Toward the Relevance of Platelet Subpopulations for Transfusion Medicine

Gravity sedimentation reveals functionally and morphologically different platelets in human blood

In contrast to red blood cells, platelets float rather than sediment when a column of blood is placed in the gravitational field. By the analogy of erythrocyte sedimentation (ESR), it can be expressed with the platelet antisedimentation rate (PAR), which quantitates the difference in platelet count between the upper and lower halves of the blood column after 1 h of 1 g sedimentation. Venous blood samples from 21 healthy subjects were analyzed for PAR. After a 1-h sedimentation, the upper and lower fractions of blood samples were analyzed for platelet count, mean platelet volume (MPV), immature platelet fraction (IPF), and high-fluorescence IPF (H-IPF). The mechanisms behind platelet flotation were explored by further partitioning of the blood column, time-dependent measurements of platelet count and comparison with ESR. The structure and function of the platelets were assessed by electron microscopy (EM) and atomic force microscopy (AFM), and platelet aggregometry, respectively. Platelet antisedimentation is driven by density differences and facilitated by a size-exclusion mechanism caused by progressive erythrocyte sedimentation. The area under the curve (AUC) of the whole blood adenosine diphosphate (ADP) aggregation curves showed significant differences between the upper and lower samples (p < .005). AUC in the upper samples of 38% of healthy subjects exceeded the top of the normal range (53-122) suggesting that ascending platelets show an intensified ADP-induced aggregability ex vivo. H-IPF was significantly higher in the upper samples (p < .05). EM and AFM revealed that platelets in the upper samples were larger in volume and contained 1.6 times more alpha granules compared to platelets in the lower samples. Our results indicate that antisedimentation is able to differentiate platelet populations based on their structural and functional properties. Therefore, PAR may be a suitable laboratory parameter in various thromboinflammatory disorders.

The Effect of Leukocyte Removal and Matrix Metalloproteinase Inhibition on Platelet Storage Lesions

The reasons for unfavorable changes in platelet concentrate (PC) quality during storage are not fully understood yet. We aimed to evaluate whether leukocytes and matrix metalloproteinases (MMPs) lead to a decrease in the quality of PCs and examine whether MMP inhibition will slow down the platelets' aging. Nine PCs were divided into three parts: (1) leukocyte-depleted (F) PCs, (2) PCs with no additional procedures (NF), and (3) PCs with the addition of an MMP inhibitor-doxycycline (D). Each PC was stored for 144 h, and a sample for testing was separated from each part on the day of preparation and after 24, 48, 72 and 144 h of storage. Blood morphological analysis, platelet aggregation, and the expression of activation markers were evaluated. MMP-2 and MMP-9 concentration, activity, and gene expression were assessed. Platelet aggregation decreased, and platelet activation marker expression increased during the storage. D concentrates showed the lowest level of platelet activation. In turn, leukocyte-depleted PCs showed the highest level of platelet activation in general. MMP-9 platelet activity was higher in leukocyte-containing concentrates at the end of the storage period. We concluded that the filtration process leads to a higher platelet activation level. The presence of doxycycline in PCs reduces the expression of the activation markers as compared to leukocyte-depleted concentrates.

High fragmentation in platelet concentrates impacts the activation, procoagulant, and aggregatory capacity of platelets

Platelets are transfused to patients to prevent bleeding. Since both preparation and storage can impact the hemostatic functions of platelets, we studied platelet concentrates (PCs) with different initial composition in regard to platelet fragmentation and its impact on storage-induced changes in activation potential. Ten whole blood derived PCs were assessed over 7 storage days. Using flow cytometry, platelet (CD41+) subpopulations were characterized for activation potential using activation markers (PAC-1, P-selectin, and LAMP-1), phosphatidylserine (Annexin V), and mitochondrial integrity (DiIC₁(5)). Aggregation response, coagulation, and soluble activation markers (cytokines and sGPVI) were also measured. Of the CD41+ events, the PCs contained a median of 82% normal-sized platelets, 10% small platelets, and 8% fragments. The small platelets exhibited procoagulant hallmarks (increased P-selectin and Annexin V and reduced DiIC₁(5)). Normal-sized platelets responded to activation, whereas activation potential was decreased for small and abolished for fragments. Five PCs contained a high proportion of small platelets and fragments (median of 28% of CD41+ events), which was significantly higher than the other five PCs (median of 9%). A high proportion of small platelets and fragments was associated with procoagulant hallmarks and decreased activation potential, but, although diminished, they still retained some activation potential throughout 7 days storage.

