The role of cytokines in lymphocyte homeostasis.

The size of lymphocyte populations is regulated by replication and death. Cytokines produced by non-lymphoid cells provide key survival and replication signals for several lymphocyte subpopulations. The availability of these cytokines thus serves as a homeostatic regulatory mechanism by determining the upper limit of the population size. IL-7 is required for survival of naive CD4+ and CD8+ cells and memory CD8+ cells. IL-15 is required for survival of memory CD8+ cells. IL-12 and IL-4 also promote memory CD8+ survivaL BAFF is required for survival of mature B cells. Antigen receptor signals, together with these cytokine signals, are required for survival of mature B cells and naive T cells. The list of extracellular survival signals for lymphocytes remains incomplete, and the intracellular pathways leading to survival are poorly understood.