Epithelial micro-invasion and the inflammatory response to colonisation by Streptococcus pneumoniae in health and in vulnerable populations

A microbe called Streptococcus pneumoniae or the pneumococcus is a leading cause of pneumonia, meningitis and sepsis across the world. Despite 20 years of the widespread use of pneumococcal vaccines, S. pneumoniae is still responsible for almost 300,000 deaths in HIV-uninfected children aged 1-59 months annually. In old people, the burden of pneumococcal pneumonia is also considerable and is associated with death in over 1 in 20.

S. pneumoniae is frequently carried at the back of the nose in healthy children and adults. Carriage is a pre-requisite for both pneumococcal disease and transmission from person to person, and is therefore a crucial target for vaccines and the generation of so-called "herd immunity". Yet remarkably, our understanding of the relationship between pneumococcal carriage, the human body's response to carriage, and how these processes relate to clearance from the nose is incomplete.

Together with our colleagues at the Liverpool School of Tropical Medicine, we have recently developed new ways to study this human-microbe relationship in the nose by infecting healthy human volunteers with S. pneumoniae without making them ill (pneumococcal controlled human infection model [PCHIM]), and by taking very small tissue samples from the nose. We have used this human model to discover that during carriage, contrary to what was thought before, S. pneumoniae gets past the normal barriers provided by the cells that line the back of the nose through a process that we have called "micro-invasion". This results in inflammation without disease, greatest around the time that the pneumococcus is cleared from the nose. These data suggest that inflammation and increased secretions from the nose mediate clearance. We have proposed that micro-invasion not only determines the outcome of carriage but that in older adults, is enhanced, resulting in disorganisation of the body's response and impaired clearance from the nose.

This new finding of micro-invasion changes the way we think about this prominent disease-causing microbe. Using well-characterised laboratory systems, we will now conduct a detailed assessment the role of several important pneumococcal components in micro-invasion and find out how these components shape the body's inflammatory response. We will then further pursue our findings using the PCHIM, focusing on the importance of the outer capsule of the pneumococcus in determining the relationship between micro-invasion, the inflammatory response to carriage and clearance of S. pneumoniae. Finally, we will test whether micro-invasion is enhanced during carriage in older adults and whether this is associated with a disordered inflammatory response by the body, resulting in impaired clearance. Overall, this work will provide a greater understanding of the relationship between micro-invasion, inflammation and bacterial clearance. We will uncover protective responses that could be used for vaccine evaluation and optimisation. Our findings will explain the vulnerability of older people to pneumococcal disease and identify pathways for intervention.

Using a novel pneumococcal controlled human infection model (PCHIM) we have recently discovered that during commensal carriage, S. pneumoniae invades the human mucosa triggering an epithelial innate-inflammatory response without causing disease. Transcriptomic analysis of the epithelial response shows that upregulation of inflammatory gene pathways is greatest around the time of pneumococcal clearance. We therefore hypothesise that this so-called micro-invasion of the mucosal epithelium and the subsequent innate-inflammatory profile determine the outcome of colonisation; and that in older adults, pneumococcal micro-invasion is enhanced and the innate-inflammatory response dysregulated, resulting in impaired clearance. We will now determine the role of the pneumococcal polysaccharide capsule and key sub-capsular components in micro-invasion using well-characterised epithelial cell systems. We will then identify the host gene expression pathways that characterise micro-invasion, discovering how pneumococcal micro-invasion is sensed in the epithelial cytosol. We will then use the PCHIM to detail the functional relationship between micro-invasive phenotype, the epithelial innate-inflammatory response and bacterial clearance. Finally, we will test whether pneumococcal micro-invasion is enhanced during colonisation in older adults and that in the context of so-called inflammaging, is associated with a dysregulated innate-inflammatory response and impaired clearance. Overall, this project will generate a fundamental understanding of the relationship between micro-invasion, the innate-inflammatory response and bacterial clearance. We will identify protective epithelial transcriptomic profiles associated with bacterial clearance that could be used for vaccine evaluation and optimisation. We will delineate the host-pathogen profiles that explain the vulnerability of older people to pneumococcal disease, and identify pathways for intervention.

The pneumococcus is still responsible for almost 300,000 deaths annually from pneumonia, meningitis and sepsis in HIV-uninfected children aged 1-59 months worldwide despite widespread use of conjugate vaccines. The mortality associated with pneumococcal pneumonia in older people exceeds 6%. This project therefore addresses an important public health need and takes advantage of newly developed laboratory and experimental medicine tools to increase the potential impact.

The public benefits of this project will therefore be through the generation of new knowledge and ultimately the development of new approaches to pneumococcal vaccination and other interventions that improve the protection of vulnerable populations such as the elderly, increasing vaccine effectiveness. This work has the potential to influence NHS policy makers and vaccine policy. Improvements to vaccine-mediated control of pneumococcal colonisation will benefit the health and wealth of people in the UK and internationally by reducing the burden of patients with pneumonia, meningitis and sepsis, increasing the effectiveness of public services; and enhancing the quality of life and health of those vaccinated. The work will also lead to the further development of the strong partnership between internationally leading researchers and public & engagement, involvement groups in London and Liverpool, involving the early career researchers at the centre of this activity. This will increase the impact of our research on societal knowledge, public awareness and engagement with biomedical research. The work with engagement groups will also raise awareness around the herd impact of vaccines and address vaccine hesitancy.

The generation of new independent researchers will further amplify this process and also enhance their future potential for impact. In addition, this work will increase the international reputation of UCL and LSTM as centres of excellence for translating basic laboratory research into experimental medicine. This will attract postgraduate students and academics from abroad, increasing inward investment.

To realise these benefits, the investigators plan to engage with: (1) policymakers (UK Department of Health and WHO); (2) international opinion leaders and advocates (PATH and the Bill & Melinda Gates Foundation with whom we have existing relationships; and (3) pharmaceutical vaccine manufacturers who have existing or are generating new pneumococcal vaccines (with whom we have existing relationships). We will leverage the external advisory group of the MPRU to maximise impact. The work will shape the future approach to pneumococcal vaccination, attract additional investment from both the public and private sectors to this research area, and greatly improve the protective effects and the longevity of pneumococcal vaccines. We will follow our "pathways to impact" with the team, specifically reviewing them at our investigator meetings and community advisory groups, refining our approach and developing new partnerships to promote the impact of the work.