DEVELOPMENT OF T-CELL IMMUNE RESPONSIVENESS IN THE CHICKEN

Chickens are highly susceptible to infection by opportunistic pathogens during the first few days after hatching. This observation has generally been attributed to an immaturity of the immune system; however, the mechanisms responsible are not known. This study investigated the ability of T cells from chickens of various ages to respond to immune stimulation. Splenic T cells were cultured in vitro and stimulated with various mitogens including Con A, PHA and monoclonal anti-CD3 antibody. T cells obtained from adult chickens proliferated extensively and produced high levels of IL-2, haemopoietic growth factors and IFN following stimulation. In contrast, it was found that T cells from 1 day old chickens failed to proliferate and secrete cytokines when similarly cultured. Reactivity to mitogens gradually developed between days 2 and 4, and by 1 week of age the level of responsiveness was equivalent to that observed with T cells obtained from adult chickens. Whereas T cells from 1 day old chicks were found to be phenotypically mature and capable of binding mitogens as effectively as T cells from adult birds, they were functionally immature as assessed by their inability to proliferate or produce cytokines following immune stimulation. In addition, cells present in the spleen of 1 day old chicks constitutively produced a soluble inhibitor that prevented the proliferation of stimulated adult T cells. The production of inhibitor decreased dramatically by the second day posthatching which coincided with an enhanced ability of T cells to respond to immune stimulation. These findings suggest that the period of transient T cell unresponsiveness to immune stimulation is mediated by a functional immaturity of T cells and the production of a soluble inhibitor(s).