Objectives Elevated expression of Siglec-1 on circulating monocytes has been reported in some inflammatory and autoinmune diseases, but its expression and role in rheumatoid arthritis (RA) has not been elucidated.Methods Siglec-1 protein and mRNA levels in 42 RA patients, 39 osteoarthritis patients, 28 SLE patients and 42 normal controls were determined by flow cytometry and QRT-PCR, respectively.In addition, ten patients with active RA were received DMARDs for 12 weeks and the frequencies of Siglec-1(+) cells and DAS28 were assessed before and after therapy.Furthermore, TNF-αα, IFN-γ and type Ⅱ collagen were used to up-regulate Siglec-1 and PBMCs from different groups were stimulated with mitogens or antigens and cells proliferation and cytokines production were determined.Results The protein and mRNA levels of Siglec-1 on PBMCs and monocytes in RA patients were significantly higher than those in OA patients and healthy controls.Moreover, the expression of Siglec-1 protein on PBMCs was positively correlated with DAS28, ESR, hs-CRP and IgM-RF, but not with anti-CCP antibody.Interestingly, Siglec-1 expression was decreased in parallel with the decrease of DAS28 after 12-week antirheumatic treatment.Furthermore, TNF-α, IFN-γ and type Ⅱ collagen can up-regulate Siglec-1 on PBMCs.Elevated PBMCs proliferation and pro-inflammatory cytokines production to collagen stimulation in RA patients reduced when Siglec-1 was inhibited by anti-Siglec-1 antibody.Conclusions Elevated Siglec-1 expression on PBMCs and monocy-tes can potentially serve as a biomarker for monitoring disease acfivity in RA.Siglec-1 may also play a proinflammatory role in stimulating lymphocytes proliferation and activation in RA.