Abstract
Recent studies have reported on the importance of RBCs in fish responses to viral infections and DNA vaccines. Surface-displaying recombinant bacterins (spinycterins) are a safe and adaptable prototype for viral vaccination of fish and represent an alternative method of aquaculture prophylaxis, since have been reported to enhance fish immune response. We evaluated the innate immune response of rainbow trout (Oncorhynchus mykiss) red blood cells (RBCs), head kidney, and spleen to spinycterins expressing a fragment of the glycoprotein G of viral haemorrhagic septicemia virus (VHSV), one of the most devastating world-wide diseases in farmed salmonids. We first selected an immunorelevant downsized viral fragment of VHSV glycoprotein G (frg16(252-450)). Then, spinycterins expressing frg16(252-450) fused to Nmistic anchor-motif (Nmistic+frg16(252-450)) were compared to spinycterins expressing frg16(252-450) internally without the anchor motif. Nmistic+frg16(252-450) spinycterins showed increased attachment to RBCs in vitro and modulated the expression of interferon- and antigen presentation-related genes in RBCs in vitro and in vivo, after intravenous injection. In contrast, the head kidney and spleen of fish injected with frg16(252-450), but not Nmistic+frg16(252-450), spinycterins demonstrated upregulation of interferon and antigen-presenting genes. Intravenous injection of Nmistic+frg16(252-450) spinycterins resulted in a higher innate immune response in RBCs while frg16(252-450) spinycterins increased the immune response in head kidney and spleen. Although more studies are required to evaluate the practicality of using spinycterins as fish viral vaccines, these results highlight the important contribution of RBCs to the fish innate immune response to antiviral prophylactics.