Evaluation of C3a receptor expression on human leucocytes by the use of novel monoclonal antibodies.

Varying results have been published in the past regarding the reactivity of different leucocyte subpopulations, including neutrophils, monocytes and B lymphocytes, to the anaphylatoxin C3a and its degradation product C3a(desArg). To better characterize the cellular distribution of C3a receptor (C3aR) expression, monoclonal antibodies against two different epitopes on the third extracellular domain of the human C3aR were generated. Quantification of C3aR as compared with C5aR densities was performed on peripheral blood leucocytes by quantitative indirect immunofluorescence. Eosinophils and basophils expressed similar numbers of C3aR and C5aR molecules/cell. On eosinophils 10 700+/-4500 (mean+/-SD) C3aR and 14 700+/-4100 C5aR were found, whereas basophils carried 8100+/-2100 C3aR and 13 500+/-3800 C5aR. Monocytes expressed approximately six times more C5aR than C3aR molecules on their surface (6000+/-2500 C3aR versus 34 100+/-9300 C5aR molecules) whereas on neutrophils, the expression of C5aR was more than 20 times higher than the expression of C3aR (3100+/-1000 C3aR versus 63 500+/-12 200 C5aR). No C3aR expression was detectable on peripheral blood-derived B lymphocytes and on tonsillar B cells before and after stimulation with interleukin-2/Staphylococcus aureus Cowan strain I. Our findings correspond well with the paucity of data on C3a-induced functional activities in monocytes and neutrophils and suggest that eosinophilic and basophilic granulocytes represent the primary effector cells in the peripheral blood which can be stimulated by C3a.