The transmembrane adaptor protein SIT is a negative regulator of TCR-mediated signaling. However, little is known about the functional role of SIT in mature T cells. In this study, we show that mice deficient for SIT display a decreased number of naive CD8+ T cells and a progressive accumulation of memory-like (CD44high) CD8+ T lymphocytes that resemble cells undergoing homeostatic proliferation. Indeed, when transferred into lymphopenic hosts, SIT−/− naive CD8+ T cells undergo enhanced homeostatic proliferation and express a higher level of CD44 in comparison to wild-type T cells. By using class-I-restricted TCR transgenic models with different ligand affinity/avidity, we show that lymphopenia-induced homeostatic proliferation is more pronounced in cells carrying low-affinity TCRs. Strikingly, the loss of SIT induces homeostatic proliferation of HY TCR transgenic cells, which are normally unable to proliferate in lymphopenic mice. Collectively, these data demonstrate that SIT negatively regulates T cell homeostasis. Finally, we show that SIT-deficient T cells develop a mechanism analogous to sensory adaptation as they up-regulate CD5, down-regulate the coreceptor, and display impaired TCR-mediated ZAP-70 activation.
Transmembrane adaptor proteins (TRAPs) organize signaling complexes at the plasma membrane, and thus function as critical linkers and integrators of signaling cascades downstream of antigen receptors. We have previously shown that the transmembrane adaptor protein SIT regulates the threshold for thymocyte selection. Moreover, T cells from SIT-deficient mice are hyperresponsive to CD3 stimulation and undergo enhanced lymphopenia-induced homeostatic proliferation, thus indicating that SIT inhibits TCR-mediated signaling. Here, we have further addressed how SIT regulates signaling cascades in T cells. We demonstrate that the loss of SIT enhances TCR-mediated Akt activation and increased phosphorylation/inactivation of Foxo1, a transcription factor of the Forkhead family that inhibits cell cycle progression and regulates T-cell homeostasis. We have also shown that CD4+ T cells from SIT-deficient mice display increased CD69 and CD40L expression indicating an altered activation status. Additional biochemical analyses further revealed that suppression of SIT expression by RNAi in human T cells resulted in an enhanced proximal TCR signaling. In summary, the data identify SIT as an important modulator of TCR-mediated signaling that regulates T-cell activation, homeostasis and tolerance.
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