Suitability of the Vero cell method for titration of diphtheria antitoxin in the United States potency test for diphtheria toxoid

Replacing the in vivo toxin challenge test with an in vitro assay for assessment of potency for diphtheria toxoid containing vaccines

Replacement of the potency tests for diphtheria vaccines is a high priority for the international initiative to reduce, refine, and replace animal use in vaccine testing. Diphtheria toxoid containing vaccine products marketed in the US currently require potency testing by the United States Public Health Service (USPHS) test, which includes an in vivo passive protection test with a diphtheria toxin challenge. Here we describe an in vitro Diphtheria Vero Cell (DVC) assay which combines the immunization approach from the USPHS test and the use of a cell based neutralization assay for serological testing of vaccine potency. The DVC assay reduces the overall number of animals used compared to other serological potency tests and eliminates the in vivo toxin challenge used in the US test. The DVC assay can be used to test vaccine products with a low or high diphtheria toxoid dose. It has been optimized and validated for use in a quality control testing environment. Results demonstrate similar sera antibody unitage as well as agreement between the serum neutralization values determined using the USPHS test and the DVC assay and thus support the use of the DVC assay for routine and stability testing for diphtheria toxoid containing vaccine products.

Human antibodies neutralizing diphtheria toxin in vitro and in vivo

Diphtheria is an infectious disease caused by Corynebacterium diphtheriae. The bacterium primarily infects the throat and upper airways and the produced diphtheria toxin (DT), which binds to the elongation factor 2 and blocks protein synthesis, can spread through the bloodstream and affect organs, such as the heart and kidneys. For more than 125 years, the therapy against diphtheria has been based on polyclonal horse sera directed against DT (diphtheria antitoxin; DAT). Animal sera have many disadvantages including serum sickness, batch-to-batch variation in quality and the use of animals for production. In this work, 400 human recombinant antibodies were generated against DT from two different phage display panning strategies using a human immune library. A panning in microtiter plates resulted in 22 unique in vitro neutralizing antibodies and a panning in solution combined with a functional neutralization screening resulted in 268 in vitro neutralizing antibodies. 61 unique antibodies were further characterized as scFv-Fc with 35 produced as fully human IgG1. The best in vitro neutralizing antibody showed an estimated relative potency of 454 IU/mg and minimal effective dose 50% (MED50%) of 3.0 pM at a constant amount of DT (4x minimal cytopathic dose) in the IgG format. The targeted domains of the 35 antibodies were analyzed by immunoblot and by epitope mapping using phage display. All three DT domains (enzymatic domain, translocation domain and receptor binding domain) are targets for neutralizing antibodies. When toxin neutralization assays were performed at higher toxin dose levels, the neutralizing capacity of individual antibodies was markedly reduced but this was largely compensated for by using two or more antibodies in combination, resulting in a potency of 79.4 IU/mg in the in vivo intradermal challenge assay. These recombinant antibody combinations are candidates for further clinical and regulatory development to replace equine DAT.

A single booster dose of diphtheria vaccine is effective for travelers regardless of time interval since previous doses

Our study showed the immune response before and after a booster against diphtheria given within the 20-year interval recommended in Sweden or after a prolonged interval. Of 40 travellers, 10/13 in recommended interval group were immune before booster and 19/27 with a delayed interval. After booster, 13/13 versus 26/27 were protected. One booster was sufficient to achieve immunity regardless of the interval.

