The sensing pattern and antitoxic response of Crassostrea gigas against extracellular products of Vibrio splendidus.

Serious juvenile oyster disease induced by pathogenic Vibrio splendidus has resulted in tremendous economic loss, but the molecular mechanisms underlying this killing mechanism remain unclear. The resistance of adult oyster to V. splendidus or its virulence factors might provide a possible access to cognize the interaction between pathogen and host. In the present study, the extracellular products (ECP) from less virulent V. splendidus JZ6 were injected into adult Pacific oyster Crassostrea gigas, and the cellular and humoral immune response induced by ECP were investigated. The phagocytosis rate of hemocytes was significantly up-regulated (30.57%) at 6 h after ECP injection compared with that (21%) of control groups. And significantly high level of ROS production was also observed from 3 h to 12 h in ECP-injected oysters, concomitant with increased apoptosis rate of hemocytes (16.4% in ECP-injected group, p < 0.01) compared with control group (6.7%). By RT-PCR analysis, the expression level of antioxidant CgSOD in hemocytes significantly increased to 6.41-fold of that in control groups (p < 0.01) at 12 h post ECP injection. The expression levels of anti-toxic metalloprotease inhibitors CgTIMP629 and CgTIMP628 were also significantly up-regulated at the early (3-6 h) and late (6-24 h) stage of immune response, respectively. Moreover, after the ECP were incubated with serum proteins isolated from the ECP-injected oysters in vitro, the metalloprotease activity of ECP significantly declined by 21.39%, and less degraded serum proteins were detected by SDS-PAGE. When the primarily cultured hemocytes were stimulated with heat-inactivated ECP or fragments derived from ECP-degraded serum proteins, the expressions of CgTIMP629 (13.64 and 7.03-fold of that in saline group, respectively, p < 0.01) and CgTIMP628 (5.07 and 6.08-fold of that in saline group, respectively, p < 0.01) in hemocytes were all significantly induced. All the results indicated that the adult oysters could launch phagocytosis, antioxidant and anti-toxic response to resist the virulence of ECP, possibly by sensing heterologous ECP and ECP-induced endogenous alarm signals. These results provided a possible clue for the resistance mechanism of adult oysters towards the ECP of less virulent V. splendidus, which might be valuable for exploring strategies for the control of oyster disease.