Natural infection induced immune response against salmonid alphavirus in farmed salmon

Pancreas disease (PD) is one of the most serious infectious diseases of salmon caused by salmonid alphavirus (SAV), and can be transmitted over distance in marine environments. Vaccination has already proven to be an important management tool for prevention and control of infectious diseases in profitable aquaculture. Currently, the inactivated whole-virus vaccine against PD is commercially available in Norway, however, its effectiveness in the field remains limited studied; lifetime protection has not been proven yet. In recent years, significant advances in molecular biology have led to the emergence of subunit vaccines. E2 protein is an envelope protein of SAV in Norway and supposed to be the carrier of neutralizing epitopes. Thus, E2 protein represents a potential candidate target for subunit vaccine-based immunotherapy against SAV. The aim of this study is to determine the natural infection induced humoral response against E2 protein in farmed salmon. Atlantic salmon reared in sea cage culture at the north western part of Norway was used in this study. Blood samples were collected monthly from March 2015 to Feb 2016. Diagnosis of SAV infection was based on the transcription-polymerase chain reaction PCR results and symptom on fishes. The serological reactivity was determined by an enzyme-linked immunosorbent assay (ELISA). A recombinant E2 protein was used as target antigen, and a recombinant ubiquitin protein was used as negative control in this study. A western blot (WB) assay was applied for the verification of specific binding. The serologic results demonstrated that no E2 specific antibody was identified in the plasma collected from March to August 2015. During August and September 2015, there was an outbreak of PD in the sea cage. In the heart tissues collected in August, the infection of SAV2 was documented by PCR assay. A significant elevated percentage of peripheral neutrophils were demonstrated during this period, which declined to the normal level afterwards. In plasma from salmon collected in September, one-month post outbreak SAV infection, a significant increase in serum antibodies against E2 protein was detected. Two months after infection, the detected antibody frequency remains 7 out of 10 samples, and the level of antibodies became relatively stable until February 2016 before slaughtering. The frequency of Antibody against E2 protein was nearly equal to the detected SAV2 infection rate. The specific bindings of salmon antibodies to the E2 protein were confirmed by the WB assay. These results demonstrate a natural infection of SAV2 induced humoral responses against E2 protein, which indicates that E2 protein can be a promising target for vaccine and diagnosis design. Furthermore, the antibodies from SAV2 infected salmon could react with SAV3, which indicates the potential cross protection of SAV3 whole virus vaccine. Further study is required to investigate into the actual protective effect of the humoral responses and to determine the targeted epitopes on E2 protein, and their correlation with fish survival. This will provide valuable information for the treatment of SAV caused salmon pancreas disease in the future.