A-type CpG ODN with higher binding affinity to LvToll1 could probably activate downstream IFN system-like antiviral response in shrimp Litopenaeus vannamei

CpG oligodeoxynucleotides (CpG ODNs) is a widely used immune adjuvant, which could activate various immune responses including antiviral response through interaction with Toll-like receptor 9 (TLR9) in mammals. In the present study, four types of CpG ODN (CpG-A, CpG-B CpG-C, and CpG-P) were synthesized and injected to the shrimp Litopenaeus vannamei in order to evaluate their immune enhancement effect in shrimp. The copy numbers of white spot syndrome virus in the shrimps treated with different types of CpG ODNs were of 3.10×10 (CpG-A), 8.32×10 (CpG-B), 9.84×10 (CpG-C), and 8.12×10 (CpG-P) copies ng DNA respectively, which were significantly lower (p < 0.01) than that in PBS group (1.70×10 copies ng DNA). Surface plasmon resonance (SPR) assay revealed that the four types of CpG ODN displayed different binding affinity to LvToll1, LvToll2 and LvToll3, and the highest binding affinity was observed between CpG-A and LvToll1. Correspondingly, the mRNA transcripts of LvTolls were up-regulated significantly in CpG-A stimulated shrimps,which was significantly higher than that in CpG-B, CpG-C and CpG-P groups (p < 0.01). The phagocytic rate and ROS level of shrimp hemocytes in CpG-A and CpG-B groups increased significantly compared with that in other groups, which were 1.63-fold, 9.98-fold (p < 0.01) in CpG-A and 1.60-fold, 4.92-fold (p < 0.01) in CpG-B higher than those in PBS group, respectively. Moreover, after CpG-A stimulation, the probable IFN level in shrimp plasma increased to 2.60-fold (p < 0.01) of that in PBS group, and the mRNA expressions of IFN system-like antiviral genes (LvIRF, LvVago4 and LvSTAT) were also significantly up-regulated in CpG-A group, displaying a stronger response than that in CpG-B, CpG-C and CpG-P groups. The results indicated that CpG-A could promote the cellular and humoral immunity in shrimp, and induce relatively higher antiviral immune response among the four CpG ODNs. It provided useful information to understand the stimulatory effects of CpG ODNs in shrimp, promoting the application of CpG ODNs in aquaculture.