Prime-boost vaccination with heterologous live vectors encoding SIV gag and multimeric HIV-1 gp160 protein: efficacy against repeated mucosal R5 clade C SHIV challenges

A single lentivector DNA based immunization contains a late heterologous SIVmac251 mucosal challenge infection

Variety of conventional vaccine strategies tested against HIV-1 have failed to induce protection against HIV acquisition or durable control of viremia. Therefore, innovative strategies that can induce long lasting protective immunity against HIV chronic infection are needed. Recently, we developed an integration-defective HIV lentiDNA vaccine that undergoes a single cycle of replication in target cells in which most viral antigens are produced. A single immunization with such lentiDNA induced long-lasting T-cell and modest antibody responses in cynomolgus macaques. Here eighteen months after this single immunization, all animals were subjected to repeated low dose intra-rectal challenges with a heterologous pathogenic SIVmac251 isolate. Although the viral set point in SIVmac-infected cynomolgus is commonly lower than that seen in Indian rhesus macaques, the vaccinated group of macaques displayed a two log reduction of peak of viremia followed by a progressive and sustained control of virus replication relative to control animals. This antiviral control correlated with antigen-specific CD4+ and CD8+ T cells with high capacity of recall responses comprising effector and central memory T cells but also memory T cell precursors. This is the first description of SIV control in NHP model infected at 18 months following a single immunization with a non-integrative single cycle lentiDNA HIV vaccine. While not delivering sterilizing immunity, our single immunization strategy with a single-cycle lentivector DNA vaccine appears to provide an interesting and safe vaccine platform that warrants further exploration.

Genetic Adjuvants in Replicating Single-Cycle Adenovirus Vectors Amplify Systemic and Mucosal Immune Responses against HIV-1 Envelope

Most infections occur at mucosal surfaces. Providing a barrier of protection at these surfaces may be a useful strategy to combat the earliest events in infection when there are relatively few pathogens to address. The majority of vaccines are delivered systemically by the intramuscular (IM) route. While IM vaccination can drive mucosal immune responses, mucosal immunization at intranasal (IN) or oral sites can lead to better immune responses at mucosal sites of viral entry. In macaques, IN immunization with replicating single-cycle adenovirus (SC-Ads) and protein boosts generated favorable mucosal immune responses. However, there was an apparent "distance effect" in generating mucosal immune responses. IN immunization generated antibodies against HIV envelope (env) nearby in the saliva, but weaker responses in samples collected from the distant vaginal samples. To improve on this, we tested here if SC-Ads expressing genetic adjuvants could be used to amplify antibody responses in distant vaginal samples when they are codelivered with SC-Ads expressing clade C HIV env immunogen. SC-Ads env 1157 was coadministered with SC-Ads expressing 4-1BBL, granulocyte macrophage colony-stimulating factor (GMCSF), IL-21, or Clostridoides difficile (C. diff.) toxin fragments by IN or IM routes. These data show that vaginal antibody responses were markedly amplified after a single immunization by the IN or IM routes, with SC-Ad expressing HIV env if this vaccine is complemented with SC-Ads expressing genetic adjuvants. Furthermore, the site and combination of adjuvants appear to "tune" these antibody responses towards an IgA or IgG isotype bias. Boosting these priming SC-Ad responses with another SC-Ad or with SOSIP native-like env proteins markedly amplifies env antibody levels in vaginal washes. Together, this data may be useful in informing the choice of route of delivery adenovirus and peptide vaccines against HIV-1.

Current advances in HIV vaccine preclinical studies using Macaque models

The macaque simian or simian/human immunodeficiency virus (SIV/SHIV) challenge model has been widely used to inform and guide human vaccine trials. Substantial advances have been made recently in the application of repeated-low-dose challenge (RLD) approach to assess SIV/SHIV vaccine efficacies (VE). Some candidate HIV vaccines have shown protective effects in preclinical studies using the macaque SIV/SHIV model but the model's true predictive value for screening potential HIV vaccine candidates needs to be evaluated further. Here, we review key parameters used in the RLD approach and discuss their relevance for evaluating VE to improve preclinical studies of candidate HIV vaccines.

