Biochemistry, physiology, and function of platelets stored as concentrates

Platelet in vitro assays: their correspondence with their in vivo hemostatic potential

Developments during the past few years have resulted in multiple kinds of platelet products for transfusion. This involves different collection methods, containers, preservative solutions, modifications of storage temperatures and durations, and additional treatments such as pathogen reduction. Much experience has been obtained testing these processes in vitro to seek indications of in vivo effectiveness. Availability of an in vitro method that correlated with in vivo effectiveness would be extremely valuable for these different kinds of platelet products and as more innovation in platelet preparation occurs in the future. This report reviews the methods for in vitro platelet testing with a view to their in vivo implications and whether such testing could be helpful in projecting the clinical effectiveness of different platelet products.

Defining the effects of storage on platelet bioenergetics: The role of increased proton leak

The quality of platelets decreases over storage time, shortening their shelf life and potentially worsening transfusion outcomes. The changes in mitochondrial function associated with platelet storage are poorly defined and to address this we measured platelet bioenergetics in freshly isolated and stored platelets. We demonstrate that the hypotonic stress test stimulates both glycolysis and oxidative phosphorylation and the stored platelets showed a decreased recovery to this stress. We found no change in aggregability between the freshly isolated and stored platelets. Bioenergetic parameters were changed including increased proton leak and decreased basal respiration and this was reflected in a lower bioenergetic health index (BHI). Mitochondrial electron transport, measured in permeabilized platelets, showed only minor changes which are unlikely to have a significant impact on platelet function. There were no changes in basal glycolysis between the fresh and stored platelets, however, glycolytic rate was increased in stored platelets when mitochondrial ATP production was inhibited. The increase in proton leak was attenuated by the addition of albumin, suggesting that free fatty acids could play a role in increasing proton leak and decreasing mitochondrial function. In summary, platelet storage causes a modest decrease in oxidative phosphorylation driven by an increase in mitochondrial proton leak, which contributes to the decreased recovery to hypotonic stress.

Preservation of Platelets by Adding Epigallocatechin-3-O-Gallate to Platelet Concentrates

The effect of epigallocatechin-3- O-gallate (EGCG), a major component of green tea, on platelet preservation was evaluated. Single donor platelets ( N = 10) were collected and preserved by the standard method. EGCG was added to the platelet concentrates before preservation and then the functional and biochemical parameters were monitored throughout the storage period. After 6 days of preservation, the aggregability of the platelets was significantly maintained by addition of 50 and 100 μg/ml of EGCG. Platelet prothrombinase activity was also significantly retained by the addition of EGCG. The accumulation of P-selectin and RANTES in the plasma preserved with EGCG was less than those preserved without EGCG, which indicated that EGCG might inhibit platelet activation. Furthermore, EGCG reduced the increase of LDH in plasma during preservation and inhibited the activation of caspase-3 and cleavage of gelsolin, thereby showing that EGCG could inhibit the apoptosis of platelets. These results suggest that EGCG may play an effective role in preserving platelets by inhibiting the activation and apoptosis of platelets.

Platelet storage solution improves the in vitro function of preserved platelet concentrate

BACKGROUND AND OBJECTIVES

Stored platelets develop biochemical lesions, manifest as depressed haemostatic function, clot retraction and wound healing. ViaCyte trade mark, a proprietary experimental preservative solution (comprising D-ribose, D-glucose, Hanks solution, Hepes solution, bovine serum albumin, tic anticoagulant peptide and sterile water), was tested in comparison with the presently accepted storage solution, citrate-dextrose-phosphate-plasma (CDP-P), to evaluate its ability to preserve platelet function during storage.

MATERIALS AND METHODS

Platelets stored in ViaCyte and platelets suspended in CDP-P were transferred to polypropylene tubes with PL732 covers and analysed for adenine nucleotide levels (ATP molecules), in vitro agonist-mediated P-selectin expression and aggregation.

