Apparent blood stream origin of endothelial and smooth muscle cells in the neointima of long, impervious carotid-femoral grafts in the dog

Functional regeneration at the blood-biomaterial interface

The use of cardiovascular implants is commonplace in clinical practice. However, reproducing the key bioactive and adaptive properties of native cardiovascular tissues with an artificial replacement is highly challenging. Exciting new treatment strategies are under development to regenerate (parts of) cardiovascular tissues directly in situ using immunomodulatory biomaterials. Direct exposure to the bloodstream and hemodynamic loads is a particular challenge, given the risk of thrombosis and adverse remodeling that it brings. However, the blood is also a source of (immune) cells and proteins that dominantly contribute to functional tissue regeneration. This review explores the potential of the blood as a source for the complete or partial in situ regeneration of cardiovascular tissues, with a particular focus on the endothelium, being the natural blood-tissue barrier. We pinpoint the current scientific challenges to enable rational engineering and testing of blood-contacting implants to leverage the regenerative potential of the blood.

A novel method for engineering autologous non-thrombogenic in situ tissue-engineered blood vessels for arteriovenous grafting

The durability of prosthetic arteriovenous (AV) grafts for hemodialysis access is low, predominantly due to stenotic lesions in the venous outflow tract and infectious complications. Tissue engineered blood vessels (TEBVs) might offer a tailor-made autologous alternative for prosthetic grafts. We have designed a method in which TEBVs are grown in vivo, by utilizing the foreign body response to subcutaneously implanted polymeric rods in goats, resulting in the formation of an autologous fibrocellular tissue capsule (TC). One month after implantation, the polymeric rod is extracted, whereupon TCs (length 6 cm, diameter 6.8 mm) were grafted as arteriovenous conduit between the carotid artery and jugular vein of the same goats. At time of grafting, the TCs were shown to have sufficient mechanical strength in terms of bursting pressure (2382 ± 129 mmHg), and suture retention strength (SRS: 1.97 ± 0.49 N). The AV grafts were harvested at 1 or 2 months after grafting. In an ex vivo whole blood perfusion system, the lumen of the vascular grafts was shown to be less thrombogenic compared to the initial TCs and ePTFE grafts. At 8 weeks after grafting, the entire graft was covered with an endothelial layer and abundant elastin expression was present throughout the graft. Patency at 1 and 2 months was comparable with ePTFE AV-grafts. In conclusion, we demonstrate the remodeling capacity of cellularized in vivo engineered TEBVs, and their potential as autologous alternative for prosthetic vascular grafts.

Biomaterial-driven in situ cardiovascular tissue engineering-a multi-disciplinary perspective

There is a persistent and growing clinical need for readily-available substitutes for heart valves and small-diameter blood vessels. In situ tissue engineering is emerging as a disruptive new technology, providing ready-to-use biodegradable, cell-free constructs which are designed to induce regeneration upon implantation, directly in the functional site. The induced regenerative process hinges around the host response to the implanted biomaterial and the interplay between immune cells, stem/progenitor cell and tissue cells in the microenvironment provided by the scaffold in the hemodynamic environment. Recapitulating the complex tissue microstructure and function of cardiovascular tissues is a highly challenging target. Therein the scaffold plays an instructive role, providing the microenvironment that attracts and harbors host cells, modulating the inflammatory response, and acting as a temporal roadmap for new tissue to be formed. Moreover, the biomechanical loads imposed by the hemodynamic environment play a pivotal role. Here, we provide a multidisciplinary view on in situ cardiovascular tissue engineering using synthetic scaffolds; starting from the state-of-the art, the principles of the biomaterial-driven host response and wound healing and the cellular players involved, toward the impact of the biomechanical, physical, and biochemical microenvironmental cues that are given by the scaffold design. To conclude, we pinpoint and further address the main current challenges for in situ cardiovascular regeneration, namely the achievement of tissue homeostasis, the development of predictive models for long-term performances of the implanted grafts, and the necessity for stratification for successful clinical translation.

