Heterologous Simian Immunodeficiency Virus Research Articles

A live-attenuated RhCMV/SIV vaccine shows long-term efficacy against heterologous SIV challenge.

Previous studies have established that strain 68-1-derived rhesus cytomegalovirus (RhCMV) vectors expressing simian immunodeficiency virus (SIV) proteins (RhCMV/SIV) are able to elicit and maintain cellular immune responses that provide protection against mucosal challenge of highly pathogenic SIV in rhesus monkeys (RMs). However, these efficacious RhCMV/SIV vectors were replication and spread competent and therefore have the potential to cause disease in immunocompromised subjects. To develop a safer CMV-based vaccine for clinical use, we attenuated 68-1 RhCMV/SIV vectors by deletion of the Rh110 gene encoding the pp71 tegument protein (ΔRh110), allowing for suppression of lytic gene expression. ΔRh110 RhCMV/SIV vectors are highly spread deficient in vivo (~1000-fold compared to the parent vector) yet are still able to superinfect RhCMV+ RMs and generate high-frequency effector-memory-biased T cell responses. Here, we demonstrate that ΔRh110 68-1 RhCMV/SIV-expressing homologous or heterologous SIV antigens are highly efficacious against intravaginal (IVag) SIVmac239 challenge, providing control and progressive clearance of SIV infection in 59% of vaccinated RMs. Moreover, among 12 ΔRh110 RhCMV/SIV-vaccinated RMs that controlled and progressively cleared an initial SIV challenge, 9 were able to stringently control a second SIV challenge ~3 years after last vaccination, demonstrating the durability of this vaccine. Thus, ΔRh110 RhCMV/SIV vectors have a safety and efficacy profile that warrants adaptation and clinical evaluation of corresponding HCMV vectors as a prophylactic HIV/AIDS vaccine.

Antibody Fab‐Fc properties outperform titer in predictive models of SIV vaccine‐induced protection

Characterizing the antigen‐binding and innate immune‐recruiting properties of the humoral response offers the chance to obtain deeper insights into mechanisms of protection than revealed by measuring only overall antibody titer. Here, a high‐throughput, multiplexed Fab‐Fc Array was employed to profile rhesus macaques vaccinated with a gp120‐CD4 fusion protein in combination with different genetically encoded adjuvants, and subsequently subjected to multiple heterologous simian immunodeficiency virus (SIV) challenges. Systems analyses modeling protection and adjuvant differences using Fab‐Fc Array measurements revealed a set of correlates yielding strong and robust predictive performance, while models based on measurements of response magnitude alone exhibited significantly inferior performance. At the same time, rendering Fab‐Fc measurements mathematically independent of titer had relatively little impact on predictive performance. Similar analyses for a distinct SIV vaccine study also showed that Fab‐Fc measurements performed significantly better than titer. These results suggest that predictive modeling with measurements of antibody properties can provide detailed correlates with robust predictive power, suggest directions for vaccine improvement, and potentially enable discovery of mechanistic associations.

Control of Heterologous Simian Immunodeficiency Virus SIVsmE660 Infection by DNA and Protein Coimmunization Regimens Combined with Different Toll-Like-Receptor-4-Based Adjuvants in Macaques.

