Frozen/thawed platelets: importance of osmotic tolerance and platelet subpopulations

Bacterial Contamination of Platelet Products

Transfusion of bacterially contaminated platelets, although rare, is still a major cause of mortality and morbidity despite the introduction of many methods to limit this over the past 20 years. The methods used include improved donor skin disinfection, diversion of the first part of donations, use of apheresis platelet units rather than whole-blood derived pools, primary and secondary testing by culture or rapid test, and use of pathogen reduction. Primary culture has been in use the US since 2004, using culture 24 h after collection of volumes of 4-8 mL from apheresis collections and whole-blood derived pools inoculated into aerobic culture bottles, with limited use of secondary testing by culture or rapid test to extend shelf-life from 5 to 7 days. Primary culture was introduced in the UK in 2011 using a "large-volume, delayed sampling" (LVDS) protocol requiring culture 36-48 h after collection of volumes of 16 mL from split apheresis units and whole-blood derived pools, inoculated into aerobic and anaerobic culture bottles (8 mL each), with a shelf-life of 7 days. Pathogen reduction using amotosalen has been in use in Europe since 2002, and was approved for use in the US in 2014. In the US, recent FDA guidance, effective October 2021, recommended several strategies to limit bacterial contamination of platelet products, including pathogen reduction, variants of the UK LVDS method and several two-step strategies, with shelf-life ranging from 3 to 7 days. The issues associated with bacterial contamination and these strategies are discussed in this review.

Platelet cryopreservation using a trehalose and phosphate formulation

Long-term storage of platelets is infeasible due to platelet activation at low temperatures. In an effort to address this problem, we evaluated the effectiveness of a formulation combining trehalose and phosphate in protecting platelet structure and function following cryopreservation. An annexin V binding assay was used to quantify the efficacy of the trehalose and phosphate formulation in suppressing platelet activation during cryopreservation. Of the platelets cryopreserved with the trehalose plus phosphate formulation, 23% +/- 1.2% were nonactivated, compared with 9.8% +/- 0.26% nonactivated following cryopreservation with only trehalose. The presence of both trehalose and phosphate in the cryopreservation medium is critical for cell survival and preincubation in trehalose plus phosphate solutions further enhances viability. The effectiveness of trehalose plus phosphate in preserving platelets in a nonactivated state is comparable to 6% dimethyl sulfoxide (Me(2)SO). Measurements of platelet metabolic activity using an alamarBlue assay also established that trehalose plus phosphate is superior to trehalose alone. Finally, platelets protected by the trehalose plus phosphate formulation exhibit similar aggregation response upon thrombin addition as fresh platelets, but an increase of cytosolic calcium concentration upon thrombin addition was not observed in the cryopreserved platelets. These results suggest that trehalose and phosphate protect several aspects of platelet structure and function during cryopreservation, including an intact plasma membrane, metabolic activity, and aggregation in response to thrombin, but not intracellular calcium release in response to thrombin.

Biopreservation of cells and engineered tissues

The development of effective preservation and long-term storage techniques is a critical requirement for the successful clinical and commercial application of emerging cell-based technologies. Biopreservation is the process of preserving the integrity and functionality of cells, tissues and organs held outside the native environment for extended storage times. Biopreservation can be categorized into four different areas on the basis of the techniques used to achieve biological stability and to ensure a viable state following long-term storage. These include in vitro culture, hypothermic storage, cryopreservation and desiccation. In this chapter, an overview of these four techniques is presented with an emphasis on the recent developments that have been made using these technologies for the biopreservation of cells and engineered tissues.

Comparison of various dimethylsulphoxide-containing solutions for cryopreservation of leucoreduced platelet concentrates

BACKGROUND AND OBJECTIVES

Leucoreduced platelet concentrates (LR-PCs) can be stored at 20-24 degrees C for 5-7 days. When LR-PCs are cryopreserved they can be stored for several years. For cryopreservation to become applicable in blood-bank practice, an off-the-shelf cryoprotectant is needed that can be added to the LR-PC in a sterile manner. For this, we varied the composition of the cryopreservation medium and studied various parameters of cryopreserved LR-PCs for up to 24 h after thawing at room temperature.

MATERIALS AND METHODS

LR-PCs in plasma or Composol were concentrated and divided into 2 units. To each unit, an equal part of 10% dimethylsulfoxide (DMSO) in plasma, Composol with or without 5% albumin, or GPO (pasteurized plasma-protein solution) was added. Freezing occurred at 1 degrees C/min and LR-PCs were placed in the vapour phase of nitrogen. LR-PCs were thawed at 37 degrees C and stored at room temperature. LR-PCs were tested for morphology, platelet recovery, swirling effect, and activation antigens at various time-points thereafter.

RESULTS

LR-PCs in 100%, 65% and 50% plasma supplemented with Composol showed good morphology scores (>250), limited CD62P expression (<35%), low CD63 expression (<20%) and a swirling effect of about 2, at 24 h after thawing. At the same time-point, platelet recovery was >80% under all conditions and CD42b expression varied between 70 and 85%. Results of LR-PCs in 15% plasma and Composol, with or without plasma substitutes, were not acceptable at 24 h after thawing, i.e. the morphology score was <200 and the CD62P expression was >40%.

CONCLUSIONS

A minimum of 50% plasma in the cryopreserved LR-PC is necessary to maintain an acceptable in vitro quality of platelets up to 24 h after thawing. Composol is a good candidate for using to prepare an off-the-shelf cryoprotectant.

Use of hollow fiber membrane filtration for the removal of DMSO from platelet concentrates

It has been hypothesized that, in addition to freezing injury, some damage to platelets may result from the cell packing that occurs during removal of the cryoprotectant. This study examined DMSO removal by fluid exchange across hollow-fiber (HF) filters as an alternative to centrifugation. The DMSO solution with or without cell suspension was passed once through the filter. The optimum exchange during unloading of DMSO was determined by varying the flow rates in the external and internal compartments of the HF filter. Initially, buffered solutions of a 5% DMSO solution in the absence of platelets were pumped into the fibers and exchanged against PBS. The residual DMSO was determined by osmometry. The exchange of DMSO across the membrane was flow dependent and also influenced by the chemical nature of the HF fibers. No protocol using a reasonable rate flow through the fibers removed more than 95% of the DMSO in a single pass. The optimum protocol was achieved with polysynthane fibers with an internal flow rate of approximately 20 mi/min and an external flow rate of 100 ml/min. Subsequently, frozen/thawed platelet concentrates in DMSO were washed using centrifugation and compared to the HF filtration method. Platelet quality was assayed by flow cytometry, cell count, morphology and osmotic stress test. Both filtration and centrifugal washing techniques resulted in comparable morphological scores and numbers of discoid cells. When agents reducing platelet activation were added, platelet quality was improved after washing by either technique. The lower platelet osmotic response with HF filtration than with centrifugation while using activation inhibitors was attributed to the remaining amount of the inhibitors. All other parameters tested were similar. The expression of CD62P was equivalent with both techniques, and centrifugation did not activate platelets more than filtration contrary to what was originally anticipated. In conclusion, platelet quality was comparable after washing by either technique but hollow fiber filtration does remove cryoprotectant more rapidly than does centrifugation.