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 cis-mechanism 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.