Efficacy of live‐attenuated and whole‐inactivated simian immunodeficiency virus vaccines against vaginal challenge with virulent SIV

Immunogenic properties of SARS-CoV-2 inactivated by ultraviolet light

Vaccination against COVID-19 is the most effective method of controlling the spread of SARS-CoV-2 and reducing mortality from this disease. The development of vaccines with high protective activity against a wide range of SARS-CoV-2 antigenic variants remains relevant. In this regard, evaluation of the effectiveness of physical methods of virus inactivation, such as ultraviolet irradiation (UV) of the virus stock, remains relevant. This study demonstrates that the UV treatment of SARS-CoV-2 completely inactivates its infectivity while preserving its morphology, antigenic properties, and ability to induce the production of virus-neutralizing antibodies in mice through immunization. Thus, the UV inactivation of SARS-CoV-2 makes it possible to obtain viral material similar in its antigenic and immunogenic properties to the native antigen, which can be used both for the development of diagnostic test systems and for the development of an inactivated vaccine against COVID-19.

Increases in NKG2C Expression on T Cells and Higher Levels of Circulating CD8+ B Cells Are Associated with Sterilizing Immunity Provided by a Live Attenuated SIV Vaccine

Vaccines based on live attenuated viruses are highly effective immunogens in the simian immunodeficiency virus (SIV)/rhesus macaque animal model and offer the possibility of studying correlates of protection against infection with virulent virus. We utilized a tether system for studying, in naive macaques and animals vaccinated with a live-attenuated vaccine, the acute events after challenge with pathogenic SIV. This approach allowed for the frequent sampling of small blood volumes without sedation or restraining of the animals, thus reducing the confounding effect of sampling stress. Before challenge, vaccinated animals presented significantly higher levels of proliferating and activated B cells than naive macaques, which were manifested by high expression of CD8 on B cells. After SIV challenge, the only changes observed in protected vaccinated macaques were significant increases in expression of the NK marker NKG2C on CD4 and CD8 T cells. We also identified that infection of naive macaques with SIV resulted in a transient peak of expression of CD20 on CD8 T cells and a constant rise in the number of B cells expressing CD8. Finally, analysis of a larger cohort of vaccinated animals identified that, even when circulating levels of vaccine virus are below the limit of detection, live attenuated vaccines induce systemic increases of IP-10 and perforin. These studies indicate that components of both the innate and adaptive immune systems of animals inoculated with a live-attenuated SIV vaccine respond to and control infection with virulent virus. Persistence of the vaccine virus in tissues may explain the elevated cytokine and B-cell activation levels. In addition, our report underpins the utility of the tether system for the intensive study of acute immune responses to viral infections.

Post-translational intracellular trafficking determines the type of immune response elicited by DNA vaccines expressing Gag antigen of Human Immunodeficiency Virus Type 1 (HIV-1)

In the current study, immune responses induced by Gag DNA vaccines with different designs were evaluated in Balb/C mice. The results demonstrated that the DNA vaccine with the full length wild type gag gene (Wt-Gag) mainly produced Gag antigens intracellularly and induced a higher level of cell-mediated immune (CMI) responses, as measured by IFN-gamma ELISPOT, intracellular cytokine staining (ICS), and cytotoxic T lymphocytes (CTL) assays against a dominant CD8(+) T cell epitope (AMQMLKETI). In contrast, the addition of a tissue plasminogen activator (tPA) leader sequence significantly improved overall Gag protein expression/secretion and Gag-specific antibody responses; however, Gag-specific CMI responses were decreased. The mutation of zinc-finger motif changed Gag protein expression patterns and reduced the ability to generate both CMI and antibody responses against Gag. These findings indicate that the structure and post-translational processing of antigens expressed by DNA vaccines play a critical role in eliciting optimal antibody or CMI responses.

