The co‐occurrence of rheumatoid factor (RF) and anti–citrullinated protein antibody (ACPA) positivity in rheumatoid arthritis (RA) is well described. However, the mechanisms underlying the potential interaction between these 2 distinct autoantibodies have not been well defined. The aim of this study was to evaluate the epidemiologic and molecular interaction of ACPAs and RF and its association with both disease activity and measures of RA‐associated inflammation.

In a cohort of 1,488 US veterans with RA, measures of disease activity and serum levels of cytokines and multiplex ACPAs were compared between the following groups of patients: double‐negative (anti–cyclic citrullinated peptide [anti‐CCP]−/RF−), anti‐CCP+/RF−, anti‐CCP−/RF+, or double‐positive (anti‐CCP+/RF+). Additional studies were performed using an in vitro immune complex (IC) stimulation assay in which macrophages were incubated with ACPA ICs in the presence or absence of monoclonal IgM‐RF, and tumor necrosis factor α production measured as a readout of macrophage activation.

Compared with the double‐negative subgroup (as well as each single‐positive subgroup), the double‐positive subgroup exhibited higher disease activity as well as higher levels of C‐reactive protein and inflammatory cytokines (all P < 0.001). In vitro stimulation of macrophages by ACPA ICs increased cytokine production, and the addition of monoclonal IgM‐RF significantly increased macrophage tumor necrosis factor α production (P = 0.003 versus ACPA ICs alone).

The combined presence of ACPAs and IgM‐RF mediates increased proinflammatory cytokine production in vitro and is associated with increased systemic inflammation and disease activity in RA. Our data suggest that IgM‐RF enhances the capacity of ACPA ICs to stimulate macrophage cytokine production, thereby providing a mechanistic link by which RF enhances the pathogenicity of ACPA ICs in RA.