Host-pathogen interactions in severe group A streptococcal infections

Abstract

Streptococcus pyogenes, also called group A Streptococcus (GAS), is an important human pathogen ranked number 9 on the list of global killer pathogens. It causes a wide spectrum of disease, including uncomplicated superficial infections of the skin and throat, to life-threatening invasive infections and post-streptococcal sequelae.

This thesis aims to increase the understanding of pathogenic mechanisms contributing to severe soft tissue infections caused by GAS. To investigate host-pathogen interactions at the local site of infection, a tissue biopsy material collected from patients with severe soft tissue infections was used. The studies revealed that tissue biopsies from the site of infection contained high bacterial load, high amounts of the streptococcal exotoxins SpeB and SpeF, and heavy infiltration of inflammatory cells. A potent pro-inflammatory cytokine response was evident in the biopsies, and the amount of Th1 cytokines was significantly associated with severity of infection. The results indicated a critical role for superantigens, the cysteine protease SpeB and pro-inflammatory cytokines in severe GAS tissue infections.

Viable bacteria could be detected in the tissue even late after onset of infection and prolonged intravenous antibiotic therapy. In the tissue biopsies GAS, which is normally considered an extracellular pathogen, could be found intracellularly in host cells, predominantly macrophages. The patient studies further implied a potential role of SpeB for intracellular survival. An in vitro infection model was used to confirm these data. The studies provided evidence that GAS could infect and survive, for a prolonged time, intracellularly in human monocytes and macrophages. Furthermore, infection with a speB-deficient mutant verified a role for SpeB in promoting intracellular survival. This intracellular persistence of GAS at the tissue site may have evolved as an immune escape mechanism as well as a strategy to avoid antibiotic eradication, which may explain the high bacterial load present even long after onset of infection.

Important components of the host immune response are the antimicrobial peptides. The antimicrobial peptide LL-37 has been reported to be important in the control of GAS infections. Analyses of LL-37 in our patient biopsy material revealed high amounts of the mature peptide in all biopsies, which correlated positively to bacterial load. Studies of interaction between the peptide and SpeB, which has previously been reported to inactive LL-37, revealed a strong co-localization of SpeB and LL-37 around the bacteria. Hence, the study provided in vivo support that SpeB-mediated inactivation of LL-37 may represent a bacterial resistance mechanism at the infected tissue site.

These studies have identified pathogenic mechanisms that likely contribute to the severity of these infections, and consequently, are targets for intervention. Improved therapeutic strategies are required to limit the morbidity and mortality associated with severe GAS infections.