The surface M
protein of group A streptococci (GAS) is one of the major
virulence factors for this pathogen.
Antibodies to the M
protein can facilitate opsonophagocytosis by phagocytic cells present in human blood. We investigated whether pooled normal
immunoglobulin G (
IVIG) contains
antibodies that can opsonize and enhance the phagocytosis of type M1 strains of GAS and whether the levels of these
antibodies vary for different
IVIG preparations. We focused on the presence of anti-M1
antibodies because the M1T1 serotype accounts for the majority of recent invasive GAS clinical isolates in our surveillance studies. The level of anti-M1
antibodies in three commercial
IVIG preparations was determined by
enzyme-linked
immunosorbent assay (ELISA), and the opsonic activity of these
antibodies was determined by neutrophil-mediated opsonophagocytosis of a representative M1T1 isolate. High levels of opsonic anti-M1
antibodies were found in all
IVIG preparations tested, and there was a good correlation between ELISA titers and opsonophagocytic activity. However, there was no significant difference in the levels of opsonic anti-M1
antibodies among the various
IVIG preparations or lots tested. Adsorption of
IVIG with M1T1 bacteria removed the anti-M1 opsonic activity, while the level of anti-M3 opsonophagocytosis was unchanged. Plasma was obtained from seven patients with streptococcal
toxic shock syndrome who received
IVIG therapy, and the level of anti-M1
antibodies was assessed before and after
IVIG administration. A significant increase in the level of type M1-specific
antibodies was found in the plasma of all patients who received
IVIG therapy (P < 0.006). The results reveal another potential mechanism by which
IVIG can ameliorate severe invasive
group A streptococcal infections.