Nrf2 alleviates spaceflight-induced immunosuppression and thrombotic microangiopathy in mice

Spaceflight-related stresses impact health via various body systems, including the haematopoietic and immune systems, with effects ranging from moderate alterations of homoeostasis to serious illness. Oxidative stress appears to be involved in these changes, and the transcription factor Nrf2, which regulates expression of a set of cytoprotective and antioxidative stress response genes, has been implicated in the response to spaceflight-induced stresses. Here, we show through analyses of mice from the MHU-3 project, in which Nrf2-knockout mice travelled in space for 31 days, that mice lacking Nrf2 suffer more seriously from spaceflight-induced immunosuppression than wild-type mice. We discovered that a one-month spaceflight-triggered the expression of tissue inflammatory marker genes in wild-type mice, an effect that was even more pronounced in the absence of Nrf2. Concomitant with induction of inflammatory conditions, the consumption of coagulation-fibrinolytic factors and platelets was elevated by spaceflight and further accelerated by Nrf2 deficiency. These results highlight that Nrf2 mitigates spaceflight-induced inflammation, subsequent immunosuppression, and thrombotic microangiopathy. These observations reveal a new strategy to relieve health problems encountered during spaceflight.

Evaluation of a novel moving threshold gating strategy for assessment of reticulated platelets in dogs using the ADVIA 2120 analyzer

BACKGROUND

A novel method using a moving threshold (r-PLTmt) to determine canine r-PLTs (reticulated platelets) has been introduced for ADVIA 2120 software v6.11.7.

OBJECTIVES

We aimed to evaluate absolute (ar-PLTmt) and percent (%r-PLTmt) prior to and after visual inspection of scattergrams (ar-PLTmtv, %rPLTmtv) compared with flow cytometry (flow) and to determine reference intervals (RIs) in 120 dogs.

METHODS

For method comparison, 42 blood samples of healthy and thrombocytopenic dogs were included. Calculation of Spearman's rho, Bland-Altman, and Passing-Bablok analysis was performed. Coefficients of variation (CVs) were determined for three concentration levels.

RESULTS

Moderate correlations between %r-PLTmt and %r-PLTmtv (r_s 0.75-0.76) were seen compared with flow cytometry. The CV for medium %r-PLTs counts assessed with flow cytometry was 12.9%. Comparable CVs were obtained for ar-PLTmt (14.4%) and %r-PLTmt (15.7%), and ar-PLTmtv and %r-PLTmtv (10.9% and 12.9%, respectively). At low and high concentration levels, CVs for % and absolute r-PLTmt/rPLTmtv ranged between 23%-30% and 15%-20%. In patients with microcytic hypochromic erythrocytes, CVs for ar-PLTmt and %r-PLTmt were 36%-66%. Visual inspection of scattergrams resulted in a marked decrease in CV ranging between 15% and 20%. A proportional bias of 10.8% between %r-PLTmt and flow cytometry became lower (9.7%) after visual validation of scattergrams. Passing-Bablok analysis showed proportional and constant error. RIs for r-PLTmt and r-PLTmtv were 0.2%-3.8% and 0.6-10.2 × 10⁹ /L and 0.3%-4.5% and 1.1-10.3 × 10⁹ /L, respectively. Median values for %r-PLTmtv were higher in young adults (≤2 years) than in older dogs (P = 0.03).

CONCLUSIONS

r-PLTmt and r-PLTmtv were moderately correlated with flow cytometry. Visual inspection of scattergrams is recommended.

Discriminating young platelets on human leukocyte antigen-I expression highlights their extremely high reactivity potential

Background

The platelet population is heterogeneous, with different subsets that differ on the basis of their function and reactivity. An intrinsic factor participating in this difference of reactivity could be the platelet age. The lack of relevant tools allowing a formal identification of young platelets prevents so far to draw solid conclusions regarding platelet reactivity. We recently reported that human leukocyte antigen-I (HLA-I) molecules are more expressed on human young platelets.

Objectives

The aim of this study was to assess platelet reactivity according to their age based on HLA-I expression level.

Methods

Platelet activation was assessed by flow cytometry (FC) for different platelet subsets based on their HLA-I expression. These populations were further cell sorted and their intrinsic properties were determined by FC and electron microscopy (EM). Statistical analyses were performed with GraphPad Prism 5.02 software using two-way ANOVA followed by a Tukey post hoc test.

Results

HLA-I expression level allowed the identification of 3 platelet subpopulations regarding to their age (HLA low, dim, and high). HLA-I was reliable to guide platelet cell sorting and highlighted the features of young platelets in the HLA-I^high population. In response to different soluble agonists, HLA-I^high platelets were the most reactive subset as shown by the level of P-selectin secretion and fibrinogen binding assessed by flow cytometry. Moreover, the highest capacity of HLA-I^high platelets to simultaneously express annexin-V and von Willebrand factor or activated αIIbβ3 after coactivation with TRAP and CRP indicated that the procoagulant feature of platelets was age-related.

Conclusion

The young HLA-I^high population is the most reactive and prone to become procoagulant. These results open up new perspectives to investigate deeply the role of young and old platelets.

