# Defining minimal criteria for peer-reviewed reporting of mesenchymal stromal cell clinical trials for autoimmune diseases

Lindsay C Davies, Lucie Biard, Damián García-Olmo, Gary Gilkeson, Massimiliano Gnecchi, Joanne Kurtzberg, Gesine Kogler, Katarina Le Blanc, Mark W. Lowdell, Yves-Marie Pers, et al.

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## To cite this version:

Lindsay C Davies, Lucie Biard, Damián García-Olmo, Gary Gilkeson, Massimiliano Gnecchi, et al. Defining minimal criteria for peer-reviewed reporting of mesenchymal stromal cell clinical trials for autoimmune diseases. Cytotherapy, 2025, 10.1016/j.jcyt.2025.06.005. hal-05239565

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## Mesenchymal Stem/Stromal Cells

# Defining minimal criteria for peer-reviewed reporting of mesenchymal stromal cell clinical trials for autoimmune diseases

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Lindsay C. Davies<sup>1,2,3,*</sup>, Lucie Biard<sup>4,5,6</sup>, Damián García-Olmo<sup>7</sup>, Gary Gilkeson<sup>8</sup>, Massimiliano Gnocchi<sup>9,10</sup>, Joanne Kurtzberg<sup>11</sup>, Gesine Kogler<sup>12</sup>, Katarina Le Blanc<sup>13,14</sup>, Mark W. Lowdell<sup>15</sup>, Yves-Marie Pers<sup>16,17</sup>, Ling-yun Sun<sup>18</sup>, Karin Tarte<sup>19,20</sup>, Antonio Uccelli<sup>21</sup>, Fermin Sanchez-Guijo<sup>22</sup>, Dominique Farge<sup>5,23,24</sup>, on behalf of the ISCT Educational Course Working Group on the Use of Mesenchymal Stromal Cells for Autoimmune Diseases (Paris 2024)

Abbreviations: AD, autoimmune disease; AT, adipose tissue; ATMP, advanced therapeutic medicinal product; BM, bone marrow; CD, Crohn's disease; CQA, critical quality attributes; DSA, donor specific antigens; DSMB, Data and Safety Monitoring Board; EMA, European Medicines Agency; ESRD, end-stage renal disease; FDA, Food and Drug Administration; GMP, good manufacturing practice; HbA1c, hemoglobin A1c; HLA, human leukocyte antigen; ICH, International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use; IDO, indoleamine 2,3 dioxygenase;

IFN-γR1, interferon gamma receptor 1; IL, interleukin; ISCT, International Society for Cell and Gene Therapy; LPS, lipopolysaccharide; MA, meta-analysis; MOA, mechanism of action; MS, multiple sclerosis; MSC, mesenchymal stromal cell; PDL, population doubling level; QC, quality control; SLE, systemic lupus erythematosus; SoHO, regulation on substances of human origin; SR, systematic review; T1D, Type 1 diabetes; UC, umbilical cord

<footnote n="1"><sup>1</sup> NextCell Pharma AB, Huddinge, Sweden</footnote>

<footnote n="2"><sup>2</sup> QVance AB, Huddinge, Sweden</footnote>

<footnote n="3"><sup>3</sup> Department of Microbiology, Tumor and Cell Biology, Karolinska Institutet, Stockholm, Sweden</footnote>

<footnote n="4"><sup>4</sup> INSERM U1342, Team ECSTRRA, Paris, France</footnote>

<footnote n="5"><sup>5</sup> Université Paris Cité, Paris, France</footnote>

<footnote n="6"><sup>6</sup> Department of Biostatistics and Medical Informatics, Assistance Publique Hôpitaux de Paris, Paris, France</footnote>

<footnote n="7"><sup>7</sup> Department of Surgery, Fundación Jiménez Díaz University Hospital, UAM, Madrid, Spain</footnote>

<footnote n="8"><sup>8</sup> Medical University of South Carolina, Charleston, South Carolina, USA</footnote>

<footnote n="9"><sup>9</sup> Department of Molecular Medicine, Unit of Cardiology, University of Pavia, Pavia, Italy</footnote>

<footnote n="10"><sup>10</sup> Cardiothoracovascular Department, Translational Cardiology Unit, Fondazione IRCCS Policlinico San Matteo, Pavia, Italy</footnote>

<footnote n="11"><sup>11</sup> Department of Pediatrics, Duke University School of Medicine, Durham, North Carolina, USA</footnote>

<footnote n="12"><sup>12</sup> Institute for Transplantation Diagnostics and Cell Therapy, Düsseldorf, Germany</footnote>

<footnote n="13"><sup>13</sup> Department of Laboratory Medicine, Karolinska Institutet, Huddinge, Sweden</footnote>

<footnote n="14"><sup>14</sup> Department of Cell Therapies and Allogeneic Stem Cell Transplantation, Karolinska University Hospital, Stockholm, Sweden</footnote>

<footnote n="15"><sup>15</sup> Department of Haematology, University College London, London, UK</footnote>

<footnote n="16"><sup>16</sup> IRMB, University of Montpellier, INSERM, CHU Montpellier, Montpellier, France</footnote>

<footnote n="17"><sup>17</sup> Clinical Immunology and Osteoarticular Diseases Therapeutic Unit, Department of Rheumatology, Lapeyronie University Hospital, Montpellier, France</footnote>

<footnote n="18"><sup>18</sup> Department of Rheumatology and Immunology, The Affiliated Nanjing Drum Tower Hospital of Nanjing University Medical School, Nanjing, Jiangsu, China</footnote>

<footnote n="19"><sup>19</sup> UMR U1236, INSERM, Univ Rennes, Etablissement Français du Sang, Immunology and Cell Therapy Lab, Rennes University, Rennes, France</footnote>

<footnote n="20"><sup>20</sup> SITI Laboratory, CHU Rennes, Etablissement Français du Sang, Rennes, France</footnote>

<footnote n="21"><sup>21</sup> Department of Neurosciences, Ophthalmology and Genetics, University of Genoa, Italy</footnote>

<footnote n="22"><sup>22</sup> University of Salamanca, IBSAL-University Hospital of Salamanca, Salamanca, Spain</footnote>

<footnote n="23"><sup>23</sup> Department of Internal Medicine, CRMR MATHEC, Maladies Auto-Immunes et Thérapie Cellulaire, Centre de Référence des Maladies Auto-Immunes Systémiques Rares d'Ile-de-France, FAI2R, Hôpital Saint-Louis, Assistance Publique des Hôpitaux de Paris, Paris, France</footnote>

<footnote n="24"><sup>24</sup> Department of Medicine, McGill University, Montreal, Quebec, Canada</footnote>

<footnote>*Correspondence: Lindsay Davies, PhD, NextCell Pharma AB, Hälsovägen 7, 141 57, Huddinge, Sweden. E-mail address: Lindsay.davies@nextcellpharma.com (L.C. Davies).</footnote>
A B S T R A C T

**Background:** Mesenchymal stromal cells (MSCs) have been employed in clinical trials for their immunomodulatory capabilities, especially in the treatment of autoimmune disorders such as type 1 diabetes. In January (25-26th) 2024, the International Society of Cell and Gene Therapy (ISCT) held a two-day workshop entitled "Cell Therapies for Autoimmune Diseases: MSCs from Biology to Clinical Application" in Paris, France.

