PECAM1 and rheumatoid arthritis: We further determined the ratios of the three cell types in the synovium by performing multiplex immunohistochemical (mIHC) staining on synovial sections from RA patients, using Vimentin (for RA FLSs), CD68 (for macrophages), and CD31 (for endothelial cells) as markers to estimate the relative abundances of these three cell types in the inflamed synovium.