Antibody-Suppressor CXCR5+CD8+ T Cells Are More Potent Regulators of Humoral Alloimmunity after Kidney Transplant in Mice Compared to CD4+ Regulatory T Cells

Adoptive cell therapy (ACT), especially with CD4+ regulatory T cells (CD4+ Tregs), is an emerging therapeutic strategy to minimize immunosuppression and promote long-term allograft acceptance, although much research remains to realize its potential. In this study, we investigated the potency of novel Ab-suppressor CXCR5+CD8+ T cells (CD8+ TAb-supp) in comparison with conventional CD25highFoxp3+CD4+ Tregs for suppression of humoral alloimmunity in a murine kidney transplant (KTx) model of Ab-mediated rejection (AMR). We examined quantity of peripheral blood, splenic and graft-infiltrating CD8+ TAb-supp, and CD4+ Tregs in KTx recipients and found that high alloantibody-producing CCR5 knockout KTx recipients have significantly fewer post-transplant peripheral blood and splenic CD8+ TAb-supp, as well as fewer splenic and graft-infiltrating CD4+ Tregs compared with wild-type KTx recipients. ACT with alloprimed CXCR5+CD8+ T cells reduced alloantibody titer, splenic alloprimed germinal center (GC) B cell quantity, and improved AMR histology in CCR5 knockout KTx recipients. ACT with alloprimed CD4+ Treg cells improved AMR histology without significantly inhibiting alloantibody production or the quantity of splenic alloprimed GC B cells. Studies with TCR transgenic mice confirmed Ag specificity of CD8+ TAb-supp-mediated effector function. In wild-type recipients, CD8 depletion significantly increased alloantibody titer, GC B cells, and severity of AMR pathology compared with isotype-treated controls. Anti-CD25 mAb treatment also resulted in increased but less pronounced effect on alloantibody titer, quantity of GC B cells, and AMR pathology than CD8 depletion. To our knowledge, this is the first report that CD8+ TAb-supp cells are more potent regulators of humoral alloimmunity than CD4+ Treg cells.

Tissue engineered vascular grafts are resistant to the formation of dystrophic calcification

Advancements in congenital heart surgery have heightened the importance of durable biomaterials for adult survivors. Dystrophic calcification poses a significant risk to the long-term viability of prosthetic biomaterials in these procedures. Herein, we describe the natural history of calcification in the most frequently used vascular conduits, expanded polytetrafluoroethylene grafts. Through a retrospective clinical study and an ovine model, we compare the degree of calcification between tissue-engineered vascular grafts and polytetrafluoroethylene grafts. Results indicate superior durability in tissue-engineered vascular grafts, displaying reduced late-term calcification in both clinical studies (p < 0.001) and animal models (p < 0.0001). Further assessments of graft compliance reveal that tissue-engineered vascular grafts maintain greater compliance (p < 0.0001) and distensibility (p < 0.001) than polytetrafluoroethylene grafts. These properties improve graft hemodynamic performance, as validated through computational fluid dynamics simulations. We demonstrate the promise of tissue engineered vascular grafts, remaining compliant and distensible while resisting long-term calcification, to enhance the long-term success of congenital heart surgeries.

Assessing the Biocompatibility and Regeneration of Electrospun-Nanofiber Composite Tracheal Grafts

OBJECTIVE

Composite tracheal grafts (CTG) combining decellularized scaffolds with external biomaterial support have been shown to support host-derived neotissue formation. In this study, we examine the biocompatibility, graft epithelialization, vascularization, and patency of three prototype CTG using a mouse microsurgical model.

STUDY DESIGN

Tracheal replacement, regenerative medicine, biocompatible airway splints, animal model.

METHOD

CTG electrospun splints made by combining partially decellularized tracheal grafts (PDTG) with polyglycolic acid (PGA), poly(lactide-co-ε-caprolactone) (PLCL), or PLCL/PGA were orthotopically implanted in mice (N = 10/group). Tracheas were explanted two weeks post-implantation. Micro-Computed Tomography was conducted to assess for graft patency, and histological analysis was used to assess for epithelialization and neovascularization.

RESULT

Most animals (greater than 80%) survived until the planned endpoint and did not exhibit respiratory symptoms. MicroCT confirmed the preservation of graft patency. Grossly, the PDTG component of CTG remained intact. Examining the electrospun component of CTG, PGA degraded significantly, while PLCL+PDTG and PLCL/PGA + PDTG maintained their structure. Microvasculature was observed across the surface of CTG and infiltrating the pores. There were no signs of excessive cellular infiltration or encapsulation. Graft microvasculature and epithelium appear similar in all groups, suggesting that CTG did not hinder endothelialization and epithelialization.

CONCLUSION

We found that all electrospun nanofiber CTGs are biocompatible and did not affect graft patency, endothelialization and epithelialization. Future directions will explore methods to accelerate graft regeneration of CTG.

LEVEL OF EVIDENCE

N/A Laryngoscope, 134:1155-1162, 2024.

CXCR5 + CD8 + T Cell-mediated Suppression of Humoral Alloimmunity and AMR in Mice Is Optimized With mTOR and Impaired With Calcineurin Inhibition

BACKGROUND

Adoptive cellular therapy (ACT) with antibody-suppressor CXCR5 + CD8 + T cells (CD8 + T Ab-supp ) inhibits alloantibody production, antibody-mediated rejection (AMR), and prolongs graft survival in multiple transplant mouse models. However, it is not known how conventional immunosuppressive agents impact the efficacy of CD8 + T Ab-supp ACT.

METHODS

We investigated the efficacy of CD8 + T Ab-supp cell ACT when combined with calcineurin inhibitor (CNi) or mammalian target of rapamycin inhibitor (mTORi) in a murine model of kidney transplant.

RESULTS

ACT-mediated decrease in germinal center B cells, posttransplant alloantibody titer, and amelioration of AMR in high alloantibody-producing CCR5 knockout kidney transplant recipients were impaired when ACT was combined with CNi and enhanced when combined with mTORi. CNi (but not mTORi) reduced ACT-mediated in vivo cytotoxicity of IgG + B cells and was associated with increased quantity of germinal center B cells. Neither CNi nor mTORi treatment impacted the expression of cytotoxic effector molecules (FasL, Lamp1, perforin, granzyme B) by CD8 + T Ab-supp after ACT. Concurrent treatment with CNi (but not mTORi) reduced in vivo proliferation of CD8 + T Ab-supp after ACT. The increase in quantity of splenic CD44 + CXCR5 + CD8 + T cells that occurs after ACT was reduced by concurrent treatment with CNi but not by concurrent treatment with mTORi (dose-dependent).

CONCLUSIONS

Impaired efficacy of ACT by CNi is attributed to reduced persistence and/or expansion of CD8 + T Ab-supp cells after ACT. In contrast, concurrent immunosuppression with mTORi preserves CD8 + T Ab-supp cells quantity, in vivo proliferation, and in vivo cytotoxic effector function after ACT and enhances suppression of humoral alloimmunity and AMR.

Quantification of Skeletal Muscle Perfusion in Peripheral Artery Disease Using 18F-Sodium Fluoride Positron Emission Tomography Imaging

BACKGROUND

Perfusion deficits contribute to symptom severity, morbidity, and death in peripheral artery disease (PAD); however, no standard method for quantifying absolute measures of skeletal muscle perfusion exists. This study sought to preclinically test and clinically translate a positron emission tomography (PET) imaging approach using an atherosclerosis-targeted radionuclide, fluorine-18-sodium fluoride (18F-NaF), to quantify absolute perfusion in PAD.

