CD4 and infection: Given that the magnitude of Gag-specific CD8 T cell responses as measured by cytokine production was maintained during the first 40 weeks of SIVΔnef vaccination, we sought to locate SIV-specific CD8 T cell responses within the female reproductive tract, the route of challenge; the gut mucosa, the anatomical site most enriched for activated CD4 memory T cells that are the most permissive target for SIV; and the secondary lymphoid tissues, where antigen-processing and presentation are most likely to occur after infection.