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BACKGROUND AND AIMS: Differential responsiveness to interleukin (IL)-2 between effector CD4+ T cells (Teff) and regulatory T cells (Treg) is a fundamental mechanism of immunoregulation. The single nucleotide polymorphism rs61839660, located within IL2RA (CD25), has been associated with the development of Crohn's disease. We sought to identify the T cell immune phenotype of IBD patients who carry this SNP. METHOD(S): Teff and Treg were isolated from individuals homozygous (TT), heterozygous (CT) or wild type (CC) for the minor allele at rs61839660, and used for phenotyping (flow cytometry, Cytometry Time Of Flight) functional assays or T cell receptor (TCR) sequencing. Phosphorylation of signal transducer and activator of transcription 5 (STAT5) was assessed in response to IL-2, IL-7 and in the presence of basiliximab, a monoclonal antibody directed against CD25. Teff proinflammatory cytokine expression levels were assessed by reverse transcription quantitative polymerase chain reaction after IL-2 and/or TCR stimulation. RESULT(S): Presence of the minor T allele enhances CD25 expression leading to increased STAT5 phosphorylation and proinflammatory cytokine transcript expression by Teff in response to IL-2 stimulation in vitro. Teff from TT individuals demonstrate a more activated gut homing phenotype. TCR sequencing analysis suggests that TT patients may have a reduced clonal capacity to mount an optimal regulatory T cell response. CONCLUSION(S): rs61839660 regulates the responsiveness of T cells to IL-2 signaling by modulating CD25 expression. As low dose IL-2 is being trialed as a selective Treg modulator in CD, these findings highlight the potential for adverse effects in patients with this genotype.Copyright © The Author(s) 2021. Published by Oxford University Press on behalf of European Crohn's and Colitis Organisation.