Examination of the shared genetic architecture between multiple sclerosis and systemic lupus erythematosus facilitates discovery of novel lupus risk loci.

Systemic Lupus Erythematosus (SLE) is an autoimmune disease with heterogeneous manifestations, including neurological and psychiatric symptoms. Genetic association studies in SLE have been hampered by insufficient sample size and limited power compared to many other diseases. Multiple Sclerosis (MS) is a chronic relapsing autoimmune disease of the central nervous system (CNS) that also manifests neurological and immunological features. Here, we identify a method of leveraging large-scale genome wide association studies (GWAS) in MS to identify novel genetic risk loci in SLE. Statistical genetic comparison methods including linkage disequilibrium score regression (LDSC) and cross-phenotype association analysis (CPASSOC) to identify genetic overlap in disease pathophysiology, traditional 2-sample and novel PPI-based mendelian randomization to identify causal associations and Bayesian colocalization were applied to association studies conducted in MS to facilitate discovery in the smaller, more limited datasets available for SLE. Pathway analysis using SNP-to-gene mapping identified biological networks composed of molecular pathways with causal implications for CNS disease in SLE specifically, as well as pathways likely causal of both pathologies, providing key insights for therapeutic selection.

Analysis of Trans-Ancestral SLE Risk Loci Identifies Unique Biologic Networks and Drug Targets in African and European Ancestries.

Systemic lupus erythematosus (SLE) is a multi-organ autoimmune disorder with a prominent genetic component. Individuals of African ancestry (AA) experience the disease more severely and with an increased co-morbidity burden compared to European ancestry (EA) populations. We hypothesize that the disparities in disease prevalence, activity, and response to standard medications between AA and EA populations is partially conferred by genomic influences on biological pathways. To address this, we applied a comprehensive approach to identify all genes predicted from SNP-associated risk loci detected with the Immunochip. By combining genes predicted via eQTL analysis, as well as those predicted from base-pair changes in intergenic enhancer sites, coding-region variants, and SNP-gene proximity, we were able to identify 1,731 potential ancestry-specific and trans-ancestry genetic drivers of SLE. Gene associations were linked to upstream and downstream regulators using connectivity mapping, and predicted biological pathways were mined for candidate drug targets. Examination of trans-ancestral pathways reflect the well-defined role for interferons in SLE and revealed pathways associated with tissue repair and remodeling. EA-dominant genetic drivers were more often associated with innate immune and myeloid cell function pathways, whereas AA-dominant pathways mirror clinical findings in AA subjects, suggesting disease progression is driven by aberrant B cell activity accompanied by ER stress and metabolic dysfunction. Finally, potential ancestry-specific and non-specific drug candidates were identified. The integration of all SLE SNP-predicted genes into functional pathways revealed critical molecular pathways representative of each population, underscoring the influence of ancestry on disease mechanism and also providing key insight for therapeutic selection.

Deconvoluting the heterogeneity of SLE: The contribution of ancestry.

Systemic lupus erythematosus (SLE) is a multiorgan autoimmune disorder with a prominent genetic component. Evidence has shown that individuals of non-European ancestry experience the disease more severely, exhibiting an increased incidence of cardiovascular disease, renal involvement, and tissue damage compared with European ancestry populations. Furthermore, there seems to be variability in the response of individuals within different ancestral groups to standard medications, including cyclophosphamide, mycophenolate, rituximab, and belimumab. Although the widespread application of candidate gene, Immunochip, and genome-wide association studies has contributed to our understanding of the link between genetic variation (typically single nucleotide polymorphisms) and SLE, despite decades of research it is still unclear why ancestry remains a key determinant of poorer outcome in non-European-ancestry patients with SLE. Here, we will discuss the impact of ancestry on SLE disease burden in patients from diverse backgrounds and highlight how research efforts using novel bioinformatic and pathway-based approaches have begun to disentangle the complex genetic architecture linking ancestry to SLE susceptibility. Finally, we will illustrate how genomic and gene expression analyses can be combined to help identify novel molecular pathways and drug candidates that might uniquely impact SLE among different ancestral populations.

