Development of immunocompetence of broiler chickens

Subpopulations of T-cells, B-cells, macrophages and ellipsoid-associated re ticular cells (EARC) could be demonstrated by immunohistochemical staining early in the development of chicken spleen. However, the typical structures of the spleen, such as the peri-arteriolar lymphoid sheath (PALS) and the ellipsoids with their surrounding ring of macrophages, were only formed aro und embryonic day (ED) 20. These structures and especially the B-cell compa rtment, i.e., the peri-ellipsoid lymphoid sheath (PELS) gradually matured d uring the first week posthatch. Therefore, we analysed at what age broiler chickens could generate a humora l response against the thymus-dependent antigen bovine serum albumin (BSA). Chickens were immunised in ovo (ED16 and ED18) and at 1, 7 and 12 days of age and subsequent BSA-specific immunoglobulin (Ig) M and IgG responses wer e measured up to 10 days postimmunisation (DPI). No major differences were observed in the relative growth rates, while hatchability was only slightly reduced. Only in chicks immunised on 12 days of age, IgM and IgG responses were high with a normal kinetic pattern. In chicks immunised on 7 days of age, responses were just detectable, but they were absent in chicks immunis ed in ovo and on the day of hatching (Day 1). In a subsequent experiment, 1-, 7- and 12-day-old chicks were BSA-immunised and Ig responses were measured for a longer period up to the age of 28 day s. The IgG response of chicks immunised at 1 day of age was lower and occur red later (from 28 DPI) than the response of chicks immunised at 7 and 14 d ays of age (from 14 DPI). It was not increased by a booster immunisation on 29 days of age, in contrast to the response of chicks immunised at 7 and 1 4 days of age. These findings indicate that vaccination at 1 day of age doe s not activate the B-cell response resulting in antibody production and sup port the idea that the immune function of the late embryonic and neonatal c hickens is not entirely developed due to the incomplete structural organisa tion of their secondary immune organs. (C) 1999 Elsevier Science B.V. All r ights reserved.