Exogenous melatonin: quantitative enhancement in vivo of cells mediating non-specific immunity.

Melatonin (MLT), a biogenic indoleamine and neuromodulator produced by the pineal gland, is known to activate T helper cells by means of direct binding to melatonin receptors on both Th1 and Th2 cells. The present in vivo study aimed to investigate the effect of exogenously administered MLT on the hemopoietic and immune cell populations of the bone marrow and spleen in healthy, young adult male mice at two distinct MLT exposure intervals. The neurohormone, administered daily through the diet (7-14 days), was homogenized into finely ground chow. Control mice received ground chow without MLT. The results revealed cell lineage-specific, quantitative, MLT exposure-time-dependent changes in both the bone marrow and spleen. NK cells and monocytes (both components of the non-specific immune system functioning as the first line of defense against neoplasia and virus infected cells) were significantly increased in the bone marrow by both 7 and 14 days of dietary melatonin. The quantitative increment in these two cell populations, in the organ of their production, i.e. the bone marrow, indicates that new cell proliferation/production may have been stimulated by MLT. In the spleen, as in the bone marrow, NK cell levels remained significantly elevated at both 7 and 14 days after melatonin exposure. However, the number of monocytes in the spleen did not maintain, at day 14 of MLT exposure, the high levels observed after 7 days of MLT, in spite of their sustained, high numbers at 14 days in the bone marrow. This suggests that the progeny of the apparently increased monocyte production in the bone marrow (elevated absolute numbers therein), had localized in anatomical sites (other than the spleen) also common to these cells. Thus, the selective, positive influences of in vivo administered, exogenous melatonin on cells mediating non-specific immunity suggests a plausible mechanism for numerous claims that it is responsible for tumor amelioration in patients.