Oral immunization of macaques with attenuated vaccine virus induces protection against vaginally transmitted AIDS

Monkey Models and HIV Vaccine Research

Since the discovery of acquired immunodeficiency syndrome (AIDS) in 1981, it has been extremely difficult to develop an effective vaccine or a therapeutic cure despite over 36 years of global efforts. One of the major reasons is due to the lack of an immune-competent animal model that supports live human immunodeficiency virus (HIV) infection and disease progression such that vaccine-induced correlates of protection and efficacy can be determined clearly before human trials. Nevertheless, rhesus macaques infected with simian immunodeficiency virus (SIV) and chimeric simian human immunodeficiency virus (SHIV) have served as invaluable models not only for understanding AIDS pathogenesis but also for studying HIV vaccine and cure. In this chapter, therefore, we summarize major scientific evidence generated in these models since the beginning of the AIDS pandemic. Hopefully, the accumulated knowledge and lessons contributed by thousands of scientists will be useful in promoting the search of an ultimate solution to end HIV/AIDS.

Attenuation of Replication-Competent Adenovirus Serotype 26 Vaccines by Vectorization

Replication-competent adenovirus (rcAd)-based vaccine vectors may theoretically provide immunological advantages over replication-incompetent Ad vectors, but they also raise additional potential clinical and regulatory issues. We produced replication-competent Ad serotype 26 (rcAd26) vectors by adding the E1 region back into a replication-incompetent Ad26 vector backbone with the E3 or E3/E4 regions deleted. We assessed the effect of vectorization on the replicative capacity of the rcAd26 vaccines. Attenuation occurred in a stepwise fashion, with E3 deletion, E4 deletion, and human immunodeficiency virus type 1 (HIV-1) envelope (Env) gene insertion all contributing to reduced replicative capacity compared to that with the wild-type Ad26 vector. The rcAd26 vector with E3 and E4 deleted and containing the Env transgene exhibited 2.7- to 4.4-log-lower replicative capacity than that of the wild-type Ad26 in vitro. This rcAd26 vector is currently being evaluated in a phase 1 clinical trial. Attenuation as a result of vectorization and transgene insertion has implications for the clinical development of replication-competent vaccine vectors.

Human immunodeficiency virus vaccine trials

More than 2 million AIDS-related deaths occurred globally in 2008, and more than 33 million people are living with HIV/AIDS. Despite promising advances in prevention, an estimated 2.7 million new HIV infections occurred in that year, so that for every two patients placed on combination antiretroviral treatment, five people became infected. The pandemic poses a formidable challenge to the development, progress, and stability of global society 30 years after it was recognized. Experimental preventive HIV-1 vaccines have been administered to more than 44,000 human volunteers in more than 187 separate trials since 1987. Only five candidate vaccine strategies have been advanced to efficacy testing. The recombinant glycoprotein (rgp)120 subunit vaccines, AIDSVAX B/B and AIDSVAX B/E, and the Merck Adenovirus serotype (Ad)5 viral-vector expressing HIV-1 Gag, Pol, and Nef failed to show a reduction in infection rate or lowering of postinfection viral set point. Most recently, a phase III trial that tested a heterologous prime-boost vaccine combination of ALVAC-HIV vCP1521 and bivalent rgp120 (AIDSVAX B/E) showed 31% efficacy in protection from infection among community-risk Thai participants. A fifth efficacy trial testing a DNA/recombinant(r) Ad5 prime-boost combination is currently under way. We review the clinical trials of HIV vaccines that have provided insight into human immunogenicity or efficacy in preventing HIV-1 infection.

Human immunodeficiency virus (HIV) immunopathogenesis and vaccine development: a review

The development of a safe, effective and globally affordable HIV vaccine offers the best hope for the future control of the HIV-1 pandemic. Since 1987, scores of candidate HIV-1 vaccines have been developed which elicited varying degrees of protective responses in nonhuman primate models, including DNA vaccines, subunit vaccines, live vectored recombinant vaccines and various prime-boost combinations. Four of these candidate vaccines have been tested for efficacy in human volunteers, but, to the exception of the recent RV144 Phase III trial in Thailand, which elicited a modest but statistically significant level of protection against infection, none has shown efficacy in preventing HIV-1 infection or in controlling virus replication and delaying progression of disease in humans. Protection against infection was observed in the RV144 trial, but intensive research is needed to try to understand the protective immune mechanisms at stake. Building-up on the results of the RV144 trial and deciphering what possibly are the immune correlates of protection are the top research priorities of the moment, which will certainly accelerate the development of an highly effective vaccine that could be used in conjunction with other HIV prevention and treatment strategies. This article reviews the state of the art of HIV vaccine development and discusses the formidable scientific challenges met in this endeavor, in the context of a better understanding of the immunopathogenesis of the disease.

BST-2 mediated restriction of simian-human immunodeficiency virus

Pathogenic simian-human immunodeficiency viruses (SHIV) contain HIV-1 Vpu and SIV Nef, both shown to counteract BST-2 (HM1.24; CD317; tetherin) inhibition of virus release in a species-specific manner. We show that human and pig-tailed BST-2 (ptBST-2) restrict SHIV. We found that sequential "humanization" of the transmembrane domain (TMD) of the pig-tailed BST-2 (ptBST-2) protein resulted in a fluctuation in sensitivity to HIV-1 Vpu. Our results also show that the length of the TMD in human and ptBST-2 proteins is important for BST-2 restriction and susceptibility to Vpu. Taken together, our results emphasize the importance of tertiary structure in BST-2 antagonism and suggests that the HIV-1 Vpu transmembrane domain may have additional functions in vivo unrelated to BST-2 antagonism.

Partial protection against SIV challenge by vaccination of adenovirus and MVA vectors in rhesus monkeys

This study explores the effect of priming rhesus monkeys with an Ad5/35 vector expressing simian immunodeficiency virus (SIV) gag and gp120, and then boosting the animals with an modified vaccinia virus Ankara (MVA) vector encoding the same antigens after a 2-month interval. The animals were intravenously challenged with 100 TCID50 of highly pathogenic SIVmac239 virus 2 months after the booster vaccination. The priming vaccination induced robust SIV-specific cell-mediated and humoral immune responses, and boosting further enhanced the cellular immunity. Vaccination reduced peak and long-term viral loads by 1-2 logs for a period of >6 months, as reflected by a reduction in both the SIV RNA and DNA levels. Of considerable interest, the immunized monkeys did not suffer from loss of CD4 T cells, particularly central memory CD4 T cells. These results demonstrate that prophylactic vaccination with Ad5/35 followed by MVA reduces viral replication and prevents CD4 T-cell loss, and that these effects may decrease the likelihood of disease progression.

