Assessment of neonatal platelet adhesion, activation, and aggregation

Platelet function in neonates and children

Platelets are major regulators of haemostasis and coagulation. The primary role of platelets in coagulation is to form a stable clot and stop bleeding. Studies of platelet phenotype and function in neonates and children have been restricted by the large volumes required for many common platelet function tests such as platelet aggregometry. Developmental changes in platelets have not been as well described as developmental changes in plasma coagulation proteins, and overall, platelet phenotype and function in neonates and children has been understudied when compared to adults. Recent developments in more sensitive platelet function testing methods requiring smaller blood volumes such as flow cytometry has enabled recent studies to further investigate platelet phenotype and function in neonates and children. In this review we will provide an overview of recent advances from the past five years in platelets in the context of developmental haemostasis, as well as the role of platelets in neonatal paediatric disease.

Transfusion of Adult, but Not Neonatal, Platelets Promotes Monocyte Trafficking in Neonatal Mice

BACKGROUND

Thrombocytopenia is common in preterm neonates. Platelet transfusions are sometimes given to thrombocytopenic neonates with the hope of reducing the bleeding risk, however, there are little clinical data to support this practice, and platelet transfusions may increase the bleeding risk or lead to adverse complications. Our group previously reported that fetal platelets expressed lower levels of immune-related mRNA compared with adult platelets. In this study, we focused on the effects of adult versus neonatal platelets on monocyte immune functions that may have an impact on neonatal immune function and transfusion complications.

METHODS

Using RNA sequencing of postnatal day 7 and adult platelets, we determined age-dependent platelet gene expression. Platelets and naive bone marrow-isolated monocytes were cocultured and monocyte phenotypes determined by RNA sequencing and flow cytometry. An in vivo model of platelet transfusion in neonatal thrombocytopenic mice was used in which platelet-deficient TPOR (thrombopoietin receptor) mutant mice were transfused with adult or postnatal day 7 platelets and monocyte phenotypes and trafficking were determined.

RESULTS

Adult and neonatal platelets had differential immune molecule expression, including Selp. Monocytes incubated with adult or neonatal mouse platelets had similar inflammatory (Ly6Chi) but different trafficking phenotypes, as defined by CCR2 and CCR5 mRNA and surface expression. Blocking P-sel (P-selectin) interactions with its PSGL-1 (P-sel glycoprotein ligand-1) receptor on monocytes limited the adult platelet-induced monocyte trafficking phenotype, as well as adult platelet-induced monocyte migration in vitro. Similar results were seen in vivo, when thrombocytopenic neonatal mice were transfused with adult or postnatal day 7 platelets; adult platelets increased monocyte CCR2 and CCR5, as well as monocyte chemokine migration, whereas postnatal day 7 platelets did not.

CONCLUSIONS

These data provide comparative insights into adult and neonatal platelet transfusion-regulated monocyte functions. The transfusion of adult platelets to neonatal mice was associated with an acute inflammatory and trafficking monocyte phenotype that was platelet P-sel dependent and may have an impact on complications associated with neonatal platelet transfusions.

Platelets and ductus arteriosus closure in neonates

Platelet plug formation is critically involved in murine ductus arteriosus closure and thrombocytopenia in preterm infants seems to negatively affect spontaneous and pharmacologically induced ductal closure. Furthermore, platelet dysfunction may contribute to ductal patency, especially in extremely immature infants. Neonatal platelets likely have multifaceted roles during ductal closure, such as secretion of several signaling molecules and facilitation of specific cell-cell interactions. The only available randomized-controlled trial on platelet transfusions in preterm infants with patent ductus arteriosus demonstrated that a liberal transfusion regimen did not promote ductal closure, but was associated with an increased rate of intraventricular hemorrhage. Herein, we discuss the available mechanistic evidence on the role of platelets in ductus arteriosus closure and their potential clinical implications in preterm infants. We further briefly outline future research directions aimed at a better understanding of platelet-endothelial interactions in neonatal health and disease.

Sex-Related Differences in Platelet Aggregation: A Literature Review Supplemented with Local Data from a Group of Generally Healthy Individuals

The process of platelet aggregation is often influenced by several factors including sex and age. A literature review confirmed the existence of sex-related differences in platelet aggregation. Although 68 out of 78 papers found such differences, there are still some controversies regarding these differences, which can be due to multiple factors (age, trigger, concomitant disease, sample handling, etc.). These outcomes are discussed in line with novel results obtained from a local study, in which blood samples from a total of 53 overall healthy women and men with ages ranging from 20 to 66 years were collected. Aggregation was induced with seven different triggers (ristocetin, thrombin receptor activating peptide 6 [TRAP-6], arachidonic acid [AA], platelet-activating factor 16 [PAF-16], ADP, collagen, or thromboxane A₂ analog U-46619) ex vivo. In addition, three FDA-approved antiplatelet drugs (vorapaxar, ticagrelor, or acetylsalicylic acid [ASA]) were also tested. In general, women had higher aggregation responses to some agonists (ADP, TRAP), as well as lower benefit from inhibitors (ASA, vorapaxar). The aggregatory responses to AA and TRAP decreased with age in both sexes, while responses to ADP, U-46619, and PAF were affected by age only in women. In conclusion, more studies are needed to decipher the biological importance of sex-related differences in platelet aggregation in part to enable personalized antiplatelet treatment.

Reduced platelet function in preterm neonates compared with term neonates

Background

A reduced platelet function might contribute to the longer bleeding time seen in preterm neonates. However, the previously used platelet function testing in neonates is limited due to methodological limitations, mainly caused by difficulties in obtaining adequate blood volume. Therefore, the platelet function in preterm neonates is sparsely investigated. The aim of this study was to compare platelet function in preterm neonates at birth and at expected term age with platelet function in term neonates at birth.

