PROJECT SUMMARY Sporadic inclusion body myositis (sIBM) is a rare disorder of aging Americans, causing asymmetric muscle weakness and severe disability and morbidity. It is currently untreatable, and poorly understood. The prevalence of sIBM is likely to increase as the proportion of the United States population above the age of 65 years continues to grow. A major barrier to clinical trials in sIBM has been the lack of full understanding of the natural history of the disease. It remains to be determined whether the rates of disease progression is uniform and whether the various biomarkers associated with sIBM (anti-NT5c1A antibodies, variant T-cell populations) influence the natural history and disease behavior. Given the slow rate of disease progression, such observations cannot be made in the context of a routine clinical trial, and such studies need to be done as a separate stand-alone observational study. To address these unmet needs, we propose a prospective study with four specific aims. Aim 1: To determine for the first time whether c1A antibodies mediate disease progression over a two year interval in patients with sIBM. Aim 2: To perform a detailed morphological, histochemical, and immunohistochemical analysis of fresh muscle biopsy specimens obtained from a subset of patients with sIBM. Aim 3: To characterize the distribution of “immunosenescent” lymphocytes in circulating blood from patients with sIBM. Aim 4: To quantify the decline in the respiratory function of sIBM patients. The significance of our proposed study is 1) to allow for a detailed characterization of the disease progression in sIBM over a two-year period, and 2) to explore the relationship of a number of biomarkers associated with sIBM, and their influence on disease behavior and disease progression. Upon completion of these aims, we will 1) understand the disease phenotype, including pattern of respiratory involvement, and disease progression in sIBM better and understand the influence of serum antibodies to NT5c1A antibodies on the natural history and disease behavior; 2) define differences in serum variant T-cells and cytokine signatures in sIBM patients and their influence on disease progression and behavior; and 3) understand muscle pathology and immune cell distribution in sIBM patients and its relationship to NT5c1A antibodies. These findings may influence future trial design in sIBM. Finally, we will have created a thirteen-site consortium of myositis treatment centers that will be ready to adopt quickly any future clinical trials aimed at changing the course of sIBM.