BioEngineering @ UQ

Baculovirus infection in pictures by Linda Lua

Overview of baculovirus infection

Baculovirus infection consist of two infection cycles

• Starting at the end of last infection, polyhedra (also called occlusion bodies) are released from the dead host larva (bottom right). In their occluded form, baculoviruses are very resilient to environmental factors and can survive for extended periods of time.
• Upon ingestion by a healthy larva, the polyhedra will reach the alkaline environment of midgut before dissolving. Solubilization releases the viruses that upon fusion with midgut cells will trigger primary infection.
• Infected cells will produce two types of progeny: a budded virus form and an occluded virus form. The budded virus collects cell membrane material as well as binding proteins when budding through the cell membrane and this virus form is responsible for secondary infection through an endocytotic process, thus spreading infection from midgut cells to cells throughout the larva.
• The occluded virus form accumulate in the animal during infection and is released upon cell lysis and ultimately through disintegration of the larva will enter the environment to start a new infection cycle.

Endocytosis of baculovirus

This TEM show the entry of a budded virus through endocytosis marking the onset of secondary infection.

Virogenic stroma formation

One of the early signs of infection is the formation in the cell nucleous of virogenic stroma, within which virus DNA is replicated and assembled into individual nucleocapsids (below).

Envelopment in the ring zone

From the virogenic stroma, the nucleocapsids move to the ring zone where they acquire an envelop (see below). In this case, we see a single nucleocapsid in the envelop and the virus in question is Helicoverpa  armigera Single Nucleocapsid nucleopolyhedrosisvirus or HaSNPV. Many baculoviruses have multiple nucleocapsids per envelop and are according termed MNPV. The functional role of the pretty swirls or whorls remains controversial.

Virion occlusion

Virus destined occlusion in polyhedra (and a future role in primary infection of larva) gradually gets embedded in a crystalline mass predominantly composed of the polyhedrin protein. From appearance, the process seems to occur through polyhedrin accumulating to supersaturated levels in the cell nucleus triggering spontaneous crystallization with enveloped nucleocapsids acting as seeds.

The polyhedron envelope

Polyhedra grow to several hundred micron in size. At the end of their formation phase, precursors to the future polyhedron envelope appears in the residual virogenic stroma. The precursors condense around the individual polyhedra eventually forming a coherent outer "skin" (below), which is critical for the resilience of the polyhedra to environmental factors.

The complete occlusion bodies

At the end, infected cells may harbour anywhere up to 200-300 occlusion bodies in the nucleus (left), which will be released as the cell lyses (left). Observed in SEM the nucleus appears pregnant with occlusion bodies (below left), while individual occlusion bodies have a crystalline appearance (below right). It is possible to see occlusion bodies that have lost their envelope with empty spaces left behind from some of the lost occluded virions.