Retroviral vectors expressing soluble CD4: a potential gene therapy for AIDS

A glycan shield on chimpanzee CD4 protects against infection by primate lentiviruses (HIV/SIV)

Pandemic HIV-1 (group M) emerged following the cross-species transmission of a simian immunodeficiency virus from chimpanzees (SIVcpz) to humans. Primate lentiviruses (HIV/SIV) require the T cell receptor CD4 to enter into target cells. By surveying the sequence and function of CD4 in 50 chimpanzee individuals, we find that all chimpanzee CD4 alleles encode a fixed, chimpanzee-specific substitution (34T) that creates a glycosylation site on the virus binding surface of the CD4 receptor. Additionally, a single nucleotide polymorphism (SNP) has arisen in chimpanzee CD4 (68T) that creates a second glycosylation site on the same virus-binding interface. This substitution is not yet fixed, but instead alleles containing this SNP are still circulating within chimpanzee populations. Thus, all allelic versions of chimpanzee CD4 are singly glycosylated at the virus binding surface, and some allelic versions are doubly glycosylated. Doubly glycosylated forms of chimpanzee CD4 reduce HIV-1 and SIVcpz infection by as much as two orders of magnitude. Full restoration of virus infection in cells bearing chimpanzee CD4 requires reversion of both threonines at sites 34 and 68, destroying both of the glycosylation sites, suggesting that the effects of the glycans are additive. Differentially glycosylated CD4 receptors were biochemically purified and used in neutralization assays and microscale thermophoresis to show that the glycans on chimpanzee CD4 reduce binding affinity with the lentiviral surface glycoprotein, Env. These glycans create a shield that protects CD4 from being engaged by viruses, demonstrating a powerful form of host resistance against deadly primate lentiviruses.

A novel gene therapy strategy using secreted multifunctional anti-HIV proteins to confer protection to gene-modified and unmodified target cells

Current human immunodeficiency virus type I (HIV) gene therapy strategies focus on rendering HIV target cells non-permissive to viral replication. However, gene-modified cells fail to accumulate in patients and the virus continues to replicate in the unmodified target cell population. We have designed lentiviral vectors encoding secreted anti-HIV proteins to protect both gene-modified and unmodified cells from infection. Soluble CD4 (sCD4), a secreted single chain variable fragment (sscFv(17b)) and a secreted fusion inhibitor (sFI(T45)) were used to target receptor binding, co-receptor binding and membrane fusion, respectively. Additionally, we designed bi- and tri-functional fusion proteins to exploit the multistep nature of HIV entry. Of the seven antiviral proteins tested, sCD4, sCD4-scFv(17b), sCD4-FI(T45) and sCD4-scFv(17b)-FI(T45) efficiently inhibited HIV entry. The neutralization potency of the bi-functional fusion proteins sCD4-scFv(17b) and sCD4-FI(T45) was superior to that of sCD4 and the Food and Drug Administration-approved fusion inhibitor T-20. In co-culture experiments, sCD4, sCD4-scFv(17b) and sCD4-FI(T45) secreted from gene-modified producer cells conferred substantial protection to unmodified peripheral blood mononuclear cells. In conclusion, continuous delivery of secreted anti-HIV proteins via gene therapy may be a promising strategy to overcome the limitations of the current treatment.

Secreted antiviral entry inhibitory (SAVE) peptides for gene therapy of HIV infection

Gene therapeutic strategies for human immunodeficiency virus type 1 (HIV-1) infection could potentially overcome the limitations of standard antiretroviral drug therapy (ART). However, in none of the clinical gene therapy trials published to date, therapeutic levels of genetic protection have been achieved in the target cell population for HIV-1. To improve systemic antiviral efficacy, C peptides, which are efficient inhibitors of HIV-1 entry, were engineered for high-level secretion by genetically modified cells. The size restrictions for efficient peptide export through the secretory pathway were overcome by expressing the C peptides as concatemers, which were processed into monomers by furin protease cleavage. These secreted antiviral entry inhibitory (SAVE) peptides mediated a substantial protective bystander effect on neighboring nonmodified cells, thus suppressing virus replication even if only a small fraction of cells was genetically modified. Accordingly, these SAVE peptides may provide a strong benefit to AIDS patients in future, and, if applied by direct in vivo gene delivery, could present an effective alternative to antiretroviral drug regimen.

Gene therapy for HIV infection: what does it need to make it work?

