THE HUMAN C3A RECEPTOR IS EXPRESSED ON NEUTROPHILS AND MONOCYTES, BUTNOT ON B-LYMPHOCYTE OR T-LYMPHOCYTE

The pathophysiological relevance of the complement split product C3a a s a proinflammatory mediator is still ill defined. The expression patt ern of the human C3a receptor (C3aR) can provide important clues for t he role of this anaphylatoxin in inflammation. There is strong evidenc e for C3aR expression on basophils, and eosinophils, but additionally, only on tumor cell lines of leukemic or hepatic origin. It is unclear whether neutrophils also express the C3aR, but need a costimulus prov ided by eosinophils for certain biological responses, or whether neutr ophils lack the C3aR and respond to C3a via a secondary stimulus gener ated by eosinophils, i.e., by an indirect mode. In the present study, polyclonal antiserum raised against the second extracellular loop of t he C3aR was used to characterize C3aR expression on peripheral blood l eukocytes. For high degree purification of neutrophils, a negative sel ection method was established that decreased the contamination with CD 9(bright+) eosinophils down to <0.2%. Flow cytometric analyses, functi onal assays, and binding assays on highly purified neutrophils confirm ed C3aR expression and coupling. Monocytes were identified as an addit ional C3aR-positive cell population of the peripheral blood. The expre ssion of the C3aR on eosinophils could be confirmed. In contrast, the receptor could not be detected on unchallenged B or T lymphocytes (or lymphocyte-derived Raji cells).