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Biologicals 40 (2012) 96e99

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Biologicals

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Application of deglycosylation to SDS PAGE analysis improves calibrationof influenza antigen standards

Ruth Harvey a, Michelle Hamill b, James S. Robertson a, Philip D. Minor a, Galina M. Vodeiko c,Jerry P. Weir c, Hitoshi Takahashi d, Yuichi Harada d, Shigeyuki Itamura d, Pearl Bamford e,Tania Dalla Pozza e, Othmar G. Engelhardt a,*aDivision of Virology, National Institute for Biological Standards and Control, Health Protection Agency, Blanche Lane, South Mimms, Potters Bar,Hertfordshire EN6 3QG, United KingdombBioStatistics, National Institute for Biological Standards and Control, Health Protection Agency, Blanche Lane, Potters Bar, Hertfordshire EN6 3QG, United KingdomcDivision of Viral Products, Center for Biologics Evaluation and Research (CBER), Food and Drug Administration (FDA), Bethesda, MD 20892, USAdCenter for Influenza Virus Research, National Institute of Infectious Diseases, Gakuen 4-7-1, Musashimurayama-shi, Tokyo 208 0011, Japane Therapeutic Goods Administration, PO Box 100, Woden ACT 2606, Australia

a r t i c l e i n f o

Article history:Received 23 September 2011Received in revised form21 November 2011Accepted 20 December 2011

Keywords:InfluenzaVaccineStandardisationHAStandardsDeglycosylationCalibration

* Corresponding author. Tel.: þ44 01707 641000; faE-mail address: othmar.engelhardt@nibsc.hpa.org.

1045-1056/$36.00 � 2012 The International Alliancedoi:10.1016/j.biologicals.2011.12.009

a b s t r a c t

Each year the production of seasonal influenza vaccines requires antigen standards to be available for thepotency assessment of vaccine batches. These are calibrated and assigned a value for haemagglutinin (HA)content. The calibration of an antigen standard is carried out in a collaborative study amongst a smallnumber of national regulatory laboratories which are designated by WHO as Essential Regulatory Labora-tories (ERLs) for the purposes of influenza vaccine standardisation. The calibration involves two steps; firstthe determination of HA protein in a primary liquid standard by measurement of total protein in a purifiedinfluenza virus preparation followed by determination of the proportion of HA as determined by PAGEanalysis of the sample; and second, the calibration of the freeze-dried reference antigen against the primarystandard by single radial immunodiffusion (SRD) assay. Herewe describe a collaborative study to assess theeffect of adding a deglycosylation step prior to the SDS-PAGE analysis for the assessment of relative HAcontent.We found that while the final agreed HAvalue of the samples tested was not significantly differentwith orwithout deglycosylation, the deglycosylation step greatly improved between-laboratoryagreement.

� 2012 The International Alliance for Biological Standardization. Published by Elsevier Ltd. All rightsreserved.

1. Introduction

Vaccination against seasonal and pandemic influenza virusremains an important strategy to reduce morbidity and mortalityassociated with the disease [1]. Inactivated seasonal trivalentvaccines produced by numerous manufacturers worldwidecurrently contain standardized amounts of the haemagglutinin(HA) from an A/H1N1, A/H3N2 and B strain of influenza virus. Dueto rapid mutation in influenza viruses (antigenic drift), the WorldHealth Organisation (WHO) makes biannual recommendations forthe strains to be used in the formulation of influenza vaccine, basedon worldwide surveillance and characterisation of circulatingstrains [2]; as a result, one or more strains in the vaccine typicallyneeds to be updated (reviewed in [3] and [4]). The potency of eachlot of vaccine produced is determined by the single radial immu-nodiffusion assay (SRD) which has been the WHO recommended

x: þ44 01707 641366.uk (O.G. Engelhardt).

for Biological Standardization. Pub

assay for influenza vaccine potency determination since 1979 [5,6].The use of SRD for quantitation of HA antigen in vaccine requiresa set of two reference reagents, an HA antigen standard and an anti-HA sheep antiserum, that match the vaccine strain. Thus, newreagents must be prepared each time there is a change in the WHOrecommended vaccine strain.

