Cross-reactivity between streptococcal M surface antigen and human skin.

Psoriasis can be triggered by haemolytic streptococcal infections. As M protein is a major pathogenic surface antigen in these streptococci, the cross-reactivity between streptococcal M protein surface antigens and human epidermis was investigated. The conserved component common to the few M proteins investigated consists of an alpha-helical 'coiled-coil' configuration, similar to sub-units of human keratin. The amino acid sequence of protein M6, one of the M proteins that has been fully sequenced, was compared with that of 4721 ubiquitous peptides, by computer-assisted analysis using a protein-sequence data bank. Of all human proteins in the data bank 50-kDa keratin type 1 showed the closest homology with protein M6. Further evaluation revealed that this homology mainly involved the heptapeptide repeat patterns, which form the alpha-helical 'coiled-coil' structure, in both M6 and 50-kDa keratin. Cryostat sections of normal, involved and uninvolved psoriatic skin were studied for cross-reactivity with rabbit antisera raised against 10 different M proteins. All these antisera reacted with the stratum corneum of normal and psoriatic epidermis to a variable extent. Staining of keratinocyte cytoplasm was also observed, but this tended to be more prominent in lesional than in uninvolved and normal skin. Some of the M antisera also stained dendritic cells in the upper dermis as well as endothelium and smooth muscle. These cross-reactivities might be relevant to the pathogenesis of post-streptococcal psoriasis.