Rheumatoid arthritis (RA) is characterised by the overproduction of autoantibodies such as rheumatoid factor (RF) and anti-cyclic citrullinated peptide (anti-CCP) antibody. T follicular helper (Tfh) cells are a specialised Th subset that provides signals to B cells, promoting the secretion of antibodies. Our previous studies showed that the frequency of circulating Tfh cells were markedly increased in RA patients and positively correlated with disease activity and the levels of anti-CCP autoantibody. Adiponectin (AD) is an adipokine secreted mainly by adipocytes. Our previous work has demonstrated that AD is highly expressed in the inflamed synovial joint tissue and correlates closely with progressive bone erosion in RA patients. However, it remains unknown whether AD aggravates the severity of RA via modulating Tfh cells. This study aims to investigate whether AD exerts effect on Tfh cells in RA.
CD4+ T cells were purified from peripheral blood mononuclear cells (PBMCs) of healthy controls (HC), and adiponectin receptor 1 (AdipoR1) expression on the surface of CD4+CXCR5+PD-1+ (Tfh) cells was detected by flow cytometry. Purified HC CD4+ T cells were cultured with different concentration fetal bovine serun (FBS) in the presence or absence of AD. The percentages of Tfh cells were analysed by flow cytometry. RA or osteoarthritis (OA) fibroblast-like synoviocytes (FLSs) were stimulated with AD for 72h and then co-cultured with HC CD4+ T cells through cell-to-cell contact or in a transwell system. The percentages of Tfh cells were analysed by flow cytometry and the levels of soluble factors such as interleukin-(IL)-6, IL-21, IL-12 and IFNγ in the supernatants were determined by Human Magnetic Bead Panel or Enzyme linked immunosorbent assay (ELISA). Then anti-IL-6 antibody and/or anti-IL-21 antibody was added to the co-culture system, and the percentages of Tfh cells were analysed by flow cytometry. The frequency of Tfh cells in the joint tissue of collagen-induced arthritis (CIA) mice was examined by flow cytometry. The mRNA expression of Tfh cell transcription factors and functional molecules such as B-cell lymphoma 6 (Bcl-6), B lymphocyte maturation protein 1 (Blimp-1), IL-6, IL-21, IL-12 and IFNγ in the joints of CIA mice were detected by real time PCR (RT-PCR).
Adiponectin receptor 1 (AdipoR1) expression was detected on the surface of Tfh cells. However, in the present study, we did not find that AD has a direct effect on Tfh cell generation in vitro. Nonetheless, AD-stimulated RA FLSs could promote Tfh cell generation, predominantly via IL-6 production. And this upregulated effect was partially abolished upon neutralising IL-6. Finally, intraarticular injection of AD aggravated synovial inflammation with increased frequency of Tfh cells in the joints of AD-treated CIA mice.
Our study demonstrated that AD-stimulated RA FLSs promote Tfh cell generation, which is mainly mediated by the secretion of soluble factor IL-6. This finding reveals a novel mechanism for AD in RA pathogenesis.