Rapid progression is associated with lymphoid follicle dysfunction in SIV-infected infant rhesus macaques.

HIV-infected infants are at an increased risk of progressing rapidly to AIDS in the first weeks of life. Here, we evaluated immunological and virological parameters in 25 SIV-infected infant rhesus macaques to understand the factors influencing a rapid disease outcome. Infant macaques were infected with SIVmac251 and monitored for 10 to 17 weeks post-infection. SIV-infected infants were divided into either typical (TypP) or rapid (RP) progressor groups based on levels of plasma anti-SIV antibody and viral load, with RP infants having low SIV-specific antibodies and high viral loads. Following SIV infection, 11 out of 25 infant macaques exhibited an RP phenotype. Interestingly, TypP had lower levels of total CD4 T cells, similar reductions in CD4/CD8 ratios and elevated activation of CD8 T cells, as measured by the levels of HLA-DR, compared to RP. Differences between the two groups were identified in other immune cell populations, including a failure to expand activated memory (CD21-CD27+) B cells in peripheral blood in RP infant macaques, as well as reduced levels of germinal center (GC) B cells and T follicular helper (Tfh) cells in spleens (4- and 10-weeks post-SIV). Reduced B cell proliferation in splenic germinal GCs was associated with increased SIV+ cell density and follicular type 1 interferon (IFN)-induced immune activation. Further analyses determined that at 2-weeks post SIV infection TypP infants exhibited elevated levels of the GC-inducing chemokine CXCL13 in plasma, as well as significantly lower levels of viral envelope diversity compared to RP infants. Our findings provide evidence that early viral and immunologic events following SIV infection contributes to impairment of B cells, Tfh cells and germinal center formation, ultimately impeding the development of SIV-specific antibody responses in rapidly progressing infant macaques.

Massively parallel de novo protein design for targeted therapeutics.

De novo protein design holds promise for creating small stable proteins with shapes customized to bind therapeutic targets. We describe a massively parallel approach for designing, manufacturing and screening mini-protein binders, integrating large-scale computational design, oligonucleotide synthesis, yeast display screening and next-generation sequencing. We designed and tested 22,660 mini-proteins of 37-43 residues that target influenza haemagglutinin and botulinum neurotoxin B, along with 6,286 control sequences to probe contributions to folding and binding, and identified 2,618 high-affinity binders. Comparison of the binding and non-binding design sets, which are two orders of magnitude larger than any previously investigated, enabled the evaluation and improvement of the computational model. Biophysical characterization of a subset of the binder designs showed that they are extremely stable and, unlike antibodies, do not lose activity after exposure to high temperatures. The designs elicit little or no immune response and provide potent prophylactic and therapeutic protection against influenza, even after extensive repeated dosing.

Transient Immune Activation in BCG-Vaccinated Infant Rhesus Macaques Is Not Sufficient to Influence Oral Simian Immunodeficiency Virus Infection.

BCG vaccination has been demonstrated to increase levels of activated CD4+ T cells, thus potentially influencing mother-to-child transmission of human immunodeficiency virus (HIV). To assess the risk of BCG vaccination in HIV infection, we randomly assigned newborn rhesus macaques to receive BCG vaccine or remain unvaccinated and then undergo oral simian immunodeficiency virus (SIV) challenges 3 weeks later. We observed elevated levels of activated peripheral CD4+ T cells (ie, HLA-DR+CD38+CCR5+ CD4+ T cells) by week 3 after vaccination. BCG was also associated with an altered immune gene expression profile, as well as with monocyte activation in both peripheral blood and the draining axillary lymph node, indicating significant BCG vaccine-induced immune activation. Despite these effects, BCG vaccination did not increase the rate of SIV oral transmission or disease progression. Our findings therefore identify patterns of T-cell and monocyte activation that occur after BCG vaccination but do not support the hypothesis that BCG vaccination is a risk factor for postnatal HIV transmission or increased pathogenesis in infants.

An Alphavirus-derived replicon RNA vaccine induces SARS-CoV-2 neutralizing antibody and T cell responses in mice and nonhuman primates.

