Galectin-8 Senses Phagosomal Damage and Recruits Selective Autophagy Adapter TAX1BP1 To Control Mycobacterium tuberculosis Infection in Macrophages

Function of autophagy genes in innate immune defense against mucosal pathogens

Mucosal immunity is posed to constantly interact with commensal microbes and invading pathogens. As a fundamental cell biological pathway affecting immune response, autophagy regulates the interaction between mucosal immunity and microbes through multiple mechanisms, including direct elimination of microbes, control of inflammation, antigen presentation and lymphocyte homeostasis, and secretion of immune mediators. Some of these physiologically important functions do not involve canonical degradative autophagy but rely on certain autophagy genes and their 'autophagy gene-specific functions.' Here, we review the relationship between autophagy and important mucosal pathogens, including influenza virus, Mycobacterium tuberculosis, Salmonella enterica, Citrobacter rodentium, norovirus, and herpes simplex virus, with a particular focus on distinguishing the canonical versus gene-specific mechanisms of autophagy genes.

Capsule-deficient group A Streptococcus evades autophagy-mediated killing in macrophages

The hyaluronic acid capsule is crucial in protecting group A Streptococcus (GAS) against phagocytic killing. However, there have been reported outbreaks caused by capsule-deficient GAS strains, and the mechanisms underlying their evasion of immune clearance remain unclear. This study demonstrated that the capsule-deficient mutant [Cap(-)] of the emm1 strain increased survival within phagocytic cells compared to the wild-type strain [Cap(+)]. Although both Cap(+) and Cap(-) strains exhibited similar abilities to disrupt the phagosome, only the Cap(+) strain was colocalized with lysosomes and acidified compartments in phagocytic cells, indicating its susceptibility to autophagosome elimination. In contrast, the Cap(-) mutant evaded the recognition of galectin-8 and ubiquitin, impairing selective autophagy-mediated elimination. These findings suggest that a deficiency in the capsule could impair the intracellular elimination of GAS in macrophages, revealing previously unknown aspects of the host's recognition of the GAS capsule in macrophages.

IMPORTANCE

Group A Streptococcus (GAS) is a Gram-positive bacterium that causes diseases ranging from mild pharyngitis to severe necrotizing fasciitis. Phagocytic cells serve as the primary defense against bacterial infections, exhibiting remarkable efficiency in eliminating intracellular pathogens. The hyaluronic acid capsule is a critical virulence factor that contributes to the resistance of phagocytosis in GAS. Nevertheless, the outbreaks caused by GAS strains that lack the hyaluronic acid capsule have been reported, and the selective advantage of capsule-deficient strains during infection is not fully understood. This study showed that the autophagic adaptor proteins recognize the capsulated GAS strain but not the capsule-deficient mutant, indicating that the hyaluronic acid capsule could be the autophagic target in macrophages. These findings imply that the hyaluronic acid capsule of GAS actually enhances its elimination within phagocytic cells, subverting the understanding of the capsule in GAS pathogenesis.

Targeting selective autophagy and beyond: From underlying mechanisms to potential therapies

BACKGROUND

Autophagy is an evolutionarily conserved turnover process for intracellular substances in eukaryotes, relying on lysosomal (in animals) or vacuolar (in yeast and plants) mechanisms. In the past two decades, emerging evidence suggests that, under specific conditions, autophagy can target particular macromolecules or organelles for degradation, a process termed selective autophagy. Recently, accumulating studies have demonstrated that the abnormality of selective autophagy is closely associated with the occurrence and progression of many human diseases, including neurodegenerative diseases, cancers, metabolic diseases, and cardiovascular diseases.

AIM OF REVIEW

This review aims at systematically and comprehensively introducing selective autophagy and its role in various diseases, while unravelling the molecular mechanisms of selective autophagy. By providing a theoretical basis for the development of related small-molecule drugs as well as treating related human diseases, this review seeks to contribute to the understanding of selective autophagy and its therapeutic potential.

KEY SCIENTIFIC CONCEPTS OF REVIEW

In this review, we systematically introduce and dissect the major categories of selective autophagy that have been discovered. We also focus on recent advances in understanding the molecular mechanisms underlying both classical and non-classical selective autophagy. Moreover, the current situation of small-molecule drugs targeting different types of selective autophagy is further summarized, providing valuable insights into the discovery of more candidate small-molecule drugs targeting selective autophagy in the future. On the other hand, we also reveal clinically relevant implementations that are potentially related to selective autophagy, such as predictive approaches and treatments tailored to individual patients.

A TBK1 variant causes autophagolysosomal and motoneuron pathology without neuroinflammation in mice

Heterozygous mutations in the TBK1 gene can cause amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). The majority of TBK1-ALS/FTD patients carry deleterious loss-of-expression mutations, and it is still unclear which TBK1 function leads to neurodegeneration. We investigated the impact of the pathogenic TBK1 missense variant p.E696K, which does not abolish protein expression, but leads to a selective loss of TBK1 binding to the autophagy adaptor protein and TBK1 substrate optineurin. Using organelle-specific proteomics, we found that in a knock-in mouse model and human iPSC-derived motor neurons, the p.E696K mutation causes presymptomatic onset of autophagolysosomal dysfunction in neurons precipitating the accumulation of damaged lysosomes. This is followed by a progressive, age-dependent motor neuron disease. Contrary to the phenotype of mice with full Tbk1 knock-out, RIPK/TNF-α-dependent hepatic, neuronal necroptosis, and overt autoinflammation were not detected. Our in vivo results indicate autophagolysosomal dysfunction as a trigger for neurodegeneration and a promising therapeutic target in TBK1-ALS/FTD.

Galectin-8 modulates human osteoclast activity partly through isoform-specific interactions

In overactive human osteoclasts, we previously identified an alternative splicing event in LGALS8, encoding galectin-8, resulting in decreased expression of the long isoform. Galectin-8, which modulates cell-matrix interactions and functions intracellularly as a danger recognition receptor, has never been associated with osteoclast biology. In human osteoclasts, inhibition of galectin-8 expression revealed its roles in bone resorption, osteoclast nuclearity, and mTORC1 signaling regulation. Galectin-8 isoform-specific inhibition asserted a predominant role for the short isoform in bone resorption. Moreover, a liquid chromatography with tandem mass spectrometry (LC-MS/MS) proteomic analysis of galectin-8 isoforms performed in HEK293T cells identified 22 proteins shared by both isoforms. Meanwhile, nine interacting partners were specific for the short isoform, and none were unique to the long isoform. Interactors specific for the galectin-8 short isoform included cell adhesion proteins and lysosomal proteins. We confirmed the interactions of galectin-8 with CLCN3, CLCN7, LAMP1, and LAMP2, all known to localize to secretory vesicles, in human osteoclasts. Altogether, our study reveals direct roles of galectin-8 in osteoclast activity, mostly attributable to the short isoform.

