The use of rodent models to investigate host-bacteria interactions related to periodontal diseases

Review of osteoimmunology and the host response in endodontic and periodontal lesions

Both lesions of endodontic origin and periodontal diseases involve the host response to bacteria and the formation of osteolytic lesions. Important for both is the upregulation of inflammatory cytokines that initiate and sustain the inflammatory response. Also important are chemokines that induce recruitment of leukocyte subsets and bone-resorptive factors that are largely produced by recruited inflammatory cells. However, there are differences also. Lesions of endodontic origin pose a particular challenge since that bacteria persist in a protected reservoir that is not readily accessible to the immune defenses. Thus, experiments in which the host response is inhibited in endodontic lesions tend to aggravate the formation of osteolytic lesions. In contrast, bacteria that invade the periodontium appear to be less problematic so that blocking arms of the host response tend to reduce the disease process. Interestingly, both lesions of endodontic origin and periodontitis exhibit inflammation that appears to inhibit bone formation. In periodontitis, the spatial location of the inflammation is likely to be important so that a host response that is restricted to a subepithelial space is associated with gingivitis, while a host response closer to bone is linked to bone resorption and periodontitis. However, the persistence of inflammation is also thought to be important in periodontitis since inflammation present during coupled bone formation may limit the capacity to repair the resorbed bone.

Development of an animal model for Aggregatibacter actinomycetemcomitans biofilm-mediated oral osteolytic infection: a preliminary study

BACKGROUND

Biofilm-induced inflammatory osteolytic oral infections, such as periodontitis and peri-implantitis, have complex etiology and pathogenesis. A significant obstacle to research has been the lack of appropriate animal models where the inflammatory response to biofilms can be investigated. The aim of this study is to develop a novel animal model to study the host response to Aggregatibacter actinomycetemcomitans (previously Actinobacillus actinomycetemcomitans)-biofilm colonizing titanium implants.

METHODS

Titanium implants were inoculated in vitro with A. actinomycetemcomitans, establishing a biofilm for 1 to 3 days. Biofilm-inoculated and control implants were transmucosally placed into rat hard palate or alveolar ridge. Analysis included documentation of clinical inflammation, polymerase chain reaction, and culture detection of A. actinomycetemcomitans and microcomputed tomography quantitation of peri-implant bone volume.

RESULTS

Viable A. actinomycetemcomitans biofilm was successfully established on titanium implants in vitro, detected by confocal laser scanning microscopy. An inflammatory response characterized by clinical inflammation, bleeding, ulceration, hyperplasia, and necrosis was observed around biofilm-inoculated implants. A. actinomycetemcomitans was detected by polymerase chain reaction and culture analysis on 100% of biofilm-inoculated implants for up to 3 weeks and 25% for up to 6 weeks. Microcomputed tomography analysis demonstrated significantly lower bone volume (P <0.05) around biofilm-inoculated implants (29.6% ± 7.6%) compared to non-inoculated implants (50.5% ± 9.6%) after 6 weeks.

CONCLUSIONS

These results describe a novel animal model where A. actinomycetemcomitans biofilm was established in vitro on titanium implants before placement in rat oral cavity, leading to an inflammatory response, osteolysis, and tissue destruction. This model may have potential use for investigation of host responses to biofilm pathogens and antibiofilm therapy.

The C5a receptor impairs IL-12-dependent clearance of Porphyromonas gingivalis and is required for induction of periodontal bone loss

The C5a anaphylatoxin receptor (C5aR; CD88) is activated as part of the complement cascade and exerts important inflammatory, antimicrobial, and regulatory functions, at least in part, via crosstalk with TLRs. However, the periodontal pathogen Porphyromonas gingivalis can control C5aR activation by generating C5a through its own C5 convertase-like enzymatic activity. In this paper, we show that P. gingivalis uses this mechanism to proactively and selectively inhibit TLR2-induced IL-12p70, whereas the same pathogen-instigated C5aR-TLR2 crosstalk upregulates other inflammatory and bone-resorptive cytokines (IL-1β, IL-6, and TNF-α). In vivo, the ability of P. gingivalis to manipulate TLR2 activation via the C5a-C5aR axis allowed it to escape IL-12p70-dependent immune clearance and to cause inflammatory bone loss in a murine model of experimental periodontitis. In the latter regard, C5aR-deficient or TLR2-deficient mice were both resistant to periodontal bone loss, in stark contrast with wild-type control mice, which is consistent with the interdependent interactions of C5aR and TLR2 in P. gingivalis immune evasion and induction of bone-resorptive cytokines. In conclusion, P. gingivalis targets C5aR to promote its adaptive fitness and cause periodontal disease. Given the current availability of safe and effective C5aR antagonists, pharmacological blockade of C5aR could act therapeutically in human periodontitis and reduce associated systemic risks.

