Treatment with adipose tissue-derived mesenchymal stem cells exerts anti-diabetic effects, improves long-term complications, and attenuates inflammation in type 2 diabetic rats
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Long-term diabetes-associated complications are the major causes of morbidity and mortality in individuals with diabetes. These diabetic complications are closely linked to immune system activation along with chronic, non-resolving inflammation, but therapies to directly reverse these complications are still not available. Our previous study demonstrated that mesenchymal stem cells (MSCs) attenuated chronic inflammation in type 2 diabetes mellitus (T2DM), resulting in improved insulin sensitivity and islet function. Therefore, we speculated that MSCs might exert anti-inflammatory effects and promote the reversal of diabetes-induced kidney, liver, lung, heart, and lens diseases in T2DM rats.
We induced a long-term T2DM complication rat model by using a combination of a low dose of streptozotocin (STZ) with a high-fat diet (HFD) for 32 weeks. Adipose-derived mesenchymal stem cells (ADSCs) were systemically administered once a week for 24 weeks. Then, we investigated the role of ADSCs in modulating the progress of long-term diabetic complications.
Multiple infusions of ADSCs attenuated chronic kidney disease (CKD), nonalcoholic steatohepatitis (NASH), lung fibrosis, and cataracts; improved cardiac function; and lowered serum lipid levels in T2DM rats. Moreover, the levels of inflammatory cytokines in the serum of each animal group revealed that ADSC infusions were able to not only inhibit pro-inflammatory cytokines IL-6, IL-1β, and TNF-α expression but also increase anti-inflammatory cytokine IL-10 systematically. Additionally, MSCs reduced the number of iNOS(+) M1 macrophages and restored the number of CD163(+) M2 macrophages.
Multiple intravenous infusions of ADSCs produced significant protective effects against long-term T2DM complications by alleviating inflammation and promoting tissue repair. The present study suggests ADSCs may be a novel, alternative cell therapy for long-term diabetic complications.
KeywordsDiabetes Diabetes complications Mesenchymal stem cells Macrophage Inflammation
Mesenchymal stem cells
Type 2 diabetes mellitus
Adipose-derived mesenchymal stem cells
Chronic kidney disease
End-stage renal disease
Systemic lupus erythaematosus
Foetal bovine serum
Phosphate buffer solution
Normal chow diet
Intraperitoneal glucose tolerance tests
Insulin tolerance tests
Fasting blood insulin
Fasting blood glucose
Low-density lipoprotein cholesterol
Blood urea nitrogen
Albumin to creatinine ratio
Homeostatic model assessment index of insulin resistance
Homeostatic model assessment index of β cells
α-Smooth muscle actin
Tissue inhibitor of metalloproteinases-1
Collagen type I
Collagen type III
Pro-surfactant protein C
Type 2 diabetes mellitus (T2DM) is a long-term metabolic disorder that represents a global public health challenge; not only does T2DM affect industrialized countries, but its impact is also increasing drastically in developing nations . Importantly, T2DM is closely associated with the long-term damage, dysfunction, and failure of various organs, such as chronic kidney failure (CKD), nonalcoholic steatohepatitis, pulmonary fibrosis, cardiovascular disease, and cataracts. Diabetic nephropathy (DN), occurring in 25–40% of individuals with T2DM, has become the single most common cause of end-stage renal disease (ESRD) in the USA, accounting for > 50% of new cases of renal failure . Notably, individuals with diabetes who develop ESRD have a poor prognosis because of a high risk of cardiovascular events . Diabetic cardiomyopathy (DCM) is one of the major causes of mortality and morbidity in diabetic patients . Currently, the first-line treatments widely prescribed for diabetic complications provide only palliative relief but no definitive cure. Hence, an effective strategy to reverse the long-term complications in these patients needs urgent investigation.
