Vol. 117 No. 3 March 2014

Mesenchymal stem cell therapy for salivary gland dysfunction and xerostomia: a systematic review of preclinical studies David Hebbelstrup Jensen, MD,a Roberto Stefan Oliveri, MD, PhD,b Stig-Frederik Trojahn Kølle, MD,c Anne Fischer-Nielsen, MD, DMSc,b Lena Specht, MD, DMSc,d Allan Bardow, DDS, PhD,e and Christian Buchwald, MD, DMSca University of Copenhagen and Copenhagen University Hospital Rigshospitalet, Copenhagen, Denmark

The most severe forms of xerostomia and salivary gland dysfunction, as well as a severely reduced quality of life, are seen in Sjo¨gren syndrome (SS) and after radiotherapy for head and neck cancer. For both conditions, no effective regenerative therapies yet exist. Thus, the aim of this article was to assess, through systematic review, the potential benefit of mesenchymal stem cell (MSC) therapy in radiation-induced and SS-related salivary gland dysfunction and xerostomia. We searched PubMed/MEDLINE, Embase, Web of Science, the Cochrane Database of Systematic Reviews, the World Health Organization Clinical Trials Registry Platform, and Google Scholar. We identified 6 separate study comparisons eligible for inclusion. Owing to the limited number of studies, we conclude that more randomized, adequately powered clinical trials are needed to validate the potential beneficial effect of MSCs on salivary gland dysfunction and xerostomia. Nonetheless, the preliminary studies identified in the present review were encouraging for further research. (Oral Surg Oral Med Oral Pathol Oral Radiol 2014;117:335-342)

OVERVIEW Xerostomia, or dry mouth syndrome, is the subjective feeling of dry mouth, which may or may not be associated with a reduced secretion of saliva. The feeling of oral dryness is generally not observed until the salivary flow is reduced by more than 50%.1 Numerous conditions can lead to a reduced secretion of saliva, and hence xerostomia; it occurs most commonly as a side effect of many drugs. Among the more severe conditions, which also have the greatest effect on the secretion rate and normally a chronic perspective, are Sjögren syndrome (SS) and effects of radiotherapy for head and neck cancers (radiation-induced xerostomia [RIX]). Both RIX and SS are characterized by a progressive loss of acinar cells in the salivary glands and thus a progressive decline in saliva production. Patients with xerostomia have a severely reduced quality of life, David H. Jensen is supported by a grant from the Candys Foundation, a nonprofit foundation. This study did not receive other grants from any funding agency in the public, commercial, or not-for-profit sectors. a Department of Otorhinolaryngology, Head & Neck Surgery and Audiology, Copenhagen University Hospital Rigshospitalet, and Faculty of Health and Medical Sciences, University of Copenhagen. b Cell Therapy Facility, Blood Bank, Department of Clinical Immunology, Copenhagen University Hospital Rigshospitalet. c Department of Plastic Surgery, Breast Surgery & Burns, Copenhagen University Hospital Rigshospitalet. d Department of Oncology, Copenhagen University Hospital Rigshospitalet. e Department of Oral Medicine, School of Dentistry, University of Copenhagen. Received for publication Jun 21, 2013; returned for revision Oct 27, 2013; accepted for publication Nov 14, 2013. Ó 2014 Elsevier Inc. All rights reserved. 2212-4403/$ - see front matter http://dx.doi.org/10.1016/j.oooo.2013.11.496

and currently no treatments are aimed at regenerating the acinar cells and thus restoring normal salivary function. However, recent studies have suggested that treatment with mesenchymal stem cells (MSCs) might be a promising regenerative treatment option for increasing saliva flow rates (SFRs) and thus relieving xerostomia. The aim of this study was to assess the potential benefit of MSC therapy in RIX and SS-related salivary gland dysfunction and xerostomia, based on a systematic review of the published literature on the subject. Although the etiologies of RIX and SS-related salivary gland dysfunction are very different, the effects of both are comparable in terms of their chronic and progressive nature and their severe effect on SFRs, which nearly always leads to pathologically reduced salivary flow (i.e., hyposalivation, International Classification of Diseases [ICD]-10 K11.7) and xerostomia (i.e., ICD-10 R68.2). Without any comparison in mind, both conditions are dealt with in tandem in the present review, because MSC therapy could become a treatment option for both. Thus, at this preclinical stage of MSC therapy for RIX and SS-related salivary gland dysfunction, all positive findings should be regarded as encouraging for future treatment strategies.

