Differential susceptibility of cell-bound streptococcal immunoglobulin Fc-receptors to autolytic degradation.

The susceptibility of IgA and IgG Fc-receptors of different streptococci to autolytic degradation was tested by incubating washed bacteria in phosphate-buffered saline at 37 degrees C for 24-72 h. Before incubation, strain M1 bound 74 per cent of 2.5 micrograms radio-labelled IgG added to 200 microliter of the suspension, and strain M4 bound 85 per cent of 2.5 micrograms radio-labelled IgA; both types of binding decreased to less than 15 per cent. Evidence was obtained that the decrease in binding was caused by digestion of the respective receptors, and not by degradation of the immunoglobulins or through inhibition by solubilized Fc-receptors. The autolytic process was inhibited by EDTA, iodoacetic acid, benzamidinium chloride, phenylmethylsulfonyl fluoride and diisopropylfluorophosphate. The IgG Fc-receptors in other types of group A streptococci (including type M4) and a group C and a group G strain were not affected by such prolonged incubation, also when strains were mixed with M-type 1 streptococci from the start of incubation. These findings, which thus revealed certain differences in stability between streptococcal Fc-receptors, are discussed in relation to the intracellular degradation of streptococci.