**Methods:** Key opinion leaders in MSC biology, clinical and regulatory backgrounds came together with European, Asian and South American delegates to discuss the need for standardisation in the design, conduct and reporting of MSC clinical trials for autoimmune diseases.

**Results:** The authors of this paper emphasise the need to comply with standard guidelines for protocols and reporting of trials, working towards greater transparency and reproducibility of research. Standardisation in reporting of clinical trials using MSCs for autoimmune disorders is essential so that the reader can interpret data correctly and ensure that meta-analyses are generated from comparable datasets, providing meaningful knowledge and guidelines for those working in the field.

**Conclusion:** In this article the key considerations for the effective design and execution of a clinical trial using MSCs for autoimmune disorders, including product characterisation and key manufacturing parameters are discussed. Recommendations are made with respect to minimal criteria for data reporting, so as to move forward the development of MSC therapy and see a successful transition of these drug products to market, thereby addressing a significant unmet clinical need in autoimmunity.

**Key Words:** mesenchymal stromal cells, autoimmune disease, immunomodulation, standardization, clinical trial design, data reporting.

Introduction

Mesenchymal stromal cells (MSCs) have been utilized in regenerative medicine and cell therapy for immune disorders for over twenty years. Despite large numbers of clinical trials claiming efficacy in several different disease and injury contexts, only a small number of these advanced therapy medicinal products (ATMPs) have reached the market [1].

MSCs were originally given the nomenclature of stem cells, isolated from the bone marrow (BM) with a bipolar morphology, able to self-renew and demonstrate the capacity to differentiate into the three main lineages associated with the mesoderm-bone, fat and cartilage [2]. Since that initial discovery, MSC research has grown exponentially with the discovery of cells in virtually all organ systems. MSCs were originally defined for their usage in tissue engineering applications, with a focus on their differentiation capacity. Today, MSCs are more often employed for their ability to modulate the immune system, and as such have been progressively applied in the treatment of autoimmune diseases (ADs).

With an incidence of 8–10% in the Western population, ADs are a group of >80 heterogeneous conditions, characterized by polyclonal activation of the immune system and a failure of the immune system to maintain adapted tolerance to auto-antigens [3–6]. Growing understanding of the genetic background of ADs has led to a revision of their proposed pathogenesis, with consideration to the presence of self-directed tissue inflammation as a component of each type of AD. Most classical ADs are polygenic diseases, resulting from a combination of auto-inflammatory and autoimmune mechanisms with a predominant autoimmune background; for example, systemic lupus erythematosus (SLE), type 1 diabetes (T1D) and autoimmune thyroiditis. In contrast, others have a predominant auto-inflammatory component, such as Crohn's disease (CD). Therefore, the optimal treatment of each AD type should consider the specific pathological continuum between autoimmunity and auto-inflammation, which invariably contributes to the phenotypic expression of the individual disease.

For these reasons, MSC immunosuppressive, angiogenic and anti-fibrotic properties, theoretically appear as ideal tools to target the respective auto-inflammatory and autoimmune components of AD. However, as recently detailed in Greco *et al.* [7], despite the high numbers of clinical trials using MSCs for the treatment of different ADs, no MSC products within Europe have been approved as a treatment for AD. Only 1 MSC product (Alofisel, Takeda Pharma A/S, Valensbæk Strand, Denmark), an allogeneic adipose derived (AT)-MSC aimed at treating a complication of CD, complex anal fistulas, not the AD itself, has been approved in Europe since 2018. This product has furthermore now been voluntarily removed from the European Union market until December 2024 as the company considered that

it was not possible to provide additional data on the effectiveness of Alofisel as requested by the European Medicines Agency (EMA) [8].

In January (25–26th) 2024, the International Society of Cell and Gene Therapy (ISCT) held a 2-day workshop entitled "Cell Therapies for Autoimmune Diseases: MSCs from Biology to Clinical Application" in Paris, France. At this meeting, key opinion leaders in MSC biology, clinical and regulatory backgrounds came together with European, Asian, North and South American delegates to discuss the need for standardization in the design, conduct and reporting of MSC clinical trials for autoimmune disorders. ISCT's Mesenchymal Stromal Cell committee (MSC-ISCT) has worked extensively to facilitate and promote the therapeutic use of MSC with successive ISCT position statements and consensus guidelines for: (i) minimal criteria for MSC identification [9,10], which was further updated in 2019 to clarify nomenclature [11], and (ii) MSC analytic methods to assess MSC mechanism(s) of action (MOA), specifically with respect to immune functional assays as a potency release criterion [12]. Most recently, ISCT's latest publication explored, using the Delphi Process, expert opinions on revised characterization profiling and definitions of MSCs. In this publication the need for clarity in reporting of clinical trials with MSC products was discussed, with suggestions on guidelines for key factors and minimal information that should be disclosed by the Sponsor of the clinical trial [13]. Despite these efforts there remains considerable heterogeneity in MSC preclinical and clinical assessments, making critical evaluation of data and interpretation challenging.

In this paper, we bring together the key factors highlighted in the discussion held at the January 2024 ISCT workshop and their relevance with application to MSC treatment for ADs. We build on the publication of Renesme *et al.* [13] underlining the importance of an end-to-end consideration from product concept and development through to trial conduct, data collection and analysis and interpretation for MSC treatment in ADs. In addition, there is a need for increased transparency for researchers, clinicians and patients when reporting clinical results to globally improve our understanding of ADs and the potential for MSC treatment of these specific conditions. We propose minimal criteria for reporting MSC clinical trials for AD, moving research towards a standard approach to clinical trial design, conduct, data collection and analysis.

Key Factors Impacting the Identity, Quality and Efficacy of MSC Products in AD Patients

1. Tissue source

Evidence from the literature teaches us that the tissue source of the MSCs directly impacts both the markers we use to identify the
cells as well as the functionality of the cell product [14]. MSCs, whether perinatal (placental, UC or amniotic fluid derived) or adult (such as BM and AT), importantly have an epigenetic memory of their tissue or origin, predisposing these cells to certain functions and cellular intellect [15,16].

