Influence of light-polymerizing units and zirconia on the physical, chemical and biological properties of self-adhesive resin cements

BACKGROUND

Self-adhesive resin cements (SARCs) are widely used for fixed prostheses, but incomplete cleaning near the gingival margin can cause inflammation. However, the factors influencing cement properties and the biological response of gingival fibroblasts to cement eluates are not well understood. This study examines the impact of two light-polymerizing units (LPUs) on the physical and chemical properties of two SARCs under simulated clinical conditions, as well as the subsequent response of human gingival fibroblasts (hGFs) to these eluates.

METHODS

Dental cement discs of SARCs were polymerized using Kerr DemiPlus and 3 M Elipar DeepCure-S LED LPUs with or without a 2-mm thick zirconia screen. Physical properties (microhardness, surface roughness, residual monomers) were evaluated. hGFs' cell viability, wound healing potency, and gene expression were assessed.

RESULTS

Both Maxcem and RelyX exhibited reduced microhardness and increased surface roughness when polymerized through zirconia or with DemiPlus LPU. Higher residual monomers (HEMA and GDMA in Maxcem; TEGDMA in RelyX) concentration was observed with DemiPlus and zirconia polymerization. Maxcem polymerized with DemiPlus exhibited lower cell viability, impaired healing, and altered gene expression in hGFs compared to those polymerized with Elipar LPU. Gene expression changes included downregulated NRF2 and HO-1 and upregulated CCR-3.

CONCLUSIONS

Light-polymerizing Maxcem through zirconia with DemiPlus LPU compromised SARCs' properties, leading to higher residual monomers and negatively impacting hGFs' viability, healing, and gene expression. Careful material selection and polymerization techniques are crucial to minimize adverse effects on surrounding tissues.

CLINICAL SIGNIFICANCE

Clinicians should exercise caution when using LPUs and SARCs, especially when polymerizing through zirconia. This will help optimize the physical and chemical properties of SARCs and minimize potential adverse effects on the surrounding gingival soft tissues.

Mitochondrial homeostasis in odontoblast: Physiology, pathogenesis and targeting strategies

Caries and pulpitis remain a major global disease burden and affect the quality of life of patients. Odontoblasts are key players in the progression of caries and pulpitis, not only secreting and mineralizing to form dentin, but also acting as a wall of defense to initiate immune defenses. Mitochondrion is an information processor for numerous cellular activities, and dysregulation of mitochondrion homeostasis not only affects cellular metabolism but also triggers a wide range of diseases. Elucidating mitochondrial homeostasis in odontoblasts can help deepen scholars' understanding of odontoblast-associated diseases. Articles on mitochondrial homeostasis in odontoblasts were evaluated for information pertinent to include in this narrative review. This narrative review focused on understanding the complex interplay between mitochondrial homeostasis in odontoblasts under physiological and pathological conditions. Furthermore, mitochondria-centered therapeutic strategies (including mitochondrial base editing, targeting platforms, and mitochondrial transplantation) were emphasized by resolving key genes that regulate mitochondrial function. Mitochondria are involved in odontoblast differentiation and function, and act as mitochondrial danger-associated molecular patterns (mtDAMPs) to mediate odontoblast pathological progression. Novel mitochondria-centered therapeutic strategies are particularly attractive as emerging therapeutic approaches for the maintenance of mitochondrial homeostasis. It is expected to probe key events of odontoblast differentiation and advance the clinical resolution of dentin formation and mineralization disorders and odontoblast-related diseases.

TRPA1 up-regulation mediates oxidative stress in a pulpitis model in vitro

BACKGROUND AND PURPOSE

Pulpitis is associated with tooth hypersensitivity and results in pulpal damage. Thermosensitive transient receptor potential (TRP) ion channels expressed in the dental pulp may be key transducers of inflammation and nociception. We aimed at investigating the expression and role of thermo-TRPs in primary human dental pulp cells (hDPCs) in normal and inflammatory conditions.

EXPERIMENTAL APPROACH

Inflammatory conditions were induced in hDPC cultures by applying polyinosinic:polycytidylic acid (poly(I:C)). Gene expression and pro-inflammatory cytokine release were measured by RT-qPCR and ELISA. Functions of TRPA1 channels were investigated by monitoring changes in intracellular Ca2+ concentration. Mitochondrial superoxide production was measured using a fluorescent substrate. Cellular viability was assessed by measuring the activity of mitochondrial dehydrogenases and cytoplasmic esterases. TRPA1 activity was modified by agonists, antagonists, and gene silencing.

KEY RESULTS

Transcripts of TRPV1, TRPV2, TRPV4, TRPC5, and TRPA1 were highly expressed in control hDPCs, whereas TRPV3, TRPM2, and TRPM3 expressions were much lower, and TRPM8 was not detected. Poly(I:C) markedly up-regulated TRPA1 but not other thermo-TRPs. TRPA1 agonist-induced Ca2+ signals were highly potentiated in inflammatory conditions. Poly(I:C)-treated cells displayed increased Ca2+ responses to H2O2, which was abolished by TRPA1 antagonists. Inflammatory conditions induced oxidative stress, stimulated mitochondrial superoxide production, resulted in mitochondrial damage, and decreased cellular viability of hDPCs. This inflammatory cellular damage was partly prevented by the co-application of TRPA1 antagonist or TRPA1 silencing.

CONCLUSION AND IMPLICATIONS

Pharmacological blockade of TRPA1 channels may be a promising therapeutic approach to alleviate pulpitis and inflammation-associated pulpal damage.

Effects of Synthetic Toll-Like Receptor 9 Ligand Molecules on Pulpal Immunomodulatory Response and Repair after Injuries

Synthetic oligodeoxynucleotides (ODNs) containing unmethylated cytosine-phosphate-guanine (CpG) motifs (CpG-ODNs) are ligand molecules for Toll-like receptor 9 (TLR9), which is expressed by odontoblasts in vitro and dental pulp cells. This study determined the effects of CpG-ODNs on pulpal immunomodulatory response and repair following injury. Briefly, the upper right first molars of three-week-old mice were extracted, immersed in Type A (D35) or B (K3) CpG-ODN solutions (0.1 or 0.8 mM) for 30 min, and then replanted. Pulpal healing and immunomodulatory activity were assessed by hematoxylin-eosin and AZAN staining, as well as immunohistochemistry. One week following the operation, inflammatory reactions occurred in all of the experimental groups; however, re-revascularization and newly formed hard tissue deposition were observed in the pulp chamber of all groups at week 2. A positive trend in the expression of immune cell markers was observed toward the CpG-ODN groups at 0.1 mM. Our data suggest that synthetic CpG-ODN solutions at low concentrations may evoke a long-lasting macrophage-TLR9-mediated pro-inflammatory, rather than anti-inflammatory, response in the dental pulp to modulate the repair process and hard tissue formation. Further studies are needed to determine the effects of current immunomodulatory agents in vitro and in vivo and develop treatment strategies for dental tissue regeneration.

Immune System of Dental Pulp in Inflamed and Normal Tissue

Teeth are vulnerable to structural compromise, primarily attributed to carious lesions, in which microorganisms originating from the oral cavity deteriorate the mineralized structures of enamel and dentin, subsequently infiltrating the underlying soft connective tissue, known as the dental pulp. Nonetheless, dental pulp possesses the necessary capabilities to detect and defend against bacteria and their by-products, using a variety of intricate defense mechanisms. The pulp houses specialized cells known as odontoblasts, which encounter harmful substances produced by oral bacteria. These cells identify pathogens at an early stage and commence the immune system response. As bacteria approach the pulp, various cell types within the pulp, such as different immune cells, stem cells, fibroblasts, as well as neuronal and vascular networks, contribute a range of defense mechanisms. Therefore, the immune system is present in the healthy pulp to restrain the initial spread of pathogens, and then in the inflamed pulp, it prepares the conditions for necrosis or regeneration, so inflammatory response mechanisms play a critical role in maintaining tissue homeostasis. This review aims to consolidate the existing literature on the immune system in dental pulp, encompassing current knowledge on this topic that explains the diverse mechanisms of recognition and defense against pathogens exhibited by dental pulp cells, elucidates the mechanisms of innate and adaptive immunity in inflamed pulp, and highlights the difference between inflamed and normal pulp tissue.

