6B, do not always correlate well with the levels of Egr2 in normal thymocytes; notably, in population A, where Egr2 expression is lowest, there are substantial effects on Socs1 expression in Egr2f/fCD4Cre mice. We suggest that the regulation of Socs1 by Egr2 is biologically significant, as events previously documented as lying downstream of Socs1 signaling during selection were also affected in Egr2-deficient thymocytes. Egr2-deficient CD4+CD8lo cells were unable to correctly upregulate Bcl2 expression as would normally occur following resumption of cytokine

signaling 30, and this was linked to lowered levels of pStat5 and a reduced ability to survive in IL-7-supplemented medium. We note that survival might be further compromised by the loss of a small population of high-level expressors of IL-7R in Egr2-deficient CD4+CD8lo subsets. BVD-523 Regulation of Socs1 might also provide an explanation for the increase in numbers of CD8SP thymocytes in Egr2-Tg animals, as this fits well with the observations that in the absence of Socs1, CD8SP T-cell differentiation

is enhanced 33. It would be of great interest to determine whether gain of Socs1 is able to rescue this aspect of the Egr2-Tg phenotype. PARP inhibitor We and others have shown that following TCR ligation, both the MAPK and calcineurin signaling pathways are required for induction of Egr2

15, 22. The convergence of these pathways on Egr2 suggests it may lie at a crucial control point in the selection process. Previously, it has been suggested that the expression levels and activity of Egr proteins combine to modulate positive selection following TCR ligation 24. Where the signal is strong, as in the highest affinity TCR interactions with peptide-MHC, Egr2 and its relatives Egr1 and Egr3 are induced at high levels and are not the rate-limiting step in the selection process. However, where the TCR is weak enough for a thymocyte to be on the boundary between positive selection and death by neglect, increased amounts of Egr proteins permit positive selection to occur, and decreased amounts cause the cell to fail selection. Our data suggest PFKL an extension of this model whereby titration of the levels of Egr2 by TCR signal strength could perhaps modulate the cytokine-mediated survival signal by regulating the level of Socs1, thus fine-tuning the process of positive selection. Egr2-Tg mice were made by microinjection into oocytes of Sfi1-linearised pVAhCD2 plasmid 40 containing LoxP-flanked DsRed and Egr2 cDNA. Details of construct are available upon request. Other strains used have been previously published as follows: CD4Cre 27; Egr2f/f28 MHC-deficient mice 41, 42, Rag2−/−43, Rag1−/−44, TCR-β F5 45, TCR-β HY 46 and TCR-β OTII 47.