Salmonella Typhimurium manipulates macrophage cholesterol homeostasis through the SseJ-mediated suppression of the host cholesterol transport protein ABCA1.

Upon infection of host cells, Salmonella enterica serovar Typhimurium resides in a modified-endosomal compartment referred to as the Salmonella-containing vacuole (SCV). SCV biogenesis is driven by multiple effector proteins translocated through two type III secretion systems (T3SS-1 and T3SS-2). While many host proteins targeted by these effector proteins have been characterised, the role of host lipids in SCV dynamics remains poorly understood. Previous studies have shown that S. Typhimurium infection in macrophages leads to accumulation of intracellular cholesterol, some of which concentrates in and around SCVs; however, the underlying mechanisms remain unknown. Here, we show that S. Typhimurium utilises the T3SS-2 effector SseJ to downregulate expression of the host cholesterol transporter ABCA1 in macrophages, leading to a ~45% increase in cellular cholesterol. Mechanistically, SseJ activates a signalling cascade involving the host kinases FAK and Akt to suppress Abca1 expression. Mutational inactivation of SseJ acyltransferase activity, silencing FAK, or inhibiting Akt prevents Abca1 downregulation and the corresponding accumulation of cholesterol during infection. Importantly, RNAi-mediated silencing of ABCA1 rescued bacterial survival in FAK-deficient macrophages, suggesting that Abca1 downregulation and cholesterol accumulation are important for intracellular survival.

The pathogenesis of systemic lupus erythematosus: Harnessing big data to understand the molecular basis of lupus.

Systemic lupus erythematosus (SLE) is a chronic, systemic autoimmune disease that causes damage to multiple organ systems. Despite decades of research and available murine models that capture some aspects of the human disease, new treatments for SLE lag behind other autoimmune diseases such as Rheumatoid Arthritis and Crohn's disease. Big data genomic assays have transformed our understanding of SLE by providing important insights into the molecular heterogeneity of this multigenic disease. Gene wide association studies have demonstrated more than 100 risk loci, supporting a model of multiple genetic hits increasing SLE risk in a non-linear fashion, and providing evidence of ancestral diversity in susceptibility loci. Epigenetic studies to determine the role of methylation, acetylation and non-coding RNAs have provided new understanding of the modulation of gene expression in SLE patients and identified new drug targets and biomarkers for SLE. Gene expression profiling has led to a greater understanding of the role of myeloid cells in the pathogenesis of SLE, confirmed roles for T and B cells in SLE, promoted clinical trials based on the prominent interferon signature found in SLE patients, and identified candidate biomarkers and cellular signatures to further drug development and drug repurposing. Gene expression studies are advancing our understanding of the underlying molecular heterogeneity in SLE and providing hope that patient stratification will expedite new therapies based on personal molecular signatures. Although big data analyses present unique interpretation challenges, both computationally and biologically, advances in machine learning applications may facilitate the ability to predict changes in SLE disease activity and optimize therapeutic strategies.

Activation of focal adhesion kinase by Salmonella suppresses autophagy via an Akt/mTOR signaling pathway and promotes bacterial survival in macrophages.