Identification of platelet subpopulations in cryopreserved platelet components using multi-colour imaging flow cytometry

Cryopreservation of platelets, at  - 80 °C with 5-6% DMSO, results in externalisation of phosphatidylserine and the formation of extracellular vesicles (EVs), which may mediate their procoagulant function. The phenotypic features of procoagulant platelets overlap with other platelet subpopulations. The aim of this study was to define the phenotype of in vitro generated platelet subpopulations, and subsequently identify the subpopulations present in cryopreserved components. Fresh platelet components (n = 6 in each group) were either unstimulated as a source of resting platelets; or stimulated with thrombin and collagen to generate a mixture of aggregatory and procoagulant platelets; calcium ionophore (A23187) to generate procoagulant platelets; or ABT-737 to generate apoptotic platelets. Platelet components (n = 6) were cryopreserved with DMSO, thawed and resuspended in a unit of thawed plasma. Multi-colour panels of fluorescent antibodies and dyes were used to identify the features of subpopulations by imaging flow cytometry. A combination of annexin-V (AnnV), CD42b, and either PAC1 or CD62P was able to distinguish the four subpopulations. Cryopreserved platelets contained procoagulant platelets (AnnV+/PAC1-/CD42b+/CD62P+) and a novel population (AnnV+/PAC1-/CD42b+/CD62P-) that did not align with the phenotype of aggregatory (AnnV-/PAC1+/CD42b+/CD62P+) or apoptotic (AnnV+/PAC1-/CD42b-/CD62P-) subpopulations. These data suggests that the enhanced haemostatic potential of cryopreserved platelets may be due to the cryo-induced development of procoagulant platelets, and that additional subpopulations may exist.

Donor variation in stored platelets: Higher metabolic rates of platelets are associated with mean platelet volume, activation and donor health

BACKGROUND

Platelets (PLTs) differ in glycolytic activity, resulting in rapid acidification of 'poor' storing PLT concentrates (PCs) in plasma, or depletion of glucose when stored in PLT additive solution (PAS). We aimed to understand why PLT glycolysis rates vary between donors and how this affects storage performance.

STUDY DESIGN AND METHODS

Buffy coats from donors <45, 45-70 and >70 years were selected and single-donor PCs in plasma or PAS-E were prepared. PCs were stored for 8 days at 22 ± 2°C and sampled regularly for analysis. Mitochondrial activity was analyzed with an Oroboros oxygraph. Age groups, or subgroups divided into quartiles based on glucose consumption, were analyzed with ANOVA.

RESULTS

In each comparison, PCs of the different groups were not different in volume and cellular composition. PLTs with the highest glucose consumption had a higher initial mean platelet volume (MPV) and developed higher CD62P expression and Annexin A5 binding during storage. Higher glycolytic activity in these PLTs was not a compensation for lower mitochondrial ATP production, because mitochondrial ATP-linked respiration of fresh PLTs correlated positively with MPV (R²  = 0.71). Donors of high glucose-consuming PLTs had more health-related issues. Storage properties of PCs from donors over 70 were not significantly different compared to PCs from donors younger than 45 years.

CONCLUSIONS

High glucose-consuming PCs developing higher activation levels, not only displayed enhanced mitochondrial activity but were also found to contain larger PLTs, as determined by MPV. Storage performance of PLTs was found to be associated with donor health, but not with donor age.

Variation in Platelet Activation State in Pre-Donation Whole Blood: Effect of Time of Day and ABO Blood Group

Background

Whilst there has been investigation into the effect of time of the day on platelet activation and function in healthy individuals, there is a lack of studies in the literature to examine this relationship among platelet donors.

Methods

We assessed the extent of platelet activation by percentage of platelets with surface-expressed P-selectin and flow cytometry in samples of whole blood from a group of qualified platelet donors (n = 84).

Results

The mean (SD) percentage of activated platelets in the pre-donation blood samples was 1.85 ± 1.57% (range 0.2–7.5%). In univariate analyses, the percentage of activated platelets was significantly and inversely correlated with the collection time (ie, the time of day blood samples were collected) (r = –0.35, p = 0.001) and positively correlated to mean platelet volume (MPV) (r = 0.29, p = 0.008). A weaker positive correlation was also observed with ABO blood group (r = 0.228, p = 0.036). Analysis of the collection time as a categorical variable showed a greater degree of activated platelets in samples collected between 8:00 h and 10:00 h than in samples collected during the hours of >10:00 h ≤14:00 h (2.5 ± 1.8 versus 1.1 ± 0.74, p < 0.001). In the adjusted linear regression model, collection time was a significant independent predictor of platelet activation state in whole blood (β = –0.26; p < 0.001), as did ABO blood group (β = 0.55; p = 0.019).

Conclusion

Our results show that collection time is the most important predictor of platelet activation state in pre-donation whole blood among platelet donors. This work may have implications for optimizing the timing of platelet donation.

An Insight into Recent Advances on Platelet Function in Health and Disease

Platelets play a variety of roles in vascular biology and are best recognized as primary hemostasis and thrombosis mediators. Platelets have a large number of receptors and secretory molecules that are required for platelet functionality. Upon activation, platelets release multiple substances that have the ability to influence both physiological and pathophysiological processes including inflammation, tissue regeneration and repair, cancer progression, and spreading. The involvement of platelets in the progression and seriousness of a variety of disorders other than thrombosis is still being discovered, especially in the areas of inflammation and the immunological response. This review represents an integrated summary of recent advances on the function of platelets in pathophysiology that connects hemostasis, inflammation, and immunological response in health and disease and suggests that antiplatelet treatment might be used for more than only thrombosis.

