A human monoclonal IgA autoantibody - 185/12-behaves like an autoimmune antiphospholipid antibody

A human monoclonal anticardiolipin autoantibody (ACA) of the IgA-k isotype, designated 185/ 12, is described. The antibody was prepared from peripheral B cells, obtained from a patient with a history of habitual abortion, by immortalization with Epstein-Barr virus (EBV). The antibody displays a strong binding activity to cardiolipin and phosphatidyl L-serine, but not to phosphatidylcholine, phosphatidylinositol, ssDNA and dsDNA. It binds to cardiolipin in a concentration-related and saturable manner (K(d) = 3.0 x 10^-8 M). This reaction is dependent upon the presence of bovine serum, and is fully inhibited by cardiolipin vesicles. The 185/12 antibody exhibits different binding patterns to the solid-phase bound cardiolipin-serum complex and to its individual components (cardiolipin and bovine serum). The B(max) of 185/12 binding to the complex (0.968 OD units) is higher than the sum of the B(max) values calculated for each one of the complex components (0.352 ± 0.179 = 0.531 OD units). Bovine serum as well as purified β₂-glycoprotein I β₂-GPI) in suspension inhibit the binding of 185/12 to the complex. 185/12 binding capacity increases in direct relation to the rising concentration of β₂-GPI. Collectively, these data may be interpreted to suggest that 185/12 antibody, which is an IgA isotype, exhibits characteristics usually attributed only to antiphospholipid autoantibodies (APA) of the IgG isotype, that are associated with the clinical spectrum of APA syndrome (APA-S). It is, therefore, possible that autoantibodies of the IgA isotype could play a pathogenic role, which may be different from that of the IgG isotype, in the development of autoimmune phenomena.