Characterization of platelet-reactive antibodies in children with varicella-associated acute immune thrombocytopenic purpura (ITP)

Biochemical analyses were performed on blood samples obtained from two children (P1, P2) who presented with acute immune thrombocytopenic purpura (ITP) following a recent varicella zoster virus (VZV) infection. Patient sera had antibodies that were reactive with normal blood-group O platelets as measured by flow-cytometric assay. Western blot analysis of electrophoretically separated normal blood-group O platelets under reducing and non-reducing conditions demonstrated that these sera were reactive with platelet antigens of ∼50 and ∼110 kD, respectively. These 50/110 kD antigens were not reactive with seven sera from acute ITP patients whose illness was not preceded by VZV infection, with serum from a patient with a prior history of VZV and no thrombocytopenia, nor with normal healthy control sera. VZV antibodies (IgG and IgM), isolated from patient sera by affinity chromatography using immobilized purified VZV glycoproteins, were found to bind to gel-filtered autologous platelets and with normal blood-group O platelets, as analysed by flow cytometry. No binding was observed using antibodies similarly prepared from healthy volunteer sera. To investigate their ability to sensitize platelets to complement activation, affinity-purified VZV antibodies were incubated with platelets and then with purified complement components C1 and ¹²⁵I-labelled C4. Platelets reacted with VZV-specific antibodies from the two patients and showed increases of 2.3–2.4-fold of platelet-surface deposition of ¹²⁵I-C4b, compared to controls. These data provide evidence that virus-specific antibodies occurring in children with varicella-associated acute ITP cross-react with normal platelet antigens, and may contribute to platelet clearance.