Cyclically stretching developing tissue in vivo enhances mechanical strength and organization of vascular grafts

Tissue-engineered vascular grafts must have qualities that rival native vasculature, specifically the ability to remodel, the expression of functional endothelial components and a dynamic and functional extracellular matrix (ECM) that resists the forces of the arterial circulation. We have developed a device that when inserted into the peritoneal cavity, attracts cells around a tubular scaffold to generate autologous arterial grafts. The device is capable of cyclically stretching (by means of a pulsatile pump) developing tissue to increase the mechanical strength of the graft. Pulsed (n=8) and unpulsed (n=8) devices were implanted for 10 days in Lovenaar sheep (n=8). Pulsation occurred for a period of 5-8 days before harvest. Thick unadhered autologous tissue with cells residing in a collagen ECM was produced in all devices. Collagen organization was greater in the circumferential direction of pulsed tissue. Immunohistochemical labelling revealed the hematopoietic origin of >90% cells and a significantly higher coexpression with vimentin in pulsed tissue. F-actin expression, mechanical failure strength and strain were also significantly increased by pulsation. Moreover, tissue could be grafted as carotid artery patches. This paper shows that unadhered tissue tubes with increased mechanical strength and differentiation in response to pulsation can be produced with every implant after a period of 10 days. However, these tissue tubes require a more fine-tuned exposure to pulsation to be suitable for use as vascular grafts.

Role of fluid dynamics on the healing of anin vivo tissue engineered vascular graft

A polyester (PET) reinforced fibrin-FN-VEGF-TGFbeta vascular graft, formed by a four-step preclotting technique of a porous PET arterial graft, shows the overlapping inflammation, proliferation, and remodeling steps of normal wound healing when implanted in the descending thoracic aorta (DTA) position in the dog, forming a surface layer of endothelial cells. While the DTA grafts readily healed (i.e., endothelialized), similar grafts implanted in the carotid-femoral artery position did not fully heal. Since the initial phases of healing were shown to be dependent upon the transport of blood-borne constituents to the graft surface, the extent of healing appears to be dependent on the fluid dynamics present in the artery-graft-artery construct. The length of the noncompliant graft, the construction of the anastomoses, bends in the construct, graft diameter, and graft compliance can affect the fluid dynamics in the implant, and thus the healing of the graft. This has clinical relevance for the testing and development of new vascular graft materials.

Mesenchymal stem cells and the artery wall

The presence of ectopic tissue in the diseased artery wall is evidence for the presence of multipotential stem cells in the vasculature. Mesenchymal stem cells were first identified in the marrow stroma, and they differentiate along multiple lineages giving rise to cartilage, bone, fat, muscle, and vascular tissue in vitro and in vivo. Transplantation studies show that marrow-derived mesenchymal stem cells appear to enter the circulation and engraft other tissues, including the artery wall, at sites of injury. Recent evidence indicates that mesenchymal stem cells are also present in normal artery wall and microvessels and that they also may enter the circulation, contributing to the population of circulating progenitor cells and engrafting other tissues. Thus, the artery wall is not only a destination but also a source of progenitor cells that have regenerative potential. Although potential artifacts, such as fusion, need to be taken into consideration, these new developments in vascular biology open important therapeutic avenues. A greater understanding of how mesenchymal stem cells from the bone marrow or artery wall bring about vascular regeneration and repair may lead to novel cell-based treatments for cardiovascular disease.

Comparison of the ovine and porcine animal models for biocompatibility testing of vascular prostheses

OBJECTIVE

Evaluation of the pig and sheep models for biocompatibility investigations of vascular prostheses (VP).

DESIGN

Comparative analysis of animal experimental investigations involving two different animal models.

MATERIALS AND METHODS

Commercially available polyester vascular prostheses (PET-VP) were implanted into two different animal models (infrarenal porcine aorta and ovine carotid artery). The costs, surgical handling, patency rate, and healing on the basis of macroscopic, microscopic, and immunohistochemical criteria were analyzed over a period of 3 months.

