Antiplatelet Antibodies Do Not Predict the Response to Intravenous Immunoglobulins during Immune Thrombocytopenia

Study of platelet kinetics in immune thrombocytopenia to predict splenectomy response

Despite the efficacy of splenectomy for chronic immune thrombocytopenia (ITP), its considerable failure rate and its possible related complications prove the need for further research into potential predictors of response. The platelet sequestration site determined by 111 In-labelled autologous platelet scintigraphy has been proposed to predict splenectomy outcome, but without standardisation in clinical practice. Here, we conducted a single-centre study by analysing a cohort of splenectomised patients with ITP in whom 111 In-scintigraphy was performed at La Paz University Hospital in Madrid to evaluate the predictive value of the platelet kinetic studies. We also studied other factors that could impact the splenectomy outcome, such as patient and platelet characteristics. A total of 51 patients were splenectomised, and 82.3% responded. The splenic sequestration pattern predicted a higher rate of complete response up to 12 months after splenectomy (p = 0.005), with 90% sensitivity and 77% specificity. Neither age, comorbidities, therapy lines nor previous response to them showed any association with response. Results from the platelet characteristics analysis revealed a significant loss of sialic acid in platelets from the non-responding patients compared with those who maintained a response (p = 0.0017). Our findings highlight the value of splenic sequestration as an independent predictor of splenectomy response.

The role of CD8+ T-cell clones in immune thrombocytopenia

Immune thrombocytopenia (ITP) is traditionally considered an antibody-mediated disease. However, a number of features suggest alternative mechanisms of platelet destruction. In this study, we use a multidimensional approach to explore the role of cytotoxic CD8+ T cells in ITP. We characterized patients with ITP and compared them with age-matched controls using immunophenotyping, next-generation sequencing of T-cell receptor (TCR) genes, single-cell RNA sequencing, and functional T-cell and platelet assays. We found that adults with chronic ITP have increased polyfunctional, terminally differentiated effector memory CD8+ T cells (CD45RA+CD62L-) expressing intracellular interferon gamma, tumor necrosis factor α, and granzyme B, defining them as TEMRA cells. These TEMRA cells expand when the platelet count falls and show no evidence of physiological exhaustion. Deep sequencing of the TCR showed expanded T-cell clones in patients with ITP. T-cell clones persisted over many years, were more prominent in patients with refractory disease, and expanded when the platelet count was low. Combined single-cell RNA and TCR sequencing of CD8+ T cells confirmed that the expanded clones are TEMRA cells. Using in vitro model systems, we show that CD8+ T cells from patients with ITP form aggregates with autologous platelets, release interferon gamma, and trigger platelet activation and apoptosis via the TCR-mediated release of cytotoxic granules. These findings of clonally expanded CD8+ T cells causing platelet activation and apoptosis provide an antibody-independent mechanism of platelet destruction, indicating that targeting specific T-cell clones could be a novel therapeutic approach for patients with refractory ITP.

How we treat primary immune thrombocytopenia in adults

Primary immune thrombocytopenia (ITP) is an immune-mediated bleeding disorder characterized by decreased platelet counts and an increased risk of bleeding. Multiple humoral and cellular immune abnormalities result in accelerated platelet destruction and suppressed platelet production in ITP. The diagnosis remains a clinical exclusion of other causes of thrombocytopenia. Treatment is not required except for patients with active bleeding, severe thrombocytopenia, or cases in need of invasive procedures. Corticosteroids, intravenous immunoglobulin, and anti-RhD immunoglobulin are the classical initial treatments for newly diagnosed ITP in adults, but these agents generally cannot induce a long-term response in most patients. Subsequent treatments for patients who fail the initial therapy include thrombopoietic agents, rituximab, fostamatinib, splenectomy, and several older immunosuppressive agents. Other potential therapeutic agents, such as inhibitors of Bruton's tyrosine kinase and neonatal Fc receptor, are currently under clinical evaluation. An optimized treatment strategy should aim at elevating the platelet counts to a safety level with minimal toxicity and improving patient health-related quality of life, and always needs to be tailored to the patients and disease phases. In this review, we address the concepts of adult ITP diagnosis and management and provide a comprehensive overview of current therapeutic strategies under general and specific situations.