Inflammation and uncoupling as mechanisms of periodontal bone loss

Periodontal disease is characterized by both inflammation and bone loss. Advances in research in both these areas have led to a new appreciation of not only each field but also the intimate relationship between inflammation and bone loss. This relationship has resulted in a new field of science called osteoimmunology and provides a context for better understanding the pathogenesis of periodontal disease. In this review, we discuss several aspects of the immuno-inflammatory host response that ultimately results in loss of alveolar bone. A proposal is made that periodontal inflammation not only stimulates osteoclastogenesis but also interferes with the uncoupling of bone formation and bone resorption, consistent with a pathologic process. Furthermore, arguments based on experimental animal models suggest a critical role of the spatial and temporal aspects of inflammation in the periodontium. A review of these findings leads to a new paradigm to help explain more fully the impact of inflammation on alveolar bone in periodontal disease so that it includes the effects of inflammation on uncoupling of bone formation from resorption.

Role of Porphyromonas gingivalis phosphoserine phosphatase enzyme SerB in inflammation, immune response, and induction of alveolar bone resorption in rats

Porphyromonas gingivalis secretes a serine phosphatase enzyme, SerB, upon contact with gingival epithelial cells in vitro. The SerB protein plays a critical role in internalization and survival of the organism in epithelial cells. SerB is also responsible for the inhibition of interleukin-8 (IL-8) secretion from gingival epithelial cells infected with P. gingivalis. This study examined the ability of a P. gingivalis SerB mutant to colonize the oral cavity and induce gingival inflammation, immune responses, and alveolar bone resorption in a rat model of periodontal disease. Both P. gingivalis ATCC 33277 and an isogenic ΔSerB mutant colonized the oral cavities of rats during the 12-week experimental period. Both of the strains induced significant (P < 0.05) systemic levels of immunoglobulin G (IgG) and isotypes IgG1, IgG2a, and IgG2b, indicating the involvement of both T helper type 1 (Th1) and Th2 responses to infection. Both strains induced significantly (P < 0.05) higher levels of alveolar bone resorption in infected rats than in sham-infected control rats. However, horizontal and interproximal alveolar bone resorption induced by the SerB mutant was significantly (P < 0.05) lower than that induced by the parental strain. Rats infected with the ΔSerB mutant exhibited significantly higher levels of apical migration of the junctional epithelium (P < 0.01) and polymorphonuclear neutrophil (PMN) recruitment (P < 0.001) into the gingival tissues than rats infected with the wild type. In conclusion, in a rat model of periodontal disease, the SerB phosphatase of P. gingivalis is required for maximal alveolar bone resorption, and in the absence of SerB, more PMNs are recruited into the gingival tissues.

Destructive and protective roles of cytokines in periodontitis: a re-appraisal from host defense and tissue destruction viewpoints

Periodontal diseases (PD) are chronic infectious inflammatory diseases characterized by the destruction of tooth-supporting structures, being the presence of periodontopathogens required, but not sufficient, for disease development. As a general rule, host inflammatory mediators have been associated with tissue destruction, while anti-inflammatory mediators counteract and attenuate disease progression. With the discovery of several T-cell subsets bearing distinct immunoregulatory properties, this pro- vs. anti-inflammatory scenario became more complex, and a series of studies has hypothesized protective or destructive roles for Th1, Th2, Th17, and Treg subpopulations of polarized lymphocytes. Interestingly, the "protective vs. destructive" archetype is usually considered in a framework related to tissue destruction and disease progression. However, it is important to remember that periodontal diseases are infectious inflammatory conditions, and recent studies have demonstrated that cytokines (TNF-α and IFN-γ) considered harmful in the context of tissue destruction play important roles in the control of periodontal infection. Therefore, in this review, the state-of-the-art knowledge concerning the protective and destructive roles of host inflammatory immune response will be critically evaluated and discussed from the tissue destruction and control-of-infection viewpoints.

