Immunogenicity of rVSVΔG-ZEBOV-GP Ebola vaccine (ERVEBO®) in African clinical trial participants by age, sex, and baseline GP-ELISA titer: A post hoc analysis of three Phase 2/3 trials.

BACKGROUND: ERVEBO®, a live recombinant vesicular stomatitis virus (VSV) vaccine containing the Zaire ebolavirus glycoprotein (GP) in place of the VSV GP (rVSVΔG-ZEBOV-GP), was advanced through clinical development by Merck & Co., Inc., Rahway, NJ, USA in collaboration with multiple partners to prevent Ebola virus disease (EVD) and has been approved for human use in several countries. METHODS: We evaluated data from three Phase 2/3 clinical trials conducted in Liberia (PREVAIL), Guinea (FLW), and Sierra Leone (STRIVE) during the 2013-2016 West African EVD outbreak to assess immune responses using validated assays. We performed a post hoc analysis of the association of vaccine response with sex, age (18-50 yrs & >50 yrs), and baseline (BL) GP-enzyme-linked immunosorbent assay (ELISA) titer (<200 & ≥200 EU/mL), including individual study (PREVAIL, FLW, or STRIVE) data and pooled data from all 3 studies. The endpoints were total IgG antibody response (EU/mL) measured by the GP-ELISA and neutralizing antibody response measured by the plaque reduction neutralization test (PRNT) to rVSVΔG-ZEBOV-GP at Days 28, 180, and 365 postvaccination. RESULTS: In the overall pooled population, in all subgroups, and in each trial independently, GP-ELISA and PRNT geometric mean titers increased from BL, generally peaking at Day 28 and persisting through Day 365. Immune responses were greater in women and participants with BL GP-ELISA ≥ 200 EU/mL, but did not differ across age groups. CONCLUSION: These data demonstrate that rVSVΔG-ZEBOV-GP elicits a robust and durable immune response through 12 months postvaccination in participants regardless of age, sex, or BL GP-ELISA titer. The higher immune responses observed in women and participants with pre-existing immunity are consistent with those described previously and for other vaccines. Trials were registered as follows: PREVAIL: ClinicalTrials.gov NCT02344407; FLW: Pan African Clinical Trials Registry PACTR201503001057193; STRIVE: ClinicalTrials.gov NCT02378753. Protocols V920-009, 011, and 018.

Estimation of the correlates of protection of the rVSVΔG-ZEBOV-GP Zaire ebolavirus vaccine: a post-hoc analysis of data from phase 2/3 clinical trials.

