Serum factors affecting the specificity of anticardiolipin antibodies.

The effects were investigated of two pretreatments of human serum and plasma test samples on their subsequent reactivity in the anticardiolipin antibody enzyme-linked immunosorbent assay (ACA-ELISA). The first treatment involved heat inactivation of test samples at 56 degrees C for 30 min, a process sometimes used to inactivate samples from suspected human immunodeficiency virus positive individuals. Such treatment significantly increased the IgG ACA unit/mL values of normal sera, but when this effect was examined further, it was found that the increase in binding occurred on both cardiolipin-coated and uncoated wells and was therefore non-specific. Heat inactivation of sera prior to ACA testing should therefore be avoided. The second treatment involved diluting immunoglobulin (Ig)G and IgM ACA-positive sera in normal human serum (NHS) or newborn calf serum (NCS); sera diluted in NHS showed a significant increase in titre, particularly IgM ACA-positive sera. This phenomenon was found to be due to a serum cardiolipin-binding cofactor which enhances antibody recognition. The cofactor is heat stable and is present in normal sera (male and female) and also in IgG ACA-positive sera. The binding of a human IgM monoclonal antibody to cardiolipin was not affected by the cofactor. The cardiolipin/cofactor complex may represent the optimal autoantigen/autoimmunogen and a re-appraisal, therefore, of the clinical relevance of antibodies to cardiolipin and other negatively charged molecules is warranted.