Modulation of mast cell function by TIM-3 signaling (HYP3P.351)

Transmembrane, immunoglobulin and mucin domain-3 (Tim-3) is an inducible negative regulator of Th1 function and anti-tumor immune responses. Recent studies found that Tim-3 is constitutively expressed on mast cells and that an anti-Tim-3 pAb enhanced secretion of pro-inflammatory cytokines by antigen-stimulated bone marrow-derived mast cells (BMMC). However, the mechanism(s) by which Tim-3 modulates mast cell function are unknown. Here we show that Tim-3 enhances FcϵRI-derived signals to co-stimulate NF-AT, AP1, NF-κB, and IL-6 promoter transcriptional activation. This activity requires specific tyrosines in the Tim-3 cytoplasmic tail. Furthermore, various antibodies to Tim-3 significantly enhanced IgE+Ag-stimulated tyrosine phosphorylation and up-regulated BMMC cytokine secretion. Conversely, we observed impaired IgE+Ag-dependent activation of mast cells in which Tim-3 expression was knocked down. Using BMMC generated from Nur77-GFP reporter mice, we found that cross-linking Tim-3 synergizes with the antigen receptor FcϵRI to enhance Nur77-GFP expression. Our work suggests that Tim-3 acts proximal to the antigen receptor, leading to enhancement of the NF-κB, NF-AT, and AP-1 pathways. Thus, these studies are consistent with Tim-3 being a positive regulator of mast cell function. Mast cells contribute to both innate and adaptive immune responses, so Tim-3 may serve as an important modulator of mast cell effector functions in autoimmune and allergic diseases.