The Influences Of Periodontal Inflammation On The Biological Properties Of Dental Pulp Stem Cells Within The Inflamed Dental Pulps

Background Stem cells constitute the source of differentiated cells for generating tissues during development and have attracted interest for regenerating tissues that are diseased or damaged postnatally. In recent years, we have witnessed major advances in both our understanding of stem cell biology and the practice of stem cell-based therapies in many clinical settings. Over this same period, stem cell research has grown exponentially due to the recognition that the use of stem cells as therapeutics has the potential to improve the quality of life of patients with a wide variety of medical conditions, ranging from Alzheimer’s disease to cardiac ischemia to bone or tooth loss. Mesenchymal stem cells(MSCs) represent a population of nonhematopoietic fibroblast-like cells that display the capacity to self-renew and differentiate into multiple lineages, including osteoblasts, adipocytes and chondrocytes, and these cells divide to replenish dying cells and regenerate damaged tissues. These unspecialized cells were originally discovered in bone marrow, but have also been found in a variety of connective tissues, including adipose tissue, the umbilical cord, synovial membranes, tendons and many dental tissues, at either mature or developing states. Although the optimal cell type for biomedical use remains debatable, the ease of isolation of dental stem cells, in particular dental pulp stem cells(DPSCs), from discarded or extracted human teeth, generally due to impaction or orthodontic reasons, makes them a promising source of autologous postnatal stem cells for numerous and varied bioengineering and biotherapeutic applications. Initially, the realization of DPSCs’ ability to differentiate into odontoblasts led to efforts in tooth tissue engineering that aim to create reparative dentin support for damaged tooth structures. However, there is mounting evidence suggesting that these stem cells are able to repair extraoral tissues(e.g., in the musculoskeletal system) owing to their similarities with bone marrow mesenchymal stem cells(BMMSCs). DPSCs are now recognized as arguably the most accessible and attractive multipotent MSCs with high proliferation rates for tissue engineering and regenerative medicine.Since the characterization of clonogenic, highly proliferative cell populations from adult human dental pulp in 2000 and human exfoliated deciduous teeth(generally termed SHED) in 2003, stem cells from dental tissues other than dental pulp, such as the periodontal ligament(PDL), apical papilla, dental follicle and gingival tissue, have also been identified and investigated for potential therapeutic applications. These dental stem cells are also relatively easy to harvest from naturally lost or surgically removed teeth. They are also generally homogenous and proliferate faster than BMMSCs and many other MSC types. In addition to their excellent capacity to differentiate into odontogenic cells, they also have the ability to give rise to other cell lineages similar to, but different in potency from, BMMSCs. Importantly, most, if not all, dental stem cells display a stable morphology and do not lose their MSC characteristics at higher passages during culture or after in vivo transplantation. Because postnatal root formation is a developmental process, some dental cell types involved in root formation(e.g., stem cells from the apical papilla and periapical follicle) are expected to be more embryonic-like than other sources of dental stem cells. As a result, dental stem cells have potential for applications in not only dentistry but also in the treatment of neurodegenerative and ischemic diseases, diabetes research, bone repair and other applications in the field of regenerative medicine.An important but often overlooked advantage of teeth as a source of stem cells is that many dental stem cells can also be harvested from inflamed/diseased dental tissues, such as inflamed dental pulp or PDL tissues, hyperplastic or inflamed gingiva and inflamed periapical tissues. Furthermore, there is increasing evidence suggesting that these cells retain, at least to some extent, the MSC properties and tissue regeneration potential, indicating that dental stem cells can be much more accessible than is currently thought and could have a more important role in future developments in regenerative medicine. In investigations on this topic, DPSCs isolated by pulpectomy from diseased dental pulp collected from teeth diagnosed with irreversible pulpitis have shown a similar, or slightly decreased, colony forming capacity, proliferation rate and osteo/dentinogenic potential compared to DPSCs isolated from normal pulp tissues, but both cell types have demonstrated the ability to form pulp/dentin complexes when transplanted into immunocompromised mice. Although these findings suggest that inflammation does not significantly affect their stem cell properties, the inflammatory environment inevitably leads to certain changes in the proliferation rate, differentiation potential and immunomodulatory properties of stem cells. These effects have been demonstrated by studying stem cells after in vitro inflammatory stimulation, as well as by studying stem cells isolated from in vivo inflamed tissues. Further characterization of these multipotent cell populations will generate important knowledge regarding the effects of inflammation on human dental stem cells and determine whether stem cells isolated from inflamed dental tissue can serve as a source of therapeutic cells for future regenerative therapies.Although DPSCs have been confirmed to exist in clinically compromised teeth with irreversible pulpitis, and these cells have demonstrated potential for application in tissue engineering and regenerative medicine, the influence of periodontal inflammation and its disease progression on human DPSCs remains unexplored. The characterization of DPSCs isolated from periodontally compromised teeth offers a new way to understand the condition of dental pulp tissue within a diseased tooth. However, millions of teeth are extracted annually due to periodontitis, and these teeth could provide an inexhaustible cell source for research and applications if DPSCs retain some of their MSC properties during the different stages of periodontitis.Materials and Methods DPSCs were isolated from discarded human teeth that were extracted due to aggressive periodontitis(Ag P) and divided into three experimental groups(Groups A, B and C) based on the degree of inflammation-induced bone resorption approaching the apex of the tooth root before tooth extraction. DPSCs derived from impacted or nonfunctional third molars of matched patients were used as a control. Mesenchymal stem cell(MSC)-like characteristics, including colony-forming ability, proliferation, cell cycle, cell surface antigens, multilineage differentiation capability and in vivo tissue regeneration potential, were all evaluated in a patient-matched comparison.Results It was found that STRO-1- and CD146-positive DPSCs can be isolated from human teeth, even in very severe cases of Ag P. Periodontal inflammation and its progression had an obvious impact on the characteristics of DPSCs isolated from periodontally affected teeth. Although all the isolated DPSCs in Groups A, B and C showed decreased colony-forming ability and proliferation rate(P < 0.05), the decreases were not consistent with the degree of periodontitis. Furthermore, the cells did not necessarily show significantly diminished in vitro multi-differentiation potential. Only DPSCs from Group A and the Control group formed dentin-like matrix in vivo when cell-seeded biomaterials were transplanted directly into an ectopic transplantation model. However, when cell-seeded scaffolds were placed in the root fragments of human teeth, all the cells formed significant dentin- and pulp-like tissues. The ability of DPSCs to generate dental tissues decreased when the cells were isolated from periodontally compromised teeth(P < 0.05). Again, increased periodontal destruction was not necessarily followed by a decrease in the amount of dentin- and pulp-like tissue formed.Conclusions This is the first exploratory study that focuses on the impact of periodontitis and its progression on the stem cell properties of DPSCs. Our data demonstrate the existence of functional MSCs in periodontally compromised teeth from patients with aggressive periodontitis, as long as part of the pulp is still viable. Further characterization is needed to determine whether these cells, or other subpopulations within the diseased pulp, can serve as a source of endogenous multipotent cells for stem cell therapy and tissue engineering. In particular, it is necessary to establish a protocol to verify the consistency of their differentiation and tissue regeneration potential. Studies should also address the molecular events underlying the changes in their stem cell properties to determine whether their reduced stemness can be reversed to make the cells more like normal DPSCs. Such investigations may also be helpful for identifying the determinants of periodontal disease progression in pulpal damage. They could also help establish clinical guidelines to protect DPSCs from inflammation insult and/or to circumvent the associated impairment of their regenerative abilities.