We recently concluded our Ask the Expert session on The Dos and Don’ts of Cryopreservation. Cryopreservation is an integral activity in most cell culture labs. Cryopreservation permits the storage and keeping of cells, tissues, and 3D systems like organoids, for future use. How cryopreservation is managed, the materials used and equipment employed can greatly impact its success. For instance, if cells are frozen too quickly ice crystals can form causing membrane damage and cell death. Thus, successful cryopreservation requires thoughtful consideration of several key items, including cryoprotective agent selection, contamination testing, storage vessels, and cooling rate, just to name a few.
With so many things to consider, it is great to have expert help at hand to answer questions about the dos and don’ts of cryopreservation. For this Ask the Expert Session, we assembled a team of experts to answer questions on cryopreservation. Experts discussed freeze/thaw troubleshooting, best practice protocols, cell therapy applications and imaging considerations. In addition, vessel selection and DMSO use was also covered.
Please see the full transcript of questions and answers below.
Dr. Kyung-A Song is currently a Scientific Support Specialist supporting NA regions at Corning Life Sciences, where she answers customer inquiries regarding specifications, selection and application of all products in the Corning Life Sciences extensive portfolio. She graduated from Sungkyunkwan University with a Ph.D. in molecular biology and cancer immunology. She completed her postdoctoral fellowship in pharmaceutical research and drug discovery at Virginia Commonwealth University. She has been with Corning for 1.5 years.
Dr. Xiaoyu Zhu is Scientific Supporting Specialist supporting North America at Corning Life Sciences. He received the Ph.D. in Cell Biology in Beijing Normal University. Dr. Zhu has once been an associate professor at Chinese Center for Disease Control and Prevention (China CDC) and a Technical Application Scientist at Life Technologies, a subsidiary of Thermo Fisher Scientific at present. He supports the Corning Life Sciences portfolio, assisting customers with product selection, application support and troubleshooting. He has been with Corning for 8 years
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The two options are a matter of preference. Some researchers choose external thread designs since nothing gets inserted into the vial which may help minimize contamination. Some researchers choose internal thread designs because they fit better in their freezer boxes. If someone uses automation, they need to consider what thread can be used with their instrument grippers.
These homemade devices may work with many cell lines. However, they don’t always give controlled, reproducible, or uniform cooling. As a result, there may be serious differences in viability among the vials upon thawing. Therefore, we do not recommend using insulated cardboard or polystyrene foam for your culture.
Cryoprotective agents (CPAs) can be divided into two groups, intracellular and extracellular cryoprotectants. An intracellular CPA is a small molecule which can penetrate the cell membrane. DMSO, glycerol, ethylene glycol, propylene glycol and cell banker series belong to intracellular CPA. An extracellular CPA is a large molecule that is added to the cryoprotectant solution. Sucrose, dextrose, methylcellulose and polyvinylpyrrolidone (PVP) are examples (1).
DMSO has been successfully used for cryopreservation in a variety of cell types, including stem cells and dendritic cells which are used for cell therapy. According to the literature(2,3), many authors used freezing media containing FBS/HSA with 10% DMSO.
The use of PVP has been investigated as an alternative to cryopreservation with DMSO and fetal calf serum (FCS) in human adipose tissue-derived adult stem cells (4). The author observed that recovery of cells cryopreserved in 10% PVP with human serum was similar to cells cryopreserved in DMSO with animal serum. In addition, they used methylcellulose either alone or combined with reduced levels of DMSO which indicated that 1% methylcellulose produced comparable results with DMSO concentrations as low as 2% during an apoptosis assay.
We are having trouble with our iPSCs generating colonies after thaw. How long should we wait for colonies to form and do you think it is a problem with our cryopreservation protocol?
Cryogenic preservation of cell cultures is widely used to maintain backups or reserves of cells without the associated effort and expense of feeding and caring for them. The success of the freezing process depends on four critical areas:
- Good cell condition. iPSC should be fed daily before cryopreservation to get the healthier condition. Cells should be frozen after being passaged for 2-4 days. Overgrowth might make poorly viability after thawing. Before the cryopreservation, cell clumps should be dissolved. The cryoprotectant may be hard to penetrate the cell cluster, which results in only a small part of cells surviving after thawing. Properly handle and gently harvest the cultures. When collecting iPSC, we recommend centrifuging at 200 -300 X g for 2min, and operating pipettors gently. 1-2 x 106 cells/ml is the typical density of cryopreservation. Too high density might reduce the cell viability.
- Correct use of the cryoprotective agent. The most common cryoprotectant is DMSO. The final concentration of around 10% is most often used. For iPSC cryopreservation, some researchers also tend to add FBS or Ficoll to the freezing media. There are also some commercial products available in the market. To gain high recovery efficiency, researchers should use fresh medium materials. The cryoprotectant mix should be prepared on the day of the experiment.
