C3b acceptors on human peripheral blood mononuclear cells; characterization and functional role.

C3b acceptors (C3bAs) of human peripheral blood mononuclcar cells (PBMC) reacting with the labile binding site of nascent C3b (C3b^x) have been investigated by the immune adherence (IA) test. In non-cellular systems some conventional chemical groups (OH^-, NH₂^-) have been reported to be the target of the covalent binding of C3b^x. Thus it should be assumed that every cell can fix C3b^x via its labile binding site and C3bAs are barely saturable. Contrary to this expectation, however, normal human PBMC were found to be heterogeneous from this point of view, as 57±4% of B cells and 21±2% of Null ceils possess C3bAs while T ceils do not. C3bAs of human PBMC are saturable and trypsin-sensitive structures. The covalent nature of the C3b^x-C3bA interaction has also been proved. Studying the effect of acceptor-bound C3b on the function of other cell-surface structures, the inhibition of the Fey receptor function and the abolishment of the enhancement of pokeweed mitogen-stimulated blastogenesis by immune complexes were found. [ABSTRACT FROM AUTHOR]