Abstract

The in vivo histamine sensitization test (HIST) has historically been performed to guarantee the safety of acellular per­tussis vaccine batches. Non-compliance of batches is primarily associated with the presence of low levels of pertussis toxin (PTx). Because of ethical, standardization and scientific reasons, a variety of alternative in vitro approaches have been studied to replace the lethal HIST. A broadly applied and partially accepted method is the CHO cell clustering test, which is based on the clustered growth pattern of CHO cells when exposed to minute amounts of PTx. One of the major hurdles for global application of the CHO clustering test is the manual assessment of the clusters, which is associated with suboptimal reproducibility of test outcomes and is time-consuming. Here, various parameters of CHO cell nuclei were evaluated in search for a reliable, objective read-out parameter. We demonstrate that the distance between each nucleus and its nearest neighbor (3N method) is the most suitable parameter to assess clustered cell growth. This method detects 2.8 mIU PTx/mL and thereby complies with the requirement set for the sensitivity of the CHO clustering test based on visual reading. In commercial acellular pertussis vaccines spiked with PTx, the method detects 45 mIU/mL PTx, which is substantially lower than the 181-725 mIU/mL PTx detected by visual interpretation. The 3N method thus allows objective and sensitive assessment of CHO clustering and thereby encourages broad and global implementation of the in vitro test as an alternative to the HIST.

Highlights

  • Pertussis toxoid (PTd) is the primary component of acellular pertussis vaccines and is considered of high importance for vaccine-induced protective immunity (Sato et al, 1984; Black et al, 1988)

  • PTd is generated by inactivation of pertussis toxin (PTx), a toxin that even at low concentrations is responsible for the occasional adverse effects of acellular pertussis (aP) vaccines (Pittman, 1984; Tamura et al, 1982)

  • Monitoring reversion of the toxoid to its native form as well as assessment of residual PTx has long relied on the lethal murine histamine sensitization test (HIST), in which exposure to PTx increases the sensitivity to histamine by 30-300 times (Kind, 1958; Bergman and Munoz, 1965)

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Summary

Introduction

Pertussis toxoid (PTd) is the primary component of acellular pertussis (aP) vaccines and is considered of high importance for vaccine-induced protective immunity (Sato et al, 1984; Black et al, 1988). Monitoring reversion of the toxoid to its native form as well as assessment of residual PTx has long relied on the lethal murine histamine sensitization test (HIST), in which exposure to PTx increases the sensitivity to histamine by 30-300 times (Kind, 1958; Bergman and Munoz, 1965). This in vivo model, causes severe animal suffering, lacks mechanistic understanding, suffers from standardization problems, and accounts for approximately 65,000 mice on a global level each year (Hoonakker et al, 2017)

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