Challenges and Strategies for Endothelializing Decellularized Small-Diameter Tissue-Engineered Vessel Grafts

Vascular diseases are the leading cause of ischemic necrosis in tissues and organs, necessitating using vascular grafts to restore blood supply. Currently, small vessels for coronary artery bypass grafts are unavailable in clinical settings. Decellularized small-diameter tissue-engineered vessel grafts (SD-TEVGs) hold significant potential. However, they face challenges, as simple implantation of decellularized SD-TEVGs in animals leads to thrombosis and calcification due to incomplete endothelialization. Consequently, research and development focus has shifted toward enhancing the endothelialization process of decellularized SD-TEVGs. This paper reviews preclinical studies involving decellularized SD-TEVGs, highlighting different strategies and their advantages and disadvantages for achieving rapid endothelialization of these vascular grafts. Methods are analyzed to improve the process while addressing potential shortcomings. This paper aims to contribute to the future commercial viability of decellularized SD-TEVGs.

Small Diameter Xenogeneic Extracellular Matrix Scaffolds for Vascular Applications

Currently, despite the success of percutaneous coronary intervention (PCI), coronary artery bypass graft (CABG) remains among the most commonly performed cardiac surgical procedures in the United States. Unfortunately, the use of autologous grafts in CABG presents a major clinical challenge as complications due to autologous vessel harvest and limited vessel availability pose a significant setback in the success rate of CABG surgeries. Acellular extracellular matrix (ECM) scaffolds derived from xenogeneic vascular tissues have the potential to overcome these challenges, as they offer unlimited availability and sufficient length to serve as "off-the-shelf" CABGs. Unfortunately, regardless of numerous efforts to produce a fully functional small diameter xenogeneic ECM scaffold, the combination of factors required to overcome all failure mechanisms in a single graft remains elusive. This article covers the major failure mechanisms of current xenogeneic small diameter vessel ECM scaffolds, and reviews the recent advances in the field to overcome these failure mechanisms and ultimately develop a small diameter ECM xenogeneic scaffold for CABG. Impact Statement Currently, the use of autologous vessel in coronary artery bypass graft (CABG) is common practice. However, the use of autologous tissue poses significant complications due to tissue harvest and limited availability. Developing an alternative vessel for use in CABG can potentially increase the success rate of CABG surgery by eliminating complications related to the use of autologous vessel. However, this development has been hindered by an array of failure mechanisms that currently have not been overcome. This article describes the currently identified failure mechanisms of small diameter vascular xenogeneic extracellular matrix scaffolds and reviews current research targeted to overcoming these failure mechanisms toward ensuring long-term graft patency.

Roles of bone-marrow-derived cells and inflammatory cytokines in neointimal hyperplasia after vascular injury

Bone-marrow-derived cells can generate vascular progenitor cells that contribute to pathological remodeling in models of restenosis after percutaneous coronary intervention (PCI). We created models of vascular injury in mice with bone marrow transplants (BMT) to determine relationships between bone-marrow-derived cells and subsequent biological factors. Mesenchymal stromal cells (MSCs) seemed to inhibit the inflammatory reaction and help stabilize injured vascular regions through mobilizing more endogenous bone-marrow-derived (EBMD) cells to the peripheral circulation. Granulocyte-colony stimulating factor (G-CSF) mobilized more EBMD cells to the peripheral circulation, and they accumulated on the injured side of the vascular lumen. The inflammatory cytokines, tumor necrosis factor (TNF)-alpha, and interleukin (IL)-6 mobilized EBMD cells that play an important role in the process of neointimal hyperplasia after vascular injury. These factors might comprise a mechanism that alters the transdifferentiation or paracrine capabilities of EBMD cells and are potential targets of treatment for patients with cardiovascular diseases.

IL-6 mobilizes bone marrow-derived cells to the vascular wall, resulting in neointima formation via inflammatory effects

AIM

Among the many factors related to bone marrow cell mobilization, local inflammation induced by cytokines may drive bone marrow cells to the vascular wall, resulting in a thickened neointima. However, the relationship between inflammatory reactions and bone marrow cell invasion has not yet been fully clarified.

