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The National Heart, Lung, and Blood Institute (NHLBI), one of 27 institutes/centers within the National Institutes of Health, has a strong interest in promoting translational research. This can be traced to its mission statement, which reads in part: “The National Heart, Lung, and Blood Institute stimulates basic discoveries about the causes of disease, enables the translation of basic discoveries into clinical practice, fosters training and mentoring of emerging scientists and physicians, and communicates research advances to the public” (http://www.nhlbi.gov/about/org/mission). Each year, the NHLBI invests more than $3 billion to support research, training, and education programs to promote the prevention and treatment of heart, lung, and blood diseases and sleep disorders. More than two thirds of this investment is allocated to investigator-initiated research, spanning the continuum from discovery through preclinical and clinical studies. For more than a decade, the need for targeted programs to foster translational research, including investigational new drug (IND)–enabling research activities, has been recognized by the National Heart, Lung, and Blood Institute (NHLBI) leadership. As summarized in Table 1, the first of these was initiated in 2004 to provide production assistance for regenerative cell preparations (Production Assistance for Cellular Therapies). A second program was launched in 2007 to support gene therapy research (Gene Therapy Resource Program), and these were followed by additional programs to support translation of promising discoveries from laboratory to clinic: Science Moving Towards Research Translation and Therapy (SMARTT) in 2010; National Institutes of Health Centers for Accelerated Innovations in 2011; and Vascular Interventions/Innovations and Therapeutic Advances in 2013. Then in 2014, (1) the Centers for Advanced Diagnostics and Experimental Therapeutics in Lung Diseases was funded to accelerate the development of novel products to treat lung diseases and sleep-disordered breathing; and (2) the Center for Translation Research and Implementation Science was formed to address gaps …

On March 25 and 26, 2015, the National Heart, Lung, and Blood Institute sponsored a meeting on the State of the Science in Transfusion Medicine on the National Institutes of Health (NIH) campus in Bethesda, Maryland, which was attended by a diverse group of 330 registrants. The meeting's goal was to identify important research questions that could be answered in the next 5 to 10 years and which would have the potential to transform the clinical practice of transfusion medicine. These questions could be addressed by basic, translational, and/or clinical research studies and were focused on four areas: the three "classical" transfusion products (i.e., red blood cells, platelets, and plasma) and blood donor issues. Before the meeting, four working groups, one for each area, prepared five major questions for discussion along with a list of five to 10 additional questions for consideration. At the meeting itself, all of these questions, and others, were discussed in keynote lectures, small-group breakout sessions, and large-group sessions with open discourse involving all meeting attendees. In addition to the final lists of questions, provided herein, the meeting attendees identified multiple overarching, cross-cutting themes that addressed issues common to all four areas; the latter are also provided. It is anticipated that addressing these scientific priorities, with careful attention to the overarching themes, will inform funding priorities developed by the NIH and provide a solid research platform for transforming the future practice of transfusion medicine.

Cell therapy products have become more sophisticated both in type and manufacturing complexity (1-3). As clinicians' and researchers' understanding of disease and cell mechanisms of action continue to grow, cell therapy products are being increasingly used or considered for use in clinical trials (4-12). The safety and efficacy of cell therapy products have been reported within individual clinical trials and through review articles (13-19). These publications are often limited to an individual cell type and also differ by how the data are being collected and reported. The Health Resources and Services Administration (HRSA) has provided funds to study the use of cell therapy in the United States with a goal of building a database similar to that established and maintained by the bone marrow transplant community for years. The European Union has pursued a similar goal. Despite these efforts, there is limited published information that describes cell therapy product administration across both cell types and indications. To this end, The National Institutes of Health, National Heart, Lung and Blood Institute (NHLBI) Production Assistance for Cellular Therapies (PACT) program has collected data over the past 10 years on the administration of cell therapy products manufactured within the program. The PACT program was formed in 2003 through funding from the NHLBI. The program provides clinical cell therapy product manufacturing support to investigators wishing to transition a novel cell therapy from the developmental stage to clinical applications within the purview of the NHLBI. PACT is not responsible for directly monitoring the clinical trials of investigators receiving PACT-manufactured products. PACT has required collection of standardized information on product manufacturing, transport, receipt, administration, and adverse reactions with product administration. The purpose of this data collection is 1) to monitor administration of PACT cell therapy products, and 2) to build a product administration database to identify trends or safety concerns that may be associated with cell therapy product administrations. All clinical trials where PACT has provided manufacturing support were approved by local ethics committees.

