Lymantria dispar nucleopolyhedrovirus hrf-1 expands the larval host range of Autographa californica nucleopolyhedrovirus

The gypsy moth (Lymantria dispar) is nonpermissive for Autographa californica nucleopolyhedrovirus (AcNPV) infection. We previously isolated a gene, host range factor 1 (hrf-1), from L. dispar nucleopolyhedrovirus that promotes AcNPV replication in Ld652Y cells, a nonpermissive L. dispar cell line (S. M. Thiem, X. Du, M. E. Quentin, and M. M. Berner, J. Virol. 70:2221- 2229, 1996). In the present study, we investigated the ability of hrf-1 to alter the larval host range of AcNPV. Bioassays using recombinant AcNPV bearing hrf-1 were conducted with insect larvae by use of oral infection. AcNPV bearing hrf-1 was infectious for neonate L. dispar larvae, with a 50% lethal concentration of 1.2 x 10⁵ polyhedral inclusion bodies/ml of diet, which is similar to that of wild-type AcNPV for permissive hosts. AcNPV can kill neonate L. dispar larvae at high doses, but it does not kill third- instar larvae. However, electron microscopy studies of AcNPV-inoculated third-instar larvae revealed virus replication in the midgut cells. PCR analyses indicated that the virus was AcNPV. These results suggest that the block for AcNPV infection of L. dispar larvae is its inability to spread systematically from primary infection sites in the midgut epithelium and that this barrier is leaky in neonates. hrf-1 allows AcNPV to overcome this barrier. AcNPV recombinants hearing hrf-1 were also significantly more infectious for Helicoverpa zea, a resistant species, suggesting that the blocks for AcNPV infection of L. dispar and H. zea larvae may be similar.