Japan. J. Microbiol. Vol. 20(4), 339-346,1976
Alteration Rabies Passage
of the In Vitro Host
Virus
after
Using Mitsue
Serial
Alkaline
HASHIMOTO
Chick
Range Embryo
Maintenance
and
Kamesaburo
of Cell
Medium
YOSHINO
Departmentof Virology,Institute of Medical Science,Universityof Tokyo, Tokyo (Received for publication, March 17, 1976)
ABSTRACT
HEP Flury strain of rabies virus maintained by 7-day chicken egg passage (parent line) and the same strain serially passaged in primary chick embryo (CE) cells using alkaline maintenance medium (AM line) were inoculated to cells of various species. Growth was negative in primary mouse embryo, L and HeLa cells, and positive in primary hamster kidney and BHK21 cells with both lines. An allor-none difference between the two lines was observed in primary monkey kidney and Vero cells. The parent line did not multiply in these monkey cells, whereas the AM line grew to high titers. In the case of Vero cells a unique cytopathic effect (CPE) was induced by the AM line. After five consecutive passages in Vero cells, the CPE-inducing agent was identified as rabies virus by a neutralization test. It was infective to intracerebrally inoculated suckling mice but not to adult mice, and its Vero cell-infective titer determined by CPE induction was about 1 log lower than the baby mouse-infective and CE plaque-forming titers. In contrast to the AM line, HEP Flury strain receiving 150 CE cell passages under neutral maintenance medium and three other strains receiving similar CE cell passages all failed to grow in Vero cells.
ease. For example, a human diploid celladapted virus was obtained after many times repeated subculturing of infected cells with normal cells, and the ability of growing to considerable titers in mouse embryo and green monkey kidney cells was acquired only after this human cell adaptation process [41]. As to the susceptibility of monkey cells to rabies virus, a subline of CV-1 cells originally derived from an African green monkey kidney was reported to be able to grow two strains of rabies virus to a high titer [42]. However, another green monkey kidney cell line Vero [43] exhibited only an equivocal growth of HEP Flury strain [13]. Our experience has also shown that this strain maintained by egg passage multiplies neither in Vero nor in primary monkey kidney cells. In the meantime, an earlier study on the growth of HEP Flury strain in CE cells [44] revealed that serial passage of this virus through CE cells using an alkaline mainte-
Rabies virus has been cultivated in vitro with primary hamster kidney [8, 23, 33], pig kidney [1], dog kidney [15, 20], fetal calf kidney [7], green monkey kidney [41], dog salivary gland [14, 15], bat brown fat [2], chick embryo (CE) [24, 28, 39, 41, 44], human diploid [18, 21, 41], mouse embryo [41], rat ganglion [29, 30], mouse ganglion [29, 30], mouse ependymoma [3] and reptilian cells [6, 32] as well as in established cell lines of baby hamster kidney [4, 5, 18], hamster embryo [16, 40], green monkey kidney [42], dog kidney [41], rabbit reticuloendothelium [18, 19, 26], mouse ependymoma [5], pig Fallopian tube [9] and cold-blooded vertebrate origins [11, 37, 40]. Adaptation of this virus to growth in these cells has not necessarily been achieved with Requests for reprints should be addressed to Dr. Kamesaburo Yoshino, Department of Virology, Institute of Medical Science, University of Tokyo, 4-6-1 Shiroganedai, Minato-ku, Tokyo 108, Japan. 339
340
nance
M. HASHIMOTO
medium
properties which virus population
resulted
in changes
of viral
suggested a reduction in of defective interfering
particles [12, 22], namely disappearance of autointerference and acquisition of the ability to grow to a high titer in CE cells under neutral medium. A further investigation was then made on the behavior of this virus in cultured cells of different species in comparison with the parent line. As described in this report, the alkaline medium passage line differed greatly from the parent virus in that an unequivocal growth occurred in primary monkey kidney and Vero cells. MATERIALS
AND
METHODS
Viruses. Two lines of HEP Flury strain of rabies virus [25] were compared in most experiments. One was 7-day egg passage line which represented the 283rd to 284th egg passages, and seed virus suspensions were prepared from infected embryos in the same manner as described in the accompanying paper [36]. The other line was maintained by serial passage in CE cells using maintenance medium adjusted at pH 8.2 in the manner stated earlier [44], and culture fluids representing the 46th to 55th CE passage levels were used. The two lines were designated as the parent line and AM (alkaline medium) line, respectively. Another CE passage line of this strain had been supplied by Dr. A. Kondo, National Institute of Health, Tokyo, which represented the 20th CE passage when received and was further passaged in CE cells using neutral maintenance medium in this laboratory up to the 150th passage level. Three other strains of rabies virus [46] had the following passage history before adaptation to CE cells : Nishigahara, 1848 rabbit brain, 4 adult mouse brain, 30 one-day egg and then 17 adult mouse brain passages; CVS, 64 adult mouse brain and 42 one-day egg passages; and Takamen, 12 adult mouse brain and 12 suckling mouse brain passages. The three strains were further passaged in CE cells using neutral maintenance medium as indicated in Results. Diluents. PBS refers to Dulbecco and Vogt's phosphate-buffered saline free of cations [17], and BS means 0.01 M phosphatebuffered saline at pH 7.2 free of potassium.
AND K. YOSHINO
BS containing fresh egg-yolk at 0.1 % (yolksaline) was used as a virus diluent in infectivity titration. Quantitation of virus infectivity. Infectivity of
HEP
Flury
count as
in
PFU
nal
strain
CE
For
[47, both
32
flow
and
media
buffer
ment
of
the
described
the
strains,
lation
[45],
old
mice
titers
were
DDy
observed
for
lethal
the
to
for
rubber-stoppered
culture
dispensing
per bottle. embryo and
were
5
Primary hamster
supplied
by
National
Institute
S3
[34],
clone
our
of
S3
Nakano, and
BHK21
ment
of
also
used.
media
[31],
for
serum
2%,
in
the
balanced
13
of
pH
in
solution
calf
were
maintenance minimum 10%
inactivated 7.2
with monkey in
medium and
calf
NaHCO3,
LE
containing
serum
Depart-
with
primary grown
M.