Autophagy has emerged as an important antimicrobial host defense mechanism that not only orchestrates the systemic immune response, but also functions in a cell autonomous manner to directly eliminate invading pathogens. Pathogenic bacteria such as Salmonella have evolved adaptations to protect themselves from autophagic elimination. Here we show that signaling through the non-receptor tyrosine kinase focal adhesion kinase (FAK) is actively manipulated by the Salmonella SPI-2 system in macrophages to promote intracellular survival. In wild-type macrophages, FAK is recruited to the surface of the Salmonella-containing vacuole (SCV), leading to amplified signaling through the Akt-mTOR axis and inhibition of the autophagic response. In FAK-deficient macrophages, Akt/mTOR signaling is attenuated and autophagic capture of intracellular bacteria is enhanced, resulting in reduced bacterial survival. We further demonstrate that enhanced autophagy in FAK(-/-) macrophages requires the activity of Atg5 and ULK1 in a process that is distinct from LC3-assisted phagocytosis (LAP). In vivo, selective knockout of FAK in macrophages resulted in more rapid clearance of bacteria from tissues after oral infection with S. typhimurium. Clearance was correlated with reduced infiltration of inflammatory cell types into infected tissues and reduced tissue damage. Together, these data demonstrate that FAK is specifically targeted by S. typhimurium as a novel means of suppressing autophagy in macrophages, thereby enhancing their intracellular survival.

Brain angiogenesis inhibitor 1 (BAI1) is a pattern recognition receptor that mediates macrophage binding and engulfment of Gram-negative bacteria.

Bacterial recognition by host cells is essential for initiation of infection and the host response. Bacteria interact with host cells via multiple pattern recognition receptors that recognize microbial products or pathogen-associated molecular patterns. In response to this interaction, host cell signaling cascades are activated that lead to inflammatory responses and/or phagocytic clearance of attached bacteria. Brain angiogenesis inhibitor 1 (BAI1) is a receptor that recognizes apoptotic cells through its conserved type I thrombospondin repeats and triggers their engulfment through an ELMO1/Dock/Rac1 signaling module. Because thrombospondin repeats in other proteins have been shown to bind bacterial surface components, we hypothesized that BAI1 may also mediate the recognition and clearance of pathogenic bacteria. We found that preincubation of bacteria with recombinant soluble BAI1 ectodomain or knockdown of endogenous BAI1 in primary macrophages significantly reduced binding and internalization of the Gram-negative pathogen Salmonella typhimurium. Conversely, overexpression of BAI1 enhanced attachment and engulfment of Salmonella in macrophages and in heterologous nonphagocytic cells. Bacterial uptake is triggered by the BAI1-mediated activation of Rac through an ELMO/Dock-dependent mechanism, and inhibition of the BAI1/ELMO1 interaction prevents both Rac activation and bacterial uptake. Moreover, inhibition of ELMO1 or Rac function significantly impairs the proinflammatory response to infection. Finally, we show that BAI1 interacts with a variety of Gram-negative, but not Gram-positive, bacteria through recognition of their surface lipopolysaccharide. Together these findings identify BAI1 as a pattern recognition receptor that mediates nonopsonic phagocytosis of Gram-negative bacteria by macrophages and directly affects the host response to infection.

Differential regulation of the interferon response in systemic lupus erythematosus distinguishes patients of Asian ancestry.

OBJECTIVES: Type I interferon (IFN) plays a role in the pathogenesis of systemic lupus erythematosus (SLE), but insufficient attention has been directed to the differences in IFN responses between ancestral populations. Here, we explored the expression of the interferon gene signatures (IGSs) in SLE patients of European ancestry (EA) and Asian ancestry (AsA). METHODS: We used gene set variation analysis with multiple IGS encompassing the response to both type 1 and type 2 IFN in isolated CD14+ monocytes, CD19+B cells, CD4+T cells and Natural Killer (NK) cells from patients with SLE stratified by self-identified ancestry. The expression of genes upstream of the IGS and influenced by lupus-associated risk alleles was also examined. Lastly, we employed machine learning (ML) models to assess the most important features classifying patients by disease activity. RESULTS: AsA patients with SLE exhibited greater enrichment in the IFN core and IFNA2 IGS compared with EA patients in all cell types examined and, in the presence and absence of autoantibodies. Overall, AsA patients with SLE demonstrated higher expression of genes upstream of the IGS than EA counterparts. ML with feature importance analysis indicated that IGS expression in NK cells, anti-dsDNA, complement levels and AsA status contributed to disease activity. CONCLUSIONS: AsA patients with SLE exhibited higher IGS than EA patients in all cell types regardless of autoantibody status, with enhanced expression of genetically associated genes upstream of the IGS potentially contributing. AsA, along with the IGS in NK cells, anti-dsDNA and complement, independently influenced SLE disease activity.