Chronic low-grade inflammation, recently referred to as ‘metaflammation’, is thought to be a relevant factor contributing to the development of diabetic complications [5, 6, 7, 8]. These immunological changes include alterations in the levels of inflammatory cytokines and chemokines [9, 10], changes in the number of infiltrating leukocytes [11, 12, 13], and the development of tissue fibrosis [14, 15]. In clinical studies, the levels of inflammatory markers appear to predict the onset and progression of diabetic complications . It is now generally accepted that tissue-resident macrophages play major roles in the regulation of tissue inflammation. Macrophages exhibit a phenotypic range that is intermediate between two extremes, M1 (pro-inflammatory) and M2 (anti-inflammatory). Treatment with immunosuppressants such as mycophenolate mofetil or sirolimus reduces renal inflammation in association with prevention of the development of glomerular injury in diabetic rats [16, 17]. Similarly, blockade of the MCP-1 receptor (CCR-2) with a selective antagonist suppresses the infiltration of interstitial macrophages and ameliorates diabetic glomerular sclerosis . The results of recent studies highlight the possibility that immunomodulation and, specifically, immunoresolvents are novel strategies to overcome several diabetic complications simultaneously.
Mesenchymal stem cells (MSCs) are fibroblast-like stem cells with the ability to self-renew and undergo multilineage differentiation. Recently, more attention has been paid to the immunomodulatory and anti-inflammatory effects of MSCs [19, 20]. Clinical studies have shown that an infusion of MSCs suppressed systemic inflammation in T2DM patients , and MSCs regulated inflammation and promoted repair of damaged tissues in inflammatory diseases such as graft-versus-host disease (GvHD)  and systemic lupus erythaematosus (SLE) . While the efficacy of MSC to ameliorate the progression of early-stage diabetic complications is relatively well established, the therapeutic effects of MSCs on advanced diabetic complications are still unknown. Furthermore, there is limited detailed information confirmed by long-term studies on the efficacy and feasibility of multiple intravenous MSC infusions for T2DM with advanced complications.
In this study, we induce a rat model to closely mimic the long-term complications occurred in T2DM. We reported that multiple intravenous adipose-derived mesenchymal stem cell (ADSC) infusions produced significant anti-diabetic effects and hindered the progression of long-term diabetic complications, such as lung, liver, kidney, and cardiovascular complications. Moreover, ADSCs attenuated systemic inflammation and altered the tissue M1/M2 ratio, demonstrating the therapeutic potential of ADSCs on long-term diabetic complications.
Isolation and identification of adipose-derived MSCs
ADSCs were isolated, purified, and identified as described previously . Male Sprague–Dawley (SD) rats weighing 80–100 g were selected and anaesthetized by an intraperitoneal injection of 2% sodium pentobarbital (40 mg/kg). After disinfection with 75% alcohol, the skin of the rats was cut along the abdominal line, and the subcutaneous groin fat was removed. Adipose tissue was washed three times with sterile phosphate buffer solution (PBS) and minced. The extracellular matrix (ECM) was digested with 0.1% type I collagenase and 0.05% trypsin (Gibco, Grand Island, NY, USA) and centrifuged at 1500 rpm for 10 min. The sediment was suspended with low-glucose Dulbecco’s modified Eagle’s medium (DMEM; Gibco) containing 10% foetal bovine serum (FBS; Gibco) and 1% penicillin–streptomycin (Gibco), then transferred into a dish and cultured at 37 °C in 5% CO2. After 48 h, no adherent cells were removed, the media were replaced, and the culture medium was replaced twice a week. Passage 3 of the ADSCs was used and phenotyped.
Animal experiment and treatment
Eight-week-old male SD rats were provided a normal chow diet (NCD) or a high-fat diet (HFD; 60% fat, Research Diets, New Brunswick, NJ) for 8 weeks. After 8 weeks’ HFD, a single dose of 25 mg/kg streptozotocin (STZ) (Sigma-Aldrich, Saint Louis, MO) dissolved in 10 mmol/l citrate buffer (pH 4.5) was intraperitoneally injected to HFD-fed rats. Random glucose was consecutively measured by monitoring tail capillary blood glucose levels. Rats with more than three random glucose level measurements ≥ 16.7 mmol/l were considered diabetic. Then, to generate a long-term T2DM complication rat model, T2DM rats were fed a HFD for 24 more weeks.