Increased cellular immune responses and CD4+ T-cell proliferation correlate with reduced plasma viral load in SIV challenged recombinant simian varicella virus - simian immunodeficiency virus (rSVV-SIV) vaccinated rhesus macaques

Background

An effective AIDS vaccine remains one of the highest priorities in HIV-research. Our recent study showed that vaccination of rhesus macaques with recombinant simian varicella virus (rSVV) vector – simian immunodeficiency virus (SIV) envelope and gag genes, induced neutralizing antibodies and cellular immune responses to SIV and also significantly reduced plasma viral loads following intravenous pathogenic challenge with SIV_MAC251/CX1.

Findings

The purpose of this study was to define cellular immunological correlates of protection in rSVV-SIV vaccinated and SIV challenged animals. Immunofluorescent staining and multifunctional assessment of SIV-specific T-cell responses were evaluated in both Experimental and Control vaccinated animal groups. Significant increases in the proliferating CD4+ T-cell population and polyfunctional T-cell responses were observed in all Experimental-vaccinated animals compared with the Control-vaccinated animals.

Conclusions

Increased CD4+ T-cell proliferation was significantly and inversely correlated with plasma viral load. Increased SIV-specific polyfunctional cytokine responses and increased proliferation of CD4+ T-cell may be crucial to control plasma viral loads in vaccinated and SIV_MAC251/CX1 challenged macaques.

Immunogenicity of viral vector, prime-boost SIV vaccine regimens in infant rhesus macaques: attenuated vesicular stomatitis virus (VSV) and modified vaccinia Ankara (MVA) recombinant SIV vaccines compared to live-attenuated SIV

In a previously developed infant macaque model mimicking HIV infection by breast-feeding, we demonstrated that intramuscular immunization with recombinant poxvirus vaccines expressing simian immunodeficiency virus (SIV) structural proteins provided partial protection against infection following oral inoculation with virulent SIV. In an attempt to further increase systemic but also local antiviral immune responses at the site of viral entry, we tested the immunogenicity of different orally administered, replicating vaccines. One group of newborn macaques received an oral prime immunization with a recombinant vesicular stomatitis virus expressing SIVmac239 gag, pol and env (VSV-SIVgpe), followed 2 weeks later by an intramuscular boost immunization with MVA-SIV. Another group received two immunizations with live-attenuated SIVmac1A11, administered each time both orally and intravenously. Control animals received mock immunizations or non-SIV VSV and MVA control vectors. Analysis of SIV-specific immune responses in blood and lymphoid tissues at 4 weeks of age demonstrated that both vaccine regimens induced systemic antibody responses and both systemic and local cell-mediated immune responses. The safety and immunogenicity of the VSV-SIVgpe+MVA-SIV immunization regimen described in this report provide the scientific incentive to explore the efficacy of this vaccine regimen against virulent SIV exposure in the infant macaque model.

Control of viremia and maintenance of intestinal CD4(+) memory T cells in SHIV(162P3) infected macaques after pathogenic SIV(MAC251) challenge

Recent HIV vaccine failures have prompted calls for more preclinical vaccine testing in non-human primates. However, similar to HIV infection of humans, developing a vaccine that protects macaques from infection following pathogenic SIV(MAC251) challenge has proven difficult, and current vaccine candidates at best, only reduce viral loads after infection. Here we demonstrate that prior infection with a chimeric simian-human immunodeficiency virus (SHIV) containing an HIV envelope gene confers protection against intravenous infection with the heterologous, highly pathogenic SIV(MAC251) in rhesus macaques. Although definitive immune correlates of protection were not identified, preservation and/or restoration of intestinal CD4(+) memory T cells were associated with protection from challenge and control of viremia. These results suggest that protection against pathogenic lentiviral infection or disease progression is indeed possible, and may correlate with preservation of mucosal CD4(+) T cells.

