Pneumococcal colonisation models of the nasopharynx: The role of virulence factors and host immunity during colonisation

The human nasopharynx is the most important ecological niche for Streptococcus pneumoniae and the establishment of nasopharyngeal carriage is an essential pre-requisite to invasive pneumococcal disease. Virulence factors which mediate long term carriage and the immunogenicity of individual bacterial components are of interest to the development of vaccines which currently fall short of protecting against >90 known serotypes of pneumococci. This thesis contains a long term mouse model of nasopharyngeal carriage conducted in outbred mice, using both wild type and attenuated isogenic mutant pneumococcal strains. Whilst serotype-2 (D39) and serotype-3 (A66) pneumococci were carried asymptomatically in the nasopharynx for at least 21 days, mutants that lacked neuraminidases, PspA and pneumolysin were cleared from the nasopharynx <14 days. Both the carriage of WT-D39 and the clearance of the pneumolysin negative (PlnA-) were associated with the generation of serum IgM anti-capsular antibody, and IgG anti-PspA, which correlated to bacterial numbers in the nasopharynx. Carriage also stimulated anti-capsular IgA in the cervical lymph nodes, and a local macrophage cellular response. Using attenuated carriage it was possible to model the effects of subsequent exposure to pneumococci, in the form of future carriage events and invasive disease. Prior colonisation significantly shortened the duration of carriage from >28 days to <14 days within the same serotype however, both the polysaccharide capsule and conserved protein antigens contribute to protection. Colonisation could also protect mice from a normally fatal invasive challenge with both D39 and A66. Immune sera alone from previously colonised mice was able to delay onset of fatal pneumonia, however the presence of both PspA and capsule antigens were not indispensible to the protection in this instance. Taken together, the results in this thesis support existing data about the immunogenicity of capsule and conserved proteins which may inform the creation of novel and more efficacious pneumococcal vaccines.