Aggregatibacter actinomycetemcomitans-induced bone loss and antibody response in three rat strains

BACKGROUND

The aim of this study is to compare the colonization, immunoglobulin (Ig) G response, and alveolar bone loss in Aggregatibacter actinomycetemcomitans (Aa)-inoculated Fawn Hooded Hypertensive (FHH), Dahl Salt-Sensitive (DSS), and Brown Norway (BN) rats.

METHODS

Each rat strain was divided into wild-type Aa-inoculated and non-inoculated control groups. Blood taken at 12 weeks after inoculation was assessed for Aa-specific IgG antibodies by an enzyme-linked immunosorbent assay. Colonization was assessed 12 weeks postinoculation. Bone loss was estimated by measuring the distance from the cemento-enamel junction (CEJ) to the alveolar bone crest (ABC) at 20 molar sites. Colonization and antibody levels were compared by using the Student t test. Diseased rats were defined as having two sites per quadrant with CEJ-ABC distances that were significantly greater than the control CEJ-ABC distances.

RESULTS

The Aa colonization of FHH rats was significantly higher than in other strains (P <0.05). The Aa-specific IgG levels in the DSS Aa-inoculated group were significantly higher than in its control group (P <0.05). Only FHH rats showed Aa disease-associated bone loss (P = 0.0021).

CONCLUSIONS

Aa colonized and caused more disease in FHH rats than in the other rat strains. The rat strains each responded differently to the same Aa strain.

"Gum bug, leave my heart alone!"--epidemiologic and mechanistic evidence linking periodontal infections and atherosclerosis

Evidence from epidemiologic studies suggests that periodontal infections are independently associated with subclinical and clinical atherosclerotic vascular disease. Although the strength of the reported associations is modest, the consistency of the data across diverse populations and a variety of exposure and outcome variables suggests that the findings are not spurious or attributable only to the effects of confounders. Analysis of limited data from interventional studies suggests that periodontal treatment generally results in favorable effects on subclinical markers of atherosclerosis, although such analysis also indicates considerable heterogeneity in responses. Experimental mechanistic in vitro and in vivo studies have established the plausibility of a link between periodontal infections and atherogenesis, and have identified biological pathways by which these effects may be mediated. However, the utilized models are mostly mono-infections of host cells by a limited number of 'model' periodontal pathogens, and therefore may not adequately portray human periodontitis as a polymicrobial, biofilm-mediated disease. Future research must identify in vivo pathways in humans that may (i) lead to periodontitis-induced atherogenesis, or (ii) result in treatment-induced reduction of atherosclerosis risk. Data from these studies will be essential for determining whether periodontal interventions have a role in the primary or secondary prevention of atherosclerosis.

Too old to fight? Aging and its toll on innate immunity

Elderly individuals display increased susceptibility to chronic inflammatory diseases and microbial infections, such as periodontitis and oral aspiration pneumonia. The resurgent interest in innate immunity in the 2000s has been accompanied by parallel studies to understand the impact of aging on the function of the innate immune system, which not only provides first-line defense but is essential for the development of adaptive immunity. This review summarizes and discusses our current understanding of age-associated molecular alterations in neutrophils and macrophages, key inflammatory phagocytes implicated in both protective and destructive host responses. The analysis of recent literature suggests that, in advanced age, phagocytes undergo significant changes in signal transduction pathways that may affect their ability to perform antimicrobial functions or regulate the inflammatory response. These abnormalities are expected to contribute to the pathology of oral infection-driven inflammatory diseases in the elderly. Moreover, the elucidation of age-associated defects in the innate immune system will facilitate the development of intervention therapeutic strategies to promote or restore innate immune function and improve the quality of health in old age.

