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Introduction

  1. Top of page
  2. Introduction
  3. Basic concepts and clinical applications of adult stem cells
  4. Overview of MSCs
  5. MSCs and the immune system
  6. Role of MSCs in autoimmune rheumatic diseases
  7. MSC therapy for autoimmune rheumatic diseases
  8. Effects of MSCs on Th17 and Treg cell populations
  9. Conclusions
  10. AUTHOR CONTRIBUTIONS
  11. REFERENCES

The process of immunologic tolerance is critical in preventing autoimmune disease and maintaining immune system homeostasis. Increased understanding regarding cytokine networks has led to the development of neutralizing antibodies against tumor necrosis factor α (TNFα) and against interleukin-1 (IL-1) and IL-6 signaling in the treatment of rheumatoid arthritis (RA). However, there remains an unmet need, given the significant number of patients not achieving remission or whose condition fails to respond to these drugs. Mesenchymal (stromal) stem cells (MSCs) are promising tools for the repair of damaged joint tissue such as cartilage, bone, and tendons. Moreover, MSCs have potent antiinflammatory and immunomodulatory properties, both in vitro and in vivo (1). Research into MSC therapy for Crohn's disease, type 1 diabetes mellitus, graft-versus-host disease (GVHD), and multiple sclerosis continues apace with ongoing phase II/III trials (http://www.clinicaltrials.gov). There have been conflicting reports regarding the effects of MSCs in autoimmune rheumatic diseases, particularly in the collagen-induced arthritis (CIA) mouse model of RA (2–8). Conversely, promising results in patients with systemic lupus erythematosus (SLE) were recently reported (9), even in the face of conflicting results in murine models of SLE.

In the present review, we examine MSCs as a possible cellular therapy for RA, SLE, and systemic sclerosis (SSc), and critically assess possible reasons for conflicting results in the literature. We also address whether MSC dysfunction could play a role in the pathogenesis of these conditions. Finally, we examine the possible mechanisms of action of MSCs at the cellular level, including their effects on Treg cells and Th17 cell populations.

Basic concepts and clinical applications of adult stem cells

  1. Top of page
  2. Introduction
  3. Basic concepts and clinical applications of adult stem cells
  4. Overview of MSCs
  5. MSCs and the immune system
  6. Role of MSCs in autoimmune rheumatic diseases
  7. MSC therapy for autoimmune rheumatic diseases
  8. Effects of MSCs on Th17 and Treg cell populations
  9. Conclusions
  10. AUTHOR CONTRIBUTIONS
  11. REFERENCES

Adult stem cells are derived from, or resident in, most adult tissues. Whereas embryonic stem cells have the ability to give rise to cells of all 3 germ layers, adult stem cells have a restricted differentiation potential. A number of adult stem cell types have now been characterized, including hematopoietic stem cells (HSCs), MSCs, neural stem cells, and intestinal stem cells, among others. Although most of these adult stem cells have been defined in vitro, there is increasing focus on the stem cell niche, i.e., the microenvironment that, in vivo, controls the fate of stem cells and that induces their tissue-specific features (10). In the clinical setting, HSCs have been used to treat hematologic disorders and some treatment-refractory autoimmune conditions (11), while MSCs have been used to treat acute GVHD following bone marrow transplantation (12) and autoimmune disorders (13).

Overview of MSCs

  1. Top of page
  2. Introduction
  3. Basic concepts and clinical applications of adult stem cells
  4. Overview of MSCs
  5. MSCs and the immune system
  6. Role of MSCs in autoimmune rheumatic diseases
  7. MSC therapy for autoimmune rheumatic diseases
  8. Effects of MSCs on Th17 and Treg cell populations
  9. Conclusions
  10. AUTHOR CONTRIBUTIONS
  11. REFERENCES

MSCs, which were originally described in human bone marrow (BM), are plastic-adherent cells that are able to form single cell–derived colonies and to differentiate into osteoblasts (14). A seminal study subsequently demonstrated that MSCs have single cell–inherent ability to differentiate into multiple mesenchymal lineages, including bone, fat, and cartilage (15). Their native counterparts in vivo are not entirely known. BM-derived MSCs are thought to be part of the stroma to support hematopoiesis, but they are also thought to provide replacement of tissue lost to physiologic turnover, senescence, injury, and disease.

Over the years, cells with the phenotype and function of MSCs have been isolated from many components of adult joint tissue, including adipose tissue, periosteum, synovium, synovial fluid, muscle, and articular cartilage (16). We have isolated multipotent MSCs from the synovial membrane of human knee joints, and these cells may play a role in the regenerative response in, and pathogenesis of, arthritic diseases (17).

