Impaired thymic selection in mice expressing altered levels of the SLP-76 adaptor protein

Abstract: Intracellular signaling initiated by liga-tion of the TCR influences cell fate at multiple points during the lifespan of a T cell. This is espe-cially evident during thymic selection, where the nature of TCR-dependent signaling helps to estab-lish a MHC-restricted, self-tolerant T cell reper-toire. The Src homology 2 domain-containing leu-kocyte-specific phosphoprotein of 76 kDa (SLP-76) adaptor protein is a required intermediate in multiple signaling pathways triggered by TCR en-gagement, several of which have been implicated in dictating the outcome of thymic selection (e.g., intracellular calcium flux and activation of ERK family MAPKs). To determine if thymocyte matu-ration and selection at later stages of development are sensitive to perturbations in SLP-76 levels, we analyzed these crucial events using several trans-genic (Tg) lines of mice expressing altered levels of SLP-76 in the thymus. In Tg mice expressing low levels of SLP-76 in preselection thymocytes, the CD4:CD8 ratio in the thymus and spleen was skewed in a manner consistent with impaired selec-tion and/or maturation of CD4 thymocytes. Low SLP-76 expression also correlated with reduced CD5 expression on immature thymocytes, consis-tent with reduced TCR signaling potential. In con-trast, reconstitution of SLP-76 at higher levels re-sulted in normal thymic CD5 expression and CD4: CD8 ratios in the thymus and periphery. It is curious that thymic deletion of TCR-Tg (HY) thy-mocytes was markedly impaired in both lines of Tg-reconstituted SLP-76/ mice. Studies using chimeric mice indicate that the defect in deletion of HY thymocytes is intrinsic to the developing thymocyte, suggesting that maintenance of suffi-cient SLP-76 expression from the endogenous lo-cus is a key element in the selection process. J