Tokyo,
Eagle's
of at
Dr.
Institute
and
were
were
2%
HeLa
by
our
hydrolysate) with
Nakano,
Health,
supplemented
salt
lactalbumin mented
of
cells
case
[38]. cells
clone
maintained
growth
adjusted
which
M.
BHK21
of
respectively,
and
suspension
method kidney
donated
cells
medium
except cells,
these
in 4 •~ 7 •~
[43] had been maintained For comparison's sake,
Cells
The
BIU
Tokyo.
Institute
C13
called
cell
of Dr.
newly
Cancer
essential and
cells National
[35].
of CE, mouse cells were pre-
Health,
L929
and
LD50
prepared
standard monkey
courtesy
[27] and Vero cells in our laboratory. HeLa
a
virus ml
bottles,
cultures kidney
according to cynomolgus
given
was
of
OIU
mouse 4-week-
0.03
were
ml
in
inocuas
Where or
of
mice
glass
plaques
determine
baby
as The
egg
were
unit).
pared Primary
form
amount
2 weeks unit
cm,
[36].
expressed
(baby mouse infective Cells. Cell monolayers
2.5
developed
one-day
strain
in
of
improve-
paper
infective units). done, 2 to 3-day-old
of
and
place
later
to
by
intracerebrally
The
unable
5%
growth
0.11%
A further
titrated
and
egg was
with the
in
was
as
incubator
and
foregoing
being
were
(one-day titration
an
equipped
NaOH.
origi-
preparation
contained
technique
in
cells,
The modified
respectively,
and
plaque
expressed.
was
employed,
NaHCO3,
Tris
CE
C
was
overlay
0.22%
48]
incubation,
at
CO2-air
by
were
monolayer
post-infection
regulated
other
titers units).
method
follows.
and
assayed
and
(plaque-forming
assay
and
was
cells,
kidney (Earle's 0.5% supple-
maintained
CELL
CULTURE
after virus infection with serum-free medium. Cell susceptibility test. Bottle cultures
OF
LE of
cells were washed once with PBS and received each 0.5 ml of a virus suspension which had been diluted with maintenance medium so as to contain 106 PFU of virus/ ml. An experimental group consisted of two bottles. After 2-hr incubation at 35 C the cell sheet was washed twice with PBS and once with maintenance medium, the last washing being saved for titration of residual infectivity. Then 5 ml of maintenance medium was added to each bottle, and subsequent incubation was made at 35 C. Daily observation was made for cytopathic effect (CPE) comparing the infected with control uninfected cultures. After 3 days the medium was renewed, whereby the previous fluids were pooled from the parallel bottles for PFU titration. The last titration of fluid virus was done usually 4 days later when the test was terminated. When virus titration was not performed immediately after harvesting, the specimens were stored at -70 C in the same fashion as previously stated [36]. Neutralizationtest. An immune serum was obtained by hyperimmunizing a rabbit with a suspension of CVS strain-infected rabbit brain, and inactivated by heating at 56 C for 30 min. Serial twofold dilutions thereof starting from 1 : 100 were made with BS in the amount of 0.5 ml. A test virus seed was diluted with BS containing 0.1 % bovine serum albumin (Armor fraction V) so as to contain approximately 4000 PFU/ml and mixed with the antiserum dilutions in equal amounts. Control was a mixture of the same virus and plain BS. After an incubation at 37 C for 1 hr in a water bath, the mixtures were inoculated each to three dishes of CE cell monolayer for PFU determination. Reciprocal of the highest serum dilution which reduced the plaque number to less than half of control count was the endpoint. Leighton tube titration. Vero cell monolayers were prepared in rubber-stoppered Leighton tubes, the fluid volume being 1 ml/tube. After washed once with PBS, the tubes received 0.1 ml of serial decimal dilutions of seed virus prepared with maintenance medium. Adsorption of virus was allowed
RABIES
341
VIRUS
to take place at 35 C for 2 hr, after which 0.9 ml per tube of maintenance medium was added. Four tubes were set per dilution. Subsequent incubation was done at 35 C for 2 weeks, renewing the medium twice weekly. Appearance of CPE was recorded by daily observation, and TCID50 (50% tissue culture infective dose) was calculated by ReedMuench formula [35] at the end of the observation period. RESULTS
Comparisonof Susceptibilityof Various Cells to the Parent and AM Line of HEP Flury Virus
Table 1 summarizes results of experiments in which growth of the two lines of HEP Flury virus was compared in cells of various species. Primary mouse embryo, HeLa and L cells all failed to exhibit a rise of fluid virus titer above the residual infectivity of the last washing with both lines. Growth of the two lines was about equally luxuriant in primary hamster kidney and BHK21 cells, titers of the AM line slightly exceeding those of the parent line on day 3. An intensive CPE characterized by rounding and shrinkage of cells was noticed in BHK21 cells and to a lesser extent in primary hamster kidney cells infected with the AM line. In contrast, the parent line induced no CPE in primary hamster kidney cells and caused CPE at a late stage in one of the two BHK21 cell lines tested. Reaction of primary monkey kidney and Vero cells was markedly different between the two seeds. No growth of the parent line was perceived in either cells, whereas an unequivocal titer uprise was seen after the infection with the AM line, which was especially conspicuous in the case of Vero cells. CPE was negative in primary monkey kidney cells. On the other hand, Vero cells infected with the AM line showed a unique CPE, which was characterized by floating-up of numerous cells in fluid phase above intact cells adhering to the glass wall (Fig. 1). After removal of such floating cells by medium change, the monolayer again yielded many floating cells within 1 to 2 days. This experiment was repeated several times and a high virus yield was consistently observed in Vero cells, appearance of CPE being either within the first week or sometimes retarded to
M. HASHIMOTO
342
Table
later
than
line
in the
1.
day same
Test
for
7, while cells
susceptibilities
growth was
of various
of the
always
AND K. YOSHINO
cells
parent
negative.