MTBVAC-Based TB-HIV Vaccine Is Safe, Elicits HIV-T Cell Responses, and Protects against Mycobacterium tuberculosis in Mice

The tuberculosis (TB) vaccine MTBVAC is the only live-attenuated Mycobacterium tuberculosis (Mtb)-based vaccine in clinical development, and it confers superior protection in different animal models compared to the current vaccine, BCG (Mycobacterium bovis bacillus Calmette-Guérin). With the aim of using MTBVAC as a vector for a dual TB-HIV vaccine, we constructed the recombinant MTBVAC.HIVA^2auxo strain. First, we generated a lysine auxotroph of MTBVAC (MTBVACΔlys) by deleting the lysA gene. Then the auxotrophic MTBVACΔlys was transformed with the E. coli-mycobacterial vector p2auxo.HIVA, harboring the lysA-complementing gene and the HIV-1 clade A immunogen HIVA. This TB-HIV vaccine conferred similar efficacy to the parental strain MTBVAC against Mtb challenge in mice. MTBVAC.HIVA^2auxo was safer than BCG and MTBVAC in severe combined immunodeficiency (SCID) mice, and it was shown to be maintained up to 42 bacterial generations in vitro and up to 100 days after inoculation in vivo. The MTBVAC.HIVA^2auxo vaccine, boosted with modified vaccinia virus Ankara (MVA).HIVA, induced HIV-1 and Mtb-specific interferon-γ-producing T cell responses and polyfunctional HIV-1-specific CD8+ T cells producing interferon-γ (IFN-γ), tumor necrosis factor alpha (TNF-α), and CD107a in BALB/c mice. Here we describe new tools to develop combined vaccines against TB and HIV with the potential of expansion for other infectious diseases.

Live-Attenuated Bacterial Vectors for Delivery of Mucosal Vaccines, DNA Vaccines, and Cancer Immunotherapy

Vaccines save millions of lives each year from various life-threatening infectious diseases, and there are more than 20 vaccines currently licensed for human use worldwide. Moreover, in recent decades immunotherapy has become the mainstream therapy, which highlights the tremendous potential of immune response mediators, including vaccines for prevention and treatment of various forms of cancer. However, despite the tremendous advances in microbiology and immunology, there are several vaccine preventable diseases which still lack effective vaccines. Classically, weakened forms (attenuated) of pathogenic microbes were used as vaccines. Although the attenuated microbes induce effective immune response, a significant risk of reversion to pathogenic forms remains. While in the twenty-first century, with the advent of genetic engineering, microbes can be tailored with desired properties.

In this review, I have focused on the use of genetically modified bacteria for the delivery of vaccine antigens. More specifically, the live-attenuated bacteria, derived from pathogenic bacteria, possess many features that make them highly suitable vectors for the delivery of vaccine antigens. Bacteria can theoretically express any heterologous gene or can deliver mammalian expression vectors harboring vaccine antigens (DNA vaccines). These properties of live-attenuated microbes are being harnessed to make vaccines against several infectious and noninfectious diseases. In this regard, I have described the desired features of live-attenuated bacterial vectors and the mechanisms of immune responses manifested by live-attenuated bacterial vectors. Interestingly anaerobic bacteria are naturally attracted to tumors, which make them suitable vehicles to deliver tumor-associated antigens thus I have discussed important studies investigating the role of bacterial vectors in immunotherapy. Finally, I have provided important discussion on novel approaches for improvement and tailoring of live-attenuated bacterial vectors for the generation of desired immune responses.

Mucosal Vaccine Approaches for Prevention of HIV and SIV Transmission

Optimal protective immunity to HIV will likely require that plasma cells, memory B cells and memory T cells be stationed in mucosal tissues at portals of viral entry. Mucosal vaccine administration is more effective than parenteral vaccine delivery for this purpose. The challenge has been to achieve efficient vaccine uptake at mucosal surfaces, and to identify safe and effective adjuvants, especially for mucosally administered HIV envelope protein immunogens. Here, we discuss strategies used to deliver potential HIV vaccine candidates in the intestine, respiratory tract, and male and female genital tract of humans and nonhuman primates. We also review mucosal adjuvants, including Toll-like receptor agonists, which may adjuvant both mucosal humoral and cellular immune responses to HIV protein immunogens.

