A protein with characteristics of factor H is present on rodent platelets and functions as the immune adherence receptor

Complement-coated particles interact with specific immune adherence recepto rs JAR). In primates, this function is served by complement receptor 1 (CR1 ) on erythrocytes. In contrast, rodent platelets bear LAR distinct from CR1 , the identity of which was studied here. A 150-kDa C3b-binding protein was isolated from rat platelets, which had immunochemical and biochemical iden tity to plasma factor H. Immunofluorescence microscopy and flow cytometry d emonstrated that factor H was present on the surface of rat and mouse plate lets, which could be removed by treatment with neuraminidase. Sheep erythro cytes bearing C3b underwent immune adherence with rat and mouse platelets, which was blocked with anti-factor H F(ab ')(2) antibodies, but not with an tibodies binding to the complement regulator, Crry, on the platelet surface . By reverse transcription-polymerase chain reaction using rat platelet RNA and primers designed from mouse factor H, a 472-base pair product was gene rated that was identical in sequence to that produced from rat liver RNA. T he translated protein product was 85% similar to mouse liver factor H. The 3 ' -nucleotide sequence from platelets predicted a soluble factor H protei n. By Northern analysis, liver and platelets had identically sized factor H mRNA. Thus, rat and mouse platelets have a membrane protein with character istics of factor H that is linked via sialic acid residues and functions as the LAR. Whether platelet factor H is acquired by passive adsorption from sera and/or is produced by platelets remains to be determined.