Prevention of infection by a granulocyte-macrophage colony-stimulating factor co-expressing DNA/modified vaccinia Ankara simian immunodeficiency virus vaccine

Generation of an attenuated Tiantan vaccinia virus by deletion of the ribonucleotide reductase large subunit

Attenuation of the virulence of vaccinia Tiantan virus (VTT) underlies the strategy adopted for mass vaccination campaigns. This strategy provides advantages of safety and efficacy over traditional vaccines and is aimed at minimization of adverse health effects. In this study, a mutant form of the virus, MVTT was derived from VTT by deletion of the ribonucleotide reductase large subunit (R1) (TI4L). Compared to wild-type parental (VTT) and revertant (VTT-rev) viruses, virulence of the mutant MVTT was reduced by 100-fold based on body weight reduction and by 3,200-fold based on determination of the intracranial 50% lethal infectious dose. However, the immunogenicity of MVTT was equivalent to that of the parental VTT. We also demonstrated that the TI4L gene is not required for efficient replication. These data support the conclusion that MVTT can be used as a replicating virus vector or as a platform for the development of vaccines against infectious diseases and for cancer therapy.

Broad and potent cellular and humoral immune responses after a second late HIV-modified vaccinia virus ankara vaccination in HIV-DNA-primed and HIV-modified vaccinia virus Ankara-boosted Swedish vaccinees

We have previously shown that an HIV vaccine regimen including three HIV-DNA immunizations and a single HIV-modified vaccinia virus Ankara (MVA) boost was safe and highly immunogenic in Swedish volunteers. A median 38 months after the first HIV-MVA vaccination, 24 volunteers received 10(8) plaque-forming units of HIV-MVA. The vaccine was well tolerated. Two weeks after this HIV-MVA vaccination, 18 (82%) of 22 evaluable vaccinees were interferon (IFN)-γ enzyme-linked immunospot (ELISpot) reactive: 18 to Gag and 10 (45%) to Env. A median minimal epitope count of 4 to Gag or Env was found in a subset of 10 vaccinees. Intracellular cytokine staining revealed CD4(+) and/or CD8(+) T cell responses in 23 (95%) of 24 vaccinees, 19 to Gag and 19 to Env. The frequency of HIV-specific CD4(+) and CD8(+) T cell responses was equally high (75%). A high proportion of CD4(+) and CD8(+) T cell responses to Gag was polyfunctional with production of three or more cytokines (40% and 60%, respectively). Of the Env-specific CD4(+) T cells 40% were polyfunctional. Strong lymphoproliferative responses to Aldrithiol-2 (AT-2)-treated subtype A, B, C, and A_E virus were demonstrable in 21 (95%) of 22 vaccinees. All vaccinees developed binding antibodies to Env and Gag. Neutralizing antibodies were detected in a peripheral blood mononuclear cell (PBMC)-based assay against subtype B and CRF01_AE viruses. The neutralizing antibody response rates were influenced by the vaccine dose and/or mode of delivery used at the previous HIV-MVA vaccination. Thus, a second late HIV-MVA boost induced strong and broad cellular immune responses and improved antibody responses. The data support further exploration of this vaccine concept.

DNA/MVA Vaccines for HIV/AIDS

Since the initial proof-of-concept studies examining the ability of antigen-encoded plasmid DNA to serve as an immunogen, DNA vaccines have evolved as a clinically safe and effective platform for priming HIV-specific cellular and humoral responses in heterologous "prime-boost" vaccination regimens. Direct injection of plasmid DNA into the muscle induces T- and B-cell responses against foreign antigens. However, the insufficient magnitude of this response has led to the development of approaches for enhancing the immunogenicity of DNA vaccines. The last two decades have seen significant progress in the DNA-based vaccine platform with optimized plasmid constructs, improved delivery methods, such as electroporation, the use of molecular adjuvants and novel strategies combining DNA with viral vectors and subunit proteins. These innovations are paving the way for the clinical application of DNA-based HIV vaccines. Here, we review preclinical studies on the DNA-prime/modified vaccinia Ankara (MVA)-boost vaccine modality for HIV. There is a great deal of interest in enhancing the immunogenicity of DNA by engineering DNA vaccines to co-express immune modulatory adjuvants. Some of these adjuvants have demonstrated encouraging results in preclinical and clinical studies, and these data will be examined, as well.

Local control of repeated-dose rectal challenges in DNA/MVA-vaccinated macaques protected against a first series of simian immunodeficiency virus challenges

Here, we report the results of a late boost and three additional series of simian immunodeficiency virus (SIV) challenges in seven DNA/modified vaccinia virus Ankara (MVA)-vaccinated rhesus macaques who resisted a first series of rectal challenges. During 29 additional challenges delivered over 2.3 years, all animals became infected. However, 13 blips of virus in six macaques and anamnestic Env-specific rectal IgA responses in three of the six suggested that local control of infections was occurring during the serial challenge.