Statement of Clinical Relevance The treatment of xerostomia after radiotherapy and Sjögren syndrome is unsatisfactory. Mesenchymal stem cell therapy has previously been evaluated in numerous clinical trials with positive results. However, a critical appraisal of this intervention for xerostomia has not been conducted. 335

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Radiation-induced salivary gland dysfunction and xerostomia Radiotherapy directly and indirectly leads to irreversible damage of the acinar cells in the salivary glands. The mean dose that represents the threshold for a significantly reduced salivary secretion is 26 to 39 Gy,2-5 but no lower limit for parotid gland dysfunction has been observed.6 Radiotherapy plays a major role in the curative treatment of most head and neck cancer, either as single-modality therapy or in combination with chemotherapy, surgery, or both. Despite the new and more advanced methods of delivery, such as intensitymodulated radiotherapy, a significant proportion of the radiation is deposited in the tissue surrounding the tumor. Of particular concern to patients’ quality of life is the co-irradiation of the salivary glands during radiotherapy. The effect of radiation on the glands is observed early in the course of radiotherapy,7 and the observed injury of the salivary secretory tissue has been estimated to be maximal at 6 days after irradiation.8 SS-related salivary gland dysfunction and xerostomia SS is a chronic, systemic autoimmune disease that generally leads to a progressive decline in salivary flow, owing to the autoimmune destruction of the acinar cells in the salivary glands. It is defined as either a primary disorder or occurring in connection with other connective tissue diseases.9 Therefore, although the sicca symptoms are one of the defining features of the disease, a great number of other organ systems could potentially be involved. However, the disease is often characterized by a lymphocytic infiltrate in the salivary glands that attacks the exocrine cells, particularly the acinar cells. The current treatment options for primary SS are largely symptomatic, in that they generally aim at stimulating the residual salivary cells with cholinergic medication, which of course is only effective if enough secretory cells remain. In addition, salivary substitutes are often prescribed. For the more heterogeneous group with SS in relation to other connective tissue diseases, systemically acting immunosuppressive agents or B celledirected therapies are treatment options.10-12 These therapies are aimed at decreasing the autoimmune destruction of exocrine tissue but not at regenerating already destroyed acinar cells. However, conflicting results exist on the effectiveness of such treatments.10-12 Therefore, a therapy aimed at regenerating the secretory cells is greatly needed for a growing number of patients. Oral sequelae of salivary gland dysfunction and xerostomia The decreased production of saliva both after radiotherapy and in SS predisposes the patient to a variety of

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conditions. This is caused either directly or indirectly by the diminished production of saliva. The sequelae include oral dryness and impairment of normal oral functions (speech, chewing, and swallowing) because of insufficient lubrication of the mucosal surfaces and of ingested food. Furthermore, the oral mucosa can become dry, leading to ulceration and easy injury. In addition, the directly reduced flow leads to a diminished clearance of bacteria and their substrates from the mouth, which in combination with other factors may result in highly increased and severe dental caries, as well as oral candidiasis.13,14 Stem cell treatment for salivary gland dysfunction and xerostomia Stem cell therapy may hold great promise for the development of novel interventions for many serious diseases and injuries and has therefore gained considerable attention among patients, advocacy groups, health care providers, and policy makers. Stem cells are able to renew themselves while at the same time being able to differentiate into more specialized cell types.15 Among the various types of embryonic, fetal, and adult stem cells, multipotent MSCs have shown particular promise in a large number of preclinical studies and clinical trials. Accordingly, ex vivo expanded MSCs have shown promising results in clinical trials for a number of conditions, including graft-versus-host disease,16 Crohn disease,17 induction therapy in organ transplantation,18 and ischemic cardiomyopathy.19 MSCs can be isolated from various adult tissues but have been most thoroughly characterized and assessed when isolated from the bone marrow, where they are thought to have a supportive role in hematopoiesis. Also, adipose tissuee derived MSCs have gained increasing and considerable interest during the past decade.20 MSCs were originally described more than 40 years ago by Friedenstein.21 Morphologically, MSCs are spindle-shaped and fibroblast-like. As there is no unique surface marker identifying MSCs, a set of minimum criteria for the identification of MSCs has been proposed by the International Society of Cellular Therapy: (1) adherence to a plastic culture surface; (2) differentiation into mesodermal cell lineages (adipocytes, chondrocytes, and osteoblasts); and (3) expression of the surface markers CD73, CD90, and CD105 with concomitant absence of the surface markers CD34, CD45, CD14 or CD11b, CD79a, or CD19 and HLA-DR.22 Four mechanisms by which MSCs could potentially improve organ function have been proposed: cell fusion, cellular transdifferentiation, vasculogenesis, and local paracrine effects.23 In vitro, MSCs have been found to be able to differentiate into specialized cell types from the mesodermal germ layers (i.e., adipocytes, osteoblasts, and chondroblasts). In addition,

