Glycan-based interactions of Streptococcus pneumoniae and the host

Abstract

Streptococcus pneumoniae is commonly found as an asymptomatic colonizer of the nasopharynx of children, but it can also translocate to normally sterile body sites and cause severe diseases, like pneumonia, septicemia or meningitis. Pneumococci spread via aerosols. Upon entry into the upper respiratory tract of the host, glycoconjugates with terminal sialic acids (Sias) are among the first structures pneumococci encounter. Hence, they play an important role in pneumococcal pathogenesis. Moreover, glycans are also implicated in the recognition of microbial pathogens by the innate immune system, as many ligands of Tolllike receptors are glycoconjugates. Both aspects of glycan-based pneumococcal-host interactions were studied in this thesis.

In most mammals, the Sia N-acetylneuraminic acid (Neu5Ac) is converted into Nglycolylneuraminic acid (Neu5Gc) by the cytidine-monophosphate-N-acetylneuraminic acid hydroxylase (CMAH). However, humans lack Neu5Gc due to a deletion in CMAH, instead they overproduce Neu5Ac. We reported a faster disease progression in Cmah-/- versus wildtype (wt) mice after pneumococcal challenge and an upregulation of pneumococcal sialidase NanA and the main sialic acid transporter SatABC in response to Neu5Ac as compared with Neu5Gc, which was mediated by the response regulator CiaR. Moreover, we detected higher pneumococcal adhesion rates to cells presenting Neu5Ac than Neu5Gc. In vitro, higher bacterial adherence downregulated IL-8 secretion, and in vivo, pneumococcal pyruvate oxidase (SpxB) and pneumolysin contributed to a reduced immune response in Cmah-/- compared with wt mice after intranasal challenge.

Influenza infections lead to changes in the pulmonary environment and sensitize for a pneumococcal infection. We observed higher protein concentrations, increased numbers of dead cells as well as upregulated hydrogen peroxide concentrations in bronchoalveolar lavages of influenza- versus mock-infected mice. The increased virus-mediated stress in the lower respiratory tract mediated an upregulation of the pneumococcal serine protease HtrA during influenza/pneumococcal coinfection. A mutant of HtrA was severely attenuated in a murine coinfection model, suggesting an important role of HtrA in pneumococcal outgrowth following primary influenza infection. Dendritic cells link the innate with the adaptive immune system. We found an RNA-mediated recognition of pneumococci by TLR3 in dendritic cells, which induced the secretion of the cytokine IL-12. Moreover, in influenza/pneumococcal coinfections, the virus upregulated TLR3 expression, which led to an enhanced production of IL-12 by dendritic cells.

In summary, we show that glycan-mediated interactions of S. pneumoniae and the host play a major role in pneumococcal host tropism and strongly affect pneumococcal virulence, as well as innate immune responses.