Development of a simplified ice-free cryopreservation method for heart valves employing VS83, an 83% cryoprotectant formulation

Cryopreserved skin epithelial cell sheet combined with acellular amniotic membrane as an off-the-shelf scaffold for urethral regeneration

BACKGROUND

Autologous tissue transplantation for urethral repair is often limited and causes donor site complications. Here, a cryopreserved rabbit skin epithelial cell sheet (SEC) combined with an acellular amniotic membrane (AM) was used to repair rabbit urethral defects.

METHODS

Abdominal skin was collected from 4-week-old New Zealand rabbits, and primary epithelial cells were extracted and cultured to form a cell sheet. Fresh SEC-AMs were constructed and cryopreserved. A cryopreservation system including optimized medium, two-pump perfusion, a programmed freezer and liquid nitrogen storage was established. Cell viability, mechanical strength, electron microscopy, and histological staining were performed in vitro after 1 month. Next, the sheets were transplanted subcutaneously for 2 weeks, and the graft was used to repair the rabbit urethral defect. Urinary function was measured and samples were collected for histological staining after 1 month.

RESULTS

We confirmed that cryopreservation damage of SECs was reduced by composition with acellular AMs in terms of high cell activity. The SEC mechanical strength was also enhanced by AMs, which was convenient for the operation. In in vivo experiments, we transplanted sheets into the groin area for two weeks and found that cryopreservation reduced inflammatory cell infiltration and significantly improved vascular density. In the urethral repair experiment, the near-normal passive urine flow rate, smooth mucosa of the gross specimen, intact epithelialization and abundant neovascularization were confirmed in the cryopreserved-SEC-AM group compared with the other groups.

CONCLUSIONS

Cryopreserved SEC-AMs demonstrated similar outcomes of rabbit urethral defect repair as fresh SEC-AMs, showing good clinical application prospects.

Effect of intracellular uptake of nanoparticle-encapsulated trehalose on the hemocompatibility of allogeneic valves in the VS83 vitrification protocol

Trehalose is a disaccharide molecule consisting of two molecules of glucose. Industrially, trehalose is derived from corn starch and utilized as a drug. This study aims to examine whether the integration of nanoparticle-encapsulated trehalose to the Ice-Free Cryopreservation (IFC) method for preserving heart valves has better cell viability, benefits to protect the extracellular matrix (ECM), and reduce immune response after storage. For the experiment to be carried out, we obtained materials, and the procedures were carried out in the following manner. The initial step was the preparation of hydroxyapatite nanoparticles, followed by precipitation to acquire Apatite colloidal suspensions. Animals were obtained, and their tissue isolation and grouping were done ethically. All samples were then divided into four groups, Control group, Conventional Frozen Cryopreservation (CFC) group, IFC group, and IFC + T (IFC with the addition of 0.2 M nanoparticle-encapsulated Trehalose) group. Histological analysis was carried out via H&E staining, ECM components were stained with Modified Weigert staining, and the Gomori Ammonia method was used to stain reticular fibers. Alamar Blue assay was utilized to assess cell viability. Hemocompatibility was evaluated, and samples were processed for immunohistochemistry (TNFα and IL-10). Hemocompatibility was quantified using Terminal Complement Complex (TCC) and Neutrophil elastase (NE) as an indicator. The results of the H&E staining revealed less formation of extracellular ice crystals and intracellular vacuoles in the IFC + T group compared with all other groups. The CFC group’s cell viability showed better viability than the IFC group, but the highest viability was exhibited in the IFC + T group (70.96 ± 2.53, P < 0.0001, n = 6). In immunohistochemistry, TNFα levels were lowest in both IFC and IFC + T group, and IL-10 expression had significantly reduced in IFC and IFC + T group. The results suggested that the nanoparticle encapsulated trehalose did not show significant hemocompatibility issues on the cryopreserved heart valves.