METHODS AND RESULTS

Eight Yorkshire pigs underwent unilateral femoral artery ligation and dynamic 18F-NaF PET/computed tomography imaging on the day of and 2 weeks after occlusion. Following 2-week imaging, calf muscles were harvested to quantify microvascular density. PET methodology was validated with microspheres in 4 additional pig studies and translated to patients with PAD (n=39) to quantify differences in calf perfusion across clinical symptoms/stages and perfusion responses in a case of revascularization. Associations between PET perfusion, ankle-brachial index, toe-brachial index, and toe pressure were assessed in relation to symptoms. 18F-NaF PET/computed tomography quantified significant deficits in calf perfusion in pigs following arterial occlusion and perfusion recovery 2 weeks after occlusion that coincided with increased muscle microvascular density. Additional studies confirmed that PET-derived perfusion measures agreed with microsphere-derived perfusion measures. Translation of imaging methods demonstrated significant decreases in calf perfusion with increasing severity of PAD and quantified perfusion responses to revascularization. Perfusion measures were also significantly associated with symptom severity, whereas traditional hemodynamic measures were not.

CONCLUSIONS

18F-NaF PET imaging quantifies perfusion deficits that correspond to clinical stages of PAD and represents a novel perfusion imaging strategy that could be partnered with atherosclerosis-targeted 18F-NaF PET imaging using a single radioisotope injection.

REGISTRATION

URL: https://www.clinicaltrials.gov; Unique identifier: NCT03622359.

Development of Novel Sutureless Balloon Expandable Fetal Heart Valve Device Using Absorbable Polycaprolactone Leaflets

Congenital heart disease (CHD) accounts for nearly one-third of all congenital defects, and patients often require repeated heart valve replacements throughout their lives, due to failed surgical repairs and lack of durability of bioprosthetic valve implants. This objective of this study is to develop and in vitro test a fetal transcatheter pulmonary valve replacement (FTPVR) using sutureless techniques to attach leaflets, as an option to correct congenital defects such as pulmonary atresia with intact ventricular septum (PA/IVS), in utero. A balloon expandable design was analyzed using computational simulations to identify areas of failure. Five manufactured valves were assembled using the unique sutureless approach and tested in the fetal right heart simulator (FRHS) to evaluate hemodynamic characteristics. Computational simulations showed that the commissural loads on the leaflet material were significantly reduced by changing the attachment techniques. Hemodynamic analysis showed an effective orifice area of 0.08 cm2, a mean transvalvular pressure gradient of 7.52 mmHg, and a regurgitation fraction of 8.42%, calculated over 100 consecutive cardiac cycles. In conclusion, the FTPVR exhibited good hemodynamic characteristics, and studies with biodegradable stent materials are underway.

A Fluid-Solid-Growth Solver for Cardiovascular Modeling

We implement full, three-dimensional constrained mixture theory for vascular growth and remodeling into a finite element fluid-structure interaction (FSI) solver. The resulting "fluid-solid-growth" (FSG) solver allows long term, patient-specific predictions of changing hemodynamics, vessel wall morphology, tissue composition, and material properties. This extension from short term (FSI) to long term (FSG) simulations increases clinical relevance by enabling mechanobioloigcally-dependent studies of disease progression in complex domains.

Evaluation of tissue-engineered human acellular vessels as a Blalock-Taussig-Thomas shunt in a juvenile primate model

Objectives

Palliative treatment of cyanotic congenital heart disease (CCHD) uses systemic-to-pulmonary conduits, often a modified Blalock-Taussig-Thomas shunt (mBTTs). Expanded polytetrafluoroethylene (ePTFE) mBTTs have associated risks for thrombosis and infection. The Human Acellular Vessel (HAV) (Humacyte, Inc) is a decellularized tissue-engineered blood vessel currently in clinical trials in adults for vascular trauma, peripheral artery disease, and end-stage renal disease requiring hemodialysis. In addition to restoring blood flow, the engineered HAV demonstrates the capacity for host cellular remodeling into native-like vasculature. Here we report preclinical evaluation of a small-diameter (3.5 mm) HAV as a mBTTs in a non-human primate model.

Methods

We implanted 3.5 mm HAVs as right subclavian artery to pulmonary artery mBTTs in non-immunosuppressed juvenile rhesus macaques (n = 5). HAV patency, structure, and blood flow were assessed by postoperative imaging from 1 week to 6 months. Histology of HAVs and surrounding tissues was performed.

Results

Surgical procedures were well tolerated, with satisfactory anastomoses, showing feasibility of using the 3.5 mm HAV as a mBTTs. All macaques had some immunological reactivity to the human extracellular matrix, as expected in this xenogeneic model. HAV mBTTs remained patent for up to 6 months in animals, exhibiting mild immunoreactivity. Two macaques displaying more severe immunoreactivity to the human HAV material developed midgraft dilatation without bleeding or rupture. HAV repopulation by host cells expressing smooth muscle and endothelial markers was observed in all animals.

Conclusions

These findings may support use of 3.5 mm HAVs as mBTTs in CCHD and potentially other pediatric vascular indications.

Functional mechanical behavior of the murine pulmonary heart valve

Genetically modified mouse models provide a versatile and efficient platform to extend our understanding of the underlying disease processes and evaluate potential treatments for congenital heart valve diseases. However, applications have been limited to the gene and molecular levels due to the small size of murine heart valves, which prohibits the use of standard mechanical evaluation and in vivo imaging methods. We have developed an integrated imaging/computational mechanics approach to evaluate, for the first time, the functional mechanical behavior of the murine pulmonary heart valve (mPV). We utilized extant mPV high resolution µCT images of 1-year-old healthy C57BL/6J mice, with mPVs loaded to 0, 10, 20 or 30 mmHg then chemically fixed to preserve their shape. Individual mPV leaflets and annular boundaries were segmented and key geometric quantities of interest defined and quantified. The resulting observed inter-valve variations were small and consistent at each TVP level. This allowed us to develop a high fidelity NURBS-based geometric model. From the resultant individual mPV geometries, we developed a mPV shape-evolving geometric model (SEGM) that accurately represented mPV shape changes as a continuous function of transvalvular pressure. The SEGM was then integrated into an isogeometric finite element based inverse model that estimated the individual leaflet and regional mPV mechanical behaviors. We demonstrated that the mPV leaflet mechanical behaviors were highly anisotropic and nonlinear, with substantial leaflet and regional variations. We also observed the presence of strong axial mechanical coupling, suggesting the important role of the underlying collagen fiber architecture in the mPV. When compared to larger mammalian species, the mPV exhibited substantially different mechanical behaviors. Thus, while qualitatively similar, the mPV exhibited important functional differences that will need to accounted for in murine heart valve studies. The results of this novel study will allow detailed murine tissue and organ level investigations of semi-lunar heart valve diseases.

Regeneration of tracheal neotissue in partially decellularized scaffolds

Extensive tracheal injury or disease can be life-threatening but there is currently no standard of care. Regenerative medicine offers a potential solution to long-segment tracheal defects through the creation of scaffolds that support the generation of healthy neotissue. We developed decellularized tracheal grafts (PDTG) by removing the cells of the epithelium and lamina propria while preserving donor cartilage. We previously demonstrated that PDTG support regeneration of host-derived neotissue. Here, we use a combination of microsurgical, immunofluorescent, and transcriptomic approaches to compare PDTG neotissue with the native airway and surgical controls. We report that PDTG neotissue is composed of native tracheal cell types and that the neoepithelium and microvasculature persisted for at least 6 months. Vascular perfusion of PDTG was established within 2 weeks and the graft recruited multipotential airway stem cells that exhibit normal proliferation and differentiation. Hence, PDTG neotissue recapitulates the structure and function of the host trachea and has the potential to regenerate.

