CA1039651A - Attenuation of cytomegalovirus - Google Patents

Abstract

ABSTRACT

An attenuated strain of cytomegalovirus is prepared by serial passage in human tissue culture cells. The strain of virus so attenuated is incorporated into vaccines and used for developing immunity in humans against infec-tions with cytomegalovirus.

Description

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This inventioll relates to attenuatcd strains of the cytomesalovirus (C.~l.V.) and to vaccines for developing immunity in humans agail1st illfections with C.M.V. In a further aspect the invention relates to a method of in~unising hwnans by the use of such vaccines.
Cytomegalovirus is the most co~non known infections cause of mental retardation in infancy. Infection by C.M.V. is particularly frequent during pregnancy, since about 40% of women, at least in Eng]and and Wales, enter this state without antibodies, and are thus sus-ceptible to infection. Infected wo~en may excrete virus in the urine, on the cervix and in the milk.
Most of these congenital infections are asymptomatic or cause only minor illnesses, and yet a substantial proportion of the infected infants suffer irreversible brain damage, which could range from some degree of sensorineural deafness to subnormal intelligenceO No chemotherapeutic treatment or vaccination against the infection has hitherto been recommended or successfully established experimentally, and the high incidence of the disease and the risk of its effects has remained a con-ider-able hazard to the population.
It ha~ b~Il found that a live attenuated c.~.-r. vaccinc can be prepared which stimulates the production of comple- ~-ment-fixing (C.F.) and neutralising antibodies in susceptible individuals with very sligllt side-effects. The attenuation of the C.M.V. strain involves a considerable nun~ber of passages in tissue cultures particularly in human fibroblast

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A:L/ 7 11 cel~s to substnllt~a]ly reduce its pathogenicity but with-out loss of sufficient immunogenicity and infectivity.
The attenuated strain prepared in this manner can be incor-porated iII injectable carriers to provide vaccines suitable for parenteral administration, for instance by the intra-dermal and subcutaneous route.
In one aspect therefore the present invention pro-vides an attenuated strain of the cytomegalovirus, which stimulates production of C.F. and neutralising antibodies in susceptible humans, when tested about 10 weeks after the subcutaneous administration of 104 T.C.D.50 units of the strain, without an~ significant side effects lasting longer than 1 to 2 weeks or detectable excretions of the virus from the test subjects. In a particular aspect such a strain is provided by serially passaging a cytomegalo-virus isolate in susceptible cell cultures, in particular human fibroblast cell cultures, to yield a strain, the pathogenicity of which is of an acceptable level, but in-fectivity and immunogenicity are retained.
Any wild strain isolated from patients, or sub-clinical carriers,may be suitable for the attenuation procedure. It is convenient to start with C.M.~r. strain Ad 169, also known as strain N.I.H. 76559, ieolated from a subclinical carrier, namely by taking some adenoid tissue, in which the virus remains latent, and ma~ing tissue culture from this. In a short time the virus mani-- - fests itself in the cultures. Alternatively it can be isolated from the urine or by taking throat swabs and culturins from these.

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The susceptiblc cell cultures suitable for serial passages according to the present invention include human cell cultures preferably without contaminating viruses or other microorganisms, such as mycoplasma, which sho~r no oncological properties under the usual standard test con-ditions, e.g. the hamster pouch test. Primary and second-ary human fibroblast cells are especially suitable for the purpose~ such as the known embryonic fibroblasts (M.A.F.), foreskin fibroblasts and diploid embryonic lung fibroblast cells (H.E.L. or M.R.C.-5 Jacobs, ~.P. et al. Nature 1970 227 168). These cultures are usually not capable of un-limited propagation and have virtually normal chromosome characteristics.
Human cells have been found to be necessary for C.M.V.
since the virus is highly species specific and grows well only in human cells. Fibroblast cells are preferred for the purpose. With increasing number of passages it was found that the virus multiplies more quickly and becomes more adapted to tissue culture. Although a relatively low number of passages may be sufficient to reduce pathogeni-city, normally at least 40 passages are carried out to ;
provide an attenuated strain which is considered to be --acceptable and safe for use. -~
It is advantageous to propagate the virus in more than one of these fibroblast cells. A passage history in-volving at least 8 passages in each of the above-mentioned types of cultures, i.e. M.A.F., human foreskin fibroblasts, .E.L. and M.R.C.-5 cells, has been preferred. The result-ing attenuated strain may be checked for.freedom from .. . ~.. .~ .