Longitudinal study to assess the safety and efficacy of a live-attenuated SHIV vaccine in long term immunized rhesus macaques

Live-attenuated viruses derived from SIV and SHIV have provided the most consistent protection against challenge with pathogenic viruses, but concerns regarding their long-term safety and efficacy have hampered their clinical usefulness. We report a longitudinal study in which we evaluated the long-term safety and efficacy of DeltavpuSHIV(PPC), a live virus vaccine derived from SHIV(PPC). Macaques were administered two inoculations of DeltavpuSHIV(PPC), three years apart, and followed for eight years. None of the five vaccinated macaques developed an AIDS-like disease from the vaccine. At eight years, macaques were challenged with pathogenic SIV and SHIV. None of the four macaques with detectable cellular-mediated immunity prior to challenge had detectable viral RNA in the plasma. This study demonstrates that multiple inoculations of a live vaccine virus can be used safely and can significantly extend the efficacy of the vaccine, as compared to a single inoculation, which is efficacious for approximately three years.

Replicating viral vectors as HIV vaccines Summary Report from IAVI Sponsored Satellite Symposium, International AIDS Society Conference, July 22, 2007

At the International AIDS Society Conference on Pathogenesis, Treatment and Prevention held in Sydney, Australia, in July 2007, the International AIDS Vaccine Initiative (IAVI) convened a satellite symposium entitled 'Accelerating the Development of Replicating Viral Vectors for AIDS Vaccines.' Its purpose was to highlight the rationale for accelerating the development of replicating viral vectors for use as vaccines against HIV-1, and to bring together vaccine scientists, regulatory officials, and public health specialists from industrialized and developing nations to discuss the major issues facing the development and testing of replicating viral vector-based vaccines.

Protection of macaques against AIDS with a live attenuated SHIV vaccine is of finite duration

Using background data that live vaccines against several viral pathogens are effective in inducing life-long protection against disease, we undertook studies in macaques to determine the duration of protection that two live SHIV vaccines could induce against AIDS. Earlier studies had established that macaques immunized once with a live vaccine and challenged 6 months later were protected, and that other macaques given two sequential inoculations of live vaccines were protected for at least 1 year. Protection was associated with persistence of the vaccine viruses. In this study, we sought to determine whether the duration of protection in macaques given a single inoculation of replication competent live vaccines would extend beyond 3 years. Two groups of four rhesus macaques were inoculated with two live SHIV vaccines, respectively. The viruses replicated transiently in all animals but at the 3-year time point, PCR analysis of PBMC did not detect DNA of either virus in any of the animals, and all were negative for CMI responses in the blood. All 8 animals succumbed to disease when challenged with pathogenic viruses.

Selective induction of cell-mediated immunity and protection of rhesus macaques from chronic SHIV(KU2) infection by prophylactic vaccination with a conserved HIV-1 envelope peptide-cocktail

Infection of Indian-origin rhesus macaques by the simian human immunodeficiency virus (SHIV) is considered to be a suitable preclinical model for directly testing efficacy of vaccine candidates based on the HIV-1 envelope. We used this model for prophylactic vaccination with a peptide-cocktail comprised of highly conserved HIV-1 envelope sequences immunogenic/antigenic in macaques and humans. Separate groups of macaques were immunized with the peptide-cocktail by intravenous and subcutaneous routes using autologous dendritic cells (DC) and Freund's adjuvant, respectively. The vaccine elicited antigen specific IFN-gamma-producing cells and T-cell proliferation, but not HIV-neutralizing antibodies. The vaccinated animals also exhibited efficient cross-clade cytolytic activity against target cells expressing envelope proteins corresponding to HIV-1 strains representative of multiple clades that increased after intravenous challenge with pathogenic SHIV(KU2). Virus-neutralizing antibodies were either undetectable or present only transiently at low levels in the control as well as vaccinated monkeys after infection. Significant control of plasma viremia leading to undetectable levels was achieved in majority of vaccinated monkeys compared to mock-vaccinated controls. Monkeys vaccinated with the peptide-cocktail using autologous DC, compared to Freund's adjuvant, and the mock-vaccinated animals, showed significantly higher IFN-gamma production, higher levels of vaccine-specific IFN-gamma producing CD4(+) cells and significant control of plasma viremia. These results support DC-based vaccine delivery and the utility of the conserved HIV-1 envelope peptide-cocktail, capable of priming strong cell-mediated immunity, for potential inclusion in HIV vaccination strategies.

Humoral immune responses to the human immunodeficiency virus type-1 (HIV-1) in the genital tract compared to other mucosal sites

Infection with the human immunodeficiency virus-1 (HIV-1) must be considered as a primarily mucosal disease. On a worldwide basis, the absolute majority of HIV infections occur through mucosal surfaces of the genital and intestinal tracts, and the earliest and most dramatic immunologic alterations are induced by the virus in mucosal tissues. However, individual compartments of mucosal components of the immune system display remarkable differences with respect to dominant antibody isotypes, virus phenotypes, densities and origins of cells involved in innate and specific immunity, presence or absence of inductive sites, and routes of immunizations that induce humoral and cellular responses. In this regard, the mucosal immune system of the female and male genital tracts exhibit several features which are distinct from other mucosal tissues, including dominance of the IgG isotype, local as well as pronounced systemic origin of antibodies, the absence of organized lymphoepithelial inductive sites and limited humoral responses stimulated by local antigen administration. Furthermore, it is evident that, irrespective of the route of infection, HIV-1 induces easily detectable IgG but not IgA specific antibody responses. These differences must be considered in the design of protective vaccines against infection with HIV and other agents of sexually transmitted diseases.

Humoral immune responses to the human immunodeficiency virus type-1 (HIV-1) in the genital tract compared to other mucosal sites

Infection with the human immunodeficiency virus-1 (HIV-1) must be considered as a primarily mucosal disease. On a worldwide basis, the absolute majority of HIV infections occur through mucosal surfaces of the genital and intestinal tracts, and the earliest and most dramatic immunologic alterations are induced by the virus in mucosal tissues. However, individual compartments of mucosal components of the immune system display remarkable differences with respect to dominant antibody isotypes, virus phenotypes, densities and origins of cells involved in innate and specific immunity, presence or absence of inductive sites, and routes of immunizations that induce humoral and cellular responses. In this regard, the mucosal immune system of the female and male genital tracts exhibit several features which are distinct from other mucosal tissues, including dominance of the IgG isotype, local as well as pronounced systemic origin of antibodies, the absence of organized lymphoepithelial inductive sites and limited humoral responses stimulated by local antigen administration. Furthermore, it is evident that, irrespective of the route of infection, HIV-1 induces easily detectable IgG but not IgA specific antibody responses. These differences must be considered in the design of protective vaccines against infection with HIV and other agents of sexually transmitted diseases.

