Journal
MICROBES AND INFECTION
Volume 11, Issue 8-9, Pages 762-769Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.micinf.2009.04.016
Keywords
Francisella tularensis; Neutrophils; Complement activation; complement C3; Humans; Phagocytosis; Respiratory burst
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Funding
- Department of Veterans Affairs, Veterans Health Administration, Office of Research and Development, Biomedical Laboratory Research and Development
- National Institutes of Health [P01-AI44642, R01-AI073835]
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The remarkable infectiousness of Francisella tularensis suggests that the bacterium efficiently evades innate immune responses that typically protect the host during its continuous exposure to environmental and commensal microbes. In our studies of the innate immune response to F. tularensis, we have observed that, unlike the live vaccine strain (LVS) of F. tularensis subsp. holarctica, F. tularensis subsp. novicida U112 opsonized in pooled human serum activated the NADPH oxidase when incubated with human neutrophils. Given previous observations that F. tularensis fixes relatively small quantities of complement component C3 during incubation in human serum and the importance of C3 to neutrophil phagocytosis, we hypothesized that F. tularensis subsp. novicida may fix C3 in human serum more readily than would LVS. We now report that F. tularensis subsp. novicida fixed approximately six-fold more. C3 than did LVS when incubated in 50% pooled human serum and that this complement opsonization was antibody-mediated. Furthermore, antibody-mediated C3 deposition enhanced bacterial uptake and was indispensable for the neutrophil oxidative response to F. tularensis subsp. novicida. Taken together, our results reveal important differences between these two strains of F. tularensis and may, in part, explain the low virulence of F. tularensis subsp. novicida for humans. Published by Elsevier Masson SAS.
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