The anti-HA sheep antiserum is raised against purified HA fromthe recommended strain. The HA antigen reagent is typicallya preparation of inactivated whole virus donated by a vaccinemanufacturer that is freeze-dried by, or on behalf of, an EssentialRegulatory Laboratory (ERL). To calibrate this reagent, a primaryliquid standard (PLS) is prepared by an ERL from a purified viruspreparation of the recommended strain. The total protein withina PLS is determined using a standard protein quantitative assay (e.g.Lowry) whilst the proportion of HA protein within the PLS isdetermined from a densitometric analysis of the viral proteinsseparated by SDS-PAGE run under both reduced and non-reducedconditions [7]. In this assay it is the protein bands of the reducedsamples that are used in the quantitation, with the non-reduced

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1 2 3 4 M 1 2 3 4 no PNGaseF + PNGaseF

HA1

M1 and HA2

NPNP

HA1

M1

HA2

Fig. 1. SDS PAGE analysis of viruses. The four viruses included in the panel to be testedby each laboratory were analysed by SDS PAGE. Sample 1, PR8; 2, NYMC X-157; 3,NYMC X-161b; 4, IVR-116. All virus samples were reduced. Viruses were comparedeither deglycosylated (PNGase F þ), or non deglycosylated (PNGase F -).

R. Harvey et al. / Biologicals 40 (2012) 96e99 97

samples run alongside to aid in identification of the protein bandswhere a degree of co-migration occurs. Densitometric analysis ofprotein bands in the non-reduced samples is not optimal foraccurate quantitation because a lot of the proteins co-migrate or arepresent as higher molecular weight aggregates.

The calibration of a PLS is performed in a collaborative studywithin WHO designated ERLs, namely the Centre for BiologicsEvaluation and Research (CBER), Food and Drug Administration,USA, the National Institute for Biological Standards and Control(NIBSC), Health Protection Agency, UK, the National Institute ofInfectious Diseases (NIID), Japan and the Therapeutic GoodsAdministration (TGA), Australia. The data from all laboratories arecollated so that an HA content value in mg can be assigned to thePLS. The primary standard is then used in SRD to calibrate thefreeze-dried secondary standard, which is the antigen referencematerial used by manufacturers as well as National Control Labo-ratories for the potency testing of an influenza vaccine.

Quantitation of peaks from the densitometric analysis of theSDS-PAGE of purified virus preparations is sometimes technicallychallenging due to various degrees of co-migration of the relevantprotein bands. This phenomenon is strain-dependent and notpredictable. Recently we described a modification of the SDS-PAGEmethod to include deglycosylation of samples before analysis [8].Because deglycosylation shifts the mobility of the HA bands withinthe SDS-PAGE gels, this modification improved the reproducibilityof the quantitation of the HA in our laboratory. Therefore,a collaborative study was set up among the ERLs to assess themodified method in comparison to standard in-house methods ofthe ERLs for the calibration of HA antigen reagent.

2. Materials and methods

2.1. Study design

A panel of purified preparations of four viruses was sent to eachof the four ERLs. Each laboratory tested the samples for HA content(% HA) using their standard in-house methodology and then testedthe samples following a prescribed method for the deglycosylationmethod. Data was returned to NIBSC, collated and statistical anal-ysis carried out to assess the reproducibility of the alternativemethods within and between laboratories. Each sample was testedby each laboratory by each method at least three times, so all datashown are the average of at least 3 individual data points.

2.2. Virus concentrates

A panel of viruses was made comprised of PR8 (H1N1), NYMC X-157 (H3N2), NYMC X-161b (H3N2) and IVR-116 (H1N1). All viruseswere grown in 11 day old embryonated hens’ eggs. Allantoic fluidwas harvested 72 h post infection and was clarified at high speed(10 000 rpm, 30 min, 4 �C, in a Beckman SW28 rotor). Virus waspelleted by centrifugation (25 000 rpm, 90min at 4 �C in a BeckmanSW28 rotor). Virus pellets were resuspended in 1 ml PBS andloaded onto an 11 ml, 10e40% continuous sucrose gradient whichwas centrifuged at 35 000 rpm for 35 min at 4 �C in a BeckmanSW41 rotor. The virus band was harvested, made up to an appro-priate volume in PBS, pelleted by centrifugation (25 000 rpm,90 min, 4 �C, in Beckman SW28 rotor) and the final virus pelletresuspended in PBS (10 ml per egg used).

