Variable susceptibility to opsonophagocytosis of group A Streptococcus M-1strains by human immune sera

Immunity to group A streptococci (GAS) is thought to be related to the acqu isition of type-specific antibody directed against the M protein. However, recent work suggests that immunity may only be strain and not M-type specif ic. Therefore, susceptibility of 70 different GAS M-1 strains to opsonizati on and killing by convalescent sera was compared by using a highly sensitiv e chemiluminescence assay and by standard bactericidal assay, Sequencing of the emm1 gene in 10 strains with variable susceptibility to opsonization r evealed 100% homology in 9 strains. Several substitutions in the N-terminal and 2 in the A, repeat regions of strain CS-190 were associated with profo und resistance to opsonization, Thus amino acid substitutions within differ ent regions of the M-l protein molecule may adversely affect opsonization b y immune sera. In addition, non-M protein factors from identical M types in fluence susceptibility to phagocytosis, These findings may in part explain the persistently high prevalence of M-l strains worldwide over the last 15 years.