Human cornea-derived mesenchymal stromal cells inhibit T cells through indoleamine 2,3 dioxygenase

Defining the mechanism of immune modulation by mesenchymal stromal cells (MSCs) from distinct anatomical tissues is of great translational interest. The human cornea is an immunologically privileged organ, and the mechanism of immunoregulation of cornea-derived MSCs (cMSCs) is currently unknown. We investigated cMSCs derived from the corneas of 5 independent human donorS for their fitness and mechanism of action in suppressing T cells. cMSCs display the immunophenotype CD45-CD73+CD105+CD90+CD44+ and robust in vitro growth. 30-plex secretome analysis identified that cMSCs innately secrete specific molecules in a dose-dependent manner. cMSCs do not express or upregulate costimulatory but do upregulate coinhibitory molecules upon stimulation with interferon γ (IFNγ). cMSCs inhibit T-cell proliferation in contact-dependent co-cultures, which can be predicted by a unique secretome signature. In addition, co-culturing in a 2-chamber transwell system has demonstrated that cMSCs also inhibit T-cell proliferation in a non-contact-dependent manner. Mechanistic analysis has demonstrated that activated T cells effectively induce indoleamine 2,3-dioxygenase (IDO) but not other enzymes of the tryptophan metabolic pathway in cMSCs. Silencing of IDO in cMSCs reduces their fitness to suppress T cells. These results provide evidence that in cMSCs, one of the principal mechanisms of immunosuppression on T cells is through IDO. These results suggest that MSCs derived from the human cornea display immunoregulatory properties and, thus, may play a role in maintaining the immune-privileged niche of the cornea.