The protein tyrosine phosphatase SHP-1 sets signaling thresholds during thymocyte development

Modulation of the signal strength of the TCR determines the outcome of positive and negative selection during thymocyte development. T_reg cells, a T cell subset previously shown to suppress the proliferation of other T cells, also develop in the thymus but the underlying mechanism is currently not well understood. It is believed that CD4+CD25+ T_reg cell development is driven by high affinity peptides, which may in turn result in relatively high TCR signaling. Previous studies have demonstrated that SHP-1 plays a role in determining signal strength from the TCR. To examine the role of SHP-1 in T cell development, we crossed the ovalbumin-specific DO11.10 TCR transgene onto the motheaten background, which lacks SHP-1 expression. Analysis of the progeny of these crosses provided evidence that SHP-1 regulates thymocyte selection. Flow cytometric analysis of thymocytes revealed enhanced positive and negative selection in the DO11:me/me mice as seen by a decreased DP population concurrent with a relative increase of the single positive population. Ex vivo assays demonstrated that DO11:me/me thymocytes were more sensitive to antigen-induced deletion and proliferation. Non-transgenic melme mice showed enhanced T_reg cell development as indicated by increased CD4+CD25+ T cells in the thymus and spleen. These CD4+CD25+ T cells were found to be functional T_reg cells by ex vivo proliferation and suppression assays. Studies of DO11:me/ me mice further confirmed our hypothesis that SHP-1 helps to set the threshold for T_reg cell selection. These mice showed increased numbers and percentages of CD4+CD25+ T_reg cells compared to their +/+ littermates. Using fetal thymic organ cultures from DO11:+/+ embryos incubated with OVA peptide, we were able to mimic the enhanced negative selection and CD4+CD25+ T_reg cell selection seen in the DO11:me/ me. Our observation that the absence of SHP-1 leads to altered selection of TCR transgenic thymocytes demonstrates that SHP-1 regulates the strength of TCR-mediated signals in vivo and, in turn, helps to set the threshold for thymocyte selection. Based on our studies we also propose a new model for CD4+CD25+ T_reg cell development in which “super-positive” selection results in the development of T_reg cells.