Staphylococcus aureus triggers inflammation through inflammasome activation and recruitment of neutrophils, responses that are critical for pathogen clearance but are associated with substantial tissue damage. We postulated that necroptosis, cell death mediated by the RIPK1/RIPK3/MLKL pathway, would function to limit pathological inflammation. In models of skin infection or sepsis, Mlkl-/- mice had high bacterial loads, an inability to limit interleukin-1b (IL-1b) production, and excessive inflammation. Similarly, mice treated with RIPK1 or RIPK3 inhibitors had increased bacterial loads in a model of sepsis. Ripk3-/- mice exhibited increased staphylococcal clearance and decreased inflammation in skin and systemic infection, due to direct effects of RIPK3 on IL-1b activation and apoptosis. In contrast to Casp1/4-/- mice with defective S. aureus killing, the poor outcomes of Mlkl-/- mice could not be attributed to impaired phagocytic function. We conclude that necroptotic cell death limits the pathological inflammation induced by S. aureus.

Klebsiella pneumoniae are Gram-negative facultative anaerobes that are found within host-associated commensal microbiomes, but they can also cause a wide range of infections that are often difficult to treat. These infections are caused by different pathotypes of K. pneumoniae , called either classical or hypervirulent strains.

Adaptive changes in the genome of a locally predominant clinical isolate of the multidrug-resistant Klebsiella pneumoniae ST258 (KP35) were identified and help to explain the selection of this strain as a successful pulmonary pathogen. The acquisition of 4 new ortholog groups, including an arginine transporter, enabled KP35 to outcompete related ST258 strains lacking these genes. KP35 infection elicited a monocytic response, dominated by Ly6C ^hi monocytic myeloid-derived suppressor cells that lacked phagocytic capabilities, expressed IL-10, arginase, and antiinflammatory surface markers. In comparison with other K . pneumoniae strains, KP35 induced global changes in the phagocytic response identified with proteomics, including evasion of Ca ²⁺ and calpain activation necessary for phagocytic killing, confirmed in functional studies with neutrophils. This comprehensive analysis of an ST258 K . pneumoniae isolate reveals ongoing genetic adaptation to host microenvironments and innate immune clearance mechanisms that complements its repertoire of antimicrobial resistance genes and facilitates persistence in the lung.

Macrophages are main effectors of heme metabolism, increasing transiently in the liver during heightened disposal of damaged or senescent RBCs (sRBCs). Macrophages are also essential in defense against microbial threats, but pathological states of heme excess may be immunosuppressive. Herein, we uncovered a mechanism whereby an acute rise in sRBC disposal by macrophages led to an immunosuppressive phenotype after intrapulmonary Klebsiella pneumoniae infection characterized by increased extrapulmonary bacterial proliferation and reduced survival from sepsis in mice. The impaired immunity to K. pneumoniae during heightened sRBC disposal was independent of iron acquisition by bacterial siderophores, in that K. pneumoniae mutants lacking siderophore function recapitulated the findings observed with the WT strain. Rather, sRBC disposal induced a liver transcriptomic profile notable for suppression of Stat1 and IFN-related responses during K. pneumoniae sepsis. Excess heme handling by macrophages recapitulated STAT1 suppression during infection that required synergistic NRF1 and NRF2 activation but was independent of heme oxygenase-1 induction. Whereas iron was dispensable, the porphyrin moiety of heme was sufficient to mediate suppression of STAT1-dependent responses in human and mouse macrophages and promoted liver dissemination of K. pneumoniae in vivo. Thus, cellular heme metabolism dysfunction negatively regulated the STAT1 pathway, with implications in severe infection.