BACKGROUND: Establishment of immune correlates of protection can provide a measurable criterion for assessing protection against infection or disease. For some vaccines, such as the measles vaccine, antibodies serve as the correlate of protection, but for others, such as human papillomavirus, the correlate of protection remains unknown. Merck & Co, Kenilworth, NJ, USA, in collaboration with multiple partners, developed a live recombinant vesicular stomatitis virus vaccine (rVSVΔG-ZEBOV-GP [ERVEBO]) containing the Zaire ebolavirus glycoprotein (GP) in place of the recombinant vesicular stomatitis virus GP to prevent Ebola virus disease. Seroresponse, defined as post-vaccination GP-ELISA of 200 ELISA units (EU) per mL or higher and two-times or more above baseline, was proposed; however, correlates of protection have not been determined. The objective of this post-hoc analysis was to infer possible correlates of protection for rVSVΔG-ZEBOV-GP. METHODS: In this post-hoc analysis we included vaccinated participants with serology data from three phase 2/3 immunogenicity trials in Guinea, Sierra Leone, and Liberia (n=2199). Two of the trials were open-label, single-arm trials (one randomised [STRIVE], one non-randomised [FLW]); and one trial was randomised, placebo-controlled with two vaccine comparators (PREVAIL). Endpoints were total IgG antibody response (EU per mL) to rVSVΔG-ZEBOV-GP measured by GP-ELISA and neutralising antibody response to rVSVΔG-ZEBOV-GP measured by plaque reduction neutralisation test at days 14, 28, 180, and 365 after vaccination. Reverse cumulative distribution curves of the antibody concentrations were used to estimate statistical correlates of protection between 70% and 100% that might be applied to vaccine efficacy and effectiveness estimates. FINDINGS: Although GP-ELISA and plaque reduction neutralisation tests showed similar response patterns, GP-ELISA provided a wider range of measurable titres and better differentiation for estimating correlates of protection compared with the plaque reduction neutralisation test. At day 14 after vaccination in the FLW trial, 1060 (100%) of 1060 participants had GP-ELISA levels at or above 68 EU per mL and 742 (70%) of 1060 had levels at or above 313 EU per mL. At day 28 after vaccination in the pooled population, 1953 (100%) of 1953 participants had levels at or above 73 EU per mL and 1368 (70%) of 1953 participants had levels at or above 735 EU per mL. GP-ELISA seroresponse 200 EU per mL or higher and two-times or more increase in antibody level from baseline occurred in 80% or higher of participants at each assessment and in 94% or higher of participants at any time after vaccination. INTERPRETATION: Our results are consistent with previous work suggesting that seroresponse defined as GP-ELISA of 200 EU per mL or higher and two-times or more from baseline associated with vaccination might be the most appropriate dichotomous correlate of protection and falls within the seroprotective threshold range described herein. FUNDING: Merck Sharp & Dohme, Biomedical Advanced Research and Development Authority, Office of the Assistant Secretary for Preparedness and Response, US Department of Health and Human Services.

A Single-Dose Recombinant Parainfluenza Virus 5-Vectored Vaccine Expressing Respiratory Syncytial Virus (RSV) F or G Protein Protected Cotton Rats and African Green Monkeys from RSV Challenge.

Human respiratory syncytial virus (RSV) is a common cause of severe respiratory disease among infants, immunocompromised individuals, and the elderly. No licensed vaccine is currently available. In this study, we evaluated two parainfluenza virus 5 (PIV5)-vectored vaccines expressing RSV F (PIV5/F) or G (PIV5/G) protein in the cotton rat and African green monkey models for their replication, immunogenicity, and efficacy of protection against RSV challenge. Following a single intranasal inoculation, both animal species shed the vaccine viruses for a limited time but without noticeable clinical symptoms. In cotton rats, the vaccines elicited RSV F- or G-specific serum antibodies and conferred complete lung protection against RSV challenge at doses as low as 103 PFU. Neither vaccine produced the enhanced lung pathology observed in animals immunized with formalin-inactivated RSV. In African green monkeys, vaccine-induced serum and mucosal antibody responses were readily detected, as well. PIV5/F provided nearly complete protection against RSV infection in the upper and lower respiratory tract at a dose of 106 PFU of vaccine. At the same dose levels, PIV5/G was less efficacious. Both PIV5/F and PIV5/G were also able to boost neutralization titers in RSV-preexposed African green monkeys. Overall, our data indicated that PIV5/F is a promising RSV vaccine candidate.IMPORTANCE A safe and efficacious respiratory syncytial virus (RSV) vaccine remains elusive. We tested the recombinant parainfluenza virus 5 (PIV5) vectors expressing RSV glycoproteins for their immunogenicity and protective efficacy in cotton rats and African green monkeys, which are among the best available animal models to study RSV infection. In both species, a single dose of intranasal immunization with PIV5-vectored vaccines was able to produce systemic and local immunity and to protect animals from RSV challenge. The vaccines could also boost RSV neutralization antibody titers in African green monkeys that had been infected previously. Our data suggest that PIV5-vectored vaccines could potentially protect both the pediatric and elderly populations and support continued development of the vector platform.

Rapid isolation of dengue-neutralizing antibodies from single cell-sorted human antigen-specific memory B-cell cultures.

