Project: Dissecting Staphylococcus aureus macrophage exitCopyright: © Weber/Wolz
Nasal carriage of Staphylococcus aureus is a major risk factor for severe and invasive infections. Clinical infection relapse even after state-of-the-art antibiotic therapy is not uncommon, suggesting that bacteria may hide inside cells. Recent studies in murine experimental models indicate that S. aureus persistence in intracellular reservoirs in macrophages in the liver or the peritoneum is critical for the pathology of sepsis development. During chronic infections such as osteomyelitis, S. aureus may also persist within cells for prolonged time. On a molecular and cellular level S. aureus dissemination and host cell exit or persistence are not well understood. After entering the bloodstream, S. aureus is readily phagocytosed by multiple types of phagocytes, e.g. macrophages. However, the pathogen is able to replicate within human macrophages and escape from within.
We could previously show that phenol soluble modulins (PSMs) and human-specific two component leukocidins LukAB and/or PVL are crucial for this process. PSMs can lyse cells receptor-independently, mediate the escape of S. aureus from phagosomes into the cytosol and function in mouse and humans. Conversely, LukAB and PVL are highly receptor dependent and restricted to humans, hampering analysis of these toxins in conventional mouse models. Our data indicate that LukAB promotes human macrophage escape and cell death, but the type and route of cell death remain elusive. Further, IL-1β triggered during infection is NLRP3-dependent but toxin-independent, despite the fact that exogenously added recombinant LukAB toxin triggers IL-1β release. Thirdly, murine macrophages do not show cell death and exit.
In good agreement with the SPP’s key aims three resulting questions will be addressed here: 1) Which form of cell death is induced by existing S. aureus and how is it precisely executed? 2) Which ‘non-toxin’ trigger is responsible for NLRP3-dependent IL-1β release and how is pyroptotic cell death via the NLRP3 inflammasome suppressed or avoided during persistence? 3) Which determinants are responsible for the species-specific differences between human vs mouse and could murine host factors be ‘humanized’ or, vice versa, bacterial toxins be ‘murinized’? By combining our expertises in the molecular and cellular immune cell signaling (Weber) and S. aureus physiology and genetic engineering (Wolz), we will employ LukAB-inducible and reporter strains, manipulation of host cells, imaging and proteomics screens to probe the interaction between bacterial and host factors. This approach will allow us to define the key determinants driving cell death or S. aureus persistence. Understanding the mechanisms employed by S. aureus to evade intracellular killing, survive and escape could pave the way for better classification and treatment of clinical S. aureus phenotypes using novel anti-infective treatment approaches.