The issue of heterogeneity of MSC-based advanced therapy medicinal products-a review

Abstract

Mesenchymal stromal stem cells (MSCs) possess a remarkable potential for numerous clinical applications due to their unique properties including self-renewal, immunomodulation, paracrine actions and multilineage differentiation. However, the translation of MSC-based Advanced Therapy Medicinal Products (ATMPs) into the clinic has frequently met with inconsistent outcomes. One of the suspected reasons for this issue is the inherent and extensive variability that exists among such ATMPs, which makes the interpretation of their clinical efficacy difficult to assess, as well as to compare the results of various studies. This variability stems from numerous reasons including differences in tissue sources, donor attributes, variances in manufacturing protocols, as well as modes of administration. MSCs can be isolated from various tissues including bone marrow, umbilical cord, adipose tissue and others, each with its unique phenotypic and functional characteristics. While MSCs from different sources do share common features, they also exhibit distinct gene expression profiles and functional properites. Donor-specific factors such as age, sex, body mass index, and underlying health conditions can influence MSC phenotype, morphology, differentiation potential and function. Moreover, variations in preparation of MSC products introduces additional heterogeneity as a result of cell culture media composition, presence or absence of added growth factors, use of different serum supplements and culturing techniques. Once MSC products are formulated, storage protocols play a pivotal role in its efficacy. Factors that affect cell viability include cell concentration, delivery solution and importantly, post-thawing protocols where applicable. Ensuing, differences in administration protocols can critically affect the distribution and functionallity of administered cells. As MSC-based therapies continue to advance through numerous clinical trials, implication of strategies to reduce product heterogeneity is imperative. Central to addressing these challenges is the need for precise prediction of clinical responses, which require well-defined MSC populations and harmonized assessment of their specific functions. By addressing these issues by meaningful approaches, such as, e.g., MSC pooling, the field can overcome barriers to advance towards more consistent and effective MSC-based therapies.

Keywords: advanced therapy medicinal product (ATMP); cell therapy; equipotency; heterogeneity; mesenchymal stromal (stem) cell; pooling.

FIGURE 1

Immunomodulatory mechanisms of MSCs. The modulation of immune responses by MSCs is exerted via numerous secreted factors and entities, such as cytokines, growth factors, extracellular vesicles and others. ATP–adenosine triphosphate; BMP–bone morphogenetic protein; CCL–chemokine (C-C motif) ligand; CX3CL–chemokine (C-X3-C motif) ligand; CXCL–chemokine (C-X-C motif) ligand; EGF–epidermal growth factor; FGF–fibroblast growth factor; HGF–hepatocyte growth factor; IL–interleukin; LIF–leukemia inhibitory factor; M1–type one macrophages; M2–type two macrophages; MIF–macrophage migration inhibitory factor; PDGF–platelet derived growth factor; TGF–transforming growth factor; VEGF–vascular endothelial growth factor.

FIGURE 2

The heterogeneity of MSC-based ATMPs can be broadly categorized into three groups: differences among donors, variations arising from the tissue source, and differences introduced by preparation and administration protocols.

FIGURE 3

An example of a matrix of assays to predict MSC potency. Morphological characteristics, cell surface marker expression, gene expression, soluble factor secretion, tri-lineage differentiation, lymphocyte proliferation, macrophage polarization and endothelial cell tube formation could be used to assess potency of MSC cell product lots.

FIGURE 4

Two strategies of MSC pooling currently used in therapeutic applications. MSCs from three donors are individually expanded and then pooled together for Stempeucel^® MSC bank generation. On the other hand, in MSC-FFM, MNCs from eight donors are pooled together before cells attach to the surface of tissue culture flasks. After expansion, cells are harvested and stored in cryovials to constitute a MSC bank.