With the importance of vitamin E adequacy to a strong immune system so clearly demonstrated, much of the research on the subject is now exploring the modes of action that are involved.
In work with broilers at the University of Arkansas, Erf et al. (1998) shed more light on the relationship between vitamin E status and the development of T cells, leukocytes that develop mainly in the thymus but also in the spleen. This work involved 1,900 male broiler chicks whose diet from hatch through seven weeks of age included four treatment levels of vitamin E supplementation: 0; 15,454; 41,818; or 79,090 IU per ton of feed (0, 17, 46 or 87 IU/kg). The highest level of supplementation was roughly three to five times the level found in many commercial broiler operations, the authors noted.
T cells' immune functions include both direct destruction of antigens and the activation of other immune cells. However, these functions belong to two separate subpopulations of T cells, commonly called killer and helper cells, which differentiate as they mature.
Killer T cells destroy specific target cells, primarily virus-infected and tumor cells. Helper T cells, by contrast, produce chemicals (cytokines) that activate B cells, macrophages and killer T cells, and also influence the class of antibody produced by the B cells. Thus, while less directly lethal to antigens, helper T cells have a more central function in regulating the immune response than killer T cells.
Erf et al. (1998) reported increases in both the percentage of T cells that had developed into helper cells and the percentage of all T cells that were mature with increased dietary vitamin E supplementation (P < 0.05). An increase in the total percentage of T cells that are mature is important because immature T cells undergo selection processes that result in the death of more than 95 percent of these cells before they differentiate into functioning helper or killer cells.
In seven-week-old broilers receiving the highest vitamin E supplementation, fully 29.5 percent of the T cells in the thymus were mature, capable of performing the helper or killer function (Figure 1). In unsupplemented birds, by contrast, mature T cells accounted for only 18.6 percent of the total T cell population at seven weeks of age. For the intermediate treatments, total mature T cell populations in the thymus at seven weeks of age increased as the vitamin E level increased.
