Platelet stimulation by antifibronectin antibodies requires the Fc region of antibody

Anti-human fibronectin antibodies produced in a goat or in rabbits stimulate the release of serotonin from washed or gelatin/Sepharose-treated human platelets in a dose-dependent manner. This finding led us to propose that fibronectin on the platelet plasma membrane might serve as a collagen receptor on these cells [Bensusan, H. B., Koh, T. L., Henry, K. G., Murray, B. A. & Culp, L. A. (1978) Proc. Natl. Acad. Sci. USA 75, 5864-5868]. To determine whether direct interaction of the antibody with platelet membrane fibronectin was responsible for this stimulation, we prepared proteolytic fragments of the antifibronectin antibodies. Purified F(ab')2 fragments had a greatly diminished ability to elicit degranulation, and rabbit F(ab')2 fragments were totally ineffective in this regard. Preimmune IgG from these sources was capable of inhibiting antibody-induced serotonin release in a dose-dependent manner. Purified antibody Fc fragments also inhibited this stimulation in doses stoichiometrically equivalent to the inhibition seen with preimmune IgG. These results suggest that (i) platelet stimulation elicited by anti-human fibronectin antibody is mediated by the platelet Fc receptor and therefore is likely to be dependent on antigen-antibody complex formation and (ii) fibronectin on the platelet surface may not be the primary receptor for platelet adherence to collagen.