Early effector T cells producing significant IFN-γ develop into memory

Currently, transition of T cells from effector to memory is believed to occur as a consequence of exposure to residual suboptimal Ag found in lymphoid tissues at the waning end of the effector phase and microbial clearance. This led to the interpretation that memory arises from slightly activated late effectors producing reduced amounts of IFN-gamma. In this study, we show that CD4 T cells from the early stage of the effector phase in which both the Ag and activation are optimal also transit to memory. Moreover, early effector T cells that have undergone four divisions expressed significant IL-7R, produced IFN-gamma, and yielded rapid and robust memory responses. Cells that divided three times that had marginal IL-7R expression and no IFN-gamma raised base level homeostatic memory, whereas those that have undergone only two divisions and produced IFN-gamma yielded conditioned memory despite low IL-7R expression. Thus, highly activated early effectors generated under short exposure to optimal Ag in vivo develop into memory, and such transition is dependent on a significant production of the cell's signature cytokine, IFN-gamma.