MSCs can protect β-cell, increase the secretion of insulin, and reduce glycemia in patients with T1DM by regulating the immune system. The application of MSCs in eliminating autoimmune diseases has been fully proven in an animal model, and MSCs have a wide range of regulatory effects on immune cells. Domouky et al. showed that MSCs could reduce hyperglycemia in diabetic rats on day 15 (56). The inhibition of T-cell proliferation in islets and the presence of increased Treg in T1DM were features of MSCs’ autoimmune properties (57). Shigemoto-Kuroda et al. developed T1DM mouse models for autoimmune diseases and discovered that MSCs could suppress type 1 helper T cell (Th1) development and delay the onset of T1DM in mice. CD4⁺ cells were found in significant numbers in the islets of mice treated with PBS, while fewer CD4⁺ cells were found in the islets of MSC-treated mice. The level of insulin in plasma was increased by MSC treatment, and there was a significant reduction in the production of IL-12, IFN-γ, p70, and tumor necrosis factor (TNF) (58). Bassi et al. isolated MSCs from epididymal fat tissue from 8-week-old male Balb/c mice and characterized through immunophenotyping its capacity to prevent the proliferation of CD4⁺ T cells (59, 60). Treatment of NOD mice with MSCs attenuated hyperglycemia of early-onset autoimmune diabetes and increased amylin levels, reflective of autoimmune diabetes improvement; reduced the amount of inflammatory cell infiltration, maintaining insulin expression in pancreatic islets by suppressing the Th1 immune response in the pancreas; and promoted the high expression of active TGF-β1 (60). Meanwhile, syngeneic MSCs were detected for a significantly longer period, albeit with diminishing persistence in immune-deficient mice model (61). In another study, van Megen et al. found that activation of MSCs can take up and process antigen and increase HLA-DR expression and immune inhibitory markers, while their metabolic profile was maintained without enhancing T-cell proliferation. MSCs can also enhance immunosuppressive capacity without stimulating alloreactive T cells (62). In an in-vitro study, Montanucci et al. provided preliminary evidence that immunoisolatory microcapsule-hHUCMS (CpS-hUCMS) may represent a functional biohybrid artificial system, where molecular products can induce effective immunomodulatory effects in vitro and in T1DM patients, making it possible to further clarify their therapeutic potential in humans (63). Montanucci et al. isolated and microencapsulated human umbilical cord Wharton jelly-derived mesenchymal stem cells (hUCMS) for xenograft (TX) in a spontaneous T1DM mouse model (NOD mice). At 10 days of TX, Treg cells did not increase, while at 216 days of TX, CD4⁺ CD25 high cells increased in terms of both percentage and number. Further research found that at 216 days of TX, only the mild T1DM NOD mice presented sustained and full alleviation of hyperglycemia, while no alleviation of hyperglycemia was observed in severe T1DM NODs. These findings suggested that the successful hUCMS therapy approach for the treatment of T1DM in NOD mice depended on the stage of the T1DM disease process, with severe T1DM NODs exhibiting a continuous decrease in residual β-cell mass (64). All these results provide encouraging first steps in the clinical translation of the use of preactivated MSCs as a cellular immune intervention therapy, which helps to treat inflammatory and autoimmune disorders, including T1DM.

MSCs can increase the mass of islets and the survival of β-cells by differentiation to β-cell-like cells. MSCs rapidly undergo mesodermal lineage differentiation to β-cell-like cells, and the transdifferentiation of MSCs into insulin-producing cells was successfully attempted in vitro, with the pancreatic and duodenal homeobox 1 (PDX-1), the key marker that was present in the transdifferentiation of MSCs to insulin-producing cells (65). Chen et al. successfully differentiated MSCs into pancreatic islet β-cell-like cells by pre-inducing in L-DMEM with 10 mmol/L nicotinamide and 1 mmol/L β-mercaptoethanol for 24 h and re-inducing in serum-free H-DMEM with 10 mmol/L nicotinamide and 1 mmol/L mercaptoethanol for another 10 h. These induced islet β-cell-like cells with similar morphological characters to pancreatic islet cells and promoted the transcription, translation, and excretion of insulin, which could effectively control the level of blood glucose in diabetic rats (66). Similar results were reported by several other groups that islet-like clusters can be formatted in vitro by cultured MSCs given the appropriate procedure (67, 68). Human umbilical cord Wharton jelly cells (hUCWJCs) are a subtype of MSCs, which were transplanted into a T1DM mouse model with renal damage, and the therapeutic effect of transplantation was evaluated. It was found that hUCWJCs can promote the level of C-peptide and insulin in mice, which certified the potential of intraperitoneal injection of hUCWJCs and the ability of hUCWJCs to migrate to damaged tissues to enhance the secretion of insulin from non-pancreatic local cells (69).