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Fig. 1. Flow chart illustrating the literature search strategy and the different phases of study eligibility assessment in the systematic review. Electronic databases were searched from dates of inception to January 2013.

some recent studies indicate a potential for true transdifferentiation into cells from other germ layers.24 The paracrine bystander effects of MSCs in the form of immunomodulatory, antiapoptotic, anti-inflammatory, and angiogenic effects have also gained clinical and therapeutic interest in recent years.25,26 Systemically infused, ex vivo expanded MSCs have been found to have a tendency to home to local inflammatory and damaged sites by a chemotactic gradient in the bloodstream,27,28 although many of the intravenously administered MSCs are transiently trapped in the lungs.29,30 MSC therapy may thus hold promise as one option for the treatment of salivary gland dysfunction and xerostomia, given the putative regenerative ability reported in preclinical studies and clinical trials.

METHODS All animal/human studies describing the assessment of MSCs vs control (placebo, no treatment, or best available care) for salivary gland dysfunction and xerostomia were considered eligible for inclusion. Furthermore, we searched clinical trials reporting the use of MSCs in these

conditions (whether single-cohort studies or controlled). Studies were included irrespective of publication status or language. The primary outcome was the mean increase in SFR, which was extracted using SFR data from figures. The figures were imported into imageJ,31 and a coordinate system was overlain. This coordinate system was used to more precisely evaluate the precise location of SFR time points. To evaluate bias in the studies, we also noted how generation of allocation sequence was done, if a concealed allocation of participants was reported, and if the study was blinded. We also set out to evaluate possible publication bias using a funnel plot and to evaluate smallstudy effects using Egger regression. Strategy for searching for relevant publications Electronic searches were performed in the Cochrane Database of Systematic Reviews (1996 to January 2013), PubMed/MEDLINE (January 1966 to January 2013), Embase (1974 to January 2013), Web of Science (1900 to January 2013), and Google Scholar using the following keywords: MSC or mesenchymal stem cell or adipose stem cell or adipose stromal cell or pre-adipocytes or processed lipoaspirate cell or stromal vascular

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Humans

NOD/Ltj mice

Humans

Xu et al. (2012), murine study34 Xu et al. (2012), human study34

Khalili et al. (2012)33

Lin et al. (2011)37

Kojima et al. (2011)36

RIX, radiation-induced xerostomia; MSC, mesenchymal stem cell.

C57BL/6-TgH/BALB/c mice Balb/c or C57BL/6-gfp NOD mice

8- to 9-week-old C57BL/6 C57/BL6 mice Bone marrow RIX mice weighing 18 to 22 g C57BL/6 mice C57BL/6-Tg(CAG-EGFP) Adipose tissue RIX mice NOD.SCID-PrkcSCID mice C57BL/6 mice Bone marrow RIX

Nonblinded, randomized, placebo-controlled trial Nonblinded, randomized, placebo-controlled trial Nonblinded, placebo-controlled trial Nonblinded, placebo-controlled trial Nonblinded, placebo-controlled trial Prospective study

