Stimulating the immune response of Litopenaeus vannamei using the phagocytosis activating protein (PAP) gene

High mortality in the shrimp farming industry is caused by several pathogens such as white spot syndrome virus (WSSV), yellow head virus (YHV) and Vibrio harveyi (V. harveyi). A PAP (Phagocytosis activating protein) gene able to activate phagocytosis of shrimp hemocytes was cloned into the eukaryotic expression vector phMGFP. In vitro expression was confirmed by transfection of PAP-phMGFP into CHO (Chinese Hamster Ovary) cells and the expression of the Green Fluorescent Protein (GFP) was observed. In order to activate the phagocytic activity of shrimp, 20, 40 and 80 μg/shrimp of this PAP-phMGFP vector were injected into Litopenaeus vannamei muscle. After challenged with WSSV, 40 μg/shrimp produced the highest relative percent survival (77.78 RPS). Analysis for the expression of the GFP gene in various tissues showed the expression mostly in the hemolymph of the immunized shrimp. The expression level of PAP and proPO (Prophenoloxidase) gene were highest at 7 days after immunization. This agreed with the efficiency of protection against WSSV that also occurred 7 days after immunization with the highest RPS of 86.61%. However there was no protection 30 days after immunization. Hemocytes of shrimp injected with PAP-phMGFP had 1.9 folds and 3 folds higher percentage phagocytosis and phagocytic index than the shrimp injected with PBS. Accordingly, copies of WSSV reduced in the PAP-phMGFP injected shrimp. In addition, PAP-phMGFP also protected shrimp against several pathogens: WSSV, YHV and V. harveyi, with RPS values of 86.61%, 63.34% and 50% respectively. This finding shows that the immune cellular defense mechanisms in shrimp against pathogens can be activated by injection of PAP-phMGFP and could indicate possible useful ways to begin to control this process.