Regulatory perspective on in vitro potency assays for human mesenchymal stromal cells used in immunotherapy

Abstract

Mesenchymal stromal cells (MSCs) are multipotent cells derived from various tissues that can differentiate into several cell types. MSCs are able to modulate the response of immune cells of the innate and adaptive immune system. Because of these multimodal properties, the potential use of MSCs for immunotherapies is currently explored in various clinical indications. Due to the diversity of potential MSC medicinal products at the level of cell source, manufacturing process and indication, distinct functionality tests may be needed to ensure the quality for each of the different products. In this review, we focus on in vitro potency assays proposed for characterization and release of different MSC medicinal products. We discuss the most used functional assays, as presented in scientific advices and literature, highlighting specific advantages and limitations of the various assays. Currently, the most proposed and accepted potency assay for release is based on in vitro inhibition of T cell proliferation or other functionalities. However, for some products, assays based on other MSC or responder cell properties may be more appropriate. In all cases, the biological relevance of the proposed assay for the intended clinical activity should be substantiated with appropriate product-specific (non-)clinical data. In case practical considerations prevent the use of the ideal potency assay at release, use of a surrogate marker or test could be considered if correlation with functionality has been demonstrated. Nevertheless, as the field of MSC immunology is evolving, improvements can be expected in relevant assays and consequently in guidance related to potency testing.

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.