Molecular pathways identified from single nucleotide polymorphisms demonstrate mechanistic differences in systemic lupus erythematosus patients of Asian and European ancestry.

Systemic lupus erythematosus (SLE) is a multi-organ autoimmune disorder with a prominent genetic component. Individuals of Asian-Ancestry (AsA) disproportionately experience more severe SLE compared to individuals of European-Ancestry (EA), including increased renal involvement and tissue damage. However, the mechanisms underlying elevated severity in the AsA population remain unclear. Here, we utilized available gene expression data and genotype data based on all non-HLA SNP associations in EA and AsA SLE patients detected using the Immunochip genotyping array. We identified 2778 ancestry-specific and 327 trans-ancestry SLE-risk polymorphisms. Genetic associations were examined using connectivity mapping and gene signatures based on predicted biological pathways and were used to interrogate gene expression datasets. SLE-associated pathways in AsA patients included elevated oxidative stress, altered metabolism and mitochondrial dysfunction, whereas SLE-associated pathways in EA patients included a robust interferon response (type I and II) related to enhanced cytosolic nucleic acid sensing and signaling. An independent dataset derived from summary genome-wide association data in an AsA cohort was interrogated and identified similar molecular pathways. Finally, gene expression data from AsA SLE patients corroborated the molecular pathways predicted by SNP associations. Identifying ancestry-related molecular pathways predicted by genetic SLE risk may help to disentangle the population differences in clinical severity that impact AsA and EA individuals with SLE.

Patient-Reported Outcome Information Collected from Lupus Patients Using a Mobile Application: Compliance and Validation.

OBJECTIVE: Patient-reported outcomes (PROs) can provide critical information concerning the impact of a disease on an individual. Mobile technology to collect PRO data in an electronic format (ePRO) allows for frequent assessment in the person's regular environment. The goal of this study was to assess the compliance with a phone application (app) and validate ePRO information in individuals with systemic lupus erythematosus (SLE). METHODS: A smartphone app that collects ePRO data from various clinical instruments was developed. Information was collected by both an ePRO and a paper-administered instrument as part of a multicenter randomized interventional clinical trial of patients meeting American College of Rheumatology (ACR) criteria for the classification of SLE. To determine agreement between PRO information collected in the different formats, intraclass correlation coefficients (ICCs), paired Student's t tests, and Bland-Altman plots were evaluated. Compliance and Cronbach's alpha were also assessed as a measure of survey reliability. RESULTS: For the 62 subjects from diverse ancestral backgrounds, compliance with ePRO completion was high (more than 75%). Cronbach alpha values for PROs indicated moderate to high survey reliability. The vast majority (73.4%) of ICC values were indicative of good to excellent reliability between measurement methods. Bland-Altman plots verified method agreement, and 87% of pairwise t tests yielded an insignificant difference between information collected with the different administration methods. CONCLUSION: The excellent compliance and the high level of consistency between data collected by paper and that collected by electronic methods indicate that the app provides a reliable means of cataloging real-time changes in PROs in SLE patients.

Mendelian randomization and pathway analysis demonstrate shared genetic associations between lupus and coronary artery disease.

Coronary artery disease (CAD) is a leading cause of death in patients with systemic lupus erythematosus (SLE). Despite clinical evidence supporting an association between SLE and CAD, pleiotropy-adjusted genetic association studies are limited and focus on only a few common risk loci. Here, we identify a net positive causal estimate of SLE-associated non-HLA SNPs on CAD by traditional Mendelian randomization (MR) approaches. Pathway analysis using SNP-to-gene mapping followed by unsupervised clustering based on protein-protein interactions (PPIs) identifies biological networks composed of positive and negative causal sets of genes. In addition, we confirm the casual effects of specific SNP-to-gene modules on CAD using only SNP mapping to each PPI-defined functional gene set as instrumental variables. This PPI-based MR approach elucidates various molecular pathways with causal implications between SLE and CAD and identifies biological pathways likely causative of both pathologies, revealing known and novel therapeutic interventions for managing CAD in SLE.

