LamB as a potential negative regulator to the immune response of shrimp Litopenaeus vannamei upon Vibrio parahaemolyticus infection

Vibrio parahaemolyticus, an important aquatic pathogen, is the causative agent of numerous high mortality disease outbreaks in shrimp industry. Recent studies have found that V. parahaemolyticus outer membrane proteins (OMPs) may homology with shrimp plasma proteins, and could potentially regulate the immune responses of shrimps against pathogens. In this study, we identified that the outer membrane protein LamB of V. parahaemolyticus reacted with anti-hemocyanin antibody using affinity proteomics approach, Western blot and Dot-ELISA analysis. CTL epitope predictions identified 11 homologous epitopes between LamB and Litopenaeus vannamei hemocyanin. In addition, seven potential LamB interaction proteins, including β-1,3-glucan-binding protein (LGBP), β-actin, Toll2, actin T2, enolase partial, arginine kinase and hemocyanin, were identified from shrimp plasma by co-immunoprecipitation (Co-IP) and LC-ESI-MS/MS. In particular, we found that rLamB could could bind HMC, rLGBP and rToll2, and down-regulate the expression level of Toll2, LGBP and hemocyanin upon pathogen infection. Moreover, rLamB increased the bacterial loading in shrimp hemolymph at 12 h and 24 h. Taken together, these findings reveal the important role of LamB in recognition and negative regulation of the host's immune system upon bacterial infection.