INHIBITION OF HOMOLOGOUS COMPLEMENT ACTIVATION BY THE HEAT-STABLE ANTIGEN

The murine heat-stable antigen (HSAg) is of particular interest due to its unique tissue distribution. HSAg is expressed on most thymocytes, bone marrow cells, immature B cells, and erythrocytes, but not on per ipheral T and mature B cells. Although HSAg has been thought to be a d ifferentiation antigen, its actual biological significance remains unk nown except for the HSAg on antigen presenting cells. Recently, a new rat anti-HSAg mAb, R13, has been developed. Here it has been found tha t the mouse complement activation on mouse erythrocytes, but not the h uman, guinea pig or rabbit complement activations, was enhanced in the presence of the Fab fragment of R13. Affinity-purified HSAg derived f rom mouse erythrocytes could be passively incorporated into rabbit ery throcytes because of its molecular characteristic of glycosyl-phosphat idyl inositol-anchored protein. Mouse complement activation, but not g uinea pig complement activation, was partially suppressed on the HSAg- incorporated rabbit erythrocytes. These findings suggest that HSAg has a homologous complement regulating activity.