Disturbances in the Complement-mediated inflammatory pathway are known to promote chronic inflammation in both the central and peripheral nervous system. The anaphylatoxin C5a is a central aspect of this, exerting pro-inflammatory effects such as chemotaxis, leukocyte degranulation and cytokine/chemokine production through interaction with its endogenous receptor, C5aR1. In comparison, understanding the role of the second C5a receptor, C5aR2, is limited, with conflicting data indicating both pro- and anti-inflammatory activity. Existing studies have relied predominantly on knockout mouse models to observe the role of C5aR2, however the impact of the receptor on trafficking and expression of C5aR1 and/or C3aRis still not fully understood and thus elimination of C5aR2 in these models does little to clarify the influence of the receptor on the reported disease outcomes. In order to further elucidate the immunomodulatory role of C5aR2 in both healthy and pathological conditions, selective ligands for the receptor are needed.

In this study, it was hypothesised that potent and selective agonists for C5a receptor 2 could be developed based on the lead molecule BM2020-7. Using FMOC-based solid phase peptide synthesis, analogues of BM2020-7 were constructed and the ability of these analogues to modulate C5aR2-mediated β-arrestin 2 recruitment was assessed in HEK-293 cells transiently expressing C5aR2. Analogues that exhibited >25% efficacy of 100 nM C5a were then counter-screened for ligand-induced ERK 1/2 phosphorylation in CHO cell lines stably expressing C3aR and C5aR1. Through analysis of the structure-activity relationship and site-directed modification of C5aR2 ligands, we have developed potent, selective full agonists of C5aR2-mediated β-arrestin 2 recruitment using BM2020-7 as a scaffold. These ligands will act as valuable tool compounds in probing of the role of C5aR2 within the complement system, alongside aiding in the further development of potent, selective, and effective peptide therapeutics targeting the receptor.