,; ~7 1~39651 extralleous or~a~isllls by culturlng tests in various media favollrillg the gro~th of such microorganisms.
The infectivityl and thus the active amount of viable ~riruses in the culture is tested by observing the cyto-pathic effect over a period, for in~tance in H.E.L. cells.
Usually 2 to 10 fold serial dilutions are used and the effect calculated by the Reed-Muench (Reed, L.V. ~ Muench, H. (1938) Amer. _. Hyg. 27 493) formula. The values are expressed as T.C.D.50 (tissue culture infective dose, which provides a cytopathic effect in 50% of the test number of cells).
Whilst the virus strain retains its ability to infect cells, its overall pathogenicity to the human host is reduced to an acceptable level. No detectable amounts of the virus must appear in throat was~ings or urine samples when tested on cell cultures within 4 weeks after the 1ntra-dermal inoculation of 3 x 10 T.C.D.50 units of the atten-uated strain or after 10 T.C.D.50 units have been given subcutaneously. Some local lesions may appear at the site of inoculation, which usually reach their maximum 10 days after intradermal inoculation. However, it is more impor-tant that with 10 T C.D.50 dosage given subcutaneously, only slight tenderness and aching may result in some patients around the 13th day, but such minimal side effects would normally disappear within a week.
It is also relevant that no disturbance of the liver-function test was observed with 105 T.G.D.50 units (subc.) or 105 T.C.D.50 (intrader.), and only a minority of _ 5 _ : , ,' ~ , : . : ~ . ' .

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1~39~51 vaccinated individllals developed a telllporary cnlargement of the lymph glands in the neck, but no other effects or per-manent damage can be observed ~ith a strain attenuated ac-cording to the present invention.
The immunogenicity of the strain so prepared can be tested by determining complement-fixing antibodies by the standard microtitre technique. Neutralising antibodies can be assayed by the plaque reduction method. For instance, the majority of test patients developed C.F. antibody titres of at least 32,frequently around 64 or more, 4 to 8 weeks after subcutaneous inoculation with 104 T.C.D.50 units of the C.M.V. attenuated strain.
The reaction of the test subjects, who developed sufficient seropositivity after inoculation~ to challenge tests with high dosages of the attenuated viruses has in-dicated that the immunogenicity conferred to the patients is similar in kind and degree to natural immunological pro- ~-tection, and would effectively prevent the re-occurrence of the disease. There is a sufficient cross-immunological relationship between the various strains of the C.M.V., ; - and the novel strain provided by the present invention, that it has therefo~e the capability of protecting the pop-ulation to a sufficiently great extent against the disease. ~-When large-scale production is required the attenuated C.M.V. strain is usually stored as a stock virus. Samples of this are then propagated, for instance on freshly pre- -pared M.R.C.-5 cells in an appropriate medium, such as the 6 -~

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~039651 the serum-frce ~agle's mini~l~un essential medium~ (M.~.M.) (Eagle H. Science 1959 130 ~32) 7 When almost complete cytopathic effect becomes apparent, after 5-7 days of in-cubation, the cells are re-suspended and sonicated with the aid of an ultrasonic generator to facilitate the dis-integration of the oells. The sonicated preparation may then - be centrifuged at low speed to get rid of gross cell debris, and fin~lly put through a sterilising filter.
- It has been found convenient to store the virus suspension in serum free medium containing a stabiliser, such as 25% sorbitol. The formulation can then be frozen in ampules containing about 0.5 ml and stored in liquid ~-nitrogen. Each batch should be tested for sterility and infectivity, and the ampules should contain about 107 T.C.D.50 units for each ml of liquid. For inocu]ation the contents may be diluted, for instance, in Hank's B.S.S.
medium (~Ianks, J.H. & Wallace, R.E. Pro~ Soc. exp. Biol.
1949 71 196) to the required dose level, and the final titre confirmed, whenever necessary. Alternatively, the required single dosage or multiples thereof are pre-~ented in an ampoule, either in suspension or a frozen, dried or freeze-dried form.
In another aspect the invention provides a vaccine for developing immunity in humans against infections with C.M.V., which comprises an attenuated C.M.V. strain, as hereinbefore defined, in an effective dosage, or multiples thereof, in a pharmaceutically acceptable carrier. The effective dosage for vaccination may be from at least 3 x 103 to 3 x 105 T.C.D.50 :i ~ 7 ~

