A critical role for macrophages in neovessel formation and the development of stenosis in tissue‐engineered vascular grafts

Abstract

The primary graft-related complication during the first clinical trial evaluating the use of tissue-engineered vascular grafts (TEVGs) was stenosis. We investigated the role of macrophages in the formation of TEVG stenosis in a murine model. We analyzed the natural history of TEVG macrophage infiltration at critical time points and evaluated the role of cell seeding on neovessel formation. To assess the function of infiltrating macrophages, we implanted TEVGs into mice that had been macrophage depleted using clodronate liposomes. To confirm this, we used a CD11b-diphtheria toxin-receptor (DTR) transgenic mouse model. Monocytes infiltrated the scaffold within the first few days and initially transformed into M1 macrophages. As the scaffold degraded, the macrophage infiltrate disappeared. Cell seeding decreased the incidence of stenosis (32% seeded, 64% unseeded, P=0.024) and the degree of macrophage infiltration at 2 wk. Unseeded TEVGs demonstrated conversion from M1 to M2 phenotype, whereas seeded grafts did not. Clodronate and DTR inhibited macrophage infiltration and decreased stenosis but blocked formation of vascular neotissue, evidenced by the absence of endothelial and smooth muscle cells and collagen. These findings suggest that macrophage infiltration is critical for neovessel formation and provides a strategy for predicting, detecting, and inhibiting stenosis in TEVGs.

Implantation of Completely Biological Engineered Grafts Following Decellularization into the Sheep Femoral Artery

“…Macrophages are likely associated with the early stages of inflammation-mediated TEVG remodeling leading to recruitment of aSMA + cells via paracrine mechanisms as in the mouse model. 31,32 At 24 weeks, there was complete recellularization with aSMA + cells and only rare CD45 + cells, consistent with the hypothesis that favorable remodeling coincides with the resolution of inflammation and an absence of immunogenicity of these grafts. Short TEVGs (2-3 cm) that were implanted interpositionally were used in these studies to evaluate graft remodeling as efficiently as possible.…”

Beyond Burst Pressure: Initial Evaluation of the Natural History of the Biaxial Mechanical Properties of Tissue-Engineered Vascular Grafts in the Venous Circulation Using a Murine Model

“…We discovered that the vascular neotissue formation is driven by migration of the host endothelial cells and smooth muscle cells (SMCs) from the neighboring blood vessel wall through an immune-mediated process orchestrated by the seeded cells using a paracrine mechanism. 9,10 So far, our studies have focused mainly on the cellular processes involved in vascular neotissue formation, less on formation of the extracellular matrix (ECM) and the associated biomechanical factors. [7][8][9][10] There is a growing literature highlighting the importance of mechanobiological principles on vascular physiology and pathophysiology.…”

“…9,10 So far, our studies have focused mainly on the cellular processes involved in vascular neotissue formation, less on formation of the extracellular matrix (ECM) and the associated biomechanical factors. [7][8][9][10] There is a growing literature highlighting the importance of mechanobiological principles on vascular physiology and pathophysiology. [11][12][13][14][15] For example, compliance mismatch between a vascular graft and the native vessel into which it is implanted has been demonstrated to be a critical factor, leading at times to the formation of neointimal hyperplasia and stenosis.…”

Comparison of the Biological Equivalence of Two Methods for Isolating Bone Marrow Mononuclear Cells for Fabricating Tissue-Engineered Vascular Grafts

“…10,11 In studies evaluating vascular neotissue formation in TEVG in animals that were macrophage depleted using clodronate liposomes, we demonstrated that 11 In this same study, we discovered that the macrophages infiltrating patent TEVG demonstrated a shift away from the M1 phenotype, while the macrophages infiltrating stenotic TEVG demonstrated increased expression of M1 phenotypic markers. We have also previously demonstrated that cell seeding is not essential for vascular neotissue formation, but instead functions to improve patency by modulating the host inflammatory response to the TEVG scaffold by decreasing the number infiltrating macrophages and shifting the macrophage phenotype away from M1 through an as yet undetermined paracrine mechanism.…”