2.3. SDS-PAGE analysis

SDS-PAGE analysis was performed in each laboratory according tostandard protocols. For the gel in Fig. 1 the following method wasused. One to 3 mL of virus concentrate (approximately 10 mg total

protein) wasmixedwithwater to a total volume of 10 ml. Loading dye(2.5ml)with2% (v/v)b-mercaptoethanol as reducing agentwas addedto each sample. Samples were heated to 95 �C for 3 min prior toloading onto the gels. Ten percent Bis-Tris gels were used and run at125 V for 90min usingMOPS buffer (Invitrogen) followed by stainingusing Colloidal Blue stain (Invitrogen). Quantitation was carried outusing an Imagescanner and associated software (GE healthcare). Thecontent of HA for each sample was calculated as follows; firstly, thetotal viral proteinvalue, in arbitrary units,was calculated by summingthe quantitated values for the HA1, HA2, NP andM1 bands on the gel(thesewere the predominant bands observed in the SDS-PAGE gels ofpurified virus). The HA1 and HA2 values were summed to give thetotal HA value. The percent HA of total protein was calculated bydividing the total HA by the total protein multiplied by 100.

2.4. Deglycosylation using PNGase F

Deglycosylation was achieved using PNGase F (New EnglandBiolabs). Approximately 10 mg (typically 1e3 ml) of each virusconcentrate was denatured according to the PNGase F manufac-turer’s instructions in a total reaction volume of 10 ml and sampleswere incubated at 37 �C overnight (approx. 16 h) with 1 ml of a 1/20dilution of PNGase F enzyme in buffer provided by the manufac-turer and 1% final concentration NP40 (provided with enzyme).Loading dye (2.5 ml) with 2% (v/v) b-mercaptoethanol as reducingagent was added to each sample. Samples were heated to 95 �C for3 min prior to loading onto SDS-PAGE gels. Gels were run andstained as above.

2.5. Statistical analysis

Statistical analysis was performed using Minitab 15 statisticalsoftware. The analysis was performed using a general linear modelwith the Tukey method for pair-wise comparisons [9].

3. Results

A panel of seasonal viruses was assembled and assessed by SDSPAGE analysis with and without deglycosylation (Fig. 1). The resultsshow that when the virus samples are run under reducing condi-tions with no PNGaseF treatment, there are varying degrees of co-migration between the viral protein bands. Of note, the NA proteinoften cannot be seen on gels, as there is not enough present toproduce a visible band and, as a glycoprotein, it is diffuse withoutdeglycosylation. When deglycosylation is used, a discrete NA bandmay be apparent for some viruses, but, as observed in this study,

Fig. 2. HA content of viruses. Each of the four laboratories measured HA content using their in-house method (Method A) or the prescribed deglycosylation protocol (Method B).Results shown are the average of �3 separate experiments for each method and each sample, except for lab 4 Method A as explained in the text. The overall average for each virus/method is also shown (average). Error bars denote standard deviations.

R. Harvey et al. / Biologicals 40 (2012) 96e9998

this is not always the case. Running the same set of virus samplesunder reducing conditions after deglycosylation leads to muchbetter separation of the viral proteins, which allows more accurateand reproducible quantitation.

We analysed %HAvalues with (method B) or without (method A)deglycosylation treatmentof samples, basedon theoriginal in-houseSDS-PAGE procedures of each laboratory. In order to compare thereproducibility of the methods among laboratories a single methodfor calculating the %HA content as described in materials andmethodswas used. One participant, laboratory 4, did not have an in-house experimental method for the quantitation of %HA and uses anassumed value of 35% instead, based on historical experience forassignment of HA content to the PLS. Therefore, no method A wasincluded in the statistical analysis for laboratory 4, but an assumedvalue 35% is included in the results of the study shown in Fig. 2.

