Multiple nucleocapsid packaging of Autographa californica nucleopolyhedrovirus accelerates the onset of systemic infection in Trichoplusia ni

Among the nucleopolyhedroviruses (Baculoviridae), the occlusion-derived vir us (ODV), which initiates infection in host insects, may contain only a sin gle nucleocapsid per virion (the SNPVs) or one to many nucleocapsids per vi rion (the MNPVs), but the significance of this difference is unclear. To ga in insight into the biological relevance of these different packaging strat egies, we compared pathogenesis induced by ODV fractions enriched for multi ple nucleocapsids (ODV-M) or single nucleocapsids (ODV-S) of Autographa cal ifornica multicapsid nucleopolyhedrovirus (AcMNPV) containing a beta-galact osidase reporter gene, In time course experiments wherein newly molted four th-instar Trichoplusia ni were challenged with doses of ODV-S or ODV-M that yielded the same final mortality (similar to 70%), we characterized viral foci as either being restricted to the midgut or involving tracheal cells ( the secondary target tissue, indicative of systemic infection). We found th at while the timing of primary infection by ODV-S and ODV-M was similar, OD V-S established significantly more primary midgut cell foci than ODV-M, but ODV-M infected tracheal cells at twice the rate of ODV-S, The more efficie nt establishment of tracheal infections by ODV-M decreased the probability that infections were lost by midgut cell sloughing, explaining why higher n umbers of primary infections established by ODV-S within larvae were needed to achieve the same final mortality. These results showed that the multipl e nucleocapsid packaging strategy of AcMNPV accelerates the onset of irreve rsible systemic infections and may indicate why MNPVs have wider individual host ranges than SNPVs.