Deletion of Biliverdin Reductase A in Myeloid Cells Promotes Chemokine Expression and Chemotaxis in Part via a Complement C5a--C5aR1 Pathway.

Biliverdin reductase (BVR)-A is a pleotropic enzyme converting biliverdin to bilirubin and a signaling molecule that has cytoprotective and immunomodulatory effects. We recently showed that biliverdin inhibits the expression of complement activation fragment 5a receptor one (C5aR1) in RAW 264.7 macrophages. In this study, we investigated the role of BVR-A in determining macrophage inflammatory phenotype and function via regulation of C5aR1. We assessed expression of C5aR1, M1-like macrophage markers, including chemokines (RANTES, IP-10), as well as chemotaxis in response to LPS and C5a in bone marrow-derived macrophages from BVR fl/fl and LysM-Cre:BVR fl / fl mice (conditional deletion of BVR-A in myeloid cells). In response to LPS, macrophages isolated from LysM-Cre:BVR fl/fl showed significantly elevated levels of C5aR1 as well as chemokines (RANTES, IP10) but not proinflammatory markers, such as iNOS and TNF. An increase in C5aR1 expression was also observed in peritoneal macrophages and several tissues from LysM-Cre:BVR fl/fl mice in a model of endotoxemia. In addition, knockdown of BVR-A resulted in enhanced macrophage chemotaxis toward C5a. Part of the effects of BVR-A deletion on chemotaxis and RANTES expression were blocked in the presence of a C5aR1 neutralizing Ab, confirming the role of C5a-C5aR1 signaling in mediating the effects of BVR. In summary, BVR-A plays an important role in regulating macrophage chemotaxis in response to C5a via modulation of C5aR1 expression. In addition, macrophages lacking BVR-A are characterized by the expression of M1 polarization-associated chemokines, the levels of which depend in part on C5aR1 signaling.