ANTI-LACTOFERRIN AUTOANTIBODIES - RELATION BETWEEN EPITOPES AND IRON-BINDING DOMAIN

Anti-neutrophil cytoplasm antibodies (ANCA) have been found in the ser a of patients presenting systemic necrotizing microscopic vasculitis, i.e. Wegener's granulomatosis and microscopic polyangiitis. Lactoferri n (LF) is one of the antigens rarely recognized by ANCA, and anti-LF a utoantibodies are found in several autoimmune conditions, including rh eumatoid vasculitis, rheumatoid arthritis, systemic lupus erythematosu s, ulcerative colitis, primary sclerosing cholangitis and Crohn's dise ase. We analysed the epitopes recognized by human anti-LF antibodies t o test whether the heterogeneity of clinical presentation might be due to a different epitope recognition profile. Several monoclonal antibo dies were raised and used in competition studies with six human sera. Four distinct epitopes were identified on LF and LF binding of only on e of six sera was inhibited by one of the monoclonals. Thus, anti-LF a utoreactivity appears to be polyclonal and not restricted to an immuno dominant epitope. Specific epitope profiles cannot be determined in th ese autoimmune conditions. We hypothesized that the interaction of ant i-LF antibodies with the LF iron binding domain might contribute to pa thogenesis by inhibiting iron chelation after neutrophil activation, t hereby providing increased iron availability for endothelial cell dama ge. The relation of anti-LF mouse monoclonals or polyclonal human or r abbit antibodies to the LF iron-binding domain was studied in competit ion assays between Fe-59 and these antibodies. Preincubation of LF wit h monoclonals or anti-LF human sera did not affect the binding of Fe-5 9 on LF Fe-59-binding kinetic studies showed that rabbit anti-LF polyc lonal, but not mouse monoclonals or human anti-LF positive sera, was c apable of inhibiting iron binding on LF. Therefore, anti-LF autoantibo dies did not appear to modulate LF iron-binding activity. We conclude that LF is a rare antigen specificity for ANCA and that the clinical a nd pathophysiological relevance of anti-LF autoreactivity remains unce rtain.