We found that while positive selection initiates normally, as measured by Erk phosphorylation and TCR-β and CD69 expression, loss of Egr2 caused a partial block in progression from the DP to the SP stages, and overexpression of Egr2 resulted in the accumulation of SP cells at the expense of DP cells, particularly affecting the CD8 lineage. Egr2 Tg thymocytes, particularly
CD8 SP, were less susceptible to glucocorticoid-induced cell death. By contrast, Egr2f/fCD4cre thymocytes showed an increased susceptibility to cell death, AZD2014 and this was likely due in part to a failure to correctly upregulate Bcl2 following positive selection. Intriguingly, excess Egr2 expression inhibited Socs1 expression, and loss of Egr2 caused upregulation of Socs1 and maintenance of its expression post-selection, together with a failure to properly upregulate the IL-7R. The inhibition of downstream Stat5 phosphorylation, and a reduction, albeit small, in IL-7-mediated survival, suggest that Egr2
may be involved in the process of cytokine-induced survival and provide one explanation for why Bcl2 levels are lowered. These conclusions confirm and extend those of Wiest and coworkers, who recently published a similar study using mice in which Egr2 had been deleted from the DN stage onwards 26. Similar to Egr2f/fCD4cre thymocytes, Egr-1 and 3 doubly-deficient thymocytes are susceptible to apoptosis 14, and in Egr-1−/− mice, recent thymic emigrants are also relatively fragile 35. Bcl-2, FasL and Id3, a regulator of E-box proteins, which when knocked out causes defects in positive selection 36, have all been suggested as target genes of all Egr Ku-0059436 price family members, and indeed,
both Bcl2 (this study, and 26) and Id3 26 are downregulated in response to Egr2 loss during positive selection. As has been discussed by other authors (for example, see 26), complementation by other Egr family members of the Egr2-specific defect in Bcl2 expression may explain why the effects we observed were relatively small. Although Egr2 has also been suggested to upregulate Acetophenone FasL in the periphery 19, 20, we did not observe any changes in FasL expression in Egr2 mutant thymocytes (D. M. and V. J. L., unpublished observations). We show in this study that post-selection cytokine-mediated survival is affected in Egr2 mutant mice, and that expression of Socs1, which must be downregulated to allow the cytokine survival pathway to function 30, is regulated by Egr2. Interestingly, Egr1 has previously been shown to bind to cognate sites within the human Socs1 promoter and to positively regulate Socs1 expression in macrophages 37. As one of the binding sites is conserved in the murine promoter, it is possible that Egr2 is able to bind to the Socs1 promoter directly and repress its activity, perhaps in combination with a member of the cofactor family of Nab proteins (for example, see 38).