Monitoring antigen-specific memory B cells and the antibodies they encode is important for understanding the specificity, breadth and duration of immune response to an infection or vaccination. The antibodies isolated could further help design vaccine antigens for raising relevant protective immune responses. However, developing assays to measure and isolate antigen-specific memory B cells is technically challenging due to the low frequencies of these cells that exist in the circulating blood. Here, we describe a flow cytometry method to identify and isolate dengue envelope-specific memory B cells using a labeled dengue envelope protein. We enumerated dengue-envelope specific memory B cells from a cohort of dengue seropositive donors using this direct flow cytometry assay. A more established and conventional assay, the cultured B ELISPOT, was used as a benchmark comparator. Furthermore, we were able to confirm the single-sorted memory B-cell specificity by culturing B cells and differentiating them into plasma cells using cell lines expressing CD40L. The culture supernatants were assayed for antigen binding and the ability of the antibodies to neutralize the cognate dengue virus. Moreover, we successfully isolated the heavy and light Ig sequences and expressed them as full-length recombinant antibodies to reproduce the activity seen in culture supernatants. Mapping of these antibodies revealed a novel epitope for dengue 2 virus serotype. In conclusion, we established a reproducible methodology to enumerate antigen-specific memory B cells and assay their encoded antibodies for functional characterization.

Comparison of T cell immune responses induced by vectored HIV vaccines in non-human primates and humans.

Following the disappointing outcome of the phase IIb test-of-concept step study in which Merck's adenovirus type 5 (Ad5) HIV-1 clade B gag/pol/nef vaccine failed to demonstrate efficacy in HIV high-risk individuals, an extensive review of the trial and preclinical studies which supported the trial is ongoing. One point of interest is how well preclinical nonhuman primate immunogenicity studies predicted what was observed in humans. Here we compare the HIV-1-specific cellular immune responses elicited in nonhuman primates and human clinical trial subjects to several HIV-1 vaccine candidates. We find that although rhesus macaques are immunologically more responsive to vaccination than humans, the hierarchy in potency of single-modality prime-boost regimens using several vector approaches (adenovirus, DNA, and pox vectors) was well predicted. Vaccine approaches using complex formulations such as novel adjuvants (DNA+CRL1005) or mixed-modality prime-boost (DNA/Ad5; Ad5/ALVAC) did not correlate as well between rhesus macaques and humans. Although the immunogenicity of the vaccines and vaccine regimens evaluated were not all accurately predicted, testing in rhesus macaques generally offers an indispensable tool for ranking the immunological potential of HIV-1 vaccine candidates.

Comparative cell-mediated immunogenicity of DNA/DNA, DNA/adenovirus type 5 (Ad5), or Ad5/Ad5 HIV-1 clade B gag vaccine prime-boost regimens.

BACKGROUND: We report composite results from the Merck phase I program of near-consensus clade B human immunodeficiency virus (HIV) type 1 gag vaccines. METHODS: Healthy HIV-uninfected adults were enrolled in 6 blinded placebo-controlled studies evaluating the immunogenicity of (1) a 4-dose regimen of a DNA vaccine, (2) a 3-dose priming regimen of the DNA vaccine with a booster dose of an adenovirus type 5 (Ad5)-vectored vaccine, or (3) a 3-dose regimen of the Ad5 vaccine. The DNA plasmid was provided with or without an aluminum phosphate or CRL1005 adjuvant. The primary end point was the unfractionated HIV-1 gag-specific interferon gamma enzyme-linked immunospot (ELISpot) response 4 weeks after the final dose. RESULTS: Overall, 254 (83%) of 307 subjects randomized to the vaccine groups were evaluable. Adjuvants did not enhance immunogenicity of the DNA vaccine. Postboost ELISpot responder frequencies were higher for Ad5-containing regimens than for the DNA/DNA regimen (33%) but were similar for DNA/Ad5 (55%) and Ad5/Ad5 (50%). DNA/DNA elicited mainly a CD4 response, whereas Ad5/Ad5 elicited mainly a CD8 response; DNA/Ad5 generated CD4 and CD8 responses comparable to those of DNA/DNA and Ad5/Ad5, respectively. CONCLUSIONS: The DNA vaccine alone or as a priming regimen for the Ad5 vaccine did not increase unfractionated ELISpot responses compared with the Ad5 vaccine alone. Qualitative T cell responses to different vaccine regimens deserve further study.