The coronavirus disease 2019 (COVID-19) pandemic, caused by infection with the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), is having a deleterious impact on health services and the global economy, highlighting the urgent need for an effective vaccine. Such a vaccine would need to rapidly confer protection after one or two doses and would need to be manufactured using components suitable for scale up. Here, we developed an Alphavirus-derived replicon RNA vaccine candidate, repRNA-CoV2S, encoding the SARS-CoV-2 spike (S) protein. The RNA replicons were formulated with lipid inorganic nanoparticles (LIONs) that were designed to enhance vaccine stability, delivery, and immunogenicity. We show that a single intramuscular injection of the LION/repRNA-CoV2S vaccine in mice elicited robust production of anti-SARS-CoV-2 S protein IgG antibody isotypes indicative of a type 1 T helper cell response. A prime/boost regimen induced potent T cell responses in mice including antigen-specific responses in the lung and spleen. Prime-only immunization of aged (17 months old) mice induced smaller immune responses compared to young mice, but this difference was abrogated by booster immunization. In nonhuman primates, prime-only immunization in one intramuscular injection site or prime/boost immunizations in five intramuscular injection sites elicited modest T cell responses and robust antibody responses. The antibody responses persisted for at least 70 days and neutralized SARS-CoV-2 at titers comparable to those in human serum samples collected from individuals convalescing from COVID-19. These data support further development of LION/repRNA-CoV2S as a vaccine candidate for prophylactic protection against SARS-CoV-2 infection.

Therapeutic conserved elements (CE) DNA vaccine induces strong T-cell responses against highly conserved viral sequences during simian-human immunodeficiency virus infection.

HIV-specific T-cell responses play a key role in controlling HIV infection, and therapeutic vaccines for HIV that aim to improve viral control will likely need to improve on the T-cell responses induced by infection. However, in the setting of chronic infection, an effective therapeutic vaccine must overcome the enormous viral genetic diversity and the presence of pre-existing T-cell responses that are biased toward immunodominant T-cell epitopes that can readily mutate to evade host immunity and thus potentially provide inferior protection. To address these issues, we investigated a novel, epidermally administered DNA vaccine expressing SIV capsid (p27Gag) homologues of highly conserved elements (CE) of the HIV proteome in macaques experiencing chronic but controlled SHIV infection. We assessed the ability to boost or induce de novo T-cell responses against the conserved but immunologically subdominant CE epitopes. Two groups of animals were immunized with either the CE DNA vaccine or a full-length SIV p57gag DNA vaccine. Prior to vaccination, CE responses were similar in both groups. The full-length p57gag DNA vaccine, which contains the CE, increased overall Gag-specific responses but did not increase CE responses in any animals (0/4). In contrast, the CE DNA vaccine increased CE responses in all (4/4) vaccinated macaques. In SIV infected but unvaccinated macaques, those that developed stronger CE-specific responses during acute infection exhibited lower viral loads. We conclude that CE DNA vaccination can re-direct the immunodominance hierarchy towards CE in the setting of attenuated chronic infection and that induction of these responses by therapeutic vaccination may improve immune control of HIV.

Multigenic DNA vaccine induces protective cross-reactive T cell responses against heterologous influenza virus in nonhuman primates.

Recent avian and swine-origin influenza virus outbreaks illustrate the ongoing threat of influenza pandemics. We investigated immunogenicity and protective efficacy of a multi-antigen (MA) universal influenza DNA vaccine consisting of HA, M2, and NP antigens in cynomolgus macaques. Following challenge with a heterologous pandemic H1N1 strain, vaccinated animals exhibited significantly lower viral loads and more rapid viral clearance when compared to unvaccinated controls. The MA DNA vaccine induced robust serum and mucosal antibody responses but these high antibody titers were not broadly neutralizing. In contrast, the vaccine induced broadly-reactive NP specific T cell responses that cross-reacted with the challenge virus and inversely correlated with lower viral loads and inflammation. These results demonstrate that a MA DNA vaccine that induces strong cross-reactive T cell responses can, independent of neutralizing antibody, mediate significant cross-protection in a nonhuman primate model and further supports development as an effective approach to induce broad protection against circulating and emerging influenza strains.

