CRYOPRESERVATION STRATEGY FOR TISSUE ENGINEERING CONSTRUCTS CONSISTING OF HUMAN MESENHYMAL STEM CELLS AND HYDROGEL BIOMATERIALS

BACKGROUND

The development of vitrification strategy for cell-biomaterial constructs, particularly biologically inspired nanoscale materials and hydrogels mimicking the in vivo environment is an active area. A cryopreservation strategy mimicking the in vivo environment for cell-hydrogel constructs may enhance cell proliferation and biological function.

OBJECTIVE

To demonstrate the efficacy of vitrification as a platform technology involving tissue engineering and human mesenchymal stem cells (hMSCs).

MATERIALS AND METHODS

Microcarriers made from alginate coated with chitosan and collagen are used. Conventional freezing and vitrification were compared. The vitrification strategy includes 10 min step-wise exposure to a vitrification solution (40% v/v EG, 0.6M sucrose) and immersion into liquid nitrogen.

RESULTS

Confocal imaging of live/dead staining of hMSCs cultured on the surface of microcarriers demonstrated that vitrified cells had excellent appearance and prolonged spindle shape morphology. The proliferation ability of post-vitrified cells arbitrated to protein Ki-67 gene expression was not significantly different in comparison to untreated control, while that of post-freezing cells was almost lost. The ability of hMSCs cultured on the surface of microcarriers to proliferate has been not affected by vitrification and it was significantly better after vitrification than after conventional freezing during continuous culture. Collagen II related mRNA expression by 4 weeks post-vitrification and post-freezing showed that ability to differentiate into cartilage was sustained during vitrification and reduced during conventional freezing. No significant difference was found between control and vitrification groups only.

CONCLUSION

Vitrification strategy coupled with advances in hMSC-expansion platform that completely preserves the ability of stem cells to proliferate and subsequently differentiate allows not only to reach a critical cell number, but also demonstrate prospects for effective utilization and transportation of cells with their support system, creating demand for novel biodegradable materials.

Effective Cryopreservation of Neural Stem or Progenitor Cells without Serum or Proteins by Vitrification

Development of effective cryopreservation protocols will be essential to realizing the potential for clinical application of neural stem and progenitor cells. Current cryopreservation protocols have been largely employed in research, which does not require as stringent consideration of viability and sterility. Therefore, these protocols involve the use of serum and protein additives, which can potentially introduce contaminants, and slow cooling with DMSO/glycerol-based cryopreservation solutions, which impairs cell survival. We investigated whether serum- and protein-free vitrification is effective for functional cryopreservation of neurosphere cultures of neural stem or progenitor cells. To protect the samples from introduction of other contaminants during handling and cryostorage, an original “straw-in-straw” method (250 μl sterile straw placed in 500 μl straw) for direct immersion into liquid nitrogen and storing the samples was also introduced. The protocol employed brief step-wise exposure to vitrification solution composed of ethylene glycol (EG) and sucrose (40% v/v EG, 0.6 M sucrose) and removal of vitrification solution at room temperature. Evaluation of the effects of vitrification revealed that there were no differences between control and vitrified neural stem or progenitor cells in expression of the neural stem or progenitor cell markers, proliferation, or multipotent differentiation. This sterile method for the xeno-free cryopreservation of murine neurospheres without animal or human proteins may have the potential to serve as a starting point for the development of cryopreservation protocols for human neural stem and progenitor cells for clinical use.

