Suppression of in vitro megakaryocyte production by antiplatelet autoantibodies from adult patients with chronic ITP

Evaluation of platelet kinetics in patients with liver cirrhosis: similarity to idiopathic thrombocytopenic purpura

BACKGROUND

Thrombocytopenia is a common manifestation of liver cirrhosis (LC), but its underlying mechanism is not fully understood. The purpose of the present paper was to evaluate the platelet kinetics in LC patients by examining several non-invasive convenient markers.

METHODS

Fifty-seven LC patients, 32 patients with idiopathic thrombocytopenic purpura (ITP), 12 with aplastic anemia (AA), and 29 healthy individuals were studied. Plasma thrombopoietin was measured by enzyme-linked immunosorbent assay. Absolute reticulated platelet (RP) count and plasma glycocalicin were used as indices for thrombopoiesis, and the indices for platelet turnover were the RP proportion and the plasma glycocalicin normalized to the individual platelet count (GCI).

RESULTS

There was no difference in thrombopoietin levels between LC patients and healthy controls. The RP proportion and GCI were significantly higher and the absolute RP count and glycocalicin significantly lower in LC patients than in healthy controls. These markers in ITP and LC patients were comparable, but significantly different from those in AA patients. The bone marrow megakaryocyte density in LC and ITP patients was similar, and significantly higher than in AA patients.

CONCLUSIONS

Cirrhotic thrombocytopenia is a multifactorial condition involving accelerated platelet turnover and moderately impaired thrombopoiesis. Thrombopoietin deficiency is unlikely to be the primary contributor to cirrhotic thrombocytopenia.

Screening for platelet antibodies in adult idiopathic thrombocytopenic purpura: a comparative study using solid phase red cell adherence assay and flow cytometry

BACKGROUND

Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disorder caused by antiplatelet autoantibodies. In this study, we compared 2 methods for screening serum platelet antibodies in patients with ITP.

METHODS

A total of 44 adult patients were clinically classified with ITP. We used 2 indirect tests to detect human leukocyte antigen antibodies and/or platelet-specific antibodies in their sera. In method I, we used solid phase red cell adherence (SPRCA) assay. In method II, by flow cytometry, platelets from plateletpheresis components were used as target cells, and fluorescein isothiocyanate-conjugated sheep anti-human IgG Fc was used as the staining reagent. Positive results were defined as any test with the percentage of fluorescence exceeding the reference range by 3% or more in method II. Direct tests detecting platelet-associated IgG on platelets of patients with ITP were done by flow cytometry.

RESULTS

Serum specimens from 44 adult patients with ITP (28 female, 16 male) were tested. SPRCA assay could only detect platelet antibodies in 22 patients (50%). By method II, 31 serum specimens (70.5%) yielded positive results. There was a difference between the results of the SPRCA test and method II, with a high degree of significance (p < 0.001) by the McNemar test. No significant difference in platelet counts was observed for patients with and without discernible platelet antibodies by SPRCA assay (p = 0.90). The direct test was positive in 12 patients (66.7%) out of 18 ITP patients tested.

CONCLUSION

Flow cytometry is more sensitive than SPRCA assay for detecting platelet antibodies. Detection of platelet antibodies is useful in explaining the immune mechanism and platelet transfusion refractoriness in ITP.

AMG 531, a thrombopoiesis-stimulating protein, for chronic ITP

BACKGROUND

Most current treatments for chronic immune thrombocytopenic purpura (ITP) act by decreasing platelet destruction. In a phase 1-2 study, we administered a thrombopoiesis-stimulating protein, AMG 531, to patients with ITP.

METHODS

In phase 1, 24 patients who had received at least one treatment for ITP were assigned to escalating-dose cohorts of 4 patients each and given two identical doses of AMG 531 (0.2 to 10 microg per kilogram of body weight). In phase 2, 21 patients were randomly assigned to receive six weekly subcutaneous injections of AMG 531 (1, 3, or 6 microg per kilogram) or placebo. The primary objective was to assess the safety of AMG 531; the secondary objective was to evaluate platelet counts during and after treatment.

