Thromboelastography clot strength profiles and effect of systemic anticoagulation in COVID-19 acute respiratory distress syndrome: a prospective, observational study

OBJECTIVE

Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV-2) infection may yield a hypercoagulable state with fibrinolysis impairment. We conducted a single-center observational study with the aim of analyzing the coagulation patterns of intensive care unit (ICU) COVID-19 patients with both standard laboratory and viscoelastic tests. The presence of coagulopathy at the onset of the infection and after seven days of systemic anticoagulant therapy was investigated.

PATIENTS AND METHODS

Forty consecutive SARS-CoV-2 patients, admitted to the ICU of a University hospital in Italy between 29th February and 30th March 2020 were enrolled in the study, providing they fulfilled the acute respiratory distress syndrome criteria. They received full-dose anticoagulation, including Enoxaparin 0.5 mg·kg-1 subcutaneously twice a day, unfractionated Heparin 7500 units subcutaneously three times daily, or low-intensity Heparin infusion. Thromboelastographic (TEG) and laboratory parameters were measured at admission and after seven days.

RESULTS

At baseline, patients showed elevated fibrinogen activity [rTEG-Ang 80.5° (78.7 to 81.5); TEG-ACT 78.5 sec (69.2 to 87.9)] and an increase in the maximum amplitude of clot strength [FF-MA 42.2 mm (30.9 to 49.2)]. No alterations in time of the enzymatic phase of coagulation [CKH-K and CKH-R, 1.1 min (0.85 to 1.3) and 6.6 min (5.2 to 7.5), respectively] were observed. Absent lysis of the clot at 30 minutes (LY30) was observed in all the studied population. Standard coagulation parameters were within the physiological range: [INR 1.09 (1.01 to 1.20), aPTT 34.5 sec (29.7 to 42.2), antithrombin 97.5% (89.5 to 115)]. However, plasma fibrinogen [512.5 mg·dl-1 (303.5 to 605)], and D-dimer levels [1752.5 ng·ml-1 (698.5 to 4434.5)], were persistently increased above the reference range. After seven days of full-dose anticoagulation, average TEG parameters were not different from baseline (rTEG-Ang p = 0.13, TEG-ACT p = 0.58, FF-MA p = 0.24, CK-R p = 0.19, CKH-R p  = 0.35), and a persistent increase in white blood cell count, platelet count and D-dimer was observed (white blood cell count p < 0.01, neutrophil count p = 0.02, lymphocyte count p < 0.01, platelet count p = 0.13 < 0.01, D-dimer levels p= 0.02).

CONCLUSIONS

SARS-CoV-2 patients with acute respiratory distress syndrome show elevated fibrinogen activity, high D-dimer levels and maximum amplitude of clot strength. Platelet count, fibrinogen, and standard coagulation tests do not indicate a disseminated intravascular coagulation. At seven days, thromboelastographic abnormalities persist despite full-dose anticoagulation.

Prevalence and significance of anti-prothrombin (aPT) antibodies in patients with Lupus Anticoagulant (LA)

OBJECTIVE

Anti-prothrombin (aPT) antibodies have been found in Lupus Anticoagulant (LA) positive patients. Their prevalence and relative contribution to thromboembolic risk in LA-positive patients is not well defined. The aim of this study was to determine their presence and association with thromboembolic events in a large series of patients with confirmed LA.

METHODS

Plasma from LA-positive patients was collected at Thrombosis Centers and sent to a reference central laboratory for confirmation. Positive plasma was tested using home-made ELISA for the presence of aPT and anti-beta(2)GPI antibodies.