Immunity and protection by live attenuated HIV/SIV vaccines

Live attenuated virus vaccines have shown the greatest potential to protect against simian immunodeficiency virus (SIV) infection, a model for human immunodeficiency virus (HIV). Immunity against the vaccine virus is thought to mediate protection. However, it is shown computationally that the opposite might be true. According to the model, the initial growth of the challenge strain, its peak load, and its potential to be pathogenic is higher if immunity against the vaccine virus is stronger. This is because the initial growth of the challenge strain is mainly determined by virus competition rather than immune suppression. The stronger the immunity against the vaccine strain, the weaker its competitive ability relative to the challenge strain, and the lower the level of protection. If the vaccine virus does protect the host against a challenge, it is because the competitive interactions between the viruses inhibit the initial growth of the challenge strain. According to these arguments, an inverse correlation between the level of attenuation and the level of protection is expected, and this has indeed been observed in experimental data.

Novel application of nonhuman primate tethering system for evaluation of acute phase SIVmac251 infection in rhesus macaques (Macaca mulatta)

Infection of rhesus macaques with simian immunodeficiency virus (SIV) is the preferred animal model for the development and testing of human immunodeficiency virus (HIV) vaccines, and animals protected from SIV challenge by live attenuated vaccines are an invaluable tool for determining immune correlates of protection. The acute phase of SIV infection, in which immune responses are most critical for slowing disease progression, occurs within the first 4 weeks of exposure. The small window of time available for observing critical immune responses makes obtaining adequate blood samples with sufficient frequency difficult. This study is the first to apply a previously reported nonhuman primate (NHP) tether system to study viral immunology. The use of the tether allows for frequent blood sampling without using restraints or sedation, thereby reducing the potentially confounding physiological changes induced by stress. We performed comparative analysis of acute phase immune responses in vaccinated and unvaccinated animals challenged with SIV-mac251. Our results demonstrate live attenuated vaccine-induced protection, which is associated with small increases in the cytotoxic T-cell (CTL) response to immunodominant epitopes, but not with increases in antibody titers. Additionally, vaccination was shown to establish a pool of antigen-specific CD8+ memory cells available for expansion after challenge. The confirmatory nature of these data indicates the validity of using the tether system for evaluation of acute phase anti-SIV responses and can be applied to the study of immune responses in other viral infections in which frequent sampling in small windows of time would be useful.

Immune mechanisms associated with protection from vaginal SIV challenge in rhesus monkeys infected with virulence-attenuated SHIV 89.6

Although live-attenuated human immunodeficiency virus-1 (HIV) vaccines may never be used clinically, these vaccines have provided the most durable protection from intravenous (IV) challenge in the simian immunodeficiency virus (SIV)/rhesus macaque model. Systemic infection with virulence attenuated-simian-human immunodeficiency virus (SHIV) 89.6 provides protection against vaginal SIV challenge. This paper reviews the findings related to the innate and adaptive immune responses and the role of inflammation associated with protection in the SHIV 89.6/SIVmac239 model. By an as yet undefined mechanism, most monkeys vaccinated with live-attenuated SHIV 89.6 mounted effective anti-viral CD8+ T cell responses while avoiding the self-destructive inflammatory cycle found in the lymphoid tissues of unprotected and unvaccinated monkeys.

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

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

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

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

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.

Mucosal DNA vaccination with highly attenuated Shigella is superior to attenuated Salmonella and comparable to intramuscular DNA vaccination for T cells against HIV

An immunization strategy using attenuated bacteria to deliver DNA vaccine plasmids to mucosal sites may induce protective T cell responses against sexual HIV transmission. In a murine intranasal (i.n.) immunization model, we demonstrate that transiently persistent Deltaasd Shigella flexneri strain 15D harboring DNA vaccines induces HIV- and SIV-specific gamma interferon (IFN-gamma) producing CD8+ T cells among splenocytes more efficiently than either a longer persisting DeltaaroD Salmonella typhimurium strain SL7207 or transiently persistent S. typhi strain Ty21a harboring DNA vaccines. Also, the frequency of antigen-specific gamma interferon (IFN-gamma) producing cells induced by Shigella 15D harboring a DNA vaccine were comparable to that induced by intramuscular (i.m.) immunization with purified DNA vaccine. Moreover, the magnitude of mucosal and systemic antigen-specific IgA and IgG responses after immunization were dependent upon the route (i.m. vs. i.n.) of inoculation, with i.n. Shigella 15D DNA vaccines generating higher levels of HIV-specific IgA in vaginal washings than i.m. purified DNA vaccine. Deltaasd S. flexneri is a promising vector for mucosal DNA vaccine immunization against HIV.

