Molecular cloning, characterization, and expression analyses of immunoglobulin tau heavy chain (IgT) in largemouth bass (Micropterus salmoides)

Immunoglobulins (Igs) first appeared in jawless fish, which are humoral mediators playing a vital function in the adaptive immunity to protect the teleosts from various pathogens. In the present study, the sequences of largemouth bass (Micropterus salmoides) membrane-bound and secreted IgT (mIgT and sIgT) were firstly cloned. Largemouth bass mIgT contained an open reading frame (ORF) of 1785 bp encoding a transmembrane protein of 594 amino acid residues, while the sIgT cDNA contained an ORF of 1641 bp encoding a secreted protein of 546 amino acid residues. Multiple alignments analysis showed that amino acid sequences of four constant regions of largemouth bass mIgT and sIgT were exactly the same, and corresponded to other reported teleost IgT/Z. Total twelve conserved cysteine (Cys) residues, eleven conserved tryptophan (W) residues, four N-linked glycosylation sites were found in largemouth bass IgT. Tissue distribution analysis of IgT showed that the highest level was detected in the head kidney, then in spleen, liver, gill, intestine, and skin, lowest in muscle. In addition, the qRT-PCR analysis showed that IgT expressions were increased significantly after immersion vaccination with Aeromonas hydrophila, and much faster and stronger responses were observed in the gill and intestine than in the spleen and head kidney. These results demonstrated that largemouth bass IgT might play a more important role in mucosal immunity than systemic immunity.