Inducible antibacterial responses in macrophages.

Macrophages destroy bacteria and other microorganisms through phagocytosis-coupled antimicrobial responses, such as the generation of reactive oxygen species and the delivery of hydrolytic enzymes from lysosomes to the phagosome. However, many intracellular bacteria subvert these responses, escaping to other cellular compartments to survive and/or replicate. Such bacterial subversion strategies are countered by a range of additional direct antibacterial responses that are switched on by pattern-recognition receptors and/or host-derived cytokines and other factors, often through inducible gene expression and/or metabolic reprogramming. Our understanding of these inducible antibacterial defence strategies in macrophages is rapidly evolving. In this Review, we provide an overview of the broad repertoire of antibacterial responses that can be engaged in macrophages, including LC3-associated phagocytosis, metabolic reprogramming and antimicrobial metabolites, lipid droplets, guanylate-binding proteins, antimicrobial peptides, metal ion toxicity, nutrient depletion, autophagy and nitric oxide production. We also highlight key inducers, signalling pathways and transcription factors involved in driving these different antibacterial responses. Finally, we discuss how a detailed understanding of the molecular mechanisms of antibacterial responses in macrophages might be exploited for developing host-directed therapies to combat antibiotic-resistant bacterial infections.

CFTR is required for zinc-mediated antibacterial defense in human macrophages.

Cystic fibrosis transmembrane conductance regulator (CFTR) is an anion transporter required for epithelial homeostasis in the lung and other organs, with CFTR mutations leading to the autosomal recessive genetic disease CF. Apart from excessive mucus accumulation and dysregulated inflammation in the airways, people with CF (pwCF) exhibit defective innate immune responses and are susceptible to bacterial respiratory pathogens such as Pseudomonas aeruginosa. Here, we investigated the role of CFTR in macrophage antimicrobial responses, including the zinc toxicity response that is used by these innate immune cells against intracellular bacteria. Using both pharmacological approaches, as well as cells derived from pwCF, we show that CFTR is required for uptake and clearance of pathogenic Escherichia coli by CSF-1-derived primary human macrophages. CFTR was also required for E. coli-induced zinc accumulation and zinc vesicle formation in these cells, and E. coli residing in macrophages exhibited reduced zinc stress in the absence of CFTR function. Accordingly, CFTR was essential for reducing the intramacrophage survival of a zinc-sensitive E. coli mutant compared to wild-type E. coli. Ectopic expression of the zinc transporter SLC30A1 or treatment with exogenous zinc was sufficient to restore antimicrobial responses against E. coli in human macrophages. Zinc supplementation also restored bacterial killing in GM-CSF-derived primary human macrophages responding to P. aeruginosa, used as an in vitro macrophage model relevant to CF. Thus, restoration of the zinc toxicity response could be pursued as a therapeutic strategy to restore innate immune function and effective host defense in pwCF.

HDAC6/aggresome processing pathway importance for inflammasome formation is context-dependent.

The inflammasome is a large multiprotein complex that assembles in the cell cytoplasm in response to stress or pathogenic infection. Its primary function is to defend the cell and promote the secretion of pro-inflammatory cytokines, including IL-1ß and IL-18. Previous research has shown that in immortalized bone marrow-derived macrophages (iBMDMs) inflammasome assembly is dependent on the deacetylase HDAC6 and the aggresome processing pathway (APP), a cellular pathway involved in the disposal of misfolded proteins. Here we used primary BMDMs from mice in which HDAC6 is ablated or impaired and found that inflammasome activation was largely normal. We also used human peripheral blood mononuclear cells and monocyte cell lines expressing a synthetic protein blocking the HDAC6-ubiquitin interaction and impairing the APP and found that inflammasome activation was moderately affected. Finally, we used a novel HDAC6 degrader and showed that inflammasome activation was partially impaired in human macrophage cell lines with depleted HDAC6. Our results therefore show that HDAC6 importance in inflammasome activation is context-dependent.

HDAC6/aggresome processing pathway importance for inflammasome formation is context dependent.

