This paper shows that melatonin is able to activate human Th1 lymphocytes by increasing the production of IL-2 and IFN-gamma in vitro. Th2 cells appear not to be affected by melatonin, since IL-4, which is mostly produced by Th2 cells, is not modified by the hormone. Melatonin also enhances IL-6 production by PBMCs. The activation by melatonin of IL-6 production is apparently related to the presence of monocytes, rather than to Th2 cells, in the cell preparation, since PBMCs depleted of monocytes (CD14+ cells) were not activated. Activation of PBMCs by melatonin was dependent on the dose and, measured by cytokine production, was observed only when cells were either not activated or only slightly activated by low concentrations of PHA, or when cell activation was achieved by incubating the cells with previously irradiated cells. Using a different approach to identify what type of cells among the PBMC subsets was activated by melatonin, the expression of CD69, a marker of cell activation, was studied. Melatonin increased the percentage of cells expressing the CD69 Ag in CD4+ but not in CD8+ cells. We have also achieved enhanced production of IL-2 and IL-6 using CGP 52608, a specific ligand of the putative nuclear melatonin receptor RZR/ROR, raising the possibility of direct effects of melatonin on gene regulation in both Th1 cells and monocytes. The results suggest that melatonin may be involved in the regulation of human immune functions by modulating the activity of Th1 cells and monocytes via nuclear receptor-mediated transcriptional control.

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