The MSC-ISCT committee's latest position statement on MSC nomenclature highlights this further, stating that the originally proposed cell surface markers outlined in 2006, designed for 2D *in vitro* expanded MSCs, are not suitable for cells derived from all tissue sources and all manufacturing protocols [11]. Therefore, caution should be placed on how a drug developer characterizes their MSC product. This represents one example of the rationale behind the committee's recommendation that in the communication of scientific and clinical works the authors should state the tissue origin as a prefix to the MSC nomenclature, such as BM-MSCs or umbilical cord MSCs (UC-MSCs) [11,17]. It was also highlighted that "*unless there is rigorous functional evidence in vitro and in vivo to demonstrate the self-renewal and differentiation properties, the term mesenchymal stem cell should not be used.*" This important clarification in the nomenclature of the MSC acronym has further been defined by Phinney *et al.* [18] who succinctly defines the role of the stromal cell in immune modulating treatments, and stem cells in regenerative therapies. With respect to the use of MSCs in ADs, the focus of this paper, we refer to the use of stromal cells throughout.

Tissue source of MSCs has been linked, although tangentially, to pro-coagulant activity after administration illustrating the importance in defining MSC starting material for the safety of the final cell therapy. In line with the inherent memory of their tissue origin and role in *in vivo* repair functions, some MSC sources such as those from the placenta, express higher levels of pro-thrombotic factors such as tissue factor and extracellular matrix components such as fibronectin and type I collagen [19,20]. This is an understudied area of MSC biology, with few centers evaluating coagulopathy *in vitro*. However, the principal safety concerns of pro-coagulant effects, and the reason why the literature reports such disparate data on platelet and coagulation cascade activation, lie evidently in the handling and processing of the cells prior to administration. Factors such as *in vitro* age/passage number, culture conditions including culture medium and attachment matrices coated to the surface of bioreactor fibers, microcarriers or plastic in 2D systems, directly impact the cell surface marker profile of MSCs and the levels of factors such as tissue factor and phosphatidylserine visible to the immune system of the patient [20,21]. The translation of these *in vitro* findings to the clinical setting and impact on patient safety requires further investigation, with a need for *robust* *in vitro* assays which can be employed robustly to analyze the impact of MSCs on the coagulation cascade. Thompson *et al.* [22] reported a meta-analysis (MA) of safety data from different MSC products with intravenous infusion and irrefutably demonstrated the safe profile of these cells with administration into patients. This links further to the discussion below on manufacturing.

It is well characterized that donor heterogeneity of MSCs is evident irrespective of tissue source and may contribute in part to the disparity seen in clinical trial results. Donor selection encompasses a wide variety of parameters for consideration in developing an MSC therapy, of which a nonexhaustive number of these have been discussed below. One of the key advantages of MSC therapy and their inherent immunomodulatory properties has been the ability to use these cells in an autologous or allogeneic setting without the needs for human leukocyte antigen (HLA) matching. Within the context of the AD that one is treating, allogeneic can be seen as a better option, as MSCs derived from AD patients can be impacted by AD related genetic and environmental factors, potentially reducing the potency or efficacy of the product. Our lack of understanding as to the role of the stroma in the pathology of AD makes allogeneic cell selection a lower risk strategy, supported by a growing body of evidence that MSCs from AD patients demonstrate functional alterations, such as in

patients with SLE [23], systemic sclerosis [24,25] and multiple sclerosis (MS) [26], that may impact their therapeutic efficacy in the autologous setting. One major consideration of allogeneic MSCs, especially when considering repeated infusions, is the risk of alloimmunization against donor specific antigens (DSA), a process that has been linked to the tissue source of MSCs, but without any proven relationship to safety or clinical efficacy [27–29]. A systematic assessment of pre-existing versus induced DSA is an essential step forward to definitively determining the impact of MSC tissue source and manufacturing process, study design, and recipient features on the incidence of DSA and whether these DSA impact response to MSC treatment.

Similarly, the patient's age at point of diagnosis or treatment may also warrant the need for an allogeneic product, with a younger, more potent profile offering the best chance of clinical improvement to the AD patient. The diminishing therapeutic properties of MSCs with donor age, irrespective of tissue source, has been documented extensively within the literature [30,31]. As such this parameter remains a constant consideration in the development and design of an MSC drug product and clinical trial and should be reported with the study design and results openly.

Gender-related immunosuppressive properties have also been reported in human BM-MSCs, with interferon-γ receptor 1 (IFN-γR1) and interleukin (IL)-6β expression higher in female than in male donor derived cells. In addition, female BM-MSCs exhibit an enhanced immunosuppressive function against T cell proliferation compared to male BM-MSCs [32]. A growing body of evidence suggests that female MSC donors may offer a better source of starting material for MSC drug products being specifically employed for immune modulation. *In vitro* studies have shown that AT-MSCs derived from female donors produce significantly higher levels of the anti-inflammatory mediators IL-1RA, prostaglandin E2 and indoleamine 2,3 dioxygenase (IDO) than their male counterparts. Female AT-MSCs also promote the downregulation of IL-2R gene expression in peripheral blood mononuclear cells, leading to higher immunosuppression levels in comparison to male AT-MSCs [33].

To overcome issues with donor heterogeneity and inconsistency in manufacturing of MSC drug products, there are now a number of developers working with pooled MSC products bringing together multiple MSC donors in the formulation of their final drug product [34–36]. Clinical trials based on pooled products for ADs, such as Carlsson *et al.* [34] who using pooled UC-MSC for the treatment of T1D, demonstrates both the safety and efficacy of this approach for standardization of MSC drug products.

## 2. *Manufacturing of MSC drug products*

Many of the clinical trials reported for MSC in ADs, remain at early stage (Phase I/II), with significant numbers of these studies originating from academic settings, and therefore the manufacturing protocols are optimized for small scale batches. Standardization of manufacturing processes for MSCs is a challenge, with issues linked directly to the stage of clinical development and changes in regulatory requirements in the transition from Phase I/II to Phase III and commercialization. Manufacturing requirements need to be considered early in process development and design of clinical trials as it will impact the progression of the product development and relevance of the early datasets generated, representing a potential failure point for an MSC market approval.

