Investigation of Staphylococcus aureus-induced immunosuppression during nasal colonisation and the implications for vaccine function

Abstract

Staphylococcus aureus (S. aureus) is a major part of the natural human microbiome, primarily colonising the anterior nares. The overall aim of this project was to determine the impacts nasal colonisation has on local and systemic immune responses to S. aureus, identify the immunosuppressive cells facilitating S. aureus survival in the nasal cavity and to investigate the potential implications this immunosuppressive response is having on subsequent vaccine function. S. aureus nasal colonisation induces a local T-cell response involving IL-22 and IL-17 which promotes bacterial clearance from the nasal tissue (NT). However, prolonged and recurrent nasal colonisation is commonplace which suggests an immunosuppressive response may be occurring to constrain local pro-inflammatory responses involved in bacterial clearance. To investigate this, Wild-type (WT) mice were intranasally colonised with S. aureus, and production of the regulatory cytokines IL-10 and IL-27 were assessed. Both regulatory cytokines were upregulated locally within 24h of nasal colonisation by myeloid and B cells. The impact of IL-10 on bacterial persistence was established using IL10-/- mice, which demonstrated enhanced bacterial clearance from the nose compared to WT mice. This improved clearance was associated with elevated levels of IL-17+ and IL-22+ T-cells within the NT. To establish a link between IL-10 and IL-27, IL-27 activity was blocked by intra-nasal administration of ?IL-27 antibodies leading to reduced IL-10 levels and enhanced bacterial elimination. S. aureus-derived lactate, IL-35 production and miR21 activity all contributed to the regulation of IL-10 production within the NT. These results indicate S. aureus promotes local anti-inflammatory responses during nasal colonisation through multiple mechanisms to create an immunosuppressive microenvironment permissive for bacterial survival. The consequence of this commensal-induced immunosuppression may have far-reaching implications on subsequent exposures to S. aureus and vaccine efficacy. The impact S. aureus-induced IL-10 production had on vaccine efficacy was also investigated. It was demonstrated that IL-10 production in macrophages could be suppressed through the use of small molecule PI3K/Akt inhibitors. Vaccination studies were carried out to determine if the use of these small molecule inhibitors or IL-10 and IL-27 blocking antibodies, given during vaccination could improve antigen-specific T cell responses against subsequent S. aureus infection. The inhibition of IL-10 with small molecule PI3K/Akt inhibitors led to improved effector T cell responses but did not translate to enhanced clearance. However, inhibition of IL-10 production using blocking antibodies during vaccination led to both enhanced T cell responses and improved bacterial clearance against systemic S. aureus infection and also during subcutaneous infection. This project has dramatically broadened our knowledge on the complexity of the immune response induced by S. aureus nasal colonisation and the implications for this on systemic responses. Specifically, it highlights that targeting immunosuppressive priming of the immune system might be a novel strategy to improve anti-S. aureus vaccine efficacy.