Calibration and commutability assessment of the 1st International Standard for Diphtheria Antitoxin Human

The 1st International Standard for Diphtheria Antitoxin Human (coded 10/262) was established by the World Health Organization Expert Committee on Biological Standardization in 2012. This paper describes the production, characterization and calibration of the new standard which is intended for use in the standardization of assays used to measure diphtheria antibody responses in human serum. The new standard was calibrated in terms of the International Standard for Diphtheria Antitoxin Equine in an international collaborative study. A total of 8 participants from 8 different countries performed in vivo and/or in vitro toxin neutralization tests and returned data that was used to assign units to the proposed new standard. The new standard has a diphtheria antitoxin potency of 2 IU/ampoule and is predicted to be stable. A follow up study was performed to assess commutability of the new standard. The follow up study was an existing external quality assessment, modified to include the new standard. Results obtained suggest that the new standard is commutable, showing comparable behaviour to native human serum samples in the majority of the assays compared, and is therefore suitable for use as a reference preparation in assays used to measure the level of anti-diphtheria antibodies in human serum.

Reduction of antibody response to an 11-valent pneumococcal vaccine coadministered with a vaccine containing acellular pertussis components

In pneumococcal conjugate vaccines (PCVs), polysaccharide antigens are often conjugated to protein carriers related to other common vaccines. It is therefore important to test PCV interaction with other pediatric vaccines when administered simultaneously. We assessed the immune response to an 11-valent PCV conjugated to diphtheria and tetanus carriers (PncD/T11), administered concomitantly, but in separate sites, with a combined vaccine containing epitopes related antigenically to the carriers: polyribosylribitol phosphate-tetanus tox oid (PRP-T), diphtheria toxoid (DT), and tetanus toxoid (TT). In addition, these combinations contained inactivated poliovirus vaccine (IPV) and either whole-cell pertussis (wP) or acellular pertussis (aP) components. After coadministration of PncD/T11 with the combined vaccine containing wP (DTwP/IPV/PRP-T), the responses to all polysaccharides in the PncD/T11 were satisfactory. In contrast, when coadministered with an aP-containing combination (DTaP/IPV/PRP-T), the response to all seven pneumococcal conjugates to TT was significantly reduced after primary and booster immunization. The pneumococcal conjugates to DT were not significantly reduced after the primary series, but were somewhat reduced after booster. It is likely that some suppression of the tetanus-mediated response occurred even when the PncD/T11 was coadministered with wP, but this suppression was masked by the adjuvant effect of wP. By replacing wP with aP, this adjuvant effect was removed, unmasking the suppression of the tetanus-mediated response. With the increasing use of multiple aP-containing vaccines in infancy, novel approaches to adjuvants and carrier protein technology are likely to be required.

Adjuvant activity of Mycobacterium bovis BCG expressing CRM197 on the immune response induced by BCG expressing tetanus toxin fragment C

In order to develop a combined recombinant Mycobacterium bovis BCG (rBCG) vaccine against diphtheria, pertussis and tetanus (DPT), we have constructed different strains of rBCG expressing tetanus toxin fragment C (FC), driven by the up-regulated M. fortuitum beta-lactamase promoter, pBlaF*. Tetanus toxin FC was expressed in comparable levels in native form or in fusion with the beta-lactamase exportation signal sequence; however, in both constructs it was localized to the cytosol. Immunization of mice with rBCG-FC or its combination with rBCG expressing CRM197, induced anti-tetanus toxin antibodies with a Th2 immunoglobulin profile. Administration of a subimmunizing dose of the diphtheria-tetanus toxoid vaccine showed that rBCG-FC primed mice for production of an intense humoral response. Interestingly, the combination of rBCG-FC and rBCG-CRM197 reduced the time required for maturation of the immune response and increased anti-tetanus toxin antibody levels, suggesting adjuvant properties for rBCG-CRM197; this combination induced 75% protection in mice challenged with 100 minimum lethal doses (MLD) of tetanus toxin. Antisera from guinea pigs immunized with this combination were shown to neutralize tetanus toxin and diphtheria toxin. Our results suggest reciprocal adjuvant effects of rBCG-FC and rBCG-CRM197, which may contribute to induction of a more effective immune response against both diseases.

