Expression of Ly6C/6G defines a novel Aire-dependent subset of medullary thymic epithelial cells

Ly6 family proteins are mainly expressed by myeloid-cell lineages such as granulocytes (expressing Ly6G) and monocytes (expressing Ly6C). However, exact function of Ly6 family protein in each cell-type remains unknown. For example, although expression of Ly6C has been used to define functionally distinct subset of monocytes (Ly6C+ monocytes vs. Ly6C− monocytes), molecular basis for the expression of Ly6C and functional difference between the two populations are largely unknown. Medullary thymic epithelial cells (mTECs) are composed of heterogeneous populations in terms of the spectrum of tissue-restricted antigens (TRAs) expression for ensuring the elimination of autoreactive T-cell repertoire. We have recently identified mTEC subset(s) that express Ly6C and Ly6G both recognized by monoclonal antibody Gr-1 (i.e., Ly6C/6G+ mTECs). Approximately 10% of total mTECs expressed Ly6C/6G and Ly6C/6G+ mTECs were preferentially localized at the cortico-medullary junction. We found that Ly6C/6G+ mTECs and Ly6C/6G− mTECs do not form distinct lineage(s). Instead, Ly6C/6G− mTECs are able to convert into Ly6C/6G+ mTECs and vice versa in a reaggregated thymic organ culture (RTOC). Interestingly, Aire-deficient mice do not possess Ly6C/6G+ mTECs. RNA-seq analyses have revealed that Ly6C/6G+ mTECs express higher levels of chemokines, integrins and extracellular matrix proteins compared with those from Ly6C/6G− mTECs, suggesting that Ly6C/6G+ mTECs might constitute a unique tolerogenic subset among heterogenous mTECs.