RESULTS

LA was confirmed in 231 patients. Sixty-one of 231 (26%, 95%CI 22-33) LA positive subjects were positive for IgG aPT and 62 (27%, 95% CI 21-33) were positive for IgM aPT antibodies. Clinical features of Antiphospholipid Syndrome (APS) were not associated with the presence of IgG aPT [43 APS in 61 (70%) positive and 109 APS in 170 (64%) negative IgG aPT subjects, p=ns] or IgM aPT. Rate of positivity of IgG and IgM a beta(2)GPI was significantly higher than that of IgG and IgM aPT. Clinical events accounting for APS occurred in 97 of 130 (75%) IgG a beta(2)GPI positive and in 55 of 101 (54%) IgG a beta(2)GPI negative patients (OR 2.4, 95% CI 1.4 to 4.3, p=0.002). No significant association with clinical events in patients positive for both IgG aPT and IgG a beta(2)GPI as compared to those positive for one or another test was found. When patients negative for both IgG aPT and IgG a beta(2)GPI (LA positive only) were compared with remaining patients, a significantly lower association with clinical events was found (OR=0.4, 95% CI: 0.2 to 0.7, p=0.004).

CONCLUSIONS

As compared to IgG a beta(2)GPI, the prevalence of IgG aPT in patients with LA is significantly lower and not associated with the clinical features of APS.

Management of bleeding and of invasive procedures in patients with platelet disorders and/or thrombocytopenia: Guidelines of the Italian Society for Haemostasis and Thrombosis (SISET)

The optimal management of bleeding or its prophylaxis in patients with disorders of platelet count or function is controversial. The bleeding diathesis of these patients is usually mild to moderate: therefore, transfusion of platelet concentrates may be inappropriate, as potential adverse effects might outweigh its benefit. The availability of several anti-hemorrhagic drugs further compounds this problem, mainly because the efficacy/suitability of the various treatment options in different clinical manifestations is not well defined. In these guidelines, promoted by the Italian Society for Studies on Haemostasis and Thrombosis (Società Italiana per lo Studio dell'Emostasi e della Trombosi [SISET]), we aim at offering the best available evidence to help the physicians involved in the management of patients with disorders of platelet count or function. Literature review and appraisal of available evidence are discussed for different clinical settings and for different available treatments, including platelet concentrates (PC), recombinant activated factor VII, desmopressin, antifibrinolytics, aprotinin and local hemostatic agents.

Defective platelet responsiveness to thrombin and protease-activated receptors agonists in a novel case of gray platelet syndrome: correlation between the platelet defect and the alpha-granule content in the patient and four relatives

BACKGROUND

We report a novel case of gray platelet syndrome (GPS). A 14-year-old boy had bleeding diathesis, mild thrombocytopenia, giant platelets with severe defect of alpha-granule secretory proteins, myelofibrosis and splenomegaly.

METHODS AND RESULTS

Platelet function studies showed a marked reduction of aggregation and Ca(2+) mobilization by thrombin, protease-activated receptor 1 (PAR1)-activating peptide (AP) and PAR4-AP, PAR1 expression at 55% of normal levels, and a more than two hundred fold reduction of in vitro whole-blood thromboxane B(2) (TXB(2)) production. Sequencing of coding regions of the PAR1 gene failed to show abnormalities. This patient was initially classified as a sporadic case of GPS, as electron microscopy failed to identify giant platelets and/or alpha-granule deficiency in his relatives. However, further studies on the father and three other relatives showed a relative lack of platelet alpha-granule proteins by immunofluorescence microscopy, a defective platelet response to PAR4-AP, and severely reduced in vitro whole-blood TXB(2) production. On this basis, we suggest that in this family, GPS was transmitted in a dominant fashion with highly variable penetrance.

CONCLUSIONS

Our study suggests that current diagnostic criteria fail to identify some patients with a mild GPS phenotype and that such patients might be identified by the methods cited above. It also better characterizes the pathogenesis of defective platelet responses to thrombin, and raises interesting questions on the correlation between abnormal PAR function and the lack of alpha-granule content in GPS.

