cancers

Review
Aging of Bone Marrow Mesenchymal Stromal Cells:
Hematopoiesis Disturbances and Potential Role in the
Development of Hematologic Cancers
Fulvio Massaro 1,2,† , Florent Corrillon 3, *,† , Basile Stamatopoulos 3 , Nathalie Meuleman 2 , Laurence Lagneaux 3
and Dominique Bron 2

1 PhD Program in Clinical and Experimental Medicine, University of Modena and Reggio Emilia,
41121 Modena, Italy; fulvio.massaro@bordet.be
2 Department of Hematology, Jules Bordet Institute (ULB), 1000 Brussels, Belgium;
nathalie.meuleman@bordet.be (N.M.); dominique.bron@bordet.be (D.B.)
3 Laboratory of Clinical Cell Therapy, ULB-Research Cancer Center (U-CRC), Jules Bordet Institute,
Université Libre de Bruxelles (ULB), 1000 Brussels, Belgium; basile.stamatopoulos@ulb.ac.be (B.S.);
laurence.lagneaux@bordet.be (L.L.)
* Correspondence: florent.corrillon@bordet.be
† These authors contributed equally to this work.

Simple Summary: As for many other cancers, the risk of developing hematologic malignancies
increases considerably as people age. In recent years, a growing number of studies have highlighted
the influence of the aging microenvironment on hematopoiesis and tumor progression. Mesenchymal
stromal cells are a major player in intercellular communication inside the bone marrow microenvi-
ronment involved in hematopoiesis support. With aging, their functions may be altered, leading to
hematopoiesis disturbances which can lead to hematologic cancers. A good understanding of the



mechanisms involved in mesenchymal stem cell aging and the consequences on hematopoiesis and


tumor progression is therefore necessary for a better comprehension of hematologic malignancies
Citation: Massaro, F.; Corrillon, F.; and for the development of therapeutic approaches.
Stamatopoulos, B.; Meuleman, N.;
Lagneaux, L.; Bron, D. Aging of Bone Abstract: Aging of bone marrow is a complex process that is involved in the development of many
Marrow Mesenchymal Stromal Cells: diseases, including hematologic cancers. The results obtained in this field of research, year after
Hematopoiesis Disturbances and
year, underline the important role of cross-talk between hematopoietic stem cells and their close
Potential Role in the Development of
environment. In bone marrow, mesenchymal stromal cells (MSCs) are a major player in cell-to-cell
Hematologic Cancers. Cancers 2021,
communication, presenting a wide range of functionalities, sometimes opposite, depending on the
13, 68. https://doi.org/10.3390/
environmental conditions. Although these cells are actively studied for their therapeutic properties,
cancers13010068
their role in tumor progression remains unclear. One of the reasons for this is that the aging of
Received: 16 November 2020 MSCs has a direct impact on their behavior and on hematopoiesis. In addition, tumor progression
Accepted: 24 December 2020 is accompanied by dynamic remodeling of the bone marrow niche that may interfere with MSC
Published: 29 December 2020 functions. The present review presents the main features of MSC senescence in bone marrow and
their implications in hematologic cancer progression.
Publisher’s Note: MDPI stays neu-
tral with regard to jurisdictional claims Keywords: mesenchymal stromal cells; aging; bone marrow niche; hematopoiesis; hematologic
in published maps and institutional malignancies; inflammation; inflammaging
affiliations.

Copyright: © 2020 by the authors. Li-

1. Introduction
censee MDPI, Basel, Switzerland. This Cancers and aging are closely linked [1]. Indeed, in most organisms, aging is ac-
article is an open access article distributed companied by multiple alterations at the cellular, tissue and systemic levels. All of these
under the terms and conditions of the alterations provide fertile ground for the development and progression of tumors, as evi-
Creative Commons Attribution (CC BY) denced by the shared hallmarks of aging and cancers [2,3]. Although the intrinsic processes
license (https://creativecommons.org/ leading to cell transformation from a normal cell into a tumor cell are now well known,
licenses/by/4.0/).

Cancers 2021, 13, 68. https://doi.org/10.3390/cancers13010068 https://www.mdpi.com/journal/cancers

Cancers 2021, 13, 68 2 of 22

it is also commonly accepted that the microenvironment surrounding cells and the inter-
actions between malignant cells and this microenvironment play crucial roles in tumor
development and growth. Malignant hemopathies represent no exception: they comprise a
wide collection of disorders, all originating from cells of the bone marrow (BM) and the
lymphatic system and accounting for almost 230,000 new cases every year in Europe [4].
The homeostasis and maintenance of BM cells and the immune system require continu-
ous renewal of all types of blood cells. This function is ensured in the BM by hematopoietic
stem cells (HSCs) that can differentiate into myeloid progenitors, giving rise to erythrocytes,
platelets, granulocytes, and monocytes, or into lymphoid progenitors, giving rise to B lym-
phocytes, T lymphocytes and NK cells. The function and regulation of HSCs are supported
by their close environment, the BM niche [5–7]. A growing number of studies point to a
clear link between aging, remodeling of the BM microenvironment and impairment of
hematopoiesis, leading, among other things, to hematologic cancers [8,9]. The BM niche is
a complex and dynamic network that is not yet fully understood and is regulated by a wide
number of cell types: endothelial cells, mesenchymal stromal cells (MSCs), perivascular
stromal cells, osteoblasts, sympathetic neurons, nonmyelinating Schwann cells, adipocytes
and regulatory T cells.
MSCs are multipotent nonhematopoietic cells able to differentiate into osteoblasts,
chondrocytes, adipocytes and fibroblasts [10–12]. They also secrete a wide variety of com-
pounds, such as growth factors, antiapoptotic factors, angiogenic factors and several
cytokines, and thus contribute to the regenerative process, wound healing, hematopoietic
support and regulation of the immune response [13,14]. MSCs also produce a large amount
of extracellular vesicles (EVs), small vesicles playing a major role in cell-to-cell communica-
tion. EVs transport different elements, such as proteins, lipids and microRNAs (miRNAs),
to target cells and are involved in many biological functions of MSCs [15–17]. It is now
known that the aging of MSCs alters their EV production and has a direct impact on their
functions and differentiation capacities [18,19]. Aging is also associated with an increased
incidence of hematologic malignancies such as chronic and acute leukemias, non-Hodgkin
lymphomas and plasma cell disorders: the mean age at diagnosis is 65–70 years, and the
incidence typically increases in groups of older subjects [4,20]. Due to their functions,
MSCs are important actors in the tumoral microenvironment, but their exact role remains
ambiguous. Indeed, different studies carried out to date show both a protumoral and an
antitumoral function of MSCs, as reviewed by Galland and Stamenkovic in The Journal of
Pathology [21]. Although the prominent role of MSCs in vivo seems to be participation in
tumor progression, further studies will be necessary to obtain a deep understanding of
their exact role inside the tumoral microenvironment.
In this review, we will start by highlighting the effects of aging on the functions of
bone marrow mesenchymal stromal cells (BM-MSCs) inside the BM niche and their effects
on hematopoiesis. Then, we will discuss the tumorigenic potential of BM-MSCs in the case
of hematologic cancers.

