Aims/hypothesis: Limited information is available on the cellular interactions between regulatory T (T(reg)) cells and mesenchymal stem cells (MSCs). In particular, a direct effect of MSCs on the survival and proliferation of T(reg) cells has not been demonstrated.
Methods: We investigated the effects of MSCs on effector T (T(eff)) cells and T(reg) cells, and the molecular mechanisms involved in the distinct regulation of these two cell populations by MSCs in vivo and in vitro.
Results: We show that MSCs are capable of selectively suppressing T(eff) cells and fostering the generation of T(reg) cells. T(eff) cells, but not T(reg) cells, fail to respond to IL-2 and undergo profound apoptosis in the presence of MSCs. The differential regulations of these two T cell subsets by MSCs are associated with their distinct expressions of CD25, with MSCs specifically reducing the expression of CD25 on T(eff) and sparing T(reg) cells intact. In vivo, the administration of MSCs significantly delays the rejection of allogeneic islet grafts in adaptive transferred recipients by favouring the induction of T(reg) cells. In this model, MSCs inhibit the proliferation and development of alloreactive T(eff) but potently enhance the induction of T(reg) cells.
Conclusions/interpretation: We demonstrate that MSCs are capable of regulating T(eff) and T(reg) cells differentially in vitro. MSCs inhibit T(eff) cells by inducing apoptosis and impairing the proliferative response to IL-2 in T(eff) cells, but favour the survival and expansion of T(reg) cells. This result is further demonstrated in mice that have undergone allogeneic islet transplantation, in which MSCs suppress alloreactive T(eff) cells while favouring the induction of T(reg) cells, thus protecting the islet allografts from rejection.