Recombinant adenovirus expressing the haemagglutinin of Peste des petits ruminants virus (PPRV) protects goats against challenge with pathogenic virus; a DIVA vaccine for PPR

Peste des petits ruminants virus (PPRV) is a morbillivirus that can cause severe disease in sheep and goats, characterised by pyrexia, pneumo-enteritis, and gastritis. The socio-economic burden of the disease is increasing in underdeveloped countries, with poor livestock keepers being affected the most. Current vaccines consist of cell-culture attenuated strains of PPRV, which induce a similar antibody profile to that induced by natural infection. Generation of a vaccine that enables differentiation of infected from vaccinated animals (DIVA) would benefit PPR control and eradication programmes, particularly in the later stages of an eradication campaign and for countries where the disease is not endemic. In order to create a vaccine that would enable infected animals to be distinguished from vaccinated ones (DIVA vaccine), we have evaluated the immunogenicity of recombinant fowlpox (FP) and replication-defective recombinant human adenovirus 5 (Ad), expressing PPRV F and H proteins, in goats. The Ad constructs induced higher levels of virus-specific and neutralising antibodies, and primed greater numbers of CD8⁺ T cells than the FP-vectored vaccines. Importantly, a single dose of Ad-H, with or without the addition of Ad expressing ovine granulocyte macrophage colony-stimulating factor and/or ovine interleukin-2, not only induced strong antibody and cell-mediated immunity but also completely protected goats against challenge with virulent PPRV, 4 months after vaccination. Replication-defective Ad-H therefore offers the possibility of an effective DIVA vaccine.

DNA Immunization for HIV Vaccine Development

DNA vaccination has been studied in the last 20 years for HIV vaccine research. Significant experience has been accumulated in vector design, antigen optimization, delivery approaches and the use of DNA immunization as part of a prime-boost HIV vaccination strategy. Key historical data and future outlook are presented. With better understanding on the potential of DNA immunization and recent progress in HIV vaccine research, it is anticipated that DNA immunization will play a more significant role in the future of HIV vaccine development.

A global approach to HIV-1 vaccine development

A global human immunodeficiency virus-1 (HIV-1) vaccine will have to elicit immune responses capable of providing protection against a tremendous diversity of HIV-1 variants. In this review, we first describe the current state of the HIV-1 vaccine field, outlining the immune responses that are desired in a global HIV-1 vaccine. In particular, we emphasize the likely importance of Env-specific neutralizing and non-neutralizing antibodies for protection against HIV-1 acquisition and the likely importance of effector Gag-specific T lymphocytes for virologic control. We then highlight four strategies for developing a global HIV-1 vaccine. The first approach is to design specific vaccines for each geographic region that include antigens tailor-made to match local circulating HIV-1 strains. The second approach is to design a vaccine that will elicit Env-specific antibodies capable of broadly neutralizing all HIV-1 subtypes. The third approach is to design a vaccine that will elicit cellular immune responses that are focused on highly conserved HIV-1 sequences. The fourth approach is to design a vaccine to elicit highly diverse HIV-1-specific responses. Finally, we emphasize the importance of conducting clinical efficacy trials as the only way to determine which strategies will provide optimal protection against HIV-1 in humans.

Immunological and virological mechanisms of vaccine-mediated protection against SIV and HIV

A major challenge for the development of a highly effective AIDS vaccine is the identification of mechanisms of protective immunity. To address this question, we used a nonhuman primate challenge model with simian immunodeficiency virus (SIV). We show that antibodies to the SIV envelope are necessary and sufficient to prevent infection. Moreover, sequencing of viruses from breakthrough infections revealed selective pressure against neutralization-sensitive viruses; we identified a two-amino-acid signature that alters antigenicity and confers neutralization resistance. A similar signature confers resistance of human immunodeficiency virus (HIV)-1 to neutralization by monoclonal antibodies against variable regions 1 and 2 (V1V2), suggesting that SIV and HIV share a fundamental mechanism of immune escape from vaccine-elicited or naturally elicited antibodies. These analyses provide insight into the limited efficacy seen in HIV vaccine trials.

Specificity and 6-month durability of immune responses induced by DNA and recombinant modified vaccinia Ankara vaccines expressing HIV-1 virus-like particles

BACKGROUND

Clade B DNA and recombinant modified vaccinia Ankara (MVA) vaccines producing virus-like particles displaying trimeric membrane-bound envelope glycoprotein (Env) were tested in a phase 2a trial in human immunodeficiency virus (HIV)-uninfected adults for safety, immunogenicity, and 6-month durability of immune responses.

METHODS

A total of 299 individuals received 2 doses of JS7 DNA vaccine and 2 doses of MVA/HIV62B at 0, 2, 4, and 6 months, respectively (the DDMM regimen); 3 doses of MVA/HIV62B at 0, 2, and 6 months (the MMM regimen); or placebo injections.

RESULTS

At peak response, 93.2% of the DDMM group and 98.4% of the MMM group had binding antibodies for Env. These binding antibodies were more frequent and of higher magnitude for the transmembrane subunit (gp41) than the receptor-binding subunit (gp120) of Env. For both regimens, response rates were higher for CD4(+) T cells (66.4% in the DDMM group and 43.1% in the MMM group) than for CD8(+) T cells (21.8% in the DDMM group and 14.9% in the MMM group). Responding CD4(+) and CD8(+) T cells were biased toward Gag, and >70% produced 2 or 3 of the 4 cytokines evaluated (ie, interferon γ, interleukin 2, tumor necrosis factor α, and granzyme B). Six months after vaccination, the magnitudes of antibodies and T-cell responses had decreased by <3-fold.

