New Insights on the Role of the Mesenchymal–Hematopoietic Stem Cell Axis in Autologous and Allogeneic Hematopoiesis

Abstract

Cytoreductive protocols are integral both as conditioning regimens for bone marrow (BM) transplantation and as part of therapies for malignancies, but their associated comorbidities represent a long-standing clinical problem. In particular, they cause myeloablation that debilitates the physiological role of mesenchymal stem and precursor cells (MSPCs) in sustaining hematopoiesis. This review addresses the damaging impact of cytoreductive regimens on MSPCs. In addition, it discusses prospects for alleviating the resulting iatrogenic comorbidities. New insights into the structural and functional dynamics of hematopoietic stem cell (HSC) niches reveal the existence of “empty” niches and the ability of the donor-derived healthy HSCs to outcompete the defective HSCs in occupying these niches. These findings support the notion that conditioning regimens, conventionally used to ablate the recipient hematopoiesis to create space for engraftment of the donor-derived HSCs, may not be a necessity for allogeneic BM transplantation. In addition, the capacity of the MSPCs to cross-talk with HSCs, despite major histocompatibility complex disparity, and suppress graft versus host disease indicates the possibility for development of a conditioning-free, MSPCs-enhanced protocol for BM transplantation. The clinical advantage of supplementing cytoreductive protocols with MSPCs to improve autologous hematopoiesis reconstitution and alleviate cytopenia associated with chemo and radiation therapies for cancer is also discussed.

Introduction

Cytoreductive therapies are currently broadly used to treat a variety of malignancies and to induce allogeneic hematopoietic chimerism for treatment of malignant [1,2] and nonmalignant [3 –5] disorders. When this approach is used for cancers, the primary therapeutic objective is control of proliferation in the malignant growths. When this approach is used for nonmalignant diseases, the primary sought-after effect is suppression of immune and autoimmune reactions. Despite the significant therapeutic benefit of cytoreductive therapies for many fatal disorders, they may also result in serious comorbidities affecting the gastrointestinal tract, reproductive organs, pulmonary, urothelial and cardiovascular systems, and hematopoiesis [6,7]. Bone marrow (BM) aplasia and pancytopenia contribute considerably to the adverse side effects of cytoreductive protocols.

With the understanding that cytoreductive regimens damage mesenchymal stem and progenitor cells' (MSPCs) ability to sustain autologous hematopoiesis impaired by anticancer chemo and radiation therapies and engraftment of the allogeneic hematopoietic stem cells (HSC) under conditioning regimens, a search has begun for a means to protect this very important function of BM stroma.

This review provides a synopsis on the current knowledge about different mesenchymal compartment subsets within HSC niches and discusses the prospect for development of the MSPC-based protocols for: (i) supplementation into myeloablative regimens to support autologous hematopoietic reconstitution and (ii) sustaining allogeneic BM engraftment. The rationale for incorporating MSPCs into cytoreductive regimens is based on new insights on the dynamic structure of BM and the critical role of MSPCs within HSC niches under physiologic, pathologic, and therapy-associated conditions [8].

HSC Niches and BM Transplantation Challenges

The unitary theory of blood cells origin from HSCs was postulated by Alexander Maximow [9] and later was confirmed experimentally by Till and McCulloch [10]. HSCs represent a small subpopulation of cells within BM originally identified as CD34⁺ cells capable of hematopoietic reconstitution. The two basic features of HSCs are their ability to self-renew and to differentiate into mature blood cells. These functionally different HSC subsets are supported by long-term (LT-HSCs) and short-term (ST-HSC) repopulating HSC niches [11 –13].

Bone marrow transplantation (BMT) has been explored for the treatment of a vast variety of disorders; however, comorbidities associated with cytoreductive conditioning regimens and the risk of graft versus host disease (GVHD) limit its broad clinical application [14]. BM is the most sensitive tissue to human leukocyte antigen (HLA) disparity. In addition to major histocompatibility complex (MHC) antigens, numerous minor histocompatibility antigens play a role in allogeneic BM engraftment [15]. HSC engraftment implies that these cells have the ability to migrate into the host's BM microenvironment, known as HSC niches, and undergo self-renewal or differentiation into mature blood cells [16 –18]. This concept led to the conclusion that the host's HSC niches must be ablated to create a space for donor-derived BM to engraft [18 –20].

In conventional BMT protocols, recipient niches are damaged by ablative conditioning regimens resulting in cytopenia [21]. Different conditioning protocols are grouped into total myeloablative conditioning (MAC) [22], nonmyeloablative conditioning (NMC) [23], and reduced-intensity conditioning (RIC) [24]. In addition, monoclonal antibody therapy (MAT) has also been used to facilitate BM engraftment [25,26]. MAT is not conventionally considered as a conditioning regimen; however, it also leads to cytopenia and its associated comorbidities.

Despite the improvements in clinical outcomes achieved by reducing the conditioning intensity, the complications caused by these regimens often outweigh their therapeutic benefits. An optimal, clinically safe, and effective protocol for BMT remains to be established. Despite the associated comorbidities, myeloablation remains a standard component of the conditioning regimen required for BMT. This review presents evidence to challenge its necessity, especially as part of treatment for nonmalignant disorders.

Mesenchymal Cell Compartment of HSC Niches During Hematopoietic Homeostasis

HSC niches are complex, functionally dynamic structures sustaining either HSC self-renewal or their differentiation into mature blood cells. These two functionally discrete types of niches populated predominantly by HSCs in the dormant and proliferative status, respectively, have distinct spatial allocations within BM. HSCs migrate between these niches or exit into peripheral circulation [27,28]. They can also switch from dormancy to proliferation during homeostasis and under different stimuli [29]. Ultimately, HSCs' fate (quiescence vs. proliferation, self-renewal vs. differentiation, migration and engraftment) is regulated by their microenvironmental niches [30 –32]. Mesenchymal stem cells (MSCs), originally identified by Friedenstein, represent one of the major cellular compartments within HSC niches [33].

Research on MSC differentiation into different tissues produced a new branch of reparative medicine. Another essential property of MSCs is their immunomodulatory function [34 –36]. The focus of this review is on yet another important role of MSCs, which is in regulating hematopoiesis. The two major types of HSCs niches are endosteal (also known as long term [LT] or periarteriolar owing to their proximity to arterioles) and perivascular (also known as short term [ST] or perisinusoidal) niches, which are located around sinusoids in the inner core of BM [37]. The minority of HSCs are in a quiescent state and reside in endosteal niches, whereas the majority of HSCs, which represent a mixture of dividing and dormant cell populations, are located in perivascular niches [38,39].

Cytopenia results from impaired hematopoiesis, and cytoreductive therapy is one of its causative factors. Understanding the role of damaged mesenchymal cells within HSC niches in pathogenesis of this complication is needed to improve the outcomes of anticancer treatments and BMT-based therapies. HSCs have a vast regenerative potential. For example, replenishment of CD4⁺ cells, destroyed by human immunodeficiency virus (HIV), is sustained for years. On the contrary, only a few weeks of cytoreductive therapy results in cytopenia [21]. The potential key mechanisms based on compromised function of the mesenchymal stroma cells compartment in the BM, which are responsible for an abrupt impairment of hematopoietic homeostasis under cytoreductive regimens are discussed in this review.

Regulation of the HSC-MSC axis is mediated through either direct cell-to-cell interactions [31] or by secreted molecules [37,38]. The subsets of mesenchymal cells in HSC niches play different roles in hematopoiesis by regulating self-renewal and lineage-specific differentiation of HSCs in homeostasis, pathology, and postconditioning reconstitution by mediating both these mechanisms [39,40]. A brief review of the current understanding of mesenchymal cell subsets and their structural components sustaining LT- and ST-HSC niches is provided hereunder and illustrated in Fig. 1A, B, D.

FIG. 1.

Schematic illustration of hematopoietic stem cell niche types in the bone marrow. (A) MSCs represent one of the major cellular components sustaining hematopoietic homeostasis in both LT-HSC niches surrounding arterioles located within the endosteal zone (left) and ST-HSC niches in the vicinity of the sinusoids located in the perivascular zone (right). (B) Hematopoiesis in physiologic condition. Left panel: MSCs in the LT-HSC niches in the endosteal zone are characterized by the NestinHi/NG2+/Sca1+/LepR−/SCFLo/CXCL12Lo phenotype. These niches predominantly harbor (i) HSCs with long-term repopulating potential sustaining self-renewal (via symmetrical division) and (ii) population of the precursor hematopoietic cells initiating lineage-specific differentiation (via asymmetrical division) and thus losing the stem cell features. Middle panel: The latter cells migrate into ST-HSC niches where they undergo further differentiation, proliferation, and maturation into blood cells. The phenotype of MSCs in ST-HSC niches is NestinLo/NG2−/Sca1−/LepR+/SCFHi/CXCL12Hi. Right panel: photomicrograph of normal BM smear, illustrating physiologic hematopoiesis. (C) Hematopoiesis after myeloablative conditioning is reduced. All types of stem and progenitor cells of both hematopoietic and mesenchymal lineages are affected. In gray are shown MSCs that have acquired a senescent state in response to myeloablative conditioning. Myeloablative conditioning is one of the factors contributing to MSCs acquiring a senescent state, resulting in their impaired ability to sustain the hematopoiesis in both LT-HSC and ST-HSC niches. Right panel: photomicrograph of postconditioning aplastic BM no longer sustaining physiologic hematopoiesis, resulting in pancytopenia [177]. (D) Figure key for cell types [178]. HSC, hematopoietic stem cell; BM, bone marrow; MSC, mesenchymal stem cell; LT-HSC, long-term hematopoietic stem cell; ST-HSC, short-term hematopoietic stem cell. Color images are available online.

Endosteal HSC niches harbor mesenchymal cells, which are responsible for sustaining LT-HSCs. This includes MSPCs and mature osteoblasts [41,42]. Osteoblasts sustain LT-HSCs subsets' self-renewal ability [38,43], providing “quiescence” signaling to HSCs by direct interaction [44] or by secreting osteopontin [45]. Depending on their stage of differentiation, MSCs promote either myeloid or lymphoid differentiation [46,47]. Osteoclasts are also involved in the regulation of HSC niches and have close interactions with osteoblasts [48]. Nestin and Leptin markers define MSC subsets in both LT- and ST-HSC niches. Nestin⁺ cells have BM fibroblast colony-forming unit (CFU-F) activity, the ability to direct MSCs into lineage-specific differentiation, and a strong self-renewal potential. Two subtypes of Nestin⁺ cells have been identified in mouse BM: Nestin^peri cells adjacent to arterioles and Nestin^retic cells (reticular in shape) associated with sinusoids [39].

