A study of the molecular pharmacological properties of C5a and C5a des Arg : defining their biased agonist profile and contrasting biological function

• Main Author: Ramsey, S. J.
• Other Authors: MacEwan, David ; Nicholas, Pullen
• Published: University of Liverpool 2017
• Subjects: 615.1
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.755464

The complement system provides an essential role in the orchestration of an effective immune response against microbial infection. Activation leads to the generation of C5a which activates receptors expressed on immune cells promoting microbial clearance. Dysregulation of the complement system that results in the excessive production of C5a can lead to host tissue damage. This has fuelled drug discovery efforts to therapeutically block the actions of C5a to treat diseases associated with dysregulation of the complement system. C5a is rapidly degraded to C5a des Arg, a mechanism which is considered to inactivate its effector functions, however, there is incomplete understanding of precise functions of C5a and C5a des Arg on the cognate C5a receptors, C5a1 and C5a2. The research conducted here attempted to improve upon this understanding as well as fuel future drug discovery efforts that target this component of the complement system. Functional assays using human isolated neutrophils or engineered cell lines along with C5a1 and C5a2 receptor selective antagonists were employed to assess the contribution of each C5a receptor to functional responses elicited by human purified C5a and C5a des Arg. For each functional endpoint investigated, the relative activity of C5a des Arg was compared with C5a using the Log(Emax/EC50) transformation. Single point mutagenesis was performed on the C5a1 receptor to relate differences in agonist functional activity to mode of agonist binding. The data presented here supports the hypothesis that C5a des Arg behaves as a biased agonist in relation to C5a. Via the C5a1 receptor, C5a des Arg produces neutrophil phenotypes that are involved in the orchestration of immune cell recruitment to sites of infection. However, unlike C5a, C5a des Arg does not induce a respiratory burst response that leads to the generation of hypochlorous acid. The biased agonism of C5a des Arg appears to be brought about by its inability to interact with amino acid residues within the seventh transmembrane domain of the C5a1 receptor, which prevents the receptor recruitment of -arrestins. Furthermore, data presented here also show that the C5a2 receptor does not directly contribute to C5a or C5a des Arg mediated activation of the human isolated neutrophil. Taken together, the data support a hypothesis whereby the generation of C5a des Arg, through C5a1 receptor, limits the ability of C5a to promote the production of potentially damaging respiratory burst while still enabling immune cell extravasation.