Subclasses of IgG Anticardiolipin Antibodies in Patients with Systemic Lupus Erythematosus

Antibodies directed to a co‐factor associated with negatively charged phospholipids, such as cardiolipin, occur in patients with systemic lupus erythematosus (SLE), and possibly more often in those with venous or arterial thrombosis, thrombocytopenia or recurrent fetal loss. They are also found in patients without any of these manifestations and their biological effect, if any, might thus be related to their IgG subclass. To investigate this possibility, we determined anticardiolipin antibodies (ACA) by enzyme immunoassay (EIA) using monoclonal antibodies (MoAb) against human IgG subclasses. A net absorbance of x +3 SD of the value of 30 blood donors was taken as the cut‐of point. The specificity of the assay was verified through inhibition experiments using cardiolipin micelles. Thirty‐three patients with SLE were studied, all of whom had been shown to have ACA by a point dilution screening assay. IgGI ACA were found in 85% of the patients, and ACA of the IgG2, IgG3 and IgG4 subclasses in 42%, 39%. and 15%. There was a significant correlation between the presence of IgG3 ACA and of anti‐DNA antibodies but none between subclass distribution and major clinical manifestations of SLE.