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Abstract

Objective

To determine whether anti–endothelial cell autoantibodies (AECAs) from systemic lupus erythematosus (SLE) patients with the antiphospholipid syndrome are involved in the initial endothelial cell (EC) membrane perturbation effect that is postulated to provide a target for antiphospholipid antibody (aPL) binding and, hence, to trigger the thrombotic cascade. To identify the AECA antigenic target on ECs and to determine the mechanism whereby the EC membrane is disrupted.

Methods

AECAs from SLE patients were assayed for binding to ECs by flow cytometry. Positive AECAs were assayed by immunoblotting, and a consensus antigen was identified by mass spectrometry. This candidate antigen was tested in recombinant form for AECA recognition. AECAs were affinity-purified on this antigen and incubated with ECs to determine their physiologic effects. Anti-Hsp60 antibody titers were determined by enzyme-linked immunosorbent assay. The relationship of anti-Hsp60 status and lupus anticoagulant (LAC) status to thrombotic manifestations between disease onset and the last followup visit were analyzed.

Results

Most of the SLE sera (73%) possessed IgG that bound to the surface of ECs. These positive IgG shared reactivity against a 60-kd EC surface polypeptide that was identified as human Hsp60. The presence of Hsp60 at the EC surface was established using anti-Hsp60 antibodies from commercial sources or affinity-purified from SLE sera that bound ECs. Incubation of ECs with these anti-Hsp60 antibodies induced apoptosis in a time- and dose-dependent manner, as determined by Hoechst 33342 dye staining of condensed nuclei and by annexin V binding to surface phosphatidylserine. Anti-Hsp60 antibodies were not restricted to SLE patients, but were found in patients with other autoimmune diseases. However, anti-Hsp60 antibodies were significantly associated with an increased frequency of thrombosis when present in combination with LAC in the SLE patients.

Conclusion

The presence of Hsp60 at the surface of ECs serves as a target for the anti-Hsp60 antibodies in SLE sera. These anti-Hsp60 antibodies bind to ECs and induce apoptosis, particularly phosphatidylserine exposure, thus providing a target for the binding of aPL and inducing the subsequent thrombotic cascade.