Interferon-γ primes macrophages for pathogen ligand-induced killing via a caspase-8 and mitochondrial cell death pathway

Abstract

Cell death plays an important role during pathogen infections. Here, we report that interferon-γ (IFNγ) sensitizes macrophages to Toll-like receptor (TLR)-induced death that requires macrophage-intrinsic death ligands and caspase-8 enzymatic activity, which trigger the mitochondrial apoptotic effectors, BAX and BAK. The pro-apoptotic caspase-8 substrate BID was dispensable for BAX and BAK activation. Instead, caspase-8 reduced pro-survival BCL-2 transcription and increased inducible nitric oxide synthase (iNOS), thus facilitating BAX and BAK signaling. IFNγ-primed, TLR-induced macrophage killing required iNOS, which licensed apoptotic caspase-8 activity and reduced the BAX and BAK inhibitors, A1 and MCL-1. The deletion of iNOS or caspase-8 limited SARS-CoV-2-induced disease in mice, while caspase-8 caused lethality independent of iNOS in a model of hemophagocytic lymphohistiocytosis. These findings reveal that iNOS selectively licenses programmed cell death, which may explain how nitric oxide impacts disease severity in SARS-CoV-2 infection and other iNOS-associated inflammatory conditions.

No longer married to inflammasome signaling: the diverse interacting pathways leading to pyroptotic cell death

“…Notably, this pyroptotic cell death was implicated in cell types that naturally express low levels of GSDMD, such as cortical neurons and mast cells, and speculatively, may also take place upon pathogen-mediated inhibition of GSDMD [ 46 , 47 , 49 ]. While the induction of mitochondrial apoptosis resulting from caspase-8 or caspase-1 signaling result, at least in part, from caspase-8 or caspase-1 cleavage of Bid to activate BAX and BAK, it is worth noting that caspase-8 can also promote BAX and BAK activity independent of Bid, by regulating the transcription of Bcl-2 family members [ 14 ]. Whether caspase-1 may also control gene transcription [ 118 , 119 ], akin to caspase-8 processing of N4BP1 to control TLR-induced transcriptional activity [ 120 ], will be of interest to determine.…”

“…Similarly, Bjanes et al [ 158 ] noted significantly reduced NINJ1 expression protected macrophages from lytic death, including forms induced by canonical and non-canonical inflammasome signaling. NINJ1 has also been documented to mediate membrane rupture and efficient LDH release following a caspase-8 and inducible nitric oxide synthase (iNOS)-mediated cell death caused by IFNγ and pathogen-ligand TLR signaling [ 14 ]. It is tempting to speculate that NINJ1 may therefore contribute to the damaging caspase-8- and iNOS-driven host response observed following SARS-CoV-2 infection [ 14 ], however how NINJ1 causes cell lysis requires further study.…”

“…This allows cytochrome c release, which nucleates the formation of the caspase-9 apoptosome that enables caspase-3 and -7 cleavage and activation, thereby promoting cellular disassembly and clearance in a manner that limits the leakage of immunogenic content [ 166 ]. The activity of BAX and BAK is tightly controlled by BCL-2 family members and, in the case of macrophages, pro-survival BCL-2, BCL-xL, MCL-1 and A1 have all been documented as playing important roles in restraining BAX and BAK activation [ 2 , 14 , 167 , 168 ].…”

“…The relevance of these in vitro genetic and biochemical experiments was highlighted by the discovery that bacterial outer membrane vesicles derived from pathogenic bacteria such as Neisseria gonorrhoeae can trigger mitochondrial apoptosis to activate NLRP3 and drive inflammatory IL-1β responses when injected into mice [ 169 ]. Similarly, iNOS and caspase-8 can induce a BAX and BAK dependent cell death that is likely to cause damaging immune responses following SARS-CoV-2 infection in mice [ 14 ].…”

“…In more recent years, the identification of new cell death effectors has highlighted the crucial role of PRRs in signaling cell death during infections, and has uncovered how cell death can act to eliminate intracellular pathogens [ 1–9 ]. Conversely, in some contexts, PRR-driven cell death can also increase pathogen infection by limiting the immune response [ 10–13 ], or can cause host-tissue damage and increase disease severity, as evidenced in cytokine shock syndromes [ 14–16 ].…”