Cryopreservation and Rapid Recovery of Differentiated Intestinal Epithelial Barrier Cells at Complex Transwell Interfaces Is Enabled by Chemically Induced Ice Nucleation

Cell-based models, such as organ-on-chips, can replace and inform in vivo (animal) studies for drug discovery, toxicology, and biomedical science, but most cannot be banked "ready to use" as they do not survive conventional cryopreservation with DMSO alone. Here, we demonstrate how macromolecular ice nucleators enable the successful cryopreservation of epithelial intestinal models supported upon the interface of transwells, allowing recovery of function in just 7 days post-thaw directly from the freezer, compared to 21 days from conventional suspension cryopreservation. Caco-2 cells and Caco-2/HT29-MTX cocultures are cryopreserved on transwell inserts, with chemically induced ice nucleation at warmer temperatures resulting in increased cell viability but crucially retaining the complex cellular adhesion on the transwell insert interfaces, which other cryoprotectants do not. Trans-epithelial electrical resistance measurements, confocal microscopy, histology, and whole-cell proteomics demonstrated the rapid recovery of differentiated cell function, including the formation of tight junctions. Lucifer yellow permeability assays confirmed that the barrier functions of the cells were intact. This work will help solve the long-standing problem of transwell tissue barrier model storage, facilitating access to advanced predictive cellular models. This is underpinned by precise control of the nucleation temperature, addressing a crucial biophysical mode of damage.

Cryopreservation of assay-ready hepatocyte monolayers by chemically-induced ice nucleation: preservation of hepatic function and hepatotoxicity screening capabilities

Cell culture plays a critical role in biomedical discovery and drug development. Primary hepatocytes and hepatocyte-derived cell lines are especially important cellular models for drug discovery and development. To enable high-throughput screening and ensure consistent cell phenotypes, there is a need for practical and efficient cryopreservation methods for hepatocyte-derived cell lines and primary hepatocytes in an assay-ready format. Cryopreservation of cells as adherent monolayers in 96-well plates presents unique challenges due to low volumes being susceptible to supercooling, leading to low recovery and well-to-well variation. Primary cell cryopreservation is also particularly challenging due to the loss of cell viability and function. In this study, we demonstrate the use of soluble ice nucleator materials (IN) to cryopreserve a hepatic-derived cell line (HepG2) and primary mouse hepatocytes, as adherent monolayers. HepG2 cell recovery was near 100% and ∼75% of primary hepatocytes were recovered 24 hours post-thaw compared to just 10% and 50% with standard 10% DMSO, respectively. Post-thaw assessment showed that cryopreserved HepG2 cells retain membrane integrity, metabolic activity, proliferative capacity and differentiated hepatic functions including urea secretion, cytochrome P450 levels and lipid droplet accumulation. Cryopreserved primary hepatocytes exhibited reduced hepatic functions compared to fresh hepatocytes, but functional levels were similar to commercial suspension-cryopreserved hepatocytes, with the added benefit of being stored in an assay-ready format. In addition, normal cuboidal morphology and minimal membrane damage were observed 24 hours post-thaw. Cryopreserved HepG2 and mouse hepatocytes treated with a panel of pharmaceutically active compounds produced near-identical dose-response curves and EC50 values compared to fresh hepatocytes, confirming the utility of cryopreserved bankable cells in drug metabolism and hepatotoxicity studies. Cryopreserved adherent HepG2 cells and primary hepatocytes in 96 well plates can significantly reduce the time and resource burden associated with routine cell culture and increases the efficiency and productivity of high-throughput drug screening assays.