Intervention Disease model Tissue origin of MSCs MSC donor Study participants Study design Reference

Table I. Characteristics of included studies

RESULTS A total of 177 unique references were identified after the removal of duplicates. After abstract screening and full-article assessment, 5 met the inclusion criteria33-37 (Figure 1). Owing to the broad inclusion criteria, only one study was excluded after the screening phase (see Figure 1). The excluded study was a conference abstract on one of the included studies (Table I). None of the studies reported how generation of allocation sequence was done or reported a concealed allocation of participants or blinding. Owing to the small number of studies, we did not evaluate potential publication bias or small-study effects using a funnel plot or Egger regression, respectively. The 5 included references comprised 6 eligible study comparisons. Three studies were on RIX and 2 studies were on SS-related salivary gland dysfunction and xerostomia. Five of the studies were on murine models of salivary gland dysfunction and included control groups, whereas one was a single-cohort study involving patients with SS. A total of 85 animals and 24 patients participated, of which outcome measures for the primary comparison were available in 85 animals and 11 patients. The effect of the intervention was in all cases evaluated by a measure of the SFR, either compared with placebo or as a before/after study. Our primary outcome of interest was the intervention effect on SFR (see Table II), but other relevant variables, such as study design, participants, tissue origin of MSCs, and disease model, were also extracted (see Table I). To evaluate the fate of the transplanted cells, information on cell tracking was also recorded, when available. We did not apply meta-analysis of data, owing to the small number and heterogeneous nature of included studies, encompassing both murine studies and a single human study.

Lim et al. (2013)35

fraction cell and xerostomia. For a full search strategy, please see the supplementary search strategy at http:// dx.doi.org/10.1016/j.oooo.2013.11.496. Manual searches included scanning of reference lists in relevant articles. Additional ongoing but unpublished trials were sought via the World Health Organization Trial Register.32 Data collection and analysis were performed independently by 2 authors (D.H.J. and R.S.O.), and disagreements were solved by consensus. Included articles were selected using the aforementioned search criteria, and abstracts were screened for relevance. All authors participated in the selection of trials for inclusion. The following data were extracted from the included articles: study design, study participants, MSC donor, tissue origin of MSCs, disease model, effect on SFR, number of participants, and time from induction of disease to intervention (if applicable) in both intervention and control groups. If an outcome was measured serially, only the final measure was reported.

1
105 MSCs were injected into each submandibular gland 24 hours after irradiation with 15 Gy 1
105 MSCs were injected into each submandibular gland 10 weeks after irradiation with 10 Gy 1
105 MSCs were injected into each submandibular gland 11 days after irradiation with 15 Gy Bone marrow Sjögren-like 1
105 MSCs were given intravenously once, when animals were 6 weeks of age or 16 weeks of age Bone marrow Sjögren-like 1
105 MSCs were given intravenously once, when animals were 6 weeks of age or 16 weeks of age Umbilical cord Sjögren 1
106 MSCs per kilogram body weight were given disease intravenously once

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The SFR reported is from a follow-up visit at 12 months 163% increase 11 patients, no control group 1.6* 0.6*

Radiation-induced salivary gland dysfunction and xerostomia Lim et al.35 used a murine model of radiation-induced salivary gland dysfunction and (RIX) in which they irradiated a group of mice with 15 Gy and obtained a measure of the stimulated salivary flow before and after the intervention. Using a local injection of MSCs directly into the submandibular gland, they observed a significantly increased salivary secretion (41%) compared with the saline-treated control, in addition to an increased weight of the glands (see Table II). Histologic analysis found several functional acini, less apoptotic cells, and increased density of microvessels. In a very similar study, Kojima et al.36 used fatty tissueederived MSCs and found improved salivary flow, proliferation of blood vessels, and other paracrine effects of the MSC in the treatment group compared with the control (see Table II). Lin et al.37 found that cell transplantation of MSCs in their RIX model increased salivary flow, gland weight, and body weight compared with the control group.

MSC, mesenchymal stem cell; SFR, salivary flow rate. *This is a before/after study without a control group.

