Gene expression in Autographa californica nuclear polyhedrosis virus infections exhibiting host specific modulation of occlusion body formation

Nuclear polyhedrosis viruses (NPVs) are enveloped, rod-shaped insect viruses containing a circular DNA genome (family: Baculoviridae). These viruses, which are being developed as biopesticides, are transmitted via polyhedral inclusion bodies (PIBs) in which viruses are embedded. In order to genetically engineer safe viral pesticides, it is necessary to understand the molecular basis of NPV host specificity and virulence.

This research deals with a host range model (System III) in which Autographa californica multicapsid NPV (AcMNPV) replicates permissively in Spodoptera frugiperda (SF) cells but semipermissively in Bombyx mori (BM) cells. Early in infection, both cell lines displayed similar nuclear hypertrophy, but subsequent stages of cytopathic effect (CPE) were very different. PIBs were produced in SF but not BM cells. Instead, BM cells exhibited a unique CPE characterized by large sac-like bodies at the cell periphery. Virus growth kinetics experiments showed that infectious progeny virus was not produced in BM cells.

Because PIBs were not found in the BM infection, gel electrophoresis was performed to determine whether polyhedrin, the major constituent protein of the PIB, was synthesized. Polyhedrin was not produced. The synthesis of another major late viral protein known as plO was also blocked. Thirteen other infected cell-specific polypeptides (ICSPs) were not detected in the semipermissive infection. Most of these appeared to be late ICSPs. Six ICSPs were found only in infected BM cells. Eight ICSPs were produced in both SF and BM infections but at different rates in each cell line. Inhibition of host proteins synthesis occurred in SF cells, but there was no clear evidence for host inhibition in BM cells.

System III demonstrates a strong host range restriction which is manifested late in the BM infection. Additional research on the detailed mechanisms of the restriction should lead to the identification of viral DNA sequences responsible for host specificity and pave the way for the engineering of safe pesticides.