On the Interaction between beta2-Microglobulin and Group A Streptococci

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Abstract
.beta.2-microglobulin (.beta.2m) interacted with many group A streptococcal strains. The interaction appeared to require multipoint attachment, since monomeric .beta.2m in solution showed no binding, whereas both .beta.2m monomers bound to liposomes, and .beta.2m in aggregates showed affinity for the bacteria. Aggregated HLA antigens (-A, -B and -C) and aggregated .beta.2m exhibited the same binding patterns when tested in binding experiments with various group A streptococcal strains. .beta.2m Aggregates in excess completely blocked the binding of aggregated HLA antigens, thereby demonstrating that .beta.2m is able to interact with streptococcal surface structures also when it is part of the HLA antigen complex. M protein-positive group A streptococcal strains bound significantly more .beta.2m than M protein-negative variants of these strains. Purified M 12 protein partly inhibited the binding of radiolabeled .beta.2m aggregates to whole streptococci, and in gel filtration and affinity chromatography experiments, the M 12 protein interacted with .beta.2m. The various data suggest that the interaction between .beta.2m and group A streptococci could be mediated by M protein. Lipoteichoic and (LTA) is a constituent of the streptococcal cell wall that has been reported to form complexes with M protein at the bacterial cell surface. However, LTA did not influence the interaction between .beta.2m and streptococci, suggesting that the binding of .beta.2m to streptococcal M protein represents a pure protein-protein interaction. In vivo such an interaction could be established between infecting streptococci and host cells. Among 45 strains of different M types large differences in .beta.2m binding were recorded, whereas among 60 strains of the classical nephritogenic M types 12 and 49, all were highly .beta.2m-reactive, which points towards a role for .beta.2m in streptococcal pathogenicity.