by reference). Recipients of live virus are vaccinated intradermally, using a sterile bifurcated needle, with 50 μl CAEV suspension to a single skin site. Vaccinations also can be performed using a killed or attenuated CAEV preparation or other CAEV immunogen such as CAEV gp135 (see, also, Table 1, supra). A sterile transparent dressing is applied to the vaccination site, then removed after a crust is formed. 
     Blood samples are drawn on or about days 14, 28 and 54 after administration of the CAEV immunogen and evaluated to determine antibody titers (humoral response) or T helper or cytotoxic T cell immune response (cellular response). Methods for examining the humoral and cellular responses are disclosed in Example I or otherwise well known in the art (see, for example, Egan et al., J. Infect. Dis. 171:1623-1627 (1995), which is incorporated herein by reference; see, also, Harlow and Lane, supra, 1988). If desired, a secondary (booster) immunization is administered on or about day 56 after the initial vaccination. Additional evaluations of humoral and cellular responses are made on or about days 70, 90, 160, 180, 270 and 355 after the initial vaccination. If desired, a tertiary (booster) immunization is administered on or about day 365. 
     For administration of a CAEV immunogen that is a haptenic peptide, approximately 1 mg of peptide is conjugated to keyhole limpet hemocyanin and administered with an adjuvant, intradermally, as above, in a volume of about 0.1 ml. Evaluations of the humoral and cellular response are made as above and, if desired, secondary or tertiary immunizations are administered. 
     Although the invention has been described with reference to the examples above, it should be understood that various modifications can be made without departing from the spirit of the invention. Accordingly, the invention is limited only by the following claims. 
     
         __________________________________________________________________________
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- Arg Met Gln Arg Lys Glu Arg His Lys                                     
1               5                                                         
- (2) INFORMATION FOR SEQ ID NO:7:                                        
-      (i) SEQUENCE CHARACTERISTICS:                                      
#acids    (A) LENGTH: 10 amino                                            
          (B) TYPE: amino acid                                            
          (D) TOPOLOGY: linear                                            
-     (ii) MOLECULE TYPE: peptide                                         
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:                             
- Val Arg Ser Ser Tyr Gly Ile Thr Ser Ala                                 
#                10                                                       
- (2) INFORMATION FOR SEQ ID NO:8:                                        
-      (i) SEQUENCE CHARACTERISTICS:                                      
#acids    (A) LENGTH: 10 amino                                            
          (B) TYPE: amino acid                                            
          (D) TOPOLOGY: linear                                            
-     (ii) MOLECULE TYPE: peptide                                         
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8:                             
- Gly Arg Ile Lys Leu Gln Gly Ile Gly Gly                                 
#                10                                                       
- (2) INFORMATION FOR SEQ ID NO:9:                                        
-      (i) SEQUENCE CHARACTERISTICS:                                      
#acids    (A) LENGTH: 10 amino                                            
          (B) TYPE: amino acid                                            
          (D) TOPOLOGY: linear                                            
-     (ii) MOLECULE TYPE: peptide                                         
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:                             
- Gln Glu Ile Leu Glu Asp Trp Ile Gln Gln                                 
#                10                                                       
- (2) INFORMATION FOR SEQ ID NO:10:                                       
-      (i) SEQUENCE CHARACTERISTICS:                                      
#acids    (A) LENGTH: 11 amino                                            
          (B) TYPE: amino acid                                            
          (D) TOPOLOGY: linear                                            
-     (ii) MOLECULE TYPE: peptide                                         
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:10:                            
- Lys Arg Ile Asn Asn Lys Tyr Asn Lys Asn Se - #r                         
#                10                                                       
- (2) INFORMATION FOR SEQ ID NO:11:                                       
-      (i) SEQUENCE CHARACTERISTICS:                                      
#acids    (A) LENGTH: 10 amino                                            
          (B) TYPE: amino acid                                            
          (D) TOPOLOGY: linear                                            
-     (ii) MOLECULE TYPE: peptide                                         
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:11:                            
- Asp Gly Leu Leu Glu Gln Glu Glu Lys Lys                                 
#                10                                                       
- (2) INFORMATION FOR SEQ ID NO:12:                                       
-      (i) SEQUENCE CHARACTERISTICS:                                      
#acids    (A) LENGTH: 10 amino                                            
          (B) TYPE: amino acid                                            
          (D) TOPOLOGY: linear                                            
