Invertebrates rely on innate immunity to respond to the entry of foreign microorganisms. The prophenoloxidase (proPO) system plays crucial roles in crustacean innate immunity. White spot syndrome virus (WSSV) infection has recently been responsible for significant economic losses in many crayfish-producing farms in China. We therefore aimed to examine the response of the proPO system to WSSV infection. The virulence of the virus, expression of the proPO gene, and phenoloxidase (PO) activity in six tissues from the red swamp crayfish Procambarus clarkii were investigated using LD₅₀ tests, real-time polymerase chain reaction and spectrophotometry, after infection with different numbers of WSSV virions. The LD₅₀ of the WSSV strain was 2.08 ×10⁷ virions/crayfish. proPO mRNA expression was increased in all studied tissues after infection with WSSV, except in the cuticle epidermis after infection with 2 × 10⁷ virions. proPO mRNA transcription was significantly increased in hemocytes and hepatopancreas, suggesting that P. clarkii might depend mainly on innate immunity for defense against viral pathogens. PO activity differed among different tissues during WSSV infection, suggesting that the proPO system might be activated by different mechanisms in different tissues in response to different viral stresses.

Key words: Expression, Phenoloxidase, Procambarus clarkii, Prophenoloxidase, White spot syndrome virus