Virus-like particles: designing an effective AIDS vaccine

Viruses that infect eukaryotic organisms have the unique characteristic of self-assembling into particles. The mammalian immune system is highly attuned to recognizing and attacking these viral particles following infection. The use of particle-based immunogens, often delivered as live-attenuated viruses, has been an effective vaccination strategy for a variety of viruses. The development of an effective vaccine against the human immunodeficiency virus (HIV) has proven to be a challenge, since HIV infects cells of the immune system causing severe immunodeficiency resulting in the syndrome known as AIDS. In addition, the ability of the virus to adapt to immune pressure and reside in an integrated form in host cells presents hurdles for vaccinologists to overcome. A particle-based vaccine strategy has promise for eliciting high titer, long-lived, immune responses to a diverse number of viral epitopes against different HIV antigens. Live-attenuated viruses are effective at generating both cellular and humoral immune responses. However, while these vaccines stimulate immunity, challenged animals rarely clear the viral infection and the degree of attenuation directly correlates with protection from disease. Further, a live-attenuated vaccine has the potential to revert to a pathogenic form. Alternatively, virus-like particles (VLPs) mimic the viral particle without causing an immunodeficiency disease. VLPs are self-assembling, non-replicating, non-pathogenic particles that are similar in size and conformation to intact virions. A variety of VLPs for lentiviruses are currently in preclinical and clinical trials. This review focuses on our current status of VLP-based AIDS vaccines, regarding issues of purification and immune design for animal and clinical trials.

Antigen expression kinetics and immune responses of mice immunized with noninfectious simian-human immunodeficiency virus DNA

In a previous report we demonstrated that three injections of an rt-deleted noninfectious genome of the simian-human immunodeficiency virus SHIV(KU2) induced protection against AIDS in macaques (D. K. Singh, Z. Liu, D. Sheffer, G. A. Mackay, M. Smith, S. Dhillon, R. Hegde, F. Jia, I. Adany, and O. Narayan, J. Virol 79:3419-3428, 2005). To make this DNA safer, we deleted two more genes, the integrase gene and vif, along with the 3' long terminal repeat. We also replaced the gag, pro, and nef genes (SIVmac239 origin) with those of human immunodeficiency virus (HIV) type 1 strain SF2. The resultant construct, designated delta4SHIV(KU2) DNA, was used in this study to evaluate gene expression and immunogenicity in BALB/c mice. DNA-transfected human embryonic kidney epithelial cells (HEK 293) produced all of the major viral proteins and released p24 in the supernatant for 12 days. Inoculation of the vaccine DNA into the gastrocnemius muscles resulted in intense mononuclear cell infiltration at the inoculated sites and the production of viral p24 in myocytes, in infiltrating mononuclear cells, and in cells in the spleen and draining lymph nodes between 3 and 10 days postinoculation. Expression of p24 in the muscle cells peaked at day 7 and became undetectable after day 12. The same 12-day period of expression of p24 was observed in mice that were given a second injection 4 weeks after the first. Evaluation of immune responses in BALB/c mice revealed that the DNA induced enzyme-linked immunospot and antigen-specific proliferative cell-mediated immunity responses. The responses were stronger in mice that were coinjected with a second plasmid expressing granulocyte-macrophage colony-stimulating factor. Since new waves of viral antigen production could be induced with each boosting injection of the vaccine DNA, this DNA could be a safe and efficient agent to induce long-term protection against HIV.

A combination DNA and attenuated simian immunodeficiency virus vaccine strategy provides enhanced protection from simian/human immunodeficiency virus-induced disease

Among the most effective vaccine candidates tested in the simian immunodeficiency virus (SIV)/macaque system, live attenuated viruses have been shown to provide the best protection from challenge. To investigate if preimmunization would increase the level of protection afforded by live attenuated SIVmac239Deltanef (Deltanef), macaques were given two priming immunizations of DNA encoding SIV Gag and Pol proteins, with control macaques receiving vector DNA immunizations. In macaques receiving the SIV DNA inoculation, SIV-specific cellular but not humoral responses were readily detectable 2 weeks after the second DNA inoculation. Following boosting with live attenuated virus, control of Deltanef replication was superior in SIV-DNA-primed macaques versus vector-DNA-primed macaques and was correlated with higher levels of CD8+/gamma-interferon-positive and/or interleukin-2-positive cells. Challenge with an intravenous inoculation of simian/human immunodeficiency virus (SHIV) strain SHIV89.6p resulted in infection of all animals. However, macaques receiving SIV DNA as the priming immunizations had statistically lower viral loads than control animals and did not develop signs of disease, whereas three of seven macaques receiving vector DNA showed severe CD4+ T-cell decline, with development of AIDS in one of these animals. No correlation of immune responses to protection from disease could be derived from our analyses. These results demonstrate that addition of a DNA prime to a live attenuated virus provided better protection from disease following challenge than live attenuated virus alone.

Perils at mucosal front lines for HIV and SIV and their hosts

HIV-1 and simian immunodeficiency virus (SIV), as well as their hosts, face perils at mucosal front lines in early infection. At these sites, 'resting' CD4+ memory T cells fuel infection (because they are hosts for virus), depleting CD4+ memory T cells throughout the lymphoid tissues, particularly in the gut, and eliciting an immunosuppressive regulatory T-cell response that impairs host defence. But HIV-1 and SIV also risk elimination at the earliest stage of infection, at the mucosal point of entry, if founder populations of infected cells do not expand sufficiently to establish a self-propagating infection. Microbicides and vaccines could increase these viral vulnerabilities at mucosal front lines.

A noninfectious simian/human immunodeficiency virus DNA vaccine that protects macaques against AIDS

Simian/human immunodeficiency virus SHIV(KU2) replicates with extremely high titers in macaques. In order to determine whether the DNA of the viral genome could be used as a vaccine if the DNA were rendered noninfectious, we deleted the reverse transcriptase gene from SHIVKU2 and inserted this DNA (DeltartSHIVKU2) into a plasmid that was then used to test gene expression and immunogenicity. Transfection of Jurkat and human embryonic kidney epithelial (HEK 293) cells with the DNA resulted in production of all of the major viral proteins and their precursors and transient export of a large quantity of the Gag p27 into the supernatant fluid. As expected, no infectious virus was produced in these cultures. Four macaques were injected intradermally with 2 mg of the DNA at 0, 8, and 18 weeks. The animals developed neutralizing antibodies and low enzyme-linked immunospot assay (E-SPOT) titers against SHIVKU2. These four animals and two unvaccinated control animals were then challenged with heterologous SHIV89.6P administered into their rectums. The two control animals developed viral RNA titers exceeding 10(6) copies/ml of plasma, and these titers were accompanied by the loss of CD4+ T cells by 2 weeks after challenge. The two control animals died at weeks 8 and 16, respectively. All four of the immunized animals became infected with the challenge virus but developed lower titers of viral RNA in plasma than the control animals, and the titers decreased over time in three of the four macaques. The fourth animal remained viremic and died at week 47. Whereas the control animals failed to develop E-SPOT responses, all four of the immunized animals developed anamnestic E-SPOT responses after challenge. The animal that died developed the highest E-SPOT response and was the only one that produced neutralizing antibodies against the challenge virus. These results established that noninfectious DNA of pathogenic SHIV could be used as a vaccine to prevent AIDS, even though the immunological assays used did not predict the manner in which the challenge virus would replicate in the vaccinated animals.