Serial Passage of HEP Flury Virus in Vero Cells The Vero cell culture fluid from the experiment recorded in Table 1 was diluted 1 100 with maintenance medium and inoculated in 0.5-ml amounts to a new set of Vero cell culture bottles, which were maintained in the same manner as in the above cell susceptibility test. The culture fluids were harvested after 1 week when CPE was distinct. Serial passage was carried out in this manner, and induction of CPE was noticed at each passage, as shown in Table 2. The fluid from the 5th passage, which corresponded to a 10-15 dilution of the original seed, contained approximately 107 PFU of virus. Identification of the passaged agent was done by a neutralization test with a hyperimmune rabbit serum, comparing neutralization endopoints against the parent line of HEP Flury virus and the passaged agent. As revealed in Table 3, the two endpoints perfectly coincided with each other.
Titration of the VeroCell Passaged HEP Flury Virus in Different Hosts HEP
Flury
virus
is known
to be infective
to the parent
and
AM
lines
of HEP
Flury
virus
to intracerebrally inoculated baby mice but innocuous to adult mice [25]. Whether or not this property was also changed after the serial Vero cell passage was examined. A new stock virus was prepared from the fifth Vero cell passage fluids and titrated for infectivities for baby mice and for 4-week mice by intracerebral inoculation. Also, the same seed was titrated for PFU and for infectivity to Vero cells. The results are combined for presentation in Table 4. The virus had about equal PFU and BIU titers, but all adult mice given the undiluted seed survived. The Vero cell TCID50 titer determined by the above-stated CPE, i.e., the capacity to produce numerous floating cells, was about 1 log lower than the PF and UBIU titers. The survival of adult mice was not due to autointerference of the massively inoculated virus, since a separate test showed that adult mice given serial dilutions of the same seed all survived. Susceptibility of Vero Cells to Various Rabies Virus Other Than the AM In our laboratory was available CE line of HEP Flury virus which derived from the same parent 7-day sage but grown in CE cells under
Strains of Line another had been egg pasneutral
CELL
CULTURE
OF
RABIES
VIRUS
343
Fig. 1. CPE induced by the AM line of HEP Flury virus in Vero cells . a, Control unifected Vero cells on day 7. b, Vero cells given the parent line on day 7. c, Vero cells infected with the AM line on day 4 (CPE grade = 1+). d, Vero cells infected with the AM line on day 7 (CPE grade =2-4). Table
maintenance going other men,
medium
a total strains, had
also
at each
of 150 CE Nishigahara, been
passaged
2.
Serial
passage
passage,
under-
passages. CVS and in CE
of HEP
Three Taka-
cells
in a
Flury
similar and
virus
in Vero
manner, 50th
These
four
lethal
effects
cells
and
CE
were
passage
viruses upon
could
at their levels, be
one-day
90th,
89th
respectively.
titrated chick
by
their
embryos.
344
M.
Table
3.
Plaque-reduction and
Table
4.
HASHIMOTO
Titration
AND K. YOSHINO
neutralization the Vero
of the
Vero
cell-passaged
cell-passaged
Each virus was diluted so as to contain 103 OIU per inoculum and given to bottle cultures of Vero cells. None of these viruses showed a significant titer increase nor distinct CPE. DISCUSSION The present experiments have demonstra ted that HEP Flury strain of rabies virus experiencing more than 46 serial passages in CE cells using alkaline maintenance medium differed from the parent line in that the in vitro host range was expanded including primary monkey kidney and Vero cells. There have been other cases in which rabies virus passaged in one host cell acquired ability to grow in unrelated cell species. As cited in Introduction, a human diploid cell-adapted strain could grow in green monkey kidney cells, in which the original virus did not multiply at all [41]. Adaptation to pig kidney cells was also successful by the use of a hamster kidney cell-adapted strain [1]. A unique point of the present study may be
test
with HEP
HEP
a hyperimmune Flury
Flury
rabbit
serum
in different
hosts
virus
virus
that the alteration of the in vitro host range depended upon the use of alkaline maintenance medium through serial cell passage. An earlier investigation [44] suggested that the progeny virus resulting from such serial passage might contain defective interfering particles [12, 22] at a low proportion. Here we are reminded of a fact that influenza virus passaged through MDBK cells, becoming free of defective particles, could grow in HeLa cells which did not support multiplication of the parent virus [10]. Hence we presume that the AM line of HEP Flury virus contained defective interfering particles in a sufficiently reduced amount to expand its in vitro host range. However, there is some contradiction in this interpretation. That is to say, the change of properties of HEP Flury virus, as evidenced by disappearance of autointerference and acquisition of ability to grow quickly in CE cells under a neutral environment, was seen after five alkaline medium passages in CE cells [44], whereas data at
CELL
hand Vero
CULTURE
OF
indicate that the ability to grow in cells is bestowed on the virus at a later
passage level. Two alternative explanations are being considered. One is that the reduction of defective particles in virus population took place gradually over many passage levels, and it is only after a practical disappearance of such particles that the in vitro host range was altered. The other is that the alkaline medium passage not only diminished the number of defective particles but also selected out a variant in a later passage level which could resist certain growthlimiting factors operative in monkey cells, and this combination contributed to the positive growth of the progeny virus in primary monkey kidney and Vero cells. Which interpretation is valid remains to be determined by further studies.
