Infusible platelet membrane microvesicles: a potential transfusion substitute for platelets

Intravenous Hemostats: Foundation, Targeting, and Controlled-Release

Uncontrollable blood loss is the greatest cause of mortality in prehospital patients and the main source of disability and death in hospital care. Compared with external hemostats, intravenous hemostats are more appropriate for preventing and treating uncontrolled bleeding in vivo and large bleeding on the body surface. This Review initially establishes intravenous hemostats' response basis, including the coagulation mechanism, fibrinolytic pathway, and protein corona. Second, the study of advancement of intravenous hemostat targeting was expanded from two perspectives, cellular hemostatic agents and synthetic hemostatic agents. Meanwhile, after discussing the progress of controlled-release intravenous hemostats with platelets as the stimuli, this Review offers insight into the possibility of controlled-release intravenous hemostats with microenvironment as the stimuli, combining the studies of controlled-release targeted thrombolysis.

Human platelet lysate-derived extracellular vesicles enhance angiogenesis through miR-126

OBJECTIVES

Extracellular vesicles (EVs) are key biological mediators of several physiological functions within the cell microenvironment. Platelets are the most abundant source of EVs in the blood. Similarly, platelet lysate (PL), the best platelet derivative and angiogenic performer for regenerative purposes, is enriched of EVs, but their role is still too poorly discovered to be suitably exploited. Here, we explored the contribution of the EVs in PL, by investigating the angiogenic features extrapolated from that possessed by PL.

METHODS

We tested angiogenic ability and molecular cargo in 3D bioprinted models and by RNA sequencing analysis of PL-derived EVs.

RESULTS

A subset of small vesicles is highly represented in PL. The EVs do not retain aggregation ability, preserving a low redox state in human umbilical vein endothelial cells (HUVECs) and increasing the angiogenic tubularly-like structures in 3D endothelial bioprinted constructs. EVs resembled the miRNome profile of PL, mainly enriched with small RNAs and a high amount of miR-126, the most abundant angiogenic miRNA in platelets. The transfer of miR-126 by EVs in HUVEC after the in vitro inhibition of the endogenous form, restored angiogenesis, without involving VEGF as a downstream target in this system.

CONCLUSION

PL is a biological source of available EVs with angiogenic effects involving a miRNAs-based cargo. These properties can be exploited for targeted molecular/biological manipulation of PL, by potentially developing a product exclusively manufactured of EVs.

The emerging therapeutic potential of extracellular vesicles in trauma

Traumatic injury is a major cause of morbidity and mortality worldwide, despite significant advances in treatments. Most deaths occur either very early, through massive head trauma/CNS injury or exsanguination (despite advances in transfusion medicine), or later after injury often through multiple organ failure and secondary infection. Extracellular vesicles (EVs) are known to increase in the circulation after trauma and have been used to limited extent as diagnostic and prognostic markers. More intriguingly, EVs are now being investigated as both causes of pathologies post trauma, such as trauma-induced coagulopathy, and as potential treatments. In this review, we highlight what is currently known about the role and effects of EVs in various aspects of trauma, as well as exploring current literature from investigators who have begun to use EVs therapeutically to alter the physiology and pathology of traumatic insults. The potential effectiveness of using EVs therapeutically in trauma is supported by a large number of experimental studies, but there is still some way to go before we understand the complex effects of EVs in what is already a complex disease process.

Evaluation of the procoagulant properties of a newly developed platelet modified lysate product

BACKGROUND

Platelet transfusion is associated with logistical problems with the national storage guidelines of platelets. This results in decreased function in vivo as a result of the platelet storage lesion, and complications such as allergic or hemolytic reactions and thrombosis. We evaluated a new, freshly prepared platelet modified lysate (PML) product designed to be more procoagulant than fresh and stored platelets.

METHODS

Fresh platelets were concentrated, sonicated, and centrifuged to produce PML. Samples of both washed and unwashed PML were evaluated for particle size, concentration, and activity, and then tested for clot kinetics and thrombin generation. PML samples were also stored at various temperatures for durations up to 6 months and evaluated for clot kinetics and thrombin generation throughout.

RESULTS

PML showed significantly higher concentration of platelet microparticles, increased procoagulant properties, and increased thrombin generation as compared to fresh and stored platelets. In addition, PML maintained its clot kinetics over a 6-month storage period with variable storage conditions.

CONCLUSIONS

The newly proposed PML product is more procoagulant, stable, and has additional potential applications than currently available platelet products. Further studies will be performed to assess its functions in vivo and to assess thrombotic potential.

