Whole blood analysis of reticulated platelets: improvements of detection and assay stability

Immature platelets and platelet reactivity in patients with acute ST-segment Elevation Myocardial Infarction using whole blood flow cytometry with SYTO-13 staining

BACKGROUND

Reduced effect of antiplatelet therapy has been reported in patients with ST-segment elevation myocardial infarction (STEMI). Multiple factors may concur to explain this, including increased amount of highly reactive immature platelets.

OBJECTIVES

To investigate the association between immature platelets and reactivity determined with multicolour flow cytometry using the SYTO-13 dye in STEMI patients.

METHODS

We conducted an observational study of 59 patients with acute STEMI. Blood samples were obtained within 24 h after admission and after loading doses of dual antiplatelet therapy. For comparison, samples were obtained from 50 healthy individuals. Immature platelets and platelet reactivity were investigated using multicolour flow cytometry including the SYTO-13 dye that binds to platelet RNA and thus provides a method for subdividing platelets into immature and mature platelets. Additionally, we assessed platelet aggregation, serum-thromboxane B2 levels and standard immature platelet markers.

RESULTS

Immature platelets were more reactive than mature platelets in both STEMI patients and healthy individuals (p-values < 0.05). STEMI patients had lower platelet aggregation and thromboxane B2 levels than healthy individuals. We found a positive association between automatically determined immature platelet markers and CD63 expression on activated platelets (Spearman's rho: 0.27 to 0.58, p-values < 0.05).

CONCLUSIONS

Our study shows that immature platelets identified with a multicolour flow cytometric method using the SYTO-13 dye are more reactive than mature platelets in patients with acute STEMI and in healthy individuals. The presence of immature platelets may be important for the overall platelet reactivity, which may have implications for the effect of antiplatelet therapy.

Role of Reticulated Platelets in Cardiovascular Disease

Human platelets differ considerably with regard to their size, RNA content and thrombogenicity. Reticulated platelets (RPs) are young, hyper-reactive platelets that are newly released from the bone marrow. They are larger and contain more RNA compared to older platelets. In comparison to more mature platelets, they exhibit a significantly higher thrombogenicity and are known to be elevated in patients with an increased platelet turnover such as, diabetics and after acute myocardial infarction. Several studies have shown that RPs correlate with an insufficient antiplatelet response to aspirin and specific P2Y₁₂ receptor inhibitors. In addition, RPs are promising novel biomarkers for the prediction of adverse cardiovascular events in cardiovascular disease. However, the reason for RPs intrinsic hyper-reactivity and their association with ischemic events is not completely understood and the biology of RPs is still under investigation. We here present a structured review of preclinical and clinical findings concerning the role of RPs in cardiovascular disease.

Reticulated platelets and immature platelet fraction: Clinical applications and method limitations

So-called "reticulated" or "immature platelets," which are newly released into the circulation, are more reactive than mature platelets, contain more RNA, and can be counted using flow cytometry after staining with thiazole orange or using some fully automated hematology analyzers, albeit with numerical disagreement. This review provides an overview of the state of the art of available technology for measuring immature or reticulated platelets (RP) with preanalytical (time stability, biological variation), analytical (methods, imprecision), and postanalytical (reference range) limitations. We also analyzed the clinical conditions in which immature/RP can be considered a diagnostic or prognostic tool (ie, differential diagnosis of thrombocytopenia, recovery after bone marrow or stem cell transplantation, risk assessment in cardiovascular diseases, response to antiplatelet drugs). They might also be of clinical utility in other settings but with lower evidence. The lack of a specific reference method and universal control material, as well as dependency of results on the measurement technique used, calls for different reference intervals and compromises comparison between clinical studies carried out using different analytical methods. To obviate lack of agreement between methods, more specific RNA dyes are necessary and the impact of the platelet size on the fluorescence signal defined. In the harmonization age, also in nomenclature field, a new definition instead of "reticulated" or "immature" platelets would be useful, and "young platelets" might be a more appropriate definition taking into account both the age and the functionality of this platelet fraction.

Reticulated platelets – clinical application and future perspectives

Reticulated platelets are immature platelets freshly released from the bone marrow into the circulation and contain vestigial amounts of ribonucleic acid. Thus, they can serve as an indicator for the activity of thrombopoiesis. Despite the current lack of a standardized reference method, two types of hematology analyzers have incorporated a fully automated measurement of reticulated platelets. The “immature platelet fraction” (IPF; Sysmex XE-/XN-series) has some clinical utility in the differential diagnosis of thrombocytopenia. This is less clear for “reticulated platelets” (retPLT; Abbott CELL-DYN Sapphire/Alinity HQ). The usefulness of these parameters in the prediction of platelet recovery after chemotherapy or stem cell transplantation and as a decision aid for platelet transfusions has not been unequivocally confirmed. Recent findings have shown an association of reticulated platelets with an adverse risk in patients with coronary artery disease and stroke as well as resistance to anti-platelet therapy. Furthermore, a role of reticulated platelets for the prediction of sepsis was indicated. However, validation in larger prospective trials is necessary to establish the clinical benefit of reticulated platelets in these conditions. This review gives an overview of the available analytical methods and summarizes the current knowledge regarding the clinical application of reticulated platelets.

