Protection of human immunodeficiency virus type 2-exposed seronegative macaques from mucosal simian immunodeficiency virus transmission

A novel approach for addressing diseases not yielding to effective vaccination? Immunization by replication-competent controlled virus

Vaccination involves inoculation of a subject with a disabled disease-causing microbe or parts thereof. While vaccination has been highly successful, we still lack sufficiently effective vaccines for important infectious diseases. We propose that a more complete immune response than that elicited from a vaccine may be obtained from immunization with a disease-causing virus modified to subject replication-essential genes to the control of a gene switch activated by non-lethal heat in the presence of a drug-like compound. Upon inoculation, strictly localized replication of the virus would be triggered by a heat dose administered to the inoculation site. Activated virus would transiently replicate with an efficiency approaching that of the disease-causing virus and express all viral antigens. It may also vector heterologous antigens or control co-infecting microbes.

Prevention of human immunodeficiency virus and AIDS: postexposure prophylaxis (including health care workers)

Postexposure prophylaxis (PEP), which is designed to prevent human immunodeficiency virus (HIV) infection after an exposure, is one of several strategies for HIV prevention. PEP was first used after occupational HIV exposures in the late 1980s, with the Centers for Disease Control and Prevention issuing the first set of guidelines that included considerations regarding the use of antiretroviral agents for PEP after occupational HIV exposures in 1990. Use of PEP has been extended to nonoccupational exposures, including after sexual contact or injection-drug use. This article provides a rationale for PEP, assessment of the need for PEP, and details of its implementation.

Understanding the "lucky few": the conundrum of HIV-exposed, seronegative individuals

It has been known for many years that not all individuals who are repeatedly exposed to HIV-1 show evidence of viral replication, seroconvert, and eventually develop disease. Quite apart from those who seroconvert but progress slowly to AIDS (ie, slow progressors, long-term nonprogressors, elite controllers), these rare, exposed seronegatives either resist infection or harbor extremely low levels of virus that may be detected only using ultrasensitive methods (occult infection). The correlates of protection that confer this unique status to a tiny minority of HIV-exposed individuals remain a subject of intense interest, investigation, and controversy, as no single genetic or immunologic parameter has yet been able to fully explain this phenomenon. However, there is general consensus that studying these individuals may provide invaluable information to aid in the design of vaccines and therapeutic approaches. This review describes the major findings on this important topic, with a focus on immunologic studies.

CD8+ lymphocytes do not mediate protection against acute superinfection 20 days after vaccination with a live attenuated simian immunodeficiency virus

In order to test the hypothesis that CD8+ cytotoxic T lymphocytes mediate protection against acute superinfection, we depleted >99% of CD8+ lymphocytes in live attenuated simian immunodeficiency virus macC8 (SIVmacC8) vaccinees from the onset of vaccination, maintained that depletion for 20 days, and then challenged with pathogenic, wild-type SIVmacJ5. Vaccinees received 5 mg per kg of humanized anti-CD8 monoclonal antibody (MAb) 1 h before inoculation, followed by the same dose again on days 3, 7, 10, 13, and 17. On day 13, peripheral CD8+ T lymphocytes were >99% depleted in three out of four anti-CD8 MAb-treated vaccinees. At this time attenuated SIVmacC8 viral RNA loads in anti-CD8 MAb-treated vaccinees were significantly higher than control vaccinees treated contemporaneously with nonspecific human immunoglobulin. Lymphoid tissue CD8+ T lymphocyte depletion was >99% in three out of four anti-CD8 MAb-treated vaccinees on the day of wild-type SIVmacJ5 challenge. All four control vaccinees and three out of four anti-CD8 MAb-treated vaccinees were protected against detectable superinfection with wild-type SIVmacJ5. Although superinfection with wild-type SIVmacJ5 was detected at postmortem in a single anti-CD8 MAb-treated vaccinee, this did not correlate with the degree of preceding CD8+ T lymphocyte depletion. Clearance of attenuated SIVmacC8 viremia coincided with recovery of normal CD8+ T lymphocyte counts between days 48 and 76. These results support the view that cytotoxic T lymphocytes are important for host-mediated control of SIV primary viremia but do not indicate a central role in protection against acute superinfection conferred by inoculation with live attenuated SIV.

Human immunodeficiency virus (HIV)-2-specific T lymphocyte proliferative responses in HIV-2-infected and in HIV-2-exposed but uninfected individuals in Guinea-Bissau

Human immunodeficiency virus (HIV)-2-specific T lymphocyte proliferative responses were determined in cultures of peripheral blood mononuclear cells from HIV-2-exposed uninfected individuals, HIV-2-infected individuals and HIV-negative controls in Guinea-Bissau. Increased HIV-2-specific T lymphocyte proliferative responses were detected in both groups compared to HIV-negative controls (healthy HIV-uninfected individuals without known exposure to an HIV-infected person); five out of 29 of the HIV-2-exposed uninfected and half (16 of 32) of the HIV-2-infected individuals had stimulation indexes >2, compared to one out of 49 of the HIV-negative controls (P = 0.003 and P < 0.0001, respectively). The exposed uninfected individuals had reactivity to a HIV-2 V3-peptide corresponding to amino acids 311-326 of the envelope glycoprotein, while the HIV-2-infected people reacted mainly to HIV-2 whole viral lysate. Thus, this study demonstrates a high degree of HIV-2-specific T helper cell activity, as measured by lymphocyte proliferation, in HIV-2-exposed uninfected individuals as well as in HIV-2-infected subjects. These immune responses could be important for resistance to the infection and for the control of established infection and, thus, play a role in the lower transmission and progression of HIV-2 compared to HIV-1.