Emerging Technologies for Understanding Platelet Diversity

This review discusses our understanding of platelet diversity with implications for the roles of platelets in hemostasis and thrombosis and identifies advanced technologies set to provide new insights. We use the term diversity to capture intrasubject platelet variability that can be intrinsic or governed by the environment and lead to a heterogeneous response pattern of aggregation, clot promotion, and external communication. Using choice examples, we discuss how the use of advanced technologies can provide new insights into the underlying causes of platelet molecular, structural, and functional diversity. As sources of diversity, we discuss the proliferating megakaryocytes with different allele-specific expression patterns, the asymmetrical formation of proplatelets, changes in platelets induced by aging and priming, interplatelet heterogeneity in thrombus organization and stability, and platelet-dependent communications. We provide indications how current knowledge gaps can be addressed using promising technologies, such as next-generation sequencing, proteomic approaches, advanced imaging techniques, multicolor flow and mass cytometry, multifunctional microfluidics assays, and organ-on-a-chip platforms. We then argue how this technology base can aid in characterizing platelet populations and in identifying platelet biomarkers relevant for the treatment of cardiovascular disease.

Atypical Roles of the Chemokine Receptor ACKR3/CXCR7 in Platelet Pathophysiology

The manifold actions of the pro-inflammatory and regenerative chemokine CXCL12/SDF-1α are executed through the canonical GProteinCoupledReceptor CXCR4, and the non-canonical ACKR3/CXCR7. Platelets express CXCR4, ACKR3/CXCR7, and are a vital source of CXCL12/SDF-1α themselves. In recent years, a regulatory impact of the CXCL12-CXCR4-CXCR7 axis on platelet biogenesis, i.e., megakaryopoiesis, thrombotic and thrombo-inflammatory actions have been revealed through experimental and clinical studies. Platelet surface expression of ACKR3/CXCR7 is significantly enhanced following myocardial infarction (MI) in acute coronary syndrome (ACS) patients, and is also associated with improved functional recovery and prognosis. The therapeutic implications of ACKR3/CXCR7 in myocardial regeneration and improved recovery following an ischemic episode, are well documented. Cardiomyocytes, cardiac-fibroblasts, endothelial lining of the blood vessels perfusing the heart, besides infiltrating platelets and monocytes, all express ACKR3/CXCR7. This review recapitulates ligand induced differential trafficking of platelet CXCR4-ACKR3/CXCR7 affecting their surface availability, and in regulating thrombo-inflammatory platelet functions and survival through CXCR4 or ACKR3/CXCR7. It emphasizes the pro-thrombotic influence of CXCL12/SDF-1α exerted through CXCR4, as opposed to the anti-thrombotic impact of ACKR3/CXCR7. Offering an innovative translational perspective, this review also discusses the advantages and challenges of utilizing ACKR3/CXCR7 as a potential anti-thrombotic strategy in platelet-associated cardiovascular disorders, particularly in coronary artery disease (CAD) patients post-MI.

Proteome and functional decline as platelets age in the circulation

BACKGROUND

Platelets circulate in the blood of healthy individuals for approximately 7-10 days regulated by finely balanced processes of production and destruction. As platelets are anucleate we reasoned that their protein composition would change as they age and that this change would be linked to alterations in structure and function.

OBJECTIVE

To isolate platelets of different ages from healthy individuals to test the hypothesis that changes in protein content cause alterations in platelet structure and function.

METHODS

Platelets were separated according to thiazole orange fluorescence intensity as a surrogate indicator of mRNA content and so a marker of platelet age and then subjected to proteomics, imaging, and functional assays to produce an in-depth analysis of platelet composition and function.

RESULTS

Total protein content was 45 ± 5% lower in old platelets compared to young platelets. Predictive proteomic pathway analysis identified associations with 28 biological processes, notably higher hemostasis in young platelets whilst apoptosis and senescence were higher in old platelets. Further studies confirmed platelet ageing was linked to a decrease in cytoskeletal protein and associated capability to spread and adhere, a reduction in mitochondria number, and lower calcium dynamics and granule secretion.

CONCLUSIONS

Our findings demonstrate changes in protein content are linked to alterations in function as platelets age. This work delineates physical and functional changes in platelets as they age and serves as a base to examine differences associated with altered mean age of platelet populations in conditions such as immune thrombocytopenia and diabetes.

Assessment of apheresis platelets during 5 days of storage: A National Cancer Institute, Cairo University experience

BACKGROUND

Platelet transfusion therapy is widely used to prevent hemorrhage in patients with thrombocytopenia and platelet disorders. The platelet concentrate (PC) quality is affected by increased storage time, as reflected in the decreased number of platelets, morphological changes, and impaired functions. This study aimed to analyze the impact of 5 days storage on platelets count and the expression of CD63, and Annexin V as activation markers during PC storage.