Methods

We included 43 preterm neonates born at gestational age (GA) 28 + 0 to 34 + 0 and 21 term neonates born at GA 38 + 0 to 41 + 0. Within the first 24 hours of life, 1-1.5 mL peripheral blood was obtained and for preterm neonates, resampling was performed at expected term age (GA 38 + 0 to 41 + 0). Platelet function testing included impedance aggregometry and platelet activation measured by flow cytometry. In addition, platelet count was determined.

Results

Platelet count and platelet activation were reduced in preterm neonates compared with term neonates at birth, but we found no difference in impedance aggregometry at birth. At expected term age, platelet count and aggregation exceeded term levels, but platelet activation remained impaired in the preterm.

Conclusion

Preterm neonatal function is decreased at birth and does not seem to reach term levels during the first 4 to 13 weeks of life.

Platelets in the neonate: Not just a small adult

Neonates, particularly those born preterm, have a high incidence of thrombocytopenia and bleeding, most commonly in the brain. Because of this, it has historically been accepted that neonates should be transfused at higher platelet counts than older children or adults, to decrease their bleeding risk. However, a number of observational studies and a recent large, randomized trial found a higher incidence of bleeding and mortality in neonates who received more platelet transfusions. The mechanisms underlying the deleterious effects of platelet transfusions in neonates are unknown, but it has been hypothesized that transfusing adult platelets into the very different physiological environment of a neonate may result in a “developmental mismatch” with potential negative consequences. Specifically, neonatal platelets are hyporeactive in response to multiple agonists and upon activation express less surface P‐selectin than adult platelets. However, this hyporeactivity is well balanced by factors in neonatal blood that promote clotting, such as the elevated hematocrit, elevated von Willebrand factor (VWF) levels, and a predominance of ultra‐long VWF polymers, with the net result of normal neonatal primary hemostasis. So far, most studies on the developmental differences between neonatal and adult platelets have focused on their hemostatic functions. However, it is now clear that platelets have important nonhemostatic functions, particularly in angiogenesis, immune responses, and inflammation. Whether equally important developmental differences exist with regard to those nonhemostatic platelet functions and how platelet transfusions perturb those processes in neonates remain unanswered questions.

Neonatal platelet physiology and implications for transfusion

The neonatal hemostatic system is different from that of adults. The differences in levels of procoagulant and anticoagulant factors and the evolving equilibrium in secondary hemostasis during the transition from fetal/neonatal life to infancy, childhood, and adult life are known as "developmental hemostasis." In regard to primary hemostasis, while the number (150,000-450,000/µl) and structure of platelets in healthy neonates closely resemble those of adults, there are significant functional differences between neonatal and adult platelets. Specifically, platelets derived from both cord blood and neonatal peripheral blood are less reactive than adult platelets to agonists, such as adenosine diphosphate (ADP), epinephrine, collagen, thrombin, and thromboxane (TXA₂) analogs. This platelet hyporeactivity is due to differences in expression levels of key surface receptors and/or in signaling pathways, and is more pronounced in preterm neonates. Despite these differences in platelet function, bleeding times and PFA-100 closure times (an in vitro test of whole-blood primary hemostasis) are shorter in healthy full-term infants than in adults, reflecting enhanced primary hemostasis. This paradoxical finding is explained by the presence of factors in neonatal blood that increase the platelet-vessel wall interaction, such as high von Willebrand factor (vWF) levels, predominance of ultralong vWF multimers, high hematocrit, and high red cell mean corpuscular volume. Thus, the hyporeactivity of neonatal platelets should not be viewed as a developmental deficiency, but rather as an integral part of a developmentally unique, but well balanced, primary hemostatic system. In clinical practice, due to the high incidence of bleeding (especially intraventricular hemorrhage, IVH) among preterm infants, neonatologists frequently transfuse platelets to non-bleeding neonates when platelet counts fall below an arbitrary limit, typically higher than that used in older children and adults. However, recent studies have shown that prophylactic platelet transfusions not only fail to decrease bleeding in preterm neonates, but are associated with increased neonatal morbidity and mortality. In this review, we will describe the developmental differences in platelet function and primary hemostasis between neonates and adults, and will analyze the implications of these differences to platelet transfusion decisions.

CAMT-MPL: congenital amegakaryocytic thrombocytopenia caused by MPL mutations - heterogeneity of a monogenic disorder - a comprehensive analysis of 56 patients

Congenital amegakaryocytic thrombocytopenia caused by deleterious homozygous or compound heterozygous mutations in MPL (CAMT-MPL) is a rare inherited bone marrow failure syndrome presenting as an isolated thrombocytopenia at birth progressing to pancytopenia due to exhaustion of hematopoietic progenitors. The analysis of samples and clinical data from a large cohort of 56 patients with CAMT-MPL resulted in a detailed description of the clinical picture and reliable genotype-phenotype correlations for this rare disease. We extended the spectrum of CAMT causing MPL mutations regarding number (17 novel mutations) and impact. Clinical courses showed great variability with respect to the severity of thrombocytopenia, the development of pancytopenia and the consequences from bleedings. The most severe clinical problems were (i) intracranial bleedings pre- and perinatally and the resulting long-term consequences, and (ii) the development of aplastic anemia in the later course of the disease. An important and new finding was that thrombocytopenia was not detected at birth in a quarter of the patients. The rate of non-hematological abnormalities in CAMT-MPL was higher than described so far. Most of the anomalies were related to the head region (brain anomalies, ocular and orbital anomalies) and consequences of intracranial bleedings. The present study demonstrates a higher variability of clinical courses than described so far and has important implications on diagnosis and therapy. The diagnosis CAMT-MPL has to be considered even for those patients who are inconspicuous in the first months of life or show somatic anomalies typical for other inherited bone marrow failure syndromes.

