Growth and expansion of human T regulatory type 1 cells are independent from TCR activation but require exogenous cytokines

Cloned T regulatory type 1 (Tr1) cells produce IL-10, TGF-beta, IFN-gamma, and very low or non-detectable levels of IL-2 and IL-4 following TCR-mediated activation. In addition, upon TCR stimulation, Tr1 cell clones up-regulate activation markers but show low proliferative responses, partially due to the suppressive effect of autocrine IL-10 and TGF-beta. Here we show that Tr1 cells have growth requirements different from those of Th1 and Th2 cells. Exogenous IL-15, and to a lesser extent IL-2, induce and support the proliferation of Tr1 cells in the absence of TCR activation. This strong cytokine response correlates with high constitutive levels of the IL-2/15Rbeta and common gamma chains expressed by Tr1 cell clones. Furthermore, suboptimal doses of IL-15, in combination with IL-2, induce a significant growth (median value: 25-fold increase in cell number) of Tr1 cell clones during a culture period of 11 days, which leads to an in vitro expansion of Tr1 cell clones comparable to that of Th1 and Th2 cell clones. Tr1 cell clones cultured in IL-15 continue to secrete immunosuppressive cytokines and to proliferate poorly upon reactivation via TCR. These findings indicate that Tr1 cells are constitutively capable of responding to cytokines and mainly to IL-15. This growth factor enables a significant in vitro expansion of Tr1 cells facilitating further biological and biochemical characterization of this unique T cell subset.