Vaccination with Ad5 vectors expands Ad5-specific CD8 T cells without altering memory phenotype or functionality.

BACKGROUND: Adenoviral (Ad) vaccine vectors represent both a vehicle to present a novel antigen to the immune system as well as restimulation of immune responses against the Ad vector itself. To what degree Ad-specific CD8(+) T cells are restimulated by Ad vector vaccination is unclear, although such knowledge would be important as vector-specific CD8(+) T cell expansion could potentially further limit Ad vaccine efficacy beyond Ad-specific neutralizing antibody alone. METHODOLOGY/PRINCIPAL FINDINGS: Here we addressed this issue by measuring human Adenovirus serotype 5 (Ad5)-specific CD8(+) T cells in recipients of the Merck Ad5 HIV-1 vaccine vector before, during, and after vaccination by multicolor flow cytometry. Ad5-specific CD8(+) T-cells were detectable in 95% of subjects prior to vaccination, and displayed primarily an effector-type functional profile and phenotype. Peripheral blood Ad5-specific CD8(+) T-cell numbers expanded after Ad5-HIV vaccination in all subjects, but differential expansion kinetics were noted in some baseline Ad5-neutralizing antibody (Ad5 nAb) seronegative subjects compared to baseline Ad5 nAb seropositive subjects. However, in neither group did vaccination alter polyfunctionality, mucosal targeting marker expression, or memory phenotype of Ad5-specific CD8(+) T-cells. CONCLUSIONS: These data indicate that repeat Ad5-vector administration in humans expands Ad5-specific CD8(+) T-cells without overtly affecting their functional capacity or phenotypic properties. This is a secondary analysis of samples collected during the 016 trial. Results of the Merck 016 trial safety and immunogenicity have been previously published in the journal of clinical infectious diseases [1]. TRIAL REGISTRATION: ClinicalTrials.gov NCT00849680[http://www.clinicaltrials.gov/show/NCT00849680].

Adenovirus-specific immunity after immunization with an Ad5 HIV-1 vaccine candidate in humans.

The immunologic basis for the potential enhanced HIV-1 acquisition in adenovirus serotype 5 (Ad5)-seropositive individuals who received the Merck recombinant Ad5 HIV-1 vaccine in the STEP study remains unclear. Here we show that baseline Ad5-specific neutralizing antibodies are not correlated with Ad5-specific T lymphocyte responses and that Ad5-seropositive subjects do not develop higher vector-specific cellular immune responses as compared with Ad5-seronegative subjects after vaccination. These findings challenge the hypothesis that activated Ad5-specific T lymphocytes were the cause of the potential enhanced HIV-1 susceptibility in the STEP study.

Baseline Ad5 serostatus does not predict Ad5 HIV vaccine-induced expansion of adenovirus-specific CD4+ T cells.

The mechanisms underlying possible increased HIV-1 acquisition in adenovirus 5 (Ad5)-seropositive subjects vaccinated with Ad5-HIV-1 vectors in the Merck STEP trial remain unclear. We find that Ad5 serostatus does not predict Ad5-specific CD4(+) T cell frequency, and we did not observe durable significant differences in Ad5-specific CD4(+) T cells between Ad5-seropositive and Ad5-seronegative subjects after vaccination. These findings indicate no causative role for Ad5-specific CD4(+) T cells in increasing HIV-1 susceptibility in the STEP trial.

