Patent Abstract:
means and methods for producing mammalian viruses , the method comprising infecting a culture of immortalized human cells with a virus , incubating the culture infected with virus to propagate the virus under conditions that permit growth of the virus , and to form a virus - containing medium , and removing the virus - containing medium . the viruses can be harvested and be used for the production of vaccines . advantages include that human cells of the present invention can be cultured under defined serum - free conditions and the cells show improved capability for propagating virus . methods are provided for producing , in cultured human cells , influenza virus and vaccines derived thereof . this method eliminates the necessity of using whole chicken embryos for the production of influenza vaccines . the method also provides for the continuous or batch - wise removal of culture media . as such , the present invention allows the large - scale continuous production of viruses to a high titer .

Detailed Description:
the invention provides a method for producing a virus and / or viral proteins , other than adenovirus or adenoviral proteins , for use as a vaccine comprising : providing a cell with at least a sequence encoding at least one gene product of the e1 gene , or a functional derivative thereof , of an adenovirus ; providing the cell with a nucleic acid encoding the virus or the viral proteins ; culturing the cell in a suitable medium and allowing for propagation of the virus or expression of the viral proteins ; and harvesting the virus and / or viral proteins from the medium and / or the cell . heretofore , few ( if any ) human cells have been found that were suitable to produce viruses and / or viral proteins for use as vaccines in any reproducible and scalable manner , in sufficiently high yields , and / or easily purifiable . we have now found that cells having adenoviral e1 sequences ( preferably in their genome ) are capable of sustaining the propagation of viruses in significant amounts . a preferred cell according to the invention is derived from a human primary cell , preferably a cell which is immortalized by a gene product of the e1 gene . in order to be able to grow a primary cell , it , of course , needs to be immortalized . a good example of such a cell is one derived from a human embryonic retinoblast . in cells according to the invention , it is important that the e1 gene sequences are not lost during the cell cycle . it is , therefore , preferred that the sequence encoding at least one gene product of the e1 gene is present in the genome of the human cell . for safety reasons , care is best taken to avoid unnecessary adenoviral sequences in the cells . it is thus another embodiment of the invention to provide cells that do not produce adenoviral structural proteins . however , in order to achieve large scale ( continuous ) virus production through cell culture , it is preferred to have cells capable of growing without needing anchorage . preferred cells according to the invention have that capability . to have a clean and relatively safe production system from which it is easy to recover and , if desired , purify the virus , it is preferred to have a method according to the invention wherein the human cell comprises no other adenoviral sequences . the most preferred cell for the methods and uses of the invention is the previously identified per . c6 cell or a derivative thereof . thus , the invention provides a method of using a cell , wherein the cell further comprises a sequence encoding e2a or a functional derivative , analogue or fragment thereof , preferably a cell wherein the sequence encoding e2a or a functional derivative , analogue or fragment thereof is present in the genome of the human cell and , most preferably , a cell wherein the e2a encoding sequence encodes a temperature - sensitive ( ts ) mutant e2a . furthermore , as previously stated , the invention also provides a method wherein the human cell is capable of growing in suspension . the invention also provides a method wherein the human cell can be cultured in the absence of serum . a cell according to the invention , in particular per . c6 , preferably has the additional advantage that it can be cultured in the absence of serum or serum components . thus , isolation is easy , safety is enhanced , and the system has good reliability ( synthetic media are the best for reproducibility ). the human cells of the invention and , in particular , those based on primary cells , particularly ones based on her cells , are capable of normal ( for humans ) post - and peri - translational modifications and assembly . this means that they are very suitable for preparing viral proteins and viruses for use in vaccines . thus , the invention provides a method wherein the virus and / or the viral proteins comprise a protein that undergoes post - translational and / or peri - translational modification , such as glycosylation . a good example of a viral vaccine that has been cumbersome to produce in any reliable manner is influenza vaccine . the invention provides a method wherein the viral proteins comprise at least one of an influenza virus neuraminidase and / or a hemagglutinin . other viral proteins ( subunits ) and viruses ( wt to be inactivated ) or attenuated viruses that may be produced in the methods according to the invention include enterovirus ( such as rhinovirus , aphtovirus , or poliomyelitis virus ), herpes virus ( such as herpes simplex virus , pseudorabies virus or bovine herpes virus ), orthomyxovirus ( such as influenza virus ), a paramyxovirus ( such as newcastle disease virus , respiratory syncitio virus , mumps virus or a measles virus ), retrovirus ( such as human immunodeficiency virus or a parvovirus or a papovavirus ), rotavirus or a coronavirus ( such as transmissible gastroenteritis virus ), a flavivirus ( such as tick - borne encephalitis virus or yellow fever virus ), a togavirus ( such as rubella virus or eastern -, western -, or venezuelan equine encephalomyelitis virus ), a hepatitis - causing virus ( such as hepatitis a or hepatitis b virus ), a pestivirus ( such as hog cholera virus ), a rhabdovirus ( such as rabies virus ), or a bunyaviridae virus ( such as hantavirus ). in one embodiment , a cell of the invention is useful in the generation of an influenza virus strain that does not grow very efficiently on embryonal eggs . the invention also includes the use of a human cell having a sequence encoding at least one adenoviral e1 protein or a functional derivative , homolog or fragment thereof in its genome , which cell does not produce structural adenoviral proteins for the production of a virus , or at least one viral protein for use in a vaccine . of course , for such a use , the cells preferred in the methods according to the invention are also preferred . the invention also provides the products resulting from the methods and uses according to the invention , especially viral proteins and viruses obtainable according to those uses and / or methods , especially when brought in a pharmaceutical composition comprising suitable excipients and , in some formats , inactivated viruses , subunits , or adjuvants . dosage and ways of administration can be sorted out through normal clinical testing in so far as they are not yet available through the already registered vaccines . thus , the invention also provides a virus or a viral protein for use in a vaccine obtainable by a method or by a use according to the invention , the virus or the viral protein being free of any non - human mammalian proteinaceous material and a pharmaceutical formulation comprising such a virus and / or viral protein . the invention further provides a human cell having a sequence encoding at least one e1 protein of an adenovirus or a functional derivative , homolog or fragment thereof in its genome , which cell does not produce structural adenoviral proteins and having a nucleic acid encoding a virus or at least one non - adenoviral viral protein . this cell can be used in a method according to the invention . in a preferred embodiment , the invention provides influenza virus obtainable by a method according to the invention or by a use according to the invention . in another embodiment , the invention provides influenza vaccines obtainable by a method according to the invention or by a use according to the invention . in