Complement and periodontitis

Although the complement system is centrally involved in host defense, its overactivation or deregulation (e.g., due to inherent host genetic defects or due to pathogen subversion) may excessively amplify inflammation and contribute to immunopathology. Periodontitis is an oral infection-driven chronic inflammatory disease which exerts a systemic impact on health. This paper reviews evidence linking complement to periodontal inflammation and pathogenesis. Clinical and histological observations show a correlation between periodontal inflammatory activity and local complement activation. Certain genetic polymorphisms or deficiencies in specific complement components appear to predispose to increased susceptibility to periodontitis. Animal model studies and in vitro experiments indicate that periodontal bacteria can either inhibit or activate distinct components of the complement cascade. Porphyromonas gingivalis, a keystone species in periodontitis, subverts complement receptor 3 and C5a anaphylatoxin receptor signaling in ways that promote its adaptive fitness in the presence of non-productive inflammation. Overall, available evidence suggests that complement activation or subversion contributes to periodontal pathogenesis, although not all complement pathways or functions are necessarily destructive. Effective complement-targeted therapeutic intervention in periodontitis would require determining the precise roles of the various inductive or effector complement pathways. This information is essential as it may reveal which specific pathways need to be blocked to counteract microbial evasion and inflammatory pathology or, conversely, kept intact to promote host immunity.

Too old to fight? Aging and its toll on innate immunity

Elderly individuals display increased susceptibility to chronic inflammatory diseases and microbial infections, such as periodontitis and oral aspiration pneumonia. The resurgent interest in innate immunity in the 2000s has been accompanied by parallel studies to understand the impact of aging on the function of the innate immune system, which not only provides first-line defense but is essential for the development of adaptive immunity. This review summarizes and discusses our current understanding of age-associated molecular alterations in neutrophils and macrophages, key inflammatory phagocytes implicated in both protective and destructive host responses. The analysis of recent literature suggests that, in advanced age, phagocytes undergo significant changes in signal transduction pathways that may affect their ability to perform antimicrobial functions or regulate the inflammatory response. These abnormalities are expected to contribute to the pathology of oral infection-driven inflammatory diseases in the elderly. Moreover, the elucidation of age-associated defects in the innate immune system will facilitate the development of intervention therapeutic strategies to promote or restore innate immune function and improve the quality of health in old age.

Periodontal inflammation and bone loss in aged mice

BACKGROUND AND OBJECTIVE

Young mice do not develop measurable periodontal bone loss, unless heavily infected with human periodontal pathogens. However, mice with a genetically altered immune system are unable to control their own oral flora and develop periodontitis early in life. Based on the potential of the indigenous oral microbiota to cause periodontitis, we hypothesized that normal mice may ultimately develop inflammatory periodontal bone loss, i.e. as a function of age. If confirmed, this could serve as an aging model of chronic periodontitis.

MATERIAL AND METHODS

Periodontal bone levels were measured as the distance from the cementoenamel junction to the alveolar bone crest in young mice (8-10 wk of age), old mice (>or= 18 mo of age) and mice of intermediate ages. Differential expression of inflammatory mediators in the gingivae of young and old mice was determined by quantitative real-time PCR.

RESULTS

In comparison with young mice, old mice displayed significantly (p < 0.05) increased periodontal bone loss, accompanied by elevated expression of proinflammatory cytokines (interleukin-1 beta, tumor necrosis factor alpha and interleukin-17A) and innate immune receptors involved in the induction or amplification of inflammation (Toll-like receptor 2, CD14, CD11b, CD18, complement C5a receptor and triggering receptor expressed on myeloid cells 3).

CONCLUSION

Mice develop naturally induced periodontal bone loss as a function of age. This aging model of periodontitis represents a genuinely chronic model to study mechanisms of periodontal tissue destruction.

Aggregatibacter actinomycetemcomitans leukotoxin: from threat to therapy

Aggregatibacter actinomycetemcomitans is a Gram-negative bacterium that colonizes the human oral cavity and is the causative agent for localized aggressive periodontitis (LAP), an aggressive form of periodontal disease that occurs in adolescents. A. actinomycetemcomitans secretes a protein toxin, leukotoxin (LtxA), which helps the bacterium evade the host immune response during infection. LtxA is a membrane-active toxin that specifically targets white blood cells (WBCs). In this review, we discuss recent developments in this field, including the identification and characterization of genes and proteins involved in secretion, regulation of LtxA, biosynthesis, newly described activities of LtxA, and how LtxA may be used as a therapy for the treatment of diseases.