RESULTS

After 5 days of storage at room temperature, 12.2% of platelets stored in ViaCyte exhibited P-selectin expression at rest, and 64.2% exhibited P-selectin expression upon activation with thrombin challenge, an increase of 52%. Platelets stored in CDP-P exhibited 44.4% P-selectin expression at rest, suggesting significant activation during storage, and thrombin stimulation resulted in P-selectin expression of 47.9%, an increase of only 2.5% (P< or =0.002, untreated vs. treated). ViaCyte also maintained ATP levels throughout the storage period, while these levels became depressed in platelets stored in CDP-P (P< or =0.02, untreated vs. treated). Storing platelets in the experimental preservative solution maintained their ability to aggregate, while control platelets lost their ability to aggregate in response to agonist.

CONCLUSIONS

ViaCyte appears to protect platelets during storage, reflected by a low level of induced lesions. Platelets stored in ViaCyte maintain energy levels at their resting state, which preserves their ability to aggregate and secrete granule contents, and ensures the availability of additional platelets for activation upon in vitro challenge.

Binding of activated platelets to WBCs in vivo after transfusion

BACKGROUND

During preparation and storage of apheresis concentrates, platelets are being activated. One of the alterations that occur during this process is an increased expression of P-selectin (CD62p) on the cytoplasmic surface of platelets. This neoepitope represents a ligand for the binding of platelets to WBCs. It has been suggested that the activation of platelets is associated with the sequestration of platelets after transfusion. In this in vivo study, the binding of platelets to WBCs was analyzed following transfusion of platelet concentrates (PCs).

STUDY DESIGN AND METHODS

Double apheresis concentrates were prepared with two different cell separators. One of the split products was stored for 1 to 2 days and the other one for 3 to 5 days. Flow cytometry was applied to analyze the degree of platelet activation in vitro, and also to measure the extent of platelet binding to WBC subclasses in vivo after transfusion into patients.

RESULTS

The results of this study show that platelet activation occurs during apheresis and storage of PCs. After transfusion of the PCs, no significant binding of platelets to T or B-cells could be detected. However, a significant binding of platelets to monocytes and neutrophil granulocytes occurs. While in Baxter PCs stored for 1-2 days the amount of platelet-leukocyte aggregates in vivo was higher compared to COBE PCs, no such difference could be detected anymore for the PCs stored for 3-5 days.

CONCLUSION

This study demonstrates that binding of activated platelets occurs to monocytes and neutrophil granulocytes but not to T- and B-cells in the circulation after transfusion. In addition, the interaction of platelets and WBCs is dependent on the degree of P-selectin expression. Platelets showing a higher degree of activation adhere to WBCs to a higher degree than nonactivated platelets.

Prevention of monocyte adhesion and inflammatory cytokine production during blood platelet storage: an in vitro model with implications for transfusion practice

A novel platelet additive solution [ThromboSoltrade mark (TS)] was designed to allow extended refrigerated platelet storage. It has been shown to preserve platelet function and prevent cytokine accumulation in platelet concentrates stored for up to 9 days. It consists of amiloride, adenosine, sodium nitroprusside, dipyridamole, quinacrine, and ticlopidine. We hypothesized that the cytokine inhibition may be due to prevention of monocyte (MC) adhesion and activation on the surfaces of platelet storage bag plastic polymers. In an in vitro model, we incubated purified peripheral blood MCs on discs of polyolefin and polyvinylchloride from platelet storage bags, and on polystyrene, in the presence of TS for up to 7 days. We found that after incubation with TS, adherent MC numbers were decreased by >80-95% compared with controls on all surfaces examined. Levels of cytokines [interleukin (IL)-1beta, IL-1RA, IL-6, IL-8, and tumor necrosis factor-alpha] were low in wells with TS but rose progressively in the controls during incubation. Amiloride alone had similar effects on adhesion and cytokine release as the complete TS preparation. Removing amiloride from TS abrogated these effects. These findings suggest an important role for TS and amiloride in monocyte function, and have implications for the development of agents designed for prolonged platelet storage.

Thrombopoietin and the TPO receptorduring platelet storage

BACKGROUND

For most cells, the addition of a specific growth factor has improved cellular viability by preventing programmed cell death (apoptosis). To determine whether the platelet-specific hematopoietic growth factor thrombopoietin (TPO) might improve platelet viability, endogenous TPO and the platelet TPO receptor were analyzed during storage, and the effect of recombinant TPO on platelet viability was assessed.

