Streptococcus pneumoniae and the host : activation, evasion and modulation of the human innate immune system

Abstract

Streptococcus pneumoniae is a major cause of severe infections such as pneumonia, septicemia and meningitis, but also a common colonizer of the nasopharynx in children. In most individuals colonization is harmless and eventually cleared by the immune system, but in rare cases pneumococci can reach deeper into the body and cause diseases. It is not understood why pneumococci cause infections in a few individuals while in most cases the bacteria are limited to the nasopharynx and eventually cleared. It is clear, however, that a well-orchestrated immune system is essential to prevent and limit pneumococcal infections. Macrophages are essential for an early clearance of pneumococci and dendritic cells are required to initiate appropriate adaptive responses. Both cell types were studied in this thesis.

Cytokine secretion by dendritic cells directs the development of T-cells, and we studied the induction of IL-12 secretion by dendritic cells in response to pneumococci. We showed that pneumococcal RNA was recognized by TLR3, which together with the adapter molecule TRIF induced secretion of IL-12. Infection of dendritic cells with influenza A virus upregulated TLR3 expression which contributed to a more efficient detection of pneumococci and enhanced IL-12 secretion.

We observed that the pneumococcal pore forming toxin pneumolysin had profound effects on cytokine responses in human dendritic cells and macrophages. We found a cell death independent inhibition of cytokine secretion in human dendritic cells and macrophages by pneumolysin expressing pneumococci. Interestingly however, cytokine secretion by macrophages derived from the human THP-1 cell line was enhanced in the presence of pneumolysin. We described pneumolysin mediated effects on these cell types and explored initial insight into the underlying mechanisms.

Clearance of pneumococci by macrophages is supported by deposition of complement on the bacterial surface. The pneumococcal surface protein PspC binds human Factor H to evade opsonophagocytosis, and can also act as an adhesin. We characterized two variants of PspC proteins present in B6 clinical isolates. The two proteins showed differential expression patters on the bacterial surface and had distinct functions as Factor H binding protein or adhesin. Small changes in surface localization impaired the protein function, indicating the importance of correct surface expression. We tested the effects of vitamin D on the activation of dendritic cells by pneumococci and the induction of T-cell responses. Vitamin D supported dendritic cell maturation and skewed T-cell responses from an inflammatory to a regulatory phenotype.

This work gives insight into the complex interactions between S. pneumoniae and human immune cells, and underlines the dynamic effects of pneumococcal virulence factors on the host. A thorough understanding of the activation and evasion of immune responses by pneumococci as well as the effects of immunomodulatory agents such as vitamin D is essential for the development of future treatment options and vaccine approaches.