Neuroendocrine-immune interactions in synovitis.

Synovial tissue lines the noncartilaginous surfaces of synovial joints and supplies these avascular structures with nutrients. In diseases such as rheumatoid arthritis, inflammation of the synovial tissue--synovitis--induces diffuse damage to the joints. The presence of functional receptors for glucocorticoids, androgens and estrogens in synoviocytes might link inflammation and the endocrine system at the local level. Synovial tissue could be regarded as an intracrine tissue, whereby active steroids influence the cells in which they are synthesized, without their release into the extracellular space. An increase in the peripheral metabolism of sex steroids is characteristic of rheumatoid synovitis, with an augmented ratio of estrogen to androgen occurring in both male and female patients. Changes in the peripheral nervous system at the site of local inflammation are also hallmarks of synovitis in rheumatoid arthritis. In the chronic phase of synovitis, sympathetic nerve fibers are lost; by contrast, sensory nerve fibers sprout into the inflamed tissue. Complex interactions occur between the endocrine, nervous and immune systems during synovitis. In particular, studying neuroendocrine-immune interactions in the inflamed synovium will potentially uncover new mechanisms in the pathophysiology of rheumatoid arthritis and might lead to new methods of therapeutic intervention.