- A controlled rate of freezing. The ideal cooling rate for cells is -1°C per minute. The best way to control cooling rates is by using electronic programmable freezing units. To control the cost, researchers can use the Corning® CoolCell™. When cells are stored into Corning cryogenic vials, the vials can be inserted into a room temperature CoolCell container. Place the CoolCell container upright into a -80°C freezer or dry ice locker.
- Storage under proper cryogenic conditions. Liquid nitrogen freezers permit storage in the vapor phase above the liquid at a temperature between -140°C and -180°C. Using vapor phase storage greatly reduces the possibility of leaky vials or ampules exploding during removal.
When thawing the iPSC:
- Cells should be thawed rapidly by placing the cryovials in a water bath set at 37°C.
- Use pipettor to transfer the cell suspension into 10 X volume of medium, drop by drop. The operation should be gentle and slow.
- The seeding density range for each 35mm well (6-well plate) is between 2x105 - 1x106 viable cells. Cells might attach to the Corning Matrigel® coated plate in 30 minutes after thawing. 70-80% confluence may be observed after 24-48 hours of plating.
We are having trouble getting liquid nitrogen right now. Do you have any suggestions of how to store our hiPSCs without it.
I would suggest cryopreserving the iPCS in the vapor phase above the liquid nitrogen in the long run. The temperature of the vapor phase can reach between -140°C and -180°C. Although -80°C is another common temperature to store mammalian cells, the cells tend to reduce viability within months of storage. Research has reported that adding 10% Ficoll 70 to the 10% DMSO containing cryoprotectant makes cells frozen at -80°C for one year without loss of viability, compared with liquid nitrogen storage. You may find more details on the paper below.
Our lab has been working to reduce DMSO in our media for hepatocyte cryopreservation. Do you have any recommendations on reducing DMSO along with the lowest percentages you’ve seen for this application.
There are many reports regarding cryopreservation of hepatocytes from different species like mouse, monkey, dog, rat, and human. According to many literature, the authors used 10-20% of DMSO for cryopreservation of hepatocytes. 10% of DMSO is most common concentration as a minimum of cryoprotectant for hepatocytes. If you are worried about cell viability, you can attempt to add oligosaccharides as a supplement in the freezing media that contains 10% of DMSO(1). The authors observed use of oligosaccharides resulted in greatest improvement in cell viability using trypan blue (TB) exclusion method. In addition, a researcher team in Sweden evaluated cell viability of hepatocytes using STEM-CELLBANKERTM (CB) which is a xeno-free cryopreservation solution containing 10% DMSO, glucose and anhydrous dextrose, comparing standard 12% DMSO-UW medium(2). They observed higher cell viability in CB protocols than DMSO-UW in hepatocytes.
I know this is a common issue, but do you have recommendations for improving cell viability post thaw. We have had varying success and I’m not sure why.
There are a variety of factors that are critical for successful cryopreservation such as cell harvesting, cryoprotective agents (CPAs), storage vessels, cooling rate, cryogenic storage and thawing. Unfortunately, we usually notice cell viability problems which are associated with cryopreservation after the thawing and plating steps. I would suggest four major check points that you need to consider for improving cell viability. First, cell health and cell density are critical when the cells are frozen. The healthier your cells are, the higher post thawing viability you may obtain. We recommend 2 x 106 cells for a typical Corning® cryogenic vial (Corning Cat. No. 430661 or 430489). If cell density is too high, the nutrients or CPAs might not be sufficient to maintain the health of cells. In addition, before freezing, the cells need to avoid excessive exposure to cell dissociation reagents or CPAs and keeping too long at room temperature during cell harvesting. Second, when you transfer the cryogenic vials containing resuspended cells with freezing media into a freezing container such as Corning CoolCell™ Container at -80°C, cooling rate (-1°C per minute) is an important factor for cell freezing. You need to use a suitable freezing container, type and concentration of CPAs during cell freezing. Third, you should avoid exposing the cells to warm temperatures during transfer to cryogenic storage (-196°C). Finally, thawing must be done quickly and CPAs removed properly in order to avoid toxicity or osmotic shock. I recently presented a webinar regarding a general guide to cryogenically storing animal cell cultures. Please refer to the webinar which is available on the Corning website, with particular attention to time mark 25:27 when I go over a troubleshooting methodology which may improve your viability.
How should we refreeze cells? We thawed lymphocytes after receiving them from another lab, then tried to freeze some of the sample again to use at a later time. When we thawed the cells that we froze again, we saw very low viability compared to the cells that were only thawed once. Is this to be expected?