Chronic immune activation associated with chronic helminthic and human immunodeficiency virus infections: role of hyporesponsiveness and anergy

Chronic immune activation is one of the hallmarks of human immunodeficiency virus (HIV) infection. It is present also, with very similar characteristics, in very large human populations infested with helminthic infections. We have tried to review the studies addressing the changes in the immune profiles and responses of hosts infected with either one of these two chronic infections. Not surprisingly, several of the immune derangements and impairments seen in HIV infection, and considered by many to be the "specific" effects of HIV, can be found in helminth-infected but HIV-noninfected individuals and can thus be accounted for by the chronic immune activation itself. A less appreciated element in chronic immune activation is the immune suppression and anergy which it may generate. Both HIV and helminth infections represent this aspect in a very wide and illustrative way. Different degrees of anergy and immune hyporesponsiveness are present in these infections and probably have far-reaching effects on the ability of the host to cope with these and other infections. Furthermore, they may have important practical implications, especially with regard to protective vaccinations against AIDS, for populations chronically infected with helminths and therefore widely anergic. The current knowledge of the mechanisms responsible for the generation of anergy by chronic immune activation is thoroughly reviewed.

Dichotomy between CD1a+ and CD83+ dendritic cells in lymph nodes during SIV infection of macaques

The prevalence and differentiation of dendritic cells (DC) in lymphoid tissue of simian immunodeficiency virus (SIV)-infected cynomolgus monkeys was studied during disease progression. Lymph node biopsies were consecutively obtained from clinical rapid and slow progressors until the development of disease consistent with simian acquired immunodeficiency syndrome (sAIDS) occurred. Quantitative evaluation of CD1a+ DC and the expression of DC antigens related to maturation (CD83, DC-LAMP and S100b) were performed at the single cell level by in situ image analysis. Despite a persistent prevalence of CD1a+ DC in lymphoid tissue during disease progression, there was a subsequent drop of mature CD83+, DC-LAMP+ and S100b+ DC, correlating with the decline of CD4+ T cells in blood. Thus, disease progression to sAIDS was associated with impaired maturation of DC, and lack of CD83, DC-LAMP and S100b expression.

HIV-1 vaccination administered intramuscularly can induce both systemic and mucosal T cell immunity in HIV-1-uninfected individuals

A vaccine regimen that can rapidly control HIV-1 replication at the site of exposure following sexual contact is likely to be the most effective in preventing HIV-1 infection. As part of a larger, phase II clinical trial, we evaluated the ability of a recombinant canarypox HIV-1 vaccine to induce CTL that can be detected in both the systemic and mucosal compartments following i.m. immunization in 12 low- and high-risk HIV-1 seronegative volunteers. In the 7 volunteers receiving four immunizations with live recombinant canarypox ALVAC-HIV vaccine with or without rgp120/SF-2, HIV-1-specific CTL were detected in the blood of 5 (71%) and in the rectum of 4 (57%). CTL responses were observed in both risk strata. In contrast, 5 volunteers receiving placebo had undetectable responses in both compartments. Vaccine-induced, HIV-1-specific effector activities included IFN-gamma secretion and class I MHC-restricted CD8(+) CTL. Rectal and systemic CD8(+) CTL clones established in 1 vaccine recipient revealed similar Env-specific responses and MHC restriction. These findings indicate that parenteral vaccination can induce HIV-1-specific CTL that localize to sites of HIV-1 acquisition, where their presence may be critical in the control of initial viral replication and eventual dissemination. Determination of the optimal strategy to induce mucosal T cells requires future clinical studies.

Greater CD8+ TCR heterogeneity and functional flexibility in HIV-2 compared to HIV-1 infection

Virus-specific CD8(+) T cells are known to play an important role in the control of HIV infection. In this study we investigated whether there may be qualitative differences in the CD8(+) T cell response in HIV-1- and HIV-2-infected individuals that contribute to the relatively efficient control of the latter infection. A molecular comparison of global TCR heterogeneity showed a more oligoclonal pattern of CD8 cells in HIV-1- than HIV-2-infected patients. This was reflected in restricted and conserved TCR usage by CD8(+) T cells recognizing individual HLA-A2- and HLA-B57-restricted viral epitopes in HIV-1, with limited plasticity in their response to amino acid substitutions within these epitopes. The more diverse TCR usage observed for HIV-2-specific CD8(+) T cells was associated with an enhanced potential for CD8 expansion and IFN-gamma production on cross-recognition of variant epitopes. Our data suggest a mechanism that could account for any possible cross-protection that may be mediated by HIV-2-specific CD8(+) T cells against HIV-1 infection. Furthermore, they have implications for HIV vaccine development, demonstrating an association between a polyclonal, virus-specific CD8(+) T cell response and an enhanced capacity to tolerate substitutions within T cell epitopes.