RESULTS

Handling and operating times (63 +/- 10 versus 76 +/- 16 min; P = 0.125) did not differ significantly. The cost of the two animal models was comparable. Integration of the VP was complete in the sheep model, but varied in the pig model (two complete, four incomplete). Complete endothelialization of all VPs was observed in the pig, which contrasted with the sheep with complete (circular) endothelialization only in the region of the anastomosis. The thickness of neointima in the region of the anastomosis differed insignificantly; immunohistochemically, only periprosthetic Ki67 was significantly reduced (28.7 +/- 9.9 versus 6 +/- 0.9%; P = 0.002) in the sheep.

CONCLUSIONS

In the porcine model, extremely good endothelialization of the VP was observed, with formation of a rapid neointimal hyperplasia. The ovine model was characterized by the fact that postoperative follow-up investigations were easy to perform. Complete endothelialization was not observed.

Involvement of Progenitor Cells in Vascular Repair

Whereas the genesis of an arterial lesion is thought to be the result of migration and proliferation of vascular cells, recent insights into the biology of progenitor cells now question this concept. Specifically, endothelial and smooth muscle cells appear to be derived from multiple sources such as circulating stem and progenitor cells, as well as tissue-resident progenitor cell populations. These cells may engraft at sites of vascular injury and play an integral role in vascular repair. In this review, experimental data from in vitro studies, animal models, and scattered human observations are reviewed in the context of emerging hypotheses regarding the response to vascular injury.

Utilizing granulocyte colony-stimulating factor to enhance vascular graft endothelialization from circulating blood cells

Cells in the blood circulating through a vascular graft can contribute to endothelialization of its flow surface. We hypothesized that granulocyte colony-stimulating factor (G-CSF) could enhance this process by increasing circulating bone marrow progenitor cells. Ten dogs received composite grafts that were shielded from any source of endothelialization other than the circulating blood. On the seventh postoperative day and for 7 days thereafter, five dogs were injected subcutaneously with 10 mg/kg/day of human G-CSF. The additional five dogs, used as controls, received no G-CSF. Grafts were retrieved at 4 weeks. All dogs recovered promptly postoperatively. White cell counts in G-CSF dogs increased by an average of 9.5-fold at the end of treatment, and had returned to normal before retrieval. All grafts remained patent. G-CSF grafts had significantly higher endothelialization than the controls (82.2 +/- 9.2% vs. 23.7 +/- 4.4%, p = 0.0004), with extensive flow surface neointima, covered with a single layer of endothelium verified by FVIII/vWF and CD34 staining. Control grafts had virtually no neointima and were covered with a thin layer of fibrin coagulum. Significantly more endothelial-lined microvessels were also found in the G-CSF grafts than in the controls. Dogs treated with G-CSF have increased endothelialization of synthetic vascular grafts due to increased circulating bone marrow progenitor cells.

Transjugular intrahepatic portosystemic shunt with an autologous vein-covered stent: results in a swine model

PURPOSE

To investigate the feasibility, safety, and efficacy of an autologous vein-covered stent (AVCS) to prevent shunt stenosis in a porcine transjugular intrahepatic portosystemic shunt (TIPS) model.

MATERIALS AND METHODS

TIPS were created with an AVCS in 12 healthy domestic swine and with a bare stent in 10 additional swine. Tissue response was compared with use of venography, histology, and computerized morphometry analysis 2 weeks after implantation. Differences between AVCS and noncovered stents (established by a t-test), as well as regional differences within a single stent (established by an f test), were considered significant at P <.05.

