This report indicates that the MVA-HIV and FPV-HIV candidate vaccines were well tolerated when administered in both homologous and heterologous vector regimens, and heterologous boosting was superior at inducing a balanced HIV-specific CD4+ and CD8+ T-cell response. FPV-HIV was poorly immunogenic when given alone, but significantly boosted CD8+ T-cell responses in participants primed with two doses of MVA-HIV. Overall, the heterologous regimen induced HIV-specific CD4+ and CD8+ T-cell responses in approximately half of the participants after the second boost, and concurrent CD4+ and CD8+ T-cell responses detected in 38% of responders. CD4+ T-cell responses tended to be directed at Gag, while CD8+ T-cell responses were more evenly directed at Gag and Env, but both tended to target a single HIV protein at a time. Although the bulk IFN-γ ELISpot assays do not distinguish CD4+ from CD8+ T-cell phenotypes, it confirmed the ICS findings that the heterologous vector regimen induced better HIV-specific cellular responses than the homologous regimens. In contrast to studies of another poxvirus vector, NYVAC-HIV-C [38
], we observed no gender differences in T-cell response rate or magnitude. It is unclear why the non-structural HIV proteins were so poorly immunogenic in our study, however, we were not able to perform a thorough assessment because only Pol and Nef peptide pools were available for use in the in-vitro assays. In addition, only responses to the HIV reverse transcriptase (RT) portion of Pol could be expected as only that part of Pol was contained in the vaccine constructs.
While the main focus of this study was to assess the tolerability of escalating doses of MVA-HIV and the cellular immunogenicity of these poxvirus-vectored HIV vaccine regimens, we also conducted a limited assessment of HIV-specific humoral immunogenicity. Interestingly, we saw that gp120 binding antibody responses were better in the group that received the homologous vector boosting regimen. However, binding assays were not performed after the priming vaccinations alone, so it is difficult to make firm conclusions about the contribution of the later boosts, aside from the fact that while the FPV-HIV preparation was able to boost CD8+ T-cell responses directed at Env peptide pools, it was not capable of doing so for gp120 binding antibodies.
A number of other studies have been conducted with poxvirus-vectored vaccine regimens in the HIV, malaria and tuberculosis fields, including the heterologous MVA and FPV vector combination [39
] and the same poxviral-vectored products in HIV-infected adolescents [16
]. However, the majority of these studies employed bulk IFN-γ ELISpot as the primary cellular immunogenicity readout and ICS and/or CD4+/CD8+ T-cell depleted INF-γ ELISpot assays were performed only for small subgroups. In addition, it is difficult to compare our results to those from HIV immunotherapy studies as their subjects had already been primed through natural infection. This existing literature does suggest that MVA-vectored constructs induce primarily a CD4+ T-cell response, but the optimal number of priming vaccinations and the precise contribution of a heterologous vector boost has not been clearly delineated. It is unknown why MVA vector regimens tend to induce stronger HIV-specific CD4+ T-cell responses to HIV inserts while replication-defective adenovirus vectors induce stronger CD8+ T-cell responses, but the heterologous pox-vector boost does appear to provide additional antigenic exposure to boost the CD8+ T-cell response. Of note, Precopio et al. demonstrated vigorous anti-vector CD8+ T-cell responses by ICS after three, but not two, intramuscular injections of the same MVA (Therion Biologics), given on the same 0, 1 and 3 month schedule [48
]. This suggests that in our study, strong vector-specific cellular immune responses induced by homologous vector boosting may have out-competed the insert-specific response and suppressed the boosting effect of additional vaccinations. It would be of interest to determine if removal of immunodominant epitopes in MVA can lead to improvement of insert-specific cellular responses.
The lack of correlates of protection from HIV infection and/or disease continues to complicate development of an efficacious HIV-1 vaccine. Nevertheless, clues may be inferred from challenge studies of analogous vaccines in the simian immunodeficiency virus (SIV) model and the results of efficacy trials in humans. A study of Therion’s MVA and FPV vectors with similarly engineered SIV inserts showed an increase in the magnitude of SIV-specific IFN-γ ELISpot responses after FPV-SIV boosting of macaques that received a heterologous regimen as compared to a homologous MVA-SIV regimen [49
]. Particularly instructive are the findings of the phase IIB study of the adenovirus-serotype 5 (Ad5)-HIV-gag/pol/nef candidate vaccine developed by Merck conducted in the Americas and the Phase III study of a combination regimen of canarypox-HIV-gag/protease/env vaccine (vCP1521) boosted by recombinant HIV glycoprotein 120 subunit vaccine (AIDSVAX B/E) in Thailand (Protocol RV144). The Ad5-HIV vaccine induced relatively strong HIV-specific CD8+ T-cell responses in most study participants but modest HIV-specific CD4+ T-cell and antibody responses, and failed to protect participants from infection or reduce setpoint HIV RNA levels [29
]. Alternatively, the canarypox-HIV plus rgp120 regimen induced modest HIV-specific CD8+ and CD4+ T-cell responses, but better T-cell help as indicated by lymphoproliferation, and binding antibody responses (all directed primarily against env) and had an efficacy of 31% against HIV infection [13
]. Although far from definitive, the results from the study in Thailand suggest that a balanced immune response including HIV-specific CD4+ T-cell and antibody responses and a more modest CD8+ T-cell response may be important characteristics of a successful vaccination strategy. In our study, CD4+ T-cells secreting IL-2 alone were seen more frequently in the M/F group, possibly indicating a T helper response similar to that seen in RV 144 [51
In conclusion, while it is unknown if the immune responses induced by the heterologous pox-vector regimen described here may recapitulate some of the important protective aspects of the canarypox-HIV plus rgp120 regimen, this regimen can augment both CD4+ and CD8+ T-cell immune responses that may be important components of an effective preventive HIV vaccine and should be considered for additional larger scale testing.