The inflammasome is a large multiprotein complex that assembles in the cell cytoplasm in response to stress or pathogenic infection. Its primary function is to defend the cell and promote the secretion of pro-inflammatory cytokines, including IL-1ß and IL-18. It was shown that in immortalized bone marrow derived macrophages (iBMDMs) inflammasome assembly is dependent on the deacetylase HDAC6 and the aggresome processing pathway (APP), a cellular pathway involved in the disposal of misfolded proteins. Here we used primary BMDMs from mice in which HDAC6 is ablated or impaired and found that inflammasome activation was largely normal. We also used human peripheral blood mononuclear cells and monocytes cell lines expressing a synthetic protein blocking HDAC6-ubiquitin interaction and impairing the APP and found that inflammasome activation was moderately affected. Finally, we used a novel HDAC6 degrader and showed that inflammasome activation was partially impaired in human macrophage cell lines with depleted HDAC6. Our results therefore show that HDAC6 importance in inflammasome activation is context dependent.

TLR4 phosphorylation at tyrosine 672 activates the ERK/c-FOS signaling module for LPS-induced cytokine responses in macrophages.

TLRs engage numerous adaptor proteins and signaling molecules, enabling a complex series of post-translational modifications (PTMs) to mount inflammatory responses. TLRs themselves are post-translationally modified following ligand-induced activation, with this being required to relay the full spectrum of proinflammatory signaling responses. Here, we reveal indispensable roles for TLR4 Y672 and Y749 phosphorylation in mounting optimal LPS-inducible inflammatory responses in primary mouse macrophages. LPS promotes phosphorylation at both tyrosine residues, with Y749 phosphorylation being required for maintenance of total TLR4 protein levels and Y672 phosphorylation exerting its pro-inflammatory effects more selectively by initiating ERK1/2 and c-FOS phosphorylation. Our data also support a role for the TLR4-interacting membrane proteins SCIMP and the SYK kinase axis in mediating TLR4 Y672 phosphorylation to permit downstream inflammatory responses in murine macrophages. The corresponding residue in human TLR4 (Y674) is also required for optimal LPS signaling responses. Our study, thus, reveals how a single PTM on one of the most widely studied innate immune receptors orchestrates downstream inflammatory responses.

Mitochondrial dynamics in macrophages: divide to conquer or unite to survive?

Mitochondria have long been appreciated as the metabolic hub of cells. Emerging evidence also posits these organelles as hubs for innate immune signalling and activation, particularly in macrophages. Macrophages are front-line cellular defenders against endogenous and exogenous threats in mammals. These cells use an array of receptors and downstream signalling molecules to respond to a diverse range of stimuli, with mitochondrial biology implicated in many of these responses. Mitochondria have the capacity to both divide through mitochondrial fission and coalesce through mitochondrial fusion. Mitochondrial dynamics, the balance between fission and fusion, regulate many cellular functions, including innate immune pathways in macrophages. In these cells, mitochondrial fission has primarily been associated with pro-inflammatory responses and metabolic adaptation, so can be considered as a combative strategy utilised by immune cells. In contrast, mitochondrial fusion has a more protective role in limiting cell death under conditions of nutrient starvation. Hence, fusion can be viewed as a cellular survival strategy. Here we broadly review the role of mitochondria in macrophage functions, with a focus on how regulated mitochondrial dynamics control different functional responses in these cells.

HDAC7 is an immunometabolic switch triaging danger signals for engagement of antimicrobial versus inflammatory responses in macrophages.

The immune system must be able to respond to a myriad of different threats, each requiring a distinct type of response. Here, we demonstrate that the cytoplasmic lysine deacetylase HDAC7 in macrophages is a metabolic switch that triages danger signals to enable the most appropriate immune response. Lipopolysaccharide (LPS) and soluble signals indicating distal or far-away danger trigger HDAC7-dependent glycolysis and proinflammatory IL-1ß production. In contrast, HDAC7 initiates the pentose phosphate pathway (PPP) for NADPH and reactive oxygen species (ROS) production in response to the more proximal threat of nearby bacteria, as exemplified by studies on uropathogenic Escherichia coli (UPEC). HDAC7-mediated PPP engagement via 6-phosphogluconate dehydrogenase (6PGD) generates NADPH for antimicrobial ROS production, as well as D-ribulose-5-phosphate (RL5P) that both synergizes with ROS for UPEC killing and suppresses selective inflammatory responses. This dual functionality of the HDAC7-6PGD-RL5P axis prioritizes responses to proximal threats. Our findings thus reveal that the PPP metabolite RL5P has both antimicrobial and immunomodulatory activities and that engagement of enzymes in catabolic versus anabolic metabolic pathways triages responses to different types of danger for generation of inflammatory versus antimicrobial responses, respectively.

