TGFβ trained immunity in mast cells: do inflammasomes matter?

Abstract

Abstract Mast cell (MC) contributions to solid cancer microenvironments are controversial and seem tissue dependent. TGFβ is a potent MC chemokine abundantly expressed by high-grade cancers. Previous work reported that TGFβ alone induces biologically relevant expression of IL-6 from MC, which is potentially important for reciprocal paracrine interactions with cancer cells and other leukocytes. Therefore, the objective of the present study was to examine MC in an immunocompetent, high-grade model of breast cancer. Using the 4T1 system in BALB/c mice, we compared mast cell infiltration into primary tumors of wild-type versus 4T1 cell-specific knockouts of IL-6. We also hypothesized that TGFβ institutes a training effect on MC like other metabolically driven mechanisms involving the NLRP3 inflammasome. Using a mixture of flow cytometric and qPCR assays we explored potential MC training in the context of TGFβ. Results: Contrary to our prediction, MC infiltration into IL-6KO tumors was significantly increased compared to WT. Studies of classic intracellular training mechanisms in response to TGFβ were mixed. Glucose uptake, glycolytic capacity, mitochondrial potential, and ASC speck formation were unaffected by TGFβ, but IL-6 expression increased as previously seen. Additionally, we were unable to detect substantive NLRP3 expression in MC regardless of condition. Despite these results, we were surprised to observe robust increases in expression of IL-1β, IL-18, and caspase-1 with TGFβ stimulus. We conclude that MC are actively recruited to 4T1 tumors dependent on IL-6 (and TGFβ), MC produce select cytokines in response to TGFβ, yet the mechanism explaining this does not involve the NLRP3 inflammasome and remains unresolved.