Histological response of inflamed pulp to hydraulic calcium silicate cements in direct pulp capping: Systematic review of pulpitis models

This systematic review aimed to compare the histological response of inflamed pulpodentinal complex to the hydraulic calcium silicate cements in experimental animal models of pulpitis. Articles that evaluated the histological response of inflamed pulp to mineral trioxide aggregate (MTA) in comparison with other restorative materials were selected and analysed in detail. The risk of bias assessment was conducted using SYRCLE's RoB tool. The GRADEpro tool was used to determine the overall quality of evidence. Out of the 2947 retrieved articles from databases, five articles fulfilled the inclusion criteria. MTA induced significantly more hard tissue formation compared to calcium hydroxide. The use of pulp-capping material containing fluocinolone acetonide and ASP/PLGA-ASP/ACP/PLLA-PLGA composite membrane was comparable. This systematic review could not demonstrate enhanced efficiency of capping materials compared to MTA. Future well-conducted animal studies are warranted for demonstrating the hard tissue formation abilities of pulp-capping materials with convenient inflammatory conditions.

An in vitro coculture approach to study the interplay between dental pulp cells and Streptococcus mutans

AIM

To develop a new coculture system that allows exposure of dental pulp cells (DPCs) to Streptococcus mutans and dentine matrix proteins (eDMP) to study cellular interactions in dentine caries.

METHODOLOGY

Dental pulp cells and S. mutans were cocultured with or without eDMP for 72 h. Cell proliferation and viability were assessed by cell counting and MTT assays, while bacterial growth and viability were determined by CFU and LIVE/DEAD staining. Glucose catabolism and lactate excretion were measured photometrically as metabolic indicators. To evaluate the inflammatory response, the release of cytokines and growth factors (IL-6, IL-8, TGF-β1, VEGF) was determined by ELISA. Non-parametric statistical analyses were performed to compare all groups and time points (Mann-Whitney U test or Kruskal-Wallis test; α = .05).

RESULTS

While eDMP and especially S. mutans reduced the number and viability of DPCs (p ≤ .0462), neither DPCs nor eDMP affected the growth and viability of S. mutans during coculture (p > .0546). The growth of S. mutans followed a common curve, but the death phase was not reached within 72 h. S. mutans consumed medium glucose in only 30 h, whereas in the absence of S. mutans, cells were able to catabolize glucose throughout 72 h, resulting in the corresponding amount of l-lactate. No change in medium pH was observed. S. mutans induced IL-6 production in DPCs (p ≤ .0011), whereas eDMP had no discernible effect (p > .7509). No significant changes in IL-8 were observed (p > .198). TGF-β1, available from eDMP supplementation, was reduced by DPCs over time. VEGF, on the other hand, was increased in all groups during coculture.

CONCLUSIONS

The results show that the coculture of DPCs and S. mutans is possible without functional impairment. The bacterially induced stimulation of proinflammatory and regenerative cytokines provides a basis for future investigations and the elucidation of molecular biological relationships in pulp defence against caries.

Approaches to vital pulp therapies

Tooth decay, which leads to pulpal inflammation due to the pulp's response to bacterial components and byproducts is the most common infectious disease. The main goals of clinical management are to eliminate sources of infection, to facilitate healing by regulating inflammation indental tissue, and to replace lost tissues. A variety of novel approaches from tissue engineering based on stem cells, bioactive molecules, and extracellular matrix-like scaffold structures to therapeutic applications, or a combination of all these are present in the literature. Shortcomings of existing conventional materials for pulp capping and the novel approches aiming to preserve pulp vitality highligted the need for developing new targeted dental materials. This review looks at the novel approches for vital pulp treatments after briefly addresing the conventional vital pulp treatment as well as the regenerative and self defense capabilities of the pulp. A narrative review focusing on the current and future approaches for pulp preservation was performed after surveying the relevant papers on vital pulp therapies including pulp capping, pulpotomy, and potential approaches for facilitating dentin-pulp complex regeneration in PubMed, Medline, and Scopus databases.

IPO7 promotes lipopolysaccharide-induced inflammatory responses in human dental pulp cells via p38 MAPK and NF-κB signaling pathways

Pulpitis is a chronic inflammatory process that greatly affects the physical, mental health and life quality of patients. Human dental pulp cells (hDPCs) are essential components of dental pulp tissue and play a significant role in pulpitis. Lipopolysaccharide (LPS) is an initiator of pulpitis and can induce the production of inflammatory cytokines in hDPCs by activating p38 MAPK and NF-κB signaling pathways. Importin7 (IPO7), a member of the importin-β family, is widely expressed in many tissues. Previous studies have shown that IPO7 mediated nuclear translocation of p-p38 after stimulation, and IPO7 homologous protein IPO8 participated in human dental pulp inflammation. This research aims to investigate whether IPO7 is involved in pulpitis and explore its underlying mechanisms. In the current study, we found the expression of IPO7 was increased in pulpitis tissue. In vitro, hDPCs treated with LPS to mimic the inflammatory environment, the expression of IPO7 was increased. Knockdown of IPO7 significantly inhibited the production of inflammatory cytokines and suppressed the p38 MAPK and NF-κB signaling pathways. Activating the p38 MAPK and NF-κB signaling pathways by the p38 activator and p65 activator reversed the inflammatory responses. IPO7 interacted with p-p38 under LPS stimulation in hDPCs. In addition, the increased binding between IPO7 and p-p38 is associated with the decreased binding ability of IPO7 to Sirt2. In conclusion, we found that IPO7 was highly expressed in pulpitis and played a vital role in modulating human dental pulp inflammation.

Expression of Toll-like Receptors in Stem Cells of the Apical Papilla and Its Implication for Regenerative Endodontics

Regenerative therapies to replace cells and tissues damaged due to trauma and dental infections require temporal and spatial controlled recruitment and the differentiation of progenitor/stem cells. However, increasing evidence shows microbial antigens can interfere with this process. Toll-like receptors (TLRs) are crucial in recognizing pathogen-associated molecular patterns. Stem cells of the apical papilla (SCAP) are required for normal dental development and are intimately involved in the reparative and regenerative capacity of developing teeth. We hypothesized that TLRs are expressed in SCAP and that the activation of TLR2/TLR4 or TLR3 by different ligands results in differential cellular fate, impacting their differentiation into a mineralizing phenotype. We found that most TLRs are expressed as detected by PCR except TLR7 and TLR8; exposure to heat-killed E. coli results in upregulating TLR2 and TLR4 and reducing mineralization capacity. In addition, bacterial exposure resulted in the upregulation of 11 genes, of which 9 were chemokines whose proteins were also upregulated and released, promoting in vitro macrophage migration. On the other hand, TLR3 activation resulted in increased proliferation and a dramatic inhibition of osteogenic and odontoblastic differentiation, which was reversed by inhibition or the knockdown of TLR3 expression. The profound effects of TLR activation resulting in different cell fates that are ligand and receptor-specific warrants further evaluation and represents an important therapeutic target to make regenerative approaches more predictable following dental infections.

Root Canal Infection and Its Impact on the Oral Cavity Microenvironment in the Context of Immune System Disorders in Selected Diseases: A Narrative Review

The oral cavity has a specific microenvironment, and structures such as teeth are constantly exposed to chemical and biological factors. Although the structure of the teeth is permanent, due to exposure of the pulp and root canal system, trauma can have severe consequences and cause the development of local inflammation caused by external and opportunistic pathogens. Long-term inflammation can affect not only the local pulp and periodontal tissues but also the functioning of the immune system, which can trigger a systemic reaction. This literature review presents the current knowledge on root canal infections and their impact on the oral microenvironment in the context of immune system disorders in selected diseases. The result of the analysis of the literature is the statement that periodontal-disease-caused inflammation in the oral cavity may affect the development and progression of autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, or Sjogren's syndrome, as well as affecting the faster progression of conditions in which inflammation occurs such as, among others, chronic kidney disease or inflammatory bowel disease.