Universally accepted criteria and specific markers for the prospective identification of MSCs remain elusive. The International Society for Cytotherapy suggested the following criteria to define human MSCs: should exhibit plastic adherence under standard tissue culture conditions; should express CD105, CD73, and CD90 and lack expression of CD45, CD34, CD14, or CD11b; should express HLA–DR molecules; and should possess differentiation capability, in order to differentiate into osteoblasts, chondrocytes, and adipocytes, under defined conditions (18). However, to date, no single isolation method is regarded as standard practice, and the above criteria allow only a retrospective classification of cultured cells as containing MSCs.

The different approaches to expansion of MSCs in culture make it difficult to directly compare experimental results, raising issues of standardization of MSC-based therapy in the clinic (10). Furthermore, transformations and an altered karyotype of the cells are a concern, particularly following long-term culture. However, carefully controlling the conditions for culture of MSCs can minimize their potential for malignant transformation (19). Of note, aneuploidy was recently reported in culture-expanded human MSC populations, but this was not associated with transformations; instead, the cells became senescent and growth arrested (20).

In human BM, MSCs seem to be an important component of the HSC niche and are involved with regulation of hematopoiesis. The MSC niche has been an increasing area of research, with a growing body of evidence suggesting that MSCs might occupy a perivascular zone in BM and some other tissues, wherein they would stabilize blood vessels and contribute to tissue and immune system homeostasis (21). This may not be the case in avascularized tissues, such as articular cartilage, or in other tissue, such as that recently described in the synovium (22).

MSCs and the immune system

  1. Top of page
  2. Introduction
  3. Basic concepts and clinical applications of adult stem cells
  4. Overview of MSCs
  5. MSCs and the immune system
  6. Role of MSCs in autoimmune rheumatic diseases
  7. MSC therapy for autoimmune rheumatic diseases
  8. Effects of MSCs on Th17 and Treg cell populations
  9. Conclusions
  10. AUTHOR CONTRIBUTIONS
  11. REFERENCES

Culture-expanded MSCs possess immunosuppressive and antiinflammatory functions both in vitro and in vivo, and therefore they serve as attractive candidates for the treatment of inflammatory autoimmune conditions. MSCs are generally considered to be immune privileged. They are known to express class I major histocompatibility complex (MHC) and were previously thought to lack expression of class II MHC. However, Western blot analysis revealed that MSC lysates do contain class II alloantigens (23). Of note, the addition to expansion medium of mitogenic growth factors, such as fibroblast growth factor 2 and platelet-derived growth factor, which was pursued to enhance the consistency of bioprocessing of the cellular product, has been reported to induce functional HLA–DR in MSCs (24), which, after transplantation, could act as nonprofessional antigen-presenting cells. However, human MSCs lack the costimulatory molecules CD80 and CD86, and therefore would not activate alloreactive T cells (1).

Given these properties, MSCs could be used in the allogeneic setting without the need for immunosuppression. That said, MSCs are not inert participants, but rather, express a wide range of receptors and can release a number of cytokines and chemokines. The investigation of the mechanisms underlying the immunomodulatory effects of MSCs may uncover a hitherto poorly understood arm of the immune system. It remains to be determined whether the immunomodulatory properties of cultured MSCs are a clinically relevant artifact from in vitro culture or whether they are a feature of native MSCs.

MSCs can inhibit, in a dose-dependent manner, the proliferation of, and cytokine production by, T cells, B cells, natural killer cells, and dendritic cells via multiple mechanisms. Cocktails of cytokines and cell-to-cell contact molecules are involved in mediating these effects in vitro and in vivo (25, 26). The scenario of MSC-induced immunosuppression in vivo is likely to be much more complex. The rationale for their use in autoimmune disease is based on the assumption that immune dysregulation could be restored by ex vivo expansion and reinfusion of cells that have an immune- regulatory function. There are now more than 90 clinical trials registered as having studied MSCs for the treatment of various human diseases, including chronic inflammatory and autoimmune disorders (http://www.clinicaltrials.gov).

Role of MSCs in autoimmune rheumatic diseases

  1. Top of page
  2. Introduction
  3. Basic concepts and clinical applications of adult stem cells
  4. Overview of MSCs
  5. MSCs and the immune system
  6. Role of MSCs in autoimmune rheumatic diseases
  7. MSC therapy for autoimmune rheumatic diseases
  8. Effects of MSCs on Th17 and Treg cell populations
  9. Conclusions
  10. AUTHOR CONTRIBUTIONS
  11. REFERENCES

While the role of T and B cells has been extensively studied in autoimmune pathogenesis, little is known of the contribution of MSCs to this process. It has been proposed that MSCs, along with Treg cells, contribute to the maintenance of self tolerance in the periphery (1), a hypothesis that is difficult to test given our current scarce knowledge of MSC niches in vivo. The immunosuppressive effect of cultured MSCs that is seen on T cells and dendritic cells in vitro and in vivo in experimental systems of MSC transplantation would operate to prevent deleterious autoreactive immune responses in healthy subjects.

