Recombinant canarypox vaccine-elicited CTL specific for dominant and subdominant simian immunodeficiency virus epitopes in rhesus monkeys

Lessons learned from HIV antiretroviral treatment interruption trials

PURPOSE OF REVIEW

Clinical trials with an antiretroviral therapy (ART) interruption remains indispensable for assessing strategies for ART-free HIV remission. This review highlights the lessons learned from ART interruption studies so far, including the risks to the participants and implications for HIV remission.

RECENT FINDINGS

Historically, analytic HIV treatment interruption (ATI) studies were commonly designed with a prolonged duration of ART interruption and with viral load set point as the primary outcome. For a variety of reasons, including participant risk, recent treatment interruption trials have frequently used time to viral rebound as the primary endpoint and have restarted ART once a predetermined viral load threshold is reached. Through treatment interruption trials, investigators have tested the efficacy of therapeutic and curative strategies that showed promise in preclinical trials, including therapeutic vaccines, latency-reversing agents, and broadly neutralizing antibodies. In most populations, ATI trials have been well tolerated, with few adverse clinical events and no significant changes to the reservoir. Several reservoir predictors of HIV-rebound timing have been reported, with a subset of trials uncovering posttreatment controllers who can maintain HIV remission despite ART discontinuation.

SUMMARY

Treatment interruption trials are a vital tool, but their optimal design remain uncertain and must balance participant risks with scientific rigor. The ability to predict the timing or extent of HIV rebound and identify mechanisms of posttreatment control may accelerate the development of novel therapeutics for sustained HIV remission.

Prime-boost immunization with poxvirus or adenovirus vectors as a strategy to develop a protective vaccine for HIV-1

Challenges in the development of an effective HIV-1 vaccine are myriad with significant hurdles posed by viral diversity, the lack of a human correlate of protection and difficulty in creating immunogens capable of eliciting broadly neutralizing antibodies. The implicit requirement for novel approaches to these problems has resulted in vaccine candidates designed to elicit cellular and/or humoral immune responses, to include recombinant DNA, viral and bacterial vectors, and subunit proteins. Here, we review data from clinical studies primarily of poxvirus and adenovirus vector vaccines, used in a heterologous prime-boost combination strategy. Currently, this strategy appears to hold the most promise for an effective vaccine based on results from immunogenicity testing and nonhuman primate challenge models, as well as the modest efficacy recently observed in the Thai prime-boost trial.

Phase III HIV vaccine trial in Thailand: a step toward a protective vaccine for HIV

The large human efficacy trail in Thailand, RV144, was concluded in the summer of 2009. This is the first Phase III trial to show limited, but significant, efficacy in preventing HIV acquisition. This trial represents the first sign that a preventive vaccine for HIV may be feasible. The vaccine regimen tested in Thailand consisted of priming with a Canarypox vector carrying three synthetic HIV genes. The priming was followed by booster inoculations with two recombinant envelope proteins from HIV, clade B and E. The need to understand the role in protection from HIV acquisition of the new responses, induced by this vaccine combination, has brought together many researchers with the common goal of improving the development of a safe and effective vaccine for HIV.

Safety, immunogenicity and efficacy of poxvirus-based vector vaccines expressing the haemagglutinin gene of a highly pathogenic H5N1 avian influenza virus in pigs

This study investigates the safety, immunogenicity and efficacy of different pox-vector vaccines expressing the haemagglutinin of a highly pathogenic (HP) H5N1 avian influenza virus (AIV) (A/chicken/Indonesia/7/03) in pigs. Pigs were vaccinated twice, with a 4-week interval, with a fowlpox (TROVAC), a canarypox (ALVAC), or a vaccinia (NYVAC) vector vaccine combined with an oil-in-water adjuvant, with the unadjuvanted NYVAC, or left unvaccinated. Six weeks after the second vaccination, all pigs were challenged intra-tracheally with low pathogenic (LP) H5N2 AIV A/chicken/Belgium/150/99. Sera were examined in haemagglutination inhibition (HI) tests against the H5N1 AIV from which the vaccine haemagglutinin derived, the challenge virus and the human A/Vietnam/1194/04 HPAIV. After challenge pigs were compared for H5N2 virus replication in the trachea and 4 lung lobes at 24 or 72h post-challenge. Vaccination was well tolerated by all animals. Antibody titres peaked 2 weeks after the second vaccination and were 2- to 4-fold higher against the vaccine virus than heterologous H5 viruses. The NYVAC and ALVAC adjuvanted vaccines consistently induced higher antibody titres than TROVAC or NYVAC without adjuvant. Following challenge, the H5N2 challenge virus was isolated from all unvaccinated pigs, while 19 out of 21 vaccinates showed complete virological protection. Pox-vector vaccines were safe, immunogenic and efficacious against challenge with a heterologous H5 AIV, offering an alternative to classical inactivated vaccines. It remains to be seen whether they would protect against a swine-adapted H5 virus, which may replicate 100-1000 times better than our challenge virus.

