Melatonin synthesis in human colostrum mononuclear cells enhances dectin‐1‐mediated phagocytosis by mononuclear cells

Many cells in the organism besides pinealocytes, synthesize melatonin. Here, we evaluate both the mechanism of zymosan‐induced melatonin synthesis and its autocrine effect in human colostral mononuclear cells. The synthesis of melatonin was induced by activation of the transcription factor nuclear factor kappa B (NF‐κB), as either the blockade of the proteasome or the binding of NF‐κB to DNA inhibits zymosan‐induced melatonin synthesis. As observed in RAW 264.7 lineage cells, the dimer involved is RelA/c‐Rel. Melatonin plays a direct role in mononuclear cell activity, increasing zymosan‐induced phagocytosis by stimulating MT₂ melatonin receptors and increasing the expression of dectin‐1. This role was confirmed by the blockade of melatonin receptors using the competitive antagonist luzindole and the MT₂‐selective partial agonist 4P‐PDOT. In summary, we show that melatonin produced by immune‐competent cells acts in an autocrine manner, enhancing the clearance of pathogens by increasing phagocyte efficiency. Given that these cells are present in human colostrum for 4 or 5 days after birth, this mechanism may be relevant for the protection of infant health.