Differential virulence and innate immune interactions of Type I and II fimbrial genotypes of Porphyromonas gingivalis

INTRODUCTION

The fimA-encoded fimbriae of the periodontal pathogen Porphyromonas gingivalis display genetic diversity. Type I fimbriated P. gingivalis (Pg-I) has been most widely studied at the molecular level, whereas Pg-II is the most frequent isolate from severe periodontitis.

METHODS

To investigate virulence differences between Types I and II fimbriae, we examined strains 33277 (Pg-I) and OMZ314 (Pg-II), reciprocal swap mutants (i.e. expressing the heterologous fimbrial type), and their respective FimA-deficient derivatives. These organisms were tested in a mouse periodontitis model and in interactions with mouse macrophages, a cell type that plays important roles in chronic infections.

RESULTS

Strain 33277 induced significantly more periodontal bone loss than OMZ314 and substitution of Type II fimbriae with Type I in OMZ314 resulted in a more virulent strain than the parent organism. However, the presence of Type II fimbriae was associated with increased proinflammatory and invasive activities in macrophages.

CONCLUSION

The inverse relationship between proinflammatory potential and ability to cause experimental periodontitis may suggest that an aggressive phenotype could provoke a host response that would compromise the persistence of the pathogen.

Pre-clinical models for oral and periodontal reconstructive therapies

The development of new medical formulations (NMF) for reconstructive therapies has considerably improved the available treatment options for individuals requiring periodontal repair or oral implant rehabilitation. Progress in tissue engineering and regenerative medicine modalities strongly depends on validated pre-clinical research. Pre-clinical testing has contributed to the recent approval of NMF such as GEM 21S and INFUSE bone grafts for periodontal and oral regenerative therapies. However, the selection of a suitable pre-clinical model for evaluation of the safety and efficacy of a NMF remains a challenge. This review is designed to serve as a primer to choose the appropriate pre-clinical models for the evaluation of NMF in situations requiring periodontal or oral reconstruction. Here, we summarize commonly used pre-clinical models and provide examples of screening and functional studies of NMF that can be translated into clinical use.

Short-term induction of thrombocytopenia delays periodontal healing in rats with periodontal disease: participation of endostatin and vascular endothelial growth factor

BACKGROUND AND OBJECTIVE

Platelets contain factors, including VEGF and endostatin, that can modulate the healing process. We evaluated the effects of severe thrombocytopenia on periodontal healing in rats and determined the contribution of VEGF and endostatin to the healing process.

MATERIAL AND METHODS

Rats were distributed into three test groups and two control groups. Cotton ligatures were placed at the gingival margin level of the lower first molar in the test groups. Sham-operated rats and rats in one of the periodontitis groups were killed 15 days later. Rats in the remaining two periodontitis groups had the ligatures removed in order to study the spontaneous recovery from the periodontal disease 15 days later, and these rats were treated with rabbit antiplatelet serum, in order to induce thrombocytopenia, or normal rabbit serum. An additional group without ligatures received antiplatet serum in the same period.

RESULTS

After ligature removal, rats treated with normal rabbit serum showed reduced myeloperoxidase activity, decreased alveolar bone loss and increased numbers of blood vessels. Thrombocytopenia caused a delay in alveolar bone regeneration, a decrease in the number of vessels and a modest decrease in myeloperoxidase activity. In the rats with periodontitis, serum endostatin concentrations were slightly decreased and serum VEGF remained unchanged compared with sham-operated animals. After ligature removal, a significant VEGF increase and endostatin decrease were observed in the rats treated with normal rabbit serum. Thrombocytopenia led to a dramatic fall in both VEGF and endostatin concentrations.

CONCLUSION

Thrombocytopenia leads to a delay of periodontal healing in the situation of experimental periodontitis, which might be mediated in part by a decrease in the serum concentration of VEGF and endostatin derived from the platelets. However, other factors derived from the platelets may also have contributed to a delay of periodontal healing in the rats with thrombocytopenia.