To evaluate this model, HFD-fed rats were randomly selected for sacrifice at the time point before HFD (HFD-b), before STZ injection (STZ-b), 1 week after STZ injection (STZ-1w), 12 weeks after STZ injection (STZ-12w), and 24 weeks after STZ injection (STZ-24w). The epididymal adipose, pancreas, and kidney were stained with haematoxylin and eosin (H&E), and body weight, fasting blood glucose (FBG), fasting blood insulin (FBI), serum C-peptide, and urinary albumin to creatinine ratio (ACR) were detected.
At 24 weeks after STZ injection, the T2DM rats were randomly treated through the tail vein with a single infusion of 3 × 106 ADSCs suspended in 0.5 ml of PBS once a week for 24 weeks (referred to as the MSC-treated group) or an infusion of 0.5 ml PBS alone (referred to as the T2DM group) once a week for 24 weeks. On week 57, intraperitoneal glucose tolerance tests (IPGTTs), insulin tolerance tests (IPITTs), and hyperinsulinaemic–euglycaemic clamp study were performed as previously described to assess the therapeutic effects of ADSCs. Whole blood was collected from the left ventricle, and plasma was obtained after centrifugation at 3000 rpm for 10 min. FBG and FBI were detected. Blood lipids (including total cholesterol [TC], low-density lipoprotein cholesterol [LDL-C], and triglyceride [TG]), hepatic enzymes (including alanine aminotransferase [ALT] and aspartate aminotransferase [AST]), serum creatinine, blood urea nitrogen (BUN), ACR, and blood cell counts were measured by the Servicebio corporation.
Homeostatic model assessment index of insulin resistance (HOMA-IR) and of β cells (HOMA-β) was calculated as follows: HOMA-IR index = (FBG, mmol/l) × (FBI, mU/l)/22.5 and HOMA-β index = 20 × (FBI, mU/l)/[(FBG, mmol/l) − 3.5)].
All animal procedures were reviewed and approved by the Institutional Animal Care and Use Committee of the Chinese People’s Liberation Army (PLA) general hospital.
ADSCs’ homing efficiency in different tissues of T2DM rats
ADSCs were labelled with chloromethyl-benzamidodialkylcarbocyanine (CM-Dil; Life technologies, Eugene, OR, USA) according to the manufacturer’s instructions. Twenty-four weeks after STZ injection, the T2DM rats were infused with ADSCs-Dil (3 × 106 ADSCs-Dil suspended in 0.5 ml PBS). Twenty-four hours and 7 days after ADSCs-Dil infusion, the T2DM rats were sacrificed and their adipose, pancreas, kidney, liver, lung, and heart were removed for preparation of sections. Labelled ADSCs were calculated under a laser scanning confocal microscope (Leica, Wetzlar, Germany).
Histology and immunofluorescence
For histological analysis, epididymal adipose, pancreas, kidney, lung, cardiac, and lens tissues were fixed in formalin and then embedded with paraffin. Tissues were sectioned into 4–6-μm slices and stained with haematoxylin and eosin (H&E), periodic acid-Schiff (PAS), Masson’s trichrome, Sirius Red, and Oil Red O according to the standard protocol. The morphological structure of each tissue sample was observed under a light microscope, and photomicrographs were taken (Olympus, Japan).
Kidney: The severity of the kidney injury includes infiltration of polymorphonuclear cells, congestion, desquamation, loss of microvilli, and swelling of tubule cells. Kidney injury was graded as follows for each criterion: 0, normal; 1, mild; 2, moderate; and 3, severe. Tubular changes including hydropic degeneration (swelling/vacuolization), desquamation, brush border loss, and peritubular infiltration were graded as follows with a maximum score of 12: 0, normal; 1, mild; 2, moderate; and 3, severe. Glomerulosclerotic injury was graded using PAS-stained sections as follows: 0, normal; 1, ≤ 25% of the glomerular area (mild sclerosis); 2, 25–50% of the glomerular area (moderate sclerosis); and 3, ≥ 75% of the glomerular area (severe sclerosis) .
