Effect of adoptive transfer of antigen-specific B cells on periodontal bone resorption

Osteoimmunology in Periodontitis and Orthodontic Tooth Movement

PURPOSE OF REVIEW

To review the role of the immune cells and their interaction with cells found in gingiva, periodontal ligament, and bone that leads to net bone loss in periodontitis or bone remodeling in orthodontic tooth movement.

RECENT FINDINGS

Periodontal disease is one of the most common oral diseases causing inflammation in the soft and hard tissues of the periodontium and is initiated by bacteria that induce a host response. Although the innate and adaptive immune response function cooperatively to prevent bacterial dissemination, they also play a major role in gingival inflammation and destruction of the connective tissue, periodontal ligament, and alveolar bone characteristic of periodontitis. The inflammatory response is triggered by bacteria or their products that bind to pattern recognition receptors that induce transcription factor activity to stimulate cytokine and chemokine expression. Epithelial, fibroblast/stromal, and resident leukocytes play a key role in initiating the host response and contribute to periodontal disease. Single-cell RNA-seq (scRNA-seq) experiments have added new insight into the roles of various cell types in the response to bacterial challenge. This response is modified by systemic conditions such as diabetes and smoking. In contrast to periodontitis, orthodontic tooth movement (OTM) is a sterile inflammatory response induced by mechanical force. Orthodontic force application stimulates acute inflammatory responses in the periodontal ligament and alveolar bone stimulated by cytokines and chemokines that produce bone resorption on the compression side. On the tension side, orthodontic forces induce the production of osteogenic factors, stimulating new bone formation. A number of different cell types, cytokines, and signaling/pathways are involved in this complex process. Inflammatory and mechanical force-induced bone remodeling involves bone resorption and bone formation. The interaction of leukocytes with host stromal cells and osteoblastic cells plays a key role in both initiating the inflammatory events as well as inducing a cellular cascade that results in remodeling in orthodontic tooth movement or in tissue destruction in periodontitis.

Interactions of B-lymphocytes and bone cells in health and disease

Bone remodeling occurs through the interactions of three major cell lineages, osteoblasts, which mediate bone formation, osteocytes, which derive from osteoblasts, sense mechanical force and direct bone turnover, and osteoclasts, which mediate bone resorption. However, multiple additional cell types within the bone marrow, including macrophages, T lymphocytes and B lymphocytes influence the process. The bone marrow microenvironment, which is supported, in part, by bone cells, forms a nurturing network for B lymphopoiesis. In turn, developing B lymphocytes influence bone cells. Bone health during homeostasis depends on the normal interactions of bone cells with other lineages in the bone marrow. In disease state these interactions become pathologic and can cause abnormal function of bone cells and inadequate repair of bone after a fracture. This review summarizes what is known about the development of B lymphocytes and the interactions of B lymphocytes with bone cells in both health and disease.

Could AMPs and B-cells be the missing link in understanding periodontitis?

Periodontal diseases are common inflammatory conditions characterized by bone loss in response to simultaneous bacterial aggression and host defenses. The etiology of such diseases is still not completely understood, however. It has been shown that specific pathogens involved in the build-up of dysbiotic biofilms participate actively in the establishment of periodontitis. This multifactorial pathology also depends on environmental factors and host characteristics, especially defenses. The immune response to the pathogens seems to be critical in preventing the disease from starting but also contributes to tissue damage. It is known that small molecules known as antimicrobial peptides (AMPs) are key actors in the innate immune response. They not only target microbes, but also act as immuno-modulators. They can help to recruit or activate cells such as neutrophils, monocytes, dendritic cells, or lymphocytes. AMPs have already been described in the periodontium, and their expression seems to be connected to disease activity. Alpha and beta defensins and LL37 are the AMPs most frequently linked to periodontitis. Additionally, leukocyte infiltrates, especially B-cells, have also been linked to the severity of periodontitis. Indeed, the particular subpopulations of B-cells in these infiltrates have been linked to inflammation and bone resorption. A link between B-cells and AMP could be relevant to understanding B-cells' action. Some AMP receptors, such as chemokines receptors, toll-like receptors, or purinergic receptors, have been shown to be expressed by B-cells. Consequently, the action of AMPs on B-cell subpopulations could participate to B-cell recruitment, their differentiation, and their implication in both periodontal defense and destruction.

