New prospects for the development of a vaccine against human immunodeficiency virus type 1. An overview

Comparison of immunogenicity between codon optimized HIV-1 Thailand subtype B gp140 and gp145 vaccines

HIV-1 pandemic posed an unprecedented challenge to the global health and it is believed that an effective vaccine will be the final solution against HIV-1. HIV-1 envelope is the primary immunogen in developing neutralization antibody based HIV vaccine. To define the suitable Env derived immunogen, we systemically compared the immunogenicity of gp140 and gp145 in a DNA vaccination alone and a prime-boost modalities. Two DNA vaccines and two recombinant Tiantan vaccinia vaccines (rTTV) were constructed for vaccination of female Balb/c mice. Elispot assay was used to read out the T cell immunity and ELISA assay was used to quantify antibody immunity. PLL (poly-L-leucine)-ELISA assay was used in linear antibody epitope mapping. Mice primed with gp145 tended to elicit more Env-specific T cells responses than those primed with gp140, significant difference was observed in DNA immunization alone. The ultimate T cell responses in prime-boost regimen tend to be determined mainly by the priming efficacy. Linear antibody epitope mapping displayed that sera raised by gp145 priming were vigorously reactive to more peptides than that by gp140. Our data demonstrated HIV-1 Thailand B-derived gp145 may raise higher T-cell responses and broader linear peptide-specific antibody responses than gp140 does. However, it remains to be determined that how these observations are relevant to the neutralization of antibody activities.

Single epitope mucosal vaccine delivered via immuno-stimulating complexes induces low level of immunity against simian-HIV

The difficulty in developing an effective vaccine to contain the HIV/AIDS epidemic coupled with the fact that primary HIV-1 infection typically occurs via mucosal sites has increased emphasis on vaccine approaches that protect at mucosal surfaces. In this study we employed HIV and simian-HIV (SHIV)-derived T helper (Th) and cytotoxic T lymphocyte (CTL) single epitopes incorporated into immuno-stimulating complexes (ISCOM) as a candidate immunogens. Immunized rhesus macaques (Macaca mulatta) were challenged with CCR5-tropic SHIV(SF162p4). On the day of challenge, low levels of virus-neutralizing antibodies (Ab) and CTLs were detected in ISCOM-immunized macaques. Greater than 10(5) viral RNA copies per ml of plasma in 2/5 immunized and 3/4 control macaques were detected within 3 weeks post-challenge. Depletion of CD4+ T cells from gut-associated lymphoid tissues (GALT) was observed by post-challenge day (PCD) 14 in all macaques regardless immunization. Nonetheless, lower viral loads and relatively better preservation of peripheral CD4+ T cells following the SHIV infection was observed in ISCOM-immunized macaques. We predict that if coadministered with additional epitopes and/or more efficacious mucosal delivery system or route, HIV/SIV-derived peptide vaccines may have potential to elicit heterologous protection.

Sustained Conservation of CD4+T Cells in Multiprotein Triple Modality-Immunized Rhesus Macaques after Intrarectal Challenge with Simian Immunodeficiency Virus

As part of a European multicenter study designed to determine the optimal combination and order of a mixed-modality vaccine against acquired immunodeficiency syndrome, rhesus monkeys received a combination of three different vectors, all expressing the same Simian Immunodeficiency Virus (SIV) genes followed by mucosal challenge with highly pathogenic SIV. In the study reported here, animals were primed with DNA followed by one booster immunization with Semliki Forest Virus (SFV) and two immunizations with modified Vaccinia Ankara (MVA). To address the relevance of mucosal immunization, we compared systemic versus a combination of systemic and mucosal antigen application. Although all vaccinees became infected after intrarectal challenge with SIV, most (six of eight) were protected from profound loss of CD4+ cells. In addition, vaccinees showed lower viral loads than did controls (p < 0.05). Overall, these protective effects were more pronounced in those animals whose schedule included immunization via the mucosa. In summary, the vaccine regimen used here achieved one important criterion of efficacy: the suppression of disease development as indicated by conservation of CD4+ cells.

