Protein-adjuvant PLX3397 clinical trial vaccines often elicit relatively Th2 skewed responses with little murine IgG2a/b production [57]. Thus the significant enhancement of IgG2a production we observed with viral vectors here may be of protective value, particularly if it generalizes to other antigens postulated to induce Fc-dependent

responses. Antibody avidity has not been demonstrated to correlate with protection against blood-stage malaria and has in fact been predicted to be unimportant in response to merozoite antigens [48] and [58]. The relationship between avidity and protection in other diseases is complex and variable, but avidity has been observed to be associated with protection against respiratory syncytial virus, HIV-1 and anthrax [59], [60], [61] and [62]. The finding of enhanced avidity with A–M and related regimes compared to protein vaccination therefore merits further study and may be of interest beyond the malaria field. There was strikingly little variation in the rate of decline of total IgG ELISA titer over the prolonged period of follow-up after vaccination.

It would therefore seem that peak ELISA titer is an adequate predictor of antibody concentration at a later time point. The presence of a correlation between splenic ASC counts and ELISA titer at both early and late time points supports this. The reliable priming of antibody responses by adenovirus prior to subsequent boosting by MVA or protein strongly suggests that adenovirus containing regimes reliably generate memory B cell responses. It remains to be see more seen whether the different vaccine modalities investigated here induce memory B cell/antigen-recall responses that vary independently of peak antibody titer/overall regime immunogenicity. It is interesting to note that in our previous studies, the viral vector PfMSP1-based antigen failed to induce detectable antigen-specific CD4+ T cell responses in BALB/c mice, even though viral vectored regimes can induce measurable CD4+ T cell responses

against other antigens [5], [6] and [63]. before This would appear at odds with our finding of a reliably primed and boosted, avid, IgG2a skewed response to A–M-containing regimes: a response which bears the hallmarks of a Th1 response to a ‘T-dependent’ antigen bearing CD4+ T cell epitopes. Quite possibly, such helper T cell responses were simply below the limit of detection of the ICS assay, or these cells secreted cytokines other than IFNγ, TNFα and IL-2. Alternatively, recent evidence shows that, in mice, IFNα- or IFNγ-activated DCs can drive T-independent immunoglobulin class-switching with either a Th1 or Th2 skew, and that T-independent type-2 antigens can induce long-lived cells capable of mounting a secondary recall response [64] and [65]. It is therefore possible that adjuvants (and viral vectors) may be able to influence class-switching in a CD4+ T cell-independent manner.