OBJECTIVEIt is well known that vitamin A can improve mucosal immunity and anti-infection immunity. But the mechanisms thereof remain to be clarified. Previous studies on the role of vitamin A in immune regulation focused on lymphocytes, whereas little had been done about dendritic cells, which play very important roles in immune response. The objective of this study was to understand the effects of retinoic acid (RA), the metabolic product of vitamin A in vivo,on the differentiation, maturation and functions of dendritic cells from cord blood.

METHODSCord blood samples were collected from nine well-nourished full-term neonates. Mononuclear cells were isolated by Ficoll-Hypaque gradient centrifugation and cultured in the presence of 1000 u/ml GM-CSF, 500 u/ml IL-4 for 6 days, then TNF-alpha 20 ng/ml was added into the medium and cultured for another 3 days. The cells were incubated with or without 1 x 10(-6) MRA. Expression of surface molecules, CD1a, CD83, HLA-DR on DC was measured by flow cytometry. The ability of DC derived from the culture to induce proliferation of T cells in the mixed lymphocyte reaction (allo-MLR) was used for the evaluation of their function. IL-12, IFN-gamma, IL-4 and IL-10 were detected at mRNA levels by RT-PCR to understand the roles of DC treated with RA in regulation of Th1/Th2 balance.

RESULTSOn the sixth day of cell culture, the percentage of DC incubated with RA (57.28 +/- 9.22) was much lower than that without RA (79.57 +/- 11.85) (P < 0.001), but on the ninth day, there were no differences between the presence or absence of RA (76.18 +/- 10.27 vs. 73.72 +/- 15.58). When RA was added to the medium and the culture was continued for nine days, the percent of immature DC (CD1a + HLA-DR+) was much higher than that of the control (absence of RA) (58.93 +/- 4.70 vs. 45.80 +/- 7.88, t = 6.575, P < 0.001); whereas, mature DC (CD83 + HLA-DR+) percentage was markedly lower than that of the control (17.25 +/- 8.49 vs. 27.92 +/- 13.94, t = 4.435, P = 0.002). The T lymphocytes proliferation induced by the DC treated with RA was reduced from 16 857 +/- 3 643 to 11 924 +/- 2 576 cpm (t = 5.598, P < 0.001) in allo-MLR. Expression of mRNA for IL-12p35, IL-12p40, IFN-gamma in the cells that had been incubated with RA declined, but IL-10, IL-4 increased significantly.

CONCLUSIONVitamin A inhibited the differentiation and maturation of cord blood DC, reduced it's ability to stimulate allo-T lymphocytes proliferation, and down-regulated Th1 cytokines, up-regulated Th2 cytokines, consequently made immune response inclined to Th2.