MSCs have been used in human clinical trials, showing that stem cell transplantation has beneficial effects on T1DM. In an open-label, non-randomized, parallel-armed prospective study, Lu et al. enrolled 53 participants including 33 adult-onset (≥18 years) and 20 juvenile-onset T1DM (ChiCTR2100045434). The results revealed that an intravenous dose of allogeneic UC-MSCs was safe in people with newly diagnosed T1DM at 12 months of follow-up, which probably led to better islet β-cell protection compared with standard treatment alone during the first year after diagnosis (70). Cai et al. proved that transplantation of UC-MSCs was safe and associated with moderate improvement of metabolic measures in patients with established T1DM too (NCT01374854) (50). Another clinical trial revealed that MSC injection through liver puncture could successfully decrease the levels of insulin, islet cells, and glutamic acid decarboxylase (GAD) antibody in two patients within 1 year, with a decreased concentration of blood glucose and HbA1c and increased concentration of C-peptide, indicating immune regulatory cell tolerance (71).

Immunomodulatory activity is the most important ability of HSCs in patients with T1DM. HSCs can inhibit the occurrence of T1DM (76–78). Patients with recent-onset T1DM have been triumphantly reverted to euglycemia by autologous hematopoietic stem and progenitor cell transplantation (AHSCT), and modulation of autologous hematopoietic stem and progenitor cells (HSPCs) with prostaglandins (PGs) in vitro enhances their immunoregulatory properties through increasing the expression of the immune checkpoint-signaling molecule PD-L1 (79). Wang et al. demonstrated a lower proportion of proliferating T conventional cells (Tcon) and a higher absolute number and percentage of Treg cells in pancreatic lymph nodes from resistant mice among the younger recipients compared to the rapid progressors among the older recipients, and older NOD mice progressed more rapidly to the end stage of diabetes (80). Although mixed chimerism with MHC-matched non-autoimmune donor bone marrow (BM) transplants did not prevent T1DM in NOD mice models, induction of either mixed or complete chimerism with MHC-mismatched BM transplants inhibited T1DM in the same mice (81). This limited the translational applications of HSCs to reshape the autoimmune response by myeloablative agents/approaches. The genetically modified HSCs were used to overcome the disadvantage. Ex-vivo genetic manipulation of NOD HSCs to encode proinsulin and transgenically target MHC class II could successfully prevent T1DM onset (78, 82). The increased CXCL12 (SDF-1) level in bone marrow-derived HSCs of NOD mice is considered to change the transport of HSCs and peripheral dendritic cells, which is conducive to the occurrence of T1DM (78, 83).

D’Addio et al. enrolled 65 individuals with newly diagnosed T1DM in three independent clinical trials, where the patients transplanted with HSCs showed enhanced C-peptide levels at 6 months after treatment compared with baseline, and the immune system showed an overall stabilization in the remaining follow-up period (84). Gu et al. performed a parallel-assignment, phase-II prospective, non-randomized trial, in which 20 patients were treated only with insulin injections and 20 received autologous hematopoietic stem cell therapy (AHSCT). The results demonstrated the beneficial effects of AHSCT in patients with recent-onset T1DM by increasing the concentration of C-peptide and inducing insulin independence, and the safety and good tolerability of AHSCT compared with conventional intensive insulin therapy was also certified (77). Another clinical trial in Ning’s research also proved that AHSCT was safe without a reduction in the diversity of T-cell receptor (TCR) repertoires, and TCR repertoires tended to be more stable after AHSCT (85). The clinical trial data also showed significant direct correlations between HSPC levels and the coefficient of variation of glucose levels or time in hypoglycemia, which were weaker in patients with long-standing diabetes than in those with short-term diabetes (86). HSC transplantation improves glycated hemoglobin levels in a time-dependent manner (87).

