Blocking of melatonin synthesis and MT(1) receptor impairs the activation of Jurkat T cells.

Melatonin has been proposed as regulating the immune system by affecting cytokine production in immunocompetent cells, enhancing the production of several T helper (Th)1 cytokines. To further investigate the melatonin's role in IL-2/IL-2R system, we established an inducible T-REx expression system in Jurkat cells in which the protein levels of HIOMT enzyme or MT(1) receptor were significantly down-regulated upon tetracycline incubation. We found that T-REx Jurkat cells with lower levels of HIOMT activity, and consequently lower content of endogenous melatonin, showed IL-2 production decrease after activation with lectin. Likewise, tetracycline-inducible stable cell line expressing MT(1) antisense produced decreased amounts of IL-2 (mRNA and protein levels) after stimulation. Moreover, in T-Rex-MT(1) cells incubated with tetracycline, a sub-optimal PHA dose failed to induce the early activation marker CD25 on the cell surface. The results shown here support the relevance of endogenous melatonin and its signaling in T cell activation.

Evidence of immune system melatonin production by two pineal melatonin deficient mice, C57BL/6 and Swiss strains.

We evaluated two pineal melatonin deficient mice described in the literature, i.e., C57BL/6 and Swiss mice, as animal models for studying the immunomodulatory action of melatonin. Plasma melatonin levels in C57BL/6 and Swiss strains were detectable, but lower than levels in control C3H/HENHSD mice. Since these strains are suppose to be pineal melatonin deficient an extrapineal melatonin synthesis may contribute to plasma levels. Regarding cells and tissues from the immune system, all of them were found to synthesize melatonin although at low levels. N-acetyltransferase (AANAT) mRNA was also amplified in order to analyze the alternative splicing between exons 3-4 described for pineal C57BL/6 mice which generates an inclusion of a pseudoexon of 102 bp. For the pineal gland, both the wild type and the mutant isoforms were present in all mice strains although in different proportions. We observed a predominant wild type AANAT mature RNA in thymus, spleen and bone marrow cells. Peripheral blood mononuclear cells (PBMC) culture shown an evident AANAT amplification in all strains studied. Although the bands detected were less intense in melatonin deficient mice, the amplification almost reached the control cell intensity after stimulation with phytohemaglutinin (PHA). In summary, melatonin detection and AANAT mRNA expression in inbred and outbred mice clearly indicate that different cells and tissues from the immune system are able to synthesize melatonin. Thus, the pineal defect seems not to be generalized to all tissues, suggesting that other cells may compensate the low pineal melatonin production contributing to the measurable plasma melatonin level.