Novel Role of T Cells and IL-6 (Interleukin-6) in Angiotensin II-Induced Microvascular Dysfunction

Hypertension is an established risk factor for subsequent cardiovascular diseases, with Ang II (angiotensin II) playing a major role in mediating thrombotic and inflammatory abnormalities. Although T cells and IL-6 (interleukin-6) play an important role in adaptive immune responses, little is known about their role(s) in the thromboinflammatory responses associated with Ang II. Here we show using intravital microscopy coupled with the light/dye injury model that Rag-1 deficient (Rag-1-/-) and IL-6 deficient (IL-6-/-) mice are afforded protection against Ang II-induced thrombosis. Blocking IL-6 receptors (using CD126 and gp130 antibodies) significantly diminished Ang II-mediated thrombosis and inflammatory cell recruitment in mice. Furthermore, the adoptive transfer of IL-6-/--derived T cells into Rag-1-/- mice failed to accelerate Ang II-induced thrombosis compared with Rag-1-/- mice reconstituted with wild-type-derived T cells, suggesting T cell IL-6 mediates the thrombotic abnormalities associated Ang II hypertension. Interestingly, adoptive transfer of WT T cells into Rag-1-/-/Ang II mice resulted in increased numbers of immature platelets, which constitutes a more active platelet population, that is, prothrombotic and proinflammatory. To translate our in vivo findings, we used clinical samples to demonstrate that IL-6 also predisposes platelets to an interaction with collagen receptors, thereby increasing the propensity for platelets to aggregate and cause thrombosis. In summary, we provide compelling evidence for the involvement of IL-6, IL-6R, and T-cell-dependent IL-6 signaling in Ang II-induced thromboinflammation, which may provide new therapeutic possibilities for drug discovery programs for the management of hypertension.

Platelets contribute to the initiation of colitis-associated cancer by promoting immunosuppression

Essentials Inflammation plays a key role in the development of colorectal cancer. Understanding mechanisms of cancer initiation might reveal new anticancer preventive strategy. Hyperactive platelets promote tumor formation by fostering immune evasion of cancer. Platelet inhibition by clopidogrel prevents carcinogenesis by restoring antitumor immunity.

SUMMARY

Background Clinical and experimental evidence support a role for inflammation in the development of colorectal cancer, although the mechanisms are not fully understood. Beyond thrombosis and hemostasis, platelets are key actors in inflammation; they have also been shown to be involved in cancer. However, whether platelets participate in the link between inflammation and cancer is unknown. Objective To investigate the contribution of platelets and platelet-derived proteins to inflammation-elicited colorectal tumor development. Methods We used a clinically relevant mouse model of colitis-associated cancer. Platelet secretion and platelet reactivity to thrombin were assessed at each stage of carcinogenesis. We conducted an unbiased proteomic analysis of releasates of platelets isolated at the pretumoral stage to identify soluble factors that might act on tumor development. Plasma levels of the identified proteins were measured during the course of carcinogenesis. We then treated the mice with clopidogrel to efficiently inhibit platelet release reaction. Results At the pretumoral stage, hyperactive platelets constituted a major source of circulating protumoral serum amyloid A (SAA) proteins. Clopidogrel prevented the early elevation of the plasma SAA protein level, decreased colitis severity, and delayed the formation of dysplastic lesions and adenocarcinoma. Platelet inhibition hindered the expansion and function of immunosuppressive myeloid cells, as well as their infiltration into tumors, but increased the number of tissue CD8+ T cells. Platelets and releasates of platelets from mice with cancer were both able to polarize myeloid cells towards an immunosuppressive phenotype. Conclusions Thus, platelets promote the initiation of colitis-associated cancer by enhancing myeloid cell-dependent immunosuppression. Antiplatelet agents may help to prevent inflammation-elicited carcinogenesis by restoring antitumor immunity.

Assessment of procoagulant potential in patients with reactive thrombocytosis and its association with platelet count

OBJECTIVE

We aimed to determine hemostatic changes and characterize the procoagulant potential among patients with reactive thrombocytosis (RT).

METHODS

Sixty patients with RT (median platelet count 718 × 10⁹ /L) and 20 healthy persons were tested for complete blood count, C-reactive protein, von Willebrand factor (VWF), factor VIII and fibrinogen, and thrombin generation. Platelet studies, including light transmission aggregometry and Cone and Plate(let) Analyzer, were also conducted. Reticulated platelets and platelet P-selectin expression were measured using flow cytometry.

RESULTS

Compared to patients with mild thrombocytosis (platelet count 500-700 × 10⁹ /L; n = 27), those with moderate-to-severe thrombocytosis (platelet count >700 × 10⁹ /L; n = 33) had significantly higher fibrinogen, factor VIII, and VWF antigen and activity levels; higher endogenous thrombin potential, peak thrombin generation and velocity index levels, and shorter time-to-peak thrombin level. VWF antigen and activity, fibrinogen, and factor VIII were positively associated with platelet count, whereas VWF activity/antigen ratio was inversely correlated. In a multivariate analysis of RT and control participants, only platelet count predicted endogenous thrombin potential with a positive-linear correlation. No patients developed acquired von Willebrand syndrome.

CONCLUSIONS

As determined by thrombin generation, RT was associated with in vitro prothrombotic tendency, which correlated with platelet count. This may explain the increased thromboembolic risk previously reported in patients with RT.

Reticulated platelets predict cardiovascular death in acute coronary syndrome patients

Reticulated platelets (RP) are newly-formed platelets with a greater mass, a residual amount of RNA and an increased prothrombotic potential. No studies investigating the association between RP and the risk of cardiovascular death in acute coronary syndrome (ACS) patients are available. In the frame of the AMI-Florence 2 study, we investigated RP in 229 (154 M/ 75 F) ACS patients (125 ST-elevation myocardial infarction [STEMI]; 104 Non-STEMI/Unstable Angina). RP were measured by using the Sysmex XE-2100 haematology analyzer and were expressed as the percentage of RP out of the total optical platelet count (immature platelet fraction; IPF) and as the percentage of RP highly fluorescent (H-IPF). At one-year follow-up, 22 out of 229 patients (9.6%) died from cardiovascular causes. Higher values of IPF (p=0.05) and H-IPF (p=0.006) were detected in dead compared to alive patients. A receiver operating characteristics curve analysis identified IPF ≥3.3% and H-IPF ≥0.9% as optimal cut-off values to predict cardiovascular death. At the multivariate model adjusted for the Global Registry of Acute Coronary Events (GRACE) risk score, the association between RP and cardiovascular death remained significant for both IPF [OR (95%CI) : 4.15 (1.24–13.91) p=0.02] and H-IPF [OR (95%CI): H-IPF 5.03 (1.38–18.38) p=0.01]. In conclusion, RP are independent predictors of cardiovascular death and may be useful in improving risk stratification for ACS patients. Future prospective studies to evaluate the role of RP in determining cardiovascular events are warranted.

