What Happens in the Thymus Does Not Stay in the Thymus: How T Cells Recycle the CD4+-CD8+ Lineage Commitment Transcriptional Circuitry To Control Their Function

Abstract

MHC-restricted CD4(+) and CD8(+) T cells are at the core of most adaptive immune responses. Although these cells carry distinct functions, they arise from a common precursor during thymic differentiation, in a developmental sequence that matches CD4 and CD8 expression and functional potential with MHC restriction. Although the transcriptional control of CD4(+)-CD8(+) lineage choice in the thymus is now better understood, less was known about what maintains the CD4(+) and CD8(+) lineage integrity of mature T cells. In this review, we discuss the mechanisms that establish in the thymus, and maintain in postthymic cells, the separation of these lineages. We focus on recent studies that address the mechanisms of epigenetic control of Cd4 expression and emphasize how maintaining a transcriptional circuitry nucleated around Thpok and Runx proteins, the key architects of CD4(+)-CD8(+) lineage commitment in the thymus, is critical for CD4(+) T cell helper functions.

Figure 1. Thymic imprint on the Cd4 locus

Cd4 expression is regulated by a combination of at least 4 cis-regulatory elements: the promoter (P4) near the TSS, the proximal enhancer (E4p) located 13kb upstream of the TSS, the silencer (S4) situated in the first Cd4 intron and a recently identified enhancer (DHS+3) located 3’ of the silencer. In CD4⁺-lineage thymocytes (top). Thpok binds the silencer and promotes sustained Cd4 expression. Locus demethylation is achieved in an E4p-dependent manner, presumably involving recruitment of Tet enzymes. E-proteins E2A and HEB, which are required for CD4⁺-lineage specification and bind E4p may be involved in these processes. In CD8⁺-lineage cells, Runx3 binds the silencer, promoting Cd4 repression and silencing. The locus is hypermethylated in a silencer-dependent manner, involving DNA methyl transferases (Dnmt), conceivably recruited by Runx3. Black boxes indicate Cd4 exons, grey rectangles cis regulatory elements, black arrow: TSS, active in CD4⁺-lineage cells.

Figure 2. Maintenance of lineage stability in thymocytes and mature T cells

(top panel) In thymocytes, Thpok and Runx3 form a dual negative regulatory loop (1), in which Runx3 inhibits Thpok expression and drives CD8⁺ lineage gene expression, whereas Thpok inhibits CD8 and Runx3 expression, protecting the CD4⁺ lineage. The mutually exclusive expression of Thpok and Runx3 decides lineage differentiation. (bottom two panels) During CD4⁺ effector T cell differentiation, Thpok serves two functions to constrain Runx activity. First, it inhibits Runx3 and cytotoxic gene expression (2) and thereby protects helper functions and Th2 differentiation. Second, Thpok antagonizes the function of Runx proteins (3) (Runx1 and Runx3) and thereby prevent cytotoxic diversion of CD4⁺ T cell expressing Runx3, including Th1 effectors (bottom panel).