Immune stimulation by inactivated Vibrio splendidus promotes the development and resistance of oyster Crassostrea gigas larvae

The oyster Crassostrea gigas larvae in embryonic development stage have been suffering high mortality rate during hatching, which have seriously hindered the sustainable development of oyster seedling industry. This study explored the immune protection and developmental effects of priming with inactivated Vibrio splendidus in oyster larvae at middle umbo stage (10 d post fertilization, dpf). The results showed that the immune system of umbo larvae was activated after pre-immune stimulation with inactivated V. splendidus. The expressions of immune recognition receptors (CTL-3, Integrin beta-1, TLR4), NF-kB signaling component (IKK), effector molecules (IL17-5, Defh2, HSP70) were significantly up-regulated, and the activities of antioxidant enzymes (superoxide dismutase and catalase), hydrolytic enzyme (lysozyme) also increased significantly. The proteins from the stimulated umbo larvae have obvious characteristics of agglutination and inhibition of V. splendidus growth. When the larvae at late umbo (18 dpf) or pediveliger stage (21 dpf) were challenged by live V. splendidus, much lower death rate was observed in the stimulation group compared to the control group. Simultaneously, the expressions of above immune related genes and the activities of antioxidant enzymes were all rapidly upregulated in pediveliger larvae of immune stimulated group. Moreover, the significantly increased shell height and shell length as well as accelerated development rate, and higher settlement rate were revealed after the umbo larvae are stimulated by inactivated Vibrio splendidus. In summary, inactivated V. splendidus stimulation in oyster umbo larvae could activate their immune system, enhance their resistance against V. splendidus infection till to pediveliger stage, and promote their following growth and development. All the results provided a theoretical basis for solving poor disease resistance and high mortality of larvae in oyster seedling industry.