Reference interval for immature platelet fraction on Sysmex XN hematology analyzer: a comparison study with Sysmex XE-2100

BACKGROUND

Recently introduced hematology analyzer, the Sysmex XN modular system (Sysmex, Kobe, Japan), has newly adopted a florescent channel to detect platelets and immature platelet fraction (IPF). This study aimed to establish new reference intervals for %-IPF and absolute number of IPF (A-IPF) on Sysmex XN. Platelet counts, %-IPF, and A-IPF were also compared between Sysmex XN and XE-2100 systems (Sysmex).

METHODS

Except outliers, blood samples from 2104 healthy individuals and 140 umbilical cord blood were analyzed using both Sysmex XN and XE-2100. The results of two systems were compared using Bland-Altman plot. The reference intervals for %-IPF and A-IPF were defined using non-parametric percentile methods according to the Clinical and Laboratory Standard Institute guideline (C28-A3).

RESULTS

The platelet counts, %-IPF, and A-IPF showed non-parametric distributions. The mean difference between Sysmex XN and XE-2100 in healthy individuals revealed a positive bias in platelets (+8.0×109/L), %-IPF (+1.2%), and A-IPF (+3.0×109/L). The reference intervals for %-IPF and A-IPF on Sysmex XN were: 1.0%-7.3% and 2.49-15.64×109/L in healthy individuals; and 1.0%-4.4% and 2.94-12.82×109/L in umbilical cord blood.

CONCLUSIONS

This large-scale study demonstrates a clear difference of platelet counts and IPF between Sysmex XN and XE-2100. The new reference intervals for IPF on Sysmex XN would provide fundamental data for clinical practice and future research.

Time-Dependent Decay of mRNA and Ribosomal RNA during Platelet Aging and Its Correlation with Translation Activity

Previous investigations have indicated that RNAs are mostly present in the minor population of the youngest platelets, whereas translation in platelets could be biologically important. To attempt to solve this paradox, we studied changes in the RNA content of reticulated platelets, i.e., young cells brightly stained by thiazole orange (TO^bright), a fluorescent probe for RNAs. We provoked in mice strong thrombocytopenia followed by dramatic thrombocytosis characterized by a short period with a vast majority of reticulated platelets. During thrombocytosis, the TO^bright platelet count rapidly reached a maximum, after which TO^dim platelets accumulated, suggesting that most of the former were converted into the latter within 12 h. Experiments on platelets, freshly isolated or incubated ex vivo at 37°C, indicated that their “RNA content”, here corresponding to the amounts of extracted RNA, and the percentage of TO^bright platelets were positively correlated. The “RNA Content” normalized to the number of platelets could be 20 to 40 fold higher when 80–90% of the cells were reticulated (20–40 fg/platelet), than when only 5–10% of control cells were TO^bright (less than 1fg/platelet). TO^bright platelets, incubated ex vivo at 37°C or transfused into mice, became TO^dim within 24 h. Ex vivo at 37°C, platelets lost about half of their ribosomal and beta actin RNAs within 6 hours, and more than 98% of them after 24 hours. Accordingly, fluorescence in situ hybridization techniques confirmed the presence of beta actin mRNAs in most reticulated-enriched platelets, but detected them in only a minor subset of control platelets. In vitro, constitutive translation decreased considerably within less than 6 hours, questioning how protein synthesis in platelets, especially in non-reticulated ones, could have a biological function in vivo. Nevertheless, constitutive transient translation in young platelets under pathological conditions characterized by a dramatic increase in circulating reticulated platelets could deserve to be investigated.

Impact of Atrial Fibrillation and Sinus Rhythm Restoration on Reticulated Platelets

OBJECTIVE

To assess the impact of nonvalvular atrial fibrillation (NVAF) and sinus rhythm restoration on the distribution of reticulated platelets (RPs), which are known to be associated with thrombotic propensity and have a greater predilection for thrombus participation.

PARTICIPANTS AND METHODS

The RP content was assessed by flow cytometry (thiazole orange/CD61) in 110 consecutive patients with NVAF before and 3 to 4 months after catheter ablation of the pulmonary veins. Results were compared with those of 55 age- and sex-matched controls with normal sinus rhythm.

RESULTS

The mean ± SD percentage of RPs was higher in patients with NVAF compared with controls (28.5%±7.3% vs 6.4%±5.3%; P<.001). The RP content did not vary by CHA2DS2-VASc score. After catheter ablation of the pulmonary veins, 63 patients were available for follow-up assessment. A significant reduction of RPs was observed compared with preintervention values (29.85%±7.1% vs 20.79%±7.6%; P<.001). During follow-up, 19% of patients (12 of 63) had confirmed AF recurrence. The mean ± SD percentage of RPs was higher in this group than in those without a recurrence (24.7%±6.5% vs 18.9%±7.5%; P=.01).

CONCLUSION

Nonvalvular atrial fibrillation affects the percentage of RPs, independent of the CHA2DS2-VASc score. After ablation, RP content dropped significantly. High RP content in patients with NVAF may explain the potential mechanism of thromboembolic complications and the lack of efficacy of currently available antiplatelet therapy for stroke prevention in this dysrhythmia.

Association of immature platelets with adverse cardiovascular outcomes

BACKGROUND

Immature platelets are less responsive to the effects of antiplatelet drugs and contain messenger ribonucleic acid that is translationally active. They can be measured easily using an automated hematoanalyzer and reported as part of the complete blood count.