Quantifying Regulated Mitochondrial Fission in Macrophages.

Mitochondria have co-evolved with eukaryotic cells for more than a billion years, becoming an important cog in their machinery. They are best known for being tasked with energy generation through the production of adenosine triphosphate, but they also have roles in several other cellular processes, for example, immune and inflammatory responses. Mitochondria have important functions in macrophages, key innate immune cells that detect pathogens and drive inflammation. Mitochondrial activity is influenced by the highly dynamic nature of the mitochondrial network, which alternates between interconnected tubular and fragmented forms. The dynamic balance between this interconnected fused network and fission-mediated mitochondrial fragmentation modulates inflammatory responses such as production of cytokines and mitochondrial reactive oxygen species. Here we describe methods to differentiate mouse bone marrow cells into macrophages and the use of light microscopy, electron microscopy, flow cytometry, and Western blotting to quantify regulated mitochondrial dynamics in these differentiated macrophages.

An alternative downstream translation start site in the non-TIR adaptor Scimp enables selective amplification of CpG DNA responses in mouse macrophages.

Toll-like receptor (TLR) signaling relies on Toll/interleukin-1 receptor homology (TIR) domain-containing adaptor proteins that recruit downstream signaling molecules to generate tailored immune responses. In addition, the palmitoylated transmembrane adaptor protein family member Scimp acts as a non-TIR-containing adaptor protein in macrophages, scaffolding the Src family kinase Lyn to enable TLR phosphorylation and proinflammatory signaling responses. Here we report the existence of a smaller, naturally occurring translational variant of Scimp (Scimp TV1), which is generated through leaky scanning and translation at a downstream methionine. Scimp TV1 also scaffolds Lyn, but in contrast to full-length Scimp, it is basally rather than lipopolysaccharide (LPS)-inducibly phosphorylated. Macrophages from mice that selectively express Scimp TV1, but not full-length Scimp, have impaired sustained LPS-inducible cytokine responses. Furthermore, in granulocyte macrophage colony-stimulating factor-derived myeloid cells that express high levels of Scimp, selective overexpression of Scimp TV1 enhances CpG DNA-inducible cytokine production. Unlike full-length Scimp that localizes to the cell surface and filopodia, Scimp TV1 accumulates in intracellular compartments, particularly the Golgi. Moreover, this variant of Scimp is not inducibly phosphorylated in response to CpG DNA, suggesting that it may act via an indirect mechanism to enhance TLR9 responses. Our findings thus reveal the use of alternative translation start sites as a previously unrecognized mechanism for diversifying TLR responses in the innate immune system.

Inhibition of the master regulator of Listeria monocytogenes virulence enables bacterial clearance from spacious replication vacuoles in infected macrophages.

A hallmark of Listeria (L.) monocytogenes pathogenesis is bacterial escape from maturing entry vacuoles, which is required for rapid bacterial replication in the host cell cytoplasm and cell-to-cell spread. The bacterial transcriptional activator PrfA controls expression of key virulence factors that enable exploitation of this intracellular niche. The transcriptional activity of PrfA within infected host cells is controlled by allosteric coactivation. Inhibitory occupation of the coactivator site has been shown to impair PrfA functions, but consequences of PrfA inhibition for L. monocytogenes infection and pathogenesis are unknown. Here we report the crystal structure of PrfA with a small molecule inhibitor occupying the coactivator site at 2.0 Å resolution. Using molecular imaging and infection studies in macrophages, we demonstrate that PrfA inhibition prevents the vacuolar escape of L. monocytogenes and enables extensive bacterial replication inside spacious vacuoles. In contrast to previously described spacious Listeria-containing vacuoles, which have been implicated in supporting chronic infection, PrfA inhibition facilitated progressive clearance of intracellular L. monocytogenes from spacious vacuoles through lysosomal degradation. Thus, inhibitory occupation of the PrfA coactivator site facilitates formation of a transient intravacuolar L. monocytogenes replication niche that licenses macrophages to effectively eliminate intracellular bacteria. Our findings encourage further exploration of PrfA as a potential target for antimicrobials and highlight that intra-vacuolar residence of L. monocytogenes in macrophages is not inevitably tied to bacterial persistence.

