Abstract & Commentary

Staphylococcus aureus Virulence — how the Alpha Hemolysin Damages the Host

By Dean L. Winslow, MD, FACP, FIDSA, Chairman, Department of Medicine, Santa Clara Valley, Medical Center; Clinical Professor, Stanford University School of Medicine, is Associate Editor for Infectious Disease Alert.

Dr. Winslow is a consultant for Siemens Diagnostic.

Synopsis: Staphylococcal a-hemolysin (Hla) is a major virulence factor in S.aureus pneumonia. It activates the nucleotide-binding domain and leucine-rich repeat containing gene family, pyrin domain containing 3 (NLRP3) inflammasome inducing production of IL-1B and programmed cell death.

Source: Kebaier C, et al. Staphylococcus aureus a-hemolysin mediates virulence in a murine model of severe pneumonia through activation of the NLRP3 inflammasome. Jrl Infect Dis 2012; 205(5): 807-17.

A wild type Newman strain of S.aureus and the hemolysin-deficient Newman strain hla::erm were studied in wild-type mice and Nlrp3-/- and IL1r1-/- mice. In addition, pyrogen-free purified hemolysin was used in several intratracheal challenge experiments. An established murine pneumonia model was employed using viable S. aureus, heat-killed S.aureus (HKSA) or hemolysin. Bronchoalveolar lavage (BAL) with determination of cell counts in BAL fluid, lung mechanics and histology were all assessed.

Heat-killed S. aureus, Hla, and HKSA+Hla did not induce IL-1 secretion in Nlrp3-/- mice. In contrast, Hla with or without HKSA induced high levels of IL-1 in wild type, but not in Nlrp3-/- mice. In a similar fashion, the histology of lungs in wild type mice challenged with HKSA+Hla showed marked neutrophil infiltration, vasculitis, vascular extravasation, epithelial sloughing and necrosis whereas histopathologic changes seen in similarly-challenged Nlrp3-/- mice were minimal. At 60 hours 80% of challenged Nlrp3-/- mice survived compared to only 30% of wild type mice.

In mice challenged with live S. aureus wild type Newman strain, mortality was higher in the wild type mice than it was in the Nlrp3-/- mice. Pulmonary function abnormalities were more severe in wild type surviving mice treated with vancomycin than observed in Nlrp3-/- mice. These differential effects were not related to S. aureus burden since S. aureus CFU/mL in BAL fluid were the same in wild type and Nlrp3-/- mice. As in the studies done with HKSA and HKSA+Hla, mortality and histopathological abnormalities were correlated with IL-1 and neutrophil levels in BAL fluid, but not with TNF in BAL fluid.

In mice challenged with hemolysin-deficient S. aureus there were no differences in neutrophil counts in BAL fluid in wild type vs. NLRP3-/- mice. In IL1r1-/- mice challenged with live S. aureus mortality was indistinguishable from that seen in wild type mice. Finally, experiments in CD11b+ cells, hemolysin induced NLRP3-dependent programmed necrosis in these cells as shown by release of the marker HMGB1.

Commentary

All S.aureus produce secreted exotoxins including the cytolysins a-hemolysin, hemolysin, and bicomponent leukocidins.1 Moreover, a-hemolysin has emerged as a key virulence factor in a murine model of S. aureus necrotizing pneumonia.2 Purified hemolysin has also been shown to induce pulmonary inflammation in rats and rabbits.3-5

The NLRP3 inflammasome is signaling complex which activates procaspase-1, processing and secretion of IL-1B and IL-18, and the initiation of programmed cell necrosis. The NLRP3 inflammasome is activated in response to a number of pathogen-derived molecules and S. aureus hemolysin has recently been shown to induce NLRP3-mediated signaling in cultured cells.6

The elegant studies reported in this paper conclusively demonstrate that hemolysin almost certainly causes pulmonary damage through activation of NLRP3 inflammasome-mediated signaling and that this effect is independent of IL-1 expression. While mice lacking NLRP3 are not completely protected from hemolysin-producing S. aureus this suggests that hemolysin promotes virulence through both NLRP3-dependent and NLRP3-independent mechanisms. The authors postulate that targeting the NLRP3 inflammasome might someday be a potential therapeutic strategy in not only S. aureus pneumonia, but also in bacterial pneumonias due to other pathogens including Streptococcus pneumoniae in which pore-forming toxins including pneumolysin have also been shown to activate NLRP3 signaling.

References

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