Factor H binds to washed human platelets

Summary.  Background: Factor H regulates the alternative pathway of complement. The protein has three heparin‐binding sites, is synthesized primarily in the liver and copurifies from platelets with thrombospondin‐1. Factor H mutations at the C‐terminus are associated with atypical hemolytic uremic syndrome, a condition in which platelets are consumed. Objectives The aim of this study was to investigate if factor H interacts with platelets. Methods: Binding of factor H, recombinant C‐ or N‐terminus constructs and a C‐terminus mutant to washed (plasma and complement‐free) platelets was analyzed by flow cytometry. Binding of factor H and constructs to thrombospondin‐1 was measured by surface plasmon resonance. Results: Factor H bound to platelets in a dose‐dependent manner. The major binding site was localized to the C‐terminus. The interaction was partially blocked by heparin. Inhibition with anti‐GPIIb/IIIa, or with fibrinogen, suggested that the platelet GPIIb/IIIa receptor is involved in factor H binding. Factor H binds to thrombospondin‐1. Addition of thrombospondin‐1 increased factor H binding to platelets. Factor H mutated at the C‐terminus also bound to platelets, albeit to a significantly lesser degree. Conclusions: This study reports a novel property of factor H, i.e. binding to platelets, either directly via the GPIIb/IIIa receptor or indirectly via thrombospondin‐1, in the absence of complement. Binding to platelets was mostly mediated by the C‐terminal region of factor H and factor H mutated at the C‐terminus exhibited reduced binding.

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