Liver: Steatosis was assessed by Kleiner et al.  grading the percentage of steatotic hepatocyte involvement as follows: grade 0, < 5%; grade 1, 5–33%; grade 2, > 33–66%; and grade 3, > 66%. In addition, Brunt’s histological scoring system was used to evaluate the degree of hepatocellular ballooning and lobular inflammation (grade of activity) as well as the stage of fibrosis . Minimal criteria for the histological diagnosis of definite nonalcoholic steatohepatitis (NASH) included the combined presence of grade 1 steatosis, hepatocyte ballooning, and lobular inflammation with or without fibrosis.
Lung: The degree of lung injury degree was evaluated according to Mikawa’s scoring standards : (1) alveolar congestion, (2) haemorrhage, (3) infiltration or aggregation of neutrophils in the airspace or vessel wall, and (4) thickness of the alveolar wall/hyaline membrane formation. Each item was scored on a 5-point scale: 0, minimal damage; 1, mild damage; 2, moderate damage; 3, severe damage; and 4, maximal damage. The final lung injury score was the sum of the four items. Tissue sections were stained with Masson’s trichrome for the evaluation of fibrosis. Glycogen content in type II pneumocytes was graded using PAS-stained sections as follows: normal, not detectable; mild, < 25% of alveolar epithelial cells; mild to moderate, 25–50% of alveolar epithelial cells; moderate to severe, 50–75% of alveolar epithelial cells; and severe, 75–100% of alveolar epithelial cells .
For immunofluorescence analysis, tissues were cut into 6-μm sections. Frozen sections were incubated for 14 h at 4 °C with primary antibodies for insulin (1:200, guinea pig, Abcam, MA), glucagon (1:2000, mouse, Abcam), collagen I (1:500, rabbit, Abcam), α-smooth muscle (1:100, mouse, Sigma-Aldrich), iNOS(1:100, rabbit, Abcam), CD163 (1:200, mouse, Bio-rad, CA, USA), albumin(1:50, mouse, Proteintech, Wuhan, China), pro-surfactant protein C (SP-C, 1:50, rabbit, Proteintech), and CD206 (1:500, rabbit, Abcam) and were then incubated with Alexa Fluor 488/594-conjugated secondary antibodies (1:500, Invitrogen, USA) at room temperature for 2 h. Slides were observed under a laser scanning confocal microscope.
Rats were anaesthesized by isoflurane inhalation. Echocardiography (VisualSonics VEVO 2100, Toronto, ON, Canada) was performed by the same cardiologist who was blind to the treatments.
Western blot analysis
Total protein was extracted from samples of epididymal adipose tissue, and the procedure was carried out as described previously. The primary antibodies were PI3K (1:1000, rabbit, Cell Signaling Technology, MA, USA), total or phosphorylated AKT (Ser473) (p-AKT) (1:1000, rabbit, Cell Signaling Technology), and β-actin (1:2000, mouse, ZSGB-Bio, Beijing, China). The secondary antibodies were goat anti-rabbit and rabbit anti-mouse IgG horseradish peroxidase (HRP) from the ZSGB-Bio company. The blots were analysed using ImageJ software (National Institutes of Health, Bethesda, MD).
Quantitative real-time PCR
Total RNA was isolated from rat adipose tissue, liver tissue, renal tissue, and lung tissue using Trizol reagent (Life technologies, Frederick, USA) and was reverse-transcribed with a reverse transcription kit (Thermo Scientific, CA, USA)) according to the manufacturer’s instructions. Real-time polymerase chain reaction (RT-PCR) was performed on ABI Prism thermal cycler model StepOnePlus (Applied Biosystems, CA, USA) using a SYBR Green PCR master mix (Applied Biosystems). The thermal cycling programme was 94 °C for 3 min, followed by 94 °C for 30 s, 60 °C for 30 s, and 72 °C for 30 s for 40 cycles. Melting curve analysis was included to ensure primer specificity. The primers are listed in Additional file 1: Table S1.