Standardization therefore lies in the quality attributes of the cells that are released for clinical use, with many of these assays not considering nuanced differences in the manufacturing protocol which may impact MSC biology, clinical efficacy or reproducibility. Changes in manufacturing procedures and handling of the cells, for example use of freshly detached versus frozen-thawed cells, can significantly change the marker profiles and the functional properties of MSCs including immunomodulatory activity [37–40]. The prelicensing of
MSCs with IFNγ alone or in combination with other inflammatory factors has been proposed as a way to counteract the effects of cryopreservation or replicative senescence, and ready the cells for intravenous infusion [41]. Such second-generation MSCs (or enhanced MSCs) are currently being evaluated in early phase clinical trials, but direct comparison with classical unprimed MSCs would be required to conclude on the efficacy and safety of such approaches, which are also associated with an increased expression of HLA molecules and potentially related immunogenicity.

Irrespective of academic or industrial production, we continue to see MSCs expanded using a wide variety of platforms including 2D cell culture flasks or stacks as open systems and growing numbers of 3D bioreactors including those involving hollow fiber-based cartridges and microcarriers [42,43]. Production protocols in 2D with tissue based starting material where explants are seeded, and cells migrate out onto the culture plastic during the initial culture limit the transition to a closed 3D based platform until later in the production process [34]. Closed systems usually involve a single culture step from initial seeding up to dissociation and formulation of the product, meaning that passage number becomes an irrelevant parameter, and the need to standardize based on cumulative population doublings comes into play. It is important to understand that the identity and potency of a well-defined MSC product will change as the number of population doublings increases, such that the number of acceptable cumulative population doublings should be considered in developing the critical quality attributes (CQA) of the drug product.

Differences in these systems render the use of passage number redundant, a system that has been used extensively to evaluate in vitro cell age in the manufacturing of MSCs. It is therefore important that we move towards an accepted measure of cellular aging for comparative purposes in our clinical trial design and reporting. The authors propose that, irrespective of the culture procedure, this parameter should be reported as population doubling levels (PDL), demonstrating the in vitro age of the expanded cell. For this information to be relevant and taken in context, there is a need to report the culture system employed such as 2D cell culture, as well as named starting material such as tissue explants, freshly dissociated cell suspension or commercial starting material. Importantly, high cumulative PDL links to replicative senescence and reduced immunomodulatory functionality in MSCs, particularly related to a loss of IDO activity [44,45].

### 3. Defining critical quality attributes

Safety, quality and clinical efficacy are at the forefront of consideration in the development of MSC products. CQA are defined in the International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use (ICH) Q8(R2) guideline as a "physical, chemical, biological, or microbiological property or characteristic that should be within an appropriate limit, range, or distribution to ensure the desired product quality" [46].

CQAs are more finely distilled in the US Code of Federal Regulations (21CFR610) as safety, purity, identity and potency [47]. Safety CQAs are standard across the cell therapy field and often rely on compound methods for testing sterility, mycoplasma and endotoxin. Bacterial contaminates such as endotoxins, including lipopolysaccharide (LPS), modulate the immune plasticity of MSCs and importantly can impact potential therapeutic efficacy [48]. Likewise, infection with mycoplasma can dramatically change the secretome of MSCs and their immunomodulatory potential, including their ability to modulate B cell function and enhance their T cell suppressive effect [49,50]. However, in defining other CQAs, MSC therapy in AD patients is challenged by the inherent biological variability and lack of clarity surrounding CQA definitions. As stated within the EU guidelines there is a need to design these CQAs on a case-by-case basis and it is the drug developer's responsibility to understand the impact of intrinsic

and extrinsic factors on the safety and quality of their product in such parameter designs and in setting limits [51].

Quality control (QC) release testing is constantly evolving with the development of somatic cell therapy products in general. The advent of new technologies that are pharmacopoeia compliant, such as PCR based mycoplasma testing and fast sterility evaluation, are paramount to the success and commercial adoption of cell therapies in AD patients. In addition to such safety parameters, additional identity features are needed for QC (largely related to cell surface markers determined by flow cytometry and closely tied to the minimal criteria for MSCs [9]). It is here that we see marked differences in the number and choice of markers used to identify the product when looking at the literature. In a recent MA of MSC preclinical studies in ADs only 6.7% of the evaluated studies reported all five ISCT criteria in the minimum definition of an MSC [52]. While this relates specifically to preclinical studies, this mix and match approach can also be seen in some reported clinical trials with MSCs for different indications.

Implementation of the new Substance of Human Origin (SoHO) legislation, that will be enforced in 2027 by the EMA, will also significantly change quality considerations for clinical trials, especially those led out of academic centers [53]. The need for long-term traceability for raw materials, including media supplemented with human components such as platelet lysate and human serum albumin will significantly impact product manufacturing and how we move these products to market. The use of human components in MSC manufacturing whether in the media, attachment solution or excipient mix for cryopreservation and delivery to the patients will result in changes to raw material suppliers and the need for comparability studies.

Potency represents one of the key CQAs of an MSC product. It should be stated that utilization of potency assays, and the interpretation of these assays alike, is considerably different between academic/early and late stage/commercial developers. Assays regarding potency evolve with the establishment and clinical development of the MSC product, greater understanding of the clinical MOA and the increasing need for assays to comply with robustness and ability to tech transfer that comes with scaling and commercialization. In 2016 the MSC-ISCT Committee released a position statement emphasizing the need to consider a matrix approach to potency assay development to encompass the multifactorial response of MSCs to administration into patients [12]. This approach is particularly relevant and of value to AD applications where the disease is heterogenous and involves multiple inflammatory and autoimmune pathways in vivo. It is widely accepted that the patient's own milieu strongly impacts responsiveness to MSC therapy and mimicking this in a meaningful way in vitro for the purposes of QC can be challenging. Potency assays could therefore be employed to evaluate MSC response to inflammatory priming, reflecting the capacity of the MSC product to respond to the inflammatory environment characteristic of a patient with a given AD [28,54]. Robb et al. [55] and Martin et al. [56] highlighted the need in recognizing the role of the patient and their individual milieu in modulating the MOA of MSCs. This also extends to the recognition that use of a potency assay for one intended application and one AD disease type may not be relevant to another disease application, even if both diseases fall under the umbrella of ADs. This is exemplified by the Food and Drug Administration's (FDA) decision on Mesoblast's remestemcel-L product for the treatment of Graft versus Host Disease requesting an additional clinical trial in order to show efficacy of the MSC product, and to provide further scientific rationale correlating the relationship of the chosen potency assay to the product's biologic activity [57].

A recent guidance document from the FDA highlights the challenges of developing potency assays in the treatment of diseases where the MOA is not defined to one pathway or linked to a particular biomarker [58]. This document sets forth a template for what a
potency assay should encompass and the quality of said assay or matrix of assays, including the need for a validated, quantitative assay(s) with predefined acceptance and rejection criteria with appropriate reference materials, standards and/or controls. Furthermore, the biological activity(ies) indicative of potency should be specific to the product. Here the guidelines state that as potency is "determined based on individual product attributes" the suitability of those tests will be evaluated on a case-by-case basis [58]. The document further highlights that that "no single test can adequately measure those product attributes that predict clinical efficacy". Rather clinical efficacy will be the result of "adequate and well-controlled investigations conducted with a consistently manufactured product" and here potency assays represent an important part of product characterization testing that are used to establish that consistent manufacturing protocol [58].