CONCLUSIONS

DDMM and MMM vaccinations with virus-like particle-expressing immunogens elicited durable antibody and T-cell responses.

Nonhuman primate models for HIV/AIDS vaccine development

The development of HIV vaccines has been hampered by the lack of an animal model that can accurately predict vaccine efficacy. Chimpanzees can be infected with HIV-1 but are not practical for research. However, several species of macaques are susceptible to the simian immunodeficiency viruses (SIVs) that cause disease in macaques, which also closely mimic HIV in humans. Thus, macaque-SIV models of HIV infection have become a critical foundation for AIDS vaccine development. Here we examine the multiple variables and considerations that must be taken into account in order to use this nonhuman primate (NHP) model effectively. These include the species and subspecies of macaques, virus strain, dose and route of administration, and macaque genetics, including the major histocompatibility complex molecules that affect immune responses, and other virus restriction factors. We illustrate how these NHP models can be used to carry out studies of immune responses in mucosal and other tissues that could not easily be performed on human volunteers. Furthermore, macaques are an ideal model system to optimize adjuvants, test vaccine platforms, and identify correlates of protection that can advance the HIV vaccine field. We also illustrate techniques used to identify different macaque lymphocyte populations and review some poxvirus vaccine candidates that are in various stages of clinical trials. Understanding how to effectively use this valuable model will greatly increase the likelihood of finding a successful vaccine for HIV.

Live attenuated Rev-independent Nef¯SIV enhances acquisition of heterologous SIVsmE660 in acutely vaccinated rhesus macaques

BACKGROUND

Rhesus macaques (RMs) inoculated with live-attenuated Rev-Independent Nef¯ simian immunodeficiency virus (Rev-Ind Nef¯SIV) as adults or neonates controlled viremia to undetectable levels and showed no signs of immunodeficiency over 6-8 years of follow-up. We tested the capacity of this live-attenuated virus to protect RMs against pathogenic, heterologous SIVsmE660 challenges.

METHODOLOGY/PRINCIPAL FINDINGS

Three groups of four RM were inoculated with Rev-Ind Nef¯SIV and compared. Group 1 was inoculated 8 years prior and again 15 months before low dose intrarectal challenges with SIVsmE660. Group 2 animals were inoculated with Rev-Ind Nef¯SIV at 15 months and Group 3 at 2 weeks prior to the SIVsmE660 challenges, respectively. Group 4 served as unvaccinated controls. All RMs underwent repeated weekly low-dose intrarectal challenges with SIVsmE660. Surprisingly, all RMs with acute live-attenuated virus infection (Group 3) became superinfected with the challenge virus, in contrast to the two other vaccine groups (Groups 1 and 2) (P=0.006 for each) and controls (Group 4) (P=0.022). Gene expression analysis showed significant upregulation of innate immune response-related chemokines and their receptors, most notably CCR5 in Group 3 animals during acute infection with Rev-Ind Nef¯SIV.

CONCLUSIONS/SIGNIFICANCE

We conclude that although Rev-Ind Nef¯SIV remained apathogenic, acute replication of the vaccine strain was not protective but associated with increased acquisition of heterologous mucosal SIVsmE660 challenges.

Vaccination with Vaxfectin(®) adjuvanted SIV DNA induces long-lasting humoral immune responses able to reduce SIVmac251 Viremia

We evaluated the immunogenicity and efficacy of Vaxfectin(®) adjuvanted SIV DNA vaccines in mice and macaques. Vaccination of mice with Vaxfectin(®) adjuvanted SIV gag DNA induced higher humoral immune responses than administration of unadjuvanted DNA, whereas similar levels of cellular immunity were elicited. Vaxfectin(®) adjuvanted SIVmac251 gag and env DNA immunization of rhesus macaques was used to examine magnitude, durability, and efficacy of humoral immunity. Vaccinated macaques elicited potent neutralizing antibodies able to cross-neutralize the heterologous SIVsmE660 Env. We found remarkable durability of Gag and Env humoral responses, sustained during ~2 y of follow-up. The Env-specific antibody responses induced by Vaxfectin(®) adjuvanted env DNA vaccination disseminated into mucosal tissues, as demonstrated by their presence in saliva, including responses to the V1-V2 region, and rectal fluids. The efficacy of the immune responses was evaluated upon intrarectal challenge with low repeated dose SIVmac251. Although 2 of the 3 vaccinees became infected, these animals showed significantly lower peak virus loads and lower chronic viremia than non-immunized infected controls. Thus, Vaxfectin(®) adjuvanted DNA is a promising vaccine approach for inducing potent immune responses able to control the highly pathogenic SIVmac251.

