Absence of complement factor c5a receptor on circulating cells reduces myocardial reperfusion injury and improves cardiac function

Purpose: Following acute myocardial infarction, infarct size is an important determinant of subsequent heart function. Infarct size is further increased by processes provoked by reperfusion itself, called myocardial ischemia-reperfusion injury (MI/R), for which clinically approved therapy is not available. The innate immune response, including activation of the complement system, is an important determinant of infarct size after reperfusion. The receptor for anaphylatoxin C5a (C5aR), however, has not been investigated yet. We studied the role of C5aR in myocardial reperfusion injury. Methods: Wildtype BALB/c mice and C5aR-/- mice underwent surgical occlusion of the proximal left coronary artery for 30 minutes, followed by reperfusion. Infarct size was calculated as a percentage of area at risk (IS/AAR; Evan's blue and TTC staining) for both groups after 24 hours of reperfusion. Cardiac function was measured at baseline and 28 days after reperfusion with echocardiography. Infarct size was also determined in chimeric mice (wildtype bonemarrow transplanted in C5aR-/- mice and vice versa, and control groups), after which IS/AAR was calculated 24 hours after reperfusion. Immunohistochemical staining for neutrophils (Ly6G) was performed on paraffin embedded mouse left ventricles. Results: At 24 hours after MI/R we observed a reduction of IS/AAR in C5aR-/- mice compared with wildtype mice (27.8% ±8.0 vs. 35.7% ±10.5, p = 0.013). Echocardiographic analysis after 28 days showed an improvement of ejection fraction of 6.6% (19.2% ±5.4 vs. 25.8% ±5.5, p = 0.001). The chimera experiment revealed that the reduction of infarct size was only present in mice with knockout bonemarow (25.3% ±8.7 vs. 34.6% ±10.1, p = 0.012), implicating that the absence of C5aR on circulating cells is responsible for this infarct size reduction. Immunohistochemical analysis showed significantly less neutrophils in the infarct area in C5aR-/- mice compared with wildtype mice (190±138 vs. 471±221 cells per mm2, p = 0.012), emphasizing their role in MI/R injury. Conclusions: Absence of the C5a receptor on circulating cells reduces infarct size and improves cardiac function in a mouse model of MI/R injury, potentially by inhibiting neutrophil chemotaxis to the injured myocardium. Blocking the C5a receptor might be a promising therapeutic option to prevent MI/R injury.