Platelet kinetics in idiopathic thrombocytopenic purpura (ITP) before and at different times after splenectomy

Immune attack on megakaryocytes in immune thrombocytopenia

A State of the Art lecture titled "Immune Attack on Megakaryocytes in ITP: The Role of Megakaryocyte Impairment" was presented at the International Society on Thrombosis and Haemostasis Congress in 2023. Immune thrombocytopenia (ITP) is an acquired autoimmune disorder caused by autoantibodies against platelet surface glycoproteins that provoke increased clearance of circulating platelets, leading to reduced platelet number. However, there is also evidence of a direct effect of antiplatelet autoantibodies on bone marrow megakaryocytes. Indeed, immunologic cells responsible for autoantibody production reside in the bone marrow; megakaryocytes progressively express during their maturation the same glycoproteins against which ITP autoantibodies are directed, and platelet autoantibodies have been detected in the bone marrow of patients with ITP. In vitro studies using ITP sera or monoclonal antibodies against platelet and megakaryocyte surface glycoproteins have shown an impairment of many steps of megakaryopoiesis and thrombopoiesis, such as megakaryocyte differentiation and maturation, migration from the osteoblastic to the vascular niche, adhesion to extracellular matrix proteins, and proplatelet formation, resulting in impaired and ectopic platelet production in the bone marrow and diminished platelet release in the bloodstream. Moreover, cytotoxic T cells may target bone marrow megakaryocytes, resulting in megakaryocyte destruction. Altogether, these findings suggest that antiplatelet autoantibodies and cellular immunity against bone marrow megakaryocytes may significantly contribute to thrombocytopenia in some patients with ITP. Finally, we summarize relevant new data on this topic presented during the 2023 International Society on Thrombosis and Haemostasis Congress. The complete unraveling of the mechanisms of immune attack-induced impairment of megakaryopoiesis and thrombopoiesis may open the way to new therapeutic approaches.

Platelet Kinetics in Idiopathic Thrombocytopenic Purpura Patients Treated with Thrombopoietin Receptor Agonists

AIM: Thrombopoietin receptor agonists (Tpo RA) increase platelet counts in the majority of chronic autoimmune thrombocytopenia (idiopathic thrombocytopenic purpura; ITP) patients. It is unknown whether this treatment may also improve platelet survival (PS) in these patients. METHODS: In order to determine platelet survival (PS), autologous platelets were labeled with (111)In oxine and retransfused in six patients under treatment with Tpo RA (romiplostim n = 3; eltrombopag n = 3). RESULTS: Stable platelet counts of greater than 100 × 10(3)/μl were observed in all 6 patients. Platelet survival was decreased in all cases (mean 2.10 days; range 0.13-3.73 days). No correlation was found between platelet count and PS. Similarly, there was no significant relationship between platelet turnover and platelet count. However, a high platelet turnover, exceeding 25 or three times the norm was observed in 2 patients who presented the lowest PS (0.13 or 0.83 days). Two patients had a moderately shortened PS (1.91 or 2.42 days), and, correspondingly, a moderately increased platelet turnover rate (63,072 or 72,872 platelets/μl/day). CONCLUSION: These results indicate that Tpo RA may not only overcompensate platelet destruction in ITP, but may interfere with other mechanisms, which, in some cases, results in a reduced platelet destruction rate.

Immune thrombocytopenia and pregnancy

Immune thrombocytopenia (ITP) is not infrequently encountered during reproductive years with an estimated incidence of 0.1-1 per 1000 pregnancies. An international consensus group recently re-defined ITP and outlined standardized response criteria and up-to-date investigation and management. The pathogenesis encompasses autoantibody platelet destruction and immune-mediated decreased platelet production. Maternal antibodies may cross the placenta and have the potential to cause fetal and/or neonatal thrombocytopenia. The diagnosis and subsequent management of ITP in pregnancy requires a multidisciplinary approach involving the midwife, obstetrician, haematologist and anaesthetist. Women with ITP diagnosed prior to pregnancy should receive preconception counselling to outline potential treatments and provide information regarding expected maternal and neonatal outcome. Management prior to 36 weeks aims to avoid treatment in the absence of bleeding and ensure an acceptable platelet count for planned procedures. At 34-36 weeks, women are generally reviewed to consider whether a tailored course of treatment is required in preparation for delivery. Further research is required to determine a suitable platelet count for neuraxial anaesthesia. The mode of delivery should be guided by obstetric indication. It is pertinent to consider both the risk of maternal bleeding and thrombosis in maternal ITP. The risk of neonatal intracranial haemorrhage in association with ITP is less than 1%. Postpartum a cord blood platelet count should be checked. Additional management is dependent upon the neonatal platelet count. Data collection using the new standardized terminology should provide robust comparable epidemiological data regarding ITP in pregnancy.