2. The Role of BM-MSCs in BM and Hematopoiesis Alterations during Aging

As mentioned above, during the aging process, changes in HSCs and hematopoiesis
disturbances occur. More precisely, the number of lymphoid progenitors decreases to
the benefit of myeloid progenitors that increase but lose some of their functions [22,23].
The result is an alteration of the immune system, leading to an increased susceptibility
to infections and to the development of autoimmune diseases and cancers. The aging of
HSCs is due in part to cell-intrinsic factors, as reviewed by Mejia-Ramirez and Florian in
Haematologica [24], but also by external signals from the aging microenvironment of which
BM-MSCs are a part. In this section, we will discuss the main features of BM-MSC aging
and its consequences on hematopoiesis and the inflammatory state of BM through the
modifications of BM-MSC secretome, the imbalance of their immunomodulation properties
and the imbalance between osteogenesis and adipogenesis leading to progressive replace-
 Cancers 2021, 13, x FOR PEER REVIEW 3 of 23

Cancers 2021, 13, 68 3 of 22

modifications of BM-MSC secretome, the imbalance of their immunomodulation proper-

ties and the imbalance between osteogenesis and adipogenesis leading to progressive re-
placement of bone by fat (Figure 1). We will also briefly examine the spatial and functional
ment of boneheterogeneity
by fat (Figure 1). We will
of BM-MSCs also
inside the briefly
BM niche examine the spatial
and its change during and
aging.functional
heterogeneity of BM-MSCs inside the BM niche and its change during aging.

Figure
Figure 1. With 1. With
aging, aging,
several several
factors inducefactors induce the
the senescence senescence
of bone marrowof bone marrow
mesenchymal mesenchymal
stromal stromal
cells (BM-MSCs) cells
that
accumulate inside the bone
(BM-MSCs) that marrow (BM) inside
accumulate niche. (A)
theThe
bonesenescent
marrowBM-MSCs adopt(A)
(BM) niche. theThe
senescent-associated
senescent BM-MSCssecretory
adopt
phenotype (SASP) enriched in particular with proinflammatory cytokines, and their extracellular vesicle (EV) morphology
and contenttheare
senescent-associated
strongly modified (seesecretory phenotype
Section 2.1). (SASP) enriched properties
(B) The immunomodulatory in particular withBM-MSCs
of aged proinflammatory
are im-
paired. For example, their ability to promote macrophage (MΦ) polarization into the M2 phenotype and theirmodified
cytokines, and their extracellular vesicle (EV) morphology and content are strongly capacity to(see
inhibit T lymphocyte proliferation
Section 2.1). are reduced (see Section properties
(B) The immunomodulatory 2.2). (C) An imbalance between osteogenesis
of aged BM-MSCs and adipogenesis
are impaired. For example,
occurs, leading to a progressive replacement of bone by fat (see Section 2.3). All of these processes are closely intercon-
their ability to promote macrophage (MΦ) polarization into the M2 phenotype and their capacity
nected and can lead to the establishment of low-grade chronic inflammation and hematopoiesis alterations (Figure created
to inhibit T lymphocyte proliferation are reduced (see Section 2.2). (C) An imbalance between
with BioRender).
osteogenesis and adipogenesis occurs, leading to a progressive replacement of bone by fat (see
Section 2.3). All2.1.
of Epigenetic and Secretome
these processes Modifications
are closely Associated
interconnected andwith BM-MSC
can lead to Aging
the establishment of
MSCs are multipotent cells with proliferative properties. However,
low-grade chronic inflammation and hematopoiesis alterations (Figure created with BioRender). similar to any
normal cell, they can only undergo a limited number of cell divisions before entering a
2.1. Epigeneticsenescent
and Secretome Modifications
state. Cellular Associated
senescence with BM-MSC
and its related cell cycle Aging
arrest were observed for the
first time by Hayflick in long-term in vitro culture of human fibroblasts [25]. Since then, a
MSCs are multipotent cells with proliferative properties. However, similar to any
wide variety of factors causing MSC senescence have also been described, such as oxida-
normal cell, they can only undergo a limited number of cell divisions before entering a
tive stress [26], telomere attrition occurring during in vitro expansion [27] or unrepaired
senescent state. Cellular
DNA damages senescence and its related
[28]. Accumulation cell cycle
of senescent arrest
cells was alsowere observed
observed for the
in several aged
first time by Hayflick
tissues, asinit long-term in vitro in
was well illustrated culture of study
a recent human fibroblasts
evaluating [25]. Sinceofthen,
the expression a
p16 and
wide variety of factors
p21, causingofMSC
two markers senescence
senescence, havefrom
in organs alsoyoung
been described,
or old donorssuch
[29].asAn
oxidative
increased
level of p21
stress [26], telomere was also
attrition observedduring
occurring in BM-MSCs fromexpansion
in vitro elderly people,
[27]suggesting that senescent
or unrepaired DNA
damages [28].BM-MSCs
Accumulationaccumulate with physiological
of senescent cells wasaging [30]. Nevertheless,
also observed in severalsome
aged experiments
tissues,
as it was wellstudying MSC senescence
illustrated in a recent dostudy
not useevaluating
cells form elderly donors but rather
the expression of p16in vitro
and stress-
p21,
two markers of senescence, in organs from young or old donors [29]. An increased level
of p21 was also observed in BM-MSCs from elderly people, suggesting that senescent
BM-MSCs accumulate with physiological aging [30]. Nevertheless, some experiments
studying MSC senescence do not use cells form elderly donors but rather in vitro stress-
induced-senescence conditions such as long-term culture expansion or senescence induced
by gamma irradiation. It is therefore necessary to remain cautious when comparing data
concerning in vitro senescence with physiological aging.
Several pathways and actors implicated in MSC senescent cell cycle arrest have
been identified: the well-established p53/p21 and p16/pRB pathways, as well as the
AKT/mTOR pathway [31], JAK/STAT pathway [32], mitogen activated protein kinase
p38MAP [33] and fibroblast growth factor FGF21 [34].
Cancers 2021, 13, 68 4 of 22