Fc receptor-mediated immune responses: new tools but increased complexity in HIV prevention

The modest success of the RV144 HIV vaccine trial in Thailand and the ensuing suggestion that a Fc-receptormediated antibody activity might have played a role in the protection observed have intensified investigations on Fcrelated immune responses. HIV neutralizing antibodies have been and continue to be the focal point of research into humoral immune protection. However, recent knowledge that their protective efficacy can be augmented by Fc-FcR interactions has increased the complexity of identifying immune correlates of protection. If anything, continued studies of both humoral and cellular immune mechanisms point to the lack of a single protective anti-HIV immune response. Here we focus on humoral immunity, analyzing the role played by Fc receptor-related responses and discussing how new knowledge of their interactions requires further investigation, but may also spur novel vaccination approaches. We initially address classical Fc-receptor mediated anti-viral mechanisms including antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent cell mediated viral inhibition (ADCVI), and antibody-dependent cellular phagocytosis (ADCP), as well as the effector cells that mediate these functions. Next, we summarize key aspects of FcR-Fc interactions that are important for potential control of HIV/SIV such as FcR polymorphisms and post-transcriptional modifications. Finally we discuss less commonly studied non-mechanistic anti-HIV immune functions: antibody avidity and envelopespecific B cell memory. Overall, a spectrum of immune responses, reflecting the immune system's redundancy, will likely be needed to prevent HIV infection and/or disease progression. Aside from elicitation of critical immune mechanisms, a successful vaccine will need to induce mature B cell responses and long-lasting immune memory.

Accelerating next-generation vaccine development for global disease prevention

Vaccines are among the greatest successes in the history of public health. However, past strategies for vaccine development are unlikely to succeed in the future against major global diseases such as AIDS, tuberculosis, and malaria. For such diseases, the correlates of protection are poorly defined and the pathogens evade immune detection and/or exhibit extensive genetic variability. Recent advances have heralded in a new era of vaccine discovery. However, translation of these advances into vaccines remains impeded by lack of understanding of key vaccinology principles in humans. We review these advances toward vaccine discovery and suggest that for accelerating successful vaccine development, new human immunology-based clinical research initiatives be implemented with the goal of elucidating and more effectively generating vaccine-induced protective immune responses.

Shaping T cell - B cell collaboration in the response to human immunodeficiency virus type 1 envelope glycoprotein gp120 by peptide priming

Prime-boost vaccination regimes have shown promise for obtaining protective immunity to HIV. Poorly understood mechanisms of cellular immunity could be responsible for improved humoral responses. Although CD4+ T-cell help promotes B-cell development, the relationship of CD4+ T-cell specificity to antibody specificity has not been systematically investigated. Here, protein and peptide-specific immune responses to HIV-1 gp120 were characterized in groups of ten mucosally immunized BALB/c mice. Protein and peptide reactivity of serum antibody was tested for correlation with cytokine secretion by splenocytes restimulated with individual gp120 peptides. Antibody titer for gp120 correlated poorly with the peptide-stimulated T-cell response. In contrast, titers for conformational epitopes, measured as crossreactivity or CD4-blocking, correlated with average interleukin-2 and interleukin-5 production in response to gp120 peptides. Antibodies specific for conformational epitopes and individual gp120 peptides typically correlated with T-cell responses to several peptides. In order to modify the specificity of immune responses, animals were primed with a gp120 peptide prior to immunization with protein. Priming induced distinct peptide-specific correlations of antibodies and T-cells. The majority of correlated antibodies were specific for the primed peptides or other peptides nearby in the gp120 sequence. These studies suggest that the dominant B-cell subsets recruit the dominant T-cell subsets and that T-B collaborations can be shaped by epitope-specific priming.

HIV/SIV DNA vaccine combined with protein in a co-immunization protocol elicits highest humoral responses to envelope in mice and macaques

Vaccination with HIV/SIV DNAs elicits potent T-cell responses. To improve humoral immune responses, we combined DNA and protein in a co-immunization protocol using in vivo electroporation in mice and macaques. DNA&protein co-immunization induced higher antibody responses than DNA or protein alone, or DNA prime/protein boost in mice. DNA&protein co-immunization induced similar levels of cellular responses as those obtained by DNA only vaccination. The inclusion of SIV or HIV Env gp120 protein did not impair the development of cellular immune responses elicited by DNA present in the vaccine regimen. In macaques, the DNA&protein co-immunization regimen also elicited higher levels of humoral responses with broader cross-neutralizing activity. Despite the improved immunogenicity of DNA&protein co-immunization, the protein formulation with the EM-005 (GLA-SE) adjuvant further increased the anti-Env humoral responses. Dissecting the contribution of EM-005, we found that its administration upregulated the expression of co-stimulatory molecules and stimulated cytokine production, especially IL-6, by dendritic cells in vivo. These terminally differentiated, mature, dendritic cells possibly promote higher levels of humoral responses, supporting the inclusion of the EM-005 adjuvant with the vaccine. Thus, DNA&protein co-immunization is a promising strategy to improve the rapidity of development, magnitude and potency of the humoral immune responses.

Protective efficacy of Modified Vaccinia virus Ankara in preclinical studies

Modified Vaccinia virus Ankara (MVA) is a tissue culture-derived, highly attenuated strain of vaccinia virus (VACV) exhibiting characteristic defective replication in cells from mammalian hosts. In the 1960s MVA was originally generated as a candidate virus for safer vaccination against smallpox. Now, MVA is widely used in experimental vaccine development targeting important infectious diseases and cancer. Versatile technologies for genetic engineering, large-scale production, and quality control facilitate R&D of recombinant and non-recombinant MVA vaccines matching today's requirements for new biomedical products. Such vaccines are attractive candidates for delivering antigens from pathogens against which no, or no effective vaccine is available, including emerging infections caused by highly pathogenic influenza viruses, chikungunya virus, West Nile virus or zoonotic orthopoxviruses. Other directions are seeking valuable vaccines against highly complex diseases such as AIDS, malaria, and tuberculosis. Here, we highlight examples of MVA candidate vaccines against infectious diseases, and review the efforts made to assess both the efficacy of vaccination and immune correlates of protection in preclinical studies.

