Stem cells in the treatment of diabetes mellitus – focus on
mesenchymal stem cells
Stem cells in the treatment of diabetes mellitus
Stem cells in the treatment of diabetes mellitus – focus on
mesenchymal stem cells – Günter Päth, Nikolaos Perakakis, Christos S. Mantzoros, Jochen
Reference: YMETA 53824
To appear in: Metabolism
Received date: 10 August 2018
Accepted date: 14 October 2018
Please cite this article as: Günter Päth, Nikolaos Perakakis, Christos S. Mantzoros, Jochen
Seufert , Stem cells in the treatment of diabetes mellitus – focus on mesenchymal stem
cells. Ymeta (2018), doi:10.1016/j.metabol.2018.10.005
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Stem cells in the treatment of diabetes mellitus – focus on mesenchymal stem cells
Günter Päth1*, Nikolaos Perakakis2, Christos S. Mantzoros2, Jochen Seufert1
1 Division of Endocrinology and Diabetology, Department of Medicine II, Medical Center –
University of Freiburg, Faculty of Medicine, University of Freiburg, Germany
2 Division of Endocrinology, Diabetes and Metabolism, Department of Internal Medicine,
Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, USA
- Corresponding author
Günter Päth, PhD
Division of Endocrinology and Diabetology
Department of Medicine II
Medical Center – University of Freiburg
Faculty of Medicine, University of Freiburg
Hugstetter Str. 55
Tel.: +49 761 270 73270
Fax: +49 761 270 33720
Stem cells, diabetes mellitus, transplantation, embryonic stem cells, induced pluripotent stem
cells, mesenchymal stromal cells
ADA, American Diabetes Association; AE, adverse event; α-SMA, alpha smooth muscle
actin; ANXA1, annexin-1; AUC, area under the curve; BAD, Bcl-2 antagonist of cell death;
Bcl-2b, B-cell lymphoma 2; bFGF, basic fibroblast growth factor; BM, bone marrow; BMI,
body mass index; CAR cell, CXCL12-abundant reticular cell; CCL5/RANTES, CCchemokine
ligand 5; CD, cluster of differentiation; CFU-F, colony forming unit-fibroblast;
CXCL12, CXC chemokine ligand 12; DC, dendritic cell; Ebf, early B-cell factor; ECM,
extracellular matrix; EMT, epithelial to mesenchymal transition; ERK, extracellular signalregulated
kinases; ESC, embryonic stem cells; FDA, Food and Drug Administration; FGFR1,
fibroblast growth factor receptor 1; GAD, glutamic acid decarboxylase; GCV, ganciclovir;
GM-CSF, granulocyte-macrophage colony-stimulating factor; GSIS, glucose-stimulated
insulin secretion; GVHD, graft-versus-host disease; HbA1c, glycated hemoglobin A1c; HGF,
hepatocyte growth factor; HLA, human leukocyte antigen; HNF1A, hepatocyte nuclear factor
1α; HOMA, homeostasis model assessment; HSC, hematopoietic stem cells; HSV-Tk, herpes
simplex virus thymidine kinase; ICAM1, intercellular adhesion molecule 1; ICOS, inducible
costimulator; IDDM, insulin-dependent diabetes mellitus; IDO, indoleamine 2,3-dioxygnase;
IGF, insulin-like growth factor; IL, interleukin; IL-1RA, interleukin-1 receptor antagonist;
IPF-1, insulin promoter factor 1 (official name, PDX1); IR, insulin resistance; iPSC, induced
pluripotent stem cells; ISC, insulin screting cell; ISL-1, insulin gene enhancer protein 1; IU,
international unit; IV, intravenous; MET, mesenchymal to epithelial transition; MHC, major
histocompatibility complex; MMP, matrix metalloproteases; MMTT, mixed-meal tolerance
test; MNC, mononuclear cells; MODY3, maturity-onset diabetes of the young type 3; MPC,
mesenchymal precursor cells; MSC, mesenchymal stem cells; NG2, neural/glial antigen 2;
NGF, nerve growth factor; NGN3 neurogenin 3; NKX6.1, NK6 homeobox 1; NO, nitric
oxide; NOD, non-obese diabetic; NSG, NOD scid gamma; PAX, paired box protein; PDGF,
platelet-derived growth factor; PD-L1, programmed cell death ligand 1; PDX1, pancreatic and
duodenal homeobox 1; PEDF, pigment epithelium-derived factor; PGE2, Prostaglandin E2;
SC, stem cells; SCF, stem cell factor; SCID, severe combined immunodeficiency; STZ,
streptozotocin; T1DM, type 1 diabetes mellitus; T2DM, type 2 diabetes mellitus; TGF,
transforming growth factor; Th, T helper; TNF, tumour necrosis factor; TRAIL, TNF-related
apoptosis inducing ligand; Treg, regulatory T cell; TSG-6, tumour necrosis factor-stimulated
gene 6; TUNEL, terminal deoxynucleotidyl transferase mediated dUTP nick end labeling;
UC, umbilical cord; VCAM1, vascular cell adhesion molecule 1; VEGF, vascular endothelial
growth factor; WHO, world health organisation; WJ, Wharton’s Jelly.
Diabetes mellitus type 1 and type 2 have become a global epidemic with dramatically
increasing incidences. Poorly controlled diabetes is associated with severe life-threatening
complications. Beside traditional treatment with insulin and oral anti-diabetic drugs, clinicians
try to improve patient’s care by cell therapies using embryonic stem cells (ESC), induced
pluripotent stem cells (iPSC) and adult mesenchymal stem cells (MSC). ESC display a
virtually unlimited plasticity, including the differentiation into insulin producing β-cells, but
they raise ethical concerns and bear, like iPSC, the risk of tumours. IPSC may further inherit
somatic mutations and remaining somatic transcriptional memory upon incomplete reprogramming,
but allow the generation of patient/disease-specific cell lines. MSC avoid such
issues but have not been successfully differentiated into β-cells. Instead, MSC and their
pericyte phenotypes outside the bone marrow have been recognized to secrete numerous
immunomodulatory and tissue regenerative factors. On this account, the term ‘medicinal
signaling cells’ has been proposed to define the new conception of a ‘drug store’ for injured
tissues and to stay with the MSC nomenclature. This review presents the biological
background and the resulting clinical potential and limitations of ESC, iPSC and MSC, and
summarizes the current status quo of cell therapeutic concepts and trials.
Human organs and tissues possess a limited capacity to completely recover their structure and
function in a number of pathologic conditions and degenerative diseases. This fact initiated
the multidisciplinary field of regenerative medicine which investigates the potential of stem
cells (SC) for tissue repair and restoration of organ function. Based on their nature and origin,
SC exhibit features of interest for cell therapies; e.g. targeting functional degeneration and
loss of insulin producing pancreatic β-cells in diabetes mellitus (DM). The diverse potential of
embryonic SC (ESC), induced pluripotent SC (iPSC) and adult mesenchymal SC (MSC) has
been exploited to restore or maintain insulin secretion as well as to investigate patient-specific
disease aspects. MSC are currently the most investigated cells in DM-related trials while
clinical testing of ESC has just started. This review summarizes the biological aspects and the
application strategies for the treatment of DM by stem cell therapy.
Patients with autoimmune DM type 1 (T1DM) experience a loss of insulin producing
pancreatic β-cells and rely on daily insulin injections. Despite modern insulin therapies,
exogenous application of insulin can never be as accurate and dynamic like insulin secretion
from endogenous β-cells and therefore can only partially reduce the risk for the development
of micro- (i.e. nephropathy, retinopathy) or macrovascular (i.e. coronary artery disease,
peripheral artery disease, cerebrovascular disease) complications. Additionally, efforts to
develop effective immunosuppressive treatments to prevent β-cell loss before disease onset
had limited success so far . Consequently, restoration of endogenous insulin secretion
represents an important aim to prevent hyper- and hypoglycemia as well as to reduce or avoid
diabetic complications and the patient’s requirement for self-management of glycemic control
by exogenous insulin administration.