Cassette-based presentation of SIV epitopes with recombinant gas vesicles from halophilic archaea

In earlier studies we demonstrated recombinant gas vesicles from Halobacterium sp. NRC-1, expressing a model six amino acid insert, or native vesicles displaying chemically coupled TNP, each were immunogenic, and antigenic. Long-lived responses displaying immunologic memory were elicited without exogenous adjuvant. Here we report the generation and expression of cassettes containing SIV derived DNA. The results indicate a cassette-based display/delivery system derived from recombinant halobacterial gas vesicle genes is highly feasible. Data specifically support four conclusions: (i) Recombinants carrying up to 705 bp of SIV DNA inserted into the gvpC gene form functional gas vesicles; (ii) SIV peptides contained as part of the expressed recombinant, surface exposed GvpC protein are recognized by antibody elicited in monkeys exposed to native SIV in vivo; (iii) in the absence of adjuvant, mice immunized with the recombinant gas vesicle (r-GV) preparations mount a solid, titratable antibody response to the test SIV insert that is long lived and exhibits immunologic memory; (iv) recombinant organelles, created through the generation of cassettes encoding epitopes inserted into the gvpC DNA, can be used to construct a multiepitope display (MED) library, a potentially cost effective vehicle to express and deliver peptides of SIV, HIV or other pathogens.

Raccoon poxvirus as a mucosal vaccine vector for domestic cats

In the present study, we evaluated both the immunogenicity and safety of recombinant raccoon poxvirus (RCN) as a mucosal vaccine vector for domestic cats. RCN is an orthopoxvirus that was isolated from healthy raccoons and has been used experimentally as a vaccine vector for rabies and other antigens in a variety of species, including raccoons, skunks, foxes, bobcats, rabbits, domestic cats, piglets, sheep and non-human primates. We evaluated the antibody response induced by a recombinant RCN vaccine expressing the rabies-G glycoprotein (RCN/rabies-G) administered to cats by the oral (PO), intranasal (IN), conjunctival (CO) or intranasal/conjunctival (IN/CO) route (dose: 10 plaque forming units or PFU). The IN route, either alone or combined with the CO route, induced the highest rabies virus neutralizing antibody (RVNA) titers. The RVNA titers remained high when measured at six months post-vaccination, demonstrating that the recombinant vaccine administered via these routes is very efficient at inducing long-lasting immunity. A dose-response was observed following IN vaccination in cats. Doses of 10 PFU induced strong antibody responses in 4 of 5 animals [geometric mean titer: 3.2 (log)]. None of the animals vaccinated with 10 PFU developed detectable RVNA titers. In this study, RCN/rabies-G viral shedding was below detectable levels. Nasal, oral and fecal swabs collected from these cats were negative for RCN by both virus isolation and by nested-PCR. In addition, no horizontal transmission of the virus could be detected. Gang-housed sentinel animals for each group did not develop detectable anti-RVNA or -RCN antibodies. To study tissue tropism of recombinant raccoon poxvirus vaccines, a RCN that can express the lacZ gene (RCN/lacZ) was constructed. Expression of beta-galactosidase (beta-gal) was validated in vitro and in mice in vivo. Cats were vaccinated IN with 10 PFU of RCN/lacZ. No histopathological lesions were detected in any of the tissues collected from these cats at 1, 4, 7 and 15 days post-vaccination. In addition, no virus or beta-gal expression was detected in any of these tissues. Controls demonstrated that virus could be reisolated from nasal swabs immediately after administration of 10 PFU to cats. These results suggest that recombinant RCN vaccines undergo limited replication after intranasal administration in cats that is sufficient to elicit strong, long-lasting systemic antibody responses.

HIV type 1-specific inter- and intrasubtype cellular immune responses in HIV type 1-infected Ugandans

