alpha beta TCR+ cells are a minimal fraction of peripheral CD8(+) pool in MHC class I-deficient mice

MHC class I molecules play a role in the maintenance of the naive periphera l CD8(+) T cell pool. The mechanisms of the peripheral maintenance and the life span of residual CD8(+) cells present in the periphery of beta(2)-micr oglobulin-deficient (beta(2)m(-/-)) mice are unknown. We here show that ver y few CD8(+) cells in beta(2)m(-/-) mice coexpress CD8 beta, a marker of th e thymus-derived CD8(+) T cells. Most of the CD8 alpha(+) cells express CD1 1c and can be found in beta(2)m/RAG-2 double-deficient mice, demonstrating that these cells do not require rearranged Ag receptors for differentiation and survival and may be of dendritic cell lineage. Rare CD8 alpha(+)CD8 be ta(+) cells can be detected following in vivo alloantigenic stimulation 2 w k after the adult thymectomy, Selective MHC class I expression by bone marr ow-derived cells does not lead to an accumulation of CD8 beta(+) cells in b eta(2)m(-/-) mice. These findings demonstrate that 1) thymic export of CD8( +) T cells in beta(2)m(-/-) mice is reduced more severely than previously t hought; 2) non-T cells expressing CD8 alpha become prominent when CD8(+) T cells are virtually absent; 3) at least some beta(2)m(-/-) CD8(+) T cells h ave a life span in the periphery comparable to wild-type CD8(+) cells; and 4) similar ligands induce positive selection in the thymus and survival of CD8(+) T cells in the periphery.