Establishment of immune competence in the avian GALT during the immediate post-hatch period.

Population dynamics of intestinal lymphocytes and the temporal development of lymphocyte functions were studied in broiler chicks during the first 2 weeks post-hatch. This period is of major immunological importance as the chick is immediately exposed to environmental antigens and pathogens. We show that the gut-associated lymphoid tissue contains functionally immature T and B lymphocytes at hatch, and that function is attained during the first 2 weeks of life as demonstrated by mRNA expression of both ChIL-2 and ChIFNgamma. Functional maturation occurred in two stages: the first-during the first week post-hatch, and the second during the second week, which was also accompanied by an increase in lymphocyte population, as determined by expression of antigen receptor genes. Evidence is presented to show that in the intestinal milieu cellular immune responses mature earlier, and are a prerequisite for humoral responses. Hence, the lack of antibody response in young chicks is primarily due to immaturity of T lymphocytes.