Safety and immunogenicity of adenovirus-vectored near-consensus HIV type 1 clade B gag vaccines in healthy adults.

Vaccines inducing pathogen-specific cell-mediated immunity are being developed using attenuated adenoviral (Ad) vectors. We report the results of two independent Phase I trials of similar replication-deficient Ad5 vaccines containing a near-consensus HIV-1 clade B gag transgene. Healthy HIV-uninfected adults were enrolled in two separate, multicenter, dose-escalating, blinded, placebo-controlled studies to assess the safety and immunogenicity of a three-dose homologous regimen of Ad5 and MRKAd5 HIV-1 gag vaccines given on day 1, week 4, and week 26. Adverse events were collected for 29 days following each intradeltoid injection. The primary immunogenicity endpoint was the proportion of subjects with a positive unfractionated Gag-specific IFN-gamma ELISPOT response measured 4 weeks after the last dose (week 30). Analyses were performed after combining data for each dose group from both protocols, stratifying by baseline Ad5 titers. Overall, 252 subjects were randomized to receive either vaccine or placebo, including 229 subjects (91%) who completed the study through week 30. Tolerability and immunogenicity did not appear to differ between the Ad5 and MRKAd5 vaccines. The frequency of injection-site reactions was dose dependent. Systemic adverse events were also dose dependent and more frequent in subjects with baseline Ad5 titers <200 versus > or =200, especially after the first dose. The percent of ELISPOT responders and the ELISPOT geometric means overall were significantly higher for all four vaccine doses studied compared to placebo, and were generally higher in vaccine recipients with baseline Ad5 titers <200 versus > or = 200. Ad5 titers increased after vaccination in a dose-dependent fashion. Both Ad5-vectored HIV-1 vaccines were generally well tolerated and induced cell-mediated immune responses against HIV Gag-peptides in the majority of healthy adults with baseline Ad5 titers <200. Preexistent and/or vaccine-induced immunity to the Ad5 vector may dampen the CMI response to HIV Gag.

DNA gag/adenovirus type 5 (Ad5) gag and Ad5 gag/Ad5 gag vaccines induce distinct T-cell response profiles.

Results from Merck's phase II adenovirus type 5 (Ad5) gag/pol/nef test-of-concept trial showed that the vaccine lacked efficacy against human immunodeficiency virus (HIV) infection in a high-risk population. Among the many questions to be explored following this outcome are whether (i) the Ad5 vaccine induced the quality of T-cell responses necessary for efficacy and (ii) the lack of efficacy in the Ad5 vaccine can be generalized to other vector approaches intended to induce HIV type 1 (HIV-1)-specific T-cell responses. Here we present a comprehensive evaluation of the T-cell response profiles from cohorts of clinical trial subjects who received the HIV CAM-1 gag insert delivered by either a regimen with DNA priming followed by Ad5 boosting (n = 50) or a homologous Ad5/Ad5 prime-boost regimen (n = 70). The samples were tested using a statistically qualified nine-color intracellular cytokine staining assay measuring interleukin-2 (IL-2), tumor necrosis factor alpha, macrophage inflammatory protein 1beta, and gamma interferon production and expression of CD107a. Both vaccine regimens induced CD4(+) and CD8(+) HIV gag-specific T-cell responses which variably expressed several intracellular markers. Several trends were observed in which the frequencies of HIV-1-specific CD4(+) T cells and IL-2 production from antigen-specific CD8(+) T cells in the DNA/Ad5 cohort were more pronounced than in the Ad5/Ad5 cohort. Implications of these results for future vaccine development will be discussed.

Safety and immunogenicity of a replication-incompetent adenovirus type 5 HIV-1 clade B gag/pol/nef vaccine in healthy adults.