The efficacy of antiviral drug therapy for HIV infection is limited by toxicity and viral resistance. Thus, alternative therapies need to be explored. Several gene therapeutic strategies for HIV infection have been developed, but in clinical testing therapeutically effective levels of the transgene product were not achieved. This review focuses on the determinants of therapeutic efficacy and discusses the potential and also the limits of current gene therapy approaches for HIV infection.

Genetic therapy for HIV/AIDS

Despite the tremendous success of highly active antiretroviral treatment (HAART) introduced nearly 8 years ago for the treatment of human immunodeficiency virus (HIV), innovative therapies, including gene transfer approaches, are still required for nearly half of the general patient population. A number of potential gene therapeutic targets for HIV have been identified and include both viral and cellular genes essential for viral replication. The diverse methods used to inhibit viral replication comprise RNA-based strategies such as ribozymes, RNA decoys, antisense messenger RNAs and small interfering RNA (siRNA) molecules. Other potential anti-HIV genes include dominant negative viral proteins, intracellular antibodies, intrakines and suicide genes, all of which have had a modicum of success in vitro. Cellular targets include CD4+ T cells, macrophages and their progenitors. The greatest gene transfer efficiency has been achieved using retroviral or, more recently, lentiviral vectors. A limited number of Phase I clinical trials suggest that the general method is safe. It is proposed that a national network for HIV gene therapy (similar to the AIDS Clinical Trial Groups) may be the best way to determine which approaches should proceed clinically.

Gene therapy of HIV-1 infection using lentiviral vectors expressing anti-HIV-1 genes

The use of vectors based on primate lentiviruses for gene therapy of human immunodeficiency virus type 1 (HIV-1) infection has many potential advantages over the previous murine retroviral vectors used for delivery of genes that inhibit replication of HIV-1. First, lentiviral vectors have the ability to transduce dividing and nondividing cells that constitute the targets of HIV-1 infection such as resting T cells, dendritic cells, and macrophages. Lentiviral vectors can also transfer genes to hematopoietic stem cells with a superior gene transfer efficiency and without affecting the repopulating capacity of these cells. Second, these vectors could be potentially mobilized in vivo by the wild-type virus to secondary target cells, thus expanding the protection to previously untransduced cells. And finally, lentiviral vector backbones have the ability to block HIV-1 replication by several mechanisms that include sequestration of the regulatory proteins Tat and Rev, competition for packaging into virions, and by inhibition of reverse transcription in heterodimeric virions with possible generation of nonfunctional recombinants between the vector and viral genomes. The inhibitory ability of lentiviral vectors can be further increased by expression of anti-HIV-1 genes. In this case, the lentiviral vector packaging system has to be modified to become resistant to the anti-HIV-1 genes expressed by the vector in order to avoid self-inhibition of the vector packaging system during vector production. This review focuses on the use of lentiviral vectors as the main agents to mediate inhibition of HIV-1 replication and discusses the different genetic intervention strategies for gene therapy of HIV-1 infection.

Retroviral vector-mediated expression in primary human T cells of an endoplasmic reticulum-retained CD4 chimera inhibits human immunodeficiency virus type-1 replication

Intracellular expression of genes that inhibit key steps in the human immunodeficiency virus (HIV-1) replicative cycle could offer an alternative therapy for AIDS treatment. One of these approaches involves the inhibition of env protein maturation through the expression of CD4 molecules with added exogenous sequences that promote their retention in the endoplasmic reticulum (ER). We have tested this strategy using a CD4 chimera (CD4epsilon10) containing an ER retention sequence derived from the TCR CD3-epsilon chain. Transfection of CD4epsilon10 in the human T cell line Jurkat made it resistant to infection with two different HIV-1 isolates, which was evaluated by measuring p24 antigen production, induction of apoptosis, and syncytia formation. Furthermore, polymerase chain reaction (PCR) analysis of genomic DNA showed no traces of the proviral HIV-1 genome in CD4epsilon10-transfected cells, suggesting it was not maintained latently in these cells. To facilitate the delivery of the CD4epsilon10 chimera to primary cells from AIDS patients, a Moloney-based retroviral vector was constructed that expresses CD4epsilon10 under the transcriptional control of the HIV-1 long terminal repeat (LTR) promoter. Transduction of the MT-2 human T cell line with this vector rendered it resistant to infection with HIV-1 by a process that involved the inhibition of gp160 proteolytic processing. Finally, transduction of the CD4epsilon10 chimera into T lymphoblasts derived from asymptomatic HIV-infected individuals demonstrated a protective effect, resulting in both an increased cellular proliferation rate and an increased percentage of CD4+ cells. These results suggest that it is feasible to use retroviral transduction of CD4epsilon10 as a gene therapy approach for AIDS treatment.