Modified horseshoe crab peptides target and kill bacteria inside host cells.

Bacteria that occupy an intracellular niche can evade extracellular host immune responses and antimicrobial molecules. In addition to classic intracellular pathogens, other bacteria including uropathogenic Escherichia coli (UPEC) can adopt both extracellular and intracellular lifestyles. UPEC intracellular survival and replication complicates treatment, as many therapeutic molecules do not effectively reach all components of the infection cycle. In this study, we explored cell-penetrating antimicrobial peptides from distinct structural classes as alternative molecules for targeting bacteria. We identified two ß-hairpin peptides from the horseshoe crab, tachyplesin I and polyphemusin I, with broad antimicrobial activity toward a panel of pathogenic and non-pathogenic bacteria in planktonic form. Peptide analogs [I11A]tachyplesin I and [I11S]tachyplesin I maintained activity toward bacteria, but were less toxic to mammalian cells than native tachyplesin I. This important increase in therapeutic window allowed treatment with higher concentrations of [I11A]tachyplesin I and [I11S]tachyplesin I, to significantly reduce intramacrophage survival of UPEC in an in vitro infection model. Mechanistic studies using bacterial cells, model membranes and cell membrane extracts, suggest that tachyplesin I and polyphemusin I peptides kill UPEC by selectively binding and disrupting bacterial cell membranes. Moreover, treatment of UPEC with sublethal peptide concentrations increased zinc toxicity and enhanced innate macrophage antimicrobial pathways. In summary, our combined data show that cell-penetrating peptides are attractive alternatives to traditional small molecule antibiotics for treating UPEC infection, and that optimization of native peptide sequences can deliver effective antimicrobials for targeting bacteria in extracellular and intracellular environments.

Nicotinamide riboside attenuates age-associated metabolic and functional changes in hematopoietic stem cells.

With age, hematopoietic stem cells (HSC) undergo changes in function, including reduced regenerative potential and loss of quiescence, which is accompanied by a significant expansion of the stem cell pool that can lead to haematological disorders. Elevated metabolic activity has been implicated in driving the HSC ageing phenotype. Here we show that nicotinamide riboside (NR), a form of vitamin B3, restores youthful metabolic capacity by modifying mitochondrial function in multiple ways including reduced expression of nuclear encoded metabolic pathway genes, damping of mitochondrial stress and a decrease in mitochondrial mass and network-size. Metabolic restoration is dependent on continuous NR supplementation and accompanied by a shift of the aged transcriptome towards the young HSC state, more youthful bone marrow cellular composition and an improved regenerative capacity in a transplant setting. Consequently, NR administration could support healthy ageing by re-establishing a more youthful hematopoietic system.

An alloy of zinc and innate immunity: Galvanising host defence against infection.

Innate immune cells such as macrophages and neutrophils initiate protective inflammatory responses and engage antimicrobial responses to provide frontline defence against invading pathogens. These cells can both restrict the availability of certain transition metals that are essential for microbial growth and direct toxic concentrations of metals towards pathogens as antimicrobial responses. Zinc is important for the structure and function of many proteins, however excess zinc can be cytotoxic. In recent years, several studies have revealed that innate immune cells can deliver toxic concentrations of zinc to intracellular pathogens. In this review, we discuss the importance of zinc status during infectious disease and the evidence for zinc intoxication as an innate immune antimicrobial response. Evidence for pathogen subversion of this response is also examined. The likely mechanisms, including the involvement of specific zinc transporters that facilitate delivery of zinc by innate immune cells for metal ion poisoning of pathogens are also considered. Precise mechanisms by which excess levels of zinc can be toxic to microorganisms are then discussed, particularly in the context of synergy with other antimicrobial responses. Finally, we highlight key unanswered questions in this emerging field, which may offer new opportunities for exploiting innate immune responses for anti-infective development.

Frontline Science: LPS-inducible SLC30A1 drives human macrophage-mediated zinc toxicity against intracellular Escherichia coli.