Age-related alterations in innate immune receptor expression and ability of macrophages to respond to pathogen challenge in vitro

The impact of ageing in innate immunity is poorly understood. Studies in the mouse model have described altered innate immune functions in aged macrophages, although these were not generally linked to altered expression of receptors or regulatory molecules. Moreover, the influence of ageing in the expression of these molecules has not been systematically examined. We investigated age-dependent expression differences in selected Toll-like and other pattern-recognition receptors, receptors involved in inflammatory amplification, and in transmembrane and intracellular regulators of inflammatory signaling. Young and aged macrophages were examined under resting conditions or upon activation with Porphyromonas gingivalis, a major pathogen in periodontal disease, the prevalence and severity of which increase in old age. We detected a limited number of age-dependent alterations, involving both reduction and increase of immune activity. Interestingly, surface expression of receptors that amplify inflammation (C5a anaphylatoxin receptor and triggering receptor expressed on myeloid cells [TREM]-1) was elevated in aged macrophages. No significant age-dependent differences were observed regarding the phagocytosis and intracellular killing of P. gingivalis, consistent with lack of significant changes in phagocytic receptor expression and induction of antimicrobial molecules. Therefore, at least at the cellular level, certain aspects of innate immune function may not necessarily decline with age.

Macrophage inflammatory protein-1alpha: a salivary biomarker of bone loss in a longitudinal cohort study of children at risk for aggressive periodontal disease?

BACKGROUND

Periodontitis develops in a time-dependent manner. Cross-sectional studies document one moment in time but fail to capture the progressive nature of disease. Radiographic measures of bone loss are relatively insensitive but are reliable markers of irreversible disease. Longitudinal studies are needed to identify biomarkers that can precede radiographic evidence of bone loss and, thus, mark the period prior to clinical evidence of irreversible disease. A longitudinal study of students susceptible to localized aggressive periodontitis (LAgP) was conducted to evaluate chemokines/cytokines found in saliva derived from periodontally healthy children who subsequently developed alveolar bone loss.

METHODS

Students were screened, sampled for Aggregatibacter actinomycetemcomitans (previously Actinobacillus actinomycetemcomitans [Aa]), and divided into a cohort of Aa+ and Aa- students. Ninety-six periodontally healthy Aa+ and Aa- students were recalled every 6 to 9 months following screening. Examinations, saliva collections, and radiographs were performed. After seven students developed bone loss, the levels of 21 cytokines were assessed and matched to saliva from seven Aa+ and seven Aa- students who remained healthy for > or =1 year. Subsequently, saliva from an additional 27 students who remained healthy was analyzed.

RESULTS

Nineteen cytokines were not detected or were detected at low levels. Macrophage inflammatory protein (MIP)-1alpha was elevated 50-fold in seven Aa+ students who developed disease 6 to 9 months prior to radiographic detection of bone loss compared to levels in 21 Aa+ and 20 Aa- students who remained healthy (P <0.001). Interleukin (IL)-1beta was also elevated (P = 0.01). MIP-1alpha had a specificity of 96.8% and a sensitivity of 100%, whereas IL-1beta showed 90.3% specificity and 85.7% sensitivity relative to bone loss. MIP-1alpha levels were also related to increasing probing depth and the number of pockets >6 mm.

CONCLUSION

The superior sensitivity and specificity of MIP-1alpha, which correlated well with probing depths and the onset of bone loss, suggested that it could be used as an early biomarker for LAgP.

An improved cost-effective, reproducible method for evaluation of bone loss in a rodent model

AIM

This study was designed to investigate the utility of two "new" definitions for assessment of bone loss in a rodent model of periodontitis.

MATERIAL AND METHODS

Eighteen rats were divided into three groups. Group 1 was infected by Aggregatibacter actinomycetemcomitans (Aa), group 2 was infected with an Aa leukotoxin knock-out, and group 3 received no Aa (controls). Microbial sampling and antibody titres were determined. Initially, two examiners measured the distance from the cemento-enamel-junction to alveolar bone crest using the three following methods; (1) total area of bone loss by radiograph, (2) linear bone loss by radiograph, (3) a direct visual measurement (DVM) of horizontal bone loss. Two "new" definitions were adopted; (1) any site in infected animals showing bone loss >2 standard deviations above the mean seen at that site in control animals was recorded as bone loss, (2) any animal with two or more sites in any quadrant affected by bone loss was considered as diseased.

RESULTS

Using the "new" definitions both evaluators independently found that infected animals had significantly more disease than controls (DVM system; p<0.05).

