MHC class I is required for peripheral accumulation of CD8+ thymic emigrants

MHC molecules influence the fate of T lymphocytes at two important stages of their differentiation. Recognition of self peptide/MHC complexes in the thymus determines whether immature T cells should live and mature into immunocompetent T cells or whether they should die. In the periphery, recognition of Ags presented by MHC molecules induces T cell activation, proliferation, and differentiation into effector/memory T cells. We describe in this work a third role that MHC molecules play in T cell physiology. CD8+ thymic emigrants require presence of MHC class I molecules in the periphery to seed the peripheral lymphoid organs. Numbers of CD8+ T cells are reduced severely in both the thymus and the periphery of beta2-microglobulin-deficient (beta2m[-/-]) mice. When grafted with wild-type (beta2m[+/+]) thymic epithelium, immature beta2m(-/-) T cells that populate the graft develop into functional mature CD8+ cells. However, significant numbers of peripheral CD8+ cells in grafted beta2m(-/-) mice can be observed only after injection of MHC class I-expressing cells in the periphery. Thus, naive T cells in the periphery do not passively await antigenic stimulation, but actively engage in interactions with self MHC molecules that may promote their survival.