In the present study, we found that the S100A8/A9 heterodimer is expressed predominantly by CD68+ macrophages in the ST of patients with RA. In addition, using the recombinant proteins, we demonstrated that heterodimeric S100A8/A9, as well as homodimeric S100A9, stimulates monocytes and in vitro-differentiated macrophage to produce proinflammatory cytokines such as TNF-??.