METHODS

We inserted a large wire into the femoral artery in male balb/c(WT), interleukin (IL)-6-knockout (KO) and bone marrow-transplanted (BMT) mice that had received bone marrow cells from KO mice. Immunohistochemistry was performed to evaluate the degree of intimal hyperplasia and inflammation following vascular injury.

RESULTS

Three days after the vascular injury, the number of CD34/Sca-1-positive cells in the blood was higher in the KO mice. The numbers of apoptotic cells in the neointima was lower in the KO and BMT mice at two hours after injury. The morphometric analysis performed at one and four weeks after injury showed that the intima/media ratio was significantly lower in the KO and BMT mice, while CD34-positive cells were much more frequent in the WT mice. Furthermore, re-endothelialization appeared earlier in the KO and BMT mice than in the WT mice. No differences in the levels of vascular endothelial growth factor or hepatocyte growth factor were observed in the mice sera between the WT, KO and BMT mice after injury. The in vitro culture of bone marrow cells showed more differentiated smooth muscle-like cells in the WT mice than in the KO mice.

CONCLUSIONS

IL-6 is involved in neointimal formation following vascular injury, possibly acting through inflammatory effects inducing the production of bone marrow cells.

Human mesenchymal stromal cells (MSCs) reduce neointimal hyperplasia in a mouse model of flow-restriction by transient suppression of anti-inflammatory cytokines

AIM

Mesenchymal stromal cells from human bone marrow (hMSCs) were observed to produce therapeutic benefits in some models for cardiac and vascular injuries but their mode of action was not defined. We tested the effects of hMSCs in models for restricted vascular flow.

METHODS

We made model for restricted vascular flow produced by permanent ligation of a carotid artery and injected hMSCs to clarify the effects of hMSCs to vascular lesions.

RESULTS

Seven, 14, and 28 days after infusion of hMSCs into the cardiac left ventricle of the mice, there was a significant reduction in neointimal hyperplasia (p<0.05). Seven days after administration of the hMSCs, macrophages infiltration into the ligated artery and serum levels of monocyte chemoattractive protein-1 (MCP-1/CCL-2) (p<0.05) were reduced. However, no hMSCs were detected in the lesions by sensitive PCR assays. We then observed that the serum level of MCP-1 was a potential biomarker for the therapeutic effects of hMSCs in a mouse model for high-fat-diet.

CONCLUSIONS

These results indicated the administration of hMSCs decreased the initial and excess inflammatory responses to carotid artery ligation. The decrease in inflammatory response apparently decreased the subsequent neointimal hyperplasia.

Beneficial effects of granulocyte-colony stimulating factor on small-diameter heparin immobilized decellularized vascular graft

Autologous recellularization of decellularized scaffolds is a promising challenge in the field of tissue-engineered vascular graft and could be boosted by endothelial progenitor cells (EPCs). The purpose of this study was to examine the effects of granulocyte-colony stimulating factor (G-CSF) treatment on this process. Heparin immobilized decellularized grafts were fabricated and implanted into 48 rats, of which 25 rats received G-CSF (50 ug/kg/day) for 14 days after operation (G-CSF group) and other 23 received saline serving as control. Five animals of each group were euthanized at 2 weeks for analysis of early graft endothelialization; whereas the rest were investigated by Doppler ultrasound to monitor the graft patency rate up to 6 months. After 14 days of G-CSF administration, the number of CD(34) (+)/CD(133) (+) progenitor cells was increased by 16.2 folds, and endothelial cell-specific immunostaining revealed an enhancement of early endothelialization in G-CSF group. After 6 months of implantation, the G-CSF treated grafts exhibited a significantly smaller hyperplastic neointima area compared with the controls, not only at midportion (0.38 ± 0.02 vs. 0.47 ± 0.07 mm(2), p < 0.0001), but also at distal anastomosis (0.42 ± 0.04 vs. 0.70 ± 0.13 mm(2), p < 0.0001). Moreover, G-CSF treated grafts had a higher patency rate compared with the control animals (19/20 vs. 12/18, p = 0.005). In conclusion, G-CSF-induced mobilization of circulating EPCs regenerated endothelium and inhibited neointimal hyperplasia of small-diameter heparinized decellularized vascular graft. This cytokine therapy may be a feasible strategy for the improvement of patency rate of the novel allogeneic graft.