Technological innovations in cellular and molecular aspects of tissue engineering ‐‐scaffolds, stem cells and 3D printed tissues ‐‐have been dramatically increased in the last decade. However, regenerative treatment still has challenges in translation to clinic. This is partly due to failure of addressing an essential element of wound healing, inflammation. It is now well-recognized that inflammation is an active process. This paradigm shift opened up a new avenue of therapeutic approaches called “host-modulation.” Host-modulation therapies capable of modulating inflammatory response at multiple levels and mimicking the natural sequence of wound healing offer a new direction and promising clinical translation.
Key points
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Inflammation is a complex reaction to irritants or harmful agents such as allergens, chemical agents, bacteria, or viruses and it includes vascular and cellular responses, primarily migration and activation of leukocytes from the venous capillary to the injury site starting as an acute reaction.
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If it persists, acute inflammation progresses into a chronic inflammation associated with lymphocyte and macrophage infiltration, blood vessel proliferation, and fibrosis and extends over a longer time and may cause morbidity or mortality in certain conditions.
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If uncontrolled, chronic inflammation leads to continuous tissue (both soft and hard tissue) destruction and ultimately loss of function or organ.
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In periodontal disease, fundamentally, lost periodontal tissues and function due to the chronic disease cannot be rebuilt with traditional periodontal therapies.
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A new avenue of therapeutic approach called “host-modulation therapy” is capable of modulating inflammatory response at multiple levels and offers a new direction in the regeneration of periodontal tissues.
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Specialized proresolution lipid meditators including lipoxin, resolvins, protectins, and maresins are highly promising endogenous molecules modulating the host response during wound healing that promote periodontal tissue regeneration.
Introduction
Periodontal disease results from a series of pathologic processes affecting the periodontium, including gingiva, periodontal ligament, cementum, and alveolar bone, ultimately leading to tooth loss, functional and esthetic complications. There are various subsets of this disease, the most prevalent of which are gingivitis and periodontitis. Periodontal disease is a complex condition; since it was first described, many etiologic models have been proposed to characterize the disease and its progression appropriately. , As a result, several classifications have been used to refer to periodontal disease and its subsets. , Periodontal disease is now recognized as a chronic oral inflammatory condition initiated by multiple oral microbial species and associated with a systemic inflammatory state.
The current pathogenesis model of periodontitis has a multilevel framework and is defined by bacterial components, environmental factors, and host and genetic variations. In contrast to previous models, this new framework incorporates genetic variations and environmental factors in the dynamic nature of disease-initiating/resolving biochemical processes to account for the patient-to-patient variations in clinical expression or the site-to-site variations within the same patient. However, the most fundamental controversy in the pathogenesis of periodontal disease is the relationship between microbial biofilm and host immune-inflammatory response. Earlier studies focusing on periodontal biofilm, its composition, and species-level information led the field to the development of preventive and therapeutic modalities based on biofilm removal or resective surgical approaches targeting environmental modifications to, once again, prevent biofilm reinstitution. However, although it has been established that the primary etiologic basis for periodontal disease is bacterial, disease progression results from the inadequate resolution of the host’s acute inflammatory response to these microbial agents. As a result, a state of chronic inflammation ensues and is itself responsible for the bone and tissue damage observed in patients with periodontitis. Evidence that the host inflammatory response changes the composition of the biofilm, selective for specific organisms , has not only resulted in a shift in focus of etiologic factors but also changed the understanding of the composition and the role of microbiological species in host-bacteria interaction. Current evidence suggests that Porphyromonas gingivalis is a keystone bacterium that causes dysbiosis, defined as significant shifts in the microbiome. The implication that organisms such as P gingivalis and Tannerella forsythia can cause dysbiosis in the microbiome due to inflammation raises a fundamental question regarding the treatment of periodontitis.
Treatment of periodontal diseases: the unchanged story of a progressive inflammatory disease
Therapeutic approaches to periodontitis, in a broader aspect, fall under 2 main categories: anti-infective and regenerative therapy. Although anti-infective therapy aims to remove etiologic factors to stop disease progression, regenerative treatments combine anti-infective approaches with procedures aiming to restore the periodontal structures destroyed by the disease. However, both treatment modalities must include maintenance procedures for sustainable efficacy.
