OBJECTIVEA growing body of evidence suggests that the pineal hormone, melatonin (MLT), has immunomodulatory properties; MLT can induce an increment of cell proliferation and an increase or a decrease of a number of cytokines in adults' peripheral blood mononuclear cells (PBMC). However, the influence of MLT on the modulation of neonatal cord blood mononuclear cells (CBMC) proliferation has not been reported. The present study aimed to investigate the possible regulatory effects of MLT on the proliferation of CBMC in vitro.

METHODSTen cord blood preparations from placenta vena umbilicalis of 10 healthy full-term normally delivered newborns and 10 healthy adult volunteers' peripheral blood preparations as controls were obtained. Cord/peripheral blood mononuclear cells suspension were prepared, 2 x 10(5) cells were added to 96-well plate and co-cultured with different stimulants (in cell cultures containing 5 microg phytohemagglutinin (PHA)/ml, 50 ng MLT/ml, 5 ng MLT/ml, 500 pg MLT/ml, 50 pg MLT/ml, 50 ng IL-2/ml, 5 ng MLT/ml + 5 microg PHA/ml or 5 ng MLT/ml + 50 ng IL-2/ml) for 72 h, while the cell suspension (with no stimulant) was used as controls, also cultured for 72 h. With the methods of microscopic examination and (3)H-TdR incorporation test, the influence of melatonin on CBMC morphology and proliferation were investigated, and the effects of MLT on the proliferation of CBMC and PBMC were compared with LSD-t T test and independent samples T test.

RESULTSIn CBMC, MLT (50 pg/ml - 50 ng/ml) increased the (3)H-TdR incorporation rates in a dose-dependent manner, the rate was the highest in the concentration of 5 ng/ml. After 72 h of cell culture, the number of cells in the MLT (5 ng/ml)-exposed group was higher than that recorded before incubation when observed under the high power microscope, including many big mononuclear cells. After adding different stimulants MLT (5 ng/ml), IL-2 (50 ng/ml), MLT plus PHA (5 microg/ml) or MLT plus IL-2 into mediums, the (3)H-TdR incorporation rates of CBMC (cpm) was 114 327 +/- 52 863, 16 087 +/- 9006, 118 360 +/- 59 207 and 17 682 +/- 7391 respectively. In comparison with the controls (14 133 +/- 8688), the incorporation rates of both MLT-exposed group and MLT + PHA-exposed group increased significantly (t = 5.9143, P < 0.001; t = 5.5078, P < 0.001); the rate of IL-2-treated group or MLT + IL-2-treated group demonstrated no significant changes (t = 0.4938, P > 0.05; t = 0.9839, P > 0.05); while the incorporation rates of MLT-exposed group or MLT + PHA-exposed group had no significant difference compared with that of PHA-exposed group (t = 0.1730, P > 0.05; t = 0.3286, P > 0.05). After adding different stimulants into the medium, the incorporation rates of CBMC were all higher than those of PBMC.

CONCLUSIONSMLT can promote not only the proliferation of PBMC, but also the proliferation of CBMC, and the effects of MLT on CBMC were stronger than those on PBMC. This suggests that MLT could be involved in the regulation of the newborn immune system and suggests a new immunotherapeutic strategy in the treatment of certain diseases of neonates.