Expression of the leukocyte functional molecule (LFA-1) on mouse platelets

Platelet involvement in adhesion, hemostasis, and immune adherence is mediated by functionally associated cell surface molecules. Platelets are also involved in cytolytic reactions, but little is known about the mechanisms or biologic significance of these processes. To further investigate cell surface molecules concerned with platelet function, antisera against mouse platelets, thymocytes, and macrophages and monoclonal antibodies against Mac-1 (complement receptor type 3) and leukocyte function-associated glycoprotein type 1 (LFA-1) were used to demonstrate LFA-1--like molecules on mouse platelets. The alpha subunits of platelet and thymocyte LFA-1 showed identical electrophoretic mobility but differed significantly from the alpha subunit of macrophage Mac-1. Peptide mapping demonstrated the identity of the beta subunits of these three molecules but showed that the alpha subunit of Mac-1 was distinct from the alpha subunits of platelet and thymocyte LFA-1. Platelet LFA-1, as demonstrated by surface iodination with lactoperoxidase and by labeling sialic acid residues with sodium borohydride, was not a major component of the platelet membrane. The functional significance of LFA-1 on mouse platelets has yet to be demonstrated, monoclonal antibodies against LFA-1 having little effect on adenosine diphosphate-induced platelet aggregation and immune adherence. In contrast, although Mac-1 could not be demonstrated on mouse platelets in immunoprecipitation studies, its presence was clearly demonstrated by low levels of antibody binding in enzyme-linked immunosorbent assays and by the ability of M1/70 monoclonal antibody to inhibit platelet immune adherence. Human platelets, which are inactive in immune adherence assays, are shown to lack LFA-1 and Mac-1.