TLR9 and rheumatoid arthritis: The classic and intermediate monocyte subpopulations in the synovium and blood of RA patients express TLR2 in large quantities, and all three monocyte subpopulations express TLR9; and the subpopulation of classical monocytes is triggered produces pro-inflammatory cytokines in response to TLR2 and TLR9 agonist stimulation (108).