Clinical islet transplantation aims to re-establish endogenous insulin secretion and has been
steadily refined since its beginning in the 1980s . An important step was the ‘Edmonton
Protocol’ from 1999 which avoids β-cell toxic glucocorticoids by using sirolimus, tacrolimus
and daclizumab for immunosuppression . Ongoing clinical research improved isolation,
culture and transplant techniques, and evaluated advanced anti-inflammatory and
immunomodulatory interventions [4-6]. As a result, a multicenter analysis with 18 diabetic
patients receiving 34 islet transplantations showed a graft survival (defined by C-peptide
concentrations of ≥ 0.3 ng/ml) of about 72.2%, 44.4% and 22.2% after 1, 2 and 5 years,
respectively . Subsequently, a multicenter phase 3 trial, which enrolled 48 participants
receiving 75 islet transplantations, successfully improved glycemic control to a median
glycated hemoglobin A1c (HbA1c) level of 5.6% after 1 and 2 years . Compared to
standard insulin therapy, islet transplantation more efficiently improved glycemic control and
progression of retinopathy, and resolved hypoglycemia even in patients with only partially
remaining graft function [9, 10].
Transplantation of whole pancreases is an established alternative to islets and both procedures
display advantages and limitations . The standard procedure of islet infusion into the liver
is much safer with less complications than pancreas transplantation which is considered a
major surgery with accordingly enhanced risks for the patient. Thus, pancreas transplantation
is rarely performed alone and is most commonly combined with a kidney transplantation in
patients with T1DM and end-stage renal disease. The major obstacle of the less risky islet
transplantation is the limited graft survival. Insulin independence after islet transplantation
initially reached barely 10% at 1 year but has been improved by the Edmonton protocol to 10-
15% at 5 years  and by later developments, such as T cell-depleting agents and blockade
of tumour necrosis factor (TNF), to 50% at 5 years . Thereby, at least experienced islet
transplantation centers have substantially improved long-term graft function towards the
reported 5 year outcomes in pancreas transplantation; 55% for pancreas alone and after
kidney, and 72% for pancreas together with kidney [2, 13].
As with other organ transplantation, there is a great scarcity of human donor material which
evoked intensive research on the generation of insulin producing β-cells from SC. Whole
pancreas transplantation strictly depends on high quality organs while generation of insulin
producing β-cells from SC has the potential to solve the problem of limited availability of
donor material for islet transplantation. However, the need for immunosuppression reserves
both pancreas and islet transplantation as a therapeutic choice to a limited patient population
such as brittle diabetics with life-threatening hypoglycemic events or subjects who anyway
receive immunosuppression; e.g. because of kidney transplantations for renal failure due to
ESC represent the inner cell mass of the blastocyst and possess a pluripotent differentiation
capacity. This makes them capable to form all three germ layers (ectoderm, endoderm and
mesoderm) which subsequently give rise to all cell types of the body. On this account, these
cells are considered the superior tool for tissue generation but they evoke ethical concerns
regarding their origin from human embryos. Moreover, they bear a clinical risk since their
pluripotent nature makes undifferentiated ESC capable to form teratomas and malignant
teratocarcinomas in vivo. The development of clinically safe differentiation protocols and
testing routines for tumorigenicity in rodents represents an important challenge in the field
The differentiation of human ESC into functional β-cells is not trivial since transforming
processes have to mimic complex embryonal organogenesis in vitro. Differentiation protocols
therefore established a number of factors and inhibitors that modulate molecular pathways in
an exact sequential timing to resemble the natural development of pancreatic β-cells.
Generally, human ESC were firstly differentiated into definitive endoderm cells and then
sequentially into primitive gut tube and posterior foregut, pancreatic endoderm and finally β-
cells using multiple specified media and supplementation in each step [16, 17]. The earlier
attempts proved the concept but did not achieve high yields; e.g. a typical early study resulted
in an average percentage of insulin+ cells in differentiated human ESC cultures of 7.3% .
Meanwhile, the exact media compositions have been further improved and current protocols
reached the level of scalable production of β-cell phenotypes with functional insulin secretion
[17-20]. However, the complexity of protocols and differences in employed ESC and iPSC
lines raise difficulties in reproducibility outside experienced laboratories since stem cell
differentiation is despite all progress not a routine procedure.
A major step in differentiation of human ESC towards β-cells is the expression of the
transcription factors pancreatic and duodenal homeobox 1 (PDX1) and NK6 homeobox 1
(NKX6.1) which are markers of pancreatic endoderm and endocrine precursor cells. It has
been shown that comparable human embryonic pancreas tissue from fetal weeks 6-9, which
contained very few β-cells at that stage, is capable to mature into functional β-cells after
transplantation into non-obese diabetic mice with severe combined immunodeficiency
(NOD/SCID mice) . Based on these findings, in vivo maturation of PDX1+/NKX6.1+
progenitors into β-cells has been recognized to be more efficient than mimicking this monthslong
process in vitro . Furthermore, scalable in vitro differentiation of ESC into endocrine
pancreatic precursor cells is to date more robust and less complicated [23, 24] than generation
of fully functional β-cell phenotypes by advanced protocols [17, 19] and therefore the
favoured strategy of the company ViaCyte .
The resulting improvements in differentiation of ESC towards PDX1+/NKX6.1+ pancreatic
progenitors during the last decade have also reduced the risk of tumour formation in vivo
which occurred in a typically early study of the company NovoCell (name changed to
ViaCyte in 2010) with a rate of 7 out of 46 transplanted mice . Since then, ViaCyte has
steadily optimized the approach to efficient large-scale production and embedded their
pancreatic endoderm and endocrine progenitor cells into a macroencapsulation device to
generate immune isolated VC-01 implants [18, 25, 26].
Macroencapsulation improved graft survival and clinical safety. Although ESC-derived
progenitors are hypoimmunogenic, transplanted cells are challenged by adaptive immune
responses such as local inflammation and rejection. In addition, once maturated into insulin
producing β-cells, graft cells will be attacked by persistent autoreactive T cells in patients
with T1DM. Several studies have demonstrated that macroencapsulation protects embedded
cells by isolation from immune responses and thereby avoids rejection and the need for
immunosuppression [27, 28]. Furthermore, macroencapsulation prevents escape of embedded
cells into the body. This is an important safety issue since any ESC transplanted in an
undifferentiated state bears the potential of malignant transformation. In this view,
subcutaneously transplanted devices could be retrieved and removed easily. Using this
concept, Viacyte has achieved a milestone by initiating the first-in-man clinical trial to test
safety and efficacy of their pancreatic endoderm implant VC-01 in patients with T1DM and
hypoglycemic unawareness (ClinicalTrials.gov: NCT02239354, currently enrolling patients,
estimated completion in January 2021).
This trial will answer the question whether the established in vivo maturation of human ESCderived
PDX1+/NKX6.1+ progenitors in rodents can be comparably recapitulated in human
subjects. In addition, two of three further trials initiated by Viacyte aim to test a modified
implantation device with reduced immuno isolation that allows vascularisation of the
macroencapsulated cells. Provided that outcome of all these trials will prove safety, efficacy
and significant long-term graft function, this approach holds the potential to pave the way
towards clinical β-cell replacement without immunosuppression and independent of the
limited availability of donor islets.
The ethical criticism related to the use of human pre-implantation embryos for extraction of
ESC inspired researchers to refine the work of John B. Gurdon. His key study demonstrated
that enucleated Xenopus laevis eggs that were transplanted with nuclei from differentiated
intestinal epithelium could, at least in small numbers, develop into living tadpoles . This
demonstrated for the first time that somatic cell nuclei have the potential to revert into a
pluripotent state. Decades passed by before this finding gained broad acceptance in the
scientific community and the progression of this idea culminated in the birth of the first
mammal, clone sheep Dolly, on July 5, 1996 .
Gurdons basic concept of reprogrammable somatic cells was further developed and one
decade after Dolly’s birth, Takahashi and Yamanaka induced embryonic-like pluripotency in
somatic mouse fibroblasts by viral overexpression of the four transcription factors Sox2,
Oct4, Klf4 and c-Myc . For their groundbreaking discoveries Gurdon and Yamanaka were
honored by the Noble Prize in 2012 (www.nobelprize.org).
As a major achievement, iPSC overcome the ethic obstacle of using embryos for harvest of
ESC. During ongoing research, the original Yamanaka protocol has been diversely modified
and viral integration was effectively replaced by treatment with recombinant proteins, small
molecules and microRNAs [32, 33]. Following the principle route of pancreatic development
as used for ESC, researchers have successfully differentiated iPSC into functional β-cell
phenotypes [34, 35] and also established scalable production of both endocrine pancreatic
progenitors and β-cells [20, 36].
Nevertheless, further refinement of procedures is still an issue and new tools were created for
the identification of compounds and conditions which enhance yield and functionality of
generated β-cells. For example, human iPSC expressing the fluorescent reporters Venus and
mCherry markers under the control of intrinsic neurogenin 3 and insulin promoters have been
generated for screening of differentiation efficiency . These cells have served to identify
an inhibitor of fibroblast growth factor receptor 1 (FGFR1) that, while blocking the early
development of pancreatic progenitors, promoted the terminal differentiation of pancreatic
endocrine progenitors into endocrine cells including β-cells.
