Treg cells mitigate inflammatory responses and symptoms via β2-AR/β-Arr2/ERK signaling in an experimental rheumatoid arthritis

β2-adrenergic receptor (β2-AR) is widely expressed on immune cells, including T cells and it has a non-canonical signaling pathway, which is β2-AR-β-arrestin 2 (β-Arr2)-extracellular signal-regulated kinase 1/2 (ERK1/2). Our previous studies have shown that the β2-AR agonist terbutaline (Terb) can activate β2-AR and significantly inhibit helper T (Th) 17 cell function in collagen-induced arthritis (CIA). However, the effects of β2-AR on regulatory T (Treg) cells in CIA have not been consistently determined. The aim of our research was to explore whether β2-AR-β-Arr2-ERK1/2 signaling in Treg cells regulates inflammatory responses and signs in CIA. DBA1/J mice were used to prepare CIA model by intradermal injection of collagen type II (CII). Inducible Treg (iTreg) cells were induced from CD4+ T cells that were isolated from the spleens of normal or CIA mice and treated with Terb, β-Arr2 gene silence or overexpression or the ERK1/2 inhibitor U0126 in vitro. β2-AR and β-Arr2 expression, pERK1/2 level, interleukin (IL)-10 and transforming growth factor (TGF)-β production and the suppression of effector T (Teff) cell proliferation mediated by nature Treg (nTreg) cells were determined. β-Arr2-overexpressed Treg cells were intravenously injected into the tail base of CIA mice. Arthritic symptoms were assessed by clinical arthritis scores. Frequencies of Th17 and Treg cells, cytokine production and osteoclast and osteoblast-specific gene expression were estimated. The increased β-Arr2 expression and pERK1/2 level induced by Terb in CIA iTregs were reduced by β-Arr2 gene silence. The increased TGF-β and IL-10 production in iTreg cells by Terb and the suppression of Teff cell proliferation mediated by Terb-treated nTregs from CIA mice were downregulated by β-Arr2 gene silence or U0126 and further enhanced by β-Arr2 gene overexpression. Adoptive transfer of β-Arr2-overexpressed Treg cells into CIA mice reduced limb inflammation, osteoclast-specific gene expression in joints and Th17 cell cytokine production in joints and serum but increased osteoblast-specific gene expression in joints and Treg cell cytokine production in joints and serum. β2-AR/β-Arr2/ERK signaling in Treg cells contributes to alleviation of inflammatory responses and symptoms in CIA.