Cholesterol-dependent cytolysins promote immune evasion by clustering host membrane lipids

Pathogens causing necrotizing soft tissue infections, such as Streptococcus pyogenes and Clostridium perfringens, use cholesterol dependent cytolysins (CDCs) to evade immune responses by blocking pro-inflammatory TNFα production in macrophages. This evasion may be an unintended consequence of membrane repair responses to the CDC. CDCs trigger lipid-dependent sequestration and shedding of CDC pores on microvesicles. However, the molecular mechanisms by which CDCs exploit membrane repair to disrupt immune signaling is unknown. One target may be cholesterol-rich microdomains (CRMs) because they are focal points for toxin binding and immune signaling. We hypothesized that when CDCs oligomerize, they cluster membrane lipids in CRMs, along with immune receptors, into microscale assemblies that are spontaneously shed as microvesicles. We tested this hypothesis by artificially clustering sphingolipids using the sphingolipid-binding protein Ostreolysin A (OlyA), fused with tandem FKBP dimerization domains. We challenged murine bone marrow derived macrophages with and without FKBP homodimerizer and OlyA prior to LPS challenge and measured intracellular TNFα by flow cytometry. We found that oligomerization reduced OlyA binding, consistent with shedding. OlyA oligomerization further suppressed TNFα production, suggesting that microscale lipid assemblies interfere with cellular signaling. Overall, these results suggest that one cost to repair by shedding toxins on microvesicles is the loss of cellular signaling needed to promote inflammation. This may represent one strategy CDC-producing pathogens use to promote immune evasion and cause disease.