Minor Antigen Vaccine-Sensitized DLI: In Vitro Responses Do Not Predict In Vivo Effects.

UNLABELLED: We reported on a pilot study of minor histocompatibility antigen vaccination using constructs expressing male-specific gene disparities of selected mouse CDNA on Y and sex determining region Y in the canine model. We performed reduced-intensity hematopoietic cell transplantation with female donors and male recipients, producing stable mixed donor-recipient hematopoietic chimeras. We then performed a vaccine series in three female transplant donors followed by donor lymphocyte infusion (DLI) into their respective mixed chimeras. One mixed chimera experienced a significant shift in the percentage of donor chimerism, but no response occurred in the other 2 recipients. We then hypothesized that inadequate donor sensitization was responsible for these results. METHODS: To test this hypothesis, we added 4 monthly booster vaccinations to 2 of the original hematopoietic cell transplantation donors, including the donor that drove the partial response, followed by a second DLI. RESULTS: Strong T cell responses were shown by ELISpot and confirmed by intracellular cytokine staining in both donors. A second DLI resulted in a further increase in donor chimerism in the same mixed chimera that experienced the previous increase, but no change in donor chimerism was again seen in the other recipient. Evaluation of RNA expression of the target antigens demonstrated that conversion occurred in the recipient that expressed both selected mouse CDNA on Y and sex determining region Y. CONCLUSIONS: T cell responses against Y chromosome-encoded disparities were not necessarily sufficient to drive in vivo female antimale responses. Other factors including the presence of specific haplotypes or the heterogeneous expression of the target antigen may affect T cell responses against minor histocompatibility antigens. These results warrant future vaccine studies in a larger transplant cohort using epigenetic modulation of the recipient to promote target gene expression.

Complex Minigene Library Vaccination for Discovery of Pre-Erythrocytic Plasmodium T Cell Antigens.

Development of a subunit vaccine targeting liver-stage Plasmodium parasites requires the identification of antigens capable of inducing protective T cell responses. However, traditional methods of antigen identification are incapable of evaluating T cell responses against large numbers of proteins expressed by these parasites. This bottleneck has limited development of subunit vaccines against Plasmodium and other complex intracellular pathogens. To address this bottleneck, we are developing a synthetic minigene technology for multi-antigen DNA vaccines. In an initial test of this approach, pools of long (150 bp) antigen-encoding oligonucleotides were synthesized and recombined into vectors by ligation-independent cloning to produce two DNA minigene library vaccines. Each vaccine encoded peptides derived from 36 (vaccine 1) and 53 (vaccine 2) secreted or transmembrane pre-erythrocytic P. yoelii proteins. BALB/cj mice were vaccinated three times with a single vaccine by biolistic particle delivery (gene gun) and screened for interferon-γ-producing T cell responses by ELISPOT. Library vaccination induced responses against four novel antigens. Naïve mice exposed to radiation-attenuated sporozoites mounted a response against only one of the four novel targets (PyMDH, malate dehydrogenase). The response to PyMDH could not be recalled by additional homologous sporozoite immunizations but could be partially recalled by heterologous cross-species sporozoite exposure. Vaccination against the dominant PyMDH epitope by DNA priming and recombinant Listeria boosting did not protect against sporozoite challenge. Improvements in library design and delivery, combined with methods promoting an increase in screening sensitivity, may enable complex minigene screening to serve as a high-throughput system for discovery of novel T cell antigens.

Development of a Minor Histocompatibility Antigen Vaccine Regimen in the Canine Model of Hematopoietic Cell Transplantation.