However, due to their origin from adult somatic cells, iPSC can inherit somatic mutations and
incomplete reprogramming can maintain somatic transcriptional memory including cancer
associated gene activity [38, 39]. These dangers currently do not define them as the first
choice for clinical use but, more importantly, iPSC enable the successful generation of
patient-specific cell phenotypes that allow to recapitulate disease processes in vitro and can
serve as platforms for drug development and testing [40-42]. For example, researchers
successfully generated an iPSC line from a patient carrying a hepatocyte nuclear factor 1α
(HNF1A) mutation resulting in maturity-onset diabetes of the young type 3 (MODY3). In the
near future, patient-specific cell lines will help to develop disease-related models that
overcome the obstacle of species differences between human subjects and animal models.
For an excellent graphical overview on MSC biology discussed in section III we recommend
the poster by Somoza et al. .
MSC within the bone marrow (BM)
The discovery of MSC has been generally attributed to A. J. Friedenstein who observed that
BM explants form plastic adherent fibroblast-like clonogenic cells with a high replicative
capacity in vitro and named them colony forming unit-fibroblasts (CFU-F) . Friedenstein
et al. further figured out that culture expanded CFU-F are capable to differentiate into
osteoblasts, chondrocytes and adipocytes, and to reconstitute a hematopoietic
microenvironment after transplantation in irradiated mice [45, 46]. These findings supported
the pioneering study of Tavassoli and Crosby who demonstrated that autologous BM
fragments transplanted into extramedullary sites can reconstitute hematopoietic and
adventitial structures in rats . The observed process started from a developing network of
proliferating reticular cells and was successively followed by the occurrence of osteoblasts,
osteoid tissue, endothelial layers of sinusoidal structure and finally hematopoietic repopulation.
These findings pointed out that CFU-F include a group of cells with the capacity of
multipotent differentiation into mesenchymal lineages. Based on these features, multipotent
CFU-F were renamed ‘mesenchymal stem cells’ by A. I. Caplan in 1991 . Caplan later
commented that the term ‘stem cell’ was provocative at that time but justified by ongoing
research displaying that CFU-F could generate bone, cartilage, fat, muscle and other
mesodermal phenotypes in vitro . Postnatally, the contribution of BM-MSC to bone
formation is associated with declining numbers of MSC after birth as indicated by the 10fold
drop of CFU-F colony numbers in BM obtained from newborn and skeletally developed
teenaged donors and a steady further decline with aging .
Thus, bone formation was considered a core function of BM-MSC in vivo and their
osteogenic potential has been investigated in further detail by using scaffolds. As an example,
porous calcium phosphate ceramic cubes of 3 mm in size were loaded with BM-MSC
expressing the genetic marker lacZ and then subcutaneously transplanted into
immunodeficient mice . The MSC monolayers formed osteoblasts, then the scaffold
became vascularized by host vessels and mineralized osteocytes developed. Importantly,
lacZ+ osteoblasts and osteocytes confirmed that new bone was formed by donor MSC.
Testings of this approach in animals and clinical settings showed that transplanted porous
scaffolds loaded with BM-MSC significantly contributed to bone repair in rodents and in
patients with large bone defects [52, 53].
The cube experiments further demonstrated that lacZ+ cells also occur around blood vessels
. In support, cultured CFU-F express the same markers [e.g. cluster of differentiation
(CD) 146] as adventitial reticular cells of sinusoids in the intact BM in vivo .
Consequently, BM-MSC which do not undergo osteogenic differentiation reside at the
abluminal surface of endothelial cells. After subcutaneous transplantation, cells sorted for
CD146 were capable to organize a hematopoietic microenvironment outside the BM. This
confirmed that skeletal progenitors are a functional part of the hematopoietic stem cell (HSC)
niche and form a specialized microenvironment as conceptually already conceived by R.
Schofield in 1978 . The use of culture expanded BM-MSC to improve outcomes of BM
transplantation in cancer patients after chemo-therapy was tested first-in-man in 1995 and was
clinically successful and safe .
Lineage tracing, using nestin (Nes)- or leptin receptor (LepR) promoter driven expression of
the fluorescence reporters GFP or tdTomato, was employed to further investigate MSCrelated
cell fates in the BM [57, 58]. One should keep in mind that characterization of BM
lineages is complicated since marker expression like that of Nes-GFP is in part variable
during cellular development and overlapping between distinct phenotypes. Besides Nes and
LepR gene activation, the in situ localization of MSC in the BM has been mainly defined by
the differential expression of CD146, CD271, neural/glial antigen 2 (NG2) and alpha smooth
muscle actin (α-SMA). Resulting findings indicated the existence of three different MSC
populations within the endosteal niche and the perivascular niches at arterioles and sinusoids.
These investigations disclosed that Nes+ MSC of the endosteal niche secrete factors or
express cell surface molecules that regulate quiescence in nearby HSC [59, 60]. Nes+ MSC in
the perivascular niches express the key niche factors CXC chemokine ligand 12 (CXCL12),
therefore also called CXCL12-abundant reticular (CAR) cells, and stem cell factor (SCF)
which both control retention and maintenance of HSC . The perivascular MSC could be
divided in rare periarteriolar NG2+ cells with high nestin expression (Nesbright/NG2+) and
abundant perisinusoidal LepR+ cells with low nestin expression (Nesdim/Lepr+). Deletion of
Cxcl12 and Scf in Nes+ MSC results in the mobilization of HSC to extramedullary organs and
a marked reduction of HSCs in the BM [57, 61, 62]. Notably, the secretion of CXCL12 is in
part regulated by direct innervation of the sympathetic nervous system and modulated by
circadian rhythms .
The very low numbers of CFU-F in BM of adult human donors points out that BM-MSC are a
minor population . In line with this, it was found that among BM-MSC ‘abundant’
CAR/LepR+ cells account for only 0.3% of mouse BM cells . This small population is the
major source of adipocytes and osteoblasts in adult mouse BM but most of these cells
remained undifferentiated to maintain the hematopoietic niche. The underlying molecular
regulation was unclear until recently Seike et al. found that CAR/LepR+ cells preferentially
express early B-cell factor (Ebf) 3 and analyzed its function . Deletion of Ebf3 in
CAR/LepR+ cells severly impaired HSC niche function and BM became osteosclerotic with
increased bone in aged mice. Additional deletion of Ebf1 further increased niche dysfunction
leading to depletion of HSC already in infant marrow. This demonstrated that CAR/LepR+
MSC-derived Ebf3 and Ebf1 are required to maintain the HSC niche by inhibition of
Pericyte-MSC outside the BM
The early view on MSC as BM stroma cells has nowadays completely changed and the
occasionally used term ‘mesenchymal stromal cell’ became misleading. Instead, it became
obvious that MSC within the BM are not part of the connective tissue stroma but are forming
the endosteal and perivascular niches. Most BM-MSC are of perivascular origin . In a
landmark study, Crisan et al. clearly documented that MSC phenotypes exist outside the BM
in multiple organs as perivascular pericytes expressing typical BM-MSC markers like CD146,
NG2 and α-SMA, and being multipotent for osteogenic, chondrogenic, adipogenic and
myogenic lineages in vitro . Further functional characterization tested whether such
pericyte-MSC possess the ability of BM-MSC to restore a hematopoietic niche in irradiated
mice . The study revealed that sorted CD146+ perivascular cells, isolated from human
adipose tissue, are capable to support the long-term persistence of transplanted human HSC
while CD146- perivascular cells did not. This observation clearly demonstrated that BM-MSC
and pericytes expressing the same markers are equivalent in function.
Various studies have meanwhile demonstrated that MSC phenotypes could be isolated from
virtually all tissues of the body including fat, muscle, cord blood, Wharton’s jelly, placenta
and others . This initiates the notion that possibly a unique MSC may exists but it became
obvious that all these MSC, beside core markers, display differential gene expression profiles
in a time and tissue-related manner and thereby affect stemness . For example, muscle
pericytes are not spontaneously osteochondrogenic while cord blood-derived MSC
phenotypes display the unique capacity to form cartilage spontaneously in vivo. Furthermore,
there is evidence that also the intrinsic mechanical properties of the extracellular matrix
influences cell fate decisions in MSC, as softer matrices that mimic muscle are myogenic
while rigid matrices that mimic collagenous bone are osteogenic . Collectively, this raises
the notion that mesenchymal stemness of MSC is adapted to or imprinted by tissue
microenvironment and that MSC from placenta, Wharton’s jelly, umbilical cord blood etc.
may display the most embryonal-like phenotype [68, 69].