Perivascular HSC niches are located in proximity to BM sinusoids and contain a mixture of cells in dormant and proliferative states. The majority of cells in the proliferative state represent a population of lineage-committed progenitors with short-term repopulating potential; these cells are incapable of hematopoietic reconstitution in vivo [49]. CXCL12-abundant reticular (CAR) cells in perivascular niches have a high expression of CXCL12 [50 –52]. LepR is another defining marker of stromal cells supporting ST-HSC niches; this population contributes to bone regeneration after irradiation or fracture [40].

The major molecules regulating the MSC-HSC interactions are NG2, Sca-1, SCF/c-Kit, and CXCL12/CXCR4. They have different levels of expression in endosteal and perivascular niches. NG2⁺/Nestin^Hi mesenchymal cells in periarteriolar niches promote HSC quiescence, and depletion of NG2⁺ cells reduces the long-term repopulating potential of HSCs in BM [53]. Sca-1, expressed on mesenchymal cells, plays a role in HSC self-renewal and cell lineage differentiation [17,54]. Nestin^Hi periarteriolar mesenchymal cells are Sca-1^Hi [39,55], whereas LepR⁺ CAR cell lack Sca-1 expression [56].

The SCF/c-Kit axis plays an essential role in HSC self-renewal [57,58]. Mutations or conditional deletion of the SCF or c-Kit locus results in hematopoietic aplasia and anemia [59]. LepR⁺ CAR cells show a high expression of SCF [58,60]. The CXCL12-CXCR4 axis is important for the homing of HSCs within BM [61]. CXCL12 plays a role in regulating both the quiescent and proliferative states of HSCs in strong correlation with the level of this molecule's expression. CXCL12 is about 10 times higher in CAR cells in perivascular HSC niches compared with osteoblasts in endosteal niches [60].

In summary, endosteal and perivascular HSC niches represent distinct sites preferentially harboring either LT- or ST-HSC niches. This is not a status quo condition: rather, HSCs can change their phenotype, niche type location, and proliferative versus dormancy status. This flexibility contributes to HSC regulation and to maintaining the cellular subsets balance in response to physiologic, pathologic, and therapeutic stimuli [38]. The MSCs in HSC niches, supporting either LT- or ST-HSC populations, have characteristic phenotypes (Table 1).

Table 1.

Mesenchymal Stem Cells with Different Profiles Are Present in the Long-Term Hematopoietic Stem Cell and Short-Term Hematopoietic Stem Cell Niches

MSCs in the LT-HSC and ST-HSC niches	MSC quiescent state profile	MSC proliferative state profile
LT-HSC	NG2⁺/Nestin^Hi/Sca1⁺	LepR⁻/SCF^Lo/CXCL12^Lo
ST-HSC	NG2⁻/Nestin^Lo/Sca1⁻	LepR⁺/SCF^Hi/CXCL12^Hi

Markers characteristic of the quiescent state profile are prevalent in the MSCs residing in the LT-HSC niches, whereas markers characteristic of the proliferative state profile are prevalent in the MSCs residing in the ST-HSC niches.

LT-HSC, long-term hematopoietic stem cell; ST-HSC, short-term hematopoietic stem cell; MSC, mesenchymal stem cell.

Impact of Myeloablation on Function of the MSC/HSC Axis

The chemo and radiation therapies for treating malignancies, the conditioning protocols for BMT, and the immunosuppressive therapies for autoimmune conditions and organ transplantation represent different modalities of cytoreductive regimens. The adverse effects of these regimens are often severe and multifactorial, affecting numerous organs. One of them is the deteriorating effect on hematopoiesis with resulting anemia, thrombocytopenia, and pancytopenia. Hematopoietic homeostasis is sustained through the balance between symmetric and asymmetric division of HSCs. Symmetrical division sustains self-renewal of HSCs. These cells primarily remain in the G₀ cell cycle phase under homeostatic condition [62]. Asymmetric division produces one cell with its original properties and another with the potential for lineage-specific differentiation [12,63,64].

Numerous factors regulating these processes are altered by cytoreductive regimens that affect the physiological balance between ST- and LT-HSCs niches [65 –67]. Stromal cells in the HSC niches regulate quiescent, nonproliferative state of the HSCs, which protect them from both direct ionizing radiation (IR)-induced killing and indirect cytokine and free radical-mediated killing [68]. A study employing genetic barcode-based technology demonstrated homogenous differentiation of all engrafted HSC populations in unconditioned mice. By contrast, in mice subjected to conditioning regimes, only a small subset of HSC clones were involved in differentiation [65].

Mesenchymal cell subsets regulating hematopoiesis are damaged by cytoreductive regimens by numerous mechanisms inducing defects at the cellular, protein, and molecular levels. Osteoblasts are reported to respond to radiation by inducing HSC migration toward endosteal regions and eliciting “quiescence” signaling [69]. Anomalies in the CXCL12-CXCR4 axis were demonstrated to contribute to the pathogenesis of pancytopenia in response to myeloablation [51].

Exposure to IR induces long-term persistent senescence status in different cell lines that contributes to reduced functionality of the affected tissues [70]. This was shown to be the case for stromal BM cells as well. It was demonstrated in the mouse model that exposure to the IR triggers expression of the senescence markers and increases expression of p16 (INK4a)/p19(ARF), and that the IR-induced damage to the BM microenvironment can produce an adverse effect on hematopoiesis in an Ink4a/arf-dependent manner [71].

The studies evaluating the effects of radiation on human bone marrow mesenchymal stromal cells also demonstrated induction of the senescence status in these cells when exposed to a low dose of IR [72]. A study by Cameron et al. identified Nrf2 as an important factor in the response of normal tissues to radiation. They showed that Nrf2-mediated gene expression regulates a hematopoietic cell compartment response to radiation, where Nrf2 increases survival of irradiated osteoblasts necessary for LT-HSC niches. The absence therein of Nrf2 elevates stem cell radiosensitivity. Loss of Nrf2 promotes LT-HSCs from a quiescent to a proliferative state, which causes hematopoietic exhaustion and hence lower engraftment after myoablative irradiation [73].

Exposure to high levels of radiation is known to cause an acute radiation syndrome causing BM failure and resulting anemia, thrombocytopenia, and pancytopenia leading to life-threatening infections. The role of damaged MSCs in such conditions is supported by data providing the evidence that coculture of human macrophages with human MSCs or with exosomes isolated from MSCs can significantly enhance survival of mice exposed to lethal irradiation [74]. These and other data support the need to further explore the potential of using the MSCs to treat radiation-induced damage to different tissues [75].

A major target of low dose radiation effects are the stem cells. These cells can accumulate radiation effect during their relatively long lifespan, ultimately affecting their function. However, the common perception about MSCs is that they are relatively radioresistant. This concept can be challenged based on the data demonstrating that the radiation sensitivity of mammalian cells does not always follow a simple linear dose–effect response, with numerous cell types showing hyper-radiosensitivity to very low levels of radiation with no correlation with the cell survival response to the high doses [76,77].

Another interesting observation revealed that in the MSC cells, the number of Ki67 ⁺ cycling cells did not reduce with an increase of the radiation dose. Also unexpected was the observed lack of direct correlation between dose of radiation and number of the apoptotic cells. It was shown in the proliferative and functional acid-beta-galactosidase assays that a large percentage of the MSCs entered senescence and exhibited radiosensitivity in response to very low dose of radiation (40 mGy) while exhibiting radioresistance in response to higher doses of radiation (up to 2000 mGy) [72].

Unlike discrepancies regarding the role of the mesenchymal components in the HSC niches in compromised hematopoiesis, the fact that radiation exposure impairs hematopoiesis is undeniable. New therapeutic modalities are being developed to protect, reverse, or, preferably, prevent BM damage associated with exposure to radiation [78]. Conventionally, the BM susceptibility to radiation-induced damage was attributed to high proliferative activity of the hematopoietic stem and precursor cells, as cells with high cell division rates are known to be more radiosensitive [79]. Numerous mechanisms reportedly contribute to the HSC damage in response to radiation, such as TP53-mediated apoptosis, telomere dysfunction affecting lineage differentiation, and induction of senescence [80 –82]. Mesenchymal stromal cells, which play a paramount role in sustaining hematopoiesis in the HSC niches in physiologic condition, are reported in some studies to be less radiosensitive compared to HSC [83,84]. Thus MSCs have been suggested as viable candidates for cell-based therapy of cytoreductive regimen-induced BM aplasia [85,86].

It was shown that MSCs can support hematopoiesis in patients with acute radiation syndrome as MSCs promote proliferation of irradiated CD34⁺ HSCs, rescue these cells from radiation-induced apoptosis, and support hematopoietic reconstitution after coculture [87]. Other studies have demonstrated that radioprotection can be delivered through MSCs-induced increased secretion of the hematopoietic growth factor Flt3L [88]. Preclinical and clinical trials show the effectiveness of the MSCs in sustaining the HSCs engraftment in BMT patients [89,90]. The mechanisms of action are suggested to be the secretion of cytokines and growth factors that stimulate hematopoiesis, combined with inhibition of T and dendritic cell-based responses [91,92].

The studies exploring the potential therapeutic applications of the donor-derived MSCs are evolving from the utilization of direct MSC-based therapeutic protocols to the utilization of MSC-educated macrophages and MSC-exosome educated macrophages as adoptive cell therapies for radiation-induced malfunction of mesenchymal stromal cells in the HSC niches in the BM [93].

Chemotherapy is another iatrogenic tool, which, like radiation, has a cytoreductive effect with potential to cause BM aplasia and associated comorbidities. Hematopoietic recovery after chemotherapy can be one of the most challenging aspects of cancer patient management. One of the potential pathogenic mechanisms of flawed hematopoiesis under this therapy is chemotherapy-induced damage of the stromal components of the BM. It was shown over 20 years ago by estimation of the frequency of the CFU-F in the BM of the recipients of BMT that the BM stromal microenvironment is seriously and irreversibly damaged after BMT (based on 12-year-long follow-up) [94].