STUDY DESIGN AND METHODS

During platelet storage, TPO stability was assessed by SDS-PAGE, TPO receptor function was measured, and the platelet TPO receptor was characterized by a (125)I-rHuTPO competitive-binding assay. A recombinant TPO, pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF), was added to platelet concentrates during storage, and its effect on pH, LDH, and metabolic activity was determined.

RESULTS

During storage, the molecular weight and concentration of endogenous TPO (125 +/- 19 pg/mL) and exogenous TPO (5720 +/- 140 pg/mL) were constant for 12 days; the number (33 +/- 4), binding affinity (149 +/- 33 pM), and function of the platelet TPO receptors were constant for 7 days. Metabolic activity measured with the MTT and MTS assays closely correlated with changes in the pH and LDH. The addition of PEG-rHuMGDF did not alter the pH, LDH, or metabolic activity of platelets during storage, but it did increase by 65 percent the uptake of (35)S-methionine into platelets. Finally, platelet concentrates obtained from donors treated with PEG-rHuMGDF retained normal metabolic activity for 12 days, as compared with 5 to 6 days for normal platelet concentrates.

CONCLUSIONS

TPO and its platelet receptor are present in normal amounts and have normal function during platelet storage. The addition of recombinant TPO increased platelet methionine transport but did not alter platelet viability during storage. Other means to prevent apoptosis during platelet storage should be considered, and the measurement of platelet metabolic activity by MTT and MTS assays may assist this effort.

Structure-activity relationships for selected sulfur-rich antithrombotic compounds

We assessed the antithrombotic activity of some simple organosulfur compounds which have some of the functionality found in the disulfide ajoene, a pharmacologically active compound isolated from garlic. The results establish that antithrombotic activity is associated with disulfides directly attached to a phenyl ring and is further enhanced by an alpha-sulfonyl group. CH(3)SO(2)CH(2)SSPh proved to be a potent inhibitor of platelet aggregation with an IC(50) of 5 microM.

Quality evaluation of plateletpheresis using the new AMICUS (Baxter) cell separator: evolution of CD 62 expression

The purpose of this study was to evaluate the new AMICUS (Baxter-Fenwal Division) cell separator in terms of donor safety, efficiency, and quality of the product obtained. One hundred eighty-three single-donor plateletpheresis procedures were performed, using a collection of 4-4.5 x 10(11) platelets as endpoint. During the first part of the study, the mean volume processed was 3,225 ml and the mean procedure duration 69.5 min. During the second part, after a software change, the mean volume and mean procedure time were 3,071 ml and 68.3 min, respectively. According to local policy, every collection bag was separated into two therapeutic units each containing a mean of 1.87 (1.83) x 10(11) platelets. The white blood cell (WBC) contamination per therapeutic unit was less than 5 x 10(6) in 91% of phereses performed in part one of the study and in 98% of phereses performed in part two. During the recommended 5 days storage, sequential in vitro analyses were performed in 27 units, showing limited platelet activation according to CD62 expression and morphological changes on electron microscopy (EM). Furthermore, there was a correlation between CD62 expression and the degree of WBC contamination (P = 0.03). In conclusion, platelet collection with the new Amicus allows for high platelet yields of adequate quality as judged by WBC content, CD62 expression, and electron microscopic morphological changes.

In vitro analysis of platelet concentrates stored in the presence of modulators of 3',5' adenosine monophosphate, and organic anions