METHODS

Fifty PCs collected from single donors were tested for platelet count on days 0, 3, and 5 using a Sysmex blood counter. CD61, CD63, and Annexin V expression was analyzed by a multicolor Navios flow cytometer.

RESULTS

There was a significant decrease in platelet count during 5 days of storage. There was a direct relationship between storage time and degree of platelet activation. CD63 had almost double increased expression on day 5 than day 3. Annexin V showed significantly increased expression on day 3 with minor differences between days 3 and 5.

CONCLUSION

According to standard blood bank conditions, PC stored for 5 days showed a degree of in vitro activation as evidenced by CD63 and Annexin V expression, may lead to reduced therapeutic efficacy. Flow cytometry monitoring platelet activation in PC offers a better understanding of the changes during PC storage and may help improve platelet products.

Platelet procoagulant potential is reduced in platelet concentrates ex vivo but appears restored following transfusion

BACKGROUND

The procoagulant profile of platelet concentrates (PCs) following transfusion has been difficult to evaluate due to lack of specific markers. This study aimed to characterize procoagulant platelets in PCs and the effect of transfusion.

STUDY DESIGN AND METHODS

Buffy coat-derived PCs from 12 donors were pooled, split, then stored conventionally, cold (2-6°C) or cryopreserved (-80°C). Procoagulant platelet profiles were assessed by flow cytometry (GSAO⁺ /P-selectin⁺ ), lactadherin-binding, and calibrated automated thrombogram, during storage, unstimulated, or after thrombin and collagen stimulation and compared with blood from healthy volunteers. Platelet activation (P-selectin) and procoagulant platelet formation potential were measured (flow cytometry) in patients receiving clinically indicated conventional PC transfusion.

RESULTS

Independent of significant increases with storage, procoagulant platelet proportions with and without agonist stimulation were significantly blunted in conventionally stored PCs (stimulated day 5 conventional PC 4.2 ± 1.3%, healthy volunteer blood 11.1 ± 2.9%; p < .0001). Cryopreserved PCs contained the highest proportion of procoagulant platelets (unstimulated: cryopreserved 25.6 ± 1.8% vs. day 5 conventional 0.5 ± 0.1% vs. day 14 cold-stored 5.8 ± 1.0%, p < .0001), but demonstrated minimal increase with agonist. Transfusion of PCs was associated with an increase in procoagulant platelets (2.2 ± 1.4% vs. 0.6 ± 0.2%; p = .004) and reversal of the blunted agonist response (15.8 ± 5.9% vs. 4.0 ± 1.6%; p < .0001). Procoagulant responses post-transfusion were significantly higher than healthy controls, suggesting a priming effect. The P-selectin agonist response was not restored upon transfusion (79.4 ± 13.9% vs. 82.0 ± 2.5%).

CONCLUSION

Storage blunts the procoagulant platelet response to agonist stimulation in PCs. Despite this, conventionally stored PCs have high procoagulant potential following transfusion, with a discordant, persistent reduction in P-selectin response.

Platelet adhesion in type 2 diabetes: impact of plasma albumin and mean platelet volume

Background

Altered mean platelet volume (MPV) and plasma albumin has been reported in type 2 diabetes (T2D). MPV is suggested to predict cardiovascular risk but there is a lack of evidence for associations between MPV and platelet adhesion. Plasma albumin and magnesium are other factors reported to influence thrombotic risk. The objectives of this study were to assess the association between platelet adhesion and plasma factors with a potential role to affect platelet activation.

Methods

Blood was collected from 60 T2D patients and 60 healthy controls. Platelet adhesion to different protein surfaces induced by various soluble activators were measured in microplates. MPV, albumin and magnesium were analysed together with additional routine tests.

Results

Despite normal levels, plasma albumin significantly correlated with adhesion of T2D platelets but not with controls. There was a significant association between MPV and platelet adhesion in both groups, but association was smaller in T2D. Levels of glucose, HbA1c or magnesium did not correlate with platelet adhesion.

Conclusions

Plasma albumin was associated with platelet adhesion in T2D suggesting that albumin may be a factor to consider upon cardiovascular risk assessment. MPV was more associated with the level of platelet adhesion in healthy individuals than in well-controlled T2D patients.

The impact of platelet indices on clinical outcome in heart failure: results from the MyoVasc study

Aims

Platelet indices have been associated with traditional cardiovascular risk factors, cardiovascular diseases and all‐cause mortality. This study aimed to investigate the role of platelet count, mean platelet volume (MPV) and platelet‐to‐leukocyte ratio, including platelet‐to‐monocyte and platelet‐to‐lymphocyte ratio with cardiac function, heart failure (HF) phenotypes and clinical outcome, worsening of HF.

