Expression of gamma interferon by simian immunodeficiency virus increases attenuation and reduces postchallenge virus load in vaccinated rhesus macaques.

Simian immunodeficiency virus (SIV) infection of macaques is a model for human immunodeficiency virus (HIV) infection. We have previously reported the construction and characterization of an SIV vector with a deletion in the nef gene (SIV(delta nef)) and expressing gamma interferon (SIV(HyIFN)) (L. Giavedoni and T. Yilma, J. Virol. 70:2247-2251, 1996). We now show that rhesus macaques vaccinated with SIV(HyIFN) have a lower viral load than a group similarly immunized with SIV(delta nef). Viral loads remained low in the SIV(HyIFN)-vaccinated group even though SIV expressing gamma interferon could not be isolated after 6 weeks postimmunization in these animals. All immunized and two naive control macaques became infected when challenged with virulent SIV(mac251), at 25 weeks postvaccination. In contrast to the two naive controls that died by 12 and 18 weeks postchallenge, all vaccinated animals remained healthy for more than 32 weeks. In addition, postchallenge cell-associated virus load was significantly lower in SIV(HyIFN)-immunized animals than in the group vaccinated with SIV(delta nef). These findings indicate that cytokine-expressing viruses can provide a novel approach for development of safe and efficacious live attenuated vaccines for AIDS.     

Antiretroviral therapy during primary immunodeficiency virus infection can induce persistent suppression of virus load and protection from heterologous challenge in rhesus macaques

A limited period of chemotherapy during primary immunodeficiency virus infection might provide a long-term clinical benefit even if treatment is initiated at a time point when virus is already detectable in plasma. To evaluate this strategy, we infected rhesus macaques with the pathogenic simian/human immunodeficiency virus RT-SHIV and treated them with the antiretroviral drug (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) for 8 weeks starting 7 or 14 days postinfection. PMPA treatment suppressed viral replication efficiently in all of the monkeys. After chemotherapy ended, virus replication rebounded and viral RNA in plasma reached levels comparable to that of the controls in four of the six monkeys. However, in the other two animals, virus loads peaked only moderately after withdrawal of the drug and then declined to low or even undetectable levels. These low levels of viremia remained stable for at least 31 weeks after cessation of therapy. At this time point, these two monkeys were challenged with SIV(8980) to evaluate whether the host responses which were able to keep RT-SHIV replication under control were also sufficient to protect against infection with a highly pathogenic heterologous virus. Both monkeys proved to be protected against the heterologous virus. In one of the two animals, low levels of SIV(8980) replication were detected. Thus, by chemotherapy during the acute phase of pathogenic virus replication, we could achieve not only persistent virus load suppression in two out of six monkeys but also protection from subsequent heterologous challenge. By this chemotherapeutic attenuation, the replication kinetics of attenuated viruses could be mimicked and a vaccination effect similar to that induced by live attenuated simian immunodeficiency virus vaccines was achieved.     

Therapeutic dendritic-cell vaccine for simian AIDS

An effective immune response against human immunodeficiency virus or simian immunodeficiency virus (SIV) is critical in achieving control of viral replication. Here, we show in SIV-infected rhesus monkeys that an effective and durable SIV-specific cellular and humoral immunity is elicited by a vaccination with chemically inactivated SIV-pulsed dendritic cells. After three immunizations made at two-week intervals, the animals exhibited a 50-fold decrease of SIV DNA and a 1,000-fold decrease of SIV RNA in peripheral blood. Such reduced viral load levels were maintained over the remaining 34 weeks of the study. Molecular and cellular analyses of axillary and inguinal node lymphocytes of vaccinated monkeys revealed a correlation between decreased SIV DNA and RNA levels and increased SIV-specific T-cell responses. Neutralizing antibody responses were augmented and remained elevated. Inactivated whole virus-pulsed dendritic cell vaccines are promising means to control diseases caused by immuno- deficiency viruses.     

