Streptococcus cristatus modulates epithelial innate immune response through regulating nuclear factor-kappa B pathway.

Abstract

Streptococci are the dominant oral commensal organisms. They, along with putative periodontal pathogens, normally colonize multiple oral tissue sites. An emerging paradigm indicates that the host can distinguish between pathogenic and nonpathogenic stimuli, and commensal bacteria could modulate expression of host genes to contribute to mucosal tolerance. Here, I hypothesized that an oral commensal Streptococcus cristatus can attenuate epithelial proinflammatory response to Fusobacterium nucleatum via regulation of Toll-Like Receptor (TLR) signaling, as a mechanism for oral mucosal tolerance to polymicrobial infection. I first demonstrated that S. cristatus itself did not provoke IL-8 production in epithelial cells, and it was able to inhibit IL-8 responses to several putative oral pathogens including F. nucleatum. The inhibitory effect of S. cristatus on IL-8 was independent of its viability and its coaggregation with F. nucleatum, was not related to soluble bacterial products, and appeared to require bacterial contact with epithelial cells. Similar effects were seen with several other species of oral streptococci. Next, I performed pathway-specific microarray analysis to specifically monitor host gene modulation by S. cristatus on a broad scale. I found that S. cristatus altered the F. nucleatum–induced expression of a number of proinflammatory cytokine genes. Profiling of TLR signaling related genes revealed that S. cristatus most significantly impacted the downstream pathways, especially NF-κB, rather than altering TLRs and their adaptors and interacting proteins. Lastly, I examined the underlying mechanism(s) involved in the modulating effect of S. cristatus by looking specifically at its impact on the nuclear factor kappaB (NF-κB) pathway under the Toll-like receptor signaling background. I found that the IL-8 suppression by S. cristatus was coincident with the inhibition of NF-κB activation and IκB-α degradation. Pre-incubation with TLR2 and TLR4 antibodies, however, did not affect the epithelial response to either species alone or in combination. I thus conclude that the oral commensal S. cristatus is not only tolerated by the host, but also able to modulate host inflammatory response to pathogenic species through inhibiting IκB-NF-κB signaling module. The anti-inflammatory effect of S. cristatus might represent a regulatory mechanism present at the epithelial surface to tolerate polymicrobial colonization.