The predominant role of mucosal immunoglobulin IgT in the gills of rainbow trout (Oncorhynchus mykiss) after infection with Flavobacterium columnare.

Columnaris disease, induced by Flavobacterium columnare, seriously affects the health of freshwater fish species and damages the mucosal tissues, such as the fins, skin, and gills. Teleosts represent the first bony vertebrate to contain both innate and adaptive immune responses against pathogens. So far, three immunoglobulin isotypes (IgM, IgD, and IgT/IgZ) have been identified in teleost fish, and IgT in mucosal tissues of teleost fish was reported to perform a similar function to IgA in mammals during parasitic infection. However, very limited information is known about the function of IgT in gill mucosal tissues during bacterial infection. In the present study, rainbow trout (Oncorhynchus mykiss) was infected with F. columnare (Fc) via immersion. After Fc infection, the gill structure of rainbow trout showed serious hyperplasia symptoms on the secondary lamellae at 12 h post infection (hpi). Moreover, the mRNA expression levels of NOS2 and cathelicidin-1 were significantly upregulated immediately at 12 hpi and showed high expression throughout the experiment. IgT and IgM showed much higher mRNA expression levels at 28 days post infection (dpi) and 75 dpi, while IgD only showed high mRNA expression levels at 28 dpi. Importantly, the accumulation of IgTB cells and strong bacteria-specific IgT responses were detected in the gill lamellae of both infected fish (28 dpi) and survivor fish (75 dpi). Overall, our results suggest that IgT and IgTB cells play a central role in the adaptive immune responses of fish gill mucosa against bacterial infection.