Effects of cytotoxic T lymphocytes (CTL) directed against a single simian immunodeficiency virus (SIV) Gag CTL epitope on the course of SIVmac239 infection

Immunogenicity and efficacy of DNA/MVA HIV vaccines in rhesus macaque models

INTRODUCTION

Despite 30 years of research on HIV, a vaccine to prevent infection and limit disease progression remains elusive. The RV144 trial showed moderate, but significant protection in humans and highlighted the contribution of antibody responses directed against HIV envelope as an important immune correlate for protection. Efforts to further build upon the progress include the use of a heterologous prime-boost regimen using DNA as the priming agent and the attenuated vaccinia virus, Modified Vaccinia Ankara (MVA), as a boosting vector for generating protective HIV-specific immunity. Areas covered: In this review, we summarize the immunogenicity of DNA/MVA vaccines in non-human primate models and describe the efficacy seen in SIV infection models. We discuss immunological correlates of protection determined by these studies and potential approaches for improving the protective immunity. Additionally, we describe the current progress of DNA/MVA vaccines in human trials. Expert commentary: Efforts over the past decade have provided the opportunity to better understand the dynamics of vaccine-induced immune responses and immune correlates of protection against HIV. Based on what we have learned, we outline multiple areas where the field will likely focus on in the next five years.

The TB-specific CD4(+) T cell immune repertoire in both cynomolgus and rhesus macaques largely overlap with humans

Non-human primate (NHP) models of tuberculosis (TB) immunity and pathogenesis, especially rhesus and cynomolgus macaques, are particularly attractive because of the high similarity of the human and macaque immune systems. However, little is known about the MHC class II epitopes recognized in macaques, thus hindering the establishment of immune correlates of immunopathology and protective vaccination. We characterized immune responses in rhesus macaques vaccinated against and/or infected with Mycobacterium tuberculosis (Mtb), to a panel of antigens currently in human vaccine trials. We defined 54 new immunodominant CD4(+) T cell epitopes, and noted that antigens immunodominant in humans are also immunodominant in rhesus macaques, including Rv3875 (ESAT-6) and Rv3874 (CFP10). Pedigree and inferred restriction analysis demonstrated that this phenomenon was not due to common ancestry or inbreeding, but rather presentation by common alleles, as well as, promiscuous binding. Experiments using a second cohort of rhesus macaques demonstrated that a pool of epitopes defined in the previous experiments can be used to detect T cell responses in over 75% of individual monkeys. Additionally, 100% of cynomolgus macaques, irrespective of their latent or active TB status, responded to rhesus and human defined epitope pools. Thus, these findings reveal an unexpected general repertoire overlap between MHC class II epitopes recognized in both species of macaques and in humans, showing that epitope pools defined in humans can also be used to characterize macaque responses, despite differences in species and antigen exposure. The results have general implications for the evaluation of new vaccines and diagnostics in NHPs, and immediate applicability in the setting of macaque models of TB.

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.

IMPORTANCE

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.

The impact of viral evolution and frequency of variant epitopes on primary and memory human immunodeficiency virus type 1-specific CD8⁺ T cell responses

It is unclear if HIV-1 variants lose the ability to prime naïve CD8(+) cytotoxic T lymphocytes (CTL) during progressive, untreated infection. We conducted a comprehensive longitudinal analysis of viral evolution and its impact on primary and memory CD8(+) T cell responses pre-seroconversion (SC), post-SC, and during combination antiretroviral therapy (cART). Memory T cell responses targeting autologous virus variants reached a nadir by 8 years post-SC with development of AIDS, followed by a transient enhancement of anti-HIV-1 CTL responses upon initiation of cART. We show broad and high magnitude primary T cell responses to late variants in pre-SC T cells, comparable to primary anti-HIV-1 responses induced in T cells from uninfected persons. Despite evolutionary changes, CD8(+) T cells could still be primed to HIV-1 variants. Hence, vaccination against late, mutated epitopes could be successful in enhancing primary reactivity of T cells for control of the residual reservoir of HIV-1 during cART.

The potential role of epitope-specific T-cell receptor diversity in the control of HIV replication

PURPOSE OF REVIEW

The purpose of this review is to assess the influence of T-cell receptor clonotype diversity on the recognition and control of chronic viral infections, and specifically in the case of HIV infection.

RECENT FINDINGS

The latest publications have examined the role of T-cell receptor repertoires specific for dominant epitopes in the ability to recognize variants and control viremia in chronic viral infections. In the hepatitis C virus and SIV models, diverse T-cell receptor repertoires appear to limit immune escape. In HIV infection, circulating clonotypes may have different functional abilities, showing another potential advantage of diverse clonotypic repertoires. A recent study suggests that at times narrow repertoires against a conserved epitope may be effective, perhaps through the ability to cross-recognize potential epitope variants.

SUMMARY

The studies discussed in this review have identified T-cell receptor diversity as an important factor for understanding the immune recognition of highly variable viruses. Further studies are needed to determine whether T-cell receptor repertoire analysis of HIV epitope-specific immune responses will provide a more accurate correlate for the control of viremia than conventional immune function assays.

CD8(+) T cells in preventing HIV infection and disease

The complex interplay between the host immune response and HIV has been the subject of intense research over the last 25 years. HIV and simian immunodeficiency virus (SIV) CD8 T cells have been of particular interest since they were demonstrated to be temporally associated with reduction in virus load shortly following transmission. Here, we briefly review the phenotypic and functional properties of HIV-specific and SIV-specific CD8 T-cell subsets during HIV infection and consider the influence of viral variation with specific responses that are associated with disease progression or control. The development of an effective HIV/AIDS vaccine combined with existing successful prevention and treatment strategies is essential for preventing new infections. In the context of previous clinical HIV/AIDS vaccine trials, we consider the challenges faced by therapeutic and vaccine strategies designed to elicit effective HIV-specific CD8 T cells.

A new antigen scanning strategy for monitoring HIV-1 specific T-cell immune responses

Delineation of the immune correlates of protection in natural infection or after vaccination is a mandatory step for vaccine development. Although the most recent techniques allow a sensitive and specific detection of the cellular immune response, a consensus on the best strategy to assess their magnitude and breadth is yet to be reached. Within the AIDS Vaccine Integrated Project (AVIP http://www.avip-eu.org) we developed an antigen scanning strategy combining the empirical-based approach of overlapping peptides with a vast array of database information. This new system, termed Variable Overlapping Peptide Scanning Design (VOPSD), was used for preparing two peptide sets encompassing the candidate HIV-1 vaccine antigens Tat and Nef. Validation of the VOPSD strategy was obtained by direct comparison with 15mer or 20mer peptide sets in a trial involving six laboratories of the AVIP consortium. Cross-reactive background responses were measured in 80 HIV seronegative donors (HIV-), while sensitivity and magnitude of Tat and Nef-specific T-cell responses were assessed on 90 HIV+ individuals. In HIV-, VOPSD peptides generated background responses comparable with those of the standard sets. In HIV-1+ individuals the VOPSD pools showed a higher sensitivity in detecting individual responses (Tat VOPSD vs. Tat 15mers or 20mers: p≤0.01) as well as in generating stronger responses (Nef VOPSD vs. Nef 20mers: p<0.001) than standard sets, enhancing both CD4 and CD8 T-cell responses. Moreover, this peptide design allowed a marked reduction of the peptides number, representing a powerful tool for investigating novel HIV-1 candidate vaccine antigens in cohorts of HIV-seronegative and seropositive individuals.