Tables 1 (method A) and 2 (method B) show individual meansfor each method, lab and virus calculated using data from replicateresponses, expressed as %HA protein of the total viral protein ina sample. Overall means for each labwere calculated as the mean ofthe four individual virus means within the lab, and overall meansfor each virus were calculated as the mean of the individual labmeans for each of the viruses.

The results for all viruses were similar for bothmethods, with anoverall mean of 44%HA for method A and a mean of 48%HA formethod B.

Within a virus, variability between labs depended on themethod, with method B appearing to have better agreement

Table 1Method A lab and virus means with between-lab %CVs.

Virus Method A Virus mean Betweenlab %CV

Lab 1 Lab 2 Lab 3

IVR-116 36.85 55.88 33.02 41.92 29.21NYMC X-157 45.21 49.32 39.76 44.76 10.71NYMC X-161b 43.10 51.07 37.56 43.91 15.47PR8 43.57 56.00 36.96 45.51 21.24Lab Mean 42.18 53.07 36.83 44.03 (overall) 18.80

Values shown are the averages of at least 3 independent experiments.Bold indicates means between viruses and/or labs, as well as CVs.

between labs than method A. This is reflected by the between labCV for individual viruses, which ranged from 10% to 30% for methodA and 5%e9% for method B. Similarly, the overall between labvariability was lower for method B than for method A (6% and 19%respectively), indicating an overall improved agreement betweenlabs for method B as compared to method A.

Between replicate variability (Tables 3 and 4) was assessed bypooling individual CVs calculated from replicate responses withineach lab and virus. Estimates were pooled both across viruseswithin a lab to give a pooled estimate of within lab, betweenreplicate variation, and across labs within a virus to give a pooledestimate of within virus, between replicate variation. An overallpooled estimate of between replicate variability for each methodwas calculated using the average of the pooled standard deviationsand the overall mean for the particular method.

Overall pooled estimates for both methods were similar (7.0%for method A and 5.8% for method B) indicating similar betweenreplicate variability for the two methods.

For method A, pooled CVs within a lab and virus were less than10%,with the exception of lab 3whichwasmarginallymore variableat 13.6%. This was due to two viruses, NYMC X-161b and NYMC X-157which had higher between replicate variability (16.1% and 18.6%respectively) than viruses PR8 and IVR-116 (3.9% and 1.3% respec-tively). This is reflected also by the pooled within virus CVs whichare higher for NYMC X-161b and NYMC X-157 (9.1% and 9.4%) thanfor PR8 and IVR-116 (3.5% and 5.9%). Comparing the pooled withinlab CVs and the between virus CVs (Tables 1 and 3) and taking into

Table 2Method B lab and virus means with between-lab %CVs.

Virus Method B Virus mean Betweenlab %CV

Lab 1 Lab 2 Lab 3 Lab 4

IVR-116 46.62 54.46 46.72 44.67 48.12 9.00NYMC X-157 49.23 50.46 47.31 42.81 47.45 7.07NYMC X-161b 45.87 50.67 44.37 46.09 46.75 5.82PR8 45.73 52.16 48.44 49.19 48.88 5.41Lab Mean 46.86 51.94 46.71 45.63 47.79 (overall) 5.91

Values shown are the averages of at least 3 independent experiments.Bold indicates means between viruses and/or labs, as well as CVs.

Table 3Method A pooled CVs (within lab and within virus e based on replicate tests).

Method A

Lab Virus

1 2 3 IVR-116 NYMC X-157 NYMC X-161b PR8

5.46 4.72 13.55 5.93 9.38 9.11 3.507.02

Table 4Method B Pooled CVs (within lab and within virus e based on replicate tests).

