The Distribution of IgT mRNA(+) Cells in the Gut of the Atlantic Salmon (Salmo salar L.)

SIMPLE SUMMARY: IgT is a specific type of antibody found in teleost fish that is crucial for protecting mucosal surfaces. However, studying IgT is challenging due to the limited available cell markers. Here, we investigated the distribution of IgT mRNA(+) cells and pIgR mRNA(+) cells (essential for Ig transport over cell membranes) in Atlantic salmon (Salmo salar) intestines. Using in situ hybridization, we examined two different sizes (developmental stages) of the fish and compared the hybridization-positive cell distribution between vaccinated and unvaccinated fish. Our findings revealed that IgT mRNA(+) cells were mainly located beneath the intestinal mucosa, specifically in the lamina propria, while pIgR mRNA(+) cells were found in both the lamina propria and mucosa. Additionally, vaccinated fish exhibited abdominal adhesions with tissue containing IgT and pIgR mRNA(+) cells. We observed regional variations in the distribution of IgT mRNA(+) cells within the salmon intestine that were unaffected by intraperitoneal vaccination but sensitive to fish age. This study provides new insights into the distribution and dynamics of IgT and pIgR mRNA(+) cells, advancing our understanding of the spatial mucosal immune system and its implications for teleosts, with potential applications in aquaculture. ABSTRACT: The newly discovered IgT(+) B cell is thought to play a dominant role in mucosal immunity, but limited studies have examined its distribution in fish species, hindering our understanding of its function. This study investigated IgT and poly Ig receptor (pIgR) mRNA(+) cell distribution in Atlantic salmon (Salmo salar) gut using RNAscope in situ hybridization (ISH) and assessed the effects of vaccination. The pyloric caeca, mid-intestine (first and second parts), and posterior segment in two weight stages (Group 1: avg. 153 g, Group 2: avg. 1717 g) were examined in both vaccinated and unvaccinated fish. ISH revealed more IgT mRNA(+) cells in the second part of the midgut compared to other intestinal segments, as well as a higher number of positive cells in Group 2 (older fish). In line with previous findings, intraperitoneal vaccination had no significant impact on the number of IgT(+) transcripts. IgT mRNA(+) cells were found mostly in the lamina propria and near capillaries, while pIgR was registered in both the lamina propria and mucosa. Interestingly, vaccinated fish presented adhesions and granulomatous tissue in the peritoneum, with both IgT and pIgR mRNA(+) cells. Taken together, these results suggest that the distribution of IgT mRNA(+) cells in the intestine of Atlantic salmon is region-specific and is not affected by intraperitoneal vaccination but varies with fish age.