Abrogation of attenuated lentivirus-induced protection in rhesus macaques by administration of depo-provera before intravaginal challenge with simian immunodeficiency virus mac239

In nonhuman primate models of acquired immunodeficiency syndrome, live attenuated lentiviruses provide the most reliable protection from systemic and mucosal challenge with pathogenic simian immunodeficiency virus (SIV). Although live attenuated lentiviruses may never be used in humans because of safety concerns, understanding the nature of the protective immune mechanisms induced by live attenuated vaccines in primate models will be useful for developing other vaccine approaches. Approximately 60% of rhesus macaques immunized with nonpathogenic simian-human immunodeficiency virus (SHIV) strain 89.6 are protected from infection or clinical disease after intravaginal (IVAG) challenge with pathogenic SIVmac239. The goal of the present study was to determine whether administration of Depo-Provera before IVAG challenge with SIV decreases the protective efficacy of infection with SHIV89.6. The rate of protection after IVAG challenge with SIVmac239 was significantly lower (P<.05), and the acute postchallenge plasma viral RNA levels were significantly higher (P<.006), in Depo-Provera-treated, SHIV89.6-immunized macaques than in Depo-Provera-naive, SHIV89.6-immunized macaques. In the primate model of sexual transmission of human immunodeficiency virus, treatment with progesterone before IVAG challenge with a pathogenic virus can decrease the efficacy of a model "vaccine."

Protection Against Late-Onset AIDS in Macaques Prophylactically Immunized with a Live Simian HIV Vaccine Was Dependent on Persistence of the Vaccine Virus

This is a 5-year follow-up study on 12 macaques that were immunized orally with two live SHIV vaccines, six with V1 and six with V2. All 12 macaques became persistently infected after transient replication of the vaccine viruses; all were challenged vaginally 6 mo later with homologous pathogenic SHIV(KU-1). Two of the V1 group developed full-blown AIDS without evidence of vaccine virus DNA in tissues. The data on the 10 vaccinated survivors showed that all 10 became infected with SHIV(KU-1) and that DNA of both vaccine and SHIV(KU-1) viruses were present 6 mo postchallenge, with minimal replication of SHIV(KU-1). During the following 5 years, these animals remained persistently infected, but with only one of the two viruses. Six animals eliminated their vaccine virus after variable periods of time and four of these succumbed to reactivation of the challenge virus and AIDS. Five years after challenge, four latently infected animals, two with V2 and two with SHIV(KU-1), were reinoculated with SHIV(KU-1.) This resulted in transient superinfection and the animals promptly returned to their prechallenge status. Immunosuppression of the four animals 1 year later with Abs to CD8+ lymphocytes resulted in transiently productive replication of their respective latent viruses, and upon recovery of CD8+ lymphocytes, they reverted to their latent virus status. The major finding was that of eight animals that eliminated the vaccine virus, six developed AIDS. The two others harboring SHIV(KU-1) remain at risk for developing late-onset disease. The primary correlate against AIDS was persistence of the vaccine virus.

Routes of inoculation and the immune response to a resolving genital flavivirus infection in a novel murine model

The prolonged, abnormal immune response patterns produced by many sexually transmitted viruses have been intensively studied. Because normal antiviral immune responses in the vagina are less well-defined, we developed a resolving murine model using vaginal inoculation with the flavivirus, West Nile virus. Infection resulted in 12% mortality, with sterile protective immunity to vaginal or systemic re-challenge. B-cell numbers increased in the vaginal mucosa from day 1-7 after primary infection, while similar increases in B220(+), CD4(+) and CD8(+) lymphocytes in the draining lymph node were delayed by 48 h. By day 4 postinfection, a MHC-II(+) dendritic cell population became depleted from the stroma and formed aggregates below the basement membrane at points of demonstrable epithelial infection. In contrast, primary systemic or intradermal inoculation resulted in 80-90% mortality, but also conferred protective sterile immunity to vaginal West Nile virus re-challenge. Intravaginal and intradermal immunization elicited comparable, accelerated accumulation of larger B-cell numbers in the mucosa and draining lymph node upon intravaginal re-challenge than systemic immunization. However, accumulation of CD4(+) T cells in both sites in the intradermally immunized group was significantly greater than in intravaginally or systemically immunized mice. Accelerated accumulation of dendritic cells occurred at periodic sub-basement membrane sites in the absence of detectable virus 1 day after vaginal re-challenge, irrespective of the route of immunization. These data illustrate the diversity of possible effective immune responses to West Nile virus in the vaginal mucosa. They show primary vaginal inoculation produces effective immunity to flavivirus infection with lower mortality than other routes and suggest a local role for vaginal mucosal dendritic cells in both primary and secondary responses.

Transmission, acute HIV-1 infection and the quest for strategies to prevent infection

By the acute stage of HIV-1 infection, the immune system already faces daunting challenges. Research on mucosal barriers and the events immediately after heterosexual transmission that precede this acute stage could facilitate the development of effective microbicides and vaccines.

Evidence for immune-mediated reduction of viral replication in Macaca nemestrina mucosally immunized with inactivated SHIV(89.6)

Although most HIV-1 infections worldwide result from heterosexual transmission, most vaccine candidates have focused on induction of systemic immunity and protection. We hypothesized that combining systemic priming with mucosal boosting would induce mucosal immunity that would protect from intravaginal challenge. Macaques were primed systemically with recombinant vaccinia viruses and boosted mucosally using inactivated SHIV(89.6) plus adjuvant. Other animals received protein boosts with adjuvant alone. Priming and boosting induced antiviral IgG and IgA antibodies. Such antibodies were induced to a lesser degree in animals receiving boosts alone. Anti-SHIV T cell responses were induced only in the prime-boost animals. Immunized animals and controls were challenged intravaginally with SHIV(89.6) and significant reductions in proviral and viral RNA loads were observed in the prime-boost animals. The boost-only animals did not have significant viral load reductions. These data suggest that cellular immunity was required for protection from intravaginal challenge. This immunization regimen provides a promising lead for vaccine development.

