IgG F(ab')2 of rabbit anti-M sera but not the unfractionated sera are bactericidal for some group A streptococci with IgG Fc-receptor activity. Opsonic effect ascribable to anti-IgG.

We have earlier reported on a group A streptococcal strain, type M12, which upon serial mouse passage acquired IgG Fc-receptor activity but lost the M-antigen. The passaged strain, 12P, was highly virulent for mice and grew well in normal human blood. The present study particularly concerns the opsonic effect on 12P of rabbit anti-M3, anti-M12 and anti-12P sera, as well as the corresponding IgG F(ab')2. Indirect bactericidal tests showed that the homologous anti-12P seum and IgG F(ab')2 were opsonic. The anti-M3 and anti-M12 had no effect on 12P; surprisingly, however, IgG F(ab')2 isolated from these sera displayed a clearcut opsonic activity. Data are presented which indicate that these "paradoxical" results can be explained by the binding of IgG F(ab')2 with anti-IgG specificity to human IgG, linked to the streptococcal surface through Fc-receptors. Only anti-12P serum, or IgG F(ab')2, were protective for mice on challenge with strain 12P.