IgM in primary biliary cirrhosis. Physicochemical and complement activating properties.

In PBC, common features beyond cholestasis and presence of mitochondrial antibodies are signs of complement activation and high levels if IgM. In order to characterize this IgM physicochemically and immunologically, we studied IgM from 15 patients with PBC in comparison with that from patients with high levels of polyclonal IgM without signs of liver disease (idiopathic hyper-IgM-emia) and normals. Agarose electrophoresis, immunofixation, gel filtration, and ultracentrifugation studies gave no evidence of any abnormal IgM populations such as complexes, aggregates, 7S IgM, or oligoclonality. However, IgM in PBC is highly cryoprecipitable, precipitable with 2.5% PEG (mol. wt. 6000) and binds to conglutinin. In addition, purified IgM from PBC patients rapidly converts complement factor C3 in fresh normal serum, mainly via the classical pathway, in contrast to IgM in the same concentration from normals or patients with idiopathic hyper-IgM-emia. IgM from PBC patients behaves like an immune complex, although it has the same molecular size and electrophoretic properties as normal IgM. No evidence of any antigen(s) bound to IgM in vivo in PBC was found in this study.