The role of ThPOK in control of CD4/CD8 lineage commitment

The enigma of CD4-lineage specification

CD4(+) T cells are essential for defenses against pathogens and affect the functions of most cells involved in the immune response. Although CD4(+) T cells generally recognize peptide antigens bound to MHC-II molecules, important subsets are restricted by other MHC or MHC-like molecules, including CD1d-restricted "invariant" iNK T cells. This review discusses recently identified nodes in the transcriptional circuits that are involved in controlling CD4(+) T-cell differentiation, notably the commitment factor Thpok and its interplay with Runx transcriptional regulators, and focuses on how transcription factors acting upstream of Thpok, including Gata3, Tox and E-box proteins, promote the emergence of CD4-lineage-specific gene expression patterns.

Regulation of GATA-3 expression during CD4 lineage differentiation

GATA-3 is necessary for the development of MHC class II-restricted CD4 T cells, and its expression is increased during positive selection of these cells. TCR signals drive this upregulation, but the signaling pathways that control this process are not well understood. Using genetic and pharmacological approaches, we show that GATA-3 upregulation during thymocyte-positive selection is the result of additive inputs from the Ras/MAPK and calcineurin pathways. This upregulation requires the presence of the transcription factor c-Myb. Furthermore, we show that TH-POK can also upregulate GATA-3 in double-positive thymocytes, suggesting the existence of a positive feedback loop that contributes to lock in the initial commitment to the CD4 lineage during differentiation.

Transcriptional and epigenetic regulation of CD4/CD8 lineage choice

The helper versus cytotoxic-lineage choice of CD4(+)CD8(+) DP thymocytes correlates with MHC restriction of their T cell receptors and the termination of either CD8 or CD4 coreceptor expression. It has been hypothesized that transcription factors regulating the expression of the Cd4/Cd8 coreceptor genes must play a role in regulating the lineage decision of DP thymocytes. Indeed, progress made during the past decade led to the identification of several transcription factors that regulate CD4/CD8 expression that are as well important regulators of helper/cytotoxic cell fate choice. These studies provided insight into the molecular link between the regulation of coreceptor expression and lineage decision. However, studies initiated by the identification of ThPOK, a central transcription factor for helper T cell development, have offered another perspective on the cross-regulation between these two processes. Here, we review advances in our understanding of regulatory circuits composed of transcription factors and their link to epigenetic mechanisms, which play essential roles in specifying and sealing cell lineage identity during the CD4/CD8 commitment process of DP thymocytes.

Directing traffic on the NKT-cell highway: a key role for ThPOK

CD1d-restricted NKT cells include CD4(+) and DN subsets, with an additional CD8(+) subset that is present in humans but not in mice. The molecular regulation of CD4/CD8 expression by NKT cells, and differentiation of these NKT-cell subsets, is poorly understood. The transcription factors GATA3 and ThPOK regulate lineage commitment of conventional MHC class II-restricted CD4(+) T cells; however, their role in CD4/CD8 expression by CD1d-restricted NKT cells is less clear. A new study in this issue of the European Journal of Immunology demonstrates a key role for ThPOK in differentiation of NKT-cell subsets. This study reveals that GATA3 and ThPOK are necessary for the development of CD4(+) NKT cells. Furthermore, ThPOK-deficient mice generate an unusual population of CD8(+) NKT cells, which are absent in control mice. This study sheds new light on the underlying molecular events leading to the emergence of distinct NKT-cell subsets.

p300-mediated acetylation stabilizes the Th-inducing POK factor

The lineage-specifying factor Th-inducing POK (ThPOK) directs the intrathymic differentiation of CD4 T cells. Although the regulation of ThPOK at the transcription level has been extensively studied, specific posttranslational modifications regulating the activity of ThPOK have not been addressed. In this paper, we show that ThPOK is an unstable protein that is more readily degraded in CD8 T cells compared with CD4 T cells. Among the various proteins that bind ThPOK, acetyltransferase p300 specifically promotes the acetylation of ThPOK at K210, K216, and K339, outcompeting ubiquitination, thereby stabilizing the protein. In CD4 T cells, attenuation of p300-mediated acetylation promotes the degradation of ThPOK. In contrast, mutation of lysines 210, 216, and 339 to arginines stabilizes ThPOK and enhances its ability to suppress the expression of CD8 molecule and cytotoxic effectors in CD8 T cells. Our results reveal an essential role of p300-mediated acetylation in regulating the stability of ThPOK and suggest that such regulation may play a part in CD4/CD8 lineage differentiation.