GagCM9-specific CD8+ T cells expressing limited public TCR clonotypes do not suppress SIV replication in vivo

Several lines of evidence suggest that HIV/SIV-specific CD8(+) T cells play a critical role in the control of viral replication. Recently we observed high levels of viremia in Indian rhesus macaques vaccinated with a segment of SIVmac239 Gag (Gag(45-269)) that were subsequently infected with SIVsmE660. These seven Mamu-A*01(+) animals developed CD8(+) T cell responses against an immunodominant epitope in Gag, GagCM9, yet failed to control virus replication. We carried out a series of immunological and virological assays to understand why these Gag-specific CD8(+) T cells could not control virus replication in vivo. GagCM9-specific CD8(+) T cells from all of the animals were multifunctional and were found in the colonic mucosa. Additionally, GagCM9-specific CD8(+) T cells accessed B cell follicles, the primary residence of SIV-infected cells in lymph nodes, with effector to target ratios between 20-250 GagCM9-specific CD8(+) T cells per SIV-producing cell. Interestingly, vaccinated animals had few public TCR clonotypes within the GagCM9-specific CD8(+) T cell population pre- and post-infection. The number of public TCR clonotypes expressed by GagCM9-specific CD8(+) T cells post-infection significantly inversely correlated with chronic phase viral load. It is possible that these seven animals failed to control viral replication because of the narrow TCR repertoire expressed by the GagCM9-specific CD8(+) T cell population elicited by vaccination and infection.

A novel candidate HIV vaccine vector based on the replication deficient Capripoxvirus, Lumpy skin disease virus (LSDV)

BACKGROUND

The Capripoxvirus, Lumpy skin disease virus (LSDV) has a restricted host-range and is being investigated as a novel HIV-1 vaccine vector. LSDV does not complete its replication cycle in non-ruminant hosts.

METHODS

The safety of LSDV was tested at doses of 104 and 106 plaque forming units in two strains of immunocompromised mice, namely RAG mice and CD4 T cell knockout mice. LSDV expressing HIV-1 subtype C Gag, reverse transcriptase (RT), Tat and Nef as a polyprotein (Grttn), (rLSDV-grttn), was constructed. The immunogenicity of rLSDV-grttn was tested in homologous prime-boost regimens as well as heterologous prime-boost regimes in combination with a DNA vaccine (pVRC-grttn) or modified vaccinia Ankara vaccine (rMVA-grttn) both expressing Grttn.

RESULTS

Safety was demonstrated in two strains of immunocompromised mice.In the immunogenicity experiments mice developed high magnitudes of HIV-specific cells producing IFN-gamma and IL-2. A comparison of rLSDV-grttn and rMVA-grttn to boost a DNA vaccine (pVRC-grttn) indicated a DNA prime and rLSDV-grttn boost induced a 2 fold (p < 0.01) lower cumulative frequency of Gag- and RT-specific IFN-γ CD8 and CD4 cells than a boost with rMVA-grttn. However, the HIV-specific cells induced by the DNA vaccine prime rLSDV-grttn boost produced greater than 3 fold (p < 0.01) more IFN- gamma than the HIV-specific cells induced by the DNA vaccine prime rMVA-grttn boost. A boost of HIV-specific CD4 cells producing IL-2 was only achieved with the DNA vaccine prime and rLSDV-grttn boost. Heterologous prime-boost combinations of rLSDV-grttn and rMVA-grttn induced similar cumulative frequencies of IFN- gamma producing Gag- and RT-specific CD8 and CD4 cells. A significant difference (p < 0.01) between the regimens was the higher capacity (2.1 fold) of Gag-and RT-specific CD4 cells to produce IFN-γ with a rMVA-grttn prime - rLSDV-grttn boost. This regimen also induced a 1.5 fold higher (p < 0.05) frequency of Gag- and RT-specific CD4 cells producing IL-2.

CONCLUSIONS

LSDV was demonstrated to be non-pathogenic in immunocompromised mice. The rLSDV-grttn vaccine was immunogenic in mice particularly in prime-boost regimens. The data suggests that this novel vaccine may be useful for enhancing, in particular, HIV-specific CD4 IFN- gamma and IL-2 responses induced by a priming vaccine.

Timing of immune escape linked to success or failure of vaccination

Successful vaccination against HIV should limit viral replication sufficiently to prevent the emergence of viral immune escape mutations. Broadly directed immunity is likely to be required to limit opportunities for immune escape variants to flourish. We studied the emergence of an SIV Gag cytotoxic T cell immune escape variant in pigtail macaques expressing the Mane-A*10 MHC I allele using a quantitative RT-PCR to measure viral loads of escape and wild type variants. Animals receiving whole Gag expressing vaccines completely controlled an SIV_mac251 challenge, had broader CTL responses and exhibited minimal CTL escape. In contrast, animals vaccinated with only a single CTL epitope and challenged with the same SIV_mac251 stock had high levels of viral replication and rapid CTL escape. Unvaccinated naïve animals exhibited a slower emergence of immune escape variants. Thus narrowly directed vaccination against a single epitope resulted in rapid immune escape and viral levels equivalent to that of naïve unvaccinated animals. These results emphasize the importance of inducing broadly directed HIV-specific immunity that effectively quashes early viral replication and limits the generation of immune escape variants. This has important implications for the selection of HIV vaccines for expanded human trials.

Recombinant yellow fever vaccine virus 17D expressing simian immunodeficiency virus SIVmac239 gag induces SIV-specific CD8+ T-cell responses in rhesus macaques

Here we describe a novel vaccine vector for expressing human immunodeficiency virus (HIV) antigens. We show that recombinant attenuated yellow fever vaccine virus 17D expressing simian immunodeficiency virus SIVmac239 Gag sequences can be used as a vector to generate SIV-specific CD8(+) T-cell responses in the rhesus macaque. Priming with recombinant BCG expressing SIV antigens increased the frequency of these SIV-specific CD8(+) T-cell responses after recombinant YF17D boosting. These recombinant YF17D-induced SIV-specific CD8(+) T cells secreted several cytokines, were largely effector memory T cells, and suppressed viral replication in CD4(+) T cells.

Tonsillar application of AT-2 SIV affords partial protection against rectal challenge with SIVmac239

BACKGROUND

Although mucosal responses are important for preventing infections with HIV, the optimal strategies for inducing them remain unclear. To evaluate vaccine strategies targeting the oral mucosal lymphoid tissue inductive sites as an approach to provide immunity at distal sites, we vaccinated healthy macaques via the palatine/lingual tonsils with aldrithiol 2 (AT-2) inactivated Simian immunodeficiency virus (SIV)mac239, combined with CpG-C immunostimulatory oligonucleotide (CpG-C ISS-ODN, C274) as the adjuvant.

METHODS

Macaques received 5 doses of C274 or control ODN C661 and AT-2 SIV on the tonsillar tissues every 6 weeks before being challenged rectally with SIVmac239, 8 weeks after the last immunization.