Poxvirus tropism for primary human leukocytes and hematopoietic cells

Poxviruses including canarypox (ALVAC) and vaccinia viruses have, in recent years, received considerable attention as live vectors for the development of vaccines against infectious diseases such as AIDS, malaria, and tuberculosis. However, the cellular targets for viral infection within the human immune system and the consequences of infection for cells involved in the generation of immune responses have not been clearly delineated. Using recombinant enhanced green fluorescence protein (EGFP)-expressing ALVAC and vaccinia viruses, we have focused here on a side-by-side comparison of ALVAC and vaccinia virus tropism for cells from human peripheral blood and bone marrow. Both ALVAC and vaccinia viruses showed a strong bias toward monocyte infection. ALVAC minimally infected CD19(+) B cells and was unable to infect ex vivo NK cells and T lymphocytes, whereas vaccinia virus could infect B lymphocytes and NK cell populations. Vaccinia virus was also able to infect T lymphocytes at low but detectable levels which could be enhanced upon their activation. Both ALVAC and vaccinia viruses could infect immature monocyte-derived dendritic cells (MDDCs), but only ALVAC infection induced their subsequent maturation. Infection in human bone marrow cells showed that ALVAC infection was restricted to a myelomonocytoid cell-specific CD33(+) cell population, while vaccinia virus showed a strong, but not exclusive, preference for these cells.

Analysis of TCRalphabeta combinations used by simian immunodeficiency virus-specific CD8+ T cells in rhesus monkeys: implications for CTL immunodominance

Immunodominance is a common feature of Ag-specific CTL responses to infection or vaccines. Understanding the basis of immunodominance is crucial to understanding cellular immunity and viral evasion mechanisms and will provide a rational approach for improving HIV vaccine design. This study was performed comparing CTLs specific for the SIV Gag p11C (dominant) and SIV Pol p68A (subdominant) epitopes that are consistently generated in Mamu-A*01(+) rhesus monkeys exposed to SIV proteins. Additionally, vaccinated monkeys were used to prevent any issues of antigenic variation or dynamic changes in CTL responses by continuous Ag exposure. Analysis of the TCR repertoire revealed the usage of higher numbers of TCR clones by the dominant p11C-specific CTL population. Preferential usage of specific TCRs and the in vitro functional TCR-alpha- and -beta-chain-pairing assay suggests that every peptide/MHC complex may only be recognized by a limited number of unique combinations of alpha- and beta-chain pairs. The wider array of TCR clones used by the dominant p11C-specific CTL population might be explained by the higher probability of generating those specific TCR chain pairs. Our data suggest that Ag-specific naive T cell precursor frequency may be predetermined and that this process dictates immunodominance of SIV-specific CD8(+) T cell responses. These findings will aid in understanding immunodominance and designing new approaches to modulate CTL responses.

Spatial and temporal expression of herpes simplex virus type 1 amplicon-encoded genes: implications for their use as immunization vectors

