Transmural macrophage migration into an arterial bioresorbable vascular graft promotes inflammatory-mediated response and collagen deposition for vascular remodeling.

Macrophages are the primary cell type orchestrating bioresorbable vascular graft (BVG) remodeling and infiltrate from three sources: the adjacent native vessel, circulating blood, and transmural migration from outer surface of the graft. To elucidate the kinetics of macrophage infiltration into the BVG, we fabricated two different bilayer arterial BVGs consisting of a macroporous sponge layer and a microporous electrospun (ES) layer. The Outer ES graft was designed to reduce transmural cell infiltration from the outer surface and the Inner ES graft was designed to reduce cell infiltration from the circulation. These BVGs were implanted in mice as infrarenal abdominal aorta grafts and extracted at 1, 4, and 8 weeks (n = 5, 10, and 10 per group, respectively) for evaluation. Cell migration into BVGs was higher in the Inner ES graft than in the Outer ES graft. For Inner ES grafts, the majority of macrophage largely expressed a pro-inflammatory M1 phenotype but gradually changed to tissue-remodeling M2 macrophages. In contrast, in Outer ES grafts macrophages primarily maintained an M1 phenotype. The luminal surface endothelialized faster in the Inner ES graft; however, the smooth muscle cell layer was thicker in the Outer ES graft. Collagen fibers were more abundant and matured faster in the Inner ES graft than that in the Outer ES graft. In conclusion, compared to macrophages infiltrating from the circulating blood, transmural macrophages from outside promote the acute inflammatory-mediated response for vascular remodeling and subsequent collagen deposition within BVGs. STATEMENT OF SIGNIFICANCE: To elucidate the kinetics of macrophage infiltration into the bioresorbable vascular graft (BVG), two different bilayer arterial BVGs were implanted in mice as infrarenal abdominal aorta grafts. Cell migration into BVGs was higher in the inner electrospun graft which cells mainly infiltrate from outer surface than in the outer electrospun graft which cells mainly infiltrate from the circulating blood. In the inner electrospun grafts, the majority of macrophages changed from the M1 phenotype to the M2 phenotype, however, outer electrospun grafts maintained the M1 phenotype. Collagen fibers matured faster in the Inner electrospun graft. Compared to macrophages infiltrating from the circulating blood, transmural macrophages from outside promote the acute inflammatory-mediated response for vascular remodeling and subsequent collagen deposition within BVGs.

Improvement of a Novel Small-diameter Tissue-engineered Arterial Graft With Heparin Conjugation.

BACKGROUND: Small diameter (<6 mm), bioabsorbable, arterial, tissue-engineered vascular grafts (TEVGs) remain limited by thromboembolism. The objective of this study was to test whether heparin-eluting (HE) TEVGs prevent early thrombosis in a large animal model. METHODS: TEVGs were created with an outer poly-ε-caprolactone electrospun nanofiber layer, with a 15-µm average pore size and an inner layer composed of a 50:50 poly(L-lactide-co-ε-caprolactone) copolymer. Adult female sheep (n = 5) underwent bilateral carotid artery interposition grafting, with a control TEVG in 1 carotid artery and an HE TEVG in the contralateral position. Animals were followed for 8 weeks with weekly Duplex ultrasonography to monitor TEVG performance. RESULTS: All sheep survived to the designated endpoint. At 8 weeks all 5 HE TEVGs were patent. Three of 5 control TEVGs had early thrombotic occlusion at <1 week. More than 97% of heparin release occurred within the first 24 hours. Histologic evaluation of the HE TEVG displayed cellularity like a native carotid artery with no evidence of calcification. Significantly fewer platelets adhered to the HE TEVG than to the control TEVG (P < .001). CONCLUSIONS: This study suggests HE TEVGs prevent acute graft thrombosis. We hypothesize that the HE properties of the HE TEVG during vascular endothelialization is useful for maintaining TEVG patency. This technique may aid in the translation of small arterial TEVGs to the clinic.

Imatinib attenuates neotissue formation during vascular remodeling in an arterial bioresorbable vascular graft.