RESULTS

Although no T-cell or B-cell responses were detected in the blood before challenge, antibody (Ab) responses were detected in the rectum. Immunization with AT-2 SIV significantly reduced the frequency of infection compared with nonimmunized controls, irrespective of adjuvant. In the vaccinated animals that became infected, peak viremias were somewhat reduced. SIV-specific responses were detected in the blood once animals became infected with no detectable differences between the differently immunized groups and the controls.

CONCLUSION

This work provides evidence that vaccine immunogens applied to the oral mucosal associated lymphoid tissues can provide benefit against rectal challenge, a finding with important implications for mucosal vaccination strategies.

Effective simian immunodeficiency virus-specific CD8+ T cells lack an easily detectable, shared characteristic

The immune correlates of human/simian immunodeficiency virus control remain elusive. While CD8(+) T lymphocytes likely play a major role in reducing peak viremia and maintaining viral control in the chronic phase, the relative antiviral efficacy of individual virus-specific effector populations is unknown. Conventional assays measure cytokine secretion of virus-specific CD8(+) T cells after cognate peptide recognition. Cytokine secretion, however, does not always directly translate into antiviral efficacy. Recently developed suppression assays assess the efficiency of virus-specific CD8(+) T cells to control viral replication, but these assays often use cell lines or clones. We therefore designed a novel virus production assay to test the ability of freshly ex vivo-sorted simian immunodeficiency virus (SIV)-specific CD8(+) T cells to suppress viral replication from SIVmac239-infected CD4(+) T cells. Using this assay, we established an antiviral hierarchy when we compared CD8(+) T cells specific for 12 different epitopes. Antiviral efficacy was unrelated to the disease status of each animal, the protein from which the tested epitopes were derived, or the major histocompatibility complex (MHC) class I restriction of the tested epitopes. Additionally, there was no correlation with the ability to suppress viral replication and epitope avidity, epitope affinity, CD8(+) T-cell cytokine multifunctionality, the percentage of central and effector memory cell populations, or the expression of PD-1. The ability of virus-specific CD8(+) T cells to suppress viral replication therefore cannot be determined using conventional assays. Our results suggest that a single definitive correlate of immune control may not exist; rather, a successful CD8(+) T-cell response may be comprised of several factors.

Infection with "escaped" virus variants impairs control of simian immunodeficiency virus SIVmac239 replication in Mamu-B*08-positive macaques

An understanding of the mechanism(s) by which some individuals spontaneously control human immunodeficiency virus (HIV)/simian immunodeficiency virus replication may aid vaccine design. Approximately 50% of Indian rhesus macaques that express the major histocompatibility complex (MHC) class I allele Mamu-B*08 become elite controllers after infection with simian immunodeficiency virus SIVmac239. Mamu-B*08 has a binding motif that is very similar to that of HLA-B27, a human MHC class I allele associated with the elite control of HIV, suggesting that SIVmac239-infected Mamu-B*08-positive (Mamu-B*08+) animals may be a good model for the elite control of HIV. The association with MHC class I alleles implicates CD8+ T cells and/or natural killer cells in the control of viral replication. We therefore introduced point mutations into eight Mamu-B*08-restricted CD8+ T-cell epitopes to investigate the contribution of epitope-specific CD8+ T-cell responses to the development of the control of viral replication. Ten Mamu-B*08+ macaques were infected with this mutant virus, 8X-SIVmac239. We compared immune responses and viral loads of these animals to those of wild-type SIVmac239-infected Mamu-B*08+ macaques. The five most immunodominant Mamu-B*08-restricted CD8+ T-cell responses were barely detectable in 8X-SIVmac239-infected animals. By 48 weeks postinfection, 2 of 10 8X-SIVmac239-infected Mamu-B*08+ animals controlled viral replication to <20,000 viral RNA (vRNA) copy equivalents (eq)/ml plasma, while 10 of 15 wild-type-infected Mamu-B*08+ animals had viral loads of <20,000 vRNA copy eq/ml (P = 0.04). Our results suggest that these epitope-specific CD8+ T-cell responses may play a role in establishing the control of viral replication in Mamu-B*08+ macaques.

Vaccine-induced, simian immunodeficiency virus-specific CD8+ T cells reduce virus replication but do not protect from simian immunodeficiency virus disease progression

Our limited understanding of the interaction between primate lentiviruses and the host immune system complicates the design of an effective HIV/AIDS vaccine. To identify immunological correlates of protection from SIV disease progression, we immunized two groups of five rhesus macaques (RMs) with either modified vaccinia Ankara (MVA) or MVADeltaudg vectors that expressed SIVmac239 Gag and Tat. Both vectors raised a SIV-specific CD8(+) T cell response, with a magnitude that was greater in mucosal tissues than in peripheral blood. After challenge with SIVmac239, all vaccinated RMs showed mucosal and systemic CD8(+) T cell recall responses that appeared faster and were of greater magnitude than those in five unvaccinated control animals. All vaccinated RMs showed a approximately 1-log lower peak and early set-point SIV viral load than the unvaccinated animals, and then, by 8 wk postchallenge, exhibited levels of viremia similar to the controls. We observed a significant direct correlation between the magnitude of postchallenge SIV-specific CD8(+) T cell responses and SIV viral load. However, vaccinated RMs showed no protection from either systemic or mucosal CD4(+) T cell depletion and no improved survival. The observation that vaccine-induced, SIV-specific CD8(+) T cells that partially control SIVmac239 virus replication fail to protect from immunological or clinical progression of SIV infection underscores both the complexity of AIDS pathogenesis and the challenges of properly assessing the efficacy of candidate AIDS vaccines.

Nef-mediated MHC class I down-regulation unmasks clonal differences in virus suppression by SIV-specific CD8(+) T cells independent of IFN-gamma and CD107a responses

CD8(+) T lymphocytes (CTL) play a role in controlling HIV/SIV infection. CTL antiviral activity is dependent on recognition of antigenic peptides associated with MHC class I molecules on infected target cells, and CTL activation can be impaired by Nef-mediated down-regulation of MHC class I molecules. We tested the ability of a series of rhesus macaque CD8(+) T-cell clones specific for the SIV Gag CM9 peptide to suppress SIV infection of autologous CD4(+) T cells. We used a set of SIV(mac)239 viruses with either wild-type Nef or Nef mutations that impair MHC class I down-regulation. All CTL clones efficiently suppressed virus replication in cells infected with mutant viruses with altered Nef function, phenotypically MHC class I(high) or MHC class I(intermediate). However, the ability of the clones to suppress virus replication was variably reduced in the presence of wild-type Nef (MHC class I(low)) despite the observations that all CTL clones showed similar IFN-gamma responses to titrated amounts of cognate peptide as well as to SIV-infected cells. In addition, the CTL clones showed variable CD107a (CTL degranulation marker) responses that did not correlate with their capacity to suppress virus replication. Thus, the clonal differences are not attributable to TCR avidity or typical effector responses, and point to a potential as yet unknown mechanism for CTL-mediated suppression of viral replication. These data emphasize that current assays for evaluating CTL responses in infected or vaccinated individuals do not fully capture the complex requirements for effective CTL-mediated control of virus replication.