Impact of Reticulated Platelets on Platelet Reactivity in Neonates

Neonatal megakaryopoiesis and platelet turnover form a developmentally unique pattern by generating a pool of newly released reticulated platelets from the bone marrow into the circulation. Reticulated platelets are more reactive and hyperaggregable compared to mature platelets, due to their high residual mRNA content, large size, increased expression of platelet surface receptors, and degranulation. The proportion of reticulated platelets in neonates is higher compared to that in adults. Due to the emergence of an uninhibited platelet subpopulation, the newly formed reticulated platelet pool is inherently hyporesponsive to antiplatelets. An elevated population of reticulated platelets is often associated with increased platelet reactivity and is inversely related to high on-treatment platelet reactivity, which can contribute to ischemia. Measurements of the reticulated platelet subpopulation could be a useful indicator of increased tendency for platelet aggregation. Future research is anticipated to define the distinct functional properties of newly formed reticulated or immature platelets in neonates, as well as determine the impact of enhanced platelet turnover and high residual platelet reactivity on the response to antiplatelet agents.

Congenital amegakaryocytic thrombocytopenia - Not a single disease

Congenital amegakaryocytic thrombocytopenia (CAMT) is a rare inherited bone marrow failure syndrome (IBMFS) that is characterized by severe thrombocytopenia at birth due to ineffective megakaryopoiesis and development towards aplastic anemia during the first years of life. CAMT is not a single monogenetic disorder; rather, many descriptions of CAMT include different entities with different etiologies. CAMT in a narrow sense, which is primarily restricted to the hematopoietic system, is caused mainly by mutations in the gene for the thrombopoietin receptor (MPL), sometimes in the gene for its ligand (THPO). CAMT in association with radio-ulnar synostosis, which is not always clinically apparent, is mostly caused by mutations in MECOM, rarely in HOXA11. Patients affected by other IBMFS - especially Fanconi anemia or dyskeratosis congenita - may be misdiagnosed as having CAMT when they lack typical disease features of these syndromes or have only mild symptoms. This article reviews scientific and clinical aspects of the various disorders associated with the term "CAMT" with a main focus on the disease caused by mutations in the MPL gene.

Age-Dependent Control of Collagen-Dependent Platelet Responses by Thrombospondin-1-Comparative Analysis of Platelets from Neonates, Children, Adolescents, and Adults

Platelet function is developmentally regulated. Healthy neonates do not spontaneously bleed, but their platelets are hypo-reactive to several agonists. The mechanisms underlying immature platelet function in neonates are incompletely understood. This critical issue remains challenging for the establishment of age-specific reference ranges. In this study, we evaluated platelet reactivity of five pediatric age categories, ranging from healthy full-term neonates up to adolescents (11–18 years) in comparison to healthy adults (>18 years) by flow cytometry. We confirmed that platelet hypo-reactivity detected by fibrinogen binding, P-selectin, and CD63 surface expression was most pronounced in neonates compared to other pediatric age groups. However, maturation of platelet responsiveness varied with age, agonist, and activation marker. In contrast to TRAP and ADP, collagen-induced platelet activation was nearly absent in neonates. Granule secretion markedly remained impaired at least up to 10 years of age compared to adults. We show for the first time that neonatal platelets are deficient in thrombospondin-1, and exogenous platelet-derived thrombospondin-1 allows platelet responsiveness to collagen. Platelets from all pediatric age groups normally responded to the C-terminal thrombospondin-1 peptide RFYVVMWK. Thus, thrombospondin-1 deficiency of neonatal platelets might contribute to the relatively impaired response to collagen, and platelet-derived thrombospondin-1 may control distinct collagen-induced platelet responses.

Role of platelets in neonatal necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is an inflammatory bowel necrosis of premature infants and is a leading cause of morbidity and mortality in infants born between 23 and 28 weeks of gestation. Fifty to 95% of all infants with NEC develop thrombocytopenia (platelet counts <150 × 109/L) within 24-72 h of receiving this diagnosis. In many patients, thrombocytopenia is severe and is treated with one or more platelet transfusions. However, the underlying mechanism(s) and biological implications of NEC-related thrombocytopenia remain unclear. This review presents current evidence from human and animal studies on the clinical features and mechanisms of platelet depletion in NEC. Anecdotal clinical experience is combined with evidence from laboratory studies and from an extensive literature search in databases PubMed, EMBASE, and Scopus and the electronic archives of abstracts presented at the annual meetings of the Pediatric Academic Societies. To avoid bias in identification of existing studies, key words were short-listed prior to the actual search both from anecdotal experience and from PubMed's Medical Subject Heading (MeSH) thesaurus. IMPACT: Fifty to 95% of infants with necrotizing enterocolitis (NEC) develop idiopathic thrombocytopenia (platelet counts <150 × 109/L) within 24-72 h of disease onset. Early clinical trials suggest that moderate thrombocytopenia may be protective in human NEC, although further work is needed to fully understand this relationship. We have developed a neonatal murine model of NEC-related thrombocytopenia, where enteral administration of an immunological stimulant, trinitrobenzene sulfonate, on postnatal day 10 induces an acute necrotizing ileocolitis resembling human NEC. In this murine model, thrombocytopenia is seen at 15-18 h due to platelet consumption and mild-moderate thrombocytopenia is protective.

