Mutations in and near the second calcium-binding domain of integrin alphaIIb affect the structure and function of integrin alphaIIbbeta3

Human platelet antigen 1a epitopes are dependent on the cation-regulated conformation of integrin α(IIb)β(3) (GPIIb/IIIa)

BACKGROUND

The HPA-1a (Leu(33)) polymorphism of platelet integrin αIIbβ3 is the target of alloantibodies in 70-80% cases of neonatal alloimmune thrombocytopenia (NAIT) in Caucasians and reliable detection of these antibodies is essential for appropriate clinical management. However, the ability to detect such antibodies is highly variable between laboratories and, in a number of clinical cases where there is a HPA-1 genotype mismatch between mother and neonate, HPA-1a antibodies are undetectable. Furthermore, some studies have not shown a consistent relationship between maternal anti-HPA-1a level and neonatal platelet count. Since the integrity and conformation of the αIIbβ3 complex are dependent on divalent cations, we investigated whether HPA-1a epitope integrity and/or conformation might be affected by the presence of the cation chelator EDTA in patient samples or in assay buffers, thus providing a possible explanation for the variable sensitivity of current assays.

PRINCIPLE FINDINGS

Exposure of the αIIbβ3 complex to EDTA resulted in reduced reactivity of three anti-HPA-1a mAbs (B2, 19-7 and 23-15). More significantly, cation chelation adversely affected detection of polyclonal anti-HPA-1a, not only in the platelet immunofluorescence assay, where alloantibody binding was reduced compared to control platelets (mean MFI reduction 44.5%, range 17.3-69.7%, n=4), but also in the commonly used monoclonal antibody specific immobilisation of platelet antigens assay (MAIPA) where both alloantibody and monoclonal capture antibody binding were reduced (mean OD reduction 82.8%, range 68.3-96.6%, n=9).

CONCLUSIONS

These data show that HPA-1a antibodies recognise epitopes on αIIbβ3 that are sensitive to EDTA treatment and that cation chelation grossly reduces the sensitivity of the MAIPA assay by diminishing not only HPA-1a alloantibody binding but also 'capture' monoclonal antibody binding. These findings may, in part, explain the current variability in antibody measurement and will guide the development of more sensitive tests for anti-integrin antibodies in NAIT and other conditions.

Sustained engraftment and resolution of bleeding phenotype after unrelated cord blood hematopoietic stem cell transplantation for severe glanzmann thrombasthenia

Glanzmann thrombasthenia is a rare, autosomal recessive, qualitative platelet disorder resulting from abnormal platelet surface glycoprotein IIb/IIIa. In phenotypically severe cases, medical management is often challenging. Although definitive hemostasis can be achieved with platelet transfusion, alloimmunization and subsequent platelet refractoriness remain a real risk. To date, only hematopoietic stem cell transplantation has been curative; however, suitable donor availability can be a barrier for some patients. We are the first to report the use of umbilical cord blood hematopoietic stem cell transplantation for Glanzmann thrombasthenia.

Qualitative disorders of platelets and megakaryocytes

Qualitative disorders of platelet function and production form a large group of rare diseases which cover a multitude of genetic defects that by and large have as a common symptom, excessive mucocutaneous bleeding. Glanzmann thrombasthenia, is enabling us to learn much about the pathophysiology of integrins and of how alphaIIb beta3 functions. Bernard-Soulier syndrome, an example of macrothrombocytopenia, combines the production of large platelets with a deficit or non-functioning of the major adhesion receptor of platelets, the GPIb-IX-V complex. Amino acid substitutions in GPIb alpha, may lead to up-regulation and spontaneous binding of von Willebrand factor as in Platelet-type von Willebrand disease. In disorders with defects in the MYH9 gene, macrothrombocytopenias are linked to modifications in kidney, eye or ear, whereas other inherited thrombocytopenias variously link a low platelet count with a propensity to leukemia, skeletal defects, learning impairment, and abnormal red cells. Defects of secretion from platelets include an abnormal alpha-granule formation as in the gray platelet syndrome (with marrow myelofibrosis), and of organelle biogenesis in the Hermansky-Pudlak and Chediak-Higashi syndromes where platelet dense body defects are linked to abnormalities of other lysosomal-like organelles including melanosomes. Finally, defects involving surface receptors (P2Y(12), TPalpha) for activating stimuli, of proteins essential for signaling pathways (including Wiskott-Aldrich syndrome), and of platelet-derived procoagulant activity (Scott syndrome) show how studies on platelet disorders are helping unravel the pathways of primary hemostasis.