Evaluation of recombinant influenza virus-simian immunodeficiency virus vaccines in macaques

There is an urgent need for human immunodeficiency virus (HIV) vaccines that induce robust mucosal immunity. Influenza A viruses (both H1N1 and H3N2) were engineered to express simian immunodeficiency virus (SIV) CD8 T-cell epitopes and evaluated following administration to the respiratory tracts of 11 pigtail macaques. Influenza virus was readily detected from respiratory tract secretions, although the infections were asymptomatic. Animals seroconverted to influenza virus and generated CD8 and CD4 T-cell responses to influenza virus proteins. SIV-specific CD8 T-cell responses bearing the mucosal homing marker beta7 integrin were induced by vaccination of naïve animals. Further, SIV-specific CD8 T-cell responses could be boosted by recombinant influenza virus-SIV vaccination of animals with already-established SIV infection. Sequential vaccination with influenza virus-SIV recombinants of different subtypes (H1N1 followed by H3N2 or vice versa) produced only a limited boost in immunity, probably reflecting T-cell immunity to conserved internal proteins of influenza A virus. SIV challenge of macaques vaccinated with an influenza virus expressing a single SIV CD8 T cell resulted in a large anamnestic recall CD8 T-cell response, but immune escape rapidly ensued and there was no impact on chronic SIV viremia. Although our results suggest that influenza virus-HIV vaccines hold promise for the induction of mucosal immunity to HIV, broader antigen cover will be needed to limit cytotoxic T-lymphocyte escape.

Vaccine-induced cellular responses control simian immunodeficiency virus replication after heterologous challenge

All human immunodeficiency virus (HIV) vaccine efficacy trials to date have ended in failure. Structural features of the Env glycoprotein and its enormous variability have frustrated efforts to induce broadly reactive neutralizing antibodies. To explore the extent to which vaccine-induced cellular immune responses, in the absence of neutralizing antibodies, can control replication of a heterologous, mucosal viral challenge, we vaccinated eight macaques with a DNA/Ad5 regimen expressing all of the proteins of SIVmac239 except Env. Vaccinees mounted high-frequency T-cell responses against 11 to 34 epitopes. We challenged the vaccinees and eight naïve animals with the heterologous biological isolate SIVsmE660, using a regimen intended to mimic typical HIV exposures resulting in infection. Viral loads in the vaccinees were significantly less at both the peak (1.9-log reduction; P < 0.03) and at the set point (2.6-log reduction; P < 0.006) than those in control naïve animals. Five of eight vaccinated macaques controlled acute peak viral replication to less than 80,000 viral RNA (vRNA) copy eq/ml and to less than 100 vRNA copy eq/ml in the chronic phase. Our results demonstrate that broad vaccine-induced cellular immune responses can effectively control replication of a pathogenic, heterologous AIDS virus, suggesting that T-cell-based vaccines may have greater potential than previously appreciated.

Prolonged survival of vaccinated macaques after oral SIVmac239 challenge regardless of viremia control in the chronic phase

To evaluate the efficacy of a multigenic vaccine and its protective immunity in the SIVmac239 challenge model, 12 rhesus macaques were divided into two groups. The vaccine group was intramuscularly immunized with multigenic DNA and recombinant adenovirus vaccine, while the control group received buffers. At 16 weeks after the last immunization, all macaques were challenged orally with pathogenic SIVmac239. The mean plasma SIV RNA loads of the vaccine group were significantly lower than those of the placebo control group up to 16 weeks post-challenge. The vaccine-induced Gag-specific IFN-gamma ELISPOT T cell responses inversely correlated with the viral loads before the chronic phase. Two out of six vaccinated macaques with strong and sustained Gag-specific T cell responses showed viremia control and maintained CD4+ T cell percentage. However, the other four vaccinated macaques showed high viral loads and reduced level of CD4+ T cell percentages during the chronic phase, comparable to those in control macaques. Five out of six vaccinated macaques survived for more than 72 weeks, while five out of six controls died of an AIDS-related disease. Therefore, the vaccination conferred not only reduction of viral loads in a portion of vaccinated macaques (2/6), but also prolonged survival of all vaccinated macaques regardless of viremia control. Our results further suggest that new experimental approaches may be needed to assess protective effects from AIDS-associated disease in the immunized macaques after oral SIV challenge.

The hope for an HIV vaccine based on induction of CD8+ T lymphocytes--a review

The only long-term and cost-effective solution to the human immunodeficiency virus (HIV) epidemic in the developing world is a vaccine that prevents individuals from becoming infected or, once infected, from passing the virus on to others. There is currently little hope for an AIDS vaccine. Conventional attempts to induce protective antibody and CD8(+) lymphocyte responses against HIV and simian immunodeficiency virus (SIV) have failed. The enormous diversity of the virus has only recently been appreciated by vaccinologists, and our assays to determine CD8(+) lymphocyte antiviral efficacy are inadequate. The central hypothesis of a CTL-based vaccine is that particularly effective CD8(+) lymphocytes directed against at least five epitopes that are derived from regions under functional and structural constraints will control replication of pathogenic SIV. This would be somewhat analogous to control of virus replication by triple drug therapy or neutralizing antibodies.

Nonhuman primate models and the failure of the Merck HIV-1 vaccine in humans

The adenovirus type 5 (Ad5)-based vaccine developed by Merck failed to either prevent HIV-1 infection or suppress viral load in subsequently infected subjects in the STEP human Phase 2b efficacy trial. Analogous vaccines had previously also failed in the simian immunodeficiency virus (SIV) challenge-rhesus macaque model. In contrast, vaccine protection studies that used challenge with a chimeric simian-human immunodeficiency virus (SHIV89.6P) in macaques did not predict the human trial results. Ad5 vector-based vaccines did not protect macaques from infection after SHIV89.6P challenge but did cause a substantial reduction in viral load and a preservation of CD4+ T cell counts after infection, findings that were not reproduced in the human trials. Although the SIV challenge model is incompletely validated, we propose that its expanded use can help facilitate the prioritization of candidate HIV-1 vaccines, ensuring that resources are focused on the most promising candidates. Vaccine designers must now develop T cell vaccine strategies that reduce viral load after heterologous challenge.

The Mamu B 17-restricted SIV Nef IW9 to TW9 mutation abrogates correct epitope processing and presentation without loss of replicative fitness

CD8(+) cytotoxic T lymphocytes (CTL) play an important role in controlling virus replication in HIV- and SIV-infected humans and monkeys, respectively. Three well-studied SIV CTL determinants are the two Mamu A()01-restricted epitopes Gag CM9 and Tat SL8, and the Mamu B()17-restricted epitope Nef IW9. Point mutations leading to amino acid replacements in these epitopes have been reported to mediate SIV escape from CTL control. We found that synthetic peptides containing mutations in SIV Gag CM9 and Tat SL8 were no longer recognized by the respective CTL. On the other hand, the described I-to-T replacement at the N-terminal amino acid residue of the SIV Nef IW9 epitope only moderately affected CTL recognition of the variant peptide, TW9. In an attempt to dissect the mechanism of escape of the Nef TW9 mutation, we investigated the effect of this mutation on CTL recognition of CD4(+)T cells infected with an engineered SIV(mac)239 that contained the TW9 mutation in Nef. Although, the wild type and mutant virus both infected and efficiently replicated in rhesus macaque CD4(+)T cells, the TW9 mutant virus failed to induce IFN-gamma expression in an SIV Nef IW9-specific CTL clone. Thus, unlike escape from Gag CM9- or Tat SL8-specfic CTL control presumably by loss of epitope binding, these results point to a defect at the level of processing and/or presentation of the variant TW9 epitope with resultant loss of triggering of the cognate TCR on CTL generated against the wild type peptide. Our data highlight the value of functional assays using virus-infected target cells as opposed to peptide-pulsed APC when assessing relevant escape mutations in CTL epitopes.