Host and bacterial factors linking periodontitis and rheumatoid arthritis

Observations from numerous clinical, epidemiological and serological studies link periodontitis with severity and progression of rheumatoid arthritis. The strong association is observed despite totally different aetiology of these two diseases, periodontitis being driven by dysbiotic microbial flora on the tooth surface below the gum line, while rheumatoid arthritis being the autoimmune disease powered by anti-citrullinated protein antibodies (ACPAs). Here we discuss genetic and environmental risk factors underlying development of both diseases with special emphasis on bacteria implicated in pathogenicity of periodontitis. Individual periodontal pathogens and their virulence factors are argued as potentially contributing to putative causative link between periodontal infection and initiation of a chain of events leading to breakdown of immunotolerance and development of ACPAs. In this respect peptidylarginine deiminase, an enzyme unique among prokaryotes for Porphyromonas gingivalis, is elaborated as a potential mechanistic link between this major periodontal pathogen and initiation of rheumatoid arthritis development.

Immunotherapy with regulatory T and B cells in periodontitis

Periodontitis (PD), also known as gum disease, is a condition causing inflammatory bone resorption and tooth loss. Regulatory T cells (Tregs) and regulatory B cells (Bregs) are vital in controlling the immune response and hence play a role in infections and peripheral tolerance adjustment. These cells have immunosuppressive and tissue-repairing capabilities that are important for periodontal health; however, in inflammatory circumstances, Tregs may become unstable and dysfunctional, accelerating tissue deterioration. In recent years, Regulatory cell-mediated immunotherapy has been shown to be effective in many inflammatory diseases. Considering the roles of Tregs and Bregs in shaping immune responses, this study aimed to review the published articles in this field to provide a comprehensive view of the existing knowledge about the role of regulatory T and B cells, as well as their therapeutic applications in PD.

Phenotypes, roles, and modulation of regulatory lymphocytes in periodontitis and its associated systemic diseases

Periodontitis is a common chronic inflammatory disease that can result in tooth loss and poses a risk to systemic health. Lymphocytes play important roles in periodontitis through multiple mechanisms. Regulatory lymphocytes including regulatory B cells (Bregs) and T cells (Tregs) are the main immunosuppressive cells that maintain immune homeostasis, and are critical to our understanding of the pathogenesis of periodontitis and the development of effective treatments. In this review, we discuss the phenotypes, roles, and modulating strategies of regulatory lymphocytes including Bregs and Tregs in periodontitis and frequently cooccurring inflammatory diseases such as rheumatoid arthritis, Alzheimer disease, diabetes mellitus, and stroke. The current evidence suggests that restoring immune balance through therapeutic targeting of regulatory lymphocytes is a promising strategy for the treatment of periodontitis and other systemic inflammatory diseases.

Application of specialized pro-resolving mediators in periodontitis and peri-implantitis: a review

Scaling and root planning is a key element in the mechanical therapy used for the eradication of biofilm, which is the major etiological factor for periodontitis and peri‐implantitis. However, periodontitis is also a host mediated disease, therefore, removal of the biofilm without adjunctive therapy may not achieve the desired clinical outcome due to persistent activation of the innate and adaptive immune cells. Most recently, even the resident cells of the periodontium, including periodontal ligament fibroblasts, have been shown to produce several inflammatory factors in response to bacterial challenge. With increased understanding of the pathophysiology of periodontitis, more research is focusing on opposing excessive inflammation with specialized pro‐resolving mediators (SPMs). This review article covers the major limitations of current standards of care for periodontitis and peri‐implantitis, and it highlights recent advances and prospects of SPMs in the context of tissue reconstruction and regeneration. Here, we focus primarily on the role of SPMs in restoring tissue homeostasis after periodontal infection.