We developed a method of simultaneous vaccination with DNA and protein resulting in robust and durable cellular and humoral immune responses with efficient dissemination to mucosal sites and protection against simian immunodeficiency virus (SIV) infection. To further optimize the DNA-protein coimmunization regimen, we tested a SIVmac251-based vaccine formulated with either of two Toll-like receptor 4 (TLR4) ligand-based liposomal adjuvant formulations (TLR4 plus TLR7 [TLR4+7] or TLR4 plus QS21 [TLR4+QS21]) in macaques. Although both vaccines induced humoral responses of similar magnitudes, they differed in their functional quality, including broader neutralizing activity and effector functions in the TLR4+7 group. Upon repeated heterologous SIVsmE660 challenge, a trend of delayed viral acquisition was found in vaccinees compared to controls, which reached statistical significance in animals with the TRIM-5α-resistant (TRIM-5α R) allele. Vaccinees were preferentially infected by an SIVsmE660 transmitted/founder virus carrying neutralization-resistant A/K mutations at residues 45 and 47 in Env, demonstrating a strong vaccine-induced sieve effect. In addition, the delay in virus acquisition directly correlated with SIVsmE660-specific neutralizing antibodies. The presence of mucosal V1V2 IgG binding antibodies correlated with a significantly decreased risk of virus acquisition in both TRIM-5α R and TRIM-5α-moderate/sensitive (TRIM-5α M/S) animals, although this vaccine effect was more prominent in animals with the TRIM-5α R allele. These data support the combined contribution of immune responses and genetic background to vaccine efficacy. Humoral responses targeting V2 and SIV-specific T cell responses correlated with viremia control. In conclusion, the combination of DNA and gp120 Env protein vaccine regimens using two different adjuvants induced durable and potent cellular and humoral responses contributing to a lower risk of infection by heterologous SIV challenge.IMPORTANCE An effective AIDS vaccine continues to be of paramount importance for the control of the pandemic, and it has been proven to be an elusive target. Vaccine efficacy trials and macaque challenge studies indicate that protection may be the result of combinations of many parameters. We show that a combination of DNA and protein vaccinations applied at the same time provides rapid and robust cellular and humoral immune responses and evidence for a reduced risk of infection. Vaccine-induced neutralizing antibodies and Env V2-specific antibodies at mucosal sites contribute to the delay of SIVsmE660 acquisition, and genetic makeup (TRIM-5α) affects the effectiveness of the vaccine. These data are important for the design of better vaccines and may also affect other vaccine platforms.

Functional effector memory T cells contribute to protection from superinfection with heterologous simian immunodeficiency virus or simian-human immunodeficiency virus isolates in Chinese rhesus macaques.

Many studies have revealed a protective effect of infection of an individual with an immunodeficiency virus against subsequent infection with a heterologous strain. However, the extent of protection against superinfection conferred by the first infection and the biological consequences of superinfection are not well understood. Here, we report that a rhesus monkey model of mucosal superinfection was established to investigate the protective immune response. Protection against superinfection was shown to correlate with the extent of the polyfunctionality of CD4+ effector memory T cells, whereas neutralizing antibody responses did not protect against superinfection in this model. Notably, immunodeficiency-virus-associated effector memory T-cell responses might significantly contribute to the suppression of virus superinfection. This provides a potential theoretical basis for the development of an HIV/AIDS vaccine.

Vaccination with Live Attenuated Simian Immunodeficiency Virus (SIV) Protects from Mucosal, but Not Necessarily Intravenous, Challenge with a Minimally Heterologous SIV.

Few studies have evaluated the impact of the viral challenge route on protection against a heterologous simian immunodeficiency virus (SIV) challenge. We vaccinated seven macaques with a live attenuated SIV that differed from SIVmac239Δnef by 24 amino acids, called m3KOΔnef. All animals were protected from an intrarectal SIVmac239 challenge, whereas only four animals were protected from subsequent intravenous SIVmac239 challenge. These data suggest that immune responses elicited by vaccination with live attenuated SIV in an individual animal can confer protection from intrarectal challenge while remaining insufficient for protection from intravenous challenge. Our study is important because we show that vaccinated animals can be protected from a mucosal challenge with a heterologous SIV, but the same animals are not necessarily protected from intravenous challenge with the same virus. This is unique because in most studies, either vaccinated animals are challenged multiple times by the same route or only a single challenge is performed. An individually vaccinated animal is rarely challenged multiple times by different routes, so protection from different challenge routes cannot be measured in the same animal. Our data imply that vaccine-elicited responses in an individual animal may be insufficient for protection from intravenous challenge but may be suitable for protection from a mucosal challenge that better approximates human immunodeficiency virus (HIV) exposure.

Breakthrough Virus Neutralization Resistance as a Correlate of Protection in a Nonhuman Primate Heterologous Simian Immunodeficiency Virus Vaccine Challenge Study.

