Anti-cardiolipin antibodies (ACLA) have been suggested to play a role in the pathogenesis of thrombocytopenia. When sera from 40 patients with chronic idiopathic thrombocytopenic purpura (ITP) were analyzed for ACLA, these autoantibodies were present in 12 (30%). In 4 sera the antibody activity was restricted to the IgG or IgM isotype, respectively, while 4 of the samples contained both IgG and IgM antibodies. To elucidate the interaction between platelets and ACLA, we studied the reactivity of sera from non-ITP patients with ACLA, with fragmented platelet membranes. None of them had a concurrent platelet deficiency, but sera from 8 (67%) of them showed increased IgG-binding to platelet membranes. Absorbtion with cardiolipin reduced membrane binding in 6 (50%). C3 levels were normal, while low C4 values occurred in both groups with significantly lower levels in ACLA-positive patients (p<0.05). Circulating immune complexes (CIC) were common in both groups.

Conclusions: The prevalence of ACLA is increased in ITP and sera from non-ITP patients with ACLA react with fragmented platelet membranes. This reactivity is often decreased by absorption with cardiolipin, suggesting that ACLA bind to phospholipid epitopes on platelet membranes.