Qualitative and Quantitative Comparison of Plasma Exosomes from Neonates and Adults

Exosomes are extracellular vesicles that contain nucleic acids, lipids and metabolites, and play a critical role in health and disease as mediators of intercellular communication. The majority of extracellular vesicles in the blood are platelet-derived. Compared to adults, neonatal platelets are hyporeactive and show impaired granule release, associated with defects in Soluble N-ethylmaleimide-sensitive fusion Attachment protein REceptor (SNARE) proteins. Since these proteins participate in biogenesis of exosomes, we investigated the potential differences between newborn and adult plasma-derived exosomes. Plasma-derived exosomes were isolated by ultracentrifugation of umbilical cord blood from full-term neonates or peripheral blood from adults. Exosome characterization included size determination by transmission electron microscopy and quantitative proteomic analysis. Plasma-derived exosomes from neonates were significantly smaller and contained 65% less protein than those from adults. Remarkably, 131 proteins were found to be differentially expressed, 83 overexpressed and 48 underexpressed in neonatal (vs. adult) exosomes. Whereas the upregulated proteins in plasma exosomes from neonates are associated with platelet activation, coagulation and granule secretion, most of the underexpressed proteins are immunoglobulins. This is the first study showing that exosome size and content change with age. Our findings may contribute to elucidating the potential “developmental hemostatic mismatch risk” associated with transfusions containing plasma exosomes from adults.

Shear-Induced Platelet Activation is Sensitive to Age and Calcium Availability: A Comparison of Adult and Cord Blood

INTRODUCTION

Antiplatelet therapy for neonates and infants is often extrapolated from the adult experience, based on limited observation of agonist-induced neonatal platelet hypoactivity and poor understanding of flow shear-mediated platelet activation. Therefore, thrombotic events due to device-associated disturbed flow are inadequately mitigated in critically ill neonates with indwelling umbilical catheters and infants receiving cardiovascular implants.

METHODS

Whole blood (WB), platelet-rich plasma (PRP), and gel-filtered platelets (GFP) were prepared from umbilical cord and adult blood, and exposed to biochemical agonists or pathological shear stress of 70 dyne/cm2. We evaluated α-granule release, phosphatidylserine (PS) scrambling, and procoagulant response using P-selectin expression, Annexin V binding, and thrombin generation (PAS), respectively. Activation modulation due to depletion of intracellular and extracellular calcium, requisite second messengers, was also examined.

RESULTS

Similar P-selectin expression was observed for sheared adult and cord platelets, with concordant inhibition due to intracellular and extracellular calcium depletion. Sheared cord platelet Annexin V binding and PAS activity was similar to adult values in GFP, but lower in PRP and WB. Annexin V on sheared cord platelets was calcium-independent, with PAS slightly reduced by intracellular calcium depletion.

CONCLUSIONS

Increased PS activity on purified sheared cord platelets suggest that their intrinsic function under pathological flow conditions is suppressed by cell-cell or plasmatic components. Although secretory functions of adult and cord platelets retain comparable calcium-dependence, PS exposure in sheared cord platelets is uniquely calcium-independent and distinct from adults. Identification of calcium-regulated developmental disparities in shear-mediated platelet function may provide novel targets for age-specific antiplatelet therapy.

Ontogeny of platelet function

Although the hemostatic potential of adult platelets has been investigated extensively, regulation of platelet function during fetal life is less clear. Recent studies have provided increasing evidence for a developmental control of platelet function during fetal ontogeny. Fetal platelets feature distinct differences in reactive properties compared with adults. These differences very likely reflect a modified hemostatic and homeostatic environment in which platelet hyporeactivity contributes to prevent pathological clot formation on the one hand but still ensures sufficient hemostasis on the other hand. In this review, recent findings on the ontogeny of platelet function and reactivity are summarized, and implications for clinical practice are critically discussed. This includes current platelet-transfusion practice and its potential risk in premature infants and neonates.

Platelet activation in experimental murine neonatal pulmonary hypertension

Serotonin (5-HT) contributes to the pathogenesis of experimental neonatal pulmonary hypertension (PH) associated with bronchopulmonary dysplasia (BPD). Platelets are the primary source of circulating 5-HT and is released upon platelet activation. Platelet transfusions are associated with neonatal mortality and increased rates of BPD. As BPD is often complicated by PH, we tested the hypothesis that circulating platelets are activated and also increased in the lungs of neonatal mice with bleomycin-induced PH associated with BPD. Newborn wild-type mice received intraperitoneal bleomycin (3 units/kg) three times weekly for 3 weeks. Platelets from mice with experimental PH exhibited increased adhesion to collagen under flow (at 300 s-1 and 1,500 s-1 ) and increased expression of the αIIbβ3 integrin and phosphatidylserine, markers of platelet activation. Platelet-derived factors 5-HT and platelet factor 4 were increased in plasma from mice with experimental PH. Pharmacologic blockade of the 5-HT 2A receptor (5-HT 2A R) prevents bleomycin-induced PH and pulmonary vascular remodeling. Here, platelets from mice with bleomycin-induced PH demonstrate increased 5-HT 2A R expression providing further evidence of both platelet activation and increased 5-HT signaling in this model. In addition, bleomycin treatment increased lung platelet accumulation. In summary, platelets are activated, granule factors are released, and are increased in numbers in the lungs of mice with experimental neonatal PH. These results suggest platelet activation and release of platelet-derived factors may increase vascular tone, promote aberrant angiogenesis, and contribute to the development of neonatal PH.

Developmental Differences in Platelet Inhibition Response to Prostaglandin E1

BACKGROUND

The mechanisms underlying neonatal platelets hyporesponsiveness are not fully understood. While previous studies have demonstrated developmental impairment of agonist-induced platelet activation, differences in inhibitory signaling pathways have been scarcely investigated.

