Monoclonal whole IgG impairs both fibrin and thrombin formation: hemostasis and surface plasmon resonance studies.

OBJECTIVES: Monoclonal gammopathies frequently associate with hemostatic alterations. Thrombotic events occur with high incidence particularly upon treatment, while in rarer cases hemorrhagic diathesis can be observed. The pathology of these tendencies could be caused by thrombocytopenia or hyperviscosity burden of circulating monoclonal antibodies. Studies also suggest interference of monoclonal antibodies with primary hemostasis. We isolated monoclonal whole IgG paraproteins from two myeloma patients to observe their effect on thrombin formation and fibrin polymerization. METHODS: Monoclonal whole IgG was prepared from sera of two newly diagnosed untreated multiple myeloma patients and control normal plasma samples. Fibrin formation was measured using thrombin time and dilute prothrombin time tests and thrombin formation was detected with a fluorimetric thrombin generation assay. In addition, molecular interactions were investigated by surface plasmon resonance (SPR). RESULTS: Thrombin time was prolonged upon addition of monoclonal IgG even at 30 g/L by 12 %, increasing up to 36 % at 60 g/L concentration. Dilute prothrombin time was prolonged by 20 % even at 30 g/L. Thrombin generation assay indicated an impairment in thrombin formation at the presence of monoclonal IgG compared to polyclonal at equivalent concentration. By an SPR assay we determined that both clonality IgG preparations interacted with fibrinogen, however interaction with human thrombin was only detected with monoclonal immunoglobulins (KD=1.03 × 10-7 M). CONCLUSIONS: Here we provide evidence that isolated monoclonal whole IgG from myeloma patients can impair both fibrin and thrombin formation and we demonstrate by SPR assay that it interacts with components of the final phase of the coagulation system.

Dasatinib Inhibits Procoagulant and Clot Retracting Activities of Human Platelets.

Tyrosine kinase inhibitors (TKI) such as the BCR-ABL inhibitor dasatinib and nilotinib are highly effective therapies for chronic myeloid leukemia (CML). However, several lines of evidence suggest that dasatinib can induce bleeding which may be due to impaired collagen-induced platelet adhesion, aggregation, and secretion. Sarcoma family kinases (SFK) play central role in the GPVI-induced signaling pathway. We aimed to investigate whether and how dasatinib can modulate SFK-mediated platelet procoagulant activity in a purified system and in dasatinib/nilotinib treated CML patients. In platelet rich plasmas of healthy volunteers, dasatinib dose-dependently reduced convulxin-induced phosphatidylserine exposure and attenuated thrombin formation. Similarly to these changes, integrin activation and clot retraction were also significantly inhibited by 100 nM dasatinib. Platelets isolated from dasatinib treated patients showed a significantly lower phosphatidylserine expression upon convulxin activation compared to premedication levels. In these samples, thrombin generation was significantly slower, and the quantity of formed thrombin was less compared to the trough sample. Western blot analyses showed decreased phosphorylation levels of the C-terminal tail and the activation loop of SFKs upon dasatinib administration. Taken together, these results suggest that dasatinib inhibits the formation of procoagulant platelets via the GPVI receptor by inhibiting phosphorylation of SFKs.

Cellular Factor XIII, a Transglutaminase in Human Corneal Keratocytes.

Cellular factor XIII (cFXIII, FXIII-A2), a transglutaminase, has been demonstrated in a few cell types. Its main function is to cross-link proteins by isopeptide bonds. Here, we investigated the presence of cFXIII in cells of human cornea. Tissue sections of the cornea were immunostained for FXIII-A in combination with staining for CD34 antigen or isopeptide cross-links. Isolated corneal keratocytes were also evaluated by immunofluorescent microscopy and flow cytometry. FXIII-A in the corneal stroma was quantified by Western blotting. FXIII-A mRNA was detected by RT-qPCR. The cornea of FXIII-A-deficient patients was evaluated by cornea topography. FXIII-A was detected in 68 ± 13% of CD34+ keratocytes. Their distribution in the corneal stroma was unequal; they were most abundant in the subepithelial tertile. cFXIII was of cytoplasmic localization. In the stroma, 3.64 ng cFXIII/mg protein was measured. The synthesis of cFXIII by keratocytes was confirmed by RT-qPCR. Isopeptide cross-links were detected above, but not within the corneal stroma. Slight abnormality of the cornea was detected in six out of nine FXIII-A-deficient patients. The presence of cFXIII in human keratocytes was established for the first time. cFXIII might be involved in maintaining the stability of the cornea and in the corneal wound healing process.