This is a consideration further extending to the use of concomitant medication that may impact the ability of the MSCs to exert their effects. In the context of AD patients, the simultaneous use of immunosuppressants or high dose steroids should be discouraged, due to the dampening effect on the patient's immune system, thereby blocking the immunomodulatory multifactorial effects of MSCs upon administration. Likewise, for those patients having received immunosuppressant or dampening treatments prior to MSC therapy, a wash out period to clear remaining drug from the body would be recommended prior to MSC administration.

## 4. Formulation, dosing and route of administration

Within an academic setting, with small cohorts of patients, closely linked to the academic hospital in which products are, in many cases, manufactured locally, there are the options for procedures such as dosing to a particular patient body weight (within a good manufacturing practice [GMP] facility) and the administration of freshly cultured cells. While it is accepted that these approaches may offer an optimal cell suspension, they are less practical when considering commercialization of such cell products. This change in manufacturing approach during upscaling and adapting manufacturing processes for Phase III clinical trials and commercialization likely contributed to some of the loss in efficacy seen with these larger trials that typically are led out of industrial settings.

Realistically for a commercial product to be financially and logistically viable for use in sites globally, cells will need to have been cryopreserved, requiring a careful validation of the impact of this cryopreservation on their functionality. Likewise, autologous MSC therapies offer limited value for ADs as discussed above. Allogeneic, off-the-shelf, therapies offer greater accessibility to AD patients with shorter lead time to patients for use in disorders where time is of the essence to treat within a certain window of disease state. Such products offer a financial solution to a costly manufacturing process by allowing upscaling of batch sizes, optimization of QC testing, and thereby reduced cost to the patient/healthcare system.

Recently, there has also been a move towards fixed dosing strategies for some intravenously delivered MSC drug products [34]. The same approach has also been evaluated in diseases where local administration has been employed (MA review [59]). It is recognized that fixed dosing offers distinct advantages in reducing the need for specialist facilities at the clinics to dose specifically to the patient, for example counting of cells, and improves standardization in handling of the product by individual clinics where staff may not be specialists in cell therapy products. The impact of this approach must be weighed against the therapeutic efficacy declared regarding the product itself. Dosing of the MSCs fits with the individual product, and an extrapolation of the literature should not be employed without further risk analysis. Phase I/II dose escalation studies with Data Safety Monitoring Board (DSMB) evaluations between each dose cohorts are the optimal way to establish the safety and most

efficacious dosage where first in man studies are being performed, with further development inside confirmatory randomized controlled trials or single arm studies (dependent on the frequency of the AD being evaluated). All the previously discussed factors including tissue cell source, donor age and manufacturing method will impact the dosage question with respect to both safety of the patient and efficacy of the product. A MA of fixed dose versus dosing to size of fistula in the treatment of perianal fistulas in CD demonstrated that efficacy directly correlated to a size-based dosing strategy [60]. Therefore, consideration must be given to how we standardize dosing strategies to improve commercialization potential and accessibility to patients, so that specialist cell therapy centers are not needed for such treatments, while ensuring the safety and efficacy of the treatment itself.

MOA strongly relates to route of administration. Animal models have demonstrated differences in persistence of MSCs with administration route and the impact of cryopreservation prior to delivery. Use of *in vivo* animal models to delineate MOA translatable to human disease are hampered due to the heterogeneity in AD pathogenesis and patient population. However, such studies are relevant in determining the *in vivo* biodistribution and toxicological profile of an individual cell therapy, and this work should be conducted using the clinical grade MSC product, not a research and development equivalent.

With respect to the clinical context, it is now widely accepted that the fate of MSCs upon intravenous infusion is to be rapidly cleared from the patient's system whether through efferocytosis, complement attack or apoptosis. The role of the patient's own immune repertoire and response to this "hit and run" effect upon exposure to the MSC product is central to the tolerogenic response that is evident in ADs such as T1D [34,61,62]. However, persistence may be longer for those cells administered intramuscularly [63]. This is a phenomenon also posed as directly relevant to the choice and consideration placed on a potency assay for the product as previously discussed by the MSC-ISCT committee [64].

## 5. The search for biomarkers

The search for biomarkers indicative of AD patient response to MSC therapy and MOA *in vivo* remains somewhat elusive. It is widely considered that data achieved from animal studies in different AD disease contexts offers limited information with respect to MOA due to species variation in the immune repertoire and inherent differences between experimental AD models and clinical disease. Robust biomarkers specific for ADs have not been reported to date, and caution is placed on the use of one relevant biomarker due to the heterogeneity of ADs. The use of inflammatory biomarkers in AD patient stratification for treatment inclusion criteria warrants further investigation.

Prediction of response and refinement of our target populations for treatment with MSCs is an area where further understanding of individual MSC products is required. A key example of this has been the recent discussion regarding the use of MSCs for the treatment of MS [65]. In this multicenter clinical trial, the authors caution the use of autologous BM-MSCs for the treatment of the disease, citing no effect on the inflammatory parameters. The authors succinctly address the need for more properly designed Phase II clinical trials for the disease addressing the autologous or allogeneic source of the MSCs, the delivery method (intravenous or intrathecal) and the dose and frequency of dosing for defining how these MSCs could be effectively used in the treatment of progressive MS.

Consideration to the sex of the patient and hormonal balance has also received attention for use of MSCs, with a recent evaluation of predictive factors influencing responsiveness to MSC therapy in type II diabetes demonstrating that male patients are more likely to respond to treatment due to higher levels of testosterone [66]. These
early, but interesting data, confirm the need to define the patient population we are investigating and consider both sex and ethnicity in our trial design and data interpretation.

## 6. Trial design and statistical considerations for the clinical evaluation of MSC in AD

The clinical evaluation of therapeutic procedures relies on clinical experiments whose design and methods aim at controlling variability to characterize true treatment effects towards reproducibility and causal inference. With the goal of the highest level of evidence, the main aspects of the evaluation approach are the experimental design of the clinical study (prospective/retrospective, controlled/uncontrolled, randomized etc.) and the standardization of any procedures in the patient's management (selection, interventions and exposures, measurements, follow-up, etc.). Both evaluation of MSCs, being an ATMP, and the clinical settings of AD challenge such an experimental approach on several aspects. Planning, conduct and eventual reporting of clinical studies evaluating MSCs for ADs should consider such variables to strengthen the level of evidence available. These considerations related to MSC therapy and AD overlap with those of rare disease and transplant procedures, which usually come with relatively small sample sizes and the requirement of multisite trials in expert centers and with pluri-disciplinary care, paving the way for center-mediated effects [67–69].

