Nature Commun. (2014)

Credit: © 2014 NPG

Progress in regenerative medicine and organ transplantation has resulted in a constant pressure on supplies of donated blood and organs. Deep-freezing, or cryopreservation, is one method of mitigating fluctuations in supply and demand, but is not routinely undertaken due to the levels of cell death induced by ice crystal formation and growth during the freezing and thawing processes. The current state-of-the-art in cryopreservation is replacement of water with organic solvents, followed by rapid freezing to achieve an ice-free state, known as vitrification. Issues with this process are the organic solvents used, which can be toxic to cells, and the possibility for devitrification — or ice formation — during thawing. However, the discovery of antifreeze proteins in Arctic fish has suggested that this vitrification process could be replaced by using compounds that mimic their action.

Now, Matthew Gibson and co-workers from the University of Warwick have demonstrated that poly(vinyl alcohol) (PVA) facilitates cryopreservation of both sheep and human blood. PVA is both non-toxic and widely available, and despite having few structural similarities to antifreeze proteins, still confers protection to cryopreserved red blood cells. Gibson and colleagues have also shown that only 0.1 wt% PVA is needed, in comparison with the 40 wt% organic solvents required in current vitrification-based cryopreservation processes.

Perhaps surprisingly, they also show that cryopreservation of the cells occurs despite the formation of small extracellular ice crystals, precisely what vitrification aims to avoid. This suggests that the mere presence of ice crystals is not a barrier to cryopreservation, but that the growth of these crystals during thawing is in fact the cause of cell death. They suggest that the success of PVA is due to its ability to inhibit ice crystal growth (pictured, right) compared with untreated cells (left). In conjunction with rapid thawing to avoid ice crystal growth instead of melting, its use may preclude the use of organic solvents and vitrification for cryopreservation in the future.