Methods and results

Univariate and multivariable linear and Cox regression analyses were used to investigate the associations between platelet indices, cardiac function and worsening of HF in 3250 subjects enrolled in the MyoVasc study. Higher MPV, lower platelet count, lower platelet‐to‐leukocyte and platelet‐to‐monocyte ratios have been associated with reduced left ventricular ejection fraction (beta estimate [β]_{MPV [fL]} = −0.05 [−0.09; −0.02], β_{platelet count (× 10/L)}⁹ = 3.4 [1.2; 5.6], β_{platelet‐to‐leukocyte ratio} = 1.4 [1.1; 1.8], β_{platelet‐to‐monocyte ratio} = 28 [20; 36]) and increased E/E' ratio (β _{MPV [fL]} = 0.04 [0.003; 0.07], β_{platelet count (× 10/L)}⁹ = −3.1 [−5.3; −0.92], β_{platelet‐to‐leukocyte ratio} = −0.83 [−1.2; −0.46], β_{platelet‐to‐monocyte ratio} = −20 [−28; −12]), independent of age and sex. Cox regression demonstrated an increased risk for worsening of HF in subjects with MPV > 75th percentile (hazard ratio [HR] = 1.47 [1.16; 1.87]), platelet count < 25th percentile (HR = 1.36 [1.07; 1.74]), platelet‐to‐leukocyte < 25th percentile (HR = 1.53 [1.20; 1.95]), platelet‐to‐monocyte < 25th percentile (HR = 1.38 [1.08; 1.77]) and platelet‐to‐lymphocyte > 75th percentile (HR = 1.50 [1.17; 1.93]) ratios, independent of potential confounders. MPV > 75th percentile and platelet count < 25th percentile were strongly related to outcome in HFpEF vs. HFrEF (P for difference = 0.040). Platelet‐to‐leukocyte ratios were associated with worse outcome in both HF phenotypes, without a significant difference between HFpEF and HFrEF.

Conclusions

Platelet indices are linked with worse cardiac function and adverse clinical outcome, independent of subjects' underlying cardiovascular profile. This study emphasizes their important value to provide additional information on pathophysiology and risk stratification in HF syndrome.

Immature platelet fraction over time and clinical outcomes in patients with acute myocardial infarction

BACKGROUND

Immature platelets in the circulation can be measured as immature platelet fraction (IPF). Limited data exist regarding IPF during the course of an acute myocardial infarction (AMI), the association between IPF and extent of cardiac damage, and the long-term prognostic implications of IPF in patients with AMI.

AIMS

To examine the temporal course of IPF during the first month after AMI, the association between IPF and extent of cardiac damage, and the long-term prognostic effect of IPF in AMI patients.

METHODS

Patients with AMI treated with percutaneous coronary intervention (PCI) were examined. IPF was evaluated by a Sysmex XN-3000 autoanalyzer, at 4 time points: baseline; one day post-PCI; 3 days post-PCI, and 30 days post-PCI. The association between peak troponin-T levels and IPF was evaluated. One-year clinical outcomes (cardiac hospitalization, urgent revascularization, or death) were assessed.

RESULTS

One hundred patients were included, mean age was 59.5 ± 11.3 years, 82 were men, 27 had diabetes, and 54 were hospitalized with ST-segment elevation myocardial infarction (STEMI) and 46 with non-ST segment elevation myocardial infarction (NSTEMI). The levels of IPF modestly decreased a day after PCI but did not change in subsequent measurements. Peak troponin-T level was significantly associated with the levels of IPF at all 4 time points. IPF levels three days post-PCI were associated with the composite clinical outcome at 1 year.

CONCLUSIONS

The levels of IPF following AMI remain relatively stable over a one-month period. Higher levels of IPF during the acute phase of AMI appear to be associated with worse cardiac outcomes at 1 year.

Platelets as messengers of early-stage cancer

Platelets have an important role in tumor angiogenesis, growth, and metastasis. The reciprocal interaction between cancer and platelets results in changes of several platelet characteristics. It is becoming clear that analysis of these platelet features could offer a new strategy in the search for biomarkers of cancer. Here, we review the human studies in which platelet characteristics (e.g., count, volume, protein, and mRNA content) are investigated in early-stage cancer. The main focus of this paper is to evaluate which platelet features are suitable for the development of a blood test that could detect cancer in its early stages.

Procoagulant platelets: Laboratory detection and clinical significance

Platelets play a critical role in both haemostasis and thrombosis, and it is now evident that not all platelets behave the same when they are called to action. A functionally distinct subpopulation of platelets forms in response to maximal agonist stimulation: the procoagulant platelet. This platelet subpopulation is defined by its ability to expose phosphatidylserine on its surface, allowing for coagulation factor complexes to form and generate bursts of thrombin and fibrin to stabilize platelet clots. Reduced levels of procoagulant platelets have been linked to bleeding in Scott's syndrome and haemophilia A patients, and elevated levels have been demonstrated in many thrombotic disorders, including identifying patients at higher risk for stroke recurrence. One obstacle for incorporating an assay for measuring procoagulant platelets into clinical management algorithms is the lack of consensus on the exact definition and markers for this subpopulation. This review will outline the biological characteristics of procoagulant platelets and the laboratory assays currently used to identify them in research settings. It will summarize the findings of clinical research demonstrating the relevance of measuring the procoagulant platelet levels in patients and will discuss how an appropriate assay can be used to elucidate the mechanism behind the formation of this subpopulation, facilitating novel drug discovery to improve upon current outcomes in cardiovascular and other thrombotic disorders.

