A monoclonal antibody to the surface membrane of human platelets which inhibits ristocetin- and collagen-induced platelet aggregation reacts with H1 histones of cell nuclei.

A murine monoclonal antibody HuPIA3, produced by immunization with human platelet membranes, reacted by radioimmunoassay with platelets, and inhibited ristocetin- and collagen-induced platelet aggregation and release of 14C-serotonin. The antibody also inhibited ristocetin-induced aggregation of washed, formaldehyde-fixed platelets by von Willebrand factor. On cultures of human and rodent fibroblasts, and on frozen sections of rabbit liver and rat kidney, the antibody gave a diffuse, homogenous immunofluorescence staining of cell nuclei which could be abolished by treatment with 0.1 M HC1 or 2 M NaCl and restored by reconstitution with histones, suggesting a reaction with nuclear histones. Absorption of the antibody with histones abolished nuclear staining and abrogated the inhibitory effect of the antibody on ristocetin- and collagen-induced platelet aggregation and 14C-serotonin release. Conversely, absorption with platelets removed antibody reactivity for platelets and for cell nuclei. In addition, the antibody reacted with H1 histones by radioimmunoassay, and immunoblotting studies showed that the antibody reacted with a protein of 199,000 daltons on platelets and with H1 histones (31,000 dalton and 32,000 dalton). These observations suggest that the antibody recognizes epitopes found on surface molecules of platelets as well as on H1 histones of cell nuclei.