Inflammation of the Dental Pulp

The dental pulp is sensitive to external factors such as microbial infection from dental caries and/or mechanical/chemical irritations during dental procedures. Dental tissue behaves differently to the other connective tissues. It is unique in a way that its soft tissues (pulp and pulp-dentin complex) are enclosed within mineralized hard tissues (enamel, dentin, and cement), and its pulp is supplied by a rich neurovascular network that regulates various inflammatory mediators In recent years, many studies have examined the pulp-dentin complex regeneration with DPSCs. While it is important to perform research on cells, scaffolds, and growth factors, it is also critical to develop animal models for preclinical trials. The development of a reproducible animal model of transplantation is essential for obtaining precise and accurate data in vivo. The efficacy of pulp regeneration should be assessed qualitatively and quantitatively using animal models. This review article sought to introduce in vivo experiments that have evaluated the potential of dental pulp stem cells for pulp-dentin complex regeneration. According to a review of various researches about DPSCs, the majority of studies have used subcutaneous mouse and dog teeth for animal models. There is no way to know which animal model will reproduce the clinical environment. If an animal model is developed which is easier to use and is useful in more situations than the currently popular models, it will be a substantial aid to studies examining pulp-dentin complex regeneration. Porphyromonas gingivalis is considered with inducing pulpal inflammation and has lipopolysaccharide (LPS) as an inflammatory stimulator. GV1001 peptide has anticancer and anti-inflammation activity due to inhibiting activation of signaling molecules after penetration into the various types of cells. Therefore, this study examined inhibitory effect of GV1001 on dental pulp cells (hDPCs) stimulated by P. gingivalis LPS. The intracellular distribution of GV1001 was analyzed by confocal microscopy. Real-time RT-PCR wasperformedtodeterminetheexpressionlevelsofTNF- 𝛼 andIL-6cytokines.TheroleofsignalingbyMAPkinases(ERKandp38) was explored using Western blot analysis. The effect of GV1001 peptide on hDPCs viability was measured by MTT assay. GV1001 was predominantly located in hDPC cytoplasm. The peptide inhibited P. gingivalis LPS-induced TNF- 𝛼 and IL-6 production in hDPCs without significant cytotoxicity. Furthermore, GV1001 treatment markedly inhibited the phosphorylation of MAP kinases (ERK and p38) in LPS-stimulated hDPCs. GV1001 may prevent P. gingivalis LPS-induced inflammation of apical tissue. Also, these findings provide mechanistic insight into how GV1001 peptide causes anti-inflammatory actions in LPS-stimulated pulpitis without significantly affecting cell viability. 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. Purpose . This study quantified the expression of substance P (SP), calcitonin gene-related peptide (CGRP), 𝛽 -endorphins ( 𝛽 -End), and methionine-enkephalin (Met-Enk) in human dental pulp following orthodontic intrusion. Methods . Eight patients were selected according to preestablished inclusion criteria. From each patient, two premolars (indicated for extraction due to orthodontic reasons) were randomly assigned to two different groups: the asymptomatic inflammation group (EXPg), which would undergo controlled intrusive force for seven days, and the control group (CTRg), which was used to determine the basal levels of each substance. Once extracted, dental pulp tissue was prepared to determine the expression levels of both neuropeptides and endogenous opioids by radioimmunoassay (RIA). Results . All samples from the CTRg exhibited basal levels of both neuropeptides and endogenous opioids. By day seven, all patients wereasymptomatic, even when all orthodontic-intrusivedevices werestillactive. In the EXPg, the SP and CGRP exhibited statistically significant different levels. Although none of the endogenous opioids showed statistically significant differences, they all expressed increasing trends in the EXPg. Conclusions . SP and CGRP were identified in dental pulp after seven days of controlled orthodontic intrusion movement, even in the absence of pain. The importance of inflammation has been underestimated in pulpal healing, and in the past, it has been considered only as an undesirable effect. Associated with moderate inflammation, necrosis includes pyroptosis, apoptosis, and nemosis. There are now evidences that inflammation is a prerequisite for pulp healing, with series of events ahead of regeneration. Immunocompetent cells are recruited in the apical part. They slide along the root and migrate toward the crown. Due to the high alkalinity of the capping agent, pulpcells display mildinflammation, proliferate,and increaseinnumberand sizeand initiatemineralization. Pulpfibroblasts become odontoblast-like cells producing type I collagen, alkaline phosphatase, and SPARC/osteonectin. Molecules of the SIBLING family, matrix metalloproteinases, and vascular and nerve mediators are also implicated in the formation of a reparative dentinal bridge, osteo/orthodentin closing the pulp exposure. Beneath a calciotraumatic line, a thin layer identified as reactionary dentin underlines the periphery of the pulp chamber. Inflammatory and/or noninflammatory processes contribute to produce a reparative dentinal bridge closing the pulp exposure, with minute canaliculi and large tunnel defects. Depending on the form and severity of the inflammatory and noninflammatory processes, and according to the capping agent, pulp reactions are induced specifically. 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 .