Regarding the notion of developmental and tissue-related differences, Chen et al. recently
proposed the concept of multiple ‘paralogous’ stem-cell niches which are progressively and
functionally transformed within an individual organism throughout its life span . In their
view, delineation of distinct cell phenotypes results from complex multiple interchangeable
events of epithelial to mesenchymal transition (EMT) and reverse mesenchymal to epithelial
transition (MET). These dynamic processes make it difficult to discern cell identities and to
define reliable markers. Therefore, the question whether all pericytes give rise to MSC, or in
the alternative view, pericyte-MSC differ from BM-MSC but may derive from a common
progenitor, is not finally answered to date [49, 71, 72]. Surely, the answer will be complex
and limited by the accuracy and composition of available marker sets; ‘true’ pericyte-MSC
may likely represent a subpopulation among all pericytes.
In vivo function of pericyte-MSC
Pericytes have been discovered in the early 1870s by C.J. Ebert and C.M.B. Rouget  and
were named by K.W. Zimmermann describing their contractile nature in 1923 . Later
characterization specified that pericytes communicate with endothelial cells by both physical
contact and secreted factors to regulate growth, stability, architecture and blood flow of
microvessels as well as they are important for the integrity of the blood brain barrier and
provide clearance and phagocytosis in the brain .
Pericytes attach to the epithelium by their tips and their contractile apparatus consisting of
microfilaments containing actin, myosin and tropomyosin enables them to regulate the
capillary diameter or to move along the microvessels . This indicated that perivascular
pericyte-MSC and their BM counterparts are not static but dynamic and their close proximity
to the vasculature enables them to readily mobilize and travel in the bloodstream to sites of
injury. Consistent with this view, pericytes respond to a series of pro-inflammatory stimuli
and are able to sense different types of tissue trauma signals by their expressed functional
pattern-recognition receptors and contribute to the onset of innate immune responses by cellcell
contact and paracrine effectors . Similarly, transplanted BM-MSC home to various
sites of injury, e.g. stroke , pancreatic islet inflammation and diabetic kidney [78, 79] and
cancer . Once on site, BM-MSC secrete a variety of immunomodulatory, antiinflammatory,
angiogenic, anti-apoptotic and tissue-regenerative trophic factors , and
fend off invading microbes by secretion of anti-microbial peptide LL36 that kills bacteria
upon contact [82, 83].
Altogether the ability of migration and humoral tissue restoration is a common feature of
MSC independent of their BM or pericyte origin. A.I. Caplan, who once coined the term
MSC, has meanwhile suggested to rename these cells ‘medicinal signalling cells’ to more
accurately reflect the new conceptional view on MSC as a ‘drug store’ for injured tissues in
vivo and to preserve the MSC nomenclature .
MSC in cancer
Besides MSC and HSC, the perivascular niche also accommodates tumour SC and its
microenvironment has been shown to regulate tumour dormancy and growth [85-87].
Tumours recruit pericytes by e.g. platelet-derived growth factor (PDGF) to maintain their
tumour vessels  and consequently, inhibition of PDGF receptor signalling causes pericyte
detachment and vessel regression, and diminishes tumour growth in several cancer models
[89-91]. Tumour cells further interact with the surrounding stroma leading to a chronically
increased release of inflammatory cytokines and growth factors  that has been described
as a ‘wound that never heals’ . The chronic inflammatory state drives the recruitment of
responsive cell types including MSC [94, 95] which account for 0.01–1.1% of total cells in
prostatectomies from human prostate tumours .
It is now understood that MSC interact with tumour cells at various stages of progression but
it is not finally clear whether their role is tumour promoting or suppressive. Several cancer
models implicated that MSC promote tumour progression and invasiveness as well as having
a role in the creation of a metastatic niche at the secondary site [97-100]. In contrast, MSC
suppressed tumour growth in several cancer models including breast cancer, Kaposi’s
sarcoma, hepatoma and melanoma [101-104]. Reasons for conflicting findings may result
from the heterogeneity of tested MSC populations, differences in experimental design and
varying responses dependent on the stimuli .
However, it seems unlikely that transplanted MSC have a significant role in inducing or
promoting tumours in human subjects, as their clinical use has been considered safe since
1995  and clinicians did not notice a tumour risk. In support of this notion, a metaanalysis
has studied 1012 participants who received MSC for treatment of ischemic stroke,
Crohn’s disease, cardiomyopathy, myocardial infarction, graft versus host disease or served as
healthy volunteers but did not find any indication of malignancy .
Meanwhile, research has employed the recruitment of MSC to tumours in order to target
malignant diseases with genetically modified MSC that, for example, overexpress pigment
epithelium-derived factor (PEDF) to reduce angiogenesis or overexpress TNF-related
apoptosis inducing ligand (TRAIL) to induce apoptosis . Despite using MSC from
different sources, different transfection methods and a wide array of expressed proteins, the
data consistently showed a reduction in tumor growth and prolonged survival in rodents.
These promising pre-clinical outcomes initiated the first-in-man trial TREAT-ME-1 which
aimed to target advanced gastrointestinal cancer (ClinicalTrials.gov: NCT02008539).
The trial used ganciclovir (GCV) in combination with autologous BM-MSC overexpressing
herpes simplex virus thymidine kinase (HSV-Tk) under the control of the CC-chemokine
ligand 5 (CCL5/RANTES) promoter. Mechanistically, engineered MSC migrate to tumours
where they become activated to express CCL5 . Subsequently induced HSV-Tk
phosphorylates GCV which then inhibits DNA polymerases and thereby induces apoptosis in
transfected cells and, due to a bystander effect, also in nearby tumour and stromal cells .
The primary study aim was to evaluate safety and tolerability, and both features were found
generally favorable with stable disease in four patients, and progressive disease in 2 patients
after one year follow-up . Slowed tumour progression and enhanced survival are of great
importance in the field and engineered MSC may contribute in the future to prolong the life of
cancer patients. Side note: the HSV-Tk/GCV suicide gene technique has also been tested as
an ’emergency switch’ that would allow to eliminate transplanted iPSC in case of malfunction
Transdifferentiation of MSC into insulin producing pancreatic β-cells
There was initially great optimism that MSC could be easily transdifferentiated across the
germ layer border into insulin producing pancreatic β-cells and thereby avoid the ethical and
tumorigenic obstacles of ESC and iPSC. Generation of insulin producing cells from MSC
employed genetic engineering including overexpression of PDX1, neurogenin 3 (NGN3) and
paired box 4 (PAX4) [111-113] and/or complex in vitro protocols using various conditions
and factors to resemble pancreatic development [114-118]. Depletion of β-cells in rodents by
a high dosage of the β-cell toxic compound streptozotocin (STZ) has frequently been used to
test the functional capacity of transplanted insulin producing cells. As a variant related to
aspects of T2DM , multiple low-dose STZ-treatment causes islet inflammation for
testing of MSC-mediated recovery of β-cell dysfunction and partial loss. Aspects related to
the immunology of T1DM have been investigated in female NOD mice with spontaneously
occurring autoimmune insulitis .
Several studies reported that transplanted MSC-derived insulin producing cells can improve
glycemia in STZ-diabetic rodents [111, 114, 118, 121]. Nevertheless, stem cell specialists
remained sceptic concerning in vitro transdifferentiation of MSC beyond mesodermal
lineages. In this regard, the efficiency of MSC transdifferentiation was generally very low and
resulting insulin producing phenotypes frequently possessed an acurate secretory capacity but
were not further expandable or vice versa. To date, transdifferentiation of MSC has not
reached clinically significant large scale production of pancreatic progenitors or β-cells as it
has been established for ESC and iPSC.
The study of Ianus et al. initiated the notion that injected BM cells contain a subpopulation of
cells that engraft into islets and are capable to transdifferentiate into insulin producing
phenotypes in vivo . It was observed that injection of BM cells with insulin gene 2
promoter driven GFP expression into sublethally irradiated mice gave rise to a small
proportion of 1.7-3% glucose-responsive GFP+/insulin+ cells within islets which, after
isolation and sorting, show a functional insulin secretion comparable to control β-cells.