TLR-inducible zinc toxicity is an antimicrobial mechanism utilized by macrophages, however knowledge of molecular mechanisms mediating this response is limited. Here, we show that E. coli exposed to zinc stress within primary human macrophages reside in membrane-bound vesicular compartments. Since SLC30A zinc exporters can deliver zinc into the lumen of vesicles, we examined LPS-regulated mRNA expression of Slc30a/SLC30A family members in primary mouse and human macrophages. A number of these transporters were dynamically regulated in both cell populations. In human monocyte-derived macrophages, LPS strongly up-regulated SLC30A1 mRNA and protein expression. In contrast, SLC30A1 was not LPS-inducible in macrophage-like PMA-differentiated THP-1 cells. We therefore ectopically expressed SLC30A1 in these cells, finding that this was sufficient to promote zinc-containing vesicle formation. The response was similar to that observed following LPS stimulation. Ectopically expressed SLC30A1 localized to both the plasma membrane and intracellular zinc-containing vesicles within LPS-stimulated THP-1 cells. Inducible overexpression of SLC30A1 in THP-1 cells infected with the Escherichia coli K-12 strain MG1655 augmented the zinc stress response of intracellular bacteria and promoted clearance. Furthermore, in THP-1 cells infected with an MG1655 zinc stress reporter strain, all bacteria contained within SLC30A1-positive compartments were subjected to zinc stress. Thus, SLC30A1 marks zinc-containing compartments associated with TLR-inducible zinc toxicity in human macrophages, and its ectopic over-expression is sufficient to initiate this antimicrobial pathway in these cells. Finally, SLC30A1 silencing did not compromise E. coli clearance by primary human macrophages, suggesting that other zinc exporters may also contribute to the zinc toxicity response.

Mammalian lipid droplets are innate immune hubs integrating cell metabolism and host defense.

Lipid droplets (LDs) are the major lipid storage organelles of eukaryotic cells and a source of nutrients for intracellular pathogens. We demonstrate that mammalian LDs are endowed with a protein-mediated antimicrobial capacity, which is up-regulated by danger signals. In response to lipopolysaccharide (LPS), multiple host defense proteins, including interferon-inducible guanosine triphosphatases and the antimicrobial cathelicidin, assemble into complex clusters on LDs. LPS additionally promotes the physical and functional uncoupling of LDs from mitochondria, reducing fatty acid metabolism while increasing LD-bacterial contacts. Thus, LDs actively participate in mammalian innate immunity at two levels: They are both cell-autonomous organelles that organize and use immune proteins to kill intracellular pathogens as well as central players in the local and systemic metabolic adaptation to infection.

Lipopolysaccharide promotes Drp1-dependent mitochondrial fission and associated inflammatory responses in macrophages.

Mitochondria have a multitude of functions, including energy generation and cell signaling. Recent evidence suggests that mitochondrial dynamics (i.e. the balance between mitochondrial fission and fusion) also regulate immune functions. Here, we reveal that lipopolysaccharide (LPS) stimulation increases mitochondrial numbers in mouse bone marrow-derived macrophages (BMMs) and human monocyte-derived macrophages. In BMMs, this response requires Toll-like receptor 4 (Tlr4) and the TLR adaptor protein myeloid differentiation primary response 88 (MyD88) but is independent of mitochondrial biogenesis. Consistent with this phenomenon being a consequence of mitochondrial fission, the dynamin-related protein 1 (Drp1) GTPase that promotes mitochondrial fission is enriched on mitochondria in LPS-activated macrophages and is required for the LPS-mediated increase in mitochondrial numbers in both BMMs and mouse embryonic fibroblasts. Pharmacological agents that skew toward mitochondrial fusion also abrogated this response. LPS triggered acute Drp1 phosphorylation at serine 635 (S635), followed by sustained Drp1 dephosphorylation at serine 656 (S656), in BMMs. LPS-induced S656 dephosphorylation was abrogated in MyD88-deficient BMMs, suggesting that this post-translational modification is particularly important for Tlr4-inducible fission. Pharmacological or genetic targeting of Tlr4-inducible fission had selective effects on inflammatory mediator production, with LPS-inducible mitochondrial fission promoting the expression and/or secretion of a subset of inflammatory mediators in BMMs and mouse embryonic fibroblasts. Thus, triggering of Tlr4 results in MyD88-dependent activation of Drp1, leading to inducible mitochondrial fission and subsequent inflammatory responses in macrophages.

Uropathogenic Escherichia coli employs both evasion and resistance to subvert innate immune-mediated zinc toxicity for dissemination.

