Cryopreservation of human T lymphocytes under fast cooling with controlled ice nucleation in cryoprotective solutions of low toxicity

Relevance of controlled cooling and freezing phases in T-cell cryopreservation

When cells are cryopreserved, they go through a freezing process with several distinct phases (i.e., cooling until nucleation, ice nucleation, ice crystal growth and cooling to a final temperature). Conventional cell freezing approaches often employ a single cooling rate to describe and optimize the entire freezing process, which neglects its complexity and does not provide insight into the effects of the different freezing phases. The aim of this work was to elucidate the impact of each freezing phase by varying different process parameters per phase. Hereto, spin freezing was used to freeze Jurkat T cells in either a Me2SO-based or Me2SO-free formulation. The cooling rates before ice nucleation and after total ice crystallization impacted cell viability, resulting in viability ranging from 26.7% to 52.8% for the Me2SO-free formulation, and 22.5%-42.6% for the Me2SO-based formulation. Interestingly, the degree of supercooling upon nucleation did not exhibit a significant effect on cell viability in this work. However, the rate of ice crystal formation emerged as a crucial factor, with viability ranging from 2.4% to 53.2% for the Me2SO-free formulation, and 0.3%-53.2% for the Me2SO-based formulation, depending on the freezing rate. A morphological study of the cells post-cryopreservation was performed using confocal microscopy, and it was found that cytoskeleton integrity and cell volume were impacted, depending on the formulation-process parameter combination. These findings underscore the importance of scrutinizing all cooling and freezing phases, as each phase impacted post-thaw viability in a distinct way, depending of the specific formulation used.

Technical pitfalls when collecting, cryopreserving, thawing, and stimulating human T-cells

The collection, cryopreservation, thawing, and culture of peripheral blood mononuclear cells (PBMCs) can profoundly influence T cell viability and immunogenicity. Gold-standard PBMC processing protocols have been developed by the Office of HIV/AIDS Network Coordination (HANC); however, these protocols are not universally observed. Herein, we have explored the current literature assessing how technical variation during PBMC processing can influence cellular viability and T cell immunogenicity, noting inconsistent findings between many of these studies. Amid the mounting concerns over scientific replicability, there is growing acknowledgement that improved methodological rigour and transparent reporting is required to facilitate independent reproducibility. This review highlights that in human T cell studies, this entails adopting stringent standardised operating procedures (SOPs) for PBMC processing. We specifically propose the use of HANC's Cross-Network PBMC Processing SOP, when collecting and cryopreserving PBMCs, and the HANC member network International Maternal Pediatric Adolescent AIDS Clinical Trials (IMPAACT) PBMC Thawing SOP when thawing PBMCs. These stringent and detailed protocols include comprehensive reporting procedures to document unavoidable technical variations, such as delayed processing times. Additionally, we make further standardisation and reporting recommendations to minimise and document variability during this critical experimental period. This review provides a detailed overview of the challenges inherent to a procedure often considered routine, highlighting the importance of carefully considering each aspect of SOPs for PBMC collection, cryopreservation, thawing, and culture to ensure accurate interpretation and comparison between studies.

Slow Cooling and Controlled Ice Nucleation Enabling the Cryopreservation of Human T Lymphocytes with Low-Concentration Extracellular Trehalose

Cryopreservation of human T lymphocytes has become a key strategy for supporting cell-based immunotherapy. However, the effects of ice seeding on the cryopreservation of cells under relatively slow cooling have not been well researched. The cryopreservation strategy with a nontoxic, single-ingredient, and injectable cryoprotective solution remains to be developed. We conducted ice seeding for the cells in a solution of normal saline with 1% (v/v) dimethyl sulfoxide (Me2SO), 0.1 M trehalose, and 4% (w/v) human serum albumin (HSA) under different slow cooling rates. With the positive results, we further applied seeding in the solution of 0.2 M trehalose and 4% (w/v) HSA under the same cooling rates. The optimal concentration of trehalose in the Me2SO-free solutions was then investigated under the optimized cooling rate with seeding, with control groups without seeding, and in a freezing container. In vitro toxicity of the cryoprotective solutions to the cells was also tested. We found that the relative viability of cells (1% [v/v] Me2SO, 0.1 M trehalose and 4% [w/v] HSA) was improved significantly from 88.6% to 94.1% with ice seeding, compared with that without seeding (p < 0.05). The relative viability of cells (0.2 M trehalose and 4% [w/v] HSA) with seeding was significantly higher than that without seeding, 96.3% and 92.0%, respectively (p < 0.05). With no significant difference in relative viability between the solutions of 0.2 M trehalose or 0.3 M trehalose with 4% (w/v) HSA (92.4% and 94.6%, respectively, p > 0.05), the solution of 0.2 M trehalose and 4% (w/v) HSA was selected as the optimized Me2SO-free solution. This strategy could cryopreserve human T lymphocytes without any toxic cryoprotectant and boost the application of cell products in humans by intravenous injection, with the osmolality of the low-concentration cryoprotective solution close to that of human plasma.

Natural Cryoprotective and Cytoprotective Agents in Cryopreservation: A Focus on Melatonin

Cryoprotective and cytoprotective agents (Cytoprotective Agents) are fundamental components of the cryopreservation process. This review presents the essentials of the cryopreservation process by examining its drawbacks and the role of cytoprotective agents in protecting cell physiology. Natural cryoprotective and cytoprotective agents, such as antifreeze proteins, sugars and natural deep eutectic systems, have been compared with synthetic ones, addressing their mechanisms of action and efficacy of protection. The final part of this article focuses melatonin, a hormonal substance with antioxidant properties, and its emerging role as a cytoprotective agent for somatic cells and gametes, including ovarian tissue, spermatozoa and spermatogonial stem cells.