Structural and functional mapping of the thrombin domain involved in the binding to the platelet glycoprotein Ib

The activation of human platelets by alpha-thrombin is mediated in part by cleavage of the protease-activated receptor (PAR) 1 and 4 and by the glycoprotein (Ib)alpha, (Gp(Ib)alpha), which binds with high affinity to alpha-thrombin. Recent studies have shown that the thrombin domain referred to as heparin binding site (HBS) is involved in the interaction with the platelet Gp(Ib)alpha. The HBS is rich in basic amino acids. To identify the key amino acid residues involved in the binding to Gp(Ib)alpha, we have performed alanine scanning mutagenesis of the basic HBS R93, R97, R101, R233, K236, K240, R233/K236/Q239, as well as of the neutral Q239 residues, located in different regions of the domain. For comparison, mutation at R67 within the fibrinogen recognition site (FRS) of thrombin was performed as well. Solid-phase binding experiments showed that the Kd of thrombin-GpIb interaction was reduced 22-fold for R93A, 8-fold for R97A, 13-fold for R101A, 29-fold for R233A, 21-fold for K236A, 5-fold for K240A, and 31-fold for the triple mutant R233A/K236A/Q239A, while the Q239A and R67A forms did not show any significant affinity change. The platelet activating capacity of these mutants was evaluated as well. Using gel-filtered platelets, the EC50 value of thrombin-induced aggregation was from 5- to 13-fold higher in the HBS mutants than in the WT form, and was linearly and positively correlated with the corresponding Kd values pertaining to thrombin binding to GpIb. Measurements of PAR-1 hydrolysis on the platelet membrane showed that the HBS mutants R233A, R101A, R93A, K236A, and R233/K236/Q239 forms had a reduction of the apparent kcat/Km value. These results are a consequence of a defective binding to GpIb, which is known to optimize the interaction with PAR-1 in situ. A confirm of this hypothesis came from the demonstration that the kcat/Km value pertaining to the hydrolysis by the HBS-mutated thrombins of the synthetic PAR-1 38-60 peptide in solution was similar to that one obtained with the WT form. In conclusion, these experiments provide a structural and functional mapping of the thrombin HBS subregions involved in the binding to the platelet Gp(Ib)alpha and in the cell activation.

Binding of thrombin to glycoprotein Ib accelerates the hydrolysis of Par-1 on intact platelets

The activation of human platelets by alpha-thrombin is mediated at least in part by cleavage of protease-activated G-protein-coupled receptors, PAR-1 and PAR-4. Platelet glycoprotein Ibalpha also has a high affinity binding site for alpha-thrombin, and this interaction contributes to platelet activation through a still unknown mechanism. In the present study the hypothesis that GpIbalpha may contribute to platelet activation by modulating the hydrolysis of PAR-1 on the platelet membrane was investigated. Gel-filtered platelets from normal individuals were stimulated by alpha-thrombin, and the kinetics of PAR-1 hydrolysis by enzyme was followed with flow cytometry using an anti-PAR-1 monoclonal antibody (SPAN 12) that recognizes only intact PAR-1 molecules. This strategy allowed measurement of the apparent k(cat)/K(m) value for thrombin hydrolysis of PAR-1 on intact platelets, which was equal to 1.5 +/- 0.1 x 10(7) m(-1) sec(-1). The hydrolysis rate of PAR-1 by thrombin was measured under conditions in which thrombin binding to GpIb was inhibited by different strategies, with the following results. 1) Elimination of GpIbalpha on platelet membranes by mocarhagin treatment reduced the k(cat)/K(m) value by about 6-fold. 2) A monoclonal anti-GpIb antibody reduced the apparent k(cat)/K(m) value by about 5-fold. 3) An oligonucleotide DNA aptamer, HD22, which binds to the thrombin heparin-binding site (HBS) and inhibits thrombin interaction with GpIbalpha, reduced the apparent k(cat)/K(m) value by about 5-fold. 4) Displacement of alpha-thrombin from the binding site on GpIb using PPACK-thrombin reduced the apparent k(cat)/K(m) value by about 5-fold, and 5) mutation at the HBS of thrombin (R98A) caused a 5-fold reduction of the apparent k(cat)/K(m) value of PAR-1 hydrolysis. Altogether these results show that thrombin interaction with GpIb enhances the specificity of thrombin cleavage of PAR-1 on intact platelets, suggesting that GpIb may function as a "cofactor" for PAR-1 activation by thrombin.

