ABSTRACT
Neisseria meningitidis is a gram-negative bacterium that often asymptomatically colonizes the human nasopharyngeal tract. These bacteria cross the epithelial barrier can cause life-threatening sepsis and/or meningitis. Antimicrobial peptides are one of the first lines of defense against invading bacterial pathogens. Human beta-defensin 2 (hBD2) is an antimicrobial peptide with broad antibacterial activity, although its mechanism of action is poorly understood. Here, we investigated the effect of hBD2 on N. meningitidis. We showed that hBD2 binds to and kills actively growing meningococcal cells. The lethal effect was evident after 2 h incubation with the peptide, which suggests a slow killing mechanism. Further, the membrane integrity was not changed during hBD2 treatment. Incubation with lethal doses of hBD2 decreased the presence of diplococci; the number and size of bacterial microcolonies/aggregates remained constant, indicating that planktonic bacteria may be more susceptible to the peptide. Meningococcal DNA bound hBD2 in mobility shift assays and inhibited the lethal effect of hBD2 in a dose-dependent manner both in suspension and biofilms, supporting the interaction between hBD2 and DNA. Taken together, the ability of meningococcal DNA to bind hBD2 opens the possibility that extracellular DNA due to bacterial lysis may be a means of N. meningitidis to evade immune defenses.
ABSTRACT
Antimicrobial peptides (AMPs) play an important role in the defense against pathogens by targeting and killing invading microbes. Some pathogenic bacteria have been shown to negatively regulate AMP expression, while several commensals may induce AMP expression. The expression of certain AMPs, such as human beta-defensin 2 (hBD2), can be induced via nuclear factor NF-κB, which, in turn, is negatively controlled by tumor necrosis factor alpha-induced protein 3 (TNFAIP3, or A20). In this work, we examined the expression of hBD1 and hBD2 during coincubation of pharyngeal epithelial cells with pathogenic Neisseria meningitidis and commensal lactobacilli. The Lactobacillus strains induced hBD2 expression in human pharyngeal cells, while the pathogen N. meningitidis did not. In coincubation experiments, meningococci were able to dampen the AMP expression induced by lactobacilli. We found that N. meningitidis induced the NF-κB inhibitor A20. Further, RNA silencing of A20 resulted in increased hBD2 expression after meningococcal infection. Since it is known that induction of A20 reduces NF-κB activity and thus hBD2 levels, meningococcal-mediated A20 induction could be a way for the pathogen to dampen AMP expression. Finally, treatment of N. meningitidis and lactobacilli with synthetic hBD2 reduced N. meningitidis viability more efficiently than Lactobacillus reuteri, explaining why maintaining low AMP levels is important for the survival of the pathogen.