Enzyme-linked immunosorbent assay
Serum from each group were collected and stored at − 80 °C until they were thawed for the assay. The concentrations of IL-1β, IL-6, IL-10, and TNF-α in serum were measured using ELISA kits from Multi Sciences LTD. (Hangzhou, China). The levels of serum insulin were assessed using ELISA kits from Elabscience (Wuhan, China). The levels of C-peptide were assessed using ELISA kits from Millipore (St. Charles, MO). All procedures were performed according to the manufacturer’s instructions.
Data are presented as mean ± standard deviation (SD) for normally distributed data and as mean [interquartile range] when non-normally distributed. Normality was assessed using the one-sample Kolmogorov–Smirnov test. Statistical differences between two groups were analysed by either unpaired Student t test (normally distributed data) or Mann–Whitney U test (non-normally distributed data), and differences between multiple groups of data were assessed by one-way analysis of variance (ANOVA) with Bonferroni’s multiple comparison test. Statistical significance was defined as p < 0.05. All analyses were accomplished using the software in GraphPad Prism 3.0 (GraphPad Software, San Diego, CA, USA) and SPSS statistical software version 25 (SPSS Inc., IBM, USA).
Establishment of the HFD/STZ-induced long-term T2DM rat model
Multiple ADSC infusions improved glucose homeostasis by improving insulin sensitivity and promoting pancreatic islet recovery in long-term T2DM complication rats
Multiple ADSC infusions attenuated T2DM-induced kidney damage in long-term T2DM complication rats
Multiple ADSC infusions attenuated T2DM-induced liver disease in long-term T2DM complication rats
Multiple ADSC infusions attenuated T2DM-induced lung disease in long-term T2DM complication rats
Multiple ADSC infusions attenuated T2DM-induced cardiac changes in long-term T2DM complication rats
Multiple ADSC infusions exerted an anti-cataract effect in long-term T2DM complication rats
Cataracts are among the common complications of diabetes mellitus. As shown in Fig. 6g, the lenses of the normal rats remained clear and transparent without any turbidity. In contrast, the rats in the T2DM group presented with cloudy lenses with nuclear opacity. Most of the lenses of the MSC-treated rats were significantly clearer than those of the rats in the T2DM group. The H&E staining results also validated the anti-cataract effect of MSC infusions in vivo (Fig. 6g).
Twenty-four hours after injection, among the organs examined, only a small number of ADSCs could be detected in the lung and liver. However, engraftment of ADSCs was barely seen in the pancreas, adipose tissue, myocardium, or kidney of T2DM rats. The trends were similar at 7 days after injection (Additional file 1: Figure S1a,b). In order to analyze whether the CM-Dil-positive ADSCs in the pancreas, liver, and lung could differentiate into β cell, hepatocyte, or pneumocyte of the alveolar epithelium, we separately labelled CM-Dil-positive cells with insulin, albumin (a marker of hepatocyte), and SP-C (a marker of pneumocyte). Results of immunofluorescence staining showed that most of the CM-Dil-positive cells in the pancreas, lung, and liver of T2DM rats did not express cellular marker of β cell, hepatocyte, or pneumocyte, indicating that ADSCs did not differentiate into those cellular types (Additional file 1: Figure S1c). Besides, the number of ADSC homing to the liver and lung can hardly explain the significant therapeutic effect on liver and lung disease. These results indicated that the effect of ADSCs on the treatment of diabetic complication does not depend on the homing of ADSCs into target organs.
Multiple ADSC infusions attenuated inflammation and changed the phenotypes of macrophages in the target organs of diabetic complications in long-term T2DM complication rats
It has been indicated that MSCs have potential as a regenerative therapy for diabetes-associated complications (for recent review, see [31, 32]). Most current studies have focused on the prevention or amelioration of early-stage diabetic complications. However, the therapeutic effects of MSCs in reversing the long-term diabetic complications have not been reported. In our study, the therapeutic effects of MSCs are particularly understudied in diabetic animals with a disease duration of more than 24 weeks. The present data suggested that multiple MSC infusions not only effectively restored glucose homeostasis and alleviated insulin resistance but also ameliorated hyperlipidaemia and altered the progression of long-term diabetic complications, such as CKD, NASH, lung fibrosis, and cataracts, and improved cardiac function in T2DM rats.