### Population

Most ADs are complex systemic diseases with high heterogeneity between patients, challenging the characterization of standardized target populations in clinical trials [70–72] as well as disease activity measurements during the trials (endpoints). ADs often exhibit disease-specific systemic manifestations, with multiple localizations with the definition of remission or relapse also varying according to the AD type. In some cases, multiple definitions co-exist for a given disease, some relying on clinical variables only, or combining biological markers. Baseline variability between participants could indeed be responsible for potential confounding bias with respect to the treatment effect later in the trial.

To address such complexity, activity measurements have usually been defined for each AD type as either global or organ-specific, for example the SELENA-SLEDAI and the BILAG indices for SLE [73–76]. The use of such indices in characterizing and selecting the trial population (eligibility criteria) contributes to the control of baseline variability between participants and provides useful information for inter-study comparability. Additionally, study reports should include minimal patient demographics including age and sex, as well as ethnicity. If relevant, outcomes may also be described by subgroups defined according to these clinical parameters, as such variables have been shown as potential predictive markers [66]. Further analyses to identify predictive biomarkers, such as interaction tests, may be conducted, provided sufficient sample size.

### Endpoints

Similar considerations apply to the definition of study efficacy endpoints quantifying the treatment effect. Within the scope of this article, a ubiquitous primary endpoint for clinical efficacy assessment in trials cannot be identified and shared across all possible AD indications and jurisdictions. Overall, the use of validated disease-specific clinical activity indices should be recommended to assess clinical efficacy, allowing reproducibility of therapeutic assessment. Moreover, in the absence of consensus for a single index, it is useful to include commonly used validated indices as secondary endpoints to allow inter-trial comparability and information sharing. Of note, the observation window, or measurement timepoint, for each endpoint is a key feature to be reported to ensure reproducibility.

We acknowledge that these disease activity indices are not specific to MSCs and, in some cases, validated or historical primary endpoints, supported by regulatory agencies, fall somewhat short from accounting for the MOA and clinical output (disease-modifying treatment versus disease-reversal, add-on to standard of care versus replacement of standard of care). For instance, hemoglobin (Hb) A1c in T1D is relevant for interventional drugs that are designed to reverse disease status or to replace insulin therapy. MSCs in such a context cannot be evaluated using the same endpoints. Where MSC therapy is designed to be used in conjunction with other treatments, such as insulin, in the case of T1D, and the cells are proposed to slow disease progression, insulin would be provided alongside MSCs, meaning that in individuals with good glycemic control, a fluctuation in HbA1c is unlikely to be seen. Biological surrogate efficacy endpoints (biomarkers) have been proposed and should be included as study endpoints to inform the specific mechanisms [77]. They should be clearly defined in study reports (biomarker, sampling timepoint, analysis methods etc.) to ensure reproducibility.

However, significant debate persists regarding the ability of disease activity indices to, as intermediate outcomes, (i) capture minimal clinically meaningful effects in trials [78] and (ii) ensure research reproducibility [79,80]. This is exemplified in the case of SLE, where numerous composite multidimensional activity indices co-exist, sometimes including physician or patient subjective assessments, leading to difficult interpretation of scale variations and limited comparability across studies [74,81–84] despite strong efforts of standardization by co-operative groups over the years [85–88]. Therefore, whenever possible, simpler and more stringent endpoints should also be included in the trial protocol and reported, such as end-stage renal disease - ESRD in SLE, other well-defined organ failure or mortality [78]. It is acknowledged that the study timeframe may often not be compatible with the required observation window for such late onset disease events, except for trials targeting very advanced patients.

Overall, in direct relation with the primary endpoint, the estimate used to evaluate the treatment effect needs to be defined explicitly with its attributions: treatment (description of the treatment [see sections above]), target population (including subgroups and handling of intercurrent events such as treatment discontinuation, dose adjustments, etc. in defining the population of analyses), endpoint (efficacy variable) and corresponding population-level summary to be computed and reported [89]. Finally, safety endpoints should be carefully collected and reported in the most informative and transparent manner (number of events per patients, grade/severity, management, outcome).

### Trial design

The combination of rare diseases with dire prognosis and costly MSC treatment challenges the clinical trial design and the conventional paradigm of drug development before market authorization (Phases I–III) on several levels. Specifically, large samples can be hard to achieve in the setting of rare AD diseases over a reasonable study timeframe, even in multicenter studies. Therefore, a lot is asked from early clinical trials, from initial safety evaluation to solid efficacy assessment. Various approaches have been proposed to tackle these issues to some extent, although a specific route needs to be tailored to the clinical AD setting taking into account rarity of disease, existing standard of care, disease prognosis, etc.

While acknowledging the constraints of such a setting in terms of ethical, logistical or economic considerations, the undeniable benefits and superiority of a randomized controlled design over single-arm studies prevail for causal inference on treatment effects [90,91]. The use of external data, such as historical controls or real-world evidence, represents another approach to enrich trial observations in the setting of rare diseases, with numerous statistical developments proposed [92–95]. Real-world data analysis resulting from registries
in academic health centers and hospital exemption settings represent an attractive option, which, if implemented at the scale of MSCs expert clinical centers, can provide valuable data and experience, complementing and enriching interventional trial data using dedicated designs and analysis methods [96]. Indeed, randomized controlled experiments do remain a gold standard for therapeutic evaluation which should be recommended in the setting of MSCs to provide the highest level of evidence [90,97].

With this in mind, adaptive designs, which aim at addressing ethical concerns and resource optimization (notably in terms of patient accrual and trial duration) may offer attractive options. Such designs usually rely on sequential analyses to inform early decisions on adapting aspects of the design, e.g., dose, early stopping for efficacy/futility/toxicity, dropping/adding arms in platform trials, randomization ratio, eligibility criteria, etc., while ensuring the design operating characteristics [98,99]. For example, group sequential designs, based on an interim analysis, allow sparing accrual and futile exposure of patients to an unpromising treatment or toxic intervention; conversely, they may also allow the early graduation of a treatment deemed promising based on accumulated evidence and moving on to the next step of development, e.g., patient crossover from a control arm, next trial or trial registration. In randomized trials, response adaptive randomization consists of adjusting the randomization ratio during patient accrual based on the previous patient's response favoring allocation to the most promising arm for the upcoming patients. Such an approach has been advocated to facilitate acceptability of randomized schemes in settings with a high therapeutic need. Specific constraints related to small samples sizes at time of adapting the randomization ratio have been recently addressed [100,101]. In the perspective of personalized medicine, adaptive enrichment designs usually rely on the assessment of some patient characteristic(s) and their interaction with a marker of response to treatment in order to adapt, during the trial itself, the inclusion criteria or treatment allocation to specific subgroup(s) of patients deemed the most treatment-sensitive based on previous patient data [102]. Of note, adaptive approaches require the use of dedicated statistical methods to control the risk of bias in treatment effect estimation [103,104]. Seamless designs (Phase I/II, Phase II/III, Phase I/II/III) aiming at expedited development with limited sample sizes have also been proposed, some relying on the joint assessment of toxicity and efficacy [105,106].