The biochemistry of gene therapy for AIDS

Gene therapy has enormous potential and could in the near future involve the clinical biochemist in monitoring its efficacy. The involvement of clinical biochemists in this field could be not only in evaluating the impact of a gene-based strategy on disease progression, but also in measuring the expression of the products of therapeutic genes in treated individuals. Indeed, gene therapy presents new possibilities for the treatment of many diseases and, in particular, merits consideration in the treatment of a fatal disease like AIDS. The aim of this paper is to review the biochemical basis and clinical relevance of the gene therapy approaches directed towards the inhibition of human immunodeficiency virus type-1. We discuss the goals which have been achieved, the problems which have occurred and the efforts that are being made to solve them. In this regard, particular attention is paid to new strategies targeting 'therapeutic' enzymes to human immunodeficiency virus type-1 nucleic acids.

Gene therapy for infectious diseases

Gene therapy is being investigated as an alternative treatment for a wide range of infectious diseases that are not amenable to standard clinical management. Approaches to gene therapy for infectious diseases can be divided into three broad categories: (i) gene therapies based on nucleic acid moieties, including antisense DNA or RNA, RNA decoys, and catalytic RNA moieties (ribozymes); (ii) protein approaches such as transdominant negative proteins and single-chain antibodies; and (iii) immunotherapeutic approaches involving genetic vaccines or pathogen-specific lymphocytes. It is further possible that combinations of the aforementioned approaches will be used simultaneously to inhibit multiple stages of the life cycle of the infectious agent.

Efficient retroviral gene transfer of a Tat-regulated herpes simplex virus thymidine kinase gene for HIV gene therapy

We have previously reported that lymphoid and monocytic CD4+ cells transfected with and expressing a herpes simplex virus thymidine kinase (HSV-TK) gene, under the transcriptional control of the HIV long terminal repeat, are protected from HIV spread in the presence of 10 microM acyclovir. Furthermore, the expression of HSV-TK was able to enhance the antiviral effect of AZT. We now investigate the ability of retroviral vectors containing the HSV-TK gene under the transcriptional control of HIV regulatory sequences to transduce target cells. Bone marrow cells or lymphocytes might be genetically modified by these vectors in an HIV gene therapy strategy. In this study, we describe high-titer retroviral producer clones expressing the HSV-TK gene from an HIV LTR. Lymphoid cells transduced with one of these retroviruses express HSV-TK at a relatively low basal level. When the same cells express the HIV regulatory protein Tat, they are 10-fold more sensitive to killing by ganciclovir. Thus, this vector has potential application for gene therapy of HIV infection.

Analysis of trans-dominant mutants of the HIV type 1 Rev protein for their ability to inhibit Rev function, HIV type 1 replication, and their use as anti-HIV gene therapeutics

The HIV-1 rev gene product facilitates the transport of singly spliced and unspliced HIV-1 transcripts and is necessary for productive HIV-1 infection. On the basis of the previously described trans-dominant Rev mutant M10, four point mutants and one frameshift mutant of the Rev protein were constructed. The mutants were inserted into retroviral expression vectors and analyzed for their ability to inhibit Rev-mediated gene expression. Transient transfection systems were used to screen these new mutants, and each was shown to inhibit expression of a Rev-dependent CAT reporter plasmid. Inhibition of HIV-1 envelope gene expression was tested in the HeLa-T4 cell line and was also shown to be inhibited by the trans-dominant Rev mutants. Retroviral vector producer cell lines were constructed and used to transduce Rev trans-dominant genes into the human T-cell line SupT1. The engineered SupT1 cell lines were then challenged with HIV-1 IIIB and HIV-1 expression was monitored by Northern blot analysis and in situ hybridization. SupT1 cells expressing either a Rev point mutant or the frameshift mutant showed greatly reduced HIV-1 mRNA accumulation and the Rev-dependent singly spliced and unspliced HIV-1 mRNAs were reduced. The kinetics of viral replication following challenge of Rev trans-dominant-engineered SupT1 cells with both HIV-1 IIIB and MN strains was significantly reduced and cells were protected from viral lysis. Viruses that emerge late in infection from Rev trans-dominant-engineered cultures are not resistant to Rev-mediated inhibition. Last, trans-dominant Rev-mediated protection of human CD4+ lymphocytes from challenge with primary HIV-1 patient isolates confirms the potential utility of this system as an anti-HIV-1 gene therapy approach.