The current and, in fact, historical “gold standard” anti-infective and “regenerative” periodontal treatment is scaling and root planing (SRP), which aims to disrupt the biofilm-associated with disease etiology and create a new environment for connective tissue reattachment. Several studies have validated the beneficial effects of SRP combined with personal plaque control. SRP has been shown to decrease clinical inflammation, result in microbial shifts to a less pathogenic subgingival flora, reduce pocket depth, improve clinical attachment levels, and halt or slow disease progression. , However, studies also showed that SRP alone might not be sufficient to eliminate the periodontal pocket presenting a considerable risk of recolonization by the pathogenic bacteria and continuation or progression of the disease. Most importantly, the reattachment without bone gain obtained by scaling and root planing procedure on the disinfected root surfaces is not long-term and can result in pocket reformation as a result of deattachment as early as 3 months following SRP. Surgical procedures that primarily provide access to the site of destruction are often required in patients whose periodontal status remains unimproved after SRP and have been used to treat chronic periodontitis for decades. The rationale for the use of surgery in periodontal treatment is based on its ability to provide better access for removal of etiologic factors, decrease deep probing depths, and regenerate or reconstruct lost periodontal tissues. However, the only advantage of surgical approaches without specific procedures designed to regenerate periodontal tissues, including the periodontal ligament and alveolar bone, is its ability to increase the efficacy of root debridement, especially at sites with deep probing depths or bifurcations. Both nonsurgical and surgical procedures frequently result in the formation of long junctional epithelium without bone formation or new connective tissue attachment, which is considered weak and unstable. , Conversely, these procedures may cause a decrease in bone height or gingival recession, leading to esthetic and functional concerns. ,
Adjunct therapies to nonsurgical and surgical approaches have also been proposed and used to overcome the limitation of these mechanical procedures. In this context, several pharmacotherapeutics, including locally administered antimicrobials such as tetracycline, metronidazole, chlorhexidine, doxycycline, and minocycline, have since then been successfully used in periodontal therapy. Local drug delivery as a monotherapy does not provide a superior result when compared with SRP. This approach is practical when used adjunct to SRP, especially in sites that do not respond to conventional therapy. However, the studies conducted with systemic or local antimicrobials revealed that the long epithelial attachment obtained by these treatments is still at high risk of demolishing and pocket formation and biofilm recolonization, especially in susceptible individuals.
Fundamentally, periodontitis is irreversible with the progressive destruction of soft and hard tissues, including bone. Once the periodontal tissues are lost, the healthy periodontal architecture could not be rebuilt. Wound healing is a dynamic process that involves many aspects that are unpredictable and can be challenging following treatment. To date, wound management was the center of the new techniques and treatments to overcome those challenges. Following surgical debridement, the wound healing process takes place with 4 distinct but overlapping phases in sequence involving various cell types, extracellular matrix, cytokines, and growth factors, hemostasis and coagulation (blot clot), inflammation, cell proliferation and migration (fibroblasts and collagen), and remodeling. It is essential to understand wound healing concerning various aspects of cells, molecules, and their functional properties to regenerate tissue functionally and structurally indistinguishable from the original tissue instead of repair that presents fibrotic scars. Under normal conditions, upon injury or surgical debridement, blood clot formation is followed by the initial inflammation, where polymorphonuclear neutrophils and monocytes are activated for efficient phagocytosis and wound debridement by eliminating necrotic tissues. The initial inflammatory response shifts to a late inflammatory phase in which macrophages migrate to the wound area with secreted cytokines or growth factors that promote the wound healing process. Granulation tissue formation is then initiated with collagen accumulation following the inflammatory stage, where cytokines and growth factors induce the migration and proliferation of fibroblasts and endothelial cells into the wound site. The formation and maturation of a new collagen-rich matrix with endothelial cells involving in angiogenesis for revascularization are then activated by this cell-rich granulation tissue. The fate of the granulation tissue maturation is determined by the essential factors, including available functional cells and receptor-mediated events or signals making distinct outcomes, “tissue regeneration” or “repair.” Studies with the fetal wounds where scarless and rapid healing occurs have highlighted the importance of proinflammatory and anti-inflammatory cytokine balance and the timely secretion of the growth factors by the cells, and a wound healing process orchestrated by highly specialized resolution molecules and pathways. However, this coordination is affected by several factors in adult tissues with changes in inflammatory response capacity and in oral tissues that continuously face bacterial and physical insults.
