A human in vitro vaccination model ("organ-on-a-chip") to study influenza vaccines

Vaccination is the cornerstone of public health policy allowing the reduction of the burden of infectious diseases. Vaccines induce the production of antigen-specific antibodies and immune memory cells capable of rapid and effective reactivation upon subsequent exposure to the same pathogen. However, the vaccine effectiveness deteriorates as pathogens mutate, particularly if this occurs at a high rate, such as in influenza.
Influenza is an acute respiratory illness causing seasonal annual epidemics and occasional widespread pandemics. Annual vaccination is the current strategy to prevent infection even if the provided protection is modest. The reason for the poor vaccine effectiveness is the high diversity and ever-changing antigenicity of influenza viruses. Mutations occur especially in the hemagglutinin surface glycoprotein, the main target of antibodies, and so an update of the vaccine strains is needed every year. Developing an influenza vaccine that improves the breadth of protection, avoiding repeated vaccination, is a high scientific priority. However, influenza studies and vaccine testing are performed in animal models that do not fully represent human immunity. This slows the process towards a universal influenza virus vaccine underlining the necessity of novel screening platform. Human in vitro models are emerging as promising tools to recapitulate a secondary lymphoid organ. Despite all the progresses made in this field, further efforts are still needed to develop an in vitro model that reflect all key properties of human immune responses for prediction of vaccine responsiveness.
Therefore, I investigated the possibility to develop an innovative in vitro model of secondary lymphoid organ using human tonsil explants in the first half of my doctoral studies. In particular, I cultured tonsils in 3D perfusion bioreactors and I showed higher viability, metabolic activity and more robust immune responses in perfusion-cultured tissues compared to those in static cultures. I also reported that tonsils cultured in 3D perfusion bioreactors were able to respond to various premanufactured vaccines, protein antigens and antigen combinations. Thus, this in vitro model could be used in vaccine research and I focused on studying influenza vaccines in the second part of my doctoral studies. I explored how the antigenic composition of influenza vaccines could affect the generation of a broad immunogenicity. In particular, I tested different combination of H3N2 influenza strains and I found that a multivalent in vitro immunization with three phylogenetically distant H3N2 influenza strains induced a broader immune response compared to combinations with more related strains. Furthermore, I demonstrated that the in vitro model was able to generate de novo immune responses. Thus, perfusion-cultured tonsil is a valuable human in vitro model for immunology research with potential application in vaccine candidate selection. Moreover, I assessed the use of peripheral blood mononuclear cells seeded in scaffold for the development of an alternative in vitro model of secondary lymphoid organ.