Comprehensive assessments of immune correlates of protection in human immunodeficiency virus (HIV) vaccine trials are essential to vaccine design. Neutralization sieve analysis compares the neutralization sensitivity of the breakthrough transmitted/founder (TF) viruses from vaccinated and control animals to infer the molecular mechanisms of vaccine protection. Here, we report a robust neutralization sieve effect in a nonhuman primate simian immunodeficiency virus (SIV) vaccine trial (DNA prime/recombinant adenovirus type 5 [rAd5] boost) (VRC-10-332) that demonstrated substantial protective efficacy and revealed a genetic signature of neutralization resistance in the C1 region of env. We found significant enrichment for neutralization resistance in the vaccine compared to control breakthrough TF viruses when tested with plasma from vaccinated study animals, plasma from chronically SIV-infected animals, and a panel of SIV-specific monoclonal antibodies targeting six discrete Env epitopes (P < 0.008 for all comparisons). Neutralization resistance was significantly associated with the previously identified genetic signature of resistance (P < 0.0001), and together, the results identify virus neutralization as a correlate of protection. These findings further demonstrate the in vivo relevance of our previous in vitro analyses of the SIVsmE660 challenge stock, which revealed a broad range of neutralization sensitivities of its component viruses. In sum, this report demonstrates proof-of-concept that phenotypic sieve analyses can elucidate mechanistic correlates of immune protection following vaccination and raises a cautionary note for SIV and SHIV (simian-human immunodeficiency virus) vaccine studies that employ challenge strains with envelope glycoproteins that fail to exhibit neutralization resistance profiles typical of TF viruses. With more than 2 million new infections annually, the development of an effective vaccine against HIV-1 is a global health priority. Understanding immunologic correlates of protection generated in vaccine trials is critical to advance vaccine development. Here, we assessed the role of vaccine-elicited neutralizing antibodies in a recent nonhuman primate study of a vaccine that showed significant protection against simian immunodeficiency virus (SIV) challenge and suggested a genetic signature of neutralization sensitivity. We found that breakthrough viruses able to establish infection in vaccinated animals were substantially more resistant to antibody-mediated neutralization than were viruses from controls. These findings suggest that vaccine-elicited neutralizing antibodies selectively blocked the transmission of more sensitive challenge viruses. Sieve analysis also corroborated a genetic signature of neutralization sensitivity and highlighted the impact of challenge swarm diversity. Our findings suggest an important role for neutralization sieve analyses as an informative component of comprehensive immune-correlates analyses.

Characterization and Implementation of a Diverse Simian Immunodeficiency Virus SIVsm Envelope Panel in the Assessment of Neutralizing Antibody Breadth Elicited in Rhesus Macaques by Multimodal Vaccines Expressing the SIVmac239 Envelope.