OBJECTIVES

The purpose of this study was to determine the prognostic significance of elevated immature platelet count (IPC) in patients with coronary artery disease (CAD).

METHODS

In this prospective cohort study in patients with CAD, patients underwent IPC measurement and were then followed up for the composite endpoint of major adverse cardiovascular events (MACE), defined as a composite of all-cause mortality, myocardial infarction, unplanned revascularization, or hospitalization for angina. For the purposes of analysis, patients were stratified into tertiles of IPC.

RESULTS

Eighty-nine patients were followed up for a median of 31 months. Stratification to the high IPC tertile was associated with higher rates of MACE compared with the intermediate and low tertiles (60% vs. 24% vs. 16%, respectively; p < 0.001). Time-dependent receiver-operating characteristic analysis revealed that an IPC level ≥7,632 platelets/μl was 70.7% sensitive and 82.1% specific for MACE. After adjustment for age, admission diagnosis, index revascularization, heart failure, smoking, hematocrit, and baseline platelet count, patients with an IPC level ≥7,632 platelets/μl were more likely to experience a MACE (hazard ratio: 4.65; 95% confidence interval: 1.78 to 12.16; p < 0.002).

CONCLUSIONS

IPC is a novel biomarker for MACE risk stratification in patients with CAD. Future studies should focus on the utilization of this marker for individualized antiplatelet therapy.

Clinical applicability of reticulated platelets

BACKGROUND

Reticulated platelets (RPs), immature platelets newly released from the bone marrow into the circulation, have a high content of ribonucleic acid and are larger and more active in thrombus formation.

OBJECTIVE

This review compiles articles that evaluated RP in order to establish their clinical significance.

DISCUSSION

RPs increase when platelet production rises and decrease when production falls. As such, the measurement of circulating RPs allows the assessment of thrombocytopenia, i.e., bone marrow production or peripheral destruction.

CONCLUSION

RPs are a promising laboratory tool for evaluation of idiopathic thrombocytopenia (differentiating hypoproduction from accelerated platelet destruction), chemotherapy and after stem cell transplantation (predicting platelet recovery) and thrombocytosis (estimating platelet turnover). Additional randomized and well controlled clinical studies are required to clearly establish the significance of circulating RPs in other clinical conditions.

Reference intervals of reticulated platelets and other platelet parameters and their associations

CONTEXT

Reticulated platelets are platelets recently released from the bone marrow, and they can serve as a noninvasive indicator of recent megakaryopoietic activity. Widespread clinical use has been hampered by laborious methods and lack of standardization. Recently, a fully automated method was released on the Abbott CELL-DYN Sapphire hematology analyzer.

OBJECTIVE

To establish reference ranges for reticulated platelets. Secondary aims were to investigate associations between reticulated platelets and other platelet parameters like mean platelet volume, plateletcrit, and platelet distribution width.

DESIGN

Reticulated platelets and other platelet parameters were measured in an unselected cohort of 8089 subjects visiting a primary health care laboratory. The reticulated platelet data were analyzed using the Bhattacharya technique. In addition, a nonparametric method was used in selected subjects with normal platelet counts for providing reference ranges.

RESULTS

Reticulated platelets ranged from 0.4% to 6.0% or from 1 to 18 × 10(3)/μL. Reticulated platelets increased significantly with the subjects' age. Statistically, males had slightly higher values than females, but the differences were negligible. Reticulated platelets were positively correlated with platelet count and negatively with mean platelet volume.

CONCLUSIONS

Reference ranges have been established for reticulated platelets as measured on the CELL-DYN Sapphire hematology analyzer. There were no relevant differences between the sexes, but there was a clear effect of age. An individual's reticulated platelets are associated with the platelet count as well as mean platelet volume.

Interleukin-6 mediates the platelet abnormalities and thrombogenesis associated with experimental colitis

Clinical studies and animal experimentation have shown that colonic inflammation is associated with an increased number and reactivity of platelets, coagulation abnormalities, and enhanced thrombus formation. The objective of this study was to define the contribution of IL-6 to the thrombocytosis, exaggerated agonist-induced platelet aggregation, and enhanced extra-intestinal thrombosis that occur during experimental colitis. The number of mature and immature platelets, platelet life span, thrombin-induced platelet aggregation response, and light/dye-induced thrombus formation in cremaster muscle arterioles were measured in wild-type (WT) and IL-6-deficient (IL-6(-/-)) mice with dextran sodium sulfate (DSS)-induced colitis. DSS colitis in WT mice was associated with thrombocytosis with an elevated number of both mature and immature platelets and no change in platelet life span. The thrombocytosis response was absent in IL-6(-/-) mice. DSS treatment also enhanced the platelet aggregation response to thrombin and accelerated thrombus development in WT mice, but not in IL-6(-/-) mice. Exogenous IL-6 administered to WT mice elicited a dose-dependent enhancement of thrombus formation. These findings indicate that IL-6 mediates the thrombocytosis, platelet hyperreactivity, and accelerated thrombus development associated with experimental colitis. The IL-6-dependent colitis-induced thrombocytosis appears to result from an enhancement of thrombopoiesis because platelet life span is unchanged.