Galectin-8 inhibition and functions in immune response and tumor biology

Galectins are among organisms' most abundantly expressed lectins (carbohydrate-binding proteins) that specifically bind β-galactosides. They act not only outside the cell, where they bind to extracellular matrix glycans, but also inside the cell, where they have a significant impact on signaling pathways. Galectin-8 is a galectin family protein encoded by the LGALS8 gene. Its role is evident in both T- and B-cell immunity and in the innate immune response, where it acts directly on dendritic cells and induces some pro-inflammatory cytokines. Galectin-8 also plays an important role in the defense against bacterial and viral infections. It is known to promote antibacterial autophagy by recognizing and binding glycans present on the vacuolar membrane, thus acting as a danger receptor. The most important role of galectin-8 is the regulation of cancer growth, metastasis, tumor progression, and tumor cell survival. Importantly, the expression of galectins is typically higher in tumor tissues than in noncancerous tissues. In this review article, we focus on galectin-8 and its function in immune response, microbial infections, and cancer. Given all of these functions of galectin-8, we emphasize the importance of developing new and selective galectin-8 inhibitors and report the current status of their development.

Interplay between group A Streptococcus and host innate immune responses

SUMMARYGroup A Streptococcus (GAS), also known as Streptococcus pyogenes, is a clinically well-adapted human pathogen that harbors rich virulence determinants contributing to a broad spectrum of diseases. GAS is capable of invading epithelial, endothelial, and professional phagocytic cells while evading host innate immune responses, including phagocytosis, selective autophagy, light chain 3-associated phagocytosis, and inflammation. However, without a more complete understanding of the different ways invasive GAS infections develop, it is difficult to appreciate how GAS survives and multiplies in host cells that have interactive immune networks. This review article attempts to provide an overview of the behaviors and mechanisms that allow pathogenic GAS to invade cells, along with the strategies that host cells practice to constrain GAS infection. We highlight the counteractions taken by GAS to apply virulence factors such as streptolysin O, nicotinamide-adenine dinucleotidase, and streptococcal pyrogenic exotoxin B as a hindrance to host innate immune responses.

Activation of the lysosomal damage response and selective autophagy: the coordinated actions of galectins, TRIM proteins, and CGAS-STING1 in providing immunity against Mycobacterium tuberculosis

Autophagy is a crucial immune defense mechanism that controls the survival and pathogenesis of M. tb by maintaining cell physiology during stress and pathogen attack. The E3-Ub ligases (PRKN, SMURF1, and NEDD4) and autophagy receptors (SQSTM1, TAX1BP1, CALCOCO2, OPTN, and NBR1) play key roles in this process. Galectins (LGALSs), which bind to sugars and are involved in identifying damaged cell membranes caused by intracellular pathogens such as M. tb, are essential. These include LGALS3, LGALS8, and LGALS9, which respond to endomembrane damage and regulate endomembrane damage caused by toxic chemicals, protein aggregates, and intracellular pathogens, including M. tb. They also activate selective autophagy and de novo endolysosome biogenesis. LGALS3, LGALS9, and LGALS8 interact with various components to activate autophagy and repair damage, while CGAS-STING1 plays a critical role in providing immunity against M. tb by activating selective autophagy and producing type I IFNs with antimycobacterial functions. STING1 activates cGAMP-dependent autophagy which provides immunity against various pathogens. Additionally, cytoplasmic surveillance pathways activated by ds-DNA, such as inflammasomes mediated by NLRP3 and AIM2 complexes, control M. tb. Modulation of E3-Ub ligases with small regulatory molecules of LGALSs and TRIM proteins could be a novel host-based therapeutic approach for controlling TB.

Host-directed therapy against mycobacterium tuberculosis infections with diabetes mellitus

Tuberculosis (TB) is caused by the bacterial pathogen Mycobacterium tuberculosis (MTB) and is one of the principal reasons for mortality and morbidity worldwide. Currently, recommended anti-tuberculosis drugs include isoniazid, rifampicin, ethambutol, and pyrazinamide. TB treatment is lengthy and inflicted with severe side-effects, including reduced patient compliance with treatment and promotion of drug-resistant strains. TB is also prone to other concomitant diseases such as diabetes and HIV. These drug-resistant and complex co-morbid characteristics increase the complexity of treating MTB. Host-directed therapy (HDT), which effectively eliminates MTB and minimizes inflammatory tissue damage, primarily by targeting the immune system, is currently an attractive complementary approach. The drugs used for HDT are repositioned drugs in actual clinical practice with relative safety and efficacy assurance. HDT is a potentially effective therapeutic intervention for the treatment of MTB and diabetic MTB, and can compensate for the shortcomings of current TB therapies, including the reduction of drug resistance and modulation of immune response. Here, we summarize the state-of-the-art roles and mechanisms of HDT in immune modulation and treatment of MTB, with a special focus on the role of HDT in diabetic MTB, to emphasize the potential of HDT in controlling MTB infection.

Vps18 contributes to phagosome membrane integrity in Mycobacterium tuberculosis-infected macrophages

Mycobacterium tuberculosis (Mtb) has evolved to be exquisitely adapted to survive within host macrophages. The capacity to damage the phagosomal membrane has emerged as central to Mtb virulence. While Mtb factors driving membrane damage have been described, host factors that repair that damage to contain the pathogen remain largely unknown. We used a genome-wide CRISPR screen to identify novel host factors required to repair Mtb-damaged phagosomal membranes. Vacuolar protein sorting-associated protein 18 (Vps18), a member of the HOPS and CORVET trafficking complexes, was among the top hits. Vps18 colocalized with Mtb in macrophages beginning shortly after infection, and Vps18-knockout macrophages demonstrated increased damage of Mtb-containing phagosomes without impaired autophagy. Mtb grew more robustly in Vps18-knockout cells, and the first-line anti-tuberculosis antibiotic pyrazinamide was less effective. Our results identify Vps18 as required for phagosomal membrane integrity in Mtb-infected cells and suggest that modulating phagosome integrity may hold promise for improving the efficacy of antibiotic treatment for TB.

