CD8 expression up to the double-positive CD3(low/intermediate) stage of thymic differentiation is sufficient for development of peripheral functionalcytotoxic T lymphocytes

Control of CD8 alpha transcription during development of alpha/beta T cell receptor (TCR) T lymphocytes is mediated by at least two distinct stage-spe cific cis-acting transcriptional mechanisms (i.e., enhancers). On the CD8 a lpha (-/-)knockout (KO) background, cis-mechanism I and cis-mechanism II to gether mediate appropriate stage- and sublineage-specific transgenic (Tg) C D8 alpha expression and "rescue" development of peripheral CD8(+) single-po sitive (SP) cytotoxic T lymphocytes (CTLs). In contrast, on the wild-type ( WT)/CD8(+/+) or CD8 alpha -/-KO backgrounds, a CD8 alpha Tg directed by cis -mechanism I alone is activated during the double negative [DN] to double p ositive [DP] transition and expressed up to the CD3(low/intermediate) DP st age but not in more mature DP or SP thymocytes or peripheral T cells. As lo ss of cis mechanism I activity occurs around the onset of positive selectio n, it is possible that events associated with TCR/major histocompatibility complex (MHC) interactions and selection are involved in initiating these c hanges in CD8 alpha transcription. To examine this issue, phenotypic and fu nctional studies were performed for thymocytes and T cells of CD8 alpha -/- KO nice that expressed a CD8 alpha Tg under control of cis-mechanism I only . Despite loss of CD8 alpha expression at the DP CD3(low/intermediate) stag e, increased populations of mature CD31(hi)CD4(-)CD8(-) thymocytes and CD3( +)CD4(-)CD8(-) peripheral T cells were detected. By several criteria, inclu ding MHC class I-restricted antigen recognition, these cells have at least partially undergone positive and negative selection. Therefore, initiation of selection and sublineage commitment are determined before loss of cis-me chanism I-mediated control of CD8 alpha transcription. Further, CD8 express ion beyond the CD3(low/intermediate) DP thymic stage is not essential for C TL development in vivo or function.