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~39651 units. ~or subcutaneous administration the dose is usually from 5 x 103 to ~ x 104 T.C.I).50 units, frequently around 104 T.C.D.50. For intradermal inoculation, the doses are higher, and are normally from 5 x 104 to 3 x 105 or more, preferably above 2 x 105 T.C.D.50 units, to provide adequate protection.
The pharmaceutically acceptable carrier can be a liquid, such as an aqueous solution containing also nutrients and stabilizers, e.g. Hank's B.S.S. or-other media. The same liquid carrier can also be present in a frozen state incor-porating the virus particles. Furthermore, the carrier may, in some instances, include a sterile sealed container, such as an ampoule or vial, containing the virus particles, for instance, in a dried or freeze-dried state.
It can also be recognised that the invention provides, in a further aspect, a method for developing immunity in suscep-tible humans against infections with C.M.V., which comprises the parenteral administration of a vaccine, as hereinbefore de- -~ined, in the form of a suspension in a carrier. I~ necessary, a dried or frozen vaccine is resuspended and may then be injec-ted under sterile conditions, for instance intradermally into the forearm, or subcutaneously into the delta region of the upper arm.
The effectiveness of the ~accine may be tested about one or two weeks later by determining the antibody titres and the absence of pathogenicity by observation of clinical symptoms and throat washings and urine tests.
Susceptible humans include many women, usually girls, who are already of child bearing age, but have not yet developed i~munity in consequence of an earlier infection with C.M.V.

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103~651 Exalllp]e 1 - Prcl)aration of the attellu~ted straiII
____ The C.M.V. strain Ad 169 had 14 passages in M.A.~.
cells (human embryonic fibroblasts, Microhiological Associates, Bethesda, U.S.A., cf. also Rowe, W.P. et al, Proc. Soc. e~. Biol. Med., 1965, 92, 418) was obtained from Dr. Rowe as strain N.I.H. 76559. A further 10 pass-ages were carried out in human foreskin fibroblasts, 4 passages in M.A.F. cells, 18 passages in diploid human embryonic lung fibroblasts (H.E.L.) and 8 passages in diploid human lung fibroblasts (M.R.C.-5 cells, cf. Jacobs, J.P. et al, Nature, 1970, 227, 168). The media used for the maintenance and propagation of these cells were Eagle's M.E.M. with 2-10% foetal calf serum, and transfers were carried out after 5 to 10 days of incubation. The resulting strain, in its 54th passage was then preserved in liquid nitrogen.
It was checked for freedom from extraneous organisms by culture on blood-agar and in nutrient broth (both aerobically and anaerobically), in mycoplasma medium, and by subculture on to various epithelial-cell tissue-cultures (on which C.M.V. does not grow) - i.e., human embryonic kidney, primary monkey kidney, and hela cells.
It was also examined, after concentration by centrifuga-tion at 40,000 rev/min for one hour, by electron-micro-scopy; bacteria or viruses other than C.~.V. were not seen.
Virus suspensions were titrated for infectivity in tube cultures of H.E.L. cells, using serial tenfold dilutions of ~he virus and five tubes per dilution. The , ~ - , : :

