Intracellular immune complexes known as inflammasomes sense breaches of cytosolic sanctity. Inflammasomes promote downstream proinflammatory events, including interleukin-1 (IL-1) family cytokine ...release and pyroptotic cell death. The nucleotide-binding leucine-rich repeat family, apoptosis inhibitory protein/nucleotide-binding leucine-rich repeat family, caspase recruitment domain (CARD) domain-containing protein 4 (NAIP/NLRC4) inflammasome is involved in a range of pathogenic and protective inflammatory processes in mammalian hosts. In particular, the NAIP/NLRC4 inflammasome responds to flagellin and components of the virulence-associated type III secretion (T3SS) apparatus in the host cytosol, thereby allowing it to be a critical mediator of host defense during bacterial infection. Notable species- and cell type-specific differences exist in NAIP/NLRC4 inflammasome responses to bacterial pathogens. With a focus on Salmonella enterica serovar Typhimurium as a model pathogen, we review differences between murine and human NAIP/NLRC4 inflammasome responses. Differences in NAIP/NLRC4 inflammasome responses across species and cell types may have arisen in part due to evolutionary pressures.
•Mice express several different NAIPs, each recognizing a specific bacterial ligand.•Humans express one functional NAIP, which broadly detects multiple bacterial ligands.•NAIP inflammasome in murine intestinal epithelial cells (IECs) controls Salmonella.•NAIP inflammasome controls Salmonella in human macrophages but not in human IECs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Salmonella enterica serovar Typhimurium is a Gram-negative pathogen that uses two distinct type III secretion systems (T3SSs), termed Salmonella pathogenicity island (SPI)-1 and SPI-2, to deliver ...virulence factors into the host cell. The SPI-1 T3SS enables Salmonella to invade host cells, while the SPI-2 T3SS facilitates Salmonella's intracellular survival. In mice, a family of cytosolic immune sensors, including NAIP1, NAIP2, and NAIP5/6, recognizes the SPI-1 T3SS needle, inner rod, and flagellin proteins, respectively. Ligand recognition triggers assembly of the NAIP/NLRC4 inflammasome, which mediates caspase-1 activation, IL-1 family cytokine secretion, and pyroptosis of infected cells. In contrast to mice, humans encode a single NAIP that broadly recognizes all three ligands. The role of NAIP/NLRC4 or other inflammasomes during Salmonella infection of human macrophages is unclear. We find that although the NAIP/NLRC4 inflammasome is essential for detecting T3SS ligands in human macrophages, it is partially required for responses to infection, as Salmonella also activated the NLRP3 and CASP4/5 inflammasomes. Importantly, we demonstrate that combinatorial NAIP/NLRC4 and NLRP3 inflammasome activation restricts Salmonella replication in human macrophages. In contrast to SPI-1, the SPI-2 T3SS inner rod is not sensed by human or murine NAIPs, which is thought to allow Salmonella to evade host recognition and replicate intracellularly. Intriguingly, we find that human NAIP detects the SPI-2 T3SS needle protein. Critically, in the absence of both flagellin and the SPI-1 T3SS, the NAIP/NLRC4 inflammasome still controlled intracellular Salmonella burden. These findings reveal that recognition of Salmonella SPI-1 and SPI-2 T3SSs and engagement of both the NAIP/NLRC4 and NLRP3 inflammasomes control Salmonella infection in human macrophages.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Inflammasomes are essential for host defense against intracellular bacterial pathogens like
, as they activate caspases, which promote cytokine release and cell death to control infection. In mice, ...interferon (IFN) signaling promotes inflammasome responses against bacteria by inducing a family of IFN-inducible GTPases known as guanylate-binding proteins (GBPs). Within murine macrophages, IFN promotes the rupture of the
-containing vacuole (LCV), while GBPs are dispensable for this process. Instead, GBPs facilitate the lysis of cytosol-exposed
. In contrast, the functions of IFN and GBPs in human inflammasome responses to
are poorly understood. We show that IFN-γ enhances inflammasome responses to
in human macrophages. Human GBP1 is required for these IFN-γ-driven inflammasome responses. Furthermore, GBP1 co-localizes with
and/or LCVs in a type IV secretion system (T4SS)-dependent manner and promotes damage to the LCV, which leads to increased exposure of the bacteria to the host cell cytosol. Thus, our findings reveal species- and pathogen-specific differences in how GBPs function to promote inflammasome responses.
Inflammasomes are multiprotein signaling complexes that activate the innate immune system. Canonical inflammasomes recruit and activate caspase-1, which then cleaves and activates IL-1β and IL-18, as ...well as gasdermin D (GSDMD) to induce pyroptosis. In contrast, non-canonical inflammasomes, caspases-4/-5 (CASP4/5) in humans and caspase-11 (CASP11) in mice, are known to cleave GSDMD, but their role in direct processing of other substrates besides GSDMD has remained unknown. Here, we show that CASP4/5 but not CASP11 can directly cleave and activate IL-18. However, CASP4/5/11 can all cleave IL-1β to generate a 27-kDa fragment that deactivates IL-1β signaling. Mechanistically, we demonstrate that the sequence identity of the tetrapeptide sequence adjacent to the caspase cleavage site regulates IL-18 and IL-1β recruitment and activation. Altogether, we have identified new substrates of the non-canonical inflammasomes and reveal key mechanistic details regulating inflammation that may aid in developing new therapeutics for immune-related disorders.