BACKGROUND: Bioresorbable vascular grafts (BVGs) can transform biologically into active blood vessels and represent an alternative to traditional synthetic conduits, which are prone to complications such as infection and thrombosis. Although platelet-derived growth factors and c-Kit positive cells play an important role in smooth muscle cell (SMC) migration and proliferation in vascular injury, atherosclerosis, or allograft, their roles in the vascular remodeling process of an arterial BVG remains unknown. Thus, we assessed the neottisue formation on arterial BVG remodeling by administrating imatinib, which is both a platelet-derived growth factor receptor kinase inhibitor and c-Kit receptor kinase inhibitor, in a murine model. METHODS: BVGs were composed of an inner poly(L-lactic-co-ε-caprolactone) copolymer sponge layer and an outer electrospun poly(L-lactic acid) nanofiber layer, which were implanted into the infrarenal abdominal aortas of C57BL/6 mice. After graft implantation, saline or 100 mg/kg of imatinib was administrated intraperitoneally daily for 2 weeks (n = 20 per group). Five mice in each group were scheduled to be humanely killed at 3 weeks and 15 at 8 weeks, and BVGs were explanted for histologic assessments. RESULTS: Graft patency during the 8-week observational period was not significantly different between groups (control, 86.7% vs imatinib, 80.0%; P > .999). Neotissue formation consisting of endothelialization, smooth muscle proliferation, and deposition of collagen and elastin was not observed in either group at 3 weeks. Similar endothelialization was achieved in both groups at 8 weeks, but thickness and percent area of neotissue formation were significantly higher in the control group than in the imatinib group, (thickness, 30. 1 ± 7.2 µm vs 19.6 ± 4.5 µm [P = .001]; percent area, 9.8 ± 2.7% vs 6.8 ± 1.8% [P = .005]). Furthermore, SMC layer and deposition of collagen and elastin were better organized at 8 weeks in the control group compared with the imatinib group. The thickness of SMC layer and collagen fiber area were significantly greater at 8 weeks in the control group than in the imatinib group (P < .001 and P = .026, respectively). Because there was no difference in the inner diameter of explanted BVGs (831.7 ± 63.4 µm vs 841.8 ± 41.9 µm; P = .689), neotissue formation was thought to advance toward the outer portion of the BVG with degradation of the polymer scaffold. CONCLUSIONS: Imatinib attenuates neotissue formation during vascular remodeling in arterial bioresorbable vascular grafts (BVGs) by inhibiting SMC layer formation and extracellular matrix deposition. CLINICAL RELEVANCE: This study demonstrated that imatinib attenuated neotissue formation during vascular remodeling in arterial Bioresorbable vascular graft (BVG) by inhibiting smooth muscle cell formation and extracellular matrix deposition. In addition, as imatinib did not modify the inner diameter of BVG, neotissue advanced circumferentially toward the outer portion of the neovessel. Currently, BVGs have not yet been clinically applied to the arterial circulation. The results of this study are helpful for the design of BVG that can achieve an optimal balance between polymer degradation and neotissue formation.

Bone marrow-derived mononuclear cell seeded bioresorbable vascular graft improves acute graft patency by inhibiting thrombus formation via platelet adhesion.

BACKGROUND: Acute thrombosis is a crucial cause of bioresorbable vascular graft (BVG) failure. Bone marrow-derived mononuclear cell (BM-MNC)-seeded BVGs demonstrated high graft patency, however, the effect of seeded BM-MNCs against thrombosis remains to be elucidated. Thus, we evaluated an antithrombotic effect of BM-MNC-seeding and utilized platelet-depletion mouse models to evaluate the contribution of platelets to acute thrombosis of BVGs. METHODS AND RESULTS: BVGs were composed of poly(glycolic acid) mesh sealed with poly(l-lactideco-ε-caprolactone). BM-MNC-seeded BVGs and unseeded BVGs were implanted to wild type C57BL/6 mice (n = 10/group) as inferior vena cava interposition conduits. To evaluate platelet effect on acute thrombosis, c-Mpl-/- mice and Pf4-Cre+; iDTR mice with decreased platelet number were also implanted with unseeded BVGs (n = 10/group). BVG patency was evaluated at 2, 4, and 8 weeks by ultrasound. BM-MNC-seeded BVGs demonstrated a significantly higher patency rate than unseeded BVGs during the acute phase (2-week, 90% vs 30%, p = .020), and patency rates of these grafts were sustained until week 8. Similar to BM-MNC-seeded BVGs, C-Mpl-/- and Pf4-Cre+; iDTR mice also showed favorable graft patency (2-week, 90% and 80%, respectively) during the acute phase. However, the patency rate of Pf4-Cre+; iDTR mice decreased gradually after DTR treatment as platelet number recovered to baseline. An in vitro study revealed BM-MNC-seeding significantly inhibited platelet adhesion to BVGs compared to unseeded BVGs, (1.75 ±â€¯0.45 vs 8.69 ±â€¯0.68 × 103 platelets/mm2, p < .001). CONCLUSIONS: BM-MNC-seeding and the reduction in platelet number prevented BVG thrombosis and improved BVG patency, and those results might be caused by inhibiting platelet adhesion to the BVG.

Ultrasonography Improves Glycemic Control by Detecting Insulin-Derived Localized Amyloidosis.