Stable Expression of Lentiviral Antigens by Quality-Controlled Recombinant Mycobacterium bovis BCG Vectors

The well-established safety profile of the tuberculosis vaccine strain, Mycobacterium bovis bacille Calmette-Guérin (BCG), makes it an attractive vehicle for heterologous expression of antigens from clinically relevant pathogens. However, successful generation of recombinant BCG strains possessing consistent insert expression has encountered challenges in stability. Here, we describe a method for the development of large recombinant BCG accession lots which stably express the lentiviral antigens, human immunodeficiency virus (HIV) gp120 and simian immunodeficiency virus (SIV) Gag, using selectable leucine auxotrophic complementation. Successful establishment of vaccine stability stems from stringent quality control criteria which not only screen for highly stable complemented BCG ΔleuCD transformants but also thoroughly characterize postproduction quality. These parameters include consistent production of correctly sized antigen, retention of sequence-pure plasmid DNA, freeze-thaw recovery, enumeration of CFU, and assessment of cellular aggregates. Importantly, these quality assurance procedures were indicative of overall vaccine stability, were predictive for successful antigen expression in subsequent passaging both in vitro and in vivo, and correlated with induction of immune responses in murine models. This study has yielded a quality-controlled BCG ΔleuCD vaccine expressing HIV gp120 that retained stable full-length expression after 10(24)-fold amplification in vitro and following 60 days of growth in mice. A second vaccine lot expressed full-length SIV Gag for >10(68)-fold amplification in vitro and induced potent antigen-specific T cell populations in vaccinated mice. Production of large, well-defined recombinant BCG ΔleuCD lots can allow confidence that vaccine materials for immunogenicity and protection studies are not negatively affected by instability or differences between freshly grown production batches.

Multimodality vaccination against clade C SHIV: partial protection against mucosal challenges with a heterologous tier 2 virus

We sought to test whether vaccine-induced immune responses could protect rhesus macaques (RMs) against upfront heterologous challenges with an R5 simian-human immunodeficiency virus, SHIV-2873Nip. This SHIV strain exhibits many properties of transmitted HIV-1, such as tier 2 phenotype (relatively difficult to neutralize), exclusive CCR5 tropism, and gradual disease progression in infected RMs. Since no human AIDS vaccine recipient is likely to encounter an HIV-1 strain that exactly matches the immunogens, we immunized the RMs with recombinant Env proteins heterologous to the challenge virus. For induction of immune responses against Gag, Tat, and Nef, we explored a strategy of immunization with overlapping synthetic peptides (OSP). The immune responses against Gag and Tat were finally boosted with recombinant proteins. The vaccinees and a group of ten control animals were given five low-dose intrarectal (i.r.) challenges with SHIV-2873Nip. All controls and seven out of eight vaccinees became systemically infected; there was no significant difference in viremia levels of vaccinees vs. controls. Prevention of viremia was observed in one vaccinee which showed strong boosting of virus-specific cellular immunity during virus exposures. The protected animal showed no challenge virus-specific neutralizing antibodies in the TZM-bl or A3R5 cell-based assays and had low-level ADCC activity after the virus exposures. Microarray data strongly supported a role for cellular immunity in the protected animal. Our study represents a case of protection against heterologous tier 2 SHIV-C by vaccine-induced, virus-specific cellular immune 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-virus exposure of vaccine-protected macaques alters the anti-HIV-1 antibody repertoire in the absence of viremia

BACKGROUND

We addressed the question whether live-virus challenges could alter vaccine-induced antibody (Ab) responses in vaccinated rhesus macaques (RMs) that completely resisted repeated exposures to R5-tropic simian-human immunodeficiency viruses encoding heterologous HIV clade C envelopes (SHIV-Cs).

RESULTS

We examined the Ab responses in aviremic RMs that had been immunized with a multi-component protein vaccine (multimeric HIV-1 gp160, HIV-1 Tat and SIV Gag-Pol particles) and compared anti-Env plasma Ab titers before and after repeated live-virus exposures. Although no viremia was ever detected in these animals, they showed significant increases in anti-gp140 Ab titers after they had encountered live SHIVs. When we investigated the dynamics of anti-Env Ab titers during the immunization and challenge phases further, we detected the expected, vaccine-induced increases of Ab responses about two weeks after the last protein immunization. Remarkably, these titers kept rising during the repeated virus challenges, although no viremia resulted. In contrast, in vaccinated RMs that were not exposed to virus, anti-gp140 Ab titers declined after the peak seen two weeks after the last immunization. These data suggest boosting of pre-existing, vaccine-induced Ab responses as a consequence of repeated live-virus exposures. Next, we screened polyclonal plasma samples from two of the completely protected vaccinees by peptide phage display and designed a strategy that selects for recombinant phages recognized only by Abs present after - but not before - any SHIV challenge. With this "subtractive biopanning" approach, we isolated V3 mimotopes that were only recognized after the animals had been exposed to live virus. By detailed epitope mapping of such anti-V3 Ab responses, we showed that the challenges not only boosted pre-existing binding and neutralizing Ab titers, but also induced Abs targeting neo-antigens presented by the heterologous challenge virus.