Immunization of macaques with live simian human immunodeficiency virus (SHIV) vaccines conferred protection against AIDS induced by homologous and heterologous SHIVs and simian immunodeficiency virus

To evaluate the vaccine potential of SHIVs attenuated by deletion of viral accessory genes, seven rhesus macaques were sequentially immunized with Delta vpu Delta nefSHIV-4 (vaccine-I) followed by Delta vpuSHIV(PPC) (vaccine-II). Despite the absence of virological evidence of productive infection with the vaccine strains, based on analysis of infectivity among peripheral blood mononuclear cells (PBMC) of the vaccinated animals, all seven animals developed binding as well as neutralizing antibodies against both vaccine-I and -II. The animals also developed vaccine virus-specific CTLs that recognized homologous as well as heterologous pathogenic SHIVs and SIV, and also soluble inhibitory factors that blocked the in vitro replication of the vaccine strains and different challenge viruses. Virus-specific cellular and humoral responses were sustained throughout a 58-week prechallenge period. To model aspects of natural transmission, the animals received a mucosal (rectal) challenge, with a mixture of three challenge viruses, SHIV(KU), SHIV(89.6)P, and SIV(mac)R71/17E. Two mock-vaccinated control animals inoculated with the same mixture of challenge viruses developed large numbers of infectious PBMC, high plasma viremia, and precipitous loss of CD4(+) T cells. The control animals did not develop any immune responses and succumbed to AIDS between 6 and 7 weeks postchallenge. All seven vaccinated animals became infected with challenge viruses as indicated by the presence of infectious cells in the PBMC and/or viral RNA in plasma. However, peak plasma viremia in vaccinates was two to nearly five logs lower than in the control animals and later plasma viral RNA became undetectable in all vaccinates. Vaccinated animals maintained normal CD4(+) T cell levels throughout the study. Challenge with pathogenic viruses caused massive anamnestic responses as determined by quantitation of virus-specific CD4(+) and CD8(+) T cells by intracellular IFN-gamma staining, and these cells persisted for at least 74 weeks. The study is still in progress and at this time DNA of SIV has become undetectable in lymph nodes of six of the seven vaccinates, SHIV(89.6)P in five of the seven, and SHIV(KU) in three of the seven animals.

Systemic infection and limited replication of SHIV vaccine virus in brains of macaques inoculated intracerebrally with infectious viral DNA

SHIV deleted in two accessory genes, DeltavpuDeltanef SHIV(PPC), functioned well as a vaccine against later challenge with highly pathogenic SHIV(KU), and it was able to reach the brain after oral inoculation of live virus. In this study, the proviral genome cloned into a plasmid was inoculated as DNA intracerebrally and spread systemically. Few regions of the brain had detectable proviral DNA by real-time PCR. Two measures of virus replication, detection of viral mRNA expression and circular proviral DNA, were negative for those brain regions, with the exception of the infection site in the right parietal lobe, whereas lymphoid tissues were positive by both measures. Histopathological analyses of all the sampled brain and spinal cord regions did not reveal any abnormalities. Despite intracerebral inoculation of the viral DNA, the brain was not targeted for high levels of virus replication.

Protection by intranasal immunization of a nef-deleted, nonpathogenic SHIV against intravaginal challenge with a heterologous pathogenic SHIV

An effective vaccine against sexual transmission of human immunodeficiency virus (HIV) should elicit both systemic and mucosal immune responses. In this study, to examine the possibility of using an attenuated virus for mucosal immunization, four female macaques were intranasally or intravenously administered with a chimeric simian-human immunodeficiency virus with a deleted nef gene (SHIV-dn). Although all the monkeys had anti-HIV-1 antibodies with neutralizing activity in the plasma, the intranasally immunized monkeys had much higher levels of HIV-1 Env-specific IgG and IgA antibodies in mucosal secretions compared with the intravenously immunized monkeys. Moreover, three of four intranasally immunized monkeys were completely protected from intravaginal challenge with a pathogenic virus, SHIV-89.6P, whereas only one intravenously immunized monkey was protected. Thus, intranasal immunization of an attenuated virus can induce the protective efficacy against intravaginal infection.

Development of feline immunodeficiency virus ORF-A (tat) mutants: in vitro and in vivo characterization

A functional ORF-A is essential for efficient feline immunodeficiency virus replication in lymphocytes. We have characterized a series of mutants of the Petaluma strain, derived from p34TF10 and having different combinations of stop codons and increasingly long deletions in ORF-A. Six clones proved fully replicative in fibroblastoid Crandell feline kidney cells and monocyte-derived macrophage cultures but failed to replicate in T cell lines and primary lymphoblasts. Cats inoculated with three selected mutants had considerably milder infections than controls given intact ORF-A virus. In vivo, the mutants maintained growth properties similar to those in vitro for at least 7 months, except that replication in lymphoid cells was strongly reduced but not ablated. One mutant underwent extensive ORF-A changes without, however, reverting to wild-type. Antiviral immune responses were feeble in all cats, suggesting that viral loads were too low to represent a sufficiently powerful antigenic stimulus.

Presence of Intact vpu and nef genes in nonpathogenic SHIV is essential for acquisition of pathogenicity of this virus by serial passage in macaques

Use of the macaque model of human immunodeficiency virus (HIV) pathogenesis has shown that the accessory genes nef and vpu are important in the pathogenicity of simian immunodeficiency virus (SIV) and simian-human immunodeficiency virus (SHIV). We examined the ability of two nonpathogenic SHIVs, SHIV(PPC) and DeltavpuDeltanefSHIV(PPC), to gain pathogenicity by rapid serial passage in macaques. In this study, each virus was passaged by blood intravenously four times at 4-week intervals in macaques. Animals were monitored for 40 weeks for levels of CD4 T cells and quantitative measures of virus infection. DeltavpuDeltanefSHIV(PPC) maintained a limited phase of productive replication in the four animals, with no loss of CD4(+) T cells, whereas SHIV(PPC) became more pathogenic in later passages, judging by plasma viral load and viral mRNA in lymph nodes, infectious peripheral blood mononuclear cells and CD4(+) T cell loss. The nef, LTR, and env of the SHIV(PPC) viruses underwent numerous mutations, compared to DeltavpuDeltanefSHIV(PPC). This study confirms the seminal role that nef, LTR, and vpu could play in regulation of pathogenesis of HIV infection.

