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Cutting Edge: Human Anaphylatoxin C4a Is a Potent Agonist of the Guinea Pig But Not the Human C3a Receptor^{1 ,2}

Stefan Lienenklaus^*, Robert S. Ames, Mark A. Tornetta, Henry M. Sarau, James J. Foley, Torsten Crass^*, Bettina Sohns^*, Ute Raffetseder^*, Melanie Grove^*, Anja Hölzer^*, Andreas Klos^*, Jörg Köhl^* and Wilfried Bautsch^3,*

^* Institute of Medical Microbiology, Hannover Medical School, Hannover, Germany; and Departments of Pulmonary Pharmacology and Molecular Biology, SmithKline Beecham Pharmaceuticals, King of Prussia, PA 19406

The interaction of human anaphylatoxin C4a with the guinea pig (gp) and human (hu) C3a receptors (C3aR) was analyzed using human rC4a, which exhibited C4a-specific activity on guinea pig platelets. A gpC3aR of 475 residues with a large second extracellular loop and a peptide sequence 60% identical to the huC3aR was isolated from a genomic DNA library and found to be expressed in guinea pig heart, lung, and spleen. HEK-293 cells cotransfected with this clone, and a cDNA encoding G-16 specifically bound (K_d = 1.6 ± 0.7 nM) and responded functionally to C3a with an intracellular calcium mobilization (ED₅₀ = 0.18 ± 0.02 nM). Human rC4a weakly bound to both the hu- and gpC3aR (IC₅₀ > 1 µM). However, only HEK-293 cells expressing the gpC3aR responded functionally to rC4a (ED₅₀ = 8.7 ± 0.52 nM), while cells expressing the huC3aR did not (c 1 µM). Thus, through an interaction with the C3aR, huC4a may elicit anaphylatoxic effects in guinea pigs but not in man.

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