Short- and long-term clinical outcomes in rhesus monkeys inoculated with a highly pathogenic chimeric simian/human immunodeficiency virus

A highly pathogenic simian/human immunodeficiency virus (SHIV), SHIV(DH12R), isolated from a rhesus macaque that had been treated with anti-human CD8 monoclonal antibody at the time of primary infection with the nonpathogenic, molecularly cloned SHIV(DH12), induced marked and rapid CD4(+) T cell loss in all rhesus macaques intravenously inoculated with 1.0 50% tissue culture infective dose (TCID(50)) to 4.1 x 10(5) TCID(50)s of virus. Animals inoculated with 650 TCID(50)s of SHIV(DH12R) or more experienced irreversible CD4(+) T lymphocyte depletion and developed clinical disease requiring euthanasia between weeks 12 and 23 postinfection. In contrast, the CD4(+) T-cell numbers in four of five monkeys receiving 25 TCID(50)s of SHIV(DH12R) or less stabilized at low levels, and these surviving animals produced antibodies capable of neutralizing SHIV(DH12R). In the fifth monkey, no recovery from the CD4(+) T cell decline occurred, and the animal had to be euthanized. Viral RNA levels, subsequent to the initial peak of infection but not at peak viremia, correlated with the virus inoculum size and the eventual clinical course. Both initial infection rate constants, k, and decay constants, d, were determined, but only the latter were statistically correlated to clinical outcome. The attenuating effects of reduced inoculum size were also observed when virus was inoculated by the mucosal route. Because the uncloned SHIV(DH12R) stock possessed the genetic properties of a lentivirus quasispecies, we were able to assess the evolution of the input virus swarm in animals surviving the acute infection by monitoring the emergence of neutralization escape viral variants.

Feline immunodeficiency virus clade C mucosal transmission and disease courses

The transmissibility and pathogenicity of a clade C feline immunodeficiency virus (FIV-C) was examined via the oral-nasal, vaginal, or rectal mucosa. FIV-C was transmissible by all three mucosal routes. Vaginal transmission was most efficient (100%), oral exposure resulted in a 80% infection rate, and rectal transmission was least effective (44%). In contrast to previous intravenous passage studies, a broader range of host-virus relationships was observed after mucosal exposure. Three categories of FIV-C infection were defined: (1) rapidly progressive infection marked by high virus burdens and rapid CD4+ cell depletion (43% of vaginally exposed animals); (2) conventional (typical) infection featuring slowly progressive CD4+ cell decline (61% of all exposed animals); and (3) regressive (transient) infection marked by low and then barely detectable virus burdens and no CD4+ cell alterations (22% of rectally inoculated cats). These disease courses appear to have parallels in mucosal HIV and SIV infections, emphasizing the importance of the virus-mucosa interface in lentiviral pathogenesis.

