We show that MVA85A is safe and immunogenic in South African adults. Our results expand on previously reported qualitative characteristics of the MVA85A-induced immune responses [26
]. In that UK-based study MVA85A vaccination induced long-lasting, polyfunctional Ag85A-specific CD4+
T cells with a relatively immature phenotype. We did not observe differences in immune responses between BCG-vaccinated and BCG-naïve individuals. This is different to the UK studies, where MVA85A vaccination induced significantly higher responses in BCG-primed compared with BCG-naïve individuals [32
]. In our cohort all subjects had detectable BCG-specific T cell responses before vaccination. This was most likely the result of BCG vaccination, in the BCG–vaccinated individuals, or of environmental mycobacterial exposure, as M.tb
infection was an exclusion criterion. Pre-vaccination, BCG-specific T cells produced either one, or multiple cytokines. We show that the polyfunctional BCG-specific CD4+
T cell population (IFN-γ+
) is boosted by MVA85A vaccination. We hypothesize that polyfunctional cells may be a better predictor of vaccine efficacy than single-cytokine readout assays. Polyfunctional T cells, and not IFN-γ+
cells, have been associated with protection against murine Leishmania major
] and with control of viremia during non-progressive human HIV infection [36
T cell cytokine response 7 days after MVA85A vaccination was strongly dominated by IFN-γ production compared with a more diverse response pre-vaccination and on day 168 post-vaccination. Therefore the proportion of polyfunctional CD4+
T cells out of the total cytokine+
cell subset was lower at day 7 than before vaccination. The day 7 bias of predominant IFN-γ production is consistent with functional characteristics of highly activated effector cells, typically observed following vaccination or during chronic viral infections [37
]. In contrast, pre-vaccination and on day 168 post-vaccination more cells produced IL-2, thought to be characteristic of a long-lived, central memory T cell population. A vaccine-induced switch from IL-2+
central memory cells to IFN-γ+
effector cells was reported after tetanus toxoid boost [39
]. The latter study revealed a switch to central memory cells later which was similar in our trial.
The cytokine expression profiles of BCG-specific CD4+
T cells in our study differed from those observed in MVA85A-boosted Ag85A-specific CD4+
T cells in the UK study [26
]. In the latter, the peak post-vaccination response was dominated by polyfunctional cells. This may reflect differences in antigen-specificity. The UK study characterised CD4+
T cells specific for a peptide pool, whereas we focused on T cells specific for the more complex M. bovis
BCG. BCG requires antigen presenting cell processing, and many additional antigenic targets not boosted by MVA85A vaccination are presented and recognized. Compared with other antigens, BCG may induce differential expression of TNF-α and IL-2, despite IFN-γ production being very similar [40
]. In the latter study the BCG-specific response was dominated by single IFN-γ+
cells, whereas polyfunctional cells dominated the PPD-specific response. The immune inhibitory effect of live BCG and its attenuating role in the expression of mature MHC class II on macrophages may also play a role in differential T cell cytokine expression [41
]. This effect may have contributed to the ELISPOT data showing lower BCG-specific responses than PPD-specific responses and highlights the importance of characterising immune responses in detail using multiple antigens.
The observed differences in MVA85A-vaccinated persons between the UK and South Africa may also reflect a difference in exposure to environmental mycobacteria. This is suggested by data comparing the immunogenicity of BCG in Malawi and the UK, which show that BCG-naïve people in the southern African country have higher levels of pre-vaccination anti-mycobacterial immunity than in the UK [33
]. The BCG-specific responses detected in our study before MVA85A vaccination in the BCG-naïve individuals support this. Finally, differences in specimen and assay systems may contribute to the different profiles; Beveridge et al. stimulated PBMC with peptides in a 6-hour assay [26
], while we analysed whole blood stimulated with BCG for 12 hours.
T cells producing Th1 cytokines were boosted, the finding that IL-17-producing BCG-specific cells were not boosted after vaccination was surprising. We propose that cytokine expression by BCG-specific Th17 cells was suppressed by the dominant Th1 response, as Th17 lineage formation in humans is inhibited by IFN-γ [21
]. Production of IL-17 by differentiated, mycobacteria-specific T cells is also inhibited by IFN-γ [40
BCG-specific CD8+ T cells were not boosted in our intracellular cytokine analysis. This is consistent with results from a UK trial, in which the BCG-specific CD8+ response did not significantly increase after MVA85A vaccination (Beveridge and Hughes et al., unpublished data). However, in the same trial, the Ag85A-specific CD8+ T cell response was significantly boosted, whether whole blood was stimulated with Ag85A peptides or with recombinant Ag85A protein.
The safety data obtained here were very similar to those obtained in the UK and The Gambia in previous Phase I clinical trials as is the safety profile of MVA85A across the three different trial sites [44
](Brookes R et al., manuscript submitted). There were no serious or severe vaccine related adverse events. Minor local and systemic reactions were common in the first week, but overall the vaccine was well tolerated and appears safe. This profile is consistent with that seen with other recombinant MVAs evaluated in clinical trials [45
Of those screened, 68% and 29% had a positive ESAT-6 or CFP-10 ELISPOT test or a positive Mantoux test, respectively. This occurred despite the fact that those with known TB contacts or previous TB exposure or disease were excluded from screening. This illustrates the difficulty of recruiting TB-naïve individuals in high-incidence areas such as the Western Cape: most of the population has been exposed. A trial of MVA85A in M.tb infected adults is also currently ongoing at the SATVI site. M.tb-infected people were excluded in the first trial to avoid the possibility of inducing a Koch reaction.
The MVA85A vaccine was found to be safe and highly immunogenic in this population of TB naïve, HIV uninfected, otherwise healthy adults in a very high TB prevalence area. MVA85A was shown to induce potent, polyfunctional T cell responses as a boost vaccine. These data are very encouraging and justify the further clinical evaluation of the protective efficacy of these immune responses in a proof-of-concept efficacy trial in infants in this population.