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Journal of Virology, June 2004, p. 6676-6681, Vol. 78, No. 12
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.12.6676-6681.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Rinderpest Virus Phosphoprotein Gene Is a Major Determinant of Species-Specific Pathogenicity
Misako Yoneda,1 Ryuichi Miura,1 Thomas Barrett,2 Kyoko Tsukiyama-Kohara,1 and Chieko Kai1*
Laboratory Animal Research Center, Institute of Medical Science, The University of Tokyo, Tokyo 108-8639, Japan,1
Institute for Animal Health, Pirbright, Woking, Surrey GU24 ONF, United Kingdom2
Received 3 July 2003/
Accepted 3 March 2004

ABSTRACT
We previously demonstrated that the rinderpest virus (RPV) hemagglutinin
(H) protein plays an important role in determining host range
but that other viral proteins are clearly required for full
RPV pathogenicity to be manifest in different species. To examine
the effects of the RPV nucleocapsid (N) protein and phosphoprotein
(P) genes on RPV cross-species pathogenicity, we constructed
two new recombinant viruses in which the H and P or the H, N,
and P genes of the cattle-derived RPV RBOK vaccine were replaced
with those from the rabbit-adapted RPV-Lv strain, which is highly
pathogenic in rabbits. The viruses rescued were designated recombinant
RPV-lapPH (rRPV-lapPH) and rRPV-lapNPH, respectively. Rabbits
inoculated with RPV-Lv become feverish and show leukopenia and
a decrease in body weight gain, while clinical signs of infection
are never observed in rabbits inoculated with RPV-RBOK or with
rRPV-lapH. However, rabbits inoculated with either rRPV-lapPH
or rRPV-lapNPH became pyrexic and showed leukopenia. Further,
histopathological lesions and high virus titers were clearly
observed in the lymphoid tissues from animals infected with
rRPV-lapPH or rRPV-lapNPH, although they were not observed in
rabbits infected with RPV-RBOK or rRPV-lapH. The clinical, virological,
and histopathological signs in rabbits infected with the two
new recombinant viruses did not differ significantly; therefore,
the RPV P gene was considered to be a key determinant of cross-species
pathogenicity.

INTRODUCTION
Rinderpest virus (RPV) is a single-stranded, negative-sense
RNA virus classified in the genus
Morbillivirus in the family
Paramyxoviridae, order
Mononegavirales. RPV is antigenically
closely related to other members of this genus, which include
human measles virus (MV), peste des petits ruminants virus in
sheep and goats, canine distemper virus (CDV), and phocid distemper
virus, which cause diseases in carnivore species, including
seals. Since the late 1980s, novel CDV and related morbillivirus
infections have occurred worldwide and have caused fatal disease
in several species previously considered immune to these viruses,
including hyenas and various marine mammals (
5,
6). Two recent
CDV epidemics in large cats have been reported, one in California
and the other in the Serengeti, Tanzania (
1,
25a). Both were
characterized by significant fatality rates (23 and 30%, respectively).
Recently, two new emerging paramyxoviruses that were identified
in cases of severe respiratory and encephalitic diseases in
animals and humans have been described; they are now known as
Hendra virus (HeV) and Nipah virus (NiV) (
12). Hendra virus
emerged in 1994 and was transmitted to humans by close contact
with horses; Nipah virus emerged in 1999 and was passed from
pigs to humans. Both are unusual among the paramyxoviruses in
their abilities to infect and cause potentially fatal disease
in a number of host species, including humans, although their
natural host is the fruit bat. From these incidents it is clear
that a high probability of cross-species infection by paramyxoviruses
exists, and so the determinants of cross-species pathogenicity
following infection with these viruses should be a high priority
for research. As a first step toward understanding these mechanisms,
we studied the role of the hemagglutinin (H) protein gene of
RPV as a pathogenic determinant in a rabbit model, since virus
entry was considered one of the key elements determining cross-species
infectivity (
29). From these experiments it was clear that the
H protein of the lapinized strain played an important role in
allowing infection to occur but was not a factor determining
pathogenicity for rabbits. It was then considered likely that
one of the internal virus protein genes, or a combination of
these genes from the lapinized virus, was essential to produce
clinical disease in rabbits. The results of experiments in which
the phosphoprotein (P) gene (recombinant RPV-lapPH [rRPV-lapPH]),
alone or together with the nucleocapsid (N) protein gene (lapNPH),
from RPV-Lv was additionally swapped into the RBOK backbone
are presented in this report.

