Selective Reduction of <scp>MAIT</scp> Cells in Peripheral Blood T Cell Subsets of Primary Sjögren's Syndrome and Their Clinical Significance

This study aims to assess the distribution of mucosal-associated invariant T (MAIT) cells across CD3+ T cell subsets in human peripheral blood and investigate their changes in patients with primary Sjögren's syndrome (pSS), exploring their potential clinical implications. Peripheral blood from 39 pSS patients and 45 healthy controls was analysed. CD3+ T cells were categorised into four subsets: CD4+CD8-, double-positive (DP), double-negative (DN), and CD8+CD4-. MAIT cells (CD161++TCRVα7.2+) were identified using flow cytometry. Proportions and absolute counts of MAIT cells were compared between pSS patients and controls. Supplementary experiments were conducted on the peripheral blood of 15 pSS patients and 17 healthy controls to analyse the expression of GZMB and perforin in MAIT cells and their four subpopulations (CD4+ MAIT, CD8+ MAIT, DN MAIT, and DP MAIT). MAIT cell-related parameters were correlated with clinical indices, and their diagnostic value was evaluated using receiver operating characteristic (ROC) curves. The proportion and absolute count of MAIT cells were significantly reduced in pSS patients compared to healthy controls. In particular, MAIT cell levels were markedly lower in the DN and CD8+CD4- subsets. Additionally, the absolute counts of MAIT cells were significantly decreased in the DP, DN, and CD8+CD4- subsets, whereas no significant change was observed in the CD4+CD8- subset. Compared with healthy controls, the proportion of GZMB+ cells in the CD8+ MAIT and DN MAIT subpopulations of pSS patients increased. The percentages of MAIT cells within CD3+ T cells were negatively correlated with inflammatory markers, including C-reactive protein (CRP), erythrocyte sedimentation rate (ESR) and anti-SSB antibodies. Notably, MAIT cells in the DN subset were also positively correlated with complement C4 levels. MAIT cell proportions and counts vary across CD3+ T cell subsets, showing significant reductions in pSS patients. These findings highlight the potential role of MAIT cells as a diagnostic biomarker, given their strong associations with clinical disease markers in pSS.