OBJECTIVE

To compare neonatal and adult platelets with regard to inhibition of platelet reactivity by prostaglandin E1 (PGE1).

METHODS

Platelet-rich plasma from umbilical cord (CB) or adult blood was incubated with PGE1 (0-1 μM). We assessed aggregation in response to adenosine diphosphate (ADP), collagen, and thrombin receptor activating peptide as well as cyclic adenosine 3'5'-monophosphate (cAMP) levels (ELISA). Gαs, Gαi2, and total- and phospho-protein kinase A (PKA) were evaluated in adult and CB ultrapure and washed platelets, respectively, by immunoblotting.

RESULTS

Neonatal (vs. adult) platelets display hypersensitivity to inhibition by PGE1 of platelet aggregation induced by ADP and collagen (PGE1 IC50: 14 and 117 nM for ADP and collagen, respectively, vs. 149 and 491 nM in adults). They also show increased basal and PGE1-induced cAMP levels. Mechanistically, PGE1 acts by binding to the prostanoid receptor IP (prostacyclin receptor), which couples to the Gαs protein-adenylate cyclase axis and increases intracellular levels of cAMP. cAMP activates PKA, which phosphorylates different target inhibitor proteins. Neonatal platelets showed higher basal and PGE1-induced cAMP levels, higher Gαs protein expression, and a trend to increased PKA-dependent protein phosphorylation compared to adult platelets.

CONCLUSION

Neonatal platelets have a functionally increased PGE1-cAMP-PKA axis. This finding supports a downregulation of inhibitory when going from neonate to adult contributing to neonatal platelet hyporesponsiveness.

Antimicrobial blue light for decontamination of platelets during storage

Platelet (PLT) storage is currently limited to 5 days in clinics in the United States, in part, due to an increasing risk for microbial contamination over time. In light of well-documented antimicrobial activity of blue light (405-470 nm), we investigated potentials to decontaminate microbes during PLT storage by antimicrobial blue light (aBL). We found that PLTs produced no detectable levels of porphyrins or their derivatives, the chromophores that specifically absorb blue light, in marked contrast to microbes that generated porphyrins abundantly. The difference formed a basis with which aBL selectively inactivated contaminated microbes prior to and during the storage, without incurring any harm to PLTs. In accordance with this, when contamination with representative microbes was simulated in PLT concentrates supplemented with 65% of PLT additive solution in a standard storage bag, all "contaminated" microbes tested were completely inactivated after exposure of the bag to 405 nm aBL at 75 J/cm2 only once. While killing microbes efficiently, this dose of aBL irradiation exerted no adverse effects on the viability, activation or aggregation of PLTs ex vivo and could be used repeatedly during PLT storage. PLT survival in vivo was also unaltered by aBL irradiation after infusion of aBL-irradiated mouse PLTs into mice. The study provides proof-of-concept evidence for a potential of aBL to decontaminate PLTs during storage.

The effect of red blood cell transfusion on platelet function in critically ill patients

INTRODUCTION

Red blood cell (RBC) transfusion is associated with an increased risk of pro-thrombotic events, but the underlying mechanism is poorly understood. We hypothesized that RBC transfusion modulates platelet activity in critically ill patients with and without sepsis.

METHODS

In a prospective cohort study, 37 critically ill patients receiving a single RBC unit to correct for anemia were sampled prior to and 1 h after transfusion. Platelet exposure of P-selectin, CD63 and binding of PAC-1 as well as formation of platelet-leukocyte complexes were measured by flow cytometry. The ability of plasma from critically ill patients to induce ex vivo platelet aggregation was assessed by flow cytometry after incubation with platelets from a healthy donor.

RESULTS

RBC transfusion neither triggered the expression of platelet activation markers nor the formation of platelet-leukocyte complexes. Plasma from critically ill patients induced more spontaneous platelet aggregation prior to RBC transfusion compared to healthy controls, which was further augmented following RBC transfusion. Also collagen-induced platelet aggregation was already increased prior to RBC transfusion compared to healthy controls, and this response was unaffected by RBC transfusion. In contrast, ristocetin-induced platelet agglutination was decreased when compared to controls, suggesting impaired vWF-dependent platelet agglutination, even in the presence of high vWF levels. Following RBC transfusion, ristocetin-induced platelet agglutination further decreased. There were no differences between septic and non-septic recipients in all assays.

CONCLUSION

Ex vivo platelet aggregation is disturbed in the critically ill. Transfusion of a RBC unit may further increase the spontaneous platelet aggregatory response.

Megakaryocyte polyploidization: role in platelet production

Mammal megakaryocytes (MK) undergo polyploidization during their differentiation. This process leads to a marked increase in the MK size and of their cytoplasm. Contrary to division by classical mitosis, ploidization allows an economical manner to produce platelets as they arise from the fragmentation of the MK cytoplasm. The platelet production in vivo correlates to the entire MK cytoplasm mass that depends both upon the number of MKs and their size. Polyploidization occurs by several rounds of DNA replication with at the end of each round an aborted mitosis at late phase of cytokinesis. As there is also a defect in karyokinesis, MKs are giant cells with a single polylobulated nucleus with a 2^xN ploidy. However, polyploidization per se does not increase platelet production because it requires a parallel development of MK organelles such as mitochondria, granules and the demarcation membrane system. MK polyploidization is regulated by extrinsic factors, more particularly by thrombopoietin (TPO), which during a platelet stress increases first polyploidization before enhancing the MK number and by transcription factors such as RUNX1, GATA1, and FLI1 that regulate MK differentiation explaining why polyploidization and cytoplasmic maturation are intermingled. MK polyploidization is ontogenically regulated and is markedly altered in malignant myeloid disorders such as acute megakaryoblastic leukemia and myeloproliferative disorders as well as in hereditary thrombocytopenia, more particularly those involving transcription factors or signaling pathways. In addition, MKs arising from progenitors in vitro have a much lower ploidy in vitro than in vivo leading to a low yield of platelet production in vitro. Thus, it is tempting to find approaches to increase MK polyploidization in vitro. However, these approaches require molecules that are able to simultaneously increase MK polyploidization and to induce terminal differentiation. Here, we will focus on the regulation by extrinsic and intrinsic factors of MK polyploidization during development and pathological conditions.