Toll-Like Receptor Signaling and Immune Regulatory Lymphocytes in Periodontal Disease

Periodontitis is known to be initiated by periodontal microbiota derived from biofilm formation. The microbial dysbiotic changes in the biofilm trigger the host immune and inflammatory responses that can be both beneficial for the protection of the host from infection, and detrimental to the host, causing tissue destruction. During this process, recognition of Pathogen-Associated Molecular Patterns (PAMPs) by the host Pattern Recognition Receptors (PRRs) such as Toll-like receptors (TLRs) play an essential role in the host–microbe interaction and the subsequent innate as well as adaptive responses. If persistent, the adverse interaction triggered by the host immune response to the microorganisms associated with periodontal biofilms is a direct cause of periodontal inflammation and bone loss. A large number of T and B lymphocytes are infiltrated in the diseased gingival tissues, which can secrete inflammatory mediators and activate the osteolytic pathways, promoting periodontal inflammation and bone resorption. On the other hand, there is evidence showing that immune regulatory T and B cells are present in the diseased tissue and can be induced for the enhancement of their anti-inflammatory effects. Changes and distribution of the T/B lymphocytes phenotype seem to be a key determinant of the periodontal disease outcome, as the functional activities of these cells not only shape up the overall immune response pattern, but may directly regulate the osteoimmunological balance. Therefore, interventional strategies targeting TLR signaling and immune regulatory T/B cells may be a promising approach to rebalance the immune response and alleviate bone loss in periodontal disease. In this review, we will examine the etiological role of TLR signaling and immune cell osteoclastogenic activity in the pathogenesis of periodontitis. More importantly, the protective effects of immune regulatory lymphocytes, particularly the activation and functional role of IL-10 expressing regulatory B cells, will be discussed.

Switched memory B cells promote alveolar bone damage during periodontitis: An adoptive transfer experiment

Periodontitis is a bacteria-induced disease that often leads to alveolar bone damage. We sought to determine the role and mechanism of switched memory B cells in alveolar bone destruction during periodontitis. Sensitized B cells were sorted and cultured, then their expression of receptor activator for nuclear factor-κB ligand (RANKL), interleukin-6 (IL-6), and interleukin-12 (IL-12) was detected. Using these cells, we prepared adoptive transfer models in which we induced periodontitis. We found that switched memory B cells produced more RANKL in terms of both protein and mRNA levels than other subpopulations. Switched memory B cells expressed more IL-6 and IL-12 mRNA than other subpopulations, but differences in respective protein levels were not significant. Moreover, we found that switched memory B cell transfer resulted in increased alveolar bone loss and periodontal osteoclastogenesis. Moreover, switched memory B cell transfer increased the proportion of Th1 and Th17 cells as well as the expression of RANKL, osteoprotegerin (OPG), tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), IL-1β, IL-6, IL-17A in gingiva, and cervical lymph nodes (CLNs). The outcomes of the present study indicate that switched memory B cells regulate alveolar bone homeostasis via enhancing cytokine expression and increasing proliferation of Th1 and Th17 cells.

Revisiting the Page & Schroeder model: the good, the bad and the unknowns in the periodontal host response 40 years later

In their classic 1976 paper, Page & Schroeder described the histopathologic events and the types of myeloid cells and lymphocytes involved in the initiation and progression of inflammatory periodontal disease. The staging of periodontal disease pathogenesis as 'initial', 'early', 'established' and 'advanced' lesions productively guided subsequent research in the field and remains fundamentally valid. However, major advances regarding the cellular and molecular mechanisms underlying the induction, regulation and effector functions of immune and inflammatory responses necessitate a reassessment of their work and its integration with emerging new concepts. We now know that each type of leukocyte is actually represented by functionally distinct subsets with different, or even conflicting, roles in immunity and inflammation. Unexpectedly, neutrophils, traditionally regarded as merely antimicrobial effectors in acute conditions and protagonists of the 'initial' lesion, are currently appreciated for their functional versatility and critical roles in chronic inflammation. Moreover, an entirely new field of study, osteoimmunology, has emerged and sheds light on the impact of immunoinflammatory events on the skeletal system. These developments and the molecular dissection of crosstalk interactions between innate and adaptive leukocytes, as well as between the immune system and local homeostatic mechanisms, offer a more nuanced understanding of the host response in periodontitis, with profound implications for treatment. At the same time, deeper insights have generated new questions, many of which remain unanswered. In this review, 40 years after Page & Schroeder proposed their model, we summarize enduring and emerging advances in periodontal disease pathogenesis.