Investigations concerning the extent and nature of subtype-specific and intersubtype immune responses in HIV-1-infected persons are necessary for the development of appropriate candidate vaccines. In the cross-sectional study described here, 26 HIV-1-positive Ugandan patients were tested for their ability to mount HIV antigen-specific cellular immune responses. Subjects were infected with either HIV-1 subtypes A, C, or D. Recombinant vaccinia virus (rVV)-based and peptide-based enzyme-linked immunospot (Elispot) assays were used to evaluate HIV-1-specific gamma-interferon (IFN-gamma) cellular responses. rVV expressing gag, pol, or env proteins derived from HIV-1 subtypes A, B, and D were evaluated for their ability to induce whole HIV-1-protein-specific IFN-gamma responses in 14 patients. A panel of previously identified HLA class I-restricted peptides based on representative sequences from HIV-1 subtypes A, B, C, and D and restricted through HLA-A2, -A29, -B42, -B53, and -B57 alleles were used to evaluate the presence of HIV-1-peptide-specific T cells in 19 patients. Using rVV, 27 of a possible 38 subtype-specific responses (71%) and 56 of a possible 110 intersubtype responses (51%) were observed. When appropriate peptides were used 18 of 39 (46.2%) subtype-specific and 13 of 39 (33.3%) intersubtype responses were observed. Peptide responses were higher quantitatively than those seen when rVV were used. In 7 patients, both rVV and specific peptides were evaluated; in 3 of 7 individuals, global responses were seen despite a lack of measurable HLA-restricted peptide-specific responses demonstrating the need to evaluate a broader range of HIV-specific immune responses.

Expression and immunogenicity of sequence-modified human immunodeficiency virus type 1 subtype B pol and gagpol DNA vaccines

Control of the worldwide AIDS pandemic may require not only preventive but also therapeutic immunization strategies. To meet this challenge, the next generation of human immunodeficiency virus type 1 (HIV-1) vaccines must stimulate broad and durable cellular immune responses to multiple HIV antigens. Results of both natural history studies and virus challenge studies with macaques indicate that responses to both Gag and Pol antigens are important for the control of viremia. Previously, we reported increased Rev-independent expression and improved immunogenicity of DNA vaccines encoding sequence-modified Gag derived from the HIV-1(SF2) strain (J. zur Megede, M. C. Chen, B. Doe, M. Schaefer, C. E. Greer, M. Selby, G. R. Otten, and S. W. Barnett, J. Virol. 74: 2628-2635, 2000). Here we describe results of expression and immunogenicity studies conducted with novel sequence-modified HIV-1(SF2) GagPol and Pol vaccine antigens. These Pol antigens contain deletions in the integrase coding region and were mutated in the reverse transcriptase (RT) coding region to remove potentially deleterious enzymatic activities. The resulting Pol sequences were used alone or in combination with sequence-modified Gag. In the latter, the natural translational frameshift between the Gag and Pol coding sequences was either retained or removed. Smaller, in-frame fusion gene cassettes expressing Gag plus RT or protease plus RT also were evaluated. Expression of Gag and Pol from GagPol fusion gene cassettes appeared to be reduced when the HIV protease was active. Therefore, additional constructs were evaluated in which mutations were introduced to attenuate or inactivate the protease activity. Nevertheless, when these constructs were delivered to mice as DNA vaccines, similar levels of CD8(+) T-cell responses to Gag and Pol epitopes were observed regardless of the level of protease activity. Overall, the cellular immune responses against Gag induced in mice immunized with multigenic gagpol plasmids were similar to those observed in mice immunized with the plasmid encoding Gag alone. Furthermore, all of the sequence-modified pol and gagpol plasmids expressed high levels of Pol-specific antigens in a Rev-independent fashion and were able to induce potent Pol-specific T- and B-cell responses in mice. These results support the inclusion of a gagpol in-frame fusion gene in future HIV vaccine approaches.

HIV-1 vaccines: the search continues

This article gives an overview about the development of an HIV-1 vaccine. Tremendous numbers of papers have been published on this topic during the last 10 years, and this article can only touch on the different directions taken toward the development of an HIV-1 vaccine, and not give a complete overview of the entire field.

