The B7 family of costimulatory molecules likely includes members distinct from B7. 1 (CD80) and B7. 2 (CD86). After stimulation with IFN-γ or TNF-α, human myoblasts selectively express BB-1, but not B7. 1 or B7. 2. BB-1 is detected by anti-BB-1, a mAb cross-reacting with B7. 1 (but not B7. 2) and an as yet undefined costimulatory molecule. The absence of B7. 1 and B7. 2 in BB-1-positive myoblasts was confirmed by RT-PCR. The molecule detected by anti-BB-1 is functional, because anti-BB-1 mAb and CTLA4Ig (but not anti-B7. 1-or anti-B7. 2-specific mAbs) completely inhibit Ag presentation by cytokine-induced myoblasts to HLA-DR-matched Ag-specific CD4+ T cell lines. Stimulation of myoblasts with IL-4 induces B7. 1 and B7. 2, as well as BB-1, but with different time kinetics. Stimulation of CD40-positive myoblasts with anti-CD40 mAb selectively induces BB-1, whereas stimulation with CD40L-transfected mouse L cells induces BB-1 and B7. 1, with different kinetics. To assess whether BB-1 is expressed in muscle tissue, we investigated 23 muscle biopsy specimens from patients with polymyositis, dermatomyositis, inclusion body myositis, Duchenne muscular dystrophy, and nonmyopathic controls by immunohistochemistry and confocal laser microscopy. We found that, in all inflammatory myopathy cases, but not in normal muscle, many muscle fibers strongly react with anti-BB-1. In contrast, muscle fibers did not react with B7. 1-or B7. 2-monospecific mAbs in any of the pathologic specimens or in normal muscle. Our results demonstrate that human muscle cells can be induced to selectively express BB-1, a functional costimulatory molecule distinct from B7. 1 and B7. 2. This molecule may play an important role in the immunobiology of muscle.