Inhibition of human immunodeficiency virus type-1 by retroviral vectors expressing antisense-TAR

The human immunodeficiency virus type-1 (HIV-1) Tat activation response (TAR) region is essential for Tat-mediated trans-activation of the HIV-1 long terminal repeat (LTR). The TAR element is present on the 5' and 3' ends of all HIV-1 transcripts and is relatively conserved among different HIV-1 isolates. These properties make it an attractive target for anti-HIV-1 gene therapy strategies. We have constructed a Moloney murine leukemia-based retroviral vector that expresses a chimeric tRNA(iMet)-antisense TAR fusion transcript complementary to the HIV-1 TAR region. The potential of this anti-TAR retroviral vector to inhibit HIV-1 was initially tested by transient transfections with an HIV-1-LTR-Tat expression plasmid into HeLa-CAT cells. Anti-TAR inhibited Tat-mediated HIV-1 LTR-driven CAT reporter gene expression in a dose-dependent fashion. The antisense-TAR vector was then used to transduce the human SupT1 T cell line. Cotransfection of these SupT1 cells with a Tat expression plasmid plus an HIV-1 LTR-CAT reporter plasmid resulted in decreased CAT gene expression in comparison to control transduced SupT1 cells. The antisense-TAR engineered SupT1 cell line was then challenged with HIV-1MN.HIV-1 viral production was inhibited in SupT1 cells transduced with the antisense-TAR retroviral vector. Greater inhibition of HIV-1 was observed with antisense-TAR as compared to antisense-Tat expressing retroviral vector. These observations suggest that antisense-TAR retroviral vectors are potentially useful for clinical anti-HIV-1 gene therapy.

Further evaluation of soluble CD4 as an anti-HIV type 1 gene therapy: demonstration of protection of primary human peripheral blood lymphocytes from infection by HIV type 1

We previously reported on the construction of retroviral vectors that produce a secreted form of the HIV-1 receptor, T cell antigen CD4 (Morgan et al., AIDS Res Hum Retroviruses 1990;6:183-191). In this article we test the ability of these sCD4-expressing retroviral vectors to protect human T-cell lines or primary T cells from HIV-1 infection. To demonstrate that protection from HIV-1 infection is mediated by the soluble nature of this protein, two coculture protection experiments were conducted. In these experiments, sCD4-expressing retroviral vectors were used to engineer mouse NIH 3T3 cells. In one coculture experiment the human SupT1 cell line was added directly to the culture of sCD4-producing NIH 3T3 cells, and in another experiment the two cell types were separated physically by a semipermeable membrane. In both coculture configurations, the T cell line was protected from HIV-1 challenge as measured by syncytium formation and indirect immunofluorescent assays. In addition, the SupT1 line was directly engineered with sCD4-expressing retroviral vectors and shown to be protected from HIV-1 challenge. As a prelude to further preclinical studies, we tested the ability of retroviral vectors to transduce primary human peripheral blood lymphocytes (PBLs). Conditions used to stimulate T cell growth resulted in significant shifts in the CD4/CD8 cell in favor of CD8 cells. Retroviral-mediated gene transfer under these conditions resulted in low levels of gene transfer (< 5%).(ABSTRACT TRUNCATED AT 250 WORDS)

Gene therapy for human immunodeficiency virus infection: genetic antiviral strategies and targets for intervention

Gene therapeutic strategies for the treatment of human immunodeficiency virus type 1 (HIV-1) infection have received increased attention due to lack of chemotherapeutic drugs or vaccines that show long-term efficacy in vivo. An emerging group, referred to here as "genetic antivirals," is reviewed. Genetic antivirals are defined as DNA or RNA elements that are transferred into cells and affect their intracellular targets either directly, or after expression as RNA or proteins. They include antisense oligonucleotides, ribozymes, RNA decoys, transdominant mutants, toxins, and immunogens. They offer the possibility to target simultaneously multiple sites in the HIV genome, thereby minimizing the production of resistant viruses. We review the molecular mechanisms of genetic antivirals, their HIV molecular targets, and discuss issues concerning their application as anti-HIV agents.

