Does Oral Endotoxin Contribute to Systemic Inflammation?

The oral microbiome, with a unique emphasis on Porphyromonas gingivalis has been associated with a constellation of inflammatory diseases such as cardiovascular disease, rheumatoid arthritis, Alzheimer's disease, type II diabetes, and non-alcoholic associated fatty liver disease. Periodontal disease has also been shown to induce "leaky gut" leading to metabolic endotoxemia. Several recent studies investigating the habitants of the blood microbiome have found the majority of species appear to be derived from oral and skin bacterial communities in otherwise healthy individuals. Many of the same pathologies associated with perturbations of oral health, such as cardiovascular disease, show alterations to the composition of the blood microbiome as well as circulating neutrophil phenotypes. Gingival inflammation is associated with activated blood neutrophil phenotypes that can exacerbate a distal inflammatory insult which may explain the connection between oral and systemic inflammatory conditions. While in the oral cavity, neutrophils encounter oral microbes that are adept in manipulating neutrophil activity which can re-enter the vasculature thereafter. Endotoxin from oral microbes can differ significantly depending on bacterial community and state of oral health to alter cellular LPS tolerance mechanisms which may contribute to the primed neutrophil phenotype seen in periodontitis and provide a mechanism by which the oral-microbes can affect systemic health outcomes. This review synthesizes the studies between inflammatory diseases and oral health with emphasis on microbiome and corresponding lipopolysaccharides in immune tolerance and activation.

Human variation in gingival inflammation.

Oral commensal bacteria actively participate with gingival tissue to maintain healthy neutrophil surveillance and normal tissue and bone turnover processes. Disruption of this homeostatic host-bacteria relationship occurs during experimental gingivitis studies where it has been clearly established that increases in the bacterial burden increase gingival inflammation. Here, we show that experimental gingivitis resulted in three unique clinical inflammatory phenotypes (high, low, and slow) and reveal that interleukin-1ß, a reported major gingivitis-associated inflammatory mediator, was not associated with clinical gingival inflammation in the slow response group. In addition, significantly higher levels of Streptococcus spp. were also unique to this group. The low clinical response group was characterized by low concentrations of host mediators, despite similar bacterial accumulation and compositional characteristics as the high clinical response group. Neutrophil and bone activation modulators were down-regulated in all response groups, revealing novel tissue and bone protective responses during gingival inflammation. These alterations in chemokine and microbial composition responses during experimental gingivitis reveal a previously uncharacterized variation in the human host response to a disruption in gingival homeostasis. Understanding this human variation in gingival inflammation may facilitate the identification of periodontitis-susceptible individuals. Overall, this study underscores the variability in host responses in the human population arising from variations in host immune profiles (low responders) and microbial community maturation (slow responders) that may impact clinical outcomes in terms of destructive inflammation.

Is the oral microbiome a source to enhance mucosal immunity against infectious diseases?

Mucosal tissues act as a barrier throughout the oral, nasopharyngeal, lung, and intestinal systems, offering first-line protection against potential pathogens. Conventionally, vaccines are applied parenterally to induce serotype-dependent humoral response but fail to drive adequate mucosal immune protection for viral infections such as influenza, HIV, and coronaviruses. Oral mucosa, however, provides a vast immune repertoire against specific microbial pathogens and yet is shaped by an ever-present microbiome community that has co-evolved with the host over thousands of years. Adjuvants targeting mucosal T-cells abundant in oral tissues can promote soluble-IgA (sIgA)-specific protection to confer increased vaccine efficacy. Th17 cells, for example, are at the center of cell-mediated immunity and evidence demonstrates that protection against heterologous pathogen serotypes is achieved with components from the oral microbiome. At the point of entry where pathogens are first encountered, typically the oral or nasal cavity, the mucosal surfaces are layered with bacterial cohabitants that continually shape the host immune profile. Constituents of the oral microbiome including their lipids, outer membrane vesicles, and specific proteins, have been found to modulate the Th17 response in the oral mucosa, playing important roles in vaccine and adjuvant designs. Currently, there are no approved adjuvants for the induction of Th17 protection, and it is critical that this research is included in the preparedness for the current and future pandemics. Here, we discuss the potential of oral commensals, and molecules derived thereof, to induce Th17 activity and provide safer and more predictable options in adjuvant engineering to prevent emerging infectious diseases.

