4). In the male mice the number of DP thymocytes was slightly increased (Fig. 5). These results demonstrate the partial impairment of positive and negative selection in LAR-deficient mice. During selection, the strength of the TCR signal plays a pivotal role in determining cell fate 3–5. In LAR-deficient thymocytes, the

level of TCR stimulation, as measured by the intracellular Ca2+ concentration, MAPK inhibitor was reduced compared with control thymocytes (Fig. 6). We think the reduction of Ca2+ responding cells is mainly attributed to DP thymocytes because neither CD4SP nor CD8SP thymocytes express LAR 17, 18 and LAR deficiency might not affect the Ca2+ mobilization in CD4SP as well as CD8SP thymocytes. This reduction in the strength of the TCR signal may result in a decrease in the efficiency of negative and positive selection (Fig. 7). Although LAR deficiency might affect T-cell development in the thymus, the animals do

not appear to be immune-compromised and there has been no report of which in the literature. LAR deficiency might not be reflected in the immunological phenotype; first, because the effect of LAR deficiency in the thymus was subtle and its effect was compensated in the periphery; second, because LAR is a member of receptor type membrane tyrosine phosphatase family including CD45 that is expressed on thymocytes as well as peripheral T cells and CD45 could compensate the effect of LAR deficiency. How does LAR deficiency affect TCR signal transduction? During TCR signal transduction, a receptor-like PTP, CD45, activates two PTK, Lck and Fyn, by dephosphorylating a tyrosine moiety 29,

30. Activated Lck and Fyn then phosphorylate the immunotyrosine-activating check details motif on CD3ξ, which leads to the activation of thymocytes as well as T cells. Tsujikawa et al. 12 reported that LAR also regulated the activity of Lck and Fyn in CD45-deficient cells. Taken together, our results suggest that LAR is also involved in Inositol monophosphatase 1 TCR signal transduction in thymocytes. The effect of LAR deficiency on TCR signal transduction seems subtle since we did not observe significant differences in the proliferation of LAR-deficient thymocytes following TCR stimulation (Supporting Information Fig. 6). Regarding the regulation of Ca2+ mobilization by LAR, Archuleta et al. have demonstrated that activated Lck and Fyn increase tyrosine phosphorylation of phospholipase C-γ1, and activated phospholipase C-γ1 increase formation of IP3, which may be responsible for the rapid increase in intracellular Ca2+ mobilization 31. Taken together, we hypothesize that LAR may regulate Ca2+ mobilization by activating Lck and Fyn, which then leads to tyrosine phosphorylation of phospholipase C-γ1 and increases formation of IP3, which finally regulate intracellular Ca2+ mobilization. Our data suggest that LAR is only expressed during the DN-to-DP transition of T cells in the thymus and that LAR plays a role in pre-TCR- or αβTCR-mediated selection during the differentiation of thymocytes.