The storage of conventional platelet concentrates (PCs) under standard blood bank conditions is limited to five days, in part because longer storage periods lead to increasing damage in platelet integrity and functionality. The growing demand of PCs for clinical use, raises the interest to develop agents that would potentially permit a more extended period of storage. We have evaluated and compared the in vitro quality of PCs treated with: (1) Modulators of levels of cAMP (PGE1, foskolin, theophylline and isobutyl-methyl-xanthine [IBMX]); and (2) organic anions that function as alternative substrates of platelets (pyruvate and acetate). Platelet rich plasma (PRP) from pools (n = 6) of PCs was distributed into storage bags, and the agents to be tested were added, using saline as a control substance. PCs were stored at 22 degrees C with continuous agitation for up to 10 days. At 0, 5 and 10 days of storage, samples were analyzed for platelet counts, mean platelet volume (MPV), metabolic markers, and expression of glycoproteins (GPs). The addition of modulators of levels of cAMP, at the concentration used in the study, did not lead to substantial improvement in the parameters being evaluated, with respect to those in control units. The supplementation with organic anions, while not affecting the surface levels of GPs, favored the maintenance of metabolic values, such as pH, PCO2, and bicarbonate concentrations, as well as the preservation of MPV (p values < 0.05 respect to control units both at 5 and 10 days of storage). Our results indicate that while the use of modulators of levels of cAMP do not provide substantial benefit in the prevention of platelet storage lesions, organic anions have some advantageous effect in the storage promoted metabolic changes of PCs. These data might be considered when designing strategies to improve PC storage.

Flow cytometric analysis of platelet function in stored platelet concentrates

Platelet activation occurs during the collection, processing and storage of platelet concentrates. The effect of the platelet activation on the functional state of stored platelets remains however undefined. We employed flow cytometric analysis to evaluate the extent of platelet activation and the physiological response to thrombin stimulation of platelets stored for up to five days under routine blood bank conditions. Platelet surface expression of the activation markers CD62 and CD63 was examined, along with modulation of platelet membrane glycoproteins (GP) Ib and IIbIIIa. Platelet dense granule content was determined using a mepacrine uptake assay and the extent of platelet microparticle generation was quantified. Thirteen random-donor platelet concentrates prepared under routine conditions by a platelet-rich-plasma protocol were examined. Platelets were found to be activated following preparation on day 1. Although a gradual increase was seen with increasing storage time, this was not statistically significant for CD62 or CD63 expression, GPIIbIIIa or GPIb modulation or dense granule release; the generation of platelet microparticles did, however, increase with increasing storage time. The characteristic increase in surface expression of CD62, CD63 and GPIIbIIIa and decrease in GPIb and dense granule content in response to thrombin stimulation was observed with all concentrates, but these measures of platelet functional reserve showed decreasing platelet function with increasing storage time. The results indicate that platelets are activated by day 1, likely as a consequence of manipulation during collection and processing, but are not further progressively activated with increasing storage time; they do, however, become relatively hypofunctional with increasing storage.

Quality assessment of platelet concentrates supplemented with second-messenger effectors

BACKGROUND

While reducing the potential for bacterial contamination, the storage of platelet concentrates (PCs) at refrigerated temperatures is not routine, because of the induction of the so-called platelet storage lesion. As the modulation of second-messenger levels might help to overcome this drawback, a quality assessment of PCs treated with a mixture of second-messengers effectors known as ThromboSol was performed.

STUDY DESIGN AND METHODS

The PCs were supplemented with ThromboSol or phosphate-buffered saline, and stored in parallel at 22 degrees C with continuous agitation or at 4 degrees C. At 1, 5, and 9 days, an in vitro quality assessment of the PCs was performed, including measurement of cell number, metabolic and integrity markers, platelet surface expression of glycoproteins, platelet response to ristocetin and thrombin, and levels of cyclic adenosine 3', 5' monophosphate (cAMP) and thromboxane B2 (TxB2).

RESULTS

Control PCs stored at 4 degrees C underwent aggregation and displayed a significant decrease in the platelet number (40% on Day 5). By contrast, the ThromboSol-treated PCs maintained 80 percent of their initial platelet concentration after 9 days of storage at 4 degrees C. Compared to PCs stored at 22 degrees C, refrigerated PCs exhibited minor changes in metabolic values throughout storage, but the addition of ThromboSol induced a rise in metabolic rate during storage at 22 degrees C. Platelet responsiveness to both ristocetin and thrombin was maximally preserved in the ThromboSol-treated PCs stored at 4 degrees C. These units also maintained high levels of cAMP and low concentrations of TxB2 during storage.

