IL-3 is essential for ICOS-L stabilization on mast cells, and sustains the IL-33-induced RORγt(+) T(reg) generation via enhanced IL-6 induction.

IL-33 is a member of the IL-1 family. By binding to its receptor ST2 (IL-33R) on mast cells, IL-33 induces the MyD88-dependent activation of the TAK1-IKK2 signalling module resulting in activation of the MAP kinases p38, JNK1/2 and ERK1/2, and of NFκB. Depending on the kinases activated in these pathways, the IL-33-induced signalling is essential for production of IL-6 or IL-2. This was shown to control the dichotomy between RORγt(+) and Helios(+) T(regs) , respectively. SCF, the ligand of c-Kit (CD117), can enhance these effects. Here, we show that IL-3, another growth factor for mast cells, is essential for the expression of ICOS-L on BMMCs, and costimulation with IL-3 potentiated the IL-33-induced IL-6 production similar to SCF. In contrast to the enhanced IL-2 production by SCF-induced modulation of the IL-33 signalling, IL-3 blocked the production of IL-2. Consequently, IL-3 shifted the IL-33-induced T(reg) dichotomy towards RORγt(+) T(regs) at the expense of RORγt(-) Helios(+) T(regs) . However, ICOS-L expression was downregulated by IL-33. In line with that, ICOS-L did not play any important role in the T(reg) modulation by IL-3/IL-33-activated mast cells. These findings demonstrate that different from the mast cell growth factor SCF, IL-3 can alter the IL-33-induced and mast cell-dependent regulation of T(reg) subpopulations by modulating mast cell-derived cytokine profiles.