Isolation and characterization of type IIa and type IIb Fc receptors from a group A streptococcus

Certain group A streptococcal strains have been reported to express two distinct type II receptors that bind to the Fc region of human IgG. In this study, we have isolated and characterized these two type II Fc receptors and characterized their reactivity with differing species of IgG. The type IIa receptor was found to be a 56,000 molecular weight protein which binds human IgG1, IgG2 and IgG4, in addition to pig and rabbit IgG. The type IIb receptor was found to be a 38,000 molecular weight protein that bound exclusively to human IgG3. Neither the type IIa nor the type IIb receptor bound to goat, cow, dog, rat, or sheep IgG. Monospecific polyclonal antibodies were prepared against both the type IIa and type IIb Fc receptors. These antibodies demonstrated that the type IIa and type IIb were antigenically closely related and could not be distinguished from each other on the basis of their reactivity with either antibody. The distribution of type IIa and type IIb Fc receptors on a variety of different nephritogenic and non-nephritogenic group A streptococcal strains was documented.