Assessment of neonatal, cord, and adult platelet granule trafficking and secretion

Despite the transient hyporeactivity of neonatal platelets, full-term neonates do not display a bleeding tendency, suggesting potential compensatory mechanisms which allow for balanced and efficient neonatal hemostasis. This study aimed to utilize small-volume, whole blood platelet functional assays to assess the neonatal platelet response downstream of the hemostatic platelet agonists thrombin and adenosine diphosphate (ADP). Thrombin activates platelets via the protease-activated receptors (PARs) 1 and 4, whereas ADP signals via the receptors P2Y1 and P2Y12 as a positive feedback mediator of platelet activation. We observed that neonatal and cord blood-derived platelets exhibited diminished PAR1-mediated granule secretion and integrin activation relative to adult platelets, correlating to reduced PAR1 expression by neonatal platelets. PAR4-mediated granule secretion was blunted in neonatal platelets, correlating to lower PAR4 expression as compared to adult platelets, while PAR4 mediated GPIIb/IIIa activation was similar between neonatal and adult platelets. Under high shear stress, cord blood-derived platelets yielded similar thrombin generation rates but reduced phosphatidylserine expression as compared to adult platelets. Interestingly, we observed enhanced P2Y1/P2Y12-mediated dense granule trafficking in neonatal platelets relative to adults, although P2Y1/P2Y12 expression in neonatal, cord, and adult platelets were similar, suggesting that neonatal platelets may employ an ADP-mediated positive feedback loop as a potential compensatory mechanism for neonatal platelet hyporeactivity.

Two-Color Analysis of Leukocytes Labeled by Modified RBCs and Their Fragments

Red blood cells (RBCs) are attractive tools for surface modification to adhere specifically to molecules, cellular fragments (e.g., microvesicles), or whole cells for potential use in bioanalytical assays or as a delivery vehicle in targeted therapy. Within this study, we have loaded RBCs with fluorochrome-conjugated antibodies (Ab) against CD45 and CD22 leukocyte markers and evaluated the conjugation process by microscopy. We have assessed the potential application of RBCs fragments generated from conjugated RBCs for targeting Cyto-Trol control cells by flow cytometric (FCM) approaches. Based on their scattering and fluorescence characteristics (FITC and PE expression), modified RBCs and their fragments, Cyto-Trol cells, and clusters of both were distinguished by two color FCM analysis. Fragments with anti-human Kallestad Ab as a nonspecific FITC conjugate had less than 20% binding to Cyto-Trol controls compared to CD45-FITC Ab conjugate with nearly 100% binding capacity. Cyto-Trol-microvesicle-clusters were more than 45% positive for either FITC or PE. Anti-CD22-PE modified RBCs fragments were also useful in staining and showing about 19.5% positively stained events in the Cyto-Trol region. The proof-of-concept shows, that specific antibody can be attached to RBCs, and generated fragments can be useful to stain target cells for FCM analysis. © 2018 International Society for Advancement of Cytometry.

A Proposed Role of Surfactant in Platelet Function and Treatment of Pulmonary Hemorrhage in Preterm and Term Infants

BACKGROUND

We evaluated the effect of surfactant on platelet function as a potential contributing mechanism to the pathogenesis of pulmonary hemorrhage (PHEM) in term and preterm infants.

METHODS

Cord blood samples were collected from neonates following delivery. Complete blood count and platelet function were measured using a cone and platelet analyzer (CPA). Increasing surfactant concentrations were added to platelets in vitro, and the adhesion molecule P-selectin and the monoclonal antibody PAC-1 were evaluated following platelet activation by flow cytometry.

RESULTS

Forty-one infants (11 preterm and 30 term) were studied. CPA revealed a significant decrease in the average size of the aggregates and in platelet adhesion when surfactant was added. In term infants, the addition of surfactant to native platelets yielded an increased binding capacity of PAC-1 but did not affect P-selectin expression. In preterm infants, platelet activation with adenosine diphosphate in the presence of a high surfactant concentration (0.5 mg/mL) resulted in increased PAC-1 binding and no change in P-selectin expression.

CONCLUSIONS

The platelets of preterm infants are less active (hyporesponsive) than those of term infants, both in their native state as well as after stimulation with various agonists. Surfactant may play an important role in treating PHEM in preterm infants.

