Cloning, expression and immune features of Sultan fish (Leptobarbus hoevenii) mannose receptor

In order to understand the structural characteristics of MR and its role in anti-infective immune responses in Leptobarbus hoevenii, we cloned and sequenced the MR of L. hoevenii. Subsequently, real-time polymerase chain reaction (qRT-PCR) assay was used to measure the expression of the MR in the tissues of L. hoevenii with or without F. columnare infection. The results showed that the MR of L. hoevenii (LhMR) open reading frame was 4 296 bp, encoding 1 431 amino acids (aa). The amino acid sequence and molecular structure of LhMR are highly similar to the MRs from other animals, i.e., containing an extracellular ricin-like β-type clover domain (RICIN), a fibronectin type II domain (FNII) and eight tandem C Lectin-like domains (CLECTs), a transmembrane region and a short intracellular region. The mRNA of LhMR was widely expressed in 11 tested tissues, including spleen, kidney, heart, brain, skin, muscle, gills, liver, post intestine, fore intestine and mid intestine, with the highest expression level in the spleen. The mRNA expression of LhMR was significantly increased in the spleen, intestine, kidney and gills of L. hoevenii infected by F. columnare measured by qRT-PCR. Furthermore, the protein expression of LhMR was also increased in the spleen, kidney and gill of L. hoevenii infected by F. columnare revealed by immunochemistry assay. By Hematoxylin eosin staining, there were various pathological changes observed in the body kidney, intestine, liver and gills. In the body kidney, there were blood cells infiltrated in the renal tubules with necrosis in epithelial cells. The wall of intestine became thinner with diffused necrosis. Numerous vacuoles were observed in the liver cells. Gill lamella showed swollen, disordered, necrosis and detached. The results revealed that LhMR plays an important role in the immune responses of L. hoevenii infected by F. columnare and this will provide some reference for prevention and therapy strategies against columnaris disease.