Hepatocellular carcinoma complicating liver cirrhosis in type IIIa glycogen storage disease

Type III glycogen storage disease (GSD III) is an autosomal recessive disorder characterized by the accumulation of abnormal glycogen in the liver and, in most patients, in the muscle. Although liver fibrosis is a well-known consequence of GSD III, until now only eight cases of liver cirrhosis and two cases of hepatocellular carcinoma have been described in patients affected by this disease. In this case report, the authors describe the clinical history of a patient affected by GSD III who developed severe liver disease during her adult life, progressing from fibrosis to cirrhosis and finally to hepatocellular carcinoma. Until now, the hepatic involvement in GSD III has been considered by most authors as mild and almost always self-limiting. This report, together with the previously published cases, clearly indicates that severe and progressive liver disease may complicate this metabolic disorder. These observations advise a careful hepatologic follow-up of patients affected by GSD III.

The Asp(272)-Glu(282) region of platelet glycoprotein Ibalpha interacts with the heparin-binding site of alpha-thrombin and protects the enzyme from the heparin-catalyzed inhibition by antithrombin III

Platelet glycoprotein Ib (GpIb) mediates interaction with both von Willebrand factor and thrombin. Thrombin binds to GpIb via its heparin-binding site (HBS) (De Candia, E., De Cristofaro, R., De Marco, L., Mazzucato, M., Picozzi, M., and Landolfi, R. (1997) Thromb. Haemostasis 77, 735-740; De Cristofaro, R., De Candia, E., Croce, G., Morosetti, R., and Landolfi, R. (1998) Biochem. J. 332, 643-650). To identify the thrombin-binding domain on GpIbalpha, we examined the effect of GpIbalpha(1-282), a GpIbalpha fragment released by the cobra venom mocarhagin on the heparin-catalyzed rate of thrombin inhibition by antithrombin III (AT). GpIbalpha(1-282) inhibited the reaction in a dose-dependent and competitive fashion. In contrast, the GpIbalpha(1-271) fragment, produced by exposing GpIbalpha(1-282) to carboxypeptidase Y, had no effect on thrombin inhibition by the heparin-AT complex. Measurements of the apparent equilibrium constant of the GpIbalpha(1-282) binding to thrombin as a function of different salts (NaCl and tetramethyl-ammonium chloride) concentration (0.1-0.2 M) indicated a large salt dependence (Gamma(+/-) = -4.5), similar to that pertaining to the heparin binding to thrombin. The importance of thrombin HBS in its interaction with GpIbalpha was confirmed using DNA aptamers, which specifically bind to either HBS (HD22) or the fibrinogen recognition site of thrombin (HD1). HD22, but not HD1, inhibited thrombin binding to GpIbalpha(1-282). Furthermore, the proteolytic derivative gamma(T)-thrombin, which lacks the fibrinogen recognition site, binds to GpIbalpha via its intact HBS in a reaction that is inhibited by HD22. Neither alpha- nor gamma(T)-thrombin bound to GpIbalpha(1-271), suggesting that the Asp(272)-Glu(282) region of GpIbalpha may act as a "heparin-like" ligand for the thrombin HBS, thereby inhibiting heparin binding to thrombin. It was also demonstrated that intact platelets may dose-dependently inhibit the heparin-catalyzed thrombin inhibition by AT at enzyme concentrations <5 nM. Altogether, these findings show that thrombin HBS binds to the region of GpIbalpha involving the Asp(272)-Glu(282) segment, protecting the enzyme from the inactivation by the heparin-AT system.

Thrombin-induced platelet activation is inhibited by high- and low-molecular-weight heparin

BACKGROUND

Thrombin binds to platelet glycoprotein Ib (Gp Ib), and this interaction contributes to platelet activation. Thrombin ligation to Gp Ib was recently shown to be inhibited by heparin, thus raising the hypothesis, investigated in this article, that heparin might inhibit thrombin-induced platelet activation.

