A Kazal-type serine proteinase inhibitor SPIPm2 from the black tiger shrimp Penaeus monodon is involved in antiviral responses

A five-domain Kazal-type serine proteinase inhibitor, SPIPm2, from Penaeus monodon has recently been implicated in antiviral responses for it is up-regulated upon viral infection and needs further studies. The SPIPm2 genomic gene was composed of seven exons and six introns. The genomic DNA segments coding for each Kazal domain were separated by introns of variable lengths supporting the hypothesis of gene duplication in the Kazal-type gene family. RT-PCR and Western blot analysis revealed that the SPIPm2 transcript and its five-domain protein product were expressed mainly in the hemocytes and less in gill, heart and antennal gland. Upon white spot syndrome virus (WSSV) infection, the SPIPm2 was only detected in the hemocytes and plasma. Immunocytochemical study of P. monodon hemocytes showed that the percentage of SPIPm2-producing hemocytes was reduced by about half after WSSV infection. Quantitative RT-PCR revealed further that the SPIPm2 was up-regulated early in the hemocytes of WSSV-infected shrimp and gradually reduced as the infection progressed. Injection of the recombinant SPIPm2 (rSPIPm2) prior to WSSV injection resulted in a significant inhibition of WSSV replication. The rSPIPm2 injection also prolonged the mortality rate of WSSV-infected shrimp. Therefore, the SPIPm2 was involved in the innate immunity against WSSV infection in shrimp.