FIV vaccine development and its importance to veterinary and human medicine: a review FIV vaccine 2002 update and review

Feline immunodeficiency virus (FIV) is a natural infection of domestic cats that results in acquired immunodeficiency syndrome resembling human immunodeficiency virus (HIV) infection in humans. The worldwide prevalence of FIV infection in domestic cats has been reported to range from 1 to 28%. Hence, an effective FIV vaccine will have an important impact on veterinary medicine in addition to being used as a small animal AIDS model for humans. Since the discovery of FIV reported in 1987, FIV vaccine research has pursued both molecular and conventional vaccine approaches toward the development of a commercial product. Published FIV vaccine trial results from 1998 to the present have been compiled to update the veterinary clinical and research communities on the immunologic and experimental efficacy status of these vaccines. A brief report is included on the outcome of the 10 years of collaborative work between industry and academia which led to recent USDA approval of the first animal lentivirus vaccine, the dual-subtype FIV vaccine. The immunogenicity and efficacy of the experimental prototype, dual-subtype FIV vaccine and the efficacy of the currently approved commercial, dual-subtype FIV vaccine (Fel-O-Vax FIV) are discussed. Potential cross-reactivity complications between commercial FIV diagnostic tests, Idexx Snap Combo Test and Western blot assays, and sera from previously vaccinated cats are also discussed. Finally, recommendations are made for unbiased critical testing of new FIV vaccines, the currently USDA approved vaccine, and future vaccines in development.

The quest for an AIDS vaccine: is the CD8+ T-cell approach feasible?

The rationale for developing anti-HIV vaccines that stimulate cytotoxic T-lymphocyte responses is given. We argue that such vaccines will work, provided that attention is paid to the development of memory T-cell responses that are strong and preferably activated. Furthermore, the vaccine should match the prevailing virus clade as closely as possible. Vaccines will have to stimulate a wide range of responses, but it is not clear how this can be achieved.

Enhanced CD8+ T cell immune response against a V3 loop multi-epitope polypeptide (TAB13) of HIV-1 Env after priming with purified fusion protein and booster with modified vaccinia virus Ankara (MVA-TAB) recombinant: a comparison of humoral and cellular immune responses with the vaccinia virus Western Reserve (WR) vector

The humoral and cytotoxic T-lymphocyte (CTL) responses have been shown to be determinant in the clearance of many viral infections and because of those characteristics, vaccine candidates against AIDS are designed to enhance both arms of the immune system. While a protocol of immunization able to confer protection in humans against HIV will have to await the results of current clinical trials, it remains important to identify protocols of immunization in animals that achieve significant levels of humoral and cellular immune responses to HIV. In this study we have carried out a comparative analysis of the immune responses elicited in mice immunized with recombinants based on the modified vaccinia virus Ankara strain (rMVA) versus the Western Reserve strain (WR) of vaccinia virus (rVV), both expressing a V3 loop multi-epitopic protein from eight different HIV isolates (TAB13). We found that during priming, rMVA elicited a two- to three-fold higher specific CD8+ T cell response than rVV. Similar enhancement was observed during priming with purified protein TAB13 followed by a booster with rMVA. The epitopes LR150, MN and IIIB, located at the ends and in the middle of the chimeric protein, were able to induce a specific CD8+ T cell response, both after priming or prime/booster with the recombinant viruses but not after prime/booster with TAB13. By examining the cytokine pattern, the immune response triggered by these vectors was of Th-1 type. Humoral immune responses were higher in animals immunized with TAB13/TAB13 or TAB13/rVV than in animals immunized with TAB13/rMVA. These findings demonstrate that during priming or in a prime/booster immunizations, rMVA is superior to rVV in the ability to enhance specific cellular responses to an HIV-1 protein, and that both humoral and cellular immune responses to theV3 loop epitope of HIV-1 Env can be obtained by priming with TAB13 followed by a booster with viral vectors.

Development of dengue virus replicons expressing HIV-1 gp120 and other heterologous genes: a potential future tool for dual vaccination against dengue virus and HIV

BACKGROUND

Toward the goals of providing an additional vector to add to the armamentarium available to HIV vaccinologists and of creating a bivalent vaccine effective against dengue virus and HIV, we have attempted to create vectors which express dengue virus non-structural proteins and HIV immunogens. Previously we reported the successful construction of dengue virus replicons which lack structural genes necessary for virion release and spreading infection in culture but which can replicate intracellularly and abundantly produce dengue non-structural proteins. Here we attempted to express heterologous genetic material from these replicons.

RESULTS

We cloned into a Deltapre-M/E dengue virus replicon genes for either green fluorescent protein (GFP), HIV gp160 or HIV gp120 and tested the ability of these constructs to express dengue virus proteins as well as the heterologous proteins in tissue culture after transfection of replicon RNA.