Senescence is accompanied by many cellular modifications at morphological and func-

tional levels: the cells become larger and resistant to apoptosis, while the autophagy process
decreases, and they are also subject to genetic and epigenetic modifications [35,36]. Epige-
netic modifications are key components of the BM niche homeostasis and can contribute
to age- and disease-associated MSC alterations. Modifications of MSC DNA methylation
patterns and hypermethylated and hypomethylated CpG sites in several genomic loci have
been observed upon aging and replicative senescence [37,38]. Some epigenetic regulators
have been identified to participate in MSCs aging. The expression and activation of Sirt1,
a NAD-dependent histone deacetylase, decrease with age and modify MSC proliferation
and differentiation [39,40]. Interestingly, miR-199b-5p, which is predicted to target Sirt1,
is deregulated in old BM-MSCs [41]. MSCs deficient in Sirt6, another histone deacetylase,
displayed accelerated cellular senescence, dysregulated redox metabolism and increased
sensitivity to oxidative stress [42]. In HSCs, identification of somatic mutations in the
epigenetic regulators DNMT3, TET2 and ASXL1 is associated with an increased risk of
developing hematologic cancers [43]. These mutations can occur as people age and their
identification in healthy people is known as clonal hematopoiesis of indeterminate poten-
tial (CHIP). These three epigenetic regulators seem to be involved in MSC aging. In human
umbilical cord blood-derived MSCs (UC-MSCs), the inhibition of DNMT1 and DNMT3b
induces cellular senescence [44]. In a mouse model used to study age-related skeletal
diseases, the expression of TET2 resulted decreased [45]. In addition, it has been shown
in mice that a loss of ASXL1 or TET2 impairs BM-MSCs fate and their ability to support
hematopoiesis [46,47]. Taken together, these two observations suggest that epigenetic
modifications of BM-MSCs occurring during aging can contribute to hematopoiesis distur-
bances.
The alterations associated with BM-MSC senescence also lead to a deep modification
of their secretome, making them adopt a new phenotype called the senescent-associated
secretory phenotype (SASP) [35] (Figure 1A). This SASP is characterized by increased secre-
tion of growth factors, proangiogenic factors, extracellular matrix remodeling factors and
especially proinflammatory cytokines such as IL-1β, IL-6 and IL-8 [48–52]. It is now well
known that EVs contribute greatly to the SASP and that senescence of MSCs has a strong
impact on them: their secretion is increased while their size is reduced and their content
is modified, especially in terms of miRNA [18,53]. For example, the activation of AKT in
aged BM-MSCs leads to increased partitioning of miR-17 and miR-34a in EVs, which, upon
transfer to HSCs, cause functional impairment via downregulation of autophagy-related
genes [54]. Terlecki-Zaniewicz and colleagues suggested that EVs of dermal fibroblasts and
their miRNAs act as cargo for novel members of the SASP that are selectively secreted or
retained in cellular senescence [55]. Although there are no similar experimental data on
MSCs, it is reasonable to assume this may also be applicable to them. Robbins suggested
that senescent cell-derived EVs could function as pro-geronic factors [56]. The SASP par-
ticipates in the establishment of the low-grade and chronic inflammation state observed
during aging, called inflammaging [57,58].
In addition, proinflammatory cytokines induce the expression of other cytokines by
BM-MSCs [59]. It has been reported that BM-MSCs secrete a significant amount of IL-6
in response to TNF-α and IFN-γ [60] and that this cytokine is secreted at higher levels by
aged BM-MSCs [51]. IL-6 is a proinflammatory cytokine implicated in inflammaging or
promoting tumor growth and metastasis formation. The serum of elderly patients with
chronic disease or cancer usually contains more IL-6 than the serum of young healthy
people [61]. The MSCs contribute thus, by this cytokine release, to the “inflammaging”
process well described by Fulop and others in age-related diseases [62]. With TGF-β,
TNF-α and GM-CSF, IL-6 also promotes the differentiation of HSCs towards the myeloid
lineage [51,63]. Interestingly, a recent analysis of the secretome of adipose tissue-derived
MSCs (AT-MSCs) showed that GM-CSF is more highly secreted by senescent MSCs obtained
by successive passaging [64]. Other proinflammatory cytokines secreted by BM-MSCs
could impair hematopoiesis. Indeed, although IFN-γ promotes stem cell factor (SCF)
Cancers 2021, 13, 68 5 of 22

expression in mouse BM-MSCs, an important factor for the support of hematopoiesis,

chronic exposure of BM-MSCs to this cytokine leads to a decrease in the total number of
BM-MSCs, diminishing their hematopoietic support [65]. In a recent paper, Gnani and
colleagues showed a clonogenic impairment of young hematopoietic stem and progenitor
cells (HSPCs) due to the activation of a proinflammatory program when they are exposed
to compounds secreted by aged BM-MSCs [52].

2.2. Imbalance between Pro- and Anti-Inflammatory Functions

In addition to their role as multipotent progenitor cells, MSCs are also endogenous
regulators of inflammation capable of immunosuppressive or proinflammatory functions
depending on environmental conditions [66]. Thus, they express several Toll-like receptors
(TLRs) whose activation influences their immunologic properties. In normal nonsenescent
MSCs, the activation of TLR3 or the presence of IFN-γ and TNF-α polarizes the MSCs
towards an anti-inflammatory state that is able to negatively regulate the proliferation
of T lymphocytes and NK cells through the secretion of nitric oxide synthase (NOS)
and prostaglandin E2 (PGE2), respectively [67]. MSCs also promote the polarization of
macrophages into the alternatively activated anti-inflammatory type 2 state (M2) at the
expense of the classical proinflammatory type 1 state (M1) [68].
These observations suggest that within the aging and inflamed microenvironment
of the BM, BM-MSCs may influence other immune actors to counteract inflammation.
However, the studies described in literature to date do not point in this direction. For
example, it has been shown that gamma-irradiated senescent BM-MSCs showed a lower
capacity to migrate in response to proinflammatory signals and, at least in part, a lower
inhibitory capacity towards T lymphocytes [69]. In addition, the priming of UC-MSCs
with IFN-γ and TNF-α induces the phosphorylation of p38MAP kinase only in aged
MSCs, which could in turn negatively regulate the COX2/PGE2 pathway and explain at
least partially the reduction in the immunomodulatory capacity of aged MSCs [70]. Other
studies have highlighted the impact of MSC aging on macrophage polarization. In mice,
indirect coculture of macrophages with BM-MSCs using transwells induces differential
gene expression in macrophages depending on the aging state of BM-MSCs. Thus, TNF-
α and iNOS, two markers of M1 proinflammatory macrophages, are upregulated after
coculture with aged BM-MSCs compared to coculture with young BM-MSCs, which instead
induce the expression of IL-10 and ARG1, two markers of the M2 state [71]. More recently,
in a lung injury murine model, Huang and colleagues showed that injections of EVs
secreted by young human BM-MSCs but not those secreted by the oldest BM-MSCs are able
to polarize macrophages towards the M2 state, reducing the severity of lung injury [72].
These differences between old and young EVs might be due to their miRNA content.
Taken together, these results suggest that the aging of MSCs impairs their ability
to adopt an immunosuppressive phenotype in response to environmental stimulation
(Figure 1B). Interestingly, TLR signaling involved in MSC immune polarization is also
implicated in the proliferation of HSCs towards the myeloid lineage and in the migration
of monocytes [73,74].

