Abstract 1043: Activated Protein C Exerts its Anti-Inflammatory Activity by Recognizing Leukocyte Integrin Mac-1
Protein C is a major physiological anticoagulant. Upon activation, the activated Protein C (APC) inhibits coagulation by degrading the coagulation Factors V and VIII. Surprisingly, APC also possesses anti-inflammatory activity and has been approved by FDA for the treatment of sepsis. In animal models, APC also protects hosts from tissue injury caused by stroke and heart attack. However, the mechanism underlying its anti-inflammatory function is not well understood. In this study, we investigated the molecular mechanism by which APC exerts its anti-inflammatory activity, using both in vitro and in vivo approaches. We found that the anti-inflammatory function of APC depends in part on its recognition of the leukocyte integrin receptor Mac-1 (CD11b/CD18). First, Mac-1- but not mock-transfected human fibroblast 293 cells adhered strongly to APC, and this Mac-1-dependent cell adhesion could be blocked by a specific Mac-1 antagonist neutrophil inhibitory factor (NIF) and by its function-blocking antibody (mAb 44). Second, soluble APC bound to WT but not Mac-1−/− neutrophils in a dose-dependent manner. Third, APC could be co-immunoprecipited with Mac-1 from total cell lysates. Finally, APC inhibited leukocyte migration in vitro using the Boyden-chamber type transwell assays. Most importantly, we found that intravenous injections of WT mice with APC (10ug/kg) led to a 4-fold reduction in neutrophil infiltration in response to tissue inflammation. In contrast, similar injections of Mac-1−/− mice with APC did not have detectable effects. These results demonstrated that APC exerts its anti-inflammatory and cyto-protective activities in part by recognizing leukocyte receptor Mac-1.