## Sample size, error rates and power considerations

Constraints on sample size, due to a rare AD population or the cost of the MSC intervention, should not preclude transparency on the level of evidence with regards to the amount of information generated by the chosen design. As in any clinical evaluation, irrespective of phase of development, planned error rates and power considerations with sample size calculation should be reported, allowing readers to assess the amount of evidence, at least for the primary endpoint. Bayesian designs may represent an alternative approach when conventional frequentist methods fail to allow feasible schemes [107], although they also require explicit evaluation of error rates [108–110]. In all designs, even small single-arm studies, precision measures should be reported in the form of estimate intervals, e.g., confidence interval and Bayesian credibility interval [108,109,111].

## Handling heterogeneity

Cell therapies, involving specialist tertiary care facilities and center-specific patient management practices fall into the category of complex interventions, often leading to center-specific effects [67]. Variability of MSC products, as well as the complexity of ADs, introduce sources of heterogeneity among patients, as detailed above. In clinical trials, heterogeneity may influence patient outcome, through 2 principal mechanisms: (i) variability in outcomes according to a specific feature independent of the treatment effect, e.g., better prognosis in a sub-population, irrespective of treatment or (ii) variability

in the treatment effect itself. These two types of heterogeneity may also co-exist. Besides inter-patient variability, heterogeneity may be quantified if its potential sources are known, allowing for the evaluation of outcomes according to proxy variables such as clinical centers, cell source etc.

Heterogeneity on the treatment effect may be difficult to detect in uncontrolled trials. In the presence of a control group, treatment related heterogeneity is seen as an interaction between treatment and factors such as treatment by center interaction or treatment by age group. This means that the effect of MSC therapy varies in magnitude (quantitative interaction) and/or direction (qualitative interaction) depending on the heterogeneity variable. It is crucial to identify such heterogeneity, as analysis models which do not take this into account will most likely average the treatment effect across heterogeneous groups, limiting the accuracy of interpretation.

While it may not be feasible to account for all sources of heterogeneity in each trial through statistical modeling due to limited sample sizes, subgroup analyses targeting a few variables, can be prespecified in the clinical protocol and statistical analysis plan. Moreover, known and relevant sources of heterogeneity which can be summarized into proxy variables, such as center, cell source or disease status, should be collected for each patient and reported, with a view towards future data sharing and meta-analyses, allowing larger sample sizes and appropriate statistical methods to address sources of heterogeneity [112–114].

## Meta-analyses

Systematic reviews (SR) and MA aim at synthesizing information across multiple studies sharing a common objective, and can be applied to interventional studies and the assessment of efficacy or safety of an intervention or to observational studies [115,116]. While SR provides a qualitative summary of evidence, MA includes a quantitative synthesis of available numerical data. MA may be conducted from aggregate quantitative data, such as summary results reported in publications (mean differences, odds ratios, hazard ratios and their confidence intervals), or individual data, that is to say, the patient's data at the individual level, when available. Individual patient data—MA is the preferred, more informative form of MA, relying on the complete information on outcomes of interest, including their variability. They represent a powerful tool for evidence synthesis to guide decisions when confronted with scattered evidence such as the rare disease setting [117–120] and for advanced therapies [121]. Moreover, patient characteristics and/or known heterogeneity factors can be accounted for in a straightforward manner with individual data. Nevertheless, to allow for the generation of quality evidence synthesis, the quality of initial reports of studies and the existence of shared data is key [122,123]. In the cases of both observational studies and clinical trials, the poor quality of reporting can directly impact the risk of bias in SR-MA and their interpretability and should be systematically assessed and reported in MA reports [124]. With the perspective of future MA and strengthened evidence [117,118], reporting standards are crucial to assess the feasibility of evidence synthesis and exchangeability of settings across studies, at multiple levels, notably, the patient population and eligibility criteria, treatments/intervention, endpoints including their observation window, statistical methods and estimators, precision measures and sample size for each analysis.

## Conclusion

Based on the discussion held at the ISCT held a 2-day workshop entitled "Cell Therapies for Autoimmune Diseases: MSCs from Biology to Clinical Application" in Paris, France the authors recommend the following guidelines for reporting of clinical trial design and product characterization when submitting for peer-reviewed publication (Table 1, Figure 1). Such recommendations cover the full
<table><caption>**Table 1**
Recommendations for minimal reporting criteria for mesenchymal stromal cell clinical trials in the treatment of autoimmune diseases.</caption><thead><tr><th>Criterion</th></tr></thead><tbody><tr><td><strong>Terminology</strong></td></tr><tr><td><ul><li>The term "mesenchymal stromal cell" should be used where MSC is being employed as an immunomodulatory drug in the treatment of an AD</li><li>Tissue source of the MSC should be clearly stated and used as a prefix in the abbreviation of the drug's active pharmaceutical ingredient, e.g., umbilical cord mesenchymal stromal cell, UC-MSC</li><li>Statement of whether the cell product is a heterogenous stromal cell population or selected, e.g., specific marker selection or method-based selection of sub-population of MSC</li><li>Relevant donor MSC characteristics such as sex of donor, age of donor, donor screening and selection criteria including health status and viral testing parameter and acceptance criteria</li><li><strong>Characterisation of MSC drug product</strong><ul><li>Positive cell markers should be used as identity markers for the product, such as CD90+, CD73+, CD105+, with acceptance criteria for release of drug product stated</li><li>Negative markers should be used as identity markers for the product, such as CD45-, with acceptance criteria for release of drug product stated</li></ul></li><li><strong>Potency of MSC drug product</strong><ul><li>Assay or assays for potency should be utilised for demonstrating functionality of drug product at the point of release. Note the chosen assay(s) should be relevant to the proposed mechanism of action for the drug product in the context of the AD indication for which the product is intended</li><li>Indicators of cell health should be reported including cell number/recovery and viability at point of release</li></ul></li><li><strong>Critical manufacturing parameters</strong><ul><li>Critical process parameters impacting quality, functionality and safety of product should be defined; this would include isolation technique, whether the cells were manufactured in a 2D or 3D system, the in vitro age of the cells, whether they are cryopreserved or fresh and the excipient mix in which they are formulated in</li><li>Batch number of cells used in clinical trial should be stated, and where multiple batches are employed, it should be stated if any comparability between batches in terms of CQAs was performed. In all cases where multiple batches are employed in the same trial it should be stated which patient received which batch in the reported raw data sets</li></ul></li><li><strong>Clinical trial design</strong></li><li>Product, formulation and administration<ul><li>Postformulation processing steps such as post-thawing washing of product, re-formulation for dosing to patient body weight should be clearly stated including any further testing parameters such as evaluation of cell viability prior to administration</li><li>Route of administration should be clearly stated</li><li>Autologous or allogeneic source of cells should be stated. For the application in AD indications, an allogeneic source is recommended</li><li>Dose, cell concentration and formulation including named excipient mix, and rate of administration should be reported, e.g., 2 (106 cells/kg BW, intravenously infused at rate of X cells/minute in a total volume of X mL</li><li>For placebo or other control groups, the vehicle used should be clearly stated</li></ul></li><li>Trial design and statistical analysis<ul><li>Clear consideration and reporting of patient population selected and eligibility criteria</li><li>Study endpoints relevant to phase of trial and study design. Consideration to specific AD guidelines where relevant should be considered here, e.g., Type 1 diabetes [126]</li><li>Clear consideration and justification of sample size and statistical power considerations based on phase of trial, endpoints and AD indication</li><li>Transparent reporting in trial design and analysis performed</li><li>Transparent reporting in line with relevant guidelines such as consort, define, ACE [109–110,123,125]</li></ul></li></tbody></table>