DNA and virus particle vaccination protects against acquisition and confers control of viremia upon heterologous simian immunodeficiency virus challenge

We have previously shown that macaques vaccinated with DNA vectors expressing SIVmac239 antigens developed potent immune responses able to reduce viremia upon high-dose SIVmac251 challenge. To further improve vaccine-induced immunity and protection, we combined the SIVmac239 DNA vaccine with protein immunization using inactivated SIVmac239 viral particles as protein source. Twenty-six weeks after the last vaccination, the animals were challenged intrarectally at weekly intervals with a titrated dose of the heterologous SIVsmE660. Two of DNA-protein coimmunized macaques did not become infected after 14 challenges, but all controls were infected by 11 challenges. Vaccinated macaques showed modest protection from SIVsmE660 acquisition compared with naïve controls (P = 0.050; stratified for TRIM5α genotype). Vaccinees had significantly lower peak (1.6 log, P = 0.0048) and chronic phase viremia (P = 0.044), with 73% of the vaccinees suppressing viral replication to levels below assay detection during the 40-wk follow-up. Vaccine-induced immune responses associated significantly with virus control: binding antibody titers and the presence of rectal IgG to SIVsmE660 Env correlated with delayed SIVsmE660 acquisition; SIV-specific cytotoxic T cells, prechallenge CD4(+) effector memory, and postchallenge CD8(+) transitional memory cells correlated with control of viremia. Thus, SIVmac239 DNA and protein-based vaccine protocols were able to achieve high, persistent, broad, and effective cellular and humoral immune responses able to delay heterologous SIVsmE660 infection and to provide long-term control of viremia. These studies support a role of DNA and protein-based vaccines for development of an efficacious HIV/AIDS vaccine.

Harnessing CD4⁺ T cell responses in HIV vaccine development

CD4(+) T cells can perform a panoply of tasks to shape an effective response against a pathogen. Limited attention has been paid to the potential importance of functional CD4(+) T cell responses in the context of the development of next-generation vaccines, including HIV vaccines. Many CD4(+) T cell functions are newly appreciated and only partially understood. A workshop was held as a forum to bring together a small group of experts to exchange ideas on the role of CD4(+) T cells in developing durable functional antibody responses, via follicular helper T cells, as well as on the roles of CD4(+) T cells in other aspects of protective immunity. Here we discuss whether CD4(+) T cell responses may represent a beneficial component of an efficacious HIV vaccine.

Relevance of long-lived CD8(+) T effector memory cells for protective immunity elicited by heterologous prime-boost vaccination

Owing to the importance of major histocompatibility complex class Ia-restricted CD8(+) T cells for host survival following viral, bacterial, fungal, or parasitic infection, it has become largely accepted that these cells should be considered in the design of a new generation of vaccines. For the past 20 years, solid evidence has been provided that the heterologous prime-boost regimen achieves the best results in terms of induction of long-lived protective CD8(+) T cells against a variety of experimental infections. Although this regimen has often been used experimentally, as is the case for many vaccines, the mechanism behind the efficacy of this vaccination regimen is still largely unknown. The main purpose of this review is to examine the characteristics of the protective CD8(+) T cells generated by this vaccination regimen. Part of its efficacy certainly relies on the generation and maintenance of large numbers of specific lymphocytes. Other specific characteristics may also be important, and studies on this direction have only recently been initiated. So far, the characterization of these protective, long-lived T cell populations suggests that there is a high frequency of polyfunctional T cells; these cells cover a large breadth and display a T effector memory (TEM) phenotype. These TEM cells are capable of proliferating after an infectious challenge and are highly refractory to apoptosis due to a control of the expression of pro-apoptotic receptors such as CD95. Also, they do not undergo significant long-term immunological erosion. Understanding the mechanisms that control the generation and maintenance of the protective activity of these long-lived TEM cells will certainly provide important insights into the physiology of CD8(+) T cells and pave the way for the design of new or improved vaccines.

Non-neutralizing antibodies in prevention of HIV infection

INTRODUCTION

One of the challenges facing the development of an AIDS vaccine is eliciting antibody (Ab) capable of preventing the acquisition of HIV. Broadly neutralizing Ab (bnAb) that can prevent HIV infection has proven to be difficult to elicit. Here, we consider the potential for protective non-neutralizing Ab (pnnAb) to provide the much needed Ab component for an HIV vaccine. Such Ab acts by "tagging" virus or infected cells for destruction by the innate immune system.

AREAS COVERED

We review interactions between the Fc region of immunoglobulin G (IgG) and Fcϒ receptors or complement that can lead to the destruction of HIV or HIV-infected cells, correlations between the presence of pnnAb and the prevention of HIV and simian immunodeficiency virus (SIV) infections, differences between classical HIV-specific bnAb and HIV-specific pnnAb, HIV envelope antigens and adjuvants which have been hypothesized to generate pnnAb, and the use of avidity as a serological correlate for pnnAb.

EXPERT OPINION

We hypothesize that selection of HIV for the poor ability to elicit bnAb has also selected it for slow entry into cells and a window of opportunity for pnnAb to tag virus for destruction by innate immune responses.