Both mucosal and systemic routes of immunization with the live, attenuated NYVAC/simian immunodeficiency virus SIV(gpe) recombinant vaccine result in gag-specific CD8(+) T-cell responses in mucosal tissues of macaques

As most human immunodeficiency virus (HIV) infection occurs via mucosal surfaces, an important goal of vaccination may be the induction of virus-specific immune responses at mucosal sites to contain viral infection early on. Here we designed a study in macaques carrying the major histocompatibility complex class I Mamu-A(*)01 molecule to assess the capacity of the highly attenuated poxvirus NYVAC/simian immunodeficiency virus (SIV) SIV(gpe) vaccine candidate administered by the intranasal, intramuscular, or intrarectal route to induce mucosal immunity. All macaques, including one naive macaque, were exposed to SIV(mac251) by the intrarectal route and sacrificed 48 h after infection. The kinetics of immune response at various time points following immunization with NYVAC/SIV(gpe) and the anamnestic response to SIV(mac251) at 48 h after challenge were assessed in blood, in serial rectal and vaginal biopsy samples, and in tissues at euthanasia with an SIV(mac) Gag-specific tetramer. In addition, at euthanasia, antigen-specific cells producing gamma interferon or tumor necrosis factor alpha from the jejunum lamina propria were quantified in all macaques. Surprisingly, antigen-specific CD8(+) T cells were found in the mucosal tissues of all immunized macaques regardless of whether the vaccine was administered by a mucosal route (intranasal or intrarectal) or systemically. In addition, following mucosal SIV(mac251) challenge, antigen-specific responses were mainly confined to mucosal tissues, again regardless of the route of immunization. We conclude that immunization with a live vector vaccine results in the appearance of CD8(+) T-cell responses at mucosal sites even when the vaccine is delivered by nonmucosal routes.

Boosting of SIV-specific T cell responses in rhesus macaques that resist repeated intravaginal challenge with SIVmac251

Despite repeated high-risk exposure to infectious HIV-1, some individuals remain HIV-1 seronegative and apparently uninfected. The use of nonhuman primate model systems to study SIVmac transmission may help to elucidate the factors responsible for protection in exposed, seronegative (ESN) humans. In an earlier vaccination study, three control rhesus macaques that were exposed to three sequential intravaginal challenges with pathogenic SIVmac251 failed to show evidence of infection after 5 years of observation. 51Cr release assay results suggested that these animals had low-level cytotoxic T lymphocyte responses to SIVmac proteins. We hypothesized that these responses might be an important component of protection from mucosal challenge. We performed an additional intravaginal challenge of all three macaques and monitored SIV-specific T cell responses in peripheral blood, using the sensitive enzyme-linked immunospot (ELISpot) assay. After the fourth challenge, one animal became infected; this animal did not mount a strong SIV-specific T cell response. Two other macaques remained uninfected as determined by peripheral blood mononuclear cell (PBMC) coculture, polymerase chain reaction (PCR), and branched DNA (bDNA) analysis of peripheral blood and lymphoid tissues, but demonstrated boosting of SIV-specific T cell responses after challenge. These results support a protective role for SIVmac-specific T cells in repeatedly exposed, persistently seronegative rhesus macaques.

SIV(mac) pathogenesis in rhesus macaques of Chinese and Indian origin compared with primary HIV infections in humans

OBJECTIVE

To develop a SIV-rhesus macaque (Rh) model of AIDS that more closely approximates HIV pathogenesis in humans.

DESIGN

The pathogenesis of SIV was compared in two different types of Rh, the Chinese (Ch) and Indian (Ind) subspecies.

METHODS

Ch Rh and Ind Rh origin were identified genetically and infected with the SIV(mac)239 molecular clone. Plasma viral loads, depletion of intestinal lymphocytes with memory phenotype, humoral immune responses and CD4/CD8 cell ratios were compared during acute and steady-state periods of infection.

RESULTS

Plasma viral loads from 7 days after infection through 240 days were significantly lower in Rh of Ch origin compared with Ind Rh. Viral loads in Ch Rh were closer to viral loads observed in untreated humans infected with HIV-1. Depletion of intestinal effector cells was less evident in SIV-infected Ch Rh compared with Ind Rh. An index of intestinal pathogenesis was devised that closely paralleled viral load and severity of infection. There were no rapid progressors to AIDS among 10 Ch Rh. In contrast, three of four Ind Rh progressed rapidly to AIDS.

CONCLUSIONS

Compared with Ind Rh, SIV(mac) pathogenesis in Ch Rh was closer to HIV-1 infections in untreated adult humans. The differences were statistically significant. The Ch Rh subspecies is a suitable AIDS model and may have advantages over the rapid and highly pathogenic Ind Rh model. Moreover, Ind Rh supplies are limited and use of Ch Rh provides a new resource.