REFERENCES
[ 1]
Abelseth, M.K. 1964. Propagation of rabies virus in pig kidney cell culture. Canad. Vet. J. 5: 84-87. [ 2 ] Allen, R., Sims, R.A., and Sulkin, S.E. 1964. Studies with cultured brown adipose tissue. I. Persistence of rabies virus in bat brown fat. Amer. J. Hyg. 80: 11-24. [ 3 ] Atanasiu, P., and Lepine, P. 1959. Multiplication du virus rabique des rues sur la tumeur ependymaire de la souris en culture de tissu, Effet cytolytique. Ann. Inst. Pasteur 96: 72-78. [ 4 ] Atanasiu, P., Lepine, P., and Dighe, P. 1963. Purification partielle et concentration de virus rabique des rues, cultive sur une souche de cellules clonales de rein de hamster. Compt. Rend. Acad. Sci. 256: 1415-1417. [ 5 ] E Atanasiu, P., Lepine, P., and Dragonas, P. 1963. tude cinetique du virus rabique en culture de tissue a l'aide des anticorps fluorescents et des coupes ultrafines. Ann. Inst. Pasteur 105: 813-824. [ 6 ] Atanasiu, P., Raynaud, J., and Raynaud, A. 1973. Developpement du virus rabique sur des cellules embryonnaires de reptiles (Orvet, Anguis fragilis L.). Compt. Rend. Acad. Sci. Ser. D 276: 2097-2100. [ 7 ] Atanasiu, P., Tsiang, H., and Gamet, A. 1974. Nouveau vaccin antirabique humain de culture cellulaire primaire. Ann. Microbiol. 125B: 419432. [ 8 ] Barth, R., and Jaeger, O. 1970. Untersuchungen mit einigen Tollwutvirusstammen in verschiedenen Gewebekultur-systemen. Zbl. Veterin.armed. B 17: 363-380. [ 9 ] Bouillant, A.M.P., Tabel, H., and Greig, A.S. 1974. Titration and neutralization of rabies virus (ERA strain) following its replication in a pig Fallopian tube cell line. Canad. J. Comp. Med. 38: 118-123.
RABIES
[10]
VIRUS
345
Choppin, P.W., and Pons, M.W. 1970. The RNAs of infective and incomplete influenza virions grown in MDBK and HeLa cells. Virology 42: 603-610. [11] Clark, H.F. 1972. Growth and attenuation of rabies virus in cell cultures of reptilian origin. Proc. Soc. Exp. Biol. Med. 139: 1317-1325. [12] Crick, J, and Brown, F. 1974. An interfering component of rabies virus which contains RNA. J. Gen. Virol. 22: 147-151. [13] Debbie, J.G., Andrulonis, J.A., and Abelseth, M.K. 1972. Rabies antibody determination by immunofluorescence in tissue culture. Infect. Immunity 5: 902-904. [14] Depoux, R. 1964. Infection des cellules de glande sous-maxillaire de chien cultivees in vitro par un virus rabique fixe. Canad. J. Microbiol. 10: 527-534. [15] Depoux, R. 1965. Etude du virus rabique fixe cultive sur diverses cellules d'origine canine (souche Louis Pasteur). Ann. Inst. Pasteur 108: 566-578. [16] Diaz, A.M., Yager, P.A., and Baer, G.M. 1973. Rapid cytopathic effect with rabies virus in fused hamster embryo cells. Arch. Ges. Virusforsch. 43: 297-303. [17] Dulbecco, R., and Vogt, M. 1954. Plaque formation and isolation of pure lines with poliomyelitis viruses. J. Exp. Med. 99: 167-182. [18] Fernandes, M.V., Wiktor, T. J., and Koprowski, H. 1963. Mechanism of the cytopathic effect of rabies virus in tissue culture. Virology 21: 128-131. [19] Fernandes, M.V., Wiktor, T. J., and Koprowski, H. 1964. Endosymbiotic relationship between animal viruses and host cells. J. Exp. Med. 120: 1099-1116. [20] Hronovsky, V., Benda, R., and Cinatl, J. 1968. Process of adaptation of street rabies virus to dog kidney cell primary cultures. Acta Virol. 12: 233240. [21] Hronovsky, V., Cinatl, J., and Benda, R. 1973. Cultivation of the fixed rabies virus strain HEP Flury in vitro. I. Adaptation of the HEP Flury strain to the human diploid cell line HEL. J. Hyg. Epidemiol. Microbiol. Immunol. 17: 285-293. [22] Kawai, A., Matsumoto, S., and Tanabe, K. 1975. Characterization of rabies viruses recovered from persistently infected BHK cells. Virology 67: 520-533. [23] Kissling, R.E. 1958. Growth of rabies virus in non-nervous tissue culture. Proc. Soc. Exp. Biol. Med. 98: 223-225. [24] Kondo, A. 1965. Growth characteristics of rabies virus in primary chick embryo cells. Virology 27: 199-204. [25] Koprowski, H., Black, J., and Nelsen, D.J. 1954. Studies on chick-embryo-adapted-rabies virus. VI. Further changes in pathogenic properties following prolonged cultivation in the developing chick embryo. J. Immunol. 72: 94-106. [26] Love, R., Fernandes, M.V., and Koprowski, H. 1964. Cytochemistry of inclusion bodies in tissue culture cells infected with rabies virus. Proc. Soc. Exp. Biol. Med. 116: 560-563.