There is great interest in developing new immunization vectors. Helper virus-free herpes amplicons, plasmid-based vectors that encode no viral gene products and have an extremely large coding capacity, are attractive viral vaccine candidates for expressing recombinant proteins in vivo for immunization. Earlier studies in mice, using amplicons encoding the gp120 protein of human immunodeficiency virus (HIV), resulted in strikingly robust cellular immune responses as measured by cytotoxicity and interferon gamma enzyme-linked immunospot assays. To begin to understand how such vectors function in vivo to generate an immune response, we used amplicons encoding reporter constructs including green fluorescent protein (GFP) and luciferase to examine the duration of expression after administration to mice. Luciferase expression, measured with the IVIS system from Xenogen/Caliper Life Sciences (Hopkinton, MA) and by enzymatic assays of tissue extracts, revealed that expression after injection of the HSVluc amplicons peaked earlier than 24 hr after injection into mice. HSVegfp injection resulted in peak accumulation of GFP 24 hr after administration in vivo. Thus, both reporter genes revealed a rather rapid and robust expression pattern of short duration. The short period of expression appears in part to be due to gene silencing. Examination of the cells transduced by amplicons encoding GFP and human B7.1 suggested that the amplicons transduce a variety of cells, including professional antigen-presenting cells. From this and previous work, we conclude that amplicons may engender a potent immune response by directly transducing dendritic cells as well as by cross-priming of antigen produced by other transduced host cells.

Ability of herpes simplex virus vectors to boost immune responses to DNA vectors and to protect against challenge by simian immunodeficiency virus

The immunogenicity and protective capacity of replication-defective herpes simplex virus (HSV) vector-based vaccines were examined in rhesus macaques. Three macaques were inoculated with recombinant HSV vectors expressing Gag, Env, and a Tat-Rev-Nef fusion protein of simian immunodeficiency virus (SIV). Three other macaques were primed with recombinant DNA vectors expressing Gag, Env, and a Pol-Tat-Nef-Vif fusion protein prior to boosting with the HSV vectors. Robust anti-Gag and anti-Env cellular responses were detected in all six macaques. Following intravenous challenge with wild-type, cloned SIV239, peak and 12-week plasma viremia levels were significantly lower in vaccinated compared to control macaques. Plasma SIV RNA in vaccinated macaques was inversely correlated with anti-Rev ELISPOT responses on the day of challenge (P value<0.05), anti-Tat ELISPOT responses at 2 weeks post challenge (P value <0.05) and peak neutralizing antibody titers pre-challenge (P value 0.06). These findings support continued study of recombinant herpesviruses as a vaccine approach for AIDS.

Antibody and IFN-γ responses induced by a recombinant canarypox vaccine and challenge infection with equine influenza virus

In horses, equine influenza virus (EIV) is a leading cause of respiratory disease. Conventional inactivated vaccines induce a short-lived immune response. By comparison, natural infection confers a long-term immunity to re-infection. An aim of new equine influenza vaccines is to more closely mimic natural infection in order to achieve a better quality of immunity. A new live recombinant vaccine derived from the canarypox virus vector and expressing haemagglutinin genes of EIV (subtype H3N8) has been developed. Stimulation of the immune system was studied after immunisation with this canarypox-based vaccine and challenge infection by exposure to a nebulised aerosol of EIV. The humoral immune response was evaluated by measuring serum antibody levels using the single radial haemolysis (SRH) assay. The cellular immune response was assessed by the measurement of interferon gamma (IFN-gamma) synthesis in peripheral blood mononuclear cells (PBMC). Clinical signs of the disease (temperature, coughing, nasal discharge, dyspnoea, depression and anorexia) and virus excretion were monitored after challenge infection. Clinical signs and virus shedding were significantly reduced in vaccinates compared with unvaccinated controls. EIV-specific immunity was stimulated by vaccination with a recombinant vaccine as serological responses were detected after immunisation. This study also provided the first evidence for increased IFN-gamma protein synthesis in vaccinated ponies following challenge infection with EIV compared with control ponies.

Virus fitness: concept, quantification, and application to HIV population dynamics

Viral fitness has been broadly studied during the past three decades, mainly to test evolutionary models and population theories difficult to analyze and interpret with more complex organisms. More recent studies, however, are focused in the role of fitness on viral transmission, pathogenesis, and drug resistance. Here, we used human immunodeficiency virus (HIV) as one of the most relevant models to evaluate the importance of viral quasispecies and fitness in HIV evolution, population dynamics, disease progression, and potential clinical implications.