RESULTS

No major adverse events that could be attributed directly to AMG 531 occurred during the treatment period; 4 of 41 patients had transient post-treatment worsening of thrombocytopenia. In phase 1, a platelet count that was within the targeted range (50,000 to 450,000 per cubic millimeter) and at least twice the baseline count was achieved in 4 of 12 patients given 3, 6, or 10 mug of AMG 531 per kilogram. Overall, a platelet count of at least 50,000 per cubic millimeter was achieved in 7 of 12 patients, including 3 with counts exceeding 450,000 per cubic millimeter. Increases in the platelet count were dose-dependent; mean peak counts were 163,000, 309,000, and 746,000 per cubic millimeter with 3, 6, and 10 microg of AMG 531 per kilogram [corrected], respectively. In phase 2, the targeted platelet range was achieved in 10 of 16 patients treated with 1 or 3 mug of AMG 531 per kilogram per week for 6 weeks. Mean peak counts were 135,000, 241,000, and 81,000 per cubic millimeter in the groups that received the 1-mug dose, the 3-mug dose, and placebo, respectively.

CONCLUSIONS

AMG 531 caused no major adverse events and increased platelet counts in patients with ITP. (ClinicalTrials.gov number, NCT00111475 [ClinicalTrials.gov].).

Some unsettled questions in childhood thrombocytopenia caused by immunologic platelet destruction (acute and chronic ITP)

The cause of idiopathic thrombocytopenia (ITP) is largely unknown, although the underlying pathophysology is an autoimmune process. Anti-idiotypic antibodies and their role on regulatory T-cells might play an important role in the switch from acute to chronic ITP. The exact interaction remains to be elucidated. The effects of the dysregulated immune system and of the autoimmune process on thrombocytopoiesis and megakaryopiesis are ill defined. Therapy of acute and chronic ITP is directed to the risk of bleeding.

Megakaryocytic dysfunction in myelodysplastic syndromes and idiopathic thrombocytopenic purpura is in part due to different forms of cell death

Platelet production requires compartmentalized caspase activation within megakaryocytes. This eventually results in platelet release in conjunction with apoptosis of the remaining megakaryocyte. Recent studies have indicated that in low-risk myelodysplastic syndromes (MDS) and idiopathic thrombocytopenic purpura (ITP), premature cell death of megakaryocytes may contribute to thrombocytopenia. Different cell death patterns have been identified in megakaryocytes in these disorders. Growing evidence suggests that, besides apoptosis, necrosis and autophagic cell death, may also be programmed. Therefore, programmed cell death (PCD) can be classified in apoptosis, a caspase-dependent process, apoptosis-like, autophagic and necrosis-like PCD, which are predominantly caspase-independent processes. In MDS, megakaryocytes show features of necrosis-like PCD, whereas ITP megakaryocytes demonstrate predominantly characteristics of apoptosis-like PCD (para-apoptosis). Triggers for these death pathways are largely unknown. In MDS, the interaction of Fas/Fas-ligand might be of importance, whereas in ITP antiplatelet autoantibodies recognizing common antigens on megakaryocytes and platelets might be involved. These findings illustrate that cellular death pathways in megakaryocytes are recruited in both physiological and pathological settings, and that different forms of cell death can occur in the same cell depending on the stimulus and the cellular context. Elucidation of the underlying mechanisms might lead to novel therapeutic interventions.

Management of patients with refractory immune thrombocytopenic purpura

In immune thrombocytopenic purpura (ITP), thrombocytopenia is a result of both increased platelet destruction and insufficient platelet production. In adults, the course is commonly chronic, but most patients never experience serious bleeding even with severe thrombocytopenia. In case series of consecutive adult patients identified at the time of diagnosis, the frequency of death from bleeding is low, < 1%. The goal of treatment is only to prevent bleeding, not to correct the platelet count to normal. All current treatments are designed to diminish the increased platelet destruction, either by immunosuppression or splenectomy. The frequency of death from complications of treatment is similar to the frequency of death from bleeding. Perhaps because of increasing recognition of both the infrequent occurrence of serious bleeding and the risks of immunosuppressive treatment and splenectomy, data from case series across the past 30 years suggest a trend toward less therapy and fewer splenectomies among patients with ITP. However treatment is necessary for patients with severe and symptomatic thrombocytopenia. Splenectomy remains the most effective treatment for ITP, with two-thirds of patients achieving durable complete remissions. Immunosuppressive agents, including rituximab and combinations of agents, may be less effective than splenectomy in achieving complete remissions and the remissions may also be less durable. New agents for patients with ITP are currently in development that enhance platelet production, rather than diminish platelet destruction. In preliminary reports, these agents have been effective in maintaining safe platelet counts in patients with chronic ITP that was refractory to splenectomy and other treatments.