Env-Independent Protection Induced by Live, Attenuated Simian Immunodeficiency Virus Vaccines

Live attenuated simian immunodeficiency viruses (SIV), such as nef deletion mutants, are the most effective vaccines tested in the SIV-macaque model so far. To modulate the antiviral immune response induced by live attenuated SIV vaccines, we had previously infected rhesus monkeys with a nef deletion mutant of SIV expressing interleukin 2 (SIV-IL2) (B. R. Gundlach, H. Linhart, U. Dittmer, S. Sopper, S. Reiprich, D. Fuchs, B. Fleckenstein, G. Hunsmann, S. Stahl-Hennig, and K. Überla, J. Virol. 71:2225–2232, 1997). In the present study, SIV-IL2-infected macaques and macaques infected with the nef deletion mutant SIVΔNU were challenged with pathogenic SIV 9 to 11 months postvaccination. In contrast to the results with naive control monkeys, no challenge virus could be isolated from the SIV-IL2- and SIVΔNU-infected macaques. However, challenge virus sequences could be detected by nested PCR in some of the vaccinated macaques. To determine the role of immune responses directed against Env of SIV, four vaccinated macaques were rechallenged with an SIV-murine leukemia virus (MLV) hybrid in which the env gene of SIV had been functionally replaced by the env gene of amphotropic MLV. All vaccinated macaques were protected from productive infection with the SIV-MLV hybrid in the absence of measurable neutralizing antibodies, while two naive control monkeys were readily infected. Since the SIV-MLV hybrid uses the MLV Env receptor Pit2 and not CD4 and a coreceptor for virus entry, chemokine inhibition and receptor interference phenomena were not involved in protection. These results indicate that the protective responses induced by live attenuated SIV vaccines can be independent of host immune reactions directed against Env.     

A partially attenuated simian immunodeficiency virus induces host immunity that correlates with resistance to pathogenic virus challenge.

Three infectious, attenuated molecular clones of simian immunodeficiency virus (SIVmac) were tested for viral and host determinants of protective immunity. The viruses differed in degree of virulence from highly attenuated to moderately attenuated to partially attenuated. Levels of immune stimulation and antiviral immunity were measured in rhesus macaques inoculated 2 years previously with these viruses. Monkeys infected with the highly attenuated or moderately attenuated viruses had minimal lymphoid hyperplasia, normal CD4/CD8 ratios, low levels of SIV-specific antibodies, and cytotoxic T-lymphocyte activity against p55gag (Gag) or gp160env (Env). Monkeys infected with the partially attenuated virus had moderate to marked lymphoid hyperplasia, normal CD4/CD8 ratios, high levels of SIV-specific antibodies, and cytotoxic T-lymphocyte activity against both Gag and Env. After pathogenic virus challenge, monkeys immunized with the partially attenuated virus had 100- to 1,000-fold-lower viral load in peripheral blood mononuclear cells and lymph node mononuclear cells than naive control animals. One of four monkeys immunized with the highly attenuated virus and two of four monkeys immunized with the moderately attenuated virus developed similarly low viral loads after challenge. These three attenuated strains of SIV induced a spectrum of antiviral immunity that was inversely associated with their degree of attenuation. Only the least attenuated virus induced resistance to challenge infection in all immunized monkeys.     

Protection by Live, Attenuated Simian Immunodeficiency Virus against Heterologous Challenge