Ponatinib Induces a Procoagulant Phenotype in Human Coronary Endothelial Cells via Inducing Apoptosis.

BCR-ABL tyrosine kinase inhibitors (TKIs) are effective drugs in the treatment of patients with chronic myeloid leukemia. However, based on clinical studies, ponatinib was associated with the development of thrombotic complications. Since endothelial cells (ECs) regulate blood coagulation, their abnormal phenotype may play a role in the development of thrombotic events. We here aimed to investigate the effect of ponatinib on the procoagulant activity of cultured endothelial cells in vitro. Human coronary artery endothelial cells (HCAECs) were incubated with 50, 150, and 1000 nM of ponatinib. Subsequently, phosphatidylserine (PS) exposure and endothelial microvesicles (EMVs) were measured by flow cytometry. In addition, EC- and EMV-dependent thrombin generation was analyzed. To investigate pro-apoptotic effects of ponatinib, the level of Bax and Bcl-xL proteins were studied using Western blot and F3, THBD, and VCAM1 mRNAs were quantified by qPCR. Therapeutic concentrations of ponatinib significantly increased PS expression on ECs and the amount of EMVs which significantly shortened the time parameters of thrombin generation. In addition, these changes were associated with an increased ratio of Bax and Bcl-xL proteins in the presence of the decreased THBD mRNA level. Overall, ponatinib enhances the procoagulant activity of ECs via inducing apoptosis, which may contribute to thrombotic events.

Activation mechanism dependent surface exposure of cellular factor XIII on activated platelets and platelet microparticles.

BACKGROUND: Platelets contain a high amount of potentially active A subunit dimer of coagulation factor XIII (cellular FXIII; cFXIII). It is of cytoplasmic localization, not secreted, but becomes translocated to the surface of platelets activated by convulxin and thrombin (CVX+Thr). OBJECTIVE: To explore the difference in cFXIII translocation between receptor mediated and non-receptor mediated platelet activation and if translocation can also be detected on platelet-derived microparticles. Our aim was also to shed some light on the mechanism of cFXIII translocation. METHODS: Gel-filtered platelets were activated by CVX+Thr or Ca2+ -ionophore (calcimycin). The translocation of cFXIII and phosphatidylserine (PS) to the surface of activated platelets and platelet-derived microparticles was investigated by flow cytometry, immunofluorescence, and immune electron microscopy. Fluo-4-AM fluorescence was used for the measurement of intracellular Ca2+ concentration. RESULTS: Receptor mediated activation by CVX+Thr exposed cFXIII to the surface of more than 60% of platelets. Electron microscopy revealed microparticles with preserved membrane structure and microparticles devoid of labeling for membrane glycoprotein CD41a. cFXIII was observed on both types of microparticles but was more abundant in the absence of CD41a. Rhosin, a RhoA inhibitor, significantly decreased cFXIII translocation. Non-receptor mediated activation of platelets by calcimycin elevated intracellular Ca2+ concentration, induced the translocation of PS to the surface of platelets and microparticles, but failed to expose cFXIII. CONCLUSIONS: The elevation of intracellular Ca2+ concentration is sufficient for the translocation of PS from the internal layer of the membrane, while the translocation of cFXIII from the platelet cytoplasm requires additional receptor mediated mechanism(s).

Protein phosphatase inhibitor calyculin-A modulates activation markers in TRAP-stimulated human platelets.