Retrospective analysis of viral load and SIV antibody responses in rhesus macaques infected with pathogenic SIV: predictive value for disease progression

The prognostic significance of SIV plasma viral load in macaques has not been well established, primarily owing to the small numbers of animals in experimental groups. In addition, many investigators have noted that animals that fail to develop an anti-SIV humoral response develop disease rapidly. To establish the prognostic significance of viral load and seroconversion, we retrospectively analyzed the plasma viral load and serology data from 74 rhesus macaques infected with SIVmac. Viral load was analyzed at three time points: in the peak (days 7-21), acute (days 30-55), and chronic (days 80-100) periods postinfection. High viral load in the peak and acute phases was associated with more rapid development of disease (p = 0.0086, p = 0.0004, respectively). We defined clinical outcome as rapid ( <1 year) or slow (> or =1 year) progression. When peak and acute viral loads were analyzed together, acute viral load was more strongly associated with rapid progression (p = 0.03). Slow progression was strongly associated with chronic viral loads below the median of 3.47 x 10(5) RNA copies/ml. Despite having preexisting anti-SIV antibodies, 7 of 23 vaccinated animals were rapid progressors. All unvaccinated animals that mounted a humoral response to SIV were slow progressors. Animals that received a formalin-fixed, microencapsulated SIV vaccine prior to infection had lower peak viral loads than unvaccinated animals (p = 0.0005), but developed disease at the same rate. Overall, in naive animals, viral load is an important prognostic indicator of the disease progression rate. We found that viral load measured during the chronic phase (days 80-100) of infection was most closely associated with disease progression. We also found that a formalin-fixed, microencapuslated SIV vaccine reduced viral load without affecting clinical outcome. This latter finding may have implications for the evaluation of HIV-1 human vaccine trials.

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.

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.

Nasal Immunization of Nonhuman Primates with Simian Immunodeficiency Virus p55gag and Cholera Toxin Adjuvant Induces Th1/Th2 Help for Virus-Specific Immune Responses in Reproductive Tissues

Female rhesus macaques were nasally immunized with p55gag (p55) of SIV and cholera toxin as a mucosal adjuvant. Nasal immunization induced Ag-specific IgA and IgG Abs in mucosal secretions (e.g., cervicovaginal secretions, rectal washes, and saliva) and serum. Furthermore, high numbers of p55-specific IgA and IgG Ab-forming cells were induced in mucosal effector sites, i.e., uterine cervix, intestinal lamina propria, and nasal passage. p55-specific CD4+ T cells in both systemic and mucosal compartments expressed IFN-γ and IL-2 (Th1-type)- as well as IL-5, IL-6, and IL-10 (Th2-type)-specific mRNA. Moreover, p55-specific CTL activity was demonstrated in lymphocytes from blood, tonsils, and other lymphoid tissues. These results show that nasal immunization with SIV p55 with cholera toxin elicits both Th1- and selective Th2-type cytokine responses associated with the induction of SIV-specific mucosal and serum Abs, and CTL activity. These results offer a promise for the development of protective mucosal immunity to SIV.

In vivo replication capacity rather than in vitro macrophage tropism predicts efficiency of vaginal transmission of simian immunodeficiency virus or simian/human immunodeficiency virus in rhesus macaques

We used the rhesus macaque model of heterosexual human immunodeficiency virus (HIV) transmission to test the hypothesis that in vitro measures of macrophage tropism predict the ability of a primate lentivirus to initiate a systemic infection after intravaginal inoculation. A single atraumatic intravaginal inoculation with a T-cell-tropic molecular clone of simian immunodeficiency virus (SIV), SIVmac239, or a dualtropic recombinant molecular clone of SIV, SIVmac239/1A11/239, or uncloned dualtropic SIVmac251 or uncloned dualtropic simian/human immunodeficiency virus (SHIV) 89.6-PD produced systemic infection in all rhesus macaques tested. However, vaginal inoculation with a dualtropic molecular clone of SIV, SIVmac1A11, resulted in transient viremia in one of two rhesus macaques. It has previously been shown that 12 intravaginal inoculations with SIVmac1A11 resulted in infection of one of five rhesus macaques (M. L. Marthas, C. J. Miller, S. Sutjipto, J. Higgins, J. Torten, B. L. Lohman, R. E. Unger, H. Kiyono, J. R. McGhee, P. A. Marx, and N. C. Pedersen, J. Med. Primatol. 21:99-107, 1992). In addition, SHIV HXBc2, which replicates in monkey macrophages, does not infect rhesus macaques following multiple vaginal inoculations, while T-cell-tropic SHIV 89.6 does (Y. Lu, P. B. Brosio, M. Lafaile, J. Li, R. G. Collman, J. Sodroski, and C. J. Miller, J. Virol. 70:3045-3050, 1996). These results demonstrate that in vitro measures of macrophage tropism do not predict if a SIV or SHIV will produce systemic infection after intravaginal inoculation of rhesus macaques. However, we did find that the level to which these viruses replicate in vivo after intravenous inoculation predicts the outcome of intravaginal inoculation with each virus.