AD, autoimmune disease; BW, body weight; CQA, critical quality attribute; MSC, mesenchymal stromal cell; UC, umbilical cord.

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Schematic presentation of major considerations in the development of mesenchymal clinical trials for the treatment of autoimmune diseases. CQAs, critical quality attributes; MSCs, mesenchymal stromal cells. Created with Biorender.com. (Color version of figure is available online.)</figcaption></figure>
spectrum of clinical trials and are agnostic to stage of clinical development. These guidelines also take into account previous recommendations from the ISCT [9, 11–13, 64].

We further emphasize the need to comply with standard guidelines for protocols and reporting of trials, towards greater transparency and reproducibility of research [121]. Nowadays, guidance and extensions exist for various specific settings, e.g., CONSORT and SPIRIT guidelines for randomized controlled trials, DEFINE and DEFINE-SPIRIT for dose finding trials, ACE for adaptive trials and CONSORT-ROUTINE for randomized controlled trials conducted using cohorts and routinely collected data.

There is an absolute need to be transparent in defining MSC products and the mode in which they are being employed for investigative clinical trials in AD patients. There is a discord between the need to maintain confidentiality of product know-how in developing a commercial product versus the transparency required for the evolution of MSC product development by providing sufficient information for MA and individual critical evaluation of peer-reviewed studies in an accurate manner.

The safety profile of MSC infusions has lulled developers into a false sense of security that safety equates to efficacy. While the safe administration of MSCs is a luxury that allows the exploration of the use of these cells in a variety of AD patient populations, and other disease contexts, it does not reduce the need for a controlled understanding of each of the variables involved in the generation, storage and utilization of said drug product.

Standardization in reporting of clinical trials using MSCs for ADs is essential so that the reader can interpret data correctly and ensure that MA are generated from comparable datasets that provide meaningful knowledge and guidelines for those working in the field. In this article the key considerations for the effective design and execution of a clinical trial using MSCs for ADs is discussed with recommendations for minimal criteria for data reporting to move forward the development of MSC therapy and see a successful transition of these drug products to market, addressing a significant unmet clinical need in ADs.

## Disclosures

LCD is an employee of NextCell Pharma AB and QVance AB. LCD is a co-founder of QVance AB and owns CellTherEx Consulting, Stockholm, Sweden. DGO is a member of the Advisory Board of TiGenix SAU and an inventor of patents 10157355957US, TiGenix/Takeda and US11/167061, TiGenix/Takeda. DGO additionally receives consulting fees from Takeda Pharmaceuticals USA, Inc. No other authors have disclosures of interest.

## Acknowledgments

The authors acknowledge all participants and speakers at the ISCT workshop entitled "Cell Therapies for Autoimmune Diseases: MSCs from Biology to Clinical Application" held in Paris, France from 25 to 16th January 2024 for their contributions during the discussions. All participants are listed here: A. Ambrosino, C. Antunez Rodriguez, Y. Avihingsanon, M. Badoglio, L. Bernard, C. Beuvon, R. Bhave, G. Boffa, D. Bogdanić, F. Bombaldi, L. Bugarel, T. Bukreieva, C. Cardoso, C. Chanswangphuwana, N. Chevallier, A. Comerford, L. Coquelin-Salsac, S. Cordeil, J. Dulong, A. Ekblond, D. Freund, I. Frument, L. Galla, V. Girault, M. Grégoire, R. Guizem, M. Haack-Soerensen, A.C. Hick, M. Hirai, L. Højgaard, N. Hung, M. Jorvid, M. Juhl, J. Kastrup, B. Khalaj, S. Kharazi, P. Le, A. Le Blanc, M. Lebrin-Serrentino, A. Lercara, A. Maria, S.M.S. Marmol, T. Marie, G. Maroun, V. Mazzone, P. Mocquot, M.Naiimi, V.D. Nguyen, R. Nica, D. Noel, S. M. Olave, A. Olearczyk, M. Ouzin, J. Park, N. Patel, D. Patrone, A. Pizzarossa, S. Preciado, G. Pugnet, M. Puyade, M. Gómez Redondo, T. Ritter, H. Rouard, B. Sadeghi, S. Shinshikova, P.

Silva Couto, T. Sriprasart, H. Stensmyren, T. A. Takahashi, M. Thirion, D. Vargas, M. Velier, L. Viala, B. Wagner, R. Yang, Y. Zahedi.

## Funding

FSG acknowledges funding from Instituto de Salud Carlos III (ISCIII) RD21/0017/0003, RD21/0017/0006, RD24/0014/0014, RD24/0014/0034, co-financed by the European Union through the Next Generation EU Funds and the Recovery, Transformation and Resilience Plan.

## Contributions

LCD, LB and DFB wrote the manuscript. All authors reviewed and approved the final manuscript.

## Article History:

Received 26 March 2025

Accepted 9 June 2025

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