Kemp et al. compared the phenotype and functional properties of BM-derived MSCs from untreated patients with hematological malignancy and those treated with high-dose chemotherapy (HDC). This study demonstrated a significant reduction in MSC expansion and MSC CD44 expression by MSCs derived from patients receiving HDC regimens, thus implicating disadvantage in the use of the autologous MSCs in chemotherapeutically treated patients [95]. Another study by this group demonstrated that the exposure of MSCs to the chemotherapeutic agents cyclophosphamide and melphalan has strong negative effect on MSC expansion and CD44 expression. In addition, the functional injury of the MSCs was reflected by their decreased ability to support hematopoietic cell migration and repopulation [96].

The commonly used chemotherapy drugs cytarabine and daunorubicin were tested for their effect on BM-residing MSCs and drug-induced alteration of hematopoiesis-supportive function of these cells. The chemotherapeutic drugs induced altered ability of the MSCs to support hematopoietic reconstitution was attributed to increased intracellular level of reactive oxygen species and induction of cell apoptosis [97]. Somaiah et al. evaluated the effect of the chemotherapeutic drugs cytarabine, daunorubicin, and vincristine on differentiation, phenotypic and gene expression changes in MSCs, and tested the anticancer effects of drug-treated MSC on leukemia cells. The findings in this study demonstrated that the assayed drugs altered the phenotype and the osteogenic and adipogenic differentiation of MSCs and rendered leukemic cells in these patients more chemo resistant [98].

Thus, the majority of studies are in agreement that the chemotherapeutic drugs have negative effects on MSCs' various functions. However, the recent in vitro study by Lopez Perez et al. demonstrated that human BM-derived MSCs are relatively resistant to treatment with the widely used anti-metabolite drugs, 5-FU and gemcitabine. In this study the expression of the MSC surface markers and their multilineage differentiation potential was shown to be irrespective of 5-FU or gemcitabine treatment. The high expression levels of enzymes involved in DNA metabolism and multidrug resistance transporters in the MSCs addressing contributing factors for the observed readouts. However, the authors did not address in this study the ability of the MSCs to support hematopoiesis [99].

An additional confirmation for the observation that MSCs in the senescent state have decreased ability to support hematopoiesis came from a study by Yuan et al. This study showed that HIV protein p55-gag could induce senescence of MSCs, resulting in their reduced capacity to support expansion of HSCs in vitro [100].

Another correlation between senescent state in the MSCs and their impaired ability to sustain hematopoiesis was demonstrated in patients with myelodysplastic syndromes (MDS). MDS-derived MSCs were found to have significantly reduced osteogenic differentiation and altered expression of key molecules involved in the interaction with HSCs, such as Osteopontin, Jagged1, Kit-ligand, and Angiopoietin, and several chemokines. In addition, the MDS-derived MSCs had significantly diminished ability to support CD34⁺ HSCs in long-term culture [101]. In line with this finding, deficient hematopoiesis was detected in myeloma BM-derived MSCs, which also was shown in association with their senescent state. Specifically, it was shown that myeloma BM mesenchymal stromal cells have an increased expression of senescence-associated β-galactosidase, increased cell size, reduced proliferation capacity, and characteristic expression of senescence-associated secretory profile members [102].

BM aplasia associated with cytoreductive regimen-induced senescence status in the mesenchymal stromal cells is given in Fig. 1C and D.

Challenging the Niche-Creation Dogma for Allogeneic HSC Engraftment: Is Myeloablation Required for Allogeneic HSC Engraftment?

A long-held perception is that ablation of autologous hematopoiesis is required for a successful allogeneic BMT [32,65]. To minimize the adverse effects, conditioning regimens have shifted from MAC to NMC and RIC. Although outcomes have improved, the complications remain significant [65,66]. BMT conditioning protocols damage HSC niches including their mesenchymal cell components resulting in different levels of pancytopenia [103,104].

A question arises as to whether BM aplasia and pancytopenia resulting from cytoreductive therapy are owing to the impaired function of MSPCs sustaining hematopoiesis. Could damage to MSPCs be the key factor responsible for both (i) impaired autologous hematopoietic reconstitution in case of chemo and radiation therapies for malignances and (ii) hindered engraftment of donor HSCs in case of BMT? Does myeloablation damage the niche-stromal components and impair their ability to support hematopoiesis rather than clearing the HSC niche space for donor BM engraftment? We would like to address these queries by discussing two intertwined questions: what is the current view on cellular and molecular level impacts of myeloablation on hematopoiesis, and what are the ablation requirements for HSC engraftment? In addition, a question about the role of MSPCs in the prevention of GVHD is discussed.

Conditioning, as a requirement for BMT, was reexamined after evidence emerged of its damaging effect on the mesenchymal compartment, which has the vital function for supporting hematopoiesis in HSC niches [65,105]. Numerous studies demonstrated the possibility of induction of hematopoietic chimerism without host myeloablation [106 –109] and the possibility that dietary valine restriction may reduce iatrogenic complications in HSC transplantation [110]. It was shown that HSC niches are dynamic, rather than invariable, structures [28,111]. Healthy donor-derived HSCs have the ability to outcompete diseased host HSCs for niche space during engraftment [107,112]. In addition, empty HSC niches appear to be available for engraftment of donor-derived HSCs [113].

Administration of massive doses of BM cells was shown to secure hematopoietic engraftment without conditioning [106,111,114]. The stem/precursor cell ratio and genetic composition of transplanted BM were also shown to be significant factors in promoting allogeneic engraftment. A competitive repopulation assays demonstrated that low numbers of HSCs successfully compete with the greater numbers of cotransplanted short-term precursor cells [115]. In addition, wild-type (healthy) donor HSCs can replace genetically defective HSCs in the recipients BM [112]. Unconditioned BMT transplantation was shown to minimally perturb natural hematopoiesis [65]. Taken together, these data support the rational for further exploration of protocols for allogeneic HSCs transplantation without conditioning.

Potential Advantage in Adaptation of the MSPCs into Therapeutic Protocols

MSPC-based protocols to support autologous hematopoiesis under cytoreductive therapy

Is it possible to avoid pancytopenia associated with cytoreductive regimens for treatment of malignancies? MSCs play a key role in LT- and ST-HSC niche integrity [46,53,116,117]. Radiation exposure, chemotherapeutic protocols, and other cytoreductive regimens can induce the senescent state and thus play a significant role in impaired ability of MSCs to sustain HSC niche integrity and resulting pancytopenia [71,118]. Incorporating MSPCs into protocols of the cytoreductive regimens may, therefore, be a key factor for treatment and prevention of cytopenia. Further research is needed to determine which subset of the mesenchymal cells or their combination would be optimal for such therapies.

MSPC-based protocol to sustain allogeneic HSC engraftment in a conditioning-free BMT

Mesenchymal cells represent a strong candidate for sustaining a conditioning-free allogeneic HSC engraftment without GVHD. As a major cellular component of HSC niches, MSPCs sustain hematopoiesis under homeostatic conditions; when they are damaged by conditioning regimens, their ability to sustain engraftment of allogeneic HSCs is impaired. The ability of MSPCs to cross-talk with HSCs despite allogeneic disparity [119 –121] allows these donor-derived MSPCs to resume the impaired hematopoiesis-supporting function of the stromal component in the HSC niches and additionally enables the use of the high doses of MSPCs from unrelated donors or commercial sources [122].

Amelioration and prevention of GVHD

Despite the accumulating evidence supporting the prospect of a clinically successful BMT protocol without conditioning [108,123], its clinical adaptation is hindered by the potential complication of GVHD [124,125]. Transplantation of purified HSCs was attempted as a venue to avoid GVHD. However, it significantly reduced HSC engraftment and induction of allogeneic chimerism [126]. To improve engraftment of allogeneic HSCs, while avoiding GVHD, a large cohort of hematopoietic [127,128] and nonhematopoietic cells were studied [52,56,129 –131], including osteoblasts, CAR cells, and MSPC populations. However, an adverse-effect-free protocol for induction of allogeneic hematopoietic chimerism remains to be developed.

The concept that MSCs can talk with HSCs across the MHC disparity was first shown by A. J. Friedenstein over 50 years ago when he identified MSCs in the bone marrow [33]. In 1982, T.M. Dexter foreseeably asked: “Is it possible that successful transplantation requires transfusion not only of haematopoietic stem cells but also transfusion and engraftment of stromal cell precursors? We will no doubt know the answer in the near future” [132].

Since then, the insight on phenotype and functions of the MSCs has advanced significantly. It was confirmed that MSCs can be used irrespective of HLA compatibility, as they do not trigger the immunologic reactions. For a while, the absence of the antigenicity of the MSCs was explained by the limited HLA class I expression and lack of expression of the HLA class II and costimulatory molecules CD40, CD40 ligand, CD80, and CD86 [133 –135]. It was suggested that, in the absence of costimulatory molecules, the secondary signal would not be activated, leading to induction of the T cell anergy [136 –138]. It was demonstrated that MSCs do not elicit proliferative responses from allogeneic T cells in culture [139] and that allogeneic MSCs transplantation is feasible and safe in various animal models [140 –144] and in humans [134,145].

Numerous clinical trials [146,147 –149] and veterinary practice studies [140,144,150] exploring the therapeutic adaptation of the MSCs in a variety of malignant [151,152] and nonmalignant [134,146,153,154] disorders prompted further studies aiming at elucidating the mechanisms underlying their toleranogenic properties. Klyushnenkova et al. showed that immunotoleranogenic properties of the MSCs are not owing to the lack of the costimulatory molecules expression, but to their ability to suppress T cell proliferation [9]. Treatment of MSCs in this study with interferon (INF), or transduction with B7-1 or B7-2, did not enhance the ability of these cells to stimulate alloreactive T cell proliferation. The ability of the T cells, removed from the culture with MSCs, to proliferate in response to stimulation with allogeneic peripheral blood mononuclear cells supported the conclusion that MSCs have immunosuppressive property irrelevant to the lack of the costimulatory molecule expression.

Suppression of alloreactivity by MSCs has been reported in humans [136] and in baboon [155] and mouse models [156]. In addition, it was demonstrated that MSCs can be immunoprivileged and immunosuppressive. They are incapable of inducing allogeneic lymphocytes proliferation in mixed lymphocyte culture, have inhibitory effect on lymphocyte proliferation, and secrete anti-inflammatory cytokines IL-10 and TGF-β [157].