Sequential immunization of macaques with two differentially attenuated vaccines induced long-term virus-specific immune responses and conferred protection against AIDS caused by heterologous simian human immunodeficiency Virus (SHIV(89.6)P)

Four rhesus macaques were sequentially immunized with live vaccines DeltavpuDeltanefSHIV-4 (vaccine-I) and Deltavpu SHIV(PPC) (vaccine-II). The vaccine viruses did not replicate productively in the peripheral blood mononuclear cells (PBMCs) of the vaccinated animals. All four animals developed binding antibodies against both the vaccine-I and -II envelope glycoproteins but neutralizing antibodies only against vaccine-I. They developed vaccine virus-specific CTLs that also recognized homologous as well as heterologous pathogenic SHIVs. Thirty weeks after the last immunization, the vaccinated animals and three unvaccinated control animals were challenged iv with a highly virulent heterologous SHIV(89.6)P. As expected, the three unvaccinated control animals developed large numbers of infectious PBMCs, high plasma viremia, and precipitous loss of CD4(+) T cells. Two controls did not develop any immune response and succumbed to AIDS in about 6 months. The third control animal developed neutralizing antibodies and had a more chronic disease course, but eventually succumbed to AIDS-related complications 81 weeks after inoculation. The four vaccinated animals became infected with challenge virus as indicated by the presence of challenge virus-specific DNA in the PBMCs and RNA in plasma. However, virus in these animals replicated approximately 200- to 60,000-fold less efficiently than in control animals and eventually, plasma viral RNA became undetectable in three of the four vaccinates. The animals maintained normal CD4(+) T-cell levels throughout the observation period of 85 weeks after a transient drop at Week 3 postchallenge. They also maintained CTL responses throughout the observation period. These studies thus showed that the graded immunization schedule resulted in a safe and highly effective long-lasting immune response that was associated with protection against AIDS by highly pathogenic heterologous SHIV(89.6)P.

Development of virus-specific immune responses in SHIV(KU)-infected macaques treated with PMPA

Therapeutic intervention with highly active antiretroviral therapy (HAART) can lead to the suppression of HIV viremia below the threshold of detection for several years. However, impact of HAART on reconstitution of virus-specific immune responses remains poorly understood. In this study, four macaques were infected with pathogenic SHIV(KU). One week postinoculation two of the four animals were treated with PMPA [9-R-(2-phosphophomethoxypropyl)adenine] daily for 83 days. Two other macaques, that did not receive treatment, exhibited explosive virus replication accompanied by a near total loss of CD4(+) T cells and succumbed to AIDS-related complications within 6 months of infection. These animals did not develop any virus-specific immune responses. On the contrary, the animals that received PMPA showed transient loss of CD4(+) T cells that recovered during the treatment period. The virus burden declined below the level of detection that rebounded soon after cessation of PMPA therapy. The virus replicated productively for several weeks before both animals controlled the productive replication of virus. This control of virus replication was found to be associated with the development of virus-specific neutralizing antibodies, T-helper cells, and CTLs. Although PMPA did not eliminate virus from the animals, it provided them with enough time to mount virus-specific immune responses that eventually controlled the virus replication in the blood. Our results suggest that antiretroviral therapy, if initiated early during infection, would help the host in mounting virus-specific immune responses that might control productive replication of the virus.

Selective transmission of R5-tropic HIV type 1 from dendritic cells to resting CD4+ T cells

In an in vitro coculture model of monocyte-derived, cultured human dendritic cells (DC) with autologous CD4(+) resting T cells, CCR5 (R5)-tropic strains of HIV-1, but not CXCR4 (X4)-tropic strains, were transmitted to resting CD4+ T cells, leading to prolific viral output, although DC were susceptible to infection with either strain. Macrophages, which were also infectable with either R5- or X4-tropic strains, did not transmit infection to CD4+ cells. Highly productive HIV infection in this model appeared to be a consequence of heterokaryotic syncytium formation between infected DC and T cells since syncytia formation developed only in R5-infected DC/CD4+ cocultures. These results suggested that the unique microenvironment derived from the fusion between the infected DC and CD4+ cell was highly permissive and selective for replication of R5-tropic viruses. The apparent selectivity for R5-tropic strains in such syncytia was attributable neither to differential DC-mediated activation nor to selective modulation of induction of alpha- or beta-chemokines in the infected DC. This model of HIV replication may provide useful insights into in vitro correlates of HIV pathogenicity.

Lasting effects of transient postinoculation tenofovir [9-R-(2-Phosphonomethoxypropyl)adenine] treatment on SHIV(KU2) infection of rhesus macaques

SHIV(KU2) replicates to high levels in inoculated macaques and reproducibly causes an acute depletion of CD4(+) T cells. We evaluated the ability of treatment with the antiretroviral drug 9-R-(2-phosphonomethoxypropyl)adenine (PMPA; tenofovir), begun 7 days postinoculation, to inhibit viral replication and associated pathogenesis. Highly productive infection (plasma viral RNA > 10(6) copy eq/mL) was present and CD4 depletion had started when treatment was initiated. PMPA treatment was associated with a rapid decline in plasma viral RNA to undetectable levels, with parallel decreases in the infectivity of plasma and infectious cells in PBMCs and CSF and stabilization of CD4(+)T-cell levels. Viral dynamics parameters were calculated for the initial phase of exponential viral replication and the treatment-related decline in plasma viremia. Following cessation of treatment after 12 weeks, plasma viral RNA was detectable intermittently at low levels, and spliced viral transcripts were detected in lymph nodes. Although treatment was begun after viral dissemination, high viremia, and CD4 decreases had occurred, following withdrawal of PMPA, CD4(+) T-cell counts normalized and stabilized in the normal range, despite persistent low-level infection. No PMPA-resistance mutations were detected. These results validate the similar viral replicative dynamics of SHIV(KU2) and HIV and SIV, and also underscore the potential for long-term modulation of viral replication patterns and clinical course by perturbation of primary infection.

Pathogenic simian/human immunodeficiency virus SHIV(KU) inoculated into immunized macaques caused infection, but virus burdens progressively declined with time

Using the simian immunodeficiency virus/human immunodeficiency virus (SHIV)-macaque model of AIDS, we had shown in a previous report that a live, nonpathogenic strain of SHIV, further attenuated by deletion of the vpu gene and inoculated orally into adult macaques, had effectively prevented AIDS following vaginal inoculation with pathogenic SHIV(KU). Examination of lymph nodes from the animals at 18 weeks postchallenge had shown that all six animals were persistently infected with challenge virus. We report here on a 2-year follow-up study on the nature of the persistent infections in these animals. DNA of the vaccine virus was present in the lymph nodes at all time points tested, as far as 135 weeks postchallenge. In contrast, the DNA of SHIV(KU) became undetectable in one animal by week 55 and in three others by week 63. These four macaques have remained negative for SHIV(KU) DNA as far as the last time point examined at week 135. Quantification of the total viral DNA concentration in lymph nodes during the observation period showed a steady decline. All animals developed neutralizing antibody and cytotoxic-T-lymphocyte responses to SHIV(KU) that persisted throughout the observation period. Vaccine-like viruses were isolated from two animals, and a SHIV(KU)-like virus was isolated from one of the two macaques that remained positive for SHIV(KU) DNA. There was no evidence of recombination between the vaccine and the challenge viruses. Thus, immunization with the live vaccine not only prevented disease but also contributed to the steady decline in the virus burdens in the animals.