CONCLUSIONS

Heterologous proteins were readily expressed from these constructs. GFP and gp120 demonstrated minimal or no toxicity. Gp160 expressing replicons were found to express proteins abundantly at 36 hours post transfection, but after 50 hrs of transfection, few replicon positive cells could be found despite the presence of cellular debris positive for replicon proteins. This suggested that gp160 expressed from dengue virus replicons is considerably more toxic than either GFP or gp120. The successful expression of heterologous proteins, including HIV gp120 for long periods in culture suggests this vector system may be useful as a vaccine vector, given appropriate delivery methods.

Rational development of prophylactic HIV vaccines based on structural and regulatory proteins

The severity of the AIDS epidemic clearly emphasises the urgent need to expedite HIV vaccine candidates into clinical trials. Prophylactic HIV vaccine candidates have been evaluated in non-human primates. Based on specific proof of principle studies the first phase III clinical studies have recently begun in humans. However, a truly effective HIV vaccine is not yet at hand and many problems related to specific properties of the virus remain to be overcome. Previously proven empirical approaches have largely failed and now rational thinking based on an understanding of immunity to lentiviral infections is needed. This review addresses the scientific problems and complications facing the development of an HIV vaccine as well as the possible strategies currently available to overcome these problems. Recent attention has focussed on identifying the immune correlates and mechanisms of protection from either HIV infection or protection from disease progression. Based on these observations, the logic and rational behind the development of multiple component vaccine strategies are highlighted.

Dual-subtype FIV vaccine protects cats against in vivo swarms of both homologous and heterologous subtype FIV isolates

OBJECTIVE

To evaluate the immunogenicity and efficacy of an inactivated dual-subtype feline immunodeficiency virus (FIV) vaccine.

DESIGN

Specific-pathogen-free cats were immunized with dual-subtype (subtype A FIV(Pet) and subtype D FIV(Shi)) vaccine and challenged with either in vivo- or in vitro-derived FIV inocula.

METHODS

Dual-subtype vaccinated, single-subtype vaccinated, and placebo-immunized cats were challenged within vivo-derived heterologous subtype B FIV(Bang) [10--100 50% cat infectious doses (CID(50))], in vivo-derived homologous FIV(Shi)(50 CID(50)), and in vitro- and in vivo-derived homologous FIV(Pet)(20--50 CID(50)). Dual-subtype vaccine immunogenicity and efficacy were evaluated and compared to single-subtype strain vaccines. FIV infection was determined using virus isolation and proviral PCR of peripheral blood mononuclear cells and lymphoid tissues.

RESULTS

Four out of five dual-subtype vaccinated cats were protected against low-dose FIV(Bang) (10 CID(50)) and subsequently against in vivo-derived FIV(Pet) (50 CID(50)) challenge, whereas all placebo-immunized cats became infected. Furthermore, dual-subtype vaccine protected two out of five cats against high-dose FIV(Bang) challenge (100 CID(50)) which infected seven out of eight single-subtype vaccinated cats. All dual-subtype vaccinated cats were protected against in vivo-derived FIV(Pet), but only one out of five single-subtype vaccinated cats were protected against in vivo-derived FIV(Pet). Dual-subtype vaccination induced broad-spectrum virus-neutralizing antibodies and FIV-specific interferon-gamma responses along with elevated FIV-specific perforin mRNA levels, suggesting an increase in cytotoxic cell activities.

CONCLUSION

Dual-subtype vaccinated cats developed broad-spectrum humoral and cellular immunity which protected cats against in vivo-derived inocula of homologous and heterologous FIV subtypes. Thus, multi-subtype antigen vaccines may be an effective strategy against AIDS viruses.