Transplantation of ESCs or iPSCs in T1DM can regenerate the islet β-cells and increase β-cell mass through differentiating to insulin-producing cells (IPCs), pancreatic progenitors, islet organoids, and interspecific pancreatic chimeras, which benefited the treatment of T1DM. Rezania et al. cultured iPSCs expressing key markers of mature β-cells such as insulin in vitro and obtained cells which have functional similarities to human islets; the iPSCs rapidly reversed hyperglycemia in streptozotocin (STZ)-induced diabetic mice through increasing the level of insulin and C-peptide when transplanted in vivo (91). iPSCs can be generated from the skin fibroblasts of T1DM patients. These iPSCs can differentiate into pancreatic cell lineages and generate T1DM SC β-cells, making autologous stem cell-derived pancreatic progeny transplantation for T1DM possible (92). Korytnikov and Nostro isolated hPSCs successfully in a lab and then transplanted them to mice models to monitor their developmental potential in vivo, and they found that mice transplanted with multipotent pancreatic progenitor cells can form all pancreatic lineages in vivo (93). Nadav et al. used single-cell RNA sequencing of differentiating β-cells and revealed that ESC differentiation toward the mature β-cell phenotype can be tracked at each stage through monitoring the expression of markers identifying each intermediate progenitor, such as the β-cell marker insulin, endocrine precursor marker neurogenin 3, NK6 homeobox (NKX6.1), and PDX-1 (89, 94). They also proved that WNT inhibition and bone morphogenetic protein (BMP) activation could modulate the ratio of progenitors and endocrine cells, which shed light on a possible gene editing target for ESC differentiation toward the mature β-cells (89).

Haque et al. characterized autoantigen-specific naturally occurring Treg-like iPSC-Tregs and proved that adoptive transfer of ovalbumin (OVA)-specific iPSC-Tregs greatly suppressed autoimmunity in the mouse model preventing the β-cells from destruction. These tissue-associated Tregs can effectively inhibit the migration and activity of the pathogenic immune cells and accumulate in the diabetic pancreas causing T1DM by downregulating the production of proinflammatory cytokine IFN-γ and suppressing the expression of ICAM-1 (95). Another report declared that pancreatic endoderm derived from hESCs can generate functional insulin-producing cells in vivo regardless of the presence of innate lymphoid cell elements, and the combination of CTLA4Ig and anti-CD40L mAbs can block hESC-PE graft rejection in immunocompetent mice, while regulatory T cells were not needed for the tolerance during hESC-PE transplantation (96).

To increase the function of stem cells, it is very important to sustain the regeneration and differentiation ability of stem cells and prevent the programmed death of stem cells in vivo (30, 97). Several approaches and conditions, including far-red light, genetic engineering, biological material scaffolds, nanofiber tubular, combination treatment with insulin or other drugs, microcapsules, and co-transplantation with more than one type of stem cells, were utilized to promote the survival, differentiation, and immunomodulatory ability of stem cells in vivo and in vitro. These preclinical attempts tried to derive pancreas islet cells, increase the number and function of Tregs, ameliorate the function of islets, and prevent β-cells more effectively ( Table 1 ). Also, stem cells have been used in human clinical trials, which showed that stem cell transplantation had beneficial effects on T1DM, with no obvious adverse reactions ( Table 2 ). Recently, an allogeneic, gene-edited, immune-evasive, stem cell-derived therapy for the treatment of T1DM was approved in Canada for clinical trial application (CTA) (CRISPR Therapeutics and ViaCyte, Inc. to start clinical trial of the first gene-edited cell replacement therapy for the treatment of T1DM, retrieved on November 16, 2021). This CRISPR therapeutics offered novel β-cell replacement therapies to address unmet T1DM needs. All these efforts are aimed at better promoting the effectiveness of stem cells, which proved to be a more viable option for the treatment of T1DM, lessening the suffering of the patients.