A minimally replicative HIV-2 live-virus vaccine protects M. nemestrina from disease after HIV-2(287) challenge

M. nemestrina immunized with an apathogenic HIV-2 molecular clone (HIV-2KR) were protected from CD4 decline and disease upon challenge with HIV-2(287), after any immunizing virus could be detected. Higher but not lower inocula of HIV-2KR were protective against intravenous inoculation of either 10(5) or 10(1) TCID50 of HIV-2(287). Protected animals displayed substantial reductions in PBMC proviral burden (1-3 logs), viral titers (1-2 logs), and plasma viral RNA (2-4 logs) compared to unprotected or naive animals as early as 1 week postinfection. Plasma viral RNA became undetectable after 24 weeks in protected animals, but remained high in unprotected animals. No viral RNA was present in the spleen of the protected animal necropsied more than a year after challenge (though viral DNA was still present). No neutralizing responses could be demonstrated, but CTL activity was detected sooner and at higher levels after challenge in protected than in unprotected macaques. In this novel HIV-2 vaccine model, protection was clearly dose-dependent, and clearance of challenge virus RNA from the plasma did not require detectable ongoing replication of the immunizing virus at the time of challenge.

Characterization of a molecular clone of HIV type 2 infectious for Macaca nemestrina

A lambda phage clone containing a full-length HIV-2 provirus, designated HIV-2KR, was obtained from the genomic DNA of Molt4 clone 8 (Molt4/8) lymphoblastic cells infected with the HIV-2PEI2 strain. HIV-2KR is genetically distinct from known HIV-2 isolates, possessing both a unique deletion in the LTR promoter region, and a long rev reading frame. It is replication competent in vitro after transfection into Molt4/8 cells, replicates in a variety of established human T lymphoblastic (Molt-3, Molt4/8, SupT1, H9, C8166) and myelomonocytic (U937) cell lines, and displays prominent cytopathic effects on infection of Molt4/8 cells, reflecting usage of both CCR5 and CXCR4 coreceptors. In addition, HIV-2KR was found to be infectious for human and Macaca nemestrina peripheral blood lymphocytes, and primary human monocyte-macrophage cultures. Intravenous inoculation of cell-free virus into M. nemestrina resulted in infection characterized by transient, low-level viremia and modest temporary decline in CD4 lymphocyte numbers, making HIV-2KR the first HIV-2 molecular clone reported to be infectious for this primate species.

Infection and AIDS in adult macaques after nontraumatic oral exposure to cell-free SIV

Unprotected receptive anal intercourse is a well-recognized risk factor for infection with human immunodeficiency virus-type 1 (HIV-1). Isolated human case reports have implicated HIV-1 transmission by oral-genital exposure. Adult macaques exposed nontraumatically to cell-free simian immunodeficiency virus (SIV) through the oral route became infected and developed acquired immunodeficiency syndrome (AIDS). The minimal virus dose needed to achieve systemic infection after oral exposure was 6000 times lower than the minimal dose required to achieve systemic infection after rectal exposure. Thus, unprotected receptive oral intercourse, even in the absence of mucosal lesions, should be added to the list of risk behaviors for HIV-1 transmission.

