The Host Specificities of Baculovirus per os Infectivity Factors

Baculoviruses are insect-specific pathogens with a generally narrow host ranges. Successful primary infection is initiated by the proper interaction of at least 8 conserved per os infectivity factors (PIFs) with the host’s midgut cells, a process that remains largely a mystery. In this study, we investigated the host specificities of the four core components of the PIF complex, P74, PIF1, PIF2 and PIF3 by using Helicoverpa armigera nucleopolyhedrovirus (HearNPV) backbone. The four pif s of HearNPV were replaced by their counterparts from a group I Autographa californica multiple nucleopolyhedrovirus (AcMNPV) or a group II Spodoptera litura nucleopolyhedrovirus (SpltNPV). Transfection and infection assays showed that all the recombinant viruses were able to produce infectious budded viruses (BVs) and were lethal to H . armigera larvae via intrahaemocoelic injection. However, feeding experiments using very high concentration of occlusion bodies demonstrated that all the recombinant viruses completely lost oral infectivity except SpltNPV pif3 substituted pif 3-null HearNPV (vHaBacΔ pif 3-Sp pif 3- ph ). Furthermore, bioassay result showed that the median lethal concentration (LC 50 ) value of vHaBacΔ pif 3-Sp pif 3- ph was 23-fold higher than that of the control virus vHaBacΔ pif 3-Ha pif 3- ph , indicating that SpltNPV pif 3 can only partially substitute the function of HearNPV pif 3. These results suggested that most of PIFs tested have strict host specificities, which may account, at least in part, for the limited host ranges of baculoviruses.