The insect tracheal system: a conduit for the systemic spread of Autographa californica M nuclear polyhedrosis virus.

Abstract

Baculoviruses establish systemic infections within susceptible insect hosts, even though host tissues are surrounded by basal lamlnae, extracellular matrices that exclude particles smaller than these viruses. Using a recombinant Autgrapha californwca M nuclear polyhedrosis virus containing a lacZ reporter gene under the control of a constitutive promoter, we followed the progression of infection in Tichoplusia ni larvae. We discovered that infection of the larval insect tracheal system (and not hemocytes, as thought previously) provides the major conduit for this virus to pass through basal laminae and to spread throughout the host. Tracheal epidermal cells, the only known cellular components of the tracheal system, share a common lymph system. Locally these cells contact one another by interdigitating cytoplasmic extensions called epidermal feet. These two features of the tracheal system are likely to facilitate the rapid systemic spread of the virus. The findings reported here have major implications for the fields of insect pathology and biological control and usher in an important consideration regarding host-range factors.Nuclear polyhedrosis viruses (NPV) (family Baculoviridae) are enveloped double-stranded DNA viruses that infect only arthropod hosts, primarily lepidopteran insect larvae. NPVs are unusual among viruses because they produce two phenotypes, one that transmits infection between hosts and one that spreads infection within the host. The first phenotype consists of one or more enveloped virions, each containing one or more nucleocapsids sequestered within a crystalline matrix ofprotein. Such viral occlusions are called polyhedra. As with the spore stages of many bacteria and fungi, polyhedra resist desiccation and allow for temporal escape from inhospitable environments. After ingestion by susceptible larval insect hosts, polyhedra rapidly dissociate in the alkaline gut juices and release occlusion-derived virus (ODV). Even though ODV lack surface spikes or peplomers, they enter midgut columnar epithelial cells by membrane fusion (1-3). These infected epithelial cells produce the second phenotype, budded virus (BV), which buds through the basal cell membrane at sites containing gp64, a viral-encoded glycoprotein that forms spikes on the surface of BV. These gp64-containing structures mediate entry of BV into target cells by promoting fusion of the BV envelope and endocytic vesicular membranes (4-6).Early investigations of baculovirus infections focused on host pathology and the potential use of baculoviruses for biological control purposes. Accurate descriptions of the progression of pathogenesis in virus-infected insects, however, were hampered by the limits of the light and electron microscopy techniques commonly used, both with and without accompanying immunohistochemistry. Rare events are difficult to capture by using these techniques because they require meticulous serial sectioning and the three-dimensional reconstruction of hundreds, if not thousands, of sections for each insect....

Analysis of p74, a PDV envelope protein of Autographa californica nucleopolyhedrovirus required for occlusion body infectivity in vivo.

“…bud through the basolateral surface and initiate systemic spread of infection (Granados & Lawler, 1981). Infection may then spread quite rapidly throughout the insect by utilizing the tracheolar system of the insect (Engelhard et al, 1994). AcLP4 BV and PDV were able to cause lethal polyhedrosis when injected into T. ni larvae, but failed to cause infection when inoculation was by an oral route even with very high doses ; hence both midgut and systemic infection (by pass-through of nucleocapsids) were blocked.…”

Structural and ultrastructural studies of Anticarsia gemmatalis midgut cells infected with the baculovirus A. gemmatalis nucleopolyhedrovirus

“…Several workers have shown the presence of virus particles in columnar cells of the midgut ( Kawanishi et al, 1972 andKeddie et al, 1989) early on infection. Other workers have shown, with the help of recombinant baculoviruses containing marker genes, that midgut epithelial cells support viral replication ( Volkman, 1997, Flipsen et al, 1995, Washburn et al, 1995, Knebel-Mörsdorf et al, 1996and Barret et al, 1998 and that the secondary infection was carried out by the insect tracheal system ( Engelhard et al, 1994 andBarret et al, 1998). We have shown previously in cell culture that AgNPV expression of very late genes (e.g.…”

“…A recent study has indicated that the insect tracheal system is responsible for the spread of the virus throughout the insect (Engelhard et al, 1994), but the passage of the virus through midgut epithelium is essential for the success of the infection.…”

“…The use of recombinant baculovirus expressing β-galactosidase (LacZ), β-glucoronidase (GUS) and green fluorescent protein (GFP) genes have shown that the midgut columnar cells are the first to appear infected (Engelhard et al, 1994, Flipsen et al, 1995, Knebel-Mörsdorf et al, 1996and Barret et al, 1998. A recent study has indicated that the insect tracheal system is responsible for the spread of the virus throughout the insect (Engelhard et al, 1994), but the passage of the virus through midgut epithelium is essential for the success of the infection.…”

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“…The BVs are produced in the nucleus of infected cells and are released to infect neighbouring cells within an infected insect. In the last phase of infection, the viruses are encapsulated in a protein envelope becoming ODVs, which are released to the environment and transmitted to other insects via oral infection 8 . Genes involved in the proliferation of NPVs are divided into three classes, early (immediateearly and delayed early), late, and very late genes, based on their expression time after virus infection.…”