Inflammatory signals may progress to rapid degeneration and necrosis, and such events could inflict very serious damage to tissues in the body [3,4].
Factors that induce inflammation in the dental pulp and the root apex are as follows [1,2,5,6]: the ingression of microorganisms through dental caries, crack or dentinal tubules of the teeth; chemical irritation from etching and/or bonding materials for adhesion of dental materials; mechanical irritation during preparation in restorative procedures; trauma from occlusion (TFO) or orthodontic movement of the teeth. Such factors may initiate the inflammatory cascades, which, in turn, further progress to pain and root resorption via neurogenic inflammation and hard tissue remodeling [7][8][9]. Therefore, a thorough understanding of the pulpal inflammatory process is essential in the development of proper dental procedures and immunotherapeutic agents.
This special issue is published with the intent of disseminating current knowledge and findings on inflammation in the dental pulp. This issue highlights the molecular mechanisms of inflammatory cascades, immunomodulatory effects of various substances, techniques used to study inflammation and regeneration of the dental pulp, and ultimately why an understanding of inflammatory process is important in the field of endodontics.
In this issue, J.-H. Jang et al. review pathogen recognition receptors (PRRs) for innate immunity in dental pulp. PRRs recognition of pathogen-associated molecular patterns (PAMPs) on pathogenic structure is important stage of initiating specific adaptive immunity. The paper reviews the various types of PRR families that include 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 authors suggest that immunomodulation via PRRs is crucial in the understanding of pathophysiology of pulp inflammation and also in the development of novel therapeutic targets in the pulp injury and/or infection.
J.-C. Farges et al. assess responses of the pulp tissues to bacterial infection at cellular and molecular levels. The pulp response is illustrated by mechanisms used by odontoblasts and specialised immune cells to resist pathogenic bacteria.

Mediators of Inflammation
The cytokine IL-8 which are upregulated in response to bacterial component is particularly important, because it is involved in the recruitment and activation of neutrophils at the site of inflammation. This paper reports that the balance between the inflammatory response and the repair process is of paramount importance, as they dictate the rate of pulpal healing.
Y.-J. Ko et al. study the anti-inflammatory effect of human telomerase-derived peptide (GV1001) stimulated by lipopolysaccharide (LPS) from P. gingivalis. Confocal microscopy was used to analyse the intracellular distribution of GV1001, and real-time RT-PCR was performed to quantify the levels of TNF-and IL-6 cytokines. The study reports that GV1001 was capable of entering the cell and caused downregulation of LPS-induced inflammatory cascades demonstrating its potential as a therapeutic agent in vital pulp therapy, regenerative endodontic fields, and tissue engineering.
S. Kim et al. review in vivo experiments with dental pulp stem cells to observe pulp-dentin complex regeneration. Most studies in this area have been conducted with mouse or dog models making their application in clinical environment unreliable. The authors suggest that where clinical trial of a study is impractical, orthotopic transplantation of dental pulp tissue cells (DPSCs) should be performed, as it would better predict impacts in human teeth. This paper claims that more emphasis in orthotopic transplantation in animal model is required to further expand our knowledge in pulp-dentin complex regeneration.
M. Goldberg et al. discuss the importance of inflammation in the pulp healing and regeneration. Despite its association with undesirable symptoms in pulpitis, inflammation should be understood as prerequisite of the pulp healing. The view is supported by inflammation-mediated proliferation of pulp cells and initiation of mineralisation. Inflammation may also be involved in the production of a reparative dentinal bridge that covers the pulp exposure.
D. Chavarria-Bolaños et al. conduct in vivo experiment to study sensory neuropeptides and endogenous opioids expression in human dental pulp with asymptomatic inflammation. Analysis of substance P (SP), calcitonin gene-related peptide (CGRP), -endorphins ( -End), and methionineenkephalin (Met-Enk) in patients' teeth with asymptomatic inflammation induced by orthodontic intrusion showed the significant increase in the level of SP and CGRP, both of which are frequently associated with pain of pulpal origin.
There is an ever growing interest in keeping teeth in a physiologically functional state in the event of injury and/or infection to the pulp, because no prosthesis can give as much stability and comfort as the normal teeth provide. Inflammatory process that takes place within a tooth is as unique as its structure compared to the other hard tissues in the body. Therefore, an understanding of inflammation of the pulp is a key to control pathophysiological status of the pulp/periapical tissues. This special issue includes reviews and research papers, which help to understand the process of inflammation in the context of dentistry and thereby expand our knowledge to potential therapeutic targets in pulp therapy.

Sang Hyuk Park Ling Ye Robert M. Love Jean-Christophe Farges
Hiromichi Yumoto