Induction of protective immunity against pathogenic simian immunodeficiency virus by a foreign receptor-dependent replication of an engineered avirulent virus

A critical analysis of the cynomolgus macaque, Macaca fascicularis, as a model to test HIV-1/SIV vaccine efficacy

The use of a number of non-rhesus macaque species, but especially cynomolgus macaques as a model for HIV-1 vaccine development has increased in recent years. Cynomolgus macaques have been used in the United Kingdom, Europe, Canada and Australia as a model for HIV vaccine development for many years. Unlike rhesus macaques, cynomolgus macaques infected with SIV show a pattern of disease pathogenesis that more closely resembles that of human HIV-1 infection, exhibiting lower peak and set-point viral loads and slower progression to disease with more typical AIDS defining illnesses. Several advances have been made recently in the use of the cynomolgus macaque SIV challenge model that allow the demonstration of vaccine efficacy using attenuated viruses and vectors that are both viral and non-viral in origin. This review aims to probe the details of various vaccination trials carried out in cynomolgus macaques in the context of our modern understanding of the highly diverse immunogenetics of this species with a view to understanding the species-specific immune correlates of protection and the efficacy of vectors that have been used to design vaccines.

Dominant induction of vaccine antigen-specific cytotoxic T lymphocyte responses after simian immunodeficiency virus challenge

Cytotoxic T lymphocyte (CTL) responses are crucial for the control of human and simian immunodeficiency virus (HIV and SIV) replication. A promising AIDS vaccine strategy is to induce CTL memory resulting in more effective CTL responses post-viral exposure compared to those in natural HIV infections. We previously developed a CTL-inducing vaccine and showed SIV control in some vaccinated rhesus macaques. These vaccine-based SIV controllers elicited vaccine antigen-specific CTL responses dominantly in the acute phase post-challenge. Here, we examined CTL responses post-challenge in those vaccinated animals that failed to control SIV replication. Unvaccinated rhesus macaques possessing the major histocompatibility complex class I haplotype 90-088-Ij dominantly elicited SIV non-Gag antigen-specific CTL responses after SIV challenge, while those induced with Gag-specific CTL memory by prophylactic vaccination failed to control SIV replication with dominant Gag-specific CTL responses in the acute phase, indicating dominant induction of vaccine antigen-specific CTL responses post-challenge even in non-controllers. Further analysis suggested that prophylactic vaccination results in dominant induction of vaccine antigen-specific CTL responses post-viral exposure but delays SIV non-vaccine antigen-specific CTL responses. These results imply a significant influence of prophylactic vaccination on CTL immunodominance post-viral exposure, providing insights into antigen design in development of a CTL-inducing AIDS vaccine.

Vaccine-induced enhancement of viral infections

Examples of vaccine-induced enhancement of susceptibility to virus infection or of aberrant viral pathogenesis have been documented for infections by members of different virus families. Several mechanisms, many of which still are poorly understood, are at the basis of this phenomenon. Vaccine development for lentivirus infections in general, and for HIV/AIDS in particular, has been little successful. Certain experimental lentiviral vaccines even proved to be counterproductive: they rendered vaccinated subjects more susceptible to infection rather than protecting them. For vaccine-induced enhanced susceptibility to infection with certain viruses like feline coronavirus, Dengue virus, and feline immunodeficiency virus, it has been shown that antibody-dependent enhancement (ADE) plays an important role. Other mechanisms may, either in the absence of or in combination with ADE, be involved. Consequently, vaccine-induced enhancement has been a major stumble block in the development of certain flavi-, corona-, paramyxo-, and lentivirus vaccines. Also recent failures in the development of a vaccine against HIV may at least in part be attributed to induction of enhanced susceptibility to infection. There may well be a delicate balance between the induction of protective immunity on the one hand and the induction of enhanced susceptibility on the other. The present paper reviews the currently known mechanisms of vaccine-induced enhancement of susceptibility to virus infection or of aberrant viral pathogenesis.

