Differential regulation of innate immune response genes in gingival epithelial cells stimulated with Aggregatibacter actinomycetemcomitans

Activation of vitamin D in the gingival epithelium and its role in gingival inflammation and alveolar bone loss

BACKGROUND AND OBJECTIVE

Both chronic and aggressive periodontal disease are associated with vitamin D deficiency. The active form of vitamin D, 1,25(OH)₂ D₃ , induces the expression of the antimicrobial peptide LL-37 and innate immune mediators in cultured human gingival epithelial cells (GECs). The aim of this study was to further delineate the mechanism by which vitamin D enhances the innate defense against the development of periodontal disease (PD).

MATERIALS AND METHODS

Wild-type C57Bl/6 mice were made deficient in vitamin D by dietary restriction. Cultured primary and immortalized GEC were stimulated with 1,25(OH)₂ D₃ , followed by infection with Porphyromonas gingivalis, and viable intracellular bacteria were quantified. Conversion of vitamin D₃ to 25(OH)D₃ and 1,25(OH)₂ D₃ was quantified by ELISA. Effect of vitamin D on basal IL-1α expression in mice was determined by topical administration to the gingiva of wild-type mice, followed by qRT-PCR.

RESULTS

Dietary restriction of vitamin D led to alveolar bone loss and increased inflammation in the gingiva in the mouse model. In primary human GEC and established human cell lines, treatment of GEC with 1,25(OH)₂ D₃ inhibited the intracellular growth of P. gingivalis. Cultured GEC expressed two 25-hydroxylases (CYP27A1 and CYP2R1), as well as 1-α hydroxylase, enabling conversion of vitamin D to both 25(OH)D₃ and 1,25(OH)₂ D₃ . Topical application of both vitamin D₃ and 1,25(OH)₂ D₃ to the gingiva of mice led to rapid inhibition of IL-1α expression, a prominent pro-inflammatory cytokine associated with inflammation, which also exhibited more than a 2-fold decrease from basal levels in OKF6/TERT1 cells upon 1,25(OH)₂ D₃ treatment, as determined by RNA-seq.

CONCLUSION

Vitamin D deficiency in mice contributes to PD, recapitulating the association seen in humans, and provides a unique model to study the development of PD. Vitamin D increases the activity of GEC against the invasion of periodontal pathogens and inhibits the inflammatory response, both in vitro and in vivo. GEC can convert inactive vitamin D to the active form in situ, supporting the hypothesis that vitamin D can be applied directly to the gingiva to prevent or treat periodontal disease.

Tainted air: The link between pollution and Alzheimer's disease

In this issue of the Biomedical Journal, we learn how air pollution may contribute to cognitive decline and even increase risk for Alzheimer's disease. We also highlight original research documenting the body's response to infection with a common oral pathogen. Finally, we learn how a cellular antioxidant protein protects against mitochondrial dysfunction in Parkinson's disease.

Oral infection of mice with Fusobacterium nucleatum results in macrophage recruitment to the dental pulp and bone resorption

BACKGROUND

Fusobacterium nucleatum is a Gram-negative anaerobic bacterium associated with periodontal disease. Some oral bacteria, like Porphyromonas gingivalis, evade the host immune response by inhibiting inflammation. On the other hand, F. nucleatum triggers inflammasome activation and release of danger-associated molecular patterns (DAMPs) in infected gingival epithelial cells.

METHODS

In this study, we characterized the pro-inflammatory response to F. nucleatum oral infection in BALB/c mice. Western blots and ELISA were used to measure cytokine and DAMP (HMGB1) levels in the oral cavity after infection. Histology and flow cytometry were used to observe recruitment of immune cells to infected tissue and pathology.

RESULTS

Our results show increased expression and production of pro-inflammatory cytokines during infection. Furthermore, we observe that F. nucleatum infection leads to recruitment of macrophages in different tissues of the oral cavity. Infection also contributes to osteoclast recruitment, which could be involved in the observed bone resorption.

CONCLUSIONS

Overall, our findings suggest that F. nucleatum infection rapidly induces inflammation, release of DAMPs, and macrophage infiltration in gingival tissues and suggest that osteoclasts may drive bone resorption at early stages of the inflammatory process.

Trends in Susceptibility to Aggressive Periodontal Disease

Aggregatibacter actinomycetemcomitans is a gram-negative microbe involved in periodontitis. Strains with varying degrees of virulence have been identified, in healthy and periodontally compromised individuals alike. Hosts mount differential immune responses to its various serotypes and virulence factors. Studies have explored host immune response in terms of antibody titers, leukocyte responses, and specific inflammatory mediators, questioning the ways in which the infectious microorganism survives. This mini-review will identify the key themes in immune response patterns of individuals both affected by and free from aggressive periodontal disease, thereby using it to understand various forms of periodontitis.