CONCLUSIONS

Anti-Env Ab responses induced by recombinant protein vaccination were altered by the multiple, live SHIV challenges in vaccinees that had no detectable viral loads. These data may have implications for the interpretation of "vaccine only" responses in clinical vaccine trials.

Identification of a highly conserved surface on Tat variants

Extracellular Tat is suspected to protect HIV-1-infected cells from cellular immunity. Seropositive patients are unable to produce neutralizing antibodies against Tat, and Tat is still secreted under antiviral treatment. In mice, the Tat OYI vaccine candidate generates neutralizing antibodies such as the mAb 7G12. A peptide called MIMOOX was designed from fragments of Tat OYI identified as the possible binding site for mAb 7G12. MIMOOX was chemically synthesized, and its structure was stabilized with a disulfide bridge. Circular dichroism spectra showed that MIMOOX had mainly β turns but no α helix as Tat OYI. MIMOOX was recognized by mAb 7G12 in ELISA only in reduced conditions. Moreover, a competitive recognition assay with mAb 7G12 between MIMOOX and Tat variants showed that MIMOOX mimics a highly conserved surface in Tat variants. Rat immunizations with MIMOOX induce antibodies recognizing Tat variants from the main HIV-1 subtypes and confirm the Tat OYI vaccine approach.

Purification of recombinant vaccinia virus-expressed monomeric HIV-1 gp120 to apparent homogeneity

Vaccinia virus (VV) has been used to express a variety of heterologous proteins, including HIV envelope (Env) glycoproteins. The Env protein is synthesized as a precursor molecule, gp160, which is cleaved into the surface antigen gp120 and the transmembrane protein gp41. Even though production of gp160 by the VV expression system has been described, its use for gp120 production is not well documented. Here we report a new procedure for the purification of gp120 from serum-containing culture supernatant of VV-infected cells. The gp120 protein was enriched to a purity better than 60% on a snowdrop (Galanthus nivalis) lectin affinity column in the presence of 0.25% zwitterionic detergent Empigen BB. After additional DEAE anion exchange and Superdex size exclusion chromatography steps, the gp120 monomer was purified free of contamination as determined by SDS-PAGE. The retention of structural integrity was confirmed by determining the affinity constant of purified gp120s to soluble CD4 and a monoclonal antibody IgG1b12, using surface plasmon resonance analysis. The purification procedure is robust and reproducible, and may find general use for glycoprotein purifications from sources where the presence of serum is desirable.

Neutralizing antibodies to HIV-1 induced by immunization

Most neutralizing antibodies act at the earliest steps of viral infection and block interaction of the virus with cellular receptors to prevent entry into host cells. The inability to induce neutralizing antibodies to HIV has been a major obstacle to HIV vaccine research since the early days of the epidemic. However, in the past three years, the definition of a neutralizing antibody against HIV has been revolutionized by the isolation of extremely broad and potent neutralizing antibodies from HIV-infected individuals. Considerable hurdles remain for inducing neutralizing antibodies to a protective level after immunization. Meanwhile, novel technologies to bypass the induction of antibodies are being explored to provide prophylactic antibody-based interventions. This review addresses the challenge of inducing HIV neutralizing antibodies upon immunization and considers notable recent advances in the field. A greater understanding of the successes and failures for inducing a neutralizing response upon immunization is required to accelerate the development of an effective HIV vaccine.

Rhesus macaques vaccinated with consensus envelopes elicit partially protective immune responses against SHIV SF162p4 challenge

