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Objective: Interactions between the family of B7 ligands and their receptors are increasingly recognized as crucial for stimulation and/or inhibition of immune responses. The present study was undertaken to examine the expression and functional relevance of B7 homolog 3 (B7-H3), a novel B7 homolog attributed significant immunoregulatory functions, in human muscle cells in vivo and in vitro., Methods: Thirty-five muscle biopsy specimens obtained from patients with polymyositis, dermatomyositis, inclusion body myositis, or noninflammatory myopathies and normal controls were analyzed by immunohistochemistry for B7-H3 expression. The expression of B7-H3 protein on primary human myoblasts and TE671 muscle rhabdomyosarcoma cells was studied by flow cytometry and Western blot analysis. B7-H3 small interfering RNA (siRNA) was used to study the impact of knockdown of B7-H3 on CD8+ cell-mediated lysis in skeletal muscle cells., Results: B7-H3 was not detectable on normal muscle fibers. In contrast, its expression was markedly increased on muscle fibers from patients with inflammatory myopathies. Cell-surface staining was most prominent in the contact areas between muscle fibers and inflammatory cells. B7-H3 protein was detected on myoblasts cultured from control and myositis patient muscle tissue as well as in TE671 muscle rhabdomyosarcoma cells. Knockdown of B7-H3 by siRNA in TE671 cells enhanced CD8+ T cell-specific lysis, indicating a functional role of B7-H3 in the protection of skeletal muscle from immune-mediated lysis., Conclusion: Our results demonstrate that human muscle cells express B7-H3, a functional coinhibitory molecule of the B7 family. B7-H3 may play an important role in muscle-immune interactions, providing further evidence of the active role of muscle cells in local immunoregulatory processes.