Potentiality and Inflammatory Marker Expression Are Maintained in Dental Pulp Cell Cultures from Carious Teeth

OBJECTIVES

This investigation aimed to isolate and culture human dental pulp cells from carious teeth (cHDPCs) and compare their growth characteristics, colony-forming efficiency, mineralization potential and gene expression of Toll-like receptors (TLR)-2, TLR-4, TLR-9, tumour necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, IL-8, IL-17A, 1L-17R, IL-23A, nuclear factor-kappa B (NF-κB), mitogen-activated protein kinase (MAPK1), dentin matrix protein (DMP)-1, dentin sialophospho protein (DSPP), sex determining region Y-box 2 (SOX2) and marker of proliferation Ki-67 (MKi67) with cells isolated from healthy or non-carious teeth (ncHDPCs).

METHODS

Pulp tissues were obtained from both healthy and carious teeth (n = 5, each) to generate primary cell lines using the explant culture technique. Cell cultures studies were undertaken by generating growth curves, a colony forming unit and a mineralization assay analysis. The expression of vimentin was assessed using immunocytochemistry (ICC), and the gene expression of above-mentioned genes was determined using quantitative real-time reverse-transcription polymerase chain reaction.

RESULTS

ncHDPCs and cHDPCs were successfully isolated and cultured from healthy and inflamed human dental pulp tissue. At passage 4, both HDPC types demonstrated a typical spindle morphology with positive vimentin expression. No statistical difference was observed between ncHDPCs and cHDPCs in their growth characteristics or ability to differentiate into a mineralizing phenotype. ncHDPCs showed a statistically significant higher colony forming efficiency than cHDPCs. The gene expression levels of TLR-2, TLR-4, TLR-9, TNF-α, IL-6, IL-8, IL-17R, IL-23A, NF-κB, MAPK1, DMP1, DSPP and SOX2 were significantly higher in cHDPCs compared with ncHDPC cultures.

CONCLUSION

cHDPCs retain their differentiation potential and inflammatory phenotype in vitro. The inflamed tooth pulp contains viable stem/progenitor cell populations which have the potential for expansion, proliferation and differentiation into a mineralizing lineage, similar to cells obtained from healthy pulp tissue. These findings have positive implications for regenerative endodontic procedures.

Influence of HEMA on LPS- and LTA-stimulated IL-6 release from human dental pulp cells

OBJECTIVE

Dental pulp cells interact with immunogenic components such as LPS (lipopolysaccharide) or LTA (lipoteichoic acid) released from microorganisms in carious lesions. In the present investigation, the formation of the pro-inflammatory cytokines TNFα and IL-6 in LPS- or LTA-stimulated cells from the dental pulp interface and pulp fibroblasts was analyzed in the presence of the resin monomer 2-hydroxyethyl methacrylate (HEMA) under varying cellular redox conditions.

METHOD

Human pulp fibroblasts (HPC) or cells from the dental pulp interface expressing an odontoblast phenotype (hOD-1) were exposed to LTA, LPS or HEMA for 1 h or 24 h. Redox homeostasis was modified by the prooxidant BSO (L-buthionine sulfoximine) or the antioxidant NAC (N-acetyl cysteine). Formation of TNFα or IL-6 was analyzed by ELISA, and cell survival was determined by a crystal violet assay. Statistical analyses were performed using the Mann-Whitney-U-test.

RESULTS

Secretion of TNFα was not detected in LPS- or LTA-stimulated HPC or hOD-1, and IL-6 was not found after a short exposure (1 h). After a 24 h exposure, LPS induced a 3-fold increase in IL-6 formation in HPC, while LTA stimulated IL-6 release about 20-fold. Likewise, LTA was more effective than LPS in hOD-1 stimulating IL-6 levels about 50-fold. HEMA inhibited the LPS- and LTA-induced IL-6 release, and this effect was enhanced by BSO but counteracted by NAC in both cell types. IL-6 release was independent of cell survival rates.

CONCLUSIONS

The protective immune response in odontoblasts and pulp fibroblasts is impaired by monomers such as HEMA through the disturbance of the redox homeostasis.

Aberrant NF-κB activation in odontoblasts orchestrates inflammatory matrix degradation and mineral resorption

Inflammation-associated proteinase functions are key determinants of inflammatory stromal tissues deconstruction. As a specialized inflammatory pathological process, dental internal resorption (IR) includes both soft and hard tissues deconstruction within the dentin-pulp complex, which has been one of the main reasons for inflammatory tooth loss. Mechanisms of inflammatory matrix degradation and tissue resorption in IR are largely unclear. In this study, we used a combination of Cre-loxP reporter, flow cytometry, cell transplantation, and enzyme activities assay to mechanistically investigate the role of regenerative cells, odontoblasts (ODs), in inflammatory mineral resorption and matrices degradation. We report that inflamed ODs have strong capabilities of matrix degradation and tissue resorption. Traditionally, ODs are regarded as hard-tissue regenerative cells; however, our data unexpectedly present ODs as a crucial population that participates in IR-associated tissue deconstruction. Specifically, we uncovered that nuclear factor-kappa b (NF-κB) signaling orchestrated Tumor necrosis factor α (TNF-α)-induced matrix metalloproteinases (Mmps) and Cathepsin K (Ctsk) functions in ODs to enhance matrix degradation and tissue resorption. Furthermore, TNF-α increases Rankl/Opg ratio in ODs via NF-κB signaling by impairing Opg expression but increasing Rankl level, which utterly makes ODs cell line 17IIA11 (A11) become Trap⁺ and Ctsk⁺ multinucleated cells to perform resorptive actions. Blocking of NF-κB signaling significantly rescues matrix degradation and resorptive functions of inflamed ODs via repressing vital inflammatory proteinases Mmps and Ctsk. Utterly, via utilizing NF-κB specific small molecule inhibitors we satisfactorily attenuated inflammatory ODs-associated human dental IR in vivo. Our data reveal the underlying mechanisms of inflammatory matrix degradation and resorption via proteinase activities in IR-related pathological conditions.

A critical analysis of research methods and experimental models to study pulpitis

Pulpitis is the inflammatory response of the dental pulp to a tooth insult, whether it is microbial, chemical, or physical in origin. It is traditionally referred to as reversible or irreversible, a classification for therapeutic purposes that determines the capability of the pulp to heal. Recently, new knowledge about dental pulp physiopathology led to orientate therapeutics towards more frequent preservation of pulp vitality. However, full adoption of these vital pulp therapies by dental practitioners will be achieved only following better understanding of cell and tissue mechanisms involved in pulpitis. The current narrative review aimed to discuss the contribution of the most significant experimental models developed to study pulpitis. Traditionally, in vitro two(2D)- or three(3D)-dimensional cell cultures or in vivo animal models were used to analyse the pulp response to pulpitis inducers at cell, tissue or organ level. In vitro 2D cell cultures were mainly used to decipher the specific roles of key actors of pulp inflammation such as bacterial by-products, pro-inflammatory cytokines, odontoblasts or pulp stem cells. However, these simple models did not reproduce the 3D organisation of the pulp tissue and, with rare exceptions, did not consider interactions between resident cell types. In vitro tissue/organ-based models were developed to better reflect the complexity of the pulp structure. Their major disadvantage is that they did not allow the analysis of blood supply and innervation participation. On the contrary, in vivo models have allowed researchers to identify key immune, vascular and nervous actors of pulpitis and to understand their function and interplay in the inflamed pulp. However, inflammation was mainly induced by iatrogenic dentine drilling associated with simple pulp exposure to the oral environment or stimulation by individual bacterial by-products for short periods. Clearly, these models did not reflect the long and progressive development of dental caries. Lastly, the substantial diversity of the existing models makes experimental data extrapolation to the clinical situation complicated. Therefore, improvement in the design and standardization of future models, for example by using novel molecular biomarkers, databased models and artificial intelligence, will be an essential step in building an incremental knowledge of pulpitis in the future.