The development of a preventative HIV/AIDS vaccine is challenging due to the diversity of viral genome sequences, especially in the viral envelope (Env₁₆₀). Since it is not possible to directly match the vaccine strain to the vast number of circulating HIV-1 strains, it is necessary to develop an HIV-1 vaccine that can protect against a heterologous viral challenge. Previous studies from our group demonstrated that a mixture of wild type clade B Env(gp160s) were able to protect against a heterologous clade B challenge more effectively than a consensus clade B Envg(p160) vaccine. In order to broaden the immune response to other clades of HIV, in this study rhesus macaques were vaccinated with a polyvalent mixture of purified HIV-1 trimerized consensus Envg(p140) proteins representing clades A, B, C, and E. The elicited immune responses were compared to a single consensus Env(gp140) representing all isolates in group M (Con M). Both vaccines elicited anti- Env(gp140) IgG antibodies that bound an equal number of HIV-1 Env(gp160) proteins representing clades A, B and C. In addition, both vaccines elicited antibodies that neutralized the HIV-1(SF162) isolate. However, the vaccinated monkeys were not protected against SHIV(SF162p4) challenge. These results indicate that consensus Env(gp160) vaccines, administered as purified Env(gp140) trimers, elicit antibodies that bind to Env(gp160s) from strains representing multiple clades of HIV-1, but these vaccines did not protect against heterologous SHIV challenge.

CD8 down-regulation and functional impairment of SIV-specific cytotoxic T lymphocytes in lymphoid and mucosal tissues during SIV infection

Functional impairment of virus-specific T cells is a hallmark of HIV/SIV infection, but the underlying mechanisms of this dysfunction are not well understood. To address this, we simultaneously analyzed the expression and intensity of CD8 and inhibitory PD-1 on CTL in blood and lymphoid tissues in SIV-infected rhesus macaques. The intensity (mean channel fluorescence) of CD8 expression was transiently down-regulated in early SIV infection (10-14 dpi), despite an increase in CD8(+) T cell proliferation. In chronic infection, CD8 expression was maintained at low levels on CD8(+) T cells in all tissues. Interestingly, Gag-specific CTLs were clearly divided into CD8high- and CD8low-expressing populations in SIV-infected macaques, and CD8low Gag-specific cells increased with disease progression, especially in lymphoid tissues when compared with peripheral blood or in Gag-vaccinated controls. Moreover, the CD8low CTL population secreted lower levels of cytokines upon SIV antigen stimulation and exhibited lower proliferative capacity during infection compared with the CD8high CTL population. Meanwhile, intensity of PD-1 expression on Gag-specific CTL in chronic infection was significantly higher than in acute SIV infection, although the frequencies of PD-1+ Gag-specific cells were similar in acute and chronic stages. In summary, down-regulation of CD8 expression and higher expression of PD-1 on SIV-specific CTLs could coordinately attenuate SIV-specific CTL responses and their ability to recognize virus-infected target cells, especially in lymphoid tissues, resulting in failure to contain viremia, and continued persistence and replication of HIV in lymphoid tissue reservoirs.

Polyinosinic-polycytidylic acid is the most effective TLR adjuvant for SIV Gag protein-induced T cell responses in nonhuman primates

Prime-boost immunization with heterologous vaccines elicits potent cellular immunity. In this study, we assessed the influence of various TLR ligands on SIV Gag-specific T cell immunity and protection following prime-boost immunization. Rhesus macaques (RMs) were primed with SIV Gag protein emulsified in Montanide ISA51 with or without TLR3 (polyinosinic-polycytidylic acid [poly-IC]), TLR4 (monophosphoryl lipid A), TLR7/8 (3M-012), TLR9 (CpG), or TLR3 (poly-IC) combined with TLR7/8 ligands, then boosted with replication defective adenovirus 5 expressing SIV Gag (rAd5-Gag). After priming, RMs that received SIV Gag protein plus poly-IC developed significantly higher frequencies of SIV Gag-specific CD4(+) Th1 responses in blood and bronchoalveolar lavage (BAL) fluid lymphocytes compared with all other adjuvants, and low-level SIV Gag-specific CD8(+) T cell responses. After the rAd5-Gag boost, the magnitude and breadth of SIV Gag-specific CD8(+) T cell responses were significantly increased in RM primed with SIV Gag protein plus poly-IC, with or without the TLR7/8 ligand, or CpG. However, the anamnestic, SIV Gag-specific CD8(+) T cell response to SIVmac251 challenge was not significantly enhanced by SIV Gag protein priming with any of the adjuvants. In contrast, the anamnestic SIV Gag-specific CD4(+) T cell response in BAL was enhanced by SIV Gag protein priming with poly-IC or CpG, which correlated with partial control of early viral replication after SIVmac251 challenge. These results demonstrate that prime-boost vaccination with SIV Gag protein/poly-IC improves magnitude, breadth, and durability of CD4(+) T cell immune responses, which could have a role in the control of SIV viral replication.