MATERIALS AND METHODS
Cells.
B95a cells, which are highly susceptible to RPV infection (
18),
were propagated in RPMI 1640 medium (Sigma, St. Louis, Mo.)
supplemented with 5% fetal calf serum in a humidified atmosphere
containing 5% CO
2. RPMI 1640 supplemented with 2% fetal calf
serum was used as maintenance medium. The antibiotics benzylpenicillin
(100 U/ml) and streptomycin (100 U/ml) were used in all media.
Construction of the full-length cDNA of RPV.
The RPV-Lv strain, which is a highly virulent for rabbits and was obtained by virus cloning from the RPV-L strain (M. Shiotani et al., submitted for publication), was used in this study, along with the rRPV-RBOK virus rescued from a full-length DNA copy (cDNA) of the genome of the RBOK vaccine strain (3). Virus was also rescued from a plasmid carrying the full genome of the rRPV-RBOK virus with the H protein gene derived from RPV-Lv (rRPV-lapH) (29). This plasmid was used for constructions of full-length cDNAs in which the P gene alone or the N and P genes combined were derived from RPV-Lv. To enable replacement of the P gene in the RPV-lapH cDNA, an FseI site was introduced just before the N-P intergenic region and a PmeI site was introduced just before the P-M intergenic sequence. The P gene from RPV-Lv was amplified by reverse transcription-PCR from total RNA of RPV-Lv-infected B95a cells by using oligonucleotide primers with these restriction enzyme sites and was inserted in place of the normal P gene in the RPV-lapH cDNA to rescue a virus designated rRPV-lapHP. Similarly, the N gene was inserted into the latter virus by using a ClaI site at the beginning of the N gene of strain RPV-RBOK and the FseI site at the N-P intergenic region.
Rabbits, virus inocula, and samples.
Two-month-old female albino rabbits (JW-NIBS strain) with an average body weight of 1.6 kg, which are highly sensitive to RPV-L (22), were obtained from the Nippon Institute for Biological Science (Tokyo, Japan). One milliliter each of the rRPV-RBOK, rRPV-lapH, rRPV-lapPH, rRPV-lapNPH, and RPV-Lv viruses, diluted to 104 50% tissue culture infective doses (TCID50)/ml with maintenance medium, was inoculated into rabbits, using two rabbits per experiment. The experiment was repeated twice. One control rabbit was inoculated with 1 ml of maintenance medium. The rabbits were euthanatized at 4 days postinoculation (dpi) by using Dormitor (Orion, Espoo, Finland) and Dormicum (Roche, Basel, Switzerland), and selected tissues were collected for further investigation.
Clinical investigations.
Inoculated rabbits were examined daily, and rectal temperatures and body weights were recorded. Total white blood cell (WBC) counts in the peripheral blood of the rabbits were determined with a commercial kit (Unopette Test 58.56; Becton Dickinson).
Virological investigations.
The rabbits were euthanatized at 4 dpi, and the lymphoid tissues, consisting of the spleen, Peyer's patches, mesenteric lymph nodes, and appendices, were collected and weighed. Virus infectivity titers in 10% (wt/vol) homogenates of these tissues were determined in B95a cells and expressed as TCID50 per milliliter.
Histopathological examination.
The various lymphoid tissues and other organs removed at autopsy were fixed in 10% formalin, dehydrated, and embedded in paraffin by using routine techniques. Thin sections were stained with hematoxylin and eosin.

