A Spodoptera exigua serpin is involved in the immunity of hemolymph induced by Heliothis virescens ascovirus 3h

Serine protease inhibitors (serpins) are a broadly distributed protease superfamily in various organisms, which participate in the negative regulation of melanization in insects. Our pervious study demonstrated that induced melanization by Heliothis virescens ascovirus 3h is helpful for HvAV-3h viral replication. In this study, the serpin-2.1 like gene from Spodoptera exigua (Seserpin-2.1 like) which is a specific serpin after HvAV-3h infection was identified. Seserpin-2.1 like has the typical characteristics of serpin proteins family. Seserpin-2.1 like widely distributed in various tissues (including hemolymph, fat body, midgut, cuticle and head), and expressed at a higher level in fat body, hemolymph and head. At different developmental stages, it tends to express at eggs and 5th instar larvae. Seserpin-2.1 like shows special phenomenon for HvAV-3h infection. On the one hand, the transcription level of Seserpin-2.1 like in hemolymph was significantly upregulated after HvAV-3h compared to other immune challenges Gram-positive bacteria (Bacillus thuringiensis), Gram-negative bacteria (Escherichia coli), and Baculovirus (Autographa californica nucleopolyhedrovirus). On the other hand, its expression disappeared during 72 to 120 h post infection. In summary, our results suggest that Seserpin-2.1 like may be involved in innate immunity of S. exigua after infection with HvAV-3h. And HvAV-3h may inhibit the expression of Seserpin-2.1 like to promote the melanization of host larvae for viral replication.