Patch Materials for Pulmonary Artery Arterioplasty and Right Ventricular Outflow Tract Augmentation: A Review

Patch augmentation of the right ventricular outflow tract (RVOT) and pulmonary artery (PA) arterioplasty are relatively common procedures in the surgical treatment of patients with congenital heart disease. To date, several patch materials have been applied with no agreed upon clinical standard. Each patch type has unique performance characteristics, cost, and availability. There are limited data describing the various advantages and disadvantages of different patch materials. We performed a review of studies describing the clinical performance of various RVOT and PA patch materials and found a limited but growing body of literature. Short-term clinical performance has been reported for a multitude of patch types, but comparisons are limited by inconsistent study design and scarce histologic data. Standard clinical criteria for assessment of patch efficacy and criteria for intervention need to be applied across patch types. The field is progressing with improvements in outcomes due to newer patch technologies focused on reducing antigenicity and promoting neotissue formation which may have the ability to grow, remodel, and repair.

A Multimodal Approach to Quantify Chondrocyte Viability for Airway Tissue Engineering

OBJECTIVES/HYPOTHESIS

Partially decellularized tracheal scaffolds have emerged as a potential solution for long-segment tracheal defects. These grafts have exhibited regenerative capacity and the preservation of native mechanical properties resulting from the elimination of all highly immunogenic cell types while sparing weakly immunogenic cartilage. With partial decellularization, new considerations must be made about the viability of preserved chondrocytes. In this study, we propose a multimodal approach for quantifying chondrocyte viability for airway tissue engineering.

METHODS

Tracheal segments (5 mm) were harvested from C57BL/6 mice, and immediately stored in phosphate-buffered saline at -20°C (PBS-20) or biobanked via cryopreservation. Stored and control (fresh) tracheal grafts were implanted as syngeneic tracheal grafts (STG) for 3 months. STG was scanned with micro-computed tomography (μCT) in vivo. STG subjected to different conditions (fresh, PBS-20, or biobanked) were characterized with live/dead assay, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), and von Kossa staining.

RESULTS

Live/dead assay detected higher chondrocyte viability in biobanked conditions compared to PBS-20. TUNEL staining indicated that storage conditions did not alter the proportion of apoptotic cells. Biobanking exhibited a lower calcification area than PBS-20 in 3-month post-implanted grafts. Higher radiographic density (Hounsfield units) measured by μCT correlated with more calcification within the tracheal cartilage.

CONCLUSIONS

We propose a strategy to assess chondrocyte viability that integrates with vivo imaging and histologic techniques, leveraging their respective strengths and weaknesses. These techniques will support the rational design of partially decellularized tracheal scaffolds.

LEVEL OF EVIDENCE

N/A Laryngoscope, 133:512-520, 2023.

Tissue Engineering of Vascular Grafts: A Case Report From Bench to Bedside and Back

For over 25 years, our group has used regenerative medicine strategies to develop improved biomaterials for use in congenital heart surgery. Among other applications, we developed a tissue-engineered vascular graft (TEVG) by seeding tubular biodegradable polymeric scaffolds with autologous bone marrow-derived mononuclear cells. Results of our first-in-human study demonstrated feasibility as the TEVG transformed into a living vascular graft having an ability to grow, making it the first engineered graft with growth potential. Yet, outcomes of this first Food and Drug Administration-approved clinical trial evaluating safety revealed a prohibitively high incidence of early TEVG stenosis, preventing the widespread use of this promising technology. Mechanistic studies in mouse models provided important insight into the development of stenosis and enabled advanced computational models. Computational simulations suggested both a novel inflammation-driven, mechano-mediated process of in vivo TEVG development and an unexpected natural history, including spontaneous reversal of the stenosis. Based on these in vivo and in silico discoveries, we have been able to rationally design strategies for inhibiting TEVG stenosis that have been validated in preclinical large animal studies and translated to the clinic via a new Food and Drug Administration-approved clinical trial. This progress would not have been possible without the multidisciplinary approach, ranging from small to large animal models and computational simulations. This same process is expected to lead to further advances in scaffold design, and thus next generation TEVGs.

Implanted Tissue-Engineered Vascular Graft Cell Isolation with Single-Cell RNA Sequencing Analysis

The advent of single-cell RNA sequencing (scRNA-Seq) has brought with it the ability to gain greater insights into the cellular composition of tissues and heterogeneity in gene expression within specific cell types. For tissue-engineered blood vessels, this is particularly impactful to better understand how neotissue forms and remodels into tissue resembling a native vessel. A notable challenge, however, is the ability to separate cells from synthetic biomaterials to generate high-quality single-cell suspensions to interrogate the cellular composition of our tissue-engineered vascular grafts (TEVGs) during active remodeling in situ. We present here a simple, commercially available approach to separate cells within our TEVG from the residual scaffold for downstream use in a scRNA-Seq workflow. Utilizing this method, we identified the cell populations comprising explanted TEVGs and compared these with results from immunohistochemical analysis. The process began with explanted TEVGs undergoing traditional mechanical and enzymatic dissociation to separate cells from scaffold and extracellular matrix proteins. Magnetically labeled antibodies targeting murine origin cells were incubated with enzymatic digests of TEVGs containing cells and scaffold debris in suspension allowing for separation by utilizing a magnetic separator column. Single-cell suspensions were processed through 10 × Genomics and data were analyzed utilizing R to generate cell clusters. Expression data provided new insights into a diverse composition of phenotypically unique subclusters within the fibroblast, macrophage, smooth muscle cell, and endothelial cell populations contributing to the early neotissue remodeling stages of TEVGs. These populations were correlated qualitatively and quantitatively with immunohistochemistry highlighting for the first time the potential of scRNA-Seq to provide exquisite detail into the host cellular response to an implanted TEVG. These results additionally demonstrate magnetic cell isolation is an effective method for generating high-quality cell suspensions for scRNA-Seq. While this method was utilized for our group's TEVGs, it has broader applications to other implantable materials that use biodegradable synthetic materials as part of scaffold composition. Impact statement Single-cell RNA sequencing is an evolving technology with the ability to provide detailed information on the cellular composition of remodeling biomaterials in vivo. This present work details an effective approach for separating nondegraded biomaterials from cells for downstream RNA-sequencing analysis. We applied this method to implanted tissue-engineered vascular grafts and for the first time describe the cellular composition of the remodeling graft at a single-cell gene expression level. While this method was effective in our scaffold, it has broad applicability to other implanted biomaterials that necessitate separation of cell from residual scaffold materials for single-cell RNA sequencing.

Computational analysis of serum-derived extracellular vesicle miRNAs in juvenile sheep model of single stage Fontan procedure

Patients with single ventricle heart defects requires a series of staged open-heart procedures, termed Fontan palliation. However, while lifesaving, these operations are associated with significant morbidity and early mortality. The attendant complications are thought to arise in response to the abnormal hemodynamics induced by Fontan palliation, although the pathophysiology underlying these physicochemical changes in cardiovascular and other organs remain unknown. Here, we investigated the microRNA (miRNA) content in serum and serum-derived extracellular vesicles (EVs) by sequencing small RNAs from a physiologically relevant sheep model of the Fontan operation. The differential expression analysis identified the enriched miRNA clusters in (1) serum vs. serum-derived EVs and (2) pre-Fontan EVs vs. post-Fontan EVs. Metascape analysis showed that the overexpressed subset of EV miRNAs by Fontan procedure target liver-specific cells, underscoring a potentially important pathway involved in the liver dysfunction that occurs as a consequence of Fontan palliation. We also found that post-Fontan EV miRNAs were associated with senescence and cell death, whereas pre-Fontan EV miRNAs were associated with stem cell maintenance and epithelial-to-mesenchymal transition. This study shows great potential to identify novel circulating EV biomarkers from Fontan sheep serum that may be used for the diagnosis, prognosis, and therapeutics for patients that have undergone Fontan palliation.

