In normal individuals there is an adaptive immune response to a foreign
antigen in which
antibodies of increasing affinity are produced with time. This is not always true of an autoimmune response. However, because only a limited number of
autoantigens have been cloned or purified, this issue has not been studied well. In
primary biliary cirrhosis the predominant manifestation of autoimmunity is antimitochondrial
antibodies that react with
dihydrolipoamide transacetylase. The availability of recombinant
dihydrolipoamide transacetylase and the development of a rapid and reproducible
enzyme-linked
immunosorbent assay for
autoantibodies has allowed us to address the affinity of
autoantibodies using
thiocyanate inhibition.
Thiocyanate is a chaotropic compound known to inhibit
antigen-antibody binding in a concentration-dependent manner. We used this property to inhibit the binding by
enzyme-linked
immunosorbent assay of human recombinant
dihydrolipoamide transacetylase with serum
autoantibodies from 55 patients with
primary biliary cirrhosis. The relative affinity and serum
autoantibody titers were then compared with the histological stage of the liver biopsy sample. Interestingly, we found a considerable heterogeneity of relative affinities. These relative affinities did not correlate with the histological stage or the serum titer of antimitochondrial
antibodies. However, the ability of serum
autoantibodies to inhibit intact
primary biliary cirrhosis enzyme activity was found to correlate highly (R2 = 0.751) with the relative affinity. Thus there are profound differences between patients with respect to qualitative expression of
autoantibodies. The significance of this data will be unclear until more is determined regarding the nature of the
epitope that drives T cells and leads to B-cell responses.