Characterization of immune escape viruses from a macaque immunized with live-virus vaccine and challenged with pathogenic SHIVKU-1

We characterized two immune escape viruses (SHIV(KU-1/105w52) and SHIV(KU-1/105w98)) from a macaque immunized with DeltavpuDeltanef SHIV-4 and challenged with pathogenic SHIV(KU-1). This macaque developed neutralizing antibodies as well as virus-specific CTLs against the challenge virus. However, the two new viruses could not be neutralized by anti-SHIV(KU-1)-specific neutralizing antibodies and were poorly recognized by challenge virus-specific CTLs. Sequence analysis of the gene encoding gp120 revealed several mutations in the protein that might have contributed to the development of the immune-escape viruses.

The chimpanzee and other non-human-primate models in HIV-1 vaccine research

Animal models are of great importance for the study of disease pathogenesis, particularly non-human-primate models of infectious diseases. The role of non-human primates in HIV-1 research is continually discussed and debated. Here, we examine three primate models: chimpanzee-HIV-1, rhesus macaque-simian immunodeficiency virus and rhesus macaque-SHIV, and discuss immunological similarities and differences, safety and monetary issues, and ethical concerns.

Simian-human immunodeficiency virus-associated nephropathy in macaques

A number of chimeric simian-human immunodeficiency virus (SHIV) viruses containing tat, rev, vpu, and env from HIV-1 (strain HXBc2) in a genetic background of simian immunodeficiency virus (SIV(mac)239) have been derived from the parental nonpathogenic SHIV-4 virus. In this article we examine the renal pathology associated with the derivation of these pathogenic SHIV strains. The first of the pathogenic SHIVs, SHIV(KU-1), is associated with rapid CD4(+) T cell loss and opportunistic infections associated with AIDS, but only one of four infected pigtail macaques examined has developed significant renal pathology. The renal pathology in this macaque consists of a diffuse increase in matrix in the core of each lobule with collapsed glomerular capillries, which is similar to the renal changes reported in HIVAN. Passage of this virus into rhesus macaques yielded SHIV(KU-2), which results in renal pathology in three of four inoculated rhesus macaques in which <10% of the glomeruli are involved. A molecular clone of SHIV(KU-2) was derived (SHIV(KU-2MC4)) that causes neurologic and renal pathology with more than 60% of the glomeruli involved and results in uremic level BUN concentrations. These results indicate that SHIV(KU-2MC4) causes severe significant glomerular pathology and should permit a detailed analysis of the molecular determinants associated with the development of SHIV-associated glomerulosclerosis in rhesus macaques.

Immune responses and viral replication in long-term inapparent carrier ponies inoculated with equine infectious anemia virus

Persistent infection of equids by equine infectious anemia virus (EIAV) is typically characterized by a progression during the first year postinfection from chronic disease with recurring disease cycles to a long-term asymptomatic infection that is maintained indefinitely. The goal of the current study was to perform a comprehensive longitudinal analysis of the course of virus infection and development of host immunity in experimentally infected horses as they progressed from chronic disease to long-term inapparent carriage. We previously described the evolution of EIAV genomic quasispecies (C. Leroux, C. J. Issel, and R. C. Montelaro, J. Virol. 71:9627-9639, 1997) and host immune responses (S. A. Hammond, S. J. Cook, D. L. Lichtenstein, C. J. Issel, and R. C. Montelaro, J. Virol. 71:3840-3852, 1997) in four experimentally infected ponies during sequential disease episodes associated with chronic disease during the first 10 months postinfection. In the current study, we extended the studies of these experimentally infected ponies to 3 years postinfection to characterize the levels of virus replication and development of host immune responses associated with the progression from chronic disease to long-term inapparent infection. The results of these studies revealed over a 10(3)-fold difference in the steady-state levels of plasma viral RNA detected during long-term inapparent infection that correlated with the severity of chronic disease, indicating different levels of control of virus replication during long-term inapparent infections. Detailed analyses of antibody and cellular immune responses in all four ponies over the 3-year course of infection revealed a similar evolution during the first year postinfection of robust humoral and cellular immunity that then remained relatively constant during long-term inapparent infection. These observations indicate that immune parameters that have previously been correlated with EIAV vaccine protection fail to provide reliable immune correlates of control of virus replication or clinical outcome in experimental infections. Thus, these data emphasize the differences between immunity to virus exposure and immune control of an established viral infection and further emphasize the need to develop and evaluate novel immunoassays to define reliable immune correlates to vaccine and infection immunity, respectively.

A molecular clone of simian-human immunodeficiency virus (DeltavpuSHIV(KU-1bMC33)) with a truncated, non-membrane-bound vpu results in rapid CD4(+) T cell loss and neuro-AIDS in pig-tailed macaques

We report on the role of vpu in the pathogenesis of a molecularly cloned simian-human immunodeficiency virus (SHIV(KU-1bMC33)), in which the tat, rev, vpu, env, and nef genes derived from the uncloned SHIV(KU-1b) virus were inserted into the genetic background of parental nonpathogenic SHIV-4. A mutant was constructed (DeltavpuSHIV(KU-1bMC33)) in which 42 of 82 amino acids of Vpu were deleted. Phase partitioning studies revealed that the truncated Vpu was not an integral membrane protein, and pulse-chase culture studies revealed that cells inoculated with DeltavpuSHIV(KU-1bMC33) released viral p27 into the culture medium with slightly reduced kinetics compared with cultures inoculated with SHIV(KU-1bMC33). Inoculation of DeltavpuSHIV(KU-1bMC33) into two pig-tailed macaques resulted in a severe decline of CD4(+) T cells and neurological disease in one macaque and a more moderate decline of CD4(+) T cells in the other macaque. These results indicate that a membrane-bound Vpu is not required for the CD4(+) T cell loss and neurological disease in SHIV-inoculated pig-tailed macaques. Furthermore, because the amino acid substitutions in the Tat and Rev were identical to those previously reported for the nonpathogenic SHIV(PPc), our results indicate that amino acid substitutions in the Env and/or Nef were responsible for the observed CD4(+) T cell loss and neurological disease after inoculation with this molecular clone.

