Metalloproteinase-mediated Shedding of Integrin β2 Promotes Macrophage Efflux from Inflammatory Sites*

Background: Efflux of macrophages limits inflammation. Results: Macrophage integrin αMβ2 is cleaved during exiting from inflammatory sites, released αMβ2 retains ligand binding capabilities, and inhibition of its metalloproteinase-mediated cleavage impairs macrophage efflux. Conclusion: Metalloproteinase-mediated proteolysis of integrin β2 promotes macrophage efflux from inflammatory sites. Significance: Regulated proteolysis of integrin β2 during inflammation demonstrates the potential of this mechanism to contribute to resolution of inflammation. Macrophage exiting from inflammatory sites is critical to limit the local innate immune response. With tissue insult, resident tissue macrophages rapidly efflux to lymph nodes where they modulate the adaptive immune response, and inflammatory macrophages attracted to the site of injury then exit during the resolution phase. However, the mechanisms that regulate macrophage efflux are poorly understood. This study has investigated soluble forms of integrin β2 whose levels are elevated in experimental peritonitis at times when macrophages are exiting the peritoneum, suggesting that its proteolytic shedding may be involved in macrophage efflux. Both constitutive and inducible metalloproteinase-dependent shedding of integrin β2 from mouse macrophages are demonstrated. Soluble integrin β2 is primarily released as a heterodimeric complex with αM that retains its ability to bind its ligands intracellular adhesion molecule-1, fibrin, and collagen and thus may serve as a soluble antagonist. In a model of accelerated exiting, administration of a metalloproteinase inhibitor prevents macrophage efflux by 50% and impedes loss of macrophage integrin β2 from the cell surface. Exiting of peritoneal macrophages in mice lacking integrin β2 is accelerated, and antibody disruption of integrin β2-substrate interactions can reverse 50% of the metalloprotease inhibitor blockade of macrophage exiting. Thus, our study demonstrates the ability of metalloproteinase-mediated shedding of integrin β2 to promote macrophage efflux from inflammatory sites, and the release of soluble integrin heterodimers may also limit local inflammation.