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(desA rg). To better characterize the cellular distribution of C3a receptor (C3aR ) expression, monoclonal antibodies against two different epitopes on the t hird extracellular domain of the human C3aR were generated. Quantification of C3aR as compared with C5aR densities was performed on peripheral blood l eucocytes by quantitative indirect immunofluorescence. Eosinophils and baso phils expressed similar numbers of C3aR and CSaR molecules/cell. On eosinop hils 10700+/-4500 (mean+/-SD) C3aR and 14700+/-4100 CSaR were found, wherea s basophils carried 8100+/-2100 C3aR and 13 500+/-3800 C5aR. Monocytes expr essed approximately six times more C5aR than C3aR molecules on their surfac e (6000+/-2500 C3aR versus 34 100+/-9300 C5aR molecules) whereas on neutrop hils, the expression of C5aR was more than 20 times higher than the express ion of C3aR (3100+/-1000 C3aR versus 63500+/-12200 C5aR). No C3aR expressio n was detectable on peripheral blood-derived B lymphocytes and on tonsillar B cells before and after stimulation with interleukin-2/Staphylococcus aur eus Cowan strain I. Our findings correspond well with the paucity of data o n C3a-induced functional activities in monocytes and neutrophils and sugges t that eosinophilic and basophilic granulocytes represent the primary effec tor cells in the peripheral blood which can be stimulated by C3a.