Animal models are a key resource to explore the pathogenesis of diabetic complications and reduce the gap between preclinical and clinical research. Although genetically (spontaneously) modified animal models, such as db/db mice or Zucker diabetic fatty (ZDF) rats, present with many of the metabolic and organic failures that occur in T2DM [33, 34, 35], the development of experimentally (non-spontaneously) induced diabetic animals can further help to mimic the different stages of T2DM progression. We developed and characterized a rat model of long-term T2DM complications that went through three stages of T2DM development: (i) the onset of obesity with insulin resistance and compensatory hyperinsulinaemia without hyperglycaemia representing a large fraction of prediabetes, (ii) a short duration of T2DM with hyperglycaemia and relative hypoinsulinaemia characterized by a decline in the secretory capacity of the pancreatic β cells, and (iii) a long duration of T2DM with significant signs of dyslipidaemia, hepatic fibrosis, and steatosis, as well as cardiac and renal dysfunction. Altogether, this alternative model is easy to generate at a relatively low cost, widely available, and useful for exploring behavioural or drug testing related to long-term diabetic complications and for studying the pathogenesis of diabetic complications, such as diabetic cardiomyopathy and nephropathy.
Regarding the treatment of T2DM, the ability of MSCs to ameliorate circulating glucose levels may be short-lasting. Our previous study showed that in T2DM rats, a single MSC infusion alleviated hyperglycaemia for only 1 week ; then, the hyperglycaemia steadily returned to the pretreatment values. However, hyperglycaemia decreased to approximately normal levels after at least three infusions. Moreover, compared to early-phase (1 week) T2DM rats, late-phase (5 week) T2DM rats exhibited slower and poorer effects after the MSC infusions . Thus far, it has been shown that hyperglycaemia in T2DM can be reversed more effectively with multiple MSC infusions. The onset of the treatment effect may be later considering the longer course of T2DM. The present study showed that the blood glucose levels of the T2DM rats exhibited a persistent and gradual decrease until the 16th MSC infusion, and after 24 MSC treatments, the hyperglycaemia decreased to approximately normal levels. The results of this study suggest that a multiple MSC infusion strategy, rather than a single infusion or several infusions, offers a superior clinical option for advanced T2DM patients.
For the first time, we infused MSCs in long-term T2DM complication rats once a week for 24 weeks, which resulted in desirable treatment effects. However, some studies have illustrated the potential mechanisms of MSC immunosuppression, and one major mechanism is that MSCs suppress T cell proliferation due to their effects on soluble factors  such as IL-10 , which was identified at a significantly higher level in our study. Taking these ideas into account, it is necessary to perform animal experiments before developing a clinical treatment to determine whether multiple infusions of MSCs are safe for recipients and whether the body’s normal immunity is suppressed after the MSC treatments. After infusing ADSCs into the rats, there was no significant effect on food intake or body weight, and the treatment did not induce anaphylaxis or altered leukocyte concentration. These findings indicate that multiple intravenous infusions of MSCs have no obvious toxic effects on liver or kidney function or T cell immune suppression in T2DM rats.
Tissue macrophages are crucial players in T2DM-associated inflammation, which is characterized by an increased abundance of macrophages in different tissues along with the production of inflammatory cytokines . Macrophages exhibit a phenotypic range between two extremes, M1 macrophages (pro-inflammatory) and M2 macrophages (anti-inflammatory). The salient finding of this study is that the MSC infusions suppressed renal, hepatic, pulmonary, and cardiac inflammation, which was accompanied by improvements of long-term complications and decreases in tissue fibrosis in the diabetic rats. According to our previous study, we found that MSCs alleviate insulin resistance and restore islet β cells by differentiating macrophages into the anti-inflammatory phenotype [41, 42]. The present results showed an increase in the M2 to M1 ratio with no significant changes in the number of F4/80(+) macrophages. These results suggest that instead of specifically fixing one target organ, MSCs systematically attenuated inflammation, and the anti-inflammatory actions involved the induction of the phenotypic switch from M1 macrophages to M2 macrophages rather than eliminating the recruitment of macrophages into renal, hepatic, pulmonary, and cardiac tissues.