346
[27]
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Macpherson, I., and Stoker, M. 1962. Polyoma transformation of hamster cell clones-an investigation of genetic factors affecting cell competence. Virology 16: 147-151. [28] Matsumoto, S., and Kawai, A. 1969. Comparative studies on development of rabies virus in different host cells. Virology 39: 449-459. [29] Matsumoto, S., Schneider, L.G., Kawai, A., and Yonezawa, T. 1974. Further studies on the replication of rabies-like viruses in organized cultures of mammalian neural tissues. J. Virol. 14: 981-996. [30] Matsumoto, S., and Yonezawa, T. 1971. Replication of rabies virus in organized cultures of mammalian neural tissues. Infect. Immunity 3: 606616. [31] McQuilkin, W.T., Evans, V.J., and Earle, W.R. 1957. The adaptation of additional lines of NCTC clone 929 (strain L) cells to chemically defined protein-free medium NCTC 109. J. Nat. Cancer Inst. 19: 885-907. [32] Michalski, F., Cohen, M.M., and Clark, H.F. 1974. Adult and embryonic gecko cells in vitro: growth characteristics, infection by rabies, Sindbis and polyoma viruses, and transformation by SV40. Proc. Soc. Exp. Biol. Med. 146: 337-348. [33] Mikhailovsky, E.M., and Selimov, M.A. 1966. Adaptation of street rabies virus to primary Syrian hamster kidney cell culture. Acta Virol. 10: 373. [34] Puck, T.T., Marcus, P.E., and Cieciura, S. J. 1956. Clonal growth of mammalian cells in vitro. Growth characteristics of colonies from single HeLa cell with and without "feeder" layer. J. Exp. Med. 103: 273-283. [35] Reed, L.J., and Muench, H. 1938. A simple method of estimating fifty per cent end-points. Amer. J. Hyg. 27: 493-497. [36] Sekine, N., and Yoshino, K. 1976. Enhanced growth and plaguing of rabies virus in static chick embryo cells. Japan J. Microbiol. 20: 331-338. [37] Solis, J., and Mora, E.C. 1970. Viral susceptibility range of the fathead minnow (Pimephales
AND K. YOSHINO
promelas) poikilothermic cell line. Appl. Microbiol. 19: 1-4. [38] Taniguchi, S., and Yoshino, K. 1964. An analysis of the plaque assay of herpes simplex virus in chick embryo monolayers. Arch. Ges. Virusforsch. 14: 537-552. [39] Ver, B.A., and Jhara, H.I. 1968. Propagation of rabies virus strains in chick embryo tissue culture. Indian J. Pathol. Bacteriol. 11: 219-224. [40] Wiktor, T.J., and Clark, H.F. 1972. Chronic rabies virus infection of cell cultures. Infect. Immunity 6: 988-995. [41] Wiktor, T.J., Fernandes, M.V., and Koprowski, H. 1964. Cultivation of rabies virus in human diploid cell strain WI-38. J. Immunol. 93: 353366. [42] Wiktor, T.J., and Koprowski, H. 1974. Rhabdovirus replication in enucleated host cells. J. Virol. 14: 300-306. [43] Yasumura, Y., and Kawakita, Y. 1963. Studies on SV40 virus in tissue culture cells. Nippon Rinsho 21: 1201-1215. (in Japanese) [44] Yoshino, K., Kishie, T., Hashimoto, M., and Yanagi, K. 1974. Effect of alkaline maintenance medium upon the growth of rabies virus in chick embryo cells. Arch. Virol. 47: 31-38. [45] Yoshino, K., Kondo, A., Kuma, N., and Kitaoka, M. 1956. Infection of the one-day-old fertile hen's egg with rabies virus. II. Application to rapid viral titration and neutralization test. Japan. J. Med. Sci. Biol. 9: 273-282. [46] Yoshino, K., Kondo, A., Kuma, N., and Taniguchi, H. 1960. Infection of the one-day-old fertile hen's egg with rabies virus. VI. Strain differences in the egg adaptability. Arch. Ges. Virusforsch. 10: 684-697. [47] Yoshino, K., and Morishima, T. 1971. An improvement in the plaque assay of rabies virus in chick embryo cells. Arch. Ges. Virusforsch. 34: 40-50. [48] Yoshino, K., Taniguchi, S., and Arai, K. 1966. Plaque assay of rabies virus in chick embryo cells. Arch. Ges. Virusforsch. 18: 370-373.
Alteration Rabies Passage
of the In Vitro Host
Virus
after
Using Mitsue
Serial
Alkaline
HASHIMOTO
Chick
Range Embryo
Maintenance
and
Kamesaburo
of Cell
Medium
YOSHINO
Departmentof Virology,Institute of Medical Science,Universityof Tokyo, Tokyo (Received for publication, March 17, 1976)
ABSTRACT
HEP Flury strain of rabies virus maintained by 7-day chicken egg passage (parent line) and the same strain serially passaged in primary chick embryo (CE) cells using alkaline maintenance medium (AM line) were inoculated to cells of various species. Growth was negative in primary mouse embryo, L and HeLa cells, and positive in primary hamster kidney and BHK21 cells with both lines. An allor-none difference between the two lines was observed in primary monkey kidney and Vero cells. The parent line did not multiply in these monkey cells, whereas the AM line grew to high titers. In the case of Vero cells a unique cytopathic effect (CPE) was induced by the AM line. After five consecutive passages in Vero cells, the CPE-inducing agent was identified as rabies virus by a neutralization test. It was infective to intracerebrally inoculated suckling mice but not to adult mice, and its Vero cell-infective titer determined by CPE induction was about 1 log lower than the baby mouse-infective and CE plaque-forming titers. In contrast to the AM line, HEP Flury strain receiving 150 CE cell passages under neutral maintenance medium and three other strains receiving similar CE cell passages all failed to grow in Vero cells.
ease. For example, a human diploid celladapted virus was obtained after many times repeated subculturing of infected cells with normal cells, and the ability of growing to considerable titers in mouse embryo and green monkey kidney cells was acquired only after this human cell adaptation process [41]. As to the susceptibility of monkey cells to rabies virus, a subline of CV-1 cells originally derived from an African green monkey kidney was reported to be able to grow two strains of rabies virus to a high titer [42]. However, another green monkey kidney cell line Vero [43] exhibited only an equivocal growth of HEP Flury strain [13]. Our experience has also shown that this strain maintained by egg passage multiplies neither in Vero nor in primary monkey kidney cells. In the meantime, an earlier study on the growth of HEP Flury strain in CE cells [44] revealed that serial passage of this virus through CE cells using an alkaline mainte-
Rabies virus has been cultivated in vitro with primary hamster kidney [8, 23, 33], pig kidney [1], dog kidney [15, 20], fetal calf kidney [7], green monkey kidney [41], dog salivary gland [14, 15], bat brown fat [2], chick embryo (CE) [24, 28, 39, 41, 44], human diploid [18, 21, 41], mouse embryo [41], rat ganglion [29, 30], mouse ganglion [29, 30], mouse ependymoma [3] and reptilian cells [6, 32] as well as in established cell lines of baby hamster kidney [4, 5, 18], hamster embryo [16, 40], green monkey kidney [42], dog kidney [41], rabbit reticuloendothelium [18, 19, 26], mouse ependymoma [5], pig Fallopian tube [9] and cold-blooded vertebrate origins [11, 37, 40]. Adaptation of this virus to growth in these cells has not necessarily been achieved with Requests for reprints should be addressed to Dr. Kamesaburo Yoshino, Department of Virology, Institute of Medical Science, University of Tokyo, 4-6-1 Shiroganedai, Minato-ku, Tokyo 108, Japan. 339
340
nance
M. HASHIMOTO
medium
properties which virus population
resulted
in changes
of viral
suggested a reduction in of defective interfering
particles [12, 22], namely disappearance of autointerference and acquisition of the ability to grow to a high titer in CE cells under neutral medium. A further investigation was then made on the behavior of this virus in cultured cells of different species in comparison with the parent line. As described in this report, the alkaline medium passage line differed greatly from the parent virus in that an unequivocal growth occurred in primary monkey kidney and Vero cells. MATERIALS
AND
METHODS
Viruses. Two lines of HEP Flury strain of rabies virus [25] were compared in most experiments. One was 7-day egg passage line which represented the 283rd to 284th egg passages, and seed virus suspensions were prepared from infected embryos in the same manner as described in the accompanying paper [36]. The other line was maintained by serial passage in CE cells using maintenance medium adjusted at pH 8.2 in the manner stated earlier [44], and culture fluids representing the 46th to 55th CE passage levels were used. The two lines were designated as the parent line and AM (alkaline medium) line, respectively. Another CE passage line of this strain had been supplied by Dr. A. Kondo, National Institute of Health, Tokyo, which represented the 20th CE passage when received and was further passaged in CE cells using neutral maintenance medium in this laboratory up to the 150th passage level. Three other strains of rabies virus [46] had the following passage history before adaptation to CE cells : Nishigahara, 1848 rabbit brain, 4 adult mouse brain, 30 one-day egg and then 17 adult mouse brain passages; CVS, 64 adult mouse brain and 42 one-day egg passages; and Takamen, 12 adult mouse brain and 12 suckling mouse brain passages. The three strains were further passaged in CE cells using neutral maintenance medium as indicated in Results. Diluents. PBS refers to Dulbecco and Vogt's phosphate-buffered saline free of cations [17], and BS means 0.01 M phosphatebuffered saline at pH 7.2 free of potassium.