Systemic immunization with an ALVAC-HIV-1/protein boost vaccine strategy protects rhesus macaques from CD4+ T-cell loss and reduces both systemic and mucosal simian-human immunodeficiency virus SHIVKU2 RNA levels

Transmission of human immunodeficiency virus type 1 (HIV-1) occurs primarily via the mucosal route, suggesting that HIV-1 vaccines may need to elicit mucosal immune responses. Here, we investigated the immunogenicity and relative efficacy of systemic immunization with two human ALVAC-HIV-1 recombinant vaccines expressing Gag, Pol, and gp120 (vCP250) or Gag, Pol, and gp160 (vCP1420) in a prime-boost protocol with their homologous vaccine native Env proteins. The relative efficacy was measured against a high-dose mucosal exposure to the pathogenic neutralization-resistant variant SHIV(KU2) (simian-human immunodeficiency virus). Systemic immunization with both vaccine regimens decreased viral load levels not only in blood but unexpectedly also in mucosal sites and protected macaques from peripheral CD4+ T-cell loss. This protective effect was stronger when the gp120 antigen was included in the vaccine. Inclusion of recombinant Tat protein in the boosting phase along with the Env protein did not contribute further to the preservation of CD4+ T cells. Thus, systemic immunization with ALVAC-HIV-1 vaccine candidates elicits anti-HIV-1 immune responses able to contain virus replication also at mucosal sites in macaques.

Induction of Positive Cellular and Humoral Immune Responses by a Prime-Boost Vaccine Encoded with Simian Immunodeficiency Virusgag/pol

It is believed likely that immune responses are responsible for controlling viral load and infection. In this study, when macaques were primed with plasmid DNA encoding SIV gag and pol genes (SIVgag/pol DNA) and then boosted with replication-deficient vaccinia virus DIs recombinant expressing the same genes (rDIsSIVgag/pol), this prime-boost regimen generated higher levels of Gag-specific CD4+ and CD8+ T cell responses than did either SIVgag/pol DNA or rDIsSIVgag/pol alone. When the macaques were i.v. challenged with pathogenic simian/HIV, the prime-boost group maintained high CD4+ T cell counts and reduced plasma viral loads up to 30 wk after viral challenge, whereas the rDIsSIVgag/pol group showed only a partial attenuation of the viral infection, and the group immunized with SIVgag/pol DNA alone showed none at all. The protection levels were better correlated with the levels of virus-specific T cell responses than the levels of neutralization Ab responses. These results demonstrate that a vaccine regimen that primes with DNA and then boosts with a replication-defective vaccinia virus DIs generates anti-SIV immunity, suggesting that it will be a promising vaccine regimen for HIV-1 vaccine development.

Vaccine-induced CD8+ central memory T cells in protection from simian AIDS

Critical to the development of an effective HIV vaccine is the identification of adaptive immune responses that prevent infection or disease. In this study we demonstrate in a relevant nonhuman primate model of AIDS that the magnitude of vaccine-induced virus-specific CD8(+) central memory T cells (T(CM)), but not that of CD8(+) effector memory T cells, inversely correlates with the level of SIVmac251 replication, suggesting their pivotal role in the control of viral replication. We propose that effective preventive or therapeutic T cell vaccines for HIV-1 should induce long-term protective central memory T cells.

A sensitive flow cytometry-based cytotoxic T-lymphocyte assay through detection of cleaved caspase 3 in target cells

We describe a highly sensitive flow cytometry-based CTL assay using the cleavage of caspase 3 in target cells as a readout. The assay involved labeling of cells with a cell tracker dye and staining permeabilized cells with an antibody recognizing cleaved caspase 3. The assay proved to be robust and reliable in measuring antigen-specific CTL activity in a number of human and murine systems, including MLR, human peptide-specific T-cell responses induced in vitro, and CTL responses following immunization of mice with viral and peptide vaccines. The assay was found to yield comparable results as 51Cr-release, but with markedly higher sensitivity. When compared to detection of antigen-specific T cells via HLA tetramer/pentamer-based methods of T-cell staining in HIV gag peptide-specific human T cell lines the caspase 3 cleavage readout assay exhibited a comparable level of sensitivity with detection of CTL function at antigen-specific T-cell frequencies of 1:15,000 or lower. A similar level of sensitivity was obtained when murine CTL assays were performed with MLR in which effector cells were highly diluted with naïve syngeneic spleen cells. Our results indicate that the caspase 3 cleavage assay may be a powerful tool to measure antigen-specific CTL responses in human vaccine trials and in pre-clinical animal models of CTL function at both high and low effector cell frequencies.