Properties of a new intravenous immunoglobulin (IGIV-C, 10%) produced by virus inactivation with caprylate and column chromatography

BACKGROUND AND OBJECTIVES

Current manufacture of intravenous immunoglobulin (Gamimune N) uses four cold-ethanol precipitation steps and solvent-detergent treatment. Our objective was to design a new manufacturing process to maximize immunoglobulin G (IgG) purity, achieve robust viral safety, preserve all the biological activities of antibody and avoid unnecessary protein loss.

MATERIALS AND METHODS

The new process combines multiple functions in single steps. Caprylate is added to precipitate non-IgG proteins and to inactivate enveloped viruses. Two successive anion-exchange columns are used to purify IgG and remove caprylate. The new product, IGIV-C (Gamunex, 10%) is formulated with glycine at 100 mg/ml IgG, pH 4.25. Vials are incubated for 21 days at 23-27 degrees C in a final virus-inactivation step.

RESULTS

Compared with the process for production of Gamimune N, that for IGIV-C requires a shorter production time, achieves more robust virus inactivation, increases IGIV yield from plasma, improves physiological IgG subclass distribution (resulting in higher levels of IgG4), and improves purity, with lower levels of IgA (40 microg/ml), IgM (< 2 microg/ml) and albumin (< 20 microg/ml). Antibody binding, opsonization and protective activities are similar.

CONCLUSIONS

Compared with the current commercial process, the new IGIV-C manufacturing process produces a more highly purified preparation that contains slightly higher levels of IgG4 and retains antibody activities required for clinical efficacy.

Potency control of diphtheria component in adsorbed vaccines by in vitro neutralization tests

Samples from 20 lots of dT vaccine and from 20 lots of DTP vaccine were used to standardize and validate the Vero cell and the toxin binding inhibition (ToBI) tests for the potency control of diphtheria component. For the Vero cell method, violet crystal solution was used to stain the cells and estimate the endpoint of diluted diphtheria antitoxin. Diphtheria anatoxin was used for performing the ToBI test instead of toxin. The results obtained by both in vitro tests were similar to those obtained by in vivo toxin neutralization test in guinea pigs. The various analysis and the chi(2) test applied to evaluate the reproducibility and homogeneity, respectively, among in vitro tests and in vivo toxin neutralization test did not detect statistical significant difference for both analysed vaccines. An excellent correlation among in vitro tests and in vivo neutralization test was observed by Spearman's correlation coefficient.

Induction of neutralizing antibodies against diphtheria toxin by priming with recombinant Mycobacterium bovis BCG expressing CRM(197), a mutant diphtheria toxin

BCG, the attenuated strain of Mycobacterium bovis, has been widely used as a vaccine against tuberculosis and is thus an important candidate as a live carrier for multiple antigens. With the aim of developing a recombinant BCG (rBCG) vaccine against diphtheria, pertussis, and tetanus (DPT), we analyzed the potential of CRM(197), a mutated nontoxic derivative of diphtheria toxin, as the recombinant antigen for a BCG-based vaccine against diphtheria. Expression of CRM(197) in rBCG was achieved using Escherichia coli-mycobacterium shuttle vectors under the control of pBlaF*, an upregulated beta-lactamase promoter from Mycobacterium fortuitum. Immunization of mice with rBCG-CRM(197) elicited an anti-diphtheria toxoid antibody response, but the sera of immunized mice were not able to neutralize diphtheria toxin (DTx) activity. On the other hand, a subimmunizing dose of the conventional diphtheria-tetanus vaccine, administered in order to mimic an infection, showed that rBCG-CRM(197) was able to prime the induction of a humoral response within shorter periods. Interestingly, the antibodies produced showed neutralizing activity only when the vaccines had been given as a mixture in combination with rBCG expressing tetanus toxin fragment C (FC), suggesting an adjuvant effect of rBCG-FC on the immune response induced by rBCG-CRM(197). Isotype analysis of the anti-diphtheria toxoid antibodies induced by the combined vaccines, but not rBCG-CRM(197) alone, showed an immunoglobulin G1-dominant profile, as did the conventional vaccine. Our results show that rBCG expressing CRM(197) can elicit a neutralizing humoral response and encourage further studies on the development of a DPT vaccine with rBCG.