Platelet abnormalities during colonic inflammation

Patients with inflammatory bowel disease are susceptible to microvascular thrombosis and thromboembolism. The increased incidence of thrombosis is accompanied by enhanced coagulation and abnormalities in platelet function. Clinical studies have revealed thrombocytosis, alterations in platelet activation, enhanced platelet-leukocyte interactions, and elevated plasma levels of prothrombotic cytokines. This study was directed toward determining whether the thrombocytosis, altered platelet functions, and enhanced platelet-leukocyte interactions observed in patients with inflammatory bowel disease can be recapitulated in the dextran sodium sulfate and T-cell transfer models of murine colonic inflammation. Flow cytometry was used to characterize platelet function in heparin-anticoagulated whole blood of control mice and in mice with colonic inflammation. Platelets were identified by characteristic light scattering and membrane expression of CD41. Thiazole orange labeling was used to differentiate between immature and mature platelets. Platelet activation was monitored using the expression of an activation epitope of GPIIb/IIIa integrin. The combination of CD41, CD45.2, Gr-1, F4/80, and isotype control antibodies was used to detect and quantify aggregates of leukocytes, neutrophils, and monocytes with platelets. Our results indicated that colonic inflammation is associated with thrombocytosis, leukocytosis, and the appearance of immature platelets. An increased number of circulating activated platelets was detected in colitic mice, along with the formation of aggregates of leukocytes (PLA), neutrophils (PNA), and monocytes (PMA) with platelets. Selectin blockade with fucoidin inhibited dextran sodium sulfate-induced PLA formation. The findings of this study indicate that many features of the altered platelet function detected in human inflammatory bowel disease can be reproduced in animal models of colonic inflammation.

A novel role of sphingosine 1-phosphate receptor S1pr1 in mouse thrombopoiesis

Sphingosine 1-phosphate guides the elongation of megakaryocytic proplatelet extensions and triggers their shedding.

Millions of platelets are produced each hour by bone marrow (BM) megakaryocytes (MKs). MKs extend transendothelial proplatelet (PP) extensions into BM sinusoids and shed new platelets into the blood. The mechanisms that control platelet generation remain incompletely understood. Using conditional mutants and intravital multiphoton microscopy, we show here that the lipid mediator sphingosine 1-phosphate (S1P) serves as a critical directional cue guiding the elongation of megakaryocytic PP extensions from the interstitium into BM sinusoids and triggering the subsequent shedding of PPs into the blood. Correspondingly, mice lacking the S1P receptor S1pr1 develop severe thrombocytopenia caused by both formation of aberrant extravascular PPs and defective intravascular PP shedding. In contrast, activation of S1pr1 signaling leads to the prompt release of new platelets into the circulating blood. Collectively, our findings uncover a novel function of the S1P–S1pr1 axis as master regulator of efficient thrombopoiesis and might raise new therapeutic options for patients with thrombocytopenia.

Platelet life span and apoptosis

Like many nucleated mammalian cells, the life and death of the anucleate platelet is regulated by Bcl-2 family proteins. Platelets depend on Bcl-x(L) for survival. Bcl-x(L) maintains platelet viability by restraining the killer protein Bak. When Bak is unleashed, it triggers classical intrinsic apoptosis by causing mitochondrial damage. The latter leads to caspase activation and phosphatidylserine (PS) exposure. Platelet apoptosis can be blocked by caspase inhibitors, or by genetic deletion of Bak and its close relative Bax. Perturbations in the platelet apoptosis program lead to changes in platelet life span in vivo. Here, we describe methods to determine platelet life span, enumerate young platelets, and measure hallmarks of platelet apoptosis, such as PS exposure, caspase activation, and mitochondrial dysfunction.

Hematological and acute-phase responses to diet-induced obesity in IL-6 KO mice

Obesity is associated with chronic inflammation and elevated levels of IL-6. The role of IL-6 in induction of acute-phase proteins and modulation of hematological responses has been demonstrated in models of inflammation and aging, but not in obesity. We hypothesized that IL-6 is necessary to regulate the acute-phase response and hematological changes associated with diet-induced obesity (DIO) in mice. Feeding a 60%kcal/fat diet for 13 weeks to C57BL6 WT male mice induced a significant increase in IL-6 expression in visceral adipose tissue (VAT), but not liver, compared to mice fed chow diet. Significantly elevated IL-6 levels were present in the peritoneal lavage fluid, but not plasma, of DIO compared to lean mice. A comparable degree of obesity, hepatomegaly, hyperleptinemia, VAT inflammation and insulin resistance was observed in DIO WT and IL-6 KO mice compared to WT and KO mice fed chow diet. Significant leukocytosis was observed in DIO WT but not DIO KO mice compared to lean groups. A significant reduction in platelet counts, without alterations in platelet size, percentage of circulating reticulated platelets and number of bone marrow megakaryocytes, was present in DIO KO mice compared to each other group. Hepatic expression of thrombopoietin was comparable in each group, with DIO WT and KO mice having reduced VAT expression compared to lean mice. Lean KO mice had significantly elevated plasma levels of thrombopoietin compared to each other group, whereas liver-associated thrombopoietin levels were comparable in each group. Deficiency of IL-6 resulted in blunted hepatic induction of the acute-phase protein serum amyloid A-1, whereas expression of hepcidin-1 and -2, LPS-binding protein, ceruloplasmin, plasminogen activator inhibitor-1 and thrombospondin-1 was IL-6-independent. In conclusion, in the absence of overt metabolic alterations, IL-6 modulates leukocytosis, thrombopoiesis and induction of SAA-1, but not other acute-phase proteins in obese mice.

Megakaryocytes possess a functional intrinsic apoptosis pathway that must be restrained to survive and produce platelets

Deletion of Bak and Bax, the effectors of mitochondrial apoptosis, does not affect platelet production, however, loss of prosurvival Bcl-x_L results in megakaryocyte apoptosis and failure of platelet shedding.

It is believed that megakaryocytes undergo a specialized form of apoptosis to shed platelets. Conversely, a range of pathophysiological insults, including chemotherapy, are thought to cause thrombocytopenia by inducing the apoptotic death of megakaryocytes and their progenitors. To resolve this paradox, we generated mice with hematopoietic- or megakaryocyte-specific deletions of the essential mediators of apoptosis, Bak and Bax. We found that platelet production was unperturbed. In stark contrast, deletion of the prosurvival protein Bcl-x_L resulted in megakaryocyte apoptosis and a failure of platelet shedding. This could be rescued by deletion of Bak and Bax. We examined the effect on megakaryocytes of three agents that activate the intrinsic apoptosis pathway in other cell types: etoposide, staurosporine, and the BH3 mimetic ABT-737. All three triggered mitochondrial damage, caspase activation, and cell death. Deletion of Bak and Bax rendered megakaryocytes resistant to etoposide and ABT-737. In vivo, mice with a Bak^−/− Bax^−/− hematopoietic system were protected against thrombocytopenia induced by the chemotherapeutic agent carboplatin. Thus, megakaryocytes do not activate the intrinsic pathway to generate platelets; rather, the opposite is true: they must restrain it to survive and progress safely through proplatelet formation and platelet shedding.

