Many alphaIIbbeta3 autoepitopes in chronic immune thrombocytopenic purpura are localized to alphaIIb between amino acids L1 and Q459

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.

Autoantibody against integrin αv β3 contributes to thrombocytopenia by blocking the migration and adhesion of megakaryocytes

Essentials The pathogenesis of immune thrombocytopenia (ITP) has not been fully clarified. We analyzed the role of anti-αvβ3 autoantibody in the pathogenesis of ITP in patients. Anti-αvβ3 autoantibody impeded megakaryocyte migration and adhesion to the vascular niche. Anti-αv β3 autoantibody potentially contributes to the pathogenesis of refractory ITP.

SUMMARY

Background The pathogenesis of immune thrombocytopenia (ITP) has not been fully clarified. Anti-αvβ3 integrin autoantibody is detected in chronic ITP patients, but its contribution to ITP is still unclear. Objectives To clarify the potential role of anti-αvβ3 integrin autoantibody in chronic ITP and the related mechanism. Methods Relationship between levels of anti-αvβ3 autoantibody and platelets in chronic ITP patients was evaluated. The influence of anti-αvβ3 antibody on megakaryocyte (MK) survival, differentiation, migration and adhesion was assessed, and the associated signal pathways were investigated. Platelet recovery and MKs' distribution were observed in an ITP mouse model pretreated with different antibodies. Result In this study, we showed that the anti-αvβ3 autoantibody usually coexists with anti-αIIbβ3 autoantibody in chronic ITP patients, and patients with both autoantibodies have lower platelets. In in vitro studies, we showed that the anti-αvβ3 antibody had no significant effect on the survival and proliferation of MKs, whereas it decreased formations of proplatelet significantly. Anti-αvβ3 antibody impeded stromal cell derived facor-1 alpha (SDF-1α)- mediated migration and inhibited the phosphorylation of protein kinase B. Anti-αvβ3 antibody significantly inhibited MKs' adhesion to endothelial cells and Fibrogen. The phosphorylation of focal adhesion kinase and proto-oncogene tyrosine-protein kinase Src induced by adhesion was inhibited when MKs were pretreated with anti-αvβ3 antibody. In in vivo studies, we showed that injection with anti-αv antibody delayed platelet recovery in a mouse model of ITP. Conclusions These findings demonstrate that the autoantibody against integrin αv β3 may aggravate thrombocytopenia in ITP patients by impeding MK migration and adhesion to the vascular niche, which provides new insights into the pathogenesis of ITP.

Emerging Concepts in Immune Thrombocytopenia

Immune thrombocytopenia (ITP) is an autoimmune disease defined by low platelet counts which presents with an increased bleeding risk. Several genetic risk factors (e.g., polymorphisms in immunity-related genes) predispose to ITP. Autoantibodies and cytotoxic CD8⁺ T cells (Tc) mediate the anti-platelet response leading to thrombocytopenia. Both effector arms enhance platelet clearance through phagocytosis by splenic macrophages or dendritic cells and by induction of apoptosis. Meanwhile, platelet production is inhibited by CD8⁺ Tc targeting megakaryocytes in the bone marrow. CD4⁺ T helper cells are important for B cell differentiation into autoantibody secreting plasma cells. Regulatory Tc are essential to secure immune tolerance, and reduced levels have been implicated in the development of ITP. Both Fcγ-receptor-dependent and -independent pathways are involved in the etiology of ITP. In this review, we present a simplified model for the pathogenesis of ITP, in which exposure of platelet surface antigens and a loss of tolerance are required for development of chronic anti-platelet responses. We also suggest that infections may comprise an important trigger for the development of auto-immunity against platelets in ITP. Post-translational modification of autoantigens has been firmly implicated in the development of autoimmune disorders like rheumatoid arthritis and type 1 diabetes. Based on these findings, we propose that post-translational modifications of platelet antigens may also contribute to the pathogenesis of ITP.

Recognition of highly restricted regions in the β-propeller domain of αIIb by platelet-associated anti-αIIbβ3 autoantibodies in primary immune thrombocytopenia

Platelet-associated (PA) IgG autoantibodies play an essential role in primary immune thrombocytopenia (ITP). However, little is known about the epitopes of these Abs. This study aimed to identify critical binding regions for PA anti-αIIbβ3 Abs. Because PA anti-αIIbβ3 Abs bound poorly to mouse αIIbβ3, we created human-mouse chimera constructs. We first examined 76 platelet eluates obtained from patients with primary ITP. Of these, 26 harbored PA anti-αIIbβ3 Abs (34%). Further analysis of 15 patients who provided sufficient materials showed that the epitopes of these Abs were mainly localized in the N-terminal half of the β-propeller domain in αIIb (L1-W235). We could identify 3 main recognition sites in the region; 2 eluates recognized a conformation formed by the W1:1-2 and W2:3-4 loops, 5 recognized W1:2-3, and 4 recognized W3:4-1. The remaining 4 eluates could not be defined by the binding sites. Within these regions, we identified residues critical for binding, including S29 and R32 in W1:1-2; G44 and P45 in W1:2-3; and P135, E136, and R139 in W2:3-4. Of 11 eluates whose recognition sites were identified, 5 clearly showed restricted κ/λ-chain usage. These results suggested that PA anti-αIIbβ3 Abs in primary ITP tended to recognize highly restricted regions of αIIb with clonality.