Attenuated poxvirus-based simian immunodeficiency virus (SIV) vaccines given in infancy partially protect infant and juvenile macaques against repeated oral challenge with virulent SIV

An infant macaque model was developed to test pediatric vaccine candidates aimed at reducing HIV transmission through breast-feeding. Infant macaques were given multiple immunizations during the first 3 weeks of life with recombinant poxvirus vaccines expressing simian immunodeficiency virus (SIV) structural proteins Gag, Pol, and Env (ALVAC-SIV or modified vaccinia virus Ankara [MVA]-SIV). After repeated daily oral inoculations with virulent SIVmac251 at 4 weeks of age, significantly fewer ALVAC-SIV-immunized infants were infected compared with unimmunized infants. Monkeys not infected after oral challenge in infancy were rechallenged at 16 months of age or older by repeated weekly oral SIV exposure; unimmunized animals were infected after fewer SIV exposures than were animals vaccinated with ALVAC-SIV or MVA-SIV. When infected, ALVAC-SIV- and MVA-SIV-vaccinated animals also had reduced viremia compared with unimmunized animals. The results of these investigations suggest that immunization of human infants with poxvirus-based HIV vaccine candidates may offer protection against early and late HIV infection through breastfeeding.

Progress Toward an HIV Vaccine

The development of an HIV vaccine is proving to be an unprecedented challenge. The difficulty in creating this vaccine arises from the enormous genetic variation of the virus and the unusual importance of cytotoxic T lymphocytes (CTL) in controlling its spread. Whereas traditional vaccine strategies are unlikely to confer safe and effective HIV protection, novel strategies for eliciting CTL have provided substantial clinical benefits in nonhuman primate model systems. These vaccine strategies, including plasmid DNA and live recombinant vectors, are currently being evaluated in human clinical trials.

Novel and promiscuous CTL epitopes in conserved regions of Gag targeted by individuals with early subtype C HIV type 1 infection from southern Africa

Characterization of optimal CTL epitopes in Gag can provide crucial information for evaluation of candidate vaccines in populations at the epicenter of the HIV-1 epidemic. We screened 38 individuals with recent subtype C HIV-1 infection using overlapping consensus C Gag peptides and hypothesized that unique HLA-restricting alleles in the southern African population would determine novel epitope identity. Seventy-four percent of individuals recognized at least one Gag peptide pool. Ten epitopic regions were identified across p17, p24, and p2p7p1p6, and greater than two-thirds of targeted regions were directed at: TGTEELRSLYNTVATLY (p17, 35%); GPKEPFRDYVDRFFKTLRAEQATQDV (p24, 19%); and RGGKLDKWEKIRLRPGGKKHYMLKHL (p17, 15%). After alignment of these epitopic regions with consensus M and a consensus subtype C sequence from the cohort, it was evident that the regions targeted were highly conserved. Fine epitope mapping revealed that five of nine identified optimal Gag epitopes were novel: HLVWASREL, LVWASRELERF, LYNTVATLY, PFRDYVDRFF, and TLRAEQATQD, and were restricted by unique HLA-Cw*08, HLA-A*30/B*57, HLA-A*29/B*44, and HLA-Cw*03 alleles, respectively. Notably, three of the mapped epitopes were restricted by more than one HLA allele. Although these epitopes were novel and restricted by unique HLA, they overlapped or were embedded within previously described CTL epitopes from subtype B HIV-1 infection. These data emphasize the promiscuous nature of epitope binding and support our hypothesis that HLA diversity between populations can shape fine epitope identity, but may not represent a constraint for universal recognition of Gag in highly conserved domains.

Influence of polypeptide size and intracellular sorting on the induction of epitope-specific CTL responses by DNA vaccines in a mouse model

