TGFB1 and osteoarthritis: It has also been shown that macrophages dominate synovial tissue and are the main immune cell population in osteoarthritis of the knee, exhibiting expression of synovial fluid-associated β1 (TGFβ1) and elastase or transforming growth factor, and that the expression levels of TGFβ1 and elastase show a trend towards a significant correlation with the severity of osteoarthritis of the knee on imaging (14).