CD4+ and CD8+ T cells, the essential mediators of cellular immune responses, are produced in the thymus following sequential maturation stages. Hematopoietic progenitors first seed the thymus and make T cell lineage specification and commitment decisions within the CD4−CD8− double negative (DN) compartment. Thymocytes then mature to the CD4+CD8+ double positive (DP) stage, followed by vigorous negative and positive selection processes. The positively selected DP thymocytes first give rise to CD4+CD8lo intermediate (IM) cells which then differentiate into MHC class II-restricted CD4+ and MHC class I-restricted CD8+ T cells, a crucial decision known as CD4+ vs. CD8+ lineage choice.
The lineage choice decision is influenced by the timing, intensity, and duration of signals derived from the TCR and cytokines, and recent studies have identified a number of transcriptional factors that intrinsically regulate this critical fate decision. Among these, Th-POK (encoded by Zbtb7b, called Thpok here for simplicity and consistency with the literature) is specifically required for CD4+ differentiation while Runx factors promote CD8+ T cell production and repress Cd4 in CD8+ lineage committed cells. Upregulation of Thpok is most evident in the CD4+8lo IM cells and is required to antagonize Runx3 activity and expression to promote CD4+ lineage commitment. Collectively, the Th-POK-Runx3 axis appears to be a critical convergence point in the CD4+ vs. CD8+ lineage choice.
After committing to either CD4+ or CD8+ thymocytes, lineage-inappropriate genes are silenced to ensure the distinct identity and functional divergence between these two cell types. Repression of the Cd4 gene on CD8+ lineage committed cells is mediated by a ~430 bp silencer sequence in its first intron. Likewise, Thpok is repressed in CD8+ T cells by a ~560 bp sequence upstream of the Thpok exon 1a, and both Cd4 and Thpok silencers contain consensus binding motifs for Runx factors, which are necessary for CD8+ lineage commitment.
T cell factor 1 (TCF-1) and lymphoid enhancer binding factor 1 (LEF-1) are members of the TCF-LEF family transcription factors and abundantly expressed in T lineage cells, and known to be necessary for the maturation of DN T cells to the DP stage. However, because germline deletion of TCF-1 and LEF-1 causes severe early T cell developmental block and embryonic lethality, respectively, their roles beyond the DP stage are unknown. In my thesis work, I overcame these obstacles by conditionally ablating both TCF-1 and LEF-1 in DP thymocytes using CD4-Cre. We observed impaired differentiation of CD4+ T cells from the bipotent DP precursors in the absence of TCF-1 and LEF-1. Mechanistically, TCF-1 promotes CD4+ T cell development by positively regulating the expression of Thpok. TCF-1 and LEF-1 deficiency also results in derepression of the CD4 co-receptor in CD8+ lineage committed cells. In CD8+ T cells, TCF-1 interacts with Runx3 to repress expression of Cd4. These findings not only broaden the spectra of TCF-LEF-mediated regulatory activities in late stages of T cell development, but also reveal new paradigms in T cell fate decision and identity maintenance.