Insulin+ phenotypes could be reproduced in a subsequent study. Hess et al. transplanted BM
cells from GFP mice in NOD/SCID mice with multiple low-dose STZ-induced islet
inflammation and noted partial recovery of diabetic blood glucose levels . GFP-BM-cells
significantly migrate to the inflamed endocrine pancreas and their occurrence within islets
was associated with enhanced local proliferation and 2.5% GFP+/insulin+ cells. Since the
insulin+/GFP+ cells did not express PDX1, a major marker of a mature and functional β-cell,
the authors concluded that amelioration of hyperglycemia was not caused by incompletely
differentiated GFP+/insulin+ cells but by the proliferative increase in β-cell mass.
In further testing, using injection of GFP-BM cells into single-dose STZ-treated mice, only 2
GFP+/insulin+ cells out of more than 100,000 screened β-cells could be retrieved .
These very rare events were considered to rather result from cell fusion than
transdifferentiation [124, 125]. Importantly, an elaborated lineage tracing study from Douglas
Melton’s group strongly suggested that new β-cells and islets only derive from pre-existing β-
cells and not from adult pancreatic stem cells or progenitors . In this regard, a later study
reported that up to 3% of injected human BM-MSC engrafted into inflamed pancreatic islets
of multiple low-dose STZ-diabetic NOD/SCID and improved hyperglycemia by reduction of
β-cell loss and partly maintained mouse insulin blood levels in the absence of detectable
human insulin . In addition, up to 11% of injected human BM-MSC engrafted into the
STZ-injured kidneys and improved glomerular morphology as well as decreased mesangial
thickening and macrophage infiltration.
Collectively, these studies exclude significant transdifferentiation of MSC into insulin
producing cells in vivo and pave the way for the new understanding that MSC migrate to and
engraft at site of injury to support tissue repair by secretion of numerous tissue regenerative
Humoral potential of MSC
After isolation, pancreatic islets suffer from hypoxic culture stress due to loss of blood supply
and consequently impeded transport of oxygen to the inner cell layers of the threedimensional
islet structure [127, 128]. After transplantation, islets were further challenged by
local inflammation and rejection processes . Fast dynamics of revascularisation and
downregulation of immune responses have been considered important for long-term graft
function and generated interest on the angiogenic and immunomodulatory potential of MSC
in the context of islet transplantation.
Kinnaird et al. displayed that human MSC express a wide array of arteriogenic cytokine genes
and that MSC conditioned media promoted smooth muscle cell proliferation and migration in
a dose-dependent manner in vitro . In vivo, using a murine hindlimb ischemia model,
murine MSC conditioned media enhanced collateral flow recovery and remodeling, improved
limb function, reduced the incidence of autoamputation, and attenuated muscle atrophy
compared with control media. In this regard, Figliuzzi et al. tested the angiogenic effects of
BM-MSC on co-transplanted islets in STZ-diabetic rats and noted that improved graft
survival and function in association with increased numbers of new capillaries and expression
of vascular endothelial growth factor (VEGF) . Upcoming studies confirmed the
angiogenic capacity of MSC and its association with VEGF [132-135]. In this regard, it was
shown in vitro that VEGF inhibition partially blocked the enhanced formation of
anastomosing tubule networks by co-cultured endothelial cells . Therefore, VEGF
appears to be an important player which is supported by other MSC-derived factors such as
nerve growth factor (NGF)  and factors inducing angiopoietin receptor Tie-2 expression
in islets . In sum, these studies established that MSC-mediated revascularisation
contributes to islet graft survival by shortening the post-transplantation ischemia period.
Improved revascularization and functional outcome of co-transplanted islet grafts have been
further associated with reduced numbers of terminal deoxynucleotidyl transferase mediated
dUTP nick end labeling (TUNEL)+ and caspase-3+ apoptotic cells [137, 138]. Angiogenic
VEGF and several other MSC-released trophic factors including hepatocyte growth factor
(HGF), insulin-like growth factor (IGF)-1, transforming growth factor (TGF)-β, basic
fibroblast growth factor (bFGF) and granulocyte-macrophage colony-stimulating factor (GMCSF)
display anti-apoptotic properties [81, 139]. The potential of MSC to mediate survival
was tested by direct interactions with β-cells in vitro in the absence of third party cells from
In line with the variety of released growth factors, we and others displayed that MSCconditioned
medium or co-cultured MSC preserve Akt signaling in cultured islets undergoing
hypoxic culture stress and additional treatment with alloxan and STZ [135, 140]. Akt
signaling promotes survival and reduces intrinsic apoptosis by its influence on B-cell
lymphoma 2 (Bcl-2) family proteins such as phosphorylation of Bcl-2 antagonist of cell death
(BAD) and caspase-9 . MSC-released factors also activate mitogenic extracellular
signal–regulated kinases (ERK)1/2 signaling which, similar to Akt, promotes survival by
inhibition of intrinsic apoptosis [142, 143]. Interestingly, MSC induced ERK1/2 signaling
only in highly proliferative endothelial cells and INS-1E insulinoma cells but not in primary
mouse islets with a low proliferation rate [135, 140, 144]. These observations indicate an
important role for the Akt pathway in MSC-mediated survival of pancreatic islets.
MSC-released factors have been further reported to improve insulin secretion and glucoseresponse
(see Table 1 in ). Experiments may indicate a beneficial effect of cell-cell
contacts since humoral improvement of glucose-stimulated insulin secretion (GSIS) in
indirect co-cultures with cells separated by membranes  was not well reproducible by
other studies unless cells were cultured in direct contact [146, 147]. In this respect, MSC
enhance GSIS in vitro by release of annexin-1 (ANXA1) while MSC from Anxa1-/- mice had
no functional capacity . Hence, heterogeneous effects of MSC on GSIS may partly result
from different expression levels of ANXA1. Likely, the very close proximity of MSC and
islets in direct co-cultures enhanced local effector levels and involves the extracellular matrix
(ECM) since MSC co-cultured with HSC maintain the vascular niche by upregulated
expression of intercellular adhesion molecule 1 (ICAM1) and vascular cell adhesion molecule
1 (VCAM1) . Such supportive processes are an important topic in tissue engineering and
it has been recognized that islets, which lost ECM during enzymatic isolation, show improved
survival and function after treatment with ECM molecules .
Moreover, numerous MSC-released factors exhibit potent immunomodulatory characteristics;
e.g. transforming growth factor-β1 (TGF-β1), indoleamine 2,3-dioxygnase (IDO), nitric oxide
(NO), human leukocyte antigen-G (HLA-G), Prostaglandin E2 (PGE2), interleukin-1 receptor
antagonist (IL-1RA) and tumour necrosis factor-stimulated gene 6 (TSG-6) [81, 139, 151]. As
a result, MSC have been described to induce regulatory T cells and anti-inflammatory M2
macrophages, and to inhibit T cells, natural killer cells and T helper (Th)17 cell differentiation
as well as maturation of dendritic cells (DC).
Consequently, MSC substantially reduced co-transplanted islet graft inflammation and
rejection in BALB/c mice [138, 152], humanized NOD scid gamma (NSG) mice  and a
cynomolgus monkey model . All these studies showed MSC-improved engraftment in
association with reduced infiltration of T cells and neutrophils, and increased numbers of
circulating regulatory T cells. Inhibitors established that MSC-mediated prevention of T cell
proliferation and islet graft rejection was not related to IDO and heme oxignase-1, partially
related to NO and profoundly mediated by matrix metalloproteases (MMP)-2 and MMP-9 via
reduction of IL-2 receptors on T cells . MSC further suppress the proliferation and
activation of T cells by interaction with IL-10-producing CD14+ monocytes .
Remarkably, systemically injected MSC in female NOD mice reduced the incidence of
spontaneous T1DM  or reversed recent-onset hyperglycemia via release of programmed
cell death ligand 1 (PD-L1) and inhibition of myeloid/inflammatory DC through an IL-6-
dependent mechanism . Moreover, treatment of NOD mice with CD4+CD62L+
regulatory T cells (Treg), which have been cocultured with cord blood-derived MSC before,
resulted in a marked reduction of spontaneous autoimmune insulitis, restored Th1/Th2
cytokine balance in blood and induced apoptosis of infiltrated leukocytes in pancreatic islets
. This concept has been translated into clinic as ‘Stem Cell Educator’ therapy (see below)
In addition, MSC have been further tested for their ability to ameliorate wound healing which
is a frequent diabetic complication. Endogenous MSC, present in the skin as dermal sheath
cells surrounding hair follicle units  and as perivascular pericytes . Skin injury
induces MSC to recruit and activate epithelial cells, fibroblasts and keratinocytes to
revascularize and re-populate the wounded area during the proliferative healing phase .