The pathogenesis of immune thrombocytopaenic purpura

Immune thrombocytopaenic purpura (ITP) is an autoimmune bleeding disease that is rarely fatal. However, in many adults treatment is unsatisfactory, with as much morbidity from the immunosuppressive effects of treatment as from bleeding. Identifying the underlying disease process should help us to identify more targeted therapies and improve not only the treatment but also the quality of life of patients with this disorder.

Cell-mediated lysis of autologous platelets in chronic idiopathic thrombocytopenic purpura

OBJECTIVES

Investigate the contribution and mechanism of cell-mediated cytotoxicity to the pathogenesis of idiopathic thrombocytopenic purpura (ITP).

METHODS

We observed the cytotoxic effect of cytotoxic T-lymphocyte (CTL) (CD8+) and natural killer cells (CD3- CD16+ CD56+) toward chronic ITP patient's autologous platelets, and investigated the expression of Fas ligand (FasL), tumor necrosis factor (TNF)-alpha and TNF-related apoptosis inducing ligand, as well as perforin and granzyme B mRNA in CD8+ cells using flow cytometry and reverse transcriptase-polymerase chain reaction.

RESULTS

We found that platelet lysis was seen only using purified CD8+ T cells as effector cells; expression of FasL and TNF-alpha in CD8+ T cells in ITP group was elevated. Moreover, the mRNA levels of granzyme B and perforin in CD8+ cells of ITP patients were increased.

CONCLUSIONS

Our findings suggest that CTLs are activated in chronic ITP and might be involved in the pathogenesis of this disorder. Apoptosis and perforin/granzyme-mediated cytotoxicity constitute an important pathway through which CTLs destruct autologous platelets. CTLs might be a reasonable target for a therapeutic strategy.

ITP in the 21st century

Immune (or idiopathic) thrombocytopenic purpura (ITP) is commonly encountered by the practicing hematologist. Clinical management decisions have traditionally been guided by individual training and past experience. Input from the literature has been more from observational reports of case series than from scientific results of hypothesis-driven research. Practice guidelines and several surveys of clinical hematology practice have highlighted important questions in the field, and in the past 5 to 10 years both clinical and laboratory investigations have produced valuable new information. Thrombopoietin levels are normal or only slightly increased in ITP, and stimulation of thrombopoiesis appears to be a promising new therapeutic approach in clinical trials. Chronic, refractory ITP in children or adults remains a challenge for the hematologist. It is this group that has the greatest risk of serious bleeding, particularly among the elderly. The anti-B-cell monoclonal antibody, anti-CD20, has shown benefit in phase I/II clinical trials in patients who had failed a number of previous therapeutic modalities. The standard for clinical research into therapy for ITP has become evidence-based medicine, and more prospective, randomized clinical trials are being completed by multi-institutional study groups.

Immune Thrombocytopenic Purpura of Childhood

Immune mediated thrombocytopenia (ITP) is a common manifestation of autoimmune disease in children. Although patients often present with bruises, petechiae, and some mucosal bleeding, the incidence of life-threatening hemorrhage is rare (0.2–0.9%) but can be fatal when presenting in vital organs. A wide range of therapeutic regimens are currently in use, including observation alone, as the majority of children recover within 4–6 months regardless of treatment. A growing understanding of the pathophysiology of acute ITP in children has not impacted the controversy surrounding treatment, but has clarified the mechanism of action of the most frequently used agents in chronic ITP. Newer monoclonal antibodies such as Rituxan have proved very useful in chronic or refractory ITP and studies are ongoing to determine the best regimens using this form of immune modulation. Splenectomy and newer agents to boost platelet production are also under study in chronic ITP. Neonates may also have a form of immune thrombocytopenia with extensive bruising and thrombocytopenia called neonatal alloimmune thrombocytopenic purpura (NATP). Rather than autoantibodies, the platelet destruction is secondary to transplacental maternal IgG alloantibodies. During pregnancy mothers may become sensitized to platelet membrane antigens present on fetal platelets. These antibodies may result in serious bleeding, including intracranial hemorrhage in the perinatal period. Once identified, these mothers may require treatment during future pregnancies to minimize serious bleeding in the fetus and neonate. Treatment in utero and immediately following delivery is focused on restoring neonatal platelets to a safe level and preventing life-threatening bleeding.