Role of pattern recognition receptors in sensing Mycobacterium tuberculosis

Mycobacterium tuberculosis is one of the major invasive intracellular pathogens causing most deaths by a single infectious agent. The interaction between host immune cells and this pathogen is the focal point of the disease, Tuberculosis. Host immune cells not only mount the protective action against this pathogen but also serve as the primary niche for growth. Thus, recognition of this pathogen by host immune cells and following signaling cascades are key dictators of the disease state. Immune cells, mainly belonging to myeloid cell lineage, recognize a wide variety of Mycobacterium tuberculosis ligands ranging from carbohydrate and lipids to proteins to nucleic acids by different membrane-bound and soluble pattern recognition receptors. Simultaneous interaction between different host receptors and pathogen ligands leads to immune-inflammatory response as well as contributes to virulence. This review summarizes the contribution of pattern recognition receptors of host immune cells in recognizing Mycobacterium tuberculosis and subsequent initiation of signaling pathways to provide the molecular insight of the specific Mtb ligands interacting with specific PRR, key adaptor molecules of the downstream signaling pathways and the resultant effector functions which will aid in identifying novel drug targets, and developing novel drugs and adjuvants.

Mitochondrial reactive oxygen species: double agents in Mycobacterium tuberculosis infection

In addition to housing the major energy-producing pathways in cells, mitochondria are active players in innate immune responses. One critical way mitochondria fulfill this role is by releasing damage-associated molecular patterns (mtDAMPs) that are recognized by innate sensors to activate pathways including, but not limited to, cytokine expression, selective autophagy, and cell death. Mitochondrial reactive oxygen species (mtROS) is a multifunctional mtDAMP linked to pro- and antimicrobial immune outcomes. Formed as a by-product of energy generation, mtROS links mitochondrial metabolism with downstream innate immune responses. As a result, altered cellular metabolism can change mtROS levels and impact downstream antimicrobial responses in a variety of ways. MtROS has emerged as a particularly important mediator of pathogenesis during infection with Mycobacterium tuberculosis (Mtb), an intracellular bacterial pathogen that continues to pose a significant threat to global public health. Here, we will summarize how Mtb modulates mtROS levels in infected macrophages and how mtROS dictates Mtb infection outcomes by controlling inflammation, lipid peroxidation, and cell death. We propose that mtROS may serve as a biomarker to predict tuberculosis patient outcomes and/or a target for host-directed therapeutics.

Atg8ylation as a host-protective mechanism against Mycobacterium tuberculosis

Nearly two decades have passed since the first report on autophagy acting as a cell-autonomous defense against Mycobacterium tuberculosis. This helped usher a new area of research within the field of host-pathogen interactions and led to the recognition of autophagy as an immunological mechanism. Interest grew in the fundamental mechanisms of antimicrobial autophagy and in the prophylactic and therapeutic potential for tuberculosis. However, puzzling in vivo data have begun to emerge in murine models of M. tuberculosis infection. The control of infection in mice affirmed the effects of certain autophagy genes, specifically ATG5, but not of other ATGs. Recent studies with a more complete inactivation of ATG genes now show that multiple ATG genes are indeed necessary for protection against M. tuberculosis. These particular ATG genes are involved in the process of membrane atg8ylation. Atg8ylation in mammalian cells is a broad response to membrane stress, damage and remodeling of which canonical autophagy is one of the multiple downstream outputs. The current developments clarify the controversies and open new avenues for both fundamental and translational studies.

A Transcriptional Analysis of Cattle Immune Cells Reveals a Central Role of Type 1 Interferon in the In Vitro Innate Immune Response against Mycobacterium bovis

Bovine tuberculosis is a chronic infectious disease primarily caused by Mycobacterium bovis, a bacterium that affects cattle and other mammals, including humans. Despite the availability of vast research about the immune response mechanisms of human tuberculosis caused by Mycobacterium tuberculosis, the knowledge of bovine tuberculosis's immunology, particularly regarding the innate immune response, still remains scarce. In this study, we compared the transcriptome of cell cultures containing lymphocytes and M. bovis infected-macrophages with two strains of variable virulence, the virulent Mb04-303 strain and the attenuated Mb534. To that end, we infected bovine macrophages at a multiplicity of infection of one, and co-cultured the infections with autologous lymphocytes. RNA obtained from the co-cultures was sequenced to identify differentially expressed gene pathways by using the database Reactome. The RNA-seq analysis showed that the Mb04-303 infection upregulated the type 1 interferon signalling pathway, while it downregulated the KEAP1-NFE2L2 pathway. According to the literature, this last pathway is involved in the activation of antioxidant genes and inflammasome. In addition, the macrophages infected with Mb04-303 recruited more Galectin 8 than those infected with Mb534. This result indicates that Mb04-303 induced higher phagosome membrane damage, with the possible concomitant release of bacterial compounds into the cytoplasm that activates the type I signalling pathway. Altogether, Mb04-303 repressed the antioxidant and anti-inflammatory responses, likely impairing interleukin-1β activation, and trigged the canonical type 1 interferon signalling. Although these responses led to the control of bacterial replication during early infection, the virulent strain eventually managed to establish a successful infection.

The ubiquitin ligase TRIM32 promotes the autophagic response to Mycobacterium tuberculosis infection in macrophages

Mycobacterium tuberculosis (Mtb) is known to evade host immune responses and persist in macrophages for long periods. A mechanism that the host uses to combat Mtb is xenophagy, a selective form of autophagy that targets intracellular pathogens for degradation. Ubiquitination of Mtb or Mtb-containing compartments is a key event to recruit the autophagy machinery and mediate the bacterial delivery to the lysosome. This event relies on the coordinated and complementary activity of different ubiquitin ligases, including PARKIN, SMURF1, and TRIM16. Because each of these factors is responsible for the ubiquitination of a subset of the Mtb population, it is likely that additional ubiquitin ligases are employed by macrophages to trigger a full xenophagic response during Mtb infection. In this study, we investigated the role TRIM proteins whose expression is modulated in response to Mtb or BCG infection of primary macrophages. These TRIMs were ectopically expressed in THP1 macrophage cell line to assess their impact on Mtb replication. This screening identified TRIM32 as a novel player involved in the intracellular response to Mtb infection, which promotes autophagy-mediated Mtb degradation. The role of TRIM32 in xenophagy was further confirmed by silencing TRIM32 expression in THP1 cells, which causes increased intracellular growth of Mtb associated to impaired Mtb ubiquitination, reduced recruitment of the autophagy proteins NDP52/CALCOCO2 and BECLIN 1/BECN1 to Mtb and autophagosome formation. Overall, these findings suggest that TRIM32 plays an important role in the host response to Mtb infection through the induction of autophagy, representing a promising target for host-directed tuberculosis therapies.