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cultures were reed twice weelsly with ~agle's M.E.M. con--taining 2S' foetal calf serum and observed for cytopathic effect over a period of three weeks. Infectivity titres were calculated by the ~eed-Muench formula, and expressed as T.C.D.50 (tissue culture infecting doses) per ml. -Complement-fixing (C.F.) antibodies were estimated by the standard microtitre technique, using cell-associated antigen prepared from the Ad 169 strain of virus, 2 units of complement, and overnight fixation of ~ C. Neutralising 10 antibodies were determined by the plaque-reduction method (Plummer, G. et al, Proc. Soc. exp. Med. 1964, 145 and Haines H.G. et aI, Ibid, 1971, 13~, 864). In this method serial dilutions of heat-inactivated serum were incubated with equal volumes of virus suspension, diluted to contain 15 approximately 300 plaque-forming units per 0.1 ml, in the presence of 2% fresh guinea pig serum, for one hour at 37C. The reaction mixtures were then inoculated on to H.E.L.-cell monolayers and overlaid with 'Methocel'-gel medium. The antibody titre was calculated as the reciprocal 20 of the highest dilution of serum which caused a 60% re-duction in the number of plaques, when compared with the virus controls. C.M-.V.-specific IgM antibodies were estimated by the indirect fluorescent-antibody technique, using Ad 169-infected H.E.L. cells, known positive and 25 negative control human sera, and anti-human IgM antiserum conjugated with fluorescein isothiocyanate.
Tests were carried out with vaccines incorporating the attenuated C.M.V. strain, as hereinafter described in Examples 2 and 3, and confirmed that the strain fully re-30 tained its capability of inducing C.F. and neutralising -- .~0 -- ' ., - . " . ~ : .
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1~39651 antibodie~ in test subjects and had lost its pathogenicity to hulnans.

Example 2 - Preparation of a vaccine Stock virus prepared according to Example 1 was used to prepare the necessary amount of virus for purposes of vaccine manufacture by infecting M.R.C.-5 cells, freshly prepared from their 16th or 17th passages.
After twenty-four hours, the infected cell-cultures - were washed three times with Hanks' balanced salt solution (B.S.S.) and then maintained on serum-free Eagle's minimum essential medium (M.E.M.). When the cultures showed

3 ~to4 + (almost complete) cytopathic effects, usually within fiveto seven days, the cells were scraped do~n into the medium and the suspension was sonicated for 15 seconds at maximum amplitude in an MSE 150 watt ultrasonic disin-tegrator. Sorbitol solution was added to give a final con-centration of 25% and the virus suspension was measured out as 0.5 ml volumes in hard-glass ampoules and stored in liquid nitrogen. Each batch was tested for sterility and wastitrated in H.E.L. cells for infectivity; this was usually about 107 T.C.D.50 per ml. Aliquots (0.1 ml) of ~
the batch were placed in sterile ampoules and sealed. When ;
ever an ampoule was used for inoculation, the contents were diluted with Hanks' B.S.S. (20 ml) and 0.2 ml portions of this pre~aration were used for the inoculation. On each occasion the final infectivity of the inoculum given to volunteers was checked by titration on tissue-cultures.
The vaccines prepared in this manner were evaluated by vaccination tests in human volunteers, as described .. . ... ---~

I~T/7~1 1~39651 l~ereinaI`ter in ~xamp:Le 3. It was found that unit dosages for vaccinating an individual gave satisfactory il~lunity when containing 2 or 3 x 105 T.D.C.50 units for intra-dermal inoculation, and 10 or 2 x 10 T.C.D.50 units for subcutancous treatment.
Ampoules, containing single doses, or multiples of 3, 5 and 10 of the required dosage, werc prepared.