Immunological responses and viral modulatory effects of vaccination with recombinant modified vaccinia virus Ankara (rMVA) expressing structural and regulatory transgenes of simian immunodeficiency virus (SIVmac32H/J5M)

BACKGROUND

The immunogenicity and protective efficacy of recombinant modified vaccinia virus Ankara (rMVA) vectors expressing structural (gag/pol, env) and regulatory (tat, rev, nef) genes of SIVmac251/32H-J5 (rMVA-J5) were assessed.

METHODS

Immunization with rMVA constructs (2.5 x 10(7) IU) 32, 20 and 8 weeks pre-challenge was compared with 32 and 20 weeks but with a final boost 8 weeks pre-challenge with 2 x 10(6) fixed-inactivated HSC-F4 cells infected with SIVmac32H. Controls received rMVA vectors expressing an irrelevant transgene or were naïve challenge controls. All received 10 MID(50) SIVmac32H/J5 intravenously.

RESULTS

Vaccinates immunized with rMVA-J5 exhibited significant, albeit transient, control of peak primary viraemia despite inconsistent and variable immune responses elicted by vaccination. Humoral and cellular responses to Env were most consistent, with lower responses to Nef, Rev and Tat. Increasing titres of anti-vaccinia neutralizing antibodies reflected the number and dose of rMVA inoculations.

CONCLUSIONS

Improved combinations of viral vectors are required to elicit appropriate immune responses to control viral replication.

Mamu-B*08-positive macaques control simian immunodeficiency virus replication

Certain major histocompatibility complex (MHC) class I alleles are associated with the control of human immunodeficiency virus and simian immunodeficiency virus (SIV) replication. We have designed sequence-specific primers for detection of the rhesus macaque MHC class I allele Mamu-B*08 by PCR and screened a cohort of SIV-infected macaques for this allele. Analysis of 196 SIV(mac)239-infected Indian rhesus macaques revealed that Mamu-B*08 was significantly overrepresented in elite controllers; 38% of elite controllers were Mamu-B*08 positive compared to 3% of progressors (P = 0.00001). Mamu-B*08 was also associated with a 7.34-fold decrease in chronic phase viremia (P = 0.002). Mamu-B*08-positive macaques may, therefore, provide a good model to understand the correlates of MHC class I allele-associated immune protection and viral containment in human elite controllers.

Clinical experience with plasmid DNA- and modified vaccinia virus Ankara-vectored human immunodeficiency virus type 1 clade A vaccine focusing on T-cell induction

Candidate human immunodeficiency virus type 1 (HIV-1) vaccines focusing on T-cell induction, constructed as pTHr.HIVA DNA and modified vaccinia virus Ankara (MVA).HIVA, were delivered in a heterologous prime-boost regimen. The vaccines were tested in several hundred healthy or HIV-1-infected volunteers in Europe and Africa. Whilst larger trials of hundreds of volunteers suggested induction of HIV-1-specific T-cell responses in <15 % of healthy vaccinees, a series of small, rapid trials in 12-24 volunteers at a time with a more in-depth analysis of vaccine-elicited T-cell responses proved to be highly informative and provided more encouraging results. These trials demonstrated that the pTHr.HIVA vaccine alone primed consistently weak and mainly CD4(+), but also CD8(+) T-cell responses, and the MVA.HIVA vaccine delivered a consistent boost to both CD4(+) and CD8(+) T cells, which was particularly strong in HIV-1-infected patients. Thus, whilst the search is on for ways to enhance T-cell priming, MVA is a useful boosting vector for human subunit genetic vaccines.

Preclinical and clinical progress of particle-mediated DNA vaccines for infectious diseases

This review provides an overview of studies employing particle-mediated epidermal delivery (PMED) or the gene gun to administer DNA vaccines for infectious diseases in preclinical studies employing large animal models and in human clinical trials. It reviews the immunogenicity and protective efficacy of PMED DNA vaccines in nonhuman primates and swine and studies that have directly compared the effectiveness of PMED in these large animal models to existing licensed vaccines and intramuscular or intradermal delivery of DNA vaccines with a needle. Various clinical trials employing PMED have been completed and an overview of the immunogenicity, safety, and tolerability of this approach in humans is described. Finally, efforts currently in progress for commercial development of particle-mediated DNA vaccines are discussed.

Evidence for CTL-mediated selection of Tat and Rev mutants after the onset of the asymptomatic period during HIV type 1 infection

The evolution of HIV-1 sequences over time is the result of the selection of mutant variants that have escaped from host immune responses or the outgrowth of mutants with increased viral replication, or both. We investigated the contribution of both selection processes to the overall evolution of the Tat and Rev regulatory gene sequences from four individuals, ranging in time from just prior to seroconversion to stable asymptomatic infection. After sequencing at least 15 clones per sample per gene, we analyzed the sequence evolution of the MHC-I motifs that were predicted from the MHC-I haplotypes of these patients. For each identified Tat sequence, we tested the activity of the corresponding encoded protein in a transactivation assay in vitro. Our results suggest that the evolution of the Tat and Rev sequences from these individuals can be explained by mutational escape of the MHC-I epitopes and that no mutations that replaced the original sequences in the viral population are associated with either an increase or decrease in Tat activity. CTL-mediated selection appears to be an important determinant of HIV-1 regulatory gene sequence evolution during the early stages of infection.

Not all cytokine-producing CD8+ T cells suppress simian immunodeficiency virus replication

Current assays of CD8+ T-lymphocyte function measure cytokine production rather than the ability of these lymphocytes to suppress viral replication. Here we show that CD8+ T-cell clones recognizing the same epitope vary enormously in the ability to suppress simian immunodeficiency virus SIVmac239 replication in an in vitro suppression assay. However, all Nef(165-173)IW9- and Vif(66-73)HW8-specific clones from elite controllers effectively suppressed SIV replication. Interestingly, in vitro suppression efficacy was not always associated with the ability to produce gamma interferon, tumor necrosis factor alpha, or interleukin-2.

SIV escape mutants in rhesus macaques vaccinated with NEF-derived lipopeptides and challenged with pathogenic SIVmac251

BACKGROUND

Emergence of viral variants that escape CTL control is a major hurdle in HIV vaccination unless such variants affect gene regions that are essential for virus replication. Vaccine-induced multispecific CTL could also be able to control viral variants replication. To explore these possibilities, we extensively characterized CTL responses following vaccination with an epitope-based lipopeptide vaccine and challenge with pathogenic SIVmac251. The viral sequences corresponding to the epitopes present in the vaccine as well as the viral loads were then determined in every macaque following SIV inoculation.

RESULTS

In most cases, the emergence of several viral variants or mutants within vaccine CTL epitopes after SIV challenge resulted in increased viral loads except for a single macaque, which showed a single escape viral variant within its 6 vaccine-induced CTL epitopes.

CONCLUSION

These findings provide a better understanding of the evolution of CD8+ epitope variations after vaccination-induced CTL expansion and might provide new insight for the development of an effective HIV vaccine.