Serine arginine-rich splicing factor (SRSF7) cooperates with the histone methyltransferase KMT5a to promote the type I interferon response via transcriptional activation of IRF7

Tight regulation of macrophage immune gene expression is required to fight infection without risking harmful inflammation. The contribution of RNA binding proteins (RBPs) to shaping the macrophage response to pathogens remains poorly understood. Transcriptomic analysis revealed that a member of the serine/arginine-rich (SR) family of mRNA processing factors, SRSF7, is required for optimal expression of a cohort of interferon stimulated genes (ISGs) in macrophages. Using genetic and biochemical assays, we discovered that in addition to its canonical role in regulating alternative splicing, SRSF7 drives transcription of interferon regulatory transcription factor 7 (IRF7) to promote antiviral immunity. At the Irf7 promoter, SRSF7 maximizes STAT1 transcription factor binding and RNA polymerase II elongation via cooperation with the H4K20me1 histone methyltransferase KMT5a (SET8). These studies define an unorthodox role for an SR protein in activating transcription and reveal an unappreciated RNA binding protein-chromatin network that orchestrates macrophage antiviral gene expression.

BCG∆BCG1419c and BCG differ in induction of autophagy, c-di-GMP content, proteome, and progression of lung pathology in Mycobacterium tuberculosis HN878-infected male BALB/c mice

The efficacy of BCG vaccines against Mycobacterium tuberculosis (Mtb) strains of lineage 2 (Beijing) in preclinical models and humans has been questioned. We have developed BCG∆BCG1419c, by deletion of BCG1419c in BCG Pasteur, which improved control of tuberculosis (TB) in preclinical models. Here, we compared the capacity of BCG and BCG∆BCG1419c to induce autophagy in murine macrophages, modify c-di-GMP content and transcript levels of BCG1416c, encoding the enzyme responsible for c-di-GMP synthesis/degradation, and of BCG1419c, encoding the phosphodiesterase involved in c-di-GMP degradation. Furthermore, we evaluated proteomic differences in vitro and compared protection against TB produced by a low dose of the HN878-Beijing strain at 3- and 6-months post-infection. We found that BCG∆BCG1419c induced more autophagy and produced different levels of c-di-GMP as well as different transcription of BCG1416c with no expression of BCG1419c. BCG∆BCG1419c differentially produced several proteins, including some involved in interaction with host cells. Vaccination with either BCG strain led to control of bacillary burden in lungs and spleen at 3- but not 6-months post-infection, whereas it reduced pneumonic areas compared with unvaccinated controls at 6 months post-infection. Vaccination with BCG∆BCG1419c delayed progression of lung necrosis as this was observed only at 6 months post-infection. Taken together, compared with BCG, BCG∆BCG1419c increased autophagy, presented different levels of c-di-GMP and transcription of BCG1416c in vitro in a growth-phase dependent manner, modified its proteome and delayed progression of lung pathology produced by a highly virulent Beijing strain.

Deficiency in Galectin-3, -8, and -9 impairs immunity to chronic Mycobacterium tuberculosis infection but not acute infection with multiple intracellular pathogens

Macrophages employ an array of pattern recognition receptors to detect and eliminate intracellular pathogens that access the cytosol. The cytosolic carbohydrate sensors Galectin-3, -8, and -9 (Gal-3, Gal-8, and Gal-9) recognize damaged pathogen-containing phagosomes, and Gal-3 and Gal-8 are reported to restrict bacterial growth via autophagy in cultured cells. However, the contribution of these galectins to host resistance during bacterial infection in vivo remains unclear. We found that Gal-9 binds directly to Mycobacterium tuberculosis (Mtb) and Salmonella enterica serovar Typhimurium (Stm) and localizes to Mtb in macrophages. To determine the combined contribution of membrane damage-sensing galectins to immunity, we generated Gal-3, -8, and -9 triple knockout (TKO) mice. Mtb infection of primary macrophages from TKO mice resulted in defective autophagic flux but normal bacterial replication. Surprisingly, these mice had no discernable defect in resistance to acute infection with Mtb, Stm or Listeria monocytogenes, and had only modest impairments in bacterial growth restriction and CD4 T cell activation during chronic Mtb infection. Collectively, these findings indicate that while Gal-3, -8, and -9 respond to an array of intracellular pathogens, together these membrane damage-sensing galectins play a limited role in host resistance to bacterial infection.

The interrelation of galectins and autophagy

Autophagy is a vital physiological process that maintains intracellular homeostasis by removing damaged organelles and senescent or misfolded molecules. However, excessive autophagy results in cell death and apoptosis, which will lead to a variety of diseases. Galectins are a type of animal lectin that binds to β-galactosides and can bind to the cell surface or extracellular matrix glycans, affecting a variety of immune processes in vivo and being linked to the development of many diseases. In many cases, galectins and autophagy both play important regulatory roles in the cellular life course, yet our understanding of the relationship between them is still incomplete. Galectins and autophagy may share common etiological cofactors for some diseases. Hence, we summarize the relationship between galectins and autophagy, aiming to draw attention to the existence of multiple associations between galectins and autophagy in a variety of physiological and pathological processes, which provide new ideas for etiological diagnosis, drug development, and therapeutic targets for related diseases.

Autophagy restricts Mycobacterium tuberculosis during acute infection in mice

Whether or not autophagy has a role in defence against Mycobacterium tuberculosis infection remains unresolved. Previously, conditional knockdown of the core autophagy component ATG5 in myeloid cells was reported to confer extreme susceptibility to M. tuberculosis in mice, whereas depletion of other autophagy factors had no effect on infection. We show that doubling cre gene dosage to more robustly deplete ATG16L1 or ATG7 resulted in increased M. tuberculosis growth and host susceptibility in mice, although ATG5-depleted mice are more sensitive than ATG16L1- or ATG7-depleted mice. We imaged individual macrophages infected with M. tuberculosis and identified a shift from apoptosis to rapid necrosis in autophagy-depleted cells. This effect was dependent on phagosome permeabilization by M. tuberculosis. We monitored infected cells by electron microscopy, showing that autophagy protects the host macrophage by partially reducing mycobacterial access to the cytosol. We conclude that autophagy has an important role in defence against M. tuberculosis in mammals.