Example 3 - Vaccinations Volunteers for test vaccinations were medical 10 students and laboratory staff, both male and female.
All volunteers were in good general health and had com-pletely normal haematological and liver-function tests; ~ -the latter included tests for serum-bilirubin, alkaline phosphatase, thymol turbidity, serum-aspartate-aminotrans-15 ferase (S.G.O.T.) and serum-alanine--am~notransferase (S.G.P.T.). Volunteers who had no detectable C.F. anti-body in their serum, at 1 in 8 dilution, were regarded as susceptible to infection. Some volunteers who had antibody titres of 32 or greater were used as controls.
20 A) Vaccine inoculations were given intradermally into the left forearm by using 3 x 105 T.C.D.50 units in 0.2 ml.
The same dose of virus, inactivated by heating at 56C
for twenty minutes was given into the right forearm. Two antibody negative, and one antibody positive, volunteers 25 were given this dosage. Some skin lesions developed in the antibody negative volunteers at the site of the injec- ~
tion with live vaccine, but these rapidly deceased in size ~-and disappeared after three weeks.
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Blood was taken at intervals of one to two wee~s for haematological and liver-I`unction tests. Throat washings and urine from volunteers were collected directly into equal volumes of transport medium and inoculated without delay in 0.1 ml amounts into three H.E.L.-cell cultures each. These were re-fed twice weekly and observed for cytopathic effect for at lea~st four weeks. On some occasions the specimens were first concentrated twenty-fold, by centrifugation at 40,000 rev/min for one hour, before being inoculated into tissue-cultures.
There was some enlargement of lymph-glands in both axillae, but those on the left side were larger and more tender. One of the volunteers also had slightly tender glands on the left side of the neck, causing some stiff-ness, and he had a "cold". This and the glandular enlarge--ment appeared at the height of the local lesion, but lasted only a few days. On the tenth day numerous reactive lymphocytes developed in the blood of both volunteers without any other obvious haematological abnormality; by the twenty-first day the haematology was again normal.
There was no disturbance of liver-function tests. Neither had detectable antibodies developed on the tenth day after inoculation, but at three weeks they had C.F. antibody titres of 32 and 64 and neutralising antibody titres of 160 and 160 respectively.
The single control volunteer with pre-existing anti-body developed only a tiny red papule at the sites of the injection of both live and heated material after twenty-four hours, which increased .slightly over the next day and then rapidly disappeared. I~is haematological tests remained - .
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~0396Sl normal and there was no change in his antibody titre.
~rirus co~ld not be isolated from any of these 3 volunteers from throat wasllings or urine collected one week, two weeks, and four weeks after vaccination.
Four months later the 2 susceptible volunteers who had become seropositive were challenged with 105 T.C.D.50 ~
of vaccine virus intradermally; the same dose of heated -virus was given in the other arm. Both live and heated virus produced only a small reddish papule within forty-eight hours, which then quickly disappeared. The response seemed to be identical to that seen in ths naturally immune volunteer. This challenge produced no significant change in antibody titre, and in the 2 successfully -~
immunised volunteers both C.F. and neutralising antibody ~-have persisted at unchanged levels for more than 2 years.
B) Four vo unteers without pre-existing antibody were next given 10 T-C-D-50 and all 4 produced antibodies ;~
without important side-effects. Over a period, 26 susce-ptible volunteers were challenged with 104 T.C.D.50 f live vaccine virus subcutaneously (see table). In 14 of them there were no local symptoms at all. The other 12 noticed some itching over the site of inoculation, starting on the twelfth to fourteenth day, followed by a little ;
aching. Palpation at that stage revealed a small area of induration and tenderness; only 1 volunteer had redness of the overlying skin. The symptoms mostly disappear within three to four days; in 3 cases they persisted for about a week. 1 of the 12 individuals with local symptoms also had slightly enlarged, tender glands in the left axilla and left side of the neck and this was associated .

~l~7ll ~39651 Wit}1 some reactive lymphocytes in the blood. The slands subsided within a week but reactive lymphocytes persisted for about six weeks. Another had some enlarged glands in the axilla which persisted for about three weeks but without associated reactive lymphocytes. Only one of those volunteers who had no local symptoms had mild axillary adenitis, which was noticed at three weeks and which had resolved within a further week. None of the volunteers demonstrated any disturbance of liver-function tests throughout the eight weeks of observation.
25 of the 26 volunteers given 104 T.C.D.50 of live virus became seropositive. C.F. antibodies were dectable within two weeks in 11 volunteers and within three to eight weeks in the other 14 (see table). In all, peak titres were achieved within six to eight weeks. Twenty-five volunteers have so far been followed up for a year and al-though l9 of them appear to have lost their C.~. anti-bodies there is no significant loss of neutralising anti-bodies. The single volunteer in whom antibodies did not develop had neither local nor general symptoms after vac-cination.
Virus excretion was not demonstrated in any of the volunteers, although throat washings and urine specimens were examined weekly for six weeks. Twelve volunteers were studied more intensively, by daily collection of specimens, for evidence of excretion. Specimens were collected daily during the second week after vaccination from four of them, from another four during the third week, and from the remaining four d~ring the fourth and .- .

