- The Baculoviridae is a family of DNA viruses which are obligate
pathogens for a variety of insects. Baculovirus strains came to the
attention of researchers because they provided a nonchemical
alternative for the biological control of insect pests of agriculture
and forestry (Ignoffo, 1968); however, recently they have been studied
for their potential as vectors for the expression of foreign genes
(Jasny, 1988). Baculoviruses are divided into three subgroups
designated A, B, and C. Subgroup A, the most studied, consists of
nuclear polyhedrosis viruses (NPVs) in which many virions are embedded
in crystals called polyhedra. This group can be further subdivided into
two morphological types (SNPV and MNPV). The SNPV feature single
enveloped nucleocapsids which are occluded, whereas the MNPV contain one
or more nucleocapsids within an envelope which is then occluded.
Subgroup B, known as the granulosis viruses (GVs), occlude single
virions in the crystalline matrix made of the protein granulin.
Subgroup C, the nonoccluded baculoviruses (NOB), do not have an
occlusion body surrounding the virions (for review, see Granados, 1980).
After the initial infection, the virus progeny may exhibit one of
two phenotypes. After encapsidation the virion may leave the nucleus
and acquire an envelope by budding through the host plasma membrane.
This budded form (BV) goes on to infect cells and spread the virus
within the original host. As the infection proceeds with time the viral
DNA can become encapsidated, enveloped and occluded within the nucleus.
This polyhedra-derived virus (PDV) form spreads the infection from
insect to insect.
The following report is a study of the sequence of events that
occur during the infection of Lymantria dispar cells in culture by the
baculovirus Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus
(OpMNPV), each of which was examined at three multiplicities of
infection (5, 10, and 100). During the time course of infection (0
to120 h), DNA was isolated from infected cell lysates and Southern blot
anaylsis demonstrated that viral DNA is first observable at 12-18 h p.i.
for this system. The total production of BV and its rate of production
were measured by titration of infected cell supernatants and both
results show the maximum increase of BV to be during the period of 24-36
h p.i. Light microscopy revealed the presence of polyhedra by 48 h p.i.
Western blot analysis was used to examine the time course of the
expression of the three viral-induced proteins (gp64, p39-capsid, and
polyhedrin) each of which represents a temporal phase of baculovirus
gene expression within the infection cycle. The m.o.i. appeared to have
little effect on the timing of all of the events studied (DNA synthesis,
BV production, PIB detection, and protein synthesis). However, the
magnitude of such events as early DNA synthesis and gp64 expression, and
the later levels of PIB production appeared to correspond to the m.o.i.