There is concern at the National Heart, Lung, and Blood Institute (NHLBI) and among transfusion medicine specialists regarding the small number of investigators and studies in the field of pediatric transfusion medicine (PTM). Accordingly, the objective of this article is to provide a snapshot of the clinical and translational PTM research considered to be of high priority by pediatricians, neonatologists, and transfusion medicine specialists. Included is a targeted review of three research areas of importance: (i) transfusion strategies, (ii) short- and long-term clinical consequences, and (iii) transfusion-transmitted infectious diseases. The recommendations by PTM and transfusion medicine specialists represent opportunities and innovative strategies to execute translational research, observational studies, and clinical trials of high relevance to PTM. With the explosion of new biomedical knowledge and increasingly sophisticated methodologies over the past decade, this is an exciting time to consider transfusion medicine as a paradigm for addressing questions related to fields such as cell biology, immunology, neurodevelopment, outcomes research, and many others. Increased awareness of PTM as an important, fertile field and the promotion of accompanying opportunities will help establish PTM as a viable career option and advance basic and clinical investigation to improve the health and wellbeing of children.

Clinical trials for pediatric diseases face many challenges, including trial design, accrual, ethical considerations for children as research subjects, and the cost of long-term follow-up studies. In September 2011, the Production Assistance for Cellular Therapies Program, funded by the National Heart, Lung, and Blood Institute of the National Institutes of Health, sponsored a workshop, “Cell Therapy for Pediatric Diseases: A Growing Frontier,” with the overarching goal of optimizing the path of discovery in research involving novel cellular therapeutic interventions for debilitating pediatric conditions with few or no available treatment options. Academic and industry investigators in the fields of cellular therapy and regenerative medicine described the obstacles encountered in conducting a clinical trial from concept to conclusion. Patient and parent advocates, bioethicists, biostatisticians, regulatory representatives from the US Food and Drug Administration, and translational scientists actively participated in this workshop, seeking to identify the unmet needs specific to cellular therapies and treatment of pediatric diseases and propose strategies to facilitate the development of novel therapies. In this article we summarize the obstacles and potential corrective strategies identified by workshop participants to maximize the speed of cell therapy translational research for childhood diseases.

Comparative effectiveness research (CER) is the study of existing treatments or ways to deliver health care to determine what intervention works best under specific circumstances. CER evaluates evidence from existing studies or generates new evidence, in different populations and under specific conditions in which the treatments are actually used. CER does not embrace one research design over another but compares treatments and variations in practice using methods that are most likely to yield widely generalizable results that are directly relevant to clinical practice. Treatments used in transfusion medicine (TM) are among the most widely used in clinical practice, but are among the least well studied. High-quality evidence is lacking for most transfusion practices, with research efforts hampered by regulatory restrictions and ethical barriers. To begin addressing these issues, the National Heart, Lung, and Blood Institute convened a workshop in June 2011 to address the potential role of CER in the generation of high-quality evidence for TM decision making. Workshop goals were to: 1) evaluate the current landscape of clinical research, 2) review the potential application of CER methods to clinical research, 3) assess potential barriers to the use of CER methodology, 4) determine whether pilot or vanguard studies can be used to facilitate planning of future CER research, and 5) consider the need for and delivery of training in CER methods for researchers.

In 2002, to determine investigator needs in cellular therapeutics, the National Heart, Lung, and Blood Institute (NHLBI) conducted a workshop entitled “Immune reconstitution and cell-based therapy following hematopoietic stem cell (HSC) transplantation.”1 The workshop addressed the biology of immune reconstitution after transplantation, but also considered newer and more complex cell-processing methods and the changing regulatory environment in which they were emerging. The workshop participants recognized that, compared with classical HSC processing and transplantation, a much broader array of cell products and tissue types was being brought from the research laboratory to the clinic and that these activities presented both opportunities and challenges that were new and very significant. Opportunities included the emerging possibilities related to specific cell selection, stimulation, expansion, and how these and other laboratory manipulations were enabling fundamentally new treatments for a range of diseases. Challenges included the substantial difficulties often encountered when translating and scaling up preclinical research projects from their beginnings in the basic research laboratory. The workshop participants also recognized that cost and availability of highly specialized technical and regulatory expertise were significant barriers to the growth and success of human cellular therapy. Relatively few academic research institutions at the time had access to experienced cell-processing personnel, manufacturing facilities, and the regulatory expertise needed to develop a cell product internally. To address these issues and foster the growth of innovative scientific concepts in cellular therapy, the NHLBI initiated the Production Assistance for Cellular Therapies (PACT) program as a scientific, regulatory, and educational resource for the cell therapy community.2