SARS-CoV-2 Mpro Protease Variants of Concern Display Altered Viral Substrate and Cell Host Target Galectin-8 Processing but Retain Sensitivity toward Antivirals

The main protease of SARS-CoV-2 (M^pro) is the most promising drug target against coronaviruses due to its essential role in virus replication. With newly emerging variants there is a concern that mutations in M^pro may alter the structural and functional properties of protease and subsequently the potency of existing and potential antivirals. We explored the effect of 31 mutations belonging to 5 variants of concern (VOCs) on catalytic parameters and substrate specificity, which revealed changes in substrate binding and the rate of cleavage of a viral peptide. Crystal structures of 11 M^pro mutants provided structural insight into their altered functionality. Additionally, we show M^pro mutations influence proteolysis of an immunomodulatory host protein Galectin-8 (Gal-8) and a subsequent significant decrease in cytokine secretion, providing evidence for alterations in the escape of host-antiviral mechanisms. Accordingly, mutations associated with the Gamma VOC and highly virulent Delta VOC resulted in a significant increase in Gal-8 cleavage. Importantly, IC50s of nirmatrelvir (Pfizer) and our irreversible inhibitor AVI-8053 demonstrated no changes in potency for both drugs for all mutants, suggesting M^pro will remain a high-priority antiviral drug candidate as SARS-CoV-2 evolves.

The IRE1α-XBP1 Signaling Axis Promotes Glycolytic Reprogramming in Response to Inflammatory Stimuli

Immune cells must be able to adjust their metabolic programs to effectively carry out their effector functions. Here, we show that the endoplasmic reticulum (ER) stress sensor Inositol-requiring enzyme 1 alpha (IRE1α) and its downstream transcription factor X box binding protein 1 (XBP1) enhance the upregulation of glycolysis in classically activated macrophages (CAMs). The IRE1α-XBP1 signaling axis supports this glycolytic switch in macrophages when activated by lipopolysaccharide (LPS) stimulation or infection with the intracellular bacterial pathogen Brucella abortus. Importantly, these different inflammatory stimuli have distinct mechanisms of IRE1α activation; while Toll-like receptor 4 (TLR4) supports glycolysis under both conditions, TLR4 is required for activation of IRE1α in response to LPS treatment but not B. abortus infection. Though IRE1α and XBP1 are necessary for maximal induction of glycolysis in CAMs, activation of this pathway is not sufficient to increase the glycolytic rate of macrophages, indicating that the cellular context in which this pathway is activated ultimately dictates the cell's metabolic response and that IRE1α activation may be a way to fine-tune metabolic reprogramming. IMPORTANCE The immune system must be able to tailor its response to different types of pathogens in order to eliminate them and protect the host. When confronted with bacterial pathogens, macrophages, frontline defenders in the immune system, switch to a glycolysis-driven metabolism to carry out their antibacterial functions. Here, we show that IRE1α, a sensor of ER stress, and its downstream transcription factor XBP1 support glycolysis in macrophages during infection with Brucella abortus or challenge with Salmonella LPS. Interestingly, these stimuli activate IRE1α by independent mechanisms. While the IRE1α-XBP1 signaling axis promotes the glycolytic switch, activation of this pathway is not sufficient to increase glycolysis in macrophages. This study furthers our understanding of the pathways that drive macrophage immunometabolism and highlights a new role for IRE1α and XBP1 in innate immunity.

Galectins and galectin-mediated autophagy regulation: new insights into targeted cancer therapy

Galectins are animal lectins with specific affinity for galactosides via the conserved carbohydrate recognition domains. Increasing studies recently have identified critical roles of galectin family members in tumor progression. Abnormal expression of galectins contributes to the proliferation, metastasis, epithelial-mesenchymal transformation (EMT), immunosuppression, radio-resistance and chemoresistance in various cancers, which has attracted cumulative clinical interest in galectin-based cancer treatment. Galectin family members have been reported to participate in autophagy regulation under physiological conditions and in non-tumoral diseases, and implication of galectins in multiple processes of carcinogenesis also involves regulation of autophagy, however, the relationship between galectins, autophagy and cancer remains largely unclear. In this review, we introduce the structure and function of galectins at the molecular level, summarize their engagements in autophagy and cancer progression, and also highlight the regulation of autophagy by galectins in cancer as well as the therapeutic potentials of galectin and autophagy-based strategies. Elaborating on the mechanism of galectin-regulated autophagy in cancers will accelerate the exploitation of galectins-autophagy targeted therapies in treatment for cancer.

Galectin-8 involves in arthritic condylar bone loss via podoplanin/AKT/ERK axis-mediated inflammatory lymphangiogenesis

OBJECTIVE

The lymphatic system plays a crucial role in the maintenance of tissue fluid homeostasis and the immunological response to inflammation. Galectin-8 (Gal-8) regulates pathological lymphangiogenesis but the effects of which on inflammation-related condylar bone loss in temporomandibular joint (TMJ) have not been well studied.

DESIGN

We used TNFα-transgenic (TNF^TG) mice and their wildtype (WT) littermates to compare their inflammatory phenotype in TMJs. Next, lymphatic endothelial cells (LECs) were used to examine the effects of which on osteoclast formation, pro-inflammatory factor expression, and inflammatory lymphangiogenesis with or without thiodigalactoside (TDG, a Gal-8 inhibitor) treatment. At last, two murine models (TNF^TG arthritic model and forced mouth opening model) were used to explore TDG as a potential drug for the treatment of inflammation-related condylar bone loss.

RESULTS

In comparison to WT mice, lymphatic areas of lymphatic vessel endothelial receptor 1 (LYVE1)⁺/podoplanin (PDPN)⁺ and Gal-8⁺/PDPN⁺, TRAP-positive osteoclast number, and condylar bone loss are increased in TNF^TG mice. Inhibition of Gal-8 in LECs by TDG, reduces TNFα-induced osteoclast formation, pro-inflammatory factor expression, and inflammatory lymphangiogenesis. In addition, Gal-8 promotes TNFα-activated AKT/ERK/NF-κB pathways by binding to PDPN. Finally, the administration of TDG attenuates inflammatory lymphangiogenesis, inhibits osteoclast activity, and reduces condylar bone loss in TNF^TG arthritic mice and forced mouth opening mice.