We examined the ability of a live, attenuated deletion mutant of simian immunodeficiency virus (SIV), SIVmac239Delta3, which is missing nef and vpr genes, to protect against challenge by heterologous strains SHIV89.6p and SIVsmE660. SHIV89.6p is a pathogenic, recombinant SIV in which the envelope gene has been replaced by a human immunodeficiency virus type 1 envelope gene; other structural genes of SHIV89.6p are derived from SIVmac239. SIVsmE660 is an uncloned, pathogenic, independent isolate from the same primate lentivirus subgrouping as SIVmac but with natural sequence variation in all structural genes. The challenge with SHIV89.6p was performed by the intravenous route 37 months after the time of vaccination. By the criteria of CD4(+) cell counts and disease, strong protection against the SHIV89.6p challenge was observed in four of four vaccinated monkeys despite the complete mismatch of env sequences. However, SHIV89.6p infection was established in all four previously vaccinated monkeys and three of the four developed fluctuating viral loads between 300 and 10,000 RNA copy equivalents per ml of plasma 30 to 72 weeks postchallenge. When other vaccinated monkeys were challenged with SIVsmE660 at 28 months after the time of vaccination, SIV loads were lower than those observed in unvaccinated controls but the level of protection was less than what was observed against SHIV89.6p in these experiments and considerably less than the level of protection against SIVmac251 observed in previous experiments. These results demonstrate a variable level of vaccine protection by live, attenuated SIVmac239Delta3 against heterologous virus challenge and suggest that even live, attenuated vaccine approaches for AIDS will face significant hurdles in providing protection against the natural variation present in field strains of virus. The results further suggest that factors other than anti-Env immune responses can be principally responsible for the vaccine protection by live, attenuated SIV.     

Effect of CD8+ lymphocyte depletion on virus containment after simian immunodeficiency virus SIVmac251 challenge of live attenuated SIVmac239delta3-vaccinated rhesus macaques

Although live attenuated vaccines can provide potent protection against simian immunodeficiency virus (SIV) and simian-human immunodeficiency virus challenges, the specific immune responses that confer this protection have not been determined. To test whether cellular immune responses mediated by CD8+ lymphocytes contribute to this vaccine-induced protection, we depleted rhesus macaques vaccinated with the live attenuated virus SIVmac239Delta3 of CD8+ lymphocytes and then challenged them with SIVmac251 by the intravenous route. While vaccination did not prevent infection with the pathogenic challenge virus, the postchallenge levels of virus in the plasmas of vaccinated control animals were significantly lower than those for unvaccinated animals. The depletion of CD8+ lymphocytes at the time of challenge resulted in virus levels in the plasma that were intermediate between those of the vaccinated and unvaccinated controls, suggesting that CD8+ cell-mediated immune responses contributed to protection. Interestingly, at the time of challenge, animals expressing the Mamu-A*01 major histocompatibility complex class I allele showed significantly higher frequencies of SIV-specific CD8+ T-cell responses and lower neutralizing antibody titers than those in Mamu-A*01- animals. Consistent with these findings, the depletion of CD8+ lymphocytes abrogated vaccine-induced protection, as judged by the peak postchallenge viremia, to a greater extent in Mamu-A*01+ than in Mamu-A*01- animals. The partial control of postchallenge viremia after CD8+ lymphocyte depletion suggests that both humoral and cellular immune responses induced by live attenuated SIV vaccines can contribute to protection against a pathogenic challenge and that the relative contribution of each of these responses to protection may be genetically determined.     

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

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

Effect of vaccination with recombinant modified vaccinia virus Ankara expressing structural and regulatory genes of SIV(macJ5) on the kinetics of SIV replication in cynomolgus monkeys

The efficacy of a multicomponent vaccination with modified vaccinia Ankara constructs (rMVA) expressing structural and regulatory genes of simian immunodeficiency virus (SIV(mac251/32H/J5)) was investigated in cynomolgus monkeys, following challenge with a pathogenic SIV. Vaccination with rMVA-J5 performed at week 0, 12, and 24 induced a moderate proliferative response to whole SIV, a detectable humoral response to all but Nef SIV antigens, and failed to induce neutralizing antibodies. Two months after the last boost, the monkeys were challenged intravenously with 50 MID50 of SIV(mac251). All control monkeys, previously inoculated with non-recombinant MVA, were infected by week two and seroconverted by weeks four to eight. In contrast a sharp increase of both humoral and proliferative responses at two weeks post-challenge was observed in vaccinated monkeys compared to control monkeys. Although all vaccinated monkeys were infected, vaccination with rMVA-J5 appeared to partially control viral replication during the acute and late phase of infection as judged by cell- and plasma-associated viral load.     

