Inhibition of Efferocytosis by Extracellular CIRP-Induced Neutrophil Extracellular Traps

Phagocytic clearance of apoptotic cells by the macrophages (efferocytosis) is impaired in sepsis, but its mechanism is poorly understood. Extracellular cold-inducible RNA-binding protein (eCIRP) is a novel damage-associated molecular pattern that fuels inflammation. We identify that eCIRP-induced neutrophil extracellular traps (NETs) impair efferocytosis through a novel mechanism. Coculture of macrophages and apoptotic thymocytes in the presence of recombinant murine CIRP (rmCIRP)-induced NETs significantly inhibited efferocytosis. Efferocytosis was significantly inhibited in the presence of rmCIRP-treated wild-type (WT), but not PAD4(-/-) neutrophils. Efferocytosis in the peritoneal cavity of rmCIRP-injected PAD4(-/-) mice was higher than WT mice. Milk fat globule-EGF-factor VIII (MFG-E8), an opsonin, increased macrophage efferocytosis, whereas the inhibition of efferocytosis by NETs was not rescued upon addition of MFG-E8, indicating disruption of MFG-E8's receptor(s) alpha(v)beta(3) or alpha(v)beta(5) integrin by the NETs. We identified neutrophil elastase in the NETs significantly inhibited efferocytosis by cleaving macrophage surface integrins alpha(v)beta(3) and alpha(v)beta(5). Using a preclinical model of sepsis, we found that CIRP-/- mice exhibited significantly increased rate of efferocytosis in the peritoneal cavity compared with WT mice. We discovered a novel role of eCIRP-induced NETs to inhibit efferocytosis by the neutrophil elastase-dependent decrease of alpha(v)beta(3)/alpha(v)beta(5) integrins in macrophages. Targeting eCIRP ameliorates sepsis by enhancing efferocytosis.

Automated summary

In sepsis, eCIRP-induced NETs inhibit phagocytosis of apoptotic cells by macrophages through neutrophil elastase-dependent reduction of integrins. This discovery reveals a potential mechanism of therapeutic failure.