Inclusion Body Myositis.

PURPOSE OF REVIEW Inclusion body myositis (IBM) is an enigmatic progressive disease of skeletal muscle. This review provides a summary of the clinical and pathophysiologic aspects of IBM. RECENT FINDINGS The development of diagnostic blood testing for IBM followed from the discovery of a B-cell pathway in IBM muscle and circulating autoantibodies against NT5C1A, further establishing IBM's status as an autoimmune disease. The key role of cytotoxic T cells in IBM is further supported by the identification of a link between IBM and T-cell large granular lymphocytic leukemia. The testing of research diagnostic criteria in patients is improving its accuracy. Increases in estimated prevalences may be due to a combination of true increases and improved recognition of disease. SUMMARY IBM has high unmet medical need. Advances in the mechanistic understanding of IBM as an autoimmune disease will drive effective therapeutic approaches. The identification of a B-cell pathway has resulted in the first identification of an IBM autoantigen and emphasized its status as an autoimmune disease. The recognition that large granular lymphocyte CD8+ T-cell expansions are present in both blood and muscle provides additional biomarkers for IBM and suggests a mechanistic relationship to the neoplastic disease T-cell large granular lymphocytic leukemia.