CONCLUSIONS

Our findings reveal the important role of Gal-8-promoted pathological lymphangiogenesis in inflammation-related condylar bone loss.

The Blessed Union of Glycobiology and Immunology: A Marriage That Worked

In this article, we discuss the main aspects regarding the recognition of cell surface glycoconjugates and the immunomodulation of responses against the progression of certain pathologies, such as cancer and infectious diseases. In the first part, we talk about different aspects of glycoconjugates and delve deeper into the importance of N-glycans in cancer immunotherapy. Then, we describe two important lectin families that have been very well studied in the last 20 years. Examples include the sialic acid-binding immunoglobulin (Ig)-like lectins (siglecs), and galectins. Finally, we discuss a topic that needs to be better addressed in the field of glycoimmunology: the impact of oncofetal antigens on the cells of the immune system. New findings in this area are of great importance for advancement, especially in the field of oncology, since it is already known that cellular interactions mediated by carbohydrate-carbohydrate and/or carbohydrate proteins are able to modulate the progression of different types of cancer in events that compromise the functionality of the immune responses.

The role of galectins in immunity and infection

The galectin family consists of carbohydrate (glycan) binding proteins that are expressed by a wide variety of cells and bind to galactose-containing glycans. Galectins can be located in the nucleus or the cytoplasm, or can be secreted into the extracellular space. They can modulate innate and adaptive immune cells by binding to glycans on the surface of immune cells or intracellularly via carbohydrate-dependent or carbohydrate-independent interactions. Galectins expressed by immune cells can also participate in host responses to infection by directly binding to microorganisms or by modulating antimicrobial functions such as autophagy. Here we explore the diverse ways in which galectins have been shown to impact immunity and discuss the opportunities and challenges in the field.

Multifaceted roles of TAX1BP1 in autophagy

TAX1BP1 is a selective macroautophagy/autophagy receptor that plays a central role in host defense to pathogens and in regulating the innate immune system. TAX1BP1 facilitates the xenophagic clearance of pathogenic bacteria such as Salmonella typhimurium and Mycobacterium tuberculosis and regulates TLR3 (toll-like receptor 3)-TLR4 and DDX58/RIG-I-like receptor (RLR) signaling by targeting TICAM1 and MAVS for autophagic degradation respectively. In addition to these canonical autophagy receptor functions, TAX1BP1 can also exert multiple accessory functions that influence the biogenesis and maturation of autophagosomes. In this review, we will discuss and integrate recent findings related to the autophagy function of TAX1BP1 and highlight outstanding questions regarding its functions in autophagy and regulation of innate immunity and host defense.Abbreviations: ATG: autophagy related; CALCOCO: calcium binding and coiled-coil domain; CC: coiled-coil; CHUK/IKKα: conserved helix-loop-helix ubiquitous kinase; CLIR: noncanonical LC3-interacting region; GABARAP: gamma-aminobutyric acid receptor associated protein; HTLV-1: human T-lymphotropic virus 1; IFN: interferon; IL1B/IL1β: interleukin 1 beta; LIR: LC3-interacting region; LPS: lipopolysaccharide; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MAPK/JNK: mitogen-activated protein kinase; mATG8: mammalian Atg8 homolog; MAVS: mitochondrial antiviral signaling protein; MEF: mouse embryonic fibroblast; MTB: Mycobacterium tuberculosis; MYD88: myeloid differentiation primary response gene 88; NBR1: NBR1, autophagy cargo receptor; NFKB/NF-κB: nuclear factor of kappa light polypeptide gene enhancer in B cells; OPTN: optineurin; Poly(I:C): polyinosinic:polycytidylic acid; PTM: post-translational modification; RB1CC1: RB1-inducible coiled-coil 1; RIPK: receptor (TNFRSF)-interacting serine-threonine kinase; RLR: DDX58/RIG-I-like receptor; RSV: respiratory syncytia virus; SKICH: SKIP carboxyl homology; SLR: SQSTM1 like receptor; SQSTM1: sequestosome 1; TAX1BP1: Tax1 (human T cell leukemia virus type I) binding protein 1; TBK1: TANK-binding kinase 1; TICAM1: toll-like receptor adaptor molecule 1; TLR: toll-like receptor; TNF: tumor necrosis factor; TNFAIP3: TNF alpha induced protein 3; TNFR: tumor necrosis factor receptor; TOM1: target of myb1 trafficking protein; TRAF: TNF receptor-associated factor; TRIM32: tripartite motif-containing 32; UBD: ubiquitin binding domain; ZF: zinc finger.

Immune evasion and provocation by Mycobacterium tuberculosis

Mycobacterium tuberculosis, the causative agent of tuberculosis, has infected humans for millennia. M. tuberculosis is well adapted to establish infection, persist in the face of the host immune response and be transmitted to uninfected individuals. Its ability to complete this infection cycle depends on it both evading and taking advantage of host immune responses. The outcome of M. tuberculosis infection is often a state of equilibrium characterized by immunological control and bacterial persistence. Recent data have highlighted the diverse cell populations that respond to M. tuberculosis infection and the dynamic changes in the cellular and intracellular niches of M. tuberculosis during the course of infection. M. tuberculosis possesses an arsenal of protein and lipid effectors that influence macrophage functions and inflammatory responses; however, our understanding of the role that specific bacterial virulence factors play in the context of diverse cellular reservoirs and distinct infection stages is limited. In this Review, we discuss immune evasion and provocation by M. tuberculosis during its infection cycle and describe how a more detailed molecular understanding is crucial to enable the development of novel host-directed therapies, disease biomarkers and effective vaccines.

SRSF6 balances mitochondrial-driven innate immune outcomes through alternative splicing of BAX

To mount a protective response to infection while preventing hyperinflammation, gene expression in innate immune cells must be tightly regulated. Despite the importance of pre-mRNA splicing in shaping the proteome, its role in balancing immune outcomes remains understudied. Transcriptomic analysis of murine macrophage cell lines identified Serine/Arginine Rich Splicing factor 6 (SRSF6) as a gatekeeper of mitochondrial homeostasis. SRSF6-dependent orchestration of mitochondrial health is directed in large part by alternative splicing of the pro-apoptosis pore-forming protein BAX. Loss of SRSF6 promotes accumulation of BAX-κ, a variant that sensitizes macrophages to undergo cell death and triggers upregulation of interferon stimulated genes through cGAS sensing of cytosolic mitochondrial DNA. Upon pathogen sensing, macrophages regulate SRSF6 expression to control the liberation of immunogenic mtDNA and adjust the threshold for entry into programmed cell death. This work defines BAX alternative splicing by SRSF6 as a critical node not only in mitochondrial homeostasis but also in the macrophage's response to pathogens.

