Background: Previous immunohistochemical studies of muscle from patients with inclusion body myositis and polymyositis found many more T cells than B cells, suggesting a role for intramuscular cell-mediated immune mechanisms rather than humoral mechanisms.
Methods: Microarray studies were performed on muscle biopsy specimens from 40 patients with inclusion body myositis (IBM; n = 23), polymyositis (PM; n = 6), and without neuromuscular disease (n = 11). Reverse transcription PCR of selected immunoglobulin gene transcripts was performed on two patient samples. Qualitative immunohistochemical studies for B-cell lineage cell surface markers were performed on 28 muscle specimens and quantitative studies performed on a subset of 19 untreated patients with IBM or PM. CD138+ cells were isolated from muscle using laser capture microdissection, and immunoglobulin transcripts were PCR amplified to determine the presence or absence of immunoglobulin gene rearrangements unique to the B-cell lineage.
Results: Immunoglobulin gene transcripts accounted for 59% in IBM and 33% in PM of the most stringently defined highest differentially expressed muscle transcripts compared with normal. Plasma cells, terminally differentiated B cells expressing CD138 but not CD19 or CD20, are present in IBM and PM muscle in numbers several times higher than B cells.
Conclusions: There are differentiated B cells in the form of CD138+ plasma cells within the muscle of patients with inclusion body myositis and polymyositis. The principle of linked recognition of B-cell activation predicts several strategies for autoantigen discovery that could not otherwise be pursued through the study of the infiltrating T-cell population alone.