Background Human CD4+ T cells can be classified as either naïve, CCR7+CD45RA− central memory (TCM), or CCR7−CD45RA− effector memory (TEM) cells. In addition to CD45RA and CCR7, CD27 and CD28 have been used as surface markers to characterize CD4+ T cells. Human regulatory T (Treg) cells play an indispensable role for the maintenance of self tolerance and immune homeostasis. Previous studies have shown that the frequency of Treg cells in synovial fluid from patients with rheumatoid arthritis (RASF) is higher than that in peripheral blood from RA patients (RAPB). In addition, Treg cells in RASF have been reported to have impaired suppressive function compared to those in RAPB. However, the cause of impaired function of Treg cells in RASF is not fully understood. Recently, it has been demonstrated that human Treg cells can be separated into three functionally unique subpopulations: (fraction (Fr.) I) CD45RA+Foxp3low naïve Treg cells; (Fr. II) CD45RA−Foxp3high activated/effector Treg cells, both of which have suppressive functions, and (Fr. III) non-suppressive cytokine-secreting CD45RA−Foxp3low non-Treg cells.
Objectives To determine the characteristics of CD4+ T cells and Foxp3+ Treg cells in RASF.
Methods Synovial fluid and peripheral blood CD4+ T cells from RA patients were classified into different subsets based on the expression of CD45RA, CCR7, CD27, and CD28. The frequency of IFN-γ-, IL-17-, or TNF-α-producing cells, and of Foxp3- or RANKL-positive cells in each subset was analyzed by eight-color flow cytometry. CD4+Foxp3+ cells were further classified into three fractions based on the expression of CD45RA and Foxp3.
Results Synovial fluid CD4+ T cells from RA patients were classified into six functionally distinct subsets based on the expression of CCR7, CD45RA, CD27 and CD28. Only a small population of naïve cells was detected in RASF, and the TEM subset accounted for about 53% of RASF, with the CD27+CD28+ TEM subset being the largest population. Foxp3+ cells were almost exclusively present in the CD27+CD28+ TCM and CD27+CD28+ TEM subsets in RASF. The frequency of Foxp3+ cells in the CD27+CD28+ TEM population was significantly increased in RASF, compared to that in RAPB (Figure 1A). The percentages of IFN-γ-, IL-17-, and TNF-α-producing cells in the CD27−CD28+ TCM and CD27−CD28+ TEM subsets of RASF were significantly higher than those of RAPB. Notably, most of the Foxp3+ cells in RASF were non-suppressive CD45RA-Foxp3low non-Treg cells (Fr. III). The frequency of CD45RA-Foxp3low non-Treg cells (Fr. III) in RASF was significantly increased compared to that in RAPB (Figure 1B). Furthermore, the CD45RA−Foxp3low non-Treg cells (Fr. III) in CD27+CD28+ TEM subset was significantly increased in RASF, compared to that in RAPB (RASF; 5.9±4.0%, RAPB; 0.7±0.5%). Collectively, increase in Foxp3+ cells in RASF was due to increased number of non-Treg cells (Fr. III) which exist in CD27+CD28+ TEM subset.
Conclusions Most of the Foxp3+ Treg cells in RASF were non-suppressive CD45RA−Foxp3low non-Treg cells, and the frequency of the non-Treg cells in the CD27+CD28+ TEM subset was significantly increased in RASF. The pro-inflammatory environment in RA joints may induce the increase of non-Treg cells in synovial fluid.
Disclosure of Interest None declared