Spontaneous production of RANTES and antigen-specific IFN-gamma production in macaques vaccinated with SHIV-4 correlates with protection against SIVsm challenge

The beta-chemokines, RANTES, MIP-1alpha and MIP-1beta, have been implicated as being some of the protective factors in the immune response against human immunodeficiency virus (HIV) infection. We have presented data previously indicating that these chemokines also play a role in protective immunity against HIV/SIV infection in macaques. The aim of this study was to investigate the production of beta-chemokines in eight cynomolgus macaques vaccinated with non-pathogenic SHIV-4 in relation to protection against pathogenic SIVsm challenge. Four control animals were also included in the study. Two of the vaccinated monkeys were completely protected and one was partially protected against the challenge virus. The monkeys that resisted infectious SIVsm virus challenge showed higher spontaneous beta-chemokine production by peripheral blood mononuclear cells and had higher numbers of antigen-induced IFN-gamma secreting cells compared to the non-protected animals. Our observations support our previous findings that the genetic background of the host and/or environmental factors are involved in the chemokine production and that beta-chemokines contribute to protection against HIV/SIV infection.

Feline immunodeficiency virus Gag- and Env-specific immune responses after vaginal versus intravenous infection

To better understand the correlation of mucosal and systemic immune responses with lentiviral containment, we contrasted the early mucosal and systemic immune responses induced by vaginal versus intravenous exposure of cats to feline immunodeficiency virus (FIV) isolates of differing pathogenicity and clade (i.e., FIV-B-2542 and FIV-A-PPR). We found that despite divergence in viral genotype, the mucosal and systemic immune responses induced differed more with route of exposure than virus isolate. In intravenously exposed cats, Gag-specific antibody (both IgG and IgA isotype) predominated in the serum, saliva, and vaginal wash fluid irrespective of infecting virus isolate. While Env-specific responses were more variable, they were more often detected in vaginally infected cats. Both IgG and IgA directed against Gag and Env were consistently present in vaginal wash fluids independent of route of infection or virus isolate. FIV Gag- and Env-specific cytotoxic lymphocytes (CTLs) were detected in blood and tissue lymphocytes of cats infected with either virus strain but were greatest in intravenously infected animals. Likewise, FIV-specific CTLs were detected in CD8(+) vaginal lymphocytes of animals infected by either route but were also more frequent in intravenously inoculated animals. In summary, we found qualitative differences in the immune responses following vaginal infection but no evidence (1) that mucosal immune responses were enhanced in vaginally exposed cats, (2) that local mucosal infection led to measurably greater immune responses in either compartment; or (3) that more prominent immune responses correlated with lower viral burden.

Emerging viral infections

The past decade has witnessed the emergence of several significant viral pathogens and the further evolution of additional viral pathogens. Transmitted by a variety of differing routes, these organisms have presented substantial intellectual challenges to medicine of the 20th and 21st centuries. As perhaps the benchmark pathogen of the past decade, HIV has provided medicine and society with a most formidable opponent, and one that has yet to be fully conquered. Nonetheless, a variety of additional viral pathogens have also perplexed medicine over the past 10-15 years.

Cross-protection against mucosal simian immunodeficiency virus (SIVsm) challenge in human immunodeficiency virus type 2-vaccinated cynomolgus monkeys

In this study we compared the efficacy of live attenuated human immunodeficiency virus type 2 (HIV-2) vaccine alone versus boosting with live non-pathogenic HIV-2 following priming with ALVAC HIV-2 (recombinant canarypox virus expressing HIV-2 env, gag and pol). Six monkeys were first inoculated intravenously with live HIV-2(SBL-6669) and 7 to 10 months later were challenged intrarectally with 10 MID(50) of cell-free simian immunodeficiency virus (SIV) strain SIVsm. One monkey was completely protected against SIV infection and all five monkeys that became SIV-infected showed a lower virus replication and an initial lower virus load as compared with a parallel group of six control animals. In another experiment five monkeys were immunized either three times with ALVAC HIV-2 alone or twice with ALVAC HIV-2 and once with purified native HIV-2 gp125. The monkeys were then challenged with HIV-2 given intravenously and finally with pathogenic SIVsm given intrarectally. After challenge with SIVsm, three of five monkeys were completely protected against SIVsm infection whereas the remaining two macaques became SIV-infected but with limited virus replication. In conclusion, vaccination with an ALVAC HIV-2 vaccine followed by exposure to live HIV-2 could induce cross-protection against mucosal infection with SIVsm and seemed to be more efficient than immunization with a live HIV-2 vaccine only.

Cellular immune responses to HIV

The cellular immune response to the human immunodeficiency virus, mediated by T lymphocytes, seems strong but fails to control the infection completely. In most virus infections, T cells either eliminate the virus or suppress it indefinitely as a harmless, persisting infection. But the human immunodeficiency virus undermines this control by infecting key immune cells, thereby impairing the response of both the infected CD4+ T cells and the uninfected CD8+ T cells. The failure of the latter to function efficiently facilitates the escape of virus from immune control and the collapse of the whole immune system.

