Mechanisms for pneumococcal meningitis and a new vaccine platform to raise a serotype-independent protection in the host

Abstract

Streptococcus pneumoniae is a highly relevant pathogenic bacterium, responsible for a large fraction of deaths and disease morbidity in the world. The pneumococcus remains the leading cause of life-threatening pneumonia, septicemia and meningitis beyond neonatal age, despite global implementation of vaccination programs. Due to its extraordinary adaptability, S. pneumoniae has developed evasion strategies against most therapeutic interventions. In addition to escaping vaccine conferred immunity, antibiotic resistance trends are continuously on the rise. The pneumococcal polysaccharide capsule is an important virulence factor with around 100 distinct capsular serotypes identified so far, that vary in invasiveness. Among other major virulence factors of the pneumococcus are the cytotoxin pneumolysin, pneumococcal pili, and adhesin factors PspA and PspC.

Vaccine-induced pressure drives capsular switching, and acquisition of resistance genes is promoted by antimicrobial pressure, complicating treatment strategies. The clinical management of pneumococcal meningitis is particularly troublesome, which is reflected in persistently high rates of permanent neurological sequalae among survivors. Therefore, there is an urgent need to scrutinize the pathogenesis of invasive pneumococcal disease (IPD), to identify new adjunctive therapeutic or prophylactic targets, and improve clinical outcomes. The work presented in the thesis aims to contribute to an improved understanding of key virulence mechanisms in the development of pneumococcal meningitis. These include bacterial invasion of the brain, bacterial interactions with fundamental cellular components of the brain, and bacteria-induced disruption of the brain’s fluid dynamics. Moreover, we propose a new vaccine platform to prevent pneumococcal colonization and infection in a serotype-independent manner.

Pneumococcal invasion of the brain through the blood-brain barrier, and the potential therapeutic effect of blocking the endothelial cell host receptors PECAM-1 and pIgR, was investigated in paper I. In combination with antibiotics, antibody blockade successfully prevented bacterial invasion of the brain, and protected the brain from damage, in a murine bacteremia-derived meningitis model. The feasibility to modulate host responses as adjunctive therapy was demonstrated. Bacteria-host communication between the pneumococcus and human neuronal cells was shown to occur directly and indirectly in paper II. Neuronal cell injury was induced by pneumolysin and pilus-I interactions with cytoskeletal β-actin. Inhibition of the interaction, using a β-actin antibody, partially protected against cellular damage. The pneumococcal-induced pathophysiology of the brain’s waste clearance system, the glymphatic system, and consequent neurofunctional damage, was characterized in paper III. A rat meningitis model, where bacteria were intracisternally administered together with a tracer dye, was employed, to study the accumulation of fluid and bacterial components in the brains CSF compartment. The findings of the study attest to the benefit of using lumbar drainage to alleviate intracranial pressure as adjunctive therapy in bacterial meningitis. Finally, in paper IV, pneumococcal vesicles were evaluated for their capacity to induce cross-protection against several pneumococcal serotypes, in a mouse immunization model. We found that the vesicles gave an excellent homologous and heterologous protection. The conserved lipoproteins MalX and PrsA were found to be the major components in the vesicles that conferred heterologous cross-protection. We suggest that vesicles represent promising novel vaccine targets to protect against pneumococcal disease.