Clinical trial in healthy malaria-naïve adults to evaluate the safety, tolerability, immunogenicity and efficacy of MuStDO5, a five-gene, sporozoite/hepatic stage Plasmodium falciparum DNA vaccine combined with escalating dose human GM-CSF DNA

When introduced in the 1990s, immunization with DNA plasmids was considered potentially revolutionary for vaccine development, particularly for vaccines intended to induce protective CD8 T cell responses against multiple antigens. We conducted, in 1997−1998, the first clinical trial in healthy humans of a DNA vaccine, a single plasmid encoding Plasmodium falciparum circumsporozoite protein (PfCSP), as an initial step toward developing a multi-antigen malaria vaccine targeting the liver stages of the parasite. As the next step, we conducted in 2000–2001 a clinical trial of a five-plasmid mixture called MuStDO5 encoding pre-erythrocytic antigens PfCSP, PfSSP2/TRAP, PfEXP1, PfLSA1 and PfLSA3. Thirty-two, malaria-naïve, adult volunteers were enrolled sequentially into four cohorts receiving a mixture of 500 μg of each plasmid plus escalating doses (0, 20, 100 or 500 μg) of a sixth plasmid encoding human granulocyte macrophage-colony stimulating factor (hGM-CSF). Three doses of each formulation were administered intramuscularly by needle-less jet injection at 0, 4 and 8 weeks, and each cohort had controlled human malaria infection administered by five mosquito bites 18 d later. The vaccine was safe and well-tolerated, inducing moderate antigen-specific, MHC-restricted T cell interferon-γ responses but no antibodies. Although no volunteers were protected, T cell responses were boosted post malaria challenge. This trial demonstrated the MuStDO5 DNA and hGM-CSF plasmids to be safe and modestly immunogenic for T cell responses. It also laid the foundation for priming with DNA plasmids and boosting with recombinant viruses, an approach known for nearly 15 y to enhance the immunogenicity and protective efficacy of DNA vaccines.

Co-expression of HIV-1 virus-like particles and granulocyte-macrophage colony stimulating factor by GEO-D03 DNA vaccine

Here, we report on GEO-D03, a DNA vaccine that co-expresses non-infectious HIV-1 virus-like particles (VLPs) and the human cytokine, granulocyte-macrophage colony-stimulating factor (GM-CSF). The virus-like particles display the native gp160 form of the HIV-1 Envelope glycoprotein (Env) and are designed to elicit antibody against the natural form of Env on virus and virus-infected cells. The DNA-expressed HIV Gag, Pol and Env proteins also have the potential to elicit virus-specific CD4 and CD8 T cells. The purpose of the co-expressed GM-CSF is to target a cytokine that recruits, expands and differentiates macrophages and dendritic cells to the site of VLP expression. The GEO-D03 DNA vaccine is currently entered into human trials as a prime for a recombinant modified vaccinia Ankara (MVA) boost. In preclinical studies in macaques using an SIV prototype vaccine, this vaccination regimen elicited both anti-viral T cells and antibody, and provided 70% protection against acquisition during 12 weekly rectal exposures with a heterologous SIV. Higher avidity of the Env-specific Ab for the native form of the Env in the challenge virus correlated with lower likelihood of SIV infection.

No acquisition: a new ambition for HIV vaccine development?

Development of a safe and effective prophylactic HIV-1 vaccine presents unique challenges. The pessimism following the failure of two HIV-1 vaccine concepts in clinical trials, HIV-1 gp120 and an adenovirus-based approach to induce only cellular immune responses, has been replaced by cautious optimism engendered by the RV144 trial outcome, the isolation of several new broadly reactive neutralizing monoclonal antibodies, and recent primate model data indicating prevention of viral acquisition by active or passive immunization. Intense efforts are underway to optimize immunogen design, adjuvants, and the tools for preclinical evaluation of candidate vaccines in primates, where correlates of protection can be examined in detail - as proof-of-concept for clinical trials.

Controlling the HIV/AIDS epidemic: current status and global challenges

This review provides an overview of the current status of the global HIV pandemic and strategies to bring it under control. It updates numerous preventive approaches including behavioral interventions, male circumcision (MC), pre- and post-exposure prophylaxis (PREP and PEP), vaccines, and microbicides. The manuscript summarizes current anti-retroviral treatment options, their impact in the western world, and difficulties faced by emerging and resource-limited nations in providing and maintaining appropriate treatment regimens. Current clinical and pre-clinical approaches toward a cure for HIV are described, including new drug compounds that target viral reservoirs and gene therapy approaches aimed at altering susceptibility to HIV infection. Recent progress in vaccine development is summarized, including novel approaches and new discoveries.

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.