Monocyte Recruitment for Vascular Tissue Regeneration

A strategy to recruit monocytes (MCs) from blood to regenerate vascular tissue from unseeded (cell-free) tissue engineered vascular grafts is presented. When immobilized on the surface of vascular grafts, the fusion protein, H2R5 can capture blood-derived MC under static or flow conditions in a shear stress dependent manner. The bound MC turns into macrophages (Mϕ) expressing both M1 and M2 phenotype specific genes. When H2R5 functionalized acellular-tissue engineered vessels (A-TEVs) are implanted into the mouse aorta, they remain patent and form a continuous endothelium expressing both endothelial cell (EC) and MC specific proteins. Underneath the EC layer, multiple cells layers are formed coexpressing both smooth muscle cell (SMC) and MC specific markers. Lineage tracing analysis using a novel CX3CR1-confetti mouse model demonstrates that fluorescently labeled MC populates the graft lumen by two and four weeks postimplantation, providing direct evidence in support of MC/Mϕ recruitment to the graft lumen. Given their abundance in the blood, circulating MCs may be a great source of cells that contribute directly to the endothelialization and vascular wall formation of acellular vascular grafts under the right chemical and biomechanical cues.

Rapid Regeneration of a Neoartery with Elastic Lamellae

Native arteries contain a distinctive intima-media composed of organized elastin and an adventitia containing mature collagen fibrils. In contrast, implanted biodegradable small-diameter vascular grafts do not present spatially regenerated, organized elastin. The elastin-containing structures within the intima-media region encompass the elastic lamellae (EL) and internal elastic lamina (IEL) and are crucial for normal arterial function. Here, the development of a novel electrospun small-diameter vascular graft that facilitates de novo formation of a structurally appropriate elastin-containing intima-media region following implantation is described. The graft comprises a non-porous microstructure characterized by tropoelastin fibers that are embedded in a PGS matrix. After implantation in mouse abdominal aorta, the graft develops distinct cell and extracellular matrix profiles that approximate the native adventitia and intima-media by 8 weeks. Within the newly formed intima-media region there are circumferentially aligned smooth muscle cell layers that alternate with multiple EL similar to that found in the arterial wall. By 8 months, the developed adventitia region contains mature collagen fibrils and the neoartery presents a distinct IEL with thickness comparable to that in mouse abdominal aorta. It is proposed that this new class of material can generate the critically required, organized elastin needed for arterial regeneration.

Tissue-engineered composite tracheal grafts create mechanically stable and biocompatible airway replacements

We tested composite tracheal grafts (CTG) composed of a partially decellularized tracheal graft (PDTG) combined with a 3-dimensional (3D)-printed airway splint for use in long-segment airway reconstruction. CTG is designed to recapitulate the 3D extracellular matrix of the trachea with stable mechanical properties imparted from the extraluminal airway splint. We performed segmental orthotopic tracheal replacement in a mouse microsurgical model. MicroCT was used to measure graft patency. Tracheal neotissue formation was quantified histologically. Airflow dynamic properties were analyzed using computational fluid dynamics. We found that CTG are easily implanted and did not result in vascular erosion, tracheal injury, or inflammation. Graft epithelialization and endothelialization were comparable with CTG to control. Tracheal collapse was absent with CTG. Composite tracheal scaffolds combine biocompatible synthetic support with PDTG, supporting the regeneration of host epithelium while maintaining graft structure.

Antibody-suppressor CXCR5+ CD8+ T cellular therapy ameliorates antibody-mediated rejection following kidney transplant in CCR5 KO mice

CCR5 KO kidney transplant (KTx) recipients are extraordinarily high alloantibody producers and develop pathology that mimics human antibody-mediated rejection (AMR). C57BL/6 and CCR5 KO mice (H-2^b ) were transplanted with A/J kidneys (H-2^a ); select cohorts received adoptive cell therapy (ACT) with alloprimed CXCR5⁺ CD8⁺ T cells (or control cells) on day 5 after KTx. ACT efficacy was evaluated by measuring posttransplant alloantibody, pathology, and allograft survival. Recipients were assessed for the quantity of CXCR5⁺ CD8⁺ T cells and CD8-mediated cytotoxicity to alloprimed IgG⁺ B cells. Alloantibody titer in CCR5 KO recipients was four-fold higher than in C57BL/6 recipients. The proportion of alloprimed CXCR5⁺ CD8⁺ T cells 7 days after KTx in peripheral blood, lymph node, and spleen was substantially lower in CCR5 KO compared to C57BL/6 recipients. In vivo cytotoxicity towards alloprimed IgG⁺ B cells was also reduced six-fold in CCR5 KO recipients. ACT with alloprimed CXCR5⁺ CD8⁺ T cells (but not alloprimed CXCR5^- CD8⁺ or third-party primed CXCR5⁺ CD8⁺ T cells) substantially reduced alloantibody titer, ameliorated AMR pathology, and prolonged allograft survival. These results indicate that a deficiency in quantity and function of alloprimed CXCR5⁺ CD8⁺ T cells contributes to high alloantibody and AMR in CCR5 KO recipient mice, which can be rescued with ACT.

The Potential Role of Regenerative Medicine on the Future Management of Hypoplastic Left Heart Syndrome

The development and translation of regenerative medicine approaches for the treatment of hypoplastic left heart syndrome (HLHS) provides a promising alternative to the current standard of care. We review the strategies that have been pursued to date and those that hold the greatest promise in moving forward. Significant challenges remain. Continued scientific advances and technological breakthroughs will be required if we are to translate this technology to the clinic and move from palliative to curative treatment.

Tracheal Macrophages During Regeneration and Repair of Long-Segment Airway Defects

OBJECTIVES/HYPOTHESIS

Tissue-engineered tracheal grafts (TETGs) offer a potential solution for repair of long-segment airway defects. However, preclinical and clinical TETGs have been associated with chronic inflammation and macrophage infiltration. Macrophages express great phenotypic heterogeneity (generally characterized as classically activated [M1] vs. alternatively activated [M2]) and can influence tracheal repair and regeneration. We quantified and characterized infiltrating host macrophages using mouse microsurgical tracheal replacement models.

STUDY DESIGN

Translational research, animal model.

METHODS

We assessed macrophage infiltration and phenotype in animals implanted with syngeneic tracheal grafts, synthetic TETGs, or partially decellularized tracheal scaffolds (DTSs).

RESULTS

Macrophage infiltration was observed following tracheal replacement with syngeneic trachea. Both M1 and M2 macrophages were present in native trachea and increased during early tracheal repair (P = .014), with an M1/M2 ratio of 0.48 ± 0.15. In contrast, orthotopic implantation of synthetic TETGs resulted in a shift to M1 predominant macrophage phenotype with an increased M1/M2 ratio of 1.35 ± 0.41 by 6 weeks following implant (P = .035). Modulation of the synthetic scaffold with the addition of polyglycolic acid (PGA) resulted in a reduction of M1/M2 ratio due to an increase in M2 macrophages (P = .006). Using systemic macrophage depletion, the M1/M2 ratio reverted to native values in synthetic TETG recipients and was associated with an increase in graft epithelialization. Macrophage ratios seen in DTSs were similar to native values.

CONCLUSIONS

M1 and M2 macrophages are present during tracheal repair. Poor epithelialization with synthetic TETG is associated with an elevation of the M1/M2 ratio. Macrophage phenotype can be altered with scaffold composition and host-directed systemic therapies. DTSs exhibit M1/M2 ratios similar to those seen in native trachea and syngeneic tracheal replacement.

