Baculoviruses are pathogenic microorganisms that infect insects and invertebrates, making them their host. The particle form of the virus is Baculoviridae (Vago et al.,1974). Its main hosts are lepidoptera (corn borer, etc.), hymenoptera (bees, etc.), and diptera (mosquitoes, etc.) (Slack et al., 2007). They are divided into four genera, namely, Alphabaculovirus, Betabaculovirus, Gammabaculovirus, and Deltabaculovirus, according to the insect hosts from which they were isolated and their biological characteristics (Jehle et al., 2006). Autographa californica multiple Nucleopolyhedrovirus (AcMNPV) is the archetypal species of the alphabaculovirus. During the infection cycle, it produces two enveloped virion phenotypes with different structures and functions, which play different roles in virus pathogenesis: budded virus (BV) and occlusion-derived virus (ODV) (Rohrmann, 2013). The nucleocapsid structures of these viruses are similar, but the origin and composition of their envelope and their roles in the virus life cycle are different.
Spodoptera frugiperda belongs to Lepidoptera and Noctuidae. It is a major migratory insect from tropical and subtropical areas of America, and also one of the major hosts for AcMNPV and the ovarian cells for Sf9.
The baculovirus establishes a strict parasitic and adaptive relationship with its insect host during the long evolutionary process, and the information of the interaction is recorded and solidified in the viral genome. For example, polyhedrins need to be alkali-interpreted by releasing virions to initiate infection in the pH-alkaline larval midgut; the alkaline environment of the insect host’s midgut and alkaline solubility of the viral polyhedrin protein crystal are the result of the long-term evolution of both.
Baculoviruses interact with their hosts at multiple levels, but the details remain unclear. The expression of baculovirus genes has a time sequence. In this process, the host will influence the expression of baculovirus genes. For example, the expression of early baculovirus genes depends on the host’s transcription system and factors. After the baculovirus unshells and enters the nucleus, DNAs that have not yet been replicated are transcribed in the first place. The early transcription of the virus is inhibited by muscarine carnitine, so host RNA polymerase Ⅱ mediates most early gene transcription, leading to the activation and expression of the late gene, thus making the expression of viral genes orderly (Tjia et al., 1979). Host RNA polymerase is the most important host component in the early gene expression of the virus and plays a dominant role in the life cycle of the virus.
The baculovirus also has an effect on its host. For example, one of the characteristics of baculovirus overexpression in early gene expression to late gene expression is significant inhibition of host transcription. Some studies have shown that in Sf9 cells infected with AcNPV, host protein synthesis begins to decline between 6 and 10 hours after infection, and seems to stop completely after 24 hours. In addition, baculovirus can also inhibit the apoptosis of host cells and affect the cycle process of host cells. Bertin J et al. demonstrated that the P35 protein of baculovirus inhibits apoptosis induced by baculovirus infection (Bertin et al., 1996). One of the functions of AcMNPV's very early gene, IE2, has been shown to encode proteins that block cell cycles in a variety of cell lines.
The interaction between baculovirus and host is reflected at multiple levels, from simple physical adsorption, invasion to complex manipulation of host metabolic system, inhibition of host cell apoptosis, and inhibition of host hormone levels, etc. Many interesting and important host-virus interaction mechanisms have been identified from fundamental studies of the biochemistry and molecular biology of baculoviruses. These studies provide new strategies for biological pest control and eukaryotic expression vector systems.
A large part of functional genes of baculoviruses have been identified thus far, but the research on the relationship between the two mainly focuses on viruses, and there is less research on the function of host factors, so the work in this field is worthy of further development. In addition, many previous studies have carried out relevant findings from the perspective of host genes or viral gene functions, while from the perspective of bioinformatics, there is no relevant report on the changes in the expression level of host genes after baculovirus infection. In this study, after the baculovirus infecting Sf9 cells, infected cells were collected at different time points for sequencing, and the sequencing results were analyzed to show the expression of host genes at different time points. Our results provide a useful resource for further analysis of baculovirus-host interactions.