-     (ii) MOLECULE TYPE: peptide                                         
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:12:                            
- Ser Val Phe Pro Ile Val Val Gln Ala Ala                                 
#                10                                                       
- (2) INFORMATION FOR SEQ ID NO:13:                                       
-      (i) SEQUENCE CHARACTERISTICS:                                      
#acids    (A) LENGTH: 8 amino                                             
          (B) TYPE: amino acid                                            
          (D) TOPOLOGY: linear                                            
-     (ii) MOLECULE TYPE: peptide                                         
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:13:                            
- Ala Ile Asp Ala Glu Pro Thr Val                                         
1               5                                                         
- (2) INFORMATION FOR SEQ ID NO:14:                                       
-      (i) SEQUENCE CHARACTERISTICS:                                      
#pairs    (A) LENGTH: 24 base                                             
          (B) TYPE: nucleic acid                                          
          (C) STRANDEDNESS: single                                        
          (D) TOPOLOGY: linear                                            
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:14:                            
#                24GCTA CTAC                                              
- (2) INFORMATION FOR SEQ ID NO:15:                                       
-      (i) SEQUENCE CHARACTERISTICS:                                      
#pairs    (A) LENGTH: 24 base                                             
          (B) TYPE: nucleic acid                                          
          (C) STRANDEDNESS: single                                        
          (D) TOPOLOGY: linear                                            
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:15:                            
#                24TCCT AGTG                                              
- (2) INFORMATION FOR SEQ ID NO:16:                                       
-      (i) SEQUENCE CHARACTERISTICS:                                      
#pairs    (A) LENGTH: 24 base                                             
          (B) TYPE: nucleic acid                                          
          (C) STRANDEDNESS: single                                        
          (D) TOPOLOGY: linear                                            
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:16:                            
#                24GGCG GCAC                                              
- (2) INFORMATION FOR SEQ ID NO:17:                                       
-      (i) SEQUENCE CHARACTERISTICS:                                      
#pairs    (A) LENGTH: 32 base                                             
          (B) TYPE: nucleic acid                                          
          (C) STRANDEDNESS: single                                        
          (D) TOPOLOGY: linear                                            
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:17:                            
#          32      AAGA TAGTGTTAGC TT                                     
- (2) INFORMATION FOR SEQ ID NO:18:                                       
-      (i) SEQUENCE CHARACTERISTICS:                                      
#pairs    (A) LENGTH: 21 base                                             
          (B) TYPE: nucleic acid                                          
          (C) STRANDEDNESS: single                                        
          (D) TOPOLOGY: linear                                            
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:18:                            
#21                CGCA G                                                 
- (2) INFORMATION FOR SEQ ID NO:19:                                       
-      (i) SEQUENCE CHARACTERISTICS:                                      
#pairs    (A) LENGTH: 25 base                                             
          (B) TYPE: nucleic acid                                          
          (C) STRANDEDNESS: single                                        
          (D) TOPOLOGY: linear                                            
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:19:                            
#               25 ACCC TAGAC                                             
- (2) INFORMATION FOR SEQ ID NO:20:                                       
-      (i) SEQUENCE CHARACTERISTICS:                                      
#pairs    (A) LENGTH: 22 base                                             
          (B) TYPE: nucleic acid                                          
          (C) STRANDEDNESS: single                                        
          (D) TOPOLOGY: linear                                            
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:20:                            
#                 22GCA AC                                                
- (2) INFORMATION FOR SEQ ID NO:21:                                       
-      (i) SEQUENCE CHARACTERISTICS:                                      
#pairs    (A) LENGTH: 28 base                                             
          (B) TYPE: nucleic acid                                          
          (C) STRANDEDNESS: single                                        
          (D) TOPOLOGY: linear                                            
-     (xi) SEQUENCE DESCRIPTION: SEQ ID NO:21:                            
#             28   GCAA TTCTAGTC                                          
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