PD-L1 Expression in Mesenchymal Stem/Stromal Cells: Impacts on Innate and Adaptive Immunity, Therapeutic Potential, and Biomarker Utility

Mesenchymal stem/stromal cells (MSCs) are multipotent progenitor cells with potent immunomodulatory properties, making them attractive candidates for treating inflammatory and autoimmune diseases. A key mediator of MSC-induced immunosuppression is programmed death-ligand 1 (PD-L1), a checkpoint molecule that interacts with PD-1 on immune cells to regulate immune responses and promote tolerance. This review synthesizes current evidence on the role of PD-L1 expression in MSCs, emphasizing its effects on both the innate and adaptive immune systems, its therapeutic potential, and its utility as a biomarker for MSC potency and clinical efficacy. We examine how PD-L1 modulates T cell activation, dendritic cell maturation, macrophage polarization, and cytokine profiles, including its role in exosomal contexts. Additionally, we highlight its synergistic interactions with other immune checkpoints and discuss its dual function as both a therapeutic effector and a dynamic biomarker. Finally, we explore its relevance in clinical contexts such as autoimmune diseases, graft-versus-host disease, sepsis, and transplantation and conclude with a discussion of challenges and future directions in harnessing PD-L1 for MSC-based therapies.