Preservation strategies for decellularized pericardial scaffolds for off-the-shelf availability

Decellularized biological scaffolds hold great promise in cardiovascular surgery. In order to ensure off-the-shelf availability, routine use of decellularized scaffolds requires tissue banking. In this study, the suitability of cryopreservation, vitrification and freeze-drying for the preservation of decellularized bovine pericardial (DBP) scaffolds was evaluated. Cryopreservation was conducted using 10% DMSO and slow-rate freezing. Vitrification was performed using vitrification solution (VS83) and rapid cooling. Freeze-drying was done using a programmable freeze-dryer and sucrose as lyoprotectant. The impact of the preservation methods on the DBP extracellular matrix structure, integrity and composition was assessed using histology, biomechanical testing, spectroscopic and thermal analysis, and biochemistry. In addition, the cytocompatibility of the preserved scaffolds was also assessed. All preservation methods were found to be suitable to preserve the extracellular matrix structure and its components, with no apparent signs of collagen deterioration or denaturation, or loss of elastin and glycosaminoglycans. Biomechanical testing, however, showed that the cryopreserved DBP displayed a loss of extensibility compared to vitrified or freeze-dried scaffolds, which both displayed similar biomechanical behavior compared to non-preserved control scaffolds. In conclusion, cryopreservation altered the biomechanical behavior of the DBP scaffolds, which might lead to graft dysfunction in vivo. In contrast to cryopreservation and vitrification, freeze-drying is performed with non-toxic protective agents and does not require storage at ultra-low temperatures, thus allowing for a cost-effective and easy storage and transport. Due to these advantages, freeze-drying is a preferable method for the preservation of decellularized pericardium. STATEMENT OF SIGNIFICANCE: Clinical use of DBP scaffolds for surgical reconstructions or substitutions requires development of a preservation technology that does not alter scaffold properties during long-term storage. Conclusive investigation on adverse impacts of the preservation methods on DBP matrix integrity is still missing. This work is aiming to close this gap by studying three potential preservation technologies, cryopreservation, vitrification and freeze-drying, in order to achieve the off-the-shelf availability of DBP patches for clinical application. Furthermore, it provides novel insights for dry-preservation of decellularized xenogeneic scaffolds that can be used in the routine clinical cardiovascular practice, allowing the surgeon the opportunity to choose an ideal implant matching with the needs of each patient.

The choice of cryopreservation method affects immune compatibility of human cardiovascular matrices

Conventional frozen cryopreservation (CFC) is currently the gold standard for cardiovascular allograft preservation. However, inflammation and structural deterioration limit transplant durability. Ice-free cryopreservation (IFC) already demonstrated matrix structure preservation combined with attenuated immune responses. In this study, we aim to explore the mechanisms of this diminished immunogenicity in vitro. First, we characterized factors released by human aortic tissue after CFC and IFC. Secondly, we analyzed co-cultures with human peripheral blood mononuclear cells, purified monocytes, T cells and monocyte-derived macrophages to examine functional immune effects triggered by the tissue or released cues. IFC tissue exhibited significantly lower metabolic activity and release of pro-inflammatory cytokines than CFC tissue, but surprisingly, more active transforming growth factor β. Due to reduced cytokine release by IFC tissue, less monocyte and T cell migration was detected in a chemotaxis system. Moreover, only cues from CFC tissue but not from IFC tissue amplified αCD3 triggered T cell proliferation. In a specifically designed macrophage-tissue assay, we could show that macrophages did not upregulate M1 polarization markers (CD80 or HLA-DR) on either tissue type. In conclusion, IFC selectively modulates tissue characteristics and thereby attenuates immune cell attraction and activation. Therefore, IFC treatment creates improved opportunities for cardiovascular graft preservation.

Vitrification of heart valve tissues

Application of the original vitrification protocol used for pieces of heart valves to intact heart valves has evolved over time. Ice-free cryopreservation by Protocol 1 using VS55 is limited to small samples where relatively rapid cooling and warming rates are possible. VS55 cryopreservation typically provides extracellular matrix preservation with approximately 80 % cell viability and tissue function compared with fresh untreated tissues. In contrast, ice-free cryopreservation using VS83, Protocols 2 and 3, has several advantages over conventional cryopreservation methods and VS55 preservation, including long-term preservation capability at -80 °C; better matrix preservation than freezing with retention of material properties; very low cell viability, reducing the risks of an immune reaction in vivo; reduced risks of microbial contamination associated with use of liquid nitrogen; improved in vivo functions; no significant recipient allogeneic immune response; simplified manufacturing process; increased operator safety because liquid nitrogen is not used; and reduced manufacturing costs.