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Actin is integral to eukaryotic physiology as a biomechanical polymer and as a structural barrier for cell-autonomous defense against infection. Some microbial pathogens exploit the actin cytoskeleton, however, to evade cell-autonomous immunity. Subversion of actin to enter host cells and for actin-based motility are often employed by intracellular pathogens to spread from cell-to-cell. Using RNA-sequencing and computational data mining, we identify the host actin-binding protein XIRP1 as commonly induced during infection. XIRP1 is expressed by fibroblasts and macrophages in response to immune cytokines such as interferon-gamma (IFN-γ) and infection with bacteria such as Listeria, Shigella, and Salmonella. Confocal and super-resolution structured illumination microscopy (SIM) found XIRP1 localizes to fibroblast focal adhesions and macrophages podosomes. Within human macrophages, XIRP1 is recruited to cytosolic Listeria monocytogenes in an ActA-dependent manner as it replicates and uses actin-based motility for host cell escape. Chromosomal removal of XIRP1 in mice impaired this dissemination and rendered them more resistant to Listeria infection than C57BL/6NJ wildtype controls in vivo. We propose that professional cytosolic pathogens like Listeria can co-opt XIRP1 to escape the hostile intracellular environment of IFN-γ-activated macrophages as part of the host-pathogen arms race during cell-autonomous immunity.

Cleansing the cytosol Most human cells, not just those belonging to the immune system, mount protective responses to infection when activated by the immune cytokine interferon-gamma (IFN-γ). How IFN-γ confers this function in nonimmune cells and tissues is poorly understood. Gaudet et al. used genome-scale CRISPR/Cas9 gene editing to identify apolipoprotein L-3 (APOL3) as an IFN-γ–induced bactericidal protein that protects human epithelium, endothelium, and fibroblasts against infection (see the Perspective by Nathan). APOL3 directly targets bacteria in the host cell cytosol and kills them by dissolving their anionic membranes into lipoprotein complexes. This work reveals a detergent-like mechanism enlisted during human cell-autonomous immunity to combat intracellular pathogens. Science , abf8113, this issue p. eabf8113 ; see also abj5637, p. 276

Linking protein misfolding and innate immunity Multiple innate immune sensors undergo rapid assembly into large complexes known as signalosomes. This is an essential step during cellular responses to microbes and danger signals. How this process is regulated to avoid accumulation of potentially toxic protein aggregates remains poorly understood. Abdel-Nour et al. identified a pathway, dependent on heme-regulated inhibitor, eukaryotic initiation factor 2α, activating transcription factor 4, and heat shock protein B8, which controls the folding and scaffolding of innate immune sensors, allowing optimal proinflammatory signaling (see the Perspective by Pierre). The pathway appears to mirror the endoplasmic reticulum unfolded protein response (UPR), and so was named the cytosolic UPR (cUPR). The cUPR may represent a general mechanism to control protein misfolding in cells. Science , this issue p. eaaw4144 ; see also p. 28

The innate immune system is the first line of defense against invasive fungal infections. As a consequence, many successful fungal pathogens have evolved elegant strategies to interact with host immune cells. For example, Candida albicans undergoes a morphogenetic switch coupled to cell wall remodeling upon phagocytosis by macrophages and then induces macrophage pyroptosis, an inflammatory cell death program. To elucidate the genetic circuitry through which C. albicans orchestrates this host response, we performed the first large-scale analysis of C. albicans interactions with mammalian immune cells. We identified 98 C. albicans genes that enable macrophage pyroptosis without influencing fungal cell morphology in the macrophage, including specific determinants of cell wall biogenesis and the Hog1 signaling cascade. Using these mutated genes, we discovered that defects in the activation of pyroptosis affect immune cell recruitment during infection. Examining host circuitry required for pyroptosis in response to C. albicans infection, we discovered that inflammasome priming and activation can be decoupled. Finally, we observed that apoptosis-associated speck-like protein containing a CARD (ASC) oligomerization can occur prior to phagolysosomal rupture by C. albicans hyphae, demonstrating that phagolysosomal rupture is not the inflammasome activating signal. Taking the data together, this work defines genes that enable fungal cell wall remodeling and activation of macrophage pyroptosis independently of effects on morphogenesis and identifies macrophage signaling components that are required for pyroptosis in response to C. albicans infection., IMPORTANCE Candida albicans is a natural member of the human mucosal microbiota that can also cause superficial infections and life-threatening systemic infections, both of which are characterized by inflammation. Host defense relies mainly on the ingestion and destruction of C. albicans by innate immune cells, such as macrophages and neutrophils. Although some C. albicans cells are killed by macrophages, most undergo a morphological change and escape by inducing macrophage pyroptosis. Here, we investigated the C. albicans genes and host factors that promote macrophage pyroptosis in response to intracellular fungi. This work provides a foundation for understanding how host immune cells interact with C. albicans and may lead to effective strategies to modulate inflammation induced by fungal infections.

Detecting Gramnegative bacteria Invariant molecules specific to different classes of microbes, but not expressed by eukaryotic cells, alert the immune system to a potential invader. Gaudet et al. identified one such molecule expressed by a variety of Gram-negative bacteria: the monosaccharide heptose-1,7-bisphosphate (HBP) (see the Perspective by Brubaker and Monack). HBP is an intermediate in the synthesis of lipopolysaccharide, a major component of bacterial cell walls. Rather than alerting the immune system through traditional pathogen detection pathways, such as Toll-like receptors, HBP signals through the host protein TIFA (TRAF-interacting protein with forkhead-associated domain), which activates both innate and adaptive immune responses to control the infection. Science , this issue p. 1251 ; see also p. 1207

Specialized adaptations for killing microbes are synonymous with phagocytic cells including macrophages, monocytes, inflammatory neutrophils, and eosinophils. Recent genome sequencing of extant species, however, reveals that analogous antimicrobial machineries exist in certain non-immune cells and also within species that ostensibly lack a well-defined immune system. Here we probe the evolutionary record for clues about the ancient and diverse phylogenetic origins of macrophage killing mechanisms and how some of their properties are shared with cells outside the traditional bounds of immunity in higher vertebrates such as mammals.