Chimeric adenovirus 5/35 vector containing the clade C HIV gag gene induces a cross-reactive immune response against HIV

Most of the recent HIV studies have focused on the clade B virus subtype. However, it is estimated that half the HIV patients in developing countries are infected with virus belonging to clade C. Therefore, a vaccine against HIV clade C is urgently required. In this study, we evaluate the immunogenicity and protective immunity of an adenovirus vector (Ad) in BALB/c mice and cynomolgus monkeys. We developed an HIV vaccine containing the HIV clade C gag gene using a replication-defective chimeric adenovirus type 5 (Ad5) vector incorporating Ad35 fiber (Ad5/35); this vector has exhibited low hepatotoxicity in animal models. We observed that immunization with the Ad5/35 vaccine generated heightened HIV-specific immune responses in both mice and monkeys. Furthermore, the Ad5/35 vector vaccine produced a cross-immunity against challenge with recombinant vaccinia viruses expressing HIV clade B gag. These results demonstrate that Ad5/35 vaccines expressing HIV clade C gag may be promising candidates for clinical trials.

Vaccine-based, long-term, stable control of simian/human immunodeficiency virus 89.6PD replication in rhesus macaques

The X4-tropic simian/human immunodeficiency virus (SHIV) 89.6P (or 89.6PD) causes rapid CD4(+) T-cell depletion leading to an acute crash of the host immune system, whereas pathogenic R5-tropic simian immunodeficiency virus (SIV) infection, like HIV-1 infection in humans, results in chronic disease progression in macaques. Recent pre-clinical vaccine trials inducing cytotoxic T lymphocyte (CTL) responses have succeeded in controlling replication of the former but shown difficulty in control of the latter. Analysis of the immune responses involved in consistent control of SHIV would contribute to elucidation of the mechanism for consistent control of SIV replication. This study followed up rhesus macaques that showed vaccine-based control of primary SHIV89.6PD replication and found that all of these controllers maintained viraemia control for more than 2 years. SHIV89.6PD control was observed in vaccinees of diverse major histocompatibility complex (MHC) haplotypes and was maintained without rapid selection of CTL escape mutations, a sign of particular CTL pressure. Despite the vaccine regimen not targeting Env, all of the SHIV controllers showed efficient elicitation of de novo neutralizing antibodies by 6 weeks post-challenge. These results contrast with our previous observation of particular MHC-associated control of SIV replication without involvement of neutralizing antibodies and suggest that vaccine-based control of SHIV89.6PD replication can be stably maintained in the presence of multiple functional immune effectors.

Cytotoxic T-lymphocyte escape does not always explain the transient control of simian immunodeficiency virus SIVmac239 viremia in adenovirus-boosted and DNA-primed Mamu-A*01-positive rhesus macaques

Adenovirus 5 (Ad5) vectors show promise as human immunodeficiency virus vaccine candidates. Indian rhesus macaques vaccinated with Ad5-gag controlled simian-human immunodeficiency virus SHIV89.6P viral replication in the absence of Env immunogens that might elicit humoral immunity. Here we immunized 15 macaques using either a homologous Ad5-gag/Ad5-gag (Ad5/Ad5) or a heterologous DNA-gag/Ad5-gag (DNA/Ad5) prime-boost regimen and challenged them with a high dose of simian immunodeficiency virus SIVmac239. Macaques vaccinated with the DNA/Ad5 regimen experienced a brief viral load nadir of less than 10,000 viral copies per ml blood plasma that was not seen in Mamu-A*01-negative DNA/Ad5 vaccinees, Mamu-A*01-positive Ad5/Ad5 vaccinees, or vaccine-naive controls. Interestingly, most of these animals were not durably protected from disease progression when challenged with SIVmac239. To investigate the reasons underlying this short-lived vaccine effect, we investigated breadth of the T-cell response, immunogenetic background, and viral escape from CD8+ lymphocytes that recognize immunodominant T-cell epitopes. We show that these animals do not mount unusually broad cellular immune response, nor do they express unusual major histocompatibility complex class I alleles. Viral recrudescence occurred in four of the five Mamu-A*01-positive vaccinated macaques. However, only a single animal in this group demonstrated viral escape in the immunodominant Gag181-189 CM9 response. These results suggest that viral "breakthrough" in vaccinated animals and viral escape are not inextricably linked and underscore the need for additional research into the mechanisms of vaccine failure.