2.3. Imbalance between Osteogenesis and Adipogenesis

Bone tissue is a dynamic tissue undergoing constant remodeling throughout its life-
time. Its homeostasis is maintained by two complementary processes: the formation of
new bone by osteoblasts and the resorption of old and damaged tissues by osteoclasts.
BM-MSCs play an important role in this balance by being recruited at the bone-resorptive
site through TGF-β1 signaling and by differentiating into osteoblasts [75]. However, dur-
ing aging, bone resorption increases, and the bone density of the organism progressively
decreases, leading to osteoporosis and increasing the risk of fractures [76]. The age-related
changes in MSC differentiation potential have been studied by several groups in mice and
humans. Although conflicting results have been reported, one cause for the imbalance
between bone and adipose tissue occurring with aging could be due to a gradual loss
Cancers 2021, 13, 68 6 of 22

of the ability of BM-MSCs to differentiate into osteoblasts, favoring differentiation into

adipocytes (Figure 1C). A study using senescent BM-MSCs obtained after long-term cul-
ture showed an increased osteogenic differentiation potential after several passages [77].
However, other studies comparing BM-MSCs harvested from young and old donors have
shown both a maintenance [78,79] or a decrease in the osteogenic differentiation of oldest
BM-MSCs [30,80–82]. In a recent study, authors analyzed the transcriptional profile of
freshly isolated BM-MSCs from young and old donors and showed the upregulation of
genes implicated in the peroxisome proliferator-activated receptor (PPAR) signaling in
the oldest group, suggesting a reinforcement of pro-adipogenic microenvironment with
aging [83].
Several factors are implicated in the control of BM-MSC differentiation: RUNX2 and
SP7 promote osteogenesis [84,85], while CEBPα, CEBPβ, CEBPγ and PPARγ promote
adipogenesis [86]. There is a growing body of data highlighting the age-dependent control
of these factors. In mice, it has been shown that FOXP1, a transcription factor interacting
with CEBPβ, is downregulated during aging [87]. Similarly, CBFβ and MAF, two cofactors
of RUNX2, are also downregulated with increasing age [88,89], while PPARγ is upregu-
lated [90]. All of these signaling pathway modifications promote adipogenesis. The miRNA
content of BM-MSCs and their EVs also seem to be involved in the imbalance between
osteogenesis and adipogenesis. Indeed, it has been shown that aging and oxidative stress
can alter the miRNA cargo of EVs, which in turn causes the suppression of cellular prolif-
eration and osteogenic differentiation of BM stromal cells [91]. It has also been reported
that miR-31a-5p level rises in aged BM-MSCs and appears to be involved in increasing
adipogenesis and decreasing osteogenesis [92]. The decrease in osteogenic differentiation
by BM-MSCs is accompanied by a reduced level of osteopontin secretion, which is known
to negatively regulate the self-renewal of HSCs [93,94].
Adipocytes in BM impair hematopoiesis by diminishing the differentiation of
hematopoietic progenitors towards the B lymphocyte lineage [95]. In a recent paper,
Aguilar-Navarro et al. observed an increase of adipocytes in BM of elderly people as-
sociated with an increase of maturing myeloid cells and they proposed a contributive
role for adipocytes in myeloid skewing [96]. Another study conducted on mice has been
shown that aging is associated with the expansion of adipogenic potential of a stem cell-
like subpopulation in the BM which, in turn, altered hematopoiesis through an excessive
production of Dipeptidyl peptidase-4 [97].
By increasing the number of adipocytes inside the BM, the aging of BM-MSCs could
also indirectly impact the inflammatory state of the BM niche. It is indeed well known
today that adipose tissue participates in the production of a large amount of soluble factors
and cytokines and that aging and metabolic diseases, like obesity, are correlated with an
increase of its proinflammatory cytokine secretion [98].

2.4. Functional and Spatial Heterogeneity of BM-MSCs

MSCs represent a complex cell population characterised by specific localisation and
functional heterogeneity that may be essential to their biological role. Several surface
markers can be used to identify the different subpopulations of MSCs [99]. In BM, CD271
antigen can be used to identify a subset of BM-MSCs able to inhibit the proliferation of
allogenic T-lymphocytes and presenting lympho-hematopoietic engraftment-promoting
properties [100]. Most of HSCs are located in intimate cell-cell contact with these CD271+
MSCs [101]. A low or negative expression of platelet derived growth factor receptor
alpha (PDGRF-α) by CD271+ MSCs is correlated with expression of key-genes for HSC
supportive function and this expression is modulated according to the different phases
of development of the organism [102]. CD271+ cells can be further divided in two cell
subgroups with different localisation depending on the expression of CD146. CD146+
status defines MSC population located in the perivascular spaces while CD146− cells are
found in the endosteal region [103]. These populations have different degree of maturity:
CD146− MSCs are more mature whereas CD146+ cells retain plasticity. Their distribution
Cancers 2021, 13, 68 7 of 22

varies with age: Maijenburg et al. showed a predominance of CD146+ subset in pediatric
and fetal BM and suggested that variation in MSC subpopulations is a dynamic process
that can change MSC functions during aging of the BM [104].
Other studies using a new method of single cell transcriptional analysis showed age-
related changes in BM-MSCs composition. Duscher et al. identified an age-related depletion
of a subpopulation characterized by a pro-vascular transcriptional profile [105]. More
recently, Khong and colleagues identified a unique quiescent subpopulation exclusively
present in MSCs from young donors and showed that this subpopulation was characterized
by a higher expression of genes involved in tissue regeneration [106].
It has also been described the existence of two populations of MSCs with neural crest
or mesoderm embryonic origins and particularly the neural crest has been proposed as a
source of MSCs with specialized hematopoietic stem cell niche function [107]. Embryonic
origin has also been shown to play an essential role in the age-related decrease in the
functional capacities of BM-MSCs [108].

