This virus takes a less-travelled cytoskeletal road both to reach its replication site in the nucleus and to get back to the plasma membrane to escape the host cell.

Diverse bacterial and viral pathogens induce actin polymerization in the cytoplasm of host cells to facilitate infection. Here, we describe a pathogenic mechanism for promoting dynamic actin assembly in the nucleus to enable viral replication. The baculovirus Autographa californica multiple nucleopolyhedrovirus induced nuclear actin polymerization by translocating the host actin-nucleating Arp2/3 complex into the nucleus, where it was activated by p78/83, a viral Wiskott-Aldrich syndrome protein (WASP)-like protein. Nuclear actin assembly by p78/83 and Arp2/3 complex was essential for viral progeny production. Recompartmentalizing dynamic host actin may represent a conserved mode of pathogenesis and reflect viral manipulation of normal functions of nuclear actin.

An immunohistochemical study was conducted to detect the temporal infection sequence of Autographa californica M nuclear polyhedrosis virus in Trichoplusia ni larvae. Staining patterns indicated that the initial infection occurred in the midgut, simultaneously in columnar epithelial and regenerative cells, but that subsequently this tissue recovered. A major envelope glycoprotein stained in a polar fashion when it was expressed in columnar epithelial cells, but not when expressed in other cells types. Systemic infection was mediated by free virus for some tissues whereas infected hemocytes appeared to spread virus to other tissues by an unknown mechanism. A cell to cell spread within several tissues was detected. These results have important implications for baculoviruses engineered for improving their pesticide potential.

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....

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