Development of a droplet digital PCR for pertussis toxin locus copy number determination in a genetically-modified Bordetella pertussis strain

To improve pertussis toxin (PT) yield in B. pertussis strains for vaccine production a genetically-engineered strain (gdPT 191-134 strain) with a second copy of the genetically detoxified PT (gdPT) locus was developed. The consistency of the production and genetic stability of the strain when used for vaccine production must be established. We developed two simplex ddPCR assays with PCR systems for ptxA, the target gene present in two copies, and pgm, the reference gene present as a single copy. The ddPCR assay had sufficient precision to discriminate the copy number of the PT locus accurately in two B. pertussis strains: one copy in the parent, non-genetically-engineered strain and two copies in the gdPT 191-134 strain. Using the ddPCR assays, we were able to show that the ratio of the ptxA to pgm genes decreased during serial culture passages, due to the loss of PT locus, which in turn, resulted in lower levels of PT production over time. We were then able to assess culture conditions that improved the stability of the double locus, as shown by non-significant reduction in gdPT toxin yield.

Regulation of Clostridium tetani Neurotoxin Expression by Culture Conditions

BACKGROUND

Ensuring consistency of tetanus neurotoxin (TeNT) production by Clostridium tetani could help to ensure consistent product quality in tetanus vaccine manufacturing, ultimately contributing to reduced animal testing. The aim of this study was to identify RNA signatures related to consistent TeNT production using standard and non-standard culture conditions.

METHODS

We applied RNA sequencing (RNA-Seq) to study C. tetani gene expression in small-scale batches under several culture conditions.

RESULTS

We identified 1381 time-dependent differentially expressed genes (DEGs) reflecting, among others, changes in growth rate and metabolism. Comparing non-standard versus standard culture conditions identified 82 condition-dependent DEGs, most of which were specific for one condition. The tetanus neurotoxin gene (tetX) was highly expressed but showed expression changes over time and between culture conditions. The tetX gene showed significant down-regulation at higher pH levels (pH 7.8), which was confirmed by the quantification data obtained with the recently validated targeted LC-MS/MS approach.

CONCLUSIONS

Non-standard culture conditions lead to different gene expression responses. The tetX gene appears to be the best transcriptional biomarker for monitoring TeNT production as part of batch-to-batch consistency testing during tetanus vaccine manufacturing.

A next-generation sequencing based method for determining genetic stability in Clostridium tetani vaccine strains

Production of tetanus and other clostridial vaccines highly depends on the stable and reproducible production of high toxin levels. This creates a need to ensure the genetic stability of seed strains. We developed a two-stage method for improved assessment of the genetic stability of Clostridium seed strains. This method is based on next-generation sequencing (NGS) of strain DNA and mapping the sequence reads to a reference sequence. The output allows analysis of global genome consistency followed, if necessary, by detailed expert judgement of potential deviations at the gene level. The limit of detection of our method is an order of magnitude better than that of the currently established pulsed-field gel electrophoresis (PFGE). Improved genetic characterization of bacterial seed lots will have a positive impact on the characterization of the production process. This will be a first step towards applying the consistency approach to vaccine batch release of established vaccines. This can contribute to the reduction and ultimately replacement of routinely used animal tests in vaccine production. This work was carried out as part of the Innovative Medicines Initiative 2 (IMI2) project VAC2VAC (Vaccine batch to vaccine batch comparison by consistency testing).