Studies on antibodies against phospholipids

Abstract

The main clinical features of the antiphospholipid syndrome (APLS) are arterial and venous thrombosis, repeated spontaneous abortions and thrombocytopenia. Diagnosed concomitant with an underlying disease the designation is secondary APLS; otherwise primary APLS. The serological requirement for the diagnosis is the presence of antiphospholipid antibodies (APLA). These constitute closely related antibody specificities, as lupus anticoagulants (LA) and anticardiolipin antibodies (ACLA).

The aim of this thesis was to study the prevalence of APLA in habitual abortion and chronic idiopathic thrombocytopenic purpura (ITP) and to investigate for immunological and metabolic derangements of potential relevance for disease manifestations in APLS. Of 99 women with habitual abortion, 42 had ACLA and the antibodies were present in all IgG subclasses. Ten women with high antibody titers had low levels of the complement component C4 and fragmentation of C2 indicative of an ongoing complement activation. Circulating immune complexes (CIC) were prevalent in the patients, and thus a factor potentially contributing to complement activation was present.

A significant increase of thromboxane A2 (TXA2) was demonstrated in 31 subjects with APLS by increased urine concentration of the metabolite 2,3-dinor-TxB2. TXA2 is synthesized by activated platelets and has potent platelet aggregating and vasoconstrictive properties. Its actions are opposed by prostacyclin (PGI2) from endothelial cells (ETC). The PGI2 metabolite 2,3-dinor-6-keto-PGF1a was also increased but not to the same extent. The disproportionate increase in TXA2 reflects platelet activation and might be of pathogenic relevance in APLS.

Twelve sera out of forty from patients with chronic ITP contained ACLA. Sera with these antibodies from 12 non-ITP patients were studied for reactivity with platelet membranes. Eight sera showed increased binding of IgG which was reduced in six after absorption with cardiolipin liposomes. Reactivity against phospholipids should thus be considered when interpreting results suggestive of anti-platelet autoantibodies. CIC from five patients with APLA were isolated and antibody distribution between serum and CIC was determined. The relative concentration and avidity of the antibodies was higher in CIC. This could have special implications with regard to the role of CIC in platelet and complement activation and to thrombocytopenia and thrombophiliain APLS.

The in vivo effect of a single oral dose of 500 mg of aspirin on the biosynthesis of TXA2 was studied in 7 subjects with IgG ACLA by monitoring the urine excretion of 2,3 dinor-TXB2 for at least seven days. The decrease in TXA2 formation was between 46% and 85%, indicating a less inhibiting effect of aspirin than in healthy individuals. This might be of relevance for the dosage of aspirin in conditions in which platelet activation is implicated as part of the pathogenic mechanism, as in the antiphospholipid syndrome.