Role of CD8+ cell-produced anti-viral factors in protective immunity in HIV-2-exposed but seronegative macaques resistant to intrarectal SIVsm challenge

The cell-mediated immune response is likely to be important in controlling HIV/SIV infection. There is evidence that beta-chemokines and other, as yet unknown, anti-viral factors play a role in host defence against HIV infection. We reported previously that HIV-2 exposed but seronegative cynomolgus macaques developed SIV-specific cytotoxic T lymphocytes and were resistant to mucosal SIV challenge. The aim of this study was to examine CD8+ cell-dependent production of beta-chemokines and other anti-viral factors in these macaques. The animals, selected from among 17 monkeys enrolled in two separate experiments, were either treated with an anti-viral drug or immunized passively with HIV-2 antibody-positive serum. Three of these monkeys were protected against repeated HIV-2 challenge and were also able to control SIV infection 3 years later. Control samples were obtained from four macaques that became SIV infected and from 39 naïve animals. The three resistant monkeys showed significantly higher production of RANTES and MIP-1alpha than the 39 naïve animals. In addition, SIV infection was suppressed by CD8+ cell culture supernatants of these monkeys. However, antibodies to chemokines only partially neutralized CD8+ cell-mediated SIV suppression indicating that the anti-viral activity observed in these monkeys was the result of combined action of several inhibitory factors.

Mucosal exposure to subinfectious doses of SIV primes gut-associated antibody-secreting cells and T cells: lack of enhancement by nonneutralizing antibody

It has been suggested that the presence of immunoglobulin and complement receptors on rectal epithelium may facilitate the entry of HIV complexed to nonneutralizing antibody. We tested this hypothesis using simian immunodeficiency virus (SIV) infection of rhesus macaques. First, in a pilot study, a nonneutralizing IgG fraction of macaque anti-SIV gp120 was shown to enhance the immunogenicity of SIV envelope following rectal immunization. The same antibody was then mixed with a subinfectious dose of SIV and the occurrence of rectal infection was compared with virus alone. Animals were not infected overtly and were rechallenged with a 10-fold higher dose of virus with and without addition of antibody. There was no evidence of antibody-mediated infection, since equal numbers of macaques became infected, regardless of the presence of antibody. In addition, the application of immune complexes did not alter significantly the subsequent virus load or the immune responses generated. In seronegative animals, in which virus and proviral DNA were undetectable in PBMC and tissues, SIV-specific T-cell responses and antibody-secreting cells were found in systemic and gut-associated sites. Our results show that nonneutralizing antibody neither facilitated nor enhanced rectal infection with SIV, in the small number of animals used, despite the consistent trend for this antibody to enhance antibody responses to gp120 following rectal immunization with immune-complexed antigen. However, mucosal exposure to subinfectious doses of virus primed both systemic and local immunity, regardless of addition of nonneutralizing antibody.

Cross-protection in NYVAC-HIV-1-immunized/HIV-2-challenged but not in NYVAC-HIV-2-immunized/SHIV-challenged rhesus macaques

OBJECTIVES

Immunization with attenuated poxvirus-HIV-1 recombinants followed by protein boosting had protected four of eight rhesus macaques from HIV-2SBL6669 challenge. The present study was designed to confirm this result and to conduct the reciprocal cross-protection experiment.

METHODS

Twenty-four macaques were primed with NYVAC (a genetically attenuated Copenhagen vaccinia strain) recombinants with HIV-1 and HIV-2 env and gag-pol or NYVAC vector alone and boosted with homologous, oligomeric gp160 proteins or adjuvant only. Binding and neutralizing antibodies, cytotoxic T-lymphocytes (CTL) and CD8 T cell antiviral activity (CD8AA) were evaluated. One half of each immunization and control group were intravenously challenged with SHIV(HXB2) the other half was challenged with HIV-2SBL6669,. Protective outcome was assessed by monitoring virus isolation, proviral DNA and plasma viral RNA.

RESULTS

Both immunization groups developed homologous binding antibodies; however, homologous neutralizing antibodies were only observed in NYVAC-HIV-2-immunized macaques. While no cross-reactive neutralizing antibodies were detected, both immunization groups displayed cross-reactive CTL. Significant CD8AA was observed for only one NYVAC-HIV-2-immunized macaque. Virological assessments verified that both NYVAC-HIV-1 and NYVAC-HIV-2 immunization significantly reduced viral burdens and partially protected against HIV-2 challenge, although cross-protection was not at the level that had been previously reported. Humoral antibody and/or CTL and CD8AA were associated with protection against homologous HIV-2 challenge, while cellular immune responses seemed more important for cross-protection. No significant protection was observed in the SHIV-challenged macaques, although NYVAC-HIV-1 immunization resulted in significantly lower viral burdens compared with controls.

CONCLUSIONS

Further delineation of cross-reactive mechanisms may aid in the development of a broadly protective vaccine.