Influence of glycosylation on the efficacy of an Env-based vaccine against simian immunodeficiency virus SIVmac239 in a macaque AIDS model

The envelope glycoprotein (Env) of human immunodeficiency viruses (HIVs) and simian immunodeficiency viruses (SIVs) is heavily glycosylated, and this feature has been speculated to be a reason for the insufficient immune control of these viruses by their hosts. In a macaque AIDS model, we demonstrated that quintuple deglycosylation in Env altered a pathogenic virus, SIVmac239, into a novel attenuated mutant virus (delta5G). In delta5G-infected animals, strong protective immunity against SIVmac239 was elicited. These HIV and SIV studies suggested that an understanding of the role of glycosylation is critical in defining not only the virological properties but also the immunogenicity of Env, suggesting that glycosylation in Env could be modified for the development of effective vaccines. To examine the effect of deglycosylation, we constructed prime-boost vaccines consisting of Env from SIVmac239 and delta5G and compared their immunogenicities and vaccine efficacies by challenge infection with SIVmac239. Vaccination-induced immune responses differed between the two vaccine groups. Both Env-specific cellular and humoral responses were higher in wild-type (wt)-Env-immunized animals than in delta5G Env-immunized animals. Following the challenge, viral loads in SIVmac239 Env (wt-Env)-immunized animals were significantly lower than in vector controls, with controlled viral replication in the chronic phase. Unexpectedly, viral loads in delta5G Env-immunized animals were indistinguishable from those in vector controls. This study demonstrated that the prime-boost Env vaccine was effective against homologous SIVmac239 challenge. Changes in glycosylation affected both cell-mediated and humoral immune responses and vaccine efficacy.

Induction of Gag-specific T-cell responses by therapeutic immunization with a Gag-expressing Sendai virus vector in macaques chronically infected with simian-human immunodeficiency virus

Recent prophylactic vaccine trials inducing virus-specific CD8+ T-cell responses have shown control of primary infections of a pathogenic simian-human immunodeficiency virus (SHIV) in macaques. In the chronic phase, therapeutic immunization replenishing virus-specific CD8+ T-cells is likely to contribute to sustained control of virus replication. In this study, we have administered a recombinant Sendai virus (SeV) vector into five rhesus macaques that had received prophylactic vaccinations and had controlled SHIV replication for more than 1 year after challenge. Our results indicate that virus-specific CD8+ T-cell responses can be expanded and broadened by therapeutic immunization with SeV vectors in the chronic phase after prophylactic vaccine-based control of primary immunodeficiency virus infections.

Loss of virus-specific CD4(+) T cells with increases in viral loads in the chronic phase after vaccine-based partial control of primary simian immunodeficiency virus replication in macaques

Virus-specific cellular immune responses play an important role in the control of immunodeficiency virus replication. However, preclinical trials of vaccines that induce virus-specific cellular immune responses have failed to contain simian immunodeficiency virus (SIV) replication in macaques. A defective provirus DNA vaccine system that efficiently induces virus-specific CD8(+) T-cell responses has previously been developed. The vaccinated macaques showed reduced viral loads, but failed to contain SIVmac239 replication. In this study, macaques that showed partial control of SIV replication were followed up to see if or how they lost this control in the chronic phase. Two of them showed increased viral loads about 4 or 8 months after challenge and finally developed AIDS. Analysis of SIV-specific T-cell levels by detection of SIV-specific gamma interferon (IFN-gamma) production revealed that these two macaques maintained SIV-specific CD8(+) T cells, even after loss of control, but lost SIV-specific CD4(+) T cells when plasma viral loads increased. The remaining macaque kept viral loads at low levels and maintained SIV-specific CD4(+) T cells, as well as CD8(+) T cells, for more than 3 years. Additional analysis using macaques vaccinated with a Gag-expressing Sendai virus vector also found loss of viraemia control, with loss of SIV-specific CD4(+) T cells in the chronic phase of SIV infection. Thus, SIV-specific CD4(+) T cells that were able to produce IFN-gamma in response to SIV antigens were preserved by the vaccine-based partial control of primary SIV replication, but were lost with abrogation of control in the chronic phase.

Protective efficacy of an AIDS vaccine, a single DNA priming followed by a single booster with a recombinant replication-defective Sendai virus vector, in a macaque AIDS model

We previously demonstrated the excellent protective efficacy of DNA priming followed by Gag-expressing Sendai virus (SeV) boosting (DNA prime/SeV-Gag boost vaccine) against a pathogenic simian-human immunodeficiency virus (SHIV89.6PD) infection in macaques. Here we show that we established a practical, safer AIDS vaccine protocol, a single DNA priming followed by a single booster with a recently developed replication-defective F deletion SeV-expressing Gag, and show its protective efficacy against SHIV89.6PD infections.