Adenoviruses are commonly used as vectors in human clinical gene therapy trials. High doses of intravenous adenovirus vectors have been associated with development of thrombocytopenia of undetermined origin. Viral internalization requires the presence cell surface integrins, alpha(v)beta(3) or alpha(v)beta(5), that can blind ligands with a arginine-glycine-aspartic acid (RGD) sequence. This sequence is found in the adenovirus penton base. Platelets express the alpha(v)beta(3) integrin and other integrins that bind the RGD sequence of ligands such as fibrinogen, laminin, vitronectin, and von Willebrand factor (vWF). Platelet aggregation is mediated, in part, by the binding of the RGD sequence of fibrinogen to a platelet surface integrin, glycoprotein IIb/IIIa (GP IIb/IIIa). We investigated whether adenovirus particles could interfere with or potentiate agonist-induced platelet aggregation. Incubation of platelet-rich plasma with adenovirus under stirred conditions did not promote spontaneous aggregation. The addition of physiological platelet agonists, ADP, collagen, or epinephrine, induced platelet aggregation. However, the presence of adenovirus in a wide range of concentrations did not inhibit or potentiate agonist-induced aggregation. These results suggest that the adenovirus-associated thrombocytopenia observed in vivo is independent of a direct effect of the virus on platelet aggregation.

We constructed a first-generation adenovirus vector (AVC3FIX5) that we used to assess the rhesus macaque as a nonhuman primate model for preclinical testing of hemophilia B gene therapy vectors. Although we succeeded in our primary objective of demonstrating expression of human factor IX we encountered numerous toxic side effects that proved to be dose limiting. Following intravenous administration of AVC3FIX5 at doses of 3.4 x 10(11) vector particles/kg to 3.8 x 10(12) vector particles/kg, the animals in our study developed antibodies against human factor IX, and dose-dependent elevations of enzymes specific for liver, muscle, and lung injury. In addition, these animals showed dose-dependent prolongation of clotting times as well as acute, dose-dependent decreases in platelet counts and concomitant elevation of fibrinogen and von Willebrand factor. These abnormalities may be caused by the direct toxic effects of the adenovirus vector itself, or may result indirectly from the accompanying acute inflammatory response marked by elevations in IL-6, a key regulator of the acute inflammatory response. The rhesus macaque may be a useful animal model in which to evaluate mechanisms of adenovirus toxicities that have been encountered during clinical gene therapy trials.

Recognizing the challenges faced by researchers and clinicians working in the field of cellular therapy, the National Heart, Lung, and Blood Institute (NHLBI), National Institutes of Health, established the Production Assistance for Cellular Therapies (PACT) program in 2003 and expanded it in 2010. The PACT program provides both clinical product manufacturing support that furthers the mission of NHLBI in the areas of cardiac, lung, and blood diseases and broad support of translational development across all disease areas to serve the entire cell therapy community. The program also provides access to expertise in project management, regulatory affairs, and quality assurance and control. Education initiatives include webinars, cell processing facility-hosted workshops, national workshops, and active participation and leadership within the cell therapy community through collaboration with other cell therapy organizations and academia. So far, over 650 PACT-manufactured cell therapy products have been administered in 32 clinical trials for a range of illnesses and diseases such as acute myocardial infarction, sickle cell disease, and graft-versus-host disease.