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

At present it is not known which form of immunity would be most effective against infection with human immunodeficiency virus (HIV). To evaluate the possible role of cellular immunity, we examined whether four HIV type 2-exposed but seronegative macaques developed cellular immune responses and determined whether these exposed macaques were resistant to mucosal transmission of simian immunodeficiency virus (SIV). Following intrarectal challenge with SIV, 2 monkeys were protected against detectable SIV replication and another showed suppressed viral replication compared to 14 persistently infected controls. The two protected monkeys demonstrated SIV-specific cytotoxic T lymphocytes before as well as after SIV challenge. Here we provide evidence that activation of the cell-mediated arm of the immune system only, without antibody formation, can control SIV replication in macaques. The results imply that vaccines that stimulate a strong and broad cellular immune response could prevent mucosal HIV transmission.     

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

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

New prospects for the development of a vaccine against human immunodeficiency virus type 1. An overview

During the past few years, definite progress has been made in the field of human immunodeficiency virus type 1 (HIV-1) vaccines. Initial attempts using envelope gp120 or gp140 from T-cell line-adapted (TCLA) HIV-1 strains to vaccinate chimpanzees showed that neutralizing antibody-based immune responses were protective against challenge with homologous TCLA virus strains or strains with low replicative capacity, but these neutralizing antibodies remained inactive when tested on primary HIV-1 isolates, casting doubts on the efficacy of gp120-based vaccines in the natural setting. Development of a live attenuated simian immunodeficiency virus (SIV) vaccine was undertaken in the macaque model using whole live SIV bearing multiple deletions in the nef, vpr and vpx genes. This vaccine provided remarkable protective efficacy against wild-type SIV challenge, but the deletion mutants remain pathogenic, notably in neonate monkeys. Study of the mechanisms of protection in the SIV model unravelled the importance of the T-cell responses, whether in the form of cytotoxic T-lymphocyte (CTL) killing activity, or in that of antiviral factor secretion of cytokines, beta-chemokines and other unidentified antiviral factors by CD8+ T-cells. Induction of such a response is being sought at this time using various live recombinant virus vaccines, either poxvirus or alphavirus vectors or DNA vectors, which can be combined together or with a gp120/gp140 boost in various prime-boost combination strategies. New vectors include attenuated vaccinia virus NYVAC, modified vaccinia strain Ankara (MVA), Semliki Forest virus, Venezuelan equine encephalitis virus, and Salmonellas. Recent DNA prime-poxvirus boost combination regimens have generated promising protection results against SIV or SIV/HIV (SHIV) challenge in macaque models. Emphasis is also put on the induction of a mucosal immune response, involving both a secretory IgA response and a mucosal CTL response which could constitute a 'first line of defence' in the vaccinated host. Finally, a totally novel vaccine approach based on the use of Tat or Tat and Rev antigens has been shown to induce efficient protection from challenge with pathogenic SIV or SHIV in vaccinated macaques. The only vaccine in phase 3 clinical trials in human volunteers is a gp120-based vaccine, AIDSVAX. A prime-boost combination of a recombinant canarypoxvirus and a subunit gp120 vaccine is in phase 2. Emphasis has been put recently on the necessity of testing prototype vaccines in developing countries using immunogens derived from local virus strains. Trial sites have thus been identified in Kenya, Uganda, Thailand and South Africa where phase I trials have begun or are expected to start presently.     

Simian immunodeficiency virus (SIV) gag DNA-vaccinated rhesus monkeys develop secondary cytotoxic T-lymphocyte responses and control viral replication after pathogenic SIV infection