Comprehensive transcriptomic analysis of COVID-19 blood, lung, and airway.

SARS-CoV2 is a previously uncharacterized coronavirus and causative agent of the COVID-19 pandemic. The host response to SARS-CoV2 has not yet been fully delineated, hampering a precise approach to therapy. To address this, we carried out a comprehensive analysis of gene expression data from the blood, lung, and airway of COVID-19 patients. Our results indicate that COVID-19 pathogenesis is driven by populations of myeloid-lineage cells with highly inflammatory but distinct transcriptional signatures in each compartment. The relative absence of cytotoxic cells in the lung suggests a model in which delayed clearance of the virus may permit exaggerated myeloid cell activation that contributes to disease pathogenesis by the production of inflammatory mediators. The gene expression profiles also identify potential therapeutic targets that could be modified with available drugs. The data suggest that transcriptomic profiling can provide an understanding of the pathogenesis of COVID-19 in individual patients.

Validation of a multicellular tumor microenvironment system for modeling patient tumor biology and drug response.

Lung cancer rates are rising globally and non-small cell lung cancer (NSCLC) has a five year survival rate of only 24%. Unfortunately, the development of drugs to treat cancer is severely hampered by the inefficiency of translating pre-clinical studies into clinical benefit. Thus, we sought to apply a tumor microenvironment system (TMES) to NSCLC. Using microvascular endothelial cells, lung cancer derived fibroblasts, and NSCLC tumor cells in the presence of in vivo tumor-derived hemodynamic flow and transport, we demonstrate that the TMES generates an in-vivo like biological state and predicts drug response to EGFR inhibitors. Transcriptomic and proteomic profiling indicate that the TMES recapitulates the in vivo and patient molecular biological state providing a mechanistic rationale for the predictive nature of the TMES. This work further validates the TMES for modeling patient tumor biology and drug response indicating utility of the TMES as a predictive tool for drug discovery and development and potential for use as a system for patient avatars.

Non-redundant functions of FAK and Pyk2 in intestinal epithelial repair.

Adhesion signaling between epithelial cells and the extracellular matrix plays a critical role in maintaining tissue homeostasis and the response to tissue damage. Focal adhesion kinase (FAK) and its close relative Pyk2 are non-receptor tyrosine kinases that mediate adhesion signaling to promote cell proliferation, motility and survival. FAK has also been shown to act as a mechanosensor by modulating cell proliferation in response to changes in tissue compliance. We previously showed that mice lacking FAK in the intestinal epithelium are phenotypically normal under homeostatic conditions but hypersensitive to experimental colitis induced by dextran sulfate sodium (DSS). Here we report that Pyk2-deficient mice are also phenotypically normal under homeostatic conditions and are similarly hypersensitive to DSS-induced colitis. These data indicate that normal intestinal development and homeostatic maintenance can occur in the presence of either FAK or Pyk2, but that both kinases are necessary for epithelial repair following injury. In contrast, mice lacking both FAK and Pyk2 develop spontaneous colitis with 100% penetrance by 4 weeks of age. Normal colonic phenotype and function are restored upon treatment of the double knockout mice with antibiotics, implicating commensal bacteria or bacterial products in the etiology of the spontaneous colitis exhibited by these mice.

Physical and functional interactions between Cas and c-Src induce tamoxifen resistance of breast cancer cells through pathways involving epidermal growth factor receptor and signal transducer and activator of transcription 5b.

