Tim-3 enhances FcεRI-proximal signaling to modulate mast cell activation

T cell (or transmembrane) immunoglobulin and mucin domain protein 3 (Tim-3) has attracted significant attention as a novel immune checkpoint receptor (ICR) on chronically stimulated, often dysfunctional, T cells. Antibodies to Tim-3 can enhance antiviral and antitumor immune responses. Tim-3 is also constitutively expressed by mast cells, NK cells and specific subsets of macrophages and dendritic cells. There is ample evidence for a positive role for Tim-3 in these latter cell types, which is at odds with the model of Tim-3 as an inhibitory molecule on T cells. At this point, little is known about the molecular mechanisms by which Tim-3 regulates the function of T cells or other cell types. We have focused on defining the effects of Tim-3 ligation on mast cell activation, as these cells constitutively express Tim-3 and are activated through an ITAM-containing receptor for IgE (FcεRI), using signaling pathways analogous to those in T cells. Using a variety of gain- and loss-of-function approaches, we find that Tim-3 acts at a receptor-proximal point to enhance Lyn kinase-dependent signaling pathways that modulate both immediate-phase degranulation and late-phase cytokine production downstream of FcεRI ligation.T cell, or transmembrane, immunoglobulin domain and mucin domain (Tim-3) is a type I membrane protein expressed on a variety of innate and adaptive immune cell types. Tim-3 is often referred to as a checkpoint receptor due to its apparent inhibitory function on T cells and its association with activation-induced T cell exhaustion in tumors and chronic viral infection (Sánchez-Fueyo et al., 2003; Jones et al., 2008; Fourcade et al., 2010; Jin et al., 2010; Sakuishi et al., 2010). Recent studies, however, suggest a more nuanced picture of Tim-3 function in T cells, depending on the setting, e.g., acute versus chronic stimulation (Ferris et al., 2014; Gorman and Colgan, 2014). In addition to CD4 and CD8 T cells, Tim-3 is also expressed on other immune cell types, such as NK cells, macrophages, DCs, and mast cells, but its function on these cell types is less clear. Tim-3 blockade was shown to enhance macrophage function in response to sepsis (Yang et al., 2013), and also to regulate antigen (Ag) presentation by DCs, partly through Btk and c-Src (Maurya et al., 2014). On the other hand, Tim-3 expression on monocytes infiltrating the CNS during EAE was shown to promote inflammation (Anderson et al., 2007).Mast cells are first-line defenders against allergens and invading pathogens as a result of their proximity to the external environment. Cross-linking of IgE bound to the high-affinity IgE receptor FcεRI by Ag leads to the release of preformed mediators and de novo synthesis of proinflammatory and antiinflammatory mediators and cytokines, which together serve to regulate hypersensitivity, autoimmunity, cardiovascular disease, and tumor progression (Kalesnikoff and Galli, 2008). In addition to their well-known pathological roles in allergic responses, mast cells also contribute to defense against bacteria, helminthes, and tumors (Abraham and St John, 2010). It was reported that mast cells constitutively express cell surface Tim-3, and that cross-linking of Tim-3 could enhance cytokine production of IgE-sensitized and Ag-stimulated BM-derived mast cells (BMMCs) and peritoneal mast cells (pMCs) without affecting degranulation (Nakae et al., 2007). TGF-β has been shown to up-regulate expression of Tim-3 in tumor-infiltrating mast cells and a human mast cell line, through a mitogen-activated protein kinase Erk-kinase (MEK)–dependent pathway (Wiener et al., 2006; Yoon et al., 2011). Although previous data suggest that Tim-3 is a positive regulator of mast cell activation, the molecular mechanisms behind the contribution of Tim-3 to mast cell function are still unknown. Importantly, there was until now no genetic evidence addressing the function of Tim-3 in these cells. Given the important role of mast cells as sentinels in both allergic and nonallergic diseases, it is of interest to explore Tim-3 activity on this cell type and how antibody (Ab) modulation can affect its function.Here, we demonstrate through multiple approaches that Tim-3 functions to enhance proximal FcεRI signaling in mast cells. Cross-linking of Tim-3 with multiple independent antibodies enhanced mast cell degranulation and cytokine release in a dose-dependent manner. Acute knock-down or genetic deficiency of Tim-3 rendered mast cells less responsive to Ag cross-linking of FcεRI, resulting in decreased degranulation and cytokine production. The cytoplasmic tail of Tim-3 was required for co-stimulatory signal transduction in mast cells, together with FcεRI signaling pathways. This was shown in part with the use of recently reported Nur77-GFP transgenic models, which have not previously been used for the study of FcεRI signaling. Collectively, our data demonstrate that Tim-3 acts at a receptor-proximal level to intensify activation of FcεRI-dependent signaling pathways upon Ag cross-linking, while maintaining the threshold for negative signaling of Lyn.