The data showing induction of sustained and predominantly polyfunctional T-cell responses agree with results from two studies of MVA85A-induced immunity in adults from the site in South Africa 25, and from the UK 32. Although BCG vaccination alone induces polyfunctional T cells, specific T cells expressing only IFN-γ are the most common T-cell subset, both in infants 33 and in adults 20. The reason for the more polyfunctional response after in vitro Ag85A peptide pool stimulation, compared with viable BCG,

is most likely related to differential signalling between Ag presenting cells and Angiogenesis inhibitor T cells. BCG is taken up by innate cells, such as monocytes and dendritic cells, which are known to become activated and secrete inflammatory cytokines, whereas no innate response to peptides is expected. This is supported by our previous observation that more polyfunctional T-cell responses are detected after PPD stimulation of whole blood from healthy, mycobacteria-exposed persons, Akt inhibitor compared with BCG stimulation 20. We hypothesize that the polyfunctional T-cell population may be the best predictor of vaccine efficacy, because polyfunctional T cells, and not T cells expressing IFN-γ alone, have been associated with protection against another intracellular infection, Leishmania major, in mice 13. As mentioned above, recent animal data from novel TB vaccination

studies also suggest that polyfunctional T-cell responses may correlate with protection against TB 14. Whether this is also true for humans is not known. PPD-specific T-cell responses in TB patients were recently shown to be more polyfunctional than responses from healthy, household TB contacts 34. Until the efficacy of novel TB vaccines are assessed in large phase III clinical trials we have to rely on surrogates,

such as vaccine take or immunogenicity, to assess these vaccines 35. Ag85A-specific CD8+ T cells were not detected after MVA85A vaccination. This contrasts with results from a Gambian 24 and a UK 23 MVA85A trial, in which the Ag85A-specific CD8+ T-cell response was boosted. In the latter trial, a dose of 1×108 plaque forming units (pfu) of MVA85A was administered to BCG-vaccinated participants, which is double the standard dose (5×107 pfu) used in Metalloexopeptidase this and in other trials until recently 25, 36. Further, in another study, low frequencies of Ag85A-specific CD8+ T cells were only detected after in vitro expansion of specific T cells in persons vaccinated with 5×107 pfu of MVA85A 37. These results suggest that a higher dose of MVA85A may lead to more readily detectable CD8+ T-cell boosting. Increased CD4+ and CD8+ T-cell responses have also been described with increasing doses of MVA using Ag other than Ag85A 38–40. Vaccination with non-recombinant MVA of humans elicited detectable virus-specific CD8+ T-cell responses, even when a low dose of 1×106 pfu was used 41.