While these studies clearly indicate a role for the PD-1/PD-L1 pathway in dampening T cell responses, there is, rather confusingly, some evidence that this pathway is important in promoting CD8+ T cell responses in murine influenza virus [32] and Listeria monocytogenes[33] infections, suggesting that the outcome of PD-1/PD-L1 interactions might be modified by other regulatory pathways. This evidence concerns the gene PDCD1 and infection.