Salmonella stimulate macrophage macropinocytosis and persist within spacious phagosomes

From Eberthella typhi to Salmonella Typhi: The Fascinating Journey of the Virulence and Pathogenicity of Salmonella Typhi

Salmonella Typhi (S. Typhi), the invasive typhoidal serovar of Salmonella enterica that causes typhoid fever in humans, is a severe threat to global health. It is one of the major causes of high morbidity and mortality in developing countries. According to recent WHO estimates, approximately 11-21 million typhoid fever illnesses occur annually worldwide, accounting for 0.12-0.16 million deaths. Salmonella infection can spread to healthy individuals by the consumption of contaminated food and water. Typhoid fever in humans sometimes is accompanied by several other critical extraintestinal complications related to the central nervous system, cardiovascular system, pulmonary system, and hepatobiliary system. Salmonella Pathogenicity Island-1 and Salmonella Pathogenicity Island-2 are the two genomic segments containing genes encoding virulent factors that regulate its invasion and systemic pathogenesis. This Review aims to shed light on a comparative analysis of the virulence and pathogenesis of the typhoidal and nontyphoidal serovars of S. enterica.

Identification of SNPs Related to Salmonella Resistance in Chickens Using RNA-Seq and Integrated Bioinformatics Approach

Potential single nucleotide polymorphisms (SNPs) were detected between two chicken breeds (Kashmir favorella and broiler) using deep RNA sequencing. This was carried out to comprehend the coding area alterations, which cause variances in the immunological response to Salmonella infection. In the present study, we identified high impact SNPs from both chicken breeds in order to delineate different pathways that mediate disease resistant/susceptibility traits. Samples (liver and spleen) were collected from Salmonella resistant (K. favorella) and susceptible (broiler) chicken breeds. Salmonella resistance and susceptibility were checked by different pathological parameters post infection. To explore possible polymorphisms in genes linked with disease resistance, SNP identification analysis was performed utilizing RNA seq data from nine K. favorella and ten broiler chickens. A total of 1778 (1070 SNPs and 708 INDELs) and 1459 (859 SNPs and 600 INDELs) were found to be specific to K. favorella and broiler, respectively. Based on our results, we conclude that in broiler chickens the enriched pathways mostly included metabolic pathways like fatty acid metabolism, carbon metabolism and amino acid metabolism (Arginine and proline metabolism), while as in K. favorella genes with high impact SNPs were enriched in most of the immune-related pathways like MAPK signaling pathway, Wnt signaling pathway, NOD-like receptor signaling pathway, etc., which could be a possible resistance mechanism against salmonella infection. In K. favorella, protein-protein interaction analysis also shows some important hub nodes, which are important in providing defense against different infectious diseases. Phylogenomic analysis revealed that indigenous poultry breeds (resistant) are clearly separated from commercial breeds (susceptible). These findings will offer fresh perspectives on the genetic diversity in chicken breeds and will aid in the genomic selection of poultry birds.

Salmonella Typhimurium PgtE is an essential arsenal to defend against the host resident antimicrobial peptides

Salmonella enterica serovar Typhimurium is a common cause of gastroenteritis in humans and occasionally causes systemic infection. Salmonella's ability to survive and replicate within macrophages is an important characteristic during systemic infection. The outer membrane protease PgtE of S. enterica is a member of the Omptin family of outer membrane aspartate proteases which has well-characterized proteolytic activities in-vitro against a wide range of physiologically relevant substrates. However, no study has been done so far that draws a direct correlation between these in-vitro observations and the biology of the pathogen in-vivo. The main goals of this study were to characterize the pathogenesis-associated functions of pgtE and study its role in the intracellular survival and in-vivo virulence of Salmonella Typhimurium. Our study elucidated a possible role of Salmonella Typhimurium pgtE in combating host antimicrobial peptide- bactericidal/ permeability increasing protein (BPI) to survive in human macrophages. The pgtE-deficient strain of Salmonella showed attenuated proliferation and enhanced colocalization with BPI in U937 and Thp1 cells. In the presence of polymixin B, the attenuated in-vitro survival of STM ΔpgtE suggested a role of PgtE against the antimicrobial peptides. In addition, our study revealed that compared to the wild type Salmonella, the pgtE mutant is replication-deficient in C57BL/6 mice. Further, we showed that PgtE interacts directly with several antimicrobial peptides (AMPs) in the host gut. This gives the pathogen a survival advantage and helps to mount a successful infection in the host.

