In vivo response to decellularized mesothelium scaffolds.

Biological surgical scaffolds are used in plastic and reconstructive surgery to support structural reinforcement and regeneration of soft tissue defects. Macrophage and fibroblast cell populations heavily regulate scaffold integration into host tissue following implantation. In the present study, the biological host response to a commercially available surgical scaffold (Meso BioMatrix Surgical Mesh (MBM)) was investigated for up to 9 weeks after subcutaneous implantation; this scaffold promoted superior cell migration and infiltration previously in in vitro studies relative to other commercially available scaffolds. Infiltrating macrophages and fibroblasts phenotypes were assessed for evidence of inflammation and remodeling. At week 1, macrophages were the dominant cell population, but fibroblasts were most abundant at subsequent time points. At week 4, the scaffold supported inflammation modulation as indicated by M1 to M2 macrophage polarization; the foreign body giant cell response resolved by week 9. Unexpectedly, a fibroblast subpopulation expressed macrophage phenotypic markers, following a similar trend in transitioning from a proinflammatory to anti-inflammatory phenotype. Also, α-smooth muscle actin-expressing myofibroblasts were abundant at weeks 4 and 9, mirroring collagen expression and remodeling activity. MBM supported physiologic responses observed during normal wound healing, including cellular infiltration, host tissue ingrowth, remodeling of matrix proteins, and immune modulation. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 716-725, 2018.

Hypoxia Impairs Mesenchymal Stromal Cell-Induced Macrophage M1 to M2 Transition.

The transition of macrophages from the pro-inflammatory M1 to the anti-inflammatory M2 phenotype is crucial for the progression of normal wound healing. Persistent M1 macrophages within the injury site may lead to an uncontrolled macrophage-mediated inflammatory response and ultimately a failure of the wound healing cascade, leading to chronic wounds. Mesenchymal stromal cells (MSCs) have been widely reported to promote M1 to M2 macrophage transition; however, it is unclear whether MSCs can drive this transition in the hypoxic environment typically observed in chronic wounds. Here we report on the effect of hypoxia (1% O2) on MSCs' ability to transition macrophages from the M1 to the M2 phenotype. While hypoxia had no effect on MSC secretion, it inhibited MSC-induced M1 to M2 macrophage transition, and suppressed macrophage expression and production of the anti-inflammatory mediator interleukin-10 (IL-10). These results suggest that hypoxic environments may impede the therapeutic effects of MSCs.

Soluble Receptor for Advanced Glycation End Products Improves Stromal Cell-Derived Factor-1 Activity in Model Diabetic Environments.

Objective: In diabetes, hyperglycemia causes the accumulation of advanced glycation end products (AGEs) that trigger reactive oxygen species (ROS) generation through binding the receptor for AGEs (RAGE). Because exogenous growth factors have had little success in enhancing chronic wound healing, we investigated whether hyperglycemia-induced AGEs interfere with cellular responses to extracellular signals. We used stromal cell-derived factor-1 (SDF-1), an angiogenic chemokine also known to promote stem cell recruitment in skin wounds. Approach: Human leukemia-60 (HL-60) cells and mouse peripheral blood mononuclear cells (PBMCs), which express the SDF-1 receptor CXCR-4, were incubated for 24 h in medium supplemented with 25 mM d-glucose. Soluble RAGE (sRAGE) was used to block RAGE activation. Response to SDF-1 was measured in cellular migration and ROS assays. A diabetic murine excisional wound model measured SDF-1 liposome and sRAGE activity in vivo. Results: Hyperglycemia led to significant accumulation of AGEs, decreased SDF-1-directed migration, and elevated baseline ROS levels; it suppressed the ROS spike normally triggered by SDF-1. sRAGE decreased the ROS baseline and restored both the SDF-1-mediated spike and cell migration. Topically applied sRAGE alone promoted healing and enhanced the effect of exogenous SDF-1 on diabetic murine wounds. Innovation: While there is interest in using growth factors to improve wound healing, this strategy is largely ineffective in diabetic wounds. We show that sRAGE may restore signaling, thus potentiating the effect of exogenously applied growth factors. Conclusion: Blocking RAGE with sRAGE restores SDF-1-mediated cellular responses in hyperglycemic environments and may potentiate the effectiveness of SDF-1 applied in vivo.

Mesenchymal stromal cells reverse hypoxia-mediated suppression of α-smooth muscle actin expression in human dermal fibroblasts.

During wound healing, fibroblasts deposit extracellular matrix that guides angiogenesis and supports the migration and proliferation of cells that eventually form the scar. They also promote wound closure via differentiation into α-smooth muscle actin (SMA)-expressing myofibroblasts, which cause wound contraction. Low oxygen tension typical of chronic nonhealing wounds inhibits fibroblast collagen production and differentiation. It has been suggested that hypoxic mesenchymal stromal cells (MSCs) secrete factors that promote wound healing in animal models; however, it is unclear whether these factors are equally effective on the target cells in a hypoxic wound environment. Here we investigated the impact of MSC-derived soluble factors on the function of fibroblasts cultured in hypoxic fibroblast-populated collagen lattices (FPCLs). Hypoxia alone significantly decreased FPCL contraction and α-SMA expression. MSC-conditioned medium restored hypoxic FPCL contraction and α-SMA expression to levels similar to normoxic FPCLs. SB431542, an inhibitor of transforming growth factor-ß1 (TGF-ß1)-mediated signaling, blocked most of the MSC effect on FPCL contraction, while exogenous TGF-ß1 at levels similar to that secreted by MSCs reproduced the MSC effect. These results suggest that TGF-ß1 is a major paracrine signal secreted by MSCs that can restore fibroblast functions relevant to the wound healing process and that are impaired in hypoxia.