Immunological characterization of the rainbow trout bursa

The bursa of Fabricius is an immune organ, located in the caudo-dorsal surface of the cloaca, responsible for the development and maturation of avian B cells. A few years ago, a lymphoepithelial tissue placed caudal to the urogenital papilla of the cloaca analogous to the bursa was identified for the first time in Atlantic salmon (Salmo salar). The salmon bursa was demonstrated to involute around sexual maturation, as in birds. However, no primary lymphoid functions were identified in this tissue. In the current study, we have identified a homologous immune organ in rainbow trout (Oncorhynchus mykiss), a different salmonid species. This lymphoepithelium covering a blind sac, caudal to the anus, was identified in rainbow trout at different stages of development and it also experienced regression in an age-dependent way. It contained abundant IgM⁺ B cells and CD3⁺ cells and especially numerous was the number of MHC II-expressing cells. In contrast to Atlantic salmon, in rainbow trout, the bursa epithelium contained quite a few IgT⁺ B cells but very few IgD⁺ B cells. Thus, by flow cytometry, we could determine that the IgM⁺ B cells identified in the trout bursa had lost surface IgD expression. Interestingly, although an immunization of rainbow trout by bath barely had effects on the bursa at a transcriptional level, when fish were immunized anally with a model antigen, there were significant changes in the levels of transcription of immune genes in this tissue. These included secreted igm, secreted and membrane igd, bcma and prdm1-a2. Altogether these results evidence the existence of a bursa-like immune structure in another teleost species and provide novel information to understand the immune role of this tissue in fish, pointing to a relation to gut immune responses.