Embryonic age influences the capacity for cytokine induction in chicken thymocytes

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Abstract
Thymocyte responses to functional activation are of relevance to the evaluation of the efficacy of in ovo immunotherapies and vaccines in chickens. In this study we have demonstrated differences in chicken thymocyte responses according to developmental age. RNA samples from stimulated and unstimulated chicken thymocytes were assayed for messenger RNA encoding the cytokines interleukin‐1β (IL‐1β), IL‐2, interferon‐α (IFN‐α), IFN‐β, IFN‐γ and transforming growth factor‐β4 (TGF‐β4), and also components of the major histocompatibility complex (MHC), β2‐microglobulin (β2M) and the MHC class I α‐chain (MHC IA). At embryonic day 14 thymocytes were least responsive to functional activation and differences existed even between thymocyte populations at embryonic day 18 and day 1 post‐hatch. The duration of proliferation in response to stimulation was found to increase with increasing embryonic age. Mitogen stimulation of embryonic day 18 and day 1 post‐hatch thymocytes induced up‐regulation of IFN‐γ, IL‐1β and TGF‐β transcripts, and down‐regulation of IFN‐α, IFN‐β and IL‐2 transcripts, with a higher induction of IFN‐γ, IL‐1β and TGF‐β transcripts in more immature T‐cell‐receptor‐negative (TCR−) than TCR+ (TCR1+, TCR2+, or TCR3+) subsets. In contrast, in the mouse and human, both mature and immature thymocytes respond to mitogen stimulation with up‐regulation of IL‐2. Thymocytes from embryonic day 14 chicks responded to mitogen with a short burst of unsustained proliferation, and transcriptional down‐regulation of the cytokines IL‐2, IL‐1β, IFN‐α, IFN‐β and IFN‐γ. These results suggest that embryonic day 14 thymocytes are largely unresponsive to mitogen. Transcripts encoding TGF‐β and type I interferons (IFN‐α and IFN‐β) were constitutively expressed at high levels in very early thymocytes at embryonic day 14. Thymocytes at embryonic days 14 and 18 and day 1 post‐hatch responded to mitogen stimulation with up‐regulation of MHC IA transcript. The pattern of β2M transcription following mitogen stimulation was distinct from that of the globally up‐regulated MHC IA transcript, with up‐regulation of β2M transcription observed at embryonic day 18 and day 1 post‐hatch but not at embryonic day 14. In thymocyte subsets, up‐regulation of β2M transcription was found to be specific to the CD8+ TCR+ population. The balance of responses in the embryonic thymus suggests that at all stages thymocytes have a reduced capacity for activation in comparison to mature thymocyte populations.