Modern bony fishes (teleosts) represent one of the first groups in the tree of evolution to harbour the molecules of the classical adaptive immune system. Even though several similarities between teleosts and mammals can be identified on the molecular level, the anatomical differences are quite striking, and still, questions regarding fundamental aspects of the gross construction are not known. For instance, the existence of lymphatic vessels in teleosts is not evident and still debated.
The primary lymphoid organs of teleosts consist of the thymus and the head-kidney, and secondary lymphoid organs include again the head-kidney but also the spleen. Together with lymph nodes, mucosa-associated lymphoid tissues such as tonsils or Peyer’s patches were not believed to exist in fish. Nevertheless, in 2008 we reported the first identification of lymphoid aggregates in the gills of the Atlantic salmon. Together with a further description followed in 2010, we showed that these aggregates are intraepithelial, they have a distinct organisation, and they consist mainly of T cells embedded in epithelial cells, in many aspects similar to the construction of the thymus. However, there are distinct differences between the gill lymphoid tissue and the thymus both in gene expression patterns and in the anatomical construction. To understand the nature of the tissue, we have infected fish with virus and studied some of its responses. In contrast to all known lymphoid tissues, the gill lymphoid tissue seems to decrease in size and not increase as all other known lymphoid tissues in the same situation do.
Our exploration of this novel tissue has just been started. The gill lymphoid tissue has a construction not described in any other species. It also seems to have functions previously not known in a lymphoid tissue.
Erling Olaf Koppang, Section of Anatomy and Pathology, Norwegian School of Veterinary Science