Thymic function in juvenile idiopathic arthritis

Objective: Thymic function declines exponentially with age. Impaired thymic function has been associated with autoimmune disease in adults but has never been formally assessed in childhood autoimmunity. We therefore determined thymic function in children with the autoimmune disease Juvenile Idiopathic Arthritis (JIA). Methods: We measured thymic function in 70 children and young adults with JIA (age range 2.1 – 30.8 (median 10.4)) and 110 healthy age matched controls using four independent assays. We quantified T-cell receptor excision circles (WBLogTREC/ml), the proportion of CD4 + CD45RA + CD31 + T-cells (representing recent thymic emigrants - %RTE) and measured intra-thymic proliferation by calculating the αTREC/ΣTREC ratio. Lastly, regulatory T-cells (T Reg ) of thymic origin (CD4 + FOXP3 + ) were quantified in peripheral blood to assess the ability of the thymus in JIA to generate this T-cell subset. Results: Thymic function was equivalent by all four parameters in JIA when compared with our control population. Furthermore, there was no consistent effect of JIA subtype on thymic function, although intra-thymic proliferation was higher in the small RF + polyarticular group. There were no significant effects of disease modifying anti-rheumatic drugs (DMARDs) or oral corticosteroids on thymic function, although those with the worst prognostic ILAR subtypes were also those most likely to be on a DMARD. Conclusions: We demonstrate that children and young adults with JIA, unlike adults with autoimmune diseases, have thymic function that is comparable with healthy controls. The varied pathologies represented by the term ‘JIA’ suggest this observation may not be disease specific and raises interesting questions about the aetiology of thymic impairment in adult autoimmunity.