Cross-protective immune responses induced in rhesus macaques by immunization with attenuated macrophage-tropic simian immunodeficiency virus

The simian immunodeficiency virus (SIV) macaque model of AIDS has provided a valuable system with which to investigate vaccine approaches for protection against human immunodeficiency virus type 1 (HIV-1) infection. In particular, the ability of macaques persistently infected with attenuated infectious molecular clones of SIV to resist challenge with the pathogenic parental swarm has conclusively demonstrated that protective immunity can be achieved by immunization prior to exposure. The breadth of these protective responses and the immunological correlates of protection, however, have not been identified. In addition, vaccine studies have mainly employed lymphocyte-tropic strains of HIV-1 and SIV. Recent studies have implicated macrophage-tropic strains in the transmission of HIV-1 and have suggested that these virus strains should be examined in vaccine strategies. Macrophage-tropic viruses may confer additional advantages in the induction of protective immunity by replication in antigen-presenting cells. In this study, the immune response of rhesus macaques inoculated with an attenuated macrophage-tropic recombinant of SIVmac239 (SIV/17E-Cl) was evaluated with respect to protective immunity by heterologous challenge at various times after infection. Vigorous type-specific neutralizing-antibody responses restricted to SIV/17E-Cl were evident by 2 weeks postinfection. By 7 months, however, cross-reactive neutralizing antibodies emerged which neutralized not only SIV/17E-Cl but also the heterologous primary isolate SIV/DeltaB670. Challenge of SIV/17E-Cl-infected monkeys with SIV/DeltaB670 at various times postinfection demonstrated that protective responses were associated with the appearance of cross-reactive neutralizing antibodies. Furthermore, passive transfer of sera from SIV/17E-Cl-infected animals passively protected two of four naive recipients.

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.

Mucosal immunization with a live, virulence-attenuated simian immunodeficiency virus (SIV) vaccine elicits antiviral cytotoxic T lymphocytes and antibodies in rhesus macaques

An effective AIDS vaccine must protect against sexual transmission of human immunodeficiency virus (HIV). Therefore, vaccine regimens which stimulate antiviral immunity in the genital tract as well as in peripheral blood and systemic lymphoid tissues are needed. Here, we describe a method of immunization by direct inoculation of the vaginal submucosa with a live attenuated SIV, SIVmac1A11. Immunization by this route generated low levels of SIV-specific IgG and IgA antibodies in serum and vaginal secretions and viral specific cytotoxic T lymphocyte (CTL) activity in peripheral blood.

Reduced virus load in rhesus macaques immunized with recombinant gp160 and challenged with simian immunodeficiency virus

As a safe alternative to inactivated and live-attenuated whole-virus SIV vaccines, we have evaluated the potential of SIVmac239 gp160 expressed by recombinant vaccinia virus (vSIVgp160) and baculovirus (bSIVgp160) to protectively immunize rhesus macaques against intravenous (i.v.) infection with pathogenic SIVmac isolates. Macaques were immunized with live vSIVgp160 and/or bSIVgp160 protein partially purified from insect cells. The challenge viruses, propagated in rhesus peripheral blood mononuclear cells, consisted of the molecular clone SIVmac239 and another genetically similar, uncloned isolate, SIVmac251. Although antibodies that bind gp130 were induced in all animals following immunization with SIVgp160, neutralizing antibodies were undetectable 1 week prior to virus challenge. These results differ from those for macaques vaccinated with inactivated, whole SIV. All animals became infected after i.v. inoculation with 1-10 AID50 of either challenge virus. For animals challenged with SIVmac251, but not those challenged with SIVmac239, the cell-free infectious virus load in plasma of vSIVgp160-primed, bSIVgp160-boosted macaques was significantly lower than in unimmunized controls at 2 weeks postchallenge. Virus virulence, immunization regimen, and challenge with homologous or heterologous virus are factors critical to the outcome of the study. Immunization with surface glycoprotein may not necessarily provide protective immunity against infection but may reduce virus load. The relationship between reduction in virus load by vaccination and delay in onset of disease remains to be determined.