The SFR reported is from a follow-up visit at 12 months 11 patients, no control group 4.3* 3.3*

29% increase

SFR was obtained at 18 weeks of age

Comments No. of participants

6 in placebo and intervention group 27 18

47% increase

22 0.20

Lin et al. (2011)37 Stimulated SFR (mL) Khalili et al. (2012)33 Unstimulated whole SFR (mL/min) Xu et al. (2012), Stimulated SFR (mg/10 min) murine study34 Xu et al. (2012), Stimulated SFR (mL/6 min) human study34 Xu et al. (2012), Unstimulated SFR human study34 (mL/10 min)

17 0.066

41% increase 51% increase 114 0.0035 81 0.0023

Effect Outcomes Study

Stimulated SFR (mL/min) Lim et al. (2013)35 Kojima et al. (2011)36 Stimulated SFR (g/g)

SFR in control SFR after group after intervention in intervention MSC-treated group

Table II. Effect of MSC therapy on SFR

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8 in placebo and intervention groups Measure obtained 12 weeks after intervention 31 in placebo and intervention group The SFR is normalized to weight of animal, which could introduce bias. SFR was obtained 10 weeks after the intervention 32% increase 35 in placebo and intervention group SFR obtained at day 55 202% increase 5 in placebo and intervention group SFR obtained at week 22 of age

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SS-related salivary gland dysfunction and xerostomia Khalili et al.33 found that intravenous injection of bone marrow MSCs prevented loss of salivary flow and reduced lymphocytic infiltration of the salivary glands, the influx of T and B cells, the frequency of FoxP3þ (Treg) cells, and inflammation (tumor necrosis factor a, transforming growth factor b). Xu et al.34 evaluated the effects of intravenously injected bone marrow MSCs in a Sjögrenlike animal model and noted that areas of inflammation in the submandibular glands were significantly reduced. In addition, the flow improved significantly in the treatment group 2 weeks after MSC infusion. Notably, the investigators observed the donor MSCs in the submandibular glands of MSC-treated mice 1 week after the intravenous injection of MSCs, indicating active chemoattraction of MSCs to the salivary glands as anticipated.34 Importantly, the same study evaluated the safety and efficacy of intravenously infused MSCs in 24 human patients with Sjögren disease (including 11 with xerostomia). In particular, all patients tolerated MSCs well, and no adverse events during or after MSC infusion were reported. Unstimulated salivary flow in all 11 patients with symptoms of xerostomia increased significantly 2 weeks after MSC infusion and increased 2-fold after 1 month. This index continued to rise in subsequent follow-up visits. Stimulated salivary flow of these 11 patients was also significantly increased at follow-up visits.34 Cell tracking In 3 of the studies, it was possible to track the fate of the transplanted cells, and a small degree of transdifferentiation of MSCs into glandular tissue was reported in 2 of the

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studies.35,36 In the study by Kojima et al.,36 green fluorescent protein (GFP)-adipose-derived stem cells (ADSCs) were transplanted after irradiation and were found in ductal and endothelial cells at 5 and 10 weeks posttransplantation. However, no GFP-ADSCs were found to transdifferentiate to acinar cells. In the study by Lim et al.,35 a few of the labeled MSCs were suggested to transdifferentiate to a-amylase-producing cells. These results would seem to be in agreement with 2 previous in vitro studies by Lin et al.38 and Maria and Tran,39 both reporting the transdifferentiation of MSCs into acinar cells using a double chamber system.