Mucosal priming with a replicating-vaccinia virus-based vaccine elicits protective immunity to simian immunodeficiency virus challenge in rhesus monkeys

Mucosal surfaces are not targeted by most human immunodeficiency virus type 1 (HIV-1) vaccines, despite being major routes for HIV-1 transmission. Here we report a novel vaccination regimen consisting of a mucosal prime with a modified replicating vaccinia virus Tiantan strain (MVTT(SIVgpe)) and an intramuscular boost with a nonreplicating adenovirus strain (Ad5(SIVgpe)). This regimen elicited robust cellular immune responses with enhanced magnitudes, sustainability, and polyfunctionality, as well as higher titers of neutralizing antibodies against the simian immunodeficiency virus SIV(mac1A11) in rhesus monkeys. The reductions in peak and set-point viral loads were significant in most animals, with one other animal being protected fully from high-dose intrarectal inoculation of SIV(mac239). Furthermore, the animals vaccinated with this regimen were healthy, while ~75% of control animals developed simian AIDS. The protective effects correlated with the vaccine-elicited SIV-specific CD8(+) T cell responses against Gag and Pol. Our study provides a novel strategy for developing an HIV-1 vaccine by using the combination of a replicating vector and mucosal priming.

Cutting edge: rapid boosting of cross-reactive memory CD8 T cells broadens the protective capacity of the Flumist vaccine

Memory CD8 T cells recognizing conserved proteins from influenza A virus (IAV), such as nucleoprotein, have the potential to provide protection in individuals who lack the proper neutralizing Abs. In this study, we show that the most potent CD8 T cell-inducing influenza vaccine on the market (Flumist) does not induce sufficient numbers of cross-reactive CD8 T cells to provide substantial protection against lethal nonhomologous IAV challenge. However, Flumist-primed CD8 T cells rapidly acquire memory characteristics and can respond to short-interval boosting to greatly enlarge the IAV-specific memory pool, which is sufficient to protect mice from nonhomologous IAV challenge. Thus, a current vaccine strategy, Flumist, may serve as a priming platform for the rapid induction of large numbers of memory CD8 T cells with the capacity for broad protection against influenza.

Novel biopanning strategy to identify epitopes associated with vaccine protection

Identifying immune correlates of protection is important to develop vaccines against infectious diseases. We designed a novel, universally applicable strategy to profile the antibody (Ab) repertoire of protected vaccine recipients, using recombinant phages encoding random peptide libraries. The new approach, termed "protection-linked (PL) biopanning," probes the Ab paratopes of protected vaccinees versus those with vaccine failure. As proof of concept, we screened plasma samples from vaccinated rhesus macaques (RMs) that had completely resisted multiple mucosal challenges with R5-tropic simian-human immunodeficiency viruses (SHIVs). The animals had been immunized with a multicomponent vaccine (multimeric HIV-1 gp160, HIV-1 Tat, and SIV Gag-Pol particles). After PL biopanning, we analyzed the phagotopes selected for amino acid homologies; in addition to the expected Env mimotopes, one recurring motif reflected the neutralizing Ab epitope at the N terminus (NT) of HIV-1 Tat. Subsequent binding and functional assays indicated that anti-Tat NT Abs were present only in completely or partially protected RMs; peak viremia of the latter was inversely correlated with anti-Tat NT Ab titers. In contrast, highly viremic, unvaccinated controls did not develop detectable Abs against the same epitope. Based upon the protective effect observed in vivo, we suggest that Tat should be included in multicomponent HIV-1 vaccines. Our data highlight the power of the new PL-biopanning strategy to identify Ab responses with significant association to vaccine protection, regardless of the mechanism(s) or targets of the protective Abs. PL biopanning is also unbiased with regard to pathogens or disease model, making it a universal tool.

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.

Co-expression of Erns and E2 genes of classical swine fever virus by replication-defective recombinant adenovirus completely protects pigs against virulent challenge with classical swine fever virus