another aspect , the invention provides a kit for determining activity of a protease in a sample comprising at least one viral protein or virus obtainable by a method or a use of the invention , the virus or the viral protein being free of any non - human mammalian proteinaceous material . this aspect of the invention is useful particularly for determining protease activity in culture medium . culture medium is noted for being a difficult context for determining activity of a protease . however , by using a viral protein or a virus of the invention as a target for the protease , it is possible to accurately determine activity of the protease also in culture medium . in a preferred embodiment , therefore , the protease activity is determined in a sample comprising culture medium . in a preferred embodiment , the protease comprises trypsin . in a preferred embodiment , the viral protein comprises ha0 . in yet another aspect , the invention provides a method for concentrating influenza virus under conditions capable of , at least in part , preserving virus infectivity , comprising obtaining a cell - cleared supernatant comprising the virus from a culture of cells , and ultra - filtrating the supernatant under low shear conditions . influenza virus preparations harvested from embryonal eggs typically need to be purified for the preparation of a vaccine . purification typically entails at least one concentration step of the virus . current technologies for the concentration of influenza virus from such relatively crude preparations of influenza virus are cumbersome . using a method of concentration of the invention , it is possible to concentrate influenza virus preparations under conditions that maintain , at least in part , infectivity of the virus . preferably , virus is concentrated that is or can be made infectious . “ can be made infectious ,” as used herein , means the generation of infectious virus through cleavage of ha0 . in a preferred embodiment , the concentration is performed using a hollow fiber . a hollow fiber is particularly suited to concentrate under low shear conditions . in a preferred embodiment , the culture of cells comprises in vitro cultured cells . particularly suited for concentration using a method of the invention is supernatant from in vitro cultured cells , particularly when the supernatant comprises serum - free culture medium . in a preferred embodiment , the ultra - filtration is performed with a filter allowing single proteins to pass while retaining the virus . preferably , the filter comprises a cut - off of 500 kd . more preferably , the filter comprises a cut - off of 750 kd . in a particularly preferred embodiment , the concentration further comprises at least a partial removal of proteins comprising a molecular weight smaller than 500 kd and , more preferably , smaller than 750 kd . preferably , the purification is achieved using a mentioned filter . in yet another aspect , the invention provides infectious influenza virus or derivatives thereof concentrated with a method of the invention . a derivative of an infectious influenza virus of the invention typically is a virus , virus particle , or viral protein or part thereof that can be used for immunization purposes . typically , this entails a virus infectivity inactivation step . to further illustrate the invention , the following examples are provided , which are not intended to limit the scope of the invention . mdck cells were cultured in dulbecco &# 39 ; s modified eagle &# 39 ; s medium ( dmem , life technologies breda , nl ) containing 10 % heat - inactivated fetal bovine serum and 1 × l - glutamine ( gibco - brl ), at 37 ° c . and 10 % co 2 . suspension cultures of per . c6 were cultured in excell 525 ( jrh biosciences ) supplemented with 1 × l - glutamine , at 37 ° c . and 10 % co 2 , in stationary cultures in six - well dishes ( greiner ) or in 490 cm 2 tissue culture roller bottles ( corning costar corp .) during continuous rotation at 1 rpm . direct immunofluorescence assays for the detection of influenza virus infection were carried out using the imagen ™ influenza virus a and b kit ( dako ) according to the standard protocol of the supplier . samples were viewed microscopically using epifluorescence illumination . infected cells were characterized by a bright apple - green fluorescence . cell pellets were resuspended in 300 μl of cold pbs / 0 . 5 % bsa + 5 μl of propidium iodide ( concentration 50 μg / ml ) in pbs / fcs / azide solution known to persons skilled in the art . viable and dead cells were then detected via flow cytofluorometric analysis . in general , hemagglutination assays for influenza virus titers were performed according to methods known to persons skilled in the art . here , 50 μl of a two - fold diluted virus solution in pbs was added to 25 μl pbs and 25 μl of a 1 % suspension of turkey erythrocytes ( biotrading benelux b . v .) in pbs and incubated in 96 - well microtiter plates at 4 ° c . for one hour . the hemagglutination pattern was examined and scored and expressed as hemagglutinating units ( haus ). the number of haus corresponded to the reciprocal value of the highest virus dilution that showed complete hemagglutination . in general , obtained influenza viruses were disrupted in a laemmli buffer according to methods known in the art and different volumes of obtained protein mixtures were separated using 10 % sds / page gels . in brief , blots were blocked for 30 minutes at room temperature with block solution ( 5 % nonfat dry milk powder ( biorad ) in tbst supplemented with 1 % rabbit serum ( rockland ), followed by three washes with tbst . then , the blots were incubated with the anti - a / sydney / 5 / 97 ha antiserum ( 98 / 768 nibsc ) diluted 1 / 500 in 1 % bsa / tbst with 5 % rabbit serum ( rockland ) o / n at room temperature . again , the blots were washed eight times with tbst . finally , the blots were incubated with the rabbit anti - sheep antiserum ( hrp - labeled , rockland ) 1 / 6000 diluted in block solution for one hour at room temperature . after eight washes with tbst , the protein - conjugate complex was visualized with ecl ( amersham pharmacia biotech ), and films ( hyperfilm , amersham life science ) were exposed . the antisera were obtained from the nibsc ( uk ) and applied in dilutions recommended by the nibsc . the concentration of hemagglutinin in supernatants , derived from influenza virus infected - per . c6 cells , was determined by the single radial immunodiffusion ( srid ) test as previously described ( wood et al . 1977 ). the assay was performed using standard nibsc ( uk ) antigens and antisera reagents . a total of 1 ml of ten - fold serially diluted viral supernatants were inoculated on mdck cells which were grown until 95 % confluence in six - well plates . after one hour at 35 ° c ., the cells were washed twice with pbs and overloaded with 3 ml of agarose mix ( 1 . 2 ml 2 . 5 % agarose , 1 . 5 ml 2 × mem , 30 ml 200 mm l - glutamine , 24 ml trypsin - edta , 250 ml pbs ). the cells were then incubated in a humid , 10 % co 2 atmosphere at 35 ° c . for approximately three days and viral plaques were visually scored . titration of infectious virus was performed on mdck cells . in brief , cells were seeded in 96 - well plates at a density of 4 × 10 4 cells / well in dmem supplemented with 2 mm l - glutamine . twenty - four hours later , cells were infected with 100 μl of ten - fold serially diluted culture supernatants , in quadruplicate , in medium containing trypsin - edta at the concentration of 4 mg / ml . two hours after infection , cell monolayers were washed two times in pbs and incubated in medium containing trypsin for seven days at 35 ° c . supernatants from these cultures were then tested in an ha assay . tcid 50 titers were calculated according to the method of karber ( 1931 ). for inactivation of the viruses to obtain whole - inactivated virus for the generation of vaccines derived from per . c6 , a mutation protocol known to persons skilled in the art was performed using b - propiolactone . b - propiolactone is a very effective agent widely used for the inactivation of viruses and well known in the art for its mutating effects . it modifies nucleic acid bases of the viral genome and the host cell genome and blocks replication thereafter . following an established protocol used to prepare the whole inactivated influenza vaccine prepared on embryonated eggs , the amount of virus corresponding to approximately 400 mg of ha per strain was inactivated and used for the final vaccine formulation . briefly , one volume of 0 . 