Apolipoprotein H (apoH)-dependent autoantibodies and apoH protein polymorphism in selected patients showing lupus anticoagulant activity.

Apolipoprotein H (apoH) is considered to be a necessary cofactor for the binding of certain antiphospholipid antibodies to anionic phospholipids. Some apoH-dependent antiphospholipid antibodies also exert lupus anticoagulant (LA) activity, which seems to depend on antiphospholipid antibody epitope specificity. The aim of this study was to evaluate whether the presence of less frequent apoH alleles may induce structural or conformational changes in these "LA-dependent" regions that may initiate more frequent autoimmune responses in subjects. We selected patients with confirmed LA activity and none or low titers of anticardiolipin antibodies that had been sent to the laboratory for routine antiphospholipid antibody determination. Many of them had some clinical manifestation of antiphospholipid syndrome. Antibodies to apoH were determined with a commercially available anticardiolipin/apoH ELISA kit. ApoH protein polymorphism (apoH phenotype) was demonstrated by isoelectric focusing and immunoblotting. Our results showed that 47/74 (63.5%) of our selected LA-positive patients also had elevated apoH-dependent antiphospholipid antibody titers. These results point to two subgroups of patients according to the LA potency of apoH-dependent antibodies. A strong positive correlation (non-linear or linear) for apoH-dependent antibody titers and LA activity was observed in both subgroups of patients. In this study, we did not find significant differences in the distribution of apoH phenotypes among control subjects and patients with apoH-dependent/LA-positive auto- antibodies.