Toll-like receptor (TLR)-inducible zinc toxicity is a recently described macrophage antimicrobial response used against bacterial pathogens. Here we investigated deployment of this pathway against uropathogenic Escherichia coli (UPEC), the major cause of urinary tract infections. Primary human macrophages subjected EC958, a representative strain of the globally disseminated multidrug-resistant UPEC ST131 clone, to zinc stress. We therefore used transposon-directed insertion site sequencing to identify the complete set of UPEC genes conferring protection against zinc toxicity. Surprisingly, zinc-susceptible EC958 mutants were not compromised for intramacrophage survival, whereas corresponding mutants in the nonpathogenic E. coli K-12 strain MG1655 displayed significantly reduced intracellular bacterial loads within human macrophages. To investigate whether the intramacrophage zinc stress response of EC958 reflected the response of only a subpopulation of bacteria, we generated and validated reporter systems as highly specific sensors of zinc stress. Using these tools we show that, in contrast to MG1655, the majority of intramacrophage EC958 evades the zinc toxicity response, enabling survival within these cells. In addition, EC958 has a higher tolerance to zinc than MG1655, with this likely being important for survival of the minor subset of UPEC cells exposed to innate immune-mediated zinc stress. Indeed, analysis of zinc stress reporter strains and zinc-sensitive mutants in an intraperitoneal challenge model in mice revealed that EC958 employs both evasion and resistance against zinc toxicity, enabling its dissemination to the liver and spleen. We thus demonstrate that a pathogen of global significance uses multiple mechanisms to effectively subvert innate immune-mediated zinc poisoning for systemic spread.

Interleukin-1ß Maturation Triggers Its Relocation to the Plasma Membrane for Gasdermin-D-Dependent and -Independent Secretion.

IL-1ß requires processing by caspase-1 to generate the active, pro-inflammatory cytokine. Acute IL-1ß secretion from inflammasome-activated macrophages requires caspase-1-dependent GSDMD cleavage, which also induces pyroptosis. Mechanisms of IL-1ß secretion by pyroptotic and non-pyroptotic cells, and the precise functions of caspase-1 and GSDMD therein, are unresolved. Here, we show that, while efficient early secretion of endogenous IL-1ß from primary non-pyroptotic myeloid cells in vitro requires GSDMD, later IL-1ß release in vitro and in vivo proceeds independently of GSDMD. IL-1ß maturation is sufficient for slow, caspase-1/GSDMD-independent secretion of ectopic IL-1ß from resting, non-pyroptotic macrophages, but the speed of IL-1ß release is boosted by inflammasome activation, via caspase-1 and GSDMD. IL-1ß cleavage induces IL-1ß enrichment at PIP2-enriched plasma membrane ruffles, and this is a prerequisite for IL-1ß secretion and is mediated by a polybasic motif within the cytokine. We thus reveal a mechanism in which maturation-induced IL-1ß trafficking facilitates its unconventional secretion.

For when bacterial infections persist: Toll-like receptor-inducible direct antimicrobial pathways in macrophages.

Macrophages are linchpins of innate immunity, responding to invading microorganisms by initiating coordinated inflammatory and antimicrobial programs. Immediate antimicrobial responses, such as NADPH-dependent reactive oxygen species (ROS), are triggered upon phagocytic receptor engagement. Macrophages also detect and respond to microbial products through pattern recognition receptors (PRRs), such as TLRs. TLR signaling influences multiple biological processes including antigen presentation, cell survival, inflammation, and direct antimicrobial responses. The latter enables macrophages to combat infectious agents that persist within the intracellular environment. In this review, we summarize our current understanding of TLR-inducible direct antimicrobial responses that macrophages employ against bacterial pathogens, with a focus on emerging evidence linking TLR signaling to reprogramming of mitochondrial functions to enable the production of direct antimicrobial agents such as ROS and itaconic acid. In addition, we describe other TLR-inducible antimicrobial pathways, including autophagy/mitophagy, modulation of nutrient availability, metal ion toxicity, reactive nitrogen species, immune GTPases (immunity-related GTPases and guanylate-binding proteins), and antimicrobial peptides. We also describe examples of mechanisms of evasion of such pathways by professional intramacrophage pathogens, with a focus on Salmonella, Mycobacteria, and Listeria. An understanding of how TLR-inducible direct antimicrobial responses are regulated, as well as how bacterial pathogens subvert such pathways, may provide new opportunities for manipulating host defence to combat infectious diseases.