Platelet activation is accompanied with the phosphorylation of a number of proteins on serine (Ser) and threonine (Thr) residues. The phosphorylation level of these proteins is dependent upon the protein kinase/phosphatase activity ratio. The aim of this study was to investigate the consequences of inhibiting protein phosphatase 1 (PP1) and 2A (PP2A) on platelet functions. Protein phosphatases were inhibited by preincubation of platelet rich plasma (PRP) samples with calyculin-A (CLA). Subsequently, platelets were activated by thrombin-receptor activating peptide (TRAP) and platelet aggregation, platelet-derived microparticle (PMP) formation, surface expressions of P-selectin (CD62), lysosome-associated membrane protein (CD63), glycoprotein Ib and IIb were examined. Phosphatase activity was determined by using phosphorylated 20 kDa myosin light chain (P-MLC20) as substrate. In CLA-treated platelets substantial decrease of P-MLC20 phosphatase activity was observed. CLA significantly suppressed TRAP-induced surface expression of P-selectin and CD63 in a concentration-dependent manner as compared to non-treated samples and moderately decreased platelet aggregation. In TRAP-activated samples, 50 nM of CLA pretreatment completely abolished the level of PMPs and the prevention of GPIb downregulation was also observed; however, no difference was found in GPIIb expression. In conclusion, PP1 and PP2A-catalyzed dephosphorylation processes have crucial roles in PMP formation and in the regulation of alpha-granule and lysosome secretion in human platelets.

L-Selectin Expression is Influenced by Phosphatase Activity in Chronic Lymphocytic Leukemia.

BACKGROUND: Adhesion receptors have important role in cellular invasiveness and L-selectin is a primary determinant in the binding of chronic lymphocytic leukemia (CLL) cells to several glycated proteins on endothelial cells. We investigated L-selectin expression on CLL cells and explored the mechanisms that lead to their shedding. METHODS: Surface and soluble L-selectin expression levels were studied by flow cytometry and immunoassay, respectively. Magnetically isolated B-cells from patients and controls were investigated for total and protein phosphatase-2A activities. Flow cytometry of permeabilized cells was utilized for the determination of phosphorylated mitogen-activated protein kinase (pp38MAPK) and surface tumor necrosis factor alpha-converting enzyme expression (TACE). RESULTS: In CLL patients elevated absolute lymphocyte cell counts, high soluble and low surface L-selectin expression were observed. Similarly, TACE surface expression was significantly lower on B-CLL cells compared to normal B-cells. Both total phosphatase and protein phosphatase-2A activities were also significantly lower in B-CLL cells compared to normal B-cells and we found a consequently higher level of pp38 MAPK in B-CLL cells. Based on in vitro experiments a MAPK inhibitor could attenuate the phosphatase inhibitor's effect on L-selectin shedding. CONCLUSIONS: The lower phosphatase activity detectable in chronic lymphocytic leukemia, results in a downstream signaling cascade with subsequent reduction of surface L-selectin expression and this effect is mediated by enhanced phosphorylation of p38MAPK and an altered TACE expression. © 2019 The Authors. Cytometry Part B: Clinical Cytometry published by Wiley Periodicals, Inc. on behalf of International Clinical Cytometry Society.

Evaluation of flow cytometric HIT assays in relation to an IgG-Specific immunoassay and clinical outcome.

BACKGROUND: Heparin-induced thrombocytopenia (HIT) is a severe side effect of heparin treatment caused by platelet activating IgG antibodies generated against the platelet factor 4 (PF4)-heparin complex. Thrombocytopenia and thrombosis are the leading clinical symptoms of HIT. METHODS: The clinical pretest probability of HIT was evaluated by the 4T score system. Laboratory testing of HIT was performed by immunological detection of antibodies against PF4-heparin complex (EIA) and two functional assays. Heparin-dependent activation of donor platelets by patient plasma was detected by flow cytometry. Increased binding of Annexin-V to platelets and elevated number of platelet-derived microparticles (PMP) were the indicators of platelet activation. RESULTS: EIA for IgG isotype HIT antibodies was performed in 405 suspected HIT patients. Based on negative EIA results, HIT was excluded in 365 (90%) of cases. In 40 patients with positive EIA test result functional tests were performed. Platelet activating antibodies were detected in 17 cases by Annexin V binding. PMP count analysis provided nearly identical results. The probability of a positive flow cytometric assay result was higher in patients with elevated antibody titer. 71% of patients with positive EIA and functional assay had thrombosis. CONCLUSIONS: EIA is an important first line laboratory test in the diagnosis of HIT; however, HIT must be confirmed by a functional test. Annexin V binding and PMP assays using flow cytometry are functional HIT tests convenient in a clinical diagnostic laboratory. The positive results of functional assays may predict the onset of thrombosis. © 2016 International Clinical Cytometry Society.