Several lines of evidence point to the potential role of nitric oxide (NO) in the pathophysiology, as well as in the therapy, of sickle cell disease (SCD). In this study, we compared the effects of NO on platelets from normal individuals and from patients with SCD. Three NO donors were used to deliver NO to platelets: sodium 2-(N, N-diethylamino)-diazenolate-2-oxide (DEANO), S-nitrosocysteine (CysNO) and sodium trioxdintrate (OXINO or Angeli's salt). ADP-induced platelet aggregation, CD62P expression, PAC-1 binding and calcium elevation were evaluated in paired studies of normal and SCD subjects. DEANO significantly reduced aggregation in SCD platelets compared with normal platelets. DEANO similarly reduced the extent of CD62P expression in SCD platelets. All NO donors reduced PAC-1 binding, but there were no significant differences between platelets from normal or SCD subjects. Calcium elevation, as induced by ADP, was not altered by the presence of NO donors. However, when platelets were stimulated with thrombin, there was an increased initial response of SCD platelets compared with normal platelets. Taken together, these data suggest that the mode of NO delivery to platelets may produce various physiological responses and the optimization of NO delivery may contribute to reducing platelet aggregation in sickle cell disease.

Glycoprotein (GP) IIb-IIIa plays a critical role in platelet aggregation and platelet-mediated clot retraction. This study examined the intramolecular relationship between GPIIb-IIIa activation and fibrinogen binding, platelet aggregation, and platelet-mediated clot retraction. To distinguish between different high-affinity activation states of GPIIb-IIIa, the properties of an antibody (D3) specific for GPIIIa that induces GPIIb-IIIa binding to adhesive protein molecules and yet completely inhibits clot retraction were used. Clot retraction inhibition by D3 was not due to altered platelet-fibrin interaction; however, combination treatments of D3 and adenosine diphosphate (ADP) inhibited full-scale aggregation and decreased the amounts of GPIIb-IIIa and talin incorporated into the core cytoskeletons. Morphologic evaluation of the D3/ADP aggregates showed platelets that were activated but to a lesser extent when compared to ADP only. ADP addition to platelets caused an increase in the number of D3 binding sites indicating that ligand had bound to the GPIIb-IIIa receptor. These data suggest that high-affinity GPIIb-IIIa– mediated ligand binding can be separated mechanistically from GPIIb-IIIa–mediated clot retraction and that clot retraction requires additional signaling through GPIIb-IIIa after ligand binding. The conformation recognized by D3 represents the expression of a GPIIb-IIIa activation state that participates in full-scale platelet aggregation, cytoskeletal reorganization, and clot retraction.

Glycoprotein (GP) IIb-IIIa plays a critical role in platelet aggregation and platelet-mediated clot retraction. This study examined the intramolecular relationship between GPIIb-IIIa activation and fibrinogen binding, platelet aggregation, and platelet-mediated clot retraction. To distinguish between different high-affinity activation states of GPIIb-IIIa, the properties of an antibody (D3) specific for GPIIIa that induces GPIIb-IIIa binding to adhesive protein molecules and yet completely inhibits clot retraction were used. Clot retraction inhibition by D3 was not due to altered platelet-fibrin interaction; however, combination treatments of D3 and adenosine diphosphate (ADP) inhibited full-scale aggregation and decreased the amounts of GPIIb-IIIa and talin incorporated into the core cytoskeletons. Morphologic evaluation of the D3/ADP aggregates showed platelets that were activated but to a lesser extent when compared to ADP only. ADP addition to platelets caused an increase in the number of D3 binding sites indicating that ligand had bound to the GPIIb-IIIa receptor. These data suggest that high-affinity GPIIb-IIIa– mediated ligand binding can be separated mechanistically from GPIIb-IIIa–mediated clot retraction and that clot retraction requires additional signaling through GPIIb-IIIa after ligand binding. The conformation recognized by D3 represents the expression of a GPIIb-IIIa activation state that participates in full-scale platelet aggregation, cytoskeletal reorganization, and clot retraction.

Conventional platelet storage in a blood bank is up to 5 days at room temperature in plasma. We investigated the optimal medium for assessing the quality of stored platelets by comparing in vitro test responses after resuspension in autologous plasma prepared from platelet-rich plasma after 5 days of storage at room temperature, autologous plasma stored cell-free for 5 days at room temperature, or autologous plasma stored cell-free for 5 days at -20 degrees C. Five-day-old platelets were prepared from aliquots of the same unit and resuspended in I of the 3 plasma preparations. The platelet-plasma mixtures were monitored for changes in pH, mean platelet volume, hypotonic shock response, P-selection expression, and aggregation. There were statistically significant differences between platelets resuspended in original plasma and platelets resuspended in either plasma stored cell-free at room temperature or frozen, with regard to hypotonic shock response, agonist-induced aggregation, and P-selectin expression. Plasma stored with platelets for 5 days yielded inferior platelet function test responses when compared with plasma stored cell-free at room temperature or frozen. Therefore, for direct comparison of platelet responses following novel storage methods, the resuspending plasma should be stored under the same conditions as the control platelet unit.