High expression of the adaptor molecule Cas has been linked to resistance to the antiestrogen tamoxifen, both in tissue culture and in human tumors. The aim of this study was to elucidate the mechanism(s) by which overexpression of Cas confers resistance to tamoxifen. Cas overexpression in MCF-7 breast cancer cells was shown to alleviate both tamoxifen-mediated growth inhibition and induction of apoptosis. This enhancement of cell proliferation/survival occurred in the absence of detectable effects on estrogen receptor (ER) transcriptional activity under conditions where tamoxifen was present, indicating that Cas-dependent tamoxifen resistance is not the result of a switch to an ER-negative phenotype or enhanced responses to the partial agonist activity of tamoxifen. Instead, we present evidence, suggesting that Cas promotes tamoxifen resistance by deregulation of alternative cell proliferation pathways, particularly those mediated through enhanced c-Src protein tyrosine kinase activity arising from Cas/c-Src interactions. Overexpression of Cas was found to drive endogenous c-Src into complex with Cas, a process that has been shown previously to cause up-regulation of c-Src tyrosine kinase activity. MCF-7 cells overexpressing Cas exhibited increased phosphorylation of two c-Src substrates, Tyr845 in the kinase domain of the epidermal growth factor receptor (EGFR) and signal transducer and activator of transcription (STAT) 5b. Importantly, Cas-dependent protection from the antiproliferative effects of tamoxifen was reversed by the expression of dominant inhibitory variants of these substrates (Y845F EGFR and COOH-terminally truncated STAT5b). Based on these findings, we suggest that the Cas/c-Src/EGFR/STAT5 signaling axis is a major regulator of tamoxifen-resistant breast cancer cell growth and survival.

Salmonella Suppresses the TRIF-Dependent Type I Interferon Response in Macrophages.

UNLABELLED: Salmonella enterica is an intracellular pathogen that causes diseases ranging from gastroenteritis to typhoid fever. Salmonella bacteria trigger an autophagic response in host cells upon infection but have evolved mechanisms for suppressing this response, thereby enhancing intracellular survival. We recently reported that S. enterica serovar Typhimurium actively recruits the host tyrosine kinase focal adhesion kinase (FAK) to the surface of the Salmonella-containing vacuole (SCV) (K. A. Owen et al., PLoS Pathog 10:e1004159, 2014). FAK then suppresses autophagy through activation of the Akt/mTORC1 signaling pathway. In FAK(-/-) macrophages, bacteria are captured in autophagosomes and intracellular survival is attenuated. Here we show that the cell-autonomous bacterial suppression of autophagy also suppresses the broader innate immune response by inhibiting production of beta interferon (IFN-ß). Induction of bacterial autophagy (xenophagy), but not autophagy alone, triggers IFN-ß production through a pathway involving the adapter TRIF and endosomal Toll-like receptor 3 (TLR3) and TLR4. Selective FAK knockout in macrophages resulted in rapid bacterial clearance from mucosal tissues after oral infection. Clearance correlated with increased IFN-ß production by intestinal macrophages and with IFN-ß-dependent induction of IFN-γ by intestinal NK cells. Blockade of either IFN-ß or IFN-γ increased host susceptibility to infection, whereas experimental induction of IFN-ß was protective. Thus, bacterial suppression of autophagy not only enhances cell-autonomous survival but also suppresses more-systemic innate immune responses by limiting type I and type II interferons. IMPORTANCE: Salmonella enterica serovar Typhimurium represents one of the most commonly identified bacterial causes of foodborne illness worldwide. S. Typhimurium has developed numerous strategies to evade detection by the host immune system. Autophagy is a cellular process that involves the recognition and degradation of defective proteins and organelles. More recently, autophagy has been described as an important means by which host cells recognize and eliminate invading intracellular pathogens and plays a key role in the production of cytokines. Previously, we determined that Salmonella bacteria are able to suppress their own autophagic capture and elimination by macrophages. Building on that study, we show here that the inhibition of autophagy by Salmonella also prevents the induction of a protective cytokine response mediated by beta interferon (IFN-ß) and IFN-γ. Together, these findings identify a novel virulence strategy whereby Salmonella bacteria prevent cell autonomous elimination via autophagy and suppress the activation of innate immune responses.