LEVEL OF EVIDENCE

NA Laryngoscope, 132:737-746, 2022.

Fibrocytes: A Critical Review and Practical Guide

Fibrocytes are hematopoietic-derived cells that directly contribute to tissue fibrosis by producing collagen following injury, during disease, and with aging. The lack of a fibrocyte-specific marker has led to the use of multiple strategies for identifying these cells in vivo. This review will detail how past studies were performed, report their findings, and discuss their strengths and limitations. The motivation is to identify opportunities for further investigation and promote the adoption of best practices during future study design.

Tissue engineering: Relevance to neonatal congenital heart disease

Congenital heart disease (CHD) represents a large clinical burden, representing the most common cause of birth defect-related death in the newborn. The mainstay of treatment for CHD remains palliative surgery using prosthetic vascular grafts and valves. These devices have limited effectiveness in pediatric patients due to thrombosis, infection, limited endothelialization, and a lack of growth potential. Tissue engineering has shown promise in providing new solutions for pediatric CHD patients through the development of tissue engineered vascular grafts, heart patches, and heart valves. In this review, we examine the current surgical treatments for congenital heart disease and the research being conducted to create tissue engineered products for these patients. While much research remains to be done before tissue engineering becomes a mainstay of clinical treatment for CHD patients, developments have been progressing rapidly towards translation of tissue engineering devices to the clinic.

Tissue engineered vascular grafts transform into autologous neovessels capable of native function and growth

Background

Tissue-engineered vascular grafts (TEVGs) have the potential to advance the surgical management of infants and children requiring congenital heart surgery by creating functional vascular conduits with growth capacity.

Methods

Herein, we used an integrative computational-experimental approach to elucidate the natural history of neovessel formation in a large animal preclinical model; combining an in vitro accelerated degradation study with mechanical testing, large animal implantation studies with in vivo imaging and histology, and data-informed computational growth and remodeling models.

Results

Our findings demonstrate that the structural integrity of the polymeric scaffold is lost over the first 26 weeks in vivo, while polymeric fragments persist for up to 52 weeks. Our models predict that early neotissue accumulation is driven primarily by inflammatory processes in response to the implanted polymeric scaffold, but that turnover becomes progressively mechano-mediated as the scaffold degrades. Using a lamb model, we confirm that early neotissue formation results primarily from the foreign body reaction induced by the scaffold, resulting in an early period of dynamic remodeling characterized by transient TEVG narrowing. As the scaffold degrades, mechano-mediated neotissue remodeling becomes dominant around 26 weeks. After the scaffold degrades completely, the resulting neovessel undergoes growth and remodeling that mimicks native vessel behavior, including biological growth capacity, further supported by fluid-structure interaction simulations providing detailed hemodynamic and wall stress information.

Conclusions

These findings provide insights into TEVG remodeling, and have important implications for clinical use and future development of TEVGs for children with congenital heart disease.

The lysosomal trafficking regulator is necessary for normal wound healing

Non-healing wounds are a major threat to public health throughout the United States. Tissue healing is complex multifactorial process that requires synchronicity of several cell types. Endolysosomal trafficking, which contributes to various cell functions from protein degradation to plasma membrane repair, is an understudied process in the context of wound healing. The lysosomal trafficking regulator protein (LYST) is an essential protein of the endolysosomal system through an indeterminate mechanism. In this study, we examine the impact of impaired LYST function both in vitro with primary LYST mutant fibroblasts as well as in vivo with an excisional wound model. The wound model shows that LYST mutant mice have impaired wound healing in the form of delayed epithelialization and collagen deposition, independent of macrophage infiltration and polarisation. We show that LYST mutation confers a deficit in MCP-1, IGF-1, and IGFBP-2 secretion in beige fibroblasts, which are critical factors in normal wound healing. Identifying the mechanism of LYST function is important for understanding normal wound biology, which may facilitate the development of strategies to address problem wound healing.

Surgery and Sample Processing for Correlative Imaging of the Murine Pulmonary Valve

The underlying causes of heart valve related-disease (HVD) are elusive. Murine animal models provide an excellent tool for studying HVD, however, the surgical and instrumental expertise required to accurately quantify the structure and organization across multiple length scales have stunted its advancement. This work provides a detailed description of the murine dissection, en bloc staining, sample processing, and correlative imaging procedures for depicting the heart valve at different length scales. Hydrostatic transvalvular pressure was used to control the temporal heterogeneity by chemically fixing the heart valve conformation. Micro-computed tomography (µCT) was used to confirm the geometry of the heart valve and provide a reference for the downstream sample processing needed for the serial block face scanning electron microscopy (SBF-SEM). High-resolution serial SEM images of the extracellular matrix (ECM) were taken and reconstructed to provide a local 3D representation of its organization. µCT and SBF-SEM imaging methods were then correlated to overcome the spatial variation across the pulmonary valve. Though the work presented is exclusively on the pulmonary valve, this methodology could be adopted for describing the hierarchical organization in biological systems and is pivotal for the structural characterization across multiple length scales.

Hemodynamic performance of tissue-engineered vascular grafts in Fontan patients

In the field of congenital heart surgery, tissue-engineered vascular grafts (TEVGs) are a promising alternative to traditionally used synthetic grafts. Our group has pioneered the use of TEVGs as a conduit between the inferior vena cava and the pulmonary arteries in the Fontan operation. The natural history of graft remodeling and its effect on hemodynamic performance has not been well characterized. In this study, we provide a detailed analysis of the first U.S. clinical trial evaluating TEVGs in the treatment of congenital heart disease. We show two distinct phases of graft remodeling: an early phase distinguished by rapid changes in graft geometry and a second phase of sustained growth and decreased graft stiffness. Using clinically informed and patient-specific computational fluid dynamics (CFD) simulations, we demonstrate how changes to TEVG geometry, thickness, and stiffness affect patient hemodynamics. We show that metrics of patient hemodynamics remain within normal ranges despite clinically observed levels of graft narrowing. These insights strengthen the continued clinical evaluation of this technology while supporting recent indications that reversible graft narrowing can be well tolerated, thus suggesting caution before intervening clinically.

On the shape and structure of the murine pulmonary heart valve

Murine animal models are an established standard in translational research and provides a potential platform for studying heart valve disease. To date, studies on heart valve disease using murine models have been hindered by a lack of appropriate methodologies due to their small scale. In the present study, we developed a multi-scale, imaging-based approach to extract the functional structure and geometry for the murine heart valve. We chose the pulmonary valve (PV) to study, due to its importance in congenital heart valve disease. Excised pulmonary outflow tracts from eleven 1-year old C57BL/6J mice were fixed at 10, 20, and 30 mmHg to simulate physiological loading. Micro-computed tomography was used to reconstruct the 3D organ-level PV geometry, which was then spatially correlated with serial en-face scanning electron microscopy imaging to quantify local collagen fiber distributions. From the acquired volume renderings, we obtained the geometric descriptors of the murine PV under increasing transvalvular pressures, which demonstrated remarkable consistency. Results to date suggest that the preferred collagen orientation was predominantly in the circumferential direction, as in larger mammalian valves. The present study represents a first step in establishing organ-level murine models for the study of heart valve disease.

