Apoptotic cell clearance in systemic lupus erythematosus: I. Opsonization by antiphospholipid antibodies

Abstract

Objective To verify whether antiphospholipid antibodies (aPL) recognize and opsonize apoptotic human cells. Methods Apoptosis was induced via CD95 crosslinking or ultraviolet irradiation. IgG and anti-β2-glycoprotein I (anti-β2-GPI) antibodies were purified from patient sera by affinity chromatography. The aPL that bound to apoptotic cells were assessed by flow cytometry, and the subdomains recognized were identified by confocal microscopy. Human macrophages were derived from monocytes, and their ability to phagocytose 3H-labeled apoptotic bodies, whether opsonized or not opsonized by aPL, was assessed. Tumor necrosis factor α (TNFα) secretion was evaluated by enzyme-linked immunosorbent assay. Results The aPL, but not control Ig or Ig from aPL-negative patients, bound to apoptotic cells, but not to viable cells. Nuclear antigens were not recognized. Opsonization of apoptotic cells by aPL substantially enhanced recognition and binding by scavenger macrophages, with massive TNFα secretion. Conclusion Antiphospholipid antibodies facilitate apoptotic cell clearance by macrophages and trigger TNFα release, possibly enhancing the immunogenicity of the autoantigens they contain.