3 m sodium phosphate buffer was added to nine volumes of influenza virus preparation . inactivation of the viruses was carried out adding one volume of 10 % of b - propiolactone ( newall design , uk ) to 100 volumes of phosphate - buffered virus preparation and incubated at 20 ° c . for 24 hours . inactivation of the viruses was checked by plaque assay and no plaques were detected for any of the inactivated batches ( data not shown ). cell line per . c6 , or derivatives thereof , were used . cell lines were banked by a two - tier cell bank system . the selected cell line was banked in a research master cell bank ( rmcb ) which was stored in different locations . from this rmcb research , working cell banks ( rwcb ) were prepared as follows : an ampoule of the rmcb was thawed and cells were propagated until enough cells are present to freeze the cells by using dry ice . up to 500 ampoules containing 1 ml ( 1 - 2 × 10 6 cells / ml ) of rwcb were stored in the vapor phase of a liquid n 2 freezer . one ampoule containing 5 × 10 6 per . c6 cells of the wcb was thawed in a water bath at 37 ° c . cells were rapidly transferred into a 50 ml tube and resuspended by adding 9 ml of the suspension medium excell 525 ( jrh biosciences ) supplemented with 1 × l - glutamine . after three minutes of centrifugation at 1000 rpm in a tabletop centrifuge , cells were resuspended in a final concentration of 3 × 10 5 cells / ml and cultured in a t80 tissue culture flask at 37 ° c . 10 % co 2 . two to three days later , cells were seeded into 490 cm 2 tissue culture roller bottles ( corning costar corp . ), with a density of 3 × 10 5 per ml and cultured in continuous rotation at 1 rpm . per . c6 as a human cell was not known for its ability to sustain influenza virus infection and replication . it was , therefore , determined whether per . c6 cells are permissive for influenza virus infection in comparison with the dog cell line mdck that served as a positive control . on the day before infection , 2 × 10 5 mdck cells per well were seeded in six - well plates . twenty - four hours later , 4 × 10 5 seeded per . c6 and the mdck cells per well were infected with the h1n1 strain a / puerto rico / 8 / 34 ( titer 3 . 6 × 10 7 pfu / ml ) ( obtained from dr . e . claas , leiden university medical center , the netherlands ). infection was performed at various multipliticies of infection ( mois ) ranging from of 0 . 1 to 10 pfu / cell . after about two hours of incubation at 37 ° c ., the inoculum was removed and replaced by fresh culture medium . a direct immunofluorescence assay for the detection of influenza virus infection was performed 24 and 48 hours post - infection . the experiment showed permissiveness of per . c6 for influenza infection , with percentages of positive cells moi - dependent and comparable with mdck ( fig1 ). it was verified whether replication and propagation of influenza virus could be supported by per . c6 . on the day of infection , per . c6 cells were seeded in 490 cm 2 tissue culture roller bottles with the density of 2 × 10 5 cells / ml in a final volume of 40 ml in the presence of 5 μg / ml of trypsin - edta ( gibco - brl ). cells were either mock inoculated or infected with the h3n2 strain a / shenzhen / 227 / 95 ( titer 1 . 5 × 10 6 pfu / ml ) ( obtained from dr . e . claas , leiden university medical centre , the netherlands ). infections were performed at moi 10 − 4 and 10 − 3 pfu / cell . after one hour of incubation at 37 ° c ., the inoculum was removed by spinning down the cells at 1500 rpm and resuspending the cells in fresh culture medium + 5 μg / ml of trypsin - edta . harvest of 1 . 3 ml of cell suspension was carried out each day , from day 1 to day 6 post - infection . supernatants were stored at − 80 ° c . and used for hemagglutination assays . cell pellets were used for direct immunofluorescence tests and for propidium iodide staining . to further investigate the permissiveness of per . c6 for propagation of various influenza strains , an infection by using the h1n1 vaccine strains a / beijing / 262 / 95 and its reassortant x - 127 , obtained from the national institute for biological standards and control ( nibsc , uk ), was performed . on the day of infection , per . c6 cells were seeded in 490 cm 2 tissue culture roller bottles with the density of approximately 1 × 10 6 cells / ml in a final volume of 50 ml . cells were inoculated with 5 μl ( 10 − 4 dilution ) and 50 μl ( 10 − 3 dilution ) of virus in the presence of 5 mg / ml trypsin - edta . in order to establish if trypsin was indeed required , one more infection was carried out by inoculating 5 μl of the strain a / beijing / 262 / 95 in the absence of the protease . after approximately one hour of incubation at 37 ° c ., the inoculum was removed by spinning down the cells at 1500 rpm and resuspending them in fresh culture medium ± 5 mg / ml of trypsin - edta . at day 2 and day 4 post - infection , more trypsin was added to the samples . harvest of 1 . 3 ml of cell suspension was carried out from day 1 to day 6 post - infection . supernatants were stored at − 80 ° c . and used for hemagglutination assays and further infections ; cell pellets were used for direct immunofluorescence tests . results obtained with the above - mentioned immunofluorescence and hemagglutination assays are shown in fig4 and 5 , respectively , illustrating the efficient replication and release of the viruses . it was verified whether the viruses grown in per . c6 were infectious and if adaptation to the cell line could increase virus yields . virus supernatants derived from per . c6 infected with the strains a / beijing / 262 / 95 and its reassortant x - 127 ( dil . 10 - 3 ) and harvested at day 6 post - infection were used . at the day of infection , per . c6 were seeded in 490 cm 2 tissue culture roller bottles , with the density of approximately 1 × 10 6 cells / ml in a final volume of 50 ml . cells were inoculated with 100 μl and 1 ml of virus supernatant in the presence of 5 mg / ml trypsin - edta . in order to establish if trypsin was still required , one more infection was carried out by inoculating 100 μl of the strain a / beijing / 262 / 95 in the absence of the protease . after approximately one hour of incubation at 37 ° c ., the inoculum was removed by spinning down the cells at 1500 rpm and resuspending them in fresh culture medium ± 5 mg / ml of trypsin - edta . at day 2 and day 4 post - infection , more trypsin was added to the samples . harvest of 1 . 3 ml of cell suspension was carried out from day 1 to day 6 post - infection . supernatants were stored at − 80 ° c . and used for hemagglutination assays and further infections ; cell pellets were used for direct immunofluorescence tests . results obtained with the above - mentioned immunofluorescence and hemagglutination assays are shown in fig6 and 7 . data obtained with the present experiment showed infectivity of the viruses grown in per . c6 as well as an increase in virus yields . the presence of cell surface receptors for influenza virus on per . c6 propagation of human influenza a and b strains in embryonated chicken eggs leads to a selection of receptor - binding variants that harbor amino acid substitutions at the distal portion of the ha globular head in the exposed and functionally important regions of the molecule . because of these mutations , the egg - adapted strains can differ from the original human viruses in their antigenic and immunogenic activities , as well as their virulence . human influenza viruses isolated from mdck cells usually present an ha protein that is identical to the ha protein present on the virus of the original clinical sample . a recent study ( govorkova 1999 ) clarified the molecular basis for the selection of variants in chicken eggs and the absence of this variant selection phenomenon in mdck cells . all human influenza a and b strains isolated from mdck cells were found to bind with high affinity and specificity for alpha2 , 6 sialic acid - galactose linkages present in oligosaccharides present in cell surface receptors , whereas their egg - grown counterparts showed an increased affinity for the alpha2 , 3 sialic acid - galactose linkages in cell surface receptors carrying oligosaccharides ( sia2 - 3gal ). using specific lectins , it was demonstrated that only sia2 - 3gal - containing receptors were present on the surface of chicken embryonic cells , whereas mdck cells expressed both sia2 - 6gal and sia2 - 3gal . the expression of the sia2 - 3gal and sia2 - 6gal moieties on the surface of per . c6 cells was studied by facs analysis , using two different digoxigenin -( dig -) labeled lectins : sambuca nigra agglutinin ( sna ) that specifically recognizes sia2 - 6gal linkages and the maackia amurensis agglutinin ( maa ), that specifically recognizes sia2 - 3gal linkages . fig8 a shows the recognition of the sna and maa lectins and their binding to the glycosylation sites . furthermore , fig8 a shows the schematic interaction between the fitc - labeled anti - dig antibody and the dig - labeled lectin that recognizes the specific sialyl bond in the glycosylation backbone of the receptor present on the cell surface . both lectins were taken from the glycan differentiation kit ( boehringer - la roche ). the experiment was carried out on per . c6 cells in suspension and adherent mdck and cho cells . mdck and cho cells were released from the solid support using trypsin - edta ( gibco - brl ). the cell suspensions were then washed once with mem - 5 % fbs and incubated in this medium for one hour at 37 ° c . after washing with pbs ( gibco - brl ), the cells were resuspended to a concentration of approximately 10 6 cells / ml in binding medium ( tris - buffered saline , ph 7 . 5 , 0 . 5 % bsa , and 1 mm each of mgcl 2 , mncl 2 and cacl 2 ). cell aliquots were incubated for one hour at room temperature with the dig - labeled lectins sna and maa . after one hour , lectin - treated cells were washed with pbs and incubated for an additional hour at room temperature with fitc - labeled anti - dig antibody ( boehringer - mannheim ). finally , the cells were washed with pbs and analyzed by fluorescence - activated cell sorting using a fac - sort apparatus ( becton dickinson ). the results shown in fig8 b demonstrate that per . c6 cells were stained by both lectins showing the presence of the sia2 - 6gal as well as the sia2 - 3gal receptors . in the same experiment , mdck cells were used as positive control for both the sialylated receptors , whereas cho cells , due to the absence of the alpha 2 - 6 sialyltransferase glycosylation enzyme in these hamster cells , represented a negative control for the sia2 - 6gal moiety . the upper panels show results with the sna lectin and the lower panels showing results with the maa lectin . from these results , it can be concluded that per . c6 expresses cell surface proteins that have both sia2 - 3gal and sia2 - 6gal linkages in their oligosaccharide chains . effect of different concentrations of trypsin - edta on the viability of per . c6 cells , on the influenza virus production in per . c6 cells and on the ha protein derived thereof due to the absolute trypsin requirement for the propagation of influenza viruses in cell cultures , the effects of different concentrations of trypsin - edta on per . c6 cell viability and virus replication in per . c6 cells after infection using several influenza strains were investigated . infection with influenza virus strain a / sydney / 5 / 97 in the presence of low concentrations of trypsin on the day of infection , per . c6 cells were seeded in 490 cm 2 tissue culture roller bottles at a density of 1 × 10 6 cells / ml in the presence of trypsin - edta at final concentrations of 0 . 5 , 1 , 2 , 3 and 5 mg / ml . these trypsin concentrations did not interfere with the growth characteristics of the cells and their viability ( data not shown ). cells were either mock infected or infected with per . c6 - grown influenza virus a / sydney / 5 / 97 at an moi of 10 − 4 pfu / cell . the viral production was monitored by direct immunofluorescence ( data not shown ), hemagglutination assays , single - radial - immunodiffusion ( srid ) above and plaque assays , all as described above . results from this experiment are depicted in fig9 and show that the ha content as measured by srid , as well as the biological activity of the virus expressed in hau , were highest when a trypsin concentration of 1 mg / ml was used . fig9 also shows that by using a plaque assay the highest number of plaque forming units ( pfu ) per ml was observed in the sample corresponding to cells grown in medium containing 2 mg / ml of trypsin . on the day of infection , per . c6 cells were seeded in 490 cm 2 tissue culture roller bottles at a density of 1 × 10 6 cells / ml in the presence of different concentrations of trypsin - edta ranging from 1 to 5 mg / ml . cells were infected with per . c6 - grown virus b / harbin / 7 / 94 at an moi of 10 − 3 pfu / cell . production of the virus was monitored by direct immunofluorescence , hemagglutination and plaque assays as shown in fig1 . the infectability of per . c6 at day 2 increased with the concentration of trypsin . at day 3 , however , no significant difference was observed in the percentage of infected cells when 1 , 2 . 5 or 5 mg / ml trypsin was present . in the absence of trypsin ( 0 μg / ml ), no influenza virus infection was detected . at the day of the last harvest ( day 4 post - infection ), the biological activity of the virus , as measured by hemagglutination assay , did not differ significantly . interestingly , the infectivity assay performed in samples that were taken at days 3 and 4 after infection showed a difference in the production of the virus . the highest titers were obtained at day 3 and day 4 when a trypsin concentration of 2 . 5 to 5 ( day 3 ) and 1 mg / ml ( day 4 ) were used . on the day of infection , per . c6 cells were seeded in t25 tissue culture flasks at a density of 1 × 10 6 cells / ml in the presence of different concentrations of trypsin - edta ranging from 0 to 7 . 5 mg / ml . cells were infected with per . c6 - grown virus x - 127 ( egg - reassortant for the strain a / beijing / 262 / 95 ) at an moi of 10 − 4 and 10 − 3 pfu / cell . viral growth was monitored by direct immunofluorescence , hemagglutination and plaque assays . as shown in fig1 and fig1 , hau titers were identical between samples , independent of the trypsin concentration and the initial moi that was used . furthermore , no significant differences were observed in the infectivity titers as measured by plaque assay . infection of per . c6 with influenza virus strain a / sydney / 5 / 97 in the presence of high concentrations of trypsin to test the effect of increasing concentrations of trypsin on viability of the cells and virus replication , per . c6 cells were seeded in roller bottles at a density of 1 × 10 6 cells / ml in the presence of various concentrations of trypsin - edta ranging from 0 to 12 . 5 μg / ml . cells were either mock infected or infected with per . c6 - grown virus a / sydney / 5 / 97 virus at an moi of 4 × 10 − 5 pfu / cell . hau &# 39 ; s presence in the obtained batches were determined as described . importantly , data depicted in fig1 clearly show that trypsin concentrations up to 10 μg / ml do not interfere with the cell viability . moreover , the biological activity of the virus obtained at day 4 after infection as measured by hau was higher when a trypsin concentration of 2 . 5 to 5 μg / ml was used . furthermore , the tcid 50 was measured ( fig1 , graph portion a ) and plaque assays were performed ( data not shown ). no relevant differences were found in these plaque assays , in the infectivity titers ( tcid 50 ), in the ha cleavage and quantity ( approximately 10 μg / ml ) as determined by western blot analysis shown in fig1 b . influenza virus production on per . c6 cells in a hollow fiber - perfusion bioreactor system the scalability of influenza virus production in suspension growing per . c6 cells was studied by using 3 - liter ( total volume ) bioreactors containing a 2 liter cell suspension volume in serum - free medium , which is also free of animal - or human - derived proteins ( excell 525 , jrh biosciences ). influenza infection was carried out at a cell density of approximately 3 × 10 6 cells / ml . cells were inoculated with per . c6 - grown a / sydney / 5 / 97 virus , at an moi of 10 − 4 pfu / cell . samples of 5 to 10 ml of cell suspensions were taken every day to perform general cell counts , to determine the viability of the cells , for glucose concentration measurements , for direct immunofluorescence , for hemagglutination and for infectivity assays . the results of these experiments are shown in fig1 . to investigate the presence and the status of the ha protein western blots using two different anti - ha antibodies obtained from nibsc were used . srid assays as described above were also performed . the results depicted in the two western blots in fig1 show that the influenza virus batch produced in this bioreactor yielded an ha content of an estimated concentration of 15 μg / ml which was confirmed by srid assays . the ha produced is comparable to reference nibsc ha in terms of subunit composition and immune reactivity with the reference subtype - specific antis era . infection of per . c6 with a / sydney / 5 / 97 in a 15 liter bioreactor followed by a specific down stream process ( dsp ) suspension growing per . c6 cells was incubated at 37 ° c . in a 15 - liter bioreactor hollow fiber perfusion system , with a cell suspension volume of 10 liters in serum - free excell 525 medium ( jrh biosciences ). influenza infection was carried out at 35 ° c . at a cellular density of approximately 3 . 