Further studies of the immunomodulatory mechanisms of the MSCs revealed that these cells have potent immunoregulatory abilities by interacting with cells of the adaptive and innate immune system. They inhibit the differentiation of T cells into T helper 17 (Th17) cells and repress their proliferation while promoting generation of T regulatory (Treg) cells. The bi-directional immunoregulatory dialog between MSCs and Th17 cells is mediated by soluble factors, extracellular vesicles (EVs), mRNAs, miRNAs, cell-to-cell contact, and the transfer of organelles [158].

Incorporation of MSPCs into BMT protocol presents additional significant clinical advantage owing to their capacity to suppress GVHD [159 –161]. GVHD is a potentially life-threatening complication of allogeneic BMT, and is one of the major factors limiting the broad application of this therapeutic modality for hematological malignancies [162] and nonmalignant disorders [163 –165]. GVHD is classified into acute and chronic GVHD (aGVHD and cGVHD, respectively) [166].

Adaptation of the MSCs has been reported to be an effective therapeutic tool for the treatment, amelioration, and even prevention of GVHD. The mechanisms of the immunomodulatory effects of the MSCs mediating these clinically beneficial outcomes are under intensive research [149,167 –169]. It was shown that amelioration of the inflammatory reactions in GVHD by MSCs is associated with increased Treg cell populations and decreased secretion of the proinflammatory cytokines. The suggested mediators of these outcomes are MSC-secreted TSG6, IL-10, PGE2, vascular endothelial growth factor, and TGF-β1 [160].

Further modifications of the MSC-based protocols for amelioration of GVHD revealed the potential for substitution of the cellular inoculum of the MSCs by BM-MSC-derived EVs, administration of which recapitulated the therapeutic effects of the cell-based protocols. Systemic infusion of human BM-MSC-derived EVs was shown to prolong the survival of mice with aGVHD in association with reduced multiorgan damage, preservation of Treg cell population, and suppression of the naive T cells differentiation from naive to effector cell phenotype [170].

It was also shown that MSCs have the ability to enhance hematopoietic engraftment and hematological recovery after both autologous [171] and allogenic [151,172] BMT. This property may become crucial in the cytoreductive regimens, specifically not only in improving the allogeneic BM engraftment, but also in supporting autologous hematopoiesis reconstitution after its damage by chemo and radiation therapies for cancers. Koc et al. were the first to report improvement of hematopoietic engraftment when autologous BM-MSCs were cotransplanted with HSCs [171]. These findings were confirmed by results from the recent preclinical studies where MSCs were able to delay the onset of GVHD [173,174].

Newly revealed additional mechanisms contributing to the preventive effect of MSCs on GVHD provide an explanation for an existing paradox of the observed immunomodulatory effect after MSC infusion in association with failure of their long-term engraftment. The proposed explanation is that MSC immunomodulation depends on interaction with cytotoxic NK and T cells of the recipient. The apoptosis of the transplanted MSCs triggered by these immune cells is, according to this study, a requirement for the immunosuppressive outcome of MSC transplantation. Additional confirmation for this theory is the finding that the cytotoxicity of the host's cells and resulting apoptosis of the transfuse MSCs directly correlate with the immunomodulatory effect of these cells [173].

In disagreement with the numerous reports regarding the effectiveness of MSCs in ameliorating GVHD, a recently published analysis of over two-dozen large randomized controlled trials with enrollment of patients with hematologic malignancies concluded that MSC-based therapy to ameliorate aGVHD was not effective [175]. Indeed, outcomes are variable in different institutions and among the patients subjected to MSC-based protocols for therapy of GVHD. To eliminate these discrepancies and improve consistency in therapeutic outcomes, new clinical protocols need to be developed that define the optimal origin of the MSCs, the dose for their administration, and the cellular composition based on research that elucidates the mechanisms underlying their GVHD suppressive effects [176].

Summary and Conclusions

MSPCs have wide therapeutic potential. This review discussed their role in sustaining autologous and allogeneic hematopoiesis in physiologic condition and under cytoreductive therapies. MSPCs have been shown to play a critical role in hematopoietic homeostasis. Cytoreductive therapies appear to damage their ability to sustain hematopoiesis and contribute to BM aplasia and the onset of cytopenia [53,62,104,160]. These data suggest that incorporating MSPCs into chemo, radiation, and immunosuppressive therapies could sustain autologous hematopoietic reconstitution and alleviate cytopenia, comorbidity almost unavoidable in association with these therapeutic regimens.

MSPCs also hold therapeutic potential for improving outcomes of allogeneic hematopoietic chimerism. HSC niches are dynamic structures. Empty niches that self- and donor-derived HSCs can occupy are available within BM [114]. Healthy hematopoietic donor cells can outcompete the diseased recipient's HSCs to occupy recipient HSC niches [107,113]. These findings support the notion that the need for “clearing space” for BM engraftment is a misconceived paradigm and that conditioning can be eliminated from BMT procedure. The ability of mesenchymal cells to: (i) sustain allogeneic hematopoiesis across MHC disparity and (ii) prevent GVHD make them a strong candidate for the development of an MSPC-supplemented, conditioning-free BMT protocol [6].

The mesenchymal cell subsets, based on their phenotype, stage of maturation, and source of origin (eg, BM, umbilical cord, adipose tissue) as well as the cell dosage and MSPC/HSC ratio for supplementation into cytoreductive regimens is yet to be identified. Further exploration of the MSPC potential to compensate the function of mesenchymal stroma damaged by cytoreductive regimens could lead to new MSPC-supplemented therapeutic regimens for malignancies. Development of the MSPC-based conditioning-free protocol for BMT could open a new venue for treatment of nonmalignant disorders.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Acknowledgments

The author thanks Paul Hunter for help with bibliography and Jennifer Wilson and Dmitri Zorine with preparation of the article.

Funding Information

No funding was received for this work.

References

Galaverna

, Ruggeri

and Locatelli

. (2018). Myelodysplastic syndromes in children. Curr Opin Oncol, 30:402–408.

Bensinger

, Buckner

, Shannon-Dorcy

, Rowley

, Appelbaum

, Benyunes

, Clift

, Martin

, Demirer

, et al. (1996). Transplantation of allogeneic CD34+ peripheral blood stem cells in patients with advanced hematologic malignancy. Blood, 88:4132–4138.

Locatelli

and Pagliara

. (2012). Allogeneic hematopoietic stem cell transplantation in children with sickle cell disease. Pediatr Blood Cancer, 59:372–376.

, Chen

, Scott

and Tredget

. (2007). Mesenchymal stem cells enhance wound healing through differentiation and angiogenesis. Stem Cells, 25:2648–2659.

Yolcu

, Shirwan

and Askenasy

. (2017). Mechanisms of tolerance induction by hematopoietic chimerism: the immune perspective. Stem Cells Transl Med, 6:700–712.

Blaise

, Devillier

, Lecoroller-Sorriano

, Boher

, Boyer-Chammard

, Tabrizi

, Chevallier

, Fegueux

, Sirvent

, et al. (2015). Low non-relapse mortality and long-term preserved quality of life in older patients undergoing matched related donor allogeneic stem cell transplantation: a prospective multicenter phase II trial. Haematologica, 100:269–274.

Mohty

, Malard

and Savani

. (2015). High-dose total body irradiation and myeloablative conditioning before allogeneic hematopoietic cell transplantation: time to rethink?. Biol Blood Marrow Transplant, 21:620–624.

Black

and Zorina

. (2020). Cell-based immunomodulatory therapy approaches for type 1 diabetes mellitus. Drug Discov Today, 25:380–391.

Konstantinov IE. (2000). In search of Alexander A. Maximow: the man behind the unitarian theory of hematopoiesis. Perspect Biol Med, 43:269–276.

10.

Till

and McCulloch

. (1961). A direct measurement of the radiation sensitivity of normal mouse bone marrow cells. Radiat Res, 14:213–222.

11.

Krause

, Ito

, Fackler

, Smith

, Collector

, Sharkis

and May

. (1994). Characterization of murine CD34, a marker for hematopoietic progenitor and stem cells. Blood, 84:691–701.

12.

Sun

, Ramos

, Chapman

, Johnnidis

, Le

, Ho

, Klein

, Hofmann

and Camargo

. (2014). Clonal dynamics of native haematopoiesis. Nature, 514:322–327.

13.

Busch

, Klapproth

, Barile

, Flossdorf

, Holland-Letz

, Schlenner

, Reth

, Hofer

and Rodewald

. (2015). Fundamental properties of unperturbed haematopoiesis from stem cells in vivo. Nature, 518:542–546.

14.

Ghimire

, Weber

, Mavin

, Wang

, Dickinson

and Holler

. (2017). Pathophysiology of GvHD and Other HSCT-related major complications. Front Immunol, 8:79.

15.

Cattina

, Bernardi

, Mantovani

, Toffoletti

, Santoro

, Pastore

, Martino

, Console

, Martinelli

and Malagola

. (2017). Single step multiple genotyping by MALDI-TOF mass spectrometry, for evaluation of minor histocompatibility antigens in patients submitted to allogeneic stem cell transplantation from HLA-matched related and unrelated donor. Hematol Rep, 9:7051.

16.

Chen

, Miyanishi

, Wang

, Yamazaki

, Sinha

, Kao

, Seita

, Sahoo

, Nakauchi

and Weissman

. (2016). Hoxb5 marks long-term haematopoietic stem cells and reveals a homogenous perivascular niche. Nature, 530:223–227.

17.

Spangrude

, Heimfeld

and Weissman

. (1988). Purification and characterization of mouse hematopoietic stem cells. Science, 241:58–62.

18.

Sakaguchi

, Muramatsu

, Hasegawa

, Kudo

, Ishida

, Yoshida

, Koh

, Noguchi

, Shiba

, et al. (2019). Comparison of conditioning regimens for autologous stem cell transplantation in children with acute myeloid leukemia: a nationwide retrospective study in Japan. Pediatr Blood Cancer, 66:e27459.

19.

Choe

, Gergis

, Mayer

, Nagar

, Phillips

, Shore

, Smith

and van Besien

. (2017). The addition of low-dose total body irradiation to fludarabine and melphalan conditioning in haplocord transplantation for high-risk hematological malignancies. Transplantation, 101:e38.