Several studies have examined the differentiation potential of MSCs derived from patients with different rheumatic diseases, but the results are conflicting. BM-derived MSCs from RA patients showed chondrogenic potential that was similar to that observed in MSCs isolated from healthy control subjects (27). However, synovial membrane–derived MSCs from patients with active RA were defective, in terms of their clonogenicity and chondrogenic differentiation potential (28).

In SLE, there appears to be more agreement on the presence of an osteoblastic niche deficiency, both in patients and in mouse models (29). MSCs derived from the BM of 2 patients with SLE showed reduced bone-forming capacity compared to those isolated from the BM of normal healthy subjects. This was accompanied by decreased expression of the osteogenic genes Runx2 and OCN in BM-derived MSCs from SLE patients (30). In the BXSB mouse model of SLE, MSCs proliferated to a lesser extent compared to MSCs from wild-type C57BL/6 mice (31), and this was followed by a lack of differentiation into osteoblasts in long-term cultures.

Additional research needs to be done to establish whether the abnormalities of MSCs in these autoimmune conditions are a result of the disease itself or whether they play a more crucial role in their pathogenesis. Further elucidation of MSC niches and their networking with the immune system in vivo will unravel any mechanistic roles of MSCs in the onset and progression of chronic inflammatory and autoimmune disorders.

MSC therapy for autoimmune rheumatic diseases

  1. Top of page
  2. Introduction
  3. Basic concepts and clinical applications of adult stem cells
  4. Overview of MSCs
  5. MSCs and the immune system
  6. Role of MSCs in autoimmune rheumatic diseases
  7. MSC therapy for autoimmune rheumatic diseases
  8. Effects of MSCs on Th17 and Treg cell populations
  9. Conclusions
  10. AUTHOR CONTRIBUTIONS
  11. REFERENCES

Treatment of RA.

CIA is an experimental autoimmune model of inflammatory arthritis that has many histologic, immunologic, and clinical similarities to RA. Studies of MSCs as cellular therapy for CIA have been dogged by conflicting results (Table 1). The first study that utilized an immortalized mesenchymal cell line, administered intravenously (IV) in mice, showed that there was no benefit of MSC therapy for the reduction of the pathogenetic development of CIA, despite the fact that a potent immunosuppressive activity of the cells was observed in vitro (2). However, another study showed that a single intraperitoneal (IP) injection of allogeneic MSCs in mice prevented the development of severe arthritis (3). Cell-tracking experiments in that study showed that MSCs did not localize to the joints, suggesting that the clinical improvement observed was not due to direct MSC repair of the joints. Furthermore, the study showed reduced levels of proinflammatory cytokines, as compared to the levels in controls, and increased levels of IL-10, an immunosuppressive cytokine produced by Treg cells; it also demonstrated that MSC therapy resulted in de novo generation of CD4+CD25+FoxP3+ Treg cells specific for type II collagen.

Table 1. Overview of the literature describing the effects of mesenchymal stem cells (MSCs) in mouse models of rheumatoid arthritis*
Author, year (ref.)ModelSource of MSCsDonor–recipient MHC matchDose of MSCsRoute of administrationTime of treatmentOutcome of treatmentDescription of treatment
  • *

    MHC = major histocompatibility complex; BM = bone marrow; IP = intraperitoneal; TGFβ = transforming growth factor β; iNOS−/− = inducible nitric oxide synthase–deficient; IL-6−/− = interleukin-6–deficient; IV = intravenous; UC = umbilical cord; IFNγR−/− = interferon-γ receptor–deficient; IA = intraarticular.

  • Time of treatment is shown as number of days or weeks before or after induction of arthritis, except where specified as number of days after disease onset (ADO).

  • Pos. = positive effect, indicated by a disease score reduction; Neg. = negative effect, indicated by a disease score increase; UA = no effect, indicated by an unaffected disease score.