Regeneration of partially decellularized tracheal scaffolds in a mouse model of orthotopic tracheal replacement

Decellularized tracheal scaffolds offer a potential solution for the repair of long-segment tracheal defects. However, complete decellularization of trachea is complicated by tracheal collapse. We created a partially decellularized tracheal scaffold (DTS) and characterized regeneration in a mouse model of tracheal transplantation. All cell populations except chondrocytes were eliminated from DTS. DTS maintained graft integrity as well as its predominant extracellular matrix (ECM) proteins. We then assessed the performance of DTS in vivo. Grafts formed a functional epithelium by study endpoint (28 days). While initial chondrocyte viability was low, this was found to improve in vivo. We then used atomic force microscopy to quantify micromechanical properties of DTS, demonstrating that orthotopic implantation and graft regeneration lead to the restoration of native tracheal rigidity. We conclude that DTS preserves the cartilage ECM, supports neo-epithelialization, endothelialization and chondrocyte viability, and can serve as a potential solution for long-segment tracheal defects.

Spontaneous reversal of stenosis in tissue-engineered vascular grafts

We developed a tissue-engineered vascular graft (TEVG) for use in children and present results of a U.S. Food and Drug Administration (FDA)-approved clinical trial evaluating this graft in patients with single-ventricle cardiac anomalies. The TEVG was used as a Fontan conduit to connect the inferior vena cava and pulmonary artery, but a high incidence of graft narrowing manifested within the first 6 months, which was treated successfully with angioplasty. To elucidate mechanisms underlying this early stenosis, we used a data-informed, computational model to perform in silico parametric studies of TEVG development. The simulations predicted early stenosis as observed in our clinical trial but suggested further that such narrowing could reverse spontaneously through an inflammation-driven, mechano-mediated mechanism. We tested this unexpected, model-generated hypothesis by implanting TEVGs in an ovine inferior vena cava interposition graft model, which confirmed the prediction that TEVG stenosis resolved spontaneously and was typically well tolerated. These findings have important implications for our translational research because they suggest that angioplasty may be safely avoided in patients with asymptomatic early stenosis, although there will remain a need for appropriate medical monitoring. The simulations further predicted that the degree of reversible narrowing can be mitigated by altering the scaffold design to attenuate early inflammation and increase mechano-sensing by the synthetic cells, thus suggesting a new paradigm for optimizing next-generation TEVGs. We submit that there is considerable translational advantage to combined computational-experimental studies when designing cutting-edge technologies and their clinical management.

A computational bio-chemo-mechanical model of in vivo tissue-engineered vascular graft development

Stenosis is the primary complication of current tissue-engineered vascular grafts used in pediatric congenital cardiac surgery. Murine models provide considerable insight into the possible mechanisms underlying this situation, but they are not efficient for identifying optimal changes in scaffold design or therapeutic strategies to prevent narrowing. In contrast, computational modeling promises to enable time- and cost-efficient examinations of factors leading to narrowing. Whereas past models have been limited by their phenomenological basis, we present a new mechanistic model that integrates molecular- and cellular-driven immuno- and mechano-mediated contributions to in vivo neotissue development within implanted polymeric scaffolds. Model parameters are inferred directly from in vivo measurements for an inferior vena cava interposition graft model in the mouse that are augmented by data from the literature. By complementing Bayesian estimation with identifiability analysis and simplex optimization, we found optimal parameter values that match model outputs with experimental targets and quantify variability due to measurement uncertainty. Utility is illustrated by parametrically exploring possible graft narrowing as a function of scaffold pore size, macrophage activity, and the immunomodulatory cytokine transforming growth factor beta 1 (TGF-β1). The model captures salient temporal profiles of infiltrating immune and synthetic cells and associated secretion of cytokines, proteases, and matrix constituents throughout neovessel evolution, and parametric studies suggest that modulating scaffold immunogenicity with early immunomodulatory therapies may reduce graft narrowing without compromising compliance.

Tissue Engineered Vascular Graft Recipient Interleukin 10 Status Is Critical for Preventing Thrombosis

Tissue engineered vascular grafts (TEVGs) are a promising technology, but are hindered by occlusion. Seeding with bone-marrow derived mononuclear cells (BM-MNCs) mitigates occlusion, yet the precise mechanism remains unclear. Seeded cells disappear quickly and potentially mediate an anti-inflammatory effect through paracrine signaling. Here, a series of reciprocal genetic TEVG implantations plus recombinant protein treatment is reported to investigate what role interleukin-10, an anti-inflammatory cytokine, plays from both host and seeded cells. TEVGs seeded with BM-MNCs from wild-type and IL-10 KO mice, plus unseeded grafts, are implanted into wild-type and IL-10 KO mice. Wild-type mice with unseeded grafts also receive recombinant IL-10. Serial ultrasound evaluates occlusion and TEVGs are harvested at 14 d for immunohistochemical analysis. TEVGs in IL-10 KO mice have significantly higher occlusion incidence compared to wild-type mice attributed to acute (<3 d) thrombosis. Cell seeding rescues TEVGs in IL-10 KO mice comparable to wild-type patency. IL-10 from the host and seeded cells do not significantly influence graft inflammation and macrophage phenotype, yet IL-10 treatment shows interesting biologic effects including decreasing cell proliferation and increasing M2 macrophage polarization. IL-10 from the host is critical for preventing TEVG thrombosis and seeded BM-MNCs exert a significant anti-thrombotic effect in IL-10 KO mice.

Effects of Braiding Parameters on Tissue Engineered Vascular Graft Development

Tissue engineered vascular grafts (TEVGs) using scaffolds fabricated from braided poly(glycolic acid) (PGA) fibers coated with poly(glycerol sebacate) (PGS) are developed. The approach relies on in vivo tissue engineering by which neotissue forms solely within the body after a scaffold has been implanted. Herein, the impact of altering scaffold braid design and scaffold coating on neotissue formation is investigated. Several combinations of braiding parameters are manufactured and evaluated in a Beige mouse model in the infrarenal abdominal aorta. Animals are followed with 4D ultrasound analysis, and 12 week explanted vessels are evaluated for biaxial mechanical properties as well as histological composition. Results show that scaffold parameters (i.e., braiding angle, braiding density, and presence of a PGS coating) have interdependent effects on the resulting graft performance, namely, alteration of these parameters influences levels of inflammation, extracellular matrix production, graft dilation, neovessel distensibility, and overall survival. Coupling carefully designed in vivo experimentation with regression analysis, critical relationships between the scaffold design and the resulting neotissue that enable induction of favorable cellular and extracellular composition in a controlled manner are uncovered. Such an approach provides a potential for fabricating scaffolds with a broad range of features and the potential to manufacture optimized TEVGs.

Evaluating the Longevity of the Fontan Pathway

Children born with single ventricle physiology who undergo Fontan palliation face a diverse set of long-term complications. However, patient follow-up has in large part been limited to single institutional experiences without uniform application of diagnostic modalities to screen for relevant outcomes. Additionally, the use of different graft materials and variable surgical technique as part of the Fontan procedure has further complicated the evaluation of single ventricle patients. The purpose of this review is to define the changes in the Fontan pathway specific to the graft material used and its relationship to patient outcomes. As a means of introduction, we briefly review the historical evolution of the Fontan procedure with a focus on the intent behind design changes and incorporation of different biomaterials. We further delineate changes to the Fontan pathway which include the development of stenosis, differential growth, thrombosis, and calcification. Ultimately, the recognition of the changes noted within the Fontan pathway need to be assessed relative to their impact on patient hemodynamics, functional capacity, and Fontan-associated comorbidities.

