Secretory immunoglobulin M (sIgM) heavy chain gene in Siniperca kneri Garman: Molecular characterization and expression analysis in response to Aeromonas hydrophila challenge

Abstract

Immunoglobulin M (IgM) is widely recognized as the major isotype of teleost immunoglobulins. Its role in protecting fish against a number of pathogens is critical. In the current study, the IgM heavy chain gene of Siniperca kneri Garman was cloned for the first time. Its IgM heavy chain cDNA is 1952 bp in length and encodes a polypeptide of 583 amino acid residues, which includes a signal peptide, a variable region (VH) and four constant regions (CH1, CH2, CH3 and CH4). This confirms the secretory pattern of IgM (sIgM). The sIgM amino acid sequence shares high identity with those of Siniperca chuatsi (88.57%), Lateolabrax japonicus (65.73%) and Latris lineata (64.92%). Analysis of phylogenetic trees shows that the sIgM of S. kneri Garman is clustered with perciformes fish homologues and constructed as one clade. QRT-PCR analysis found the highest sIgM expression in the head kidney, with higher expressions in the spleen and liver than in the brain, intestine, mesonephros, gill, skin and heart. After Aeromonas hydrophila injection, the sIgM expression pattern was up-regulated and peaks occurred in the head kidney, spleen, intestine and gill on days 5, 7 and 21 post-challenge. Changes in the peak expression of sIgM compared to controls were most significant in the head kidney (38.59-fold) and spleen (23.75-fold), with smaller differences in the gill (9.54-fold) and intestine (7.75-fold). This suggests that it is possible that sIgM is of greater importance to systemic immunity than mucosal immunity. Its importance to the adaptive immune response is thereby confirmed.