Despite our best efforts to optimize CPA and the other numerous variables within a freezing/thawing protocol, cryopreservation is a traumatic process for cells. Therefore, observing some loss of viability after the freezing/thawing process is to be expected, and repeated cycles may result in progressive cell loss. Generally speaking, there are several factors that you need to consider when observing low cell viability: cell health and density when freezing, avoiding prolonged exposure to CPA or room temperature during harvesting, controlled and appropriate cooling rate, avoiding exposure to warm temperatures during transfer to cryogenic storage, and quick thawing and proper removal of CPAs. We have outlined this in more detail in a previous question. I would also suggest that you also check cell viability under microscopy before freezing in order to check dead cell populations in your vessels. To remove the dead cells, you can spin the PBMC with low speed centrifugation during the washing step.
Specifically for PBMCs, progressive loss of cell viability can occur as storage time increases(1). In addition, the disease of the individual from whom the PBMCs come also affects cryopreservation. According to literature(2), PBMCs from pediatric patients who are naturally infected with Dengue virus showed reduction of cell viability after cryopreservation, compared to fresh PBMCs from healthy children using the TB exclusion method.
I have included some PBMC protocols for your review. I would pay particular attention to PBMC centrifugation, DMSO and serum concentration, and freezing medium temperature(3,4).
Due to covid-19 we shut down our lab quickly and we were not able to freeze all our cells using LN2. How long would you expect iPSCs to be able to be stored at -80?
The impact of storage at -80°C to cells varies depending on the cell type. Days to months’ storage will affect the post-thawing cell status, including viability, property, or gene expression. A standard 10% DMSO freezing medium is insufficient to protect the pluripotency of cells. Three days of storage at -80 °C might cause a 50% loss of live cells and a 90% loss of Oct-4 expression. If the cryopreservation lasts three months, the damage turns worse. There is still the opportunity to wait for the recovery of pluripotency after thawing. But even 14 days culture after thawing won’t lead to full recovery.
Researchers also make efforts to develop better protocols to improve the long-term cryopreservation at -80 °C. It was reported that 10% Ficoll 70 in the freezing medium might extend the iPSC storage at -80 °C to one year with comparable viability, plating efficiency, and pluripotent phenotype versus LN2 storage. The improvements have also been achieved in red blood cells and peripheral hematopoietic stem cells.
The two papers below may help you to learn more details about this topic.
Although DMSO is an extensively applied cryoprotectant, there are still some disadvantages of this compound, such as neurological toxicity and genotoxicity. In cases of tissue freezing, researchers would like to avoid DMSO as the cryoprotectant, especially for downstream clinical applications. The selection of alternative compounds varies from tissue origin and the following application. Many DMSO alternatives have been proven on tissue freezing, including non-penetrating cryoprotectants (e.g., glucose, sucrose, galactose, or trehalose) and intracellular cryoprotectants (e.g., ethylene glycol, propylene glycol, glycerol, formamide, methanol, and butanediol). Some commercial products are also available in the market. You may find more details on the following references.
What is the best method for thaw of T cell samples from multiple labs. I don’t know each freezing protocol, so is there a best one size fits all method?
Several cryopreservation protocols have been successfully reported for T-cells. However, the best protocol of cryopreservation for your T-cell needs to be modified by your own testing since there are a variety of factors for successful cell freezing (please refer to Question #7 and #8 for detailed information). For cryoprotectant, many authors have used 5-10% DMSO with Fetal Calf Serum (FCS) or Human Serum Albumin (HAS), and they emphasized an appropriate cooling rate (-1°C per minute or slower) in T-cells(1,2,3). Please note that “There are reports on loss of response and reduced T-cell functions in frozen cells compared to freshly isolated cells and loss of antigen recognition of T-cells that increases with prolongation of freezing time”(4).
Is there a best way to prepare tissue samples for freezing if they will be heavily imaged post thaw? Also do you have recommendations for thaw when the tissue sample will be imaged?
There are some principles you should pay attention to when freezing tissues. Firstly, tissues should be frozen or fixed promptly to avoid morphological distortions. Secondly, be sure to keep fresh tissues on ice and process them as quickly as possible. Thirdly, the smaller size of the sample is preferred. Fourthly, keep the sample dry and avoid too much liquid before freezing. Thawing of tissue samples should be done rapidly in a 37°C water bath. For some tissues, blood vessels, and bone, controlling the thawing speeds may reduce tissue fragility. Besides these, the cryoprotectant or process will vary according to the organ. You may find more particular methods on the reference below.
Can I transfer cells that have been stored at -80 to liquid nitrogen directly or do I have to take interim steps?
In general, cryovials can be transferred from -80°C to -196°C directly for long-term storage.