Group A streptococci (GAS), a common human pathogen, secrete streptokinase (SK), which activates the host’s plasminogen (PLG). SK is highly specific for human PLG, exhibiting little or no activity against other mammalian species. We demonstrated the major role of the PLG/SK interaction in GAS pathogenicity using a transgenic murine model expressing human plasminogen with increased susceptibility to human pathogenic streptococci. We hypothesize that GAS hijack the host fibrinolytic system in order to circumvent local thrombosis for systemic spread. Markedly increased mortality was also observed following GAS injection in C57BL/6J mice treated with the snake venom Ancrod, which proteolytically degrades plasma fibrinogen, supporting the critical roles of coagulation in host/pathogen interaction.

However, fibrinogen also plays important roles in inflammation and immune response, it is necessary to use independent genetic models to further test the impact of coagulation in host defense against bacterial infection. The effect of variations in FV on mouse susceptibility to streptococcal infection was tested. We established a mouse model with low plasma FV level. These mice have a slightly increased bleeding time, though otherwise phenotypically normal. Subjected to streptococcal infection, these mice exhibited significantly increased mortality than wildtype controls, suggesting that the decreased thrombotic tendency in the low FV mice increases host susceptibility to infection. FV Leiden is a common prothrombotic mutation among Caucasian population with an incidence between 4% and 6%. Previous studies from Kerlin et al demonstrated the FV Leiden conferred survival advantage in patients with severe sepsis and in mice challenged with endotoxin. This may be an example of balanced gene polymorphism that maintains the FV Leiden mutant in the general gene pool by selection of bacterial infections. In order to identify the selective agents responsible for the prevalence of FV Leiden mutation, we took advantage of our plasminogen transgenic murine model for streptococcal infection to test whether streptococci is one of the selective agents. Human plasminogen transgene was introduced into FV Leiden background and the susceptibility to streptococcal infection was measured. No significant improvement of survival was observed in the FV Leiden mouse comparing with the wildtype control. Thus, streptococcal infection is not the selective agent for the prevalence of FV Leiden mutation.

These observations highlight the potential role of variations in blood coagulation factors in host susceptibility to bacterial infection.

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