Immunising humanised mice with outer membrane vesicles from Acinetobacter baumannii to study antigen recognition and vaccine development

Summary

Most vaccines that work in humans work through antibodies. Identifying antibodies present in human sera after previous exposure to a pathogen or a toxin has been a valuable way to identify new vaccine candidates (antigens) recognised by humans. This has been invaluable for guiding vaccine design as part of a process called “reverse vaccinology”. Nevertheless, it has significant limitations, not least because vaccination artificially “skews” the antibody response, something not possible to detect in normal human sera after natural infection. The Kymab mouse, overcomes this limitation by replacing mouse antibody genes with human antibody genes. This means we can immunise mice to identify what parts of an antigen are recognised by humanised antibodies. This will lead us to examine how human antibodies “see” antigen to provide protection from a vaccine. This can accelerate vaccine design and development and reduce costs in vaccine development. This is a major reason Kymab has attracted significant investment from organisations like Wellcome and BMGF. We will test the potential of outer membrane vesicles from Acinetobacter baumannii to induce protective antibodies against infections with this organism. This pathogen is a significant cause of hospital-acquired infections in high and LMICs and shows significant resistance to multiple antibiotics. We will examine the strength of binding of the antibodies induced to distinct antigens within the OMV and the comparative capacity of the antibodies induced to kill Acinetobacter baumannii bacteria. This is the first step to developing a new vaccine targeted to populations most at risk of Acinetobacter baumannii infection.

Project outcomes

Use of human sera to identify protective antigens recognised after natural infection has been pioneered under the term “reverse vaccinology”. This has proven an effective approach as demonstrated by the development of vaccines against MenB, such as Bexsero. Nevertheless, after any natural infection there are a myriad of antigens recognised, most of which are not protective and so this can naturally constrain this approach. Moreover, since vaccination is an artificial skewing of the antibody response towards protective antigens, the reverse vaccinology approach does not inform on how the human antibody repertoire develops to vaccines, at least not until phase I, which is significantly downstream in the developmental process. 

It would be helpful to examine the human antibody repertoire in an animal model, where we can assess this in much greater mechanistic detail. Kymab have developed a mouse where this is achieved through transferring the human B cell repertoire across into the murine host. Immunising these mice has provided a fantastic opportunity to examine the targeting of antigens by “human” antibodies at the supraphysiological levels induced after vaccination with purified subunit vaccines. Her, we have immunised the Kymab mouse alongside wild-type (WT) control mice to compare how antibody responses develop after immunisation.

We performed extensive analyses of the microarchitecture of the spleen in humanised mice to examine whether this was affected by the chimerisation and its relationship to WT mice. Overall, standard architecture was maintained. We observed normal B cell follicle and T zone formation and distribution across the tissue. Moreover, we observed that immunisation with a prototypical vaccine induced significant numbers of structures called germinal centres – the powerhouse of antibody selection and essential for the generation of optimal responses to vaccines. Through modifying specific technologies we could localise IgM and IgG plasma cells in humanised mice. 

These studies provide a route through which to study the development and mechanisms of selection of humanised antibody responses to vaccines that are used in humans. It provides enhanced validation of the Kymab mouse as a tool to study the development of vaccines of relevance to humans.