Date Published: April 18, 2013
Publisher: Public Library of Science
Author(s): Hiroki Takeuchi, Takanori Hirano, Sarah E. Whitmore, Ichijiro Morisaki, Atsuo Amano, Richard J. Lamont, Ralph R. Isberg.
Porphyromonas gingivalis is a major pathogen in severe and chronic manifestations of periodontal disease, which is one of the most common infections of humans. A central feature of P. gingivalis pathogenicity is dysregulation of innate immunity at the gingival epithelial interface, including suppression of IL-8 production by epithelial cells. NF-κB is a transcriptional regulator that controls important aspects of innate immune responses, and NF-κB RelA/p65 homodimers regulate transcription of IL8. Phosphorylation of the NF-κB p65 subunit protein on the serine 536 residue affects nuclear translocation and transcription of target genes. Here we show that SerB, a haloacid dehalogenase (HAD) family serine phosphatase secreted by P. gingivalis, is produced intracellularly and can specifically dephosphorylate S536 of p65 in gingival epithelial cells. A P. gingivalis mutant lacking SerB was impaired in dephosphorylation of p65 S536, and ectopically expressed SerB bound to p65 and co-localized with p65 in the cytoplasm. Ectopic expression of SerB also resulted in dephosphorylation of p65 with reduced nuclear translocation in TNF-α-stimulated epithelial cells. In contrast, the p105/50 subunit of NF-κB was unaffected by SerB. Co-expression of a constitutively active p65 mutant (S536D) relieved inhibition of nuclear translocation. Both the activity of the IL8 promoter and production of IL-8 were diminished by SerB. Deletion and truncation mutants of SerB lacking the HAD-family enzyme motifs of SerB were unable to dephosphorylate p65, inhibit nuclear translocation or abrogate IL8 transcription. Specific dephosphorylation of NF-κB p65 S536 by SerB, and consequent inhibition of nuclear translocation, provides the molecular basis for a bacterial strategy to manipulate host inflammatory pathways and repress innate immunity at mucosal surfaces.
Many of the mucosal surfaces of humans are colonized by a diverse and abundant microbiota. In most instances the host remains healthy, in large part due to numerous innate and acquired immune mechanisms that limit microbial intrusion and rapidly kill organisms that traverse epithelial barriers. In the periodontal tissues of the oral cavity the epithelium of the subgingival compartment plays a central role in orchestration of innate immunity. While this tissue is relatively porous, gingival epithelial cells secrete high levels of IL-8 and consequently large numbers of neutrophils are recruited into the periodontal area where they serve to constrain the microbial challenge . Successful periodontal pathogens, such as Porphyromonas gingivalis, are capable of disrupting innate defenses, and indeed one virulence determinant of P. gingivalis is inhibition of IL-8 production by gingival epithelial cells, a strategy known as localized chemokine paralysis , . Moreover, periodontal diseases are multispecies infections involving pathogenic communities in which the microbial constituents exhibit polymicrobial synergy. Consistent with this, P. gingivalis can antagonize IL-8 secretion in the presence of stimulatory organisms , a property that will allow P. gingivalis to enhance the pathogenicity of the entire multispecies periodontal community and which contributes to its designation as a keystone pathogen . The P. gingivalis serine phosphatase SerB is required for IL-8 suppression, and in a murine model of disease a mutant lacking SerB induces higher levels of neutrophil recruitment into gingival tissues compared to the parental strain . Additionally, loss of SerB attenuates alveolar bone destruction in animal infection models demonstrating that SerB, and its associated anti-inflammatory action, is required for P. gingivalis to realize its full pathogenic potential . The mechanistic basis for the SerB-dependent inhibition of IL-8 remains undetermined.
Periodontal diseases are characterized by destruction of the gingival tissues including the alveolar bone, with eventual exfoliation of the tooth. These are among the most common infections of humans, and in developed countries over half of the adult population will experience some form of periodontitis . The disease initiates at the epithelial surfaces of the subgingival compartment, and gingival epithelial cells play a central role in responding to microbial infection and orchestrating immune responses . Mucosal inflammatory responses involve coordinated expression of cytokines and chemokines, and the NF-κB transcriptional regulator exerts control over a number of inflammation-associated genes.