Wounds treated with MSC show acceleration of angiogenesis and re-epithelialisation 
which, according to the notion of paracrine factors, could also be achieved by treatment with
MSC-conditioned medium [164-166]. Wound healing is impaired in DM patients which show
degraded micro- and macrovessels in association with early occurring detachment and loss of
vascular pericytes at capillaries [167, 168]. Importantly, MSC-treatment successfully
improved wound healing under diabetic conditions such as in diabetic db/db mice with
mutated leptin receptor  and a rat model of diabetic foot ulceration .
MSC in clinical trials
The complex and wide-ranged humoral potential of MSC attracted much attention among
researchers and clinicians. MSC can be isolated from various tissues, frequently from BM and
adipose tissue, by minimal invasive puncture and they also allow noninvasive retrieval from
often discarded ‘medical waste’ such as placenta, cord blood and umbilical cord [68, 171]. It is
further known from early CFU-F studies and numerous studies since then that MSC could be
easily expanded in vitro without significant loss of their mesenchymal differentiation capacity
or their humoral secretion. Moreover, MSC are immune-privileged because they express very
low levels of major histocompatibility complex (MHC) class I and no MHC class II which
normally prevents or strongly reduces immune responses [172, 173]. In clinical use since
1995 , MSC are considered clinically safe  and both administration of autologous
and also allogenic MHC-mismatched MSC is generally well tolerated and clinically effective
To date, CinicalTrials.gov listed over 850 therapeutic approaches using MSC to target a broad
array of diseases including hematological disease, graft-versus-host disease (GVHD), organ
transplantation, cardiovascular and neurological diseases, bone and cartilage repair as well as
inflammatory and autoimmune diseases . Among these, more than 60 trials address
T1DM and T2DM, and from these we have summarized all trials with reported outcome in
Table 1. In these trials, the various humoral features of MSC address different disease aspects
(Fig. 1). In many patients with T1DM a minor portion of insulin producing β-cells survive but
can not recover unless thereby induced autoimmune responses are blocked . MSC
mediate immune tolerance that aims to enable partial recovery of remaining β-cell mass [158,
159] or to reduce and delay the β-cell destruction during new-onset of T1DM [179, 180]. In
T2DM, the anti-inflammatory features of MSC were used to ameliorate chronic low-grade
inflammation which has been recognized as an important cause of insulin resistance and β-
cell dysfunction . These features in combination with secretion of pro-angiogenic factors
should improve engraftment and survival of transplanted islets .
Interestingly, there is only one completed trial investigating the effect of MSC cotransplantation
on islet graft survival and function. Potentially, there are concerns on the
additional expense needed for generation, testing and application of clinical-grade MSC since
established immunosuppression regimes should prevent graft rejection. In this regard, Wang
et al. tested combined autotransplantation of BM-MSC and islets in chronic pancreatitis
patients undergoing pancreatectomy without immunosuppression . Patients showed
reduced insulin requirement in the peritransplantation period, reduced decline of C-peptide
levels after 6 month and lowered fasting blood glucose levels after 12 month. This suggests
that co-transplanted MSC reduced loss of islet graft function. Additional studies and longterm
observations are needed to verify these very limited results from 3 patients.
The other studies addressed T1DM (7 trials) as well as severe T2DM (8 trials) in patients who
required insulin and/or oral anti-diabetic drugs to control glycemia. Currently the total number
of investigated patients is relatively small. In total, 276 patients were investigated in small
groups of 6-22 subjects and 3 studies [159, 174, 182] analyzed groups of 31-45 subjects. The
enrolled patients show diversity regarding age, BMI and other aspects as well as duration and
severeness of the disease. In this view, also applied MSC came from different sources
including BM, adipose tissue and umbilical cord, have been differentially processed and
applied in different dosages. Though MSC did not cure the disease and despite much
heterogeneity regarding applied MSC, it is quite astonishing that studies reported varying
positive aspects of partially improved glycemia. Only two T2DM trials reported on diabetic
complications. Jiang et al. noted without further details that renal and cardiac functions
showed varying degrees of improvement  and Hu et al. found no rise of diabetic
complications in MSC-treated patients while placebo-treated patients displayed higher
incidences of diabetic retinopathy, neuropathy and nephropathy during 36 months follow-up
. In general, MSC were well tolerated and it could be noted as the quintessence of
outcome that all trials except one  reported reduced requirement for exogenous insulin
and/or anti-diabetic drugs.
Two studies addressed the question whether MSC treatment could delay development of
newly-onset T1DM. Hu et al. reported that both the HbA1c and C-peptide were improved
compared to the pre-therapy values and to control patients during 2 years follow-up .
Consequently, MSC-patients required smaller insulin dosages. This outcome indicated a
reduced loss of insulin producing β-cells but was not fully reproduced by Carlsson et al. who
observed improved C-peptide levels in response to a mixed-meal tolerance test but no changes
in HbA1c, fasting C-peptide and daily insulin dosages after 1 year . More trials with
prolonged observation periods are needed to clarify the potential of MSC to delay the
development of T1DM.
An interesting aspect is the so-called ‘Stem Cell Educator’ [158, 159]. While studies normally
apply MSC by intravenous injection, the ‘Stem Cell Educator’ approach routed the patient’s
blood through a closed-loop system that separates lymphocytes from the whole blood and
briefly co-cultures them with adherent cord blood-derived MSC before returning them into
the patient’s circulation. Though MSC are not delivered into the body, their temporary contact
to patient’s lymphocytes was sufficient to induce immune tolerance which ameliorates the
disturbed Th1/Th2/Th3 cytokine balance with increased Treg numbers in type 1 diabetic
patients and decreased CD86+/CD14+ monocytes and reduced markers of inflammation in
type 2 diabetic patients. As a result, all patients displayed a generally reduced requirement for
insulin and metformin and improved HbA1c values after 10-12 months follow-up. In addition,
homeostasis model assessment of insulin resistance (HOMA-IR) demonstrated that insulin
sensitivity was improved post-treatment in type 2 diabetics. The potential of the ‘Stem Cell
Educator’ is currently further investigated by 2 further trials (ClinicalTrials.com:
NCT02624804 and NCT03390231).
Collectively, the trials show that even heterogeneous MSC populations could be clinically
effective. The question, however, remains whether clinical outcome could be improved by
optimized MSC batches since actually no standardized method exists for isolation,
characterization, expansion, potency testing or pathogen screening [184-186]. A basic issue is
the donor heterogeneity which has the potential to dramatically influence therapeutical
properties of MSC. This includes the question whether autologous MSC are a good choice for
cell therapy of DM since, for eample, hyperglycemia induces pericyte dysfunction via
activation of p75 neurotrophin receptor/NF-kB-mediated release of microparticles carrying
miR-503 from neighbouring endothelial cells . Also subclinical inflammation present in
subjects with metabolic syndrome and T2DM  impairs the vascular stem cell niche and
leads to MSC dysfunction . MSC batches could be further influenced by isolation and
expansion since several studies suggested that outgrowth from donor tissue generates less
heterogeneous cell populations with increased proliferation rates and cell viability than
isolation by enzymatic tissue digestion [189-191]. Moreover, MSC may lose their functions
due to increased cellular senescence during longer expansion and passaging since a direct
positive link between early passages of MSC and clinical outcomes in GVHD has been
demonstrated . One should finally keep in mind that only a small proportion of
systemically injected MSC engraft at site of injury while most rapidly embolise in the lungs
and disappear with a half-life of about 24 h to an unclear fate [193, 194].
The International Society for Cell Therapy (ISCT) defined clinically useful MSC by
mesenchymal differentiation (into bone, cartilage and fat), plastic-adherent growth in vitro
and expression of CD73, CD90 and CD105 in the absence of hematopoietic surface markers
. Notably, expansion of plastic-adherent BM cells favours the expansion of nonclonal
stromal cell-enriched populations, often misinterpreted as pure SC fractions, which contain
varying percentages of true MSC, e.g. depending on donor age , and thereby plausibly
exhibit different clinical effectiveness [196, 197]. This demands a reliable assay, such as the
CFU-F assay, and careful evaluation of each MSC batch may allow the identification of the
percentage of stem cells and their multilineage potential in each batch of nonclonal MSC.
Potentially, efficacy of MSC populations could be further enhanced by selection via
additional markers such as stromal (STRO)-1, CD146, alkaline phosphatase, CD49a, CD271
and VCAM1 . Against this background, the US Food and Drug Administration (FDA)
demands registering of tissue processing facilities which should report on (i) prevention of
transmitting communicable disease via contaminated tissue, (ii) proper handling and
processing of tissue and (iii) demonstration of clinical safety and effectiveness of cells,
especially after extensive manipulation.