Immunogenicity and protective efficacy of orally administered recombinant Lactococcus lactis expressing surface-bound HIV Env

This study investigates whether genetically modified orally administered Lactococcus lactis (L lactis) could be used as an HIV vaccine. L lactis is immunogenic and extremely safe when delivered orally. We created a recombinant L lactis vector expressing the envelope protein of HIV on its cell surface. Oral immunization with this vector induced high levels of HIV-specific serum IgG and fecal IgA antibodies. Cell-mediated immune responses also were generated in both the regional lymph nodes and the spleen. Dendritic cells are readily infected by L lactis and appear to play a potential role in mediating the development of these immune responses. The protective efficacy of this vaccine strategy was demonstrated by challenging mice intraperitoneally with an HIV Env-expressing vaccinia virus. Their viral loads were 350-fold lower than those of control mice. These findings support the further development of L lactis-based HIV vaccines.

nef gene is required for robust productive infection by simian immunodeficiency virus of T-cell-rich paracortex in lymph nodes

The pathogenesis of AIDS virus infection in a nonhuman primate AIDS model was studied by comparing plasma viral loads, CD4(+) T-cell subpopulations in peripheral blood mononuclear cells, and simian immunodeficiency virus (SIV) infection in lymph nodes for rhesus macaques infected with a pathogenic molecularly cloned SIVmac239 strain and those infected with its nef deletion mutant (Deltanef). In agreement with many reports, whereas SIVmac239 infection induced AIDS and depletion of memory CD4(+) T cells in 2 to 3 years postinfection (p.i.), Deltanef infection did not induce any manifestation associated with AIDS up to 6.5 years p.i. To explore the difference in SIV infection in lymphoid tissues, we biopsied lymph nodes at 2, 8, 72, and 82 weeks p.i. and analyzed them by pathological techniques. Maximal numbers of SIV-infected cells (SIV Gag(+), Env(+), and RNA(+)) were detected at 2 weeks p.i. in both the SIVmac239-infected animals and the Deltanef-infected animals. In the SIVmac239-infected animals, most of the infected cells were localized in the T-cell-rich paracortex, whereas in the Deltanef-infected animals, most were localized in B-cell-rich follicles and in the border region between the paracortex and the follicles. Analyses by double staining of CD68(+) macrophages and SIV Gag(+) cells and by double staining of CD3(+) T cells and SIV Env(+) cells revealed that SIV-infected cells were identified as CD4(+) T cells in either the SIVmac239 or the Deltanef infection. Whereas the many functions of Nef protein were reported from in vitro studies, our finding of SIVmac239 replication in the T-cell-rich paracortex in the lymph nodes supports the reported roles of Nef protein in T-cell activation and enhancement of viral infectivity. Furthermore, the abundance of SIVmac239 infection and the paucity of Deltanef infection in the T-cell-rich paracortex accounted for the differences in viral replication and pathogenicity between SIVmac239 and the Deltanef mutant. Thus, our in vivo study indicated that the nef gene enhances SIV replication by robust productive infection in memory CD4(+) T cells in the T-cell-rich region in lymphoid tissues.

Oral administration of recombinant adeno-associated virus elicits human immunodeficiency virus-specific immune responses

Oral vaccines can induce both systemic and mucosal immunity. Mucosal immunity, especially regional cell-mediated immunity, plays an important role in protecting individuals from infectious diseases such as acquired immunodeficiency syndrome. In this study, a recombinant adeno-associated virus vector expressing human immunodeficiency virus type 1 env gene (AAV-HIV) was orally administered to BALB/c mice. Systemic and regional immunity was induced in the mice. Furthermore, the immunization significantly reduced viral load after an intrarectal challenge with a recombinant vaccinia virus expressing HIV env gene. Moreover, we also show that dendritic cells might contribute to the AAV-HIV vector-induced immune responses.

Preclinical AIDS vaccine research: survey of SIV, SHIV, and HIV challenge studies in vaccinated nonhuman primates

This current supplementary and systematic survey of 237 preclinical AIDS vaccine challenge/protection studies in nonhuman primates enumerates and broadly describes the recent status of different vaccine strategies in macaque and chimpanzee experimental models. Published studies since the previous survey were compiled and categorized by their vaccine types, challenge parameters, and challenge results. These models have supportively verified that some prophylactic vaccine approaches, though rarely preventing infection (which is observed in these models with some passively administered antibody-based vaccines), can control to some degree primate lentivirus replication and disease development, and this is encouraging because it places more potentially effective immunogens on the precipice for early clinical studies. Many of these promising approaches may benefit from more testing in mucosal challenge models, and resources will be needed to follow more of these partially protected vaccinees for longer periods.

