NATURAL AND DISEASE-ASSOCIATED AUTOANTIBODIES TO THE AUTOANTIGEN, DIHYDROLIPOAMIDE ACETYLTRANSFERASE, RECOGNIZE DIFFERENT EPITOPES

Naturally occurring autoantibodies are ubiquitous and may serve physio logical functions. We examined the relationship of natural and disease -associated autoantibodies in the context of autoantibodies to dihydro lipoamide acetyltransferase, the 74 kDa E2 sub-unit of the mitochondri al pyruvate dehydrogenase complex (PDC-E2), characteristic of primary biliary cirrhosis (PBC). We tested for natural autoantibodies to PDC-E 2 in normal sera, and compared epitopes recognised by natural and dise ase-associated autoantibodies. Methods included affinity purification of anti-PDC-E2 from normal and PBC sera, ELISA and immunoblotting, cap acity of antibodies to inhibit the enzyme function of the pyruvate deh ydrogenase complex (PDC), use of F(ab)(2) fragments of anti-PDC-E2 in inhibition assays, and testing affinity purified anti-PDC-E2 on peptid e fragments of PDC-E2. We found that natural autoantibodies to PDC-E2 of IgG class were demonstrable in all healthy human sera (10/10). Howe ver, their reactivity differed from that of disease-associated autoant ibodies, in that anti-PDC-E2 from normal serum failed to inhibit the c atalytic activity of PDC; and F(ab)(2) fragments from PBC sera potentl y blocked the binding of anti-PDC-E2 from PBC sera to PDC-E2, but not the binding of natural anti-PDC-E2 to PDC-E2. Immunoblotting on fragme nts of PDC-E2 using affinity-purified preparations from PBC sera and n ormal sera failed to provide evidence for gross differences in epitope reactivity. We conclude that normal human sera contain natural IgG au toantibodies to the immunodominant inner lipoyl domain of PDC-E2, as s een characteristically in PBC. However, there is evidence for differen ces in fine epitope recognition. (C) 1998 Academic Press Limited.