CD4 + T cells are required for secondary expansion and memory in CD8 + T lymphocytes

A long-standing paradox in cellular immunology concerns the conditional requirement for CD4+ T-helper (TH) cells in the priming of cytotoxic CD8+ T lymphocyte (CTL) responses in vivo. Whereas CTL responses against certain viruses can be primed in the absence of CD4+ T cells, others, such as those mediated through 'cross-priming' by host antigen-presenting cells, are dependent on TH cells. A clearer understanding of the contribution of TH cells to CTL development has been hampered by the fact that most TH-independent responses have been demonstrated ex vivo as primary cytotoxic effectors, whereas TH-dependent responses generally require secondary in vitro re-stimulation for their detection. Here, we have monitored the primary and secondary responses of TH-dependent and TH-independent CTLs and find in both cases that CD4+ T cells are dispensable for primary expansion of CD8+ T cells and their differentiation into cytotoxic effectors. However, secondary CTL expansion (that is, a secondary response upon re-encounter with antigen) is wholly dependent on the presence of TH cells during, but not after, priming. Our results demonstrate that T-cell help is 'programmed' into CD8+ T cells during priming, conferring on these cells a hallmark of immune response memory: the capacity for functional expansion on re-encounter with antigen.