Mycobacterium tuberculosis Rv0790c inhibits the cellular autophagy at its early stage and facilitates mycobacterial survival

Rv0790c is predicted to be a conserved hypothetical protein encoded by Mycobacterium tuberculosis (Mtb). However, its function in Mtb infection remains largely unknown. In this study, we found that Rv0790c promoted bacillary survival of M. smegmatis (Ms), both in vitro and in vivo. The bacillary burden of Ms exogenously expressing Rv0790c increased, whereas in Rv0790c-knockouts the bacillary burden decreased in infected macrophages. Multiple cellular processes were analyzed to explore the underlying mechanisms. We found that neither inflammatory regulation nor apoptotic induction were responsible for the promotion of bacillary survival mediated by Rv0790c. Interestingly, we found that Rv0790c facilitates mycobacterial survival through cellular autophagy at its early stage. Immunoprecipitation assay of autophagy initiation-related proteins indicated that Rv0790c interacted with mTOR and enhanced its activity, as evidenced by the increased phosphorylation level of mTOR downstream substrates, ULK-1, at Ser757 and P70S6K, at Thr389. Our study uncovers a novel autophagy suppressor encoded by mycobacterial Rv0790c, which inhibits the early stage of cellular autophagy induction upon Mtb infection and takes an important role in maintaining intracellular mycobacterial survival. It may aid in understanding the mechanism of Mtb evasion of host cellular degradation, as well as hold the potential to develop new targets for the prevention and treatment of tuberculosis.

A patatin-like phospholipase mediates Rickettsia parkeri escape from host membranes

Rickettsia species of the spotted fever group are arthropod-borne obligate intracellular bacteria that can cause mild to severe human disease. These bacteria invade host cells, replicate in the cell cytosol, and spread from cell to cell. To access the host cytosol and avoid immune detection, they escape membrane-bound vacuoles by expressing factors that disrupt host membranes. Here, we show that a patatin-like phospholipase A2 enzyme (Pat1) facilitates Rickettsia parkeri infection by promoting escape from host membranes and cell-cell spread. Pat1 is important for infection in a mouse model and, at the cellular level, is crucial for efficiently escaping from single and double membrane-bound vacuoles into the host cytosol, and for avoiding host galectins that mark damaged membranes. Pat1 is also important for avoiding host polyubiquitin, preventing recruitment of autophagy receptor p62, and promoting actin-based motility and cell-cell spread.

Pathogenic Rickettsia species are arthropod-borne, obligate intracellular bacteria that invade host cells, replicate in the cell cytosol, and spread from cell to cell. Here, Borgo et al. identify a Rickettsia phospholipase enzyme that is important for infection by helping the bacteria escape from host cell vacuoles into the host cytosol, preventing targeting by autophagy, and promoting bacterial motility and spread to other cells.

Mechanisms underlying ubiquitin-driven selective mitochondrial and bacterial autophagy

Selective autophagy specifically eliminates damaged or superfluous organelles, maintaining cellular health. In this process, a double membrane structure termed an autophagosome captures target organelles or proteins and delivers this cargo to the lysosome for degradation. The attachment of the small protein ubiquitin to cargo has emerged as a common mechanism for initiating organelle or protein capture by the autophagy machinery. In this process, a suite of ubiquitin-binding cargo receptors function to initiate autophagosome assembly in situ on the target cargo, thereby providing selectivity in cargo capture. Here, we review recent efforts to understand the biochemical mechanisms and principles by which cargo are marked with ubiquitin and how ubiquitin-binding cargo receptors use conserved structural modules to recruit the autophagosome initiation machinery, with a particular focus on mitochondria and intracellular bacteria as cargo. These emerging mechanisms provide answers to long-standing questions in the field concerning how selectivity in cargo degradation is achieved.

When the Phagosome Gets Leaky: Pore-Forming Toxin-Induced Non-Canonical Autophagy (PINCA)

Macrophages remove bacteria from the extracellular milieu via phagocytosis. While most of the engulfed bacteria are degraded in the antimicrobial environment of the phagolysosome, several bacterial pathogens have evolved virulence factors, which evade degradation or allow escape into the cytosol. To counter this situation, macrophages activate LC3-associated phagocytosis (LAP), a highly bactericidal non-canonical autophagy pathway, which destroys the bacterial pathogens in so called LAPosomes. Moreover, macrophages can also target intracellular bacteria by pore-forming toxin-induced non-canonical autophagy (PINCA), a recently described non-canonical autophagy pathway, which is activated by phagosomal damage induced by bacteria-derived pore-forming toxins. Similar to LAP, PINCA involves LC3 recruitment to the bacteria-containing phagosome independently of the ULK complex, but in contrast to LAP, this process does not require ROS production by Nox2. As last resort of autophagic targeting, macrophages activate xenophagy, a selective form of macroautophagy, to recapture bacteria, which evaded successful targeting by LAP or PINCA through rupture of the phagosome. However, xenophagy can also be hijacked by bacterial pathogens for their benefit or can be completely inhibited resulting in intracellular growth of the bacterial pathogen. In this perspective, we discuss the molecular differences and similarities between LAP, PINCA and xenophagy in macrophages during bacterial infections.

The molecular regulation of autophagy in antimicrobial immunity

Autophagy is a catabolic process that can degrade worn-out organelles and invading pathogens. The activation of autophagy regulates innate and adaptive immunity, playing a key role in the response to microbial invasion. Microbial infection may cause different consequences such as the elimination of invaders through autophagy or xenophagy, host cell death, and symbiotic relationships. Pathogens adapt to the autophagy mechanism and further relieve intracellular stress, which is conducive to host cell survival and microbial growth. The regulation of autophagy forms a complex network through which host immunity is modulated, resulting in a variety of pathophysiological manifestations. Modification of the autophagic pathway is an essential target for the development of antimicrobial drugs.