Primate models of AIDS

The primate models of AIDS provide insights into pathogenesis, transmission, and immune responses to infection and are useful in testing vaccines and drugs. The HIV-1/chimpanzee, SIV(mac)/macaque, and SHIV/macaque models are the most widely used. The advantages and drawbacks of these and other models are discussed.

Protection of macaques against a SHIV with a homologous HIV-1 Env and a pathogenic SHIV-89.6P with a heterologous Env by vaccination with multiple gene-deleted SHIVs

To evaluate the potential of SHIVs as anti-HIV-1 live vaccines, we constructed two gene-deleted SHIVs, designated SHIV-drn and SHIV-dxrn. The former lacks vpr/nef and the latter lacks vpx/vpr/nef. Four macaques that had been vaccinated with SHIV-drn were challenged with SHIV-NM-3rN, which has an HIV-1 Env that is the same as that of SHIV-drn. No challenge virus was detected by DNA PCR in, or recovered from, two of the macaques. In the other two, challenge virus was detected once and twice, respectively. Plasma viral loads were much lower than those in unvaccinated controls. Another four macaques were vaccinated with SHIV-dxrn. These macaques showed resistance but less than that of SHIV-drn-vaccinated macaques. When the two SHIV-drn-vaccinated macaques were challenged with pathogenic SHIV-89.6P, which has an HIV-1 Env that is antigenically different from that of SHIV-drn, replication of the challenge virus was restricted, and the usual decrease in the number of CD4(+) cells was prevented. In this protection, it is noteworthy that protection involved not only neutralizing antibodies and killer cell activity, but also other unknown specific and nonspecific immunity elicited by the infection.

Use of herpesvirus saimiri-immortalized macaque CD4(+) T cell clones as stimulators and targets for assessment of CTL responses in macaque/AIDS models

Herpesvirus saimiri (HVS), a nonhuman primate gamma herpes virus, was used to immortalize pig-tailed macaque CD4(+) T lymphocytes. The HVS-immortalized T cell lines were used to develop CD4(+) T cell clones from two animals. Three CD4(+) T cell clones were further characterized for the expression of cell surface markers. All expressed CD2, CD4, CD58, CD69 and CD80 and therefore resembled activated T cells. These clones required exogenous IL-2 for efficient growth and were found to be highly susceptible to infection by the challenge virus, Chimeric simian/human immunodeficiency virus (SHIV(KU-1)). They could also be productively infected not only by the quasispecies of the challenge virus (SHIV(KU-1/PDJ) and SHIV(KU-1/PNA), isolated from macaque PDj and PNa, respectively) but also by a different chimeric simian/human immunodeficiency virus (SHIV(89.6P)) and simian immunodeficiency virus (SIV(MAC239)). The virus-infected CD4(+) T cell clones were also used as stimulators for generation of CTL effectors. These effectors exhibited excellent virus-specific lysis in chromium-release assays when syngenic SHIV(KU-1) infected autologous CD4(+) T cell clones were used as targets. The target cell lysis was virus specific, as uninfected control cells showed no or minimal lysis.

Evaluation of antibody parameters as potential correlates of protection or enhancement by experimental vaccines to equine infectious anemia virus

We previously demonstrated in trials of a variety of experimental vaccines to equine infectious anemia virus (EIAV) a remarkable spectrum of efficacy ranging from sterilizing protection to severe enhancement of virus replication and disease, depending on the immunization strategy used. This range of vaccine efficacy observed in vivo offers a unique opportunity for evaluating potential in vitro immune correlates of protection and enhancement. We describe here a comprehensive analysis and comparison of EIAV envelope-specific antibody responses elicited by attenuated, inactivated whole virus and envelope subunit vaccines to EIAV, and we evaluate the potential of in vitro antibody assays as correlates of protection or enhancement. Thus vaccine-induced serum antibody responses in experimentally immunized ponies at the day of challenge were assayed using a panel of quantitative, qualitative, and functional in vitro assays, including end-point titer of total and isotypic IgG, serum antibody avidity, conformational dependence, and serum neutralization. The results of these studies revealed substantial differences in the EIAV envelope-specific antibody responses elicited by the different vaccines, indicating the importance of envelope glycoprotein antigen presentation in determining the specificity of vaccine immunity. Although no single in vitro parameter provided a statistically significant correlate of protection or enhancement, the use of multiple parameters (titer, avidity index, and conformation ratio) could be used as a reliable correlate of vaccine protection and that the level of vaccine protection was closely associated with the development of mature antibody responses. These studies demonstrate the importance of using multiple antibody assays to evaluate lentiviral vaccine responses and emphasize the need for the development of new in vitro antibody assays that may provide more insight into vaccine protection and enhancement.

Factors associated with slow disease progression in macaques immunized with an adenovirus-simian immunodeficiency virus (SIV) envelope priming-gp120 boosting regimen and challenged vaginally with SIVmac251

Rhesus macaques were immunized with a combination vaccine regimen consisting of adenovirus type 5 host range mutant-simian immunodeficiency virus envelope (Ad5hr-SIVenv) recombinant priming and boosting with native SIV gp120. Upon intravaginal challenge with SIVmac251, both persistently and transiently viremic animals were observed (S. L. Buge, E. Richardson, S. Alipanah, P. Markham, S. Cheng, N. Kalyan, C. J. Miller, M. Lubeck, S. Udem, J. Eldridge, and M. Robert-Guroff, J. Virol. 71:8531-8541, 1997). Long-term follow-up of the persistently viremic immunized macaques, which displayed significantly reduced viral burdens during the first 18 weeks postchallenge compared to controls, has now shown that one of four became a slow progressor, clearing virus from plasma and remaining asymptomatic with stable CD4 counts for 134 weeks postchallenge. Reboosting of the transiently viremic macaques did not reactivate latent virus. Rechallenge with two sequential SIVmac251 intravaginal exposures again resulted in partial protection of one of two immunized macaques, manifested by viral clearance and stable CD4 counts. No single immune parameter was associated with partial protection. Development of a strong antibody response capable of neutralizing a primary SIVmac251 isolate together with SIV-specific cytotoxic T lymphocytes were implicated, while CD8(+) T-cell antiviral activity and mucosal immune responses were not associated with delayed disease progression. Our data show that even a third immunization with the same Ad5hr-SIVenv recombinant can elicit significant immune responses to the inserted gene product, suggesting that preexisting Ad antibodies may not preclude effective immunization. Further, the partial protection against a virulent, pathogenic SIV challenge observed in two of six macaques immunized with a vaccine regimen based solely on the viral envelope indicates that this vectored-vaccine approach has promise and that multicomponent vaccines based in the same system merit further investigation.