Method B

Lab Virus

1 2 3 4 IVR-116 NYMC X-157 NYMC X-161b PR8

3.53 5.84 6.10 7.90 6.14 5.71 6.32 5.545.75

R. Harvey et al. / Biologicals 40 (2012) 96e99 99

account the range of variability between replicates in lab 3, there isno evidence to suggest that the variability between viruses isanything more than assay variation. However, the pooled withinvirus CVs are much lower (3.5%e9.4%) than the between lab CVs(10%e30% fromTable 1), suggesting that the variability between labsfor method A is greater than assay variation alone. This wasconfirmed by two-way analysis of variance where all labs weresignificantly different from all other labs (p < 0.001 in all cases).

For method B, all pooled CVs were less than 10% and werecomparable with the between lab and between virus figures fromTable 2 indicating that the %CVs from Table 2 are similar to assayvariation. However, a two-way analysis of variance showed themean for lab 2 to be higher than all other labs; although thedifference in means was small, it was statistically significant(p < 0.001). No significant differences were found between anyother labs (p > 0.05 in all cases).

From the data provided, both methods A and B give similaroverall mean %HA results (44%, Table 1, and 48%HA, Table 2,respectively). Overall assay variation, as measured by the overallpooled %CV’s was similar for bothmethods. The variability betweenviruses was low for both methods and comparable with betweenreplicate variability, however method B showed improved agree-ment between labs compared to method A.

4. Conclusion

Wehave previously demonstrated that amodification of the SDS-PAGE method to include deglycosylation of samples before analysisimproved the reproducibility of the quantitation of the HA of H5N1viruses in our laboratory [8]. A study by Li et al. [10] also showed thatthemodifiedmethod led to very reproducible data for HA content (%HA) in pandemic H1N1 viruses. Herewe have demonstrated that thenew protocol for determination of HA content, which involvesdeglycosylation of the samples prior to running SDS-PAGE, improvedthe agreement between the laboratories participating in the study.The study involved 4 ERL laboratories assessing the HA content fora panel of seasonal influenza viruses using their current standard in-house method and the new prescribed method for deglycosylation.The overall results for percentage HA content, while displayinga trend towards slightly higher HA content values, were not statis-tically significantly changed by use of themodifiedmethod, which isa reflection of the robustness of the methods currently used;however, amove towards even closer agreement between the resultsgenerated is a significant improvement. This study was carried outusing a small selection of seasonal influenza A virus samples. Thelevel of glycosylation can vary (significantly) between different

influenza viruses and so it may be found in the future that thismethod is even more useful for particularly problematic viruses. Forexample, someH5N1 strains have been shownpreviously [8] to havea significant problem of co-migration of proteins due to the level ofglycosylationof theHA. Itmaybeofvalue to continue the comparisonof currently used methods alongside the deglycosylation protocolduring the calibration of reference reagents over a number of influ-enza seasons to obtainmore data and to fully assess the suitability ofsuch a method for the calibration of reagents in the future. Thedeglycosylation stepmay need to be optimised for each new strain atthe start of a season as we have seen some differences betweenviruses in the sensitivity to the PNGase F enzyme, with some virusesrequiring longer periods of treatment, or more enzyme in the reac-tion (data not shown). In the future we can also investigate ways toimprove agreement for the total protein determination, which is theother part of the collaborative process for calibration of potencyreference reagents. There is potential to optimise this part of theassay as well as the HA quantitation procedure and so improve thereproducibility of calibration further. In the future it might be worthinvestigating the standardisation of SDS-PAGE gel consumables,equipment and protocols as well as standardisation of the quantita-tion procedure between the ERLs; however, as the present studyshows, this is not needed urgently. Other physicochemical methodssuch asmass spectrometry can also be used to measure the absoluteamountofaprotein ina sample;ultimatelyamethodsuchas thismaybe even more accurate. However, a lot of assay development andvalidationwould be required before a newmethod like that could beemployed, and a method like mass spectrometry requires moreexpertise and specific equipment, so it may be some time before itcould be used routinely in all ERLs. Overall this study shows that thenew method is more reproducible and thus has the potential toimprove the calibration of influenza reference antigen reagents.

Acknowledgements

This work was funded in part by an MRC grant (numberG0600509) awarded to Dr J.S. Robertson. We would like to thankRobert Newman for providing the panel of virus concentrates.

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