Immunohistochemistry staining of synovium. The top four images show the expression of CD147 (RA CD147), matrix metalloproteinase-1 (RA MMP-1), tissue inhibitor of metalloproteinases-1 (RA TIMP-1) and blank contrast (RA blank) in the lining and sublining layers in RA synovium (× 400). The bottom two images show the expression of CD147 in ankylosing spondylitis (AS CD147) and osteoarthritis (OA CD147) synovium. CD147 was predominantly expressed by macrophage-like cells and fibroblast-like synovial cells in the lining layer and partly by endothelial cells in the rheumatoid arthritis (RA) synovium. The primary antibody used for the blank contrast for streptavidin/peroxides staining was PBS (× 400).