Inhibition of homologous complement activation by the heat-stable antigen

The murlne heat-stable antigen (HSAg) Is of particular Interest due to Its unique tissuedistribution. HSAg Is expressed on most thymocytes, bone marrow cells, immature B cells, and erythrocytes, but not on peripheral T and mature B cells. Although HSAg has been thought to be a differentiation antigen, its actual biological significance remains unknown except for the HSAg on antigen presenting cells. Recently, a new rat antl-HSAg mAb, R13, has been developed. Here It has been found that the mouse complement activation on mouse erythrocytes, but not the human, guinea pig or rabbit complement activations, was enhanced In the presence of the Fab fragment of R13. Afflnlty-purlfled HSAg derived from mouse erythrocytes could be passively Incorporated Into rabbit erythrocytes because of Its molecular characteristic of glycosylphosphatidyl Inosltol-anchored protein. Mouse complement activation, but not guinea pig complement activation, was partially suppressed on the HSAg-incorporated rabbiterythrocytes.These findings suggest that HSAg has a homologous complement regulating activity.