Poly (I:C)-Potentiated Vaccination Enhances T Cell Response in Olive Flounder ( Paralichthys olivaceus ) Providing Protection against Viral Hemorrhagic Septicemia Virus (VHSV).

Viral hemorrhagic septicemia (VHS), caused by viral hemorrhagic septicemia virus (VHSV), is a viral disease affecting teleosts, and is the major cause of virus-related deaths in olive flounder ( Paralichthys olivaceus ). Research has focused on ways to control VHS, and recently, the use of polyinosinic-polycytidylic acid poly (I:C)-potentiated vaccination has been investigated, whereby fish are injected with poly (I:C) and then with live pathogenic virus, resulting in a significant decrease in VHSV-related mortality. T cell responses were investigated in the present study after vaccinating olive flounder with poly (I:C)-potentiated vaccination to understand the ability of poly (I:C) to induce T cell immunity. Stimulation of T cell responses with the poly (I:C)-potentiated vaccination was confirmed by examining levels of CD3 ⁺ T cells, CD4-1 ⁺ T cells and CD4-2 ⁺ T cells. Higher levels of CD4-2 ⁺ T cells were found in vaccinated fish than CD4-1 ⁺ T cells, believed to result from a synergistic effect between poly (I:C) administration and pathogenic VHSV immunization. More importantly, the role of CD4-2 ⁺ T cells in the antiviral response was clearly evident. The results of this study suggest that the outstanding protection obtained with the poly (I:C)-potentiated vaccination is due to the robust immune response initiated by the CD4-2 ⁺ T cells.