CONCLUSION

The pharmacologic supplementation of PCs with ThromboSol significantly favors the maintenance of in vitro integrity and responsiveness of platelets during extended storage at refrigerated temperature. This protective effect seems to be a consequence of the ability of ThromboSol's components to sustain high levels of cAMP and to inhibit TxB2 production during the entire extended-storage period.

Leukocyte depletion and storage of single-donor platelet concentrates

BACKGROUND AND OBJECTIVES

Because of widespread use of leukocyte reduction in platelet concentrates (PCs) and the need to store such concentrates, we investigated the effects of leukocyte depletion on the quality of stored PCs.

MATERIALS AND METHODS

Ten double-sized PCs were divided into 2 equal units which were tested simultaneously. One half was stored for 5 days after filtration through a polyester filter, the other one was stored unfiltered.

RESULTS

The volume of the 10 "oversized' PCs was 483 +/- 40 ml (mean +/- standard deviation) and they contained 5.9 +/- 1.5 x 10(11) platelets and 80 +/- 23 x 10(6) leukocytes. Filtration significantly reduced the leukocyte concentration (168 +/- 56/microliter before, 6 +/- 4 /microliter after filtration) and leukocyte count (39.9 +/- 11.3 x 10(6) vs. 1.3 +/- 0.9 x 10(6); p < 0.0005). Filtration caused a platelet loss of 16%, the platelet count decreasing not significantly from 2.91 +/- 0.75 x 10(11) to 2.40 +/- 0.94 x 10(11) (p = 0.26). After 5 days of storage all parameters of platelet function (platelet aggregation to several stimuli, hypotonic shock reaction [HSR] and platelet retraction), mean platelet volume, and pH and pCO2 showed no advantage for PCs filtered prior to storage compared to PCs stored unfiltered. Moreover, platelet aggregation on day 5 using 4 agonists at 10 concentrations showed worse results in 4 assays in prestorage filtered PCs (collagen [4 micrograms/ml: p < 0.05, ADP [0.2 mM]: p < 0.05, ADP [0.3 mM]: p < 0.05, thrombin [0.6 E/ml]: p < 0.05). But there is no convincing trend in all aggregation tests, and HSR, presumably the most useful parameter, was not different or day 5.

CONCLUSIONS

There is no advantage in terms of improved quality for prestorage leuko-depletion of PCs. Taking into account the obvious disadvantages of filtration, such as platelet loss and increasing costs per transfusion, we conclude that pre- or post-storage filtration of single-donor PCs should be done only for patients who have a clear indication for the transfusion of leukocyte-poor blood products.

Platelets stored in a glucose-free additive solution or in autologous plasma--an ultrastructural and morphometric evaluation

We evaluated a new glucose-free citrate-acetate-NaCl platelet additive solution (PAS 2). In series I, platelet concentrates (PC) were prepared by apheresis and subsequently stored either in plasma (n = 16) or in PAS 2 (n = 15). In series II, PCs were prepared from pools of four buffy coats (BC) and stored in plasma (n = 12) or in PAS 2 (n = 11). By means of ultrastructural morphometry, the volume fractions of alpha-granules, the open canalicular system (OCS) and the fraction of storage granules secreted into the OCS were analyzed during storage for up to 8 days. Additionally, we determined pH, glucose, lactate, pCO2, HCO3-, lactate dehydrogenase and platelet factor 4. Apheresis platelets stored in plasma showed no changes in their contents of alpha-granules and in the fractions of the OCS. In contrast, apheresis platelets stored in PAS 2 displayed a decrease of their relative volume fraction of alpha-granules from 9.1 +/- 1% on day 1 to 3.7 +/- 0.9% on day 5. The fraction of the OCS increased from 7.4 +/- 0.8% on day 1 to 17.1 +/- 1.4% on day 3. On day 8, 93 +/- 9% of all platelets were lysed. Levels of glucose were significantly lower in these preparations and after day 3 glucose consumption decreased to zero. Among PC derived from pooled BC, differences between storage in PAS 2 or plasma were less striking. Only the fraction of alpha-granules secreted into the OCS was significantly greater in BC derived PC stored in PAS 2 on all days. These PCs stored in PAS 2 had a higher plasma carryover (30%) in comparison to apheresis PC stored in PAS 2 (10%). We conclude that plasma is superior to PAS 2 for storage of both apheresis and buffy coat platelets. For preservation of the structural integrity of platelets, the use of PAS 2 requires a minimum of 30% plasma carryover.

