Expression of the costimulatory molecule BB-1 and its receptors in patients with scleroderma-polymyositis overlap syndrome.

We investigated expression of costimulatory molecules BB-1, B7-1 (CD80), B7-2 (CD86), and their counter-receptors CD28 and CTLA-4 (CD152) in muscle biopsy specimens of patients with scleroderma-polymyositis overlap syndrome (SSc-PM), primary polymyositis (PM), and other related diseases to examine whether the muscle fibers in patients with SSc-PM behave as antigen-presenting cells (APCs). The major histocompatibility (MHC) class II-positive muscle fibers of SSc-PM patients reacted with monoclonal antibodies (mAb) against BB-1 but not against B7-1 or B7-2. The CD4+ T cells expressed the counter-receptors CD28 and CTLA-4, and bound with the BB-1-positive muscle fibers in cell-to-cell contact. Our findings show that muscle fibers in patients with SSc-PM function as "professional" APCs in a way distinct from muscle fibers in patients with primary PM.