Pathogenesis of chronic immune thrombocytopenia: increased platelet destruction and/or decreased platelet production

Chronic immune thrombocytopenia (ITP) is a haematological disorder in which patients predominantly develop skin and mucosal bleeding. Early studies suggested ITP was primarily due to immune-mediated peripheral platelet destruction. However, increasing evidence indicates that an additional component of this disorder is immune-mediated decreased platelet production that cannot keep pace with platelet destruction. Evidence for increased platelet destruction is thrombocytopenia following ITP plasma infusions in normal subjects, in vitro platelet phagocytosis, and decreased platelet survivals in ITP patients that respond to therapies that prevent in vivo platelet phagocytosis; e.g., intravenous immunoglobulin G, anti-D, corticosteroids, and splenectomy. The cause of platelet destruction in most ITP patients appears to be autoantibody-mediated. However, cytotoxic T lymphocyte-mediated platelet (and possibly megakaryocyte) lysis, may also be important. Studies supporting suppressed platelet production include: reduced platelet turnover in over 80% of ITP patients, morphological evidence of megakaryocyte damage, autoantibody-induced suppression of in vitro megakaryocytopoiesis, and increased platelet counts in most ITP patients following treatment with thrombopoietin receptor agonists. This review summarizes data that indicates that the pathogenesis of chronic ITP may be due to both immune-mediated platelet destruction and/or suppressed platelet production. The relative importance of these two mechanisms undoubtedly varies among patients.

A dispersion model for cellular signal transduction cascades

PURPOSE

The purpose of this study was to evaluate the ability of the dispersion model to describe pharmacokinetic-pharmacodynamic data containing contributions from signal transduction cascades.

METHODS

The partial differential equations and appropriate boundary conditions describing the dispersion model for signal transduction were obtained. Explicit analytical solutions to the dispersion equation were not available, and a numerical approach was necessary. Solutions were obtained by numerical inversion of the output Laplace transform. Generalized least square fitting was used to obtain parameter estimates for a variety of experimental data sets.

RESULTS

The parameters of the dispersion model estimate the relative roles of diffusion, convection, and chemical reaction in signal transduction. The model is capable of describing messenger RNA and protein expression kinetics induced by drug action.

CONCLUSIONS

The dispersion model may find potential applications in pharmacokinetic-pharmacodynamic models involving delayed drug effects mediated by transcriptional changes.

Transit compartments versus gamma distribution function to model signal transduction processes in pharmacodynamics

Delayed effects for pharmacodynamic responses can be observed for many signal transduction processes. Three approaches are summarized in this report to describe such effects caused by cascading steps: stochastic process model, gamma distribution function, and transit compartment model. The gamma distribution function, a probability density function of the waiting time for the final step in a stochastic process model, is a function of time with two variables: number of compartments N, and the expected number of compartments occurring per unit time k. The parameter k is equal to 1/tau, where tau is the mean transit time in the stochastic process model. Effects of N and k on the gamma distribution function were examined. The transit compartment model can link the pharmacokinetic profile of the tested compound, receptor occupancy, and cascade steps for the signal transduction process. Time delays are described by numbers of steps, the mean transit time tau, and the amplification or suppression of the process as characterized by a power coefficient gamma. The effects of N, tau, and gamma on signal transduction profiles are shown. The gamma distribution function can be utilized to estimate N and k values when the final response profile is available, but it is less flexible than transit compartments when dose-response relationships, receptor dynamics, and efficiency of the transduction process are of concern. The transit compartment model is useful in pharmacokinetic/pharmacodynamic modeling to describe precursor/product relationships in signal transduction process.

Antiplatelet antibodies in childhood idiopathic thrombocytopenic purpura

Antiplatelet antibodies were shown by the Handin and Stossel method in the sera of all 103 patients with acute idiopathic thrombocytopenic purpura (ITP) and in 100 cases following recovery from it. These antibodies were also shown in the sera of all 46 patients with chronic ITP and 32 cases after recovery. The decrease in level of antiplatelet antibodies was significant in all these children following recovery (P less than 0.001 for acute ITP, P less than 0.05 for chronic ITP). Antiplatelet antibodies could be determined in 67 acute and 21 chronic ITP cases in thrombocytopenic phase and following recovery, which showed very significant decreases in levels in each case in a later period. Antiplatelet antibody levels corresponding to the thrombocytopenic phase and recovery in acute and chronic ITP were significantly higher than normal and thrombocytopenic control values (P less than 0.001 for each).

Non-parametric analysis of platelet lifespan

A model-independent and elementary method of analysis of platelet survival is proposed. The method is based on the finding that the mean and standard deviation of the platelet lifespan can be expressed in the terms of the slope at time 0 and the area under the empirical platelet survival curve. The method is tested using Monte-Carlo simulations and then applied to a set of clinical data.

