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* Centre for Respiratory Research, Department of Medicine, Royal Free and University College Medical School, Rayne Institute, London, United Kingdom;
Rheumatology Section, Faculty of Medicine, Imperial College, Hammersmith Campus, London, United Kingdom;
School of Molecular and Biomedical Science, University of Adelaide, Adelaide, Australia; and
Centre for Molecular Microbiology and Infection, Imperial College London, London, United Kingdom
Streptococcus pneumoniae is a common cause of septicemia in the immunocompetent host. To establish infection, S. pneumoniae has to overcome host innate immune responses, one component of which is the complement system. Using isogenic bacterial mutant strains and complement-deficient immune naive mice, we show that the S. pneumoniae virulence factor pneumolysin prevents complement deposition on S. pneumoniae, mainly through effects on the classical pathway. In addition, using a double pspA/ply mutant strain we demonstrate that pneumolysin and the S. pneumoniae surface protein PspA act in concert to affect both classical and alternative complement pathway activity. As a result, the virulence of the pspA/ply strain in models of both systemic and pulmonary infection is greatly attenuated in wild-type mice but not complement deficient mice. The sensitivity of the pspA/ply strain to complement was exploited to demonstrate that although early innate immunity to S. pneumoniae during pulmonary infection is partially complement-dependent, the main effect of complement is to prevent spread of S. pneumoniae from the lungs to the blood. These data suggest that inhibition of complement deposition on S. pneumoniae by pneumolysin and PspA is essential for S. pneumoniae to successfully cause septicemia. Targeting mechanisms of complement inhibition could be an effective therapeutic strategy for patients with septicemia due to S. pneumoniae or other bacterial pathogens.
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