20.

Chhabra

, Ring

, Weiskopf

, Schnorr

, Gordon

, Le

, Kwon

, Ring

, Volkmer

, et al. (2016). Hematopoietic stem cell transplantation in immunocompetent hosts without radiation or chemotherapy. Sci Transl Med, 8:351ra105.

21.

Weycker

, Doroff

, Hanau

, Bowers

, Belani

, Chandler

, Lonshteyn

, Bensink

and Lyman.

. Use and effectiveness of pegfilgrastim prophylaxis in US clinical practice: a retrospective observational study. BMC Cancer, 19:792.

22.

Waldhuter

, Kohler

, Hemmati

, Jehn

, Peceny

, Vuong

, Arnold

and Kuhl

. (2019). Allogeneic hematopoietic stem cell transplantation with myeloablative conditioning for adult cerebral X-linked adrenoleukodystrophy. J Inherit Metab Dis, 42:313–324.

23.

, Kim

, Liney

, Milford

, Gribben

, Cutler

, Lee

, Antin

, Soiffer

and Alyea

. (2006). HLA-C mismatch is associated with inferior survival after unrelated donor non-myeloablative hematopoietic stem cell transplantation. Bone Marrow Transplant, 37:845–850.

24.

Madden

, Hayashi

, Chan

, Pulsipher

, Douglas

, Hale

, Chaudhury

, Haut

, Kasow

, et al. (2016). Long-term follow-up after reduced-intensity conditioning and stem cell transplantation for childhood nonmalignant disorders. Biol Blood Marrow Transplant, 22:1467–1472.

25.

Cabrero

, Martin

, Briones

, Gayoso

, Jarque

, Lopez

, Grande

, Heras

, Arranz

, et al. (2017). Phase II Study of Yttrium-90-Ibritumomab Tiuxetan as Part of Reduced-Intensity Conditioning (with Melphalan, Fludarabine +/- Thiotepa) for Allogeneic Transplantation in Relapsed or Refractory Aggressive B Cell Lymphoma: A GELTAMO Trial. Biol Blood Marrow Transplant, 23:53–59.

26.

Epperla

, Ahn

, Ahmed

, Jagasia

, DiGilio

, Devine

, Jaglowski

, Kennedy

, Rezvani

, et al. (2017). Rituximab-containing reduced-intensity conditioning improves progression-free survival following allogeneic transplantation in B cell non-Hodgkin lymphoma. J Hematol Oncol, 10:117.

27.

Wright

, Wagers

, Gulati

, Johnson

and Weissman

. (2001). Physiological migration of hematopoietic stem and progenitor cells. Science, 294:1933–1936.

28.

Voog

and Jones

. (2010). Stem cells and the niche: a dynamic duo. Cell Stem Cell, 6:103–115.

29.

Wilson

, Oser

, Jaworski

, Blanco-Bose

, Laurenti

, Adolphe

, Essers

, Macdonald

and Trumpp

. (2007). Dormant and self-renewing hematopoietic stem cells and their niches. Ann N Y Acad Sci, 1106:64–75.

30.

Crane

, Jeffery

and Morrison

. (2017). Adult haematopoietic stem cell niches. Nat Rev Immunol, 17:573–590.

31.

Morrison

and Scadden

. (2014). The bone marrow niche for haematopoietic stem cells. Nature, 505:327–334.

32.

Schofield

. (1978). The relationship between the spleen colony-forming cell and the haemopoietic stem cell. Blood Cells, 4:7–25.

33.

Friedenstein

, Chailakhyan

, Latsinik

, Panasyuk

and Keiliss-Borok

. (1974). Stromal cells responsible for transferring the microenvironment of the hemopoietic tissues. Cloning in vitro and retransplantation in vivo. Transplantation, 17:331–340.

34.

Cras

, Farge

, Carmoi

, Lataillade

, Wang

and Sun

. (2015). Update on mesenchymal stem cell-based therapy in lupus and scleroderma. Arthritis Res Ther 17:. 301.

35.

Gharravi

, Jafar

, Ebrahimi

, Mahmodi

, Pourhashemi

, Haseli

, Talaie

and Hajiasgarli

. (2018). Current status of stem cell therapy, scaffolds for the treatment of diabetes mellitus. Diabetes Metab Syndr, 12:1133–1139.

36.

Maria

, Maumus

, Le Quellec

, Jorgensen

, Noel

and Guilpain

. (2017). Adipose-derived mesenchymal stem cells in autoimmune disorders: state of the art and perspectives for systemic sclerosis. Clin Rev Allergy Immunol, 52:234–259.

37.

and Scadden

. (2016). Hematopoietic stem cell and its bone marrow niche. Curr Top Dev Biol, 118:21–44.

38.

Khlusov

, Litvinova

, Khlusova

and Yurova

. (2018). Concept of hematopoietic and stromal niches for cell-based diagnostics and regenerative medicine (a Review). Curr Pharm Des, 24:3034–3054.

39.

Mendez-Ferrer

, Michurina

, Ferraro

, Mazloom

, Macarthur

, Lira

, Scadden

, Ma'ayan

, Enikolopov

and Frenette

. (2010). Mesenchymal and haematopoietic stem cells form a unique bone marrow niche. Nature, 466:829–834.

40.

Zhou

, Yue

, Murphy

, Peyer

and Morrison

. (2014). Leptin-receptor-expressing mesenchymal stromal cells represent the main source of bone formed by adult bone marrow. Cell Stem Cell, 15:154–168.

41.

Askmyr

, Sims

, Martin

and Purton

. (2009). What is the true nature of the osteoblastic hematopoietic stem cell niche?. Trends Endocrinol Metab, 20:303–309.

42.

Askenasy

, Zorina

, Farkas

and Shalit

. (2002). Transplanted hematopoietic cells seed in clusters in recipient bone marrow in vivo. Stem Cells, 20:301–310.

43.

Haylock

, Williams

, Johnston

, Liu

, Rutherford

, Whitty

, Simmons

, Bertoncello

and Nilsson

. (2007). Hemopoietic stem cells with higher hemopoietic potential reside at the bone marrow endosteum. Stem Cells, 25:1062–1069.

44.

Purton

and Scadden

. (2008). The hematopoietic stem cell niche. In: StemBook. Purton LE, David T. Scadden, eds. Cambridge, MA.

45.

Stier

, Ko

, Forkert

, Lutz

, Neuhaus

, Grunewald

, Cheng

, Dombkowski

, Calvi

, Rittling

and Scadden

. (2005). Osteopontin is a hematopoietic stem cell niche component that negatively regulates stem cell pool size. J Exp Med, 201:1781–1791.

46.

Ding

and Morrison

. (2013). Haematopoietic stem cells and early lymphoid progenitors occupy distinct bone marrow niches. Nature, 495:231–235.

47.

Ferraro

, Lymperi

, Mendez-Ferrer

, Saez

, Spencer

, Yeap

, Masselli

, Graiani

, Prezioso

, et al. (2011). Diabetes impairs hematopoietic stem cell mobilization by altering niche function. Sci Transl Med, 3:104ra101.

48.

Martin

and Sims

. (2005). Osteoclast-derived activity in the coupling of bone formation to resorption. Trends Mol Med, 11:76–81.

49.

Hogan

, Shpall

and Keller

. (2002). Differential long-term and multilineage engraftment potential from subfractions of human CD34+ cord blood cells transplanted into NOD/SCID mice. Proc Natl Acad Sci U S A, 99:413–418.

50.

Omatsu

, Sugiyama

, Kohara

, Kondoh

, Fujii

, Kohno

and Nagasawa

. (2010). The essential functions of adipo-osteogenic progenitors as the hematopoietic stem and progenitor cell niche. Immunity, 33:387–399.

51.

Tzeng

, Li

, Kang

, Chen

, Cheng

and Lai

. (2011). Loss of Cxcl12/Sdf-1 in adult mice decreases the quiescent state of hematopoietic stem/progenitor cells and alters the pattern of hematopoietic regeneration after myelosuppression. Blood, 117:429–439.

52.

Greenbaum

, Hsu

, Day

, Schuettpelz

, Christopher

, Borgerding

, Nagasawa

and Link

. (2013). CXCL12 in early mesenchymal progenitors is required for haematopoietic stem-cell maintenance. Nature, 495:227–230.

53.

Kunisaki

, Bruns

, Scheiermann

, Ahmed

, Pinho

, Zhang

, Mizoguchi

, Wei

, Lucas

, et al. (2013). Arteriolar niches maintain haematopoietic stem cell quiescence. Nature, 502:637–643.

54.

Bradfute

, Graubert

and Goodell

. (2005). Roles of Sca-1 in hematopoietic stem/progenitor cell function. Exp Hematol, 33:836–843.

55.

Sugiyama

, Omatsu

and Nagasawa

. (2018). Niches for hematopoietic stem cells and immune cell progenitors. Int Immunol, 31:5–11.

56.

Ding

, Saunders

, Enikolopov

and Morrison

. (2012). Endothelial and perivascular cells maintain haematopoietic stem cells. Nature, 481:457–462.

57.

Zhang

, Zhu

, Zhou

, Sheng

, Hong

, Xiang

, Qian

, Mosenson

and Wu

. (2017). A novel slug-containing negative-feedback loop regulates SCF/c-Kit-mediated hematopoietic stem cell self-renewal. Leukemia, 31:403–413.

58.

Driessen

, Johnston

and Nilsson

. (2003). Membrane-bound stem cell factor is a key regulator in the initial lodgment of stem cells within the endosteal marrow region. Exp Hematol, 31:1284–1291.

59.

Lacombe

, Krosl

, Tremblay

, Gerby

, Martin

, Aplan

, Lemieux

and Hoang

. (2013). Genetic interaction between Kit and Scl. Blood, 122:1150–1161.

60.

Balduino

, Mello-Coelho

, Wang

, Taichman

, Krebsbach

, Weeraratna

, Becker

, de Mello

, Taub

and Borojevic

. (2012). Molecular signature and in vivo behavior of bone marrow endosteal and subendosteal stromal cell populations and their relevance to hematopoiesis. Exp Cell Res, 318:2427–2437.

61.

Mendez-Ferrer

, Lucas

, Battista

and Frenette

. (2008). Haematopoietic stem cell release is regulated by circadian oscillations. Nature, 452:442–447.