The new frontier: gene and oligonucleotide therapy

Gene and oligonucleotide therapy are emerging as clinically viable therapeutic regimens for genetic, neoplastic, and infectious diseases. Approaches include insertion of human genes in viral vectors including recombinant retrovirus, adenovirus, adeno-associated virus, and herpes simplex virus-1, or recombinant bacterial plasmids. Viral vectors transfect cells directly; plasmid DNA is delivered with the help of cationic liposomes (lipofection), polylysine conjugates, gramicidin S, artificial viral envelopes or other such intracellular carriers. Major areas of interest include replacement of the cystic fibrosis transmembrane regulator gene and the alpha 1-antitrypsin gene; arrest of human immunodeficiency virus infection; and reversal of tumorigenicity and cancer immunization, among others. Oligonucleotide therapy is principally focusing on the same areas, although the approach is to halt DNA transcription or messenger RNA translation with code-blocking triple-helix-forming or "antisense" oligomers. Contributions from the pharmaceutical sciences are expected in pharmaceutical chemistry, drug delivery systems design, analytical chemistry, and biopharmaceutics.

Present status and future prospects for HIV therapies

Since the discovery of human immunodeficiency virus (HIV) in 1983, significant progress has been made toward the discovery, development, and licensing of anti-HIV drugs. In vitro screens against whole virus are now being complemented by screens against specific viral targets, resulting in the development of clinical candidates acting at several critical stages of the viral life cycle. Despite these advances, clinical therapy remains largely palliative. In addition, it has recently been recognized that HIV resistance to most drugs may pose even greater obstacles. Moreover, emerging data on immunopathogenesis raise the possibility that even if virus was eliminated from an infected individual, the patient's immune system might not be capable of restoration to normal function. In the face of such obstacles, deeper insights into the pathogenic mechanisms of disease, aggressive exploitation of those mechanisms for therapeutic gain, and continued commitment of both public and private sectors to support and collaborate in this research are needed.

The basic science of gene therapy

The development over the past decade of methods for delivering genes to mammalian cells has stimulated great interest in the possibility of treating human disease by gene-based therapies. However, despite substantial progress, a number of key technical issues need to be resolved before gene therapy can be safely and effectively applied in the clinic. Future technological developments, particularly in the areas of gene delivery and cell transplantation, will be critical for the successful practice of gene therapy.

Inhibition of replication of HIV-1 by retroviral vectors expressing tat-antisense and anti-tat ribozyme RNA

A ribozyme was constructed that specifically cleaves RNA that contains the first coding exon of the tat gene of HIV-1. This anti-tat ribozyme was incorporated into a Moloney murine leukemia virus vector. A sequence containing only the 48-nucleotide antisense region of the ribozyme was also inserted into the retroviral vector. Human T-cell lines constitutively producing the tat-antisense and the anti-tat ribozyme RNA were created by transduction into Jurkat cells. When challenged with HIV-1, both the tat-antisense and anti-tat ribozyme-producing cells inhibited the replication of HIV-1. The antisense vector conferred a greater resistance to HIV-1 replication than did the anti-tat ribozyme vector.

Retroviral vectors containing putative internal ribosome entry sites: development of a polycistronic gene transfer system and applications to human gene therapy

Recombinant retroviral vectors producing multicistronic mRNAs were constructed. Picornavirus putative internal ribosome entry sites (IRES) were used to confer cap-independent translation of an internal cistron. Internal cistrons were engineered by ligation of various lengths of the IRES of encephalomyocarditis (EMC) virus or polio virus to the E. coli chloramphenicol acetyltransferase (CAT) gene. The IRES/CAT fusions were introduced into retroviral vectors 3' to the translation stop codon of the neomycin phosphotransferase (NEO) gene, and the molecular constructs transfected into retroviral vector packaging lines. Retroviral vector producer cells efficiently express the internal CAT gene product only when the full length IRES is used. Both the EMC/CAT and polio/CAT retroviral vectors produced high titer vector supernatant capable of productive transduction of target cells. To test the generality of this gene transfer system, a retroviral vector containing an IRES fusion to the human adenosine deaminase (ADA) gene was constructed. Producer cell supernatant was used to transduce NIH/3T3 cells, and transduced cells were shown to express NEO, and ADA. Novel three-gene-containing retroviral vectors were constructed by introducing the EMC/ADA fusion into either an existing internal-promoter-containing vector, or a polio/CAT bicistronic vector. Producer cell clones of the three-gene vectors synthesize all three gene products, were of high titer, and could productively transduce NIH/3T3 cells. By utilizing cap-independent translation units, IRES vectors can produce polycistronic mRNAs which enhance the ability of retroviral-mediated gene transfer to engineer cells to produce multiple foreign proteins.