3. BM-MSCs and Hematologic Malignancies

BM-MSCs play a dual role in tumor cell growth in vitro and in vivo: they suppress
tumor cell proliferation and inhibit tumor growth, but they also suppress tumor cell
apoptosis and promote tumor growth [21,109]. We will examine these different mechanisms
in the context of hematologic malignancies below, but it is also important to note that
several studies have also highlighted the link between MSCs and metastatic process in
solid tumors. In breast cancer, MSC activity through CCL5 release and Tac1 upregulation
markedly increased tumoral metastatic capacity [110,111]. In neuroblastoma, differences in
both qualitative and quantitative features of MSCs affect tumoral progression in BM [112].
A MSC subpopulation expressing stemness, endothelial and pericytic cell markers seems
to impair neoplastic cells homing to BM in breast and prostate cancer models [113]. These
findings, even if impossible to apply to hematologic malignancies, demonstrate that MSCs
are implicated in the regulation of the interactions between neoplastic cells and BM niche.

3.1. Antitumoral Role of BM-MSCs in Hematologic Cancers

The role of the BM microenvironment in the pathogenic process of several hematologic
malignancies has been demonstrated in several settings [114]. Concerning the specific
role of MSCs, their action towards neoplastic clones is still unclear. Several studies have
underlined the antiproliferative action of MSCs, almost all in in vitro experiments with
oncohematologic cell lines, not primary patient cells. MSCs from BM, adipose tissue and
umbilical cord were used, showing the same activity independent of tissue origin.
Cell cycle arrest is one of the most accepted mechanisms explaining MSC antineoplas-
tic activity: an experiment conducted using murine B-cell lymphoma, acute lymphoblastic
leukemia (ALL) and erythroleukemia cells reported reduced cell cycle proliferation and
IL-10 levels in the presence of murine BM-MSCs [115]. Similar results were obtained us-
ing human BM-MSCs in coculture with chronic myeloid leukemia (CML), acute myeloid
leukemia (AML) and T-ALL cell lines, showing significantly reduced cyclin D2 activity
and subsequent G1 phase blockade [116]. The use of AT-MSCs was associated with the
same results [117]. Both cell-to-cell contact and paracrine signals were used to explain the
antiproliferative effect [118–120].
Some of these findings were confirmed in murine models, in the majority of cases
utilizing human MSCs. An in vivo demonstration of the antitumoral effect derived from
coculture of human MSCs with hematologic malignancy cell lines was reported in CML:
AT-MSCs inhibited CML cell line proliferation by interfering with the Wnt pathway and
β-catenin production, blocking cell cycle progression to the G1/S checkpoint through the
production of Dickkopf-related protein (Dkk-1) [121]. Similar antineoplastic activity was
also reported towards non-Hodgkin lymphoma cell lines in a mouse xenograft model: BM-
MSCs were capable of slowing tumor size increases and determining extensive intralesional
necrosis, probably in relation to reduced angiogenesis [122].
Cancers 2021, 13, 68 8 of 22

3.2. Protumoral Role of BM-MSCs in Hematologic Cancers

On the other hand, the antiapoptotic activity driven by MSCs towards neoplastic
cells has been widely documented in in vitro oncohematologic models. Among the most
important described mechanisms, direct contact between MSCs and neoplastic cells seems
to play a crucial role in the regulation of specific signaling pathways, mostly conferring
increased survival capacity. This evidence was reported in two studies using primary B-
ALL and chronic lymphocytic leukemia (CLL) cells, in which direct contact with BM-MSCs
resulted in reduced apoptosis of leukemic cells, with a particular advantage for CLL cells
adhering to the MSC layer [123,124]. Our group described increased B cell lymphoma-
2 (BCL-2) expression in CLL patient cells cocultured with BM-MSCs [125]. In B-ALL,
similar antiapoptotic activity was related to Notch-3 and Notch-4 pathway activation and
increased PGE2 secretion [126,127]. Another significant pathway seems to be related to
phosphatidylinositol-3-kinase (PI3K)/protein kinase B (AKT)-BAD hyperexpression [119].
Increased tumor cell stemness could play a key role in the survival promotion of
hematologic malignancies: a model of multiple myeloma (MM) cell lines cocultured with
BM-MSCs was associated with the overexpression of Bruton tyrosine kinase (BTK) and
increased mRNA and protein expression of NANOG, OCT4 and SOX2, key genes for stem
cell self-renewal [128].
BM-MSCs could induce drug resistance, as reported in several studies performed in
AML, ALL and CML settings. Concerning AML, the possibility of conferring resistance
towards multiple key drugs, such as cytarabine and the majority of anthracyclines (mitox-
antrone, idarubicin and doxorubicin), acting on several signaling pathways, such as nuclear
factor (NF)-кB, c-Myc, and Notch, has been demonstrated [129–131]. Similarly, the resis-
tance to cytarabine, methotrexate, idarubicin and dexamethasone in ALL seems to involve
MSC activity on extracellular signal-regulated kinase (ERK), p21 and Jagged1/Notch1
pathways. TGF-β1 and CXCL12 were reported to be two major regulators implicated in
AML proliferation and chemoresistance [132]. Interestingly, BM-MSCs are also related to re-
sistance to biological drugs, such as the BCR/ABL tyrosine kinase inhibitor (TKI) imatinib:
three studies demonstrated the involvement of the upregulation of the CXCR4/CXCL12
and IL-7 pathways [133–135]. It is noteworthy that through CXCL12-targeted deletion
in the BM hematopoietic niche, MSCs are capable of promoting CML leukemic stem cell
expansion despite TKI treatment [136].
In coculture with BM-MSCs, CLL cells display marked chemoresistance towards both
classic chemotherapy and new biologic drugs. Reduced antileukemic fludarabine activity
seems to be related to the upregulation of antiapoptotic Mcl-1 and Bcl-2 [137]. BM-MSCs
also protected CLL cells from dexamethasone- and cyclophosphamide-induced apoptosis
by maintaining Mcl-1 and protecting cells from PARP cleavage [138]. Stamatopoulos et al.
demonstrated that antagonizing the SDF-1α/CXCR4 axis can increase drug-related apopto-
sis of CLL cells when in contact with BM-MSCs [139]. Interestingly, BM-MSCs can modulate
the redox status of CLL cells, promoting cell survival and drug resistance. Additionally,
EVs derived from BM-MSCs have been shown to increase the resistance of CLL cells to
different drugs, including purine nucleoside analogs, corticosteroids, ibrutinib, idelalisib
or venetoclax [140]. BM-MSCs also play a key role in the induction of drug resistance in
MM cells [141]. The protective effect of BM-MSCs against bortezomib activity may be
mediated by IL-8 secretion [142], miRNA transfer [143] or exosomes [144]. The production
of fibroblast activation proteins (FAPs) by BM-MSCs has also been implicated in drug
resistance via the activation of the β-catenin signaling pathway [145].

