Modulation of C3a activity: Internalization of the human C3a receptor and its inhibition by C5a

The C3a receptor (C3aR) is expressed on most human peripheral blood leukocy tes with the exception of resting lymphocytes, implying a much higher patho physiological relevance of the anaphylatoxin C3a as a proinflammatory media tor than previously thought, The response to this complement split product must be tightly regulated in situations with sustained complement activatio n to avoid deleterious effects caused by overactivated inflammatory cells, Receptor internalization, an important control mechanism described for G pr otein-coupled receptors, was investigated, Using rabbit polyclonal anti-ser um directed against the C3aR second extracellular loop, a flow cytometry-ba sed receptor internalization assay was developed. Within minutes of C3a add ition to human granulocytes, C3aR almost completely disappeared from the ce ll surface. C3aR internalization could also be induced by PMA, an activator of protein kinase C. Similarly, monocytes, the human mast cell line HMC-1, and differentiated monocyte/macrophage-like U937-cells exhibited rapid ago nist-dependent receptor internalization Neither C5a nor FMLP stimulated any cross-internalization of the C3aR, On the contrary, costimulation of granu locytes with C5a, but not FMLP, drastically decreased C3aR internalization. This effect could be blocked by a C5aR-neutralizing mAb, HEK293-cells tran sfected with the C3aR, with or without G alpha 16, a pertussis toxin-resist ant G protein alpha subunit required for C3aR signal transduction in these cells, did not exhibit agonist dependent C3aR internalization. Additionally , preincubation with pertussis toxin had no effect on C3a-induced internali zation on PMNs, C3aR internalization is a rapid negative control mechanism and is influenced by the C5aR pathway.