Live imaging of baculovirus infection of midgut epithelium cells: a functional assay of per os infectivity factors (PIFs)

The occlusion-derived viruses (ODVs) of baculoviruses are responsible for oral infection of insect hosts, whereas budded viruses (BVs) are responsible for the systemic infection within the host. The ODV membrane proteins play crucial roles in mediating virus entry into midgut epithelium cells to initiate infection and are important factors in host range determination. For Autographa californica multiple nucleopolyhedrovirus (AcMNPV), seven conserved ODV membrane proteins have been shown to be essential for oral infectivity and are called per os infectivity factors (PIFs). Information on the function of the individual PIF proteins in virus entry is limited, partly due to the lack of a good in vitro system for monitoring ODV entry. Here, we constructed a baculovirus with an enhanced green fluorescent protein (EGFP) fused to the nucleocapsid to monitor the entry of virus into primary midgut epithelium cells ex vivo by confocal fluorescence microscopy. The EGFP-labeled virus showed the same BV virulence and ODV infectivity as wild type virus. The ability to bind and enter host cells was then visualized for wild type AcMNPVs and viruses with mutations in P74 (PIF0), PIF1 or PIF2, showing that P74 is required for ODV binding, while PIF1 and PIF2 play important roles in entry of ODV after binding to midgut cells. This is the first live imaging of ODV entry into midgut cells and complements the genetic and biochemical evidence for the role of PIFs in the oral infection process.