Review ArticleRegulatory perspective on in vitro potency assays for human mesenchymal stromal cells used in immunotherapy
Introduction
Mesenchymal stromal cells (MSCs) are multipotent cells that can differentiate into several cell types. MSCs can be isolated from various tissues and although their numbers are low and decrease with age, their potential to expand ex vivo may allow production of sufficient amounts for therapeutic use [1]. Human tissues from which MSCs have been isolated include bone marrow, adipose tissue, placental tissue, umbilical cords and embryonic stem cells [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. Importantly, these MSCs differ in their expression of surface markers and in their functional capacity after stimulation with proinflammatory mediators [2], [13]. As a consequence, comparison of the clinical effects of the various MSC products is hampered. To create a broader consensus for more uniform characterization of MSCs, the International Society for Cellular Therapy (ISCT) proposed three minimal criteria to define human MSCs: (i) MSCs should be plastic-adherent when maintained in standard culture conditions; (ii) MSCs should express CD105, CD73 and CD90 and should lack CD45, CD34, CD14/CD11b, CD79α/CD19 and human leukocyte antigen (HLA)-DR expression; and (iii) MSCs should be able to differentiate (in vitro) into osteoblasts, adipocytes and chondroblasts [14].
However, the nonspecificity of these criteria has been a topic of discussion ever since [15], [16]. One should at least bear in mind that, in spite of above minimal criteria, MSCs remain heterologous populations of cells with a variety of gene expression profiles, differentiation and expansion potential and phenotype, which are influenced by tissue origin, cell isolation and expansion procedures [14], [17], [18], [19].
In vivo, MSCs play a crucial role in peripheral tissue homeostasis (including blood vessels) and maintenance of the (hematopoietic) stem cell niche, mainly due to their potential to differentiate into various cell types and their secretion of several growth-promoting factors [20], [21]. It has been shown that MSCs preferentially home to damaged tissue, where they are thought to exert their biological action by direct cell-cell interactions through their surface receptors, but also by producing soluble factors [18], [22], [23], [24].
Apart from their tissue homeostasis and regeneration capacities, MSCs also have immunomodulatory abilities with potential therapeutic applications. This review article focuses on the in vitro testing of the functionality of these MSC medicinal products.
Section snippets
Immunomodulation by MSCs
The general immunomodulatory role of MSCs involves the orchestration of immunologic tolerance, next to the role of regulatory T and B cells and innate suppressor cells [25]. MSCs function via direct suppression of the activation, proliferation and effector functions of proinflammatory cells and the stimulation of various anti-inflammatory cell types to indirectly augment immune response regulation [20]. The most prominent MSC functions are exerted locally, although also some systemic effects
Clinical application
As said, MSCs have a diversity of physiological functions including the ability to migrate to inflamed tissue, differentiate into various cell types and secrete anti-inflammatory and tissue-renewing factors. Due to their broad immunoregulatory potential, clinical application of these cells is explored for a wide range of disorders with high immune activation, including graft-versus-host disease (GvHD), transplant rejection and autoimmunity [26]. Because MSCs harbor low immunogenicity (despite
In vitro assays to test for MSC functionality
As said, three minimal criteria for defining human MSCs have been proposed [14]. In addition, Wuchter et al. posed several criteria for GMP-grade MSCs [74]. However, although the criteria address MSC functionality, no concluding recommendations regarding potency assays are provided. This is in line with the notion that every MSC product is unique and that quality control assays should be justified on a case-by-case basis. Nevertheless, the following overview of potency assays to test for MSC
Selection and design of potency assays
Despite the common use of immune cell inhibition and suppressor cell activation assays, there is still a lot of uncertainty with respect to which assay would be most appropriate to assess potency of MSC products. For instance, using a potency assay combining both MLR and mitogen stimulation, Ketterl et al. observed that some MSC batches showed little suppression in the mitogen-stimulated assay, while showing a much higher suppression in the MLR [5]. This would indicate that a specific batch
Future perspectives and conclusion
The majority of MSC medicinal products in clinical development are indicated for GvHD or transplant rejection. Nevertheless, several MSC products are currently tested for treatment of chronic inflammation in autoimmune diseases, for example, systemic lupus erythematosus, systemic sclerosis, type 1 diabetes mellitus, multiple sclerosis, Crohn's disease and rheumatoid arthritis [2], [19], [21], [32], [88], [89]. Although most potential MSC products for these indications are still in experimental
Acknowledgments
We thank Dr. Carla Herberts for useful comments and suggestions on the manuscript. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Disclosure of interests: The authors have no commercial, proprietary or financial interest in the products described in this article.
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