We have analysed the influence of size, intracellular localisation, and sorting of various human immunodeficiency virus type 1 (HIV-1)-derived Gag and Env polypeptides containing well defined H2(d)-restricted cytotoxic T lymphocyte (CTL) epitopes on the induction of a humoral and cellular immune response after DNA vaccination. Thus, expression vectors were generated based on RNA- and codon-optimised genes encoding (i). budding competent full-length Gag, (ii). a myristylation defect mutant GagMyr(-), (iii). the isolated p24 capsid moiety of Gag as well as variants of these proteins, which were C-terminally fused HIV gp120-derived V3 epitope (R10I), respectively. These constructs were compared to different minitopes each encoding one of the H2(d)-restricted Gag epitopes A9I and E10F or the V3 epitope R10I that were directly linked to the C-terminus of an Ad2-E3 protein-derived ER signal peptide. Immunological evaluation of these constructs in BALB/c mice revealed that both, the budding competent as well as the intracellular Gag proteins were-irrespective of their molecular weights-equally efficient in the priming of Gag-specific humoral and cellular immune responses. In addition, the capacity of these constructs to stimulate Gag-specific humoral as well as H2-K(d) and H2-L(d) restricted cellular immune responses was not influenced by C-terminal fusion of the immunodominant H2-D(d) restricted V3 epitope. Chimeric GagV3 polyproteins encoding all three major CTL epitopes within a continuous polyprotein were more efficient to stimulate epitope-specific cellular immune responses than the selected minitopes. In addition, the minitopes failed to induce epitope-specific antibody responses. These results clearly show the advantages of complex polypeptides over minitopes regarding the induction of strong humoral and cellular immune responses.

Recombinant poxvirus boosting of DNA-primed rhesus monkeys augments peak but not memory T lymphocyte responses

Although a consensus has emerged that an HIV vaccine should elicit a cytotoxic T lymphocyte (CTL) response, the characteristics of an effective vaccine-induced T lymphocyte response remain unclear. We explored this issue in the simian human immunodeficiency virus/rhesus monkey model in the course of assessing the relative immunogenicity of vaccine regimens that included a cytokine-augmented plasmid DNA prime and a boost with DNA or recombinant pox vectors. Recombinant vaccinia virus, recombinant modified vaccinia Ankara (MVA), and recombinant fowlpox were comparable in their immunogenicity. Moreover, whereas the magnitude of the peak vaccine-elicited T lymphocyte responses in the recombinant pox virus-boosted monkeys was substantially greater than that seen in the monkeys immunized with plasmid DNA alone, the magnitudes of recombinant pox boosted CTL responses decayed rapidly and were comparable to those of the DNA-alone-vaccinated monkeys by the time of viral challenge. Consistent with these comparable memory T cell responses, the clinical protection seen in all groups of experimentally vaccinated monkeys was similar. This study, therefore, indicates that the steady-state memory, rather than the peak effector vaccine-elicited T lymphocyte responses, may be the critical immune correlate of protection for a CTL-based HIV vaccine.

Avipox-based simian immunodeficiency virus (SIV) vaccines elicit a high frequency of SIV-specific CD4+ and CD8+ T-cell responses in vaccinia-experienced SIVmac251-infected macaques

The ability of ALVAC- or fowlpox-based simian immunodeficiency virus (SIV) vaccines to boost SIV-specific CD4+ and CD8+ T-cell responses was tested in 10 vaccinia-experienced macaques infected with SIVmac251. The CD8+ T-cell response to the dominant Gag(181-189) CM9 was quantitated in seven Mamu-A*01-positive macaques by tetramer staining, by ex vivo cytotoxic T-lymphocyte (CTL) activity, and by intracellular cytokine staining (ICS) with the specific Gag(181-189) CM9 peptide. The overall CD8+ T-cell response to Gag was assessed using a peptide pool encompassing the entire Gag protein followed by measurement of TNF-alpha production in ICS assay. Similarly, virus-specific CD4+ T-cell responses were measured by ICS for TNF-alpha following stimulation with the Gag-overlapping peptide and by proliferative response following stimulation with purified p27 Gag. The two vaccine modalities effectively boosted both CD4+ and CD8+ SIV-specific T-cell response despite prior exposure to the vaccinia-derivative NYVAC vector, suggesting that sequential boosting with either avipox-based vector vaccine candidate is a realistic approach in immune therapy of human immunodeficiency virus type 1 (HIV-1)-infected individuals.