Primary replication of a recombinant Sendai virus vector in macaques

An efficient antigen expression system using a recombinant Sendai virus (SeV) has been established recently and its potential to induce resistance against immunodeficiency virus infections in macaques has been shown. SeV replication has been well characterized in mice, the natural host, but not in primates, including humans. Here, primary SeV replication was investigated in macaques. After intranasal immunization with a recombinant SeV expressing simian immunodeficiency virus Gag protein, SeV-Gag, robust gag expression was observed in the nasal mucosa and much lower but significant levels of gag expression were observed in the local retropharyngeal and submandibular lymph nodes (LN). Expression peaked within a week and lasted at least up to 13 days after immunization. SeV-Gag was isolated from nasal swabs consistently at day 4 but not at all at day 13. Gag expression was undetectable in the lung as well as in remote lymphoid tissues, such as the thymus, spleen and inguinal LN, indicating that the spread of the virus was more restricted in macaques than in mice. SeV-specific T cells were detectable in SeV-immunized macaques at day 7. Finally, no naive macaques showed significant levels of anti-SeV antibodies in the plasma, even after living in a cage together with an acutely SeV-infected macaque for 5 weeks, indicating that SeV transmission from SeV-infected macaques to naive ones was inefficient. None of the SeV-immunized macaques displayed appreciable clinical manifestations. These results support the idea that this system may be used safely in primates, including humans.

Rapid appearance of secondary immune responses and protection from acute CD4 depletion after a highly pathogenic immunodeficiency virus challenge in macaques vaccinated with a DNA prime/Sendai virus vector boost regimen

Heterologous prime/boost regimens are AIDS vaccine candidates because of their potential for inducing cellular immune responses. Here, we have developed a prime/boost regimen leading to rapid control of highly pathogenic immunodeficiency virus infection in macaques. The strategy, priming by an env and nef deletion-containing simian-human immunodeficiency virus (SHIV) proviral DNA followed by a single booster with a Gag-expressing Sendai virus (SeV-Gag), efficiently induced virus-specific T cells, which were maintained for more than 3 months until challenge. While all naive control macaques showed acute CD4(+) T-cell depletion at week 2 after an intravenous SHIV89.6PD challenge, all the macaques vaccinated with the prime/boost regimen were protected from depletion and showed greatly reduced peak viral loads compared with controls. Vaccination with the DNA alone or SeV-Gag alone was not enough to confer the consistent protection from the depletion, although it led to efficient secondary CD8(+) T-cell responses at week 2 after challenge. At week 1, a difference in the secondary responses between the protected and the unprotected macaques was clear; rapid augmentation of virus-specific CD8(+) T cells was detected in the former but not in the latter. Thus, our results indicate the importance of rapid secondary responses for reduction in the peak viral loads and protection from acute CD4(+) T-cell depletion.

A novel recombinant adeno-associated virus vaccine induces a long-term humoral immune response to human immunodeficiency virus

Recombinant adeno-associated virus (AAV) has attracted tremendous interest as a promising vector for gene delivery. In this study we have developed an HIV-1 vaccine, using an AAV vector expressing HIV-1 env, tat, and rev genes (AAV-HIV vector). A single injection of the AAV-HIV vector induced strong production of HIV-1-specific serum IgG and fecal secretory IgA antibodies as well as MHC class I-restricted CTL activity in BALB/c mice. The titer of HIV-1-specific serum IgG remained stable for 10 months. When AAV-HIV vector was coadministered with AAV-IL2 vector, the HIV-specific cell-mediated immunity (CMI) was significantly enhanced. Boosting with AAV-HIV vector strongly enhanced the humoral response. Furthermore, the mouse antisera neutralized an HIV-1 homologous strain, and BALB/c mice immunized via the intranasal route with an AAV vector expressing the influenza virus hemagglutinin (HA) gene showed protective immunity against homologous influenza virus challenge. These results demonstrate that AAV-HIV vector immunization may provide a novel and promising HIV vaccination strategy.