METHODS AND RESULTS

Aggregation of gel-filtered platelets by 1 nmol/L thrombin was reduced by both high-molecular-weight (MW) (14 500-Da) and low-MW (4500-Da) heparin, with IC50 values of 1.65+/-0.26 and 5.13+/-0.8 micromol/L, respectively. Homogeneous-MW fractions (16 000- to 13 000-Da range) were used to evaluate the heparin effect on intracytoplasmic calcium release by thrombin. Calcium mobilization by 1 nmol/L thrombin was reduced as a function of heparin concentration, and the inhibitory effect was correlated to the MW of heparin fractions (IC50 values were 1.9+/-0.39, 6.07+/-0.83, and 14. 8+/-0.43 micromol/L for 16 000-, 9000-, and 3000-Da heparin, respectively). Platelet aggregation and calcium mobilization by ADP and by the thrombin receptor-activating peptide were not affected by heparin. The activation of Gp Ib-depleted platelets by alpha-thrombin was not inhibited by heparin. Moreover, platelet stimulation by heparin binding site phosphopyridoxylated thrombin, which has a severe impairment of Gp Ib ligation, was not affected by heparin. Finally, heparin did not interfere with the hydrolysis by thrombin of the protease-activated receptor 1.

CONCLUSIONS

These results demonstrated that heparin, by inhibiting the thrombin-Gp Ib interaction, is able to interfere with thrombin-induced platelet activation. The extent of the inhibitory effect is directly related to the MW of heparin fractions.

Thrombin interaction with platelet GpIb: structural mapping and effects on platelet activation (review)

Platelet glycoprotein Ib (GpIb) is an integral platelet membrane glycoprotein which plays a major role in haemostasis, being involved in both von Willebrand factor (vWF) and alpha-thrombin high affinity binding. Such interactions contribute to the early adhesion of platelets to exposed subendothelium and to the process of platelet activation. Glycoprotein Ib belongs to the so called <leucine rich repeat> (LRR) family of proteins, characterized by a structural motif consisting of the presence of one or more tandem LRRs, flanked by conserved sequences. Several experimental strategies have recently documented the involvement of the thrombin domain referred to as 'heparin binding site' in the binding to GpIb. This review is aimed at reporting on the structural mapping of both alpha-thrombin and GpIb domains involved in such interaction and on possible roles of thrombin-GpIb binding on the mechanisms supporting the platelet activation.

Effect of high- and low-molecular-weight heparins on thrombin-thrombomodulin interaction and protein C activation

BACKGROUND

Thrombin-thrombomodulin (TM) interaction, which is critical for accelerating the protein C anticoagulant pathway, involves the heparin-like domain of TM. This study was aimed at investigating the possible effect of heparin on thrombin-TM binding and protein C activation.

METHODS AND RESULTS

The affinity of thrombin-TM interaction was studied by a functional method, based on the ability of thrombin-TM adduct to activate protein C, and by evaluation of the binding of thrombin to immobilized TM. Both experimental approaches showed that the affinity of thrombin-TM interaction was decreased by micromolar heparin concentrations. Heparin had no significant effect when a recombinant TM form, lacking the chondroitin sulfate moiety, was used. Furthermore, it was also shown that the inhibitory effect of heparin was directly proportional to the heparin molecular mass (molecular weight range, 3 to 16 kDa), which suggests that the effect was mediated by formation of electrostatic bonds between heparin and thrombin.

CONCLUSIONS

These results indicate that heparin at therapeutic concentrations reduces the affinity of thrombin for TM and the rate of protein C activation. The magnitude of this effect is proportionally linked to the molecular mass of heparin.

