| Popis: |
The M protein radiating from the surface of group A streptococci is the principal virulence factor of these organisms. Streptococci lacking M protein are readily opsonized by complement through the alternate pathway (1) and as a result are rapidly ingested and killed by blood phagocytes. Surface M protein renders the organisms resistant to opsonization, ingestion and killing by phagocytic cells (2). Two hypotheses have been advanced to account for the antiphagocytic properties of M protein: it may be directly toxic to phagocytic cells, or it may prevent opsonization by complement. A mechanism for direct toxicity has been proposed by Manjula and Fischetti (3), who noted structural similarities between type 6 M protein and alpha-tropomyosin and postulated that M protein might disable the phagocyte by interfering with its contractile proteins. To date, however, no evidence of toxicity of M protein for phagocytes has been obtained. Hot acid extracts of group A streptococci do, indeed, exert cytotoxic effects on human polymorphonuclear leukocytes, but these appear to be mediated by the formation of immune complexes involving streptococcal antigens other than M protein (4). Furthermore, M-positive streptococci do not attach to phagocytes in whole blood (5), nor do they inhibit the phagocytosis of M-negative organisms mixed with them (6). |