3 × 10 6 cells / ml in medium containing 5 mg / ml trypsin - edta ( life technologies ). cells were inoculated with per . c6 - grown a / sydney / 5 / 97 virus ( passage number 3 ) at an moi of 10 − 4 pfu / cell . perfusion with serum - free excell 525 medium containing trypsin - edta was continued during the first 24 hours upon infection . two days post - infection , cells were fed with a fed - batch solution containing glucose , essential amino acids and extra glutamine : 82 ml per liter suspension containing 50 m / v % glucose ( npbi - the netherlands ), 50 × essential amino acids without gln ( gibco - brl - life technologies ) and 200 mm glutamine ( gibco - brl - life technologies ). cell suspension samples of 10 ml were taken every day in order to perform standard cell counts ( results shown in fig1 , left graph ), glucose concentration measurements ( results shown in fig1 , right graph ), direct immunofluorescence ( fig1 ), hemagglutination ( fig1 ) and infectivity assays ( data not shown ). furthermore , the ha protein was investigated by western blot analysis and compared to an nibsc standard ha control ( fig2 ). on the day of the final harvest of the entire cell suspension ( 92 hours post - infection ), a cell debris clarification was performed in a continuous flow at 20 , 000 g using the powerfuge ™ separation system ( carr , jm separations ) according to the protocols provided by the manufacturer . clarified supernatant was then concentrated twenty - fold using a hollow fiber membrane cartridge of 500 kd cut - off ( a / g technology , jm separations ). the results depicted in fig2 show that the quantitative recovery of live influenza virus after concentration by hollow fiber as measured by hemagglutination and infectivity assays is very significant . to determine the immunogenicity of per . c6 - grown influenza viruses , an in vivo study and challenging model in ferrets was designed . two batches of trivalent whole - inactivated influenza vaccine ( composed of a / sydney / 5 / 97 , a / beijing / 262 / 95 and b / harbin / 7 / 94 ), containing 15 μg ha of each of the three strains , were used . the first batch was obtained from fertile hens &# 39 ; eggs and the second was obtained from per . c6 cells . production , purification , inactivation and formulation of the trivalent whole - inactivated per . c6 - derived influenza vaccines were performed as described below . production of all three influenza viral batches were performed in three separate 3 - liter hollow fiber fed - batch bioreactor systems with cell suspension volumes of 2 liters . fed - batch was performed with the addition of the following solution : a total volume of 96 ml containing 50 m / v % glucose ( npbi ), 50 × essential amino acids without gln ( gibco - brl - life technologies ), 200 mm glutamine ( gibco - brl - life technologies ) and 7 . 5 m / v % nahco 3 ( merck ) was added once . influenza infections were carried out at cell densities ranging from 1 . 8 × 10 6 to 2 . 6 × 10 6 viable cells / ml , in excell 525 serum - free medium containing 5 mg / ml trypsin - edta . per . c6 cells were inoculated with the per . c6 - grown a / sydney / 5 / 97 , a / beijing / 262 / 95 and b / harbin / 7 / 94 virus batches at different mois : 10 − 4 ( a / sydney / 5 / 97 ) or 10 − 3 ( a / beijing / 262 / 95 and b / harbin / 7 / 94 ) pfu / cell . during the virus production period , samples of 10 ml were taken every day to perform standard cell and viability counts , glucose concentration measurements , direct immunofluorescence and hemagglutination assays . fig2 ( results from the a / sydney / 5 / 97 - infected per . c6 cells ) shows the total and viability cell counts after infection with the virus ( upper left panel ), the glucose consumption ( upper right panel ), the percentage of positive cells in the direct immunofluorescence detection ( lower left panel ) and the haus ( lower right panel ). fig2 ( results from the a / beijing / 262 / 95 - infected per . c6 cells ) shows the total and viability cell counts after infection with the virus ( upper left panel ), the glucose consumption ( upper right panel ), the percentage of positive cells in the direct immunofluorescence detection ( lower left panel ) and the haus ( lower right panel ). fig2 ( results from the b / harbin / 7 / 94 - infected per . c6 cells ) shows the total and viability cell counts after infection with the virus ( upper left panel ), the glucose consumption ( upper right panel ), the percentage of positive cells in the direct immunofluorescence detection ( lower left panel ) and the haus ( lower right panel ). virus - containing concentrates were stored at − 80 ° c . until dsp . in all three cases , the glucose consumption , viability and total cell counts of the per . c6 cells were comparable . also , the production levels of the three viruses , as measured by direct immunofluorescence , were similar . although the hau and infectivity titers differed between different strains , per . c6 sustained replication of all influenza viruses that were tested here . on the day of harvest of the entire batch ( either at day 3 or at day 4 post - infection ), viral supernatants were clarified by centrifugation at 2000 rpm in a table top centrifuge and concentrated ten - fold by ultra - filtration using a hollow fiber membrane cartridge of 750 kd cut - off ( a / g technology , jm separations ) following the protocols provided by the manufacturer . influenza viruses were purified from the concentrated supernatants via two subsequent density centrifugation steps : a 25 - 70 % block sucrose gradient ( 1 . 5 hours at 27k ) followed by a continuous 25 - 70 % sucrose gradient ( four hours at 23k ). viral bands were diluted in approximately 50 ml of a phosphate buffer and finally pelleted at 24 , 000 rpm in an ultracentrifuge . viral pellets were dissolved in 1 . 5 to 2 . 3 ml of a phosphate buffer , aliquoted and frozen at − 80 ° c . the formulation of inactivated influenza vaccines is based on the amount ( in micrograms ) of the “ immunologically active ” ha protein , as measured by the srid assay ( wood et al . 1977 ). the test was performed to characterize the ha content of the batches . at the same time , total amount of proteins was measured using the lowry - based dc - protein assay kit ( biorad ) following the procedures suggested by the manufacturer . it was found that ha constitutes about 20 to 30 % of the total protein content of the virus preparation . in vivo immunogenicity of inactivated vaccines produced in eggs and on per . c6 ferrets and mice represent two well - established animal models for studying influenza infection and have been used to determine the efficacy and immunogenicity of influenza vaccines . using the mouse model test system , the immunogenicity produced by the per . c6 and egg - derived trivalent vaccines containing a / sydney / 5 / 97 , a / beijing / 262 / 95 and b / harbin / 7 / 94 are compared by analyzing sera of vaccinated animals by hemagglutination inhibition assay . using the ferret infection model , immunization is followed by a challenge with a / sydney / 5 / 97 . virus recovery on mdck cells and hemagglutination inhibition assay performed on the sera are used to compare the immunogenicity and efficacy of the two vaccines . ninety female balb / c mice are divided into nine groups of ten mice . on day 0 , up to 100 ml of blood is collected . the serum is separated and stored at − 20 ° c . each mouse is then vaccinated with the appropriate vaccine according to the schedule in table i . on day 28 , a further 100 ml of blood is taken . serum is stored at − 20 ° c . each mouse is again vaccinated according to the schedule in table i . on day 42 , a 100 ml blood sample is taken and all mice are sacrificed . serum is separated and frozen at − 20 ° c . hemagglutination inhibition ( hi ) assays are conducted on serum samples from day 0 , 28 and 42 . all these assays are conducted in parallel for each day for both egg - and cell - grown viruses . table i immunogenicity test in mice . immuni - group zation vacci - total numb - antigen volume nation mg ha er type ( ml ) route per dose 1 egg trivalent whole virion 0 . 