Regulation of multiple infection in alphabaculoviruses: critical factors that determine success

The physical structure of multiple nucleopolyhedroviruses (family Baculoviridae, genus Alphabaculovirus) has been demonstrated to be a key factor in the maintenance of diversity in natural isolates during host infection and in situations of low pathogen density. The physical association of genomes in nucleopolyhedroviruses is also highly relevant when insects are infected by distinct species of these viruses. Co-occlusion of two different virus species within the same occlusion body (OB) was demonstrated in this thesis after co-infection with two closely related viruses, the Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) and a non-per os infective genotype of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV). The phenomenon was also observed in infections involving two phylogenetically distant alphabaculoviruses, the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) and SfMNPV. In the latter case, co-envelopment of both viruses within the same occlusion-derived-virion (ODV) was observed, with approximately 50% of ODVs comprising genomes of both viruses following simultaneous inoculation of S. frugiperda larvae. However, infection of a cell/organism by two different alphabaculoviruses is not always possible. In S. frugiperda cells, successful infection by a second virus is only permitted within 20-24 hours following first infection. This exclusion to superinfection is a progressive process and involves both homologous and heterologous interference. Disruption of actin filaments by treatment with cytochalasin D (CD), a drug known to inhibit actin polymerization, resulted in the suppression of the exclusion and, consequently, permitted a successful second infection. A temporal window during which infection by two different virus species is possible was also observed when superinfections were performed in S. frugiperda larvae. Although total exclusion to a second infection was not detected in vivo, evidence of superinfection exclusion was observed. When larvae were first infected by AcMNPV and superinfected by SfMNPV, larval mortality, the presence of SfMNPV genomes in the progeny OBs, and the prevalence of ODVs containing only the second-infecting virus were lower than expected. Hence, superinfection exclusion prevents competition with a better adapted, i.e. more host-specific, competitor. The blockage was established in the early stages of infection, within 12 hours of inoculation of the first virus. The time interval between infections can be manipulated in order to produce mixed OBs with a particular viral proportion or prevalence of mixed ODVs. Similar OB pathogenicity (50% lethal concentration) and viral progeny production were observed for co-occluded OBs containing AcMNPV+SfMNPV and an equal mixture of OBs in S. frugiperda and S. exigua second instar larvae, whereas no positive or negative interactions between viruses were detected. This suggests that each virus infects host larvae in an independent manner, irrespective of their physical association in co-enveloped ODVs within co-occluded OBs or in mixtures of OBs of each virus.

Bibliographic Details

Main Author:	Beperet, Inés
Other Authors:	Caballero, Primitivo
Format:	tesis doctoral biblioteca
Published:	Universidad Pública de Navarra 2014-04
Online Access:	http://hdl.handle.net/10261/142655
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