Pathogenic stromal cells as therapeutic targets in joint inflammation.

Knowledge of how the joint functions as an integrated unit in health and disease requires an understanding of the stromal cells populating the joint mesenchyme, including fibroblasts, tissue-resident macrophages and endothelial cells. Knowledge of the physiological and pathological mechanisms that involve joint mesenchymal stromal cells has begun to cast new light on why joint inflammation persists. The shared embryological origins of fibroblasts and endothelial cells might shape the behaviour of these cell types in diseased adult tissues. Cells of mesenchymal origin sustain inflammation in the synovial membrane and tendons by various mechanisms, and the important contribution of newly discovered fibroblast subtypes and their associated crosstalk with endothelial cells, tissue-resident macrophages and leukocytes is beginning to emerge. Knowledge of these mechanisms should help to shape the future therapeutic landscape and emphasizes the requirement for new strategies to address the pathogenic stroma and associated crosstalk between leukocytes and cells of mesenchymal origin.