Protective immune response against cutaneous leishmaniasis by prime/booster immunization regimens with vaccinia virus recombinants expressing Leishmania infantum p36/LACK and IL-12 in combination with purified p36

In susceptible mice Leishmania infection triggers a CD4(+) Th2 response that has been correlated with evasion of the host immune system. To develop approaches that might trigger a Th1 response leading to protection against Leishmania we generated vaccinia virus recombinants (VVr) expressing the relevant p36/LACK protein of Leishmania infantum (VVp36) or co-expressing p36/LACK and interleukin-12 (VVp36IL12). Susceptible BALB/c mice were immunized with the VVr in various prime/booster protocols that included purified p36/LACK protein, followed 3 weeks later by a challenge with live L. major promastigotes. The course of the infection was monitored by measuring lesion development, parasite load and immunological parameters (IFN-gamma and IL-10 secretion by in vitro-stimulated lymphocytes, and specific IgG isotypes), before and after challenge. We found protocols of prime/booster immunization (VVp36/VVp36; VVp36IL12/p36; p36/VVp36IL12) that elicited different levels of protection in infected animals. The protocol of priming with purified p36 followed by a booster with VVp36IL12 induced 52% reduction in lesion size and a two-log unit reduction in parasite load. This partial protection correlated with activation of a specific Th1 type of immune response. These protocols could be of interest in the prophylaxis against Leishmania spp. and other parasitic diseases.

Expression and characterization of a single-chain polypeptide analogue of the human immunodeficiency virus type 1 gp120-CD4 receptor complex

The infection of CD4(+) host cells by human immunodeficiency virus type 1 (HIV-1) is initiated by a temporal progression of interactions between specific cell surface receptors and the viral envelope protein, gp120. These interactions produce a number of intermediate structures with distinct conformational, functional, and antigenic features that may provide important targets for therapeutic and vaccination strategies against HIV infection. One such intermediate, the gp120-CD4 complex, arises from the interaction of gp120 with the CD4 receptor and enables interactions with specific coreceptors needed for viral entry. gp120-CD4 complexes are thus promising targets for anti-HIV vaccines and therapies. The development of such strategies would be greatly facilitated by a means to produce the gp120-CD4 complexes in a wide variety of contexts. Accordingly, we have developed single-chain polypeptide analogues that accurately replicate structural, functional, and antigenic features of the gp120-CD4 complex. One analogue (FLSC) consists of full-length HIV-1BaL gp120 and the D1D2 domains of CD4 joined by a 20-amino-acid linker. The second analogue (TcSC) contains a truncated form of the gp120 lacking portions of the C1, C5, V1, and V2 domains. Both molecules exhibited increased exposure of epitopes in the gp120 coreceptor-binding site but did not present epitopes of either gp120 or CD4 responsible for complex formation. Further, the FLSC and TcSC analogues bound specifically to CCR5 (R5) and blocked R5 virus infection. Thus, these single-chain chimeric molecules represent the first generation of soluble recombinant proteins that mimic the gp120-CD4 complex intermediate that arises during HIV replication.

Immunization with a novel HIV-1-Tat multiple-peptide conjugate induces effective immune response in mice

We report here a novel, highly immunogenic synthetic, multiple-peptide conjugate comprising functional domains Tat(21-40) and Tat(53-68) from HIV-1 group M plus Tat(9-20) from HIV-1 group O of the HIV-Tat protein (HIV-1-Tat-MPC). Vaccination of mice with HIV-1-Tat-MPC induced an effective immune response to all three functional domains. The anti-HIV-1-Tat-MPC antibodies efficiently inhibited Tat-induced viral activation in monocytes infected with HIV(Ba-L) as well as with various clinical HIV-1 isolates, and reduced Tat-mediated cytopathicity in infected cells by 60-75%. Our results indicate that anti-HIV-1-Tat-MPC antibodies inhibit viral pathogenesis, possibly by blocking functional determinants of Tat and disrupting autocrine and paracrine actions of secreted Tat protein. This epitope-specific, synthetic Tat construct may, therefore, provide a subunit AIDS vaccine candidate for inducing an effective immunoprophylaxis response to reduce progression of HIV infection.

HIV gag mRNA transfection of dendritic cells (DC) delivers encoded antigen to MHC class I and II molecules, causes DC maturation, and induces a potent human in vitro primary immune response

Dendritic cells (DC) are the major APCs involved in naive T cell activation making them prime targets of vaccine research. We observed that mRNA was efficiently transfected, resulting in superior translation in DC compared with other professional APCs. A single stimulation of T cells by HIV gag-encoded mRNA-transfected DC in vitro resulted in primary CD4(+) and CD8(+) T cell immune responses at frequencies of Ag-specific cells (5-12.5%) similar to primary immune responses observed in vivo in murine models. Additionally, mRNA transfection also delivered a maturation signal to DC. Our results demonstrated that mRNA-mediated delivery of encoded Ag to DC induced potent primary T cell responses in vitro. mRNA transfection of DC, which mediated efficient delivery of antigenic peptides to MHC class I and II molecules, as well as delivering a maturation signal to DC, has the potential to be a potent and effective anti-HIV T cell-activating vaccine.