Antibodies that can neutralize diverse viral strains are likely to be an important component of a protective human immunodeficiency virus type 1 (HIV-1) vaccine. To this end, preclinical simian immunodeficiency virus (SIV)-based nonhuman primate immunization regimens have been designed to evaluate and enhance antibody-mediated protection. However, these trials often rely on a limited selection of SIV strains with extreme neutralization phenotypes to assess vaccine-elicited antibody activity. To mirror the viral panels used to assess HIV-1 antibody breadth, we created and characterized a novel panel of 14 genetically and phenotypically diverse SIVsm envelope (Env) glycoproteins. To assess the utility of this panel, we characterized the neutralizing activity elicited by four SIVmac239 envelope-expressing DNA/modified vaccinia virus Ankara vector- and protein-based vaccination regimens that included the immunomodulatory adjuvants granulocyte-macrophage colony-stimulating factor, Toll-like receptor (TLR) ligands, and CD40 ligand. The SIVsm Env panel exhibited a spectrum of neutralization sensitivity to SIV-infected plasma pools and monoclonal antibodies, allowing categorization into three tiers. Pooled sera from 91 rhesus macaques immunized in the four trials consistently neutralized only the highly sensitive tier 1a SIVsm Envs, regardless of the immunization regimen. The inability of vaccine-mediated antibodies to neutralize the moderately resistant tier 1b and tier 2 SIVsm Envs defined here suggests that those antibodies were directed toward epitopes that are not accessible on most SIVsm Envs. To achieve a broader and more effective neutralization profile in preclinical vaccine studies that is relevant to known features of HIV-1 neutralization, more emphasis should be placed on optimizing the Env immunogen, as the neutralization profile achieved by the addition of adjuvants does not appear to supersede the neutralizing antibody profile determined by the immunogen. Many in the HIV/AIDS vaccine field believe that the ability to elicit broadly neutralizing antibodies capable of blocking genetically diverse HIV-1 variants is a critical component of a protective vaccine. Various SIV-based nonhuman primate vaccine studies have investigated ways to improve antibody-mediated protection against a heterologous SIV challenge, including administering adjuvants that might stimulate a greater neutralization breadth. Using a novel SIV neutralization panel and samples from four rhesus macaque vaccine trials designed for cross comparison, we show that different regimens expressing the same SIV envelope immunogen consistently elicit antibodies that neutralize only the very sensitive tier 1a SIV variants. The results argue that the neutralizing antibody profile elicited by a vaccine is primarily determined by the envelope immunogen and is not substantially broadened by including adjuvants, resulting in the conclusion that the envelope immunogen itself should be the primary consideration in efforts to elicit antibodies with greater neutralization breadth.

CD40L-adjuvanted DNA/modified vaccinia virus Ankara simian immunodeficiency virus SIV239 vaccine enhances SIV-specific humoral and cellular immunity and improves protection against a heterologous SIVE660 mucosal challenge.

It remains a challenge to develop a successful human immunodeficiency virus (HIV) vaccine that is capable of preventing infection. Here, we utilized the benefits of CD40L, a costimulatory molecule that can stimulate both dendritic cells (DCs) and B cells, as an adjuvant for our simian immunodeficiency virus (SIV) DNA vaccine in rhesus macaques. We coexpressed the CD40L with our DNA/SIV vaccine such that the CD40L is anchored on the membrane of SIV virus-like particle (VLP). These CD40L containing SIV VLPs showed enhanced activation of DCs in vitro. We then tested the potential of DNA/SIV-CD40L vaccine to adjuvant the DNA prime of a DNA/modified vaccinia virus Ankara (MVA) vaccine in rhesus macaques. Our results demonstrated that the CD40L adjuvant enhanced the functional quality of anti-Env antibody response and breadth of anti-SIV CD8 and CD4 T cell responses, significantly delayed the acquisition of heterologous mucosal SIV infection, and improved viral control. Notably, the CD40L adjuvant enhanced the control of viral replication in the gut at the site of challenge that was associated with lower mucosal CD8 immune activation, one of the strong predictors of disease progression. Collectively, our results highlight the benefits of CD40L adjuvant for enhancing antiviral humoral and cellular immunity, leading to enhanced protection against a pathogenic SIV. A single adjuvant that enhances both humoral and cellular immunity is rare and thus underlines the importance and practicality of CD40L as an adjuvant for vaccines against infectious diseases, including HIV-1. Despite many advances in the field of AIDS research, an effective AIDS vaccine that can prevent infection remains elusive. CD40L is a key stimulator of dendritic cells and B cells and can therefore enhance T cell and antibody responses, but its overly potent nature can lead to adverse effects unless used in small doses. In order to modulate local expression of CD40L at relatively lower levels, we expressed CD40L in a membrane-bound form, along with SIV antigens, in a nucleic acid (DNA) vector. We tested the immunogenicity and efficacy of the CD40L-adjuvanted vaccine in macaques using a heterologous mucosal SIV infection. The CD40L-adjuvanted vaccine enhanced the functional quality of anti-Env antibody response and breadth of anti-SIV T cell responses and improved protection. These results demonstrate that VLP-membrane-bound CD40L serves as a novel adjuvant for an HIV vaccine.

