[Anti-platelet antibodies in patients with autoimmune disorders]

The levels of platelet-associated IgG (PAIgG) are reported to be elevated in patients with systemic lupus erythematosus (SLE). However, the nature of PAIgG is unclear. We investigated whether PAIgGs of SLE are anti-platelet autoantibodies or immune complexes (IC). PAIgG levels measured by flow cytometry in patients with SLE (n = 25) were 12.7 +/- 41.0 (expressed by mean channel), slightly high (but not significant) compared with healthy subjects (2.5 +/- 0.7). Three of 6 SLE patients with thrombocytopenia had high level of PAIgG (mean channels > 10). To determine whether elevated PAIgGs are composed of anti-platelet antibody or IC, we applied elution technique. Preliminary experiments showed that the eluate prepared form platelets sensitized with anti-HPA-4a antibody reacted with normal platelets, while the eluates prepared from platelets sensitized both with heat aggregated IgG and with model IC did not react with normal platelets. These results indicated that the reactivity of eluates could discriminate between platelet-bound antibody and IC. We applied this method for the analyses of PAIgG of SLE platelets. The eluates from SLE platelets (mean channels were more than 10) reacted with normal platelets, indicating that PAIgGs of SLE platelets are anti-platelet autoantibody, not IC. Furthermore, we investigated the target antigens which bound PAIgGs of SLE, using the direct immunoprecipitation and modified antigen capture ELISA (MACE). Both methods identified GPIIb/IIIa as target antigens. We concluded that 1) ether elution technique could differentiate between anti-platelet antibody and IC, 2) PAIgGs of SLE had a nature of anti-platelet autoantibody, 3) the target antigens of PAIgG were GPIIb/IIIa.