Introduction: We studied monocyte transendothelial migration and subsequent polarization into M1/M2 macrophages in response to C-reactive protein (CRP) with two disease-related ligands: (1) phosphocholine (PC) and (2) multilamellar liposomes containing both unoxidized and oxidized forms of the lipid, phosphatidylcholine. These ligands differ in biological origin: PC is present on bacterial cell walls while oxidized lipids are present in atherogenic lipids.
Methods: We used an in vitro model of human monocyte transendothelial migration and assessed the polarization of monocytes and T cells and signaling through Fcγ receptors in monocytes.
Results: CRP without ligands did not promote M2 macrophage differentiation over background levels. However, when paired with either ligand, it increased M2 numbers. M2 differentiation was dependent on IL-13, and in the case of CRP with PC, was associated with a Th2 response. Paradoxically, while CRP with PC initiated a Th2 response, the combination of liposomes with CRP resulted in a Th1 response without any change in Th2 numbers despite association with M2 macrophage polarization. To resolve the conundrum of an anti-inflammatory macrophage response coexisting with a proinflammatory T cell response, we investigated signaling of CRP and its ligands through Fcγ receptors, which leads to macrophage activation independent of T cell signaling. We found that CRP plus PC acted via FcγRI, whereas CRP with liposomes bound to FcγRII. Both were activating signals as evidenced by SYK phosphorylation.
Conclusion: We conclude that CRP with ligands can promote M2 macrophage differentiation to fibroblasts through FcγR activation, and this may result in an anti-inflammatory influence despite a proinflammatory T cell environment caused by oxidized lipids. The potential relationship of this mechanism to chronic inflammatory disease is discussed.
Keywords: C‐reactive protein; FcγR; macrophage.