CONCLUSIONS

The DVM method provides a simple, cost effective, and reproducible method for studying periodontal disease in rodents.

Involvement of SOCS3 in regulation of CD11c+ dendritic cell-derived osteoclastogenesis and severe alveolar bone loss

To investigate the role of suppressor of cytokine signaling (SOCS) molecules in periodontal immunity and RANKL-mediated dendritic cell (DC)-associated osteoclastogenesis, we analyzed SOCS expression profiles in CD4(+) T cells and the effect of SOCS3 expression in CD11c(+) DCs during periodontal inflammation-induced osteoclastogenesis and bone loss in nonobese diabetic (NOD) versus humanized NOD/SCID mice. Our results of ex vivo and in vitro analyses showed that (i) there is significantly higher SOCS3 expression associated with RANKL(+) T-cell-mediated bone loss in correlation with increased CD11c(+) DC-mediated osteoclastogenesis; (ii) the transfection of CD11c(+) DC using an adenoviral vector carrying a dominant negative SOCS3 gene significantly abrogates TRAP and bone-resorptive activity; and (iii) inflammation-induced TRAP expression, bone resorption, and SOCS3 activity are not associated with any detectable change in the expression levels of TRAF6 and mitogen-activated protein kinase signaling adaptors (i.e., Erk, Jnk, p38, and Akt) in RANKL(+) T cells. We conclude that SOCS3 plays a critical role in modulating cytokine signaling involved in RANKL-mediated DC-derived osteoclastogenesis during immune interactions with T cells and diabetes-associated severe inflammation-induced alveolar bone loss. Therefore, the development of SOCS3 inhibitors may have therapeutic potential as the target to halt inflammation-induced bone loss under pathological conditions in vivo.

Activation of the acquired immune response reduces coupled bone formation in response to a periodontal pathogen

Osteoimmunolgy involves the interaction of the immune system with skeletal elements. This interaction can lead to the formation of osseous lesions. To investigate how the acquired immune response could contribute to osteolytic lesions, we injected the periodontal pathogen Porphyromonas gingivalis adjacent to calvarial bone with or without prior immunization against the bacterium. Activation of the acquired immune response increased osteoclastogenesis and decreased coupled bone formation. The latter was accompanied by an increase in nuclear translocation of the transcription factor FOXO1 in vivo, increased apoptosis of bone-lining cells measured by the TUNEL assay and number of activated caspase-3 positive cells and a decrease in bone lining cell density. Further studies were conducted with MC3T3 osteoblastic cells. Apoptosis and increased FOXO1 DNA binding activity were induced when a combination of cytokines was tested, IL-beta, TNF-alpha, and IFN-gamma. Knockdown of FOXO1 by small interfering RNA significantly reduced cytokine stimulated apoptosis, cleaved caspase-3/7 activity and decreased mRNA levels of the proapoptotic genes, TNF-alpha, FADD, and caspase-3, -8, and -9. These results indicate that activation of the acquired immunity by a periodontal pathogen reduces the coupling of bone formation and resorption. This may occur by enhancing bone lining cell apoptosis through a mechanism that involves increased FOXO1 activation. These studies give insight into inflammatory bone diseases such as periodontal disease and arthritis were the formation of lytic lesions occurs in conjunction with deficient bone formation and activation of an acquired immune response.

Toll gates to periodontal host modulation and vaccine therapy

Toll-like receptors (TLRs) are central mediators of innate antimicrobial and inflammatory responses and play instructive roles in the development of the adaptive immune response. Thus when stimulated by certain agonists, TLRs serve as adjuvant receptors that link innate and adaptive immunity. However, when excessively activated or inadequately controlled during an infection, TLRs may contribute to immunopathology associated with inflammatory diseases, such as periodontitis. Moreover, certain microbial pathogens appear to exploit aspects of TLR signalling in ways that enhance their adaptive fitness. The diverse and important roles played by TLRs suggest that therapeutic manipulation of TLR signalling may have implications in the control of infection, attenuation of inflammation, and the development of vaccine adjuvants for the treatment of periodontitis. Successful application of TLR-based therapeutic modalities in periodontitis would require highly selective and precisely targeted intervention. This would in turn necessitate precise characterization of TLR signalling pathways in response to periodontal pathogens, as well as development of effective and specific agonists or antagonists of TLR function and signalling. This review summarizes the current status of TLR biology as it relates to periodontitis, and evaluates the potential of TLR-based approaches for host-modulation therapy in this oral disease.

