Analysis of the molecular interplay between Streptococcus pyogenes and its human host

Abstract: The common human pathogen Streptococcus pyogenes is the causative agent of numerous mild and severe clinical conditions. It expresses a number of secreted or cell wall-anchored proteins that modulate the human immune system and facilitate colonization and spread of the pathogen in the human host. During S. pyogenes infections, human plasma leaks into the site of infection as a consequence of inflammation. This thesis shows that S. pyogenes rapidly alters its expression of extracellular and intracellular proteins in response to human plasma. In addition, the pathogen also expresses multiple variants of its important virulence factors, M1 protein and C5a peptidase, when exposed to plasma. The function of modified M1 protein and C5a peptidase is not yet elucidated but is suggested to have important implications for the pathogenicity of S. pyogenes. Opsonizing IgG recognizes and mediates the elimination of bacteria during infection. Here, the identification and characterization of a novel IgG cleaving cysteine proteinase of S. pyogenes, denoted IdeS, is described. IdeS facilitates S. pyogenes evasion of Fc-mediated phagocytosis by specifically cleaving the hinge region of IgG1, IgG2, IgG3, and IgG4. Moreover, data suggesting that neutrophil proteinases release immunogenic epitopes from IdeS are presented. This is a novel mechanism by which S. pyogenes exploits the human immune system and prevents its virulence factors from being eliminated by opsonizing immunoglobulins.