Construction of a recombinant bacterial human CD4 expression system producing a bioactive CD4 molecule

The CD4 protein expressed on helper T lymphocytes is a restriction element for major histocompatibility class II immune responses. This molecule is also used by the human immunodeficiency virus as its specific cellular receptor facilitating binding of virus to cells. As soluble forms of CD4 inhibit HIV infection in tissue culture, attention has focused on this molecule. Bacterially produced CD4 would facilitate studies of the biology of the CD4 molecule. However, bacterially expressed CD4 must be refolded for assumption of its interaction with conformationally dependent anti-CD4 monoclonal antibodies as well as the HIV-1 envelope protein gp120. We report here the engineering of an external domain construct of the CD4 gene into a novel expression vector containing the nucleotide sequence encoding the pelB leader peptide of Erwinia carotovara (pDABL), to facilitate correct folding of CD4 in bacteria. Monoclonal antibodies specific for important conformational epitopes of the CD4 molecule were able to bind bacterial colonies containing the pDABL/CD4 vector but not colonies with vector alone. Importantly, recombinant gp120 produced in baculovirus bound specifically to bacterial colonies expressing the CD4 recombinant molecule. This system presents a simple screening mechanism for molecules that bind to the external domain of the CD4 glycoprotein. Vectors such as pDABL will also facilitate the production of large amounts of biologically active proteins in bacteria.

Regulatory concerns in human gene therapy

Gene therapy in humans is now being undertaken in an investigational setting. Such therapy involves the administration of biological products to human patients. A document entitled, "Points to Consider in Human Somatic Cell Therapy and Gene Therapy" has been prepared by the Center for Biologics Evaluation and Research (CBER) of the Food and Drug Administration (FDA) and is published elsewhere in this issue. This paper provides explanatory material about the CBER regulatory process and the scientific and regulatory basis for the requests for data in that document.

Prospects for homologous recombination in human gene therapy

The ideal approach to gene therapy of hereditary diseases or gene correction therapy is considered. The advantages, disadvantages and limits of gene targeting by homologous recombination are discussed with regard to its possible application in gene correction therapy and in comparison with retroviral-mediated gene complementation therapy.

Simian and feline immunodeficiency viruses: animal lentivirus models for evaluation of AIDS vaccines and antiviral agents

Infection of captive macaques with simian immunodeficiency virus (SIV) and domestic cats with feline immunodeficiency virus (FIV), both discovered in the last five years, represent excellent animal models for infection of humans with the human immunodeficiency virus (HIV). Protection against challenge infection and protection against development of simian and feline acquired immunodeficiency syndrome has been achieved in each model by use of inactivated whole virus or virus-cell vaccines. A recombinant SIV envelope peptide vaccine has also proved efficacious. These vaccines have protected against 10-100 animal infectious doses of the homologous cell-free virus given systemically, and, in the simian model, apparently show cross protection against a heterologous strain of SIV. Protected animals appear free of any latent infection although late breakthroughs of infection in a few animals imply that not all vaccinated animals are completely protected. The mechanism of protection in the simian model apparently involves envelope antibody but the role of neutralizing antibody remains unclear. Questions remaining to be answered in both SIV and FIV models are: (1) the duration of immunity, (2) the extent of protection against heterologous strains and mucosal infection, (3) protection against infection with cell-associated virus and (4) the role, if any, of cellular immunity in vaccine protection. Initial attempts at post-infection immunotherapy with SIV vaccines have not yet been successful. The inactivated whole SIV and FIV vaccines offer a promising start and provide hope that a prophylactic AIDS vaccine will be developed. Use of these animal models for antiviral therapy is just now getting underway. Both models should prove especially useful for studies of prophylaxis and therapy, especially during the early stages of infection and for investigations on drug pharmacokinetics or toxicity that can not be done as well in HIV-infected humans. The animals will also be ideal for testing the pathogenicity of drug-induced mutant forms of SIV and FIV. For these purposes it will be necessary to create self-sustaining specific pathogen-free macaque and cat breeding colonies and provide increased housing facilities for infected animals. The future of AIDS research is crucially dependent on the long term availability of these animal models.