Park et al, 2011 (6)DBA/1DBA/1 mouse BMSyngeneic1 × 106IPWeek 7Pos.TGFβ-transduced
Bouffi et al, 2010 (8)DBA/1Mouse BM: DBA/1, iNOS−/− C57BL/6, IL-6−/− C57BL/6Syngeneic, allogeneic, allogeneic1 × 106IVDay 18, 24, 28, 32Pos.Multiple injections
Liu et al, 2010 (7)DBA/1Human UCXenogeneic5 × 106IPDay 31 ADOPos.Multiple injections
Schurgers et al, 2010 (5)DBA/1Mouse BM: DBA/1J, IFNγR−/− DBA/1, C57BL/6Syngeneic, syngeneic, allogeneic1 × 106IV or IPDay −1, 16, 19, 23, 30Neg.Multiple injections
Chen et al, 2010 (34)DBA/1DBA/1 mouse BMSyngeneic1–2 × 106IVDay 0 or 21Neg.Nontransduced, single injection
Gonzalez et al, 2009 (4)DBA/1Adipose tissue: human, C57BL/6 mouseXenogeneic, allogeneic1 × 106IP or IAOnce per day for 5 days ADOPos.Multiple injections
Choi et al, 2008 (33)DBA/1DBA/1 mouse BMSyngeneic1 × 106IVDay 21, 28, 35UA, Pos.Nontransduced, IL-10–transduced
Augello et al, 2007 (3)DBA/1C57BL/10 mouse BMAllogeneic5 × 106IPDay 0 or 21Pos.Nontransduced, single injection
Djouad et al, 2005 (2)DBA/1C3 mouse cell lineAllogeneic1 × 106, 1 × 106, 4 × 106IVDay 0 or 21, day 0, day 0 or 21UA (day 21)Nontransduced, IL-10–transduced, higher dose nontransduced

Further positive results from MSC therapy were reported in a later mouse study (4), in which it was shown that daily IP injection of human or murine allogeneic and syngeneic adipose (AD)–derived MSCs, which share some of the immunomodulatory properties of their BM counterparts (32), for 5 days after the onset of disease significantly reduced the incidence and severity of arthritis in the CIA model. Intraarticular (IA) injection of AD-derived MSCs was less effective than the IP route, adding weight to the argument that the positive effects of MSCs are not simply due to direct tissue repair in the joints.

AD-derived MSCs can be easily obtained in lipoaspirates from healthy individuals, and can be expanded in vitro to provide large batches of cells for off-the-shelf treatment. As was shown in the above-mentioned study by Augello et al (3), the earlier the treatment, the lower the mean arthritis score. In draining lymph nodes from MSC-treated mice, fewer cells proliferated in response to type II collagen, and lower levels of Th1 cytokines (interferon-γ [IFNγ], IL-2, and TNFα) and Th17 cytokines (IL-17) were produced, whereas the levels of the regulatory cytokines IL-10 and transforming growth factor β (TGFβ) were increased. Again, there was expansion of CD4+CD25+FoxP3+ Treg cells following MSC treatment, showing that MSCs are capable of suppressing the autoreactive effector T cell response (4). MSCs overexpressing IL-10 have also been shown to attenuate CIA (33). Results from recent studies (6–8) further support the potential of MSC-based treatment in autoimmune inflammatory arthritis, in that immune modulation and reduction of articular damage following treatment with MSCs have been observed.

Taken together, these findings suggest that MSC therapy is able to reset the immune system by reducing the deleterious Th1/Th17 response and enhancing the protective Treg cell response. The ability of MSCs to generate de novo Treg cells may be advantageous therapeutically when compared to neutralizing antibodies against single-cytokine signaling, in terms of both safety and efficacy.

Other studies, however, have failed to demonstrate any improvement in experimental CIA with MSC treatment. One study found that Flk-1+ MSCs, a cell population that has been used to study treatment effects in phase I clinical trials, exacerbated the arthritis in mice, by promoting the secretion of IL-6 and IL-17 (34). Schurgers et al (5) were unable to demonstrate any benefit from MSC therapy in CIA. Those investigators used both an IV route and an IP route to administer the MSCs, and also used allogeneic MSCs, given the lack of response with syngeneic MSCs. None of these changes in MSC therapeutic approaches made any difference to the negative outcome. In contrast, injection of Treg cells before or after disease onset led to a dramatic improvement in arthritis.

Experimental protocols differed between all of these studies (see Table 1), which may, in part, explain the inconsistencies in results. The potential reasons for discrepancies include the following: source of MSCs (murine, syngeneic versus human, allogeneic), tissue of origin (BM versus AD), timing of treatment, number of stem cells injected, route of injection (IV, IP, or IA), and treatment regimen (e.g., a single injection of MSCs [3] versus daily injections for 5 days [4]). The studies that demonstrated beneficial effects of MSC therapy also had a number of differences in their protocols. They used IV, IP, and IA routes and used syngeneic, allogeneic, and human sources of MSCs, but the dose of MSCs was fairly consistent between studies. Other, nonmeasurable variables that may have contributed to study differences and could be relevant might include differences in the culture medium used for stem cells, lack of standardization of MSC culture conditions, and differences in animal strain and animal-housing conditions.