Platelet-derived- Extracellular Vesicles Promote Hemostasis and Prevent the Development of Hemorrhagic Shock

Every year more than 500,000 deaths are attributed to trauma worldwide and severe hemorrhage is present in most of them. Transfused platelets have been shown to improve survival in trauma patients, although its mechanism is only partially known. Platelet derived-extracellular vesicles (PEVs) are small vesicles released from platelets upon activation and/or mechanical stimulation and many of the benefits attributed to platelets could be mediated through PEVs. Based on the available literature, we hypothesized that transfusion of human PEVs would promote hemostasis, reduce blood loss and attenuate the progression to hemorrhagic shock following severe trauma. In this study, platelet units from four different donors were centrifuged to separate platelets and PEVs. The pellets were washed to obtain plasma-free platelets to use in the rodent model. The supernatant was subjected to tangential flow filtration for isolation and purification of PEVs. PEVs were assessed by total count and particle size distribution by Nanoparticle Tracking Analysis (NTA) and characterized for cells of origin and expression of EV specific-surface and cytosolic markers by flow cytometry. The coagulation profile from PEVs was assessed by calibrated automated thrombography (CAT) and thromboelastography (TEG). A rat model of uncontrolled hemorrhage was used to compare the therapeutic effects of 8.7 × 10⁸ fresh platelets (FPLT group, n = 8), 7.8 × 10⁹ PEVs (PEV group, n = 8) or Vehicle (Control, n = 16) following severe trauma. The obtained pool of PEVs from 4 donors had a mean size of 101 ± 47 nm and expressed the platelet-specific surface marker CD41 and the EV specific markers CD9, CD61, CD63, CD81 and HSP90. All PEV isolates demonstrated a dose-dependent increase in the rate and amount of thrombin generated and overall clot strength. In vivo experiments demonstrated a 24% reduction in abdominal blood loss following liver trauma in the PEVs group when compared with the control group (9.9 ± 0.4 vs. 7.5 ± 0.5 mL, p < 0.001>). The PEV group also exhibited improved outcomes in blood pressure, lactate level, base excess and plasma protein concentration compared to the Control group. Fresh platelets failed to improve these endpoints when compared to Controls. Altogether, these results indicate that human PEVs provide pro-hemostatic support following uncontrolled bleeding. As an additional therapeutic effect, PEVs improve the outcome following severe trauma by maintaining hemodynamic stability and attenuating the development of ischemia, base deficit, and cardiovascular shock.

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.

Liposome-chaperoned cell-free synthesis for the design of proteoliposomes: Implications for therapeutic delivery

Cell-free (CF) protein synthesis has emerged as a powerful technique platform for efficient protein production in vitro. Liposomes have been widely studied as therapeutic carriers due to their biocompatibility, biodegradability, low toxicity, flexible surface manipulation, easy preparation, and higher cargo encapsulation capability. However, rapid immune clearance, insufficient targeting capacity, and poor cytoplasmic delivery efficiency substantially restrict their clinical application. The incorporation of functional membrane proteins (MPs) or peptides allows the transfer of biological properties to liposomes and imparts them with improved circulation, increased targeting, and efficient intracellular delivery. Liposome-chaperoned CF synthesis enables production of proteoliposomes in one-step reaction, which not only substantially simplifies the production procedure but also keeps protein functionality intact. Building off these observations, proteoliposomes with integrated MPs represent an excellent candidate for therapeutic delivery. In this review, we describe recent advances in CF synthesis with emphasis on detailing key factors for improving CF expression efficiency. Furthermore, we provide insights into strategies for rational design of proteoliposomal nanodelivery systems via CF synthesis.

STATEMENT OF SIGNIFICANCE

Liposome-chaperoned CF synthesis has emerged as a powerful approach for the design of recombinant proteoliposomes in one-step reaction. The incorporation of bioactive MPs or peptides into liposomes via CF synthesis can facilitate the development of proteoliposomal nanodelivery systems with improved circulation, increased targeting, and enhanced cellular delivery capacity. Moreover, by adapting lessons learned from natural delivery vehicles, novel bio-inspired proteoliposomes with enhanced delivery properties could be produced in CF systems. In this review, we first give an overview of CF synthesis with focus on enhancing protein expression in liposome-chaperoned CF systems. Furthermore, we intend to provide insight into harnessing CF-synthesized proteoliposomes for efficient therapeutic delivery.

Intravenous synthetic platelet (SynthoPlate) nanoconstructs reduce bleeding and improve 'golden hour' survival in a porcine model of traumatic arterial hemorrhage

Traumatic non-compressible hemorrhage is a leading cause of civilian and military mortality and its treatment requires massive transfusion of blood components, especially platelets. However, in austere civilian and battlefield locations, access to platelets is highly challenging due to limited supply and portability, high risk of bacterial contamination and short shelf-life. To resolve this, we have developed an I.V.-administrable 'synthetic platelet' nanoconstruct (SynthoPlate), that can mimic and amplify body's natural hemostatic mechanisms specifically at the bleeding site while maintaining systemic safety. Previously we have reported the detailed biochemical and hemostatic characterization of SynthoPlate in a non-trauma tail-bleeding model in mice. Building on this, here we sought to evaluate the hemostatic ability of SynthoPlate in emergency administration within the 'golden hour' following traumatic hemorrhagic injury in the femoral artery, in a pig model. We first characterized the storage stability and post-sterilization biofunctionality of SynthoPlate in vitro. The nanoconstructs were then I.V.-administered to pigs and their systemic safety and biodistribution were characterized. Subsequently we demonstrated that, following femoral artery injury, bolus administration of SynthoPlate could reduce blood loss, stabilize blood pressure and significantly improve survival. Our results indicate substantial promise of SynthoPlate as a viable platelet surrogate for emergency management of traumatic bleeding.

