T cell homeostasis in steady state and lymphopenic conditions.

Homeostasis in the immune system is an important principle ensuring that the numbers of peripheral lymphocytes are kept more or less constant despite numerous disturbances in the immune system during the lifetime of an organism. Mechanisms relating to maintenance of homeostasis have mainly been investigated in experimental systems exhibiting extreme lymphopenic conditions in which the behavior of adoptively transferred lymphocytes is assessed in the absence of endogenous lymphocytes. While these experimental systems have yielded important insight into mechanisms that shape the survival and expansion potential of T cells, their extrapolation to normal steady state conditions that do not involve extreme lymphopenia has sometimes been difficult. We review here the different scenarios of homeostatic control mechanisms in steady state as well as severely or partially lymphopenic environments.