Perhaps a major factor was the length of culture of the MSCs. Schurgers et al used MSCs that had been cultured for several weeks prior to use in these experiments (5), and the investigators did not observe any therapeutic effect of MSC administration. Allogeneic MSCs, which were used in the later study by Augello et al (3), were from primary culture of mouse MSCs obtained after the first in vitro passage, as opposed to the immortalized cell line that was used in an earlier study (2). MSCs lose their homing ability in BM after a few hours in culture (35), and this may be a reason for their failure to ameliorate arthritis despite their retention of immunosuppressive ability in vitro. It would be interesting to assess the effect of MSCs on CIA after different numbers of cell-population doublings.

There are no reports, to date, of MSCs being used in the treatment of patients with RA. Autologous hematopoietic stem cell transplantation (HSCT) has been used in severe treatment-refractory cases, although the responses have been relatively short lived (36). However, one study examined the effect of human allogeneic AD-derived MSCs on collagen-reactive T cells isolated from patients with RA in vitro (37). Similar to the findings in CIA (4), those investigators found that human AD-derived MSCs inhibited the proliferation and expression of proinflammatory cytokines (IFNγ and TNFα) by collagen-activated T cells, while increasing the numbers of IL-10–producing T cells. Treatment with the human AD-derived MSCs also led to the generation of antigen-specific CD4+CD25+FoxP3+ Treg cells. Finally, these AD-derived MSCs also inhibited the production of the matrix-degrading enzymes collagenase and gelatinase, which are involved in the inflammatory response of the resident cells of the synovium. Only 12 of the 22 peripheral blood mononuclear cells from RA patients responded positively to type II collagen, thus providing evidence that type II collagen has not been definitively established as an autoantigen in RA. As with the other experiments, we should remain cautious in concluding that these positive results in vitro can be translated into the clinical setting in vivo, since the results from in vitro assays assessing the immunomodulatory potency of MSCs do not always show a correlation with the in vivo outcomes.

Treatment of SLE.

SLE is an archetypical autoimmune disease, with clinical manifestations ranging from minor skin symptoms and joint disease to life-threatening, major organ involvement, such as nephritis and neuropsychiatric disease. It is characterized by the presence of autoreactive T and B lymphocytes and immune complexes to a number of nuclear antigens.

Studies have been carried out in 2 mouse models of SLE, using MPR/lpr mice and (NZB × NZW)F1 mice, with conflicting results (as summarized in Table 2). There was a discrepancy between the SLE mouse studies, in that those studies conducted in vitro demonstrated an inhibition of antigen-dependent proliferation of B cells from (NZB × NZW)F1 mice after treatment with BM-derived MSCs. However, treatment with MSCs in vivo had no effect on autoantibody production, proteinuria, or mortality in these mice, despite an apparent improvement in histologic features of the kidney (38). In the same model, another group of investigators found that allogeneic BM-derived MSCs exacerbated the disease and increased autoantibody levels, characterized by more severe histopathologic features in the kidney and increased proteinuria (39). In vitro coculturing of MSCs with plasma cells from this model led to increased IgG antibody production, which was hypothesized to account for the clinical deterioration, presumably by causing increased immune complex deposition in vivo.

Table 2. Overview of the literature describing effects of mesenchymal stem cells (MSCs) in mouse models of systemic lupus erythematosus*
Author, year (ref.)ModelSource of MSCsDose of MSCsRoute of administrationTime of treatmentOutcome of treatment
Anti-dsDNA levelsANA levelsRenal
  • *

    Anti-dsDNA = anti–double-stranded DNA; ANA = antinuclear antibody; BM = bone marrow; IV = intravenous; UC = umbilical cord; IP = intraperitoneal.