Oral Cavity as a Source of Mesenchymal Stem Cells Useful for Regenerative Medicine in Dentistry

The use of mesenchymal stem cells (MSCs) for regenerative purposes has become common in a large variety of diseases. In the dental and maxillofacial field, there are emerging clinical needs that could benefit from MSC-based therapeutic approaches. Even though MSCs can be isolated from different tissues, such as bone marrow, adipose tissue, etc., and are known for their multilineage differentiation, their different anatomical origin can affect the capability to differentiate into a specific tissue. For instance, MSCs isolated from the oral cavity might be more effective than adipose-derived stem cells (ASCs) for the treatment of dental defects. Indeed, in the oral cavity, there are different sources of MSCs that have been individually proposed as promising candidates for tissue engineering protocols. The therapeutic strategy based on MSCs can be direct, by using cells as components of the tissue to be regenerated, or indirect, aimed at delivering local growth factors, cytokines, and chemokines produced by the MSCs. Here, the authors outline the major sources of mesenchymal stem cells attainable from the oral cavity and discuss their possible usage in some of the most compelling therapeutic frontiers, such as periodontal disease and dental pulp regeneration.

Review of Animal Models to Study Pulp Inflammation

Human dental pulp is a highly dynamic tissue equipped with a network of resident immunocompetent cells that play a major role in the defense against pathogens and during tissue injury. Animal studies are mandatory and complementary to in vitro experiments when studying the physiopathology of dental pulp, new diagnostic tools, or innovative therapeutic strategies. This animal approach makes it possible to define a benefit-risk ratio necessary to be subsequently tested in humans. Among the animal kingdom, rodents, rabbits, ferrets, swine, dogs, and non-human primates have been used to model human pulpitis. The diversity of animals found in studies indicate the difficulty of choosing the correct and most efficient model. Each animal model has its own characteristics that may be advantageous or limiting, according to the studied parameters. These elements have to be considered in preclinical studies. This article aims to provide a thorough understanding of the different animal models used to study pulp inflammation. This may help to find the most pertinent or appropriate animal model depending on the hypothesis investigated and the expected results.

EZH2 Promotes Extracellular Matrix Degradation via Nuclear Factor-κB (NF-κB) and p38 Signaling Pathways in Pulpitis

Pulpitis is a complicated chronic inflammatory process which can be in a dynamic balance between damage and repair. The extracellular matrix plays an important regulatory role in wound healing and tissue repair. The aim of this study was to explore the role of the epigenetic mark, enhancer of zeste homolog 2 (EZH2) on the degradation of extracellular matrix during pulpitis. Quantitative polymerase chain reaction was used to assess the expression of matrix metalloproteinases (MMPs) and type I collagen in human dental pulp cells (HDPCs) upon EZH2 and EI1 (EZH2 inhibitor) stimulation. The mechanism of EZH2 affecting extracellular matrix was explored through quantitative polymerase chain reaction and Western blot. A rat model of dental pulp inflammation was established, and the expression of type I collagen in dental pulp under EZH2 stimulation was detected by immunohistochemical staining. EZH2 upregulated the expression of MMP-1, MMP-3, MMP-8, and MMP-10 and decreased the production of type I collagen in HDPCs, while EI1 had the opposite effect. EZH2 activated the nuclear factor-kappa B (NF-κB) and p38 signaling pathways in HDPCs, the inhibition of which reversed the induction of MMPs and the suppression of type I collagen. EZH2 can downregulate the type I collagen levels in an experimental model of dental pulpitis in rats. EZH2 promotes extracellular matrix degradation via nuclear factor-κB (NF-κB) and P38 signaling pathways in pulpitis. EZH2 can decrease the type I collagen levels in vivo and in vitro.

Inflammatory Response Mechanisms of the Dentine-Pulp Complex and the Periapical Tissues

The macroscopic and microscopic anatomy of the oral cavity is complex and unique in the human body. Soft-tissue structures are in close interaction with mineralized bone, but also dentine, cementum and enamel of our teeth. These are exposed to intense mechanical and chemical stress as well as to dense microbiologic colonization. Teeth are susceptible to damage, most commonly to caries, where microorganisms from the oral cavity degrade the mineralized tissues of enamel and dentine and invade the soft connective tissue at the core, the dental pulp. However, the pulp is well-equipped to sense and fend off bacteria and their products and mounts various and intricate defense mechanisms. The front rank is formed by a layer of odontoblasts, which line the pulp chamber towards the dentine. These highly specialized cells not only form mineralized tissue but exert important functions as barrier cells. They recognize pathogens early in the process, secrete antibacterial compounds and neutralize bacterial toxins, initiate the immune response and alert other key players of the host defense. As bacteria get closer to the pulp, additional cell types of the pulp, including fibroblasts, stem and immune cells, but also vascular and neuronal networks, contribute with a variety of distinct defense mechanisms, and inflammatory response mechanisms are critical for tissue homeostasis. Still, without therapeutic intervention, a deep carious lesion may lead to tissue necrosis, which allows bacteria to populate the root canal system and invade the periradicular bone via the apical foramen at the root tip. The periodontal tissues and alveolar bone react to the insult with an inflammatory response, most commonly by the formation of an apical granuloma. Healing can occur after pathogen removal, which is achieved by disinfection and obturation of the pulp space by root canal treatment. This review highlights the various mechanisms of pathogen recognition and defense of dental pulp cells and periradicular tissues, explains the different cell types involved in the immune response and discusses the mechanisms of healing and repair, pointing out the close links between inflammation and regeneration as well as between inflammation and potential malignant transformation.

Lipoteichoic acid stimulates the proliferation, migration and cytokine production of adult dental pulp stem cells without affecting osteogenic differentiation

AIM

To model in vitro the contact between adult dental pulp stem cells (DPSCs) and lipoteichoic acid (LTA), a cell wall component expressed at the surface of most Gram-positive bacteria.

METHODOLOGY

Human DPSCs obtained from impacted third molars were cultured and exposed to various concentrations of S. aureus LTA (0.1, 1.0 and 10 µg mL^-1 ). The effects of LTA on DPSCs proliferation and apoptosis were investigated by MTT assay and flow cytometry. Mineralization of DPSCs was evaluated by alizarin red staining assay. Migration was investigated by microphotographs of wound-healing and Transwell migration assays. Reverse transcription polymerase chain reaction was used to examine the effects of LTA on p65 NF-κB translocation and TLR1, TLR2 or TLR6 regulation. Enzyme-linked immunosorbent assay was used to investigate LTA-stimulated DPSCs cytokine production. One-way or two-way ANOVA and Tukey post hoc multiple comparison were used for statistical analysis.

RESULTS

DPSCs expressed TLR1, TLR2 and TLR6 involved in the recognition of various forms of LTA or lipoproteins. Exposure to LTA did not up- or down-regulate the mRNAs of TLR1, TLR2 or TLR6 whilst LPS acted as a potent inducer of them [TLR1 (P ≤ 0.05), TLR2 (P ≤ 0.001) and TLR6 (P ≤ 0.001)]. Translocation of p65 NF-κB to the nucleus was detected in LTA-stimulated cells, but to a lesser extent than LPS-stimulated DPSCs (P ≤ 0.001). The viability of cells exposed to LTA was greater than unstimulated cells, which was attributed to an increased proliferation and not to less cell death [LTA 1 μg mL^-1 (P ≤ 0.001) and 10 μg mL^-1 (P ≤ 0.01)]. For specific doses of LTA (1.0 µg mL^-1 ), adhesion of DPSCs to collagen matrix was disturbed (P ≤ 0.05) and cells enhanced their horizontal mobility (P ≤ 0.001). LTA-stimulated DPSCs released IL-6 and IL-8 in a dose-dependent manner (P ≤ 0.0001). At all concentrations investigated, LTA did not influence osteogenic/odontoblastic differentiation.

CONCLUSIONS

Human DPSCs were able to sense the wall components of Gram-positive bacteria likely through TLR2 signalling. Consequently, cells modestly proliferated, increased their migratory behaviour and contributed significantly to the local inflammatory response through cytokine release.

Endodontic regeneration: hard shell, soft core

A loss of organs or the destruction of tissue leaves wounds to which organisms and living things react differently. Their response depends on the extent of damage, the functional impairment and the biological potential of the organism. Some can completely regenerate lost body parts or tissues, whereas others react by forming scars in the sense of a tissue repair. Overall, the regenerative capacities of the human body are limited and only a few tissues are fully restored when injured. Dental tissues may suffer severe damage due to various influences such as caries or trauma; however, dental care aims at preserving unharmed structures and, thus, the functionality of the teeth. The dentin-pulp complex, a vital compound tissue that is enclosed by enamel, holds many important functions and is particularly worth protecting. It reacts physiologically to deleterious impacts with an interplay of regenerative and reparative processes to ensure its functionality and facilitate healing. While there were initially no biological treatment options available for the irreversible destruction of dentin or pulp, many promising approaches for endodontic regeneration based on the principles of tissue engineering have been developed in recent years. This review describes the regenerative and reparative processes of the dentin-pulp complex as well as the morphological criteria of possible healing results. Furthermore, it summarizes the current knowledge on tissue engineering of dentin and pulp, and potential future developments in this thriving field.

