The memory of a killer T cell: models of CD8(+) T cell differentiation.

CD8(+) T cells have an important role in protection against infections and reinfections of intra-cellular pathogens like viruses. Naive CD8(+) T cells circulating in blood or lymphoid tissues can get activated upon stimulation by cognate antigen. The activated T cells undergo rapid proliferation and can expand more than 10(4)-folds comprising largely of effector T cells. Upon antigen clearance, the CD8(+) T-cell population contracts due to apoptosis, leaving behind a small population of memory T cells. The timing and mechanisms underlying the differentiation of naive cells into effector cells and memory cells is not yet clear. In this article, we review the recent quantitative studies that support different hypotheses of CD8(+) T-cell differentiation.