BACKGROUND: Minor histocompatibility antigen (miHA) vaccines have the potential to augment graft-versus-tumor effects without graft-versus-host disease (GVHD). We used mixed hematopoietic chimerism in the canine model of major histocompatibility complex-matched allogeneic hematopoietic cell transplantation as a platform to develop a miHA vaccination regimen. METHODS: We engineered DNA plasmids and replication-deficient human adenovirus type 5 constructs encoding large sections of canine SMCY and the entire canine SRY gene. RESULTS: Priming with replication-deficient human adenovirus type 5 constructs and boosting with ex vivo plasmid-transfected dendritic cells and cutaneous delivery of plasmids with a particle-mediated epidermal delivery device (PMED) in 2 female dogs induced antigen-specific T-cell responses. Similar responses were observed after a prime-boost vaccine regimen in three female hematopoietic cell transplantation donors. Subsequent donor lymphocyte infusion resulted in a significant change of chimerism in 1 of 3 male recipients without any signs of graft-versus-host disease. The change in chimerism in the recipient occurred in association with the development of CD4+ and CD8+ T-cell responses to the same peptide pools detected in the donor. CONCLUSIONS: These studies describe the first in vivo response to miHA vaccination in a large, outbred animal model without using recipient cells to sensitize the donor. This model provides a platform for ongoing experiments designed to define optimal miHA targets and develop protocols to directly vaccinate the recipient.

Optimizing particle-mediated epidermal delivery of an influenza DNA vaccine in ferrets.

Particle-mediated DNA delivery technologies ("gene guns") have been shown in both animal and clinical studies to be an effective means of increasing the immunogenicity and protective efficacy of DNA vaccines. The primary goal in optimizing particle-mediated epidermal delivery (PMED) vaccination in different animal models is to achieve delivery of DNA-coated gold beads into the viable epidermis. Two key para-meters that influence this outcome include the delivery pressure, which controls the penetrative force of the beads into the skin, and the anatomical site of DNA delivery. Although the ferret has been extensively used as an experimental model for influenza infection in humans, very few studies have investigated the capacity for PMED DNA vaccination to induce protective immune responses in ferrets. Here we describe methods to optimize DNA vaccine delivery using the PowderJect XR1 gene delivery in ferrets. We first assess the effects of firing pressure on both the delivery of DNA-coated gold beads into the desired epidermal layer and the degree of DNA vaccine reporter gene expression at the target site. Second, we evaluate the impact of vaccination site (skin or tongue) on DNA vaccine immunogenicity by measuring serum antibody responses to the model antigens influenza virus hemagglutinin and hepatitis B core antigen. Results from these studies support the use of the PowderJect XR1 device in ferrets for the study of prophylactic and therapeutic DNA vaccines against clinically important diseases such as influenza virus infection.

A novel tetrameric gp350 1-470 as a potential Epstein-Barr virus vaccine.

Infectious mononucleosis and B-cell transformation in response to infection with Epstein-Barr virus (EBV) is dependent upon binding of the EBV envelope glycoprotein gp350 to CD21 on B-cells. Gp350-specific antibody comprises most of the EBV neutralizing activity in the serum of infected patients, making this protein a promising target antigen for a prophylactic EBV vaccine. We describe a novel, tetrameric gp350-based vaccine that exhibits markedly enhanced immunogenicity relative to its monomeric counterpart. Plasmid DNA was constructed for synthesis, within transfected CHO cells, of a tetrameric, truncated (a.a. 1-470) gp350 protein (gp350(1-470)). Tetrameric gp350(1-470) induced ≈ 20-fold higher serum titers of gp350(1-470)-specific IgG and >19-fold enhancements in neutralizing titers at the highest dose, and was >25-fold more immunogenic on a per-weight basis than monomeric gp350(1-470). Further, epidermal immunization with plasmid DNA encoding gp350(1-470) tetramer induced 8-fold higher serum titers of gp350(1-470)-specific IgG relative to monomer. Tetrameric gp350(1-470) binding to human CD21 was >24-fold more efficient on a per-weight basis than monomer, but neither tetramer nor monomer mediated polyclonal human B-cell activation. Finally, the introduction of strong, universal tetanus toxoid (TT)-specific CD4+ T-cell epitopes into the tetrameric gp350(1-470) had no effect on the gp350(1-470)-specific IgG response in naïve mice, and resulted in suppressed gp350(1-470)-specific IgG responses in TT-primed mice. Collectively, these data suggest that tetrameric gp350(1-470) is a potentially promising candidate for testing as a prophylactic EBV vaccine, and that protein multimerization, using the approach described herein, is likely to be clinically relevant for enhancing the immunogenicity of other proteins of vaccine interest.

