Summary The development of immunocompetence in chicks after hatching is not fully understood. However, detailed knowledge of immunocompetence and maturation processes thereof in day-old chicks and juvenile chickens is necessary to implement enhanced immunization strategies. In the case of viral diseases, this especially includes the development of cellular immunity focusing on T cell-dependent responses. In the current study, we investigated T cell subsets in blood and lymphoid tissue of 1- to 21-day-old chickens with respect to their cellular composition and localization. We detected an increase of T cell frequencies in blood and spleen and a shift of the CD8α dimer expression towards a CD8αβ expression on the surface of T cells with increasing age. A relocalization of lymphocytes into antigen presentation structures within the spleen was affirmed. In addition, changes in basal mRNA level with increasing IL2 and IFNγ mRNA levels at different ages were measured. These detected changes suggest an improved T cell-dependent antiviral response with increasing age in chicken. To confirm this finding on a functional level, we conducted a transfer experiment: adult and, as negative control, neonatal naïve lymphocytes were transferred into day-old chicks. Afterwards the protection induced by these transferred cells was verified by a sublethal infection using a highly pathogenic avian influenza (HPAI) mutant carrying a deleted neuraminidase, H5Ndel. Previous experiments have shown that adult animals survive infection with this virus strain, while naïve day-old chicks show severe symptoms or even die. As a result the transfer of adult, but not neonatal lymphocytes, confers protection to day-old chicks against the infection, demonstrating functional differences in lymphocytes from chicks of different ages. Collectively, these data reveal the inability of chicks to mount an effective, cellular antiviral response in the first three weeks of life. Therefore, we propose that the observed maturation of both the innate and the adaptive arms of the immune system early in development is mandatory for controlling influenza infection in the chicken, as well as for an effective vaccination with replication-competent viral vaccine strains.

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