DISCUSSION To our knowledge, this is the first attempt to systematically review the effect of MSC therapy for the treatment of both SS- and RIX-related salivary gland dysfunction and xerostomia. Although Tran et al.40 have reviewed the use of “bone-marrow-derived stem cells,” a very heterogeneous population of cells, for salivary gland dysfunction, ex vivo expanded MSCs represent a quite different treatment modality. All included studies found a significant increase in SFR after MSC therapy, in both RIX and SS-like induced salivary gland dysfunction and xerostomia (see Table II). The increase in SFR ranged from 29% to 202%, with the lowest benefit observed in the one human study. In terms of RIX, only murine studies were available, and all involved injection of MSCs directly into the glands, all with a reported significant increase in SFR compared with the control group (see Table II). Radiation-induced salivary gland dysfunction and xerostomia To translate the promising findings from the murine studies on RIX to the clinic, an appropriate follow-up of the animals and a relevant time of treatment must be evaluated thoroughly. Also, the nature of the radiation therapy must be kept in mind. In the radiotherapy regimens for head and neck cancer, the deposition of radiation in the salivary glands is very patient-specific, depending on the clinical situation, unlike in the murine studies. Given that the induced salivary gland dysfunction in the murine studies was very standardized, it could be argued that the effect of radiation on the salivary gland cells could be different in the clinic. Specific areas in the salivary glands have thus been found to be more important for the development for salivary gland dysfunction than others.41 For example, van Luijk et al.41 found that sparing putative stem cell regions in the salivary glands during radiotherapy leads to less pronounced salivary gland dysfunction. This could also be of importance when trying to translate the findings from the murine studies to future clinical trials, in that the beneficial effects of MSCs are

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generally believed to be mediated by their bystander effects42 and thus maybe also in the current studies are mediated by supporting the proliferation of otherwise “dormant” salivary gland stem cells. Another important aspect when trying to translate the murine studies on RIX to the clinic is the different regimens used in the treatment for head and neck cancer. The radiotherapy regimens for the treatment of head and neck cancer are fractionated (usually spanning several weeks), not single-dose regimens as in the murine studies. In addition, the time from completed radiotherapy to the start of MSC treatment could be of significant importance. During the acute phase of radiotherapy for head and neck cancer, an inflammatory infiltrate is found in the salivary glands. In many different situations, MSCs have been found to be able to decrease an inflammatory reaction, thus reducing the immune-mediated destruction of the parenchyma.43 Therefore it could be envisaged that a “window of opportunity” in employing MSC treatment for RIX exists, keeping in mind the generally large increase in SFR after MSC treatment and the modest degree of transdifferentiation of MSCs to acinar cells observed in the murine studies reviewed. In the study by Lin et al.,37 the MSC treatment was begun 24 hours after the radiation, and in the study by Lim et al.,35 the MSC treatment was begun 11 days after radiation. This would not be possible to implement in the clinic, for several reasonsdnot least because of the length of the radiation treatment. However, in the study by Kojima et al.,36 the MSC treatment was begun 10 weeks after the irradiation of the salivary glands, and in keeping with the expected life span of the used mice (around 2 years), this might represent a more relevant model of MSC for RIX. It is therefore encouraging that even in the study by Kojima et al.,36 a relevant increase in SFR of 51% was observed. Another important aspect of using MSCs for the treatment of RIX is the potential risk of stimulating cancer growth, as some in vitro studies have suggested. Still, MSCs have previously been used in patients with previous cancer,16 without any increased incidence of cancer. In addition, MSC therapy has been used in numerous other clinical settings, and no increased incidence of cancer has been reported. However, a relevant recurrence-free time frame before employing MSC therapy should be considered. In patients with head and neck cancer, those that recur after primary treatment generally do so quickly. It has been estimated that 50% of all recurrences will occur within 6 months, 70% within 1 year, and 80% to 90% within 2 years.44

SS-related salivary gland dysfunction and xerostomia In all the studies for SS-related salivary gland dysfunction, more relevant time frames before MSC therapy

