CD8 expression up to the double-positive CD3^{low/intermediate} stage of thymic differentiation is sufficient for development of peripheral functional cytotoxic T lymphocytes

Control of CD8α transcription during development of α/β T cell receptor (TCR) T lymphocytes is mediated by at least two distinct stage-specific cis-acting transcriptional mechanisms (i.e., enhancers). On the CD8α^-/-knockout (KO) background, cis-mechanism I and cismechanism II together mediate appropriate stage- and sublineage-specific transgenic (Tg) CD8α expression and "rescue" development of peripheral CD8⁺ single-positive (SP) cytotoxic T lymphocytes (CTLs). In contrast, on the wild-type (WT)/CD8^+/+ or CD8α^-/-KO backgrounds, a CD8α Tg directed by cis-mechanism I alone is activated during the double negative [DN] to double positive [DP] transition and expressed up to the CD3^{low/intermediate} DP stage but not in more mature DP or SP thymocytes or peripheral T cells. As loss of cis mechanism I activity occurs around the onset of positive selection, it is possible that events associated with TCR/major histocompatibility complex (MHC) interactions and selection are involved in initiating these changes in CD8α transcription. To examine this issue, phenotypic and functional studies were performed for thymocytes and T cells of CD8α^-/-KO mice that expressed a CD8α Tg under control of cis-mechanism I only. Despite loss of CD8α expression at the DP CD3^{low/intermediate} stage, increased populations of mature CD3^hiCD4^-CD8^- thymocytes and CD3⁺CD4^-CD8^- peripheral T cells were detected. By several criteria, including 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-mechanism I-mediated control of CD8α transcription. Further, CD8 expression beyond the CD3^{low/intermediate} DP thymic stage is not essential for CTL development in vivo or function.