Cryopreservation of Liver-Cell Spheroids with Macromolecular Cryoprotectants

Spheroids are a powerful tool for basic research and to reduce or replace in vivo (animal) studies but are not routinely banked nor shared. Here, we report the successful cryopreservation of hepatocyte spheroids using macromolecular (polyampholyte) cryoprotectants supplemented into dimethyl sulfoxide (DMSO) solutions. We demonstrate that a polyampholyte significantly increases post-thaw recovery, minimizes membrane damage related to cryo-injury, and remains in the extracellular space making it simple to remove post-thaw. In a model toxicology challenge, the thawed spheroids matched the performance of fresh spheroids. F-actin staining showed that DMSO-only cryopreserved samples had reduced actin polymerization, which the polyampholyte rescued, potentially linked to intracellular ice formation. This work may facilitate access to off-the-shelf and ready-to-use frozen spheroids, without the need for in-house culturing. Readily accessible 3-D cell models may also reduce the number of in vivo experiments.

Imaging of antitubercular dimeric boronic acids at the mycobacterial cell surface by click-probe capture

Dimeric boronic acids kill Mycobacterium tuberculosis (Mtb) by targeting mycobacterial specific extracellular glycans, removing the requirement for a therapeutic agent to permeate the complex cell envelope. Here we report the successful development and use of new ‘clickable’ boronic acid probes as a powerful method to enable the direct detection and visualisation of this unique class of cell-surface targeting antitubercular agents.

Antitubercular ‘clickable’ diboronic acid agents are directly incorporated into the mycobacterial cell envelope through glycan-targeting.

Assay-ready Cryopreserved Cell Monolayers Enabled by Macromolecular Cryoprotectants

Cell monolayers underpin the discovery and screening of new drugs and allow for fundamental studies of cell biology and disease. However, current cryopreservation technologies do not allow cells to be stored frozen while attached to tissue culture plastic. Hence, cells must be thawed from suspension, cultured for several days or weeks, and finally transferred into multiwell plates for the desired application. This inefficient process consumes significant time handling cells, rather than conducting biomedical research or other value-adding activities. Here, we demonstrate that a synthetic macromolecular cryoprotectant enables the routine, reproducible, and robust cryopreservation of biomedically important cell monolayers, within industry-standard tissue culture multiwell plates. The cells are simply thawed with media and placed in an incubator ready to use within 24 h. Post-thaw cell recovery values were >80% across three cell lines with low well-to-well variance. The cryopreserved cells retained healthy morphology, membrane integrity, proliferative capacity, and metabolic activity; showed marginal increases in apoptotic cells; and responded well to a toxicological challenge using doxorubicin. These discoveries confirm that the cells are “assay-ready” 24 h after thaw. Overall, we show that macromolecular cryoprotectants can address a long-standing cryobiological challenge and offers the potential to transform routine cell culture for biomedical discovery.

Degradable Polyampholytes from Radical Ring-Opening Copolymerization Enhance Cellular Cryopreservation

Macromolecular cryoprotectants based on polyampholytes are showing promise as supplemental cryoprotectants alongside conventional DMSO-based freezing. Here we exploit radical ring-opening (ter)polymerization to access ester-containing cryoprotective polyampholytes, which were shown to be degradable. Using a challenging cell monolayer cryopreservation model, the degradable polyampholytes were found to enhance post-thaw recovery when supplemented into DMSO. This demonstrates that degradable macromolecular cryoprotectants can be developed for application in biotechnology and biomedicine.

Asymmetric trehalose analogues to probe disaccharide processing pathways in mycobacteria

The uptake and metabolism of the disaccharide trehalose by Mycobacterium tuberculosis is essential for the virulence of this pathogen. Here we describe the chemoenzymatic synthesis of new azido-functionalised asymmetric trehalose probes that resist degradation by mycobacterial enzymes and are used to probe trehalose processing pathways in mycobacteria.