Biomaterials and Advanced Technologies for Hemostatic Management of Bleeding

Bleeding complications arising from trauma, surgery, and as congenital, disease-associated, or drug-induced blood disorders can cause significant morbidities and mortalities in civilian and military populations. Therefore, stoppage of bleeding (hemostasis) is of paramount clinical significance in prophylactic, surgical, and emergency scenarios. For externally accessible injuries, a variety of natural and synthetic biomaterials have undergone robust research, leading to hemostatic technologies including glues, bandages, tamponades, tourniquets, dressings, and procoagulant powders. In contrast, treatment of internal noncompressible hemorrhage still heavily depends on transfusion of whole blood or blood's hemostatic components (platelets, fibrinogen, and coagulation factors). Transfusion of platelets poses significant challenges of limited availability, high cost, contamination risks, short shelf-life, low portability, performance variability, and immunological side effects, while use of fibrinogen or coagulation factors provides only partial mechanisms for hemostasis. With such considerations, significant interdisciplinary research endeavors have been focused on developing materials and technologies that can be manufactured conveniently, sterilized to minimize contamination and enhance shelf-life, and administered intravenously to mimic, leverage, and amplify physiological hemostatic mechanisms. Here, a comprehensive review regarding the various topical, intracavitary, and intravenous hemostatic technologies in terms of materials, mechanisms, and state-of-art is provided, and challenges and opportunities to help advancement of the field are discussed.

Bio-inspired nanomedicine strategies for artificial blood components

Blood is a fluid connective tissue where living cells are suspended in noncellular liquid matrix. The cellular components of blood render gas exchange (RBCs), immune surveillance (WBCs) and hemostatic responses (platelets), and the noncellular components (salts, proteins, etc.) provide nutrition to various tissues in the body. Dysfunction and deficiencies in these blood components can lead to significant tissue morbidity and mortality. Consequently, transfusion of whole blood or its components is a clinical mainstay in the management of trauma, surgery, myelosuppression, and congenital blood disorders. However, donor-derived blood products suffer from issues of shortage in supply, need for type matching, high risks of pathogenic contamination, limited portability and shelf-life, and a variety of side-effects. While robust research is being directed to resolve these issues, a parallel clinical interest has developed toward bioengineering of synthetic blood substitutes that can provide blood's functions while circumventing the above problems. Nanotechnology has provided exciting approaches to achieve this, using materials engineering strategies to create synthetic and semi-synthetic RBC substitutes for enabling oxygen transport, platelet substitutes for enabling hemostasis, and WBC substitutes for enabling cell-specific immune response. Some of these approaches have further extended the application of blood cell-inspired synthetic and semi-synthetic constructs for targeted drug delivery and nanomedicine. The current study provides a comprehensive review of the various nanotechnology approaches to design synthetic blood cells, along with a critical discussion of successes and challenges of the current state-of-art in this field. WIREs Nanomed Nanobiotechnol 2017, 9:e1464. doi: 10.1002/wnan.1464 For further resources related to this article, please visit the WIREs website.

State-of-the-Art-Review: Microvesicles in Blood Components: Laboratory and Clinical Aspects

There is ample evidence for the presence of microvesicles (MV) of different sizes and functions in various blood components. A variety of mechanisms have been proposed for the formation of MV. These include mechanical injury, shear stress, cell activation, activation of complements, hypoxia, and the cell aging process. While MV share many biological properties and surface receptors of their parental cells, they demonstrate significant differences in membrane asymmetry of the inner membrane phospholipid, in particular phosphatidylserine (PS). This provides high-affinity binding sites for the components of the prothrombinase complex. To what extent these MV contribute to hemostatic effectiveness, immudomodulation, and some untoward effects of the transfused blood components remains to be fully elucidated. Several methods for qualitative and semiquantitative characterization of MV are now available. Although in most cases it is necessary to separate MV from the intact cells for improved characterization, recent advances in flow cytometry make it possible to accurately differentiate MV in the presence of their parental cells on the basis of light scattering and fluorescent intensity. This review focuses on four main areas of MV in blood components: (1) the proposed mechanisms of platelet vesiculation, (2) factors influencing the formation of MV, (3) laboratory analysis of MV, and (4) the clinical impact of the presence of MV in blood components. Key Words: Microvesicte—Vesicutation—Biood component—Ptatelets—Transfusion.

Cryopreservation alters the membrane and cytoskeletal protein profile of platelet microparticles

BACKGROUND

Cryopreservation of platelets (PLTs) in dimethyl sulfoxide (DMSO) and storage at -80 °C extends the PLT shelf life to at least 2 years, allowing greater accessibility in military and rural environments. While cryopreserved PLTs have been extensively characterized, the microparticles formed as a result of cryopreservation are yet to be fully described.

STUDY DESIGN AND METHODS

Apheresis PLTs were cryopreserved at -80 °C with 5% DMSO and sampled before freezing and after thawing. Microparticle number, size, surface receptor phenotype, and function were assessed by microscopy, flow cytometry, dynamic light scattering, and thrombin-generating capacity. Proteomic changes were examined using two-dimensional gel electrophoresis and Western blotting.

