Chemotactic effect of melatonin on leukocytes

Melatonin seems to be an important stimulatory factor of the immune system. This indolamine is capable of inducing activation of leukocytes. Tissue leukocyte infiltration is a key feature of inflammatory and immune responses; however, there is no information about the effect of melatonin on leukocyte chemotaxis. Therefore, the aim of this study was to examine the in vitro and in vivo effects of melatonin on leukocyte chemotaxis, on modulation of leukocyte chemotaxis to other chemoattractants and on the in vivo induction of leukocyte chemokines. Neutrophils and mononuclear leukocytes (PBMC) were isolated by a discontinuous gradient on Hystopaque. Chemotaxis was performed in blind well Boyden's chambers. In vivo chemotaxis was determined after intraperitoneal injection of melatonin into rats. Leukocyte chemotactic response and leukocyte chemokine expression were determined in human volunteers treated with 20 mg daily of melatonin. Increased neutrophils and PBMC chemotaxis in response to 1.2 nm melatonin was observed in vitro. Peritoneal leukocytes were found increased after melatonin injection. Humans treated with melatonin showed an increased neutrophil chemotactic response to a physiological chemoattractant and increased expression of intracellular chemokines; however, decreased chemotactic response and no chemokine expression were observed in PBMC. These data suggest that melatonin could have a relevant role during the tissue leukocyte infiltration in inflammatory and immune responses.