C3a(desArg) does not bind to and signal through the human C3a receptor

Contradictory results have been published in the past regarding the functional responses of different cell types to the anaphylatoxin C3a and its natural catabolite C3a(desArg). To elucidate the interaction of the C3a receptor (C3aR) with its ligand(s) we studied the binding of human recombinant C3a (rC3a) and rC3a(desArg) to RBL-2H3 transfectants which express the C3aR. As the addition of 11 aminoterminal amino acids did not alter the functional activity of the recombinant C3a as compared to serum-derived C3a the specific binding of rC3a and rC3a(desArg) to the transfectants could be determined by flow cytometry using a monoclonal antibody (mab) against their N-terminal histidine tag. Recombinant C3a bound to the C3aR with a half maximal concentration of about 3 nM whereas no evidence for a binding of rC3a(desArg) could be obtained. Furthermore, rC3a(desArg) did not signal through the C3aR. Neither the release of lysosomal N-acetyl-beta-D-glucosaminidase nor the directional migration of C3aR-expressing RBL-2H3 transfectants could be detected in response to rC3a(desArg) whereas rC3a was highly active in both assays. Our data demonstrate a defined ligand specificity of the C3aR for the anaphylatoxin C3a. Its natural catabolite C3a(desArg), however, does not signal through the C3aR. Modulating effects of C3a(desArg) on the synthesis of cytokines in human monocytes and B lymphocytes may therefore be induced by receptor-independent mechanisms while their in vivo relevance remains as yet undefined.