RESULTS

PLT cryopreservation resulted in a 15-fold increase in the number of microparticles compared to fresh PLTs. The surface receptor phenotype of these microparticles differed to microparticles from fresh PLTs, with more microparticles expressing glycoprotein (GP)IV, GPIIb, and the GPIb-V-IX complex. Cryopreservation drastically altered the abundance of many cytoskeletal proteins in the PLT microparticles, including actin, filamin, gelsolin, and tropomyosin. Despite these changes, PLT microparticles were functional and contributed to phosphatidylserine- and tissue factor- induced thrombin generation.

CONCLUSION

This study demonstrates that PLT microparticles formed by cryopreservation are phenotypically distinct from those present before freezing. These differences may be associated with the procoagulant properties of cryopreserved PLTs.

Red cell-derived microparticles (RMP) as haemostatic agent

Among circulating cell-derived microparticles, those derived from red cells (RMP) have been least well investigated. To exploit potential haemostatic benefit of RMP, we developed a method of producing them in quantity, and here report on their haemostatic properties. High-pressure extrusion of washed RBC was employed to generate RMP. RMP were identified and enumerated by flow cytometry. Their size distribution was assessed by Doppler electrophoretic light scattering analysis (DELSA). Interaction with platelets was studied by platelet aggregometry, and shear-dependent adhesion by Diamed IMPACT-R. Thrombin generation and tissue factor (TF) expression was also measured. The effect of RMP on blood samples of patients with bleeding disorders was investigated ex vivo by thromboelastography (TEG). Haemostatic efficacy in vivo was assessed by measuring reduction of blood loss and bleeding time in rats and rabbits. RMP have mean diameter of 0.45 µm and 50% of them exhibit annexin V binding, a proxy for procoagulant phospholipids (PL). No TF could be detected by flow cytometry. At saturating concentrations of MPs, RMP generated thrombin robustly but after longer delay compared to PMP and EMP. RMP enhanced platelet adhesion and aggregation induced by low-dose ADP or AA. In TEG study, RMP corrected or improved haemostatic defects in blood of patients with platelet and coagulation disorders. RMP reduced bleeding time and blood loss in thrombocytopenic rabbits (busulfan-treated) and in Plavix-treated rats. In conclusion, RMP has broad haemostatic activity, enhancing both primary (platelet) and secondary (coagulation) haemostasis, suggesting potential use as haemostatic agent for treatment of bleeding.

Blood component therapy: Which, when and how much

Blood transfusion refers to the perioperative administration of blood and blood components. Adherence to proper indications for blood component therapy is essential because of its potential adverse effects and costs of transfusion. Over the years, the significance of blood components in treating certain diseases or conditions has been recognized. In this article, the most commonly used blood components along with the new developments in component therapy have been discussed. Recommendations by different academic and clinical trials and studies have been presented for quick reference. The individual coagulation factors are discussed in brief.

Glycoproteins of GpIbα and GpIIbIIIa on the Synthetic or Naturally Occurred Platelet-Derived Microparticles

Infusible platelet membranes (IPM) prepared from new or outdated human platelets have been developed as an alternative to standard platelet concentrates (PCs), with the additional advantage of long shelf life and increased viral safety. Lack of gpIIb/IIIa has been reported as one of the properties of the IPM microparticles. In re-examining this issue, we studied the molecules of gpIIb/IIIa and gpIbα on the surface of IPM microparticles. These molecules could better evaluate the functional efficacy of these microparticles. In comparison, we also surveyed the expression of these molecules on the surface of the naturally occurred platelet-derived microparticles during 7 days storage of PC. Unlike the previous reports, the results of this study illustrated that the produced IPM retained gpIIb/IIIa molecules. Besides, gpIbα and gpIIb/IIIa were also present on the nPMPs and their levels were significantly decreased during 7 days storage of PCs (P value = 0.001).

HLA Antigens Shed from the Surface of Synthetic or Naturally Occurred Platelet-Derived Microparticles During Storage of Platelet Concentrate

The demand for standard platelet concentrates (PCs) has continued to increase in the recent years. Infusible platelet membranes (IPM) prepared from new or outdated human platelets have been developed as an alternative to standard PCs, with the additional advantage of long shelf life and increased viral safety. Reduction of HLA antigens on the IPM has been assigned as one of the probable advantages of this product. In re-examining this issue, we studied the existence of HLA class I on the surface of IPM microparticles. In comparison we also surveyed HLA expression on the surface of the naturally occurred platelet-derived microparticles (nPMPs) during 7 days storage. Intended for producing IPM, PCs obtained from Iranian blood transfusion organization were lysed; virally inactivated with wet heat in the presence of a heat stabilizer and then sonicated. IPMs were separated using centrifugation and liquid-stored in 4°C. The expression of HLA class I antigens was surveyed using flow cytometry technique. HLA molecules were present on the microparticles. Shedding of HLA antigens was demonstrated from the surface of the both liquid-stored IPM and nPMPs during storage. Storage of IPM in 4°C was accompanied with significant reduction of HLA molecules. It seemed that achievement of HLA-free IPM could be impossible unless chloroquine treated platelets were used to prepare these microvesicles.