However, how does increasing the M2 to M1 ratio promote recovery from long-term diabetic complications?
One possibility is that M2 macrophages antagonize the functions of the M1 macrophages that exacerbate tissue damage. The M2 macrophages are often linked with tissue repair because they can antagonize the functions of M1 macrophages that exacerbate tissue damage [43, 44]. The secretion of the inflammatory cytokine IL-1β by M1 macrophages has been shown to be a major driver of persistent inflammation and the pathogenesis of diabetes, atherosclerosis, and sterile inflammation [45, 46]. This finding is consistent with our results that revealed that the level of IL-1β in the target organs of the T2DM group was high, while the level decreased after MSC treatment. Another possible explanation is the anti-fibrotic roles of M2 macrophages. Tissue fibrosis is a sign of irreversible damage in age-related diseases and diabetic complications. Recent studies have suggested that M2 macrophages can also exhibit potent anti-fibrotic activity, particularly when the tissue-repair response becomes chronic. Indeed, mechanistic studies investigating the role of M2 macrophages in chronic models of fibrosis and cancer have suggested that M2 macrophages slow the suppression of local CD4+ T cell responses and reduce ECM production by myofibroblasts . Furthermore, the nutrient competition between macrophages and neighbouring myofibroblast has been identified as an additional potent anti-fibrotic mechanism . Our results indicated that MSC treatment resulted in improvements in inflammation, which were paralleled by a reduction in ECM deposition in renal, hepatic, pulmonary, and cardiac tissues. Overall, the increased M2 macrophage levels in various target organs may assist, enhance, or at least partly explain the therapeutic effects of MSCs on long-term T2DM complications.
A limitation of our study is that we did not directly test whether macrophage activation was necessary for the MSC-mediated protective effects. Because macrophage depletion alone delays the onset of diabetes  and attenuates diabetic complications, it may be difficult to determine whether macrophage depletion abrogated the therapeutic effects of MSCs on the long-term diabetic complications that occurred in our experimental setting. Hence, future experiments will need to address this issue.
Multiple intravenous infusions of MSCs produced significant anti-diabetic effects. Moreover, MSCs attenuated systemic inflammation and altered the tissue M1/M2 ratio. These actions might be related to the alterations in the progression of long-term diabetic complications especially tissue fibrosis that leads to diseases, such as lung, liver, kidney, and cardiovascular complications, demonstrating the therapeutic potential of MSCs for long-term diabetic complications. These results can not only provide novel insights into intervention methods and therapeutic targets for the treatment of advanced stage of T2DM but may also provide crucial information related to other age-related diseases.
We thank the technical assistance from Jiejie Liu, Dongdong Ti, and other members of the Mu laboratories for insightful discussions over this work.
SY and YC contributed to the conception and design, provision of study material, collection of data, data analysis and interpretation, and manuscript writing. LZ, YY, and JX contributed to the provision of study material and collection of data. ZX, BL, and ZG contributed to the collection of data and data analysis and interpretation. JG contributed to the provision of study material and data analysis and interpretation. YM contributed to the conception and design, financial support, manuscript writing, and final approval of manuscript. SY and YC contributed equally to this article. All authors read and approved the final manuscript.
This work was supported in part by the 863 Projects of Ministry of Science and Technology of China [2013AA020105 and 2012AA02050] and the National Basic Science and Development Program [81700680 and 8187032381].
Ethics approval and consent to participate
All animal experiment protocols were approved by the medical ethics committee of the Chinese PLA General Hospital, Medical School of Chinese PLA.
Consent for publication
The authors declare that they have no competing interests.
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