Pore-forming toxins (PFTs) are used by both the immune system and by pathogens to disrupt cell membranes. Cells attempt to repair this disruption in various ways, but the exact mechanism(s) that cells use are not fully understood, nor agreed upon. Current models for membrane repair include (1) patch formation (e.g., fusion of internal vesicles with plasma membrane defects), (2) endocytosis of the pores, and (3) shedding of the pores by blebbing from the cell membrane. In this study, we sought to determine the specific mechanism(s) that cells use to resist three different cholesterol-dependent PFTs: Streptolysin O, Perfringolysin O, and Intermedilysin. We found that all three toxins were shed from cells by blebbing from the cell membrane on extracellular microvesicles (MVs). Unique among the cells studied, we found that macrophages were 10 times more resistant to the toxins, yet they shed significantly smaller vesicles than the other cells. To examine the mechanism of shedding, we tested whether toxins with engineered defects in pore formation or oligomerization were shed. We found that oligomerization was necessary and sufficient for membrane shedding, suggesting that calcium influx and patch formation were not required for shedding. However, pore formation enhanced shedding, suggesting that calcium influx and patch formation enhance repair. In contrast, monomeric toxins were endocytosed. These data indicate that cells use two interrelated mechanisms of membrane repair: lipid-dependent MV shedding, which we term 'intrinsic repair', and patch formation by intracellular organelles. Endocytosis may act after membrane repair is complete by removing inactivated and monomeric toxins from the cell surface.

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At mucosal sites such as the intestine, the immune system launches robust immunity against invading pathogens while maintaining a state of tolerance to commensal flora and ingested food Ags. The molecular mechanisms underlying this phenomenon remain poorly understood. In this study, we report that signaling by GPR81, a receptor for lactate, in colonic dendritic cells and macrophages plays an important role in suppressing colonic inflammation and restoring colonic homeostasis. Genetic deletion of GPR81 in mice led to increased Th1/Th17 cell differentiation and reduced regulatory T cell differentiation, resulting in enhanced susceptibility to colonic inflammation. This was due to increased production of proinflammatory cytokines (IL-6, IL-1β, and TNF-α) and decreased expression of immune regulatory factors (IL-10, retinoic acid, and IDO) by intestinal APCs lacking GPR81. Consistent with these findings, pharmacological activation of GPR81 decreased inflammatory cytokine expression and ameliorated colonic inflammation. Taken together, these findings identify a new and important role for the GPR81 signaling pathway in regulating immune tolerance and colonic inflammation. Thus, manipulation of the GPR81 pathway could provide novel opportunities for enhancing regulatory responses and treating colonic inflammation.