Conclusion on the current state of stem cell therapy of diabets mellitus
Stem cell therapy has to deal with a wide array of limitations which are still the subject of
current research. See Table 2 for summary and comparison of SC in cell therapy of DM.
Differentiation of ESC and iPSC has meanwhile reached clinical large-scale production and
current developments of macroencapsulation may provide clinical safe usage of these cells
that demonstrate otherwise potentials for tumor development. Macroencapsulation prevents
the escape of embedded cells into the body and subcutaneously transplanted devices could be
retrieved and removed easily. The outcome of Viacyte’s first-in-man trial of their pancreatic
endoderm implant VC-01 will clarify whether the use of ESC and iPSC is an option in DM
therapy in the near future.
MSC are clinically safe and several trials exist though they are limited in number and
investigated patients. Currently, MSC-based therapy is no cure but shows a potential to
ameliorate DM since most studies report decreased requirement of exogenous insulin and/or
anti-diabetic drugs. In this regard, MSC may be best used with diabetic patients that have
severe problems in controlling glycemia by conventional therapies; e.g. patients with brittle
DM. We see potential in optimizing their therapeutical performance by standardizing
isolation, characterization, selection, expansion, potency testing and pathogen screening.
However, increased efforts will increase costs and MSC have to prove their value.
Declarations of interest
All authors have participated in manuscript preparation and have approved the final article.
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Table 1. Clinical trials with reported outcome using MSC to treat DM.
Section A. T1DM
Patient characteristics Treatment Main outcomes
Wang et al. 2018
n = 3
Testing safety and effects of
BM-MSC and islet
patients with chronic
pancreatitis undergoing total
Aged 26, 29 and 42
years with chronic
Infusion of 20 ± 2.6 x 106
BM-MSC together with
5,107 ± 1,920 islet
equivalents/kg via the
101 patients with
pancreatectomy due to
chronic pancreatitis who
received islets alone
served as historical
No AE directly related to MSC
Insulin requirement lowered in the peritransplantation
Fasting blood glucose lowered
Fasting C-peptide with smaller declines
during months 1-6 (mean C-peptide levels
comparable to control values at 6 months)
HbA1c not different
Improved life quality
Cai et al. 2016
n = 21
Testing safety and effects of
combined UC-MSC plus
autologous BM-MNC without
immunotherapy on C-peptide
Aged 18-40 years with
established T1DM for
Fasting serum C-peptide
< 0.1 pmol/ml
Daily insulin requirement
< 100 IU
Infusion of UC-MSC (1.1 x
106/kg) and BM-MNC
(106.8 x 106/kg) through
standard care (n = 21)
MSC-treatment was well tolerated
1 patient with puncture site bleeding
1 patient with abdominal pain
Serum C-Peptide AUC 105.7% increase,
controls 7.7% decrease
Serum insulin AUC 49.3% increase, controls
HbA1c decreased 12.6%, controls increased
Reduced insulin requirement (-29.2%) with
no change in controls
Carlsson et al. 2015 Testing safety and effects of Aged 18-40 years IV infusion of 2.1–3.6 x No side effects of MSC treatment
n = 9
autologous BM-MSC in
treatment of patients with
recently diagnosed T1DM
T1DM newly diagnosed
< 3 weeks before
C-peptide ˃ 0.1 nmol/l
106 autologous BMMSC/
Sham controls (n = 9)
HbA1c, fasting C-peptide and insulin
requirements not different compared to
MMTT-induced C-peptide AUC and peak
values were preserved/increased by MSC
Dave et al. 2015
n = 10
Testing co-infusion of ISC (in
vitro differentiated from
adipose tissue-derived MSC)
together with BM-HSC
Aged 8-45 years
IDDM for at least 6
C-peptide levels < 0.5
Infusion into portal
circulation, thymus and
ISC: mean of 3.34 ml cell
inoculums with 5.25 x
104 cells/μl; ISC
expressed ISL1, PAX6
and IPF1 with mean Cpeptide
of 1.03 ng/ml
and 17.48 l IU/l after 2 h
in glucose medium
HSC: mean of 103.5 mL
with 2.66 x 106/μL
No untoward effect of stem cell infusion
Improved serum C-peptide, Hb1Ac, blood
Reduced exogenous insulin requirement
Patients returned to normal lifestyle and
Thakkar et al. 2015
n = 20
Testing co-infusion of ISC (in
vitro differentiated from
adipose tissue-derived MSC)
together with BM HSC
Comparison of autologous vs.
allogenic stem cells
Aged 8-45 years
Group 1: mean age 20.2
years, mean disease
duration 8.1 years
Group 2: mean age 19.7
years, mean disease
duration 7.9 years
Infusion into portal
circulation, thymus and
Group 1 received 2.65 ±
0.8 x 104 autologous
ISC/kg (n = 10); ISC
expressed ISL1, PAX6
Group 2 received 2.07 ±
0.67 x 104 allogenic
ISC/kg (n = 10)
HSC not reported
No untoward effect, morbidity (pulmonary
embolism, sepsis) or mortality caused by
Reduced insulin requirement
Sustained improvement in serum C-peptide
Hu et al. 2013
Testing the long-term effects of
WJ-MSC for newly-onset
Aged 17.6 ± 8.7 years
T1DM according to ADA
criteria for less than 6
2 IV infusions with a mean
of 1.5-3.2 × 107 WJMSC
with an interval of
No acute or chronic side effects compared to
Fasting plasma glucose levels not different
n = 15
Fasting C-peptide ≥ 0.3
Control group received
saline (n = 14)
Improved HbA1c, fasting C-peptide and
postprandial blood glucose levels
Reduced insulin requirement
Zhao et al. 2012
n = 12
Testing safety and efficacy of
Stem Cell Educator therapy
Aged 15 to 41 years
with a diabetic history
of 1 to 21 years
Group A: with some
residual pancreatic β-
cell function (n = 6)
Group B: without (n = 6)
Stem Cell Educator
therapy: patient’s blood
circulated through a
closed-loop system that
from the whole blood
and briefly co-cultures
them with adherent CBMSC
them to the patient’s
No AE, minimal pain from two venipunctures
Reduced insulin requirement (24 weeks)
Improved fasting C-peptide levels and
reduced HbA1C (12 weeks) in groups A
Increased in basal and glucose-stimulated Cpeptide
levels (40 weeks, group B)
Increased expression of co-stimulating
molecules CD28 and ICOS, increased
numbers of Tregs and restored
Th1/Th2/Th3 cytokine balance (4 weeks,
groups A and B)
Sham controls (n = 3) without significant
Vanikar et al. 2010
n = 11
Testing co-infusion of allogenic
ISC (in vitro differentiated
from adipose tissue-derived
MSC) together with BM HSC
Aged 5-45 years
IDDM for 1-24-years
Low serum C-peptide
levels < 0.5 ng/ml
ISC: mean of 1.5 ml with
2.1 x 103/μL; ISC
expressed ISL1, PAX6
HSC: mean of 96.3 ml
with 28.1 × 103/μL
No adverse side effect related to stem cell
infusion or therapy
Reduced insulin requirement
Serum C-peptide levels increased
Patients became free of diabetic ketoacidosis
Section B. T2DM
Patient characteristics Treatment Main outcomes
Bhansali et al. 2017 Comparison of safety and Aged 30-60 years with Infusion of 106 BM-MSC 1 patient with local extravasation of blood
n = 10
effects of autologous BMMSC
and BM-MNC in
reducing on insulin
T2DM ≥ 5 years and
failure to achieve HbA1c
≤ 7.5% while receiving
triple oral anti-diabetic
drugs in optimal doses
along with insulin for the
last 6 months
(group I, n = 10) or 109
BM-MNC (group II, n =
10) via transfemoral
route into the celiac
Sham controls were
infused into the femoral
artery (group III, n = 10)
Reduced insulin requirement in BM-MSC and
BM-MNC groups (6 of 10 patients in both
BM-MSC and BM-MNC groups, but none in
the control group achieved the primary end
point of ≥ 50% reduced insulin requirement)
Increased 2nd-phase C-peptide response in
Improvement in insulin sensitivity index with
increased insulin receptor substrate-1 gene
expression in ABM-MSC group
Hu et al. 2016
n = 31
Testing safety and long-term
effects of WJ-MSC on
Aged 18-60 years with
T2DM according to ADA
2 infusions of 106 WJ-MS
with an interval of 1
month (group I) through
veins in the back of the
Controls (group II)
received normal saline
(n = 30)
No serious AE
No chronic side effects or lingering effects
Fasting plasma glucose almost unchanged
Reduced insulin requirement
