Despite the high frequency of CD4⁺ T cells with a regulatory phenotype (CD25⁺CD127^lowFoxP3⁺) in the joints of patients with rheumatoid arthritis (RA), inflammation persists. Regulatory T cells (Tregs) can be converted into pro-inflammatory IL-17-producing cells by inflammatory mediators, particularly IL-1β.

To investigate whether activated monocytes, which are abundantly present in the rheumatic joint and potent producers of IL-1β, induce pro-inflammatory cytokine expression in Tregs and whether this impairs Treg function.

The presence and phenotype of CD4⁺CD45RO⁺CD25⁺CD127^low T cells (memory Tregs) and CD14⁺ monocytes in the peripheral blood (PB) and synovial fluid (SF) from patients with RA was investigated by flow cytometry. FACS-sorted memory Tregs from healthy controls were co-cultured with autologous in vitro-activated monocytes and anti-CD3 monoclonal antibody. Intracellular cytokine expression, phenotype and function were determined by flow cytometry, ELISA and proliferation assays.

Patients with RA showed higher frequencies of CD4⁺CD45RO⁺CD25⁺CD127^low Tregs and activated CD14⁺ monocytes in SF relative to PB. We demonstrate that activated monocytes induced an increase in the percentage of IL-17⁺, IFNγ⁺ and TNF-α⁺, but also IL-10⁺ Tregs. Blocking and reconstitution experiments revealed that the observed increase in IL-17⁺ and IFNγ⁺ Tregs was driven by monocyte-derived IL-1β, IL-6 and TNF-α and was mediated by both CD14⁺CD16^- and CD14⁺CD16⁺ monocyte subsets. Despite enhanced cytokine expression, cells maintained their CD25⁺FoxP3⁺CD39⁺ Treg phenotype and showed enhanced capacity to suppress proliferation and IL-17 production by effector T cells.

Tregs exposed to a pro-inflammatory environment show increased suppressive activity.