The critical role of apoptosis in mesenchymal stromal cell therapeutics and implications in homeostasis and normal tissue repair

Abstract

Mesenchymal stromal cells (MSCs) have been extensively tested for the treatment of numerous clinical conditions and have demonstrated good safety but mixed efficacy. Although this outcome can be attributed in part to the heterogeneity of cell preparations, the lack of mechanistic understanding and tools to establish cell pharmacokinetics and pharmacodynamics, as well as the poorly defined criteria for patient stratification, have hampered the design of informative clinical trials. We and others have demonstrated that MSCs can rapidly undergo apoptosis after their infusion. Apoptotic MSCs are phagocytosed by monocytes/macrophages that are then reprogrammed to become anti-inflammatory cells. MSC apoptosis occurs when the cells are injected into patients who harbor activated cytotoxic T or NK cells. Therefore, the activation state of cytotoxic T or NK cells can be used as a biomarker to predict clinical responses to MSC treatment. Building on a large body of preexisting data, an alternative view on the mechanism of MSCs is that an inflammation-dependent MSC secretome is largely responsible for their immunomodulatory activity. We will discuss how these different mechanisms can coexist and are instructed by two different types of MSC "licensing": one that is cell-contact dependent and the second that is mediated by inflammatory cytokines. The varied and complex mechanisms by which MSCs can orchestrate inflammatory responses and how this function is specifically driven by inflammation support a physiological role for tissue stroma in tissue homeostasis, and it acts as a sensor of damage and initiator of tissue repair by reprogramming the inflammatory environment.

Keywords: Cell death; Mesenchymal stromal cells; immunosuppression.

Fig. 1

Mechanisms of MSC-mediated immunosuppression. Licensing of MSCs is essential to induce their immunosuppressive effects and can occur via stimulation with cytokines such as IFN-γ, TNF-α, IL-1β or TLR ligands present in the inflammatory microenvironment, or via direct cell–cell contact with activated cytotoxic T and NK cells. 1) MSCs licensed by soluble factors become potent immunomodulatory agents by producing an immunosuppressive secretome characterized by molecules such as IDO, PGE2, TGF-β, TSG-6, IL-10, IL-6, HGF, soluble HLA-G and IL-1RA and the activation of PD-1/PD-L1 and Fas/FasL signaling. In addition, the combination of soluble molecules present in the microenvironment can activate apoptotic pathways in MSCs and their subsequent efferocytosis by monocytes/macrophages. 2) MSCs licensed by cytotoxic cells undergo rapid caspase-dependent apoptosis and are efferocytosed by monocytes/macrophages, which become anti-inflammatory cells releasing similar immunosuppressive molecules. Apoptotic MSCs further contribute to immunosuppression by releasing caspase-dependent immunomodulatory factors. These licensing mechanisms result in efficient immunomodulation through the inhibition of T cells, NK cells, and DCs, M2 macrophage polarization and Treg expansion. Figure generated using BioRender.com

Fig. 2

Stromal cell apoptosis as an innate mechanism of tissue homeostasis. Following injury, monocytes and macrophages rapidly migrate to the site of injury, where they become activated and start recruiting additional immune cells. Subsequently, the tissue is infiltrated by CTLs and NK cells that release perforin/granzyme B-loaded cytolytic granules to induce the surrounding tissue stromal cells to undergo apoptosis. The phagocytosis of apoptotic stromal cells efficiently reprograms macrophages into a pro-resolving phenotype that orchestrates the resolution of inflammation and tissue repair responses. Figure generated using BioRender.com