Sequential evolution and escape from neutralization of simian immunodeficiency virus SIVsmE660 clones in rhesus macaques

Simian immunodeficiency virus (SIV) infection of rhesus macaques has become an important surrogate model for evaluating HIV vaccine strategies. The extreme resistance to neutralizing antibody (NAb) of many commonly used strains, such as SIVmac251/239 and SIVsmE543-3, limits their potential relevance for evaluating the role of NAb in vaccine protection. In contrast, SIVsmE660 is an uncloned virus that appears to be more sensitive to neutralizing antibody. To evaluate the role of NAb in this model, we generated full-length neutralization-sensitive molecular clones of SIVsmE660 and evaluated two of these by intravenous inoculation of rhesus macaques. All animals became infected and maintained persistent viremia that was accompanied by a decline in memory CD4(+) T cells in blood and bronchoalveolar lavage fluid. High titers of autologous NAb developed by 4 weeks postinoculation but were not associated with control of viremia, and neutralization escape variants were detected concurrently with the generation of NAb. Neutralization escape was associated with substitutions and insertion/deletion polymorphisms in the V1 and V4 domains of envelope. Analysis of representative variants revealed that escape variants also induced NAbs within a few weeks of their appearance in plasma, in a pattern that is reminiscent of the escape of human immunodeficiency virus type 1 (HIV-1) isolates in humans. Although early variants maintained a neutralization-sensitive phenotype, viruses obtained later in infection were significantly less sensitive to neutralization than the parental viruses. These results indicate that NAbs exert selective pressure that drives the evolution of the SIV envelope and that this model will be useful for evaluating the role of NAb in vaccine-mediated protection.

Immunogenicity of DNA vaccines encoding simian immunodeficiency virus antigen targeted to dendritic cells in rhesus macaques

BACKGROUND

Targeting antigens encoded by DNA vaccines to dendritic cells (DCs) in the presence of adjuvants enhances their immunogenicity and efficacy in mice.

METHODOLOGY/PRINCIPAL FINDINGS

To explore the immunogenicity of this approach in non-human primates, we generated a single chain antibody to the antigen uptake receptor DEC-205 expressed on rhesus macaque DCs. DNA vaccines encoding this single chain antibody fused to the SIV capsid protein were delivered to six monkeys each by either intramuscular electroporation or conventional intramuscular injection co-injected or not with poly ICLC, a stabilized poly I: C analogue, as adjuvant. Antibodies to capsid were induced by the DC-targeting and non-targeting control DNA delivered by electroporation while conventional DNA immunization at a 10-fold higher dose of DNA failed to induce detectable humoral immune responses. Substantial cellular immune responses were also observed after DNA electroporation of both DNAs, but stronger responses were induced by the non-targeting vaccine. Conventional immunization with the DC-targeting DNA at a 10-fold higher dose did not give rise to substantial cellular immune responses, neither when co-injected with poly ICLC.

CONCLUSIONS/SIGNIFICANCE

The study confirms the potent immunogenicity of DNA vaccines delivered by electroporation. Targeting the DNA via a single chain antibody to DEC-205 expressed by DCs, however, does not improve the immunogenicity of the antigens in non-human primates.

Gene-based vaccination with a mismatched envelope protects against simian immunodeficiency virus infection in nonhuman primates

The RV144 trial demonstrated that an experimental AIDS vaccine can prevent human immunodeficiency virus type 1 (HIV-1) infection in humans. Because of its limited efficacy, further understanding of the mechanisms of preventive AIDS vaccines remains a priority, and nonhuman primate (NHP) models of lentiviral infection provide an opportunity to define immunogens, vectors, and correlates of immunity. In this study, we show that prime-boost vaccination with a mismatched SIV envelope (Env) gene, derived from simian immunodeficiency virus SIVmac239, prevents infection by SIVsmE660 intrarectally. Analysis of different gene-based prime-boost immunization regimens revealed that recombinant adenovirus type 5 (rAd5) prime followed by replication-defective lymphocytic choriomeningitis virus (rLCMV) boost elicited robust CD4 and CD8 T-cell and humoral immune responses. This vaccine protected against infection after repetitive mucosal challenge with efficacies of 82% per exposure and 62% cumulatively. No effect was seen on viremia in infected vaccinated monkeys compared to controls. Protection correlated with the presence of neutralizing antibodies to the challenge viruses tested in peripheral blood mononuclear cells. These data indicate that a vaccine expressing a mismatched Env gene alone can prevent SIV infection in NHPs and identifies an immune correlate that may guide immunogen selection and immune monitoring for clinical efficacy trials.

Viral vectored granulocyte-macrophage colony stimulating factor inhibits vaccine protection in an SIV challenge model: protection correlates with neutralizing antibody

In a previous vaccine study, we reported significant and apparently sterilizing immunity to high-dose, mucosal, simian immunodeficiency virus (SIV) quasi-species challenge. The vaccine consisted of vectors based on vesicular stomatitis virus (VSV) expressing simian immunodeficiency virus (SIV) gag and env genes, a boost with propagating replicon particles expressing the same SIV genes, and a second boost with VSV-based vectors. Concurrent with that published study we had a parallel group of macaques given the same doses of vaccine vectors, but in addition, we included a third VSV vector expressing rhesus macaque GM-CSF in the priming immunization only. We report here that addition of the vector expressing GM-CSF did not enhance CD8 T cell or antibody responses to SIV antigens, and almost completely abolished the vaccine protection against high-dose mucosal challenge with SIV. Expression of GM-CSF may have limited vector replication excessively in the macaque model. Our results suggest caution in the use of GM-CSF as a vaccine adjuvant, especially when expressed by a viral vector. Combining vaccine group animals from this study and the previous study we found that there was a marginal but significant positive correlation between the neutralizing antibody to a neutralization resistant SIV Env and protection from infection.

