Platelets in ITP: Victims in Charge of Their Own Fate?

Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder. The pathophysiological mechanisms leading to low platelet levels in ITP have not been resolved, but at least involve autoantibody-dependent and/or cytotoxic T cell mediated platelet clearance and impaired megakaryopoiesis. In addition, T cell imbalances involving T regulatory cells (Tregs) also appear to play an important role. Intriguingly, over the past years it has become evident that platelets not only mediate hemostasis, but are able to modulate inflammatory and immunological processes upon activation. Platelets, therefore, might play an immuno-modulatory role in the pathogenesis and pathophysiology of ITP. In this respect, we propose several possible pathways in which platelets themselves may participate in the immune response in ITP. First, we will elaborate on how platelets might directly promote inflammation or stimulate immune responses in ITP. Second, we will discuss two ways in which platelet microparticles (PMPs) might contribute to the disrupted immune balance and impaired thrombopoiesis by megakaryocytes in ITP. Importantly, from these insights, new starting points for further research and for the design of potential future therapies for ITP can be envisioned.

Positive Association between Endothelium–Platelet Microparticles and Urinary Concentration of Lead and Cadmium in Adolescents and Young Adults

(1) Background: In previous research, higher levels of urine heavy metals, especially lead and cadmium, have been associated with increased cardiovascular risk. However, there is no information linking exposure to heavy metal to endothelial and platelet microparticles (EMPs and PMPs), particularly in the younger population, which are novel biomarkers of endothelial dysfunction. (2) Methods: From a nationwide database, which was incepted in 1992–2000, screening for renal health among Taiwanese school children, a total of 789 subjects were recruited. Cross-sectional analysis was performed to evaluate the association between serum EMPs/PMPs and urine iron, nickel, copper, cadmium, lead, chromium, manganese, and zinc levels in the adolescent and young adult population. (3) Results: After we adjusted the conventional cardiovascular risk factors, CD31+/CD42a− and CD31+/CD42a+ counts, in subjects’ serum, respective markers of EMP and PMP displayed a significant positive dose-response relationship with urinary lead and cadmium levels. Higher quartiles of urine lead and cadmium levels were associated with an increased risk of higher EMPs/PMPs (≥75th percentile) in a multivariate logistic regression model. (4) Conclusion: Higher urinary lead and cadmium concentrations are strongly associated with endothelium–platelet microparticles in this adolescent and young adult population, which could help explain, in part, the mechanism through which heavy metal exposure results in cardiotoxicity.

Platelet Microparticles Decrease Daunorubicin-Induced DNA Damage and Modulate Intrinsic Apoptosis in THP-1 Cells

Platelets can modulate cancer through budding of platelet microparticles (PMPs) that can transfer a plethora of bioactive molecules to cancer cells upon internalization. In acute myelogenous leukemia (AML) this can induce chemoresistance, partially through a decrease in cell activity. Here we investigated if the internalization of PMPs protected the monocytic AML cell line, THP-1, from apoptosis by decreasing the initial cellular damage inflicted by treatment with daunorubicin, or via direct modulation of the apoptotic response. We examined whether PMPs could protect against apoptosis after treatment with a selection of inducers, primarily associated with either the intrinsic or the extrinsic apoptotic pathway, and protection was restricted to the agents targeting intrinsic apoptosis. Furthermore, levels of daunorubicin-induced DNA damage, assessed by measuring gH2AX, were reduced in both 2N and 4N cells after PMP co-incubation. Measuring different BCL2-family proteins before and after treatment with daunorubicin revealed that PMPs downregulated the pro-apoptotic PUMA protein. Thus, our findings indicated that PMPs may protect AML cells against apoptosis by reducing DNA damage both dependent and independent of cell cycle phase, and via direct modulation of the intrinsic apoptotic pathway by downregulating PUMA. These findings further support the clinical relevance of platelets and PMPs in AML.

Platelet Microparticles Protect Acute Myelogenous Leukemia Cells against Daunorubicin-Induced Apoptosis

The role of platelets in cancer development and progression is increasingly evident, and several platelet–cancer interactions have been discovered, including the uptake of platelet microparticles (PMPs) by cancer cells. PMPs inherit a myriad of proteins and small RNAs from the parental platelets, which in turn can be transferred to cancer cells following internalization. However, the exact effect this may have in acute myelogenous leukemia (AML) is unknown. In this study, we sought to investigate whether PMPs could transfer their contents to the THP-1 cell line and if this could change the biological behavior of the recipient cells. Using acridine orange stained PMPs, we demonstrated that PMPs were internalized by THP-1 cells, which resulted in increased levels of miR-125a, miR-125b, and miR-199. In addition, co-incubation with PMPs protected THP-1 and primary AML cells against daunorubicin-induced cell death. We also showed that PMPs impaired cell growth, partially inhibited cell cycle progression, decreased mitochondrial membrane potential, and induced differentiation toward macrophages in THP-1 cells. Our results suggest that this altering of cell phenotype, in combination with decrease in cell activity may offer resistance to daunorubicin-induced apoptosis, as serum starvation also yielded a lower frequency of dead and apoptotic cells when treated with daunorubicin.

