Immune response of DNA vaccine against lymphocystis disease virus and expression analysis of immune‐related genes after vaccination

In this study, we found that an intramuscular injection of Japanese flounder (Paralichthys olivaceus, 60–80 g in weight and 15–20 mL in length) with 5 μg of a DNA vaccine (pEGFP‐N2‐LCDV‐cn‐MCP 0.6 kb, containing lymphocystis disease virus major capsid protein gene) induced a strong immune response. Subsequent real‐time polymerase chain reaction showed that the expression of immune‐related genes e.g., major histocompatibility complex (MHC) class I α, MHC II α, T‐cell receptor (TCR), tumour necrosis factor (TNF), tumour necrosis factor receptor (TNFR), Mx, interleukin (IL)‐1β, CXC and IL‐8R] was significantly changed after DNA vaccination. The most remarkable alternation was the expression of MHC I α and MHC II α genes: MHC II α reached the maximum on day 8 in different tissues, and MHC I α on day 2 in the intestine and gills. The expression of TCR increased and reached a plateau in 2 days in the spleen, gills, kidney and liver after vaccination and then decreased after day 8. In contrast, the expression of TCR in the intestine increased and reached a plateau in 8 days. The expression of IL‐8R reached the maximum on day 2 in different tissues and then decreased on day 8. Mx increased in the gills, kidney, spleen and liver on days 2, 8, 2 and 2, but decreased in the intestine, gills, spleen and liver on days 2, 8, 8 and 8 respectively. The TNFR expression increased in the spleen, kidney and gills on days 2, 8 and 8, but decreased in intestine, liver and gills on days 2, 8 and 8 respectively. The expression of TNF, CXC and IL‐1β increased 2 and 8 days after the injection of DNA vaccine. However, the expression of TNF, CXC and IL‐1β altered on days 2 and 8 with different patterns in different tissues respectively. The fish responded to the DNA vaccine by yielding a specific immunoglobulin against lymphocystis disease virus (LCDV) as observed with indirect ELISA. The DNA vaccine induced a unique humoral response, suggesting that the DNA vaccine activated both cellular and humoral defences of the specific immune system of Japanese flounder.