CD4+ and CD8+ CD28null T cells are Cytotoxic to Autologous Muscle Cells in Polymyositis Patients

Objective. Inflammatory T cell infiltrates in the skeletal muscle tissue of patients with polymyositis are dominated by CD28-negative effector (CD28(null)) T cells of both the CD4 and CD8 lineage. These cells are potentially cytotoxic, and the aim of the present study was to develop a fully autologous cell culture system in which to investigate the functional contribution of such CD28(null) T cells to myotoxicity. Methods. In vitro cocultures of autologous skeletal muscle cells and T cell subsets obtained from 5 polymyositis patients were performed. Myotoxicity of T cells was quantified by calcein release and flow cytometric analyses. T cell degranulation was blocked with concanamycin A. Specific blocking of perforin, cytokines, and HLA was performed using antibodies. Results. Both CD4+CD28(null) and CD8+CD28(null) T cells induced more muscle cell death than did their CD28+ counterparts. Differentiated muscle cells (myotubes) were more sensitive to T cell-mediated cell death than were their precursors (myoblasts). Both CD8+ and CD4+ CD28(null) T cells displayed perforin polarization toward muscle cells and secreted higher levels of granzyme B and interferon-gamma (IFN gamma) in coculture than did CD28+ T cells. The myotoxic effects of CD28(null) T cells were reduced upon the blocking of perforin, cytokines, and HLA. Addition of IFN gamma or tumor necrosis factor did not induce skeletal muscle cell death in the absence of T cells; however, it did up-regulate HLA expression on muscle cells. Conclusion. Myotoxicity of CD4+ and CD8+ CD28(null) T cells is mediated by directed perforin-dependent killing and can be further influenced by IFN gamma-induced HLA expression on muscle cells. The data suggest that CD28(null) T cells are key effector cells that contribute to the muscle cell damage in polymyositis.