Cell-mediated immunity to low doses of SIVsm in cynomolgus macaques did not confer protection against mucosal rechallenge

Simian immunodeficiency virus (SIV) infection of macaques is a useful model for studies of the roles of different immune responses against viruses that cause (AIDS). In this study, six cynomolgus macaques were inoculated intrarectally with subinfectious or infectious doses of SIVsm to assess the SIV specific immunity, in particular protective immunity against subsequent challenge with a higher dose of SIVsm. Following the first inoculation with SIVsm, the two monkeys given the highest doses of cell-free SIVsm stock and one monkey given the intermediate dose became infected. In the three remaining animals, one animal inoculated with an intermediate dose and two animals given low doses of SIVsm, no overt infection occurred. Nevertheless, SIV specific cytotoxic T-cells against Gag/Pol and Nef proteins and T-cell proliferative responses against HIV-2 whole viral lysate, native HIV-2 gp125, recombinant SIV gp140 and SIV Env synthetic peptides were detected. After intrarectal rechallenge of the uninfected macaques with a higher dose of SIVsm all the animals became infected. These results demonstrate that cell mediated immunity can occur in the absence of detectable infection in monkeys inoculated with a low dose of SIVsm. Despite the presence of cellular immune responses, the animals were not protected when challenged with a higher dose of virus later.

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

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

Approaches to improve engineered vaccines for human immunodeficiency virus and other viruses that cause chronic infections

We used several approaches to develop enhanced vaccines for chronic viral infections such as human immunodeficiency virus (HIV) and hepatitis C virus (HCV). 1) Selected epitopes were used to avoid potentially harmful immune responses. 2) Linkage between helper and cytotoxic T-lymphocyte (CTL) epitopes was found to be important. 3) We developed an "epitope enhancement" approach modifying the sequences of epitopes to make more potent vaccines, including examples for HIV and HCV epitopes presented by murine class II and human class I major histocompatibility complex (MHC) molecules. 4) CTL avidity was found to be important for clearing viral infections in vivo, and the mechanism was examined. High-avidity CTLs, however, were found to undergo apoptosis when confronted with high-density antigen, through a mechanism involving tumor necrosis factor (TNF), TNF-RII, and a permissive state induced through the T-cell receptor. 5) We employed cytokines in the adjuvant to steer immune responses toward desired phenotypes, and showed synergy between cytokines. 6) Intrarectal immunization with peptide vaccine induced mucosal and systemic CTL. Local mucosal CTL were found to be critical for resistance to mucosal viral transmission and this resistance was enhanced with mucosally delivered interleukin-12. 7) We used an asymmetry in induction of mucosal and systemic immune responses to circumvent pre-existing vaccinia immunity for use of recombinant vaccinia vaccines.

Highly attenuated vaccine strains of simian immunodeficiency virus protect against vaginal challenge: inverse relationship of degree of protection with level of attenuation

Three different deletion mutants of simian immunodeficiency virus (SIV) that vary in their levels of attenuation were tested for the ability to protect against mucosal challenge with pathogenic SIV. Four female rhesus monkeys were vaccinated by intravenous inoculation with SIVmac239Delta3, four with SIVmac239Delta3X, and four with SIVmac239Delta4. These three vaccine strains exhibit increasing levels of attenuation: Delta3 < Delta3X <Delta4. The vaccinated monkeys were challenged by vaginal exposure to uncloned, pathogenic SIVmac251 at 61 weeks after the time of vaccination. On the basis of viral RNA loads in plasma, cell-associated virus loads in peripheral blood, and CD4 cell counts, strong protective effects were observed in all three groups of vaccinated monkeys. However, the degree of protection correlated inversely with the level of attenuation; the least-attenuated strain, SIVmac239Delta3, gave the greatest protection. One monkey in the Delta3X group and two in the Delta4 group clearly became superinfected by the challenge virus, but these animals had levels of SIV RNA in plasma that were considerably lower than those of naive animals that were challenged in parallel. Protection against vaginal challenge appears easier to achieve than protection against intravenous challenge, since four other SIVmac239Delta4-vaccinated monkeys showed no protection when challenged intravenously with a much lower inoculum of the same challenge virus stock. Protection against vaginal challenge in the Delta4-vaccinated group occurred in the absence of detectable serum neutralizing activities and appeared to be associated with the development of an early SIV-specific cytotoxic-T-lymphocyte response. Our results demonstrate that mucosal protection can be achieved by systemic immunization with the highly attenuated SIVmac239Delta4 more than 1 year prior to the time of challenge.

Protection of macaques against intrarectal infection by a combination immunization regimen with recombinant simian immunodeficiency virus SIVmne gp160 vaccines