Mechanisms of thrombocytopenia in chronic autoimmune thrombocytopenic purpura. Evidence of both impaired platelet production and increased platelet clearance

Mechanisms of thrombocytopenia were studied in 38 patients with mild to moderately severe chronic autoimmune thrombocytopenia (AITP). 51Cr and 111In-labeled autologous platelet turnover studies and in vitro analysis of committed megakaryocyte progenitors (CFU-Meg) were used as independent measures of platelet production. Autologous 111In-labeled platelet localization studies were performed to assess platelet clearance. Although there was no increase in the frequency of marrow CFU-Meg, a specific increase in the CFU-Meg [3H]TdR suicide rate was seen which was inversely correlated with the platelet count (P less than 0.001). Platelet turnover studies showed significant numbers of patients had inappropriate thrombopoietic responses to their reduced platelet counts. Platelet-associated antibody levels correlated inversely with platelet turnover suggesting that antiplatelet antibody impairs platelet production. The circulating platelet count was best predicted by an index relating platelet production (i.e., turnover) to the spleen-liver platelet clearance that correlated directly with platelet survival (P less than 0.001). In summary, both depressed platelet production and increased platelet clearance by the liver and spleen contribute to the thrombocytopenia of AITP.

Labeled cells in the investigation of hematologic disorders

Radiolabeling techniques for white cells, platelets, and erythrocytes are reviewed. The early studies using diisopropylfluoro-32P contributed to an understanding of the production and circulation of the blood elements, and 51Cr proved useful in localizing sites of cell migration or destruction. 111In-oxine has further improved the understanding of blood cell organ sequestration, and permitted combined kinetic and organ imaging studies. Radionuclide labels have been essential for the elucidation of various hematologic disorders, such as the neutropenias, thrombocytopenias, anemias, and polycythemia. Many new treatments, including monoclonal antibodies, have been evaluated with radionuclides.

Long-term follow-up study of 168 patients with immune thrombocytopenia. Implications for therapy

A total of 168 patients (90 adults, 78 children) with immune thrombocytopenia (ITP) and a median follow-up of 75 months were treated with a sequential regimen of corticosteroids (n = 125), splenectomy (n = 83) and immunosuppressives (n = 25). In 43 patients an observation period of a minimum of 1 month preceded the therapy. It is concluded that: (i) withholding therapy in the expectation of spontaneous recovery is at least in children justified in case of limited bleeding tendency; (ii) corticosteroids should be limited in adults to a maximum of 3 weeks and in children to a maximum of 6 weeks; (iii) if corticosteroids fail, splenectomy remains the therapy of choice, especially in patients aged less than or equal to 30 years, and should be performed within 6 months after diagnosis; (iv) after failure of the aforementioned forms of therapy, there is rarely a beneficial effect of treatment with azathioprine and vincristine; (v) no reaction to any form of therapy can be followed by an acceptable long-term outcome.

[Enzyme activities in platelets of different specific gravity in thrombocytosis of various etiology (author's transl)]

Platelets of patients with thrombocytosis following splenectomy, in chronic granulocytic leukaemia and in polycythaemia vera were separated into five fractions by centrifugation in discontinuous Ficoll density gradient. Platelet volume, content of protein and enzyme activities of lactic dehydrogenase, phosphoglycerate kinase and glyceraldehyde phosphate dehydrogenase were distinctly higher for the three groups in the heavy fraction IV compared with the light fraction I. With regard to the platelet volume, however, these differences were compensated almost completely like in the normal persons.

The pathogenesis of immune thrombocytopenic purpura

Chronic idiopathic thrombocytopenic purpura (ITP) is a human disease manifested by destructive thrombocytopenia due to a circulating antiplatelet antibody. The antibody is of IgG type and is produced primarily in the spleen and bone marrow. After binding of the antibody to a platelet-associated antigen, phagocytosis is triggered either via the Fc portion of the attached antibody or as a consequence of fixation of the third component of complement (C3). The spleen is the prime area of platelet destruction due to its unique milieu. The large intrasplenic platelet pool is subjected to high local antiplatelet antibody concentrations in an environment teeming with phagocytic cells. In addition, the stagnant blood-flow characteristics of the spleen allow ample time for antibody sensitization and phagocytosis. Similar circumstances may occur in the bone marrow. In patients with "severe" disease and high antibody titers, the liver also becomes an important area of platelet destruction. The nature of the platelet-associated antigen is presently unknown but may differ from patient to patient. A pathogenetic model is proposed on the basis of presently available data.

Platelet survival in childhood idiopathic thrombocytopenic purpura in remission

Mean platelet survival was significantly decreased (P less than 0.01) in three children with chronic and five children with acute ITP in remission. Sera from these patients sensitized normal platelets, as demonstrated by increased phagocytosis by autologous granulocytes. These results may suggest that the remission in some patients with ITP is obtained by compensatory platelet production.