Overlapping synthetic peptides as vaccines

Several vaccine strategies aim to generate cell-mediated immunity (CMI) against microorganisms or tumors. While epitope-based vaccines offer advantages, knowledge of specific epitopes and frequency of major histocompatibility complex (MHC) alleles is required. Here we show that using promiscuous overlapping synthetic peptides (OSP) as immunogens generated peptide-specific CMI in all vaccinated outbred mice and in different strains of inbred mice; CMI responses also recognized viral proteins. OSP immunogens also induced CMI ex vivo in dendritic cell/T-cell cocultures involving cells from individuals with different HLA haplotypes. Thus, broad CMI was induced by OSP in different experimental settings, using different immunogens, without identifying either epitopes or MHC backgrounds of the vaccinees.

Comparative studies on mucosal and intravenous transmission of simian immunodeficiency virus (SIVsm): the kinetics of evolution to neutralization resistance are related to progression rate of disease

The kinetics of appearance of autologous neutralizing antibodies were studied in cynomolgus macaques infected with simian immunodeficiency virus (SIVsm) by the intravenous (IV) route (six monkeys) or the intrarectal (IR) route (ten monkeys). The SIVsm inoculum virus and reisolates obtained at 2 weeks, 3 or 4 months and later than 1 year were tested in a GHOST(3) cell line-based plaque-reduction assay with autologous sera collected at the same sampling times. All monkeys developed a neutralizing-antibody response to the inoculum virus, those infected by the IV route earlier than monkeys infected by the IR route. Animals were divided into progressor (P), slow-progressor (SP) and long-term non-progressor (LTNP) monkeys, based on progression rate. In P monkeys, neutralization escape could be demonstrated by 3 months post-infection. Neutralization-resistant variants also emerged in SP and LTNP monkeys, but were much delayed compared with P monkeys. Evolution of neutralization resistance was also demonstrated by a positive-control serum in the heterologous reaction. Pooled sera from four LTNP monkeys showed a broad neutralizing capacity, including neutralization of escape variants. These results from a large group of infected monkeys showed that SIV evolves to neutralization resistance in the infected host and that the kinetics of this evolution are related to the route of transmission and the progression rate of SIV disease. The results suggest an important role for neutralizing antibodies in controlling viraemia. Although this control is transient in the infected host, neutralization resistance is relative and variant viruses may be neutralized by a broadly cross-neutralizing serum pool.

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

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

Induction of Positive Cellular and Humoral Immune Responses by a Prime-Boost Vaccine Encoded with Simian Immunodeficiency Virusgag/pol

It is believed likely that immune responses are responsible for controlling viral load and infection. In this study, when macaques were primed with plasmid DNA encoding SIV gag and pol genes (SIVgag/pol DNA) and then boosted with replication-deficient vaccinia virus DIs recombinant expressing the same genes (rDIsSIVgag/pol), this prime-boost regimen generated higher levels of Gag-specific CD4+ and CD8+ T cell responses than did either SIVgag/pol DNA or rDIsSIVgag/pol alone. When the macaques were i.v. challenged with pathogenic simian/HIV, the prime-boost group maintained high CD4+ T cell counts and reduced plasma viral loads up to 30 wk after viral challenge, whereas the rDIsSIVgag/pol group showed only a partial attenuation of the viral infection, and the group immunized with SIVgag/pol DNA alone showed none at all. The protection levels were better correlated with the levels of virus-specific T cell responses than the levels of neutralization Ab responses. These results demonstrate that a vaccine regimen that primes with DNA and then boosts with a replication-defective vaccinia virus DIs generates anti-SIV immunity, suggesting that it will be a promising vaccine regimen for HIV-1 vaccine development.

A peptide-loaded dendritic cell based cytotoxic T-lymphocyte (CTL) vaccination strategy using peptides that span SIV Tat, Rev, and Env overlapping reading frames

CTL based vaccine strategies in the macaque model of AIDS have shown promise in slowing the progression to disease. However, rapid CTL escape viruses can emerge rendering such vaccination useless. We hypothesized that such escape is made more difficult if the immunizing CTL epitope falls within a region of the virus that has a high density of overlapping reading frames which encode several viral proteins. To test this hypothesis, we immunized macaques using a peptide-loaded dendritic cell approach employing epitopes in the second coding exon of SIV Tat which spans reading frames for both Env and Rev. We report here that autologous dendritic cells, loaded with SIV peptides from Tat, Rev, and Env, induced a distinct cellular immune response measurable ex vivo. However, conclusive in vivo control of a challenge inoculation of SIVmac239 was not observed suggesting that CTL epitopes within densely overlapping reading frames are also subject to escape mutations.

A computational resource for the prediction of peptide binding to Indian rhesus macaque MHC class I molecules

Non-human primates, in general, and Indian rhesus macaques, specifically, play an important role in the development and testing of vaccines and diagnostics destined for human use. To date, several frequently expressed macaque MHC molecules have been identified and their binding specificities characterized in detail. Here, we report the development of computational algorithms to predict peptide binding and potential T cell epitopes for the common MHC class I alleles Mamu-A*01, -A*02, -A*11, -B*01 and -B*17, which cover approximately two thirds of the captive Indian rhesus macaque populations. We validated this method utilizing an SIV derived data set encompassing 59 antigenic peptides. Of all peptides contained in the SIV proteome, the 2.4% scoring highest in the prediction contained 80% of the antigenic peptides. The method was implemented in a freely accessible and user friendly website at . Thus, we anticipate that our approach can be utilized to rapidly and efficiently identify CD8+ T cell epitopes recognized by rhesus macaques and derived from any pathogen of interest.

Evaluation of SIV-lipopeptide immunizations administered by the intradermal route in their ability to induce antigen specific T-cell responses in rhesus macaques

Numerous clinical and experimental observations have shown that cellular immunity, in particular CD8+ T-lymphocytes, plays an important role in the control of HIV infection. We have focused on a lipopeptide vaccination strategy that has been shown to induce polyepitopic T-cell responses in both animals and humans, in order to deliver simian immunodeficiency virus (SIV) antigens to rhesus macaques. Given the relevance of antigen administration route in the development of an effective cellular immune response, this study was designed to assess SIV lipopeptide immunizations administered either by the intradermal (ID) or the intramuscular (IM) routes in their ability to elicit GAG and NEF multispecific T-lymphocytes in the rhesus macaque. Antigen specific T-cell responses were observed between 7 and 11 weeks following vaccination in both groups. Macaques immunized by the IM route yielded antigen-specific IFN-gamma secreting lymphocytes in response to no more than two pools of peptides derived from SIV-NEF. In contrast, among the four ID-immunized macaques, two presented multi-specific T-cell responses to as many as four pools of SIV-NEF and/or GAG peptides. Responses persisted 16 weeks following the vaccination protocol in one of the ID-vaccinated macaques. The induction of such responses is of great clinical relevance in the development of an effective HIV vaccine. Given the crucial role of CD8+ T-lymphocytes in HIV/SIV containment, vaccination through the intradermal route should merit high consideration in the development of an AIDS vaccine.