We previously reported that immunization with recombinant simian immunodeficiency virus SIVmne envelope (gp160) vaccines protected macaques against intravenous challenge by the cloned homologous virus E11S but that this protection was only partially effective against the uncloned virus, SIVmne. In the present study, we examine the protective efficacy of this immunization regimen against infection by a mucosal route. We found that the same gp160-based vaccines were highly effective against intrarectal infection not only with the E11S clone but also with the uncloned SIVmne. Protection against mucosal infection is therefore achievable by parenteral immunization with recombinant envelope vaccines. Protection appears to correlate with high levels of SIV-specific antibodies and, in animals protected against the uncloned virus, the presence of serum-neutralizing activities. To understand the basis for the differential efficacies against the uncloned virus by the intravenous versus the intrarectal routes, we examined viral sequences recovered from the peripheral blood mononuclear cells of animals early after infection by both routes. We previously showed that the majority (85%) of the uncloned SIVmne challenge stock contained V1 sequences homologous to the molecular clone from which the vaccines were made (E11S type), with the remainder (15%) containing multiple conserved changes (the variant types). In contrast to intravenously infected animals, from which either E11S-type or the variant type V1 sequences could be recovered in significant proportions, animals infected intrarectally had predominantly E11S-type sequences. Preferential transmission or amplification of the E11S-type viruses may therefore account in part for the enhanced efficacy of the recombinant gp160 vaccines against the uncloned virus challenge by the intrarectal route compared with the intravenous route.

The importance of local mucosal HIV-specific CD8(+) cytotoxic T lymphocytes for resistance to mucosal viral transmission in mice and enhancement of resistance by local administration of IL-12

Although crucial to mucosal vaccine development, the mechanisms of defense against mucosal viral infection are still poorly understood. Protection, cytotoxic T lymphocytes (CTL), and neutralizing antibodies have all been observed, but cause and effect have been difficult to determine. The ability of CTL in the mucosa to mediate protection against mucosal viral transmission has never been proven. Here, we use an HIV peptide immunogen and an HIV-1 gp160-expressing recombinant vaccinia viral intrarectal murine challenge system, in which neutralizing antibodies do not play a role, to demonstrate for the first time that long-lasting immune resistance to mucosal viral transmission can be accomplished by CD8(+) CTL that must be present in the mucosal site of exposure. The resistance is ablated by depleting CD8(+) cells in vivo and requires CTL in the mucosa, whereas systemic (splenic) CTL are shown to be unable to protect against mucosal challenge. Furthermore, the resistance as well as the CTL response can be increased by local mucosal delivery of IL-12 with the vaccine. These results imply that induction of local mucosal CTL may be critical for success of a vaccine against viruses transmitted through a mucosal route, such as HIV.

Occult systemic infection and persistent simian immunodeficiency virus (SIV)-specific CD4(+)-T-cell proliferative responses in rhesus macaques that were transiently viremic after intravaginal inoculation of SIV

The intact cervicovaginal mucosa is a relative barrier to the sexual transmission of human immunodeficiency virus type 1 (HIV-1). In the simian immunodeficiency virus (SIV) macaque model of HIV infection, seronegative transient viremia (STV; virus isolation positive followed by repeated negative cultures) occurs after intravaginal inoculation of a low dose of pathogenic SIVmac251 (C. J. Miller, M. Marthas, J. Torten, N. Alexander, J. Moore, G. Doncel, and A. Hendrickx, J. Virol. 68:6391-6400, 1994). Thirty-one adult female macaques that had been inoculated intravaginally with pathogenic SIVmac251 became transiently viremic. One monkey that had been culture negative for a year after SIV inoculation became persistently viremic and developed simian AIDS. No other STV monkey developed persistent viremia or disease. Results of very sensitive assays showed that 6 of 31 monkeys had weak SIV-specific antibody responses. SIV-specific antibodies were not detected in the cervicovaginal secretions of 10 STV monkeys examined. Twenty of 26 monkeys had lymphocyte proliferative responses to p55(gag) and/or gp130(env) antigens; 3 of 6 animals, including the monkey that became persistently viremic, had detectable cytotoxic T-lymphocyte (CTL) responses to SIV. At necropsy, lymphoid tissues and vaginal mucosa were virus culture negative, but in 10 of 10 animals, SIV provirus was detected by PCR using gag-specific primer pairs. Fifty percent of the PCR-positive tissue samples were also positive for SIV gag RNA by reverse transcriptase PCR. Thus, transient viremia following intravaginal inoculation of pathogenic SIV is associated with persistent, systemic infection, either latent or very low level productive. Atypical immune responses, characterized by lymphocyte proliferation and some CTL responses in the absence of conventionally detectable antibodies, develop in transiently viremic monkeys.

Cytotoxic T cell responses to multiple conserved HIV epitopes in HIV-resistant prostitutes in Nairobi

Many people who remain persistently seronegative despite frequent HIV exposure have HIV-specific immune responses. The study of these may provide information about mechanisms of natural protective immunity to HIV-1. We describe the specificity of cytotoxic T lymphocyte responses to HIV in seronegative prostitutes in Nairobi who are apparently resistant to HIV infection. These women have had frequent exposure to a range of African HIV-1 variants, primarily clades A, C, and D, for up to 12 yr without becoming infected. Nearly half of them have CTL directed towards epitopes previously defined for B clade virus, which are largely conserved in the A and D clade sequences. Stronger responses are frequently elicited using the A or D clade version of an epitope to stimulate CTL, suggesting that they were originally primed by exposure to these virus strains. CTL responses have been defined to novel epitopes presented by HLA class I molecules associated with resistance to infection in the cohort, HLA-A*6802 and HLA-B18. Estimates using a modified interferon-gamma Elispot assay indicate a circulating frequency of CTL to individual epitopes of between 1:3,200 and 1:50,000. Thus, HIV-specific immune responses-particularly cross-clade CTL activity- may be responsible for protection against persistent HIV infection in these African women.

