Mast Cell Phenotypic Heterogeneity Impacts the Interplay with Pathogenic Salmonella Typhimurium Bacteria

Mast cells (MCs) lodge within barrier tissues and respond to infectious microbes. Recent work demonstrated that MCs differentiate their cytokine response to extracellular versus invasive Gram-negative enterobacteria by a two-step activation mechanism that integrates Toll-like-receptor (TLR) sensing with signals elicited by type-III-secretion-system (TTSS) effectors during bacterial invasion. However, multiple MC subtypes exist, and it remains unclear how their phenotypic heterogeneity impacts microbial interactions. We find that murine MCs maintained in IL-3, or differentiated toward a connective-tissue phenotype (CT-MCs), respond potently to the enteropathogen Salmonella enterica Typhimurium (S.Tm) through two-step activation, with the TLR component explained by functional TLR4 and TLR2. By contrast, murine mucosal mast cells (M-MCs) express insignificant levels of these TLRs, therefore being unresponsive to extracellular S.Tm, but still mounting a response to invasive bacteria. Following invasion, MC granule maintenance by serglycin restricts S.Tm vacuolar and cytosolic colonization. Notably, this has no impact on the cytokine release from infected MCs, thus uncoupling S.Tm´s intracellular life-cycle from the MC cytokine response. Finally, human LUVA MCs employ a variant of two-step activation where TLR2/6 signaling combines with the TTSS-elicited signals. Together, this study explains how MC subtypes can respond differently to S.Tm-infection depending on their TLR expression and granule features.