Immune-mediated polymyositis in a dog

Evidence for MHC I–restricted CD8+ T-cell–mediated immunopathology in canine masticatory muscle myositis and polymyositis

Masticatory muscle myositis (MMM) is the most common inflammatory myopathy (IM) in dogs, associated with antibodies against myosin. To further elucidate the immunopathogenesis, we investigated muscles of 53 dogs with MMM, 32 dogs with polymyositis (PM), and 4 dogs suffering from both, with regard to the presence and location of CD4(+) and CD8(+)T cells, B cells, macrophages, major histocompatibility complex (MHC) class I and class II antigens, and autoantibodies. CD8(+)T cells were found in MMM (91%) and PM (75%), mostly paralleled (68% and 61%) by enhanced expression of MHC class I antigen on muscle fibers. CD8(+)T cells invading intact and neighboring necrotic muscle fibers were present in MMM (39%) and PM (42%). Dogs with MMM lacking intramuscular (26%) and circulating (36%) autoantibodies also had CD8(+) T-cell infiltrations and muscle-fiber lesions. Since MHC class I antigen and CD8(+) T cells were detected in the presence of CD4(+) T cells, regardless of antimuscular antibodies, we consider MMM and PM in the dog as a CD8(+) T-cell-mediated immunopathological disease that initiates muscle-fiber destruction and leads to production of myosin autoantibodies.

Selected disorders of muscle and the neuromuscular junction

A variety of disorders affect the muscles or the neuromuscular junction of dogs and cats, most often causing weakness, exercise intolerance, and muscular pain or atrophy. The myopathies are infectious, immune-mediated, inherited or acquired secondary to systemic disease. Acquired myasthenia gravis is a common disorder of the neuromuscular junction, which clinically resembles a myopathy. Reaching a specific diagnosis is essential to determine optimal therapy and prognosis for each of the commonly recognized disorders.