Red Blood Cell Cryopreservation with Minimal Post-Thaw Lysis Enabled by a Synergistic Combination of a Cryoprotecting Polyampholyte with DMSO/Trehalose

From trauma wards to chemotherapy, red blood cells are essential in modern medicine. Current methods to bank red blood cells typically use glycerol (40 wt %) as a cryoprotective agent. Although highly effective, the deglycerolization process, post-thaw, is time-consuming and results in some loss of red blood cells during the washing procedures. Here, we demonstrate that a polyampholyte, a macromolecular cryoprotectant, synergistically enhances ovine red blood cell cryopreservation in a mixed cryoprotectant system. Screening of DMSO and trehalose mixtures identified optimized conditions, where cytotoxicity was minimized but cryoprotective benefit maximized. Supplementation with polyampholyte allowed 97% post-thaw recovery (3% hemolysis), even under extremely challenging slow-freezing and -thawing conditions. Post-thaw washing of the cryoprotectants was tolerated by the cells, which is crucial for any application, and the optimized mixture could be applied directly to cells, causing no hemolysis after 1 h of exposure. The procedure was also scaled to use blood bags, showing utility on a scale relevant for application. Flow cytometry and adenosine triphosphate assays confirmed the integrity of the blood cells post-thaw. Microscopy confirmed intact red blood cells were recovered but with some shrinkage, suggesting that optimization of post-thaw washing could further improve this method. These results show that macromolecular cryoprotectants can provide synergistic benefit, alongside small molecule cryoprotectants, for the storage of essential cell types, as well as potential practical benefits in terms of processing/handling.

Covalent cell surface recruitment of chemotherapeutic polymers enhances selectivity and activity

Synthetic macromolecular chemotherapeutics inspired by host defence peptides can disrupt cell membranes and are emerging as agents for the treatment of cancer and infections. However, their off-target effects remain a major unmet challenge. Here we introduce a covalent recruitment strategy, whereby metabolic oligosaccharide engineering is used to label targeted cells with azido glycans, to subsequently capture chemotherapeutic polymers by a bio-orthogonal click reaction. This results in up to 10-fold reduction in EC50 and widening of the therapeutic window. Cell death is induced by not only membrane leakage, but also by apoptosis due to the conjugated chemotherapeutic being internalised by glycan recycling. Covalent recruitment also lead to increased penetration and significant cell death in a 3-D tumour model in just 3 hours, whereas doxorubicin required 24 hours. This conceptual approach of 'engineering cells to capture polymers' rather than 'engineering polymers to target cells' will bring new opportunities in non-traditional macromolecular therapeutics.

Low DMSO Cryopreservation of Stem Cells Enabled by Macromolecular Cryoprotectants

Mesenchymal stromal (stem) cells have potential in regenerative medicine and modulating the immune system. To deliver any cell-based therapy to the patient, it must be cryopreserved, most commonly in DMSO, which impacts cell function and causes clinical side effects. Here we report the use of a synthetically scalable polyampholyte to rescue the cryopreservation of mesenchymal stromal cells in low [DMSO] cryopreservation. Flow cytometry showed retention of key markers of multipotency comparable to 10% (v/v) DMSO, and the cells could be differentiated, showing this polymer material can be used to improve, or replace, current cryopreservation strategies.

100th Anniversary of Macromolecular Science Viewpoint: Re-Engineering Cellular Interfaces with Synthetic Macromolecules Using Metabolic Glycan Labeling

Cell-surface functionality is largely programmed by genetically encoded information through modulation of protein expression levels, including glycosylation enzymes. Genetic tools enable control over protein-based functionality, but are not easily adapted to recruit non-native functionality such as synthetic polymers and nanomaterials to tune biological responses and attach therapeutic or imaging payloads. Similar to how polymer-protein conjugation evolved from nonspecific PEGylation to site-selective bioconjugates, the same evolution is now occurring for polymer-cell conjugation. This Viewpoint discusses the potential of using metabolic glycan labeling to install bio-orthogonal reactive cell-surface anchors for the recruitment of synthetic polymers and nanomaterials to cell surfaces, exploring the expanding therapeutic and diagnostic potential. Comparisons to conventional approaches that target endogenous membrane components, such as hydrophobic, protein coupling and electrostatic conjugation, as well as enzymatic and genetic tools, have been made to highlight the huge potential of this approach in the emerging cellular engineering field.