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were used (e.g., 16 weeks for both Xu et al.34 and Khalili et al.33). Also, it was found in the study by Xu et al.34 that patients with SS with poor response to glucocorticoid or glucocorticoid combined with immunosuppressant (mean disease duration, 76.7  82.5 months) could still be responsive to MSC treatment. However, as previously mentioned, the study did not have a control group for comparison. To evaluate a potential treatment benefit of MSC for SS, a diagnosis of SS has to be made. The diagnosis of SS is complicated by the fact that many different classification criteria are in use, including the Preliminary European Classification criteria for SS (which were revised in 2002), the American-European Consensus Group criteria set, and the newly suggested Sjögren’s Syndrome International Collaborative Clinical Alliance criteria.45 These differing classification criteria, in conjunction with the widely varying clinical manifestations in addition to the sicca symptoms, would clearly lead to challenges in the interpretation of MSC treatment for xerostomia/SS, given that different criteria in establishing the diagnosis may be used. In addition, the difficulty in establishing stringent criteria for the diagnosis of SS underscores that SS is a very heterogeneous disease. Therefore, identifying the specific patient group that could potentially benefit from MSC treatment would be the first challenge in translating the preliminary results to the clinic. In addition, SS is a chronic disease, and therefore the optimal time to initiate treatment would have to be evaluated; this is in addition to choosing potential maintenance therapy and determining in what intervals it should be employed. Potential biases in the review process We conducted a thorough literature search of multiple databases and the references of the relevant studies. Because relatively few studies were found, we did not limit the inclusion to randomized controlled trials. The decisions to include or exclude studies were clearly objective. Nonetheless, our findings should be interpreted with the usual precautions concerning internal and external validity of animal studies, because translation to clinical studies is not straightforward.46-49 Implications for current research and future practice Currently, only one single-cohort clinical trial evaluated the effect of MSC therapy on SS-related xerostomia,34 and no clinical trial has been performed to evaluate the effect of MSC therapy on RIX. Taken together, the available data provide no conclusive evidence to recommend MSC treatment for either radiation-induced or SS-related salivary gland dysfunction and xerostomia. Much further research on both radiation-induced and SSrelated salivary gland dysfunction and xerostomia is

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needed to provide conclusive evidence for the benefit of MSC therapy in the treatment of xerostomia; such research needs to use appropriate control groups; evaluate the optimal MSC dose, MSC type (bone marrow, umbilical cord, adipose tissue), and time to begin treatment; and account for the different patient characteristics and previous treatments. The preliminary murine studies and the single human study, however, show encouraging results and invite further studies. Even though the clinical manifestations with sicca symptoms from both SS and RIX are similar, they have different etiologies, and the natural histories differ markedly. Therefore, a potential MSC treatment for either disease would be expected to show a considerable variability in treatment response (both initially and long-term). REFERENCES

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APPENDIX The Cochrane Database of Systematic Reviews: MSC OR mesenchymal stem cell OR mesenchymal stromal cell OR adipose stem cell OR adipose stromal cell OR pre-adipocytes OR processed lipoaspirate cells OR stromal vascular fraction cell AND xerostomia. PubMed/Medline: (((((((((((mesenchymal stromal cell[MeSH Terms]) OR MSC) OR mesenchymal stem cell) OR mesenchymal stromal cell) OR adipose stem cell) OR adipose stromal cell) OR adipose derived stromal cells) OR adsc) OR preadipocytes) OR processed lipoaspirate cells) OR stromal vascular fraction cells) AND xerostomia. EMBASE: ((MSC or mesenchymal stem cell or mesenchymal stromal cell or adipose stem cell or adipose stromal cell or adipose derived stromal cells or adsc or preadipocytes or processed lipoaspirate cells or stromal vascular fraction cells) and xerostomia).mp. [mp¼title, abstract, subject headings, heading word, drug trade name, original title, device

REVIEW ARTICLE Jensen et al. 342.e1

manufacturer, drug manufacturer, device trade name, keyword] Web of Science, advanced search: (TS¼(MSC) OR TS¼(Mesenchymal stem cell) OR TS¼(mesenchymal stromal cell) OR TS¼(adipose stem cell) OR TS¼(adipose stromal cell) OR TS¼(adipose derived stromal cell) OR TS¼(adsc) OR TS¼(preadipocytes) OR TS¼(processed lipoaspirate cells) OR TS¼(stromal vascular fraction cell)) AND TS¼(xerostomia). Google Scholar Advanced: “mesenchymal stem cell” OR “mesenchymal stromal cell” OR “adipose stem cell” OR “adipose stromal cell” OR “adipose derived stromal cell” OR ADSC OR preadipocytes OR “processed lipoaspirate cells” “xerostomia.” WHO Clinical Trial Registry Platform: mesenchymal stem cell OR mesenchymal stromal cell OR adipose stem cell OR adipose stromal cell OR adipose derived stromal cell OR ADSC OR preadipocytes OR processed lipoaspirate cells OR stromal vascular fraction cell AND xerostomia.