Immune responses but no protection against SHIV by gene-gun delivery of HIV-1 DNA followed by recombinant subunit protein boosts

The efficacy of combining immunization with human immunodeficiency vitus type 1 (HIV-1) DNA and HIV-1 recombinant proteins to obtain protection from chimeric simian/human immunodeficiency virus (SHIV) was determined. Four cynomolgus monkeys received four gene-gun immunizations intraepidermally of plasmid DNA encoding HIV-1lai env (gp160), gag, tat, nef, and rev proteins. Ten micrograms of DNA was used per immunization. The animals were boosted twice intramuscularly with 50 microgram of HIV-1lai Env (MicroGeneSys), Gag, Tat, Nef, and Rev recombinant proteins mixed in Ribi adjuvant. The antibody responses were amplified following the administration of the recombinant subunit boosts. One month after the final subunit immunization, the vaccinated animals together with four control animals were challenged intravenously with 10 monkey infectious doses of SHIV that expresses the env, tat and rev genes of HIV-1 and gag and nef from SIV. However, only low titers of neutralizing antibodies were present at the day of challenge. The consecutive HIV-1 DNA and recombinant protein immunizations induced B- and T-cell responses but not protection against SHIV replication nor reduction of the viral load.

Protective immunity induced by live attenuated simian immunodeficiency virus

Lack of information on the mechanisms of protective immunity to AIDS virus infection represents a major obstacle to the development of a rational strategy for an effective HIV vaccine. In macaques, immunization with live attenuated simian immunodeficiency viruses has induced the most potent protective immunity and continued study promises a better understanding of the nature of protective immune responses. Recent evidence supports involvement of both cytotoxic T lymphocytes and neutralizing antibodies in protective immunity against infection by simian immunodeficiency virus, but more detailed studies are needed to document their relative importance.

HIV-2 and T cell recognition

HIV-2 is less pathogenic and less transmissible than HIV-1. Recent research in relation to deletions in the HIV nef gene and to immune cross-reactions between infections by HIV-2, HIV-1 and simian immunodeficiency virus suggests that T cell recognition and the control of viral replication may be more efficient in HIV-2 infection than in HIV-1 infection. These insights may be crucial to the design of effective vaccines.

Characterization of the Peptide Binding Motif of a Rhesus MHC Class I Molecule (Mamu-A*01) That Binds an Immunodominant CTL Epitope from Simian Immunodeficiency Virus

The majority of immunogenic CTL epitopes bind to MHC class I molecules with high affinity. However, peptides longer or shorter than the optimal epitope rarely bind with high affinity. Therefore, identification of optimal CTL epitopes from pathogens may ultimately be critical for inducing strong CTL responses and developing epitope-based vaccines. The SIV-infected rhesus macaque is an excellent animal model for HIV infection of humans. Although a number of CTL epitopes have been mapped in SIV-infected rhesus macaques, the optimal epitopes have not been well defined, and their anchor residues are unknown. We have now defined the optimal SIV gag CTL epitope restricted by the rhesus MHC class I molecule Mamu-A*01 and defined a general peptide binding motif for this molecule that is characterized by a dominant position 3 anchor (proline). We used peptide elution and sequencing, peptide binding assays, and bulk and clonal CTL assays to demonstrate that the optimal Mamu-A*01-restricted SIV gag CTL epitope was CTPYDINQM181–189. Mamu-A*01 is unique in that it is found at a high frequency in rhesus macaques, and all SIV-infected Mamu-A*01-positive rhesus macaques studied to date develop an immunodominant gag-specific CTL response restricted by this molecule. Identification of the optimal SIV gag CTL epitope will be critical for a variety of studies designed to induce CD8+ CTL responses specific for SIV in the rhesus macaque.

Presence of circulating CTL induced by infection with wild-type or attenuated SIV and their correlation with protection from pathogenic SHIV challenge

The aim of this study was to evaluate the role of CTLs in the protection from challenge with pathogenic SHIV in macaques vaccinated with attenuated virus. More specifically, we have analyzed the CTL response in cynomolgus macaques vaccinated/infected with the attenuated SIVmacC8 or the wild-type SIVmacJ5 and correlated these responses to the protection from SHIV89.6P challenge. SIVmacC8-vaccinated monkeys demonstrated a broader CTL response than the SIVmacJ5-infected animals. Nevertheless, CTL against some proteins in SIVmacC8-vaccinated monkeys became progressively more difficult to detect through the day of challenge. In regards to protection from superinfection with SHIV89.6P, neither the presence of circulating CTL nor the CTL precursor frequency against any of the tested proteins correlated with the outcome of the challenge when SIVmacJ5- and SIVmacC8-infected animals were analyzed together. By analyzing the SIVmacC8-vaccinated animals separately, only the protected animal had detectable CTL precursors with moderate frequencies against all three tested proteins at the day of challenge.