Stem Cells and Healing: Impact on Inflammation

Abstract

Significance: The number of patients with nonhealing wounds has rapidly accelerated over the past 10 years in both the United States and worldwide. Some causative factors at the macro level include an aging population, epidemic numbers of obese and diabetic patients, and an increasing number of surgical procedures. At the micro level, chronic inflammation is a consistent finding.

Recent advances: A number of treatment modalities are currently used to accelerate wound healing, including energy-based modalities, scaffoldings, the use of mechano-transduction, cytokines/growth factors, and cell-based therapies. The use of stem cell therapy has been hypothesized as a potentially useful adjunct for nonhealing wounds. Specifically, mesenchymal stem cells (MSCs) have been shown to improve wound healing in several studies. Immune modulating properties of MSCs have made them attractive treatment options.

Critical issues: Current limitations of stem cell therapy include the potentially large number of cells required for an effect, complex preparation and delivery methods, and poor cell retention in targeted tissues. Comparisons of published in-vitro and clinical trials are difficult due to cell preparation techniques, passage number, and the impact of the micro-environment on cell behavior.

Future directions: MSCs may be more useful if they are preactivated with inflammatory cytokines such as tumor necrosis factor alpha or interferon gamma. This article will review the current literature with regard to the use of stem cells for wound healing. In addition the anti-inflammatory effects of MSCs will be discussed along with the potential benefits of stem cell preactivation.

William J. Ennis, DO, MBA

Figure 1.

Potential therapies for reducing scar formation during wound repair. To manipulate wound repair to become more regenerative than scar forming, strategies include the use of biomimetic scaffolds, the manipulation of the mechanical environment (e.g., negative pressure wound therapy to increase healing) or the electrical environment, the administration of small molecules, the use of gene-therapy approaches, and the use of cell-based strategies (including administration of epithelial stem cells.) All of these elements have been demonstrated to have an effect on in vitro and in vivo models of wound healing as single-agent therapies. In theory, many of these elements could be combined to recreate a receptive environment (or soil) to promote regeneration. Combining these with the appropriate stem cells (or “seed”) will undoubtedly alter the result of wound healing in humans. (Reprinted with permission from Gurtner et al.) To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/wound

Figure 2.

Paracrine effects of cultured mesenchymal stem cells (MSCs). The secretion of a broad range of bioactive molecules is now believed to be the main mechanism by which MSCs achieve their therapeutic effect. This mechanism can be divided into six main actions: immunomodulation, antiapoptosis, angiogenesis, support of the growth and differentiation of local stem and progenitor cells, antiscarring, and chemoattraction. Although the number of molecules known to mediate the paracrine action of MSCs increases every day, only a few factors that are secreted by cultured MSCs are shown. The immunomodulatory effects of MSCs consist of inhibition of the proliferation of CD8+ and CD4+ T lymphocytes and natural killer (NK) cells, suppression of immunoglobulin production by plasma cells, inhibition of maturation of dendritic cells (DCs), and stimulation of the proliferation of regulatory T cells. The secretion of prostaglandin E2 (PGE2), human leukocyte antigen G5(HLA-G5), hepatocyte growth factor (HGF), inducible nitric oxide synthase (iNOS), indoleamine 2,3-dioxygenase (IDO), transforming growth factor Beta (GFFB), leukemia-inhibitory factor (LIF), and interleukin (IL)-10 contributes to this effect. MSCs can also limit apoptosis, and the principal bioactive molecules responsible for this process are HGF, TGF-B, vascular endothelial growth factor (VEGF), insulin like growth factor (IGF)-1, stanniocalcin 1, and granulocyte macrophage colony-stimulating factor (GM-CSF). MSCs stimulate local angiogenesis by secretion of extracellular matrix (ECM) molecules, VEGF, IGF-1, phosphatidylinositol-glycan biosynthesis class F protein (PIGF), monocyte chemoattractant protein 1 (MCP-1), basic fibroblast growth factor (bFGF), and IL-6; they also stimulate mitosis of tissue-intrinsic progenitor or stem cells by secretion of stem cell factor (SCF), macrophage colony-stimulating factor (MCSF), stromal cell-derived factor (SDF-1), LIF, and angiopoietin 1. Moreover, HGF and bFGF (and possibly adrenomedullin) produced by MSCs contribute to the inhibition of scarring caused by ischemia. Finally, a group of at least 15 chemokines produced by MSCs can elicit leukocyte migration to the injured area, which is important for normal tissue maintenance. (Reprinted with permission from Singer and Caplan.) To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/wound

Figure 3.

Strategies for mesenchymal cell delivery to cutaneous wounds. Traditional techniques include local injection of cells into the soft tissue, direct topical application, and systemic delivery via injection into the peripheral circulation. These methods have resulted in improved wound healing but are limited by sub-optimal cell survival and engraftment. Novel delivery methods are being developed, utilizing tissue scaffolds to optimize stem cell function and maximize the therapeutic potential for cellular therapy. (Reprinted with permission from Chen et al.) To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/wound

Figure 4.

MSCs activated with interferon gamma provided increased tensile strength at lower cell doses (6,250 cells/cm²) when compared with naive MSC or fibroblasts. At higher cell doses (62,500 cells/cm²), there were no statistical differences between interferon gamma or naive MSCs, suggesting that the feasibility of lower cell doses could be achieved if the cells were actuated before administration.

Figure 5.

Attraction and activation of the immune system after myocardial infarction. The first cells activated after myocardial infarction are the local macrophages. Soon after, damage associated proteins (DAMPs) complement system and IL-1 attract neutrophils, which enter the damage tissue to clear the debris. Within days, large numbers of macrophages infiltrate the tissue, clearing both the debris and activating the reparative pathways. The M1 macrophages are the first to arrive and have a pro-inflammatory character. They are followed by the anti-inflammatory M2 macrophage. Lymphocytes arrive relatively late on the scene, due to the lengthy process of activation. (Reprinted with permission from van den Akker et al.) To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/wound

Figure 6.

Time course of gene expression levels in the cornea after transplantation surgery. Real-time reverse transcription (RT-PCR) showed that the levels of proinflammatory cytokines (interleukin IL-6,IL-1B, and IL-2) were up-regulated at similar levels in (a) autografts and (b) allografts at days 3 and 7 after transplantation, which defines the early phase of surgery-induced inflammation. The levels of T-cell derived cytokines (IFN-gamma) were elevated through day 28 in allografts, but not in autografts, which indicates the late phase of the allogeneic immune rejection. In allografts that received intravenous human MSCs (c), levels of IL-6, IL-1b, and IL-12a were significantly lower at days 3 and 7 and levels of IFN-gamma were markedly decreased at day 28 compared with autografts or allografts that did not receive hMSCs. n=5 at each time-point in all experimental groups. (Reprinted with permission from Oh et al.)