Continuous passage of undiluted supernatant fluids from cultures
of a bovine kidney cell line MDBK infected with the Herts 33
strain of NDV, led to the establishment of a carrier state
[MDBKcs].The general properties of this virus-cell system were compared
with those of a regulated-type of NDV persistent infection of MDBK
cells [MDBKpi] which was accidentally induced in this laboratory
in 1962 [ Edwards, 1972] .The intracellular polypeptide composition of the cell associated
virus in MDBKpi was different from that in MDBKcs cells. In MDBKcs
virus, the usual pattern of composition and distribution of NDV
polypeptides was observed whereas the MDBKpi virus lacked the M
protein which is necessary for viral assembly.Experiments using both RNA and protein synthesis inhibitors
[actinomycin D and cycloheximide, respectively] failed to induce an
increased release of infectious virus from MDBKpi cell cultures.
This indicates that the blockage is not dependent on cellular
RNA or protein synthesis.The presence of a cellular control mechanism of NDV replication
was detected in the persistently infected MDBKcs cultures as well
as in MDBK cells primarily infected with NDV and there is evidence
that a similar mechanism occurs in MDBKpi cells. It is emphasised,
however, that the cellular block does not prevent the release of
fully infectious virus from primary or carrier state cultures.Although the virus released from MDBKpi does not undergo a productive replication cycle in permissive cell culture systems or
in embryonated eggs, it is able to attach, replicate, haemadsorb
and induce cell fusion at the first passage level. The available
evidence suggests that a defect in the synthesis of M protein
miight account for failure to assemble complete viral particles at
the cell surface.Besides promoting the release of non -infectious particles,
the absence of the viral Iii polypeptide may also be responsible for
the intracellular accumulation of viral nucleocapsids. These
structures were found to be of two different types, namely granular
and smooth, according to the presence or absence of a sheath - covering the RNP component. The accumulation' observed in the
nucleus of some cells in aged cultures was always restricted to
smooth nucleocapsids. Furthermore, only the granular nucleocapsids
were shown conclusively to be NDV-specific by immunoperoxidase
techniques.Biochemical experiments on Herts NDV and MDBKcs virus, propagated
in embryonated eggs, showed that both types consist of a mixture of
two distinct kinds of virus particles. The first of these
sedimented at a density of 1.12.in sucrose or tartrate gradients
and revealed an unusual polypeptide composition including a
significantly reduced NP/F peak, low haemagglutinin and neuraminidase
activities, and law infectivity titres; whereas the virus sedimenting
at a density of 1.18 was of the standard type.A viral inhibitory factor [VIA was detected in the supernatant
fluids of MDBKcs cell cultures. This viral-induced component was
NDV-specific, could not be sedimented by ultracentrifugation, and
was able to protect indicator cell monolayers against infection by
homologous virus but not by related or unrelated viruses.On the other hand, absence of the viral inhibitory factor in
MDBKpi cell culture fluids suggested that a different mechanism is
involved in the maintenance of the regulated type of infection.Unsuccessful attempts were made to transfect the putative
integrated viral information in DIVA from MDBK cells persistently
infected with NDV. However, the results of several experiments
involving DNA analogues and the fact that a transient "cure" of
MDBKpi monolayers was obtained after prolonged propagation seem to
suggest that viral integration is a possible hypothesis, at least
so far as regulated infections are concerned