62.

Seita

and Weissman

. (2010). Hematopoietic stem cell: self-renewal versus differentiation. Wiley Interdiscip Rev Syst Biol Med, 2:640–653.

63.

Rodriguez-Fraticelli

, Wolock

, Weinreb

, Panero

, Patel

, Jankovic

, Sun

, Calogero

, Klein

and Camargo

. (2018). Clonal analysis of lineage fate in native haematopoiesis. Nature, 553:212–216.

64.

Kondo

, Scherer

, Miyamoto

, King

, Akashi

, Sugamura

and Weissman

. (2000). Cell-fate conversion of lymphoid-committed progenitors by instructive actions of cytokines. Nature, 407:383–386.

65.

, Czechowicz

, Seita

, Jiang

and Weissman

. (2019). Clonal-level lineage commitment pathways of hematopoietic stem cells in vivo. Proc Natl Acad Sci U S A, 116:1447–1456.

66.

Pietras

, Reynaud

, Kang

, Carlin

, Calero-Nieto

, Leavitt

, Stuart

, Gottgens

and Passegue

. (2015). Functionally distinct subsets of lineage-biased multipotent progenitors control blood production in normal and regenerative conditions. Cell Stem Cell, 17:35–46.

67.

Klamer

, Dorland

, Kleijer

, Geerts

, Lento

, van der Schoot

, von Lindern

and Voermans

. (2018). TGFBI expressed by bone marrow niche cells and hematopoietic stem and progenitor cells regulates hematopoiesis. Stem Cells Dev, 27:1494–1506.

68.

Greenberger

and Epperly

. (2009). Bone marrow-derived stem cells and radiation response. Semin Radiat Oncol, 19:133–139.

69.

Nilsson

, Johnston

, Whitty

, Williams

, Webb

, Denhardt

, Bertoncello

, Bendall

, Simmons

and Haylock

. (2005). Osteopontin, a key component of the hematopoietic stem cell niche and regulator of primitive hematopoietic progenitor cells. Blood, 106:1232–1239.

70.

, Rodier

, Fontaine

, Coppe

, Campisi

, DeGregori

, Laverdiere

, Kokta

, Haddad

and Beausejour

. (2010). Ionizing radiation-induced long-term expression of senescence markers in mice is independent of p53 and immune status. Aging Cell, 9:398–409.

71.

Carbonneau

, Despars

, Rojas-Sutterlin

, Fortin

, Le

, Hoang

and Beausejour

. (2012). Ionizing radiation-induced expression of INK4a/ARF in murine bone marrow-derived stromal cell populations interferes with bone marrow homeostasis. Blood, 119:717–726.

72.

Alessio

, Del Gaudio

, Capasso

, Di Bernardo

, Cappabianca

, Cipollaro

, Peluso

and Galderisi

. (2015). Low dose radiation induced senescence of human mesenchymal stromal cells and impaired the autophagy process. Oncotarget, 6:8155–8166.

73.

Cameron

, Sekhar

, Ofori

and Freeman

. (2018). The role of Nrf2 in the response to normal tissue radiation injury. Radiat Res, 190:99–106.

74.

Kink

, Forsberg

, Reshetylo

, Besharat

, Childs

, Pederson

, Gendron-Fitzpatrick

, Graham

, Bates

, et al. (2019). Macrophages educated with exosomes from primed mesenchymal stem cells treat acute radiation syndrome by promoting hematopoietic recovery. Biol Blood Marrow Transplant, 25:2124–2133.

75.

Kiang JG. (2016). Adult mesenchymal stem cells and radiation injury. Health Phys, 111:198–203.

76.

Martin

, Marples

, Davies

, Atzberger

, Edwards

, Lynch

, Hollywood

and Marignol

. (2013). DNA mismatch repair protein MSH2 dictates cellular survival in response to low dose radiation in endometrial carcinoma cells. Cancer Lett, 335:19–25.

77.

Martin

, Marples

, Lynch

, Hollywood

and Marignol

. (2013). Exposure to low dose ionising radiation: molecular and clinical consequences. Cancer Lett, 338:209–218.

78.

DiCarlo

, Tamarat

, Rios

, Benderitter

, Czarniecki

, Allio

, Macchiarini

, Maidment

and Jourdain

. (2017). Cellular therapies for treatment of radiation injury: report from a NIH/NIAID and IRSN Workshop. Radiat Res, 188:e54–e75.

79.

Bolus NE. (2017). Basic review of radiation biology and terminology. J Nucl Med Technol, 45:259–264.

80.

Shao

, Sun

, Zhang

, Feng

, Gao

, Cai

, Wang

, Look

and Wu

. (2010). Deletion of proapoptotic Puma selectively protects hematopoietic stem and progenitor cells against high-dose radiation. Blood, 115:4707–4714.

81.

Wang

, Sun

, Morita

, Jiang

, Gross

, Lechel

, Hildner

, Guachalla

, Gompf

, et al. (2012). A differentiation checkpoint limits hematopoietic stem cell self-renewal in response to DNA damage. Cell, 148:1001–1014.

82.

Wang

, Liu

, Pazhanisamy

, Li

, Meng

and Zhou

. (2010). Total body irradiation causes residual bone marrow injury by induction of persistent oxidative stress in murine hematopoietic stem cells. Free Radic Biol Med, 48:348–356.

83.

Singh

, Kloss

, Brunauer

, Schimke

, Jamnig

, Greiderer-Kleinlercher

, Klima

, Rentenberger

, Auberger

, et al. (2012). Mesenchymal stem cells show radioresistance in vivo. J Cell Mol Med, 16:877–887.

84.

Sugrue

, Brown

, Lowndes

and Ceredig

. (2013). Multiple facets of the DNA damage response contribute to the radioresistance of mouse mesenchymal stromal cell lines. Stem Cells, 31:137–145.

85.

Cuende

, Rasko

JEJ

, Koh

MBC

, Dominici

and Ikonomou

. (2018). Cell, tissue and gene products with marketing authorization in 2018 worldwide. Cytotherapy, 20:1401–1413.

86.

Galipeau

, Krampera

, Barrett

, Dazzi

, Deans

, DeBruijn

, Dominici

, Fibbe

, Gee

, et al. (2016). International Society for Cellular Therapy perspective on immune functional assays for mesenchymal stromal cells as potency release criterion for advanced phase clinical trials. Cytotherapy, 18:151–159.

87.

Drouet

, Mourcin

, Grenier

, Delaunay

, Mayol

, Lataillade

, Peinnequin

and Herodin

. (2005). Mesenchymal stem cells rescue CD34+ cells from radiation-induced apoptosis and sustain hematopoietic reconstitution after coculture and cografting in lethally irradiated baboons: is autologous stem cell therapy in nuclear accident settings hype or reality?. Bone Marrow Transplant, 35:1201–1209.

88.

Shim

, Lee

, Jang

, Lee

, Park

and Lee

. (2013). Mitigating effects of hUCB-MSCs on the hematopoietic syndrome resulting from total body irradiation. Exp Hematol, 41:346–353.e2.

89.

Kurtzberg

, Abdel-Azim

, Carpenter

, Chaudhury

, Horn

, Mahadeo

, Nemecek

, Neudorf

, Prasad

, et al. (2020). A Phase 3, Single-Arm, Prospective Study of Remestemcel-L, ex vivo culture-expanded adult human mesenchymal stromal cells for the treatment of pediatric patients who failed to respond to steroid treatment for acute graft-versus-host disease. Biol Blood Marrow Transplant, 26:845–854.

90.

Kurtzberg

, Prockop

, Chaudhury

, Horn

, Nemecek

, Prasad

, Satwani

, Teira

, Hayes

, Burke

and MSB-275 Study Group. (2020). Study 275: updated expanded access program for remestemcel-L in steroid-refractory acute graft-versus-host disease in children. Biol Blood Marrow Transplant, 26:855–864.

91.

Sher

and Ofir

. (2018). Placenta-derived adherent stromal cell therapy for hematopoietic disorders: a case study of PLX-R18. Cell Transplant, 27:140-150.

92.

Papait

, Vertua

, Magatti

, Ceccariglia

, De Munari

, Silini

, Sheleg

, Ofir

and Parolini

. (2020). Mesenchymal stromal cells from fetal and maternal placenta possess key similarities and differences: potential implications for their applications in regenerative medicine. Cells, 9:127.

93.

Chinnadurai

, Forsberg

, Kink

, Hematti

and Capitini

. (2020). Use of MSCs and MSC-educated macrophages to mitigate hematopoietic acute radiation syndrome. Curr Stem Cell Rep, 6:77–85.

94.

Galotto

, Berisso

, Delfino

, Podesta

, Ottaggio

, Dallorso

, Dufour

, Ferrara

, Abbondandolo

, et al. (1999). Stromal damage as consequence of high-dose chemo/radiotherapy in bone marrow transplant recipients. Exp Hematol, 27:1460–1466.

95.

Kemp

, Morse

, Wexler

, Cox

, Mallam

, Hows

and Donaldson

. (2010). Chemotherapy-induced mesenchymal stem cell damage in patients with hematological malignancy. Ann Hematol, 89:701–713.

96.

Kemp

, Morse

, Sanders

, Hows

and Donaldson

. (2011). Alkylating chemotherapeutic agents cyclophosphamide and melphalan cause functional injury to human bone marrow-derived mesenchymal stem cells. Ann Hematol, 90:777–789.

97.

Tang

, Li

, Chen

, Sun

, Liu

, Zheng

, Zhang

, Li

, Fu

, et al. (2018). Chemotherapy-induced niche perturbs hematopoietic reconstitution in B-cell acute lymphoblastic leukemia. J Exp Clin Cancer Res, 37:204-018-0859-3.

98.

Somaiah

, Kumar

, Sharma

, Anand

, Bhattacharyya

, Das

, Deka Talukdar

and Jaganathan

. (2018). Mesenchymal stem cells show functional defect and decreased anti-cancer effect after exposure to chemotherapeutic drugs. J Biomed Sci, 25:5.

99.

Lopez Perez

, Munz

, Vidoni

, Ruhle

, Trinh

, Sisombath

, Zou

, Wuchter

, Debus

, et al. (2019). Mesenchymal stem cells preserve their stem cell traits after exposure to antimetabolite chemotherapy. Stem Cell Res, 40:101536.

100.