RESULTS

Twenty of 22 TIPS procedures were technically successful. Ten of 12 shunts with an AVCS (83%) and two of 10 with bare stents (20%) remained patent (<50% diameter narrowing) at euthanasia 2 weeks later (P <.01). Histologic evaluation of harvested bare stents showed marked intimal hyperplasia (IH), composed of smooth muscle cells, myofibroblasts, and fibroblasts. In contrast, the AVCS were remarkably free of IH and thromboses. In patent TIPS in both groups, endothelial coverage of the luminal surface was present histologically. IH accounted for 57% (26.27/45.79) of total stent cross-sectional lumen area in the control group and 21% (8.34/39.54) in the AVCS group (P <.01), with no intrashunt differences (P >.05).

CONCLUSION

Based on short-term follow-up, AVCS significantly improved TIPS patency by prevention of both IH and in-stent thrombosis. TIPS created with an AVCS was feasible and safe in our porcine model.

Origin of neointimal endothelium and alpha-actin-positive smooth muscle cells in transplant arteriosclerosis

The development of transplant arteriosclerosis (TA) is today's most important problem in clinical organ transplantation. Histologically, TA is characterized by perivascular inflammation and progressive intimal thickening. Current thought on this process of vascular remodeling assumes that neointimal vascular smooth muscle (VSM) cells and endothelium in TA are graft-derived, holding that medial VSM cells proliferate and migrate into the subendothelial space in response to signals from inflammatory cells and damaged graft endothelium. Using MHC class I haplotype-specific immunohistochemical staining and single-cell PCR analyses, we show that the neointimal alpha-actin-positive VSM cells in rat aortic or cardiac allografts are of recipient and not of donor origin. In aortic but not in cardiac allografts, recipient-derived endothelial cells (ECs) replaced donor endothelium. Cyclosporine treatment prevents neointima formation and preserves the vascular media in aortic allografts. Recipient-derived ECs do not replace graft endothelium after cyclosporine treatment. We propose that, although it progresses beyond the needs of functional repair, TA reflects the activity of a normal healing process that restores vascular wall function following allograft-induced immunological injury.

Vascular graft healing. II. FTIR analysis of polyester graft samples from implanted bi-grafts

FTIR-ATR analysis has shown that the 4-step process for preclotting polyester vascular grafts results in a uniform and reproducible fibrin coating of the polyester fibers. Western blot analyses have shown that FN and VEGF are also present in this fibrin coating. FTIR-ATR analyses of explanted grafts indicate that, while the in vivo healing of these preclotted polyester grafts proceed through the inflammation, proliferation, and remodeling phases of normal wound healing, these phases are modified. Because the fibrin coating provides a nonporous barrier between peri-graft tissue and the flowing blood, these molecular changes are controlled by the interactions of blood-borne constituents with the lumenal surface of the preclotted graft. Also, a well prepared preclotted polyester graft shows a minimal inflammatory response. After implantation, the fibrin preclot is more than 90% gone by the fifth day. However, the proliferation phase, involving synthesis of new protein and polysaccharide materials to replace the fibrin, appears to have begun by the third day. Detection of collagen I in the 5-day explants suggests that the overlapping remodeling phase of healing has begun. Protein and saccharide materials continue to be synthesized and remodeled, and, by the tenth day, collagen IV is detected. By 14-days post-implantation, there is an increase in collagen IV and cellular membrane lipids. Because collagen IV is an indicator of the presence of endothelial cells, some of these cellular membranes must be of endothelial origin. Thus, it appears that FTIR-ATR can be a useful tool in the study of vascular healing.

Administration of granulocyte colony-stimulating factor enhances endothelialization and microvessel formation in small-caliber synthetic vascular grafts

OBJECTIVE

The purpose of this study was to determine whether systemic administration of granulocyte colony-stimulating factor (G-CSF) would promote endothelialization for small-caliber Dacron vascular grafts.

