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From the * Institute of Medical Microbiology, The pathophysiological relevance of the complement split product C3a as a proinflammatory
mediator is still ill defined. The expression pattern of the human C3a receptor (C3aR) can provide important clues for the role of this anaphylatoxin in inflammation. There is strong evidence 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 provided by eosinophils for certain biological responses, or whether neutrophils lack the C3aR and respond to C3a via a secondary stimulus generated by eosinophils, i.e., by an indirect mode. In the present study, polyclonal antiserum raised against the second
extracellular loop of the C3aR was used to characterize C3aR expression on peripheral blood
leukocytes. For high degree purification of neutrophils, a negative selection method was established that decreased the contamination with CD9bright+ eosinophils down to <0.2%. Flow
cytometric analyses, functional assays, and binding assays on highly purified neutrophils confirmed C3aR expression and coupling. Monocytes were identified as an additional C3aR-positive cell population of the peripheral blood. The expression 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).
Department of
Hematology and Oncology, Hannover, 30625, Germany; and the § Department of Molecular
Immunology, SmithKline Beecham Pharmaceuticals, King of Prussia, PA 19406-0939
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