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Objective: To investigate the effects of leptin on different T-cell populations, in order to gain more insight into the link between leptin and obesity., Methods: Three hundred and nine RRMS patients and 322 controls participated in a cross-sectional survey, to confirm whether excess weight/obesity in adolescence or early adulthood increased the risk of MS. Serum leptin levels were determined by ELISA. MBP 83-102 , and MOG 63-87 peptide-specific T cells lines were expanded from peripheral blood mononuclear cells. Leptin receptor expression was measured by RT-PCR and flow cytometry. Bcl-2, p-STAT3, pERK1/2, and p27 kip1 expression were assayed using ELISA, and apoptosis induction was determined by Annexin V detection. Cytokines were assessed by ELISPOT and ELISA, and regulatory T cells (Tregs) by flow cytometry., Results: Logistic regression analysis, showed excess weight at age 15, and obesity at 20 years of age increased MS risk (OR = 2.16, P = 0.01 and OR = 3.9, P = 0.01). Leptin levels correlated with BMI in both groups. The addition of Leptin increased autoreactive T-cell proliferation, reduced apoptosis induction, and promoted proinflammatory cytokine secretion. Obese patients produced more proinflammatory cytokines compared to overweight/normal/underweight subjects. Inverse correlation was found between leptin levels and circulating Treg cells (r = -0.97, P < 0.0001). Leptin inhibited Treg proliferation. Effects of leptin on CD4 + CD25 - effector T cells were mediated by increased STAT3 and ERK1/2 phosphorylation, and down modulation of the cell cycle inhibitor P27 kip1 . In contrast, leptin effects on Tregs resulted from decreased phosphorylation of ERK1/2 and upregulation of p27 kip1 ., Interpretation: Leptin promotes autoreactive T-cell proliferation and proinflammatory cytokine secretion, but inhibits Treg-cell proliferation., (© 2020 The Authors. Annals of Clinical and Translational Neurology published by Wiley Periodicals LLC on behalf of American Neurological Association.)