Characterisation of Treg and Th17 cells in nasopharynx-associated lymphoid tissue and their association with pneumococcal carriage in children and adults

Streptococcus pneumoniae (pneumococcus) is a gram-positive bacterium that can cause significant morbidity and mortality in humans especially in children and elderly. T regulatory cells (Treg) have an important role in modulation of immune responses to microbial infection. Although Th17 cells are involved in autoimmune diseases, these cells may play a protective role against pathogens. In this PhD project, Th17 and Treg cells in nasal-associated lymphoid tissue were characterised and their relationship with nasopharyngeal carriage of pneumococcus studied in children and adults. Frequencies of Th17 and Treg in tonsillar tissue and peripheral blood samples obtained from children and adults were analysed for intracellular expression of IL17A and Foxp3 by flow cytometry. Also, tonsillar MNC and PBMC were stimulated by pneumococcal culture supernatant (CCS) derived from wild type stain D39. The ratio of Th17/Treg cells in NALT was studied in both children and adults together with their association with pneumococcal carriage. Numbers of Th17 and Treg cells in in tonsillar tissues were shown to be significantly higher than in peripheral blood in both children and adults. The ratio of tonsillar Th17/Treg was shown to increase with age and tended to be higher in pneumococcal culture negative children than in culture-positive. It is suggested that the balance of Th17/Treg is a crucial determinant of pneumococcal clearance or persistence/carriage in human nasopharynx. A significant increase in numbers of Th17 and Treg cells were shown following pneumococcal CCS stimulation. CCS derived from isogenic mutant strains (i.e., Ply- and CbpA-) elicited lower numbers of Th17 and Treg cells. It is suggested that pneumococcal proteins including Ply and CbpA may activate Th17 and Treg cells in human NALT, and therefore may contribute to the regulation of pneumococcal carriage or clearance in human nasopharynx. Induction of Th17 and Treg from tonsillar MNC were studied using tonsillar MNC depleted of activated and memory T cells. Stimulation with pneumococcal CCS induced Th17 from naive T cells in tonsillar MNC in the presence of exogenous cytokines (i.e., TGF-β/IL21/IL1-β). TGFβ was shown to be crucial in Treg induction. Thus, the induction of both Th17 and Treg in human tonsillar tissue may be common in humans especially in children during natural infection/carriage, and the balance of the two may determine the clearance or carriage of pneumococcus in nasopharynx. Pneumococcal proteins including pneumolysin (Ply), its toxoid (PdB) and choline binding proteins (CBP) were shown to activate and promote Treg and Th17 cells in tonsillar MNC, thus they may play an important part in modulation of pneumococcal carriage in human nasopharynx. Understanding the development of natural immunity to pneumococcus and to pneumococcal proteins in particular may provide important information in the development of protein-based vaccines against pneumococcal infection in humans. Key words: Pneumococcus, Tonsillar MNC, Treg cells and Th17 cells.