Abstract

CD5 is a type I transmembrane glycoprotein acting as a pleiotropic functional receptor in the mammalian immune response system, mainly presents on the surface of cells associated with the immune system, and is essential for the classification of B cells. In this study, we identify a CD5 homologue in Nile tilapia ( Oreochromis niloticus ). The open reading frame of OnCD5 is 507 bp, encoding 168 amino acids . The deduced amino acid sequence contains a signal peptide region, a transmembrane region and a conserved portion of the cytoplasmic region. Expression analysis indicates that the OnCD5 exhibits constitutive expression in the tested tissues, with the highest expression in thymus . Analysis of the OnCD5 transcription in the classified IgM + and IgM − lymphocytes from anterior kidney, spleen and peripheral blood, and IgM lo and IgM hi lymphocytes from peripheral blood, indicates that the OnCD5 is highly expressed in the IgM  +  lymphocytes, especially in the IgM hi B lymphocytes. Furthermore, the OnCD5 expression is up-regulated significantly in anterior kidney and spleen following challenges of Aeromonas hydrophila and Streptococcus agalactiae in vivo and in vitro , likewise in IgM + B lymphocytes sorted from peripheral blood upon stimulation with LPS . Further, the recombinant OnCD5 protein has the bacteria-binding activity. Taken together, these results reveal that OnCD5 participates in host's defense during pathogen infection, and may play an important role in tilapia B cells.