Preliminary Study On Immune Priming And The Related Mechanism In Zhikong Scallop Chlamys Farreri

Zhikong scallop Chlamys farreri is an important cultured species in China and contributes enormously to the economic development of coastal provinces. Since the summer of 1997, large-scale mortalities have been causing catastrophic losses to scallop aquaculture. Investigation on scallop immune defense mechanism and searching for ways to enhance the disease resistance would be important to the disease control of scallop and the sustainable development of aquaculture in China. In this paper, comparisons between the immune responses after two sequential stimulations with the same amounts of Listonella anguillarum was performed in order to find out whether there was immunological memory in Zhikong scallops. To ascertain the previous immune experience has positive effect for the scallops to resist a secondary similar immune attack, the scallops were challenged twice by the same bacteria with same concentration, and cumulative mortality and several important immune parameters were recorded. This will help to better understand the internal defense mechanisms of scallop immunity and provide basic data for diseases prevention in shellfish aquaculture.Fluorescent real-time quantitative RT-PCR was used to examine the expression patterns of two important pathogen recognition receptors (PRRs) CfPGRP-S1 and LGBP in haemocytes of scallops after sequential challenges by Gram-negative bacteria L. anguillarum. Both genes were found up-regulated after the twice challenges and the up-regulation of CfPGRP-S1 after the secondary challenge was higher and earlier than that after the first challenge. The up-regulation of LGBP after the secondary challenge appeared earlier than that after the first challenge. It suggested that the pathogen recognition by the PRRs after the secondary challenge was enhanced and more rapidly responded than that after the previous challenge. Meanwhile, there was no significant difference in phagocytic rate (PR), antibacterial activity of hemolymph, SOD and MDA contents of hepatopancreas after the twice challenges, but the up-regulation of phagocytic index (PI) was higher and earlier after the secondary challenge than that after the first challenge. It suggested that phagocytes responded more powerfully and earlier after the secondary challenge than that after the first challenge. The enhanced pathogen recognition and the antibacterial resistance were known as two important characteristics of immunological memory in vertebrates. The enhanced recognition and phagocytosis after the secondary challenge suggested that it was possible for the scallops to have immune priming which was functionally similar to immunological memory in vertebrates. To ascertain immune priming in scallops, the scallops were challenged by a short and a long immersion in L. anguillarum solution. And the cumulative mortality of the scallops received a previous challenge was significant (P<0.01) lower than that of the control, implying that the first short-term immersion challenge endowed the scallops an enhanced resistance to the secondary challenge. Comparison of the immune parameters during the secondary challenge suggested that phagocytosis, ACP and AKP activity but not POL and SOD activities of the pre-challenged scallops were significantly higher than that of the non-challenged scallops. It implied that the enhanced phagocytosis and hydrolytic enzymes, such as ACP and AKP, were related to the enhanced immune resistance to the secondary immune challenge by L. anguillarum. All the results suggested that there was immune priming in the scallops and a short-term stimulation could give the scallops an enhanced immune ability to the secondary stimulation by the same pathogens.