DNA/Ad5 vaccination with SIV epitopes induced epitope-specific CD4⁺ T cells, but few subdominant epitope-specific CD8⁺ T cells.

The goals of a T cell-based vaccine for HIV are to reduce viral peak and setpoint and prevent transmission. While it has been relatively straightforward to induce CD8(+) T cell responses against immunodominant T cell epitopes, it has been more difficult to broaden the vaccine-induced CD8(+) T cell response against subdominant T cell epitopes. Additionally, vaccine regimens to induce CD4(+) T cell responses have been studied only in limited settings. In this study, we sought to elicit CD8(+) T cells against subdominant epitopes and CD4(+) T cells using various novel and well-established vaccine strategies. We vaccinated three Mamu-A*01(+) animals with five Mamu-A*01-restricted subdominant SIV-specific CD8(+) T cell epitopes. All three vaccinated animals made high frequency responses against the Mamu-A*01-restricted Env TL9 epitope with one animal making a low frequency CD8(+) T cell response against the Pol LV10 epitope. We also induced SIV-specific CD4(+) T cells against several MHC class II DRBw*606-restricted epitopes. Electroporated DNA with pIL-12 followed by a rAd5 boost was the most immunogenic vaccine strategy. We induced responses against all three Mamu-DRB*w606-restricted CD4 epitopes in the vaccine after the DNA prime. Ad5 vaccination further boosted these responses. Although we successfully elicited several robust epitope-specific CD4(+) T cell responses, vaccination with subdominant MHC class I epitopes elicited few detectable CD8(+) T cell responses. Broadening the CD8(+) T cell response against subdominant MHC class I epitopes was, therefore, more difficult than we initially anticipated.

GM-CSF increases mucosal and systemic immunogenicity of an H1N1 influenza DNA vaccine administered into the epidermis of non-human primates.

BACKGROUND: The recent H5N1 avian and H1N1 swine-origin influenza virus outbreaks reaffirm that the threat of a world-wide influenza pandemic is both real and ever-present. Vaccination is still considered the best strategy for protection against influenza virus infection but a significant challenge is to identify new vaccine approaches that offer accelerated production, broader protection against drifted and shifted strains, and the capacity to elicit anti-viral immune responses in the respiratory tract at the site of viral entry. As a safe alternative to live attenuated vaccines, the mucosal and systemic immunogenicity of an H1N1 influenza (A/New Caledonia/20/99) HA DNA vaccine administered by particle-mediated epidermal delivery (PMED or gene gun) was analyzed in rhesus macaques. METHODOLOGY/PRINCIPAL FINDINGS: Macaques were immunized at weeks 0, 8, and 16 using a disposable single-shot particle-mediated delivery device designed for clinical use that delivers plasmid DNA directly into cells of the epidermis. Significant levels of hemagglutination inhibiting (HI) antibodies and cytokine-secreting HA-specific T cells were observed in the periphery of macaques following 1-3 doses of the PMED HA DNA vaccine. In addition, HA DNA vaccination induced detectable levels of HA-specific mucosal antibodies and T cells in the lung and gut-associated lymphoid tissues of vaccinated macaques. Importantly, co-delivery of a DNA encoding the rhesus macaque GM-CSF gene was found to significantly enhance both the systemic and mucosal immunogenicity of the HA DNA vaccine. CONCLUSIONS/SIGNIFICANCE: These results provide strong support for the development of a particle-mediated epidermal DNA vaccine for protection against respiratory pathogens such as influenza and demonstrate, for the first time, the ability of skin-delivered GM-CSF to serve as an effective mucosal adjuvant for vaccine induction of immune responses in the gut and respiratory tract.