RANKL deletion in periodontal ligament and bone lining cells blocks orthodontic tooth movement

The bone remodeling process in response to orthodontic forces requires the activity of osteoclasts to allow teeth to move in the direction of the force applied. Receptor activator of nuclear factor-κB ligand (RANKL) is essential for this process although its cellular source in response to orthodontic forces has not been determined. Orthodontic tooth movement is considered to be an aseptic inflammatory process that is stimulated by leukocytes including T and B lymphocytes which are presumed to stimulate bone resorption. We determined whether periodontal ligament and bone lining cells were an essential source of RANKL by tamoxifen induced deletion of RANKL in which Cre recombinase was driven by a 3.2 kb reporter element of the Col1α1 gene in experimental mice (Col1α1.CreERTM+.RANKLf/f) and compared results with littermate controls (Col1α1.CreERTM-.RANKLf/f). By examination of Col1α1.CreERTM+.ROSA26 reporter mice we showed tissue specificity of tamoxifen induced Cre recombinase predominantly in the periodontal ligament and bone lining cells. Surprisingly we found that most of the orthodontic tooth movement and formation of osteoclasts was blocked in the experimental mice, which also had a reduced periodontal ligament space. Thus, we demonstrate for the first time that RANKL produced by periodontal ligament and bone lining cells provide the major driving force for tooth movement and osteoclastogenesis in response to orthodontic forces.

B cell subset distribution is altered in patients with severe periodontitis

Several studies have recently highlighted the implication of B cells in physiopathogenesis of periodontal disease by showing that a B cell deficiency leads to improved periodontal parameters. However, the detailed profiles of circulating B cell subsets have not yet been investigated in patients with severe periodontitis (SP). We hypothesised that an abnormal distribution of B cell subsets could be detected in the blood of patients with severe periodontal lesions, as already reported for patients with chronic inflammatory diseases as systemic autoimmune diseases. Fifteen subjects with SP and 13 subjects without periodontitis, according to the definition proposed by the CDC periodontal disease surveillance work group, were enrolled in this pilot observational study. Two flow cytometry panels were designed to analyse the circulating B and B1 cell subset distribution in association with the RANKL expression. A significantly higher percentage of CD27⁺ memory B cells was observed in patients with SP. Among these CD27⁺ B cells, the proportion of the switched memory subset was significantly higher. At the same time, human B1 cells, which were previously associated with a regulatory function (CD20⁺CD69^-CD43⁺CD27⁺CD11b⁺), decreased in SP patients. The RANKL expression increased in every B cell subset from the SP patients and was significantly greater in activated B cells than in the subjects without periodontitis. These preliminary results demonstrate the altered distribution of B cells in the context of severe periodontitis. Further investigations with a larger cohort of patients can elucidate if the analysis of the B cell compartment distribution can reflect the periodontal disease activity and be a reliable marker for its prognosis (clinical trial registration number: NCT02833285, B cell functions in periodontitis).

miR-146a regulates inflammatory cytokine production in Porphyromonas gingivalis lipopolysaccharide-stimulated B cells by targeting IRAK1 but not TRAF6

It has been suggested that microRNAs (miRs) are involved in the immune regulation of periodontitis. However, it is unclear whether and how miRs regulate the function of B cells in the context of periodontitis. This study is to explore the role of miR-146a on the inflammatory cytokine production of B cells challenged by Porphyromonas gingivalis (P. gingivalis) lipopolysaccharide (LPS). Primary B cells were harvested from mouse spleen. Quantitative real-time polymerase chain reaction (qPCR), enzyme-linked immunosorbent assay (ELISA) were used to detect the expression of inflammatory cytokines in B cells in the presence or absence of P. gingivalis LPS and/or miR-146a. Bioinformatics, luciferase reporter assay and overexpression assay were used to explore the binding target of miR-146a. Our results showed that miR-146a level in B cells was elevated by P. gingivalis LPS stimulation, and the mRNA expressions of interleukin (IL)-1β, 6 and 10, and IL-1 receptor associated kinase-1 (IRAK1), but not TNF receptor associated factor 6 (TRAF6), were also upregulated. The expression levels of IL-1β, 6, 10 and IRAK1 were reduced in the presence of miR-146a mimic, but were elevated by the addition of miR-146a inhibitor. MiR-146a could bind with IRAK1 3' untranslated region (UTR) but not TRAF6 3'-UTR. Overexpression of IRAK1 reversed the inhibitory effects of miR-146a on IL-1β, 6 and 10. In summary, miR-146a inhibits inflammatory cytokine production in B cells through directly targeting IRAK1, suggesting a regulatory role of miR-146a in B cell-mediated periodontal inflammation.