We examined the feasibility of ultrasound diagnosis of insulin-derived localized amyloidosis (IDLA). In addition to ultrasound detectability and findings, the insulin absorption rate, insulin dosage and hemoglobin A1c (HbA1c) levels before and after shifting the insulin injection site were investigated for 22 cases of IDLA. The detectability of IDLA on ultrasound was 100%; 59.1% was palpable lumps and 40.9% was not palpable. The palpable type had lower echo intensity and were harder than the non-palpable type. Blood flow decreased in IDLA, especially in the palpable type. IDLA, especially the palpable type, had a low insulin absorption rate. HbA1c level and insulin dosage decreased after shifting the injection site. The palpable type had more insulin reduction than the non-palpable type. Characteristic ultrasound images of IDLA were acquired. As the non-palpable type could be identified by ultrasound, its diagnosis encourages changing the insulin injection site; hence, ultrasound diagnosis of IDLA can enhance insulin treatment.

Localized Subcutaneous Insulin-Derived Amyloidosis Excised after Evaluation Using Ultrasonography in a Patient with Type 2 Diabetes Mellitus.

A 62-year-old man with type 2 diabetes mellitus, who had been on insulin therapy for the past 20 years, was found to have subcutaneous mass formation in the abdomen during a workup of worsened glycemic control. Because of suspected amyloid deposition, he was advised to avoid injections to the mass, which led to improvement of glycemic control. However, he strongly requested mass excision and was hospitalized. After evaluation using ultrasonography and computed tomography, a total mass excision was performed, and a diagnosis of insulin-derived amyloidosis was made. Comparison of the ultrasonographic and histopathological findings demonstrated that the location of the amyloid deposition nearly corresponded to the hypoechoic region. This case highlights that ultrasonography, which is a noninvasive imaging modality, can be useful for detection of insulin-derived amyloidosis.

Fast-degrading bioresorbable arterial vascular graft with high cellular infiltration inhibits calcification of the graft.

OBJECTIVE: Bioresorbable vascular grafts are biologically active grafts that are entirely reconstituted by host-derived cells through an inflammation-mediated degradation process. Calcification is a detrimental condition that can severely affect graft performance. Therefore, prevention of calcification is of great importance to the success of bioresorbable arterial vascular grafts. The objective of this study was to test whether fast-degrading (FD) bioresorbable arterial grafts with high cellular infiltration will inhibit calcification of grafts. METHODS: We created two versions of bioresorbable arterial vascular grafts, slow-degrading (SD) grafts and FD grafts. Both grafts had the same inner layer composed of a 50:50 poly(l-lactic-co-ε-caprolactone) copolymer scaffold. However, the outer layer of SD grafts was composed of poly(l-lactic acid) nanofiber, whereas the outer layer of FD grafts was composed of a combination of poly(l-lactic acid) and polyglycolic acid nanofiber. Both grafts were implanted in 8- to 10-week-old female mice (n = 15 in the SD group, n = 10 in the FD group) as infrarenal aortic interposition conduits. Animals were observed for 8 weeks. RESULTS: von Kossa staining showed calcification in 7 of 12 grafts in the SD group but zero in the FD group (P < .01, χ2 test). The cell number in the outer layer of FD grafts was significantly higher than in the SD grafts (SD, 0.87 ± 0.65 × 103/mm2; FD, 2.65 ± 1.91 × 103/mm2; P = .02). CONCLUSIONS: The FD bioresorbable arterial vascular graft with high cellular infiltration into the scaffold inhibited calcification of grafts.

Novel Bioresorbable Vascular Graft With Sponge-Type Scaffold as a Small-Diameter Arterial Graft.

BACKGROUND: Current commercialized small-diameter arterial grafts have not shown clinical effectiveness due to their poor patency rates. The present study evaluated the feasibility of an arterial bioresorbable vascular graft, which has a porous sponge-type scaffold, as a small-diameter arterial conduit. METHODS: The grafts were constructed by a 50:50 poly (1-lactic-co-ε-caprolactone) copolymer (PLCL) scaffold reinforced by a poly (1-lactic acid) (PLA) nanofiber. The pore size of the PLCL scaffold was adjusted to a small size (12.8 ± 1.85 µm) or a large size (28.5 ± 5.25 µm). We compared the difference in cellular infiltration, followed by tissue remodeling, between the groups. The grafts were implanted in 8- to 10-week-old female mice (n = 15 in each group) as infrarenal aortic interposition conduits. Animals were monitored for 8 weeks and euthanized to evaluate neotissue formation. RESULTS: No aneurysmal change or graft rupture was observed in either group. Histologic assessment demonstrated favorable cell infiltration into scaffolds, neointimal formation with endothelialization, smooth muscle cell proliferation, and elastin deposition in both groups. No significant difference was observed between the groups. Immunohistochemical characterization with anti-F4/80 antibody demonstrated that macrophage infiltration into the grafts occurred in both groups. Staining for M1 and M2, which are the two major macrophage phenotypes, showed no significant difference between groups. CONCLUSIONS: Our novel bioresorbable vascular grafts showed well-organized neointimal formation in the high-pressure arterial circulation environment. The large-pore scaffold did not improve cellular infiltration and neotissue formation compared with the small-pore scaffold.