An imbalance between blood CD4⁺CXCR5⁺Foxp3⁺ Tfr cells and CD4⁺CXCR5⁺Tfh cells may contribute to the immunopathogenesis of rheumatoid arthritis

Background: Follicular helper T (Tfh) cells are a subgroup of activated CD4⁺ T cells which can assist the formation and maintenance of germinal centers. Follicular regulatory T (Tfr) cells are a new class of regulatory T cells which play a major role in suppressing cells in humoral immunity. In contrast to the role of Tfh cells, Tfr cells can inhibit the function of Tfh cells and B cells. Imbalance of blood Tfr/Tfh ratio resulted in the expansion of auto-reactive B cells and auto-antibody production (). However, the effect of Tfr cells and Tfh cells in the pathogenesis of RA (rheumatoid arthritis) is unclear. The purpose of this study was to investigate the function of Tfr cells and Tfh cells in the pathogenesis of RA.

Methods: We recruited 20 patients fulfilled the the American College of Rheumatology diagnosis criteria and 20 healthy controls (HCs). The number of CD4⁺CXCR5⁺Foxp3⁺ Tfr cells and CD4⁺CXCR5⁺ Tfh cells in 20 RA patients were measured by flow cytometry analysis. Furthermore, the correlations between the Tfr/Tfh ratio and the characteristic clinical parameters were assessed. The serum levels of IL-21(interleukin-21), CXCL13 (chemokine (C-X-C motif) ligand 13) and TGF-β (Transforming growth factor-β) were measured by ELISA. The formation of ectopic germinal center (GC) of synovial membrane was examined by H&E staining. The transcriptional levels of CXCR5 (C-X-C chemokine receptor type 5), CXCL13, ICOS (inducible co-stimulater) and TGF-β mRNA were also analyzed. In addition, the expression of Bcl-6 (B-cell lymphoma 6), CXCR5, CXCL13 and ICOS in synovial membrane were examined by immunohistochemistry.

Results: RA patients had more Tfh cells in peripheral blood, conversely, the frequency of blood Tfr cells (p < 0.05) and the ratio of Tfr/Tfh were significantly decreased compared to healthy controls (p < 0.05, p < 0.01). Furthermore, the ratio of Tfr/Tfh was negatively correlated with values of ESR (r=-0.57, p < 0.05), RF (r=-0.5275, p < 0.001), CRP (r=-0.4486, p < 0.001), IgG (r=-0.4631, p < 0.05), DAS28 scores (r=-0.5645, p < 0.01), as well as the levels of IL-21(r=-0.7398, p < 0.01), CXCL13 (r=-0.4832, p < 0.05). However, the ratio of Tfr/Tfh was positively with the serum level of TGF-β (r=0.5115, p < 0.05). Higher mRNA expression of CXCR5, CXCL13, ICOS and lower TGF-β mRNA expression were observed in RA patients. The serum expression level of IL-21, CXCL13 was significantly increased and expression of TGF-β was significantly decreased in RA patients. Furthermore, ectopic germinal center formation and higher expression of Bcl-6, CXCR5, ICOS, CXCL13 in the synovial membrane of the joints in RA patients were observed.

Conclusions: The decreased blood CD4⁺CXCR5⁺Foxp3⁺ Tfr cells/CD4⁺CXCR5⁺ Tfh cells may be responsible for the immunopathogenesis of RA.