Platelet variability index: a measure of platelet count fluctuations in patients with immune thrombocytopenia

Fluctuations in platelet count levels over time may help distinguish immune thrombocytopenia (ITP) from other causes of thrombocytopenia. We derived the platelet variability index (PVI) to capture both the fluctuations in platelet count measurements and the severity of the thrombocytopenia over time. Raw PVI values, ranging from negative (less severe thrombocytopenia and/or low fluctuations) to positive (more severe thrombocytopenia and/or high fluctuations) were converted to an ordinal PVI score, from 0 to 6. We evaluated the performance characteristics of the PVI score for consecutive adults with thrombocytopenia from the McMaster ITP Registry. We defined patients with definite ITP as those who achieved a platelet count response after treatment with intravenous immune globulin or high-dose corticosteroids and possible ITP as those who never received ITP treatment or did not respond to treatment. Of 841 patients with thrombocytopenia, 104 had definite ITP, 398 had possible ITP, and 339 had non-ITP thrombocytopenia. For patients with definite ITP, the median PVI score was 5 [interquartile range (IQR) 5, 6] for patients with possible ITP, the median PVI score was 3 (1, 5); and for patients with non-ITP thrombocytopenia, the median PVI score was 0 (0, 2). A high PVI score correlated with the diagnosis of definite ITP even when calculated at the patient's initial assessment, before any treatment had been administered. Platelet count fluctuations alone contributed to the specificity of the overall PVI score. The PVI score may help clinicians diagnose ITP among patients who present with thrombocytopenia for evaluation.

An update on the pathophysiology of immune thrombocytopenia

: Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder mediated by antiplatelet autoantibodies and antigen-specific T cells that either destroy platelets peripherally in the spleen or impair platelet production in the bone marrow. There have been a plethora of publications relating to the pathophysiology of ITP and since January of 2019, at least 50 papers have been published on ITP pathophysiology.

PURPOSE OF REVIEW

To summarize the literature relating to the pathophysiology of ITP including the working mechanisms of therapies, T-cell and B-cell physiology, protein/RNA/DNA biochemistry, and animal models in an attempt to unify the perceived abnormal immune processes.

RECENT FINDINGS

The most recent pathophysiologic irregularities associated with ITP relate to abnormal T-cell responses, particularly, defective T regulatory cell activity and how therapeutics can restore these responses. The robust literature on T cells in ITP points to the notion that ITP is a disease initiated by faulty self-tolerance mechanisms very much like that of other organ-specific autoimmune diseases. There is also a large literature on new and existing animal models of ITP and these will be discussed. It appears that understanding how to specifically modulate T cells in patients with ITP will undoubtedly lead to effective antigen-specific therapeutics.

CONCLUSIONS

ITP is predominately a T cell disorder which leads to a breakdown in self tolerance mechanisms and allows for the generation of anti-platelet autoantibodies and T cells. Novel therapeutics that target T cells may be the most effective way to perhaps cure this disorder.

Immune Thrombocytopenia: Recent Advances in Pathogenesis and Treatments

Immune thrombocytopenia (ITP) is a rare autoimmune disease due to both a peripheral destruction of platelets and an inappropriate bone marrow production. Although the primary triggering factors of ITP remain unknown, a loss of immune tolerance-mostly represented by a regulatory T-cell defect-allows T follicular helper cells to stimulate autoreactive splenic B cells that differentiate into antiplatelet antibody-producing plasma cells. Glycoprotein IIb/IIIa is the main target of antiplatelet antibodies leading to platelet phagocytosis by splenic macrophages, through interactions with Fc gamma receptors (FcγRs) and complement receptors. This allows macrophages to activate autoreactive T cells by their antigen-presenting functions. Moreover, the activation of the classical complement pathway participates to platelet opsonization and also to their destruction by complement-dependent cytotoxicity. Platelet destruction is also mediated by a FcγR-independent pathway, involving platelet desialylation that favors their binding to the Ashwell-Morell receptor and their clearance in the liver. Cytotoxic T cells also contribute to ITP pathogenesis by mediating cytotoxicity against megakaryocytes and peripheral platelets. The deficient megakaryopoiesis resulting from both the humoral and the cytotoxic immune responses is sustained by inappropriate levels of thrombopoietin, the major growth factor of megakaryocytes. The better understanding of ITP pathogenesis has provided important therapeutic advances. B cell-targeting therapies and thrombopoietin-receptor agonists (TPO-RAs) have been used for years. New emerging therapeutic strategies that inhibit FcγR signaling, the neonatal Fc receptor or the classical complement pathway, will deeply modify the management of ITP in the near future.