The surface expression of prion protein (PrP(C)) on human platelets, as detected by flow cytometry with the monoclonal antibody 3F4, increased more than two-fold (4300 v 1800 molecules/platelet) after full activation. Maximal surface expression of PrP(C) occurred within 3 min of platelet activation and declined to approximately half of maximal levels by 2 h at 37 degrees C. In comparison, PrP(C) on the surface of platelets, activated at 22 degrees C took 10 min to reach maximum but then remained constant for 2 h. In sonicated resting platelets, PrP(C) and P-selectin remained in intact granules after subcellular fractionation. Both glycoproteins were found in the ruptured membranes of activated platelets, suggesting that the PrP(C) was translocated from internal granules to the plasma membrane during activation, as is P-selectin. Platelet PrP(C) was not removed from the surface of platelets by phosphatidylinositol-specific phospholipase C (PIPLC) treatment but was degraded by proteinase K. Platelets may serve as a useful model for following the cellular processing of PrP(C).

Chemically modified hemoglobins are potential oxygen-carrying blood substitutes, but their in vivo administration has been associated with a variety of unexpected side events, including increased platelet reactivity. We studied the effects of hemoglobin A0 (HbA0) and α-crosslinked hemoglobin (α-DBBF) on platelets in vitro. Neither hemoglobin A0 nor α-DBBF activated platelets when added alone, but both proteins potentiated submaximal agonist-induced platelet aggregation without increasing other markers of platelet activation such as serotonin secretion. Only agonists that are known to cause release of platelet arachidonic acid (AA) were potentiated while aggregation induced by ADP, which does not release AA, was not potentiated. Blockade of the thromboxane receptor with SQ-29,548 prevented the HbA0-induced and the α-DBBF-induced potentiation suggesting that the AA/thromboxane signaling pathway mediates the interaction of platelets with hemoglobin.

ACTERIAL CONTAMINATION of blood products is a recurring problem that can pose a significant risk to the transfusion recipient. With the advent of the closed system for platelet apheresis collections, the cause for concern has decreased, 1 but it has not been completely alleviated. For platelet products, bacterial contamination is especially problematic because they are stored in plasma at room temperature where bacterial pathogens can rapidly multiply to produce a significant bioburden. This was highlighted in 1986 when a group of cases of platelet transfusion-associated bacteremia was reported, and the storage period, which previously had been increased to 7 days, was reduced to 5 days to limit the time for bacterial growth in inadvertently contaminated units. 2 This has not fully eliminated the problem, and in 1990, the Food and Drug Administration received reports of six deaths as a result of bacterial sepsis; five were caused by contaminated platelets and one was caused by contaminated red blood cells) In addition, there may be the occurrence of nonfatal sepsis in as many as 1 in 1,700 transfusions of pooled platelet concentrates, 1 making platelet transfusionassociated bacteremia the most common transfusion-associated infection challenge today. 4 A simple way to decrease the growth of bacteria in stored platelets would be to store them at 4~ which retards the growth of most strains of bacteria. Although these conditions were easily adapted for storage of red blood cells, platelets were found to be significantly and irreversibly affected by exposure to cold temperatures. The coldqnduced changes negatively impact on in vivo platelet recovery and survival. It was for this reason that cold storage of platelets was abandoned. Recent

Peroxynitrite (ONOO-) induces nitration of tyrosine residues and inhibits tyrosine phosphorylation in cell free systems. We investigated the effect of peroxynitrite on protein tyrosine nitration and phosphorylation in resting or thrombin-activated platelets. Peroxynitrite (150 microM) rapidly induced tyrosine nitration of 187, 164, 113, 89, and 61 kDa proteins in gel-filtered platelets which persisted up to 4.5 h. Repeated exposure of platelets to peroxynitrite produced increasing levels of nitration. Peroxynitrite also rapidly increased tyrosine phosphorylation of 120, 117, 95, 80-85, and 70 kDa platelet proteins, but this decreased by 5 min. The same pattern of tyrosine phosphorylation, but with higher intensity, was induced by thrombin in control platelets. Pretreatment of platelets with peroxynitrite decreased thrombin-induced tyrosine phosphorylation at 0.05 and 1 U/ml thrombin but not at 2 U/ml thrombin. Platelet activation responses such as P-selectin expression, serotonin secretion, and aggregation were also decreased by peroxynitrite treatment at low thrombin concentrations. Peroxynitrite exposure and tyrosine nitration decreased platelet sensitivity to thrombin but did not absolutely prevent tyrosine phosphorylation and other platelet responses.

