Survival of Streptococcus suis in porcine blood is limited by the antibody- and complement- dependent oxidative burst response of granulocytes
Bacteremia is a hallmark of invasive Streptococcus suis (S. suis) infections of pigs often leading to septicemia, meningitis or arthritis. An important defense mechanism of neutrophils is the generation of reactive oxygen species (ROS). In this study, we report high levels of ROS production by blood granulocytes post intravenous infection of a pig with high levels of S. suis-specific antibodies and comparatively low levels of bacteremia. This prompted us to investigate the working hypothesis that immunoglobulin-mediated oxidative burst contributes to killing of S. suis in porcine blood. Several S. suis strains representing serotypes 2, 7 and 9 proofed to be highly susceptible to the oxidative burst intermediate hydrogen peroxide, already at concentrations of 0.001%. The induction of ROS in granulocytes in ex vivo infected reconstituted blood showed association with pathogen-specific antibody levels. Importantly, inhibition of ROS production by the NADPH oxidase inhibitor apocynin led to significantly increased bacterial survival in the presence of high specific antibody levels. The oxidative burst rate of granulocytes partially depended on complement activation as shown by specific inhibition. Furthermore, treatment of an IgG-depleted serum with a specific IgM protease or heat to inactivate complement resulted in a more than 3-fold decreased oxidative burst activity and increased bacterial survival in reconstituted porcine blood in accordance with an IgM-complement-oxidative burst axis. In conclusion, this study highlights an important control mechanism of S. suis bacteremia in the natural host: the induction of ROS in blood granulocytes via specific immunoglobulins such as IgM.
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