Background Primary Sjögren's syndrome (pSS) is a systemic autoimmune disease characterized by lymphocytic infiltration of the exocrine glands. Within circulating CD4⁺ T cells, the proportion of Tfh cells (CD45RA^–FoxP3^–CXCR5⁺PD-1⁺) is increased in pSS patients, compared with healthy controls (HCs). Increased levels of circulating Tfh cells are associated with higher systemic disease activity, as measured by serum IgG titers and the European League Against Rheumatism (EULAR) Sjögren's syndrome Disease Activity Index (ESSDAI). However, it is unknown if the proportion of T follicular regulatory (Tfr) cells is also altered. Tfr cells may play an important role in pSS pathogenesis, since the ratio between T follicular regulatory (Tfr) cells (CD45RA^–FoxP3⁺CXCR5⁺PD-1⁺) and Tfh cells seems to dictate B cell responses.¹

Objectives To analyze the Tfr/Tfh ratio in pSS patients and HCs and to assess the functional suppressive capability of Tfr cells by means of their expression of the inhibitory immune receptor cytotoxic T-lymphocyte-associated protein 4 (CTLA-4).

Methods Twenty-four pSS patients (23 female, mean age 44, median ESSDAI 8), diagnosed according to the revised AECG criteria, and 24 sex- and age-matched healthy controls were included. Cryopreserved peripheral blood mononuclear cells were analyzed by flow cytometry for expression of CD3, CD4, CD45RA, FoxP3, CXCR5, PD-1 and CTLA-4. Besides the frequency of positive cells, median expression levels per cell of CTLA-4 were measured using median fluorescence intensity. Statistical analysis were performed using the Mann–Whitney U or the Spearman's Rho test.

Results In addition to the increase in Tfh cells, we found an increased proportion of Tfr cells within the pool of circulating CD4⁺ T cells in pSS patients, compared with HCs (p<0.001). Unexpectedly, the Tfr/Tfh ratio was significantly higher in pSS patients, compared with HCs (p<0.001), pointing towards an immune suppressive equilibrium. However, expression levels of CTLA-4 on Tfr cells from pSS patients were significantly decreased (p<0.001), which suggests that Tfr cells from pSS patients have a lower suppressive capability. Furthermore, lower expression levels of CTLA-4 on Tfr cells were associated with an increase in the Tfh cell proportion (r²=-0.382, p=0.066). CTLA-4 expression levels were also decreased on total Treg cells (CD45RA^–FoxP3⁺) from pSS patients (p<0.001).

Conclusions Although both the proportion of Tfr cells and the Tfr/Tfh ratio are increased in pSS patients, the decreased expression of CTLA-4 by Tfr cells suggests that Tfr cells from pSS patients have a lower suppressive capacity. This may contribute to the expansion of Tfh cells and consequently to hyperactivity of B cells in pSS. Limitation of Tfh cell expansion by immunomodulatory treatment may reduce this B cell hyperactivity.

1. Sage et al. Trends Immunol. 2015; 410–8.

Disclosure of Interest None declared