Lack of Herpes Virus Entry Mediator Signals in Thymocytes Impairs Conventional CD8 T Cell Selection and Promotes Memory-like CD8 T Cell Development.

Thymocytes having diverse Ag specificities are selected in response to self-MHC-peptide expressed in thymic epithelial cells, which contributes to the formation of a T cell repertoire. However, it is not well understood whether additional signals from epithelial cells are required to drive positive selection. In this study, we found that one of the TNFR superfamily members, herpes virus entry mediator (HVEM), when expressed on thymocytes provides signals for positive selection. HVEM deficiency in double-positive (DP) thymocytes impaired positive selection of CD8 thymocytes. HVEM-deficient thymocytes in OT-1 TCR transgenic mice exhibited significant defects in positive selection and impaired CD69 upregulation of selected thymocytes. HVEM ligands (lymphotoxin-like, exhibits inducible expression, and competes with HSV glycoprotein D for HVEM, a receptor expressed by T lymphocytes, and B and T lymphocyte attenuator) were expressed in cortical thymic epithelial cells. Weak TCR ligation combined with HVEM signals enhanced ERK activation in DP thymocytes developed in vitro. Insufficient signals for positive selection in HVEM-deficient DP thymocytes led to the development of innate memory-like CD8 T cells expressing high levels of CD122, along with the increased development of PLZF+ NKT cells. These results suggest that thymocytes receive activation signals through HVEM during positive selection. Thus, our findings provide evidence that the threshold of thymocyte positive selection is set by signals from TCR in association with HVEM.