Bacteriologic status of non-cavitated proximal enamel caries lesions. A histologic and histobacteriologic study

OBJECTIVES

This histologic and histobacteriologic study evaluated non-cavitated interproximal white spot lesions in human teeth for the 1) presence and morphology of bacterial aggregations at the enamel-dentinal junction (EDJ), and 2) reactions in the subjacent pulp tissue.

METHODS

The material comprised 16 third molars diagnosed with early interproximal caries lesions obtained consecutively in a single clinical practice. Four third molars with clinically intact proximal surfaces served as controls. Caries activity was categorized as active or arrested. Teeth were prepared for histologic and histobacteriologic analyses.

RESULTS

Control teeth exhibited normal tissue conditions with no bacteria. Macroscopic discoloration of the superficial dentin occurred in 14/16 teeth with early caries lesions. Bacterial aggregations resembling biofilms were observed in 10/16 teeth (six with active and four with arrested lesions). Bacterial cells were observed superficially in some of the underlying dentinal tubules, while in a few cases a deeper penetration was evident. Pathologic changes of varying degrees were observed in the pulps of all 16 teeth, regardless of the caries activity (active or arrested). Pulp reactions varied from disruption of the odontoblast layer, with loss of odontoblasts, to formation of tertiary dentin.

CONCLUSIONS

Bacterial biofilms associated with white-spot caries lesions may traverse the enamel and reach the underlying dentin in both active and arrested lesions. In all teeth with early lesions, the pulps showed changes in response to the very superficial biofilm challenge.

CLINICAL SIGNIFICANCE

In non-cavitated, active or arrested, enamel caries lesions, bacteria traverse the enamel and may establish structured biofilms at the enamel-dentinal junction, causing early pulp changes. These new findings may stimulate clinicians to rethink the rationale for treatment methods that are based on the assumption that bacteria are absent in white-spot lesions.

Human closed and open apex premolar teeth express different toll-like receptor

Background

The innate immune activation which promotes inflammation responses in the dental pulp tissue leads to the progression of dentin caries. Accordingly, toll‐like receptors (TLRs) are key molecules of the innate immune system that identify pathogen‐associated molecular patterns (PAMPs) on microorganisms and may have a critical role in a dental injury. Therefore, this study aimed to investigate the expression of TLR2, TLR3, and TLR4 in the human dental pulp of opened and closed apex teeth.

Methods

Human dental pulps were derived from the healthy opened and closed apex premolar, in which extraction was indicated for orthodontic reasons. The extraction of RNA was performed and the gene expression determined by real‐time polymerase chain reaction (RT‐PCR). The result from real‐time PCR was confirmed using western blot analysis.

Results

Real‐time PCR data analysis showed that the expression TLR2 and TLR4 were significantly increased in closed apex premolar teeth compared to open apex teeth, whereas TLR3 expression was not significantly different in these two groups (p < .05).

Conclusion

The results of the present study suggested increased expression of TLR2 and TLR4 by the maturation of the apex, which may be due to the presence of microorganisms in the normal or destructed dental pulp tissue. Thus, identifying the expression of TLRs molecules in dental pulp tissue helps to develop a deeper knowledge of the immune responses in the oral cavity.

Melatonin antagonizes lipopolysaccharide-induced pulpal fibroblast responses

Background

Pulpal inflammation is known to be mediated by multiple signaling pathways. However, whether melatonin plays regulatory roles in pulpal inflammation remains unclear. This study aimed at elucidating an in situ expression of melatonin and its receptors in human pulpal tissues, and the contribution of melatonin on the antagonism of lipopolysaccharide (LPS)-infected pulpal fibroblasts.

Methods

Melatonin expression in pulpal tissues harvested from healthy teeth was investigated by immunohistochemical staining. Its receptors, melatonin receptor 1 (MT1) and melatonin receptor 2 (MT2), were also immunostained in pulpal tissues isolated from healthy teeth and inflamed teeth diagnosed with irreversible pulpitis. Morphometric analysis was subsequently performed. After LPS infection of cultured pulpal fibroblasts, cyclo-oxygenase (COX) and interleukin-1 β (IL-1 β) transcripts were examined by using reverse transcription-polymerase chain reaction (RT-PCR). Analysis of mRNA expression was performed to investigate an antagonism of LPS stimulation by melatonin via COX and IL-1 β induction. Mann-Whitney U test and One-way ANOVA were used for statistical analysis to determine a significance level.

Results

Melatonin was expressed in healthy pulpal tissue within the odontoblastic zone, cell-rich zone, and in the pulpal connective tissue. Furthermore, in health, strong MT1 and MT2 expression was distributed similarly in all 3 pulpal zones. In contrast, during disease, expression of MT2 was reduced in inflamed pulpal tissues (P-value< 0.001), but not MT1 (P-value = 0.559). Co-culturing of melatonin with LPS resulted in the reduction of COX-2 and IL-1 β expression in primary pulpal fibroblasts, indicating that melatonin may play an antagonistic role to LPS infection in pulpal fibroblasts.

Conclusions

Human dental pulp abundantly expressed melatonin and its receptors MT1 and MT2 in the odontoblastic layers and pulpal connective tissue layers. Melatonin exerted antagonistic activity against LPS-mediated COX-2 and IL-1 β induction in pulpal fibroblasts, suggesting its therapeutic potential for pulpal inflammation and a possible role of pulpal melatonin in an immunomodulation via functional melatonin receptors expressed in dental pulp.

Fisiopatología de los odontoblastos: una revisión

Los odontoblastos son células post-mitóticas de origen mesenquimal dispuestas en forma de palizada en la periferia de la pulpa dental y responsables de la formación de la dentina. Los odontoblastos derivan de la cresta neural y su diferenciación es la consecuencia de las interacciones epitelio-mesénquima entre las células de la papila dental y el epitelio dental interno. Este trabajo tiene como objetivo revisar los aspectos fisiológicos y patológicos de los odontoblastos, comprendiendo su origen, mecanismos de diferenciación y propiedades funcionales. Se realizó una búsqueda electrónica de literatura desde el año 2000 hasta Febrero de 2018, seleccionando 2889 artículos, de los cuales 52 artículos fueron analizados y discutidos. Los resultados exponen el origen, etapas y los factores relacionados con la diferenciación odontoblástica, junto con los aspectos principales de la organización estructural y funciones que desempeñan los odontoblastos. Esta revisión demuestra mediante la evidencia científica actual como los estudios concernientes a los odontoblastos se focalizan en comprender los mecanismos en la formación de la dentina reparativa, la respuesta inmunitaria y su rol en los procesos de inflamación y dolor. Trabajos futuros deberán esclarecer las diferentes señales involucradas en los procesos fisiopatológicos celulares y moleculares llevados a cabo por los odontoblastos. 

Next-generation sequencing predicts interaction network between miRNA and target genes in lipoteichoic acid-stimulated human neutrophils

Toll-like receptors (TLRs), which are a class of pattern-recognition receptors, can sense specific molecules of pathogens and then activate immune cells, such as neutrophils. The regulation of TLR signaling in immune cells has been investigated by various studies. However, the interaction of TLR signaling-activated microRNAs (miRNAs) and genes has not been well investigated in a specific type of immune cells. In the present study, neutrophils were isolated from peripheral blood of a healthy donor, and then treated for 16 h with Staphylococcus aureus lipoteichoic acid (LTA), which is an agonist of TLR2. The miRNA and mRNA expression profiles were analyzed via next-generation sequencing and bioinformatics approaches. A total of 290 differentially expressed genes between LTA-treated and vehicle-treated neutrophils were identified. Gene ontology analysis revealed that various biological processes and pathways, including inflammatory responses, defense response, positive regulation of cell migration, motility, and locomotion, and cell surface receptor signaling pathway, were significantly enriched. In addition, 38 differentially expressed miRNAs were identified and predicted to be involved in regulating signal transduction and cell communication. The interaction of 4 miRNAs (hsa-miR-34a-5p, hsa-miR-34c-5p, hsa-miR-708-5p, and hsa-miR-1271-5p) and 5 genes (MET, CACNB3, TNS3, TTYH3, and HBEGF) was proposed to participate in the LTA-induced signaling network. The present findings may provide novel information for understanding the detailed expression profiles and potential networks between miRNAs and their target genes in LTA-stimulated healthy neutrophils.