Thrombocytopenic conditions-autoimmunity and hypercoagulability: commonalities and differences in ITP, TTP, HIT, and APS

Immune thrombocytopenia purpura (ITP), thrombotic thrombocytopenia purpura (TTP), heparin-induced thrombocytopenia (HIT), and antiphospholipid syndrome (APS) are clinical conditions associated with significant morbidity and mortality. These well-defined clinical syndromes have in common several properties: (1) their pathogenesis is immune mediated, specifically by autoantibodies; (2) thrombocytopenia is a hallmark in these four conditions; (3) except for the case of ITP, platelet and endothelial cell activation occurs in TTP, HIT, and APS, resulting in a prothrombotic state and an increased risk of thrombosis. Although these four immune-mediated syndromes are well-defined diseases, several case reports and studies have documented the association of two diseases in the same patient, illustrating the concept of the kaleidoscope of autoimmunity.

Autoimmune thrombocytopenia

Idiopathic thrombocytopenic purpura (ITP) is an autoimmune disorder in which platelets coated with mainly antibodies against platelet GPIIb/IIIa and GPIb/IX are destroyed in the spleen. Recent evidence suggests that platelets are also destroyed by cytotoxic T cells. The diagnosis is made by exclusion for other causes of thrombocytopenia. As routine blood counts are becoming more available, many mild cases of ITP (platelets >30 x 10(9) L(-1)) are being diagnosed and they usually do not require treatment. In patients with platelet counts persistently <30 x 10(9) L(-1), treatment with corticosteroids, and/or intravenous immunoglobulin (IVIG) or anti-D may be required. The primary goal of treatment is to maintain the platelet count at a safe level with minimal side effects. After 3-6 months, if spontaneous remission has not occurred and if the side effects are significant, splenectomy is recommended. This is the single most effective treatment of ITP. The refractory patients who fails splenectomy and subsequently first- and second-line therapies, is a management dilemma. Therapeutic options are limited, available treatments potentially toxic and the chances of sustained response low. Observation with no active treatment is a reasonable option. With the increased availability of the thrombopoietic agents in the future, there may be a good prospect of keeping the platelet counts of these refractory patients at a safe long-term level with one of these drugs.

Multi-dysfunctional pathophysiology in ITP

Idiopathic thrombocytopenic purpura (ITP) is an organ-specific autoimmune disorder characterized by a low platelet count and mucocutaneous bleeding. The decrease of platelets is caused by increased autoantibodies against self-antigens, particularly IgG antibodies against GPIIb/IIIa. The production of these autoantibodies by B cells depends on a number of cellular mechanisms that form a network of modulation, with T cells playing a pivotal role in pathophysiology. Delineation of the dysfunction of cellular immunity has recently been attempted. This review will focus on these recent advances applicable to ITP and to highlight how these may translate into novel approaches to treatment in the future. Multi-dysfunction in these networks may include a failure of self-antigen recognition and tolerance, involvement of abnormal cell surface molecules, altered Th1/Th2 cytokine profiles, impaired megakaryocytopoiesis and impaired cell-mediated cytotoxicity. In ITP, multi-step dysfunctions in these networks may take place that finally lead to the occurrence of the disease. Therefore, unveiling these dysfunctions is vital in understanding the pathophysiology of ITP and will finally lead to the development of new therapies to fight the disease.

The Effect of Antiplatelet Autoantibodies on Megakaryocytopoiesis

Immune thrombocytopenic purpura (ITP) is a disorder manifested by isolated thrombocytopenia. In vivo infusion studies in the 1950s and 1960s provided evidence that the thrombocytopenia was due to autoantibody-induced platelet destruction. However, there is mounting evidence that platelet production in this disorder may also be suppressed by antibodies. Early morphologic studies showed megakaryocytic damage in ITP, and these results have been confirmed by ultrastructural studies. Autologous platelet turnover studies in the 1980s showed that most ITP patients have either normal or reduced platelet turnover rather than increased turnover, as would be expected if platelet destruction were the only pathogenetic mechanism. More recently, in vitro culture studies of both adult and pediatric ITP have shown that some ITP plasmas suppress both megakaryocytopoiesis and thrombopoiesis. In view of these findings, both platelet destruction and suppression of platelet production seem likely to be involved in the pathogenesis of ITP.

