New research published in Immunity indicates that the ATP-dependent chromatin remodeller Mi-2β — which was previously thought to be a negative regulator of gene expression — is a positive regulator of CD4 expression by double-positive (DP) thymocytes and is required for thymocyte transition from the late double-negative (DN) stage to the DP stage of development.

The nucleosome-remodelling histone deacetylase (NuRD) complex associates with Ikaros proteins, which are known regulators of T-cell development. The complex comprises several proteins, including Mi-2. There are two homologues of Mi-2, of which Mi-2β is the most highly expressed in thymocytes. So, to study the role of the Mi-2-containing chromatin-remodelling complex in T-cell development, Williams et al. generated mice in which Mi-2β was specifically inactivated at the DN stage of thymocyte development and remained inactive in mature T cells.

In the Mi-2β-deficient mice, thymic cellularity was reduced, as was the absolute number of DP thymocytes. By contrast, the number of DN thymocytes was normal, and further analysis revealed an accumulation of DN4 thymocytes (that is, those cells in the final stages of DN development), indicating that Mi-2β regulates the DN4 to DP transition during thymocyte differentiation.

In addition to the decreased numbers of DP thymocytes in the thymi of the Mi-2β deficient mice, the percentage of these cells in each thymus was also markedly reduced. By contrast, the proportion of CD8+ cells was increased. However, the phenotype of these CD8+ T cells resembled that of wild-type DP thymocytes, leading the authors to suggest that the CD8+ cells that accumulate in the absence of Mi-2β are DP cells that fail to express CD4 appropriately. Consistent with the hypothesis that Mi-2β regulates Cd4 gene expression, in mice transgenic for a reporter driven by the Cd4 enhancer and promoter, expression levels of the reporter in thymocytes were reduced in the absence of Mi-2β, with the lowest levels of expression being detected in CD8+ cells — the cells in which CD4 was not expressed appropriately. Furthermore, in wild-type thymocytes Mi-2β was shown to directly associate with the Cd4 proximal enhancer — a regulatory element known to be required for Cd4 gene expression during thymocyte development.

The Cd4 proximal enhancer normally shows high levels of histone H3-acetylation, but this was reduced in the thymi of Mi-2β-deficient mice, probably as a result of the observed decrease in association of the histone acetyltransferase p300 with the enhancer. Subsequent analysis showed that p300 and Mi-2β associate with the E-box-binding protein HEB. Because the Cd4 enhancer contains E-box binding sites, the authors propose that Mi-2β is recruited to the enhancer through its interactions with HEB. Through its ATP-dependent chromatin-modelling function Mi-2β stabilizes HEB binding, such that associated/recruited p300 increases local histone acetylation, locking the chromatin in an open conformation and thereby leading to Cd4 gene expression.

This study identifies Mi-2β as crucial for thymocyte differentiation at the transition from the DN4 to the DP stage of development and for regulation of Cd4 gene expression in DP thymocytes. As Mi-2β has previously been defined as a negative regulator of gene expression, this study uncovers a new role for Mi-2β, and future studies will probably characterize more situations in which it can act positively.