Oxidative stress induced by self-adhesive resin cements affects gene expression, cellular proliferation and mineralization potential of the MDPC-23 odontoblast-like cells

OBJECTIVE

Clinical issues have been raised about problems related to cytotoxic effects caused when applying self-adhesive cement. It was hypothesized that byproducts eluted from self-adhesive cements modulate oxidative stress response, the gene expression of signaling pathways of inflammatory process/transcriptional activators, and the expression and activity of interstitial collagenases, and modify the phenotypic characteristics of cellular proliferation and mineral deposition in odontoblastic-like cells.

METHODS

Cements (MaxCem Elite [MAX] and RelyX U200 [U200)]) were mixed, dispensed into moulds, and photoactivated according to the manufacturers' instructions. Immortalized rat odontoblast-like cells (MDPC-23) were cultured and exposed to polymerized specimens of cements for 4 h. Reactive oxidative specimen production and quantification of gene expression were evaluated. Cell proliferation assay and alizarin red staining were also performed to evaluate the disturbance induced by the cements on cellular proliferation and mineralization.

RESULTS

Despite their cytotoxic effects, both self-adhesive cements influenced the metabolism in the odontoblast cells on different scales. MAX induced significantly higher oxidative stress in odontoblast cells than U200. Gene expression varied as a function of exposure to self-adhesive cements; MAX induced the expression of pro-inflammatory cytokines such as TNF-α, whereas U200 downregulated, virtually depleted TNF-α expression, also inducing overexpression of the transcriptional factor Runx2. Overexpression of heme oxygenase-1 (HO-1) and thioredoxin reductase 1 (TRXR1) occurred after exposure to both cements, antioxidant genes that are downstream of Keap1-Nrf2-ARE system. MAX significantly induced the overexpression of collagenase MMP-1, and U200 induced the expression of gelatinase MMP-2. MAX significantly inhibited cell proliferation whereas U200 significantly activated cell proliferation. Alizarin red staining revealed significantly decreased mineral deposition especially when exposed to MAX.

SIGNIFICANCE

These results support the hypothesis that byproducts of different self-adhesive cements play important roles in the highly orchestrated process which ultimately affect the cellular proliferation and the mineral deposition in odontoblastic-like cells, possibly delaying the reparative dentin formation after cementation of indirect restorations, especially on recently exposed dentin preparations.

Microbial Modulation of Stem Cells and Future Directions in Regenerative Endodontics

Regenerative endodontic procedures (REPs) have been shown to promote the resolution of signs and symptoms of disease and increase survival compared with traditional treatment procedures. However, there is still variable predictability of continued root development and evidence that the tissues formed do not recapitulate the native pulp-dentin complex. There is growing evidence that the apical papilla is capable of surviving prolonged endodontic infection and apical periodontitis and that it represents a rich source of undifferentiated mesenchymal stem cells in REPs. The survival and proper differentiation of stem cells transferred into infected root canals are fraught with challenges. Residual antigens, such as lipopolysaccharides, have been shown to be present in dentin even after adequate chemomechanical debridement. These antigens have a profound effect on stem cell fate by modulating their proliferative capacity and postdifferentiation phenotype. Thus, root canals must be detoxified in addition to disinfection. There is a strong need for translational studies that incorporate all aspects of tissue engineering in endodontics in models that include an existing infection to promote further advancement of the field. This is particularly important to make REPs more predictable when treating immature teeth in young patients. Importantly, regenerative procedures could eventually promote tooth longevity in our aging population. Lessons learned from translational studies that best mimic the clinical challenges could be evaluated in pragmatic clinical trials to determine the effectiveness of these procedures to promote desirable patient-centered outcomes.

The roles of odontoblasts in dental pulp innate immunity

Odontoblasts located in the outermost layer of dental pulp form a natural barrier between mineralized tissues, dentin, and soft tissues, dental pulp, of the vital tooth, and they first recognize caries-related pathogens and sense external irritations. Therefore, odontoblasts possess a specialized innate immune system to fight oral pathogens invading into dentin. Generally, the rapid initial sensing of microbial pathogens, especially pathogen-associated molecular patterns (PAMPs) shared by microorganisms, are mediated by pattern recognition receptors (PRRs), such as Toll-like receptor and the nucleotide-binding oligomerization domain (NOD). The innate immune responses in odontoblasts initiated by sensing oral pathogens provide host protective events, such as inflammatory reactions, to produce a variety of pro-inflammatory mediators, including chemokines and cytokines. These attract various inflammatory cells and cause antibacterial reactions, such as the production of defensins, to kill microorganisms in the proximal region of the odontoblast layer. This review focuses on innate immunity, especially cellular and molecular mechanisms regarding the sensing of PAMPs from oral pathogens by PRRs, in odontoblasts and provides information for future studies for the development of novel therapeutic strategies, including diagnosis and treatment, to prevent exceeding dental pulp inflammation and preserve the dental pulp tissues.

Inflammatory cytokines in normal and irreversibly inflamed pulps: A systematic review

OBJECTIVE

To review the available literature in regard to the inflammatory process and pulpitis. Setting forth to evaluate if differences in the levels of various cytokines (TNF-α, IL-1β, IL-2, IL-6 and IL-8) can be observed in clinically diagnosed normal and irreversibly inflamed pulps that could serve as possible markers and/or diagnostic tools to predict and differentiate between certain states of inflammation. Methods used to measure and assess levels of cytokines have been limited to two protein quantification methods ELISA and/or Multiplex Array.

DESIGN

The databases PubMed, EMBASE/Ovid, The Cochrane Central Register of Controlled Trials, Cochrane Reviews and Scopus were consulted for the electronic literature search. Screening of titles and abstracts followed the PRISMA guidelines while data extraction and the assessment of the full texts were carried out in accordance to the GRADES assessment.

RESULTS

The review showed that significant increases in levels of IL-1β, IL-2, IL-6, IL-8 and TNF-α in irreversible pulpitis samples exist, in comparison to normal pulp samples which serve as a good basis for potential markers. Due to larger discrepancies in available literature, IL-2 seems rather unsuitable at the moment, while IL-6 and TNF alpha seem to be more promising.

CONCLUSION

It may be concluded that even by combining two protein quantification methods inconsistencies between studies exist. At the moment it is difficult to select just one specific cytokine suitable for testing, rather it supports the rationale that further high-quality clinical studies are needed.

Pulpal Tissue Inflammatory Reactions after Experimental Pulpal Exposure in Mice

INTRODUCTION

The purpose of this study was to establish a stable experimental mice pulpal inflammatory model and to evaluate inflammatory reactions of pulpal tissue after pulpal exposure.

METHODS

Pulpal inflammation was induced in 80 C57BL/6 mice by occlusal exposure of the pulp of the maxillary first molar. The mice were sacrificed randomly at 0, 1, 6, 12, 24, 48, and 72 hours after pulpal exposure. Mice without pulpal exposure served as controls. Maxillary teeth were obtained and prepared for histologic analyses and real-time polymerase chain reaction analyses.

RESULTS

As the duration of pulpal exposure increases, the inflammatory reaction is exacerbated. Within 6 to 12 hours after pulpal exposure, pulp tissues experienced red blood cell extravasation to the destruction of the odontoblast layer. After 24 hours, necrosis was observed in the pulpal tissue; until 72 hours, necrosis spread to the whole coronal pulpal tissue, and a large number of inflammatory cells were found in the radicular pulpal tissue. The results of histomorphologic scores have the same trend; samples from the 72-hour group possessed the highest score followed by samples from other groups (P < .01). The expression levels of inflammatory cytokines increased over the 72 hours, and there was a high rate of inflammatory cytokine expression at 6 and 12 hours after pulpal exposure.