AND K. YOSHINO
BS containing fresh egg-yolk at 0.1 % (yolksaline) was used as a virus diluent in infectivity titration. Quantitation of virus infectivity. Infectivity of
HEP
Flury
count as
in
PFU
nal
strain
CE
For
[47, both
32
flow
and
media
buffer
ment
of
the
described
the
strains,
lation
[45],
old
mice
titers
were
DDy
observed
for
lethal
the
to
for
rubber-stoppered
culture
dispensing
per bottle. embryo and
were
5
Primary hamster
supplied
by
National
Institute
S3
[34],
clone
our
of
S3
Nakano, and
BHK21
ment
of
also
used.
media
[31],
for
serum
2%,
in
the
balanced
13
of
pH
in
solution
calf
were
maintenance minimum 10%
inactivated 7.2
with monkey in
medium and
calf
NaHCO3,
LE
containing
serum
Depart-
with
primary grown
M.
Tokyo,
Eagle's
of at
Dr.
Institute
and
were
were
2%
HeLa
by
our
hydrolysate) with
Nakano,
Health,
supplemented
salt
lactalbumin mented
of
cells
case
[38]. cells
clone
maintained
growth
adjusted
which
M.
BHK21
of
respectively,
and
suspension
method kidney
donated
cells
medium
except cells,
these
in 4 •~ 7 •~
[43] had been maintained For comparison's sake,
Cells
The
BIU
Tokyo.
Institute
C13
called
cell
of Dr.
newly
Cancer
essential and
cells National
[35].
of CE, mouse cells were pre-
Health,
L929
and
LD50
prepared
standard monkey
courtesy
[27] and Vero cells in our laboratory. HeLa
a
virus ml
bottles,
cultures kidney
according to cynomolgus
given
was
of
OIU
mouse 4-week-
0.03
were
ml
in
inocuas
Where or
of
mice
glass
plaques
determine
baby
as The
egg
were
unit).
pared Primary
form
amount
2 weeks unit
cm,
[36].
expressed
(baby mouse infective Cells. Cell monolayers
2.5
developed
one-day
strain
in
of
improve-
paper
infective units). done, 2 to 3-day-old
of
and
place
later
to
by
intracerebrally
The
unable
5%
growth
0.11%
A further
titrated
and
egg was
with the
in
was
as
incubator
and
foregoing
being
were
(one-day titration
an
equipped
NaOH.
origi-
preparation
contained
technique
in
cells,
The modified
respectively,
and
plaque
expressed.
was
employed,
NaHCO3,
Tris
CE
C
was
overlay
0.22%
48]
incubation,
at
CO2-air
by
were
monolayer
post-infection
regulated
other
titers units).
method
follows.