Emerging Therapies in Immune Thrombocytopenia

Immune thrombocytopenia (ITP) is a rare autoimmune disorder caused by peripheral platelet destruction and inappropriate bone marrow production. The management of ITP is based on the utilization of steroids, intravenous immunoglobulins, rituximab, thrombopoietin receptor agonists (TPO-RAs), immunosuppressants and splenectomy. Recent advances in the understanding of its pathogenesis have opened new fields of therapeutic interventions. The phagocytosis of platelets by splenic macrophages could be inhibited by spleen tyrosine kinase (Syk) or Bruton tyrosine kinase (BTK) inhibitors. The clearance of antiplatelet antibodies could be accelerated by blocking the neonatal Fc receptor (FcRn), while new strategies targeting B cells and/or plasma cells could improve the reduction of pathogenic autoantibodies. The inhibition of the classical complement pathway that participates in platelet destruction also represents a new target. Platelet desialylation has emerged as a new mechanism of platelet destruction in ITP, and the inhibition of neuraminidase could dampen this phenomenon. T cells that support the autoimmune B cell response also represent an interesting target. Beyond the inhibition of the autoimmune response, new TPO-RAs that stimulate platelet production have been developed. The upcoming challenges will be the determination of predictive factors of response to treatments at a patient scale to optimize their management.

Evolution and Utility of Antiplatelet Autoantibody Testing in Patients with Immune Thrombocytopenia

To this day, immune thrombocytopenia (ITP) remains a clinical diagnosis made by exclusion of other causes for thrombocytopenia. Reliable detection of platelet autoantibodies would support the clinical diagnosis, but the lack of specificity and sensitivity of the available methods for platelet autoantibody testing limits their value in the diagnostic workup of thrombocytopenia. The introduction of methods for glycoprotein-specific autoantibody detection has improved the specificity of testing and is acceptable for ruling in ITP but not ruling it out as a diagnosis. The sensitivity of these assays varies widely, even between studies using comparable assays. A review of the relevant literature combined with our own laboratory's experience of testing large number of serum and platelet samples makes it clear that this variation can be explained by variations in the characteristics of the tests, including in the glycoprotein-specific monoclonal antibodies, the glycoproteins that are tested, the platelet numbers used in the assay and the cutoff levels for positive and negative results, as well as differences in the tested patient populations. In our opinion, further standardization and optimization of the direct autoantibody detection methods to increase sensitivity without compromising specificity seem possible but will still likely be insufficient to distinguish the often very weak specific autoantibody signals from background signals. Further developments of autoantibody detection methods will therefore be necessary to increase sensitivity to a level acceptable to provide laboratory confirmation of a diagnosis of ITP.

[Immune thrombocytopenia: From pathogenesis to treatment]

Immune thrombocytopenia (ITP) is a rare autoimmune disease due to an immune peripheral destruction of platelets and an inappropriate platelet production. The pathogenesis of ITP is now better understood: it involves a humoral immune response which dependents on the stimulation of B cells by specific T cells called T follicular helper cells, leading to their differentiation into plasma cells that produce antiplatelet antibodies thus promoting the phagocytosis of platelets mainly by splenic macrophages. The deciphering of ITP pathogenesis has led to a better understanding of the inefficiency of treatments such as rituximab, although it has not provided yet the determination of biological predictive factor of response to treatments. Moreover, new therapeutic perspectives have been opened in the last few years with the development of molecules targeting Fcγ receptor signalling such as Syk inhibitor, or molecules increasing the clearance of pathogenic autoantibodies such as inhibitors of the neonatal Fc receptor (FcRn).