A novel approach to improving recombinant factor VIIa activity with a preserved platelet preparation

Recombinant activated factor VII (NovoSeven, rFVIIa) is used to abrogate bleeding in haemophiliacs with inhibitors and is hypothesised to work by increasing activated factor X generation on the platelet surface. We hypothesised that rFVIIa activity could be increased by the co-addition of platelet procoagulant surface. This study characterised the ability of a rehydrated, lyophilised (RL) platelet preparation to increase rFVIIa activity in haemophilic conditions. RL platelets supported thrombin generation in the presence of factors VIII and IX but, in the absence of factors VIII and IX, thrombin generation was significantly reduced. RL platelets supported rFVIIa-mediated thrombin generation in a rFVIIa-concentration dependent manner. In a cell-based in vitro model of haemophilia, the presence of RL platelets increased the rFVIIa-dependent thrombin generation rate 2.8-fold compared with rFVIIa alone. Similarly, the addition of RL platelets plus rFVIIa to the in vitro model of haemophilia and to haemophilic platelet-rich plasma shortened the onset of clot formation and increased clot stability in a fibrinolytic environment versus rFVIIa alone. These results suggest that RL platelets can support rFVIIa-mediated thrombin generation, and that co-administration of RL platelets with rFVIIa may increase the efficacy of rFVIIa in some patients.

Platelet membrane fragments enhance the procoagulant effect of recombinant factor VIIa in studies with circulating human blood under conditions of experimental thrombocytopenia

The mechanism of action of recombinant factor VIIa (rFVIIa), which is being considered as an alternative treatment for the control of bleeding episodes in patients with thrombocytopenia, has not been fully characterized. This study was undertaken to explore the effects of rFVIIa and platelet microvesicles on hemostasis in an experimental model of thrombocytopenia. Damaged arterial segments were exposed to thrombocytopenic blood (shear rate 600 s(-1)) either with or without the addition of rFVIIa and/or platelet microvesicles. The presence of fibrin and platelets on the subendothelium were morphometrically quantified and immunolocalization techniques and electron microscopy were used for a more detailed analysis. Both rFVIIa and platelet microvesicles consistently improved fibrin formation on the damaged vascular subendothelium, and microvesicles were shown to be localized at different levels of the fibrin lattice. Further, under conditions of moderate thrombocytopenia, addition of platelet microvesicles potentiated the procoagulant action of rFVIIa. This effect may be due to the phospholipid surface provided by the platelet microvesicles. These studies support the concept that, under conditions of thrombocytopenia, both rFVIIa and platelet microvesicles enhance fibrin formation at sites of vascular damage.

Substitutes and alternatives to platelet transfusions in thrombocytopenic patients

Over the past decade, there have been many improvements in both the safety profile and quality of liquid-stored allogeneic platelet concentrates. However, significant problems with the clinical use of such products remain. Efforts to overcome some of these have resulted in the development of an array of novel therapeutic strategies for the manufacture of platelet products and platelet substitutes, as well as other approaches using alternatives to platelet concentrates. These various products or procedures are at various stages of clinical development. This review summarizes some recent advancements in the preparation of liquid and frozen stored platelets, as well as approaches used for the pathogen inactivation of platelets. Thus, the status of lyophilized platelets, infusible platelet membranes, red blood cells (RBCs) bearing RGD ligands, fibrinogen-coated albumin microcapsules, and liposome-based agents are discussed. Pre-clinical studies and phase 1-3 clinical trials have been encouraging for several of these; however, to date, very few have been licensed for clinical use. Potential alternatives to allogeneic platelet transfusions including correction of anemia by RBC transfusions, recombinant activated factor VII and HLA-reduced platelets are also reviewed. With the ongoing technical and scientific development of such diverse products, those properties that may be necessary for such agents to have hemostatic efficacy will become apparent. However, safety and efficacy must be demonstrable in preclinical studies and clinical trials, before novel platelet concentrates, platelet substitutes and alternatives to platelets can be used in patients with thrombocytopenia.

Thrombus formation with rehydrated, lyophilized platelets

Stored human platelets are frequently used in hemorrhagic emergencies, but have limited immediate utility for controlling bleeding due to storage lesion and are frequently contaminated with microorganisms. The development of paraformaldehyde-treated, lyophilized and rehydrated (RL) platelets, which are sterile and have a prolonged shelf life (years), ameliorate the efficacy and sterility problems with stored platelets. RL platelets have been shown to have many native functions of fresh platelets in vitro and to mediate hemostasis in vivo in large animal models of hemorrhagic shock and cardiopulmonary bypass induced platelet dysfunction. To further evaluate the functional properties of this transfusion product, we studied the role of RL platelets in three aspects of thrombus formation and lysis. First, the interaction between RL platelets and fibrinogen was investigated. The surface density of unligated GPIIb-IIIa on RL and fresh platelets were, respectively 30000 and 70000 molecules per cell as detected with the monoclonal antibody 10E-5. Freezing, lyophilization and rehydration steps in the preparation of RL platelets resulted in the surface presentation of 120000 molecules of fibrinogen per cell from alpha granule sources. After ADP activation, RL platelets bound exogenous 125I-labeled fibrinogen in a dose-dependent manner with an affinity that is similar to that of fresh platelets and was inhibited by RGD peptides. 125I-Labeled fibrinogen binding to RL and fresh platelets, respectively, saturated at 14000 and 32000 molecules per cell. Scanning electron microscopic ultrastructural analysis showed that fibrin strands interacted with the surface of RL platelets in a normal manner. The second set of studies investigated the ability of RL platelets to catalyze and amplify the clot formation process in an activation-dependent manner. We showed that RL platelets undergo degranulation in fibrin in clots and functioned as thrombogenic surfaces for the generation of activated coagulation factors and fibrin generation. A final set of studies was performed to investigate fibrin of clots that contained RL platelets. RL platelet clots were lysed in the presence of tissue plasminogen activator with a similar time course as clots without platelets, and lysis occurred faster than when fresh platelets were included in the fibrin mass. The results of these three studies demonstrate that RL platelets are capable of mediating thrombus formation and do not inhibit lysis. Our results help explain how RL platelets restore hemostasis in vivo, and indicate that these cells might be a viable alternative to fresh stored platelets in transfusion medicine.