Management of Adult Idiopathic Thrombocytopenic Purpura

Idiopathic thrombocytopenic purpura (ITP) is a common hematologic disorder manifested by immune-mediated thrombocytopenia. The diagnosis remains one of exclusion, after other thrombocytopenic disorders are ruled out based on history, physical examination, and laboratory evaluation. The goal of treatment is to raise the platelet count into a hemostatically safe range. The disorder is usually chronic, although there is considerable variation in the clinical course and most patients eventually attain safe platelet counts off treatment. However, a subset of patients has severe disease refractory to all treatment modalities, which is associated with considerable morbidity and mortality. This article focuses on the management of primary ITP in adults. We discuss criteria for treatment, the roles of splenectomy and other treatment options along with their side effects, and the management of ITP during pregnancy.

Long-term outcomes in adults with chronic ITP after splenectomy failure

Adult chronic immune thrombocytopenic purpura (ITP) is an autoimmune disorder manifested by thrombocytopenia from the effects of antiplatelet autoantibodies and T lymphocyte–mediated platelet cytotoxicity. Multiple studies show that corticosteroid treatment and splenectomy, alone or together, increase platelet counts to safe levels in 60% to 70% of patients. However, there is little information on the outcomes of ITP patients refractory to splenectomy. We studied 114 patients with ITP for whom splenectomy failed and who required additional therapy; long-term follow-up was available on 105 (92%) patients. Seventy-five (71.4%) patients attained stable partial (platelet count greater than 30 × 109/L) or complete (normal platelet count) remission; 51 patients remained in remission after therapy was discontinued, whereas 24 patients required continued treatment. Median time to remission after splenectomy failure was 46 months (range, 1-437 months). Median remission durations were 60 months (range, 10-212 months) for patients off therapy and 48 months (range, 2-167 months) for patients on therapy. Thirty (29.6%) patients remained unresponsive to treatment. Thirty-two patients died, 17 (15.7%) of ITP (bleeding, 11 patients; therapy complications, 6 patients) and 15 (13.9%) of unrelated causes. We conclude that most patients with refractory ITP attain stable remission, though on average this occurs slowly. However, a subpopulation with severe, resistant disease experiences significant morbidity and mortality.

Congenital and Acquired Thrombocytopenia

The diagnosis and management of thrombocytopenia is a growing component in the practice of hematology. The frequency with which hematologists are called in consultation for thrombocytopenia continues to increase with the advent of routine automated platelet determinations and the introduction of new medications. For most patients, such as those with inherited and auto-immune thrombocytopenia, emphasis is focused on efforts to treat or forestall bleeding without excess drug-induced toxicity or burden to the patient. However, in disorders such as heparin-induced thrombocytopenia (HIT), avoidance of thrombotic complications is the key to management. In this chapter, we provide the pediatric and adult hematologist with new insights into the pathogenesis and recognition of congenital inherited thrombocytopenias (CTP), a hitherto difficult to comprehend constellation of clinical entities. We also highlight new approaches to the diagnosis and treatment of two of the more common thrombocytopenic conditions encountered in practice, autoimmune or idiopathic thrombocytopenic purpura (ITP) and HIT.

In Section I, Dr. James Bussel discusses CTPs and their distinction from childhood ITP. He emphasizes the clinical features that enable the pediatrician and hematologist to suspect the diagnosis of CTP and those that are of use to subcategorize the various entities, where possible. He also emphasizes newer molecular markers that afford definitive diagnosis in some cases and provide insight into platelet production. This section highlights the characteristic associated findings and differences in the natural history and approaches to management of the various entities.

In Section II, Dr. Robert McMillan discusses adult chronic ITP. He revisits the utility of platelet antibody determination in diagnosis and review new insights into pathogenesis. The role of Helicobacter pylori infection and the timing of splenectomy in the management of acute and emergent ITP are examined. New insights into the natural history of ITP post-splenectomy and management strategies for patients with severe, chronic, refractory ITP are discussed.

In Section III, Dr. James Zehnder updates us on HIT. He emphasizes new insights into the clinical presentation and pathogenesis of this condition. He critically reviews the utility of laboratory testing for heparin-dependent antibodies. Recent studies on the use of direct thrombin inhibitors are examined and the management of cardiopulmonary bypass surgery in patients with HIT is discussed.