Effective cryopreservation of neural stem or progenitor cells without serum or proteins by vitrification

Development of effective cryopreservation protocols will be essential to realizing the potential for clinical application of neural stem and progenitor cells. Current cryopreservation protocols have been largely employed in research, which does not require as stringent consideration of viability and sterility. Therefore, these protocols involve the use of serum and protein additives, which can potentially introduce contaminants, and slow cooling with DMSO/glycerol-based cryopreservation solutions, which impairs cell survival. We investigated whether serum- and protein-free vitrification is effective for functional cryopreservation of neurosphere cultures of neural stem or progenitor cells. To protect the samples from introduction of other contaminants during handling and cryostorage, an original "straw-in-straw" method (250 microl sterile straw placed in 500 microl straw) for direct immersion into liquid nitrogen and storing the samples was also introduced. The protocol employed brief step-wise exposure to vitrification solution composed of ethylene glycol (EG) and sucrose (40% v/v EG, 0.6 M sucrose) and removal of vitrification solution at room temperature. Evaluation of the effects of vitrification revealed that there were no differences between control and vitrified neural stem or progenitor cells in expression of the neural stem or progenitor cell markers, proliferation, or multipotent differentiation. This sterile method for the xeno-free cryopreservation of murine neurospheres without animal or human proteins may have the potential to serve as a starting point for the development of cryopreservation protocols for human neural stem and progenitor cells for clinical use.

Vitrification Successfully Preserves Hepatocyte Spheroids

This is the first report on low-temperature preservation of self-assembled cell aggregates by vitrification, which is both a time- and cost-effective technology. We developed an effective protocol for vitrification (ice-free cryopreservation) of hepatocyte spheroids that employs rapid stepwise exposure to cryoprotectants (10.5 min) at room temperature and direct immersion into liquid nitrogen (-196°C). For this, three vitrification solutions (VS) were formulated and their effects on vitrified-warmed spheroids were examined. Cryopreservation using ethylene glycol (EG)-sucrose VS showed excellent preservation capability whereby highly preserved cell viability and integrity of vitrified spheroids were observed, through confocal and scanning electron microscopy imaging, when compared to untreated control. The metabolic functions of EG-sucrose VS-cryopreserved spheroids, as assessed by urea production and albumin secretion, were not significantly different from those of control within the same day of observation. In both the vitrification and control groups, albumin secretion was consistently high, ranging from 47.57 ± 14.39 to 70.38 ± 11.29 μg/106 cells and from 56.84 ± 14.48 to 71.79 ± 16.65 μg/106 cells, respectively, and urea production gradually increased through the culture period. The efficacy of vitrification procedure in preserving the functional ability of hepatocyte spheroids was not improved by introduction of a second penetrating cryoprotectant, 1,2-propanediol (PD). Spheroids cryopreserved with EG-PD-sucrose VS showed maintained cell viability; however, in continuous culture, levels of both metabolic functions were lower than those cryopreserved with EG-sucrose VS. EG-PD VS, in which nonpenetrating cryoprotectant (sucrose) was excluded, provided poor protection to spheroids during cryopreservation. This study demonstrated that sucrose plays an important role in the effective vitrification of self-assembled cell aggregates. In a broad view, the excellent results obtained suggest that the developed vitrification strategy, which is an alternative to freezing, may be effectively used as a platform technology in the field of cell transplantation.

Vitrification as a prospect for cryopreservation of tissue-engineered constructs

Cryopreservation plays a significant function in tissue banking and will presume yet larger value when more and more tissue-engineered products will routinely enter the clinical arena. The most common concept underlying tissue engineering is to combine a scaffold (cellular solids) or matrix (hydrogels) with living cells to form a tissue-engineered construct (TEC) to promote the repair and regeneration of tissues. The scaffold and matrix are expected to support cell colonization, migration, growth and differentiation, and to guide the development of the required tissue. The promises of tissue engineering, however, depend on the ability to physically distribute the products to patients in need. For this reason, the ability to cryogenically preserve not only cells, but also TECs, and one day even whole laboratory-produced organs, may be indispensable. Cryopreservation can be achieved by conventional freezing and vitrification (ice-free cryopreservation). In this publication we try to define the needs versus the desires of vitrifying TECs, with particular emphasis on the cryoprotectant properties, suitable materials and morphology. It is concluded that the formation of ice, through both direct and indirect effects, is probably fundamental to these difficulties, and this is why vitrification seems to be the most promising modality of cryopreservation.