Comparison of four serological methods for the detection of diphtheria anti-toxin antibody

The aim of this study was to compare four serological methods for the detection of Corynebacterium diphtheriae IgG anti-toxin antibodies (IgG-DTAb) in human serum. One hundred serum samples were evaluated for C. diphtheriae IgG-DTAb by four different methods: passive haemagglutination (PHA), latex agglutination test (LA), toxoid enzyme-linked immunosorbent assay (Toxoid-ELISA), and toxin-binding inhibition enzyme-linked immunosorbent assay (ToBI-ELISA). As the external standardisation the neutralisation test for C. diphtheriae toxin in Vero cells (TN Vero) was used. For internal standardisation of IgG-DTAb titres, the WHO standard serum of human diphtheria antitoxin was used. The study revealed a poor correlation between the reference test and the PHA (r=0.34 Pearson's correlation coefficient), an acceptable correlation for the LA (r=0.74), a good correlation for the Toxoid-ELISA (r=0.81) and a very good correlation for ToBI-ELISA (r=0.93). The sensitivity measurements of PHA, LA, Toxoid-ELISA and ToBI-ELISA tests, were 14, 100, 94, 96% respectively and the corresponding specificity characteristics were 86, 76, 94, 90 respectively. Of the four evaluated methods, the ToBI-ELISA could be recommended for scientific and precise laboratory assays of diphtheria antibody levels in humans. For screening purposes the Toxoid-ELISA could be used, but the accuracy of antibody titres below 0.1 IU/ml, considered as the limits of protection, is questionable. Both tests offer very useful alternatives to the in vitro diphtheria toxin neutralisation test in Vero cells. Because of their unsatisfactory correlation and sensitivity as compared to the reference method, PHA and LA should be avoided and replaced by one of the two enzyme immunoassays.

European sero-epidemiology network: standardisation of the results of diphtheria antitoxin assays

A European Sero-Epidemiological Network (ESEN) was established with the aim to co-ordinate and harmonise serological surveillance of immunity to communicable diseases in Europe. In this study the inter-laboratory standardisation of diphtheria toxin antibody measurements is reported. A standard panel of 162 sera was tested by the participating laboratories using an in vitro assay of their choice: VERO cell toxin neutralisation assay (NT), double-antigen delayed time-resolved fluorescence immuno-assay (DA-DELFIA), double-antigen enzyme-linked immunosorbent assay (DAE), toxin binding inhibition test (ToBI) and an indirect enzyme-linked immunosorbent assay (ELISA). The results were standardised using regression against the NT. The variations due to inter-laboratory and inter-assay variation, which would otherwise make it difficult directly to compare the main serum bank results by the different laboratories and the various assays were successfully minimised by the standardisation. The regression equations obtained will be used to transform the respective local results of testing the main serum bank into the reference test unitages. This study also gave the opportunity to compare the various assays within and between laboratories. This demonstrated a very high correlation between DA-DELFIA, DAE, ToBI and the NT. The ELISA showed a good correlation, too, however sera below some 0.1 IU/ml were overestimated.

Evaluation of a guinea pig model to assess interference in the immunogenicity of different components of a combination vaccine comprising diphtheria, tetanus and acellular pertussis (DTaP) vaccine and haemophilus influenzae type b capsular polysaccharide conjugate vaccine