The role of viscoelastic testing in the management of the parturient

Viscoelastic testing is the measurement of how the viscoelastic properties of blood change as blood clots. In pregnancy, unique changes in clotting function occur overall to make the pregnant woman hypercoagulable. Viscoelastic testing may be able to play a role in guiding placement of epidural and spinal anesthetics by demonstrating enhanced coagulation function, even when there are individual parts of the coagulation system that are abnormal, for example, thrombocytopenia. These tools may also play a role in addressing the management of maternal hemorrhage, which in the United States is on the rise. This review discusses the opportunity to use viscoelastic testing in the parturient.

Advances in Platelet Subpopulation Research

Although lacking a nucleus, platelets are increasingly recognized not only for their complexity, but also for their diversity. Some 50 years ago platelet subpopulations were characterized by size and density, and these characteristics were thought to reflect platelet aging. Since, our knowledge of platelet heterogeneity has grown to recognize that differences in platelet biochemistry and function exist. This includes the identification of vanguard and follower platelets, platelets with differing procoagulant ability including "COAT-platelets" which enhance procoagulant protein retention on their surface, and most recently, the identification of platelet subpopulations with a differential ability to generate and respond to nitric oxide. Hence, in this mini-review, we summarize the current knowledge of platelet subpopulation diversity focusing on their physical, biochemical, and functional heterogeneity. In addition, we review how platelet subpopulations may change between health and disease and how differences among platelets may influence response to anti-platelet therapy. Finally, we look forward and discuss some of the future directions and challenges for this growing field of platelet research.

Platelets in Sepsis: An Update on Experimental Models and Clinical Data

Beyond their important role in hemostasis, platelets play a crucial role in inflammatory diseases. This becomes apparent during sepsis, where platelet count and activation correlate with disease outcome and survival. Sepsis is caused by a dysregulated host response to infection, leading to organ dysfunction, permanent disabilities, or death. During sepsis, tissue injury results from the concomitant uncontrolled activation of the complement, coagulation, and inflammatory systems as well as platelet dysfunction. The balance between the systemic inflammatory response syndrome (SIRS) and the compensatory anti-inflammatory response (CARS) regulates sepsis outcome. Persistent thrombocytopenia is considered as an independent risk factor of mortality in sepsis, although it is still unclear whether the drop in platelet count is the cause or the consequence of sepsis severity. The role of platelets in sepsis development and progression was addressed in different experimental in vivo models, particularly in mice, that represent various aspects of human sepsis. The immunomodulatory function of platelets depends on the experimental model, time, and type of infection. Understanding the molecular mechanism of platelet regulation in inflammation could bring us one step closer to understand this important aspect of primary hemostasis which drives thrombotic as well as bleeding complications in patients with sterile and infectious inflammation. In this review, we summarize the current understanding of the contribution of platelets to sepsis severity and outcome. We highlight the differences between platelet receptors in mice and humans and discuss the potential and limitations of animal models to study platelet-related functions in sepsis.

Romiplostim (Nplate®) as an effective radiation countermeasure to improve survival and platelet recovery in mice

Purpose: Rapid depletion of white blood cells, platelets, and reticulocytes are hallmarks of hematopoietic injury of acute radiation syndrome (H-ARS) and, if left untreated, can lead to severe health consequences including death. While the granulocyte colony stimulating factors (G-CSF) filgrastim (Neupogen^®), pegfilgrastim (Neulasta^®), and sargramostim (Leukine^®) are approved to increase survival in patients exposed to a myelosuppressive dose of radiation, no medical countermeasure is currently available for treatment of the thrombocytopenia that also results following radiation exposure. Romiplostim (Nplate^®), a thrombopoietin receptor agonist, is the first FDA-approved thrombopoiesis-stimulating protein for the treatment of low platelet (PLT) counts in adults with chronic immune thrombocytopenia. Herein, we present the results of an analysis in mice of romiplostim as a medical countermeasure to improve survival and PLT recovery following acute radiation.Materials and methods: Male and female C57BL/6J mice (11 - 12 weeks of age, n = 21/sex/group) were total body irradiated (TBI) with 6.8 Gy X-rays that reduces 30-day survival to 30% (LD70/30). Vehicle, romiplostim, and/or pegfilgrastim were administered subcutaneously beginning 24 h after TBI for 1-5 days. Evaluation parameters included 30-day survival, pharmacokinetics, and hematology.Results: Full or maximal efficacy with an ∼40% increase in survival was achieved after a single 30 µg/kg dose of romiplostim. No further survival benefit was seen with higher (100 µg/kg) or more frequent dosing (3 or 5 once daily doses at 30 µg/kg) of romiplostim or combined treatment with pegfilgrastim. Pharmacodynamic analysis revealed that the platelet nadir was not as low and recovery was faster in the irradiated mice treated with romiplostim when compared with irradiated control animals (Day 8 versus 10 nadir; Day 22 versus 29 recovery to near baseline). Platelet volume also increased more rapidly after romiplostim injection. Kinetic profiles of other hematology parameters were similar between TBI romiplostim-treated and control mice. Peak serum levels of romiplostim in TBI mice occurred 4 - 24 h (T_max) after injection with a t_1/2 of ∼24 h. Cmax values were at ∼6 ng/ml after 30 µg/kg ± TBI and ∼200 ng/ml after 300 µg/kg. A 10-fold higher romiplostim dose increased the AUC_last values by ∼35-fold.Conclusion: A single injection of romiplostim administered 24 h after TBI is a promising radiation medical countermeasure that dramatically increased survival, with or without pegfilgrastim, and hastened PLT recovery in mice.

