Host responses and bacterial virulence factors in Neisseria infections

Abstract

Neisseria meningitidis and Neisseria gonorrhoeae are human specific pathogens. N. meningitidis is a major cause of meningitis and sepsis, whilst N. gonorrhoeae causes many millions of cases of gonorrhea in the world every year. A key factor in neisserial pathogenesis is the ability of the bacteria to attach to human host cell receptors. The initial binding is mediated by pili, long hairlike structures that extend from the bacterial cell surface. Pili bind to the cellular pilus receptor, CD46.

Lipopolysaccharide (LPS) of N. meningitidis is an extremely potent endotoxin that is largely responsible for tissue damage and shock in infected individuals. To examine the basis for bacterium-host cell contact, an LPS-deficient N. meningitidis mutant was constructed. LPS deficiency was without exception accompanied by altered colony opacity and morphology. It was also observed that LPS was essential for pilus-associated adherence but dispensable for fiber formation and twitching motility. The absence of attachment to host cells could not be attributed to altered levels of piliation or defects in the pilus adhesion phenotype. Furthermore, the LPS mutants did not invade epithelial cells and lost the natural competence for genetic transformation.

In order to develop an improved experimental infection model that mimics the human host, transgenic mice expressing the human pilus receptor CD46 were used. These mice were found to be highly susceptible to meningococcal disease after intraperitoneal infection. Crossing of the bloodbrain barrier by bacteria occurred in CD46 transgenic mice but not in nontransgenic mice. CD46 transgenic mice challenged with wild-type bacteria had high levels of pro- and antiinflammatory cytokines in sera whilst LPS-deficient bacteria only induced low cytokine levels at early time points post-challenge. Intranasal infection of CD46 mice required piliation of the bacteria for development of disease, supporting that CD46 facilitates pilus-dependent interactions at the epithelial mucosa. Taken together, these data demonstrate a crucial role of CD46 in meningococcal disease and reveal a novel experimental system for rapid consideration of vaccine candidates, as well as a model to study Neisseria pathogenesis. Antimicrobial peptides are part of the innate immunity and can be considered as endogenous antibiotics.

The antimicrobial peptide LL-37 was shown to exhibit potent bactericidal activity against N. gonorrhoeae and real-time quantitative PCR and immunocytochemistry showed that gonococcal infection caused a consistent down-regulation of LL-37 in a human cervical cell line. These data suggest that down-regulation of the expression of LL-37 by live N. gonorrhoeae may be a part of an invasive strategy in the female genital tract.