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•Human CASP4/5 directly process IL-18 at D36 to generate the activated cytokine•CASP4/5 cleave IL-1β at D27 into a p27 species that inactivates downstream IL-1R signaling•The tetrapeptide sequence of IL-18 and IL-1β regulates their processing by caspases•CASP11 can process IL-1β into the deactivating p27 species but does not process IL-18
Exconde et al. report that the non-canonical inflammasomes directly process IL-18 and IL-1β. The tetrapeptide sequence regulates IL-18 and IL-1β processing to generate an active or inactive cytokine respectively. These results suggest that the non-canonical inflammasomes directly modulate inflammation and may have a broader substrate repertoire than previously known.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Persistent bacteria contribute to antibiotic treatment failure, potentially fueling resistance. Persisters must survive host defenses while retaining infective capacity after antibiotic cessation. In ...this issue of Cell Host and Microbe, Hill et al. identify bacterial RecA as a DNA repair factor that allows intracellular bacteria survival, regrowth, and relapse.
Persistent bacteria contribute to antibiotic treatment failure, potentially fueling resistance. Persisters must survive host defenses while retaining infective capacity after antibiotic cessation. In this issue of Cell Host and Microbe, Hill et al. identify bacterial RecA as a DNA repair factor that allows intracellular bacteria survival, regrowth, and relapse.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Microbiology courses are often designed as either a lecture class with a laboratory component or a seminar-style class. Each type of course provides students with unique learning opportunities. Lab ...courses allow students to perform simple experiments that relate to fundamental concepts taught in the corresponding lectures, while seminar courses challenge students to read and discuss primary literature. Microbiology courses offering a combination of seminar-style discussions and laboratory procedures are rare. Our goal in the "Microbial Diversity and Pathogenesis" undergraduate course is to integrate experiences of a seminar class with those of a discovery-driven lab course, thereby strengthening students' learning experiences through diversified didactic approaches. In the first half of the course, students read and discuss published peer-reviewed articles that cover major topics in both basic and applied microbiology, including antibiotic resistance, pathogenesis, and biotechnology applications. Complementing this primary literature, students perform microbiology experiments related to the topics covered in the readings. The assigned readings, discussions, and experiments provide a foundation in the second half of the course for inquiry-based exploratory research using student-designed transposon screens and selections. The course culminates in each student drafting a hypothesis-driven research proposal based on their literature review, their learned experimental techniques, and the preliminary data generated as a class. Through such first-hand experimental experience, students gain fundamental lab skills that are applicable beyond the realm of microbiology, such as sterile technique and learning how to support conclusions with scientific evidence. We observed a tremendous synergy between the seminar and lab aspects of our course. This unique didactic experience allows students to understand and connect primary literature to their experiments, while the discovery-driven aspect of this approach fosters active engagement of students with scientific research.
The vast bacteriophage population harbors an immense reservoir of genetic information. Almost 2000 phage genomes have been sequenced from phages infecting hosts in the phylum Actinobacteria, and ...analysis of these genomes reveals substantial diversity, pervasive mosaicism, and novel mechanisms for phage replication and lysogeny. Here, we describe the isolation and genomic characterization of 46 phages from environmental samples at various geographic locations in the U.S. infecting a single Arthrobacter sp. strain. These phages include representatives of all three virion morphologies, and Jasmine is the first sequenced podovirus of an actinobacterial host. The phages also span considerable sequence diversity, and can be grouped into 10 clusters according to their nucleotide diversity, and two singletons each with no close relatives. However, the clusters/singletons appear to be genomically well separated from each other, and relatively few genes are shared between clusters. Genome size varies from among the smallest of siphoviral phages (15,319 bp) to over 70 kbp, and G+C contents range from 45-68%, compared to 63.4% for the host genome. Although temperate phages are common among other actinobacterial hosts, these Arthrobacter phages are primarily lytic, and only the singleton Galaxy is likely temperate.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
As an enteric bacterial pathogen, Salmonella enterica serovar Typhimurium causes diarrheal disease, leading to morbidity and mortality worldwide. Once inside the host, Salmonella uses specialized ...machinery known as type III secretion systems (T3SSs) to inject effectors into the host cell cytosol. Subsequently, these effectors remodel host cellular processes to facilitate bacterial colonization. Salmonella infects a variety of cell types, including innate immune cells such as macrophages. It is important to characterize how innate immune cells sense and respond to Salmonella infection, as these cells represent the critical first line of host defense against invading pathogens. While immune responses to infection are often studied in mice, innate immune genes differ between mice and humans. Thus, it is imperative to define human-specific innate immune responses to Salmonella. Host cells harbor multiprotein signaling complexes known as inflammasomes, which become activated upon detecting the cytosolic presence of pathogens, like Salmonella. Inflammasome activation leads to pro-inflammatory cytokine release, which recruits immune cells to the site of infection, and a lytic form of cell death, known as pyroptosis, which eliminates the pathogen’s intracellular replicative niche. In mice, Salmonella triggers inflammasome activation, which controls the infection and protects the host. In this dissertation, we studied inflammasome responses to Salmonella in human macrophages. We found that human macrophages undergo a multifaceted inflammasome response to Salmonella, as the NAIP/NLRC4, NLRP3, and caspase-4/5 inflammasomes are all activated during infection. Importantly, we also discovered that inflammasome activation restricts Salmonella’s intracellular replication in human macrophages. Upon investigating how inflammasomes restrict Salmonella replication, we found that caspase activation and cell death factors are critical for controlling Salmonella in human macrophages. While Salmonella is considered a vacuolar pathogen, we surprisingly observed both vacuolar and cytosolic populations of Salmonella in human macrophages. Moreover, inflammasome responses appear to primarily restrict cytosolic Salmonella replication. Altogether, our findings highlight how Salmonella infection is detected and subsequently controlled by inflammasome responses in human macrophages. Importantly, these studies offer insight into human innate immune responses to bacterial infections.
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