Rainbow trout and Atlantic salmon interleukin-4/13A (IL-4/13A) genes were identified. They were found expressed at high level in thymus, gill, and skin, in concert with the transcription factor gene GATA-3. High expression levels of IL-4, IL-13, and GATA-3 were also detected in murine thymus, suggesting similar importance of the fish and mammalian homologues for early T cell development. In mammals, combined high expression of IL-4/13 and GATA-3 in tissues other than thymus is mostly indicative of Th2 responses. Th2-skewage may protect fish skin and gill from parasites and from damage by inflammatory Th1 and Th17 responses. The immune milieus of fish gill and skin are relevant to aquaculture, because these tissues are preferred sites for vaccine administration. The similarities between the immune milieus of fish gill and thymus may reflect an evolutionary relationship, since these tissues map close together lining the gill cavity. Expression patterns of IL-4/13A and interferon gamma (IFN-γ) in isolated trout gill cells and pronephrocytes were consistent with Th2 identity of IL-4/13A.
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