Tumor necrosis factor-α (TNF-α) is a cytokine central to many aspects of the inflammatory response. Macrophages, mast cells, and activated TH cells (especially TH1 cells) secrete TNF-α. TNF-α stimulates macrophages to produce cytotoxic metabolites, thereby increasing phagocytic killing activity.
TNF-α has been implicated in numerous autoimmune diseases. Rheumatoid arthritis, psoriasis, and Crohn’s disease are three disorders in which inhibition of TNF-α has demonstrated therapeutic efficacy. Rheumatoid arthritis illustrates the central role of TNF-α in the pathophysiology of autoimmune diseases. Although the initial stimulus for joint inflammation is still debated, it is thought that macrophages in a diseased joint secrete TNF-α, which activates endothelial cells, other monocytes, and synovial fibroblasts. Activated endothelial cells up-regulate adhesion molecule expression, resulting in recruitment of inflammatory cells to the joint. Monocyte activation has a positive feedback effect on T-cell and synovial fibroblast activation. Activated synovial fibroblasts secrete interleukins, which recruit additional inflammatory cells. With time, the synovium hypertrophies and forms a pannus that leads to destruction of bone and cartilage in the joint, causing the characteristic deformity and pain of rheumatoid arthritis.