Interaction of streptococci with the Fc fragment of IgG.

The capacity of human IgG to interact with beta-haemolytic streptococci was studied in order to localize the site of interaction on the IgG molecule. The reactivity of different proteolytic fragments of IgG with streptococci group A, type M 1 and type M 56, group C and group G, was investigated by measuring their inhibitory effect on the uptake of 125I labelled IgG myeloma protein by the streptococci. Equivalent molar amounts of Fc fragment and undigested IgG inhibited the uptake of 125I labelled IgG myeloma protein equally well while only slight inhibition was obtained by F(ab')2 preparations. No reactivity was found with IgM, Fab or chymotrypsin produced fragment Fc', of IgG. The reactivity of IgG with the streptococci was localized to the Fc fragment. Since the Fc' fragment was non-reactive, the CH2 domain was probably carrying the IgG structures involved in the interaction with streptococci.