The effect of pore diameter on neo-tissue formation in electrospun biodegradable tissue-engineered arterial grafts in a large animal model

To date, there has been little investigation of biodegradable tissue engineered arterial grafts (TEAG) using clinically relevant large animal models. The purpose of this study is to explore how pore size of electrospun scaffolds can be used to balance neoarterial tissue formation with graft structural integrity under arterial environmental conditions throughout the remodeling process. TEAGs were created with an outer poly-ε-caprolactone (PCL) electrospun layer and an inner sponge layer composed of heparin conjugated 50:50 poly (l-lactide-co-ε-caprolactone) copolymer (PLCL). Outer electrospun layers were created with four different pore diameters (4, 7, 10, and 15 µm). Fourteen adult female sheep underwent bilateral carotid artery interposition grafting (n = 3-4 /group). Our heparin-eluting TEAG was implanted on one side (n = 14) and ePTFE graft (n = 3) or non-heparin-eluting TEAG (n = 5) on the other side. Twelve of the fourteen animals survived to the designated endpoint at 8 weeks, and one animal with 4 µm pore diameter graft was followed to 1 year. All heparin-eluting TEAGs were patent, but those with pore diameters larger than 4 µm began to dilate at week 4. Only scaffolds with a pore diameter of 4 µm resisted dilation and could do so for up to 1 year. At 8 weeks, the 10 µm pore graft had the highest density of cells in the electrospun layer and macrophages were the primary cell type present. This study highlights challenges in designing bioabsorbable TEAGs for the arterial environment in a large animal model. While larger pore diameter TEAGs promoted cell infiltration, neotissue could not regenerate rapidly enough to provide sufficient mechanical strength required to resist dilation. Future studies will be focused on evaluating a smaller pore design to better understand long-term remodeling and determine feasibility for clinical use. STATEMENT OF SIGNIFICANCE: In situ vascular tissue engineering relies on a biodegradable scaffold that encourages tissue regeneration and maintains mechanical integrity until the neotissue can bear the load. Species-specific differences in tissue regeneration and larger mechanical forces often result in graft failure when scaling up from small to large animal models. This study utilizes a slow-degrading electrospun PCL sheath to reinforce a tissue engineered arterials graft. Pore size, a property critical to tissue regeneration, was controlled by changing PCL fiber diameter and the resulting effects of these properties on neotissue formation and graft durability was evaluated. This study is among few to report the effect of pore size on vascular neotissue formation in a large animal arterial model and also demonstrate robust neotissue formation.

Electrospun Tissue-Engineered Arterial Graft Thickness Affects Long-Term Composition and Mechanics

Wall stress is often lower in tissue-engineered constructs than in comparable native tissues due to the use of stiff polymeric materials having thicker walls. In this work, we sought to design a murine arterial graft having a more favorable local mechanical environment for the infiltrating cells; we used electrospinning to enclose a compliant inner core of poly(glycerol sebacate) with a stiffer sheath of poly(caprolactone) to reduce the potential for rupture. Two scaffolds were designed that differed in the thickness of the core as previous computational simulations found that circumferential wall stresses could be increased in the core toward native values by increasing the ratio of the core:sheath. Our modified electrospinning protocols reduced swelling of the core upon implantation and eliminated residual stresses in the sheath, both of which had contributed to the occlusion of implanted grafts during pilot studies. For both designs, a subset of implanted grafts occluded due to thrombosis or ruptured due to suspected point defects in the sheath. However, there were design-based differences in collagen content and mechanical behavior during early remodeling of the patent samples, with the thinner-core scaffolds having more collagen and a stiffer behavior after 12 weeks of implantation than the thicker-core scaffolds. By 24 weeks, the thicker-core scaffolds also became stiff, with similar amounts of collagen but increased smooth muscle cell and elastin content. These data suggest that increasing wall stress toward native values may provide a more favorable environment for normal arterial constituents to form despite the overall stiffness of the construct remaining elevated due to the absolute increase in load-bearing constituents.

Spatial and Temporal Analysis of Host Cells in Tracheal Graft Implantation

OBJECTIVES/HYPOTHESIS

The ideal trachea replacement would be a living graft that is genetically identical to the host, avoiding the need for immunosuppression. We have developed a mouse model of syngeneic tracheal transplant that results in long-term survival without graft stenosis or delayed healing. To understand how host cells contribute to tracheal transplant integration, we quantified the populations of host cells in the graft and native trachea following implant.

STUDY DESIGN

Tracheal transplant, tracheal replacement, regenerative medicine, animal model.

METHODS

Tracheal grafts were obtained from female C57BL/6 mice and orthotopically transplanted into syngeneic male recipients. Cohorts were euthanized on day 14, day 45, and day 90 post-transplantation. Host and graft tracheas were explanted and analyzed by histology. Male host cells were quantified using fluorescence in situ hybridization, and macrophages were quantified with immunofluorescence.

RESULTS

Evidence of host-derived cells was found in the midgraft at the earliest time point (14 days). Host-derived cells transiently increased in the graft on day 45 and were predominantly found in the submucosa. By day 90, the population of host-derived cells population declined to a similar level on day 14. Macrophage infiltration of host and graft tissue was observed at all time points and was greatest on day 90.

CONCLUSIONS

Tracheal graft integration occurs by way of subacute transient host-cell infiltration and is primarily inflammatory in nature. Host-cell contribution to the graft epithelium is limited. These data indicate that creation of living, nonimmunogenic tracheal graft could serve as a viable solution for long-segment tracheal defects.

LEVEL OF EVIDENCE

3 Laryngoscope, 131:E340-E345, 2021.

The evaluation of a tissue-engineered cardiac patch seeded with hips derived cardiac progenitor cells in a rat left ventricular model

BACKGROUND

Ventricular septal perforation and left ventricular aneurysm are examples of potentially fatal complications of myocardial infarction. While various artificial materials are used in the repair of these issues, the possibility of associated infection and calcification is non-negligible. Cell-seeded biodegradable tissue-engineered patches may be a potential solution. This study evaluated the feasibility of a new left ventricular patch rat model to study neotissue formation in biodegradable cardiac patches.

METHODS

Human induced pluripotent stem cell-derived cardiac progenitor cells (hiPS-CPCs) were cultured onto biodegradable patches composed of polyglycolic acid and a 50:50 poly (l-lactide-co-ε-caprolactone) copolymer for one week. After culturing, patches were implanted into left ventricular walls of male athymic rats. Unseeded controls were also used (n = 10/group). Heart conditions were followed by echocardiography and patches were subsequently explanted at 1, 2, 6, and 9 months post-implantation for histological evaluation.

RESULT

Throughout the study, no patches ruptured demonstrating the ability to withstand the high pressure left ventricular system. One month after transplantation, the seeded patch did not stain positive for human nuclei. However, many new blood vessels formed within patches with significantly greater vessels in the seeded group at the 6 month time point. Echocardiography showed no significant difference in left ventricular contraction rate between the two groups. Calcification was found inside patches after 6 months, but there was no significant difference between groups.

CONCLUSION

We have developed a surgical method to implant a bioabsorbable scaffold into the left ventricular environment of rats with a high survival rate. Seeded hiPS-CPCs did not differentiate into cardiomyocytes, but the greater number of new blood vessels in seeded patches suggests the presence of cell seeding early in the remodeling process might provide a prolonged effect on neotissue formation. This experiment will contribute to the development of a treatment model for left ventricular failure using iPS cells in the future.

