Immunoglobulin M gene expression analysis of orange-spotted grouper, Epinephelus coioides, following heat shock and Vibrio alginolyticus challenge

The present study aimed to examine the expression of immunoglobulin M (IgM) gene in orange-spotted grouper (Epinephelus coioides) following thermal stress, bacterial infection, and immunization with formalin-killed Vibrio alginolyticus, a kind of bacterial pathogen that causes septicemia. In heat shock experiments, twenty-five healthy orange-spotted grouper were kept in tanks with seawater at 37 ± 0.5 °C for one hour heat-shock treatment, and then returned to 27 ± 0.5 °C seawater tanks. In bacterial challenge experiments, two hundred healthy orange-spotted grouper were infected or immunized intraperitoneally with 0.1 mL V. alginolyticus resuspended in PBS at 5 × 104 cells mL−1. Blood and organ samples (head kidney, spleen, and thymus gland) were collected and frozen immediately in liquid nitrogen for subsequent real-time PCR analyses at various times. IgM mRNA expression decreased significantly in gill, head kidney, spleen, intestine, and thymus gland from the 3rd hour after heat stress (37 °C), and consistently declined until the 48th hour, but increased in blood cells from the 3rd hour to 48th hour. There was a significant increase of IgM gene transcripts in head kidney, spleen, thymus gland and blood cells of the infected and immunized grouper. There was a clear time-dependent expression pattern of IgM mRNA expression after V. alginolyticus infection and vaccination, with a significant increase at 2 weeks post-challenge and a peak at 4 weeks or 5 weeks for the infection or vaccination group, respectively. The level of IgM mRNA expression in the infected grouper was not only higher, but also earlier than that of the immunized group. These data demonstrated that IgM mRNA expression of the grouper was influenced by acute thermal stress and V. alginolyticus challenge.