Vaccination with inactivated virus but not viral DNA reduces virus load following challenge with a heterologous and virulent isolate of feline immunodeficiency virus

It has been shown that cats can be protected against infection with the prototypic Petaluma strain of feline immunodeficiency virus (FIV(PET)) using vaccines based on either inactivated virus particles or replication-defective proviral DNA. However, the utility of such vaccines in the field is uncertain, given the absence of consistent protection against antigenically distinct strains and the concern that the Petaluma strain may be an unrepresentative, attenuated isolate. Since reduction of viral pathogenicity and dissemination may be useful outcomes of vaccination, even in the absence of complete protection, we tested whether either of these vaccine strategies ameliorates the early course of infection following challenge with heterologous and more virulent isolates. We now report that an inactivated virus vaccine, which generates high levels of virus neutralizing antibodies, confers reduced virus loads following challenge with two heterologous isolates, FIV(AM6) and FIV(GL8). This vaccine also prevented the marked early decline in CD4/CD8 ratio seen in FIV(GL8)-infected cats. In contrast, DNA vaccines based on either FIV(PET) or FIV(GL8), which induce cell-mediated responses but no detectable antiviral antibodies, protected a fraction of cats against infection with FIV(PET) but had no measurable effect on virus load when the infecting virus was FIV(GL8). These results indicate that the more virulent FIV(GL8) is intrinsically more resistant to vaccinal immunity than the FIV(PET) strain and that a broad spectrum of responses which includes virus neutralizing antibodies is a desirable goal for lentivirus vaccine development.

Accelerating the development and future availability of HIV-1 vaccines: why, when, where, and how?

An HIV-1 vaccine offers the best long-term hope to control the AIDS pandemic, especially in less-developed countries. To ensure its future availability we need to increase our research efforts today, including clinical trials. Although small-scale clinical trials of HIV-1 vaccines have been underway since 1987, the first phase III efficacy trials started only recently in the USA and Thailand. Initial results from these trials will be available within the next 2-3 years, and we must start planning now how vaccines should be used if found to be effective. In the meantime, the continuing promotion of the parallel development and assessment of other candidate vaccines is important. Financial mechanisms should also be developed as an incentive to industry and to ensure equitable distribution of future vaccines in less-developed countries. Moreover, a concerted effort is needed to ensure the development and future availability of appropriate vaccines for Africa.

Challenges in the development of an effective HIV vaccine: current approaches and future directions

OBJECTIVE

The intent of this review is to investigate and discuss why developing a successful HIV vaccine has been so challenging, first by examining the molecular biology of the virus and how HIV interacts with the immune system, and then reviewing past viral vaccine successes as well as future directions for HIV vaccine research.

BACKGROUND

Since HIV appeared in the United States in the early 1980s, an estimated 40 million people worldwide have been infected with the virus. Despite promising advances in the pharmacotherapy of HIV infection, it is apparent that the best, most cost-effective strategy for controlling the further spread of the virus is through synthesis of a protective vaccine. Almost 2 decades into the epidemic, there are few prospects for a truly effective vaccine entering the market in the foreseeable future.

METHODS

MEDLINE was searched for articles written between 1966 and June 1999. Search terms used were AIDS, HIV vaccine, HIV-1, HIV-2, vaccines, and human immunodeficiency virus.

RESULTS

Only 2 candidates for an HIV vaccine are currently in phase III clinical trials (1 in the United States and 1 in Thailand). The efficacy of these vaccines when applied to the population as a whole is widely questioned, largely because they induce protection by an antibody response only. Several studies have suggested that this approach will likely be ineffective in providing any real protection from viral infection. It appears that a strong cellular immune response is necessary in addition to a strong antibody response.