In the late 1980 early 90s, a novel surgical approach, namely “guided tissue regeneration (GTR),” was introduced as an alternative to nonsurgical or resective surgical techniques in interproximal areas with vertical or angular bone loss such as class II furcation defects. , This novel approach was based on the biological concepts of periodontal cellular structures, including fibroblasts, periodontal ligament cells, cementoblasts, and osteoblasts, and their complex interactions. Introducing a physical barrier over the bony defect to prevent the early and rapid down-migration of epithelial cells provided opportunity and time for connective tissue fibroblasts, the periodontal ligament cells, cementoblasts, and alveolar bone cells to migrate to the disinfected and freshly wounded area by mechanical treatment, to initiate a periodontal regenerative wound healing. This approach was the first to result in new bone formation and new tissue attachment that comprises all essential tissue compartments of the periodontium. The ultimate goal was to create a preserved space perfectly sealed from the outside environment, including microbial species. Initially, periodontal regeneration has focused predominantly on bone substitutes and/or barrier membranes to maintain the space and repopulate the cells responsible for defect fill. Within this context, numerous studies testing different bone substitute materials, physical barriers as occluding membranes, root cementum conditioning, and combination of these with adjunct use of systemic and local antibiotics for adjunctive antimicrobial therapies, have reported varying levels of success and failures over the last 30 years since the concept was first introduced. Systematic reviews on regenerative periodontal treatments show evidence that GTR has a more significant effect on improved attachment gain, reduced pocket depth, more gain in hard tissue with only a minor increase in gingival recession than open flap debridement. However, they also highlight marked variability between studies and the clinical relevance of these changes that is unknown.
Thus, today the treatment of periodontitis in the regeneration of lost tissues, including gingiva, periodontal ligament, cementum, and alveolar bone, is still challenging with unpredictable outcomes and at under the desirable levels, or significant side effects hinder their success. Most importantly, as chronic adult periodontitis often results in horizontal bone loss around the teeth, supra alveolar bone growth on denuded root surfaces has not yet been achieved with any of the therapeutic approaches, which requires a combination of tissue engineering, biologicals, and immune modulation with specifically targeted pathways and molecules that drive tissue formation.
Host modulatory therapies directed at the regeneration of the periodontium
As described in the previous section, the classical “GTR” promotes the critical cell and tissue compartments to regenerate lost tissues under a guided strategy. It is in theory, and in rare instances, it accomplishes the goal of the treatment with a complete regeneration. However, this is not only a rare event; it is also unpredictable, and many factors including flap design, space, type of materials used, location of the tooth, defect size, number of defect walls remained, pulp vitality, patient’s age, smoking, and medical conditions such as diabetes mellitus could influence it.
More recently, several technologies, including growth factors, biologicals, and scaffolds, have evolved to be viewed as emerging therapeutic approaches for periodontal regeneration. In attempts to promote certain cell types for proliferation or tissue formation, several biologicals, including growth factors, collagen, platelet-rich plasma (PRP), plasma rich growth factor (PRGF), fibrin sealant, have been used with varying capacities in regenerating periodontal tissues lost to the disease. Among these approaches, enamel matrix derivates (EMDs) have been used extensively in periodontal regeneration in vertical interproximal defects or furcation involvements. EMD is considered a tissue healing agent derived from proteins during cementogenesis in tooth development to enrich the cellular layers and stimulate tissue regeneration around teeth. Amelogenin is the main protein of EMD known to inhibit epithelial growth, which is the key component of the GTR strategy. In addition, amelogenin has been shown to exert osteoprotective properties and anti-inflammatory actions that enhance soft tissue healing. Furthermore, several in-vivo and in vitro studies have shown EMD’s role on cytokine balance by reducing the secretion of chemokines and proinflammatory cytokines, providing a smooth transition to hemostasis and complete healing.
PRP and PRGF are autologous bioactive substances that contain biologically active proteins which bind to the fibrin mesh or extracellular matrix, promoting the recruitment of stem cells and wound healing. However, although these factors have been proven to promote tissue healing and bone formation in extraction sockets, sinus augmentation, and around dental implants, their use in periodontal defects and capabilities of promoting periodontal regenerations are limited.
Recent consensus reports , reviewed the current FDA-approved and nonapproved emerging therapeutic approaches focusing on host modulation and tissue engineering such as protein and peptide therapy, cell-based therapy, scaffolds, bone anabolics, and lasers. FDA-approved products evaluated included EMD; recombinant human platelet-derived growth factor; and an anorganic bone matrix, whereas nonapproved therapeutic modalities included recombinant human fibroblast growth factor-2; recombinant human growth differentiation factor-5; bone morphogenetic proteins (BMP-2, BMP-7, BMP-6, and BMP-12); parathyroid hormone/teriparatide; brain-derived neurotrophic factor; and sclerostin antibodies. Mesenchymal stem cells, bone marrow stromal cells, periodontal ligament cells, embryonic stem cells, and induced pluripotent stem cells were reviewed among cell-based therapeutic approaches, viral and nonviral vectors as genetic therapies and scaffolds that show promising results for delivery of growth factors and gene therapy and combination of either natural or synthetic polymeric materials in periodontal regeneration. However, although these therapies appear viable as emerging regenerative approaches for periodontal hard and soft tissue regeneration with the potential of reconstructing the entire periodontium, the cost-to-benefit ratio and safety issues still need to be overcome, and most importantly, there is insufficient evidence with those therapies to warrant definitive clinical recommendations.
