Hallmarks of MSCs: Key quality attributes for pharmacology and clinical use.

Marketing approval for allogenic mesenchymal stromal cells (MSCs) by international regulatory jurisdictions including the US have been granted. Notwithstanding, the long-heralded clinical and commercial breakthrough for MSC products has never fully manifested. The withdrawal of an allogenic MSC product in Europe, based on inefficacious phase 3 results along with setbacks in industry-sponsored, advanced clinical trials of MSCs for COVID-19-related acute respiratory distress syndrome (ARDS) have dampened enthusiasm for MSC products. In this perspective, we highlight the hallmarks of MSC identity and potency, and how these can inform surrogate, sensitive critical quality attributes that correlate with clinical effectiveness in a variety of indications. We further highlight host-dependent pharmacological attributes of MSCs, which together with their critical quality attributes drive the observed clinical responses and thus impact the translational utility of MSCs. We provide a rational pathway to additional MSC regulatory approval and deployment for disorders with unmet medical needs.