Basic biology and role of interleukin-17 in immunity and inflammation.

Interleukin-17 (also known as interleukin-17A) is a key cytokine that links T-cell activation to neutrophil mobilization and activation. As such, interleukin-17 can mediate protective innate immunity to pathogens or contribute to the pathogenesis of inflammatory diseases, such as psoriasis and rheumatoid arthritis. This review summarizes the basic biology of interleukin-17 and discusses its emerging role in periodontal disease. The current burden of evidence from human and animal model studies suggests that the net effect of interleukin-17 signaling promotes disease development. In addition to promoting neutrophilic inflammation, interleukin-17 has potent pro-osteoclastogenic effects that are likely to contribute to the pathogenesis of periodontitis, rheumatoid arthritis and other diseases involving bone immunopathology. Systemic treatments with anti-interleukin-17 biologics have shown promising results in clinical trials for psoriasis and rheumatoid arthritis; however, their impact on the highly prevalent periodontal disease has not been investigated or reported. Future clinical trials, preferably using locally administered interleukin-17 blockers, are required to implicate conclusivelyinterleukin-17 in periodontitis and, more importantly, to establish an effective adjunctive treatment for this oral inflammatory disease.

Porphyromonas gingivalis virulence factors involved in subversion of leukocytes and microbial dysbiosis.

The oral bacterium Porphyromonas gingivalis has special nutrient requirements due to its asaccharolytic nature subsisting on small peptides cleaved from host proteins. Using proteases and other virulence factors, P. gingivalis thrives as a component of a polymicrobial community in nutritionally favorable inflammatory environments. In this regard, P. gingivalis has a number of strategies that subvert the host immune response in ways that promote its colonization and facilitate the outgrowth of the surrounding microbial community. The focus of this review is to discuss at the molecular level how P. gingivalis subverts leukocytes to create a favorable environment for a select community of bacteria that, in turn, adversely affects the periodontal tissues.

Defective neutrophil recruitment in leukocyte adhesion deficiency type I disease causes local IL-17-driven inflammatory bone loss.

Leukocyte adhesion deficiency type I (LAD-I), a disease syndrome associated with frequent microbial infections, is caused by mutations on the CD18 subunit of ß2 integrins. LAD-I is invariably associated with severe periodontal bone loss, which historically has been attributed to the lack of neutrophil surveillance of the periodontal infection. We provide an alternative mechanism by showing that the cytokine interleukin-17 (IL-17) plays a major role in the oral pathology of LAD-I. Defective neutrophil recruitment in LAD-I patients or in LFA-1 (CD11a/CD18)-deficient mice--which exhibit the LAD-I periodontal phenotype--was associated with excessive production of predominantly T cell-derived IL-17 in the periodontal tissue, although innate lymphoid cells also contributed to pathological IL-17 elevation in the LFA-1-deficient mice. Local treatment with antibodies to IL-17 or IL-23 in LFA-1-deficient mice not only blocked inflammatory periodontal bone loss but also caused a reduction in the total bacterial burden, suggesting that the IL-17-driven pathogenesis of LAD-I periodontitis leads to dysbiosis. Therefore, our findings support an IL-17-targeted therapy for periodontitis in LAD-I patients.

Porphyromonas gingivalis lipid A phosphatase activity is critical for colonization and increasing the commensal load in the rabbit ligature model.