The pathogenesis of chronic immune thrombocytopenic purpura

Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disorder in which the patient's immune system reacts with a platelet autoantigen(s) resulting in thrombocytopenia due to immune-mediated platelet destruction and/or suppression of platelet production. Platelet membrane proteins, for reasons that are unclear, become antigenic and stimulate the immune system to produce autoantibodies and cytotoxic T cells. The initial antigenic response probably occurs in the spleen followed by stimulation of other antibody-producing tissues, particularly the bone marrow. Autoantibodies against platelet glycoprotein (GP) IIb-IIIa and/or GPIb-IX are produced by the majority of ITP patients and can be detected using antigen-specific assays. Many patients produce multiple antibodies; this has been attributed to the phenomenon of epitope spreading. Once produced, autoantibody may either bind to platelets, causing their destruction by either phagocytosis or possibly complement activation and lysis, or bind to megakaryocytes, resulting in decreased thrombopoiesis. Evidence for platelet destruction in ITP includes the following: (1) infusion of ITP blood or plasma into normal recipients may result in thrombocytopenia; (2) there is decreased intravascular survival of radiolabeled platelets in most ITP patients; (3) morphologic and in vitro evidence of platelet phagocytosis can be demonstrated; and (4) cytotoxic T cells can induce lysis of autologous platelets. Evidence for suppressed platelet production in ITP includes the following: (1) morphologic studies show megakaryocyte damage in most ITP patients; (2) there is normal or decreased platelet turnover in the majority of patients; (3) in vitro studies show antibody-induced inhibition of megakaryocyte production and maturation; and (4) an increase in the platelet count occurs in many ITP patients receiving treatment with thrombopoietin mimetics. In summary, activation of the immune system by platelet autoantigens in ITP may result in platelet destruction and/or inhibition of platelet production. The importance of each mechanism in the individual patient probably varies.

Autoantigenic Epitopes on Platelet Glycoproteins

Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disorder characterized by early platelet destruction mediated by antiplatelet autoantibodies. Platelet membrane glycoproteins (GP), especially GPIIb-IIIa and GPIb-IX, contain major autoantigenic determinants in chronic ITP. Recent advances in the localization of autoantigens as well as in the detection of GP-specific antibodies have improved our understanding of the pathophysiology of the disease. The N-terminal globular head of GPIIb-IIIa, particularly the beta-propeller domain in GPIIb, seems to play an important role as a hot spot for autoantigenic epitopes in chronic ITP.

Antiplatelet antibodies in chronic adult immune thrombocytopenic purpura: assays and epitopes

Chronic immune thrombocytopenic purpura (ITP) is an autoimmune disorder characterized by thrombocytopenia due to autoantibody-induced platelet destruction. The majority of these autoantibodies are directed to epitopes on either glycoprotein (GP) IIb-IIIa or GPIb-IX. The newer antigen-specific autoantibody assays are capable of detecting both platelet-associated and plasma autoantibodies and have a definite role in the diagnosis of immune thrombocytopenia. A positive assay provides strong evidence for the presence of immune thrombocytopenia both in chronic ITP and in other diseases where immune thrombocytopenia may occur, such as collagen vascular disease and lymphoproliferative disorders. However, a negative assay does not rule out the presence of ITP. Somewhat concerning is the large number of patients who have negative assays. Several possible explanations for these observations are discussed. Recent studies have localized some ITP autoepitopes to specific regions of GPIIb-IIIa and GPIb-IX. Most autoepitopes on GPIIb-IIIa are conformational, in view of their dependence on divalent cations, and are localized to the N-terminal portion of GPIIb, while the GPIb-IX autoepitopes that have been identified are localized to GPIb amino acids 333-341.

The etiology of childhood immune thrombocytopenic purpura: how complex is it?

Recent developments in genomics and basic immunology have provided a new set of tools for investigation into the etiology and treatment of childhood immune thrombocytopenia purpura (ITP). The genomic revolution is generating a catalog of germ-line common genetic variants, some of which could influence the susceptibility or outcome of ITP. Similarly, in vitro analyses and animal models have been employed to probe the basic alterations underlying ITP. The emergence of a more refined understanding of complex diseases such as ITP has important implications for prevention, therapy, and follow-up. The relative contribution of the genetic component and its interaction with the strong environmental stimulus, such as an acute, antecedent viral infection, remains to be determined.