Increased CCL22 production in early rheumatoid arthritis development with potential impact on mucosal autoimmunity, joint inflammation and T cell recruitment to LAMP3+ dendritic cells.

OBJECTIVES: Understanding alterations in immune homeostasis can help early diagnosis and prevention in rheumatoid arthritis (RA). As joint pain often precedes RA onset, we analysed pain-associated immune mediators in individuals at risk of RA (RISK-RA) and in early untreated RA. Serum CCL22 levels were elevated in both groups, prompting us to further study its sources and effects in RA. METHODS: CCL22 expression was examined in RISK-RA and RA cohorts, healthy controls, BAL cells of healthy smokers and cigarette smoke-exposed mice. Spatial transcriptomics, single-cell RNA sequencing, and flow cytometry were used to assess CCL22 and CCR4 expression in joints. The CCR4 inhibitor C-021 was evaluated in collagen antibody-induced arthritis (CAIA). RESULTS: Increased CCL22 levels were detected in current smokers, in all tested cohorts, and in cigarette-smoke exposed mice. Smoking was associated with CCL22 gene hypomethylation in BAL cells. CCL22 was also expressed in arthritic joints, potentially induced by inflammatory mediators such as M-CSF and GM-CSF. ScRNA-seq and synovial fluid cell analyses identified LAMP3+CCR7+ dendritic cells (DCs) as the most prominent producers of CCL22. CCR4 was expressed by Tregs and subsets of activated, proliferating and effector CD4+ T cells. Beyond T cells, CCL22 may affect innate mechanisms as CCR4 inhibition altered cytokine expression in macrophages and DCs and decreased CAIA severity. CONCLUSIONS: CCL22-mediated T cell recruitment may contribute to the mucosal breach of tolerance in smokers. In joints, CCL22 could promote inflammation and recruit activated and regulatory T cells to LAMP3+ DCs, a subset characterised by robust T cell modulatory potential.