Synthetically Scalable Poly(ampholyte) Which Dramatically Enhances Cellular Cryopreservation

The storage and transport of frozen cells underpin the emerging/existing cell-based therapies and are used in every biomedical research lab globally. The current gold-standard cryoprotectant dimethyl sulfoxide (DMSO) does not give quantitative cell recovery in suspension or in two-dimensional (2D) or three-dimensional (3D) cell models, and the solvent and cell debris must be removed prior to application/transfusion. There is a real need to improve this 50-year-old method to underpin emerging regenerative and cell-based therapies. Here, we introduce a potent and synthetically scalable polymeric cryopreservation enhancer which is easily obtained in a single step from a low cost and biocompatible precursor, poly(methyl vinyl ether-alt-maleic anhydride). This poly(ampholyte) enables post-thaw recoveries of up to 88% for a 2D cell monolayer model compared to just 24% using conventional DMSO cryopreservation. The poly(ampholyte) also enables reduction of [DMSO] from 10 wt % to just 2.5 wt % in suspension cryopreservation, which can reduce the negative side effects and speed up post-thaw processing. After thawing, the cells have reduced membrane damage and faster growth rates compared to those without the polymer. The polymer appears to function by a unique extracellular mechanism by stabilization of the cell membrane, rather than by modulation of ice formation and growth. This new macromolecular cryoprotectant will find applications across basic and translational biomedical science and may improve the cold chain for cell-based therapies.

Optimization and Stability of Cell-Polymer Hybrids Obtained by "Clicking" Synthetic Polymers to Metabolically Labeled Cell Surface Glycans

Re-engineering of mammalian cell surfaces with polymers enables the introduction of functionality including imaging agents, drug cargoes or antibodies for cell-based therapies, without resorting to genetic techniques. Glycan metabolic labeling has been reported as a tool for engineering cell surface glycans with synthetic polymers through the installation of biorthogonal handles, such as azides. Quantitative assessment of this approach and the robustness of the engineered coatings has yet to be explored. Here, we graft poly(hydroxyethyl acrylamide) onto azido-labeled cell surface glycans using strain-promoted azide-alkyne "click" cycloaddition and, using a combination of flow cytometry and confocal microscopy, evaluate the various parameters controlling the outcome of this "grafting to" process. In all cases, homogeneous cell coatings were formed with >95% of the treated cells being covalently modified, superior to nonspecific "grafting to" approaches. Controllable grafting densities could be achieved through modulation of polymer chain length and/or concentration, with longer polymers having lower densities. Cell surface bound polymers were retained for at least 72 h, persisting through several mitotic divisions during this period. Furthermore, we postulate that glycan/membrane recycling is slowed by the steric bulk of the polymers, demonstrating robustness and stability even during normal biological processes. This cytocompatible, versatile and simple approach shows potential for re-engineering of cell surfaces with new functionality for future use in cell tracking or cell-based therapies.

Photochemical "In-Air" Combinatorial Discovery of Antimicrobial Co-polymers

There is an urgent need to identify new, non‐traditional antimicrobials. The discovery of new polymeric antimicrobials is limited by current low‐throughput synthetic tools, which means that limited chemical space has been explored. Herein, we employ photochemical “in‐air” reversible addition–fragmentation chain‐transfer (RAFT) polymerization with microwell plates, using liquid‐handling robots to assemble large libraries of cationic polymers, without the need for degassing or purification steps, facilitating transfer to screening. Several lead polymers were identified including a co‐polymer with propylene glycol side chains with significantly enhanced antimicrobial activity and increased therapeutic window. Mechanistic studies showed that this polymer was bacteriostatic, and surprisingly did not lyse the cell membranes, implying an alternative mode of action. This versatile method using simple robotics will help to develop new biomaterials with emergent properties.