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fifth weel~s. A final specimen was obtained from all twelve after eisht weelcs. In no case was C.M.V. isolated.
A sinsle vol~ulteer with a pre~existing low level of C.F. antibody ~C.F. titre 16) was also challenged with 10 T.C.D.50 of virus subcutaneously. He demonstrated no ]ocal reaction, but his antibody titre increased to 64 after three weeks.
A still higher dose of 105 T.C.D.50 was tested for four susceptible volunteers (see table). Three of them acquired antibodies within three to four weeks and two of these were the only volunteers, so far, in whom significant amounts of C.M.V.-specific IgM antibody developed; the titres were 10 and 40 respectively three weeks after ino-culation and they persisted for at least a further three weeks. The symptoms in these three volunteers were no more severe than in those given 10 T.C.D.50. They had some local aching and tenderness at twelve to fourteen days and one also had some enlargement of the homolateral axillary -~
lymph-glands. The symptoms had gone within another seven ~ -to ten days and there were no abnormalities in the haemato-logical or liver-function tests. The one volunteer in whom antibody did not develop, even after three months, was the only one of the four to have no local reaction at all.
It was therefore concluded that the vaccine gave adequate protection in at least 96% of the subjects, with acceptable minimal side effects and no evidence of excre-tion in any form.
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C.~. ANTIBODY RESPONS~S IN SUSC~PTIBL~ VOLUNTEERS GIV~N
VARIOUS DOSES OF C.M.V. VACCINE SUBCUTANEOUSLY
_ -C F antibody titres at Vaccine dose . .
(T.C.D.50) _ 2 l~k ~ a wk < 8 ~ 8 32 32 , 8 ~ 8 32 32 c 8 ~8 8 64 8 ~8 32 32 < 8 '' 32 32 ~- 8 .. 64 32 < 8 ~ 8 64 128 c 8 32 12B 64 8 3~ 128 128 8 ~ 8 32 128 ~- 8 32 32 32 8 ~ 8 16 32 8 ~ 8 32 128 . ~ 8 ~ 8 128 128 ~ 8 ~ 8 128 128 C 8 <8 32 32 ~8 <8 ~8 c 8 ~8 <8 ~ 8 8 - 105 ~ 8 ~ 8 128 64 ._. ~8 - <8 < 8 ~ 8 c 8 ~ 8 3232 8 <8 1 321 128 ~' ' ' - ~ .

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Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method of preparing an attenuated strain of cyto-megalovirus comprising serially passaging a cytomegalovi-rus isolate in susceptible human cell cultures to obtain a strain which stimulates the production of complement fixing and neutralising antibodies in susceptible humans, when tested about 10 weeks after the subcutaneous admini-stration of 104 T.C.D.50 units of the strain, without any significant side effects lasting longer than 1 to 2 weeks, or detectable excretions of the virus from the test sub-jects.

2. A method as claimed in claim 1 wherein the human cells are of the fibroblastic type.

3. A method as claimed in claim 1 or 2, wherein the virus is serially passaged at least 40 times.

4. A method as claimed in claim 1 or 2, wherein the virus is passaged at least 8 times in each of a human embryonic fibroblast cell culture, a human foreskin fibroblast cell culture and a diploid human embryonic lung fibroblast cell culture.

5. A method as claimed in claim 1 or 2 in which said cytomegalovirus isolate is C.M.V. strain Ad 169 (N.I.H. 76559).

6. An attenuated strain of cytomegalovirus which stimulates production of complement fixing and neutralising antibodies in susceptible humans, when tested about 10 weeks after the subcutaneous administration of 104 T.C.D.50 units of the strain, without any significant side effects lasting longer than 1 to 2 weeks, or detectable excretions of the virus from the test subjects.

7. An attenuated strain according to claim 6 derived by serial passage of a cytomegalovirus isolate in susceptible human cell cultures.

8. An attenuated strain according to claim 7 in which said cytomegalovirus isolate is C.M.V. strain Ad 169 (N.I.H. 76559).