Yuan

, Zhao

, Wang

, Teng

, Li

and Zeng

. (2017). HIV-1 p55-gag protein induces senescence of human bone marrow mesenchymal stem cells and reduces their capacity to support expansion of hematopoietic stem cells in vitro. Cell Biol Int, 41:969–981.

101.

Geyh

, Oz

, Cadeddu

, Frobel

, Bruckner

, Kundgen

, Fenk

, Bruns

, Zilkens

, et al. (2013). Insufficient stromal support in MDS results from molecular and functional deficits of mesenchymal stromal cells. Leukemia, 27:1841–1851.

102.

Andre

, Meuleman

, Stamatopoulos

, De Bruyn

, Pieters

, Bron

and Lagneaux

. (2013). Evidences of early senescence in multiple myeloma bone marrow mesenchymal stromal cells. PLoS One, 8:e59756.

103.

Mehta

, Saliba

, Cao

, Kaur

, Rezvani

, Chen

, Olson

, Parmar

, Shah

, et al. (2017). Ex vivo mesenchymal precursor cell-expanded cord blood transplantation after reduced-intensity conditioning regimens improves time to neutrophil recovery. Biol Blood Marrow Transplant, 23:1359–1366.

104.

Buckley

, Othus

, Vainstein

, Abkowitz

, Estey

and Walter

. (2014). Prediction of adverse events during intensive induction chemotherapy for acute myeloid leukemia or high-grade myelodysplastic syndromes. Am J Hematol, 89:423–428.

105.

Abbuehl

, Tatarova

, Held

and Huelsken

. (2017). Long-term engraftment of primary bone marrow stromal cells repairs niche damage and improves hematopoietic stem cell transplantation. Cell Stem Cell, 21:241–255.e6.

106.

Nilsson

, Dooner

, Weier

, Frenkel

, Lian

, Stein

and Quesenberry

. (1999). Cells capable of bone production engraft from whole bone marrow transplants in nonablated mice. J Exp Med, 189:729–734.

107.

Migliaccio AR. (2016). To condition or not to condition-That is the question: the evolution of nonmyeloablative conditions for transplantation. Exp Hematol, 44:706–712.

108.

Brecher

, Ansell

, Micklem

, Tjio

and Cronkite

. (1982). Special proliferative sites are not needed for seeding and proliferation of transfused bone marrow cells in normal syngeneic mice. Proc Natl Acad Sci U S A, 79:5085–5087.

109.

Bhattacharya

, Czechowicz

, Ooi

, Rossi

, Bryder

and Weissman

. (2009). Niche recycling through division-independent egress of hematopoietic stem cells. J Exp Med, 206:2837–2850.

110.

Taya

, Ota

, Wilkinson

, Kanazawa

, Watarai

, Kasai

, Nakauchi

and Yamazaki

. (2016). Depleting dietary valine permits nonmyeloablative mouse hematopoietic stem cell transplantation. Science, 354:1152–1155.

111.

Cronkite

, Bullis

and Brecher

. (1985). Marrow transfusions increase pluripotential stem cells in normal hosts. Exp Hematol, 13:802.

112.

Nguyen

, Wang

, Chowdhury

, Chu

, Eerdeng

, Jiang

and Lu

. (2018). Functional compensation between hematopoietic stem cell clones in vivo. EMBO Rep, 19:e45702.

113.

Shimoto

, Sugiyama

and Nagasawa

. (2017). Numerous niches for hematopoietic stem cells remain empty during homeostasis. Blood, 129:2124–2131.

114.

Purton

and Scadden

. (2007). Limiting factors in murine hematopoietic stem cell assays. Cell Stem Cell, 1:263–270.

115.

Harrison

, Jordan

, Zhong

and Astle

. (1993). Primitive hemopoietic stem cells: direct assay of most productive populations by competitive repopulation with simple binomial, correlation and covariance calculations. Exp Hematol, 21:206–219.

116.

Acar

, Kocherlakota

, Murphy

, Peyer

, Oguro

, Inra

, Jaiyeola

, Zhao

, Luby-Phelps

and Morrison

. (2015). Deep imaging of bone marrow shows non-dividing stem cells are mainly perisinusoidal. Nature, 526:126–130.

117.

Kfoury

and Scadden

. (2015). Mesenchymal cell contributions to the stem cell niche. Cell Stem Cell, 16:239–253.

118.

Rana

, Schultz

, Freeman

and Biswas

. (2012). Loss of Nrf2 accelerates ionizing radiation-induced bone loss by upregulating RANKL. Free Radic Biol Med, 53:2298–2307.

119.

Chertkov

, Gurevitch

and Udalov

. (1980). Role of bone marrow stroma in hemopoietic stem cell regulation. Exp Hematol, 8:770–778.

120.

Najar

, Fayyad-Kazan

, Faour

, Merimi

, Sokal

, Lombard

and Fahmi

. (2019). Immunological modulation following bone marrow-derived mesenchymal stromal cells and Th17 lymphocyte co-cultures. Inflamm Res, 68:203–213.

121.

Varas

, Grande

, Ramirez

and Bueren

. (2000). Implantation of bone marrow beneath the kidney capsule results in transfer not only of functional stroma but also of hematopoietic repopulating cells. Blood, 96:2307–2309.

122.

Brewer

, Chu

, Chin

and Lu

. (2016). Transplantation dose alters the differentiation program of hematopoietic stem cells. Cell Rep, 15:1848–1857.

123.

Nilsson

, Dooner

, Tiarks

, Weier

and Quesenberry

. (1997). Potential and distribution of transplanted hematopoietic stem cells in a nonablated mouse model. Blood, 89:4013–4020.

124.

Blazar

, Murphy

and Abedi

. (2012). Advances in graft-versus-host disease biology and therapy. Nat Rev Immunol, 12:443–458.

125.

Pasquini

, Wang

, Horowitz

and Gale

. (2010). 2010 report from the Center for International Blood and Marrow Transplant Research (CIBMTR): current uses and outcomes of hematopoietic cell transplants for blood and bone marrow disorders. Clin, Transpl:87–105.

126.

Bhattacharya

, Rossi

, Bryder

and Weissman

. (2006). Purified hematopoietic stem cell engraftment of rare niches corrects severe lymphoid deficiencies without host conditioning. J Exp Med, 203:73–85.

127.

Kaufman

, Colson

, Wren

, Watkins

, Simmons

and Ildstad

. (1994). Phenotypic characterization of a novel bone marrow-derived cell that facilitates engraftment of allogeneic bone marrow stem cells. Blood, 84:2436–2446.

128.

Leventhal

, Elliott

, Yolcu

, Bozulic

, Tollerud

, Mathew

, Konieczna

, Ison

, Galvin

, et al. (2015). Immune reconstitution/immunocompetence in recipients of kidney plus hematopoietic stem/facilitating cell transplants. Transplantation, 99:288–298.

129.

Omatsu

, Seike

, Sugiyama

, Kume

and Nagasawa

. (2014). Foxc1 is a critical regulator of haematopoietic stem/progenitor cell niche formation. Nature, 508:536–540.

130.

Itkin

, Gur-Cohen

, Spencer

, Schajnovitz

, Ramasamy

, Kusumbe

, Ledergor

, Jung

, Milo

, et al. (2016). Distinct bone marrow blood vessels differentially regulate haematopoiesis. Nature, 532:323–328.

131.

Ludin

, Itkin

, Gur-Cohen

, Mildner

, Shezen

, Golan

, Kollet

, Kalinkovich

, Porat

, et al. (2012). Monocytes-macrophages that express alpha-smooth muscle actin preserve primitive hematopoietic cells in the bone marrow. Nat Immunol, 13:1072–1082.

132.

Dexter TM. (1982). Is the marrow stroma transplantable?. Nature, 298:222–223.

133.

Dominici

, Le Blanc

, Mueller

, Slaper-Cortenbach

, Marini

, Krause

, Deans

, Keating

, Prockop

and Horwitz

. (2006). Minimal criteria for defining multipotent mesenchymal stromal cells. The International Society for Cellular Therapy position statement. Cytotherapy, 8:315–317.

134.

Balan

, Lucchini

, Schmidt

, Schneider

, Tramsen

, Kuci

, Meisel

, Bader

and Lehrnbecher

. (2014). Mesenchymal stromal cells in the antimicrobial host response of hematopoietic stem cell recipients with graft-versus-host disease—friends or foes?. Leukemia, 28:1941–1948.

135.

Huaman

, Bahamonde

, Cahuascanco

, Jervis

, Palomino

, Torres

and Peralta

. (2019). Immunomodulatory and immunogenic properties of mesenchymal stem cells derived from bovine fetal bone marrow and adipose tissue. Res Vet Sci, 124:212–222.

136.

Le Blanc

, Tammik

, Rosendahl

, Zetterberg

and Ringden

. (2003). HLA expression and immunologic properties of differentiated and undifferentiated mesenchymal stem cells. Exp Hematol, 31:890–896.

137.

Javazon

, Beggs

and Flake

. (2004). Mesenchymal stem cells: paradoxes of passaging. Exp Hematol, 32:414–425.

138.

Ryan

, Barry

, Murphy

and Mahon

. (2005). Mesenchymal stem cells avoid allogeneic rejection. J Inflamm (Lond), 2:8-9255-2-8.

139.

Klyushnenkova

, Mosca

, Zernetkina

, Majumdar

, Beggs

, Simonetti

, Deans

and McIntosh

. (2005). T cell responses to allogeneic human mesenchymal stem cells: immunogenicity, tolerance, and suppression. J Biomed Sci, 12:47–57.

140.

Almeida-Porada

, Porada

, Tran

and Zanjani

. (2000). Cotransplantation of human stromal cell progenitors into preimmune fetal sheep results in early appearance of human donor cells in circulation and boosts cell levels in bone marrow at later time points after transplantation. Blood, 95:3620–3627.

141.

Noort

, Kruisselbrink

, in't Anker

, Kruger

, van Bezooijen

, de Paus

, Heemskerk

, Lowik

, Falkenburg

, Willemze

and Fibbe

. (2002). Mesenchymal stem cells promote engraftment of human umbilical cord blood-derived CD34(+) cells in NOD/SCID mice. Exp Hematol, 30:870–878.

142.

