Pathophysiology and thrombokinetics in autoimmune thrombocytopenia

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 thrombocytopenic purpura

Immune thrombocytopenic purpura (ITP) is an autoantibody-mediated thrombocytopenic disorder in which accelerated destruction of platelets occurs; platelet production may also be impaired by these antibodies. ITP is characterized by mucocutaneous bleeding. Rarely, more severe hemorrhages, such as intracranial hemorrhage, may occur. Traditional therapies, such as steroids, immunoglobulin therapy, and splenectomy, generally reduce peripheral destruction of platelets. More recently, with a better understanding of the immunopathologic mechanisms underlying thrombocytopenia, several new treatments have been developed, including thrombopoietic agents, specific inhibitors of Fcgamma receptor (FcgammaR) signaling, and B-cell depletion therapies. This article outlines current understanding of the epidemiology, etiology, diagnosis, and treatment of ITP. The focus is on recent pathophysiologic insights and areas of potential controversy in which studies are ongoing.

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.

Protein C Inhibitor and Serum Amyloid a in Immune Thrombocytopaenic Purpura

In immune thrombocytopaenic purpura (ITP), phagocytic cells prematurely destroy platelets opsonized by anti-platelet auto-antibodies, while residual platelets rescued from these autoimmune attacks are hyperfunctioning. The exact pathobiological basis of this phenomenon is unknown. Protein C inhibitor (PCI), a platelet α-granule pro-coagulant molecule, is released on activation of platelets. Serum amyloid A (SAA; an acute phase protein), however, inhibits platelet aggregation and modulates platelet adhesion. We aimed to assess circulating soluble plasma PCI and SAA concentrations in 17 patients with newly diagnosed ITP and ten healthy volunteers. Plasma PCI concentrations tended to be higher in ITP patients, despite absolute thrombocytopaenia, than in normal controls. SAA levels were significantly higher in ITP patients compared with the control group. We conclude that secretion of the α-granule PCI content of platelets could result from platelet activation, and that PCI may be the link between platelet microparticles and haemostatically active ITP platelets. Increased concentrations of SAA and PCI may interfere with the disordered and compensatory pro-coagulant mechanisms of ITP.