Objective: Graves' disease (GD) and Hashimoto's thyroiditis (HT) are characterized by lymphocytic infiltrates partly resembling secondary lymphoid follicles in the thyroid. CXCR5 and its ligand CXCL13 regulate compartmentalization of B-and T-cells in secondary lymphoid organs. The aim of the study was to elucidate the role of this chemokine receptor -ligand pair in thyroid autoimmunity. Methods: Peripheral blood and thyroid-derived lymphocyte subpopulations were examined by flow cytometry for CXCR5. CXCR5 and CXCL13 cDNA were quantified in thyroid tissues by real-time RT-PCR. Results: We found no differences between the percentages of peripheral blood CXCR5þ T-and B-cells in GD patients (n ¼ 10) and healthy controls (n ¼ 10). In GD patients, the number of memory CD4 þ cells expressing CXCR5 which are functionally characterized as follicular B helper T-cells is higher in thyroidderived (18^3%) compared with peripheral blood T-lymphocytes (8^2%). The highest CXCL13 mRNA levels were found in HT (n ¼ 2, 86.1^1.2 zmol (10 221 mol) cDNA/PCR) followed by GD tissues (n ¼ 16, 9.6^3.5). Only low amounts were determined in thyroid autonomy (TA) (n ¼ 11) thyroid tissues, irrespective of whether the autonomous nodule (0.5^0.1) or the surrounding normal tissue (1.8^0.7) had been analyzed. The same differences were found for CXCR5 (HT: 179.1^6.8; GD: 17.4^10.6; TA nodule : 0.8^0.5; TA normal : 4.4^3.6). In GD, there is a correlation between CXCL13 and CXCR5 mRNA levels and the number of focal lymphocytic infiltrates and germinal centers as well as anti-thyroperoxidase but not anti-TSH receptor autoantibodies. Conclusions: CXCR5 and CXCL13 play an essential role in maintaining B-and T-cells in lymphocytic infiltrates and ectopic follicles in thyroid tissue from patients affected by autoimmunity. 150 225-234 European Journal of Endocrinology

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