Macrophages are the major source of inflammation mediators that are important in the pathogenesis of rheumatoid arthritis (RA). This study was undertaken to analyze macrophages obtained from the joints of RA patients in order to characterize the expression of Toll-like receptor 2 (TLR-2) and TLR-4 and the responses to TLR ligation.
Cells were isolated from the synovial fluid (SF) of RA patients or patients with other forms of inflammatory arthritis. Cell surface TLR-2 and TLR-4 expression and intracellular tumor necrosis factor α (TNFα) and interleukin-8 (IL-8) expression by CD14+ macrophages were determined by flow cytometry. Peptidoglycan (PG) and lipopolysaccharide (LPS) were used as ligands for TLR-2 and TLR-4, respectively.
The expression of TLR-2 and TLR-4 was increased on CD14+ macrophages from the joints of RA patients compared with that on control in vitro–differentiated macrophages or control peripheral blood monocytes. Neither TLR-2 expression nor TLR-4 expression differed between RA and other forms of inflammatory arthritis. However, PG- and LPS-induced TNFα expression and IL-8 expression were greater with RA SF macrophages than with those obtained from the joints of patients with other forms of inflammatory arthritis or with control macrophages. PG-induced TNFα expression and IL-8 expression were highly correlated with TLR-2 expression in normal macrophages, but not with that in macrophages obtained from joints of RA patients or patients with other forms of inflammatory arthritis.
TLR-2 and TLR-4 ligation resulted in increased activation of RA synovial macrophages compared with those from patients with other forms of inflammatory arthritis or compared with control macrophages. Factors other than the level of TLR-2 and TLR-4 expression contributed to the increased activation of RA SF macrophages. These observations support the notion of a potential role for activation through TLR-2 and TLR-4 in the inflammation and joint destruction of RA.