Mei M. Ho

WHO Informal Consultation on standardization and evaluation of BCG vaccines Geneva, Switzerland 22-23 September 2009.

The current World Health Organization Requirements for BCG vaccine are in need of revision to address the diversity of sub-strains used for production, potential improvements of quality control assays for lot release, and the establishment of sub-strain specific Reference Reagents. A consultation meeting was organized to discuss issues regarding the standardization and evaluation of BCG vaccines in the forum of regulators, BCG vaccine manufacturers, developers of selected new live tuberculosis (TB) vaccines and researchers. The development of new recombinant BCG and live attenuated TB vaccines and the characterisation of different BCG sub-strains using state-of-the-art technologies were also reviewed. The objective of the meeting was to revise and update the current recommendations focused on the scope, terminology, manufacturing issues, and the incorporation of new reference reagents and new quality control tests.

Report of an international collaborative study to evaluate the suitability of multiplex PCR as an identity assay for different sub-strains of BCG vaccine.

Current methods for the identification of BCG vaccine in quality control settings involve acid-fast staining with microscopic examination. However, this method is unable to distinguish the many different sub-strains of BCG, or to differentiate BCG strains from virulent members of the Mycobacterium tuberculosis complex. A multiplex PCR (mPCR) which uses six target regions in mycobacteria has been developed to identify specific sub-strains of BCG. This study reports the findings from an international collaborative study to assess the accuracy, robustness and reproducibility of this mPCR method to differentiate BCG sub-strains. The method was found to fulfil these criteria successfully and was able to distinguish BCG sub-strains in vaccine preparations. The majority of the participants in the study generated the expected PCR product profiles indicating the method is also robust.

Report of an international collaborative study to establish the suitability of using modified ATP assay for viable count of BCG vaccine.

As part of the World Health Organisation (WHO) initiative to update the current requirements for BCG vaccine a collaborative study was carried out to establish the robustness, reproducibility and the suitability of the modified ATP assay. This assay was developed by Statens Serum Institut, Denmark, as a potential replacement of the method for detection of viable counts of BCG vaccine which is routinely used as a quality control test for lot release. Two BCG preparations, of same strain but different production methods, were tested. For each preparation, two different storage conditions of -20 or 37 degrees C were used in order to establish the suitability of this assay for testing heat-treated BCG vaccine as in the temperature stability test. The lyophilised BCG samples were tested using the ATP reagents from the same source and same principle of testing but some procedural modifications were allowed to accommodate different equipment and resource availability in different laboratories. Data from four laboratories showed that the heat-treated BCG samples contained significantly lower ATP content per sample than the untreated control stored at -20 degrees C. Three laboratories gave consistent mean ATP contents, especially for control samples, even with variations in testing protocol. The present study showed that this modified ATP assay is very robust and can be reproducible. Once the correlation of cultural viable count and ATP content of a BCG vaccine product has been established, this rapid alternative assay may be used to monitor BCG viable count. Due to the fact that this study was small, further investigation is planned. A collaborative study will be carried out using this modified ATP assay in parallel with the cultural viable count method in the establishment of the replacement of the WHO International Reference Preparation of BCG vaccine.

Preclinical laboratory evaluation of a bivalent Staphylococcus aureus saccharide-exotoxin A protein conjugate vaccine.

A bivalent, unadjuvanted conjugate vaccine composed of Staphylococcus aureus capsular polysaccharides type 5 and 8 (T5 and T8 PS) conjugated to a novel carrier protein, the mutant non-toxic recombinant Pseudomonas aeruginosa exotoxin A (rEPA), has been the subject of recent clinical trials. A programme of preclinical laboratory evaluation was carried out in support of the clinical trials conducted by the National Vaccine Evaluation Consortium. This involved physical chemical characterisation and limited assessment of toxicity and immunogenicity. The carrier protein showed good stability and its conformation was essentially maintained when conjugated. The T5- and T8-rEPA conjugates were of a size range (1-3 x 106 g/mol) consistent with polysaccharide conjugates. Fluorescence spectroscopy and molecular sizing showed good batch-to-batch consistency. Although all batches of final fill preparations elicited positive immune responses in the mouse model with three schedule doses of 0.25 mg of each T5/ 8 conjugate per dose, the mouse serum IgG response to T8 PS varied from batch to batch. Storage temperature at 37?C or below or with repetitive temperature fluctuations did not significantly affect the IgG responses to T5 or T8 PS. Storage at 56?C, however, diminished the mouse serum IgG response to T5 PS. The conformation of the conjugated protein and size of the conjugates correlated well with mouse immunogenicity in the thermal stability samples; significant unfolding of the protein and downshifts in molecular size of the conjugate were only observed when stored at 56?C. The relatively high stability of the novel carrier protein when conjugated to large polysaccharides makes this an attractive candidate carrier protein for other conjugate vaccines. When assayed for serum IgG concentration, the bivalent T5/ 8 conjugate was found to evoke an IgG response well over the threshold value of 10 mg/ml anti-T5 and -T8 IgG established for the ELISA immunogenicity assay.

