Characterization of factor H binding to human polymorphonuclear leukocytes.

Previous studies indicate that factor H (fH) binds to a number of cell types and may have functions other than C regulation. We have examined for fH binding to PMN using flow cytometry and radiolabeled binding assays. Binding of fH was demonstrated to be specific and saturable with approximately 6 x 10(4) binding sites/polymorphonuclear leukocytes (PMN) and a Ka value of 3.3 x 10(8) L/M. Binding of fH to PMN was ionic strength dependent, required divalent cations, and was enhanced by PMN stimulation with FMLP and calcium ionophore, A23187. The 38-kDa N-terminal tryptic fragment of fH bound to PMN and blocking experiments with mAb suggested a receptor binding site was located within the fifth SCR of fH. fH binding was not due to associations with surface-bound C3 or to CR3. Binding of fH to U937 and Raji cells, but not to T cells was also demonstrated. These studies provide presumptive evidence for a fHR on PMN. Binding of fH by fHR could enhance recognition of opsonized targets, trigger secondary intracellular events or contribute to intrinsic protection of cells against C.