Maintenance and loss of endocytic organelle integrity: mechanisms and implications for antigen cross-presentation

The membranes of endosomes, phagosomes and macropinosomes can become damaged by the physical properties of internalized cargo, by active pathogenic invasion or by cellular processes, including endocytic maturation. Loss of membrane integrity is often deleterious and is, therefore, prevented by mitigation and repair mechanisms. However, it can occasionally be beneficial and actively induced by cells. Here, we summarize the mechanisms by which cells, in particular phagocytes, try to prevent membrane damage and how, when this fails, they repair or destroy damaged endocytic organelles. We also detail how one type of phagocyte, the dendritic cell, can deliberately trigger localized damage to endocytic organelles to allow for major histocompatibility complex class I presentation of exogenous antigens and initiation of CD8⁺ T-cell responses to viruses and tumours. Our review highlights mechanisms for the regulation of endocytic organelle membrane integrity at the intersection of cell biology and immunology that could be co-opted for improving vaccination and intracellular drug delivery.

Quantitative proteomics reveals the selectivity of ubiquitin-binding autophagy receptors in the turnover of damaged lysosomes by lysophagy

Removal of damaged organelles via the process of selective autophagy constitutes a major form of cellular quality control. Damaged organelles are recognized by a dedicated surveillance machinery, leading to the assembly of an autophagosome around the damaged organelle, prior to fusion with the degradative lysosomal compartment. Lysosomes themselves are also prone to damage and are degraded through the process of lysophagy. While early steps involve recognition of ruptured lysosomal membranes by glycan-binding galectins and ubiquitylation of transmembrane lysosomal proteins, many steps in the process, and their interrelationships, remain poorly understood, including the role and identity of cargo receptors required for completion of lysophagy. Here, we employ quantitative organelle capture and proximity biotinylation proteomics of autophagy adaptors, cargo receptors, and galectins in response to acute lysosomal damage, thereby revealing the landscape of lysosome-associated proteome remodeling during lysophagy. Among the proteins dynamically recruited to damaged lysosomes were ubiquitin-binding autophagic cargo receptors. Using newly developed lysophagic flux reporters including Lyso-Keima, we demonstrate that TAX1BP1, together with its associated kinase TBK1, are both necessary and sufficient to promote lysophagic flux in both HeLa cells and induced neurons (iNeurons). While the related receptor Optineurin (OPTN) can drive damage-dependent lysophagy when overexpressed, cells lacking either OPTN or CALCOCO2 still maintain significant lysophagic flux in HeLa cells. Mechanistically, TAX1BP1-driven lysophagy requires its N-terminal SKICH domain, which binds both TBK1 and the autophagy regulatory factor RB1CC1, and requires upstream ubiquitylation events for efficient recruitment and lysophagic flux. These results identify TAX1BP1 as a central component in the lysophagy pathway and provide a proteomic resource for future studies of the lysophagy process.

What is the Sugar Code?

A code is defined by the nature of the symbols, which are used to generate information‐storing combinations (e. g. oligo‐ and polymers). Like nucleic acids and proteins, oligo‐ and polysaccharides are ubiquitous, and they are a biochemical platform for establishing molecular messages. Of note, the letters of the sugar code system (third alphabet of life) excel in coding capacity by making an unsurpassed versatility for isomer (code word) formation possible by variability in anomery and linkage position of the glycosidic bond, ring size and branching. The enzymatic machinery for glycan biosynthesis (writers) realizes this enormous potential for building a large vocabulary. It includes possibilities for dynamic editing/erasing as known from nucleic acids and proteins. Matching the glycome diversity, a large panel of sugar receptors (lectins) has developed based on more than a dozen folds. Lectins ‘read’ the glycan‐encoded information. Hydrogen/coordination bonding and ionic pairing together with stacking and C−H/π‐interactions as well as modes of spatial glycan presentation underlie the selectivity and specificity of glycan‐lectin recognition. Modular design of lectins together with glycan display and the nature of the cognate glycoconjugate account for the large number of post‐binding events. They give an entry to the glycan vocabulary its functional, often context‐dependent meaning(s), hereby building the dictionary of the sugar code.

The opportunistic intracellular bacterial pathogen Rhodococcus equi elicits type I interferon by engaging cytosolic DNA sensing in macrophages

Rhodococcus equi is a major cause of foal pneumonia and an opportunistic pathogen in immunocompromised humans. While alveolar macrophages constitute the primary replicative niche for R. equi, little is known about how intracellular R. equi is sensed by macrophages. Here, we discovered that in addition to previously characterized pro-inflammatory cytokines (e.g., Tnfa, Il6, Il1b), macrophages infected with R. equi induce a robust type I IFN response, including Ifnb and interferon-stimulated genes (ISGs), similar to the evolutionarily related pathogen, Mycobacterium tuberculosis. Follow up studies using a combination of mammalian and bacterial genetics demonstrated that induction of this type I IFN expression program is largely dependent on the cGAS/STING/TBK1 axis of the cytosolic DNA sensing pathway, suggesting that R. equi perturbs the phagosomal membrane and causes DNA release into the cytosol following phagocytosis. Consistent with this, we found that a population of ~12% of R. equi phagosomes recruits the galectin-3,-8 and -9 danger receptors. Interestingly, neither phagosomal damage nor induction of type I IFN require the R. equi’s virulence-associated plasmid. Importantly, R. equi infection of both mice and foals stimulates ISG expression, in organs (mice) and circulating monocytes (foals). By demonstrating that R. equi activates cytosolic DNA sensing in macrophages and elicits type I IFN responses in animal models, our work provides novel insights into how R. equi engages the innate immune system and furthers our understanding how this zoonotic pathogen causes inflammation and disease.

Rhodococcus equi is a facultative intracellular bacterial pathogen of horses and other domestic animals, as well as an opportunistic pathogen of humans. In human patients, Rhodococcus pneumonia bears some pathological similarities to pulmonary tuberculosis, and poses a risk for misdiagnosis. In horses, R. equi infection has a major detrimental impact on the equine breeding industry due to a lack of an efficacious vaccine and its ubiquitous distribution in soil. Given the prevalence of subclinical infection and high false positive rate in current screening methods, there exists a critical need to identify factors contributing to host susceptibility. Here, we use a combination of bacterial genetics and animal models to investigate innate immune responses during R. equi infection. We found that R. equi modulates host immune sensing to elicit a type I interferon response in a manner resembling that of M. tuberculosis. We also found that the danger sensors galectin-3, -8, and -9 are recruited to a population of R. equi-containing vacuoles, independent of expression of VapA. Our research identifies innate immune sensing events and immune transcriptional signatures that may lead to biomarkers for clinical disease, more accurate screening methods, and insight into susceptibility to infection.