Fetal Transcatheter Trileaflet Heart Valve Hemodynamics: Implications of Scaling on Valve Mechanics and Turbulence

The scarcity of data available on the best approach for pulmonary fetal valve replacement or implantation necessitate an investigation on whether practices using adult transcatheter valves could be translated to fetal applications. The objective of this study is to evaluate the hemodynamic characteristics and the turbulent properties of a fetal sized trileaflet transcatheter pulmonary valve in comparison with an adult balloon-expandable valve in order to assess the possibility of designing valves for fetal applications using dynamic similarity. A 6 mm fetal trileaflet valve and a 26 mm SAPIEN 3 valve were assessed in a pulse duplicator. Particle image velocimetry was performed. Pressure gradient (ΔP), effective orifice area (EOA), regurgitant fractions (RF), pinwheeling indices (PI) and turbulent stresses were evaluated. ΔP was 8.56 ± 0.139 and 7.76 ± 0.083 mmHg with fetal valve and SAPIEN respectively (p < 0.0001); EOA was 0.10 ± 0.0007 and 2.1 ± 0.025 cm2 with fetal valve and SAPIEN respectively (p < 0.0001); RF with the fetal valve was 2.35 ± 1.99% and with SAPIEN 10.92 ± 0.11% (p < 0.0001); PI with fetal valve was 0.404 ± 0.01 and with SAPIEN 0.37 ± 0.07; The flow regime with the fetal valve was turbulent and Reynolds numbers reached about 7000 while those with the SAPIEN reached about 20,000 at peak velocity. Turbulent stresses were significantly higher with fetal valve compared with SAPIEN. Instantaneous viscous shear stresses with fetal valve were 5.8 times higher than those obtained with SAPIEN and Reynolds shear stresses were 2.5 times higher during peak systole. The fetal valve implantation leads to a turbulent flow (specific to this particular type and design of valve) regime unlike what is expected of a small valve with different flow properties compared to adult valves.

Antibody-suppressor CD8+ T cells ameliorate antibody-mediated rejection following kidney transplant in mice

Purpose

CCR5 KO kidney transplant (KT) mice produce high titer alloantibody (alloAb) associated with severe antibody-mediated rejection (AMR) that reproduces AMR histology observed in human KT recipients. We previously reported the novel alloAb suppressor activity of alloprimed CXCR5+IFNγ+CD8+ T (Tab-supp) cells following hepatocellular transplant in mice. Here, we investigated the biologic impact of alloprimed CD8+ Tab-supp cells and their capacity to suppress alloAb following adoptive cell transfer (ACT) into CCR5 KO KT mice.

Methods

CCR5 KO (H-2b) mice were transplanted with allogeneic A/J (H-2a) kidneys, and on postoperative day (POD) 5 underwent ACT of alloprimed CD8+ Tab-supp cells (retrieved from C57BL/6 mice alloprimed with A/J alloantigen). A second cohort received concomitant bilateral native nephrectomy, and allograft survival was monitored with serial serum creatinine (&gt;100 μmol/L defines KT graft loss). Untreated CCR5 KO KT mice served as controls.

Results

CCR5 KO KT mice developed high alloAb titer compared to wild-type (WT) recipients (5,800±700 vs 1,200±100; p=0.0003). ACT significantly inhibited alloAb production by 5-fold (1,200±200; p&lt;0.0001) and POD14 AMR pathology (peritubular capillary margination and C4d deposition, arteritis) in CCR5 KO KT mice (composite histologic score 3.6±1.5 vs 8.4±0.2 in untreated controls; p=0.006). Following ACT, alloAb remained suppressed beyond POD 30, correlating with enhanced allograft survival (MST 52 vs 14 days; p=0.006).

Conclusion

ACT of CD8+ Tab-supp cells effectively inhibits alloAb production and AMR not only after allogeneic hepatocellular transplant, but also after vascularized solid organ transplant such as in CCR5 KO KT recipients.

Novel insights into high alloantibody production in CCR5 KO recipient mice

Purpose

Conventional immunosuppressants do not effectively control development of humoral alloimmunity in a substantial proportion of kidney allograft recipients. CCR5 KO (H-2b) mice are a useful model to study antibody-mediated rejection: A/J (H-2a) kidney transplant (KT) recipients produce 10x more alloantibody (alloAb) versus wild-type (WT, H-2b) mice. CXCR5+IFNγ+CD8+ T (Tab-supp) cells are a novel regulatory CD8+ T cell subset that can kill alloprimed IgG+ B cells and mediate suppression of in vivo alloAb in an allogeneic hepatocellular transplant (HcT) model. We hypothesize that high alloAb-producing CCR5 KO KT mice have reduced CD8+ Tab-supp cells.

Methods

Investigate CD8+ Tab-supp cells in peripheral blood and splenocytes of CCR5 KO compared to WT HcT and KT recipients using flow cytometry.

Results

CCR5 KO compared to WT HcT recipients had fewer splenic Tab-supp cells (8.75±2.73×103 vs 19.1±3.07×103; p=0.03). CCR5 KO compared to WT KT mice produce higher alloAb by postoperative day (POD) 14 (5,800±700 vs 1,200±100; p=.0003) and have reduced peripheral CD8+ Tab-supp cells (650±400 vs 11,500±450 per 106 PBMC; p=0.002). Alloprimed CD8+ Tab-supp cells injected into CCR5 KO KT mice are detected in the peripheral blood, spleen, and lymph nodes, but not in the allograft. CCR5 KO KT mice that receive adoptive transfer of CD8+ Tab-supp cells on POD5 produce significantly less alloAb than WT KT mice (1,200±200 vs 5,800±700; titer assayed on POD 14; p&lt;0.0001).

Conclusion

Deficiency of CD8+ Tab-supp cells is likely an important mechanism driving high alloAb production in CCR5 KO KT recipients. This subset exerts antibody suppressor function in lymphoid depots rather than in the kidney allograft.

Different degradation rates of nanofiber vascular grafts in small and large animal models

Nanofiber vascular grafts have been shown to create neovessels made of autologous tissue, by in vivo scaffold biodegradation over time. However, many studies on graft materials and biodegradation have been conducted in vitro or in small animal models, instead of large animal models, which demonstrate different degradation profiles. In this study, we compared the degradation profiles of nanofiber vascular grafts in a rat model and a sheep model, while controlling for the type of graft material, the duration of implantation, fabrication method, type of circulation (arterial/venous), and type of surgery (interposition graft). We found that there was significantly less remaining scaffold (i.e., faster degradation) in nanofiber vascular grafts implanted in the sheep model compared with the rat model, in both the arterial and the venous circulations, at 6 months postimplantation. In addition, there was more extracellular matrix deposition, more elastin formation, more mature collagen, and no calcification in the sheep model compared with the rat model. In conclusion, studies comparing degradation of vascular grafts in large and small animal models remain limited. For clinical translation of nanofiber vascular grafts, it is important to understand these differences.

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.

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.

Optimization of Tissue-Engineered Vascular Graft Design Using Computational Modeling

Tissue-engineered vascular grafts hold great promise in many clinical applications, especially in pediatrics wherein growth potential is critical. A continuing challenge, however, is identification of optimal scaffold parameters for promoting favorable neovessel development. In particular, given the countless design parameters available, including those related to polymeric microstructure, material behavior, and degradation kinetics, the number of possible scaffold designs is almost limitless. Advances in computationally modeling the growth and remodeling of native blood vessels suggest that similar simulations could help reduce the search space for candidate scaffold designs in tissue engineering. In this study, we meld a computational model of in vivo neovessel formation with a surrogate management framework to identify optimal scaffold designs for use in the extracardiac Fontan circulation while comparing the utility of different objective functions. We show that evolving luminal radius and graft compliance can be matched to that of the native vein by the end of the simulation period with judicious combinations of scaffold parameters, although the inability to match these metrics at all times reveals constraints engendered by current materials. We emphasize further that there is yet a need to examine additional metrics, and combinations thereof, when seeking to optimize functionality and reduce the potential for adverse outcomes. Impact Statement Tissue-engineered vascular grafts have considerable promise for treating myriad conditions, and multiple designs are now in FDA-approved trials. Nevertheless, the search continues for the optimal design of the underlying polymeric scaffold. We present a novel melding of a computational model of vascular adaptation and a formal method of optimization that can aid in identifying optimal design parameters, with potential to save development time and costs while improving clinical outcomes.