The emerging roles of B cells as partners and targets in periodontitis

Initial studies of periodontal disease suggested that T cell-mediated immunity against oral Gram-negative microorganisms is a key player in the pathogenesis of this inflammatory disease. Recent investigations, however, revealed that B cells are also engaged. Given their chief role in innate-like and adaptive immune responses, B cells could exert protective functions in periodontitis. However, the periodontal bacteria-specific antibody response is generally unable to halt disease progression in affected subjects, suggesting that the antibodies produced could exhibit low anti-bacterial blocking functions or opsonophagocytic potential, and/or unfavorable effects. Moreover, although microbial antigens are involved in the induction of the inflammatory responses in human adult periodontitis, endogenous antigens also may contribute to the chronicity of this common disease. Not only antibodies to self-antigens, such as collagen, are locally produced, but the autoreactivities observed in aggressive periodontitis are more severe and diverse than those observed in chronic periodontitis, suggesting that autoimmune reactivity could play a role in the tissue destruction of periodontal disease. Further support for a pathological role of B cells in periodontitis comes from the finding that B cell-deficient mice are protected from bacterial infection-induced alveolar bone loss. Studies in patients indicate that B cells and plasma cells, together with osteoclastogenic factors (RANKL and osteoprotegerin) and specific cytokines involved in their growth and differentiation (BAFF and APRIL) participate in the induction of the pathological bone loss in periodontitis. This novel insight suggests that selective targeting of B cells could represent a future therapeutic avenue for severe periodontal disease.

The B Cell-Stimulatory Cytokines BLyS and APRIL Are Elevated in Human Periodontitis and Are Required for B Cell-Dependent Bone Loss in Experimental Murine Periodontitis

B-lineage cells (B lymphocytes and plasma cells) predominate in the inflammatory infiltrate of human chronic periodontitis. However, their role in disease pathogenesis and the factors responsible for their persistence in chronic lesions are poorly understood. In this regard, two cytokines of the TNF ligand superfamily, a proliferation-inducing ligand (APRIL) and B-lymphocyte stimulator (BLyS), are important for the survival, proliferation, and maturation of B cells. Thus, we hypothesized that APRIL and/or BLyS are upregulated in periodontitis and contribute to induction of periodontal bone loss. This hypothesis was addressed in both human and mouse experimental systems. We show that, relative to healthy controls, the expression of APRIL and BLyS mRNA and protein was upregulated in natural and experimental periodontitis in humans and mice, respectively. The elevated expression of these cytokines correlated with increased numbers of B cells/plasma cells in both species. Moreover, APRIL and BLyS partially colocalized with κ L chain-expressing B-lineage cells at the epithelial-connective tissue interface. Ligature-induced periodontitis resulted in significantly less bone loss in B cell-deficient mice compared with wild-type controls. Ab-mediated neutralization of APRIL or BLyS diminished the number of B cells in the gingival tissue and inhibited bone loss in wild-type, but not in B cell-deficient, mice. In conclusion, B cells and specific cytokines involved in their growth and differentiation contribute to periodontal bone loss. Moreover, APRIL and BLyS have been identified as potential therapeutic targets in periodontitis.