The Phosphatase Inhibitor Calyculin-A Impairs Clot Retraction, Platelet Activation, and Thrombin Generation.

The aim of this study was to investigate the effect of the serine/threonine protein phosphatase inhibitor, calyculin-A (CLA), on clot formation and on the procoagulant activity of human platelets. Platelet-rich plasma (PRP) samples were preincubated with buffer or CLA and subsequently platelets were activated by the protease-activated receptor 1 (PAR-1) activator, thrombin receptor activating peptide (TRAP). Clot retraction was detected by observing clot morphology up to 1 hour, phosphatidylserine- (PS-) expression was studied by flow cytometry, and thrombin generation was measured by a fluorimetric assay. For the intracellular Ca2+ assay, platelets were loaded with calcium-indicator dyes and the measurements were carried out using a ratiometric method with real-time confocal microscopy. CLA preincubation inhibited clot retraction, PS-expression, and thrombin formation. TRAP activation elicited Ca2+ response and PS-expression in a subset of platelets. The activated PRP displayed significantly faster and enhanced thrombin generation compared to nonactivated samples. CLA pretreatment abrogated PS-exposure and clot retraction also in TRAP-activated samples. As a consequence of the inhibitory effect on calcium elevation and PS-expression, CLA significantly downregulated thrombin generation in PRP. Our results show that CLA pretreatment may be a useful tool to investigate platelet activation mechanisms that contribute to clot formation and thrombin generation.

Hyperglycaemia suppresses microRNA expression in platelets to increase P2RY12 and SELP levels in type 2 diabetes mellitus.

Megakaryocyte (MK)-derived miRNAs have been detected in platelets. Here, we analysed the expression of platelet and circulating miR-223, miR-26b, miR-126 and miR-140 that might be altered with their target mRNAs in type 2 diabetes mellitus (DM2). MiRNAs were isolated from leukocyte-depleted platelets and plasma samples obtained from 28 obese DM2, 19 non-DM obese and 23 healthy individuals. The effect of hyperglycaemia on miRNAs was also evaluated in MKs using MEG-01 and K562 cells under hyperglycaemic conditions after 8 hours up to four weeks. Quantitation of mature miRNA, pre-miRNAs and target mRNA levels (P2RY12 and SELP) were measured by RT-qPCR. To prove the association of miR-26b and miR-140 with SELP (P-selectin) mRNA level, overexpression or inhibition of these miRNAs in MEG-01 MKs was performed using mimics or anti-miRNAs, respectively. The contribution of calpain substrate Dicer to modulation of miRNAs was studied by calpain inhibition. Platelet activation was evaluated via surface P-selectin by flow cytometry. Mature and pre-forms of investigated miRNAs were significantly reduced in DM2, and platelet P2RY12 and SELP mRNA levels were elevated by two-fold at increased platelet activation compared to controls. Significantly blunted miRNA expressions were observed by hyperglycaemia in MEG-01 and K562-MK cells versus baseline values, while the manipulation of miR-26b and miR-140 expression affected SELP mRNA level. Calpeptin pretreatment restored miRNA levels in hyperglycaemic MKs. Overall, miR-223, miR-26b, miR-126 and miR-140 are expressed at a lower level in platelets and MKs in DM2 causing upregulation of P2RY12 and SELP mRNAs that may contribute to adverse platelet function.