The potential contribution of a plasmid DNA construct to vaccine-elicited protective immunity was explored in the simian immunodeficiency virus (SIV)/macaque model of AIDS. Making use of soluble major histocompatibility class I/peptide tetramers and peptide-specific killing assays to monitor CD8(+) T-lymphocyte responses to a dominant SIV Gag epitope in genetically selected rhesus monkeys, a codon-optimized SIV gag DNA vaccine construct was shown to elicit a high-frequency SIV-specific cytotoxic T-lymphocyte (CTL) response. This CTL response was demonstrable in both peripheral blood and lymph node lymphocytes. Following an intravenous challenge with the highly pathogenic viral isolate SIVsm E660, these vaccinated monkeys developed a secondary CTL response that arose with more rapid kinetics and reached a higher frequency than did the postchallenge CTL response in control plasmid-vaccinated monkeys. While peak plasma SIV RNA levels were comparable in the experimentally and control-vaccinated monkeys during the period of primary infection, the gag plasmid DNA-vaccinated monkeys demonstrated better containment of viral replication by 50 days following SIV challenge. These findings indicate that a plasmid DNA vaccine can elicit SIV-specific CTL responses in rhesus monkeys, and this vaccine-elicited immunity can facilitate the generation of secondary CTL responses and control of viral replication following a pathogenic SIV challenge. These observations suggest that plasmid DNA may prove a useful component of a human immunodeficiency virus type 1 vaccine.     

Use of simian immunodeficiency virus for vaccine research

Rhesus monkeys were immunized with purified, disrupted, noninfectious simian immunodeficiency virus (SIV) in adjuvant induced SIV neutralizing antibodies. Two of six previously vaccinated macaques were protected against infection when challenged with 200-1,000 animal infectious doses of uncloned, pathogenic SIV and both have remained free of signs of virus infection for 19 and 30 months. Prior vaccination appeared to be of benefit in decreasing the virus load and in delaying the onset of AIDS in animals that became infected. Nonetheless, two of four previously vaccinated monkeys that became infected following challenge eventually developed AIDS and died 505 and 538 days after infection. Thus, for a vaccine to be truly effective against AIDS, it may have to protect absolutely against initial infection.     

Control of viremia and prevention of simian-human immunodeficiency virus-induced disease in rhesus macaques immunized with recombinant vaccinia viruses plus inactivated simian immunodeficiency virus and human immunodeficiency virus type 1 particles

An effective vaccine against the human immunodeficiency virus type 1 (HIV-1) will very likely have to elicit both cellular and humoral immune responses to control HIV-1 strains of diverse geographic and genetic origins. We have utilized a pathogenic chimeric simian-human immunodeficiency virus (SHIV) rhesus macaque animal model system to evaluate the protective efficacy of a vaccine regimen that uses recombinant vaccinia viruses expressing simian immunodeficiency virus (SIV) and HIV-1 structural proteins in combination with intact inactivated SIV and HIV-1 particles. Following virus challenge, control animals experienced a rapid and complete loss of CD4(+) T cells, sustained high viral loads, and developed clinical disease by 17 to 21 weeks. Although all of the vaccinated monkeys became infected, they displayed reduced postpeak viremia, had no significant loss of CD4(+) T cells, and have remained healthy for more than 15 months postinfection. CD8(+) T-cell and neutralizing antibody responses in vaccinated animals following challenge were demonstrable. Despite the control of disease, virus was readily isolated from the circulating peripheral blood mononuclear cells of all vaccinees at 22 weeks postchallenge, indicating that immunologic control was incomplete. Virus recovered from the animal with the lowest postchallenge viremia generated high virus loads and an irreversible loss of CD4(+) T-cell loss following its inoculation into a na?ve animal. These results indicate that despite the protection from SHIV-induced disease, the vaccinated animals still harbored replication-competent and pathogenic virus.     

Complement-mediated, infection-enhancing antibodies in plasma from vaccinated macaques before and after inoculation with live simian immunodeficiency virus.

Rhesus monkeys vaccinated against infection with simian immunodeficiency virus (SIV) were examined, in retrospect, for the presence of infection-enhancing antibodies and possible consequences associated with the presence of these antibodies. At the time of experimental inoculation with live virus, complement-mediated, infection-enhancing antibodies were detected in plasma specimens from six of six animals vaccinated with detergent-inactivated whole virus, from nine of nine animals vaccinated with Formalin-inactivated whole virus, and from seven of eight animals immunized with two SIV subunit preparations. The presence of infection-enhancing antibodies at the time of viral challenge gave no indication of predicting vaccine success or failure. After live-virus challenge, titers of infection-enhancing antibodies, like enzyme-linked immunosorbent assay titers, increased in unprotected animals and decreased or became undetectable in animals protected by vaccination. Thus, vaccine protection against SIV infection can still be achieved in the presence of detectable complement-mediated, infection-enhancing antibodies.     

