Role of occlusion derived virus binding and fusion to midgut cells in AcMNPV and SfMNPV pathogenesis in noctuid larvae

Occlusion derived virus (ODV) binding to and fusion with the primary cellular targets within the midgut epithelium are essential early steps during baculovirus infection of lepidopteran larvae. To investigate Autographa californica M nucleopolyhedrovirus (AcMNPV) ODV binding and fusion, an in vivo assay using fluorescent octadecyl rhodamine B (R18) incorporated into the ODV envelope (ODV_R) was developed. The assay was validated in time course, saturation, and competition experiments using larvae of the permissive host, Heliothis virescens. To determine whether the AcMNPV ODV envelope protein P74 was involved in ODV adsorption to midgut cells, binding and fusion levels of the ODV_R of an AcMNPV p74-deficient mutant were compared with those of the ODV_R of the parental wild-type virus in H. virescens larvae. The results suggested that P74 was responsible for mediating specific binding of AcMNPV ODV to the midgut cells.; Because AcMNPV has potential utility as a biological control agent, I characterized infection and pathogenesis of AcMNPV in, Spodoptera frugiperda, a significant agricultural pest. Larvae were resistant to fatal infection when inoculated orally with ODV but were susceptible when injected intrahemocoelically with budded virus. Resistance stemmed primarily from the failure of the ODV to establish infections in midgut cells; a second barrier involved the loss of infected tracheal cells associated with the midgut. To evaluate the efficiency of AcMNPV ODV_R binding and fusion to mature columnar cells in the midgut of S. frugiperda larvae, ODV that would readily infect S. frugiperda was needed as a positive control. Using in vivo techniques, I cloned an S. frugiperda MNPV that was highly infectious in S. frugiperda larvae both orally and intrahemocoelically. AcMNPV ODV_R bound to the midgut of S. frugiperda with only 15% the efficiency of SfMNPV ODV_R. Unlabeled SfMNPV ODV, but not AcMNPV ODV, competed with SfMNPV ODV_R for binding in the 2 midgut. These results suggested that SfMNPV ODV bound to a receptor not recognized by AcMNPV ODV, which accounts, at least in part, for the low oral infectivity of AcMNPV ODV in S. frugiperda larvae.