Rheumatoid arthritis fibroblast‐like synoviocytes co‐cultured with PBMC increased peripheral CD4+CXCR5+ICOS+ T cell numbers

‘Circulating’ T follicular helper cells (Tfh), characterized by their surface phenotypes CD4⁺chemokine receptor 5 (CXCR5)⁺ inducible co‐stimulatory molecule (ICOS)⁺, have been identified as the CD4⁺ T cell subset specialized in supporting the activation, expansion and differentiation of B cells. Fibroblast‐like synoviocytes (FLS) are critical in promoting inflammation and cartilage destruction in rheumatoid arthritis (RA), and the interaction between FLS and T cells is considered to facilitate FLS activation and T cell recruitment. However, it remains unknown whether RA‐FLS co‐cultured with activated peripheral blood mononuclear cells (PBMC) has immunoregulatory effects on peripheral Tfh. In the present study, we co‐cultured RA‐FLS with or without anti‐CD3/CD28‐stimulated PBMC. The results showed that RA‐FLS co‐cultured with stimulated PBMC could increase the numbers of CD4⁺CXCR5⁺ICOS⁺ T cells of RA PBMC possibly via the production of interleukin (IL)‐6, a critical cytokine involved in the differentiation of Tfh cells. We also observed increased reactive oxygen species (ROS) levels in the co‐culture system of RA‐FLS and PBMC. The percentage of CD4⁺CXCR5⁺ICOS⁺ T cells was decreased when ROS production was inhibited by N‐acetyl‐L‐cysteine (NAC), a specific inhibitor which can decrease ROS production. In addition, we showed that the higher levels of tumour necrosis factor (TNF)‐α and IL‐1β in the co‐culture system and the blocking of TNF receptor 2 (TNF‐R2) and IL‐1β receptor (IL‐1βR) both decreased the numbers of CD4⁺CXCR5⁺ICOS⁺ T cells. Our study reveals a novel mechanistic insight into how the interaction of RA‐FLS and PBMC participates in the RA pathogenesis, and also provides support for the biologicals application for RA.