Binding ability of complement receptor CR1 to C3 bound on the surface of M+ group A streptococci.

A previous study demonstrated that although M+ bacteria bound C3, mainly C3b and iC3b, via the classical pathway of complement activation, they were not phagocytosed by polymorphonuclear leucocytes (PMN). To elucidate this mechanism, we attempted to distinguish between the possibilities that M+ bacteria are effectively adhering to PMN but are not being endocytosed, or that the C3 deposited on M+ bacteria is not able to interact with the complement receptors on PMN. In the present study, we studied the interaction of C3-coated M+ bacteria with complement receptor CR1, which was isolated from the stroma of human erythrocytes. We show that the isolated complement receptor CR1 can associate with C3-coated M+ bacteria as well as with C3-coated M- bacteria, and the C3 deposited on M+ bacteria is cleaved and releases a C3 fragment in the presence of factor I and liquid-phase CR1. These results suggest that the C3 bound on the surface of M+ bacteria is able to promote adherence to the complement receptor CR1 on PMN. We also studied the distribution of C3 deposited on M+ bacteria in normal human serum (NHS) or normal human plasma (NHP). By immunofluorescence, we show that the C3 bound to M+ bacteria in NHS was deposited uniformly over the surface of the bacteria. On the other hand, the C3 bound to M+ bacteria in NHP was deposited at both ends between adjacent daughter cocci. The results suggest that an additional factor contained in NHP is related to the enhancement of anti-phagocytic activity of M+ bacteria.