Correlation between viral RNA levels but not immune responses in plasma and tissues of macaques with long-standing SIVmac251 infection

Plasma virus in human immunodeficiency virus type 1/simian immunodeficiency virus (HIV-1/SIV) infection most likely results from the combination of viruses produced in different tissues. As immunological pressure may be higher in effector sites than secondary lymphoid tissues, we investigated quantitative and qualitative changes in viral RNA in blood and tissues of 10 Mamu-A*01-positive SIV-infected macaques in parallel with the frequency of CD8+ T cells recognizing the dominant Gag181-189 CM9 epitope. The plasma virus level in these macaques directly correlated with the viral RNA levels in lymph nodes, spleen, lungs, colon, and jejunum. In contrast, the frequency of the Gag181-189 CM9 tetramer did not correlate with SIV RNA levels in any compartment. We investigated the presence of viral immune escape in RNA from several tissues. The complete substitution of wild-type genotype with viral immune-escape variant within the Gag181-189 CM9 epitope was associated with low tetramer response in all tissues and blood of two macaques. In one macaque, the replacement of wild type with an immune-escape mutant was asynchronous. While the mutant virus was prevalent in blood and effector tissues (lungs, jejunum, and colon), secondary lymphoid organs such as spleen and lymph nodes still retained 80% and 40%, respectively, of the wild-type virus. These results may imply that there are differences in the immunological pressure exerted by cytotoxic T lymphocytes (CTLs) in tissue compartments of SIVmac251-infected macaques.

Characterization of the peptide-binding specificity of Mamu-A*11 results in the identification of SIV-derived epitopes and interspecies cross-reactivity

The SIV-infected Indian rhesus macaque is the most established model of HIV infection, providing insight into pathogenesis and a system for testing novel vaccines. However, only a limited amount of information is available regarding the peptide-binding motifs and epitopes bound by their class I and class II MHC molecules. In this study, we utilized a library of over 1,000 different peptides and a high throughput MHC-peptide binding assay to detail the binding specificity of the rhesus macaque class I molecule Mamu-A*11. These studies defined the fine specificity of primary anchor positions, and dissected the role of secondary anchors, for peptides of 8-11 residues in length. This detailed information was utilized to develop size-specific polynomial algorithms to predict Mamu-A*11 binding capacity. Testing SIVmac239-derived Mamu-A*11 binding peptides for recognition by peripheral blood mononuclear cells (PBMC) from Mamu-A*11-positive, SIV-infected macaques, identified five novel SIV-derived Mamu-A*11 epitopes. Finally, we detected extensive cross-reactivity at the binding level between Mamu-A*11 and the mouse H-2 class I molecule Kk. Further experiments revealed that three out of four Mamu-A*11 binding peptides which bound Kk and were immunogenic in Kk mice were also recognized in Mamu-A*11-infected macaques. This is the first detailed description of mouse-macaque interspecies cross-reactivity, potentially useful in testing novel vaccines in mice and macaques.

Analysis of pigtail macaque major histocompatibility complex class I molecules presenting immunodominant simian immunodeficiency virus epitopes

Successful human immunodeficiency virus (HIV) vaccines will need to induce effective T-cell immunity. We studied immunodominant simian immunodeficiency virus (SIV) Gag-specific T-cell responses and their restricting major histocompatibility complex (MHC) class I alleles in pigtail macaques (Macaca nemestrina), an increasingly common primate model for the study of HIV infection of humans. CD8+ T-cell responses to an SIV epitope, Gag164-172KP9, were present in at least 15 of 36 outbred pigtail macaques. The immunodominant KP9-specific response accounted for the majority (mean, 63%) of the SIV Gag response. Sequencing from six macaques identified 7 new Mane-A and 13 new Mane-B MHC class I alleles. One new allele, Mane-A*10, was common to four macaques that responded to the KP9 epitope. We adapted reference strand-mediated conformational analysis (RSCA) to MHC class I genotype M. nemestrina. Mane-A*10 was detected in macaques presenting KP9 studied by RSCA but was absent from non-KP9-presenting macaques. Expressed on class I-deficient cells, Mane-A*10, but not other pigtail macaque MHC class I molecules, efficiently presented KP9 to responder T cells, confirming that Mane-A*10 restricts the KP9 epitope. Importantly, naive pigtail macaques infected with SIVmac251 that respond to KP9 had significantly reduced plasma SIV viral levels (log10 0.87 copies/ml; P=0.025) compared to those of macaques not responding to KP9. The identification of this common M. nemestrina MHC class I allele restricting a functionally important immunodominant SIV Gag epitope establishes a basis for studying CD8+ T-cell responses against AIDS in an important, widely available nonhuman primate species.

A dominant role for CD8+-T-lymphocyte selection in simian immunodeficiency virus sequence variation

CD8(+) T lymphocytes (CD8-TL) select viral escape variants in both human immunodeficiency virus and simian immunodeficiency virus (SIV) infections. The frequency of CD8-TL viral escape as well as the contribution of escape to overall virus diversification has not been assessed. We quantified CD8-TL selection in SIV infections by sequencing viral genomes from 35 SIVmac239-infected animals at the time of euthanasia. Here we show that positive selection for sequences encoding 46 known CD8-TL epitopes is comparable to the positive selection observed for the variable loops of env. We also found that >60% of viral variation outside of the viral envelope occurs within recognized CD8-TL epitopes. Therefore, we conclude that CD8-TL selection is the dominant cause of SIV diversification outside of the envelope.

Novel and promiscuous CTL epitopes in conserved regions of Gag targeted by individuals with early subtype C HIV type 1 infection from southern Africa

Characterization of optimal CTL epitopes in Gag can provide crucial information for evaluation of candidate vaccines in populations at the epicenter of the HIV-1 epidemic. We screened 38 individuals with recent subtype C HIV-1 infection using overlapping consensus C Gag peptides and hypothesized that unique HLA-restricting alleles in the southern African population would determine novel epitope identity. Seventy-four percent of individuals recognized at least one Gag peptide pool. Ten epitopic regions were identified across p17, p24, and p2p7p1p6, and greater than two-thirds of targeted regions were directed at: TGTEELRSLYNTVATLY (p17, 35%); GPKEPFRDYVDRFFKTLRAEQATQDV (p24, 19%); and RGGKLDKWEKIRLRPGGKKHYMLKHL (p17, 15%). After alignment of these epitopic regions with consensus M and a consensus subtype C sequence from the cohort, it was evident that the regions targeted were highly conserved. Fine epitope mapping revealed that five of nine identified optimal Gag epitopes were novel: HLVWASREL, LVWASRELERF, LYNTVATLY, PFRDYVDRFF, and TLRAEQATQD, and were restricted by unique HLA-Cw*08, HLA-A*30/B*57, HLA-A*29/B*44, and HLA-Cw*03 alleles, respectively. Notably, three of the mapped epitopes were restricted by more than one HLA allele. Although these epitopes were novel and restricted by unique HLA, they overlapped or were embedded within previously described CTL epitopes from subtype B HIV-1 infection. These data emphasize the promiscuous nature of epitope binding and support our hypothesis that HLA diversity between populations can shape fine epitope identity, but may not represent a constraint for universal recognition of Gag in highly conserved domains.