Maitra

, Szekely

, Gjini

, Laughlin

, Dennis

, Haynesworth

and Koc

. (2004). Human mesenchymal stem cells support unrelated donor hematopoietic stem cells and suppress T-cell activation. Bone Marrow Transplant, 33:597–604.

143.

Hillmann

, Ahrberg

, Brehm

, Heller

, Josten

, Paebst

and Burk

. (2016). Comparative characterization of human and equine mesenchymal stromal cells: a basis for translational studies in the equine model. Cell Transplant, 25:109–124.

144.

Ivanovska

, Grolli

, Borghetti

, Ravanetti

, Conti

, De Angelis

, Macchi

, Ramoni

, Martelli

, Gazza

and Cacchioli

. (2017). Immunophenotypical characterization of canine mesenchymal stem cells from perivisceral and subcutaneous adipose tissue by a species-specific panel of antibodies. Res Vet Sci, 114:51–58.

145.

Gonzalo-Daganzo

, Regidor

, Martin-Donaire

, Rico

, Bautista

, Krsnik

, Fores

, Ojeda

, Sanjuan

, et al. (2009). Results of a pilot study on the use of third-party donor mesenchymal stromal cells in cord blood transplantation in adults. Cytotherapy, 11:278–288.

146.

Kawabori

, Shichinohe

, Kuroda

and Houkin

. (2020). Clinical trials of stem cell therapy for cerebral ischemic stroke. Int J Mol Sci, 21:7380.

147.

Kabat

, Bobkov

, Kumar

and Grumet

. (2020). Trends in mesenchymal stem cell clinical trials 2004-2018: is efficacy optimal in a narrow dose range?. Stem Cells Transl Med, 9:17–27.

148.

Bernardo

and Fibbe

. (2012). Safety and efficacy of mesenchymal stromal cell therapy in autoimmune disorders. Ann N Y Acad Sci, 1266:107–117.

149.

Le Blanc

, Frassoni

, Ball

, Locatelli

, Roelofs

, Lewis

, Lanino

, Sundberg

, Bernardo

, et al. (2008). Mesenchymal stem cells for treatment of steroid-resistant, severe, acute graft-versus-host disease: a phase II study. Lancet, 371:1579–1586.

150.

Barberini

, Freitas

, Magnoni

, Maia

, Listoni

, Heckler

, Sudano

, Golim

, da Cruz Landim-Alvarenga

and Amorim

. (2014). Equine mesenchymal stem cells from bone marrow, adipose tissue and umbilical cord: immunophenotypic characterization and differentiation potential. Stem Cell Res Ther, 5:25.

151.

Ball

, Bernardo

, Roelofs

, Lankester

, Cometa

, Egeler

, Locatelli

and Fibbe

. (2007). Cotransplantation of ex vivo expanded mesenchymal stem cells accelerates lymphocyte recovery and may reduce the risk of graft failure in haploidentical hematopoietic stem-cell transplantation. Blood, 110:2764–2767.

152.

Chulpanova

, Kitaeva

, Tazetdinova

, James

, Rizvanov

and Solovyeva

. (2018). Application of mesenchymal stem cells for therapeutic agent delivery in anti-tumor treatment. Front Pharmacol, 9:259.

153.

Lalu

, McIntyre

, Pugliese

, Fergusson

, Winston

, Marshall

, Granton

, Stewart and Canadian Critical Care Trials Group

. (2012). Safety of cell therapy with mesenchymal stromal cells (SafeCell): a systematic review and meta-analysis of clinical trials. PLoS One, 7:e47559.

154.

Al-Khawaga

and Abdelalim

. (2020). Potential application of mesenchymal stem cells and their exosomes in lung injury: an emerging therapeutic option for COVID-19 patients. Stem Cell Res Ther, 11:437.

155.

Potian

, Aviv

, Ponzio

, Harrison

and Rameshwar

. (2003). Veto-like activity of mesenchymal stem cells: functional discrimination between cellular responses to alloantigens and recall antigens. J Immunol, 171:3426–3434.

156.

Bartholomew

, Sturgeon

, Siatskas

, Ferrer

, McIntosh

, Patil

, Hardy

, Devine

, Ucker

, Deans

, Moseley

and Hoffman

. (2002). Mesenchymal stem cells suppress lymphocyte proliferation in vitro and prolong skin graft survival in vivo. Exp Hematol, 30:42–48.

157.

Liu

, Kemeny

, Heng

, Ouyang

, Melendez

and Cao

. (2006). The immunogenicity and immunomodulatory function of osteogenic cells differentiated from mesenchymal stem cells. J Immunol, 176:2864–2871.

158.

Terraza-Aguirre

, Campos-Mora

, Elizondo-Vega

, Contreras-Lopez

, Luz-Crawford

, Jorgensen

and Djouad

. (2020). Mechanisms behind the immunoregulatory dialogue between mesenchymal stem cells and Th17 cells. Cells, 9:1660.

159.

Ball

, Bernardo

, Roelofs

, van Tol

, Contoli

, Zwaginga

, Avanzini

, Conforti

, Bertaina

, et al. (2013). Multiple infusions of mesenchymal stromal cells induce sustained remission in children with steroid-refractory, grade III-IV acute graft-versus-host disease. Br J Haematol, 163:501–509.

160.

Dander

, Lucchini

, Vinci

, Introna

, Masciocchi

, Perseghin

, Balduzzi

, Bonanomi

, Longoni

, et al. (2012). Mesenchymal stromal cells for the treatment of graft-versus-host disease: understanding the in vivo biological effect through patient immune monitoring. Leukemia, 26:1681–1684.

161.

Introna

, Lucchini

, Dander

, Galimberti

, Rovelli

, Balduzzi

, Longoni

, Pavan

, Masciocchi

, et al. (2014). Treatment of graft versus host disease with mesenchymal stromal cells: a phase I study on 40 adult and pediatric patients. Biol Blood Marrow Transplant, 20:375–381.

162.

Ferrara

, Levine

, Reddy

and Holler

. (2009). Graft-versus-host disease. Lancet, 373:1550–1561.

163.

, Park

, Kim

, Lim

, Park

, Lee

, Nam

, Lee

, Cho

and Cho

. (2014). Induction of mixed chimerism using combinatory cell-based immune modulation with mesenchymal stem cells and regulatory T cells for solid-organ transplant tolerance. Stem Cells Dev, 23:2364–2376.

164.

Sachs DH. (2018). Transplantation tolerance through mixed chimerism: from allo to xeno. Xenotransplantation, 25:e12420.

165.

Nti

, Markman

, Bertera

, Styche

, Lakomy

, Subbotin

, Trucco

and Zorina

. (2012). Treg cells in pancreatic lymph nodes: the possible role in diabetogenesis and β cell regeneration in a T1D model. Cell Mol Immunol, 9:455.

166.

Jagasia

, Greinix

, Arora

, Williams

, Wolff

, Cowen

, Palmer

, Weisdorf

, Treister

, et al. (2015). National Institutes of Health Consensus Development Project on Criteria for Clinical Trials in Chronic Graft-versus-Host Disease: I. The 2014 Diagnosis and Staging Working Group report. Biol Blood Marrow Transplant, 21:389–401.e1.

167.

Le Blanc

, Rasmusson

, Sundberg

, Gotherstrom

, Hassan

, Uzunel

and Ringden

. (2004). Treatment of severe acute graft-versus-host disease with third party haploidentical mesenchymal stem cells. Lancet, 363:1439–1441.

168.

Bonig

, Kuci

, Bakhtiar

, Basu

, Bug

, Dennis

, Greil

, Barta

, et al. (2019). Children and adults with refractory acute graft-versus-host disease respond to treatment with the mesenchymal stromal cell preparation “MSC-FFM”-Outcome Report of 92 Patients. Cells, 8:1577.

169.

Uccelli

and de Rosbo

. (2015). The immunomodulatory function of mesenchymal stem cells: mode of action and pathways. Ann N Y Acad Sci, 1351:114–126.

170.

Fujii

, Miura

, Fujishiro

, Shindo

, Shimazu

, Hirai

, Tahara

, Takaori-Kondo

, Ichinohe

and Maekawa

. (2018). Graft-versus-host disease amelioration by human bone marrow mesenchymal stromal/stem cell-derived extracellular vesicles is associated with peripheral preservation of naive T cell populations. Stem Cells, 36:434–445.

171.

Koc

, Gerson

, Cooper

, Dyhouse

, Haynesworth

, Caplan

and Lazarus

. (2000). Rapid hematopoietic recovery after coinfusion of autologous-blood stem cells and culture-expanded marrow mesenchymal stem cells in advanced breast cancer patients receiving high-dose chemotherapy. J Clin Oncol, 18:307–316.

172.

Le Blanc

, Samuelsson

, Gustafsson

, Remberger

, Sundberg

, Arvidson

, Ljungman

, Lonnies

, Nava

and Ringden

. (2007). Transplantation of mesenchymal stem cells to enhance engraftment of hematopoietic stem cells. Leukemia, 21:1733–1738.

173.

Cheung

, Bertolino

, Giacomini

, Bornhauser

, Dazzi

and Galleu

. (2020). Mesenchymal stromal cells for graft versus host disease: mechanism-based biomarkers. Front Immunol, 11:1338.

174.

Alzahrani

, Saadeldin

, Ahmad

, Kumar

, Azhar

, Siddiqui

, Kurdi

, Sajini

, Alrefaei

and Jahan

. (2020). The potential use of mesenchymal stem cells and their derived exosomes as immunomodulatory agents for COVID-19 patients. Stem Cells Int, 2020:8835986.

175.

Fisher

, Cutler

, Doree

, Brunskill

, Stanworth

, Navarrete

and Girdlestone

. (2019). Mesenchymal stromal cells as treatment or prophylaxis for acute or chronic graft-versus-host disease in haematopoietic stem cell transplant (HSCT) recipients with a haematological condition. Cochrane Database Syst Rev, 1:CD009768.

176.

Elgaz

, Kuci

, Bonig

and Bader

. (2019). Clinical use of mesenchymal stromal cells in the treatment of acute graft-versus-host disease. Transfus Med Hemother, 46:27–34.

177.

Rubin

, Strayer

and Rubin

. (2012). Rubin's pathology: Clinicopathologic foundations of medicine. , 6th edn. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins.

178.

Haider

, ed. Stem Cells: From Hype to Hope and MORE. Cambridge Scholar Publishing. 2020, in print.