Signaling transduction analysis in gingival epithelial cells after infection with Aggregatibacter actinomycetemcomitans

Periodontal diseases result from the interaction of bacterial pathogens with the host's gingival tissue. Gingival epithelial cells are constantly challenged by microbial cells and respond by altering their transcription profiles, inducing the production of inflammatory mediators. Different transcription profiles are induced by oral bacteria and little is known about how the gingival epithelium responds after interaction with the periodontopathogenic organism Aggregatibacter actinomycetemcomitans. In the present study, we examined the transcription of genes involved in signaling transduction pathways in gingival epithelial cells exposed to viable A. actinomycetemcomitans. Immortalized gingival epithelial cells (OBA-9) were infected with A. actinomycetemcomitans JP2 for 24 h and the transcription profile of genes encoding human signal transduction pathways was determined. Functional analysis of inflammatory mediators positively transcribed was performed by ELISA in culture supernatant and in gingival tissues. Fifteen of 84 genes on the array were over-expressed (P < 0.01) after 24 h of infection with viable A. actinomycetemcomitans. Over-expressed genes included those implicated in tissue remodeling and bone resorption, such as CSF2, genes encoding components of the LDL pathway, nuclear factor-κB-dependent genes and other cytokines. The ELISA data confirmed that granulocyte-macrophage colony-stimulating factor/colony-stimulating factor 2, tumor necrosis factor-α and intercellular adhesion molecule-1 were highly expressed by infected gingival cells when compared with control non-infected cells, and presented higher concentrations in tissues from patients with aggressive and chronic periodontitis than in tissues from healthy controls. The induction in epithelial cells of factors such as the pro-inflammatory cytokine CSF2, which is involved in osteoclastogenesis, may help to explain the outcomes of A. actinomycetemcomitans infection.

Vitamin D-mediated induction of innate immunity in gingival epithelial cells

Human gingival epithelial cells (GEC) produce peptides, such as β-defensins and the cathelicidin LL-37, that are both antimicrobial and that modulate the innate immune response. In myeloid and airway epithelial cells, the active form of vitamin D(3) [1,25(OH)(2)D(3)] increases the expression and antibacterial activity of LL-37. To examine the activity of vitamin D on the innate immune defense of the gingival epithelium, cultured epithelial cells were treated with either 10(-8) M 1,25(OH)(2)D(3) or ethanol for up to 24 h. A time-dependent induction of LL-37 mRNA up to 13-fold at 24 h in both standard monolayer and three-dimensional cultures was observed. Induction of the vitamin D receptor and the 1-α-hydroxylase genes was also observed. The hydroxylase was functional, as LL-37 induction was observed in response to stimulation by 25(OH)D(3). Through microarray analysis of other innate immune genes, CD14 expression increased 4-fold, and triggering receptor expressed on myeloid cells-1 (TREM-1) was upregulated 16-fold after 24 h of treatment with 1,25(OH)(2)D(3). TREM-1 is a pivotal amplifier of the innate immune response in macrophages, leading to increased production by inflammatory response genes. Activation of TREM-1 on the GEC led to an increase in interleukin-8 (IL-8) mRNA levels. Incubation of three-dimensional cultures with 1,25(OH)(2)D(3) led to an increase in antibacterial activity against the periodontal pathogen Aggregatibacter actinomycetemcomitans when the bacteria were added to the apical surface. This study is the first to demonstrate the effect of vitamin D on antibacterial defense of oral epithelial cells, suggesting that vitamin D(3) could be utilized to enhance the innate immune defense in the oral cavity.

Epithelial cells and innate antifungal defense

The human pathogenic fungus Candida albicans is the predominant cause of both superficial and invasive forms of candidiasis. Clinical observations indicate that mucocutaneous Candida infections are commonly associated with defective cell-mediated immune responses. The importance of the innate immune system as a first-line defense against pathogenic challenge has long been recognized. Over the last decade, many key molecules mediating innate host defense have been identified. Central to these developments is the discovery of pattern recognition receptors such as Toll-like receptors and C-type lectin-receptors that induce innate immune responses and also modulate cellular and humoral adaptive immunity during Candida infections. Although a large amount of information is now available in systemic infections, little is known about localized infections. We address the most relevant pattern recognition receptors and their signaling mechanisms in oral epithelial cells, to gain a better understanding of their contributions to antifungal innate immunity.

The roles of antimicrobial peptides in innate host defense

Antimicrobial peptides (AMPs) are multi-functional peptides whose fundamental biological role in vivo has been proposed to be the elimination of pathogenic microorganisms, including Gram-positive and -negative bacteria, fungi, and viruses. Genes encoding these peptides are expressed in a variety of cells in the host, including circulating phagocytic cells and mucosal epithelial cells, demonstrating a wide range of utility in the innate immune system. Expression of these genes is tightly regulated; they are induced by pathogens and cytokines as part of the host defense response, and they can be suppressed by bacterial virulence factors and environmental factors which can lead to increased susceptibility to infection. New research has also cast light on alternative functionalities, including immunomodulatory activities, which are related to their unique structural characteristics. These peptides represent not only an important component of innate host defense against microbial colonization and a link between innate and adaptive immunity, but also form a foundation for the development of new therapeutic agents.

Host defense peptides in the oral cavity and the lung: similarities and differences

Peptides with broad-spectrum antimicrobial activity are found in the mucosal surfaces at many sites in the body, including the airway, the oral cavity, and the digestive tract. Based on their in vitro antimicrobial and other immunomodulatory activities, these host defense peptides have been proposed to play an important role in the innate defense against pathogenic microbial colonization. The genes that encode these peptides are up-regulated by pathogens, further supporting their role in innate immune defense. However, the differences in the local microbial environments between the generally sterile airway and the highly colonized oral cavity suggest a more complex role for these peptides in innate immunity. For example, beta-defensin genes are induced in the airway by all bacteria and Toll-like receptor (TLR) agonists primarily through an NF-kappaB-mediated pathway. In contrast, the same genes are induced in the gingival epithelium by only a subset of bacteria and TLR ligands, via different pathways. Furthermore, the environments into which the peptides are secreted--specifically saliva, gingival crevicular fluid, and airway surface fluid--differ greatly and can effect their respective activities in host defense. In this review, we examine the differences and similarities between host defense peptides in the oral cavity and the airway, to gain a better understanding of their contributions to immunity.