Express and protect yourself: the potential role of HLA-G on muscle cells and in inflammatory myopathies.

Muscle is the site or the target of immunologic injury in several diseases. Whereas under physiologic conditions muscle fibers are negative for major histocompatibility complex (MHC) class I antigens, these are upregulated under pathologic conditions, thus rendering muscle a possible target for the recognition by cytotoxic CD8 T cells. Cultured muscle cells are capable of presenting antigens to CD4 and CD8 T cells, further indicating that muscle fibers in vivo are critically involved in the initiating or perpetuating steps of inflammatory responses. The finding that muscle fibers in autoimmune inflammatory myopathies in vivo and cultured muscle cells in vitro express the nonclassical major histocompatibility complex molecule HLA-G raises several hypothesis concerning its possible pathophysiologic role. We review present knowledge on the functional consequences of muscle-related HLA-G and provide concepts of its relevance under pathologic conditions. We further speculate on the potential therapeutic implications of HLA-G that relate to special approaches such as myoblast transplantation or strategies against inflammatory aggression in general.