Splenic clearance mechanisms of rehydrated, lyophilized platelets

A variety of platelet substitutes (e.g., rehydrated, lyophilized (RL) platelets, thromboerythrocytes, plateletsomes, infusible platelet membranes, synthocytes, fibrinogen-coated microcapules) are potentially useful as hemostatic agents in transfusion medicine. However, as "foreign" particles, platelet substitutes interact to varying extents with elements of the reticulo-endothelial system for clearance, reducing hemostatic efficacy. Experiments were performed to better understand the interaction of RL platelets with elements of the innate and acquired immune systems. The infusion of heterologous RL platelets into rats resulted in rapid clearance from the free circulation with half-life values of minutes. The clearance of RL platelets was inhibited when macrophages were rendered apoptotic with gadolinium. Transmission EM analysis of splenic tissue after infusion of lyophilized cells, as well as in vitro mixing studies with splenic macrophages and RL platelets, indicated that macrophage-mediated phagocytosis mechanisms were operant in RL platelet clearance by the reticulo-endothelial system. Studies with IV IgG, as a competitive inhibitor of the macrophage Fc receptor, provides evidence that RL platelet destruction is in part mediated by platelet surface bound IgG. This hypothesis was further supported by the finding that RL platelets react with IgG class antibodies that are pre-existing in naïve animals. These studies provide a rational basis for prolonging the circulation time of RL platelets and other platelet substitutes.

Novel platelet products, substitutes and alternatives

Despite the many advances in the safety, processing, and storage of conventional 22 degrees C liquid-stored allogeneic platelet concentrates, there still are significant drawbacks to the use of such products. Efforts to overcome these shortcomings have resulted in an array of novel platelet products, substitutes, and alternatives; which are currently at various stages of development. This review summarizes the recent developments in the frozen and cold storage of platelets; their pathogen inactivation; as well as the status of lyophilized platelets, infusible platelet membranes (IPMs), red cells bearing arginine-glycine-aspartic acid (RGD) ligands, fibrinogen-coated albumin microcapsules, and liposome-based agents; as potential alternatives to the use of conventional platelet transfusions. Pre-clinical studies have been encouraging for several of these novel products; however, to date, very few have entered human trials. Nonetheless, with the ongoing development of diverse products, those properties that may be necessary for their hemostatic effectiveness will become apparent. However, safety and efficacy must be demonstrated in pre-clinical and phase I to III clinical trials before these novel agents, substitutes and alternatives can be used clinically for patients with thrombocytopenia.

Infusible platelet membranes retain partial functionality of the platelet GPIb/IX/V receptor complex

Infusible platelet membranes (IPMs) prepared from fresh or outdated human platelets have been shown to correct prolonged bleeding times in thrombocytopenic rabbits. In previous trials, IPMs did not seem to be immunogenic and lacked dose-limiting toxicity. The present study was undertaken to explore whether the platelet glycoprotein (GP) Ib/IX/V complex might retain functionality in the IPM preparation. IPMs did not spontaneously bind von Willebrand factor (vWF), but saturable binding could be induced by ristocetin, with a dissociation constant (Kd) of 0.31 +/- 0.03 microgram/mL at 1.0 mg/mL of ristocetin. Of 4 anti-GPIb-alpha monoclonal antibodies tested, AN-51 inhibited vWF binding 67.8% +/- 5.8%, whereas AS-2, AS-7, and SZ-2 were ineffective. Maximal vWF binding induced by botrocetin was only 10% to 15% of that observed with ristocetin. Retention of partial functionality of the GPIb/IX/V receptor allowing vWF binding in a modulated manner seems to represent a critical mechanism by which IPMs may provide hemostatic efficacy.

New technologies for the inactivation of infectious pathogens in cellular blood components and the development of platelet substitutes

Despite the increased safety of blood components, achieved through improved donor selection and testing, transfusion recipients remain at risk of transfusion-associated diseases. Transfusion of cellular blood components has been implicated in transmission of viral, bacterial and protozoan diseases. While it is commonly recognized that hepatitis B virus (HBV), hepatitis C virus (HCV), cytomegalovirus (CMV), and retroviruses, such as human immunodeficiency virus (HIV) and the human lymphotrophic viruses (HTLV), can be transmitted through cellular components, other pathogens are emerging as potentially significant transfusion-associated infectious agents. For example, transmission of protozoan infections due to trypanosomes and Babesia have been reported. In addition to viral and protozoan infectious agents, cases of bacterial contamination of platelet and red cell concentrates continue to be reported and may be an under-reported transfusion complication. More importantly, new infectious agents continue to enter the donor population, and there is an inherent time delay before the new pathogens are definitively identified and new tests implemented in order to maintain consistent safety of the blood supply. The paradigm for this problem is the HIV pandemic. During the past decade a number of methods for inactivating infectious pathogens in platelet concentrates have been investigated as a strategy to improve the safety of platelet transfusion therapy. One method of treating platelet concentrates to inactive pathogens has now reached the advanced clinical trial phase in the United States and Europe. Similar efforts with a new class of compounds are underway for red cell concentrates, and two of these are in early phase trials. In addition to studies with allogeneic platelets and red cells, a number of laboratories have described methods for developing platelet substitutes or modified platelets to avoid the use of traditional platelet concentrates as a means to improve safety.

