Studies with semi-permissive infections of nuclear polyhedrosis viruses: evidence for intracellular restriction of host-range

Nuclear polyhedrosis viruses (Family: Baculoviridae) show great promise in agricultural pest control applications. However, in order for these viruses to be used most effectively, basic questions regarding the molecular basis of their replication, evolution, and host-specificity must be addressed. This thesis is concerned with delineating mechanisms of nuclear polyhedrosis virus host-specificity.

An in vitro model was used to compare the importance of membrane-level and intracellular host-range mechanisms for nuclear polyhedrosis viruses. This model was composed of two systems: Autographa califomica NPV (Acal NPV) , strain Gl (Gerwig, 1981), which at low multiplicity produces semi-permissive infections in Spodoptera frugiperda cells, formed system I of the model. System II was composed of semi-permissive, S_. frugiperda NPV (SF NPV) infections in Trichoplusia ni cells. Transfection studies with Acal NPV Gl (System 1) showed that naked DNA does not enhance this semi-permissive infection. In addition, electron microscopy revealed that 50 percent of these infections produced progeny virus, but not polyhedral inclusion bodies, indicating that there is a host-range block in this system between virion morphogenesis and PIB formation. Infection kinetic studies with system II revealed that progeny virus was produced, but the yield was 700-fold less than positive controls. However, electron microscopy showed that the progeny virus was not enveloped, and was found in only five percent of infected TN cells; the remaining 95 percent of the cells contained forms of cytopathic effect such as virogenic stroma, but no virions. This suggests a host-range block at, or prior to, virion morphogenesis. These data strongly indicate intracellular, and perhaps intranuclear sites of host-range restriction for the above model infection systems. Further experiments based on these data should eventually lead to a better understanding of the genetic and biochemical mechanisms of host-range systems for the NPV. Once these mechanisms are understood, highly effective and specific strains of these viruses can be manufactured for the purpose of controlling agriculturally important pests.