Ligand-induced binding sites (LIBS) are neoantigenic regions of glycoprotein (GP)IIb-IIIa that are exposed upon interaction of the receptor with the ligand fibrinogen or the ligand recognition sequence (RGDS). LIBS have been suggested to contribute to postreceptor occupancy events such as full-scale platelet aggregation, adhesion to collagen, and clot retraction. This study examined the induction requirements of a GPIIIa LIBS with regard to ligand specificity. Through the use of the anti-LIBS D3, we report that this complex- activating antibody induces fibrinogen-and von Willebrand factor-binding to GPIIb-IIIa on intact platelets. Bound ligand was detected by flow cytometric analysis and platelet aggregation assays. These bound ligands increased the number of D3-binding sites and altered the affinity of D3 for GPIIb-IIIa on platelets. In contrast, activation of platelet GPIIb-IIIa by D3 did not increase the binding of another RGD- containing ligand, vitronectin. Furthermore, bound vitronectin on thrombin-stimulated platelets did not cause the expression of the D3 LIBS epitope. We conclude direct activation of GPIIb-IIIa in the absence of platelet activation results in selective ligand interaction and that D3 LIBS induction requires the binding of the multivalent ligands, fibrinogen or von Willebrand factor. Thus, the region of GPIIIa recognized by D3 may be an important regulatory domain in ligand- receptor interactions that directly mediate platelet aggregation.

Scientific advances have provided new and unprecedented opportunities for the therapeutic use of stem, progenitor, and differentiated cells for the future treatment of heart, lung, blood, and sleep disorders. Stem and progenitor cells have the potential to replace cells that are damaged or diseased, restore vital functions, and offer the promise of curing disease and ending disabilities. The potential for safe new treatments can only be realized if preclinical and clinical research programs provide the scientific and clinical basis to establish new therapies for regenerative medicine. NHLBI seeks to catalyze translational efforts in this area by supporting key efforts needed for the field’s development. This chapter discusses NHLBI support for cellular therapies and illustrates this support with descriptions of two key programs, one for research centers and the other providing key resources.

Comparative effectiveness research (CER) is the study of existing treatments or ways to deliver health care to determine what intervention works best under specific circumstances. CER evaluates evidence from existing studies or generates new evidence, in different populations and under specific conditions in which the treatments are actually used. CER does not embrace one research design over another but compares treatments and variations in practice using methods that are most likely to yield widely generalizable results that are directly relevant to clinical practice. Treatments used in transfusion medicine (TM) are among the most widely used in clinical practice, but are among the least well studied. High-quality evidence is lacking for most transfusion practices, with research efforts hampered by regulatory restrictions and ethical barriers. To begin addressing these issues, the National Heart, Lung, and Blood Institute convened a workshop in June 2011 to address the potential role of CER in the generation of high-quality evidence for TM decision making. Workshop goals were to: 1) evaluate the current landscape of clinical research, 2) review the potential application of CER methods to clinical research, 3) assess potential barriers to the use of CER methodology, 4) determine whether pilot or vanguard studies can be used to facilitate planning of future CER research, and 5) consider the need for and delivery of training in CER methods for researchers.

Many significant events have occurred in the recent past that beg a broad audience to address the question "What is pediatric transfusion medicine?" Herein, we list some of these events and their relevance below and attempt to provide an answer for this question. Indeed, several issues regarding the subspecialty of pediatric transfusion medicine (PTM) are particularly timely, and it appears that a critical mass, or a nidus capable of becoming a critical mass, is developing in PTM.