Thrombin interaction with platelet GpIB: role of the heparin binding domain

The platelet membrane glycoprotein Ib (GpIb) has a high affinity binding site for alpha-thrombin whose occupancy is thought to positively modulate the thrombin-induced platelet activation. In this study, aimed at further characterizing the thrombin-GpIb interaction, two thrombin anion exosites referred to as "heparin binding site" (HBS) and "fibrinogen recognition site" (FRS) were investigated as the possible domains involved in GpIb binding. The role of thrombin HBS was explored by performing binding measurements of 125I-alpha-thrombin to purified glycocalicin (GC), the extracytoplasmic portion of GpIb, in the presence of heparin as well as after chemical modifications of the thrombin heparin binding site (thrombin-HBS phosphopyridoxylation). These studies showed that a) thrombin binding to GC could be competitively inhibited by heparin and b) the equilibrium association constant for thrombin-GC interaction was reduced up to ten-fold by chemical modifications at the HBS. On the other hand, the role of FRS in the thrombin-GC interaction could be excluded by other experiments showing that GC in solution could not influence the interaction of alpha-thrombin with two substrates which bind to both the catalytic site and the fibrinogen recognition site: 1) the thrombin receptor peptide 38-60 (TR, L38-E60) and 2) the A alpha-chain of fibrinogen. Altogether these results demonstrated that GC interaction with thrombin involves the enzyme heparin binding site, whereas the fibrinogen recognition site does not play a significant role.

Thrombin-thrombomodulin interaction: energetics and potential role of water as an allosteric effector

The interaction of rabbit lung thrombomodulin (TM) and C-terminal hirudin 54-65 fragment (Hir54-65) with human alpha-thrombin were investigated by exploiting their competitive inhibition of thrombin-fibrinogen interaction. Measurements of Ki values for TM and Hir54-65 interactions with human alpha-thrombin performed over a temperature range spanning from 10 to 40 degrees C showed a constant enthalpy for both ligands. The enthalpic and entropic contributions to the free energy of binding, however, are different for TM and the hirudin peptide. The calculated values of delta H and delta S, in fact, were -47.3 +/- 2.51 kJ (-11.3 +/- 0.6 kcal)/mol and -42.7 +/- 7.9 J (-10.2 +/- 1.9 cal)/mol.K for the hirudin peptide, while being -22.9 +/- 2.09 kJ (-5.47 +/- 0.5 kcal)/mol and 102.50 +/- 6.69 J (24.5 +/- 1.6 cal)/mol.K respectively for TM binding. These findings indicate that the interaction between thrombin and Hir54-65 is largely driven by the enthalpic contribution, whereas the positive entropy change is the driving force for the formation of the thrombin-TM complex. In other experiments performed in the presence of various concentrations of either sorbitol or sucrose it could be demonstrated that the value of the equilibrium association constant for thrombin-TM interaction increases as a function of the osmotic pressure, while the thrombin-Hir54-65 interaction was not affected by the same conditions. Moreover, control experiments showed that no major conformational changes are produced on TM by osmotic pressures used in the present study. From these experiments it was calculated that roughly 35 water molecules are released into the bulk water upon TM binding. Such a phenomenon, which is likely to be responsible for the entropic change described above, indicates the relevance of hydration processes for the formation of the thrombin-TM adduct.

Conformational transitions linked to active site ligation in human thrombin: effect on the interaction with fibrinogen and the cleavable platelet receptor

An experimental strategy based on solution viscosity perturbation allowed us to study the energetics of amide-substrates, p-aminobenzamidine (p-ABZ) and proflavin binding to the catalytic site of two proteolyzed forms of alpha-thrombin, i.e. zeta- and gamma T-thrombin. These thrombin derivatives are cleaved at the Leu144-Gly150 loop and at the fibrinogen recognition exosite (FRS), respectively. A phenomenological analysis of thermodynamic data showed that the amide substrates and p-ABZ interactions with zeta-thrombin were respectively, associated with a chemical compensation (i.e. the linear relationship between entropy and enthalpy of binding) and a hydrophobic phenomenon (i.e. a change in the standard heat capacity). The latter was slightly lower than that previously observed for a alpha-thrombin (0.78 +/- 0.25 versus 1.01 +/- 0.17 kcal/mol K). Both phenomenon were absent in gamma T-thrombin. The interaction of a alpha-, zeta- and gamma T-thrombin with macromolecular substrates that "bridge-bind" to both the catalytic site (CS) and fibrinogen recognition exosite (FRS), such as fibrinogen and the cleavable platelet receptor (CPR), was also evaluated. These interactions were studied by following fibrinopeptide A (FpA) release and by measuring intraplatelet Ca2+ changes induced by thrombin-CPR interaction. It was found that the free energy of activation (RT ln Kcat/Km) for both fibrinogen and CPR hydrolysis followed the same hierarchy, i.e. alpha > zeta > gamma. Moreover, the values of delta Cp for alpha-, zeta- and gamma T-thrombin interaction with p-ABZ were found to be linearly correlated to the free energy of activation for both fibrinogen and CPR cleavage. In conclusion, these data demonstrate that: (1) the Leu144-Gly150 loop and the FRS are both involved in the conformational transition linked to the binding of p-aminobenzamidine to the thrombin active site; (2) the extent of thrombin's capacity to undergo conformational transitions in alpha-, zeta- and gamma T forms is positively correlated to the free energy of activation for hydrolysis of macromolecular substrates interacting with both the catalytic domain and the FRS.

