Identification of correlates of protection against pneumococcal colonization

Summary

Pneumococcal disease is a public health threat that affects particularly vulnerable populations, such as children, the elderly, and those living in resource-limited conditions. Pneumococcus is an encapsulated bacterium and based on this capsule, it is classified in serotypes. There are >95 serotypes and licenced vaccines, based on capsular components, only protects against 13. These vaccines are expensive to produce, which limits the affordability and availability, especially in low and middle-income countries where the burden of the disease is the highest. Pneumococcus is found in the microflora that colonises the nasopharynx of healthy adults and children. This colonisation is the pre-requisite for disease and the reservoir for transmission. Novel vaccines must confer protection against colonisation. The correlates of protection against colonisation in humans are unknown, and this information is needed to accelerate the development of improved vaccines.

Protein-based vaccines could overcome the limitations of polysaccharide-based vaccines by protecting against all serotypes and reducing production costs. Together with the Boston Children’s Hospital (USA) and Instituto Butantan (Brazil), we generated the largest library of purified pneumococcal proteins and tested their capacity to elicit protective immune responses using samples derived from our Human Infection Model with Pneumococcus. Preliminary results have shown that a subset of these proteins elicit an immune response that correlates with protection against pneumococcal acquisition and control of density in those individuals colonised. We proposed a new partnership with Antigen Discovery, Instituto Butantan and Liverpool John Moores University to comprehensively identify correlates of protection against colonisation in a larger cohort and further develop the top protective antigens as vaccine candidates. Success will i) identify the cellular mechanisms that are involved in protection against pneumococcal colonisation, ii) identify novel protein-based vaccine candidates and iii) provide data to support funding applications to test the top candidates as nanoparticles formulation for a lung-delivered vaccine against pneumonia.

Project outcomes

Current vaccines against pneumococcus are based on capsular components of the bacteria protecting against few of the >95 different serotypes. Novel vaccine strategies aim to use unencapsulated whole cell bacteria or conserved antigens among serotypes that could confer universal protection. 

In murine models, vaccination with whole cell bacteria or protein antigens induces high levels of the cytokine IL-17A. This cytokine mediates neutrophil recruitment to the nasopharynx to clear colonisation. In humans, the role of IL-17A or any other serotype independent correlate of protection have not been well described and this is key to develop the next generation of pneumococcal vaccines. 

Using a library of 70 purified pneumococcal proteins and baseline samples derived from the human infection model with pneumococcus, we aimed to identify serotype-independent correlates of protection against pneumococcal nasal colonisation. We measured baseline blood responses of healthy young adults against the protein library and unencapsulated whole cell bacteria. The results showed similar IL-17A levels for those volunteers susceptible and those protected against experimental pneumococcal colonisation. This indicates that IL-17A levels at baseline do not associate with protection against pneumococcal acquisition. 

In addition, we observed that those volunteers protected against colonisation had higher levels of the chemokine MCP-1 than those susceptible to colonisation, highlighting monocyte recruitment as a key mediator of colonisation clearance. The results also showed that high levels of the chemokine RANTES at baseline associates with susceptibility to colonisation. To corroborate the results obtained in healthy adults, we measured protein-specific responses in blood of children, target group of novel vaccines, colonised and non-colonised with pneumococcus. 

The results showed that IL-17A levels were higher in those children colonised with pneumococcus, confirming IL-17A role as a marker of exposure to pneumococcus. Colonisation of the nasopharynx by pneumococcus is important both as reservoir for transmission and a determinant for disease. Therefore, it was important to contrast the results obtained in blood with mucosal responses.  We assessed protein-specific cytokine levels produced by nasal cells after incubation with 4 pneumococcal proteins. As observed in blood samples, nasal cells from volunteers susceptible to colonisation produced higher levels of RANTES than those protected. Levels of IL-17A were undetectable and MCP-1 levels were similar between volunteers susceptible and protected. We are currently investigating the possible role of RANTES as a marker of susceptibility.  

The results obtained from this pump-priming award suggest that contrary to the key mechanism in mouse models, in humans, systemic and mucosal protein-specific IL-17A do not correlate with protection against acquisition. These data together suggest that in humans other mechanisms may be responsible for serotype-independent protective responses and underly the importance of confirming mouse findings with human data.