Intracellular function in rehydrated lyophilized platelets

This study aimed to evaluate the effect of cross-linking and lyophilization on intracellular signalling processes in rehydrated, lyophilized (RL) platelets, which are under development as a platelet substitute for transfusion. Exposure of RL platelets to thrombin resulted in enhanced phosphorylation of several proteins, including 18 kDa and 42 kDa kinase substrates that were shown to be the substrates of myosin light chain and protein kinase C respectively. Cross-linking and lyophilization depleted the platelets of free cytoplasmic ADP and ATP, but had less effect on protein-bound nucleotides. The surface membrane of RL platelets was found to be permeable to poly dT probes less than approximately 3 kDa in size; larger nucleotide probes and proteins did not penetrate the surface membrane. Taken together, our results indicate that RL platelets retain some of the haemostatic stimulus-response functions of fresh platelets and are capable of feedback amplification in coagulation.

Infusible platelet membranes improve hemostasis in thrombocytopenic blood: experimental studies under flow conditions

BACKGROUND

The potential hemostatic effect of infusible platelet membranes (IPM; Cyplex, Cypress Bioscience) prepared from outdated human platelets is investigated.

STUDY DESIGN AND METHODS

Increasing concentrations of IPM were added to blood samples anticoagulated with low-molecular-weight heparin, in which platelets and WBC counts had been experimentally reduced by a filtration procedure. Thrombocytopenic blood with IPM was circulated in a perfusion chamber at various shear rates (300, 600, and 1200/sec(-1)), and platelet and fibrin deposition on the surface of a damaged vessel was measured. Prothrombin fragments 1 and 2 (F1+2) levels were also monitored.

RESULTS

Under conditions of severe thrombocytopenia (<6000 platelets/microL) IPM did not increase platelet deposition. However, a dose-dependent increase in fibrin deposition was observed with concentrations of IPM ranging from 0.5 to 2 mg per kg in perfusions at 300 and 600 per sec(-1) (p<0.05 vs. thrombocytopenic blood). Experimental studies performed under conditions of moderate thrombocytopenia and higher shear rates (25, 000-30,000 platelets/microL; at 600 and 1200/sec(-1)) showed that IPM concentrations equivalent to 0.5 or 1 mg per kg improved fibrin deposition (33.5 +/- 9.5% and 37.7 +/- 12.8%, respectively, vs. 22.7 +/- 5.2% in controls) and also promoted a moderate increase in platelet deposition, with a concomitant significant increase in the size of platelet aggregates (p<0.05). Exposure of thrombocytopenic blood to a damaged vessel resulted in an increase of F1+2 levels from 0.8 +/- 0.15 to 1.7 +/- 0.22 nM at 300 per sec(-1) and 1.94 +/- 0.46 nM at 600 per sec(-1). Postperfusion levels of F1+2 after the addition of IPM were always similar to levels in untreated controls.

CONCLUSION

IPM promotes local procoagulant activity at sites of vascular damage under conditions of severe and moderate thrombocytopenia. IPM also appears to facilitate platelet cohesive functions under conditions of moderate thrombocytopenia.

Platelet substitutes and novel platelet products

Despite many advances in the safety, processing and storage of conventional 22 degrees C liquid-stored allogeneic platelet concentrates, there are still significant drawbacks to standard platelet concentrates used in transfusions for patients with thrombocytopenia. Efforts to overcome these shortcomings have been undertaken in both academic and commercial settings, resulting in an array of novel platelet products and substitutes that are currently at various stages of development. This review summarises the recent developments in lyophilised platelets, infusible platelet membranes (IPM), red cells bearing arginine-glycine-aspartic acid (RGD) ligands, fibrinogen-coated albumin microcapsules and liposome-based agents as putative alternatives to conventional transfusions involving allogeneic platelet concentrates. These various products are designed to replace the use of allogeneic donor platelets with modified or artificial platelets, to augment the function of existing platelets and/or provide a procoagulant material capable of achieving primary haemostasis in patients with thrombocytopenia. Preclinical studies have been encouraging for several of these platelet substitutes and novel platelet products, however, to date, only a few of these products have entered human trials. With the ongoing development of these diverse products, properties necessary for haemostatic effectiveness will become apparent. Safety and efficacy, however, must be demonstrated in preclinical and Phase I - III clinical trials, before these novel agents can be used clinically for patients with thrombocytopenia.