Effects of unfractionated and low molecular weight heparins on platelet thromboxane biosynthesis "in vivo"

Several "in vitro" and "in vivo" studies indicate that heparin administration may affect platelet function. In this study we investigated the effects of prophylactic heparin on thromboxane (Tx)A2 biosynthesis "in vivo", as assessed by the urinary excretion of major enzymatic metabolites 11-dehydro-TxB2 and 2,3-dinor-TxB2. Twenty-four patients who were candidates for cholecystectomy because of uncomplicated lithiasis were randomly assigned to receive placebo, unfractionated heparin, low molecular weight heparin or unfractionated heparin plus 100 mg aspirin. Measurements of daily excretion of Tx metabolites were performed before and during the treatment. In the groups assigned to placebo and to low molecular weight heparin there was no statistically significant modification of Tx metabolite excretion while patients receiving unfractionated heparin had a significant increase of both metabolites (11-dehydro-TxB2: 3844 +/- 1388 vs 2092 +/- 777, p < 0.05; 2,3-dinor-TxB2: 2737 +/- 808 vs 1535 +/- 771 pg/mg creatinine, p < 0.05). In patients randomized to receive low-dose aspirin plus unfractionated heparin the excretion of the two metabolites was largely suppressed thus suggesting that platelets are the primary source of enhanced thromboxane biosynthesis associated with heparin administration. These data indicate that unfractionated heparin causes platelet activation "in vivo" and suggest that the use of low molecular weight heparin may avoid this complication.

Lymph node blast crisis in chronic myeloid leukemia mimicking T-immunoblastic lymphoma

BACKGROUND

Chronic myeloid leukemia arises from a somatic mutation in a pluripotent stem cell. It generally terminates with a blastic crisis (BC). One third of BC are lymphoid, and most have a pre-B phenotype. Few cases of T-lymphoid BC have been reported. Here we describe a lymph node blast crisis mimicking T-immunoblastic lymphoma.

METHODS

Bone marrow and lymph nodes were histologically examined by standard methods and by an immunoperoxidase technique. Cytogenetic studies were also performed on lymph node and blood cells. Analysis of T-cell receptor genes and BCR rearrangements were performed on DNA extracted from both frozen bone marrow and lymph-node cells.

RESULTS

Lymph-node histology showed an infiltration by large lymphoid blasts, consistent with a diagnosis of immunoblastic lymphoma. Blast cells were CD2, CD7, TDT positive, and negative for myeloid and mature lymphoid antigens. The Ph1 chromosome was found in both bone marrow and lymph-node cells. BCR rearrangement was found in the DNA from both bone marrow and lymph-node cells. TCR genes were not rearranged.

DISCUSSION

The present study provides strong evidence that the lymph-node blast crisis of CML can assume the morphological appearance of immunoblastic lymphoma and may retain the immunological phenotype and genetic features of early T cells with BCR rearrangements.

