Immune response to Aeromonas hydrophila and molecular characterization of polymeric immunoglobulin receptor in juvenile Megalobrama amblycephala.

Polymeric immunoglobulin receptor (pIgR) is an important immune factor in the mucosal immune system of fish, which plays a key role in mediating the secretion and transport of immunoglobulin into mucus. In this study, the full-length cDNA sequence of Megalobrama amblycephala pIgR gene was firstly cloned and the immune response to Aeromonas hydrophila was detected. After being challenged by Aeromonas hydrophila at 3 d, significantly pathological features were observed in intestine, head kidney, spleen, liver and gill of Megalobrama amblycephala. The content of lysozyme (Lys) and the activities of acid phosphatase (ACP) and alkaline phosphatase (AKP) increased significantly at 1 d and reached the peak at 3 d, and the activities of total superoxide dismutase (T-SOD), glutathione peroxidase (GSH-PX) and catalase (CAT) in serum reached the peak at 5 d and 7 d after infection, respectively. The expression level of IL-1β gene reached the peak at 3 d in intestine, 5 d in gill and spleen, 7 d in head kidney and liver of Megalobrama amblycephala after infected by Aeromonas hydrophila, respectively. The TNF-α gene expression reached the peak at 3 d in intestine and gill, 5 d in head kidney and spleen, 7 d in liver after infection, respectively. The experimental results showed that the infection of Aeromonas hydrophila caused the pathological changes of immune-related tissues and triggered the inflammation responses. The full-length cDNA sequence of Megalobrama amblycephala pIgR was 1828 bp, and its open reading frame (ORF) was 1023 bp, encoding 340 amino acids. The pIgR of Megalobrama amblycephala has a signal peptide sequence, followed by extracellular region, transmembrane region and intracellular region. The extracellular region includes two Ig-like domains (ILDs), and its tertiary structure is twisted "L". The phylogenetic tree was constructed using the adjacency method, and the pIgR genes of Megalobrama amblycephala and cyprinidae fish were clustered into a single branch. Quantitative real-time PCR (qRT-PCR) was used to detect the expression of pIgR gene in different tissues of Megalobrama amblycephala. The expression level of pIgR gene was the highest in liver, followed by intestine, head kidney, skin, middle kidney and spleen, lower in heart, gill and brain, and the lowest in muscle. After being infected by Aeromonas hydrophila, the expression level of Megalobrama amblycephala pIgR gene in intestine, head kidney, spleen, liver and gill showed a trend of increasing first and then decreasing within 28 d. The pIgR gene expression reached the peak in mucosal immune-related tissues (gill and intestine) was earlier than that in systemic immune-related tissues (head kidney and spleen), and the relative expression level of pIgR gene at peak in intestine (12.3 fold) was higher than that in head kidney (3.73 fold) and spleen (7.84 fold). These results suggested that Megalobrama amblycephala pIgR might play an important role in the mucosal immune system to against Aeromonas hydrophila infection.