Lyophilized platelets: continued development

In our initial investigation of functionality of platelets freeze-dried after stabilization with 1.8% paraformaldehyde, we found that the rehydrated cells were morphologically intact and retained adhesive and procoagulant properties. Further testing of fixed, washed freeze-dried platelets has demonstrated the physiologic nature of their adhesion in vitro and their hemostatic efficacy in vivo in correcting the bleeding time in thrombocytopenic animal models. Binding studies with monoclonal antibodies and radiolabelled ligands indicate an intact GpIb vonWillebrands factor receptor as on fresh platelets, but a somewhat attenuated GpIIbIIIa fibrinogen receptor. Repeated infusion of canine lyophilized platelet preparations in a single recipient over several months has shown no incipient cytopenia upon infusion of new doses nor accelerated clearance of platelets. These findings suggest minimal risk of systemic thrombosis or severe immunogenic reaction and support the notion of approaching clinical trials as soon as possible.

Alternatives to standard blood transfusion: availability and promise

Largely due to concerns over safety, a wide variety of alternatives to the conventional blood bank products of red cells, platelet concentrates, plasma and fractionated plasma products are under development. This review attempts to survey the alternative therapies that are being developed, whether they provide viable solutions and what impact they might have on transfusion practice.

Platelets: preparations, transfusion, modifications, and substitutes

OBJECTIVE

To summarize current knowledge and recent progress pertaining to platelet concentrate preparations, modifications, and future prospects for platelet substitutes.

METHODS

Current publications identified through a search of an electronic literature database were evaluated and reviewed. Relevant data were abstracted into this article. Abstractions of the data were made depending on their relevance. This review starts with standard methods of platelet preparation and goes on to describe different modifications intended to optimize the product and increase its safety. The article concludes with a discussion of the use of hematopoietic growth factors and novel kinds of platelet components for future use.

CONCLUSIONS

Many modifications in the preparation of platelet transfusions have occurred in recent years. Platelets prepared by standard techniques contain significant numbers of donor leukocytes, which are responsible for several adverse effects. Awareness of this problem has lead to the development of effective means for their removal. Several methods to reduce the risk of viral and bacterial transmission through platelet transfusions are emerging. New technologies in the use of platelet substitutes have attempted to prolong the platelet storage potential and prevent the development of recipient alloimmunization. As the biological activities of growth factors become better understood, the clinical applications of novel recombinant products may redefine the concept of future platelet transfusions. It is important that research continues into the optimal methods for the preparation and use of platelet transfusions to provide maximal clinical benefits with minimal risk of complications.

Reduction of blood loss by infusion of human platelets in a rabbit kidney injury model

BACKGROUND

As a first step toward testing the efficacy of stored platelets or platelet substitutes in vivo, a kidney injury model was developed to assess the hemostatic properties of human platelets in normal and thrombocytopenic rabbits.

STUDY DESIGN AND METHODS

New Zealand white rabbits were made thrombocytopenic by two consecutive injections of busulfan. Two weeks later, human platelets were transfused to animals whose reticuloendothelial systems were inhibited by the administration of ethyl palmitate. The left kidney was exposed and a slice excised from the anterior pole. The blood was contained in a parafilm boat and absorbed by preweighed gauze to assess blood loss. The percentage of human platelets transfused to the rabbit was determined by flow cytometry on blood collected from the cut site using anti-CD42a (marker for human platelets). The degree of activation of the human platelets was determined using anti-CD62a (marker specific for human p-selectin).

RESULTS

Blood loss was similar in normal animals treated with saline alone (35.4 +/- 5.8 g; n = 4); ethyl palmitate and saline (42.5 +/- 5.7 g; n = 6, p = 0.4); or ethyl palmitate and fresh human platelets (45.7 +/- 7.9 g; n = 6, p = 0.3). Bleeding in thrombocytopenic rabbits infused with saline was increased (75.6 +/- 3.9 g; n = 7) as compared with nonthrombocytopenic animals. A significant reduction in blood loss was noted in thrombocytopenic rabbits given fresh human platelets (51.6 +/- 4.5 g; n = 6, p = 0.0023). Transfusion of human platelets to rabbits did not cause activation of the platelets. Furthermore, transfusion of thrombin-activated platelets (60-98% activated) to thrombocytopenic rabbits reduced blood loss (54 +/- 7.3 g; n = 7) to the same extent as fresh platelets.

CONCLUSIONS

This is the first report describing a kidney injury model developed to assess the efficacy of fresh and activated human platelets in reducing blood loss in thrombocytopenic rabbits. This model could monitor the efficacy of human platelets prepared by various preservation protocols in suppressing bleeding in rabbits.

Platelet-like particle production from cultured human megakaryocytic cell line

The human megakaryocytic cell line CMK was positive for the expression of platelet glycoprotein(GP) IIb/IIIa but few cells were positive for GPIb. To establish a GPIb-positive CMK subclone, GPIb-positive CMK cells were purified, micromanipulated and cloned, but resulting sublines were all negative for GPIb expression. The combined presence of interleukin-3 (IL-3) and the anti-virus agent ribavirin strongly induced expression of GPIb on the cells. However, the megakaryocyte growth and development factor (MGDF)/thrombopoietin (TPO) failed to enhance GPIb expression. CMK themselves and particles are negative for GPIb expression. To utilize these particles as platelet substitutes, further appropriate culture systems are necessary.