CONCLUSIONS

Our study represents a stable mice model for studying pulpal inflammation in vivo. Mouse pupal inflammation progresses rapidly, with dramatic changes evident in just a few hours.

Functional Roles of NOD1 in Odontoblasts on Dental Pulp Innate Immunity

Caries-related pathogens are first recognized by odontoblasts and induce inflammatory events that develop to pulpitis. Generally, initial sensing of microbial pathogens is mediated by pattern recognition receptors, such as Toll-like receptor and nucleotide-binding oligomerization domain (NOD); however, little is known about NODs in odontoblasts. In this study, the levels of NODs expressed in rat odontoblastic cell line, KN-3, were assessed by flow cytometry and the levels of chemokines in NOD-specific ligand-stimulated KN-3 cells were analyzed by real-time PCR and ELISA. The signal transduction pathway activated with NOD-specific ligand was assessed by blocking assay with specific inhibitors and reporter assay. In KN-3 cells, the expression level of NOD1 was stronger than that of NOD2 and the production of chemokines, such as CINC-1, CINC-2, CCL20, and MCP-1, was upregulated by stimulation with NOD1-specific ligand, but not with NOD2-specific ligand. CINC-2 and CCL20 production by stimulation with NOD1-specific ligand was reduced by p38 MAPK and AP-1 signaling inhibitors. Furthermore, the reporter assay demonstrated AP-1 activation in NOD1-specific ligand-stimulated KN-3 cells. These findings indicated that NOD1 expressed in odontoblasts functions to upregulate the chemokines expression via p38-AP-1 signaling pathway and suggested that NOD1 may play important roles in the initiation and progression of pulpitis.

Odontoblasts: Specialized hard-tissue-forming cells in the dentin-pulp complex

Odontoblasts are specialized cells that produce dentin and exhibit unique morphological characteristics; i.e., they extend cytoplasmic processes into dentinal tubules. While osteoblasts, which are typical hard-tissue-forming cells, are generated from mesenchymal stem cells during normal and pathological bone metabolism, the induction of odontoblasts only occurs once during tooth development, and odontoblasts survive throughout the lives of healthy teeth. During the differentiation of odontoblasts, signaling molecules from the inner enamel epithelium are considered necessary for the differentiation of odontoblast precursors, i.e., peripheral dental papilla cells. If odontoblasts are destroyed by severe external stimuli, such as deep caries, the differentiation of dental pulp stem cells into odontoblast-like cells is induced. Various bioactive molecules, such as non-collagenous proteins, might be involved in this process, although the precise mechanisms responsible for odontoblast differentiation have not been fully elucidated. Recently, our knowledge about the other functional activities of odontoblasts (apart from dentin formation) has increased. For example, it has been suggested that odontoblasts might act as nociceptive receptors, and surveillance cells that detect the invasion of exogenous pathogens. The regeneration of the dentin-pulp complex has recently gained much attention as a promising future treatment modality that could increase the longevity of pulpless teeth. Finally, congenital dentin anomalies, which are concerned with the disturbance of odontoblast functions, are summarized.

An Overview of Pathogen Recognition Receptors for Innate Immunity in Dental Pulp

Pathogen recognition receptors (PRRs) are a class of germ line-encoded receptors that recognize pathogen-associated molecular patterns (PAMPs). The activation of PRRs is crucial for the initiation of innate immunity, which plays a key role in first-line defense until more specific adaptive immunity is developed. PRRs differ in the signaling cascades and host responses activated by their engagement and in their tissue distribution. Currently identified PRR families are the Toll-like receptors (TLRs), the C-type lectin receptors (CLRs), the nucleotide-binding oligomerization domain-like receptors (NLRs), the retinoic acid-inducible gene-I-like receptors (RLRs), and the AIM2-like receptor (ALR). The environment of the dental pulp is substantially different from that of other tissues of the body. Dental pulp resides in a low compliance root canal system that limits the expansion of pulpal tissues during inflammatory processes. An understanding of the PRRs in dental pulp is important for immunomodulation and hence for developing therapeutic targets in the field of endodontics. Here we comprehensively review recent finding on the PRRs and the mechanisms by which innate immunity is activated. We focus on the PRRs expressed on dental pulp and periapical tissues and their role in dental pulp inflammation.

Can Pulp Fibroblasts Kill Cariogenic Bacteria? Role of Complement Activation

Complement system activation has been shown to be involved in inflammation and regeneration processes that can be observed within the dental pulp after moderate carious decay. Studies simulating carious injuries in vitro have shown that when human pulp fibroblasts are stimulated by lipoteichoic acid (LTA), they synthetize all complement components. Complement activation leads to the formation of the membrane attack complex (MAC), which is known for its bacterial lytic effect. This work was designed to find out whether human pulp fibroblasts can kill Streptococcus mutans and Streptococcus sanguinis via complement activation. First, histological staining of carious tooth sections showed that the presence of S. mutans correlated with an intense MAC staining. Next, to simulate bacterial infection in vitro, human pulp fibroblasts were incubated in serum-free medium with LTA. Quantification by an enzymatic assay showed a significant increase of MAC formation on bacteria grown in this LTA-conditioned medium. To determine whether the MAC produced by pulp fibroblasts was functional, bacteria sensitivity to LTA-conditioned medium was evaluated using agar well diffusion assay and succinyl dehydrogenase (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide [MTT]) assay. Both assays showed that S. mutans and S. sanguinis were sensitive to LTA-conditioned medium. Finally, to evaluate whether MAC formation on cariogenic bacteria, by pulp fibroblasts, can be directly induced by the presence of these bacteria, a specific coculture model of human pulp fibroblasts and bacteria was developed. Immunofluorescence revealed an intense MAC labeling on bacteria after direct contact with pulp fibroblasts. The observed MAC formation and its lethal effects were significantly reduced when CD59, an inhibitor of MAC formation, was added. Our findings demonstrate that the MAC produced by LTA-stimulated pulp fibroblasts is functional and can kill S. mutans and S. sanguinis. Taken together, these data clearly highlight the function of pulp fibroblasts in destroying cariogenic bacteria.

Dental Pulp Defence and Repair Mechanisms in Dental Caries

Dental caries is a chronic infectious disease resulting from the penetration of oral bacteria into the enamel and dentin. Microorganisms subsequently trigger inflammatory responses in the dental pulp. These events can lead to pulp healing if the infection is not too severe following the removal of diseased enamel and dentin tissues and clinical restoration of the tooth. However, chronic inflammation often persists in the pulp despite treatment, inducing permanent loss of normal tissue and reducing innate repair capacities. For complete tooth healing the formation of a reactionary/reparative dentin barrier to distance and protect the pulp from infectious agents and restorative materials is required. Clinical and in vitro experimental data clearly indicate that dentin barrier formation only occurs when pulp inflammation and infection are minimised, thus enabling reestablishment of tissue homeostasis and health. Therefore, promoting the resolution of pulp inflammation may provide a valuable therapeutic opportunity to ensure the sustainability of dental treatments. This paper focusses on key cellular and molecular mechanisms involved in pulp responses to bacteria and in the pulpal transition between caries-induced inflammation and dentinogenic-based repair. We report, using selected examples, different strategies potentially used by odontoblasts and specialized immune cells to combat dentin-invading bacteria in vivo.

Human odontoblast-like cells produce nitric oxide with antibacterial activity upon TLR2 activation

The penetration of cariogenic oral bacteria into enamel and dentin during the caries process triggers an immune/inflammatory response in the underlying pulp tissue, the reduction of which is considered a prerequisite to dentinogenesis-based pulp regeneration. If the role of odontoblasts in dentin formation is well known, their involvement in the antibacterial response of the dental pulp to cariogenic microorganisms has yet to be elucidated. Our aim here was to determine if odontoblasts produce nitric oxide (NO) with antibacterial activity upon activation of Toll-like receptor-2 (TLR2), a cell membrane receptor involved in the recognition of cariogenic Gram-positive bacteria. Human odontoblast-like cells differentiated from dental pulp explants were stimulated with the TLR2 synthetic agonist Pam2CSK4. We found that NOS1, NOS2, and NOS3 gene expression was increased in Pam2CSK4-stimulated odontoblast-like cells compared to unstimulated ones. NOS2 was the most up-regulated gene. NOS1 and NOS3 proteins were not detected in Pam2CSK4-stimulated or control cultures. NOS2 protein synthesis, NOS activity and NO extracellular release were all augmented in stimulated samples. Pam2CSK4-stimulated cell supernatants reduced Streptococcus mutans growth, an effect counteracted by the NOS inhibitor L-NAME. In vivo, the NOS2 gene was up-regulated in the inflamed pulp of carious teeth compared with healthy ones. NOS2 protein was immunolocalized in odontoblasts situated beneath the caries lesion but not in pulp cells from healthy teeth. These results suggest that odontoblasts may participate to the antimicrobial pulp response to dentin-invading Gram-positive bacteria through NOS2-mediated NO production. They might in this manner pave the way for accurate dental pulp healing and regeneration.

