C5aR2 Activation Broadly Modulates the Signaling and Function of Primary Human Macrophages

The complement activation fragment C5a is a potent proinflammatory mediator that is increasingly recognized as an immune modulator. C5a acts through two C5a receptors, C5aR1 (C5aR, CD88) and C5aR2 (C5L2, GPR77), to powerfully modify multiple aspects of immune cell function. Although C5aR1 is generally acknowledged to be proinflammatory and immune-activating, the potential roles played by C5aR2 remain poorly defined. Despite studies demonstrating C5aR2 can modulate C5aR1 in human cells, it is not yet known whether C5aR2 functionality is limited to, or requires, C5aR1 activation or influences immune cells more broadly. The present study, therefore, aimed to characterize the roles of C5aR2 on the signaling and function of primary human monocyte-derived macrophages, using a C5aR2 agonist (Ac-RHYPYWR-OH; P32) to selectively activate the receptor. We found that although C5aR2 activation with P32 by itself was devoid of any detectable MAPK signaling activities, C5aR2 agonism significantly dampened C5aR1-, C3aR-, and chemokine-like receptor 1 (CMKLR1)-mediated ERK signaling and altered intracellular calcium mobilization mediated by these receptors. Functionally, selective C5aR2 activation also downregulated cytokine production triggered by various TLRs (TLR2, TLR3, TLR4, and TLR7), C-type lectin receptors (Dectin-1, Dectin-2, and Mincle), and the cytosolic DNA sensor stimulator of IFN genes (STING). Surprisingly, activity at the C-type lectin receptors was particularly powerful, with C5aR2 activation reducing Mincle-mediated IL-6 and TNF-α generation by 80-90%. In sum, this study demonstrates that C5aR2 possesses pleiotropic functions in primary human macrophages, highlighting the role of C5aR2 as a powerful regulator of innate immune function.