RESULTS
Rescue of rRPV-lapPH and rRPV-lapNPH.
rRPV-lapPH and rRPV-lapNPH were successfully rescued from their
respective full-length cDNAs in 293 cells (Fig.
1). After 3
days of coculture of the transfected 293 cells with B95a cells,
a number of small syncytia were observed. No difference in virus
phenotype was observed in B95a cells infected with these new
recombinant viruses, and the cytopathic effect was similar to
that shown by rRPV-lapH and rRPV-RBOK (data not shown).
Clinical signs following virus inoculation into rabbits.
Rabbits inoculated with the RPV-Lv strain show severe disease,
with clinical signs such as pyrexia, leukopenia, and failure
to gain body weight. Eight rabbits (two per virus strain) were
inoculated with the four strains of RPV, and the experiment
was carried out three times. The clinical disease seen with
each virus type was very reproducible, as shown in Fig.
2. Rabbits
inoculated with 10
4TCID
50 of RPV-Lv per ml became pyrexic from
1 dpi, while body weight loss and leukopenia were observed from
2 dpi. Rabbits inoculated with either rRPV-RBOK or rRPV-lapH
showed no observable clinical signs of infection, as observed
in our previous study (
29). In contrast, rabbits inoculated
with rRPV-lapPH or rRPV-lapNPH showed high fever on 2 dpi and
also mild leukopenia, although body weight loss was not observed.
Virus growth in lymphoid tissues.
The virus present in each of the lymphoid tissues examined was
determined by titration in B95a cells, and the data are summarized
in Table
1. High virus titers, ranging from 3.0 to 4.9 log TCID
50/ml,
were detected in the mesenteric lymph nodes, superficial lymph
nodes, appendices, spleens, and Peyer's patches of all rabbits
inoculated with RPV-Lv. In contrast, no virus was detected in
lymphoid tissues of rabbits inoculated with either rRPV-RBOK
or rRPV-lapH, as previously noted (
29). High virus titers were
detected in all of the tissues of one rabbit inoculated with
rRPV-lapPH. In the other rabbit, virus was detected only in
the spleen and in Peyer's patches. Similarly, high virus titers
were measured in the lymphoid tissues collected from rabbits
infected with rRPV-lapNPH.
Histopathology of lymphoid tissues.
All of the lymphoid tissues collected were examined for gross
pathological changes, and those from rabbits inoculated with
rRPV-RBOK were normal in every respect. In agreement with the
clinical disease seen, severe necrotic lesions, infiltration
of inflammatory cells, and multinuclear giant cells, which are
characteristics of RPV-Lv infection, were also observed in tissues
from rabbits infected with rRPV-lapPH or rRPV-lapNPH (Fig.
3).
The "starry-sky" effect, which was observed in the spleens of
rabbits infected with rRPV-lapH (
29), was also seen in the present
experiments.