Zhou et al, 2008 (41)MRL/lprHuman BM1 × 106IVAge 16–20 weeksReducedNo effectProteinuria reduced, increased survival
Sun et al, 2009 (30)MRL/lprC3H/HeJ mouse BM0.1 × 106/10 gm body weightIVAge 9 or 16 weeksReducedReduced (increased C3)Histologic features improved, proteinuria improved, creatinine levels normalized
Gu et al, 2010 (40)MRL/lprUnspecified UC1 × 106IVAge 18, 19, and 20 weeksReducedUnspecifiedProteinuria and creatinine levels reduced
Schena et al, 2010 (38)(NZB × NZW)F1C57BL/6 mouse BM1.25 × 106IVAge 27, 28, and 29 weeksNo effectNo effectNo effect on proteinuria, histologic features improved
Youd et al, 2010 (39)(NZB × NZW)F1BALB/c mouse BM1 × 106IPBiweekly, starting at age 21 or 32 weeksIncreasedIncreasedWorsening of histologic features and proteinuria

In the MRL/lpr mouse model of SLE, treatment of the mice with allogeneic BM-derived MSCs and umbilical cord (UC)–derived MSCs (40, 41), but not administration of cyclophosphamide, led to clinical improvements, with a concomitant reduction in anti–double-stranded DNA (anti-dsDNA) antibodies, proteinuria, and creatinine levels. In another study, allogeneic MSC transplantation ameliorated the osteoporosis-like phenotype and improved the osteoblastic niche deficiency in MRL/lpr mice. Of note, BM-derived MSCs isolated from mice that had undergone MSC transplantation showed a restored osteogenic potency. MSC transplantation led to increased levels of CD4+FoxP3+ Treg cells, while the levels of CD4+IL-17+ (Th17) T cells were significantly reduced (30).

There are several reports in the literature describing MSC therapy in patients with SLE (Table 3). The first study, involving autologous MSCs administered IV to 2 patients with SLE, showed no clinical improvements in disease activity, despite an increased number of Treg cells in the peripheral blood (42). This is perhaps unsurprising, given the data showing that MSCs from patients with SLE have a deficiency of the osteoblastic lineage, the restoration of which might be achieved with heterologous MSCs (29).

Table 3. Overview of the literature describing outcomes of transplantation of mesenchymal stem cells (MSCs) in human patients with systemic lupus erythematosus*
Author, year (ref.)No. of patientsSource of MSCsDose of MSCs/kgDuration of followupOutcome of treatment
Change in SLEDAIImmunologicRenal
  • *

    SLEDAI = Systemic Lupus Erythematosus Disease Activity Index; BM = bone marrow; anti-dsDNA = anti–double-stranded DNA; ANA = antinuclear antibody; UC = umbilical cord; CYC = cyclophosphamide.

Sun et al, 2009 (30)4BM cells from family members1 × 10612–18 months∼10-point drop in mean value at 12 monthsC3 levels increasedProteinuria reduced at 12 months, 2 patients taken off CYC at 6 months
Liang et al, 2010 (9)15BM cells from family members1 × 10617.2 ± 9.5 monthsFrom mean 12.2 to mean 3.2 at 12 monthsAnti-dsDNA levels reduced, ANA levels reduced (not significantly)Proteinuria reduced from 2,505 mg to 858 mg, 2 patients experienced relapse of proteinuria at 1 year
Sun et al, 2010 (45)16Unspecified UC1 × 10618 monthsFrom mean 19 to mean 7.3 at 6 monthsUnspecifiedProteinuria reduced from 3,121 mg to 1,338 mg at 3 months
Carrion et al, 2010 (42)2Autologous BM cells1 × 10614 weeksNo changeNo changeRenal flare in 1 patient at 4 months

Other recent studies assessing MSC therapy in patients with SLE have shown some benefit. The first study to demonstrate significant therapeutic benefits utilized transplantation of MSCs from healthy relatives into 4 patients with severe SLE whose condition had been refractory to treatment for 6 months with pulsed cyclophosphamide and prednisolone at dosages higher than 20 mg/day (30). These patients all met the American College of Rheumatology 1997 criteria for SLE (43) and had SLE Disease Activity Index (SLEDAI) scores (44) higher than 8, as well as having lupus nephritis (classes III, IV, or V). After treatment with a single IV infusion of allogeneic BM-derived MSCs, 2 patients were completely taken off cyclophosphamide treatment at 5–6 months, and all 4 patients were taking doses of ≤10 mg of prednisolone. There were also improvements in secondary outcomes, in that a significant decline in the SLEDAI score and decrease in proteinuria were observed in all patients at 12 months after MSC therapy.