The establishment of sub-strain specific WHO Reference Reagents for BCG vaccine

As the latest addition to the sub-strain specific WHO Reference Reagents of BCG vaccine, an international collaborative study was completed to evaluate the suitability of a candidate BCG Moreau-RJ sub-strain as a WHO Reference Reagent of BCG vaccine. This follows the recent replacement of the WHO 1st International Reference Preparation for BCG vaccine, by three sub-strain specific WHO Reference Reagents of BCG vaccine (Danish 1331, Tokyo 172-1 and Russian BCG-I) in order to complete the coverage of most predominant sub-strains used for BCG vaccine production and distribution for use worldwide. The study used cultural viable count and modified ATP assays to quantify the preparation and multiplex PCR to confirm the identity of the sub-strain. The establishment of this WHO Reference Reagent of BCG vaccine of Moreau-RJ sub-strain was approved by the WHO Expert Committee on Biological Standardization meeting in October 2012. This preparation is available for distribution by NIBSC-MHRA, UK. The data from real-time stability monitoring demonstrated that these Reference Reagents of BCG vaccine are very stable in storage condition at −20 °C. They serve as the valuable source of BCG Reference Reagents for use as comparators (1) for viability assays (such as cultural viable count and modified ATP assays); (2) for in vivo assays (such as the absence of virulent mycobacteria, dermal reactivity and protection assays) in the evaluation of candidate TB vaccines in non-clinical models; (3) for identity assays using molecular biology techniques.

Report of an International collaborative study to establish the first WHO reference reagents for BCG vaccines of three different sub-strains

The WHO First International Reference Preparation for BCG vaccine is over forty years old and is no longer available for distribution due to stock depletion and its significant loss of viability. International consultations identified a demand for replacement with sub-strain specific BCG preparations. An International collaborative study was carried out to evaluate three candidates for WHO Reference Reagent for BCG vaccine of Danish 1331, Russian BCG-I and Tokyo 172-1 sub-strains. These candidates were quantified for viability using both cultural viable count and modified ATP assays. The proposal for the establishment of these First WHO Reference Reagents for BCG vaccines was discussed in the WHO Expert Committee on Biological Standardization meeting, October 2009.

Tuberculin purified protein derivative (PPD) immunoassay as an in vitro alternative assay for identity and confirmation of potency.

Tuberculin purified protein derivative (PPD) currently can only be standardised bydelayed hypersensitivity skin reactions in sensitised guinea pigs. An in vitro dot blotimmunoassay was developed for both identity and possibly potency estimation of PPD.Polyclonal antibodies (mainly IgG) were generated and reacted with human, bovine and, tolesser extent, avian PPD preparations in immunoblotting assays. Various PPD preparationswere analysed using SDS-PAGE coupled with Western blot and size exclusionchromatography (FPLC-SEC). The results showed that PPD preparations were mixtures ofvery heterogeneous tuberculoproteins ranging in size from very large aggregates to very smalldegraded molecules. All individual fractions of PPD separated by size were immunoreactive,although those of the largest molecular sizes appeared the most immunoreactive in this invitro dot blot immunoassay. This method is very sensitive and specific to tuberculoproteinsand can be an in vitro alternative for the in vivo intradermal skin assay which uses guinea pigsfor identity of PPD preparations. Although the capacity of PPD to elicit cell-mediatedimmune responses on intradermal testing has to be confirmed by in vivo assay, the dot blotimmunoassay offers a rapid, sensitive and animal-free alternative to in vivo testing forconfirming the identity and approximate quantification of PPD preparations. The procedurewould be particularly useful for national control laboratories for confirming the data ofmanufacturers and thus reducing the use of animals.

An assessment of thermal stability of Clostridium difficile toxoid formulations.

Strains of Clostridium difficile produce toxins A and B that can cause diarrhoea and pseudomembranous colitis. Currently, there is no preventative therapy for this infection but antibodies to the toxins provide protection, therefore a toxoid-based vaccine is needed. To evaluate thermal stability, a lyophilized and liquid formulation of toxoids A and B were stored at a range of temperatures for 5 weeks. Changes in toxoid structures and immune responses in an animal model before and after the incubation period were assessed. The structural integrity and the immune responses to liquid formulations were affected when stored at 56°C but the lyophilized formulation was thermally stable and same treatment did not result in significant loss of immunological responses when immunized in an animal model.

Don’t Shoot the [Carrier]

Response to Letter Is Pseudomonas aeruginosa exotoxin A a good carrier protein for conjugate vaccines? Gerald Pier