Production and quality testing of multipotent mesenchymal stromal cell therapeutics for clinical use

I n 1867 Cohnheim proposed bone marrow (BM) as a source of fibroblasts involved in wound-healing processes. In the 1970s, Friedenstein reported the colonyforming unit fibrobast describing nonhematopoietic fibroblast-like cells from the BM with self-renewal capacity, multilineage differentiation potential in vitro, and hematopoietic support in vivo referring to them as stromal cells. Analogous to the hematopoietic system, Caplan popularized the term mesenchymal stem cells, and the mesodermal lineage differentiation prompted their clinical evaluation in cancer and osteogenesis imperfecta patients. Increasing interest stimulated a number of diverse studies yielding heterogeneous results. Minimal criteria, therefore, have been suggested to try to characterize MSCs, then depicted as “multipotent mesenchymal stromal cells,” based on a variety of properties including adherence (plastic-adherent fibroblastoid cells), cell marker expression (positive for CD105, CD73, CD90; negative for CD45, CD34, CD14 or CD11b, CD79 or CD19; and human leukocyte antigen [HLA] Class II), and differentiation in vitro (i.e., osteoblasts, chondrocytes, and adipocytes). MSC therapies have been applied in numerous clinical trials targeting various diseases such as ischemic (i.e., stroke, myocardial infarction, limb ischemia), pulmonary (i.e., respiratory distress syndrome, emphysema), and neurologic pathologies (i.e., amyotrophic lateral sclerosis, spinal cord injury), as well as skeletal degeneration (i.e., osteoarthritis, degenerative disk disease), with a specific focus on immunopathologies such as graft-versus-host disease (GvHD), Crohn disease, or multiple sclerosis. Additionally, MSCs have emerged as a versatile and promising candidate in the field of regenerative medicine and tissue engineering (e.g., to be seeded on scaffolds before implantation). Despite the increasing application of MSC-based therapies in a wide range of disease settings, sustainable translation and marketability may be limited, due to the complexity of MSC products. Variable tissue sources and heterogeneity between donors as well as within the MSC preparations (see below) are main drivers of the complexity that is hampering the identification of their molecular mechanism(s) of action (MoAs). Current concepts of the MoAs include production and/or secretion of cytokines and trophic factors, as well as secretion of microvesicles that contribute to the immunomodulatory and regenerative potential of MSC. Without clear evidence of specific MoAs that can be linked to clinical effects, it is difficult to establish consistent manufacturing conditions, quality criteria, and assays that predict efficacy. Potency is a therapeutically relevant biologic function that can be measured quantitatively by in vitro or in vivo assays that could be linked to properly controlled clinical efficacy data. Potency assays are critical to control for quality and consistency of MSC-based products; however, to date, there are few published data on potency assays for MSCs. The majority of proposed potency assays refer to their immunomodulatory and proangiogenic capabilities; however, a direct link to clinical efficacy is still lacking. We examine the current challenges and concepts as well as discuss possible future options for the production and quality testing of MSC therapeutics.