The 2 larger studies also demonstrated beneficial effects. Sun et al (45) examined the effects of infusion of UC-derived MSCs in 16 patients with SLE who had life-threatening organ involvement and whose condition was refractory to standard treatment. There were improvements in the SLEDAI and renal parameters; 2 patients had SLEDAI scores of <4 at 2 years, and another 2 patients who had central nervous system involvement with seizures remained seizure free. Liang et al (9) examined the effects of allogeneic MSC transplantation in 15 patients with SLE whose condition was refractory to standard treatment. All of the patients showed clinical improvement after MSC transplantation, accompanied by stepwise tapering of the prednisolone dose in 13 patients who were followed up for 12 months. Moreover, proteinuria decreased significantly in 7 of the patients, and the levels normalized in 5 patients. At 12 months, 4 patients had a SLEDAI score of 0, and 1 patient discontinued immunosuppressive treatment and was being treated with only 5 mg prednisolone. There were also improvements in the antinuclear antibody and anti-dsDNA antibody levels. However, the majority of patients continued to take cyclophosphamide at 12 months. Given the relatively short length of followup, there remains uncertainty as to whether disease activity may have started increasing over time. Furthermore, it remains to be determined whether a single treatment with MSCs is sufficient or whether re-treatment may be necessary, as, for instance, with rituximab.

In all of these studies, an attempt was made to detect the mechanism of the beneficial response seen following MSC transplantation. There was an increase in the percentage of CD4+FoxP3+ Treg cells in the peripheral blood of SLE patients at 3 months, and this continued to increase by 6 months. This was accompanied by an increase in the levels of TGFβ and IL-10, which are important in Treg cell activation and function (30). UC-derived MSC transplantation depressed the serum concentration of IL-4 (elevated in patients with active SLE) and exerted a concomitant suppression of autoantibody production (45). The results of these studies suggest that the mechanism by which MSC transplantation protects against pathogenesis in SLE involves expansion of Treg cell populations and suppression of humoral immunity, which is consistent with the findings in the CIA mouse model showing that MSCs protect against disease pathogenesis. However, other mechanisms linked to the multipotentiality of MSCs may also be involved; apoptosis of nephrons is believed to be an initiating factor in lupus nephritis (46), and it was hypothesized that MSCs could differentiate into endothelial cells in nephrons, which, in turn, could ameliorate lupus nephritis.

In all 4 studies described above (9, 30, 42, 45), there were no reports of any serious adverse side effects during or after the MSC infusions, which were generally well tolerated. Clearly, a longer term of followup and data on treatment safety are required. Each MSC preparation was screened for various pathogenic organisms prior to clinical use, but rigorous purification strategies and strict quality controls for safety and efficacy, e.g., potency assessment, are paramount for such treatments.

Treatment of SSc.

SSc is a complex disease characterized by fibrosis of the skin and various internal organs. A pathologic triad has been described for SSc, in which increased deposition of extracellular matrix and collagen in tissues, microvascular damage and dysfunction, and immune activation as demonstrated by the development of inflammation and frequent occurrence of autoantibodies have been observed. There are complex interactions between endothelial cells, macrophages, lymphocytes, and fibroblasts, through mediators such as cytokines, chemokines, and growth factors.

BM-derived MSCs from patients with SSc appeared similar to those from healthy donors in their phenotype and capacity to differentiate into adipogenic and osteogenic lineages (47), but they displayed impaired endothelial cell differentiation and hematopoietic support, as well as early senescence (48, 49). Importantly, these cells were found to have immunosuppressive properties similar to those in healthy donors, raising the prospect that MSCs may be useful in an autologous setting (47).

There is only a single case report of MSCs being used to treat a patient with SSc (50). Infusion of MSCs from a haploidentical allogeneic cell donor, who was a relative of the patient, led to a marked improvement in the modified Rodnan skin thickness score (51). Prior to MSC transplantation, the patient had diffuse SSc without internal organ involvement for 4 years, and the patient's disease was refractory to standard treatment. The modified Rodnan skin thickness score was reduced from 25 to 11 by 6 months following MSC transplantation. Caution should be applied before attributing this improvement entirely to MSC transplantation, as spontaneous improvement in skin disease is a recognized feature of late disease. Clearly, further work remains to be done in this area.

Although the use of MSCs in patients with SSc is limited, another stem cell type is currently being assessed as therapy for severe SSc. The Autologous Stem Cell Transplantation International Scleroderma trial (http://www.astistrial.com) is a multicenter, randomized phase III study seeking to compare the efficacy and safety of high-dose immunoablation and autologous HSCT with IV pulse cyclophosphamide for the treatment of patients with severe SSc. This trial will hopefully help to establish the place of HSCT in the treatment algorithm of SSc.

