CD4 and infection: Although those contrasting results may be related to the different methodologies used (natural infection versus challenge; live virus versus peptides for identifying specific T cells with restimulation cell cultures) [29], the second study revealed a potential role of CD4+ T cells in protection, perhaps because these cells had direct cytotoxic activity or because they supported a CD8+ T-cell response through the expression of IFN-γ [28, 51].