Cytotoxic T lymphocyte-based control of simian immunodeficiency virus replication in a preclinical AIDS vaccine trial

Recently, encouraging AIDS vaccine trials in macaques have implicated cytotoxic T lymphocytes (CTLs) in the control of the simian human immunodeficiency virus SHIV89.6P that induces acute CD4⁺ T cell depletion. However, none of these vaccine regimens have been successful in the containment of replication of the pathogenic simian immunodeficiency viruses (SIVs) that induce chronic disease progression. Indeed, it has remained unclear if vaccine-induced CTL can control SIV replication. Here, we show evidence suggesting that vaccine-induced CTLs control SIVmac239 replication in rhesus macaques. Eight macaques vaccinated with DNA-prime/Gag-expressing Sendai virus vector boost were challenged intravenously with SIVmac239. Five of the vaccinees controlled viral replication and had undetectable plasma viremia after 5 wk of infection. CTLs from all of these five macaques rapidly selected for escape mutations in Gag, indicating that vaccine-induced CTLs successfully contained replication of the challenge virus. Interestingly, analysis of the escape variant selected in three vaccinees that share a major histocompatibility complex class I haplotype revealed that the escape variant virus was at a replicative disadvantage compared with SIVmac239. These findings suggested that the vaccine-induced CTLs had “crippled” the challenge virus. Our results indicate that vaccine induction of highly effective CTLs can result in the containment of replication of a highly pathogenic immunodeficiency virus.

Engineering RENTA, a DNA prime-MVA boost HIV vaccine tailored for Eastern and Central Africa

For the development of human immunodeficiency virus type 1 (HIV-1) vaccines, traditional approaches inducing virus-neutralizing antibodies have so far failed. Thus the effort is now focused on elicitation of cellular immunity. We are currently testing in clinical trials in the United Kingdom and East Africa a T-cell vaccine consisting of HIV-1 clade A Gag-derived immunogen HIVA delivered in a prime-boost regimen by a DNA plasmid and modified vaccinia virus Ankara (MVA). Here, we describe engineering and preclinical development of a second immunogen RENTA, which will be used in combination with the present vaccine in a four-component DNA/HIVA-RENTA prime-MVA/HIVA-RENTA boost formulation. RENTA is a fusion protein derived from consensus HIV clade A sequences of Tat, reverse transcriptase, Nef and gp41. We inactivated the natural biological activities of the HIV components and confirmed immunogenicities of the pTHr.RENTA and MVA.RENTA vaccines in mice. Furthermore, we demonstrated in mice and rhesus monkeys broadening of HIVA-elicited T-cell responses by a parallel induction of HIVA- and RENTA-specific responses recognizing multiple HIV epitopes.

Repeated low-dose mucosal simian immunodeficiency virus SIVmac239 challenge results in the same viral and immunological kinetics as high-dose challenge: a model for the evaluation of vaccine efficacy in nonhuman primates

Simian immunodeficiency virus (SIV) challenge of rhesus macaques provides a relevant model for the assessment of human immunodeficiency virus (HIV) vaccine strategies. To ensure that all macaques become infected, the vaccinees and controls are exposed to large doses of pathogenic SIV. These nonphysiological high-dose challenges may adversely affect vaccine evaluation by overwhelming potentially efficacious vaccine responses. To determine whether a more physiologically relevant low-dose challenge can initiate infection and cause disease in Indian rhesus macaques, we used a repeated low-dose challenge strategy designed to reduce the viral inoculum to more physiologically relevant doses. In an attempt to more closely mimic challenge with HIV, we administered repeated mucosal challenges with 30, 300, and 3,000 50% tissue culture infective doses (TCID(50)) of pathogenic SIVmac239 to six animals in three groups. Infection was assessed by sensitive quantitative reverse transcription-PCR and was achieved following a mean of 8, 5.5, and 1 challenge(s) in the 30, 300, and 3,000 TCID(50) groups, respectively. Mortality, humoral immune responses, and peak plasma viral kinetics were similar in five of six animals, regardless of challenge dose. Interestingly, macaques challenged with lower doses of SIVmac239 developed broad T-cell immune responses as assessed by ELISPOT assay. This low-dose repeated challenge may be a valuable tool in the evaluation of potential vaccine regimes and offers a more physiologically relevant regimen for pathogenic SIVmac239 challenge experiments.

Multispecific vaccine-induced mucosal cytotoxic T lymphocytes reduce acute-phase viral replication but fail in long-term control of simian immunodeficiency virus SIVmac239

Given the current difficulties generating vaccine-induced neutralizing antibodies to human immunodeficiency virus (HIV), the focus of the vaccine community has shifted toward creating cytotoxic-T-lymphocyte (CTL)-based vaccines. Recent reports of CTL-based vaccine trials in macaques challenged with simian/human immunodeficiency virus SHIV-89.6P have supported the notion that such vaccines can ameliorate the course of disease. However, almost all of these studies included Env as an immunogen and since SHIV-89.6P is sensitive to neutralizing antibodies it is difficult to determine the mechanism(s) of protection. Consequently, SHIV-89.6P challenge of macaques may be a poor model for determining vaccine efficacy in humans. To ascertain the effect of vaccine-induced multispecific mucosal CTL, in the absence of Env-specific antibody, on the control of an immunodeficiency virus challenge, we vaccinated Mamu-A*01(+) macaques with constructs encoding a combination of CTL epitopes and full-length proteins (Tat, Rev, and Nef) by using a DNA prime/recombinant modified vaccinia virus Ankara (rMVA) boost regimen. The vaccination induced virus-specific CTL and CD4(+) helper T lymphocytes with CTL frequencies as high as 20,000/million peripheral blood mononuclear cells. The final rMVA vaccination, delivered intravenously, engendered long-lived mucosal CTL. At 16 weeks after the final rMVA vaccination, the vaccinees and naive, Mamu-A*01(+) controls were challenged intrarectally with SIVmac239. Massive early anamnestic cellular immune responses controlled acute-phase viral replication; however, the three vaccinees were unable to control virus replication in the chronic phase. The present study suggests that multispecific mucosal CTL, in the absence of neutralizing antibodies, can achieve a modicum of control over early viral replication but are unable to control chronic-phase viral replication after a high-dose mucosal challenge with a pathogenic simian immunodeficiency virus.

Major histocompatibility complex class I alleles associated with slow simian immunodeficiency virus disease progression bind epitopes recognized by dominant acute-phase cytotoxic-T-lymphocyte responses

Certain major histocompatibility complex class I (MHC-I) alleles are associated with delayed disease progression in individuals infected with human immunodeficiency virus (HIV) and in macaques infected with simian immunodeficiency virus (SIV). However, little is known about the influence of these MHC alleles on acute-phase cellular immune responses. Here we follow 51 animals infected with SIV(mac)239 and demonstrate a dramatic association between Mamu-A*01 and -B*17 expression and slowed disease progression. We show that the dominant acute-phase cytotoxic T lymphocyte (CTL) responses in animals expressing these alleles are largely directed against two epitopes restricted by Mamu-A*01 and one epitope restricted by Mamu-B*17. One Mamu-A*01-restricted response (Tat(28-35)SL8) and the Mamu-B*17-restricted response (Nef(165-173)IW9) typically select for viral escape variants in early SIV(mac)239 infection. Interestingly, animals expressing Mamu-A*1 and -B*17 have less variation in the Tat(28-35)SL8 epitope during chronic infection than animals that express only Mamu-A*01. Our results show that MHC-I alleles that are associated with slow progression to AIDS bind epitopes recognized by dominant CTL responses during acute infection and underscore the importance of understanding CTL responses during primary HIV infection.