The adhesion GPCR BAI1 mediates macrophage ROS production and microbicidal activity against Gram-negative bacteria.

The detection of microbes and initiation of an innate immune response occur through pattern recognition receptors (PRRs), which are critical for the production of inflammatory cytokines and activation of the cellular microbicidal machinery. In particular, the production of reactive oxygen species (ROS) by the NADPH oxidase complex is a critical component of the macrophage bactericidal machinery. We previously characterized brain-specific angiogenesis inhibitor 1 (BAI1), a member of the adhesion family of G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptors (GPCRs), as a PRR that mediates the selective phagocytic uptake of Gram-negative bacteria by macrophages. We showed that BAI1 promoted phagosomal ROS production through activation of the Rho family guanosine triphosphatase (GTPase) Rac1, thereby stimulating NADPH oxidase activity. Primary BAI1-deficient macrophages exhibited attenuated Rac GTPase activity and reduced ROS production in response to several Gram-negative bacteria, resulting in impaired microbicidal activity. Furthermore, in a peritoneal infection model, BAI1-deficient mice exhibited increased susceptibility to death by bacterial challenge because of impaired bacterial clearance. Together, these findings suggest that BAI1 mediates the clearance of Gram-negative bacteria by stimulating both phagocytosis and NADPH oxidase activation, thereby coupling bacterial detection to the cellular microbicidal machinery.

Salmonella Manipulates Autophagy to "Serve and Protect".

Many intracellular pathogens, including Salmonella typhimurium, trigger autophagy in host cells, which is widely thought to restrict intracellular growth and survival. In this issue of Cell Host & Microbe, Kreibich et al. (2015) demonstrate a role for the autophagic machinery in the repair of damaged Salmonella-containing vacuoles (SCVs).

ELMO1 has an essential role in the internalization of Salmonella Typhimurium into enteric macrophages that impacts disease outcome.

BACKGROUNDS AND AIMS: 4-6 million people die of enteric infections each year. After invading intestinal epithelial cells, enteric bacteria encounter phagocytes. However, little is known about how phagocytes internalize the bacteria to generate host responses. Previously, we have shown that BAI1 (Brain Angiogenesis Inhibitor 1) binds and internalizes Gram-negative bacteria through an ELMO1 (Engulfment and cell Motility protein 1)/Rac1-dependent mechanism. Here we delineate the role of ELMO1 in host inflammatory responses following enteric infection. METHODS: ELMO1-depleted murine macrophage cell lines, intestinal macrophages and ELMO1 deficient mice (total or myeloid-cell specific) was infected with Salmonella enterica serovar Typhimurium. The bacterial load, inflammatory cytokines and histopathology was evaluated in the ileum, cecum and spleen. The ELMO1 dependent host cytokines were detected by a cytokine array. ELMO1 mediated Rac1 activity was measured by pulldown assay. RESULTS: The cytokine array showed reduced release of pro-inflammatory cytokines, including TNF-α and MCP-1, by ELMO1-depleted macrophages. Inhibition of ELMO1 expression in macrophages decreased Rac1 activation (~6 fold) and reduced internalization of Salmonella. ELMO1-dependent internalization was indispensable for TNF-α and MCP-1. Simultaneous inhibition of ELMO1 and Rac function virtually abrogated TNF-α responses to infection. Further, activation of NF-κB, ERK1/2 and p38 MAP kinases were impaired in ELMO1-depleted cells. Strikingly, bacterial internalization by intestinal macrophages was completely dependent on ELMO1. Salmonella infection of ELMO1-deficient mice resulted in a 90% reduction in bacterial burden and attenuated inflammatory responses in the ileum, spleen and cecum. CONCLUSION: These findings suggest a novel role for ELMO1 in facilitating intracellular bacterial sensing and the induction of inflammatory responses following infection with Salmonella.