DISCUSSION
The H protein of a neuroadapted mouse strain of MV has been
shown to determine virus neurotropism (
11), and in a previous
study we showed that the H protein of RPV-Lv was required to
allow the bovine cell culture-derived PRV-RBOK vaccine strain
to infect rabbits; however, the virus produced was not capable
of causing disease in that species (
29). It was therefore clear
that virus entry alone was not sufficient to ensure that a virus
is pathogenic in the nonnatural host. We then focused on two
of the internal virus protein genes, the N and P genes, as possible
determinants of virus virulence for rabbits, as the other envelope-associated
proteins, the F and M proteins were not likely to be involved
in this function. The genomes of single-stranded, negative-sense
RNA viruses are tightly encapsidated by the N protein, and the
N-RNA complex serves as the template for both virus transcription
and replication. Within the virion, the RNA-dependent RNA polymerase
(L protein) is associated with the N-RNA template through an
interaction with the P protein to form the transcribing ribonucleoprotein
(RNP) complex (
21). Upon entry, virus transcription and replication
of the RNP take place in the cytoplasm of infected cells. During
transcription (as opposed to replication, where a full-length
antigenome RNA is transcribed), the gene start and stop signals
are recognized, and monocistronic, capped, and polyadenylated
mRNAs for each virus protein are produced. The intracellular
concentration of unassembled N protein, which exists as a soluble
N-P complex, is considered one factor which regulates the switch
from transcription to replication (
2,
7,
19,
24).
The P proteins of paramyxoviruses are known to perform multiple functions during virus replication, and many of these functions are mediated through its association with the N and L proteins. The P proteins of negative-stranded RNA viruses are activated by phosphorylation and, in association with the N and L proteins and genome RNA, form the replicase complex, which is essential for progeny virus production. It has been shown for vesicular stomatitis virus and Sendai virus that the N protein must be complexed with P to keep it soluble for RNA encapsidation and prevent the N protein from aggregating to form RNP-like structures lacking RNA (8, 9, 20). In addition, Sendai virus P protein was shown to form tetramer (27).
We succeeded in rescuing recombinants in which the H and P genes or the N, H, and P genes of the RPV-RBOK strain were replaced with those derived from the RPV-Lv strain (rRPV-lapPH and rRPV-lapNPH), and we compared their pathogenicities for rabbits with those of the two parent strains and the rRPV-LapH recombinant, which was previously rescued. While neither rRPV-RBOK nor rRPV-lapH was virulent in rabbits, those inoculated with either RPV-Lv, rRPV-lapPH, or rRPV-lapNPH became pyrexic and showed a significant reduction in WBC counts. While rRPV-lapH could not be detected in homogenates of lymphoid organs from infected rabbits, high titers of both rRPV-lapPH and rRPV-lapNPH were detected in almost all of the lymphoid tissues from infected rabbits analyzed. These data supported a role for the P gene as an important factor determining efficient species-specific virus replication. Only the starry-sky effect, which indicated the occurrence of immune responses such as activation of macrophages and propagation of lymphocytes (13), was observed in lymphoid tissues of rRPV-lapH-infected rabbits, which, as reported previously, showed not other observable signs of infection.
Since the replication of rRPV-lapPH and rRPV-lapNPH occurred to similar degrees in rabbit organs, species-specific interaction of the RPV-Lv N and P proteins does not appear to be required for pathogenesis, and other, possibly host cell, factors may be implicated. An important fact to bear in mind is that the P genes of most paramyxoviruses, including all morbilliviruses, additionally code for the two nonstructural proteins C and V (19), which are not essential for replication in tissue culture cells (4, 25, 26) but which are known to be virulence determinants through their ability to counteract the innate immune responses to virus infection. Compared to the parent MV, the V-defective virus causes milder clinical symptoms and lower mortality rates in a human CD46-transgenic mouse model (23) and prolonged thymocyte survival in a mouse-engrafted human thymus-liver implant model (28). In particular, C and V have been shown to be necessary to counteract the effects of interferon induction in response to virus infection (10, 14-17). The identities between predicted amino acid sequences of the P, C, and V proteins of RPV-RBOK and those of RPV-L were 80 to 86% (unpublished data). Which protein among these three plays the key role in cross-species pathogenicity is now under investigation.
While the replacement of the P gene of RPV-RBOK, along with the H gene, with those from RPV-Lv resulted in a level of virulence for rabbits that was very close to that caused by RPV-Lv itself, it was not identical in that there was no decrease in the body weights of infected rabbits, and so another virus gene(s), most probably the L gene, might also be required to confer full pathogenicity in this species. Experiments to produce an rRPV-lapNPHL recombinant to test this hypothesis and to investigate the host cell factors that may interact with P, and the P gene-associated C and V proteins, to determine virulence for rabbits are under way.

ACKNOWLEDGMENTS
This study was supported by grants-in-aid from the Ministry
of Education, Science, Culture, and Sports, Japan, and by a
grant from the Program for Promotion of Basic Research Activities
for Innovative Biosciences, Japan.

FOOTNOTES
* Corresponding author. Mailing address: Laboratory Animal Research Center, Institute of Medical Science, The University of Tokyo, 4-6-1 Sirokanedai, Minato-ku, Tokyo 108-8639, Japan. Phone: 81-3-5449-5497. Fax: 81-3-5449-5397. E-mail:
ckai{at}ims.u-tokyo.ac.jp.


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Journal of Virology, June 2004, p. 6676-6681, Vol. 78, No. 12
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.12.6676-6681.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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