DNA and virus particle vaccination protects against acquisition and confers control of viremia upon heterologous simian immunodeficiency virus challenge.

We have previously shown that macaques vaccinated with DNA vectors expressing SIVmac239 antigens developed potent immune responses able to reduce viremia upon high-dose SIVmac251 challenge. To further improve vaccine-induced immunity and protection, we combined the SIVmac239 DNA vaccine with protein immunization using inactivated SIVmac239 viral particles as protein source. Twenty-six weeks after the last vaccination, the animals were challenged intrarectally at weekly intervals with a titrated dose of the heterologous SIVsmE660. Two of DNA-protein coimmunized macaques did not become infected after 14 challenges, but all controls were infected by 11 challenges. Vaccinated macaques showed modest protection from SIVsmE660 acquisition compared with naïve controls (P = 0.050; stratified for TRIM5α genotype). Vaccinees had significantly lower peak (1.6 log, P = 0.0048) and chronic phase viremia (P = 0.044), with 73% of the vaccinees suppressing viral replication to levels below assay detection during the 40-wk follow-up. Vaccine-induced immune responses associated significantly with virus control: binding antibody titers and the presence of rectal IgG to SIVsmE660 Env correlated with delayed SIVsmE660 acquisition; SIV-specific cytotoxic T cells, prechallenge CD4(+) effector memory, and postchallenge CD8(+) transitional memory cells correlated with control of viremia. Thus, SIVmac239 DNA and protein-based vaccine protocols were able to achieve high, persistent, broad, and effective cellular and humoral immune responses able to delay heterologous SIVsmE660 infection and to provide long-term control of viremia. These studies support a role of DNA and protein-based vaccines for development of an efficacious HIV/AIDS vaccine.

SIVmac239 MVA vaccine with and without a DNA prime, similar prevention of infection by a repeated dose SIVsmE660 challenge despite different immune responses.

Vaccine regimens using different agents for priming and boosting have become popular for enhancing T cell and Ab responses elicited by candidate HIV/AIDS vaccines. Here we use a simian model to evaluate immunogenicity and protective efficacy of a recombinant modified vaccinia Ankara (MVA) vaccine in the presence and absence of a recombinant DNA prime. The simian vaccines and regimens represent prototypes for candidate HIV vaccines currently undergoing clinical testing. Recombinant DNA and MVA immunogens expressed simian immunodeficiency virus (SIV)mac239 Gag, PR, RT, and Env sequences. Vaccine schedules tested inoculations of MVA at months 0, 2, and 6 (MMM regimen) or priming with DNA at months 0 and 2 and boosting with MVA at months 4 and 6 (DDMM regimen). Twelve weekly rectal challenges with the heterologous SIV smE660 were initiated at 6 months following the last immunization. Both regimens elicited similar 61-64% reductions in the per challenge risk of SIVsmE660 transmission despite raising different patterns of immune responses. The DDMM regimen elicited higher magnitudes of CD4 T cells whereas the MMM regimen elicited higher titers and greater avidity Env-specific IgG and more frequent and higher titer SIV-specific IgA in rectal secretions. Both regimens elicited similar magnitudes of CD8 T cells. Magnitudes of T cell responses, specific activities of rectal IgA Ab, and the tested specificities for neutralization and antibody-dependent cellular cytotoxicity did not correlate with risk of infection. However, the avidity of Env-specific IgG had a strong correlation with the per challenge risk of acquisition, but only for the DDMM group. We conclude that for the tested immunogens in rhesus macaques, the simpler MMM regimen is as protective as the more complex DDMM regimen.

