Identification of Horse Anti-Ferritin Autoantibody

Ferritin-binding proteins (FBPs) in horse serum were characterized by immunoblotting and ferritin-binding experiments. FBPs purified from horse serum by horse spleen ferritin-Sepharose 4B affinity chromatography were separated into two fractions by Sephacryl S-300 gel chromatography: FBP 1 and FBP 2. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions, FBP 1 separated into 79.0- and 25.3-kDa bands, and FBP 2 separated into 62.7- and 25.3-kDa bands. Immunoblotting analysis using antibodies specific for horse IgM and IgA heavy chains and IgG F(c) fragment showed that the 79.0- and 62.7-kDa bands were IgM and IgG heavy chains, respectively. After forming complexes of horse FBPs with horse spleen ferritin, rabbit anti-horse ferritin antibody was used to form immune complexes against ferritin, allowing co-precipitation and subsequent identification by monoclonal antibodies to horse immunoglobulin (IgM, IgA, IgGa and IgGb). Horse serum was positive for IgM, IgGa, IgGb and IgA complexes and affinity-purified FBPs for IgM and IgGb complexes. FBP 1 was identified as an IgM complex that forms with ferritin but FBP 2 did not form any complexes with ferritin. These results suggest that circulating horse serum ferritin is in the form of IgM, IgG (IgGa and IgGb) and IgA complexes.