Stability validation of paraformaldehyde-fixed samples for the assessment of the platelet PECAM-1, P-selectin, and PAR-1 thrombin receptor by flow cytometry

Sample fixation for storage and/or transportation represents an unsolved challenge for multicenter clinical trials assessing serial changes in platelet activity, or monitoring various antiplatelet regimens. Whole blood flow cytometry represents a major advance in defining platelet function, although special training and expensive equipment is required. We sought to determine how fixation with 2% paraformaldehyde (PFA), and storage of blood samples over 1 week affects the flow cytometry readings for both intact and thrombin-activating four major surface platelet receptors. Whole blood platelet expression of PECAM-1, P-selectin, PAR-1 inactive receptor (SPAN-12), and cleaved (WEDE-15) epitope was assessed immediately after blood draw, after staining with 2% PFA, and at day 1, 3, 5, and 7. The study was performed in 6 volunteers with multiple risk factors for vascular disease, not receiving any antiplatelet agents. Staining with PFA resulted in a slight decrease of fluorescence intensity, especially for PECAM-1, while antigen expression at day 1, 3 and 5 remains consistent, and highly reproducible. At day 7 there was a small but inconsistent trend towards diminished fluorescence intensity. The platelet data were consistent while validated with the isotype-matched irrelevant antibody. These data suggest that there is a 5 day window to perform final flow cytometry readings of whole blood PFA-fixed inactivated platelet samples. In contrast, thrombin activation cause gradual loss of flow cytometry signal, and cannot be recommended for long-term storage. This is critical logistic information for conducting multicenter platelet substudies within the framework of major clinical trials.

"Message in the platelet"--more than just vestigial mRNA!

Although mammalian platelets are anucleated cells, a number of studies have shown that they retain a pool of messenger RNA (mRNA) carried over from the megakaryocyte during thrombopoiesis. Platelet mRNA was originally thought to be relatively unstable and short-lived within the youngest cells and has been used as a potential marker of platelet turnover. In this article we will discuss both theoretical and methodological issues related to the measurement of these younger, "reticulated platelets". A key question relating to platelet mRNA is also whether it has any functional relevance other than a marker of platelet immaturity. Evidence going back more than 30 years suggests that platelets can biosynthesize proteins. However, it is only very recently that the nature and specificity of platelet mRNA has been examined in any detail. Difficulties in obtaining pure platelet mRNA, free of contamination from other cells has added to the complexity of unravelling this story. However, there is now clear evidence that platelets contain small but significant levels of message for a variety of proteins. The platelet mRNA pool is much richer and more diverse than previously thought and recent data suggests that regulated synthesis of a selected number of proteins can be induced on platelet activation. The full complexity of the platelet genome is now just being revealed and may open the possibility for improved diagnosis and therapy of many haemostatic and thrombotic disorders.

Evaluation of platelet turnover by flow cytometry

The number of circulating newly produced platelets depends on the thrombopoietic capacity of bone marrow as well as platelet removal from the bloodstream. Flow cytometric analysis with thiazole orange (TO), a fluorescent dye that crosses platelet membranes and binds intracellular RNA, has been used to measure circulating reticulated platelets (RPs) with high RNA content as an index of platelet turnover. We first assessed the specificity of TO flow cytometry and then applied this method in the diagnosis of thrombocytopenia caused by impaired platelet production or increased destruction. We also explored the utility of TO flow cytometry to predict thrombocytopoiesis after chemotherapy-induced bone marrow aplasia. Venous blood, anticoagulated with K(2)EDTA, was incubated with 0.6 microg/ml TO plus an anti-GPIIIa monoclonal antibody. The mean percentage of RPs in control subjects (n = 23) was 6.13 +/- 3.09%. RPs were 10.41 +/- 9.02% in patients (n = 10) with hematological malignancies during aplasia induced by chemotherapy and a significant increase in RPs (35.45 +/- 6.11%) was seen in the recovery phase. In 10 patients with idiopathic thrombocytopenic purpura, the percentage of TO positive platelets was 67.81 +/- 18.79 (P < 0.001 vs. controls). In patients with thrombocytopenia associated with hepatic cirrhosis (n = 21; 21.04 +/- 16.21%, P < 0.001 vs. controls) or systemic lupus erythematosus (n = 6, 29.08 +/- 15.57%; P < 0.001 vs. controls) increases in TO-stained platelets were also observed. Measurement of TO positive platelets may be a reliable tool for the laboratory identification of platelet disorders, with a higher sensitivity than measurement of platelet volume. Measurement of RPs may also prove useful to recognize the underlying pathogenetic mechanisms in thrombocytopenia.

[Relationship between reticulated platelets and megacaryocyte number in thrombocytopenic patients]

BACKGROUND AND INTRODUCTION

Marrow aspiration is a test that helps the pathogenic diagnosis of thrombocytopenia. Our goal was to analyze the correlation between reticulated platelets (RP) values in peripheral blood with megakaryocytic number in bone marrow in a group of thrombocytopenic patients.

PATIENTS AND METHODS

Prospective observational study in thrombocytopenic patients, between June 2002 and June 2005. RP determination was performed by flow cytometry using whole blood. We used a monoclonal anti-glycoprotein-IIIa antibody (CD61PerCP) for platelet identification and orange thiazole (Retic-count) as platelet mRNA stain. Marrow study was conducted by marrow aspiration.