BACKGROUND: The safety and immunogenicity of the MRK adenovirus type 5 human immunodeficiency virus type 1 clade B gag/pol/nef vaccine, a replication-incompetent adenovirus type 5-vectored vaccine designed to elicit cell-mediated immunity against conserved human immunodeficiency virus proteins, was assessed in a phase 1 trial. METHODS: Healthy adults not infected with human immunodeficiency virus were enrolled in a multicenter, dose-escalating, blind, placebo-controlled study to evaluate a 3-dose homologous prime-boost regimen of the trivalent MRK adenovirus type 5 human immunodeficiency virus type 1 vaccine containing from 3 x 10(6) to 1 x 10(11) viral particles per 1-mL dose administered on day 1, during week 4 and during week 26. Adverse events were recorded for 29 days after each intradeltoid injection. The primary immunogenicity end point was the proportion of study participants with a positive unfractionated Gag-, Pol-, or Nef-specific interferon-gamma enzyme-linked immunosorbent spot response measured 4 weeks after administration of the last dose. RESULTS: Of 259 randomized individuals, 257 (99%) received > or = 1 dose of vaccine or placebo and were included in the safety analyses. Enzyme-linked immunosorbent spot results were available for 217 study participants (84%) at week 30. No serious vaccine-related adverse events occurred. No study participant discontinued participation because of vaccine-related adverse events. The frequency of injection-site reactions was dose dependent. Vaccine doses of > or = 3 x 10(9) viral particles elicited positive enzyme-linked immunosorbent spot responses to > or = 1 vaccine component in > 60% of recipients. High baseline antibody titers against adenovirus type 5 diminished enzyme-linked immunosorbent spot responses at all doses except the 3 x 10(10) viral particle dose. CONCLUSIONS: The vaccine was generally well tolerated and induced cell-mediated immune responses against human immunodeficiency virus type 1 peptides in most healthy adults. Despite these findings, vaccination in a proof-of-concept trial with use of this vaccine was discontinued because of lack of efficacy.

Evaluation of cellular immune responses in subjects chronically infected with HIV type 1.

The importance of host cellular immune responses, particularly CD8(+) cytotoxic T-lymphocyte (CTL) responses, in control of human immunodeficiency virus type 1 (HIV-1) infection has been demonstrated in many clinical studies. These studies, along with vaccination challenge studies in rhesus macaques, indicate the importance of cellular immune responses against HIV-1. Toward this end, we evaluated anti-HIV-1 cellular immune responses in a cohort of 54 subjects who were chronically infected with HIV-1. By validation of IFN-gamma ELISpot assay, we established a dual cut-off criterion for scoring a positive response. The magnitude and frequency of cellular immune responses were measured against HIV-1 antigens (Gag, Pol, Nef, Rev, and Tat), using synthetic peptides as antigens in ELISpot assay. Here we showed that HIV-1 Gag, Pol, and Nef were frequent targets of T cell responses in these subjects, whereas Tat and Rev were less frequently recognized. We further evaluated the possible association between host cellular immune responses and corresponding plasma viral loads in this cohort. By performing ranking correlation analysis, we demonstrated a positive correlation between host viral loads and ELISpot responses of HIV Gag and Pol in untreated subjects. For the subjects under antiviral regimens, however, we did not find any significant association. Our findings suggest that the high levels of ELISpot responses in chronically infected subjects were reflective of their persistent viral infection.

A comparison of standard immunogenicity assays for monitoring HIV type 1 gag-specific T cell responses in Ad5 HIV Type 1 gag vaccinated human subjects.

Currently, there are numerous candidate HIV vaccines aimed at inducing T-cell mediated immune responses against HIV. To assess the immunogenicity of such vaccines, a reliable T cell assay must be utilized and typically one of the following assays is chosen for this purpose: bulk culture CTL, MHC I tetramer staining, IFN-gamma ELISPOT, or IFN-gamma intracellular cytokine staining. In this paper we report a comparison of the T cell responses detected by each assay in a large cohort of healthy normal volunteers vaccinated with adenovirus serotype 5 expressing HIV gag. Using stringently validated formats of each of these assays and pools of overlapping HIV gag peptides, we demonstrate that there is a high degree of correlation between all four of the common T cell assays, but inherent differences in the sensitivity of each assay to detect responders. In this study, the ELISPOT assay is shown to have the greatest sensitivity in detecting vaccine responses, while the ICS assay, although less sensitive, has the advantage of providing additional information on the phenotype of the responding cells.