Interaction of Treponema pallidum, the syphilis spirochete, with human platelets

Extracellular bacteria that spread via the vasculature employ invasive mechanisms that mirror those of metastatic tumor cells, including intravasation into the bloodstream and survival during hematogenous dissemination, arrestation despite blood flow, and extravasation into distant tissue sites. Several invasive bacteria have been shown to exploit normal platelet function during infection. Due to their inherent ability to interact with and influence other cell types, platelets play a critical role in alteration of endothelial barrier permeability, and their role in cancer metastasis has been well established. The highly invasive bacterium and causative agent of syphilis, Treponema pallidum subspecies pallidum, readily crosses the endothelial, blood-brain and placental barriers. However, the mechanisms underlying this unusual and important aspect of T. pallidum pathogenesis are incompletely understood. In this study we use darkfield microscopy in combination with flow cytometry to establish that T. pallidum interacts with platelets. We also investigate the dynamics of this interaction and show T. pallidum is able to activate platelets and preferentially interacts with activated platelets. Platelet-interacting treponemes consistently exhibit altered kinematic (movement) parameters compared to free treponemes, and T. pallidum-platelet interactions are reversible. This study provides insight into host cell interactions at play during T. pallidum infection and suggests that T. pallidum may exploit platelet function to aid in establishment of disseminated infection.

Generation of procoagulant collagen- and thrombin-activated platelets in platelet concentrates derived from buffy coat: the role of processing, pathogen inactivation, and storage

BACKGROUND

Collagen- and thrombin-activated (COAT) platelets (PLTs), generated by dual-agonist stimulation with collagen and thrombin (THR), enhance THR generation at the site of vessel wall injury. There is evidence that higher amounts of procoagulant COAT PLTs are associated with stroke, while a decreased ability to generate them is associated with bleeding diathesis. Our aim was to study PLT functions, particularly the ability to generate COAT PLTs, in PLT concentrates (PCs) from buffy coat. Thus, we investigated the effect of processing, pathogen inactivation treatment (amotosalen-UVA), and PC storage.

STUDY DESIGN AND METHODS

Two PCs from five donors each were pooled and split in two bags; one of them was pathogen inactivated and the other one was left untreated (n = 5). Flow cytometric analyses were performed immediately after PC preparation (Day 1) and thereafter on Days 2, 5, 7, and 9 in treated and untreated PCs to measure the reactivity of PLTs (CD62P and PAC-1), the content and secretion of dense granule after stimulation with different agonists, and the percentage of COAT PLTs after dual stimulation with convulxin (agonist of the collagen receptor GPVI) and THR.

RESULTS

Preparation of PCs resulted in a significant decrease of COAT PLTs and in an impaired response to adenosine 5'-diphosphate sodium (ADP). Storage further decreased ADP response. Minor differences were observed between untreated or amotosalen-UVA-treated PCs.

CONCLUSION

Preparation of PCs from buffy coats decreased the ability to generate COAT PLTs and impaired PLT response to ADP.

Tumor-Educated Platelets as a Noninvasive Biomarker Source for Cancer Detection and Progression Monitoring

Liquid biopsies represent a potential revolution in cancer diagnostics as a noninvasive method for detecting and monitoring diseases, complementary to or even replacing current tissue biopsy approaches. Several blood-based biosources and biomolecules, such as cell-free DNA and RNA, proteins, circulating tumor cells, and extracellular vesicles, have been explored for molecular test development. We recently discovered the potential of tumor-educated blood platelets (TEP) as a noninvasive biomarker trove for RNA biomarker panels. TEPs are involved in the progression and spread of several solid tumors, and spliced TEP RNA surrogate signatures can provide specific information on the presence, location, and molecular characteristics of cancers. So far, TEP samples from patients with different tumor types, including lung, brain, and breast cancers, have been tested, and it has been shown that TEPs from patients with cancer are distinct from those with inflammatory and other noncancerous diseases. It remains to be investigated how platelets are "educated," which mechanisms cause intraplatelet RNA splicing, and whether the relative contribution of specific platelet subpopulations changes in patients with cancer. Ultimately, TEP RNA may complement currently used biosources and biomolecules employed for liquid biopsy diagnosis, potentially enhancing the detection of cancer in an early stage and facilitating noninvasive disease monitoring. Cancer Res; 78(13); 3407-12. ©2018 AACR.