Immune thrombocytopenia and thrombosis. Hematological paradox or clinical reality?

One of the features of primary immune thrombocytopenia is the development of thrombosis in patients, the incidence of which exceeds that in the population. The review provides epidemiological data on thromboembolic events, pathogenetic mechanisms and risk factors for their development, therapeutic tactics in this category of patients. In patients with immune thrombocytopenia, the following factors are important for the development of thromboembolic events: the pathogenetic features of the disease itself (increased functional activity of young platelets, platelet microparticles, antiphospholipid antibodies, activation of complement, an increase in the level of procoagulants, a decrease in the activity of ADAMTS13), the presence of comorbid risk factors for thrombosis, thrombogenic adverse effects of drugs used to treat immune thrombocytopenia. Therapeutic tactics of managing patients with immune thrombocytopenia who develop thromboembolic events consists in correcting the number of platelets (glucocorticoids, intravenous immunoglobulin) and administration of antithrombotic agents, given the clinical situation and the number of platelets.

Platelet activation in rabbits with decompression sickness

Platelets are the most easily altered type of peripheral blood cell in decompression sickness (DCS), which can feature decreased platelet count and the appearance of platelet microparticles in plasma. We hypothesized that DCS results in platelet activation in the bloodstream. The present study was carried out on 45 rabbits. The platelet count and concentration of plasma platelet markers were determined in 35 rabbits; the platelet shape was observed under scanning electron microscope in 10 rabbits. All indexes were collected at two points: 24 hours before the simulated dive and 30 minutes after the simulated dive. Platelet count decreased noticeably after DCS, from 380.10 ± 73.61 (G/L) to 330.23 ± 115.72 (G/L), a change of approximately -13.49 ± 25.57 (%). Platelet count was further decreased in the severe DCS group (a change of -45.99 ± 18.57%). Platelet count after DCS was proportional to the survival time of the rabbits after DCS. The concentration of two plasma platelet markers (PF4 and BTG) did not demonstrate statistically significant change at 30 minutes after DCS. However, platelet shape was changed, and the following features were observed: oblong, distortion, flattening shape, sticking together, mixing of membrane, and abundance of pseudopods with a 100- to 200-nm diameter. We conclude there is platelet activation in the bloodstream in cases of DCS.

Impact of Epicatechin on the Procoagulant Activities of Microparticles

Microparticles play a role in cardiovascular disease pathology. The flavanol-like epicatechin is increasingly considered due to its cardioprotective effects. The aim of this study was to investigate the impact of epicatechin on microparticle generation, phenotype and procoagulant properties. Plasma samples from 15 healthy subjects were incubated with increasing concentrations of epicatechin (1 to 100 μM). Then, the expression of glycoprotein IIb, phosphatidylserine (PS), glycoprotein Ib (GPIb) and P-selectin was assessed by flow cytometry analysis after (or not) platelet stimulation. Microparticle procoagulant activity was determined using ZymuphenTM MP and ZymuphenTM MP-TF for phospholipid and tissue factor content, and with thrombin generation (TG) assays for procoagulant function. Platelet microparticles that express GPIb (/µL) decreased from 20,743 ± 24,985 (vehicle) to 14,939 ± 14,333 (p = 0.6), 21,366 ± 16,949 (p = 0.9) and 15,425 ± 9953 (p < 0.05) in samples incubated with 1, 10 and 100 µM epicatechin, respectively. Microparticle concentration (nM PS) decreased from 5.6 ± 2.0 (vehicle) to 5.1 ± 2.2 (p = 0.5), 4.5 ± 1.5 (p < 0.05) and 4.7 ± 2.0 (p < 0.05) in samples incubated with 1, 10 and 100µM epicatechin, respectively. Epicatechin had no impact on tissue factor-positive microparticle concentration. Epicatechin decreased TG (endogenous thrombin potential, nM.min) from 586 ± 302 to 509 ± 226 (p = 0.3), 512 ± 270 (p = 0.3) and 445 ± 283 (p < 0.05). These findings indicate that epicatechin affects microparticle release, phenotype and procoagulant properties.