Serum antiplatelet
IgG and platelet-associated
IgG (PAIgG) were studied in 68
AIDS-free human immunodeficiency virus (HIV)-infected patients with severe immunologic
thrombocytopenic purpura (
ITP), for the presence of platelet
autoantibodies. Serum
IgG with antiplatelet activity was found in 72% of the sera. However, the presence of
autoantibodies against platelet
surface glycoproteins was not found in these sera by means of Western blot and immunoprecipitation procedures. Nevertheless, an immunoblot immunoassay and an indirect immunofluorescence test against semi-permeabilized platelets demonstrated the presence of
antibodies in the patient sera, that reacted with intracytoplasmic platelet components, and which might participate in the elimination of platelet fragments. Direct immunofluorescence tests demonstrated an increased amount of PAIgG in 75% of the patients; the bound
antibodies could be eluted with
ether in 44% of the cases. These eluates were found to bind to normal platelets but not to Glanzmann type I platelets. Finally, immunoprecipitation procedures demonstrated the presence of platelet
autoantibodies in six of the 35 eluates studied. These
antibodies recognized GPIIb in two cases,
GPIIIa in one case, and an unidentified platelet
protein of 150 kDa in the three other cases. The discrepancy between sera and platelet eluates was interpreted as being due to the low titre of the
antibodies and to their dilution in polyclonal hypergammaglobulinaemia. The present study provides direct evidence that isolated
ITP in some HIV-positive patients is due to the presence of platelet
autoantibodies. These results, however, do not exclude either direct or indirect involvement of HIV in the platelet destruction.