and
assayed
and
(plaque-forming
assay
and
was
cells,
kidney (Earle's 0.5% supple-
maintained
CELL
CULTURE
after virus infection with serum-free medium. Cell susceptibility test. Bottle cultures
OF
LE of
cells were washed once with PBS and received each 0.5 ml of a virus suspension which had been diluted with maintenance medium so as to contain 106 PFU of virus/ ml. An experimental group consisted of two bottles. After 2-hr incubation at 35 C the cell sheet was washed twice with PBS and once with maintenance medium, the last washing being saved for titration of residual infectivity. Then 5 ml of maintenance medium was added to each bottle, and subsequent incubation was made at 35 C. Daily observation was made for cytopathic effect (CPE) comparing the infected with control uninfected cultures. After 3 days the medium was renewed, whereby the previous fluids were pooled from the parallel bottles for PFU titration. The last titration of fluid virus was done usually 4 days later when the test was terminated. When virus titration was not performed immediately after harvesting, the specimens were stored at -70 C in the same fashion as previously stated [36]. Neutralizationtest. An immune serum was obtained by hyperimmunizing a rabbit with a suspension of CVS strain-infected rabbit brain, and inactivated by heating at 56 C for 30 min. Serial twofold dilutions thereof starting from 1 : 100 were made with BS in the amount of 0.5 ml. A test virus seed was diluted with BS containing 0.1 % bovine serum albumin (Armor fraction V) so as to contain approximately 4000 PFU/ml and mixed with the antiserum dilutions in equal amounts. Control was a mixture of the same virus and plain BS. After an incubation at 37 C for 1 hr in a water bath, the mixtures were inoculated each to three dishes of CE cell monolayer for PFU determination. Reciprocal of the highest serum dilution which reduced the plaque number to less than half of control count was the endpoint. Leighton tube titration. Vero cell monolayers were prepared in rubber-stoppered Leighton tubes, the fluid volume being 1 ml/tube. After washed once with PBS, the tubes received 0.1 ml of serial decimal dilutions of seed virus prepared with maintenance medium. Adsorption of virus was allowed
RABIES
341
VIRUS
to take place at 35 C for 2 hr, after which 0.9 ml per tube of maintenance medium was added. Four tubes were set per dilution. Subsequent incubation was done at 35 C for 2 weeks, renewing the medium twice weekly. Appearance of CPE was recorded by daily observation, and TCID50 (50% tissue culture infective dose) was calculated by ReedMuench formula [35] at the end of the observation period. RESULTS
Comparisonof Susceptibilityof Various Cells to the Parent and AM Line of HEP Flury Virus
Table 1 summarizes results of experiments in which growth of the two lines of HEP Flury virus was compared in cells of various species. Primary mouse embryo, HeLa and L cells all failed to exhibit a rise of fluid virus titer above the residual infectivity of the last washing with both lines. Growth of the two lines was about equally luxuriant in primary hamster kidney and BHK21 cells, titers of the AM line slightly exceeding those of the parent line on day 3. An intensive CPE characterized by rounding and shrinkage of cells was noticed in BHK21 cells and to a lesser extent in primary hamster kidney cells infected with the AM line. In contrast, the parent line induced no CPE in primary hamster kidney cells and caused CPE at a late stage in one of the two BHK21 cell lines tested. Reaction of primary monkey kidney and Vero cells was markedly different between the two seeds. No growth of the parent line was perceived in either cells, whereas an unequivocal titer uprise was seen after the infection with the AM line, which was especially conspicuous in the case of Vero cells. CPE was negative in primary monkey kidney cells. On the other hand, Vero cells infected with the AM line showed a unique CPE, which was characterized by floating-up of numerous cells in fluid phase above intact cells adhering to the glass wall (Fig. 1). After removal of such floating cells by medium change, the monolayer again yielded many floating cells within 1 to 2 days. This experiment was repeated several times and a high virus yield was consistently observed in Vero cells, appearance of CPE being either within the first week or sometimes retarded to
M. HASHIMOTO
342
Table
later
than
line
in the
1.
day same
Test
for
7, while cells
susceptibilities
growth was
of various
of the
always
AND K. YOSHINO
cells
parent
negative.
Serial Passage of HEP Flury Virus in Vero Cells The Vero cell culture fluid from the experiment recorded in Table 1 was diluted 1 100 with maintenance medium and inoculated in 0.5-ml amounts to a new set of Vero cell culture bottles, which were maintained in the same manner as in the above cell susceptibility test. The culture fluids were harvested after 1 week when CPE was distinct. Serial passage was carried out in this manner, and induction of CPE was noticed at each passage, as shown in Table 2. The fluid from the 5th passage, which corresponded to a 10-15 dilution of the original seed, contained approximately 107 PFU of virus. Identification of the passaged agent was done by a neutralization test with a hyperimmune rabbit serum, comparing neutralization endopoints against the parent line of HEP Flury virus and the passaged agent. As revealed in Table 3, the two endpoints perfectly coincided with each other.
Titration of the VeroCell Passaged HEP Flury Virus in Different Hosts HEP
Flury
virus
is known
to be infective
to the parent
and
AM
lines
of HEP
Flury
virus
to intracerebrally inoculated baby mice but innocuous to adult mice [25]. Whether or not this property was also changed after the serial Vero cell passage was examined. A new stock virus was prepared from the fifth Vero cell passage fluids and titrated for infectivities for baby mice and for 4-week mice by intracerebral inoculation. Also, the same seed was titrated for PFU and for infectivity to Vero cells. The results are combined for presentation in Table 4. The virus had about equal PFU and BIU titers, but all adult mice given the undiluted seed survived. The Vero cell TCID50 titer determined by the above-stated CPE, i.e., the capacity to produce numerous floating cells, was about 1 log lower than the PF and UBIU titers. The survival of adult mice was not due to autointerference of the massively inoculated virus, since a separate test showed that adult mice given serial dilutions of the same seed all survived. Susceptibility of Vero Cells to Various Rabies Virus Other Than the AM In our laboratory was available CE line of HEP Flury virus which derived from the same parent 7-day sage but grown in CE cells under
Strains of Line another had been egg pasneutral
CELL
CULTURE
OF
RABIES
VIRUS
343
Fig. 1. CPE induced by the AM line of HEP Flury virus in Vero cells . a, Control unifected Vero cells on day 7. b, Vero cells given the parent line on day 7. c, Vero cells infected with the AM line on day 4 (CPE grade = 1+). d, Vero cells infected with the AM line on day 7 (CPE grade =2-4). Table
maintenance going other men,
medium
a total strains, had
also
at each
of 150 CE Nishigahara, been
passaged
2.
Serial
passage
passage,
under-
passages. CVS and in CE
of HEP
Three Taka-
cells
in a
Flury
similar and
virus
in Vero
manner, 50th
These
four
lethal
effects
cells
and
CE
were
passage
viruses upon
could
at their levels, be
one-day
90th,
89th
respectively.
titrated chick
by
their
embryos.
344
M.
Table
3.
Plaque-reduction and
Table
4.
