Atypical signaling, ligand recognition and selective agonist discovery of complement receptor C5aR2.

C5a, the most potent anaphylatoxin in the complement system, exerts its effects through the canonical G protein-coupled receptor C5aR1 and the arrestin-coupled receptor C5aR2. Despite the critical role of C5aR2 in immunomodulation, the molecular mechanisms underlying its biased signaling, ligand recognition, and associated pathophysiology remain poorly understood. Here, we report cryo-electron microscopy structures of β-arrestin 1-bound C5aR2 and C5aR1 stimulated by C5a or its metabolite C5adesArg. By combining structural analysis with functional assays, we identified the key structural determinants that prevent G protein coupling and confer intrinsic bias toward β-arrestins. Comparative analysis elucidated the distinct ligand recognition mechanism of C5aR2 and explained the retained affinity of C5adesArg for C5aR2. These findings guided the rational design of ZQ105, a highly selective C5aR2 agonist. Leveraging ZQ105 as a chemical probe, functional studies revealed that selective C5aR2 activation induces distinct pro-inflammatory responses and receptor internalization in neutrophils. This study provides novel structural insights into transducer engagement and ligand recognition by C5aR2, yielding a valuable pharmacological tool for exploring C5aR2-related pathophysiological processes.