Preservation of microplate-attached human hepatoma cells and their use in cytotoxicity tests

Adherent cell thawing by infrared radiation

Cryopreservation of adherent cells is crucial for commercial cell therapy technology, including effective distribution and storage. Fast thawing has been shown to increase cell recovery in vitrified samples. Previously, radiofrequency (RF) has been investigated as a heating source on large samples, either with or without magnetic particles. Also, laser heating with the aid of dye or nanoparticles has been utilized on sub-millimeter samples successfully. For slow freezing cryopreservation methods, the influence of rate of thawing on viability is less clear. Cryopreservation of surface adhered cells result in many cases in detachment from the surface. We illustrate how intense infrared radiation from a focused halogen illuminator accelerates thawing. We show that two epithelial cell lines, retinal pigment epithelium cells and heterogeneous human epithelial colorectal adenocarcinoma cells, can be effectively cryopreserved and recovered using a combination of slow freezing and fast thawing under infrared illumination. We were able to successfully thaw samples, of 2-4 mm thick, including the media, on the order of a second, providing a heating rate of thousands of Kelvin per minute. Under optimal conditions, we observed higher post-thawing cell viability rates and higher cell adhesion with infrared thawing than with water bath thawing. We suggest that bulk warming with infrared radiation has an advantage over surface warming of surface-attached cells, as it alleviates cell stress during the process of thawing. These findings will pave the way for novel approaches to treating substrate-adhered cells and 3D scaffolds with cells and organoids. This technology may serve as a crucial component in lab-on-chip systems for medical testing and therapeutic use.

Directional freezing for the cryopreservation of adherent mammalian cells on a substrate

Successfully cryopreserving cells adhered to a substrate would facilitate the growth of a vital confluent cell culture after thawing while dramatically shortening the post-thaw culturing time. Herein we propose a controlled slow cooling method combining initial directional freezing followed by gradual cooling down to -80°C for robust preservation of cell monolayers adherent to a substrate. Using computer controlled cryostages we examined the effect of cooling rates and dimethylsulfoxide (DMSO) concentration on cell survival and established an optimal cryopreservation protocol. Experimental results show the highest post-thawing viability for directional ice growth at a speed of 30 μm/sec (equivalent to freezing rate of 3.8°C/min), followed by gradual cooling of the sample with decreasing rate of 0.5°C/min. Efficient cryopreservation of three widely used epithelial cell lines: IEC-18, HeLa, and Caco-2, provides proof-of-concept support for this new freezing protocol applied to adherent cells. This method is highly reproducible, significantly increases the post-thaw cell viability and can be readily applied for cryopreservation of cellular cultures in microfluidic devices.

Use of high concentrations of dimethyl sulfoxide for cryopreservation of HepG2 cells adhered to glass and polydimethylsiloxane matrices

Animal cells are generally cryopreserved in cryovials in a cell suspension state containing 5%-10% v/v dimethyl sulfoxide (DMSO) used as a cryoprotective agent. However, cryopreservation of cells in an attached state has not been intensively studied, and the effective freezing solution remains unknown. Here we determined the suitable DMSO concentration for the cryopreservation of human hepatoma HepG2 cells attached to glass and polydimethylsiloxane (PDMS) matrices coated with poly-l-lysine. With the use of the glass matrix, the rate of cell adhesion increased with the DMSO concentration up to 30% v/v in the freezing solution. In contrast, the cell-adhesion rate remained constant in the case of the PDMS matrix irrespective of the DMSO concentration between 10% v/v and 30% v/v. The viability of post-thawed cells attached to glass or PDMS matrix was also investigated. The viability was highest at the DMSO concentration of 20% v/v in the freezing solution. The DMSO concentration of 30% v/v, however, had a cytotoxic effect on the cell viability. Thus, the 20% v/v DMSO concentration was found to be most suitable for the cryopreservation of HepG2 cells in the attached state. This dose is high compared to the DMSO concentration used for the cryopreservation of cells in the suspended state.

Cryopreservation of adhered mammalian cells on a microfluidic device: Toward ready-to-use cell-based experimental platforms

In this paper, we describe cryopreservation of mammalian cells in the adhered state on a microfluidic device (microdevice) for the first time. HeLa, NIH3T3, MCF-7, and PC12 cells were cultured on a microdevice in which a commercial polystyrene dish surface was used as the cell adhesion surface. Without cell-detaching treatment, the microdevice was stored in a freezer at -80°C. After thawing, we observed a greater number of live cells on the microdevice than those on a control culture dish. Although the effectiveness of the microdevice varied depending on the cell type and surface coating, the trend was consistent. We confirmed that the phenotype of the PC12 cells to differentiate into neuron-like cells was kept after the on-chip cryopreservation, and that the results of cytotoxicity test of cisplatin against the HeLa cells were essentially unchanged by the on-chip cryopreservation. These findings will open up a new possibility of ready-to-use cell-based experimental platforms.

The 104-well microplate

The 96-well microplate is a ubiquitous tool in the laboratory; its use is so extensive that in a limited number of situations it can be restrictive. Consider the situation where 96 samples need analysis or a downstream process in which the 96-well format leaves no space for additional standards or controls in the upstream 96-well processing. Consequently, plates are split or sample number reduced thereby incurring additional cost for plates, reagents, standards, controls, sample tracking, data files, and time to analyze the entire plate. A simple solution is proposed with the development of a companion 8 × 13-array microplate. The 104-well microplate was developed within the American National Standards Institute/Society for Biomolecular Science standards as to plate geometry and dimension, including well spacing (9 mm) with the exception that the columns have been shifted 4.5mm to the left to accommodate the 13th column. The extra column allows for additional standards/controls without modifying chemistry, incorporating additional plates or changing to a 384-well plate. We show negligible difference (-0.0003 optical density) when comparing mean absorbance readings in 96- and 104-well format. We demonstrate use of the 104-well plate in a 96-well environment by incorporating it in an enzyme-linked immunosorbent assay on a standard liquid handler. Results from the assay show no difference between formats (y=1.039x-0.004, r=0.997). Although the 104 plate was not created to supplant the 96-well standard, we conclude that the 104 plate can be incorporated into the 96-well environment without significant change in existing systems.

Optimization, application, and interpretation of lactate dehydrogenase measurements in microwell determination of cell number and toxicity

The lactate dehydrogenase (LDH) assay was addressed for its sensitivity, disturbances by foaming, and cell number and size. Cells were from a U-251 MG grade IV human glioblastoma brain tumor cell line used in 100-microl well volumes. Cells were counted by microscopy and Coulter counting; assays were LDH or trypan blue. The results indicate increased 490 nm signals (level, variance) by using phenol red or by increasing fetal bovine serum from 5% to 10%. The data also indicate that defoaming results in reduced variances ranging from a factor of 2 at 1-3 units of absorption, up to a factor of 4-5 at <1 units of absorption. Coulter counting indicated a decrease in cell volume with increasing end-point cell density, attributed to general shrinking at increasing density. In comparisons, total LDH was considered relative to both cell total volume and cell numbers. The result suggests that total LDH should be regarded as reflecting cell total volume rather than cell numbers. In a comparative Cu exposure test, signals of both LDH and a sodium salt of 4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3-benzene disulfonate (WST-1) decreased with increasing Cu supply, while bromodeoxyuridine signals remained largely unaffected. The data show the differences in responses in cell viability and proliferation, but, above all, indicate that LDH should be expressed on a per cell volume basis rather than per cell, to avoid the problem that mere density effects contribute to signals on compound or metal toxicity.