5 s . c . 9 . 0 2 egg trivalent whole virion 0 . 5 s . c . 3 . 0 3 egg trivalent whole virion 0 . 5 s . c . 1 . 5 4 egg trivalent whole virion 0 . 5 s . c . 0 . 15 5 per . c6 trivalent whole virion 0 . 5 s . c . 9 . 0 6 per . c6 trivalent whole virion 0 . 5 s . c . 3 . 0 7 per . c6 trivalent whole virion 0 . 5 s . c . 1 . 5 8 per . c6 trivalent whole virion 0 . 5 s . c . 0 . 15 9 pbs 0 . 5 s . c . 0 eighteen adult female ferrets ( albino or polecat ) were divided in three groups of six divided as follows : group 1 received the egg - derived test vaccine intramuscularly ( im ), the animals were challenged with a / sydney / 5 / 97 . group 2 received the per . c6 - derived test vaccine im , the animals were challenged with a / sydney / 5 / 97 . group 3 received the test vaccine diluent only and were challenged with a / sydney / 5 / 97 . on days 0 and 28 , the test vaccines were administered . on day 56 , all the ferrets were infected intranasally with 0 . 5 ml of the a / sydney / 5 / 97 challenge virus at tcid 50 10 3 . nasal washes were performed and inflammatory cell counts , temperature and weights of the ferrets were monitored once daily from day 57 to 63 . all animals were sacrificed on day 63 . serum was separated and stored at − 20 ° c . the nasal wash samples were stored on ice and a nasal wash recovery cell count was performed using trypan blue exclusion assay . the titer of the virus obtained from the nasal wash samples was determined by measuring the virus recovery on mdck cells . the spearman and karber ( 1931 ) calculation was used to calculate tcid 50 values . hemagglutination inhibition analyses were conducted on serum samples taken on day 0 , 28 , 56 and 63 . from this experiment , it was concluded that the per . c6 - derived test vaccine was effective . characterization of ha protein derived from influenza virus produced on per . c6 in order to study the glycosylation of ha in per . c6 cells , a batch of uncleaved ha ( ha0 ) was generated . per . c6 cells were infected with virus a / sydney / 5 / 97 ( passage number 5 on per . c6 ) at mois of 1 , 0 . 1 and 0 . 01 pfu / cell in excell 525 medium containing trypsin - edta at the final concentration of 5 mg / ml . after one hour of incubation at 35 ° c ., cells were washed twice with pbs to remove trypsin and incubated o / n at 35 ° c . and 10 % co 2 , in the absence of trypsin . the next day , cell suspensions were harvested and centrifuged ( 500 g ) and cell pellets were washed twice with medium . viral supernatants were frozen at − 80 ° c . and samples thereof were used in western blot assays as described to investigate the presence or absence of uncleaved ha protein . uncleaved ha protein ( ha0 ) consists of the two subunits : ha1 and ha2 , that are connected via a disulfide bond . since this disulfide bond can be disrupted by reduction with dtt , ha1 and ha2 can be separated and visualized on a reducing gel followed by western blots using antisera that recognize the subunits . if the ha protein consists only of ha0 , one band will be visible that migrates slower through an sds / page gel as compared to the ha1 subunit and the fastest migrating ha2 subunit . the results shown in fig2 suggest the presence of mainly uncleaved ha0 from per . c6 infections when compared to the egg - derived positive control that was obtained from the nibsc ( uk ). to confirm that the band detected was indeed uncleaved hemagglutinin , an ha0 sample was digested with different concentrations of trypsin ranging from 2 . 5 to 10 μg / ml in medium o / n at 37 ° c . the digested proteins were then loaded under reducing conditions on an sds / page gel followed by western blot analysis using the same antisera as described for fig1 . as shown in fig2 a , cleavage of the ha0 could be achieved , confirming the generation of uncleaved ha protein on per . c6 . based on these results , an assay to determine trypsin activity in culture medium , using influenza ha0 as substrate is developed . to determine whether trypsin , present in the culture medium of an influenza production run is still active , a trypsin activity assay has been developed . this assay is based on the measurement of the enzymatic activity of trypsin to cleave the substrate that is most relevant for influenza vaccine production : the ha0 . it was determined whether , in a culture of per . c6 inoculated with influenza b / harbin / 7 / 94 ( moi 10 − 3 / 10 − 4 pfu / cell ), the trypsin remained active over the entire production run . to this end , 10 μl of supernatant taken at day 1 , 2 and 3 post - infection were used to cleave 68 ng of the substrate that consisted of ha0 of influenza a sydney / 5 / 97 virus , o / n at 37 ° c . following digestion , protease inhibitors were added to a final concentration of 1 × ( complete protease inhibitor cocktail , boehringer mannheim ) in 3 × laemli buffer with 150 mm dtt ( fluka ). the samples were loaded on a 10 % tris - hcl sds / page gel ( biorad ) and run . the western blot was performed as described . the results are shown in fig2 b , and demonstrate that in cultures of per . c6 inoculated with influenza b / harbin virus , trypsin remained active during the entire production run as culture supernatants were able to cleave ha0 completely . the influenza virus ha protein is a glycoprotein that contains three to nine n - linked glycosylation oligosaccharide sites . the number of sites depends on the virus strain . the location of these sites is determined by the nucleotide sequence of the ha gene and since the viral genome of influenza is replicated by an error - prone rna polymerase , mutations that generate the addition or removal of glycosylation sites occur at high frequency . the composition and structure of the oligosaccharide chains present on the ha is then determined by the array of biosynthetic and trimming glycosylation enzymes provided by the host cell . since glycosylation of ha plays an important role in virulence and vaccine efficacy , the properties of ha produced on influenza infected per . c6 was investigated . a digestion of a / sydney / 5 / 97 uncleaved ha0 protein with the n - glycosydase f enzyme was performed using protocols provided by the manufacturer to remove the seven oligosaccharides expected to be present on the a / sydney / 5 / 97 ha polypeptide . influenza a / sydney / 5 / 97 was lysed with 1 % triton x - 100 ( merck ). protease inhibitor was added to an aliquot of this lysed virus corresponding to 68 ng of ha , to a final concentration of 1 × ( complete protease inhibitor cocktail boehringer mannheim ). this sample was incubated in the presence of 100 mm napo 4 ph 7 , 10 mm edta ( j . t . baker ), 1 % sds ( j . t . baker ) and 1 % b - mercaptoethanol ( merck ). this was incubated for ten minutes at room temperature . the sample was diluted five times in mm napo 4 ph 7 , 10 mm edta ( j . t . baker ), 0 . 625 % np - 40 and 1 % b - mercaptoethanol ( merck ). of this , 40 μl was used for the glyco - f digestion . for this , 2 μl 1 u / μl of glyco - f ( n - glycosidase f , boehringer ) was added and incubated for a minimum period of 16 hours at 37 ° c . then 3 × laemli buffer with 150 mm dtt ( fluka ) was added to a final concentration of 1 ×. the samples were run on a 7 . 