Unencapsulated and pspA/ply mutants have

been reported which also have shorter duration of colonisation at lower densities than the parent WT strain [6]. These were however Natural Product Library still able to induce protective immune responses in C57BL/6 mice [6]. This may reflect a greater propensity to induce stronger protection in this inbred strain, which may explain the greater protection seen following WT D39 colonisation of CBA/Ca mice [5] than the CD1 mice reported here. It may be more challenging to achieve protection in outbred mice due to multiple genetic differences between individual mice including the MHC. Protection has been shown for a pneumolysin-deficient D39 strain in outbred MF1 mice [7], but colonisation with this strain persisted for 7–14 days and was not dissimilar to the duration of WT D39 in CD1 mice reported here. Colonisation with the WT D39 strain induced high titres of anti-bacterial serum IgG, yet no detectable anti-capsular IgG. This was also BGB324 research buy found following D39 colonisation of CBA/Ca mice [5] and MF1 mice [7]. We have also found that colonisation of CD1 mice with the TIGR4 strain did not induce anti-capsular serum IgG (unpublished data). Together, these data suggest that, in mice, a single nasopharyngeal colonisation event is not sufficient to induce a serum anti-CPS IgG response, at least for serotype 2 and 4 capsules. Colonisation has a variable effect on induction of serum

anti-CPS IgG responses in humans. In a longitudinal family study, serotypes 9V, 14, 18C, 19F and 23F induced anti-CPS responses, but serotype 6B did not [19]. Following carriage in a childhood MTMR9 study, responses were detected

to serotypes 11A and 14, but not to serotypes 6B, 19F and 23F [20]. Furthermore, experimental human colonisation did not induce an anti-capsular serum IgG response [21]. Immunogenicity of capsule following colonisation events is likely to reflect a complex interaction of bacterial strain, CPS type, host genetics, as well as the current and previous constituents of the nasopharyngeal microbiome. Ongoing longitudinal studies correlating detailed carriage history with serological data may elucidate this further. The absence of anti-capsular serum IgG did not prevent colonisation with WT D39 from inducing protection against lethal challenge, albeit at a weaker level in these CD1 mice compared to results with in-bred strains [5]. Immunity to non-capsular antigens induced through colonisation is known to be sufficient to protect [6]. Our data imply that whilst capsular antigens are not dominant during colonisation, the presence of capsule does not impede the development of anti-protein mediated protective immunity. On the contrary, the increased level and duration of colonisation with encapsulated compared to unencapsulated bacteria resulted in an increased antibody response to protein antigens and improved protection to subsequent challenge.