Previously, our group has shown that the interbranchial lymphoid tissue (ILT) is a distinct structure largely consisting of T cells embedded in a meshwork of epithelial cells, with no direct resemblance to previously described lymphoid tissues. In this study, we aim to focus on the T cell population and the possibility of the ILT being a thymus analog. By characterizing structural responsiveness to Ag challenge, the presence of recombination activating genes, and different T cell–related transcripts, we attempt to further approach the immunological function of the ILT in salmonid gills. In addition to eight healthy individuals, a group of eight infectious salmon anemia virus–challenged fish were included to observe T cell responses related to infection. The results showed reduced size of ILT in the infected group, no expression of RAG-1 and -2, and a high degree of T cell diversity within the ILT. Taking into account that the ILT can be regarded as a strategically located T cell reservoir and possibly an evolutionary forerunner of mammalian MALTs right at the border to the external environment, the alteration in transcription observed may likely represent a shift in the T cell population to optimize local gill defense mechanisms.

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In the version of this article initially published, labeling of the x-axis was missing from Fig. 4. The corrected Fig. 4 is shown below. The figure legend was correct as published and is shown below for reference. The figure has been corrected in the online version of the article, which now differs from the print version as originally published. www.jimmunol.org/cgi/doi/10.4049/jimmunol.1490047 FIGURE 4. T cell-related transcript levels in the control group. Analysis of T cell-related markers show that all transcripts investigated were present in all organs examined. Transcript levels for the control group (n 5 8) are displayed with mean 6 SEM. Transcript levels are normalized with EF1A B and displayed relative to the mean of control gills.