Macaques Vaccinated with Simian Immunodeficiency Virus SIVmac239Δnef Delay Acquisition and Control Replication after Repeated Low-Dose Heterologous SIV Challenge

An effective human immunodeficiency virus (HIV) vaccine will likely need to reduce mucosal transmission and, if infection occurs, control virus replication. To determine whether our best simian immunodeficiency virus (SIV) vaccine can achieve these lofty goals, we vaccinated eight Indian rhesus macaques with SIVmac239Delta nef and challenged them intrarectally (i.r.) with repeated low doses of the pathogenic heterologous swarm isolate SIVsmE660. We detected a significant reduction in acquisition of SIVsmE660 in comparison to that for naïve controls (log rank test; P = 0.023). After 10 mucosal challenges, we detected replication of the challenge strain in only five of the eight vaccinated animals. In contrast, seven of the eight control animals became infected with SIVsmE660 after these 10 challenges. Additionally, the SIVsmE660-infected vaccinated animals controlled peak acute virus replication significantly better than did the naïve controls (Mann-Whitney U test; P = 0.038). Four of the five SIVsmE660 vaccinees rapidly brought virus replication under control by week 4 postinfection. Unfortunately, two of these four vaccinated animals lost control of virus replication during the chronic phase of infection. Bulk sequence analysis of the circulating viruses in these animals indicated that recombination had occurred between the vaccine and challenge strains and likely contributed to the increased virus replication in these animals. Overall, our results suggest that a well-designed HIV vaccine might both reduce the rate of acquisition and control viral replication.

T-Cell Correlates of Vaccine Efficacy after a Heterologous Simian Immunodeficiency Virus Challenge

Determining the "correlates of protection" is one of the challenges in human immunodeficiency virus vaccine design. To date, T-cell-based AIDS vaccines have been evaluated with validated techniques that measure the number of CD8(+) T cells in the blood that secrete cytokines, mainly gamma interferon (IFN-gamma), in response to synthetic peptides. Despite providing accurate and reproducible measurements of immunogenicity, these methods do not directly assess antiviral function and thus may not identify protective CD8(+) T-cell responses. To better understand the correlates of vaccine efficacy, we analyzed the immune responses elicited by a successful T-cell-based vaccine against a heterologous simian immunodeficiency virus challenge. We searched for correlates of protection using a viral suppression assay (VSA) and an IFN-gamma enzyme-linked immunospot assay. While the VSA measured in vitro suppression, it did not predict the outcome of the vaccine trial. However, we found several aspects of the vaccine-induced T-cell response that were associated with improved outcome after challenge. Of note, broad vaccine-induced prechallenge T-cell responses directed against Gag and Vif correlated with lower viral loads and higher CD4(+) lymphocyte counts. These results may be relevant for the development of T-cell-based AIDS vaccines since they indicate that broad epitope-specific repertoires elicited by vaccination might serve as a correlate of vaccine efficacy. Furthermore, the present study demonstrates that certain viral proteins may be more effective than others as vaccine immunogens.

Partial Protection of Simian Immunodeficiency Virus (SIV)-Infected Rhesus Monkeys against Superinfection with a Heterologous SIV Isolate

Partial Protection of Simian Immunodeficiency Virus (SIV)-Infected Rhesus Monkeys against Superinfection with a Heterologous SIV Isolate

Partial Protection of Simian Immunodeficiency Virus (SIV)-Infected Rhesus Monkeys against Superinfection with a Heterologous SIV Isolate

Although there is increasing evidence that individuals already infected with human immunodeficiency virus type 1 (HIV-1) can be infected with a heterologous strain of the virus, the extent of protection against superinfection conferred by the first infection and the biologic consequences of superinfection are not well understood. We explored these questions in the simian immunodeficiency virus (SIV)/rhesus monkey model of HIV-1/AIDS. We infected cohorts of rhesus monkeys with either SIVmac251 or SIVsmE660 and then exposed animals to the reciprocal virus through intrarectal inoculations. Employing a quantitative real-time PCR assay, we determined the replication kinetics of the two strains of virus for 20 weeks. We found that primary infection with a replication-competent virus did not protect against acquisition of infection by a heterologous virus but did confer relative control of the superinfecting virus. In animals that became superinfected, there was a reduction in peak replication and rapid control of the second virus. The relative susceptibility to superinfection was not correlated with CD4(+) T-cell count, CD4(+) memory T-cell subsets, cytokine production by virus-specific CD8(+) or CD4(+) cells, or neutralizing antibodies at the time of exposure to the second virus. Although there were transient increases in viral loads of the primary virus and a modest decline in CD4(+) T-cell counts after superinfection, there was no evidence of disease acceleration. These findings indicate that an immunodeficiency virus infection confers partial protection against a second immunodeficiency virus infection, but this protection may be mediated by mechanisms other than classical adaptive immune responses.

