Advances in the Arms Race Between Silkworm and Baculovirus

Abstract

Insects are the largest group of animals. Nearly all organisms, including insects, have viral pathogens. An important domesticated economic insect is the silkworm moth Bombyx mori. B. mori nucleopolyhedrovirus (BmNPV) is a typical baculovirus and a primary silkworm pathogen. It causes major economic losses in sericulture. Baculoviruses are used in biological pest control and as a bioreactor. Silkworm and baculovirus comprise a well-established model of insect-virus interactions. Several recent studies have focused on this model and provided novel insights into viral infections and host defense. Here, we focus on baculovirus invasion, silkworm immune response, baculovirus evasion of host immunity, and enhancement of antiviral efficacy. We also discuss major issues remaining and future directions of research on silkworm antiviral immunity. Elucidation of the interaction between silkworm and baculovirus furnishes a theoretical basis for targeted pest control, enhanced pathogen resistance in economically important insects, and bioreactor improvement.

Keywords: Bombyx mori nucleopolyhedrovirus; antiviral immunity; baculovirus; immune evasion; silkworm.

Figure 1

Schematic diagram of baculovirus entry. Occlusion-derived viruses (ODVs) are released from occlusion bodies in the alkaline environment of larval gut juice after ingestion. Several insect gut juice proteins have strong antiviral capacity against ODVs. Intact ODVs pass through the peritrophic membrane and nucleocapsids enter the midgut epithelial cells via envelope-mediated membrane fusion to cause primary infection. Progeny budded viruses (BVs) spread through the host via the tracheal system to cause secondary infection. Binding and penetration into host cells by BV of both Bombyx mori nucleopolyhedrovirus (BmNPV) and Autographa californica multiple nucleopolyhedrovirus (AcMNPV) are mediated by the GP64 envelope glycoprotein which is specific to BV. BmNPV BV penetrates nonmidgut host cells by multiple strategies, including clathrin-independent macropinocytic endocytosis mediated by cholesterol on the cell membrane and clathrin- and dynamin-dependent endocytosis pathways. Successful BV entry also requires low pH. Nucleocapsid uncoating in the nucleus results in the subsequent virus infection process.

Figure 2

Model of the arms race of silkworm and baculovirus. Silkworms have several antiviral immune responses which they use against baculovirus infections. These include global protein synthesis shutdown, rRNA degradation, inactivation by gut juice antiviral proteins, host gene-encoded resistance, NF-κB antiviral pathway, apoptosis, melanization, and RNAi-based antiviral response. The prophenoloxidase (PPO) activation cascade causes melanization to block baculovirus infection, which is negatively regulated by serpins. RNAi antiviral defense of insects includes the major mechanism of the siRNA pathway and the minor contribution of the miRNA pathway. The silkworm-encoded miRNA bmo-miR-2819 and bmo-miRNA-390 inhibit BmNPV proliferation by downregulating viral genes. As a confrontation, baculovirus have developed several strategies to escape host immunity and promote their own replication and proliferation, including inhibition of antiviral apoptosis, melanization, RNAi and regulation of the cell cycle. For example, Bombyx mori nucleopolyhedrovirus (BmNPV) induces Bmserpin2 to inhibit host melanization. Meanwhile, Autographa californica multiple nucleopolyhedrovirus (AcMNPV) p35 inhibits siRNA pathway. Additionally, baculoviruses exploit the miRNA pathway to encode their own miRNAs (such as BmNPV-miR-1 and BmNPV-miR-3) for viral propagation.

Figure 3

Antiviral apoptosis and its modification by baculoviruses. The apoptotic caspase cascade comprises upstream initiator caspases (ICs) and downstream effector caspases (ECs). The cellular inhibitor of apoptosis (cIAP) binds caspases and blocks apoptosis in normal cells. Apoptotic signaling is initiated upon baculovirus infection, which causes iap-Antagonist (iap-A) to bind cIAP and release free caspases that facilitate apoptosis. Viral DNA replication triggers host p53 pro-apoptosis, which accelerates IC and EC activity. Progression of antiviral apoptotic signaling cascades is prevented by baculovirus-encoded apoptosis suppressors such as viral IAP (vIAP), p35, p49, and Apsup. When the apoptotic signal is initiated, vIAP blocks apoptosis by interacting with unstable cIAP such that the cIAP levels and antiapoptotic activity are maintained. Viral p35 binds ECs and p49 binds ICs and ECs to block apoptosis. Apsup inhibits apoptosis by preventing IC activity. BmNPV induces the pattern recognition receptor protein PGRP2-2 to suppress PTEN and prevent it from inhibiting PI3K/Akt signaling and activating p-Akt. In this manner, cell apoptosis is inhibited. The resultant increase in cell survival is conducive to NPV proliferation.