Intestinal epithelial cells (IECs) act as sentinels for incoming pathogens. Cytosol-invasive bacteria, such as Shigella flexneri, trigger a robust pro-inflammatory nuclear factor κB (NF-κB) response from IECs that is believed to depend entirely on the peptidoglycan sensor NOD1. We found that, during Shigella infection, the TRAF-interacting forkhead-associated protein A (TIFA)-dependent cytosolic surveillance pathway, which senses the bacterial metabolite heptose-1,7-bisphosphate (HBP), functions after NOD1 to detect bacteria replicating free in the host cytosol. Whereas NOD1 mediated a transient burst of NF-κB activation during bacterial entry, TIFA sensed HBP released during bacterial replication, assembling into large signaling complexes to drive a dynamic inflammatory response that reflected the rate of intracellular bacterial proliferation. Strikingly, IECs lacking TIFA were unable to discriminate between proliferating and stagnant intracellular bacteria, despite the NOD1/2 pathways being intact. Our results define TIFA as a rheostat for intracellular bacterial replication, escalating the immune response to invasive Gram-negative bacteria that exploit the host cytosol for growth.

Host recognition of pathogen-associated molecular patterns (PAMPs) initiates an innate immune response that is critical for pathogen elimination and engagement of adaptive immunity. Here we show that mammalian cells can detect and respond to the bacterial-derived monosaccharide heptose-1,7-bisphosphate (HBP). A metabolic intermediate in lipopolysaccharide biosynthesis, HBP is highly conserved in Gram-negative bacteria, yet absent from eukaryotic cells. Detection of HBP within the host cytosol activated the nuclear facto κB pathway in vitro and induced innate and adaptive immune responses in vivo. Moreover, we used a genome-wide RNA interference screen to uncover an innate immune signaling axis, mediated by phosphorylation-dependent oligomerization of the TRAF-interacting protein with forkhead-associated domain (TIFA) that is triggered by HBP. Thus, HBP is a PAMP that activates TIFA-dependent immunity to Gram-negative bacteria.

Clinical and epidemiological synergy exists between the globally important sexually transmitted infections, gonorrhea and HIV. Neisseria gonorrhoeae , which causes gonorrhea, is particularly adept at driving HIV-1 expression, but the molecular determinants of this relationship remain undefined. N. gonorrhoeae liberates a soluble factor that potently induces expression from the HIV-1 LTR in coinfected cluster of differentiation 4-positive (CD4 + ) T lymphocytes, but this factor is not a previously described innate effector. A genome-wide mutagenesis approach was undertaken to reveal which component(s) of N. gonorrhoeae induce HIV-1 expression in CD4 + T lymphocytes. A mutation in the ADP-heptose biosynthesis gene, hldA , rendered the bacteria unable to induce HIV-1 expression. The hldA mutant has a truncated lipooligosaccharide structure, contains lipid A in its outer membrane, and remains bioactive in a TLR4 reporter-based assay but did not induce HIV-1 expression. Mass spectrometry analysis of extensively fractionated N. gonorrhoeae- derived supernatants revealed that the LTR-inducing fraction contained a compound having a mass consistent with heptose-monophosphate (HMP). Heptose is a carbohydrate common in microbes but is absent from the mammalian glycome. Although ADP-heptose biosynthesis is common among Gram-negative bacteria, and heptose is a core component of most lipopolysaccharides, N. gonorrhoeae is peculiar in that it effectively liberates HMP during growth. This N. gonorrhoeae- derived HMP activates CD4 + T cells to invoke an NF-κB–dependent transcriptional response that drives HIV-1 expression and viral production. Our study thereby shows that heptose is a microbial-specific product that is sensed as an innate immune agonist and unveils the molecular link between N. gonorrhoeae and HIV-1.

Antibody preparations have a long history of providing protection from infectious diseases. Although antibodies remain the only natural host-derived defense mechanism capable of completely preventing infection, as products, they compete against inexpensive therapeutics such as antibiotics, small molecule inhibitors and active vaccines. The continued discovery in the monoclonal antibody (mAb) field of leads with broadened cross neutralization of viruses and demonstrable synergy of antibody with antibiotics for bacterial diseases, clearly show that innovation remains. The commercial success of mAbs in chronic disease has not been paralleled in infectious diseases for several reasons. Infectious disease immunotherapeutics are limited in scope as endemic diseases necessitate active vaccine development. Also, the complexity of these small markets draws the interest of niche companies rather than big pharmaceutical corporations. Lastly, the cost of goods for mAb therapeutics is inherently high for infectious agents due to the need for antibody cocktails, which better mimic polyclonal immunoglobulin preparations and prevent antigenic escape. In cases where vaccine or convalescent populations are available, current polyclonal hyperimmune immunoglobulin preparations (pIgG), with modern and highly efficient purification technology and standardized assays for potency, can make economic sense. Recent innovations to broaden the potency of mAb therapies, while reducing cost of production, are discussed herein. On the basis of centuries of effective use of Ab treatments, and with growing immunocompromised populations, the question is not whether antibodies have a bright future for infectious agents, but rather what formats are cost effective and generate safe and efficacious treatments to satisfy regulatory approval.