Cell-mediated and humoral immune responses in broiler chickens maintained on diets containing different levels of vitamin A

Broiler chickens were examined for the effects of low (400 IU/kg), standard (1,500 IU/kg), or high (15,000 IU/kg) dietary vitamin A (VitA) levels on immune responsiveness postimmunization to Newcastle disease virus (NDV). A control pair-fed group (1,500 IU/kg) was included to compensate for the reduced feed intake associated with diet containing the low level of VitA. Interdigital skin reactions to phytohemagglutinin (PKA) and CD4:CD8 T lymphocyte ratios were significantly reduced in chickens fed the low VitA diet whereas their antibody responses to NDV were significantly increased as compared to birds that consumed the 1,500 to 15,000 VitA diet ad libitum. On the other hand, birds on the high VitA diet had reduced lymphocyte responses to concanavalin A and pokeweed, but not to PHA. No effect of dietary VitA was observed for natural killer activity, nor on levels of percentage of cells expressing Class II MHC antigens among groups that consumed feed ad libitum. The results indicated that both humoral and cellular immune responses were modulated by levels of VitA in the diet, and suggest that VitA-deficient chickens developed a T helper (Th)2 immune response, whereas the chickens fed highly enriched VitA diet showed a Th1 immune response.