Academic journal article Bulletin of the World Health Organization

Transgenic Mice as an Alternative to Monkeys for Neurovirulence Testing of Live Oral Poliovirus Vaccine: Validation by a WHO Collaborative Study

Academic journal article Bulletin of the World Health Organization

Transgenic Mice as an Alternative to Monkeys for Neurovirulence Testing of Live Oral Poliovirus Vaccine: Validation by a WHO Collaborative Study

Article excerpt

Introduction

The neurovirulence test (NVT) for oral poliovirus vaccine (OPV) is a key test for monitoring the consistency of vaccine production (1), and following WHO guidelines is required for each monovalent bulk lot of OPV produced. The WHO NVT (2) is a standardized procedure. If consecutive lots of monovalent bulks consistently meet the specifications of the WHO test, there is a high level of assurance that the vaccines will be safe when used for human immunizations (3, 4). So far, the test for neurovirulence safety of OPV has been performed using monkeys, because only primates are naturally susceptible to poliovirus. In 1990-91, two laboratories (5, 6) with the support of WHO, established lines of transgenic mice carrying a human receptor to poliovirus. In 1992, WHO recommended that a comparison be made of the sensitivity of TgPVR mice (7) with that of monkeys for type-3 poliovirus strains with different degrees of neurovirulence, and an evaluation of TgPVR mice as a possible alternative to monkeys for the neurovirulence testing of OPV (8). Initial experiments were performed in Japan and the USA that were aimed at selecting the most suitable TgPVR mouse line and route of inoculation, developing basic test methodology, and accumulating initial data. The results obtained with TgPVR21 mice (9, 10) indicated the capacity of the test to discriminate between acceptable batches of OPV and preparations of high neurovirulence. A collaborative study was therefore launched by WHO in 1993 (11) to investigate in more detail the suitability of the method for batch release of bulk OPV. Investigators at the Central Institute for Experimental Animals (CIEA, Japan) succeeded in developing TgPVR21 mice from a limited research tool into a reliable supply of standard animals available in large numbers (12, 13). Eleven institutions from Asia, Europe, and the USA participated in the study. (a)

The study started with type-3 OPV, the least stable strain in terms of its neurovirulence, and was completed for all three serotypes in October 2000. The results of the collaborative study up to 1999 have recently been published (14). The present paper presents the final results of the collaborative study and validation of the mouse NVT. A statistical model was developed for acceptance or rejection of OPV batches in the mouse test. It has previously been shown that the WHO monkey NVT was a reproducible and sensitive assay, ensuring the safety of OPV (3, 4). Numerous data, obtained in this collaborative study, have proven that the mouse NVT is as reliable as the WHO monkey NVT for OPV. In 1999 the WHO Expert Committee on Biological Standardization therefore approved the mouse NVT as an alternative to the monkey test for poliovirus type-3 (15) and in 2000 for poliovirus type-1 and type-2 (16).

Materials and methods

Vaccines

The type-l, type-2, and type-3 OPV virus samples used in the study had been tested previously using the monkey NVT according to the WHO requirements for OPV (2) by six manufacturers and three national control authorities. Vaccines of all three types produced in each of the three currently permissible cell substrates (primary monkey kidney, Vero monkey kidney, and human diploid cells) were obtained from nine manufacturers, including six UNICEF suppliers. In all, 75 commercial samples and one experimental sample that passed the monkey NVT were evaluated in mice. In addition, the following vaccine virus samples that failed the monkey NVT were used in this study: nine type-3 commercially produced vaccines and three samples of each serotype that were either experimental vaccines or derivatives of commercially produced vaccines additionally passaged in African green monkey kidney (AGMK) or Vero cells at 37-38 [degrees]C (a temperature favouring reversion to neurovirulence). The experimental samples of type-1 generated by passage of vaccine lots at elevated temperature were used as surrogates for commercially produced vaccines that consistently failed the monkey NVT, samples of which could not be located despite intensive worldwide searches. …

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