3.3. Impact of MSC Senescence in MM, CLL and Myelodysplastic Syndrome (MDS)
As described before, aging MSCs deeply modify many of their genetic and epigenetic
activities, acquiring the so-called SASP, a peculiar condition characterized by increased pro-
duction of proinflammatory molecules. Whether this phenotype promotes oncogenesis is
still debated, as conflicting reports are present in the literature, suggesting a key regulatory
role for cancer cells towards MSCs [35]. However, several reports have underlined how
Cancers 2021, 13, 68 9 of 22

MSCs show a higher tendency to undergo senescence in some hematologic malignancy

models, according to morphology and gene expression profile alterations.
Several studies have demonstrated that MM-MSCs display important differences
compared to healthy donor MSCs (HD-MSCs): differences in gene and protein expression,
functional alterations such as lower proliferation, decreased osteoblastic differentiation
potential or impaired immunomodulation. Significant differences observed between BM-
MSCs in an environment with or without the presence of MM cells demonstrated the ability
of MM cells to modify BM-MSCs to increase their tumor-promoting effects [146]. However,
some studies suggest that MM-MSCs are inherently abnormal, and these abnormalities
remain even in the absence of MM cells.
Various studies have reported senescence-associated constitutive abnormalities in
MM-MSCs leading to abnormal cell characteristics and increased tumor support. In a
coculture model of MM cell lines and senescent MSCs, it has been described how cancer
cell priming switches MSC secretory activity, reducing the production of prosenescent
and apoptotic molecules and favoring angiogenesis and proliferation [147]. André et al.
demonstrated that MM-MSCs display a SASP, lower proliferative rate, higher cell size,
increased β-galactosidase activity and retention of cells in the S phase of the cell cycle [148].
This increased senescence was associated with reduced cyclin E1 and increased cyclin
D1 expression as well as the overexpression of miRNAs related to aging compared to
HD-MSCs [149]. In comparison to HD-MSCs, MM-MSCs express increased basal levels
of IL-1β and TNF-α [150]. As reported by Zdzisinska et al., cytokines overexpressed by
MM-MSCs can function as growth factors for MM cells and induce migration, adhesion,
osteoclastogenesis and angiogenesis [151]. MM-MSCs exhibited long-term hematopoietic
support and produced abnormally high amounts of IL-6 in the absence of any detectable
MM cells [152]. IL-6 is a known protumoral cytokine favoring cancer cell formation and
disease progression, which has also been associated with dexamethasone resistance and
plasma cell retention in BM [150,153]. A recent study indicated that myeloma-associated
elongation of telomere length of BM-MSCs may be a key element contributing to increased
IL-6 expression, by which MSCs may facilitate MM development [154].
It has also been reported that the pathogenesis of bone lesions in this disease comes
partly from senescent MSCs and their active impairment of osteoblast activity. Indeed,
the fivefold higher expression of the osteoblast inhibitor DKK1, both at the transcript and
protein levels, in MM-MSCs than in HD-MSCs suggests a direct role in osteolytic lesion
propagation through autocrine and paracrine signaling [155]. The low rate of osteogenic
differentiation is in part due to increased expression of inflammatory cytokines, such
as TNF-α, able to suppress expression of TAZ, a Runx2/Cbfa1 transcriptional coactiva-
tor [156]. EphrinB2 and EphB4 expression in MM-MSCs was lower than in HD-MSCs
and this dysregulated signaling may also decrease their osteogenic potential [157]. The
MSC gene expression profile in MM patients seems to differ according to disease status
after treatment: minimal residual disease (MRD)-negative patients express a completely
different profile compared to the pretreatment phase. This observation suggests that MSC
activity could play an important role in sustaining neoplastic proliferation, but a direct
connection between the two elements has to be demonstrated [83]. Several gene expression
profiling studies have extracted many genes differentially expressed by MM-MSCs and HD-
MSCs. These genes are principally involved in tumor-microenvironment cross-talk, coding
for proteins involved in MM cell growth, angiogenesis and osteoblast differentiation [158].
Furthermore, other significant differences involve important biological processes, such
as the cell cycle, DNA repair, cell adhesion and metabolism [77,148]. In the study of Fer-
nando et al., the downregulated genes were related to cell cycle progression, immune
system activation and bone metabolism, suggesting that MM-MSCs might contribute to
immune evasion and play a role in bone lesions [159]. Other genes related to different path-
ways of the immune system, including antigen processing and presentation, are altered in
MM-MSCs compared to HD-MSCs [159]. MM-MSCs present reduced inhibitory efficiency
towards T lymphocyte proliferation and reduced production of TGF-β, which could lead
Cancers 2021, 13, 68 10 of 22

both to escape immune control and to reduced apoptosis of MM cells [152]. The studies
evaluating the impact of MSCs in MM are summarized in Table 1.
In CLL, the relationship between MSCs and neoplastic cells has been widely docu-
mented, as discussed previously. CLL-MSCs display intrinsic qualitative and quantitative
abnormalities that may be implicated in disease development and/or progression. The
impaired proliferative potential of CLL-MSCs can be attributed, at least in part, to increased
cell apoptosis. BM-MSCs from CLL patients seem to be less numerous, to present reduced
proliferation potential and to express SASP, particularly characterized by increased pro-
duction of IL-6, IL-8 and VEGF [160]. The documented abnormal production of CXCL12
and TGF-β from MSCs could represent a key mechanism for leukemic progression [161].
However, CLL-MSCs display normal immunosuppressive properties in terms of their
capacity to suppress T-cell proliferative responses [161].
Most of these MSC features seem to be induced by the interaction with CLL cells,
as demonstrated by a coculture experience reported by Ding and colleagues: a transcrip-
tome analysis revealed an altered expression profile concerning genes mostly involved in
senescence and cell cycle regulation, such as LIF, CDKN2B, DKK2, HGF, and FOXQ1 [162].
EVs also play a key role in this cross-talk mechanism among CLL cells and MSCs, as
demonstrated in several studies [163]. In comparison to HD-MSCs, CLL-MSCs produced
more EVs able to rescue CLL cells from apoptosis and induce higher migration activity
and gene modifications than healthy Evs [140]. Moreover, CLL-derived exosomal proteins
and miRNAs can induce an inflammatory phenotype in MSCs, enhancing the proliferation,
migration and secretion of inflammatory cytokines [164]. All of the discussed studies on
MSC aging and CLL are listed in Table 2.
Similar findings were reported in MDS, in which MSCs display reduced colony-
forming and proliferation capacities and activation of the p53-p21 pathway, promoting the
formation of a BM environment hostile towards normal hematopoiesis and finally favoring
oncogenesis [165]. Alterations in cell cycle control have been found in MDS-MSCs: higher
expression of cyclin-dependent kinase inhibitor 2B (CDKN2B) could be responsible for
the low proliferative capacity of MSCs, favoring clonal progression [166]. MDS-MSCs also
displayed a shift towards increased apoptosis, lower expression of VEGF, SCF and ANGPT,
aberrant expression patterns of the Notch signaling pathway and increases in Wnt signal-
ing inhibitors [167]. Among MSCs subpopulations, CD271+ MSCs are expanded in MDS
and are in tight contact with HSCs in perivascular regions: these MSCs express abnormal
levels of CXCL12, a chemokine promoting HSCs homing, and could be responsible for the
abnormal localization of immature precursors (ALIP), a typical feature of the disease [101].
Senescent MSCs, through the increased production of cytokines such as IL-6, show the
ability to stimulate HSC proliferation and differentiation, decreasing stemness capacity and
promoting genome instability [51]. In addition, a recent review emphasizes the deleterious
influence of an inflammatory environment on the selection of mutant HSCs carrying CHIP,
with evident consequences for tumorigenesis [168]. The immunomodulatory capacity of
MDS-MSCs is deeply modified under physiological conditions: the capacity of MDS-MSCs
to inhibit T lymphocyte activation and proliferation is impaired in vitro [169]. Moreover,
global activation of inflammatory patterns (NF-кB, EGF, TGF-β, and TNF signaling) and
overexpression of negative regulators of hematopoiesis were described [170,171]. Epige-
netic regulation, such as hypermethylation, seems to confer reduced growth capacity and
osteogenic differentiation [172]. Hypomethylating agents, often used in high-risk MDS
treatment, have been found to restore a normal MSC phenotype in patients achieving
complete hematologic remission [173]. The level of expression of DICER-1 was lower in
MSCs from MDS patients, altering their miRNA content [174]. Interestingly, some miRNAs
were overexpressed in EVs derived from MDS-MSCs, such as miR-10a and miR-15a, which
are involved in cell cycle proliferation and apoptosis and are able to modify hematopoietic
cell properties [175]. Studies on the role of senescent activity in MDS pathogenesis are
summarized in Table 3.
Cancers 2021, 13, 68 11 of 22