The objective of this study was to construct a recombinant adenovirus for future CSFV vaccines used in the pig industry for the reduction of losses involved in CSF outbreaks. The Erns and E2 genes of classical swine fever virus (CSFV), which encode the two main protective glycoproteins from the "Shimen" strain of CSFV, were combined and inserted into the replication-defective human adenovirus type-5 and named the rAd-Erns-E2. Nine pigs were randomly assigned to three treatment groups (three pigs in each group) including the rAd-Erns-E2, hAd-CMV control and DMEM control. Intramuscular vaccination with 2×10(6) TCID(50) of the rAd-Erns-E2 was administered two times with an interval of 21 days. At 42 days post inoculation, pigs in all groups were challenged with a lethal dose of 1×10(3) TCID(50) CSFV "Shimen" strain. Observation of clinical signs was made and the existence of CSFV RNA was detected. Animals in the hAd-CMV and DMEM groups showed severe clinical CSF symptoms and were euthanized from 7 to 10 days after the challenge. However, no adverse clinical CSF signs were observed in vaccinated pigs after the administration of rAd-Erns-E2 and even after CSFV challenge. Neither CSFV RNA nor pathological changes were detected in the tissues of interest of the above vaccinated pigs. These results implied that the recombination adenovirus carrying the Erns-E2 genes could be used to prevent swine from classical swine fever.

Animal models in virus research: their utility and limitations

Viral diseases are important threats to public health worldwide. With the number of emerging viral diseases increasing the last decades, there is a growing need for appropriate animal models for virus studies. The relevance of animal models can be limited in terms of mimicking human pathophysiology. In this review, we discuss the utility of animal models for studies of influenza A viruses, HIV and SARS-CoV in light of viral emergence, assessment of infection and transmission risks, and regulatory decision making. We address their relevance and limitations. The susceptibility, immune responses, pathogenesis, and pharmacokinetics may differ between the various animal models. These complexities may thwart translating results from animal experiments to the humans. Within these constraints, animal models are very informative for studying virus immunopathology and transmission modes and for translation of virus research into clinical benefit. Insight in the limitations of the various models may facilitate further improvements of the models.

Improved SIV DNA vaccine can be effectively used as a boost for Ad5 in prime-boost immunization strategy

In the study under evaluation, optimized SIV DNA were used to boost T-cell responses induced by a highly immunogenic SIV Ad5-prime in Chinese rhesus macaques. A regular prime-boost regimen (SIV DNA-prime and rAd boost) and naive macaques were used as the control. After vaccination, the animals were challenged intrarectally with SIVmac251, and partial protection was observed in the macaques immunized by the Ad5-prime DNA-boost regimen. SIV-specific T-cell responses in the enzyme-linked immunospot assay were significantly higher in the Ad5-prime DNA-boost, compared with the responses in the control macaques. Viral control correlated with the generation of HLA-DR+ T cells 2 weeks after the viral challenge. Further studies using prime and boost strategies and alternative routes of vaccination (including a simultaneous approach) are warranted to fully explore the potential of prime and boost regimens for HIV-1 vaccine development.

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.

Neutralizing antibodies and control of HIV: moves and countermoves

It is now evident that powerful antibodies directed to conserved regions of HIV-1 envelope protein develop during chronic infection in some individuals and that these antibodies can neutralize a broad array of diverse isolates in vitro, so termed broadly neutralizing antibodies (bNAbs). A great deal of effort is directed internationally at understanding the ontogeny of NAbs during infection as well as in designing and testing immunogens that can elicit bNAbs in animal models and in humans. Given the parrying tactics of Env, multiple approaches, along with high-resolution structural studies, will be needed to reach a degree of understanding sufficient to design an effective vaccine. We discuss and note here some of the most important recent advances in our knowledge of how neutralizing antibodies develop in vivo, the recent discovery of extremely powerful neutralizing monoclonal antibodies isolated from natural infection, enhanced methodologies that have accelerated discoveries on both fronts, and the progress made in eliciting potent NAbs with limited breadth by vaccination.

Replicating adenovirus-simian immunodeficiency virus (SIV) vectors efficiently prime SIV-specific systemic and mucosal immune responses by targeting myeloid dendritic cells and persisting in rectal macrophages, regardless of immunization route

Although priming with replicating adenovirus type 5 host range mutant (Ad5hr)-human immunodeficiency virus (HIV)/simian immunodeficiency virus (SIV) recombinants, followed by HIV/SIV envelope boosting, has proven highly immunogenic, resulting in protection from SIV/simian-human immunodeficiency virus (SHIV) challenges, Ad5hr recombinant distribution, replication, and persistence have not been examined comprehensively in nonhuman primates. We utilized Ad5hr-green fluorescent protein and Ad5hr-SIV recombinants to track biodistribution and immunogenicity following mucosal priming of rhesus macaques by the intranasal/intratracheal, sublingual, vaginal, or rectal route. Ad recombinants administered by all routes initially targeted macrophages in bronchoalveolar lavage (BAL) fluid and rectal tissue, later extending to myeloid dendritic cells in BAL fluid with persistent expression in rectal mucosa 25 weeks after the last Ad immunization. Comparable SIV-specific immunity, including cellular responses, serum binding antibody, and mucosal secretory IgA, was elicited among all groups. The ability of the vector to replicate in multiple mucosal sites irrespective of delivery route, together with the targeting of macrophages and professional antigen-presenting cells, which provide potent immunogenicity at localized sites of virus entry, warrants continued use of replicating Ad vectors.

