HearNPV Pseudotyped with PIF1, 2, and 3 from MabrNPV: Infectivity and Complex Stability.

Effective oral infection is set off by interaction of a group of conserved per os infectivity factors (PIFs) with larval midgut columnar epithelial cells. We constructed pseudotyped viruses by substituting pif1, pif2 or pif3 genes of Helicoverpa armigera nucleopolyhedrovirus (HearNPV) with their homologs from Mamestra bracissae multiple nucleopolyhedrovirus and tested their infectivity to tissue culture cells and to larvae. Transfection and infection assays revealed that all recombinant viruses generated infectious budded virus in both cell culture and in larvae. Electron microscopy showed synthesized occlusion body and occlusion derived virus (ODV) were morphologically indistinguishable from those of the parental virus. By contrast, feeding assays revealed that pseudotyped viruses could not rescue oral infectivity except for pif3 pseudotyped virus that only partially rescued oral infectivity but at a mortality rate much lower than that of the parental HearNPV. Consistent with the bioassay result, PIF complex was detected in ODVs of pif3 pseudotyped virus only but not in pif1 or pif2 pseudotyped viruses. Our results suggest that PIF complex is essential for oral infectivity, and in the formation of the PIF complex, PIF1, 2 are virus-specific while PIF3 does not appear to be as specific and can function in heterologous environment, albeit to a much more limited extent.

Per os infectivity factors: a complicated and evolutionarily conserved entry machinery of baculovirus.

Baculoviruses are a family of arthropod-specific large DNA viruses that infect insect species belonging to the orders Lepidoptera, Hymenoptera and Diptera. In nature, occlusion-derived viruses (ODVs) initiate baculovirus primary infection in the midgut epithelium of insect hosts, and this process is largely dependent on a number of ODV envelope proteins designated as per os infectivity factors (PIFs). Interestingly, PIF homologs are also present in other invertebrate large DNA viruses, which is indicative that per os infection is an ancient and phylogenetically conserved entry mechanism shared by these viruses. Here, we review the advances in the knowledge of the functions of individual PIFs and recent discoveries about the PIF complex, and discuss the evolutionary implications of PIF homologs in invertebrate DNA viruses. Furthermore, future research highlights on the per os infection mechanism are also prospected.