In this study, we compared the effect of signal transduction inhibitors on fibrinogen binding, aggregation, the activation state of GPIIb-IIIa, and cytosolic calcium levels in cold and room temperature-stored platelets. Cold-stored platelets have a higher sensitivity to agonist-induced aggregation when compared to room temperature-stored platelets. We also found that cold-stored platelets had a significantly higher aggregation response to ADP and epinephrine, while platelets stored at room temperature responded poorly to these agonists (mean values of 61 vs. 18%, n = 14). Four inhibitors were selected to target various signaling pathways. Cold-stored platelets were more resistant to disaggregation by promethazine, prostaglandin D2, yohimbine, and echistatin. The effects of cold temperatures on stored platelets are targeted to activation pathways as there was no spontaneous aggregation or spontaneous fibrinogen binding as measured in this study. PAC-1 binding was not inhibited to the same degree as aggregation or fibrinogen binding responses, suggesting that the disaggregation was not caused by a change in the conformation of GPIIb-IIIa. Cytosolic calcium levels did not decrease in cold-stored platelets after inhibitor addition. The inhibitors are likely acting after the establishment of the GPIIb-IIIa activation state and may affect the post-occupancy signaling by the fibrinogen-occupied integrin. Differences between aggregation and disaggregation responses of cold- and room temperature-stored platelets suggest that cold-stored platelets may have different mechanisms to stabilize platelet aggregates during their formation.

Prostaglandin E 1 (PGE 1 ) inhibits tyrosine phosphorylation induced by low thrombin concentration (0.05 U/ml), but this is overcome by a high thrombin (2.0 U/ml) concentration. Thromboxane A 2 and ADP are endogenous platelet agonists released during platelet activation which potentiate platelet responses. We investigated how these endogenous agonists influenced the effects of PGE 1 on thrombin (2.0 U/ml)-induced tyrosine phosphorylation by removing released ADP with apyrase (2.0 U/ml) and by inhibiting thromboxane A 2 synthesis with indomethacin (1 μM). Adding PGE 1 (1 μM) before thrombin in apyrase/indomethacin(A/I)-treated platelets selectively prevented thrombin-induced tyrosine phosphorylation of a 117 kDa protein while other substrates were not affected. This selective effect was evident only in the presence of apyrase and was not dependent on indomethacin. Addition of PGE 1 to A/I-treated platelets after thrombin also caused selective tyrosine dephosphorylation of the 117 kDa protein. Conditions which prevented thrombin-induced 117 kDa protein tyrosine phosphorylation also decreased fibrinogen binding to platelets. The 117 kDa protein was identified as the focal adhesion kinase (FAK) by immunoprecipitation with a monoclonal antibody to FAK and by absence of its tyrosine phosphorylation in the presence of RGDS peptide which inhibits fibrinogen binding and platelet aggregation. Thus, released endogenous ADP selectively prevents PGE 1 -mediated tyrosine dephosphorylation of platelet FAK most likely by stabilizing fibrinogen binding to platelets.

ABSTRACT Platelet glycoprotein IIb-IIIa (GPIIb-IIIa, α IIb β 3 ) is expressed on the cell surface of the human erythroleukemia (HEL) cell line. Previous studies have demonstrated differences in GPIIb-IIIa ligand binding properties of HEL cells when compared to platelets. Although the mRNA sequences for GPIIb and GPIIIa are identical in platelets and HEL cells, cell specific differences in the conformation states of the GPIIb-IIIa complex may exist and may explain in part the contrasting functional properties. Two monoclonal antibodies (mAbs), an anti-GPIIb mAb C3 and an anti-GPIIIa mAb D3, were used to determine whether differences in GPIIb-IIIa conformational states could be measured. Initial studies in a purified system showed that the mAbs' binding to isolated GPIIb-IIIa conformers was increased to the active GPIIb-IIIa and to dissociated receptor subunits when compared to the inactive form. Furthermore, soluble active GPIIb-IIIa was a much better inhibitor of D3 binding to the immobilized receptor compared to soluble inactive GPIIb-IIIa. Extending these studies with intact cells, we detected at least two classes of binding sites for each mAb on each cell type. Differences in B max and in the relative affinities of the mAbs were identified and may represent subpopulations of GPIIb-IIIa conformations. Total HEL cell and platelet GPIIb-IIIa was determined in our binding assays using a radiolabeled GPIIb-IIIa complex specific mAb, 10E5. HEL cells express approximately five times more GPIIb-IIIa on a per cell basis. The percent of total GPIIb-IIIa that represented each class of mAb binding sites was determined. In summary, the relative differences in GPIIb-IIIa conformation found on platelets and HEL cells may be related to cell-specific ligand binding properties and activation states of the receptor.