Viral escape from dominant simian immunodeficiency virus epitope-specific cytotoxic T lymphocytes in DNA-vaccinated rhesus monkeys.

Virus-specific cytotoxic T lymphocytes (CTL) are critical for control of human immunodeficiency virus type 1 replication. However, viral escape from CTL recognition can undermine this immune control. Here we demonstrate the high frequency and pattern of viral escape from dominant epitope-specific CTL in SIV gag DNA-vaccinated rhesus monkeys following a heterologous simian immunodeficiency virus (SIV) challenge. DNA-vaccinated monkeys exhibited initial effective control of the SIV challenge, but this early control was lost by serial breakthroughs of viral replication over a 3-year follow-up period. Increases in plasma viral RNA correlated temporally with declines of dominant SIV epitope-specific CD8(+) T-lymphocyte responses and the emergence of viral mutations that escaped recognition by dominant epitope-specific CTL. Viral escape from CTL occurred in a total of seven of nine vaccinated and control monkeys, including three animals that initially controlled viral replication to undetectable levels of plasma viral RNA. These data suggest that CTL exert selective pressure on viral replication and that viral escape from CTL may be a limitation of CTL-based AIDS vaccine strategies.

Role of CD8(+) lymphocytes in control of simian immunodeficiency virus infection and resistance to rechallenge after transient early antiretroviral treatment.

Transient antiretroviral treatment with tenofovir, (R)-9-(2-phosphonylmethoxypropyl)adenine, begun shortly after inoculation of rhesus macaques with the highly pathogenic simian immunodeficiency virus (SIV) isolate SIVsmE660, facilitated the development of SIV-specific lymphoproliferative responses and sustained effective control of the infection following drug discontinuation. Animals that controlled plasma viremia following transient postinoculation treatment showed substantial resistance to subsequent intravenous rechallenge with homologous (SIVsmE660) and highly heterologous (SIVmac239) SIV isolates, up to more than 1 year later, despite the absence of measurable neutralizing antibody. In some instances, resistance to rechallenge was observed despite the absence of detectable SIV-specific binding antibody and in the face of SIV lymphoproliferative responses that were low or undetectable at the time of challenge. In vivo monoclonal antibody depletion experiments demonstrated a critical role for CD8(+) lymphocytes in the control of viral replication; plasma viremia rose by as much as five log units after depletion of CD8(+) cells and returned to predepletion levels (as low as <100 copy Eq/ml) as circulating CD8(+) cells were restored. The extent of host control of replication of highly pathogenic SIV strains and the level of resistance to heterologous rechallenge achieved following transient postinoculation treatment compared favorably to the results seen after SIVsmE660 and SIVmac239 challenge with many vaccine strategies. This impressive control of viral replication was observed despite comparatively modest measured immune responses, less than those often achieved with vaccination regimens. The results help establish the underlying feasibility of efforts to develop vaccines for the prevention of AIDS, although the exact nature of the protective host responses involved remains to be elucidated.

Post-exposure chemoprophylaxis (PECP) against SIV infection of macaques as a model for protection from HIV infection.

