The many roles of TOX in the immune system

Abstract

TOX is a member of an evolutionarily conserved DNA-binding protein family and is expressed in several immune-relevant cell subsets. Here, we review the key role of TOX in regulating development of CD4 T cells, natural killer cells and lymphoid tissue inducer cells, the latter responsible for the generation of lymph nodes. Although the exact molecular mechanism of action of TOX remains to be elucidated, the role of TOX in establishment of gene programs in the thymus and the potential of TOX as a regulator of E protein activity are discussed.

Figure 1

The TOX subfamily of HMG-box proteins. TOX contains an HMG-box DNA binding domain that is highly conserved in three additional proteins TOX2, TOX3, and TOX4. An adjacent lysine-rich region may serve as the nuclear localization signal (NLS) as well as influence the interaction with DNA. The N-terminal domains of these proteins show approximately 30–40% sequence identity and have transactivation activity, while the C-terminal domains differ greatly between family members.

Figure 2

TOX is a key regulator of T cell development in the thymus. TCR signaling in DP thymocytes undergoing positive selection, through interaction with thymic epithelial (TE) cells, causes downregulation of CD4 and CD8 resulting in a double dull (DD) phenotype followed by re-expression of CD4 to yield CD4⁺CD8^lo cells. The latter subpopulation of cells contains the immediate precursors of both CD4⁺CD8⁻ (CD4SP) and CD4⁻ CD8⁺ (CD8SP) thymocytes, although some CD8SP can derive directly from DD cells. TOX is induced by TCR signals early in positive selection, first detected in DD cells and further upregulated at the CD4⁺CD8^lo stage, before returning to baseline levels in SP thymocytes. In the absence of TOX (TKO), the DD to CD4⁺CD8^lo progression is severely inhibited, blocking development of CD4 T cells but with a more modest effect on development of CD8 T cells.

Figure 3

TOX is required for development of NK, LTi and CD4 T cells, possibly by modulating E protein activity. Both NK and LTi lineages share dependence on expression of Id2 as well as TOX. TKO NK cells (and thymocytes) have reduced expression of Id2, suggesting that TOX may be an upstream regulator of this E protein inhibitor. However, it is likely that TOX also regulates other events required for development of these cell lineages, as reconstitution of Id2 did not rescue NK cell development in TKO bone marrow precursors. Id2 is also expressed in the thymus, but its deletion does not cause a block in T cell development possibly due to compensation by Id3 [36,37]. However, TKO thymocytes express lower levels of Id2, and there is failure to upregulate some genes that are repressed by E proteins [13,21]. Whether this is causally linked remains to be proven. Thus, the role of Id2 during thymocyte development may need to be revisited. Interestingly, as E protein activity has also been shown to have a negative effect on TOX expression [24], a feed forward regulatory circuit might enforce TOX expression in these different cell types. (In this Figure, dashed lines refer to hypotheticals).