Cross-clade reactivity of HIV-1-specific T-cell responses in HIV-1-infected individuals from Botswana and Cameroon.

An effective HIV type 1 (HIV-1) vaccine will likely require elicitation of broadly reactive cell-mediated immune (CMI) responses against divergent HIV-1 clades. We compared anti-HIV-1 T-cell immune responses among 363 unvaccinated adults infected with diverse HIV-1 clades. Response rates to clade B Gag and/or clade B Nef in Botswana (95%) and Cameroon (98%) were similar when compared with those in countries previously studied, including Brazil (92%), Thailand (96%), South Africa (96%), Malawi (100%), and the United States (100%). Substantial cross-clade cell-mediated immune responses in Botswana and Cameroon confirm previous findings in a larger, more genetically diverse collection of HIV-1 samples.

Broad cross-clade T-cell responses to gag in individuals infected with human immunodeficiency virus type 1 non-B clades (A to G): importance of HLA anchor residue conservation.

We aimed to identify cross-clade human immunodeficiency virus type 1 (HIV-1) specific T-cell responses among 10 HLA-typed individuals who were infected with non-B HIV-1 strains (A, AG, C, D, G, or F) and to correlate these responses with genetic variation in documented T-cell epitopes. T-cell reactivity was tested against peptide pools spanning clade B Gag, Pol, Nef, Rev, and Tat consensus, with Gag and Nef providing the highest responses. Nine individuals who responded to clade B Gag demonstrated cross-reactive T-cell responses against clade A and C Gag pools, while six of seven responders to Nef-B reacted to clade A and C Nef pools. An inverse correlation between the height of the T-cell responses and the sequence divergence of the HLA class I-restricted epitopes was identified when we compared autologous Gag and Nef sequences with the reactive consensus pools. This could be explained for the Gag sequences through observed variations in the HLA anchor residues. Through mapping of 30 amino acid cross-clade-reactive regions using Gag-B pools, we were able to link 58% (14/24) of the T-cell responses to regions containing previously described HLA class I-restricted epitopes. Forty-two percent (10/24) of the responses were directed to regions containing new epitopes, for which predicted HLA class I motifs could be recognized in 70% (7/10) of individuals. We demonstrate here that cross-clade T-cell responses are frequently induced in individuals infected with distinct HIV-1 clades, suggesting that interclade variation outside of HLA anchor residues may have less impact on vaccine-induced T-cell reactivity than previously thought.

Replication-incompetent adenoviral vaccine vector elicits effective anti-immunodeficiency-virus immunity.

Recent studies of human immunodeficiency virus type 1 (HIV-1) infection in humans and of simian immunodeficiency virus (SIV) in rhesus monkeys have shown that resolution of the acute viral infection and control of the subsequent persistent infection are mediated by the antiviral cellular immune response. We comparatively assessed several vaccine vector delivery systems-three formulations of a plasmid DNA vector, the modified vaccinia Ankara (MVA) virus, and a replication incompetent adenovirus type 5 (Ad5) vector-expressing the SIV gag protein for their ability to elicit such immune responses in monkeys. The vaccines were tested either as a single modality or in combined modality regimens. Here we show that the most effective responses were elicited by a replication-incompetent Ad5 vector, used either alone or as a booster inoculation after priming with a DNA vector. After challenge with a pathogenic HIV-SIV hybrid virus (SHIV), the animals immunized with Ad5 vector exhibited the most pronounced attenuation of the virus infection. The replication-defective adenovirus is a promising vaccine vector for development of an HIV-1 vaccine.