Interactions between bacterial surface proteins and components of the fibrinolytic and contact phase systems

Sammanfattning: This thesis describes novel molecular host-parasite interactions with potential significance for the understanding of pathological processes manifested with bacterial infections. Bacterial surface proteins and virulence determinants are shown to mediate the accumulation at the surface of the pathogen of key components of the fibrinolytic and contact phase systems. Plasmin(ogen) binding surface proteins from human pathogenic group C and G streptococci were purified and the genes responsible for the binding were cloned, sequenced and expressed in Escherichia coli. Sequence data demonstrated that these proteins, which were also found to bind human fibrinogen and albumin, are members of the large family of M and M-like proteins, streptococcal surface proteins and major virulence determinants due to their antiphagocytic properties. The plasminogen bound to the streptococcal surface could be activated by streptokinase, supporting the notion of an important functional relationship between the secreted plasminogen activator and proteins exposed on the bacterial surface. Most M proteins expressed by strains of the important human pathogen Streptococcus pyogenes were found to bind human kininogens, especially high molecular weight kininogen (HK), the precursor to the vasoactive kinins and a key component of the contact phase system. The interaction has been found to involve the variable NH2-terminal region of the M proteins and the complementary binding of M1 protein on HK was mapped to the two most COOH-terminal domains; D5H which interacts with subendothelial and negatively charged surfaces and D6H which forms equimolar complexes with plasma prekallikrein or factor XI. M protein expressing streptococci were found to capture HK from human plasma. The absorbed HK was partially degraded with release of bradykinin (BK), a proinflammatory vasoactive peptide hormone. The M protein and plasma prekallikrein, the proenzyme to the BK releasing enzyme plasma kallikrein, were found not to compete for binding to the light chain of HK. Moreover, addition of activated prekallikrein to bacteria preincubated with human plasma resulted in further digestion of surface bound HK with release of the residual BK. Binding of kininogens was also found to be common among the Gram-negative human pathogens E. coli and Salmonella spp. Binding to E. coli is mediated by curli, a fibrous surface organelle which is also shown to mediate the assembly of all members of the contact phase system. Interestingly, addition of activated plasma prekallikrein releases bradykinin from HK bound on the surface of curli expressing bacteria preincubated with human plasma. It is proposed that accumulation of the contact phase system at the focus of infection, and subsequent release of bradykinin is a novel mechanism that could promote the inflammatory reaction and contribute to bacterial virulence.