Flow cytometry for pediatric platelets

The ability of platelets to carry out their hemostatic function can be impaired in a wide range of inherited and acquired conditions: trauma, surgery, inflammation, pre-term birth, sepsis, hematological malignancies, solid tumors, chemotherapy, autoimmune disorders, and many others. Evaluation of this impairment is vitally important for research and clinical purposes. This problem is particularly pronounced in pediatric patients, where these conditions occur frequently, while blood volume and the choice of blood collection methods could be limited. Here we describe a simple flow cytometry-based screening method of comprehensive whole blood platelet function testing that was validated for a range of pediatric and adult samples (n = 31) in the hematology hospital setting including but not limited to: classic inherited platelet function disorders (Glanzmann's thrombasthenia; Bernard-Soulier, Wiscott-Aldrich, and Hermasky-Pudlak syndromes, MYH9-dependent thrombocytopenia), healthy and pre-term newborns, acute and chronic immune thrombocytopenia, chronic lympholeukemia, effects of therapy on platelet function, etc. The method output includes levels of forward and side scatter, levels of major adhesion and aggregation glycoproteins Ib and IIb-IIIa, active integrins' level based on PAC-1 binding, major alpha-granule component P-selectin, dense granule function based on mepacrine uptake and release, and procoagulant activity quantified as a percentage of annexin V-positive platelets. This analysis is performed for both resting and dual-agonist-stimulated platelets. Preanalytical and analytical variables are provided and discussed. Parameter distribution within the healthy donor population for adults (n = 72) and children (n = 17) is analyzed.

Effect of Pneumatic Tubing System Transport on Platelet Apheresis Units

Platelet apheresis units are transfused into patients to mitigate or prevent bleeding. In a hospital, platelet apheresis units are transported from the transfusion service to the healthcare teams via two methods: a pneumatic tubing system (PTS) or ambulatory transport. Whether PTS transport affects the activity and utility of platelet apheresis units is unclear. We quantified the gravitational forces and transport time associated with PTS and ambulatory transport within our hospital. Washed platelets and supernatants were prepared from platelet apheresis units prior to transport as well as following ambulatory or PTS transport. For each group, we compared resting and agonist-induced platelet activity and platelet aggregate formation on collagen or von Willebrand factor (VWF) under shear, platelet VWF-receptor expression and VWF multimer levels. Subjection of platelet apheresis units to rapid acceleration/deceleration forces during PTS transport did not pre-activate platelets or their ability to activate in response to platelet agonists as compared to ambulatory transport. Platelets within platelet apheresis units transported via PTS retained their ability to adhere to surfaces of VWF and collagen under shear, although platelet aggregation on collagen and VWF was diminished as compared to ambulatory transport. VWF multimer levels and platelet GPIb receptor expression was unaffected by PTS transport as compared to ambulatory transport. Subjection of platelet apheresis units to PTS transport did not significantly affect the baseline or agonist-induced levels of platelet activation as compared to ambulatory transport. Our case study suggests that PTS transport may not significantly affect the hemostatic potential of platelets within platelet apheresis units.

Significant Hypo-Responsiveness to GPVI and CLEC-2 Agonists in Pre-Term and Full-Term Neonatal Platelets and following Immune Thrombocytopenia

Neonatal platelets are hypo-reactive to the tyrosine kinase-linked receptor agonist collagen. Here, we have investigated whether the hypo-responsiveness is related to altered levels of glycoprotein VI (GPVI) and integrin α2β1, or to defects in downstream signalling events by comparison to platelet activation by C-type lectin-like receptor 2 (CLEC-2). GPVI and CLEC-2 activate a Src- and Syk-dependent signalling pathway upstream of phospholipase C (PLC) γ2. Phosphorylation of a conserved YxxL sequence known as a (hemi) immunotyrosine-based-activation-motif (ITAM) in both receptors is critical for Syk activation. Platelets from human pre-term and full-term neonates display mildly reduced expression of GPVI and CLEC-2, as well as integrin αIIbβ3, accounted for at the transcriptional level. They are also hypo-responsive to the two ITAM receptors, as shown by measurement of integrin αIIbβ3 activation, P-selectin expression and Syk and PLCγ2 phosphorylation. Mouse platelets are also hypo-responsive to GPVI and CLEC-2 from late gestation to 2 weeks of age, as determined by measurement of integrin αIIbβ3 activation. In contrast, the response to G protein-coupled receptor agonists was only mildly reduced and in some cases not altered in neonatal platelets of both species. A reduction in response to GPVI and CLEC-2, but not protease-activated receptor 4 (PAR-4) peptide, was also observed in adult mouse platelets following immune thrombocytopenia, whereas receptor expression was not impaired. Our results demonstrate developmental differences in platelet responsiveness to GPVI and CLEC-2, and also following immune platelet depletion leading to reduced Syk activation. The rapid generation of platelets during development or following platelet depletion is achieved at the expense of signalling by ITAM-coupled receptors.

Pilot study of novel lab methodology and testing of platelet function in adolescent women with heavy menstrual bleeding

BackgroundApproximately 40% of adolescent women experience heavy menstrual bleeding (HMB), and 10-62% of them have an underlying bleeding disorder (BD). Diagnosing a BD remains challenging because of limitations of available clinical platelet function assays. The aim of this study was to characterize platelet function in a population of adolescent women with HMB using small-volume whole-blood assays.MethodsAnticoagulated whole blood was used to assess platelet GPIIbIIIa activation, α-granule secretion, and aggregation in response to multiple agonists. Platelet adhesion on collagen or von Willebrand Factor (VWF) under static and shear flow was also assessed.ResultsFifteen participants with HMB were included in the study, of which eight were diagnosed with a clinically identifiable BD. Platelet activation was blunted in response to calcium ionophore in participants without a BD diagnosis compared with that in all other participants. Impaired GPIIbIIIa activation was observed in response to all GPCR agonists, except adenosine diphosphate (ADP), in participants with qualitative platelet disorders. Our assays detected platelet aggregation in the majority of participants with a BD in response to ADP, collagen-related peptide (CRP), thrombin receptor activator 6 (TRAP-6), or U46619. Platelet adhesion and aggregation on collagen and VWF was decreased for participants with VWD.ConclusionParticipants with and without BD exhibited aberrant platelet function in several assays in response to select agonists.

