Sulfated galactans isolated from the red seaweed Gracilaria fisheri target the envelope proteins of white spot syndrome virus and protect against viral infection in shrimp haemocytes

The present study was aimed at evaluating an underlying mechanism of the antiviral activity of the sulfated galactans (SG) isolated from the red seaweed Gracilaria fisheri against white spot syndrome virus (WSSV) infection in haemocytes of the black tiger shrimp Penaeus monodon. Primary culture of haemocytes from Penaeus monodon was performed and inoculated with WSSV, after which the cytopathic effect (CPE), cell viability and viral load were determined. Haemocytes treated with WSSV-SG pre-mix showed decreased CPE, viral load and cell mortality from the viral infection. Solid-phase virus-binding assays revealed that SG bound to WSSV in a dose-related manner. Far Western blotting analysis indicated that SG bound to VP 26 and VP 28 proteins of WSSV. In contrast to the native SG, desulfated SG did not reduce CPE and cell mortality, and showed low binding activity with WSSV. The current study suggests that SG from Gracilaria fisheri elicits its anti-WSSV activity by binding to viral proteins that are important for the process of viral attachment to the host cells. It is anticipated that the sulfate groups of SG are important for viral binding.