Induction of mucosal protection against primary, heterologous simian immunodeficiency virus by a DNA vaccine

An effective vaccine against human immunodeficiency virus (HIV) should protect against mucosal transmission of genetically divergent isolates. As a safe alternative to live attenuated vaccines, the immunogenicity and protective efficacy of a DNA vaccine containing simian immunodeficiency virus (SIV) strain 17E-Fr (SIV/17E-Fr) gag-pol-env was analyzed in rhesus macaques. Significant levels of cytotoxic T lymphocytes (CTL), but low to undetectable serum antibody responses, were observed following multiple immunizations. SIV-specific mucosal antibodies and CTL were also detected in rectal washes and gut-associated lymphoid tissues, respectively. Vaccinated and naive control monkeys were challenged intrarectally with SIV strain DeltaB670 (SIV/DeltaB670), a primary isolate whose env is 15% dissimilar to that of the vaccine strain. Four of seven vaccinees were protected from infection as determined by the inability to identify viral RNA or DNA sequences in the peripheral blood and the absence of anamnestic antibody responses postchallenge. This is the first report of mucosal protection against a primary pathogenic, heterologous isolate of SIV by using a commercially viable vaccine approach. These results support further development of a DNA vaccine for protection against HIV.     

Gag-Specific Cellular Immunity Determines In Vitro Viral Inhibition and In Vivo Virologic Control following Simian Immunodeficiency Virus Challenges of Vaccinated Rhesus Monkeys

A comprehensive vaccine for human immunodeficiency virus type 1 (HIV-1) would block HIV-1 acquisition as well as durably control viral replication in breakthrough infections. Recent studies have demonstrated that Env is required for a vaccine to protect against acquisition of simian immunodeficiency virus (SIV) in vaccinated rhesus monkeys, but the antigen requirements for virologic control remain unclear. Here, we investigate whether CD8(+) T lymphocytes from vaccinated rhesus monkeys mediate viral inhibition in vitro and whether these responses predict virologic control following SIV challenge. We observed that CD8(+) lymphocytes from 23 vaccinated rhesus monkeys inhibited replication of SIV in vitro. Moreover, the magnitude of inhibition prior to challenge was inversely correlated with set point SIV plasma viral loads after challenge. In addition, CD8 cell-mediated viral inhibition in vaccinated rhesus monkeys correlated significantly with Gag-specific, but not Pol- or Env-specific, CD4(+) and CD8(+) T lymphocyte responses. These findings demonstrate that in vitro viral inhibition following vaccination largely reflects Gag-specific cellular immune responses and correlates with in vivo virologic control following infection. These data suggest the importance of including Gag in an HIV-1 vaccine in which virologic control is desired.     

Prior infection with a nonpathogenic chimeric simian-human immunodeficiency virus does not efficiently protect macaques against challenge with simian immunodeficiency virus.

Prior infection with a nef-deleted simian immunodeficiency virus (SIV) protects macaques not only against a homologous pathogenic SIV challenge but also against challenge with a chimeric SIV expressing a human immunodeficiency virus type 1 env gene (SHIV). Since this SHIV is itself nonpathogenic, we sought to explore the use of a nonpathogenic SHIV as a live, attenuated AIDS virus vaccine. Four cynomolgus monkeys infected for greater than 600 days with a chimeric virus composed of SIVmac 239 expressing the human immunodeficiency virus type 1 HXBc2 env, tat, and rev genes were challenged intravenously with 100 animal infectious doses of the J5 clone of SIVmac 32H, an isolate derived by in vivo passage of SIVmac 251. Three of the four monkeys became infected with SIVmac. This observation underlines the difficulty, even with a live virus vaccine, in protecting against an AIDS virus infection.