Distinct characteristics of neonatal platelet reactivity

Platelets undergo a process of developmental maturation, and hence its regulation of vascular integrity and control of hemostasis at various stages of neonatal ages deserves better characterization. Functional assays for platelets require a larger volume of blood than what is feasible to collect in neonates, creating a technical hurdle that has been a challenge to investigate neonatal platelets. For this reason, the current knowledge of neonatal platelet function has been based on studies from cord blood-derived platelets as a surrogate for neonatal peripheral blood. Studies indicate that neonatal platelets are hypofunctional to various agonists, although neonates tend to maintain normal hemostasis. This apparently paradoxical finding may be due to several factors, such as elevated functionally potent von Willebrand factor multimers or hematocrit levels, in the neonatal blood that enhance the platelet and vessel wall interaction, and counteract platelet hyporeactivity. This review describes the functional characteristics of neonatal platelets, differences in platelet reactivity between neonates and adults, and potential biomarkers of platelet activation.

Platelet function testing in pediatric patients

INTRODUCTION

Platelets play a key role in primary hemostasis and are also intricately linked to secondary hemostasis. Investigation of platelet function in children, especially in neonates, is seriously challenged by the volumes required to perform the majority of platelet function tests and due to the lack of standardization of these tests for use in children. Areas covered: The present review summarizes developmental hemostasis with a focus on the differences in platelet adhesion, activation and aggregation, between preterm neonates, full-term neonates, during childhood and in adults. Some of the most widely used platelet function tests are presented, including novel tests requiring only a small blood volume. Expert commentary: Currently available platelet function tests are limited as regards to investigation of neonates due to difficulties in obtaining adequate blood volume, poor standardization, lack of reference intervals for neonates and children, and an incomplete understanding of the functional phenotype of neonatal platelets, especially preterm neonatal platelets.

Dynamics of blood flow and thrombus formation in a multi-bypass microfluidic ladder network

The reaction dynamics of a complex mixture of cells and proteins, such as blood, in branched circulatory networks within the human microvasculature or extravascular therapeutic devices such as extracorporeal oxygenation machine (ECMO) remains ill-defined. In this report we utilize a multi-bypass microfluidics ladder network design with dimensions mimicking venules to study patterns of blood platelet aggregation and fibrin formation under complex shear. Complex blood fluid dynamics within multi-bypass networks under flow were modeled using COMSOL. Red blood cells and platelets were assumed to be non-interacting spherical particles transported by the bulk fluid flow, and convection of the activated coagulation factor II, thrombin, was assumed to be governed by mass transfer. This model served as the basis for predicting formation of local shear rate gradients, stagnation points and recirculation zones as dictated by the bypass geometry. Based on the insights from these models, we were able to predict the patterns of blood clot formation at specific locations in the device. Our experimental data was then used to adjust the model to account for the dynamical presence of thrombus formation in the biorheology of blood flow. The model predictions were then compared to results from experiments using recalcified whole human blood. Microfluidic devices were coated with the extracellular matrix protein, fibrillar collagen, and the initiator of the extrinsic pathway of coagulation, tissue factor. Blood was perfused through the devices at a flow rate of 2 µL/min, translating to physiologically relevant initial shear rates of 300 and 700 s^-1 for main channels and bypasses, respectively. Using fluorescent and light microscopy, we observed distinct flow and thrombus formation patterns near channel intersections at bypass points, within recirculation zones and at stagnation points. Findings from this proof-of-principle ladder network model suggest a specific correlation between microvascular geometry and thrombus formation dynamics under shear. This model holds potential for use as an integrative approach to identify regions susceptible to intravascular thrombus formation within the microvasculature as well as extravascular devices such as ECMO.

Dynamic platelet function on von Willebrand factor is different in preterm neonates and full-term neonates: changes in neonatal platelet function

Essentials It is unclear if platelet function differs between preterm and full-term neonates. Platelet behavior was characterized using a flow-based assay on von Willebrand Factor (VWF). Preterms had increased platelet interaction with VWF and glycoprotein Ibα expression. Platelets from preterm neonates behave differently on VWF compared to full-term neonates.

SUMMARY

Background Very low birth weight (VLBW) preterm neonates have an increased risk of hemorrhage-related morbidity and mortality as compared with their full-term counterparts. It is unclear whether platelet function differs between preterm and full-term neonates. This is partly because of the large volumes of blood required to perform standard platelet function tests, and the difficulty in obtaining such samples in neonates. Objectives This study was designed to characterize platelet behavior in neonates with a physiologic flow-based assay that quantifies platelet function in microliter volumes of blood under arterial shear. Methods Blood from VLBW preterm neonates of ≤ 32 weeks' gestation (n = 15) and full-term neonates (n = 13) was perfused under arterial shear over surface-immobilized von Willebrand factor (VWF). Platelet behavior was recorded by digital-image microscopy and analyzed. Surface expression of platelet glycoprotein (GP) Ibα and GPIIIa of VLBW preterm and full-term neonates was also measured. Results VLBW preterm neonates had increased numbers of platelets interacting with VWF, and increased GPIbα expression on the platelet surface. Despite the increased numbers of VWF interactions as reflected by flow-driven platelet translocation along the protein surface, no significant differences were observed in the numbers of platelets that adhered in a stationary fashion to VWF. Platelets from VLBW preterm neonates and those from full-term neonates behaved differently on VWF. Conclusions These differences in platelet function may contribute to the higher incidence of bleeding observed in VLBW preterm neonatal populations, or may represent a compensatory mechanism to counteract this risk of bleeding.