RESULTS

RP were measured in 54 thrombocytopenic patients with bone marrow study. Three were excluded from the study. Thirty-two patients had central thrombocytopenia with diminished megakaryocytes (MK) and/or dysplasia, mean of RP 9.5% (CI 95%:5.6%-13.4%). Thirteen patients had high MK,mean of RP 25.7%(CI 95%:13.1%-38.3%). Six patients had normal MK, mean of RP 13.6% (CI 95%:0.6%-26.8%). There were differences between the group of increased MK and the group of central thrombocytopenias (p=0001). A value of RP>or=11% showed a sensitivity of 70% and specificity of 81% for the diagnosis of marrow aspirate with increased MK.

CONCLUSIONS

RP are an indirect marker of megakaryocyte number in bone marrow. A value of RP>or=11% in patients with thrombocytopenia, especially with an acute onset, would indicate regenerative thrombocytopenia, while in the presence of low levels of RP a marrow aspiration should be performed.

Timed feeding of mice modulates light-entrained circadian rhythms of reticulated platelet abundance and plasma thrombopoietin and affects gene expression in megakaryocytes

Circadian (c. 24 h) rhythms of physiology are entrained to either the environmental light-dark cycle or the timing of food intake. In the current work the hypothesis that rhythms of platelet turnover in mammals are circadian and entrained by food intake was explored in mice. Mice were entrained to 12 h light-dark cycles and given either ad libitum (AL) or restricted access (RF) to food during the light phase. Blood and megakaryocytes were then collected from mice every 4 h for 24 h. It was found that total and reticulated platelet numbers, plasma thrombopoietin (TPO) concentration and the mean size of mature megakaryocytes were circadian but not entrained by food intake. In contrast, a circadian rhythm in the expression of Arnt1 in megakaryocytes was entrained by food. Although not circadian, the expression in megakaryocytes of Nfe2, Gata1, Itga2b and Tubb1 expression was downregulated by RF, whereas Ccnd1 was not significantly affected by the feeding protocol. It is concluded that circadian rhythms of total platelet number, reticulated platelet number and plasma TPO concentration are entrained by the light-dark cycle rather than the timing of food intake. These findings imply that circadian clock gene expression regulates platelet turnover in mammals.

Aberrant increase in the immature platelet fraction in patients with myelodysplastic syndrome: a marker of karyotypic abnormalities associated with poor prognosis

OBJECTIVES

Some patients with myelodysplastic syndrome (MDS) show a marked increase in the percentage of immature platelet fraction (IPF%) despite the absence of severe thrombocytopenia. To determine the significance of such an unbalanced increase in the IPF%, we investigated the IPF% and other laboratory findings of 51 patients recently diagnosed with MDS.

METHOD

Subjects consisted of 80 healthy males, 90 healthy females, and 51 patients with MDS and 20 patients with idiopathic thrombocytopenic purpura (ITP). The IPF and IPF% were determined using a Sysmex XE-2100 system loaded with IPF Master software (XE IPF Master, Sysmex). Platelet counts were measured simultaneously.

RESULTS

IPF% and platelet counts of these patients ranged from 1.1% to 25.1% (median, 5.3%) and from 6 to 260 x 10(9)/L (median, 71 x 10(9)/L), respectively. Twelve patients showed platelet counts more than 50 x 10(9)/L with 10% or more IPF%. All of the 12 patients had chromosome abnormalities including monosomy 7 and complex abnormalities involving 7 or 5q. In the other 39 patients who did not show the aberrant IPF% increase, chromosomal abnormalities were seen only in seven patients and none of them had chromosome 7 abnormalities. The IPF% of two patients increased to more than 10% in association with the appearance of monosomy 7.

CONCLUSIONS

These findings suggest that a high IPF% in MDS patient may be a marker for karyotypic abnormalities with a poor prognosis, including chromosome 7 abnormalities.

Reticulated platelets as a screening test to identify thrombocytopenia aetiology

BACKGROUND

Thrombocytopenia is a common haematological abnormality and no simple diagnostic test is available to diagnose thrombocytopenia pathogenesis.

AIM

To evaluate sensitivity and specificity of reticulated platelets (RP) as a diagnostic test for thrombocytopenia with increased thrombopoietic activity.

DESIGN

Prospective observational study in thrombocytopenic patients.

METHODS

A direct, whole-blood, dual-labelling flow cytometric method was used. Direct, whole-blood double coverage was achieved using a monoclonal anti-glycoprotein (GP)-III antibody (CD61 PerCP) for platelet identification and thiazole orange (Retic-count) as platelet mARN stain.

RESULTS

RP were measured in 101 thrombocytopenic patients and 104 non-thrombocytopenic controls. The mean RP percentage in 60 thrombocytopenic patients with no increased thrombopoietic activity was 7.5% (CI for 95%: 5.2-9.7) and RP absolute number was 3.2 x 10(9)/l (CI for 95%: 2.1-4.3). The mean RP percentage in 41 thrombocytopenic patients with increased thrombopoietic activity was 30.3% (CI for 95%: 25.1-35.5) and RP absolute number was 6.2 (CI for 95%: 4.8-7.7). The RP percentage cut-off for a diagnosis of thrombocytopenia with increased thrombopoietic activity was 11% [sensitivity 93%, specificity 85%, positive predictive value (PPV) 83%, negative predictive value (NPV) 95%].

CONCLUSION

RP measurement by flow cytometry, directly from whole-blood, is a useful screening test to differentiate between thrombocytopenia with high or low thrombopoietic activity. A RP percentage in excess of 11%, has a high sensitivity and good specificity for a diagnosis of thrombocytopenia with increased thrombopoietic activity.

