Morphological evidence for different types of IgG-Fc receptors in group A streptococci

The binding of immunoglobulin G (IgG) from man and 14 animal species to Group A streptococcal cells was studied by electron microscopy. Only the IgGs of rhesus monkey and guinea pig did not bind to any of the streptococcal strains tested. Fc receptors were seen located on filamentous protrusions of the streptococcal cell wall either regularly or in focal distribution. On the inner surface of isolated cell walls no Fc receptors were detected. Both the different labelling patterns for different IgG and the results of inhibition studies with homologous and heterologous IgGs suggested the existence of different types of Fc receptors in the same strain. Further characterization of the receptors, including their location, was performed by studying their susceptibility to several proteolytic enzymes.