Three different
immunoglobulin (
Ig) isotypes can be found in teleost fish,
IgM,
IgD, and the teleost-specific IgT.
IgM is considered to have a systemic activity, and IgT is attributed a mucosal role, similar to mammalian
IgA. In this study, the complete sequence of gilthead sea bream
IgM and IgT in their membrane (m) and soluble (s) forms are described for the first time in a perciform fish. Their constitutive gene expression is analyzed in different tissues, and their regulation upon viral, bacterial, parasitic, mucosal vaccination and dietary challenges are studied. GCB
IgM and IgT have the prototypical structure when compared to other fish Igs. The constitutive expression of sIgM was the highest overall in all tissues, whereas mIgT expression was highest in mucosal tissues, such as gills and intestine.
IgM and IgT were differentially regulated upon
infection. IgT was highly upregulated locally upon
infection with the intestinal parasite Enteromyxum leei or systemically after Nodavirus
infection. Long-term intestinal
parasitic infections increased the serum titer of both isotypes. Mucosal vaccination against Photobacterium damselae subsp. piscicida finely regulated the Ig response inducing a systemic increase of
IgM titers in serum and a local IgT response in skin mucus when animals were exposed to the pathogen by bath challenge. Interestingly,
plant-based diets inhibit IgT upregulation upon intestinal parasitic challenge, which was related to a worse disease outcome. All these results corroborate the mucosal role of IgT and emphasize the importance of a finely tuned regulation of
Ig isotypes upon
infection, which could be of special interest in vaccination studies.