Studies Into β-Glucan Recognition in Fish Suggests a Key Role for the C-Type Lectin Pathway

Immune-modulatory effects of beta-glucans are generally considered beneficial to fish health. Despite the frequent application of beta-glucans in aquaculture practice, the exact receptors and downstream signalling remains to be described for fish. In mammals, Dectin-1 is a member of the C-type lectin receptor (CLR) family and the best-described receptor for beta-glucans. In fish genomes, no clear homologue of Dectin-1 could be identified so far. Yet, in previous studies we could activate carp macrophages with curdlan, considered a Dectin-1-specific beta-(1,3)-glucan ligand in mammals. It was therefore proposed that immune-modulatory effects of beta-glucan in carp macrophages could be triggered by a member of the CLR family activating the classical CLR signalling pathway, different from Dectin-1. In the current study, we used primary macrophages of common carp to examine immune modulation by beta-glucans using transcriptome analysis of RNA isolated 6 h after stimulation with two different beta-glucan preparations. Pathway analysis of differentially expressed genes (DEGs) showed that both beta-glucans regulate a comparable signalling pathway typical of CLR activation. Carp genome analysis identified 239 genes encoding for proteins with at least one C-type Lectin Domains (CTLD). Narrowing the search for candidate beta-glucan receptors, based on the presence of a conserved glucan-binding motif, identified 13 genes encoding a WxH sugar-binding motif in their CTLD. These genes, however, were not expressed in macrophages. Instead, among the beta-glucan-stimulated DEGs, a total of six CTLD-encoding genes were significantly regulated, all of which were down-regulated in carp macrophages. Several candidates had a protein architecture similar to Dectin-1, therefore potential conservation of synteny of the mammalian Dectin-1 region was investigated by mining the zebrafish genome. Partial conservation of synteny with a region on the zebrafish chromosome 16 highlighted two genes as candidate beta-glucan receptor. Altogether, the regulation of a gene expression profile typical of a signalling pathway associated with CLR activation and, the identification of several candidate beta-glucan receptors, suggest that immune-modulatory effects of beta-glucan in carp macrophages could be a result of signalling mediated by a member of the CLR family.