We report that simian immunodeficiency virus (SIV) infection in macaques is a valuable animal model for studying post-exposure chemoprophylaxis (PECP). PECP with the acyclic nucleoside reverse transcriptase inhibitors 9-(2-phosphonylmetho-xyethyl)adenine (PMEA) and (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) at early viral infection can provide long-term protection against subsequent heterologous SIV challenge. Eight macaques previously treated with PECP (called PECP macaques) and four naive controls were challenged intravenously with the most virulent form of SIV, SIV(PBj14). All controls showed signs of SIV(PBj14)-induced acute disease syndrome on days 6 and 7 post-inoculation (PI). One had a fatal viral infection and two surviving controls had persistent infection and decreased CD4+ cell count. Virologic studies of the three surviving controls revealed SIV in multiple lymphoid tissues and peripheral blood mononuclear cells (PBMCs) at necropsy. In contrast, the PECP macaques showed none to mild signs of acute disease syndrome at day 9 PI and exhibited only transient SIV infection in PBMCs between weeks 1 and 8 PI. In virologic studies of five PECP macaques necropsied, two macaques were SIV-negative and the other three were SIV-positive only in either lymph node or bone marrow. Three SIV(PBj14)-challenged PECP macaques, that were randomly reserved for a follow-up study for > 4.0 years PI showed extremely low to undetectable levels of PBMC-associated viremia and normal to increased levels of CD4 + and CD8 + cell counts throughout the study. Our results indicate that early PECP could activate immune responses to protect against subsequent infection with heterologous challenge virus.

Gene gun-based nucleic acid immunization alone or in combination with recombinant vaccinia vectors suppresses virus burden in rhesus macaques challenged with a heterologous SIV.

Gene gun-based DNA immunization alone or in combination with recombinant vaccinia vectors was evaluated for the ability to elicit protective immune responses in rhesus macaques challenged with a pathogenic, heterologous simian immunodeficiency virus (SIV). Six monkeys primed with seven consecutive doses of DNA encoding SIVmac239 gp120 and gp160 (DNA + DNA) were divided into two groups. Three of these animals received another DNA booster immunization and the remaining three received a booster immunization containing a homologous, live recombinant vaccinia virus expressing SIVmac251 gp160 (DNA + VAC). In addition, a group of 15 animals primed with recombinant vaccinia vectors were divided into two groups. One group of six monkeys received another immunization of vaccinia (VAC + VAC) and the other nine animals received a DNA (mac239) booster immunization (VAC + DNA). Geometric mean end-point IgG titres in the DNA + VAC and VAC + DNA groups were substantially higher than the responses seen in the VAC + VAC and DNA + DNA groups, demonstrating a synergistic relationship between DNA-based vaccines and recombinant vaccinia virus-based vaccines. All vaccinates and five naive controls were challenged 19 weeks after the final booster immunization with 10 animal infectious doses of SIVDelta/B670. The vaccines did not prevent infection. However, all vaccine groups showed significant virus load reductions from seven to 56 days post challenge when compared to controls. Although the DNA + DNA group developed the lowest prechallenge antibody responses, the most significant reduction (200-fold) in virus load was associated with this group. In addition, a significant delay in CD4+ T cell loss relative to controls was observed in the DNA + DNA group. These results demonstrate that a gene gun-based DNA vaccine provided some attenuation of infection and CD4+ T cell loss after a heterologous challenge.

Inactivated whole-virus vaccine derived from a proviral DNA clone of simian immunodeficiency virus induces high levels of neutralizing antibodies and confers protection against heterologous challenge.

We tested the ability of macaques vaccinated with inactivated whole simian immunodeficiency virus (SIV) to resist challenge with either homologous or heterologous cell-free uncloned SIV administered by the intravenous route. The vaccine virus was derived from a proviral DNA clone and thus was considered genetically homogeneous. Sixteen macaques received either hepatitis B surface antigen (n = 6) or the inactivated whole-SIV vaccine (n = 10) at weeks 0, 4, and 49 of the study. All SIV vaccine recipients developed high levels of homologous and heterologous neutralizing antibodies in response to vaccination. At the time of challenge (week 53), vaccinees were further stratified to receive either homologous (n = 10) or heterologous (n = 6) uncloned live SIV. The envelope glycoproteins of the homologous and heterologous challenge viruses were 94% and 81% identical to the vaccine virus, respectively. Regardless of challenge inoculum, all vaccinees in the control group (hepatitis B surface antigen) became infected, whereas all SIV vaccinees were protected against detectable infection. These data support the concept that an efficacious vaccine for HIV might be possible, and suggest that genetic variation of HIV might not be an insurmountable obstacle for vaccine development.