Modeling of trabecular bone and lamina dura following selective alveolar decortication in rats

BACKGROUND

Modifying the balance between resorption and apposition through selectively injuring the cortical plate of the alveolus has been an approach to speed tooth movement and is referred to as periodontally accelerated osteogenic orthodontics. The aim of this study was to investigate the alveolar response to corticotomy as a function of time and proximity to the surgical injury in a rat model.

METHODS

Maxillary buccal and lingual cortical plates were injured in 36 healthy adult rats adjacent to the upper left first molars. Twenty-four animals were euthanized at 3, 7, or 11 weeks. In one group, the maxillae were removed and stripped of soft tissues, and histomorphometric analysis was performed to study alveolar spongiosa and periodontal ligament (PDL) modeling dynamics. Catabolic activity was analyzed with tartrate-resistant acid phosphatase-positive osteoclasts and preosteoclasts. Anabolic actions were measured using a fluorescent vital bone stain series followed by sacrifice at 30 and 51 days. To further analyze the new bone formation, a separate group of animals were fed with calcein fluorescent stain and processed for non-decalcified fluorescent stain histology.

RESULTS

At 3 weeks, the surgery group had significantly (P <0.05) less calcified spongiosa bone surface, greater periodontal ligament surface, higher osteoclast number, and greater lamina dura apposition width. The catabolic activity (osteoclast count) and anabolic activity (apposition rate) were three-fold greater, calcified spongiosa decreased by two-fold, and PDL surface increased by two-fold. Surgical injury to the alveolus that induced a significant increase in tissue turnover by week 3 dissipated to a steady state by postoperative week 11. The impact of the injury was localized to the area immediately adjacent to the decortication injury.

CONCLUSION

Selective alveolar decortication induced increased turnover of alveolar spongiosa, and the activity was localized; dramatic escalation of demineralization-remineralization dynamics is the likely biologic mechanism underlying rapid tooth movement following selective alveolar decortication.

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.

The interleukin-17 receptor plays a gender-dependent role in host protection against Porphyromonas gingivalis-induced periodontal bone loss

Interleukin-17 (IL-17) is a proinflammatory cytokine secreted by the newly described CD4(+) Th17 subset, which is distinct from classic Th1 and Th2 lineages. IL-17 contributes to bone destruction in rheumatoid arthritis but is essential in host defense against pathogens that are susceptible to neutrophils. Periodontal disease (PD) is a chronic inflammatory condition initiated by anaerobic oral pathogens such as Porphyromonas gingivalis, and it is characterized by host-mediated alveolar bone destruction due primarily to the immune response. The role of IL-17 in PD is controversial. Whereas elevated IL-17 levels have been found in humans with severe PD, we recently reported that female C57BL/6J mice lacking the IL-17 receptor (IL-17RA(KO)) are significantly more susceptible to PD bone loss due to defects in the chemokine-neutrophil axis (J. J. Yu, M. J. Ruddy, G. C. Wong, C. Sfintescu, P. J. Baker, J. B. Smith, R. T. Evans, and S. L. Gaffen, Blood 109:3794-3802, 2007). Since different mouse strains exhibit differences in susceptibility to PD as well as Th1/Th2 cell skewing, we crossed the IL-17RA gene knockout onto the BALB/c background and observed a similar enhancement in alveolar bone loss following P. gingivalis infection. Unexpectedly, in both strains IL-17RA(KO) female mice were much more susceptible to PD bone loss than males. Moreover, female BALB/c-IL-17RA(KO) mice were defective in producing anti-P. gingivalis immunoglobulin G and the chemokines KC/Groalpha and MIP-2. In contrast, male mice produced normal levels of chemokines and anti-P. gingivalis antibodies, but they were defective in granulocyte colony-stimulating factor upregulation. This study demonstrates a gender-dependent effect of IL-17 signaling and indicates that gender differences should be taken into account in the preclinical and clinical safety testing of anti-IL-17 biologic therapies.