Priming approaches to improve the efficacy of mesenchymal stromal cell-based therapies

Abstract

Multipotent mesenchymal stromal cells (MSC) have been widely explored for cell-based therapy of immune-mediated, inflammatory, and degenerative diseases, due to their immunosuppressive, immunomodulatory, and regenerative potentials. Preclinical studies and clinical trials have demonstrated promising therapeutic results although these have been somewhat limited. Aspects such as low in vivo MSC survival in inhospitable disease microenvironments, requirements for ex vivo cell overexpansion prior to infusions, intrinsic differences between MSC and different sources and donors, variability of culturing protocols, and potency assays to evaluate MSC products have been described as limitations in the field. In recent years, priming approaches to empower MSC have been investigated, thereby generating cellular products with improved potential for different clinical applications. Herein, we review the current priming approaches that aim to increase MSC therapeutic efficacy. Priming with cytokines and growth factors, hypoxia, pharmacological drugs, biomaterials, and different culture conditions, as well as other diverse molecules, are revised from current and future perspectives.

Pro-Inflammatory Priming of Umbilical Cord Mesenchymal Stromal Cells Alters the Protein Cargo of Their Extracellular Vesicles

“…The cargo of USMSC-EVs in this study contained many chemokines (CXCL5, CCL3, CCL4, CCL11, CCL20), which might attract immune cells to the site of injury. However, research has strongly shown that MSCs primed with pro-inflammatory cytokines (IFN-γ, TNF-α, IL-1β) show enhanced immunosuppressive abilities [14,[21][22][23]. Specifically, research using EVs secreted from pro-inflammatory primed MSCs has shown positive immunosuppressive effects by polarising pro-inflammatory immune cells into anti-inflammatory immune cells in vitro [24][25][26][27][28][29].…”

“…This study found no significant difference in growth kinetics of protein cargo between UCMSCs grown in normoxia and hypoxia, and research was divided on whether hypoxia has a stimulatory [30] or inhibitory [31,32] effect on MSC proliferation; albeit studies varied between the use of 1%-5% O 2 . Some groups have advocated for the use of hypoxia when growing cells in culture as it more closely mimics the oxygen levels that MSCs are exposed to in vivo [22], with neonatal-derived tissues rarely exceeding 5% in vivo [23]. The lack of change in growth may be because the cells were first isolated under normal oxygen (21%) conditions, and exposure to low oxygen (5%) at an earlier stage may be an influencing factor in cell proliferation.…”

Priming with inflammatory cytokines is not a prerequisite to increase immune-suppressive effects and responsiveness of equine amniotic mesenchymal stromal cells

“…In recent years, priming approaches to increase the therapeutic potential of MSCs have been investigated. Priming (also referred to as licensing or preconditioning) with pro-inflammatory mediators consists of preparing cells for some specific function or lineage-specific differentiation, which involves cell activation, molecular signaling, genetic, or epigenetic modifications [16]. This strategy is effective in promoting immunoregulatory properties when compared to unprimed cells; indeed, adipose tissue-derived MSCs are not able to inhibit proliferation of PBMCs without exposure to IFN-γ indicating that inhibitory factors are released on MSC activation [34].…”

“…Indeed, several studies indicate that MSCs need to be licensed by inflammatory stimuli, such as tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), and interleukin-1 α/β, to exert or improve their immunosuppressive actions [14]. Thus, in recent years, several studies have evaluated whether preconditioning bone marrow-derived MSCs cultured in vitro with IFN-γ and TNF-α could elicit a stronger immunosuppressive activity [4,[15][16][17]. IFN-γ induces a transient expression of indoleamine-2,3 dioxygenase (IDO) which inhibits T cell activation, proliferation, and functionality due to L-tryptophane depletion [9].…”

“…Preconditioning with TNF-α has also been shown to enhance the production and release of these molecules, although to a lesser extent than IFN-γ priming. Bone marrow MSCs primed with a combination of IFN-γ and TNF-α had an even stronger immunosuppressive phenotype compared to those primed with a single factor, although significant differences were observed in the inhibitory potential of MSCs based on donor source [16]. However, priming with IFN-γ amplifies the expression of MHCI and MHCII in equine bone marrow-derived cells [18] increasing their immunogenicity.…”