Microbial factors and host responses affecting severity of pneumococcal disease and pneumococcal carriage

Abstract

Streptococcus pneumoniae, also known as the pneumococcus, is a human specific bacterium and causes infections like otitis media, sinusitis, pneumonia, sepsis and meningitis. However, these bacteria are also frequent colonizers of the nasopharynx of healthy individuals, especially children. The major virulence factor in pneumococci is the polysaccharide capsule, which protects against phagocytosis. The death of pneumococci releases high amounts of potent mediators of inflammation. The innate immune system constitutes the first line of host defense against microbial infections. Pattern recognition receptors such as the Toll-Like Receptors (TLRs) mediate innate immune responses upon recognition of bacterial components through adaptor molecules such as MyD88.

The overarching goal of the present work was to understand underlying mechanisms in the microbe as well as in the host that contribute to the transition from a local commensal microbe-host interaction to a systemic infection, and the innate immune responses associated with both local microbial control and development of invasive disease.

Clinical isolates from patients with invasive disease and from healthy children attending day-care centers were collected in the Stockholm area during 1997. The isolates were characterized using molecular fingerprinting methods such as Pulsed Field Gel Electrophoresis and Multi Locus Sequence Typing. Further, we characterized the virulence of these isolates in mouse infection models. By using wild type mice and mice deficient in the different TLRs and MyD88 we investigated the role of these molecules for resistance against pneumococcal infection. A novel technique using biophotonic real-time in vivo imaging has been applied to understand the infection process within living animals.

We found that 40 % of the children attending day-care centers harbored pneumococci; 20 % of the isolates were non-susceptible to penicillin. We could observe cases of spread between children of pneumococcal clones known to be able to cause invasive disease. Further, we verified that pneumococci can be divided into three major groups: the first group mainly found in patients with invasive disease, the second group mainly found among carriers and a third group found both among carriers and in patients with invasive disease. Not only serotype but also properties associated with particular clones (genetically related isolates) are important for the potential to cause disease in humans. Clonal properties in addition to the capsular serotype may be important factors in the ability of pneumococci to cause invasive disease also in mice.

Further, we demonstrate the biological importance of innate immune activation and the central role of MyD88 in the host protection during pneumococcal infections. We report that wild type but not MyD88-/- mice inhibit bacterial growth by IL-6-mediated induction of the iron-regulatory antimicrobial protein hepcidin and subsequent iron deprivation. TLR9-/- but not TLR2-/-, TLR4-/-or ICE-/- (IL-1/IL-18 deficient) mice were shown to be more susceptible than wild type mice. The role of MyD88 cannot be attributed to a single TLR; instead, the most probable situation is a combination and cooperation between different TLRs and/or other recognition receptors. However, TLR9 plays a non-redundant role in the protection against invasive pneumococcal disease.