B cells promote obesity-associated periodontitis and oral pathogen-associated inflammation

Individuals with T2D and PD suffer significantly from the ability of one disease to intensify the other. Disease-associated inflammation is one mechanism thought to fuel this pathogenic feed-forward loop. Several lines of evidence indicate that proinflammatory B cells promote T2D and PD; thus, B cells are top candidates for a cell type that predisposes PD in T2D. To test directly the role of B cells in T2D-associated PD, we compared outcomes from oral Porphyromonas gingivalis challenge of lean WT or B cell-null mice with outcomes from mice that were obese and insulin-resistant before challenge. Obese WT mice responded to oral P. gingivalis challenge with significant periodontal bone loss, whereas obese B cell-null mice were protected completely from PD. By contrast, lean WT and B cell-null mice suffer similar periodontal bone loss in response to oral pathogen. B cells from obese/insulin-resistant hosts also support oral osteoclastogenesis and both oral and systemic production of inflammatory cytokines, including pro-osteoclastogenic TNF-α and MIP-2, an ortholog of human IL-8. B cells furthermore impact AT inflammation in obese, P. gingivalis-infected hosts. Taken together, these data show that fundamentally different mechanisms regulate PD in lean and obese hosts, with B cells able to promote PD only if the hosts are "primed" by obesity. These results justify more intense analysis of obesity-associated changes in B cells that predispose PD in human T2D.

Role of periodontal pathogenic bacteria in RANKL-mediated bone destruction in periodontal disease

Accumulated lines of evidence suggest that hyperimmune responses to periodontal bacteria result in the destruction of periodontal connective tissue and alveolar bone. The etiological roles of periodontal bacteria in the onset and progression of periodontal disease (PD) are well documented. However, the mechanism underlying the engagement of periodontal bacteria in RANKL-mediated alveolar bone resorption remains unclear. Therefore, this review article addresses three critical subjects. First, we discuss earlier studies of immune intervention, ultimately leading to the identification of bacteria-reactive lymphocytes as the cellular source of osteoclast-induction factor lymphokine (now called RANKL) in the context of periodontal bone resorption. Next, we consider (1) the effects of periodontal bacteria on RANKL production from a variety of adaptive immune effector cells, as well as fibroblasts, in inflamed periodontal tissue and (2) the bifunctional roles (upregulation vs. downregulation) of LPS produced from periodontal bacteria in a RANKL-induced osteoclast-signal pathway. Future studies in these two areas could lead to new therapeutic approaches for the management of PD by down-modulating RANKL production and/or RANKL-mediated osteoclastogenesis in the context of host immune responses against periodontal pathogenic bacteria.

Rheumatoid arthritis and periodontal disease

The prevalence of periodontal disease has increased two-fold among patients with rheumatoid arthritis (RA) compared to the general population. This increased prevalence is unrelated to secondary Sjögren's syndrome but instead reflects shared pathogenic mechanisms, including an increased prevalence of the shared epitope HLA-DRB1-04; exacerbated T-cell responsiveness with high tissue levels of IL-17; exaggerated B-cell responses, with plasma cells being the predominant cell type found within gingival tissue affected with periodontitis and B cells being twice as numerous as T cells; RANK overexpression; and an increase in the ratio of RANK-L over osteoprotegerin with a high level of RANK-L expression on gingival B cells, most notably those capable of recognizing Porphyromonas gingivalis. Other factors conducive to periodontitis include smoking and infection with the Epstein-Barr virus or cytomegalovirus, which act by promoting the growth of organisms such as P. gingivalis, whose DNA is often found in synovial tissue from RA patients. P. gingivalis produces the enzyme peptidylarginine deiminase that induces citrullination of various autoantigens, and levels of anti-CCP antibodies are considerably higher in RA patients with than without periodontal disease, suggesting that periodontitis may contribute to the pathogenesis of RA. Further support for this hypothesis comes from evidence that other antigens involved in RA, such as HC-gp39, are also present in gingival tissue. TNFα antagonists slow alveolar resorption but may perpetuate infection of periodontal pockets. Therefore, rheumatology patients, including those taking biotherapies, are likely to benefit from increased referral to dental care (e.g., scaling, root planing and, if needed, dental surgery), particularly as periodontitis is also associated with an increased risk of premature atheroma.