5 % sds / page gel . the sds - page and western blot were performed as follows . in brief , the blot was blocked for 30 minutes at room temperature with block solution ( 5 % nonfat dry milk powder , biorad in tbst supplemented with 1 % rabbit serum ( rockland ) followed by three washes with tbst . then , the blot was incubated with the anti - a / sydney / 5 / 97 ha antiserum ( 98 / 768 nibsc ) diluted 1 / 500 in 1 % bsa / tbst with 5 % rabbit serum ( rockland ) overnight at room temperature . again , the blot was washed eight times with tbst . finally , the blot was incubated with the rabbit anti - sheep antiserum - hrp - labeled ( rockland ) 1 / 6000 diluted in block solution for one hour at room temperature . after eight washes with tbst , the protein - conjugate complex was visualized with ecl ( amersham pharmacia biotech ) and films ( hyperfilm , amersham life science ) were exposed . as shown in fig2 , treatment with the glycosidase - f enzyme clearly reduced the size of the protein with approximately 28 - 30 kd , being approximately the predicted loss of about 4 kd per oligosaccharide . the protein band depicted with an asterisk (*) is the de - glycosylated ha0 that migrates similarly to the ha1 subunit product obtained after cleavage of ha0 into ha1 and ha2 subunits ( right lanes ). the possibility of replacing the mammalian - derived trypsin - edta with non - mammalian or recombinant proteins was investigated . recently , a mixture of proteolytic and collagenolytic enzymes ( accutase ™, paa ) from invertebrate species became available for routine cell culture . due to its non - mammalian source , accutase is free of prions , parvovirus , and other components that potentially can contaminate trypsin - edta solutions . no information regarding the type of proteases present in accutase could be obtained to date . the cleavage of ha0 was studied using western blot analysis . a constant amount of ha0 protein , obtained by per . c6 infected with a / sydney / 5 / 97 at an moi 1 pfu per cell without trypsin , was digested with serial dilutions of accutase , o / n at 37 ° c . as a positive control , the same amount of ha0 digested with 100 ng of trypsin - edta was used . the digested proteins were then loaded on a 10 % sds - page gel , under reducing conditions , for western blot analysis . as shown in fig2 , digestion with 2 ml of accutase treatment resulted in complete cleavage of ha0 ; partial cleavage was observed using 0 . 2 ml . these results suggest that treatment with accutase during influenza replication and production can replace trypsin - edta during influenza infections on per . c6 . transmission electron microscopy studies were done on per . c6 cells that were infected with the influenza strain a / sydney / 5 / 97 , as well as on viral - containing supernatants and sucrose purified material , to determine the phenotype of this influenza virus produced on per . c6 . all methods that were used are well known to persons skilled in the art . fig2 shows that the last stages of the virus life cycle are represented by budding and release of enveloped virions from the cytoplasmic membrane . spikes corresponding to the ha and na viral proteins were detected , ornamenting the periphery of the virion particles . the figure also shows the characteristic pleiomorphism of influenza viruses . infection of per . c6 with a large variety of influenza a and b virus strains the use of per . c6 as a platform technology for the production of influenza vaccine would preferably require per . c6 to support the growth of a wide range of strains of different influenza subtypes . static suspension cultures of per . c6 cells that were grown in t25 flasks and / or in six - well plates in excell 525 medium , were infected at a cell density of 10 6 cells / ml with 16 different strains of influenza viruses ( fig3 a ). these strains comprised several h3n2 , h1n1 , b type and avian strains . infections were performed in the presence of 5 μg / ml of trypsin . the viruses were obtained from nibsc as egg - passaged wt or reassortant strains and are noted . infection was performed with a virus dilution recommended by the nibsc in the product sheets that were delivered with the different strains . all viruses tested were capable of propagation on per . c6 as visualized by immunofluorescence ( data not shown ) and titration of supernatant fluids in pfu assay ( fig3 b ). these results show that even influenza strains ( depicted by an asterisk ), such as a / johannesburg / 33 / 94 , b / beijing / 184 / 93 and a / duck / singapore - q / f119 - 3 / 97 , which are normally very difficult to produce on embryonated eggs , can replicate and be produced on per . c6 cells . generation of herpes simplex type 1 ( hsv - 1 ) virus , herpes simplex type 2 ( hsv - 2 ) virus and measles virus on per . c6 it was tested whether viruses other than influenza virus and adenovirus , for example , herpes simplex virus type 1 and 2 and measles virus , could also replicate on per . c6 . vaccines that are derived from these per . c6 - grown viruses and that induce neutralizing effects in humans for protection against wt infections , are generated from the per . c6 - grown virus batches . the strains that were obtained from atcc and used for infection of per . c6 cells are depicted in table ii . the atcc and that were used for infection of per . c6 cells . to test whether hsv - 1 , hsv - 2 and measles viruses obtained from the atcc could replicate and be produced on per . c6 , passage number 46 cells were seeded in labtek chambers , coated with poly - l - lysine using known methods , at 10 5 cells / well . monkey - derived vero cells ( obtained from atcc ) were cultured at passage number 137 and were used as positive controls and seeded at a density of 2 . 5 × 10 4 cells / well . at day 0 , when wells with per . c6 cells were 60 % and vero cells 80 % confluent , cells were infected with different mois ( 10 − 3 , 10 − 2 , 10 − 1 and 1 tcid 50 per cell ). at daily intervals upon infection , cells were fixed and assayed in immunofluorescence using fitc - conjugated type - specific monoclonal antibodies using a kit ( imagen herpes simplex virus ( hsv ) type 1 and 2 , dako - and fitc - conjugated antibodies against the ha and matrix protein of measles virus ( measles ifa kit , light diagnostics ), following the procedures suggested by the manufacturer . the antisera are directed against hsv - 1 and − 2 and measles virus antigens . the results summarized in fig3 show that per . c6 is permissive for hsv - 1 ( fig3 , portion d ), hsv - 2 ( fig3 , portion e ) and measles virus ( fig3 , portion a ) infections . furthermore , the kinetics suggest that these viruses replicate on per . c6 in an moi - dependent manner . next , it was investigated whether hsv - 1 , - 2 and measles virus could be propagated on per . c6 . to this end , cells were infected with moi of 0 . 01 , 0 . 1 and 1 tcid 50 / cell for hsv - 1 ( fig3 lower portion ) and hsv - 2 ( fig3 upper portion ) and an moi of 0 . 001 tcid 50 / cell for measles virus ( fig3 middle portion ) ( passage number 1 ). at the occurrence of almost complete cpe , cells and supernatants were harvested , quickly frozen in liquid n 2 , and thawed . after this , clarified supernatants were passaged blindly using approximately 100 μl to per . c6 ( this is passage number 2 ). after reaching almost complete cpe again , a third passage ( passage number 3 ) was performed in a similar manner . the mois of the passage number 2 and 3 were determined in retrospect by tcid 50 assays . the results of these experiments show that herpes simplex virus type 1 and − 2 and measles viruses can be replicated on per . c6 and that replication and propagation can even occur when mois as low as 10 − 7 are used . to test whether per . c6 could also support the replication of a rotavirus , per . c6 cells were infected with a rhesus rotavirus ( mmu 18006 ; atcc # vr - 954 ; strain s : usa : 79 : 2 ; lot # 2181153 ). per . c6 cells ( passage number 41 ) were cultured at a density of 1 × 10 5 per ml and monkey - derived vero cells ( obtained from atcc , passage number 139 ) were cultured at a density of 2 . 