Morphological and functional platelet abnormalities in Berkeley sickle cell mice

Berkeley sickle cell mice are used as animal models of human sickle cell disease but there are no reports of platelet studies in this model. Since humans with sickle cell disease have platelet abnormalities, we studied platelet morphology and function in Berkeley mice (SS). We observed elevated mean platelet forward angle light scatter (FSC) values (an indirect measure of platelet volume) in SS compared to wild type (WT) (37+/-3.2 vs. 27+/-1.4, mean+/-SD; p<0.001), in association with moderate thrombocytopenia (505+/-49 x 10(3)/microl vs. 1151+/-162 x 10(3)/microl; p<0.001). Despite having marked splenomegaly, SS mice had elevated levels of Howell-Jolly bodies and "pocked" erythrocytes (p<0.001 for both) suggesting splenic dysfunction. SS mice also had elevated numbers of thiazole orange positive platelets (5+/-1% vs. 1+/-1%; p<0.001), normal to low plasma thrombopoietin levels, normal plasma glycocalicin levels, normal levels of platelet recovery, and near normal platelet life spans. Platelets from SS mice bound more fibrinogen and antibody to P-selectin following activation with a threshold concentration of a protease activated receptor (PAR)-4 peptide compared to WT mice. Enlarged platelets are associated with a predisposition to arterial thrombosis in humans and some humans with SCD have been reported to have large platelets. Thus, additional studies are needed to assess whether large platelets contribute either to pulmonary hypertension or the large vessel arterial occlusion that produces stroke in some children with sickle cell disease.

Reticulated platelet counts correlate with treatment response in patients with idiopathic thrombocytopenic purpura and help identify the complex causes of thrombocytopenia in patients after allogeneic hematopoietic stem cell transplantation

BACKGROUND

In thrombocytopenic conditions of unknown origin, quantification of reticulated platelets (RP) in the peripheral blood by flow cytometry has been shown to differentiate increased platelet (Plt) turnover from insufficient Plt production.

METHODS

We used a whole blood flow cytometry method combining thiazole orange and anti-CD41a-staining to assess RP in 71 healthy subjects, six with thrombocytopenic myelodysplastic syndrome (MDS), nine with liver cirrhosis, 14 patients with idiopathic thrombocytopenic purpura (ITP), and 12 patients who had undergone hematopoietic stem cell transplantation (HSCT).

RESULTS

Patients with MDS had normal, patients with liver cirrhosis had slightly elevated RP counts compared to healthy subjects. ITP patients had elevated RP counts, and RP >15% were associated with treatment response (P = 0.015). In 7/10 patients after HSCT, an increase of RP preceded Plt recovery, whereas in patients with secondary thrombocytopenia after normal regeneration, the assessment of RP allowed the differentiation between conditions with high Plt turnover, such as GvHD and microangiopathy, indicated by high RP counts, and graft failure, indicated by low RP counts.

CONCLUSIONS

Our data provide the rationale for prospective studies on the diagnostic and prognostic value of RP counts in larger patient populations with ITP and after HSCT.

The megakaryocyte lineage originates from hemangioblast precursors and is an integral component both of primitive and of definitive hematopoiesis

In the adult, platelets are derived from unipotential megakaryocyte colony-forming cells (Meg-CFCs) that arise from bipotential megakaryocyte/erythroid progenitors (MEPs). To better define the developmental origin of the megakaryocyte lineage, several aspects of megakaryopoiesis, including progenitors, maturing megakaryocytes, and circulating platelets, were examined in the murine embryo. We found that a majority of hemangioblast precursors during early gastrulation contains megakaryocyte potential. Combining progenitor assays with immunohistochemical analysis, we identified 2 waves of MEPs in the yolk sac associated with the primitive and definitive erythroid lineages. Primitive MEPs emerge at E7.25 along with megakaryocyte and primitive erythroid progenitors, indicating that primitive hematopoiesis is bilineage in nature. Subsequently, definitive MEPs expand in the yolk sac with Meg-CFCs and definitive erythroid progenitors. The first GP1bbeta-positive cells in the conceptus were identified in the yolk sac at E9.5, while large, highly reticulated platelets were detected in the embryonic bloodstream beginning at E10.5. At this time, the number of megakaryocyte progenitors begins to decline in the yolk sac and expand in the fetal liver. We conclude that the megakaryocyte lineage initially originates from hemangioblast precursors during early gastrulation and is closely associated both with primitive and with definitive erythroid lineages in the yolk sac prior to the transition of hematopoiesis to intraembryonic sites.

Measurement of reticulated platelets by simple flow cytometry: An indirect thrombocytopoietic marker

BACKGROUND

The aim of this study was to determine whether measurement of reticulated platelets (RP) by flow cytometry directly from whole blood, with no fixation or manipulation, is as useful a thrombocytopoietic marker as other more complex techniques.

METHODS

RP percentage was prospectively assessed in thrombocytopenic patients (platelets <100x10(9)/L) and non-thrombocytopenic controls using a direct, whole-blood, dual-labelling flow cytometric method. Direct, whole-blood double coverage was achieved using a monoclonal antiglycoprotein (GP)-III antibody (CD61-PerCP(R)) for platelet identification and thiazol orange (Retic-count(R)) as platelet mARN stain. After establishing thrombocytopenia etiology, patients were grouped according to whether their rate of thrombopoiesis was increased or decreased.

RESULTS

RP were measured in 53 thrombocytopenic patients with several etiologies and in 53 non-thrombocytopenic controls. The mean RP in 14 thrombocytopenic patients with no increased thrombopoietic activity was 4.8% (95% CI: 3.2-6.4) and the RP absolute number was 1.98x10(9)/L (95% CI: 1.3-2.6). The mean RP in 17 thrombocytopenic patients with increased thrombopoietic activity was 29.4% (95% CI: 24.7-34.1) and the RP absolute number was 7.24x10(9)/L (95% CI: 4.9-9.5).

CONCLUSIONS

RP measurement by flow cytometry, directly from whole blood without manipulation, is a useful screening test to differentiate thrombocytopenia with high or low thrombopoietic activity.