Efficacy of a canarypox virus-vectored vaccine against feline leukaemia

Canarypox virus recombinant vaccines have a unique efficacy and safety profile for the vaccinated host because the canarypox virus is non-replicative in mammalian hosts. After the vaccination of a mammalian species, recombinant canarypox viruses express the inserted genes but cannot multiply in the host. They stimulate a strong immune response in the absence of any virus amplification in the host or any viral spread into the environment. A new canarypox-based recombinant vaccine is the canarypox-feline leukaemia virus (FeLV) vaccine (EURIFEL FeLV; Merial) that expresses the FeLV env and gag protective genes. This paper describes experiments which demonstrate that it is effective against any oronasal FeLV challenge. The protection was shown to be solid against an oronasal challenge one year after the initial vaccination, and was effective against a very severe 'in-contact' challenge. Furthermore, the canarypox virus-FeLV vaccine was effective without an adjuvant.

Potent, persistent induction and modulation of cellular immune responses in rhesus macaques primed with Ad5hr-simian immunodeficiency virus (SIV) env/rev, gag, and/or nef vaccines and boosted with SIV gp120

Immunity elicited by multicomponent vaccines delivered by replication-competent Ad5hr-simian immunodeficiency virus (SIV) recombinants was systematically investigated. Rhesus macaques were immunized mucosally at weeks 0 and 12 with Ad5hr-SIV(smH4) env/rev, with or without Ad5hr-SIV(mac239) gag or Ad5hr-SIV(mac239) nef, or with all three recombinants. The total Ad5hr dosage was comparably adjusted among all animals with empty Ad5hr-DeltaE3 vector. The macaques were boosted with SIV gp120 in monophosphoryl A-stable emulsion adjuvant at 24 and 36 weeks. Controls received Ad5hr-DeltaE3 vector or adjuvant only. By ELISPOT analysis, all four SIV gene products elicited potent cellular immune responses that persisted 42 weeks post-initial immunization. Unexpectedly, modulation of this cellular immune response was observed among macaques receiving one, two, or three Ad5hr-SIV recombinants. Env responses were significantly enhanced throughout the immunization period in macaques immunized with Ad5hr-SIV env/rev plus Ad5hr-SIV gag and tended to be higher in macaques that also received Ad5hr-SIV nef. Macaques primed with all three recombinants displayed significant down-modulation in numbers of gamma interferon (IFN-gamma)-secreting cells specific for SIV Nef, and the Env- and Gag-specific responses were also diminished. Modulation of antibody responses was not observed. Down-modulation was seen only during the period of Ad5hr-recombinant priming, not during subunit boosting, although SIV-specific IFN-gamma-secreting cells persisted. The effect was not attributable to Ad5hr replication differences among immunization groups. Vaccine delivery via replication-competent live vectors, which can persistently infect new cells and continuously present low-level antigen, may be advantageous in overcoming competition among complex immunogens for immune recognition. Effects of current multicomponent vaccines on individual immune responses should be evaluated with regard to future vaccine design.

Prior DNA immunization enhances immune response to dominant and subdominant viral epitopes induced by a fowlpox-based SIVmac vaccine in long-term slow-progressor macaques infected with SIVmac251

A therapeutic vaccine for individuals infected with HIV-1 and treated with antiretroviral therapy (ART) should be able to replenish virus-specific CD4+ T-cells and broaden the virus-specific CD8+ T-cell response in order to maintain CD8+ T-cell function and minimize viral immune escape after ART cessation. Because a combination of DNA and recombinant poxvirus vaccine modalities induces high levels of virus-specific CD4+ T-cell response and broadens the cytolytic activity in naive macaques, we investigated whether the same results could be obtained in SIVmac251-infected macaques. The macaques studied here were long-term nonprogressors that naturally contained viremia but were nevertheless treated with a combination of antiviral drugs to assess more carefully the effect of vaccination in the context of ART. The combination of a DNA expressing the gag and pol genes (DNA-SIV-gp) of SIVmac239 followed by a recombinant fowlpox expressing the same SIVmac genes (FP-SIV-gp) was significantly more immunogenic than two immunizations of FP-SIV-gp in SIVmac251-infected macaques treated with ART. The DNA/FP combination significantly expanded and broadened Gag-specific T-cell responses measured by tetramer staining, ELISPOT, and intracellular cytokine staining and measurement of ex vivo cytolytic function. Importantly, the combination of these vaccine modalities also induced a sizeable expansion in most macaques of Gag-specific CD8-(CD4+) T-cells able to produce TNF-alpha. Hopefully, this modality of vaccine combination may be useful in the clinical management of HIV-1-infected individuals.