Thrombopoietin: biology, clinical applications, role in the donor setting

Thrombopoietin (c-Mpl ligand) is the hematopoietic growth factor that is responsible for regulating the production of platelets from bone marrow megakaryocytes. This approximately 90 kd protein has recently been isolated and is comprised of an erythropoietin domain that is approximately 50% homologous to erythropoietin and a carbohydrate domain that is highly glycosylated and appears to stabilize the protein in the circulation. Thrombopoietin is produced in the liver and blood levels are determined by the mass of circulating platelets. However, there is no platelet "sensor." Rather platelets contain high affinity thrombopoietin receptors that bind and remove thrombopoietin from the circulation and thereby directly determine circulating levels. In vitro thrombopoietin stimulates both early and late megakaryocyte precursors as well as some erythroid and multipotential progenitor cells. When administered to normal animals, it stimulates platelet production up to six-fold without affecting other lineages. However, when given to animals following chemotherapy or irradiation, it stimulates erythroid and myeloid as well as platelet recovery. Several different recombinant thrombopoietin proteins are now entering clinical trials in humans and all preliminary reports confirm a potent thrombopoietic stimulus and apparent lack of toxicity. Thrombopoietin shows great promise in preventing the thrombocytopenia associated with chemotherapy, bone marrow transplantation, and other acute or chronic thrombocytopenic disorders. In transfusion medicine, thrombopoietin may help mobilize peripheral blood progenitor cells, stimulate donors for plateletpheresis, and enhance platelet survival and function during storage, Many studies are currently underway in all these areas and should soon establish the role of thrombopoietin in clinical medicine.

The Pasteur effect in human platelets: implications for storage and metabolic control

The Pasteur effect and the associated acidosis have long been considered a major cause of platelet death during storage. We have investigated this phenomenon using a defined platelet preparation and a system whereby the oxidative and glycolytic contributions to total ATP production can be measured over a range of oxygen concentrations from saturating (pO2 = 158 mmHg) to anoxic (pO2 = 0 mmHg). Platelets do not show a Pasteur effect until the pO2 decreases to < 2.0 mmHg, whereupon lactate production increases 1.5-fold. The Pasteur effect is therefore not a likely cause of platelet death during storage where pO2 in a storage bag typically drops to no less than 50 mmHg. The data also have implications for the role of oxygen diffusion in oxidative metabolism, and for the compensatory nature of the Pasteur effect. As platelets are relatively small cells, and the onset of the Pasteur effect occurs at a relatively low oxygen concentration, diffusion may limit the rate of oxygen consumption in most other (larger) cells. The Pasteur effect is only fully compensative if the P/O2 ratio used for the calculations is lower than the conventional one. Since recent research strongly suggests that the conventional P/O2 ratio is too high, examples of fully compensative Pasteur effects may be more common than the literature suggests.

[Quality of platelet products: current methods of in vitro assessment]

Efficiency of platelet transfusion is closely related to the quality of the preparation and also to the optimization of storage conditions at 20 degrees C. During this last years, processes for obtaining platelet suspensions became different (platelet rich plasma platelet, pooled or single buffy coat platelet, apheresis platelet), process for purification were developed (filtration, gamma or UV-irradiation, synthetic media for storage, virus inactivation), duration of storage was extended to 5 or 7 days and clinical applications were intensified. According to this advance, noted in some European countries, multicenter consensus on methods of quality control must be defined by different study groups. In the recent past, many publications have described a lot of in vitro tests for estimating functions, morphology, metabolic activity, lysis and activation of platelets. Current available methods for routine quality control or for development of new procedures, such as pH measurement inspection of swirling, percent of discoid shape, mean platelet volume, hypotonic shock response or total ATP level should be considered, in accordance with the in vivo viability.