Cell-dependent production of polyhedra and virion occlusion of Autographa californica multiple nucleopolyhedrovirus fp25k mutants in vitro and in vivo

Members of the family Baculoviridae are insect-specific dsDNA viruses that have been used for biological control of insect pests in agriculture and forestry, as well as in research and pharmaceutical protein expression in insect cells and larvae. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is the type species of the family Baculoviridae. During infection of AcMNPV in permissive cells, fp25k mutants are positively selected, leading to the formation of the few polyhedra (FP) phenotype with reduced yield of polyhedra and reduced virion occlusion efficiency, which leads to decreased oral infectivity for insects. Here we report that polyhedra of AcMNPV fp25k mutants produced from different insect cell lines and insects have differences in larval per os infectivity, and that these variations are due to different virion occlusion efficiencies in these cell lines and insects. Polyhedra of AcMNPV fp25k mutants produced from Sf cells (Sf21 and Sf9, derived from Spodoptera frugiperda) and S. frugiperda larvae had poorer virion occlusion efficiency than those from Hi5 cells (derived from Trichoplusia ni) and T. ni larvae, based on immunoblots, DNA isolation and larval oral infection analysis. AcMNPV fp25k mutants formed clusters of FP and many polyhedra (MP) in the fat body cells of both T. ni and S. frugiperda larvae. Transmission electron microscopy revealed that the nature of virion occlusion of AcMNPV fp25k mutants was dependent on the different cells of the T. ni fat body tissue. Taken together, these results indicate that the FP phenotype and virion occlusion efficiency of fp25k mutants are influenced by the host insect cells.