Slow cell infection, inefficient primary infection and inability to replicate in the fat body determine the host range of Thysanoplusia orichalcea nucleopolyhedrovirus.

Thysanoplusia orichacea multicapsid nucleopolyhedrovirus (ThorMNPV) carrying an enhanced green fluorescent protein (EGFP) gene expression cassette (vThGFP) was used to study host-range mechanisms. Infection kinetics showed that vThGFP replication in Sf21 cells was too slow to suppress cell growth. Wide-host-range Autographa californica MNPV (AcMNPV) could speed up vThGFP infection and enhance the vThGFP infection rate in Sf21 cells. The enhancement was not due to recombination, as no recombinant virus was isolated from co-infection by plaque assay. No improvement of vThGFP infection in Sf21 was found by AcMNPV cosmid transactivation assay. However, culture medium from Sf21 cells infected with AcMNPV did enhance vThGFP replication in Sf21. Third-instar larvae of Spodoptera frugiperda, S. exigua and Helicoverpa zea were not killed by feeding with vThGFP polyhedra but were killed by intrahaemocoelic injection using budded viruses (BVs). This suggested that insufficient BVs were generated during the primary infection in the midgut. vThGFP infected haemocytes, tracheae and Malpighian tubules but not fat bodies of larvae of S. frugiperda, S. exigua and H. zea. Third-instar S. frugiperda larvae co-infected by injection with vThGFP and vAcDsRed2, an AcMNPV expressing a red fluorescent protein gene, showed EGFP expression in the fat body. This result suggests that vAcDsRed2 could help vThGFP to replicate in the fat body or trans-activate EGFP expression in the fat body. All these results suggested that slow cell infection, insufficient primary infection and inability to replicate in the fat body control the host range of ThorMNPV.