HASHIMOTO
Titration
AND K. YOSHINO
neutralization the Vero
of the
Vero
cell-passaged
cell-passaged
Each virus was diluted so as to contain 103 OIU per inoculum and given to bottle cultures of Vero cells. None of these viruses showed a significant titer increase nor distinct CPE. DISCUSSION The present experiments have demonstra ted that HEP Flury strain of rabies virus experiencing more than 46 serial passages in CE cells using alkaline maintenance medium differed from the parent line in that the in vitro host range was expanded including primary monkey kidney and Vero cells. There have been other cases in which rabies virus passaged in one host cell acquired ability to grow in unrelated cell species. As cited in Introduction, a human diploid cell-adapted strain could grow in green monkey kidney cells, in which the original virus did not multiply at all [41]. Adaptation to pig kidney cells was also successful by the use of a hamster kidney cell-adapted strain [1]. A unique point of the present study may be
test
with HEP
HEP
a hyperimmune Flury
Flury
rabbit
serum
in different
hosts
virus
virus
that the alteration of the in vitro host range depended upon the use of alkaline maintenance medium through serial cell passage. An earlier investigation [44] suggested that the progeny virus resulting from such serial passage might contain defective interfering particles [12, 22] at a low proportion. Here we are reminded of a fact that influenza virus passaged through MDBK cells, becoming free of defective particles, could grow in HeLa cells which did not support multiplication of the parent virus [10]. Hence we presume that the AM line of HEP Flury virus contained defective interfering particles in a sufficiently reduced amount to expand its in vitro host range. However, there is some contradiction in this interpretation. That is to say, the change of properties of HEP Flury virus, as evidenced by disappearance of autointerference and acquisition of ability to grow quickly in CE cells under a neutral environment, was seen after five alkaline medium passages in CE cells [44], whereas data at
CELL
hand Vero
CULTURE
OF
indicate that the ability to grow in cells is bestowed on the virus at a later
passage level. Two alternative explanations are being considered. One is that the reduction of defective particles in virus population took place gradually over many passage levels, and it is only after a practical disappearance of such particles that the in vitro host range was altered. The other is that the alkaline medium passage not only diminished the number of defective particles but also selected out a variant in a later passage level which could resist certain growthlimiting factors operative in monkey cells, and this combination contributed to the positive growth of the progeny virus in primary monkey kidney and Vero cells. Which interpretation is valid remains to be determined by further studies.
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[ 1]
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[10]
VIRUS
345
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Macpherson, I., and Stoker, M. 1962. Polyoma transformation of hamster cell clones-an investigation of genetic factors affecting cell competence. Virology 16: 147-151. [28] Matsumoto, S., and Kawai, A. 1969. Comparative studies on development of rabies virus in different host cells. Virology 39: 449-459. [29] Matsumoto, S., Schneider, L.G., Kawai, A., and Yonezawa, T. 1974. Further studies on the replication of rabies-like viruses in organized cultures of mammalian neural tissues. J. Virol. 14: 981-996. [30] Matsumoto, S., and Yonezawa, T. 1971. Replication of rabies virus in organized cultures of mammalian neural tissues. Infect. Immunity 3: 606616. [31] McQuilkin, W.T., Evans, V.J., and Earle, W.R. 1957. The adaptation of additional lines of NCTC clone 929 (strain L) cells to chemically defined protein-free medium NCTC 109. J. Nat. Cancer Inst. 19: 885-907. [32] Michalski, F., Cohen, M.M., and Clark, H.F. 1974. Adult and embryonic gecko cells in vitro: growth characteristics, infection by rabies, Sindbis and polyoma viruses, and transformation by SV40. Proc. Soc. Exp. Biol. Med. 146: 337-348. [33] Mikhailovsky, E.M., and Selimov, M.A. 1966. Adaptation of street rabies virus to primary Syrian hamster kidney cell culture. Acta Virol. 10: 373. [34] Puck, T.T., Marcus, P.E., and Cieciura, S. J. 1956. Clonal growth of mammalian cells in vitro. Growth characteristics of colonies from single HeLa cell with and without "feeder" layer. J. Exp. Med. 103: 273-283. [35] Reed, L.J., and Muench, H. 1938. A simple method of estimating fifty per cent end-points. Amer. J. Hyg. 27: 493-497. [36] Sekine, N., and Yoshino, K. 1976. Enhanced growth and plaguing of rabies virus in static chick embryo cells. Japan J. Microbiol. 20: 331-338. [37] Solis, J., and Mora, E.C. 1970. Viral susceptibility range of the fathead minnow (Pimephales
AND K. YOSHINO
promelas) poikilothermic cell line. Appl. Microbiol. 19: 1-4. [38] Taniguchi, S., and Yoshino, K. 1964. An analysis of the plaque assay of herpes simplex virus in chick embryo monolayers. Arch. Ges. Virusforsch. 14: 537-552. [39] Ver, B.A., and Jhara, H.I. 1968. Propagation of rabies virus strains in chick embryo tissue culture. Indian J. Pathol. Bacteriol. 11: 219-224. [40] Wiktor, T.J., and Clark, H.F. 1972. Chronic rabies virus infection of cell cultures. Infect. Immunity 6: 988-995. [41] Wiktor, T.J., Fernandes, M.V., and Koprowski, H. 1964. Cultivation of rabies virus in human diploid cell strain WI-38. J. Immunol. 93: 353366. [42] Wiktor, T.J., and Koprowski, H. 1974. Rhabdovirus replication in enucleated host cells. J. Virol. 14: 300-306. [43] Yasumura, Y., and Kawakita, Y. 1963. Studies on SV40 virus in tissue culture cells. Nippon Rinsho 21: 1201-1215. (in Japanese) [44] Yoshino, K., Kishie, T., Hashimoto, M., and Yanagi, K. 1974. Effect of alkaline maintenance medium upon the growth of rabies virus in chick embryo cells. Arch. Virol. 47: 31-38. [45] Yoshino, K., Kondo, A., Kuma, N., and Kitaoka, M. 1956. Infection of the one-day-old fertile hen's egg with rabies virus. II. Application to rapid viral titration and neutralization test. Japan. J. Med. Sci. Biol. 9: 273-282. [46] Yoshino, K., Kondo, A., Kuma, N., and Taniguchi, H. 1960. Infection of the one-day-old fertile hen's egg with rabies virus. VI. Strain differences in the egg adaptability. Arch. Ges. Virusforsch. 10: 684-697. [47] Yoshino, K., and Morishima, T. 1971. An improvement in the plaque assay of rabies virus in chick embryo cells. Arch. Ges. Virusforsch. 34: 40-50. [48] Yoshino, K., Taniguchi, S., and Arai, K. 1966. Plaque assay of rabies virus in chick embryo cells. Arch. Ges. Virusforsch. 18: 370-373.
