ThPOK acts late in specification of the helper T cell lineage and suppresses Runx-mediated commitment to the cytotoxic T cell lineage

The transcription factor ThPOK is required and sufficient for the generation of CD4⁺CD8^- thymocytes, yet the mechanism by which ThPOK orchestrates differentiation into the CD4⁺ helper T cell lineage remains unclear. Here we used reporter mice to track the expression of transcription factors in developing thymocytes. Distal promoter-driven expression of the gene encoding the transcription factor Runx3 was restricted to major histocompatibility complex (MHC) class I-selected thymocytes. In ThPOK-deficient mice, such expression was derepressed in MHC class II-selected thymocytes, which contributed to their redirection to the CD8⁺ T cell lineage. In the absence of both ThPOK and Runx, redirection was prevented and cells potentially belonging to the CD4⁺ lineage, presumably specified independently of ThPOK, were generated. Our results suggest that MHC class II-selected thymocytes are directed toward the CD4⁺ lineage independently of ThPOK but require ThPOK to prevent Runx-dependent differentiation toward the CD8⁺ lineage.