Development and adaptations of innate immunity in the gastrointestinal tract of the newly hatched chick

The intestinal immune system in Gallus species must rapidly adapt to the omnivorous onset of an adult diet and to colonization by commensal bacteria. Yet, acquired immune functions in Gallus digestive tract fully develop only towards the end of the first week post-hatch. This raises the question of immune protection in the digestive tract during the first week of life. We postulated that in addition to protection conferred by maternal antibodies, the gut is protected by a functionally sufficient innate immune system at hatch. We studied granulocyte distribution in the gut as well as expression of functional genes representing different cells and activities of the innate immune system in chicken hatchlings. These included pro-inflammatory cytokines and chemokines (IL-1β, IL-8, K203), antibacterial β-defensins, Gallinacin 1 and 2, and presenilin 1. We demonstrate innate preparedness in the developing chick gut in two circumstances: The first is independent of intestinal exposure to feed and bacteria and is manifested by heterophil maturation in situ. This gut-specific extramedullary granulopoietic process is reported for the first time in the chick, and is supported by β-defensin and presenilin 1 gene expression. The second is responsive to environmental stimuli, and is demonstrated by gradual development of pro-inflammatory functions: Exposure of the gut to feed and bacteria triggered a low but significant increase in IL-1β, IL-8 and K203. This resulted in the possible recruitment of bone marrow-derived heterophils as demonstrated by elevation of β-defensin gene expression. The pro-inflammatory activity in the developing gut also explains the later recruitment of lymphocytes.