Investigating the pathogenic consequences of Streptococcus pneumoniae lung infection in chronic respiratory diseases
Creators: name: Papanicolaou, Angelica
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 320103
subject: Respiratory diseases

description: Independently, lower respiratory tract infections caused by Streptococcus pneumoniae (SPN, the pneumococcus) and chronic inflammatory respiratory diseases, such as chronic obstructive pulmonary disease (COPD) and asthma, cause significant morbidity and mortality across the globe. Exposure to noxious particles in cigarette smoke (CS) is a leading risk factor for developing COPD and perpetuates the accumulation of leukocytes, such as macrophages and neutrophils within the airways. COPD encompasses several overlapping respiratory pathologies, including emphysema, chronic bronchitis and small airways disease. By contrast, asthma is a chronic airway inflammatory disease characterised by pathological levels of airway remodelling that leads to variable and reversible airflow obstruction manifesting as bronchial hyperresponsiveness. House dust mite (HDM) aeroallergen exposure is one of the most common causes of asthma and typically drives a dominant CD4+ T-helper type 2 (Th2) response resulting in type-2 inflammation.

In 2017, S. pnuemoniae was listed as a priority pathogen in critical need of additional treatment strategies by the World Health Organisation (WHO). Despite an abundance of airway leukocytes, COPD patients and asthmatics are highly susceptible to developing pneumococcal pneumonia; a paradox that remains poorly understood. In both chronic respiratory diseases, alveolar macrophage (AM) phenotype and function is altered and could lead to impaired pneumococcal clearance mechanisms. Moreover, the efficacy of currently available SPN vaccines appear to be blunted in COPD and asthma cohorts.
 
In this study, the pathogenic consequences of SPN infection in COPD and asthma was investigated, focusing on key cellular and immune-mediated pathways. The efficacy of currently available SPN vaccines in murine models of COPD and asthma was also examined and the benefit of the specialised pro-resolving mediator (SPM) Aspirin-triggered Resolvin D1 (AT-RvD1) assessed within the severe-asthma setting.
 
In Chapter 3, SOD3-/- mice were exposed to CS for eight weeks in order to model the highly oxidative lung environment observed in COPD patients. CS-exposed SOD3-/- mice developed significant bronchoalveolar lavage (BAL) fluid cellularity and hyperinflated airways that was associated with the accumulation of foamy alveolar macrophages. Although alveolitis scores significantly increased in CS-exposed wild type (WT) and SOD3-/- mice following SPN infection (7-days post-infection), combined CS exposure and genetic deletion did not impair pneumococcal lung clearance. This suggests that accumulation of foamy alveolar macrophages in murine airways does not hinder the resolution of pneumococcal lung infections in vivo. Further to this, PPV23-antibody responses were unaltered irrespective of chronic CS exposure and/or genetic deletion of SOD3.

In Chapter 4, how prior HDM exposure impacts pneumococcal lung clearance and the pathophysiology observed in allergic airway disease (AAD) was investigated. Prior HDM exposure impaired pneumococcal clearance in the BAL fluid at both Day 2 and Day 7 post infection. It was also observed that acute pneumococcal infection (Day 2) exacerbated airway hyperresponsiveness, increased lung inflammation and mucus production and led to increased recruitment of innate inflammatory cells in the airways. A major finding from Chapter 4 was that AMs isolated from HDM-exposed animals were less capable at specifically phagocytosing SPN. In addition, it wase observed that during acute SPN infection, prior HDM-exposure led to a significant reduction in the expression of NETosis markers; an important facet of neutrophil-mediated pneumococcal control in the airways. The pneumococcal conjugate vaccine (PCV13) modestly enhanced pneumococcal clearance in HDM-challenged mice. This data supports the use of vaccines in this high-risk group, but also shows that alternative strategies are needed to combat defective SNP clearance mechanisms in asthma.

In Chapter 5, the pro-resolving actions of AT-RvD1 on the pathological features associated with AAD and SPN clearance from the lower airways were assessed. AT-RvD1 treatment markedly improved pneumococcal clearance from the lungs at Day 4 post-infection with the combined effect of decreasing airway hyperresponsiveness and BAL fluid cellularity. The expression of common Th2 markers (IL-5 and IL-13) was significantly attenuated in murine lungs following AT-RvD1 treatment. Although, AT-RvD1 did not reduce histological lung inflammation or the number of mucin-positive cells in our model, it remains clear that AT-RvD1 is a compelling therapeutic approach for targeting both pneumococcal infections and airway hyperresponsiveness in AAD and warrants further investigation in the future.
 
This thesis builds upon the body of knowledge that relates to the pathophysiological and inflammatory consequences associated with pneumococcal lung infections in chronic lung diseases. Importantly, the present study showed convincing and novel evidence of impaired key innate-immune cell pathways in response to SPN within the asthmatic lung microenvironment. Together, the work in this thesis provides a foundational platform that can be harnessed to further evaluate and develop therapies that promote pathogen clearance in the lower airways with the combined benefit of attenuating exuberant host immune responses. As the pneumococcus continues to evade current preventative strategies, therapeutics that treat respiratory tract infections caused by SPN remain paramount, particularly in the chronic respiratory-disease setting.