Impact of freezing/thawing procedures on the post-thaw viability of cryopreserved human saphenous vein conduits

Monitoring the quality of frozen-thawed venous segments using bioimpedance spectroscopy

OBJECTIVE

Storage at temperatures as low as -80 °C and below (cryopreservation) is considered a method for long-term preservation of cells and tissues, and especially blood vessel segments, which are to be used for clinical operations such as transplantation. However, the freezing and thawing processes themselves can induce injuries to the cells and tissue by damaging the structure and consequently functionality of the cryopreserved tissue. In addition, the level of damage is dependent on the rate of cooling and warming used during the freezing-thawing process. Current methods for monitoring the viability and integrity of cells and tissues after going through the freezing-thawing cycle are usually invasive and destructive to the cells and tissues. Therefore, employing monitoring methods which are not destructive to the cryopreserved tissues, such as bioimpedance measurement techniques, is necessary. In this study we aimed to design a bioimpedance measurement setup to detect changes in venous segments after freezing-thawing cycles in a noninvasive manner.

APPROACH

A bioimpedance spectroscopy measurement technique with a two-electrode setup was employed to monitor ovine jugular vein segments after each cycle during a process of seven freezing-thawing cycles.

MAIN RESULTS

The results demonstrated changes in the impedance spectra of the measured venous segments after each freezing-thawing cycle.

SIGNIFICANCE

This indicates that bioimpedance spectroscopy has the potential to be developed into a novel method for non-invasive and non-destructive monitoring of the viability of complex tissue after cryopreservation.

Human cadaver multipotent stromal/stem cells isolated from arteries stored in liquid nitrogen for 5 years

Introduction

Regenerative medicine challenges researchers to find noncontroversial, safe and abundant stem cell sources. In this context, harvesting from asystolic donors could represent an innovative and unlimited reservoir of different stem cells. In this study, cadaveric vascular tissues were established as an alternative source of human cadaver mesenchymal stromal/stem cells (hC-MSCs). We reported the successful cell isolation from postmortem arterial segments stored in a tissue-banking facility for at least 5 years.

Methods

After thawing, hC-MSCs were isolated with a high efficiency (12 × 10⁶) and characterized with flow cytometry, immunofluorescence, molecular and ultrastructural approaches.

Results

In early passages, hC-MSCs were clonogenic, highly proliferative and expressed mesenchymal (CD44, CD73, CD90, CD105, HLA-G), stemness (Stro-1, Oct-4, Notch-1), pericyte (CD146, PDGFR-β, NG2) and neuronal (Nestin) markers; hematopoietic and vascular markers were negative. These cells had colony and spheroid-forming abilities, multipotency for their potential to differentiate in multiple mesengenic lineages and immunosuppressive activity to counteract proliferation of phytohemagglutinin-stimulated blood mononuclear cells.

Conclusions

The efficient procurement of stem cells from cadaveric sources, as postmortem vascular tissues, demonstrates that such cells can survive to prolonged ischemic insult, anoxia, freezing and dehydration injuries, thus paving the way for a scientific revolution where cadaver stromal/stem cells could effectively treat patients demanding cell therapies.

Cryopreservation of vascular tissues

Cryopreservation of human blood vessels may become an important tool in bypass surgery and peripheral vascular reconstruction. Ideally cryopreservation of a blood vessel should preserve functional characteristics comparable to those of fresh controls. The key advantage of cryopreservation is the fact that storage at deep subzero temperatures allows storage of structurally intact living vascular tissues for virtually infinite time. Originally developed for long-time storage of isolated cells, the techniques of cryopreservation of tissues are challenged by the fact that these are complex multicellular systems containing diverse types of cells with differing requirements for optimal preservation. Therefore, the post-thaw functional activity of vascular tissues is determined by the type of blood vessel and, in addition, by the cell packing effect. Moreover, evidence from pharmacological studies suggests that cryopreservation induces tissue specific changes in transmembrane signaling and the mechanisms coupling intracellular calcium release, sensitivity and calcium entry into the smooth muscle cells.

Inhibiting ice recrystallization and optimization of cell viability after cryopreservation

The ice recrystallization inhibition activity of various mono- and disaccharides has been correlated with their ability to cryopreserve human cell lines at various concentrations. Cell viabilities after cryopreservation were compared with control experiments where cells were cryopreserved with dimethylsulfoxide (DMSO). The most potent inhibitors of ice recrystallization were 220 mM solutions of disaccharides; however, the best cell viability was obtained when a 200 mM d-galactose solution was utilized. This solution was minimally cytotoxic at physiological temperature and effectively preserved cells during freeze-thaw. In fact, this carbohydrate was just as effective as a 5% DMSO solution. Further studies indicated that the cryoprotective benefit of d-galactose was a result of its internalization and its ability to mitigate osmotic stress, prevent intracellular ice formation and/or inhibit ice recrystallization. This study supports the hypothesis that the ability of a cryoprotectant to inhibit ice recrystallization is an important property to enhance cell viability post-freeze-thaw. This cryoprotective benefit is observed in three different human cell lines. Furthermore, we demonstrated that the ability of a potential cryoprotectant to inhibit ice recrystallation may be used as a predictor of its ability to preserve cells at subzero temperatures.

Activated Rho/Rho kinase and modified calcium sensitivity in cryopreserved human saphenous veins

BACKGROUND

We have shown previously that cryopreservation of human internal mammary arteries activates protein kinase C and enhances intracellular Ca(2+) [Ca(2+)](i). We now present evidence that in human saphenous veins (HSV) cryoinjury is associated with activation of the Rho/Rho kinase signaling pathways and enhanced [Ca(2+)](i).

METHODS

HSV were investigated in vitro either unfrozen within 12h after removal or after storage at -196 degrees C in a cryomedium containing 1.8M dimethyl sulfoxide and 0.1M sucrose as cryoprotectant additives.

RESULTS

Cryostorage diminished responses to receptor-mediated contractile agonists such as noradrenaline, 5-HT and endothelin-1 by up to 30% whereas responses to KCl were attenuated by about 50%. Concentration-response curves for CaCl(2) on unfrozen and cryopreserved HSV revealed similar inhibitory activities of both blocking 1,4-dihydropyridine derivatives nifedipine and the (-)-(R) enantiomer of SDZ 202-791 whereas the Ca(2+) channel activating (+)-(S) enantiomer of SDZ 202-791 was 10 times less effective at enhancing contractions to CaCl(2) when tested after cryostorage. These functional effects were reflected by changes in [Ca(2+)](i) as demonstrated by fluorescence of Fluo-3AM loaded veins. The diminished activity of (+)-(S) SDZ 202-791 in cryopreserved HSV was reversed partially when the potassium channel opener pinacidil (1 microM) was present during the freezing/thawing process. Blockade of Rho kinase by HA-1077 proved to be significantly more effective at attenuating contractile responses to both endothelin-1 and KCl after cryostorage.

CONCLUSIONS

Data suggested that cryopreservation modified [Ca(2+)](i) of venous smooth muscle cells (1) through depolarization-induced changes in Ca(2+) influx and (2) through activation of Rho kinase signaling pathways.