SIVmac239 MVA vaccine with and without a DNA prime, similar prevention of infection by a repeated dose SIVsmE660 challenge despite different immune responses

BACKGROUND

Vaccine regimens using different agents for priming and boosting have become popular for enhancing T cell and Ab responses elicited by candidate HIV/AIDS vaccines. Here we use a simian model to evaluate immunogenicity and protective efficacy of a recombinant modified vaccinia Ankara (MVA) vaccine in the presence and absence of a recombinant DNA prime. The simian vaccines and regimens represent prototypes for candidate HIV vaccines currently undergoing clinical testing.

METHOD

Recombinant DNA and MVA immunogens expressed simian immunodeficiency virus (SIV)mac239 Gag, PR, RT, and Env sequences. Vaccine schedules tested inoculations of MVA at months 0, 2, and 6 (MMM regimen) or priming with DNA at months 0 and 2 and boosting with MVA at months 4 and 6 (DDMM regimen). Twelve weekly rectal challenges with the heterologous SIV smE660 were initiated at 6 months following the last immunization.

RESULTS

Both regimens elicited similar 61-64% reductions in the per challenge risk of SIVsmE660 transmission despite raising different patterns of immune responses. The DDMM regimen elicited higher magnitudes of CD4 T cells whereas the MMM regimen elicited higher titers and greater avidity Env-specific IgG and more frequent and higher titer SIV-specific IgA in rectal secretions. Both regimens elicited similar magnitudes of CD8 T cells. Magnitudes of T cell responses, specific activities of rectal IgA Ab, and the tested specificities for neutralization and antibody-dependent cellular cytotoxicity did not correlate with risk of infection. However, the avidity of Env-specific IgG had a strong correlation with the per challenge risk of acquisition, but only for the DDMM group.

CONCLUSIONS

We conclude that for the tested immunogens in rhesus macaques, the simpler MMM regimen is as protective as the more complex DDMM regimen.

Vaccine protection against acquisition of neutralization-resistant SIV challenges in rhesus monkeys

Preclinical studies of HIV-1 vaccine candidates have typically shown post-infection virologic control, but protection against acquisition of infection has previously only been reported against neutralization-sensitive virus challenges^1–3. Here we demonstrate vaccine protection against acquisition of fully heterologous, neutralization-resistant virus challenges in rhesus monkeys. Adenovirus/poxvirus and adenovirus/adenovirus vector-based vaccines expressing SIVsmE543 Gag, Pol, and Env antigens resulted in a ≥80% reduction in the per-exposure probability of infection^4,5 against repetitive, intrarectal SIVmac251 challenges in rhesus monkeys. Protection against acquisition of infection exhibited distinct immunologic correlates as compared with post-infection virologic control and required the inclusion of Env in the vaccine regimen. These data demonstrate the first proof-of-concept that optimized HIV-1 vaccine candidates can block acquisition of stringent, heterologous, neutralization-resistant virus challenges in rhesus monkeys.

'Omics investigations of HIV and SIV pathogenesis and innate immunity

In the 30 years since the advent of the AIDS epidemic, the biomedical community has put forward a battery of molecular therapies that are based on the accumulated knowledge of a limited number of viral targets. Despite these accomplishments, the community still confronts unanswered foundational questions about HIV infection. What are the cellular or biomolecular processes behind HIV pathogenesis? Can we elucidate the characteristics that distinguish those individuals who are naturally resistant to either infection or disease progression? The discovery of simian immunodeficiency viruses (SIVs) and the ensuing development of in vivo, nonhuman primate (NHP) infection models was a tremendous advance, especially in abetting the exploration of vaccine strategies. And while there have been numerous NHP infection models and vaccine trials performed, fundamental questions remain regarding host-virus interactions and immune correlates of protection. These issues are, perhaps, most starkly illustrated with the appreciation that many species of African nonhuman primates are naturally infected with strains of SIV that do not cause any appreciable disease while replicating to viral loads that match or exceed those seen with pathogenic SIV infections in Asian species of nonhuman primates. The last decade has seen the establishment of high-throughput molecular profiling tools, such as microarrays for transcriptomics, SNP arrays for genome features, and LC-MS techniques for proteins or metabolites. These provide the capacity to interrogate a biological model at a comprehensive, systems level, in contrast to historical approaches that characterized a few genes or proteins in an experiment. These methods have already had revolutionary impacts in understanding human diseases originating within the host genome such as genetic disorders and cancer, and the methods are finding increasing application in the context of infectious disease. We will provide a review of the use of such 'omics investigations as applied to understanding of HIV pathogenesis and innate immunity, drawing from our own research as well as the literature examples that utilized in vitro cell-based models or studies in nonhuman primates. We will also discuss the potential for systems biology to help guide strategies for HIV vaccines that offer significant protection by either preventing acquisition or strongly suppressing viral replication levels post-infection.

Selected approaches for increasing HIV DNA vaccine immunogenicity in vivo

The safety, stability, and ability for repeat homologous vaccination makes the DNA vaccine platform an excellent candidate for an effective HIV-1 vaccine. However, the immunogenicity of early DNA vaccines did not translate from small animal models into larger non-human primates and was markedly lower than viral vectors. In addition to improvements to the DNA vector itself, delivery with electroporation, the inclusion of molecular adjuvants, and heterologous prime-boost strategies have dramatically improved the immunogenicity of DNA vaccines for HIV and currently makes them a leading platform with many areas warranting further research and clinical development.