Blood- and skin-derived monocytes/macrophages respond to C3a but not to C3a(desArg) with a transient release of calcium via a pertussis toxin-sensitive signal transduction pathway.

Controversial results have been published in the past regarding the functional reactivity of monocytes (Mo) and macrophages (M phi) to the anaphylatoxin C3a and its degradation product C3a(desArg). In this study we performed binding and calcium mobilization experiments with recombinant human C3a (rC3a) and rC3a(desArg). Blood Mo displayed non-inhibitable binding of FITC-labeled rC3a (rC3aFITC) but responded to rC3a with a transient release of the intracellular calcium concentration ([Ca2+]i), whereas rC3a(desArg) was completely inactive. In contrast, binding of rC3aFITC to eosinophilic granulocytes and the mast cell line HMC-1 which have been shown previously to express C3a binding sites could be blocked by a monoclonal anti-C3a antibody. The rC3a-induced [Ca2+]i release in blood Mo was pertussis toxin (PTX)-sensitive suggesting the involvement of G-proteins in the signal transduction pathway. Skin-derived Mo/M phi reacted similarly to blood Mo as no specific binding of rC3aFITC to these cells could be demonstrated, whereas an intracellular release of calcium ions in response to the anaphylatoxin was observed. Homologous desensitization to rC3a but not heterologous desensitization to rC5a was detected in further experiments. The functional effect of C3a, but not the unspecific binding of rC3aFITC to blood Mo and skin-derived Mo/M phi could be blocked by the monoclonal anti-C3a antibody. These results suggest the expression of the recently cloned G-protein-coupled receptor for C3a on human blood Mo and skin-derived Mo/M phi. However, the total number of specific C3a binding sites on these cells is distinctly lower as compared to eosinophilic granulocytes and cells of the mast cell line HMC-1. The small number of C3a receptors on Mo/M phi may be masked by a pronounced non-inhibitable binding of rC3aFITC. This binding, however, may contribute to the recently described biological effects of C3a(desArg) on Mo.