Direct effects of β-glucans
Dietary supplementation of fish with β-glucans is commonly associated with immune modulation and is often considered beneficial for fish health. The exact mechanisms of immunomodulation by β-glucans in fish are stull under debate, however. The prototypical C-type lectin receptor (CLR) named Dectin-1 which directly senses the presence of β-glucans in mammals, appears missing from fish genomes. In our studies on common carp, direct immunomodulating effects of β-glucans on macrophages exist, at least in vitro, and could be the result of signalling mediated by one or more candidate β-glucan receptors, all members of the CLR family. Also, β-glucan-induced regulation of gene expression profiles typical of the CLR signalling pathway confirm C-type lectin receptors could indeed play a role in the direct sensing of β-glucans in fish.
Long-lived effects on innate immunity
Trained immunity is a form of innate immune memory which can possibly be induced by feeding β-glucans. Given the evolutionary position of fish as early vertebrates with a fully developed immune system, trained immunity should apply and include the ability of fish macrophages to mount heightened responses to a secondary stimulus in a non-specific manner. At least in vitro, carp macrophages can indeed be trained by β-glucans. Trained unstimulated macrophages display evidence of metabolic reprogramming as well as heightened phagocytosis and increased expression of the pro-inflammatory cytokines. Trained and re-stimulated macrophages showed heightened production of reactive oxygen and nitrogen species. Trained immunity could play a role in long term health-promoting effects of β-glucans in fish, but requires in vivo evidence, which is difficult to provide outside the laboratory.
Indirect effects of short chain fatty acids
Accumulating evidence supports a role for intestinal microbiota and their metabolites in the immunomodulatory effects of β-glucans. Yet, a proven relation in fish between types of diets including β-glucans, and immunomodulation via intestinal microbiota and associated metabolites requires more detailed investigations. Initial 16S rRNA sequencing of the active naive microbiota of carp intestine suggests the presence of bacteria with the capacity to degrade and ferment carbohydrates. This suggests that carp intestinal microbiota can ferment dietary β-glucans, a process leading to the production of short-chain fatty acids (SCFA). This important class of metabolites has been shown to affect the mammalian immune system in different ways. We could show that SCFA such as acetate, butyrate, and propionate, have immunomodulatory effects on carp leukocytes, including nitric oxide production and expression of several cytokines, at least in vitro.
Role of short chain fatty acid receptors
Not only in vitro data but also preliminary in vivo observations provide support to a role of SCFA in β-glucan-induced immunomodulation. A single oral gavage of carp with β-glucans modulated the local expression in the intestine of putative SCFA receptors. This could help explain immunomodulation by β-glucan via SCFA receptors present on intestinal leukocytes. In mammals, SCFAs can affect host cells via ligation to G-protein coupled receptors (GPR). Using genomic resources and cDNA cloning we identified several coding sequences for gpr40-like genes of carp. Phylogenetic analysis shows the carp gpr40L genes have a closer phylogenetic relationship to mammalian GPR43 than to mammalian GPR41.
Future developments
A first in silico screen of genomes of 25 different fish species largely confirms the observations for gpr40L of carp with regards to both, subdivision in three subclasses and conservation of synteny. Our data provide a further step towards an understanding of the function of SCFA receptors and their role in immunomodulation by β-glucans in fish.
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