Early innate immune responses of Nile tilapia (Oreochromis niloticus) during tilapia lake virus (TiLV) infection

Tilapia lake virus (TiLV) poses a serious threat to global tilapia aquaculture industry, causing massive mortality and huge economic losses. In the present study, we describe an experimental challenge of Nile tilapia using tilapia lake virus with emphasis on the expression profile of immune genes IPS-1 and TLR-7. Healthy Oreochromis niloticus (Nile tilapia) fingerlings were intraperitoneally injected with 50 μl of 1×105 TCID50/ml of TiLV prepared by growing the virus in EPC cells. Infection was followed with histopathology and gene expression analysis in the main target organs viz. brain, liver and kidney at 12, 24, 48, 72 and 96 h post challenge (hpc). Microscopic examination revealed localized blood clotting in the brain cortex, syncytial cell formation and presence of putative intracytoplasmic inclusion bodies in the liver tissue. Innate immune gene expression of IPS-1 was down regulated in the liver while slightly upregulated in kidney and brain during the early infection period. The innate sensor, TLR-7 showed downregulation in liver and upregulation in the brain, which was also evident by the very little pathology noticed in the brain. Overall, the lack of upregulation of innate immune genes in immunocompetent tissue like kidney could be due to increased multiplication of the virus in these tissues by interfering in the immune signalling pathway.