Gene regulation in the differentiation of CD4+ and CD8+ T cells

The mechanism of T-cell differentiation, by which precursor double positive thymocytes give rise to MHC class I-specific CD8+ T cytotoxic cells and MHC class II-specific CD4+ T helper cells, has been a topic of long standing interest in the fields of immunology and developmental biology. Understanding the regulation of the genes that are required for these developmental programs to be initiated and maintained is a central component of our understanding of this lineage choice. The transcription factor ThPOK and the RUNX family members RUNX1 and RUNX3 have been implicated as critical for the differentiation of CD4+ and CD8 + T cells, respectively. To better understand the role of ThPOK in CD4+ T cell differentiation, microarray profiling was performed on ThPOK-expressing (CD4-fated) and non-expressing (CD8-fated) precursor thymocytes, and cyclin D2, a component of the cell cycle regulatory machinery, was identified as being enriched in CD4-fated thymocytes. It was further established that a greater percentage of CD4-fated thymocytes are in cell cycle compared to their CD8-fated counterparts, and achieve lineage specification faster after the common developmental checkpoint of positive selection. This difference in cell cycle kinetics was independent of ThPOK function and perhaps even functions upstream as part of the regulatory mechanism controlling ThPOK expression itself. The Cd4 and Cd8 genes, whose expression is first overlapping and subsequently mutually exclusive during thymocyte differentiation, are also tightly regulated by enhancer and silencer elements. As other coordinately regulated genes have been shown to participate in long-range chromosomal interactions that are involved in regulating their expression, we asked whether such a long-range interaction could be detected between the Cd4 and Cd8 genes. Cd4 and Cd8 were indeed found to be closely associated in a stage-specific manner during thymocyte development; this association requires the presence of the Cd4 silencer element and is dependent on the expression of RUNX proteins. These data represent a novel mechanism by which the Cd4 and Cd8 genes are regulated, and serve to further our understanding of gene regulation in the differentiation of CD4 + and CD8+ T cells.