Anaphylatoxin receptor promiscuity for commonly used complement C5a peptide agonists.

The complement system is an essential component of innate immunity. Its activation generates the effector cleavage proteins, anaphylatoxins C3a and C5a, that exert activity by interacting with three structurally related seven-transmembrane receptors. C3a activates C3aR, whilst C5a interacts with both C5aR1 and C5aR2 with equal potency. Of the three receptors, C5aR1 in particular is considered the most functionally potent inflammatory driver and has been the major target for pharmacological development. Multiple peptidic C5a agonists have been developed to target C5aR1, with the full agonists EP54 (YSFKPMPLaR) and EP67 (YSFKDMP(MeL)aR), and the partial agonist C028 (C5a, NMe-FKPdChaChadR) being the most commonly utilised in research. Recent studies have indicated that small complement peptide ligands may lack selectivity amongst the three anaphylatoxin receptors, however this has not been uniformly confirmed for these commonly used C5a agonists. In the present study, we therefore characterised the pharmacological activity of EP54, EP67, and C5a at human C5aR1, C5aR2 and C3aR, by conducting signalling assays in transfected cell lines, and in human primary macrophages. Our results revealed that none of the compounds tested were selective for human C5aR1. Both EP54 and EP67 were potent, full C3aR agonists, and EP54 and C5a potently and partially activated human C5aR2. Therefore, we caution against the usage of these ligands, particularly EP54 and EP67, as C5a surrogates in C5a/ C5aR research.