THE HUMAN MAST-CELL LINE HMC-1 BINDS AND RESPONDS TO C3A BUT NOT C3A(DESARG)

Controversial results have been published in the past regarding the fu nctional reactivity of different cell types to the anaphylatoxin C3a a nd its degradation product C3a(desArg). To understand better the effec ts of C3a and C3a(desArg) on human mast cells, the authors performed b inding experiments and calcium mobilization studies on the human mast cell line HMC-1 which has been shown previously to express C3a binding sites. For this purpose, functionally active, recombinant C3a (rC3a) was constructed with an 11 amino acid peptide attached to the N-termin us of the molecule. Using a monoclonal antibody (MoAb) against this ta g, binding of rC3a to HMC-1 cells could be demonstrated by flow cytome try. Its binding was specific as it could be blocked with serum-derive d C3a. In contrast, no binding of rC3a(desArg) to HMC-1 cells was dete ctable. Recombinant C3a led to a transient mobilization of intracellul ar calcium [Ca2+](i) in HMC-1 which was inhibitable by the C3a-specifi c MoAb K13/16. No increase of [Ca2+](i) was detected when the cells we re treated with C3a(desArg). The authors found C3a receptor (C3aR)-spe cific mRNA in HMC-1 cells indicating that this receptor represents the binding site for C3a on these cells. These results demonstrate a spec ific binding for C3a but not for C3a(desArg) on cells of the human mas t cell line HMC-1. As a consequence, functional activity was restricte d to C3a with C3a(desArg) being completely inactive. Therefore, the da ta strongly suggest that the recently cloned high affinity C3aR which is assumed to represent the binding site for the anaphylatoxin on HMC- 1 cells is unresponsive to C3a(desArg).