METHODS

We implanted 4-mm preclotted Dacron grafts in both carotids of 12 dogs. For a fair comparison, all dogs had a comparable platelet aggregation profile with platelet aggregation scores less than 30. Five dogs served as controls, and the others were given 7-day subcutaneous injections of G-CSF (10 microg/kg per day), starting on the seventh postoperative day. The effect of G-CSF was evaluated by white blood cell count, which showed a 3.7-fold (+/- 2.7-fold) increase at the end of treatment. Grafts were harvested at 4 weeks. All G-CSF grafts were patent, and one control occluded. Endothelial-like cell coverage averaged 80.8% on G-CSF grafts, but only 35.6% for control grafts (P <.0004). With the exclusion of the anastomotic pannus healing factor, the difference in endothelial-like cell coverage was even greater (68.5% vs 9.8%; P <.0001). Immunocytochemical staining and electron microscopy studies demonstrated endothelial cells. Light microscopy also showed that there were more microvessels on and in the G-CSF grafts than in the control grafts. This study suggests that G-CSF can enhance early endothelialization of small-caliber vascular grafts. Further studies to determine the proper dosage and timing are needed before clinical application can be recommended.

Enhanced endothelialization and microvessel formation in polyester grafts seeded with CD34+ bone marrow cells

The authors have shown accelerated endothelialization on polyethylene terephthalate (PET) grafts preclotted with autologous bone marrow. Bone marrow cells have a subset of early progenitor cells that express the CD34 antigen on their surfaces. A recent in vitro study has shown that CD34+ cells can differentiate into endothelial cells. The current study was designed to determine whether CD34+ progenitor cells would enhance vascular graft healing in a canine model. The authors used composite grafts implanted in the dog's descending thoracic aorta (DTA) for 4 weeks. The 8-mm × 12-cm composite grafts had a 4-cm PET graft in the center and 4-cm standard ePTFE grafts at each end. The entire composite was coated with silicone rubber to make it impervious; thus, the PET segment was shielded from perigraft and pannus ingrowth. There were 5 study grafts and 5 control grafts. On the day before surgery, 120 mL bone marrow was aspirated, and CD34+ cells were enriched using an immunomagnetic bead technique, yielding an average of 11.4 ± 5.3 × 106. During surgery, these cells were mixed with venous blood and seeded onto the PET segment of composite study grafts; the control grafts were treated with venous blood only. Hematoxylin and eosin, immunocytochemical, and AgNO3staining demonstrated significant increases of surface endothelialization on the seeded grafts (92% ± 3.4% vs 26.6% ± 7.6%; P = .0001) with markedly increased microvessels in the neointima, graft wall, and external area compared with controls. In dogs, CD34+ cell seeding enhances vascular graft endothelialization; this suggests practical therapeutic applications.

Early flow surface endothelialization before microvessel ingrowth in accelerated graft healing, with BrdU identification of cellular proliferation

The purpose of this report was to determine if flow surface endothelialization could precede microvessel ingrowth from the perigraft area in porous Dacron grafts, by using an accelerated graft healing model with short implant periods. Dacron grafts were implanted in the abdominal aorta of 22 dogs and wrapped in autogenous inferior vena cava (IVC), which provided excellent conditions for extramural angiogenesis, microvessel development, and ingrowth toward the graft. Retrieval times were 7 days (n = 4), 8 days (n = 5), 9 days (n = 4), 10 days (n = 3), 11 days (n = 4) and 12 days (n = 3) postoperatively. Graft surfaces were evaluated for thrombus coverage, cell coverage, and the number of micro-ostia. Components and cellular types in the graft wall and on the surface were studied and characterized with H&E, histochemical, and immunocytochemical staining. BrdU labeling was also used, to identify the areas where cells were actively proliferating. All grafts were patent. Although the degree of IVC/graft attachment varied, isolated islands of endothelial-like cells were found at the midgraft areas at each time period, and immunocytochemically confirmed as endothelial cells. There were two healing patterns: (1) surface endothelialization before microvessel/tissue ingrowth from the perigraft areas, and (2) surface endothelialization with full wall microvessel and tissue presence. Surface endothelialization was observed before perigraft tissue ingrowth, indicating that fallout healing is an independent source of endothelialization for porous grafts.