Effects of MSCs on Th17 and Treg cell populations

  1. Top of page
  2. Introduction
  3. Basic concepts and clinical applications of adult stem cells
  4. Overview of MSCs
  5. MSCs and the immune system
  6. Role of MSCs in autoimmune rheumatic diseases
  7. MSC therapy for autoimmune rheumatic diseases
  8. Effects of MSCs on Th17 and Treg cell populations
  9. Conclusions
  10. AUTHOR CONTRIBUTIONS
  11. REFERENCES

The Th1/Th2 paradigm divided CD4+ T helper cells into 2 distinct subsets on the basis of their cytokine expression, with Th1 postulated to provide help for cell-mediated immunity and Th2 to provide help for humoral immunity and allergy. This paradigm was enduring, despite the contradictory experimental evidence. Based on the paradigm, in CIA, IFNγ-knockout mice would be predicted not to develop arthritis, when, in fact, they developed more severe arthritis than that in wild-type mice. Such findings have led to a paradigm shift, in which Th17 cells and Treg cells have been incorporated (52).

Increasing evidence indicates that a reciprocal relationship exists between Th17 cells and Treg cells, and this has been shown in studies of the inverse relationship between transcription factors retinoic acid receptor–related orphan receptor C (RORC) (for Th17 cells) and FoxP3 (for Treg cells) in the joints of patients with juvenile idiopathic arthritis (53). If MSCs were to be brought into this equation, the mechanism by which MSC transplantation might ameliorate autoimmune rheumatic disease could be that treatment with MSCs leads to reduced RORγt (Th17) levels and increased FoxP3 (Treg) levels, reflecting the change in T cell populations in vivo (Figure 1). Autoimmunity might therefore result from T cells biased toward Th17 cells and away from Treg cells. In some of the above-described studies, MSC therapy reduced the deleterious Th17 cell response and increased the protective Treg cell response, reflecting the change in T cell populations in vivo (4, 6–8, 30). These findings support the notion of a role for MSCs in resetting the immune system, with adoptive transfer of Treg cells derived from animals treated with MSCs being able to ameliorate CIA (4).

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Figure 1. Possible effects of mesenchymal stem cells (MSCs) on Treg cell and Th17 cell populations in autoimmune rheumatic diseases. RA = rheumatoid arthritis; CIA = collagen-induced arthritis; SLE = systemic lupus erythematosus; MHC-I = class I major histocompatibility complex; IL-2 = interleukin-2; IFNγ = interferon-γ; TNFα = tumor necrosis factor α; RORγt = retinoic acid receptor–related orphan receptor γt; TGFβ = transforming growth factor β.

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Conclusions

  1. Top of page
  2. Introduction
  3. Basic concepts and clinical applications of adult stem cells
  4. Overview of MSCs
  5. MSCs and the immune system
  6. Role of MSCs in autoimmune rheumatic diseases
  7. MSC therapy for autoimmune rheumatic diseases
  8. Effects of MSCs on Th17 and Treg cell populations
  9. Conclusions
  10. AUTHOR CONTRIBUTIONS
  11. REFERENCES

The conflicting results obtained from studies using MSC therapy emphasize the need for standardized culture conditions to achieve robust production of consistent and reliable cellular products. It is evident that in vitro assays of immunomodulatory function are not predictive of the in vivo outcomes. Therefore, it is important to develop clinically relevant assays to standardize MSC therapy. This will also allow direct comparison between clinical studies. In this review, we have highlighted the fact that the findings from animal models of autoimmune rheumatic diseases may not be predictive of outcomes in clinical studies. Preclinical experimentation is a scientific and regulatory requirement, and therefore proper screening and selection of clinically relevant animal models that specifically reproduce the biologic features of a disease under study will be critical. At present, the future use of MSCs in clinical practice is likely to be restricted to patients with severe disease refractory to standard current therapies, although such treatments might be more effective if administered earlier, in order to reset the immune system by inducing regulatory networks. It will be necessary to gather stronger preclinical evidence before randomized controlled trials can be conducted, with the ultimate goal of establishing the place of MSC therapy in the treatment algorithm of autoimmune rheumatic diseases.

AUTHOR CONTRIBUTIONS

  1. Top of page
  2. Introduction
  3. Basic concepts and clinical applications of adult stem cells
  4. Overview of MSCs
  5. MSCs and the immune system
  6. Role of MSCs in autoimmune rheumatic diseases
  7. MSC therapy for autoimmune rheumatic diseases
  8. Effects of MSCs on Th17 and Treg cell populations
  9. Conclusions
  10. AUTHOR CONTRIBUTIONS
  11. REFERENCES

All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be published. Dr. De Bari had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

REFERENCES

  1. Top of page
  2. Introduction
  3. Basic concepts and clinical applications of adult stem cells
  4. Overview of MSCs
  5. MSCs and the immune system
  6. Role of MSCs in autoimmune rheumatic diseases
  7. MSC therapy for autoimmune rheumatic diseases
  8. Effects of MSCs on Th17 and Treg cell populations
  9. Conclusions
  10. AUTHOR CONTRIBUTIONS
  11. REFERENCES