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Infect. Immun. doi:10.1128/IAI.00195-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The stages of meningococcal sepsis simulated in vitro with emphasis on complement and Toll-like receptor activation

Department of Pediatrics, Ullevål University Hospital, and University of Oslo, Norway; Institute of Immunology, Rikshospitalet University Hospital, and University of Oslo, Norway; Institute of Cancer Research and Molecular Medicine, Norwegian University of Science and Technology, Trondheim, Norway; Laboratory of Protein Chemistry, Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA, USA

^* To whom correspondence should be addressed. Email: b.c.hellerud{at}medisin.uio.no.

	Abstract

The clinical presentation of meningococcal disease is closely related to the number of meningococci in the circulation. This study aimed to examine the activation of the innate immune system when exposed to increasing and clinical relevant concentrations of meningococci. We incubated representative Neisseria meningitidis serogroup B (ST-32), serogroup C (ST-11), and the LPS deficient mutant 44/76lpxA- in human serum and whole blood measuring complement activation and cytokine secretion and the effect of blocking these systems. The HEK293 cells transfected with Toll-like receptors were examined for activation of NF-B. The threshold for cytokine secretion and activation of NF-B was 10³-10⁴ meningococci/mL and up to 10⁵-10⁶ meningococci/mL LPS was the sole inflammation inducing molecule. The activation was dependent on TLR4/MD2/CD14. Complement contributed to the inflammatory response at 10⁵-10⁶ meningococci/mL and complement activation increased exponentially at 10⁷ bacteria/mL. Non-LPS components initiated TLR2 mediated activation at 10⁷ bacteria/mL. As the bacterial concentration exceeded 10⁷/mL TLR4 and TLR2 were activated increasingly independent of CD14. In this model, mimicking human disease, the inflammatory response to N. meningitidis was closely associated with the bacterial concentration. Therapeutically, CD14 inhibition alone was most efficient at low bacterial concentration whereas addition of a complement inhibitor may be beneficial when the bacterial load increases.