Periodontitis is a disease of polymicrobial etiology characterized by inflammation, degradation of host tissue, and bone that irreversibly destroys the supporting apparatus of teeth. Porphyromonas gingivalis contains lipid A with structural heterogeneity that has been postulated to contribute to the initiation of dysbiosis in oral communities by modulating the host response, thereby creating a permissive environment for its growth. We examined two P. gingivalis lipid A phosphatase mutants which contain different "locked" lipid A structures that induce different host cellular responses for their ability to induce dysbiosis and periodontitis in rabbits. Lipopolysaccharide (LPS) preparations obtained from these strains were also examined. After repeated applications of all strains and their respective LPS preparations, P. gingivalis wild type, but not the lipid A mutants, had a significant impact on both the oral commensal microbial load and composition. In contrast, in rabbits exposed to the mutant strains or the LPS preparations, the microbial load did not increase, and yet significant changes in the oral microbial composition were observed. All strains and their respective LPS preparations induced periodontitis. Therefore, the ability to alter the lipid A composition in response to environmental conditions by lipid A phosphatases is required for both colonization of the rabbit and increases in the microbial load. Furthermore, the data demonstrate that multiple dysbiotic oral microbial communities can elicit periodontitis.

Defensins and LL-37: a review of function in the gingival epithelium.

Antimicrobial peptides represent an important aspect of the innate defense system that contributes to the control of bacterial colonization and infection. As studies have progressed it has become clear that antimicrobial peptides manifest other functions in addition to their antimicrobial effects. These functions include chemotaxis of numerous types of host cells involved in both the innate and adaptive immune responses. In this review, the antimicrobial activity, the regulation and the contribution to host homeostasis of alpha-defensins and LL-37, as well as of beta-defensins, are discussed in the context of their specific tissue locations in the junctional epithelium and oral epithelium, respectively.

Commensal bacteria-dependent select expression of CXCL2 contributes to periodontal tissue homeostasis.

The oral and intestinal host tissues both carry a heavy microbial burden. Although commensal bacteria contribute to healthy intestinal tissue structure and function, their contribution to oral health is poorly understood. A crucial component of periodontal health is the recruitment of neutrophils to periodontal tissue. To elucidate this process, gingival tissues of specific-pathogen-free and germ-free wild-type mice and CXCR2KO and MyD88KO mice were examined for quantitative analysis of neutrophils and CXCR2 chemoattractants (CXCL1, CXCL2). We show that the recruitment of neutrophils to the gingival tissue does not require commensal bacterial colonization but is entirely dependent on CXCR2 expression. Strikingly, however, commensal bacteria selectively upregulate the expression of CXCL2, but not CXCL1, in a MyD88-dependent way that correlates with increased neutrophil recruitment as compared with germ-free conditions. This is the first evidence that the selective use of chemokine receptor ligands contributes to neutrophil homing to healthy periodontal tissue.

The relationship of the oral microbiotia to periodontal health and disease.

The oral microbial community represents the best-characterized consortium associated with the human host. There are strong correlations between the qualitative composition of the oral microbiota and clinically healthy or diseased states. However, additional studies are needed to elucidate the mechanisms that define these microbial/host relationships.

Computational design of epitope-scaffolds allows induction of antibodies specific for a poorly immunogenic HIV vaccine epitope.

Broadly cross-reactive monoclonal antibodies define epitopes for vaccine development against HIV and other highly mutable viruses. Crystal structures are available for several such antibody-epitope complexes, but methods are needed to translate that structural information into immunogens that re-elicit similar antibodies. We describe a general computational method to design epitope-scaffolds in which contiguous structural epitopes are transplanted to scaffold proteins for conformational stabilization and immune presentation. Epitope-scaffolds designed for the poorly immunogenic but conserved HIV epitope 4E10 exhibited high epitope structural mimicry, bound with higher affinities to monoclonal antibody (mAb) 4E10 than the cognate peptide, and inhibited HIV neutralization by HIV+ sera. Rabbit immunization with an epitope-scaffold induced antibodies with structural specificity highly similar to mAb 4E10, an important advance toward elicitation of neutralizing activity. The results demonstrate that computationally designed epitope-scaffolds are valuable as structure-specific serological reagents and as immunogens to elicit antibodies with predetermined structural specificity.