Immunogenicity of hybrid DNA vaccines expressing hepatitis B core particles carrying human and simian immunodeficiency virus epitopes in mice and rhesus macaques.

An effective HIV vaccine will likely need to induce broad and potent CTL responses. Epitope-based vaccines offer significant potential for inducing multi-specific CTL, but often require conjugation to T helper epitopes or carrier moieties to induce significant responses. We tested hybrid DNA vaccines encoding one or more HIV or SIV CTL epitopes fused to a hepatitis B core antigen (HBcAg) carrier gene as a means to improve the immunogenicity of epitope-based DNA vaccines. Immunization of mice with a HBcAg-HIV epitope DNA vaccine induced CD8(+) T cell responses that significantly exceeded levels induced with DNA encoding either the whole HIV antigen or the epitope alone. In rhesus macaques, a multi-epitope hybrid HBcAg-SIV DNA vaccine induced CTL responses to 13 different epitopes, including 3 epitopes that were previously not detected in SIV-infected macaques. These data demonstrate that immunization with hybrid HBcAg-epitope DNA vaccines is an effective strategy to increase the magnitude and breadth of HIV-specific CTL responses.

DNA immunization in combination with effective antiretroviral drug therapy controls viral rebound and prevents simian AIDS after treatment is discontinued.

DNA immunization in conjunction with antiretroviral therapy was evaluated in SIV-infected rhesus macaques treated with [R]-9-[2-phosphonylmethoxypropyl]adenine (PMPA). Macaques were immunized monthly with DNA vaccines expressing either SIV gag/tat or SIV gag/tat and 19 CD8+ T cell epitopes during 7 months of therapy. Half the animals from each group were additionally immunized before infection. Only 60% of the animals (4 controls, 20 vaccinated) responded to PMPA (ART responders). All 4 ART responder controls demonstrated viral rebound or CD4 decline after PMPA was withdrawn. In contrast, 17 of 20 vaccinated ART responders contained viral rebound for over 7 months after PMPA was withdrawn. Viral control correlated with stable CD4 counts, higher lymphoproliferation and an increase in the magnitude and breadth of the CD8+ T cell response. Immunizing before infection or with multi-epitopes enhanced these effects. These results demonstrate that DNA immunization during antiretroviral therapy may be an effective strategy to treat HIV infection.

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.

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

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

Immunization of rhesus macaques with a DNA prime/modified vaccinia virus Ankara boost regimen induces broad simian immunodeficiency virus (SIV)-specific T-cell responses and reduces initial viral replication but does not prevent disease progression following challenge with pathogenic SIVmac239.

Producing a prophylactic vaccine for human immunodeficiency virus (HIV) has proven to be a challenge. Most biological isolates of HIV are difficult to neutralize, so that conventional subunit-based antibody-inducing vaccines are unlikely to be very effective. In the rhesus macaque model, some protection was afforded by DNA/recombinant viral vector vaccines. However, these studies used as the challenge virus SHIV-89.6P, which is neutralizable, making it difficult to determine whether the observed protection was due to cellular immunity, humoral immunity, or a combination of both. In this study, we used a DNA prime/modified vaccinia virus Ankara boost regimen to immunize rhesus macaques against nearly all simian immunodeficiency virus (SIV) proteins. These animals were challenged intrarectally with pathogenic molecularly cloned SIVmac239, which is resistant to neutralization. The immunization regimen resulted in the induction of virus-specific CD8(+) and CD4(+) responses in all vaccinees. Although anamnestic neutralizing antibody responses against laboratory-adapted SIVmac251 developed after the challenge, no neutralizing antibodies against SIVmac239 were detectable. Vaccinated animals had significantly reduced peak viremia compared with controls (P < 0.01). However, despite the induction of virus-specific cellular immune responses and reduced peak viral loads, most animals still suffered from gradual CD4 depletion and progressed to disease.