Fasting C-peptide improved
HbA1c and postprandial plasma glucose
improved by trend after 36 months
HOMA of β-cell secretory function improved
Incidence of diabetic retinopathy, neuropathy
and nephropathy only increased in controls
Skyler et al. 2015
n = 45
Testing safety, tolerability,
and feasibility of allogeneic
selected subset of BMderived
Mesoblast Inc.; product
expressed MSC markers
STRO-1 and CD146) in
controlled with metformin or
one additional oral anti-
Aged < 80 years with
T2DM for 10.1 ± 6 years
Metformin either alone or
in combination with one
other oral antidiabetic
medication (except a
thiazolidinedione) for at
least 3 months
IV infusion of 0.3, 1 or 2 x
106 BM-derived MPC (n
= 15 each)
Sham controls (n = 15)
No serious acute AE due to infusion
No serious hypoglycemia events or
discontinuations due to AE, comparable AE
in MSC and placebo groups, 1 subject with
severe abdominal pain in MSC group
No immunologic responses to MSC
HbA1c reduced by trend, more pronounced
in patients with baseline HbA1c ≥ 8%
Insulin requirement reduced by trend
Guan et al. 2015
n = 6
Testing safety and effects of
Aged 27-51 years
Time from hyperglycemia
to first infusion was 4-
Patients treated with
insulin and poorly
controlled blood glucose
levels and HbA1c
2 IV infusions of 106 UCMSC/
kg through the
cubital vein with an
interval of 14-17 days
No safety issues during infusion and the
long-term monitoring period
Reduced insulin requirement (significant
during months 1-6), 3 patients became
insulin-free for 25 to 43 months
Insulin-free patients displayed reduced
HbA1c and increased fasting C-peptide
during months 1-24
Relative stable fasting plasma glucose and 2
h postprandial blood glucose levels
Liu et al. 2014
Chinese Clinical Trial
n = 22
Testing safety and effects of
treatment with allogenic
Aged 18–70 years
T2DM according to ADA
Poor glycemic control with
drugs and/or insulin
injection for at least 3
GAD antibody negative
Fasting blood glucose
level ≥ 7.0 mmol/L
HbA1c ≥ 7%
2 infusions of 106/kg WJMSC
1st infusion via peripheral
vein on day 5
2nd infusion directly
delivered to the
pancreas via the splenic
on day 10
3 patients with fever after operative day
1 patient with subcutaneous hematoma
1 patient with nausea, vomiting and
Improved HbA1c, fasting C-peptide levels,
HOMA of β-cell secretory function,
postprandial blood glucose levels
Reduced insulin requirement and oral
Reduced serum levels of IL-1β and IL-6, and
reduced numbers of CD3+ and CD4+ T
lymphocyte numbers at 6 months
Kong et al. 2014
n = 18
Testing safety and effects of
Aged 23-65 years
T2DM according to WHO
Patients received insulin
and oral anti-diabetic
drugs to control
3 IV infusions of 1-3 x 106
UC-MSC/kg with an
interval of 1 week
4 patients with slight transient fever
8 patients respond to treatment (efficacy
group); these show: reduced fasting and
postprandial blood glucose levels, and by
trend increased plasma C-peptide levels
and Treg numbers
All patients had a feeling of well-being and
are more active
Zhao et al. 2013
Testing safety and efficacy of
Stem Cell Educator
Aged 29-68 years with
Group A: oral medications
Stem Cell Educator
therapy: patient’s blood
circulated through a
No AE, mild discomfort during venipunctures
Improved metabolic control and reduced
n = 36
(n = 18)
Group B: oral medications
- insulin injections (n =
Group C: impaired β-cell
function with oral
medications + insulin
injections (n = 7)
closed-loop system that
from the whole blood
and briefly co-cultures
them with adherent CBMSC
them to the patient’s
Reduced HbA1C in groups A and B
Improved insulin sensitivity (HOMA-IR, 4
Recovery of fasting C-peptide levels in Group
C (56 weeks) and HOMA of β-cell secretory
function/C-peptide (12 weeks)
Improved serum TGF-β, reduced
CD86+CD14+ monocytes, no effect on
Treg numbers and restored Th1/Th2/Th3
cytokine balance (4 weeks)
Jiang et al. 2011
n = 10
Testing safety and effects of
placenta-derived MSC in
patients with longer
duration of disease
Aged 30-85 years
Duration of DM ≥ 3 years
Insulin requirement for
optimal glycemic control
of ≥ 0.7 IU/kg/day at
least for 1 year
3 IV infusions of 1.22-1.51
x 106/kg placentaderived
MSC with an
interval of 1 month
No fever, chills, liver damage and other side
Insulin and C-peptide levels increased
Insulin requirement decreased, 4 of 10
patients achieved reduction of ˃ 50%
Improved renal and cardiac function (no
Abbreviations. ADA, American Diabetes Association; AE, adverse event; AUC, area under the curve; BM, bone marrow; BMI, body
mass index; CD, cluster of differentiation; DM, diabetes mellitus; GAD, glutamic acid decarboxylase; HbA1c, glycated hemoglobin A1c;
HOMA, homeostasis model assessment; HSC, hematopoietic stem cells; IDDM, insulin-dependent diabetes mellitus; IL, interleukin;
ICOS, inducible costimulator; ISL-1, insulin gene enhancer protein 1; IPF1, insulin promoter factor 1 (official name, PDX1, pancreatic
and duodenal homeobox 1); IR, insulin resistance; ISC, insulin secreting cell; IU, international unit; IV, intravenous; MNC, mononuclear
cells; MMTT, mixed-meal tolerance test; MPC, mesenchymal precursor cells; MSC, mesenchymal stem cells; PAX6, paired box
protein 6; T1DM, type 1 diabetes mellitus; T2DM, type 2 diabetes mellitus; TGF, transforming growth factor; Th, T helper cell; Treg,
regulatory T cell; UC, umbilical cord; WHO, world health organisation; WJ, Wharton’s Jelly.
Table 2. Summary and comparison of SC in cell therapy of DM.
ESC iPSC MSC
Cell type and origin Embryonic SC
Inner cell mass of the blastocyst
Adult somatic cells
Reprogramming in vitro
Endosteal (BM) and perivascular niches (all
Generates all germ layers:
ectoderm, endoderm and
Generates all germ layers:
ectoderm, endoderm and
Generates mesenchymal lineages: bone,
cartilage, fat and muscle
Maintain HSC niche and hematopoiesis
Ethical concerns Use of embryos No No
pancreatic β cells
Insulin+ cells with limited secretory or
proliferative capacity (experimental)
Cell therapeutic options β-cell replacement β-cell replacement
Patient-specific cell lines
Secreted factors with immunomodulatory,
angiogenic and tissue regenerative properties
Advantages Large-scale production of
pancreatic endoderm, endocrine
progenitors and fully functional β-
Large-scale production of
pancreatic endoderm, endocrine
progenitors and fully functional β-
Easy isolation and in vitro expansion
Low immunogenicity allows allogenic
transplantation without immunosuppression
Clinical safe and well tolerated
Limitations Tumorigenic if incompletely
Tumorigenic if incompletely
maintains somatic transcriptional
ISCT minimal criteria for clinical MSC favor
expansion of nonclonal stromal cell-enriched
populations with varying proportions of true SC
Current clinical protocols are not standardized
and exhibit potential for improvements
Only a small proportion of systemically injected
cells engrafts in injured target tissues
Status First-in-man trial currently
investigates clinical safety and
efficacy of macroencapsulated
Currently not safe enough for
Patient-specific cell lines allow
investigation of disease
processes in vitro and represent a
platform for drug testing
Completed clinical trials collectively report on
reduced requirement for exogenous insulin
Greatest benefit for patients with problems in
controlling glycemia by conventional therapy
More clinical trials in progress
tumorigenicity by preventing the
escape of embedded cells into the
body and allows easy graft
removal if necessary
Abbreviations. BM, bone marrow; DM, diabetes mellitus; HSC, hematopoietic stem cell; ISCT, International Society for Cell Therapy;
SC, stem cells.
Large-scale production of pancreatic endoderm and β-cells from embryonic stem cells
First-in-man trial investigates macroencapsulated embryonic stem cell-derived β-cells
Induced pluripotent stem cells allow patient/disease-specific cell lines
Mesenchymal stem cells secrete immunomodulatory and tissue regenerative factors
Transplanted mesenchymal stem cells ameliorate human type 1 and type 2 diabetes