B Cell IgD Deletion Prevents Alveolar Bone Loss Following Murine Oral Infection

Periodontal disease is one of the most common infectious diseases of humans. Immune responses to infection trigger loss of alveolar bone from the jaw and eventual tooth loss. We investigated the contribution of B cell IgD to alveolar bone loss by comparing the response of B cell normal BALB/cJ mice and IgD deficient BALB/c-Igh-5^−/−J mice to oral infection with Porphyromonas gingivalis, a gram-negative periodontopathic bacterium from humans. P. gingivalis-infected normal mice lost bone. Specific antibody to P. gingivalis was lower and oral colonization was higher in IgD deficient mice; yet bone loss was completely absent. Infection increased the proportion of CD69⁺ activated B cells and CD4⁺ T cells in immune normal mice compared to IgD deficient mice. These data suggest that IgD is an important mediator of alveolar bone resorption, possibly through antigen-specific coactivation of B cells and CD4⁺ T cells.

A new inflammatory cytokine on the block: re-thinking periodontal disease and the Th1/Th2 paradigm in the context of Th17 cells and IL-17

For almost two decades, the Th1/Th2 paradigm has offered a productive conceptual framework for investigating the pathogenesis of periodontitis. However, as with many other inflammatory diseases, the observed role of T-cell-mediated immunity in periodontitis did not readily fit this model. A new subset of CD4+ T-cells was recently discovered that explains many of the discrepancies in the classic Th1/Th2 model, and has been termed "Th17" based on its secretion of the novel pro-inflammatory cytokine IL-17. The identification of Th17 cells as a novel effector T-cell population compels re-examination of periodontitis in the context of the new subset and its signature cytokines. This review aims to offer a clarifying insight into periodontal pathogenesis under the extended Th1/Th2/Th17 paradigm, and is predicated on the principle that periodontal disease activity is determined by a complex interplay between the immune system and periodontal pathogens. The re-examination of existing periodontal literature and further studies in the light of these new discoveries may help explain how the inflammatory response results in damage to the periodontium while generally failing to control the pathogens. This knowledge is essential for the development of immunomodulatory intervention strategies for fine-tuning the host response to maximize the protective and minimize the destructive aspects of the periodontal host response. Moreover, with the advent of anti-cytokine biologic drugs that target the Th1 and Th17 pathways in autoimmunity, the potential consequences to periodontal disease susceptibility in humans need to be understood.

Effect of l-N6-(1-iminoethyl)-lysine, an inducible nitric oxide synthase inhibitor, on murine immune response induced by Actinobacillus actinomycetemcomitans lipopolysaccharide

BACKGROUND AND OBJECTIVES

Inducible nitric oxide synthase (iNOS) activity is known to regulate the immune response. The present study was carried out to determine the effect of L-N6-(1-iminoethyl)-lysine (L-NIL), an iNOS inhibitor, on the induction of immune response to Actinobacillus actinomycetemcomitans lipopolysaccharide in mice.

MATERIAL AND METHODS

BALB/c mice were sham-immunized (group I), immunized with A. actinomycetemcomitans lipopolysaccharide (group II) or treated with L-NIL and immunized with A. actinomycetemcomitans lipopolysaccharide (group III). All animals were then challenged with viable A. actinomycetemcomitans. The levels of serum nitric oxide (NO), specific immunoglobulin G (IgG) isotypes and both interferon-gamma and interleukin-4, as well as spleen cell-derived iNOS activity, before and after bacterial challenge, were assessed. The diameter of skin lesions was also determined. Serum and spleen cells from the above groups were adoptively transferred to the recipients that were then subsequently challenged with live bacteria.

RESULTS

Treatment with L-NIL suppressed serum NO and splenic iNOS activity, but enhanced serum-specific IgG2a antibody and interferon-gamma levels. The lesions in L-NIL-treated mice healed much more rapidly. Transfer with serum and cells from L-NIL-treated and A. actinomycetemcomitans lipopolysaccharide-immunized donors resulted in rapid healing of the lesions in the recipients.

CONCLUSION

It is suggested that treatment with L-NIL in mice immunized with A. actinomycetemcomitans lipopolysaccharide may shift the immune response towards a protective T helper 1-like immunity against A. actinomycetemcomitans-induced infection.