Probing CD8 T cell responses with Listeria monocytogenes infection

CD8 T cells play a critical role in the control and eradication of intracellular pathogens. Increased understanding of CD8 T cell biology provides insight that can be translated into improved vaccination strategies. The intracellular bacterium, Listeria monocytogenes, has been used as a model organism to study every phase of the CD8 T cell response to intracellular bacterial infection. Infection of laboratory mice with L. monocytogenes has provided insight into the factors that are involved in primary T cell responses, memory CD8 T cell generation, maintenance, functionality, and diversification following repeated pathogenic challenges. In this review, we will focus on work from our laboratories utilizing the murine model of L. monocytogenes to investigate the characteristics of CD8 T cell responses to infection. This model has profoundly advanced our understanding of the CD8 T cell response to infection and is likely to continue to provide invaluable basic insights that can be translated into the development of effective vaccination strategies to protect against pathogens.

Distinct Humoral and Cellular Immunity Induced by Alternating Prime-boost Vaccination Using Plasmid DNA and Live Viral Vector Vaccines Expressing the E Protein of Dengue Virus Type 2

BACKGROUND

Dengue virus, which belongs to the Flavivirus genus of the Flaviviridae family, causes fatal dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) with infection risk of 2.5 billion people worldwide. However, approved vaccines are still not available. Here, we explored the immune responses induced by alternating prime-boost vaccination using DNA vaccine, adenovirus, and vaccinia virus expressing E protein of dengue virus type 2 (DenV2).

METHODS

Following immunization with DNA vaccine (pDE), adenovirus (rAd-E), and/or vaccinia virus (VV-E) expressing E protein, E protein-specific IgG and its isotypes were determined by conventional ELISA. Intracellular CD154 and cytokine staining was used for enumerating CD4+ T cells specific for E protein. E protein-specific CD8+ T cell responses were evaluated by in vivo CTL killing activity and intracellular IFN-γ staining.

RESULTS

Among three constructs, VV-E induced the most potent IgG responses, Th1-type cytokine production by stimulated CD4+ T cells, and the CD8+ T cell response. Furthermore, when the three constructs were used for alternating prime-boost vaccination, the results revealed a different pattern of CD4+ and CD8+ T cell responses. i) Priming with VV-E induced higher E-specific IgG level but it was decreased rapidly. ii) Strong CD8+ T cell responses specific for E protein were induced when VV-E was used for the priming step, and such CD8+ T cell responses were significantly boosted with pDE. iii) Priming with rAd-E induced stronger CD4+ T cell responses which subsequently boosted with pDE to a greater extent than VV-E and rAd-E.

CONCLUSION

These results indicate that priming with live viral vector vaccines could induce different patterns of E protein- specific CD4+ and CD8+ T cell responses which were significantly enhanced by booster vaccination with the DNA vaccine. Therefore, our observation will provide valuable information for the establishment of optimal prime-boost vaccination against DenV.

Efficiency of neutralizing antibodies targeting the CD4-binding site: influence of conformational masking by the V2 loop in R5-tropic clade C simian-human immunodeficiency virus

In R5-tropic clade C simian-human immunodeficiency viruses (SHIV-Cs), we identified a 3-asparagine (3N) deletion mutation in the V2 loop stem of gp120 as the major determinant of neutralization escape of the anti-CD4-binding site (anti-CD4-bs) neutralizing monoclonal antibody (nMAb) b12. However, the more potent anti-CD4-bs nMAbs VRC01 and VRC03 were not sensitive to this mutation. Using isogenic tier 1 or tier 2 proviruses differing only in the 3N mutation, we showed that this mutation might result in selective conformational b12 epitope masking. Therefore, human immunodeficiency virus (HIV) Env immunogens targeting the CD4-bs and designed to neutralize tier 2 viruses should take conformational masking by the V2 loop into account.