Role of Themis in peripheral CD8 T cells in hapten-induced allergic skin inflammation

Themis (thymocyte-expressed molecule involved in selection) is a T-cell-specific protein that is critically required for positive selection in the thymus. However, its function in T-cell receptor (TCR) responses during allergic skin inflammation remains unclear. To investigate the function of Themis in peripheral T cells, we generated tamoxifen-induced Themis conditional knockout (cKO) mice. The deletion of Themis by tamoxifen treatment significantly reduced ear swelling and CD8 T-cell infiltration induced by hapten 2,4-dinitrofluorobenzene (DNFB) challenge, which activates CD8 T cells. The CD8 T cells in the inflamed skin from Themis cKO mice showed decreased interferon gamma (IFNγ) production and T-bet and Eomes expression. Furthermore, the transgenic overexpression of Themis enhanced DNFB-induced allergic skin responses. However, Themis cKO mice showed unaltered skin inflammation induced by fluorescein isothiocyanate/dibutyl phthalate, which activates CD4 T cells. The TCR-stimulated proliferation and IFNγ production of Themis cKO naïve CD8 T cells were significantly decreased in vitro, whereas the proliferation and cytokine production of CD4 T cells were not altered. As expected, the administration of the SHP-1/2 inhibitor restored the reduced IFNγ production in Themis cKO CD8 T cells in vitro. Mice harboring mutant Themis lacking the Grb2-binding site showed a similar phenotype to Themis cKO mice, indicating that the function of Themis in peripheral CD8 T cells is dependent on Grb2 binding. Collectively, these results suggest that Themis regulates the threshold of TCR signaling in peripheral CD8 T cells, but not in CD4 T cells.