A replication competent adenovirus 5 host range mutant-simian immunodeficiency virus (SIV) recombinant priming/subunit protein boosting vaccine regimen induces broad, persistent SIV-specific cellular immunity to dominant and subdominant epitopes in Mamu-A*01 rhesus macaques

CTL are important in controlling HIV and SIV infection. To quantify cellular immune responses induced by immunization, CD8(+) T cells specific for the subdominant Env p15m and p54m epitopes and/or the dominant Gag p11C epitope were evaluated by tetramer staining in nine macaques immunized with an adenovirus (Ad) 5 host range mutant (Ad5hr)-SIVenv/rev recombinant and in four of nine which also received an Ad5hr-SIVgag recombinant. Two Ad5hr-SIV recombinant priming immunizations were followed by two boosts with gp120 protein or an envelope polypeptide representing the CD4 binding domain. Two mock-immunized macaques served as controls. IFN-gamma-secreting cells were also assessed by ELISPOT assay using p11C, p15m, and p54m peptide stimuli and overlapping pooled Gag and Env peptides. As shown by tetramer staining, Ad-recombinant priming elicited a high frequency of persistent CD8(+) T cells able to recognize p11C, p15m, and p54m epitopes. The presence of memory cells 38 wk postinitial immunization was confirmed by expansion of tetramer-positive CD8(+) T cells following in vitro stimulation. The SIV-specific CD8(+) T cells elicited were functional and secreted IFN-gamma in response to SIV peptide stimuli. Although the level and frequency of response of peripheral blood CD8(+) T cells to the subdominant Env epitopes were not as great as those to the dominant p11C epitope, elevated responses were observed when lymph node CD8(+) T cells were evaluated. Our data confirm the potency and persistence of functional cellular immune responses elicited by replication competent Ad-recombinant priming. The cellular immunity elicited is broad and extends to subdominant epitopes.

Immune intervention strategies for HIV-1 infection of humans in the SIV macaque model

Studies in the SIVmac macaque model have demonstrated that the extent of virus-specific CD4+ and CD8+ T-cell responses induced by vaccination prior to virus-challenge exposure correlate with viremia containment following establishment of infection. These findings led to the hypothesis that active immunization with vaccines able to induce virus-specific T-cell responses following the establishment of infection could also ameliorate the virological outcome. Here, we will review the relative effect of ART and vaccination during primary SIVmac infection of macaques.

Comparative vaccine studies in HLA-A2.1-transgenic mice reveal a clustered organization of epitopes presented in hepatitis C virus natural infection

A polyepitopic CD8(+)-T-cell response is thought to be critical for control of hepatitis C virus (HCV) infection. Using transgenic mice, we analyzed the immunogenicity and dominance of most known HLA-A2.1 epitopes presented during infection by using vaccines that carry the potential to enter clinical trials: peptides, DNA, and recombinant adenoviruses. The vaccines capacity to induce specific cytotoxic T lymphocytes and interferon gamma-producing cells revealed that immunogenic epitopes are clustered in specific antigens. For two key antigens, flanking regions were shown to greatly enhance the scope of epitope recognition, whereas a DNA-adenovirus prime-boost vaccination strategy augmented epitope immunogenicity, even that of subdominant ones. The present study reveals a clustered organization of HCV immunogenic HLA.A2.1 epitopes and strategies to modulate their dominance.

Prior vaccination increases the epitopic breadth of the cytotoxic T-lymphocyte response that evolves in rhesus monkeys following a simian-human immunodeficiency virus infection

Although recent evidence has confirmed the importance of cytotoxic T-lymphocyte (CTL) responses in controlling human immunodeficiency virus type 1 and simian immunodeficiency virus replication, the relevance of the epitopic breadth of those CTL responses remains unexplored. In the present study, we sought to determine whether vaccination can expand CTL populations which recognize a repertoire of viral epitopes that is greater than is typically generated in the course of a viral infection. We demonstrate that potent secondary CTL responses to subdominant epitopes are rapidly generated following a pathogenic simian-human immunodeficiency virus challenge of rhesus monkeys vaccinated with plasmid DNA or recombinant modified vaccinia virus Ankara vaccines. These data indicate that prior vaccination can increase the breadth of the CTL response that evolves after an AIDS virus infection.