Fiber type‐specific autoantibodies in a dog with eosinophilic myositis

Serum from a 2‐year‐old male Belgian sheepdog with eosinophilic myositis, which particularly affects the masticatory muscles, was tested for the presence of muscle‐specific autoantibodies. Control type 2 temporalis muscle fibers were selectively stained following incubation with the patient's serum and staphylococcal protein A conjugated to horseradish peroxidase (SPA‐HRPO). Likewise, type 2 fibers in the patient's temporalis muscle were selectively stained with SPA‐HRPO. The same staining procedures applied to limb muscle did not result in fiber staining. Proteins isolated from the temporalis and triceps brachii muscles of a normal dog were separated under denaturing conditions by onedimensional sodium dodecyl sulfate‐polyacrylamide gel electrophoresis. The separated proteins were transferred onto nitrocellulose paper and incubated with either sera from the patient, normal dogs, or neuromuscular disease controls. Subsequent incubation with peroxidase‐conjugated goat anti‐dog lgG demonstrated antibodies to at least four proteins of the temporalis muscle (myosin heavy chain and three unidentified proteins) when incubated with the patient's serum but not with the controls. Under all conditions, antibodies to the proteins of the triceps brachii were not detected. These findings establish the presence of autoantibodies to specific temporalis muscle proteins that may initiate the myonecrosis and inflammatory response as well as limit the distribution of the response.