Effect of fibrinogen concentration on the velocity of platelet aggregation

The relationship between fibrinogen binding to its receptor and platelet aggregation has been investigated by comparing 125I-fibrinogen binding and aggregation velocities of gel-filtered platelets in the presence of adenosine diphosphate (ADP). Aggregometric responses at various fibrinogen concentrations are found to be bell-shaped and show a maximum at fibrinogen concentrations (Fmax) similar to the 125I-fibrinogen hemisaturating doses. At higher and lower fibrinogen concentrations, platelet aggregation velocities decrease in a parallel manner. Lowering ADP concentration increases Fmax, in agreement with the modulatory effect of ADP on fibrinogen binding to platelets. Variations of fibrinogen in the range of physiopathologic plasma concentrations affect platelet aggregation induced by any ADP dose. These results clarify the relationship between the fibrinogen binding process and aggregation and demonstrate that plasma fibrinogen concentration has a major influence on the velocity of platelet aggregation.

Inhibition of fibrinogen binding to human platelets by blockage of Na+/H+ exchange

Binding of ADP to platelets enhances the binding of fibrinogen to Gp IIb-IIIa, the specific platelet receptor for adhesive proteins. The linkage between ADP and fibrinogen binding is indirect since ADP does not bind to the same receptor as fibrinogen. We have recently proposed that a third component, once affected by ADP binding, induces a conformational transition of the fibrinogen receptor from the low to the high affinity state, which is responsible for platelet aggregation [De Cristofaro, R., Landolfi, R., Castagnola, M., De Candia, E., Di Cera, E., & Wyman, J. (1988) Proc. Natl. Acad. Sci. USA 85, 8473-8476]. In the present study we provide evidence that this component should be identified with the platelet Na+/H+ antiport. Inhibition of the antiport by pharmacological agents such as amiloride, or else by decreasing extracellular Na+, results in a marked decrease of fibrinogen binding to platelets.

Meningeal hematopoiesis causing exophthalmus and hemiparesis in myelofibrosis: effect of radiotherapy. A case report

Meningeal myeloid metaplasia (MM) is very rarely observed in patients with myelofibrosis. We report the occurrence of meningeal MM causing exophthalmus and fever in a patient with myelofibrosis secondary to polycythemia vera. A computerized tomography (CT) scan showed multiple intracranial and intraorbital enhancing masses. A needle aspirate of retrobulbar space confirmed the diagnosis of extramedullary hematopoiesis. The patient subsequently developed a rapidly worsening tumor-like syndrome with hemiparesis, aphasia, and loss of sphinteric control. The administration of radiotherapy caused a complete and stable regression of clinical symptoms and a marked reduction of MM masses.

Allosteric equilibria in the binding of fibrinogen to platelets

The binding of fibrinogen to platelets occurs according to the law of mass action. The platelet receptor binds reversibly a single fibrinogen molecule and undergoes a conformational transition between two allosteric states, T and R, that differ in their affinity for fibrinogen. The equilibrium between the two forms is shifted by ADP toward the R (high-affinity) state, thus promoting the aggregation process. This model opens the way to consideration of allosteric modulation of the binding of fibrinogen to its platelet receptor.

Increased platelet-fibrinogen affinity in patients with myeloproliferative disorders

Patients with myeloproliferative disorders (MPD) are known to have some abnormalities of platelet glycoproteins (Gp). Quantitative changes of the Gp Ib, IIb-IIIa, and/or their glucidic content have been reported. Since the Gp IIb-IIIa complex plays a major role in fibrinogen binding by activated platelets, we measured the platelet fibrinogen affinity in nine patients with polycythemia vera (PV) and one subject with chronic myeloid leukemia (CML) by the aggregometric method of Marguerie. In all patients the Kd of the platelet fibrinogen reaction was significantly decreased as compared to controls, with evidence in two cases with PV of a heterogeneity of platelet-fibrinogen receptor sites. The measurement of 125I-labeled fibrinogen-platelet binding, performed in seven patients (five PV and two CML), showed receptor populations with increased (Kd1 = 0.58 + 0.3 X 10(7) mol/L) and normal affinity (Kd2 = 5.12 + 3.1 X 10(7) mol/L). These results demonstrate a heterogeneity of platelet-fibrinogen receptors in these patients and may explain the thrombotic diathesis of MPD subjects.