Multidirectionality of Anti-Platelet Antibodies in Idiopathic Thrombocytopenic Purpura

Chronic idiopathic thrombocytopenic purpura (chronic ITP) is an autoimmune disorder characterized by bleeding tendency and the presence of auto-antibody to platelets. Quantitative analysis of the level of anti-platelet antibodies and platelet associated antibodies is now possible. Furthermore, recent immunological techniques have enabled us to detect target platelet antigens in chronic ITP.

In this study, (1) anti-platelet antibodies, contained in serum, were examined by an indirect platelet suspension immunofluorescence test (indirect PSIFT) for IgG+A+M, and platelet bind-able IgG (PB-IgG) was measured quantitatively by enzyme linked immunosolvent assay (ELISA). Indirect PSIFT was positive in 4.2%, and PB-IgG was elevated in 12.5% of ITP patients. (2) Platelet associated antibodies were detected by direct PSIFT and platelet associated IgG (PA-IgG) were assayed by solid phase competitive ELISA. Direct PSIFT was positive in 34.1 % and PA-IgG was elevated in 37.8% (3) IgG antibodies against platelet lipid compounds (cardiolipin and sulfatides) were also measured by ELISA. Anti-cardiolipin antibody was elevated in 56.3% and anti-sulfatides antibody was elevated in 31.3%. Anti-cardiolipin antibodies correlated well with anti-sulfatides antibody, and also correlated with PB-IgG. (4) Target platelet antigens were detected by immunoprecipitation and Western blotting. Immunoprecipitation showed multiple antigenic bindings in one out of 19 ITP patients with positive serum anti-platelet antibody. On Western blotting analysis, multiple antigenic binding sites were detected in six out of 48 patients.

The data in this study are consistent with the idea that auto-antibodies in ITP are not directed towards a single protein, but to multiple platelet antigens which are included in proteins and lipid compounds.