**4. Dental-related stem cell based tissue regeneration**

Although DSCs are newborn in the field of stem cell therapy and tissue engineering, they have opened the promising windows not only in tooth repair and regeneration studies but also in other organs. To date, most of DSCs mediated cell therapies and tissue-engineering studies have been focused on the animal models. However, more recently, a few clinical trial studies have also been accomplished. Meanwhile, the immunogenicity of these cells should be more considered for their allogeneic transplantation.

#### **4.1. DSCs-based tooth engineering and regeneration of dental related tissue**

In general, in the field of dentistry, the final goal of tissue engineering is to develop toothreplacement therapies using the whole-bioengineered-tooth technology. To fulfill this, some authors have conducted the remarkable investigations. In this regard, there is some experiment in which either dissociated tooth germs or mesenchymal and epithelial cells derived from the tooth germs were prepared and loaded on to the prefabricated tooth-shaped scaffolds in order to fabricate a bioengineered tooth. Based on these investigations, tooth germ cells possessed a high potential to form dentin, enamel, pulp, and periodontal tissues in vivo [81-84]. However, using embryonic tooth germ and problems related to immunogenicity of animal transplanta‐ tion, make this an unfeasible approach to clinical application of tooth regeneration [84].

Alternative cells would be DSCs. Using these cells, some preliminary studies have been accomplished to examine whether DSCs in combination with appropriate scaffold are able to promote regeneration of tooth tissues [85, 86]. For instance, transplantation of PDLSCs

loaded onto HA/TCP have been shown to result in the production of cementum in mice while using gelfoam (collagen based gelatin sponge) as the carrier has been reported to fail creating cementum [31, 87]. Sonoyama and co-workers have reported the creation of a root/periodontal complex being capable of supporting a porcelain crown in swine by applying both SCAP and PDLSCs in HA/TCP as a carrier [38]. In a study on canine model, we have recently succeeded to achieve regeneration on an experimentally-created defect in peridontium using autologous DPSCs loaded onto Bio-Oss scaffolds [88]. Furthermore, Nakashima et al. have reported a successful induction of whole-pulp regeneration after pulpectomy in a dog model using autologous DPSCs loaded onto 3D scaffold of collagen-I & II and SDF-1 (stromal-cell-derived factor-1) as the morphogene [89, 90]. Moreover, a culture system termed as organ-engineering methods using PDLSCs have been devel‐ oped to generate not only tooth root, but also the surrounding periodontal tissues, including PDL and alveolar bone in mouse model [84]. In addition to the above-men‐ tioned animal studies, there is a published retrospective pilot study in human. Accord‐ ing to this trial, autologous transplantation of PDL stem/progenitor cells might provide therapeutic improvement for the periodontal defects without any adverse effects during 32–72 months of follow-up [91].

#### **4.2. DSCs-based tissue engineering and regeneration for other organs**

Apart from the potential applications in dental tissue engineering and therapy, DSCs have been opened a dynamic field in repair and regeneration of non-dental tissues. In this context, there are many investigations indicating applicability of dental related stem cells in variety of disease models in the animal. For instance, SHED has been reported to exhibit a potential to improve parkinson's disease in rat by differentiating into dopaminergic neuron-like cells. Based on the different investigations, these cells have also the ability to promote wound healing in mice [92-94]. It has been demonstrated that SHED also contribute to repair of the criticalsized calvarial defects in mice model [95].

Transplantation of DPSCs has been shown to improve alzheimer's and parkinson's disease as well as acute myocardial infarction in a rat model [96-98]. Similarly, transplantation of hIDPSCs in canine model with golden retriever muscular dystrophy (GRMD) resulted in some improvement [99]. In addition, it has been reported that transplantation of tissue-engineered hIDPSC sheet was successfully reconstructed the corneal epithelium in rabbits with total limbal stem cell deficiency (LSCD) [100]. Applying PDLSCs with collagen based gelatin sponge carrier have been found to improve facial wrinkles by generating large amounts of collagen fibers in the mouse indicating the potential capacity of PDLSC in the field of plastic surgery. In the case of GSCs, *Intra-peritoneal (ip)* administration of the cells displayed improvement of inflamma‐ tion-related tissue destruction in experimental colitis [43].

#### **4.3. Immunogenicity of allogeneic transplantation of DSCs**

According to the promising place of DSCs in clinical treatments in future, considering the immunogenicity of DSC transplantation is critical. Although autologous DSCs rather than allogenous ones are preferred for repair and regeneration purposes, several limitations such as inadequate cell numbers and donor site morbidity carry out problems. Thus, more studies will be needed to evaluate immunogenicity of allogenous DSCs for the future clinical appli‐ cations. There are some evidences suggested that DSCs, as the mesenchymal stem cells, have immunomodulatory properties both in vitro and in vivo. These studies are almost considering in vitro immunogenicity of DSCs by evaluating the expression of pro/anti-inflammatory mediators, such as MHC classes, TGF-β, ODO, HGF, nitric oxide (NO), prostaglandin, immunosuppressive minor H antigen (HLA-G), and interferon (IFN)-γ as well as effect of DSCs in suppression of T cells proliferation [28, 101, 102].

In addition, in vivo studies also showed DSCs immunomodulatory functions. For instance, SHED transplantation up-regulated the ratio of Treg (regulatory T cells) and Th17 in MRL/ lpr mice model compared to the BM-MSCs; meanwhile, no change in the expression of IL-6 & IL-10 was detected. On the other hand, transplantation of human DPSCs into the rat model didn't initiate the immunologic responses in recipients [103]. Moreover, in another study, it was demonstrated that transplantation of hIDPSCs didn't show the immune reaction in the canine [99].

In spite of existing evidences, which suggested the immunomodulatory effect of DSCs, there are some data proposing that there is different expression of immune receptors, such as tolllike receptors (TLRs) on the cells. Presence of this receptor could affect the immumomodula‐ tory phenotypes of DPSCs and DFPCs [101, 104, 105]; On the other hand, since some studies suggested that BM-MSCs may regain immunogenic property upon differentiation [106], the subject of immunogenicity of DSCs must be examined not only in an undifferentiated state but also in differentiated form.
