Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved. Objective. The risk of AS is associated with genomic variants related to antigen presentation and specific cytokine signalling pathways, suggesting the involvement of cellular immunity in disease initiation/progression. The aim of the present study was to explore the repertoire of TCR sequences in healthy donors and AS patients to uncover AS-linked TCR variants. Methods. Using quantitative molecular-barcoded 50-RACE, we performed deep TCR β repertoire profiling of peripheral blood (PB) and SF samples for 25 AS patients and 108 healthy donors. AS-linked TCR variants were identified using a new computational approach that relies on a probabilistic model of the VDJ rearrangement process. Results. Using the donor-agnostic probabilistic model, we reveal a TCR β motif characteristic for PB of AS patients, represented by eight highly homologous amino acid sequence variants. Some of these variants were previously reported in SF and PB of patients with ReA and in PB of AS patients. We demonstrate that identified AS-linked clones have a CD8+phenotype, present at relatively low frequencies in PB, and are significantly enriched in matched SF samples of AS patients. Conclusion. Our results suggest the involvement of a particular antigen-specific subset of CD8+T cells in AS pathogenesis, confirming and expanding earlier findings. The high similarity of the clonotypes with the ones found in ReA implies common mechanisms for the initiation of the diseases.