IL-7R-dependent survival and differentiation of early T-lineage progenitors is regulated by the BTB/POZ domain transcription factor Miz-1

T cells originate from early T lineage precursors (ETPs) that have entered the thymus and differentiate through well-defined steps. Mice deficient for the BTB/POZ domain of zinc finger protein-1 (Miz-1) almost entirely lack ETPs and have a CD4(-)CD8(-) to CD4(+)CD8(+) block causing a strong reduction in thymic cellularity. Miz-1(ΔPOZ) pro-T cells cannot differentiate in vitro and are unable to relay signals from the IL-7R. Both STAT5 phosphorylation and Bcl-2 up-regulation are perturbed. The high expression levels of SOCS1 found in Miz-1(ΔPOZ) cells most likely cause these alterations. Moreover, Miz-1 can bind to the SOCS1 promoter suggesting that Miz-1 deficiency causes a deregulation of SOCS1. Transgenic overexpression of Bcl-2 or inhibition of SOCS1 restored pro-T cell numbers and their ability to differentiate, supporting the hypothesis that Miz-1 is required for the regulation of the IL-7/IL-7R/STAT5/Bcl-2 signaling pathway by monitoring the expression levels of SOCS1.