Previous studies indicated a potential role for mucosal-associated invariant T (MAIT) cells in the pathogenesis of ankylosing spondylitis (AS). 1,2 Active peripheral arthritis and extra-articular manifestations may influence the presence of circulating MAIT cells in AS. Objectives: To investigate circulating MAIT cells in a homogenous group of axial spondyloarthritis (SpA) patients with only axial involvement in comparison to ageand sex-matched healthy controls (HC). Secondly, to explore the

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... ociation of MAIT cells with symptom duration and disease activity. Methods: Consecutive axial SpA patients from the Groningen Leeuwarden axial SpA (GLAS) cohort without active peripheral arthritis, inflammatory bowel disease, psoriasis or uveitis were included. Patients with active infections or current use of biologics were excluded to rule out possible influence on the presence of circulating MAIT cells. Disease activity was assessed using ASDAS, BASDAI, and serum CRP levels. The frequencies and absolute numbers of circulating MAIT cells were examined in peripheral blood of all studied samples by 5-color flow cytometry. Immediately after sampling, EDTA-blood was stained with anti-CD3, anti-CD8, anti-TCRVa7.2, anti-CD161, and anti-TCRgd. After staining, the cells were washed, fixed, and analyzed immediately on FACS. MAIT cells were identified phenotypically as CD3 + CD8 + TCRγδ -Vα7.2 + CD161 high cells. Results: Of the 41 included axial SpA patients, mean age was 46±16 years, 73% were male, mean symptom duration was 23±14 years, and 78% were HLA-B27 positive. Mean ASDAS was 2.6±1.0, mean BASDAI was 4.4±2.5, and median CRP was 3 (range 2-30). 70%, 54% and 37% of axial SpA patients had ASDAS ≥2.1, BASDAI≥4 or CRP≥5, respectively. HC had exactly the same age and sex distribution. Both the percentages and absolute numbers of circulating MAIT cells were comparable between axial SpA patients and HC (Figure 1 ). In axial SpA patients, absolute numbers of MAIT cells correlated negatively (rho=-0.339) with symptom duration. No significant associations were found between MAIT cells and disease activity, except for a negative correlation (rho=-0.332) between frequency of MAIT cells and BASDAI (Table 1 ). There were no significant differences in MAIT cells between axial SpA patients with and without active disease according to ASDAS, BASDAI or CRP.