Table 1. Studies evaluating the activity of MSCs in MM setting, with focus on MSCs senescence-like modifications.

MSCs Source Coculture Mechanisms and Reference

HD ARH-77 cell lines Secretome from MSCs showed impaired antitumor activity [147].
↑ SA-βGalA, cell size and hematopoietic support.
MM patients,
- ↓ proliferative capacity, osteoblastogenesis and immunomodulatory activity.
MGUS patients
Expression of senescence-associated secretory phenotype (SASP) [148].
↑ SA-βGalA and overexpression of miR-485-5p and miR-519d.
KMS12-PE
MM patients Cell cycle arrest in S phase.
cell lines
MM cells decreased SA-βGalA and influenced cell cycle characteristics of MM-MSCs [149].
MM patients - ↑ basal levels of IL-1β and TNF-α [150].
RPMI-8226 MM ↑ IL-6, IL-10, TNF-α, OPN, and especially HGF and BAFF production in response to MM cells.
MM patients
cell lines MSCs significantly enhanced the production of sIL-6R by MM cells [151].
MM patients, ↑ IL-6 production.
MGUS patients, - ↓ inhibitory capacity towards T lymphocyte proliferation.
Plasma cell leukemia patients Characteristics also observed in the absence of any detectable tumor plasma cell [152].
↑ SA-βGalA and tumor-supporting capacity.
NCI-H929 MM ↓ MSCs proliferation and differentiation potential.
MM patients
cell lines Dicer1 overexpression reversed the effects on differentiation and reduced cellular senescence.
MM cells could induce the senescence of MSCs from HD. [153].
↑ IL-6 and MIP-1α expression and telomere length.
MM patients -
Telomere length is positively associated with the expressions of IL-6 and MIP-1α at the mRNA level in MM-MSCs [154].
MM patients - ↑ DKK1 expression at transcript and protein levels [155].
RPMI-8226 and
MM patients MM cells inhibited osteogenesis of MSCs from HD, which were associated to a reduced TAZ expression, partially restored by neutralization of TNF-α [156].
U-266 cell lines
↓ levels of EFNB2 and EPHB4.
MM patients,
- EPHB4-Fc treatment inhibited MM growth, osteoclastosis, angiogenesis and stimulated osteoblastogenesis in vivo. EFNB2-Fc stimulated angiogenesis and
SCID-Hu MM murine model
osteoblastogenesis but had no effect on osteoclastogenesis and MM growth [157].
MM patients, XG-1 and MOLP-6 GDF15 induced dose-dependent growth of MM cells.
MGUS patients MM cell lines ↓ MM-MSCs osteogenic differentiation capacity [158].
Distinct gene expression profile between MM-MSCs and HD-MSCs (485 differentially expressed genes).
MM patients -
In particular: ZNF521 and SEMA3A, involved in bone metabolism and, HLA-DRA and CHIRL1, implicated in the activation of immune response [159].
Only 3 genes: DUSP2, MZB1, and TSPAN7, were significantly altered in MSCs isolated from MRD+ patients as compared to diagnosis.
MM patients -
By contrast, 56 genes were significantly deregulated in MRD- MSCs compared to the time of diagnosis [83].
MSCs = mesenchymal stromal cells; MM = multiple myeloma; HD = healthy donors; MGUS = monoclonal gammopathy of uncertain significance; ↑ = increase or upregulation; SA-βGalA = senescence-associated
β-galactosidase activity; ↓ = decrease or downregulation; SASP = senescence-associated secretory phenotype; MRD = minimal residual disease
Cancers 2021, 13, 68 12 of 22

Table 2. Studies evaluating the activity of MSCs in CLL setting, with focus on MSCs senescence-like modifications.

MSCs Source Coculture Mechanisms and Reference

↑ SA-βGalA.
CLL patients - ↓ CFU-F and proliferative capacity.
Polygonal aspect and expression of SASP [160].
B and T CLL-MSCs presented impaired reserves, defective cellular growth and aberrant production of SDF-1 and TGF-β1, crucial cytokines for leukemic
CLL patients
lymphocytes cells survival [161].
↑ of cycle inhibitors p16 and p57 expression, both key markers of cell senescence in CLL-MSCs.
CLL ↑ of Wnt inhibitors DKK1/DKK2 and Wnt5b expression in CLL-MSCs.
CLL patients
patients’ cells MSCs co-culture with CLL cells induced altered expression of ~1500 genes mostly involved in regulation of cell growth and senescence
(CDKN2B, DKK2, LIF, HGF, FOXQ1) and determined increased production of cytokines associated to SASP (MCP-1/IL-8/IL-6/IL-1Ra) [162].
MSC-EVs decreased apoptosis of CLL cells and increased chemoresistance towards several drugs, including fludarabine, ibrutinib, idelalisib and
CLL patients CLL venetoclax. Enhanced both spontaneous and SDF-1α -induced migration capacities of CLL cells.
(MSCs-EVs) patients’ cells Different gene expression profile between CLL cells cultured with or without EVs: overexpression of genes involved in the BCR pathway such as
CCL3/4, EGR1/2/3, and MYC [140].
CLL patients
The transfer of CLL exosomal protein and microRNA induced an inflammatory phenotype in MSCs, determining increased proliferation,
(MSCs-EVs)
- migration and secretion of inflammatory cytokines, contributing to a tumor-supportive microenvironment.
MEC-1-eGFP
Coinjection of CLL-derived exosomes and CLL cells promoted tumor growth in immunodeficient mice [164].
CLL murine model
MSCs = mesenchymal stromal cells; CLL = chronic lymphocytic leukemia; ↑ = increase or upregulation; SA-βGalA = senescence-associated β-galactosidase activity; ↓ = decrease or downregulation; CFU-F =
colony-forming unit-fibroblast; SASP = senescence-associated secretory phenotype; BCR = B-cell receptor; MSC-EVs = MSCs-derived extracellular vesicles
Cancers 2021, 13, 68 13 of 22