5 × 10 4 per ml and subsequently seeded in labtek chambers that had been pre - coated with poly - l - lysine as previously identified . cells were infected with an moi of 1 tcid 50 / cell of rhesus rotavirus in the presence and absence of 2 μg / ml of trypsin - edta . after 90 minutes of infection , cells were washed with excell 525 medium and further incubated at 37 ° c . at 10 % co 2 in a humidified atmosphere . on five consecutive days following infection , samples of supernatants were harvested , clarified from cells and cell debris by centrifugation at 2000 rpm in a table top centrifuge and analyzed in an elisa specific for rotavirus ( ideia rotavirus , dako ). the results depicted in fig3 clearly show that rhesus rotavirus replicates on per . c6 . in fig3 a , it was outlined which influenza strains could be grown on mammalian el - transformed cells , such as per . c6 ® cells . clearly , the h3n2 and h1n1 strains as well as the influenza b strains and the avian strains could all be grown to significant titers on per . c6 cells . it is known that only a limited number of influenza strains generally cause disease in humans . however , it is now well established that also strains that generally only occur in birds can , through re - assortment in man , become infectious for humans and be transferred from one human being to another . such bird - derived influenza strains can cause so - called “ pandemics ” and infect millions of people around the world and cause countless numbers of casualties . one major example of such pandemic was the spanish flu that killed millions around 1918 . in table iii ( partly taken from : “ avian influenza : assessing the pandemic threat ” publication from the world health organization ( january 2005 )), previous worldwide outbreaks of highly pathogenic avian influenza strains are given . in table iv ( from the same source ), documented human infections with avian influenza viruses are listed . from these tables , it is at least clear that at least the following virus strains are a threat to the well being of birds and humans worldwide : h5n1 , h5n2 , h5n8 , h5n9 , h7n1 , h7n3 , h7n4 , h7n7 , and h9n2 . on top of this , it has recently been established through reverse genetics that the spanish flu in 1918 - 1919 was caused by the h1n1 variant ( tumpey t m et al . ( 2005 ) science 310 : 77 - 80 ). to date , no vaccines exist against the pandemic strains listed in table iii and iv . it is therefore highly preferred to generate vaccines against influenza strains that are not only involved in the yearly influenza outbreaks for which numerous vaccines from different vaccine producers are available , albeit only produced on embryonated hens &# 39 ; eggs , but also against the strains that are associated with pandemic outbreaks or that have the potential to be associated with a pandemic outbreak ( as listed above ). generally , these strains have a h5 , h7 or a h9 haemagglutinin . clearly , it cannot be excluded that new combinations of haemagglutinin and neuraminidase will arise and mutate to become highly pathogenic in addition to the combinations listed above . here , it was tested whether a number of these strains could be grown on e1 - transformed mammalian cells , such as the human retinoblast - derived per . c6 cells . the strains that were used as examples were h5n1 , h7n7 and h9n2 . for this , h5n1 ( a / hong kong / 97 ) was initially grown on mdck cells and subsequently inoculated in per . c6 cell cultures . h7n7 ( a / chicken / netherlands / 03 ) and h9n2 ( a / chicken / saudi arabia / 00 ) strains were initially grown on embryonated eggs and subsequently inoculated in per . c6 cell cultures , generally as described herein . fluorescence was determined as outlined above . fig3 indicates that h5n1 and h9n2 infected almost all cells as nearly 100 % of the cells expressed the virus proteins . h7n7 was only determined in approximately 10 % of the cells . since the titers of the viruses were not known , it might be that the titer of the h7n7 batch was too low to obtain a proper result . subsequent experiments on per . c6 cells were generally performed as described above , in which the inoculation titers were m . o . i . 0 . 0001 ( h5n1 and h7n7 ) and 0 . 0006 ( h9n2 ). infection rates were determined by tcid50 . this experiment was performed twice . the tcid50 was determined using methods known to the person skilled in the art , on mdck cells , as described above . the results are given in fig3 . clearly , the e1 - transformed per . c6 cells are able to sustain the growth of all three strains that are associated with a pandemic outbreak , indicating that per . c6 cells are a suitable substrate to produce influenza viruses based on these strains and to make vaccines against pandemic influenza strains . in a next experiment , different m . o . i .&# 39 ; s were applied using the h5n1 strain . per . c6 cells were inoculated with m . o . i . 1 , 0 . 1 , 0 . 01 , 0 . 001 , 0 . 0001 and 0 . 00001 ( given as 1 , − 1 , − 2 , − 3 , − 4 and − 5 ). tcid50 values were again determined with mdck cells . the results are shown in fig3 and clearly indicate that even at very low inoculation titers good results can be achieved , as an m . o . i . of 0 . 01 gave the best results after three days upon inoculation . these results clearly show that per . c6 cells are very suitable for the growth of pandemic influenza strains in tissue culture thereby circumventing the need for vaccine production using embryonated hen &# 39 ; s eggs . the invention therefore also relates to vaccines comprising influenza viruses that are replicated and grown on e1 - transformed cells , exemplified by per . c6 cells . influenza vaccines may be produced by attenuating produced viruses , inactivating produced viruses or disintegrating produced viruses , wherein it is preferred that the viruses are split and the two major immunogenic components of the virus , haemagglutinin and neuraminidase are further purified and finally mixed with a therapeutically acceptable excipient , such as a therapeutically acceptable buffer or salt solution . such therapeutically acceptable excipients or carriers are well known in the art . vaccines may comprise adjuvants , such as aluminum - based adjuvants ( aluminum hydroxide and / or aluminum phosphate ). preferably , the split influenza virus containing vaccines , according to the invention , also comprise an adjuvant to stimulate the immune response . the viruses obtained by the methods applying adenovirus e1 - transformed cells are typically harvested after 2 to 10 days after inoculation and after harvest attenuated , inactivated or split by methods known to the person skilled in the art . harvest may be accompanied by sonication to release more virus . also , contaminating nucleic acids are typically removed by a nuclease treatment , such as benzonase . formalin or bpl may be used for inactivation . triton is preferably used for obtaining a split virus . the split - virus based vaccines typically comprise the antigenic determinants based on the ha and the na components of the viral coat , which components are typically purified by one or more chromatography steps , including anion and / or cation exchange resins and several concentration , zonal centrifugation and dia - and / or ultra - filtration steps . the formulation of the vaccine is such that the components are therapeutically acceptable in mammals , and preferably in humans . although the invention has been described with a particular amount of detail and with respect to particular examples , the scope of the invention is to be determined by the appended claims . bachmayer h . selective solubilization of hemagglutinin and neuraminidase from influenza virus . intervirology 1975 ; 5 : 260 - 272 . brands r ., a . m . palache , and g . j . m . van scharrenburg . madin darby canine kidney ( mdck )- cells for the production of inactivated influenza subunit vaccine . safety characteristics and clinical results in the elderly . in : brown l . e ., e . w . hampson , r . g . webster , editors . option for the control of influenza iii . amsterdam elsevier , 1996 . p . 683 - 693 . brands r ., a . m . palache , and g . j . m . van scharrenburg . development of influenza subunit vaccine produced using mammalian cell culture technology . in m . j . t . carrondo , b . griffths , j . l . p . moreira , editors . animal cell technology : from vaccines to genetic medicine . dordrecht : kluwer academic publishers , 1997 : 165 - 167 . gubareva l . v ., j . m . wood , w . j . meyer , j . m . katz , j . s . robertson , d . major , and r . g . webster . co - dominant mixtures of viruses in strains of influenza virus due to host cell variation . virol . 1994 ; 199 : 89 - 97 . govorkova e . a ., m . n . matrosovich , a . b . tuzikov , n . v . bovin , c . gerdil , b . fanget , and r . g . webster . selection of receptor - binding variants of human influenza a and b viruses in baby hamster kidney cells . virology 1999 15 ; 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