Biological properties of human C5a: selected in vitro and in vivo studies

During the activation of complement, a variety of cleavage fragments are produced which display potent inflammatory, immunomodulatory, and immunological properties. One of these fragments is C5a, a 74-residue glycopeptide produced by cleavage of the amino terminal end of the C5 alpha chain. In man, serum carboxypeptidase N rapidly converts C5a to the less potent though biologically active derivative, C5a des Arg, by removal of the carboxy terminal arginine. C5a and C5a des Arg have a variety of important biological properties which are mediated through the interaction of these glycopeptides with peripheral blood leucocytes and specific cells resident in tissue. This cellular interaction is mediated by the binding of C5a or C5a des Arg to plasma membrane receptors on these effector cells. A large number of studies have probed the immunochemical character of this ligand and the receptor to which it binds, examined the various biological responses produced by C5a, and attempted to assess the role of this molecule in various biological reactions and disease states in man. The purpose of this paper is to briefly summarize selected aspects of these areas of investigation. In summary, C5a, the most potent anaphylatoxin, elicits many important biological reactions. These reactions in large part develop from the interaction of C5a with specific plasma membrane receptors on leucocytes and other selected cells. This anaphylatoxin notably amplifies inflammatory reactions and has a potential role in immunomodulatory processes as well. For these reasons, C5a is recognized as a unique and important mediator produced by the activation of either the classical or alternative pathways of complement.