Oxygen Saturation in Dental Pulp of Permanent Teeth: Difference between Children/Adolescents and Adults

INTRODUCTION

The objective of this study was to use pulse oximetry to measure oxygen saturation in permanent maxillary central incisors with normal pulp in 2 different age groups: children/adolescents and adults.

METHODS

Blood oxygen saturation levels were measured using a pulse oximeter in 110 maxillary central incisors of 57 individuals, in 1 of 2 possible age bands, as follows: 28 children/adolescents (7-13 years old) and 29 adults (22-36 years old). The following factors were also analyzed: (1) heart rate (beats/min); (2) oxygen saturation rate measured at the patient's index finger, also using a pulse oximeter; (3) tooth crown dimensions; and (4) the time taken by the oximeter to provide a reading.

RESULTS

The mean oxygen saturation level in normal central incisors was higher among children/adolescents (84.35%) than adults (77.88%, P = .003). Oxygen saturation rates measured at the patients' fingers were not correlated with saturation obtained at the teeth (r = 0.10). There was no correlation between oxygen saturation readings and tooth dimensions (buccal surface area), heart rate, or oximeter reading time (P > .05).

CONCLUSIONS

Oxygen saturation values measured in maxillary central incisors using a pulse oximeter revealed differences between children/adolescents and adults, showing that children/adolescents have higher oxygen saturation levels. There was no correlation between oxygen saturation levels in patients' fingers and values from their teeth or between oxygen saturation readings from central incisors and tooth dimensions (buccal surface), heart rate, or oximeter reading time.

Activation of the NLRP3/caspase-1 inflammasome in human dental pulp tissue and human dental pulp fibroblasts

The NLRP3/caspase-1 inflammasome pathway plays an important role in cellular immune defence against bacterial infection; however, its function in human dental pulp tissue and human dental pulp fibroblasts remains poorly understood. We demonstrate that NLRP3 protein expression occurs to a greater extent in pulp tissue with irreversible pulpitis than in normal pulp tissue and in tissue with reversible pulpitis. Caspase-1 is present in its active (cleaved) form only in pulp tissue with irreversible pulpitis. NLRP3 and caspase-1 are expressed in the odontoblast layers in normal human dental pulp tissue, whereas in inflamed pulp tissue, the odontoblast layers are disrupted and dental pulp cells are positive for NLRP3 and caspase-1. Additionally, we investigate the role of the NLRP3/caspase-1 inflammasome pathway in human dental pulp fibroblasts and show that ATP activates the P2X7 receptor on the cell membrane triggering K(+) efflux and inducing the gradual recruitment of the membrane pore pannexin-1. Extracellular lipopolysaccharide is able to penetrate the cytosol and activate NLRP3. Furthermore, the low intracellular K(+) concentration in the cytosol triggers reactive oxygen species generation, which also induces the NLRP3 inflammasome. Thus, the NLRP3/caspase-1 pathway has a biological role in the innate immune response mounted by human dental pulp fibroblasts.

Wnt5a promotes inflammatory responses via nuclear factor κB (NF-κB) and mitogen-activated protein kinase (MAPK) pathways in human dental pulp cells

Wnt5a has been found recently to be involved in inflammation regulation through a mechanism that remains unclear. Immunohistochemical staining of infected human dental pulp and tissue from experimental dental pulpitis in rats showed that Wnt5a levels were increased. In vitro, Wnt5a was increased 8-fold in human dental pulp cells (HDPCs) after TNF-α stimulation compared with control cells. We then investigated the role of Wnt5a in HDPCs. In the presence of TNF-α, Wnt5a further increased the production of cytokines/chemokines, whereas Wnt5a knockdown markedly reduced cytokine/ chemokine production induced by TNF-α. In addition, in HDPCs, Wnt5a efficiently induced cytokine/chemokine expression and, in particular, expression of IL-8 (14.5-fold) and CCL2 (25.5-fold), as assessed by a Luminex assay. The cytokine subsets regulated by Wnt5a overlap partially with those induced by TNF-α. However, no TNF-α and IL-1β was detected after Wnt5a treatment. We then found that Wnt5a alone and the supernatants of Wnt5a-treated HDPCs significantly increased macrophage migration, which supports a role for Wnt5a in macrophage recruitment and as an inflammatory mediator in human dental pulp inflammation. Finally, Wnt5a participates in dental pulp inflammation in a MAPK-dependent (p38-, JNK-, and ERK-dependent) and NF-κB-dependent manner. Our data suggest that Wnt5a, as an inflammatory mediator that drives the integration of cytokines and chemokines, acts downstream of TNF-α.

Changes in cell density and morphology of selected cells of the ageing human dental pulp

OBJECTIVES

The aim of this study was to determine the changes in cell density and morphology of selected cells of the ageing human dental pulp.

BACKGROUND

Changes in cell density and morphology of dental pulp cells over time may affect their capability to respond to tooth injury.

MATERIALS AND METHODS

One hundred thirty-one extracted teeth were obtained from individuals between the ages of 6 and 80 years. The apical 1/3 of the root region was removed from all teeth prior to routine processing for producing histological slides. The histology slides were used to study the changes in cell density and morphology of selected pulp cells; odontoblasts, subodontoblasts and fibroblasts in the crown and root regions of the dental pulp. Student's t-test and one-way anova were used for statistical analyses.

RESULTS

In all age groups, the cell density for all types of cells was found to be higher in the crown than in the root (p < 0.05). In general, the pulp cell density was found to decrease with age in both the crown and root regions. However, it was noted that the reduction of coronal odontoblasts occurred later in life (40-49 years) when compared to that of subodontoblasts or fibroblasts (30-39 years).

CONCLUSIONS

The density of the coronal pulp cells reduces and these cells undergo morphological changes with ageing of individuals and this may affect the pulp's ability to resist tooth injury.

Distinctive genetic activity pattern of the human dental pulp between deciduous and permanent teeth

Human deciduous and permanent teeth exhibit different developmental processes, morphologies, histological characteristics and life cycles. In addition, their pulp tissues react differently to external stimuli, such as the pulp sensitivity test, dental trauma and pulp therapy materials. These suggest differences in gene expression and regulation, and in this study we compared gene-expression profiles of the human dental pulp from deciduous and permanent teeth. Pulp tissues from permanent premolars and deciduous molars aged 11–14 years were extirpated and mRNA was isolated for cDNA microarray analysis, and quantitative real-time PCR (qPCR). Other teeth were used for immunohistochemical analysis (IHC). Microarray analysis identified 263 genes with a twofold or greater difference in expression level between the two types of pulp tissue, 43 and 220 of which were more abundant in deciduous and permanent pulp tissues, respectively. qPCR analysis was conducted for eight randomly selected genes, and the findings were consistent with the cDNA microarray results. IHC confirmed that insulin-like growth factor 2 mRNA-binding protein 1 (IGF2BP1) was broadly expressed in deciduous dental pulp tissue, but minimally expressed in permanent dental pulp tissue. Immunohistochemical analysis showed that calbindin 1 (CALB1), leucine-rich repeat-containing G-protein-coupled receptor 5 (LGR5), and gamma-aminobutyric acid A receptor beta 1 (GABRB1) were abundantly expressed in permanent predentin/odontoblasts, but only minimally expressed in deciduous dental pulp tissue. These results show that deciduous and permanent pulp tissues have different characteristics and gene expression, suggesting that they may have different functions and responses to therapies focused on pulp or dentin regeneration.