THEMIS increases TCR signaling in CD4 ⁺ CD8 ⁺ thymocytes by inhibiting the activity of the tyrosine phosphatase SHP1

Abstract

The T cell lineage-restricted protein THEMIS plays a critical role in T cell development at the positive selection stage. In the SHP1 activation model, THEMIS is proposed to enhance the activity of the tyrosine phosphatase SHP1 (encoded by <i>Ptpn6</i>), thereby dampening T cell antigen receptor (TCR) signaling and preventing the inappropriate negative selection of CD4⁺CD8⁺ thymocytes by positively selecting ligands. In contrast, in the SHP1 inhibition model, THEMIS is proposed to suppress SHP1 activity, rendering CD4⁺CD8⁺ thymocytes more sensitive to TCR signaling initiated by low-affinity ligands to promote positive selection. We sought to resolve the controversy regarding the molecular function of THEMIS. We found that the defect in positive selection in <i>Themis^-/-</i> thymocytes was ameliorated by pharmacologic inhibition of SHP1 or by deletion of <i>Ptpn6</i> and was exacerbated by SHP1 overexpression. Moreover, overexpression of SHP1 phenocopied the <i>Themis^-/-</i> developmental defect, whereas deletion of <i>Ptpn6</i>, <i>Ptpn11</i> (encoding SHP2), or both did not result in a phenotype resembling that of <i>Themis</i> deficiency. Last, we found that thymocyte negative selection was not enhanced but was instead impaired in the absence of THEMIS. Together, these results provide evidence favoring the SHP1 inhibition model, supporting a mechanism whereby THEMIS functions to enhance the sensitivity of CD4⁺CD8⁺ thymocytes to TCR signaling, enabling positive selection by low-affinity, self-ligand-TCR interactions.