Age-Related Changes in the Inflammatory Status of Human Mesenchymal Stem Cells: Implications for Cell Therapy

Abstract

Human mesenchymal stem/stromal cell (hMSC)-based cell therapies are promising for treating a variety of diseases. The unique immunomodulatory properties of hMSCs have extended their therapeutic potential beyond tissue regeneration. However, extensive pre-clinical culture expansion inevitably drives cells toward replicative "aging" and a consequent decline in quality. These "in vitro-aged" hMSCs resemble biologically aged cells, which have been reported to show senescence signatures, diminished immunosuppressive capacity, and weakened regenerative potential as well as pro-inflammatory features. In this review, we have surveyed the literature to explore the intimate relationship between the inflammatory status of hMSCs and their in vitro aging process. We posit that a shift from an anti-inflammatory to a pro-inflammatory phenotype of culture-expanded hMSCs contributes to a deterioration in their therapeutic efficacy. Potential molecular and cellular mechanisms underpinning this phenomenon have been discussed. We have also highlighted studies that leverage these mechanisms to make culture-expanded hMSCs more amenable for clinical use.

Keywords: SASP; aging; allogeneic stem cell therapy; extracellular matrix; glycosaminoglycan; heparan sulfate; immunomodulation; inflammation; rejuvenating mesenchymal stem cells; secretome.

Graphical abstract

Figure 1

Clinical Trials on MSC-Based Cell Therapies Listed in clinicaltrials.gov (a Total of 1,025 as of November 2020) (A) Cumulative number of allogeneic (342) and autologous (389) trials. A further 294 trials failed to report the source of cells (mesenchymal stem cell and allogeneic or autologous were used as search strings). (B) Percentage of trials at different phases of clinical research (mesenchymal stem cell and clinical research phase number were used as search strings). (C) A timeline showing MSC products that have cleared regulatory approval in the stated countries. AD-MSC, adipose-derived MSC; BM-MSC, bone marrow-derived MSC; UC-MSC, umbilical cord-derived MSC; GvHD, graft-versus-host disease. ^∗A licensee of Mesoblast. Sources of information are as follows: ¹Korea Ministry of Food and Drug Safety website, https://www.mfds.go.kr/eng/brd/m_30/view.do?seq=71337; ²Korea Ministry of Food and Drug Safety website, https://www.mfds.go.kr/eng/brd/m_30/view.do?seq=70957; ³Korea Ministry of Food and Drug Safety website, https://www.mfds.go.kr/eng/brd/m_30/view.do?seq=69798; ⁴conditional approval obtained in New Zealand in 2012, approval lapsed in 2016, New Zealand Medicines and Medical Devices Safety Authority website, https://medsafe.govt.nz/regulatory/ProductDetail.asp?ID=15063; ⁵conditional approval obtained in Canada in 2012, approval granted in 2014, Canada Drug Product Database, https://health-products.canada.ca/dpd-bdpp/info.do?lang=en&code=87195; ⁶Korea Ministry of Food and Drug Safety website, https://www.mfds.go.kr/eng/brd/m_30/view.do?seq=70956; ⁷Pharmaceuticals and Medical Devices Agency (Japan) website, https://www.pmda.go.jp/files/000215658.pdf; ⁸Nature India website, https://www.natureasia.com/en/nindia/article/10.1038/nindia.2016.61; ⁹Pharmaceuticals and Medical Devices Agency (Japan) website, https://www.pmda.go.jp/files/000231946.pdf; ¹⁰European Medical Agency website, https://www.ema.europa.eu/en/medicines/human/EPAR/alofisel.

Figure 2

Terms that Describe In Vitro- and In Vivo-Aged Cells (A) Early- and late-passage cells describes the in vitro age of cells according to the number of serial passages they have undergone (early passage ≤5; late passage ≥10). (B) Young and old cells describes the in vivo age of cells depending on donor age (young donor ≤30; old donor ≥60). (C) Youthful cells is a general term that collectively refers to early-passage (in vitro age) and young (in vivo age) cells. (D) Aged cells is a general term that collectively refers to late-passage (in vitro age) and old (in vivo age) cells.

Figure 3

Effects of the SASP on the hMSC Microenvironment The SASP contains a wide range of soluble factors, such as reactive oxygen species (ROS), monocyte and leukocyte chemotactic proteins (MIP1a, MCP2, CXCL9, and CXCL10), pro-inflammatory cytokines (IL-1, IL-6, and IL-8), and proteases (MMP1 and MMP3). These molecules act on both hMSCs and immune cells, as well as the extracellular matrix, to create an inflammatory microenvironment. Abbreviations: MIP, macrophage inflammatory protein; MCP, monocyte chemoattractant protein; CXCL, C-X-C motif chemokine ligand; IL, interleukin; MCP, monocyte chemotactic protein; MMP, matrix metalloproteinase.

Figure 4

The Opposing Effects of Inflammation on the Immunomodulatory Function of hMSCs MSCs can be induced to acquire distinct phenotypes (MSC2 and MSC1) depending on stimuli presented in an inflammatory microenvironment. In acute inflammation, high concentrations of pro-inflammatory cytokines (IL-1, IL-6, TNF-α, IFN-γ) and low levels of anti-inflammatory cytokines (TGF-β, IL-10) trigger hMSCs to acquire immune-suppressive features (MSC2). MSC2 cells secrete high amounts of IDO and anti-inflammatory cytokines (TGF-β, IL-10) and inhibit the activity of macrophages, neutrophils, and lymphocytes. By contrast, hMSCs acquire an immune-activation phenotype (MSC1) under chronic inflammatory conditions. Persistently low and comparable levels of pro-inflammatory and anti-inflammatory cytokines in the microenvironment lead to the production of low levels of IDO and pro-inflammatory cytokines (TNF-α, IFN-γ), respectively. However, the amount of IDO secreted by MSC1 cells is significantly lower in comparison to that produced by MSC2 cells during acute inflammation. At these reduced levels, IDO is inadequate in suppressing the activity of immune cells, and an overall pro-inflammatory state persists in the microenvironment.

Figure 5

Strategies to Target the Interplay between hMSC Aging and Environmental Inflammation for Improved Therapeutic Potential hMSC senescence can occur as a result of natural aging or be induced by external stressors, such as ROS and cytotoxic chemicals. Moreover, SASP from senescent cells causes chronic, low-level inflammatory stress. This process drives the self-perpetuating cycle of inflammaging, where inflammation induces cellular senescence and senescent cells in turn exacerbate pro-inflammatory features of the microenvironment. Strategies to dampen inflammatory signals could potentially slow down the inflammaging process. In addition, targeted removal of senescent cells from an hMSC population could be an approach to maintaining cells in a rejuvenated state.