Effects of unfractionated and low molecular weight heparin on antiphospholipid antibody binding in vitro.

To compare the efficacy of unfractionated heparin and low molecular weight heparin in the in vitro binding of antiphospholipid antibodies obtained from the sera of patients with recurrent pregnancy loss. Women with immunoglobulin (Ig) G antibodies to the phospholipids cardiolipin and phosphatidylserine were selected based on a positive test by a standard enzyme-linked immunosorbent assay (ELISA). The sera were reassayed for antiphospholipid antibodies in a modified ELISA using increasing doses of unfractionated heparin or low molecular weight heparin (0, 16, 32, 64, 128, and 256 IU). Sera were fractionated by unfractionated and low molecular weight heparin affinity chromatography to compare the binding avidity and antiphospholipid antibody activity. All sera demonstrated a dose-dependent inhibition in measured antiphospholipid antibody activity with the addition of unfractionated or low molecular weight heparin. Levels of IgG cardiolipin and IgG phosphatidylserine were significantly inhibited in the presence of 32 IU of low molecular weight heparin (P <.001 and P <.05, respectively) and in the presence of 64 IU of unfractionated heparin (P <.001 and P <.05, respectively). Antiphospholipid antibody binding activity in serum as measured in the ELISA was maximally reduced 76-89% with 256 IU of either heparin derivative. Affinity chromatography with unfractionated or low molecular weight heparin columns absorbed 72% and 66% of IgG cardiolipin activity, respectively, and 46% and 54% of IgG phosphatidylserine activity, respectively. These data suggest that low molecular weight heparin and unfractionated heparin reduce the in vitro binding of antiphospholipid antibodies on a per unit basis. Both heparins demonstrate binding activity similar to that of antiphospholipid antibodies in vitro.