A guinea pig model to assess the immunogenicity of a combination vaccine containing diphtheria, tetanus and acellular pertussis (DTaP) vaccine and Haemophilus influenzae type b (Hib) capsular polysaccharide conjugated to tetanus toxoid (HibT) was evaluated comparatively with the mouse immunogenicity test to study the effect of combining these antigens on the immunogenicity of various components. The immunogenicity test in mice was performed by subcutaneous injection of groups of 10 animals twice at an interval of four weeks with 1/10 of a single human dose of various formulations of combination vaccines, DTaP or HibT vaccine. The animals were bled at 4 and 6 weeks and IgG or total antibodies to various components were determined by ELISA or RIA. The guinea pig immunogenicity model included groups of animals injected subcutaneously twice at an interval of six weeks with 1.5 times the single human dose of various formulations. The animals were bled at 4, 6 and 8 weeks and serum samples were tested for antibodies to various components by ELISA, RIA and/or neutralization tests. Additionally, potency of tetanus and diphtheria components was assessed as per the US Food and Drug Administration's regulations. Aluminium phosphate (AIPO(4)) adsorbed HibT vaccine or HibT as a combination with AIPO(4)adsorbed DTaP vaccine showed significant increases in IgG antibodies to tetanus toxin in mice as well increased tetanus antitoxin levels in guinea pigs as compared to soluble HibT vaccine. In general, combining DTaP and HibT vaccines did not affect the antibody levels to tetanus and diphtheria toxoids whereas DTaP-HibT combination vaccine elicited significantly lower IgG antibodies to pertussis toxin and filamentous haemagglutinin than DTaP vaccine alone, particularly after first injection. Mice showed similar Hib antibody responses for the combination and HibT alone whereas guinea pigs consistently showed lower anamnestic responses to Hib for combination formulations than for HibT alone. Reducing the amount of HibT and/or tetanus toxoid in the combination formulations reduced this suppression of Hib antibody response in guinea pigs. Suppression of Hib antibody response in combination vaccines has also been reported from recent clinical trials. Based on the results from this study, it appears that the guinea pig model may be able to predict the human response to various components of combination vaccines.

Differences in the immunogenicity of native and formalinized cross reacting material (CRM197) of diphtheria toxin in mice and guinea pigs and their implications on the development and control of diphtheria vaccine based on CRMs

Immunogenicity of native and formalinized cross reacting material (CRM197) of diphtheria toxin (DTx) was assessed in mice and guinea pigs. For the primary response, mice produced similar levels of diphtheria toxoid (DTxd) IgG antibodies to both native and formalinized preparations of CRM197 though the antibody levels were significantly lower than those elicited by conventional DTxd (P < 0.05). In contrast, guinea pigs showed significantly higher levels of DTxd IgG antibodies to the formalinized CRM197 preparation than the native preparation (P < 0.001) after single injection. These differences in the immunogenicity of mice and guinea pigs to native and formalinized CRM197 preparations have implications for the development and control of diphtheria or other vaccines involving the use of CRMs.

Booster vaccination against diphtheria and tetanus in man. Comparison of three different vaccine formulations--III

Adverse reactions and antibody levels were compared following a booster vaccination of 177 Danish military recruits with a plain, an aluminium hydroxide (0.5 mg Al per human dose, HD) and a calcium phosphate (0.25 mg Ca per HD) adsorbed diphtheria-tetanus (D-T) vaccine. The calcium phosphate adsorbed vaccine was given in a HD of 3 Lf of D and T toxoids and proved to be of equal efficacy as the aluminium hydroxide adsorbed vaccine which was injected in a dose containing twice the antigen amount. The calcium phosphate vaccine caused fewer adverse reactions than the one adsorbed to aluminium hydroxide. The plain vaccine (6 Lf per HD of D and T toxoid) had the highest efficacy with a similar low occurrence of adverse reactions as the calcium phosphate adsorbed vaccine. Potency assays in mice were in accordance with these immunogenicity results in man if a two dose immunization schedule was followed, but not if the vaccines were compared after a single immunization as requested by the procedure for potency testing according to current WHO and European Pharmacopoeia requirements. Both of the adsorbed vaccines primed mice for specific IgE antibody formation. This could be detected after a second immunization with either of the adsorbed vaccines or with the plain D-T vaccine. Also in humans, immunization with the plain vaccine boosted specific IgE formation to a detectable level. This may be ascribed to adjuvant priming during the primary vaccination series some 20 years previously.

