Inducing the T cell fates required for immunity.

A naïve T cell recruited into an immune response receives its critical inductive signals in the lymph node. The selected T cell must then produce cellular progeny empowered with new functions. Differentiated effector cells divide and migrate to infected tissues with an imprint of the instructions delivered to their progenitor. Some progeny of a selected naïve T cell, however, must remain undifferentiated and persist as lymph node-dwelling memory cells to replace and fortify defense should the intruder return. The integration of cell division, differentiation, diversification, and four-dimensional navigation make the clonal burst of a T cell in reaction to microbial invasion an exciting problem of developmental biology, cellular adaptation to environmental cues, and the propagation of signaling pathways through space and time. Epigenetic control of gene expression and an ancient cellular diversification mechanism called asymmetric cell division have recently been proposed to explain how a selected T cell can accomplish its imposing tasks. Future investigations will be directed toward understanding the mechanisms that allow a selected T cell to produce daughter T cells that are different, that are capable of remembering their inductive history, and that fulfill the demand for acute function and regeneration.