Background <p>Juvenile Idiopathic Arthritis (JIA) is the most common rheumatic disease in children and is characterized by chronic joint inflammation driven by immune dysregulation. A disturbed regulatory T (Treg) cell-effector T cell axis has been demonstrated in these patients which contributes to a dysregulated immune tolerance in JIA. Therefore, strategies to enhance Treg numbers and suppressive capacity to restore immune homeostasis represent a promising therapeutic approach. Nicotinamide (NAM), a form of vitamin B₃ and precursor of nicotinamide adenine dinucleotide (NAD⁺), has emerged as an immunomodulator capable of promoting Treg cell function.</p> Methods <p>Here, we investigate the effects of NAM on Treg percentages and suppressive capacity in mononuclear cells derived from peripheral blood (PBMCs) and synovial fluid of JIA patients (SFMCs). In an exploratory pilot phase II clinical study, NAM concentrations were measured in paired blood plasma and SF from 6 JIA patients after oral administration of daily 1.8&#xa0;g/m² for three days.</p> Results <p>NAM treatment significantly increased the proportion of FOXP3-expressing CD4⁺ T cells in both PBMCs and SFMCs and Treg suppressive capacity in PBMCs from healthy controls in vitro. Comparable NAM concentrations were detected in plasma and SF following oral administration, demonstrating its ability to penetrate the inflamed joint environment.</p> Conclusion <p>Given its established safety in both adults and children, NAM represents a promising future candidate for adjunctive therapy in JIA by restoring the balance between regulatory and effector T cells. Further clinical and mechanistic studies are warranted to define optimal dosing regimens and its place in the long-term treatment strategy of JIA.</p> Trial registration <p>Assessing penetration of high dose nicotinamide (Vitamin B3) in synovial fluid. EudraCT Number: 2018-002245-11, start date Jan 16th, 2019.</p>