Simultaneous quantitation of diphtheria and tetanus antibodies by double antigen, time-resolved fluorescence immunoassay

A dual, double antigen, time-resolved fluorescence immunoassay (DELFIA) for the simultaneous detection and quantitation of diphtheria (D) and tetanus (T) antibodies in sera has been developed. In the double antigen format one arm of the antibody binds to antigen coated microtitre wells and the other arm binds to labelled antigen to provide a fluorescent signal. This assay was found to be functionally specific for IgG antibodies and showed a good correlation with established toxin neutralization assays. Furthermore, the double antigen set-up was species independent, permitting the direct use of existing international references of animal origin to measure protective antibody levels in humans in international units (IU/ml). The detection limit corresponded to 0.0003 IU/ml with Eu(3+)-labelled toxoids and to 0.0035 IU/ml using Sm(3+)-labelled toxoids. The assay was fast with a high capacity making it a suitable method for serological surveillance studies.

Adjuvant properties of non-phospholipid liposomes (Novasomes) in experimental animals for human vaccine antigens

Non-phospholipid liposomes composed of dioxyethylene cetyl ether, cholesterol and oleic acid were evaluated as adjuvants with human vaccine antigens, tetanus toxoid (TT) and diphtheria toxoid (DT), in mice and rabbits. Antigens encapsulated in or mixed with liposomes elicited antitoxin levels similar to those elicited by antigens given with Freund's adjuvant or adsorbed onto aluminum phosphate. All liposomal antigen preparations, antigen given with Freund's adjuvant or adsorbed onto aluminum phosphate, elicited significantly higher IgG antibodies and antitoxin levels than soluble antigens in mice after a single injection and in rabbits after each of three injections. TT encapsulated in liposomes elicited sustained anti-TT IgG antibody levels in mice after a single injection as compared to TT mixed with liposomes. TT mixed with or encapsulated within liposomes containing monophosphoryl lipid A/squalene or squalene alone, as well as aluminum phosphate adsorbed TT elicited greater primary responses in mice than TT mixed with or encapsulated within plain liposomes. Liposomal TT preparations produced a slightly higher anamnestic response in mice than aluminum phosphate adsorbed TT. Subclass analysis of anti-TT antibodies showed that the majority of the antibodies belong to IgG1 subclass. Liposomal TT preparations, particularly those with encapsulated monophosphoryl lipid A/squalene or squalene alone, consistently elicited higher levels of anti-TT IgG2a and IgG2b than aluminum phosphate adsorbed or soluble TT. None of the preparations elicited IgG3 or IgM antibodies. It appears that non-phospholipid liposomes are as potent adjuvants as the currently employed adjuvant for human vaccines (aluminum phosphate) or a benchmark adjuvant for experimental immunology (Freund's adjuvant), and may be able to modulate the immune response towards the Th1 type.

Humoral response in rabbits immunized with calcium phosphate adjuvanted HIV-1 gp160 antigen

Rabbits were immunized with either calcium phosphate adjuvanted purified HIV-1 gp160 or a fluid preparation. Circulating antibodies were detected by ELISA, RIPA and Western Blot tests. Sera of rabbits immunized with the adjuvanted preparation contained high levels of anti-gp160 antibodies, as well as antibodies recognizing p55, p25 and p18. On the contrary, rabbits immunized with the fluid preparation contained only anti-p18 antibodies. Neutralizing antibodies were also detected. It is concluded that the calcium phosphate adjuvant could be used for preparation of candidate anti-HIV vaccines, since it permits one to induce high levels of circulating antibodies, in the absence of untoward reactions as observed when aluminium adjuvants or water in oil emulsions are used.