Host modulation therapies directed at resolution of inflammation in regenerative periodontal treatment
In the event of uncontrolled host defense mechanisms, tissue engineering, regeneration, and reconstruction of both diseased and injured tissues are significantly hampered. Inflammation is a host defense mechanism orchestrated by key cellular and molecular events leading to activation of defensive immune subsets to limit detrimental injury, eliminate pathogenic agents, and remove infected cells. Under normal conditions, a parallel host mechanism operates to contain inflammatory response leading to health or stability. Thus, resolution of inflammation is an effective suspension on the proinflammatory pathways to avoid the tissue damage inside the host and leads to the reestablishment of tissue homeostasis. However, uncontrolled inflammation may cause tissue damage by perturbing homeostasis toward immune dysregulation and chronic inflammation. Dysregulation of the resolution pathways can negatively impact tissue functionality and contribute to the disease state.
In contaminated traumatic wounds involving bone, an uncontrolled inflammatory response causes neutrophil-mediated tissue injury that, in turn, leads to irreversible bone loss. Neutrophils are essential in microbial host defense, but chronic neutrophil activation, due to failure of a timely switch from inflammatory to resolution pathways, can release noxious materials leading to host tissue injury and loss of function. Likewise, in chronic osteolytic inflammatory diseases such as periodontitis, , a failure of endogenous resolution pathways and removal of microbial challenge result in tissue destruction. Therefore, one of the most crucial elements determining the fate of the wound healing process and success of the regeneration is the inflammatory response to the treatment and endogenous anti-inflammatory, and proresolving mechanisms that activate wound healing with tissue regeneration instead of fibrosis and scarring , are crucial.
Modulation of inflammatory response and control of inflammation is, however, a challenging issue. To this end, anti-inflammatory agents such as nonsteroidal anti-inflammatory drugs (NSAIDs), although logical, do not provide predictable outcomes, partly due to side effects and compliance issues. In addition, anti-inflammatory agents can downregulate essential immune cell functions critical for wound healing; therefore, they could be toxic to regeneration.
The discovery of the new families of lipoxins, EPA- and DHA-derived chemical mediators, namely resolvins and protectins, open new avenues to design resolution-targeted therapies to control unwanted side-effects of aberrant inflammation. The findings of series of preclinical in vitro and in vivo , , studies demonstrated the beneficial role of the lipid mediators in temporal resolution of inflammation and returned to hemostasis in experimental periodontitis. Mediators of resolution of inflammation also have actions beyond the control of neutrophils; modulation of osteoclast and osteoblast function in a receptor-mediated fashion in wound healing and bone regeneration. , Furthermore, exciting findings with animal models proved an old hypothesis known as “self-healing capacity if promoted” and revealed that endogenous control of inflammation directly impacts bone healing and regeneration. , Studies with lipoxin and resolvins in experimental periodontitis in mice, rats, and rabbits demonstrated that specialized proresolution lipid mediators (SMPs) resolve the inflammation and promote regeneration of soft and hard tissues around teeth. Furthermore, benzo lipoxin A4 (a stable analog of lipoxin A4) resulted in a complete periodontal regeneration with the periodontal ligament, new cementum, and alveolar bone formation in chronic periodontal defects compared to conventional flap surgery.
The incorporation of proresolving mediators into nanoparticles constructed from natural microparticles has proven to be a practical approach for promoting survival in animal models of sepsis and reducing inflammation and stimulating resolution. Using this “proresolving nanomedicine” approach, a recent study demonstrated that complete regeneration of soft and hard tissues lost to inflammatory periodontal disease in a large animal model mimicking human periodontal disease and wound healing process following surgical debridement was clinically possible ( Fig. 1 ). Furthermore, this approach not only resulted in a resolution of inflammation at the local wound but also promoted systemic endogenous biosynthesis of SPMs and dampened production of proinflammatory mediators, indicating that nanoparticles constructed with the stable lipoxin analog, benzo-lipoxin A4 (bLXA4) potentiates the anabolic actions of bLXA4 in the regeneration of tissues lost to inflammatory disease by activating distinct proresolving and tissue-protective pathways. Currently, bLXA4 is in the process of drug development to treat periodontal diseases ( ClinicalTrials.gov NCT02342691).