Table 3. Studies evaluating the activity of MSCs in MDS, with focus on MSCs senescence-like modifications.

MSCs Source Coculture Mechanisms and Reference

↑ cell size, SA-βGalA and p53 and p21 expression.
↓ proliferative capacity, colony-forming potential and hematopoietic supporting function.
MDS patients -
Alteration of cytoskeleton.
Osteogenic differentiation potential of MDS-MSCs from lower risk MDS was impaired [165].
↑ CDKN2B expression 8–11 times higher in MDS-MSCs compared to HD-MSCs.
MDS patients -
↓ proliferative capacity [166].
↑ apoptosis and Wnt signaling inhibitory ligands Dkk-1 and Dkk-2 expression.
MDS patients - ↓ VEGF, SCF and ANGPT expression with no change in the expression of CXCL12A and LIF.
Significantly altered cell cycle status and aberrant expression pattern of Notch signaling components [167].
↓ hematopoietic cytokine expression.
MDS patients -
↓ of the capacity of MDS-MSCs to inhibit T lymphocyte activation and proliferation in vitro [169].
Functional activation of NF-κB pathway in MDS-MSCs, resulting in impaired proliferation of MSCs, contributing to the reduced support for HSPCs
MDS patients HSPCs
in vitro [170].
↑ EGF and TGF-β expression.
MDS patients -
↑ TNF signaling [171].
↓ osteogenic differentiation.
MDS patients -
Altered expression of key molecules involved in HSPCs supportive function, in particular osteopontin, Jagged1, Kit-ligand and Angiopoietin [172].
MDS patients - Hypomethylating agents restore a normal MSC phenotype in patients achieving hematologic complete remission [173].
↓ DICER1 expression.
MDS patients -
↓ mir-155, miR-181a and miR-222 expression [174].
Some microRNAs were overexpressed in MSCs-EVs and two of them, miR-10a and miR-15a, were confirmed by polymerase chain reaction. If
MDS patients
transferred to CD34+ cells, these microRNAs modify the expression of MDM2 and P53 genes. Higher cell viability and clonogenic capacity after
(MSCs-EVs)
MSCs-EVs inclusion in CD34+ cells [175].
MSCs = mesenchymal stromal cells; MDS = myelodysplastic syndrome; HD = healthy donors; ↑ = increase or upregulation; SA-βGalA = senescence-associated β-galactosidase activity; ↓ = decrease or
downregulation; HSPCs = hematopoietic stem and progenitor cells; MSC-EVs = MSCs-derived extracellular vesicles
Cancers 2021, 13, 68 14 of 22

The supportive role exerted by the microenvironment, particularly MSCs, appears

crucial in the pathogenesis and progression of certain hematologic malignancies, as was
recently demonstrated in murine models of MDS [176]. The direct contribution of MSCs
to leukemogenesis remains largely unknown. The characterization of “pathologic” MSCs
requires in vitro expansion, which may alter their biological functions. In culture, MSCs
undergo an aging process due to extensive proliferation. Several reports suggest that
CLL-MSCs and MM-MSCs are probably dependent on leukemic clones for their long-term
survival, as leukemic clones are dependent on MSCs for their own survival. Senescent
MSCs in the BM of different hemopathies have never been directly demonstrated in vivo.
In vivo tracking of MSC aging has been performed only in animal models and suggests
a decline in MSC frequency with aging [177]. Age-related changes in BM-MSCs have
been recently evaluated in uncultured BM-MSCs. A reduction in MSC colony number
and density was confirmed in older donors, but multilineage gene expression profiles
showed no age-related differences [178]. Recently, Alameda et al. [83] evaluated the impact
of aging on healthy MSCs and determined how MM cells influence their functions: the
transcriptional profile of “native” MSCs showed differences with the age of donors and in
MM patients. Interestingly, fresh MM-MSCs are transcriptionally different from HD-MSCs
and partially influenced by aging. The aging transcriptional changes are thus exacerbated
by the cross-talk between MSCs and tumor cells. However, it has still not been clarified
which actor in this cross-talk process is the first protein responsible for the pathway leading
ultimately to hematopoietic niche alteration.

4. Conclusions
MSCs represent a key component of the BM microenvironment, exerting multiple
functions that are fundamental for tissue homeostasis, such as the renewal of bone, adipose
and connective tissues; the support of the hematopoietic niche; and the modulation of
the immune system response. These activities are carried out through the secretion of
a wide variety of compounds, such as growth factors, cytokines and EVs. The aging
process determines profound modifications of both the morphology and functions of
MSCs, of which the main modification is represented by the acquisition of the SASP, which
strongly contributes to the development of a proinflammatory environment. Senescent
MSCs play a key role in the development and progression of several solid tumors, and there
is increasing evidence that they provide the inflammatory microenvironment supporting
the progression of hematologic malignancies: indeed, MSCs reduce the apoptosis of
cancer cells, induce chemoresistance and reduce the support of the hematopoietic niche,
as comprehensively demonstrated in three oncohematologic models, MM, CLL and MDS.
Whether MSC protumoral activity is the primum movens of clonal development or the
effect of neoplastic stimulation still needs to be clarified. However, this activity seems to be
of crucial importance for tumoral progression, opening the field for better comprehension
of these diseases and potential therapeutic approaches.

Author Contributions: Conceptualization, F.M., F.C., L.L., D.B.; writing and editing, F.M., F.C., L.L.,
D.B.; editing, B.S., N.M. All authors have read and agreed to the published version of the manuscript.
Funding: This research was supported by “Fonds Yvonne Boël”, “Les Amis de l’Institut Bordet” and
the FOCA.
Conflicts of Interest: The authors declare no conflict of interest.

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