Inclusion body myositis (IBM) is one of a group of muscle diseases known as the inflammatory myopathies, which are characterized by chronic muscle inflammation accompanied by muscle weakness. There is no cure for IBM, nor is there a standard course of treatment. We have determined that previously unappreciated components of the immune system, namely antibody-secreting plasma cells, infiltrate the muscle tissue in this disease. Both the role of these cells and the target(s) of the antibody they produce are not known, yet their identification is a critical first step to understanding the mechanisms of IBM immunopathology. We set out to identify the targets of the autoimmune response in both the tissue and periphery of patients with IBM. To determine the identity of the antigen target, single tissue-associated plasma cells were micro-dissected from IBM muscle tissue acquired through biopsy. Single cell PCR was then used to amplify the immunoglobulin heavy and light chain from a series of single cells. Recombinant immunoglobulin was then prepared and used to isolated muscle antigens. We discovered that a muscle intermediate filament protein was recognized by IBM tissue-derived immunoglobulin. Our proposed research is designed to examine both the prevalence and relevance of this newly identified autoimmune target in the disease. First, single plasma cells, will be isolated, using laser-assisted microdissection, from a large set of IBM muscle biopsy tissues. Then, recombinant immunoglobulin derived from these single cells will be tested for reactivity to muscle intermediate filament proteins and other candidate autoantigens. In the second part of the project, the sero-prevalence of IBM autoantibodies will be determined. Here we will identify targets of IBM serum autoantibodies using high throughput antigen arrays. The presence of serum autoantibodies will be correlated with disease features as a prerequisite for identifying these molecules as useful clinical diagnostic or prognostic biomarkers.