Sacramento PM, et al. - Researchers investigated whether exogenous serotonin (5-HT) is capable of modulating the T-cell behavior of patients with MS. They observed 5-HT caused a reduction in T-cell proliferation and Th1 and Th17 cytokines production in cell cultures from MS patients. By influencing CD8⁺ T cells, 5-HT also attenuated the release of IFN-γ and IL-17. By contrast, the impact on CD4⁺ T cells resulted in an increased IL-10 production. Data also showed that the expansion of FoxP3⁺CD39⁺ regulatory T cells (Tregs) and type 1 regulatory T cells was favored by 5-HT. Elevation in the frequency of Treg17 cells, a novel regulatory T-cell subset, was also noted. Overall, a possible protective role of serotonin in the pathogenesis of MS was suggested.