Analysis of Salmonella lineage-specific traits upon cell sorting

Microbial cell individuality is receiving increasing interest in the scientific community. Individual cells within clonal populations exhibit noticeable phenotypic heterogeneity. The advent of fluorescent protein technology and advances in single-cell analysis has revealed phenotypic cell variant in bacterial populations. This heterogeneity is evident in a wide range of phenotypes, for example, individual cells display variable degrees of gene expression and survival under selective conditions and stresses, and can exhibit differing propensities to host interactions. Last few years, numerous cell sorting approaches have been employed for resolving the properties of bacterial subpopulations. This review provides an overview of applications of cell sorting to analyze Salmonella lineage-specific traits, including bacterial evolution studies, gene expression analysis, response to diverse cellular stresses and characterization of diverse bacterial phenotypic variants.

Seneca Valley Virus Enters PK-15 Cells via Caveolae-Mediated Endocytosis and Macropinocytosis Dependent on Low-pH, Dynamin, Rab5, and Rab7

Seneca Valley virus (SVV), a new pathogen resulting in porcine vesicular disease, is prevalent in pig herds worldwide. Although an understanding of SVV biology pathogenesis is crucial for preventing and controlling this disease, the molecular mechanisms for the entry and post-internalization of SVV, which represent crucial steps in viral infection, are not well characterized. In this study, specific inhibitors, Western blotting, and immunofluorescence detection revealed that SVV entry into PK-15 cells depends on low-pH conditions and dynamin. Furthermore, results showed that caveolae-mediated endocytosis (CavME) contributes crucially to the internalization of SVV, as evidenced by cholesterol depletion, downregulation of caveolin-1 expression by small interfering RNA knockdown, and overexpression of a caveolin-1 dominant negative (caveolin-1-DN) in SVV-infected PK-15 cells. However, SVV entry into PK-15 cells did not depend on clathrin-mediated endocytosis (CME). Furthermore, treatment with specific inhibitors demonstrated that SVV entry into PK-15 cells via macropinocytosis depended on the Na+/H+ exchanger (NHE), p21-activated kinase 1 (Pak1), and actin rearrangement, but not phosphatidylinositol 3-kinase (PI3K). Electron microscopy showed that SVV particles or proteins were localized in CavME and macropinocytosis. Finally, knockdown of GTPase Rab5 and Rab7 by siRNA significantly inhibited SVV replication, as determined by measuring viral genome copy numbers, viral protein expression, and viral titers. In this study, our results demonstrated that SVV utilizes caveolae-mediated endocytosis and macropinocytosis to enter PK-15 cells, dependent on low pH, dynamin, Rab5, and Rab7. IMPORTANCE Entry of virus into cells represents the initiation of a successful infection. As an emerging pathogen of porcine vesicular disease, clarification of the process of SVV entry into cells enables us to better understand the viral life cycle and pathogenesis. In this study, patterns of SVV internalization and key factors required were explored. We demonstrated for the first time that SVV entry into PK-15 cells via caveolae-mediated endocytosis and macropinocytosis requires Rab5 and Rab7 and is independent of clathrin-mediated endocytosis, and that low-pH conditions and dynamin are involved in the process of SVV internalization. This information increases our understanding of the patterns in which all members of the family Picornaviridae enter host cells, and provides new insights for preventing and controlling SVV infection.

Klf10 favors Mycobacterium tuberculosis survival by impairing IFN-γ production and preventing macrophages reprograming to macropinocytosis

Mycobacterium tuberculosis has developed diverse mechanisms to survive inside phagocytic cells, such as macrophages. Phagocytosis is a key process in eliminating invading pathogens; thus, M. tuberculosis efficiently disrupts phagosome maturation to ensure infection. However, inflammatory cytokines produced by macrophages in response to early M. tuberculosis infection are key to promoting bacterial clarification. IFN-γ enhances M. tuberculosis engulfment and destruction by reprogramming macrophages from phagocytosis to macropinocytosis. Here, we show that the transcription factor Krüppel-like factor 10 (Klf10) plays a positive role in M. tuberculosis survival and infection by negatively modulating IFN-γ levels. Naïve Klf10-deficient macrophages produce more IFN-γ upon stimulation than wild-type macrophages, thus enhancing bacterial uptake and bactericidal activity achieved by macropinocytosis. Moreover, Klf10⁻^/ ⁻ macrophages showed cytoplasmic distribution of coronin 1 correlated with increased pseudopod count and length. In agreement with these observations, Klf10⁻^/ ⁻ mice showed improved bacterial clearance from the lungs and increased viability. Altogether, our data indicate that Klf10 plays a critical role in M. tuberculosis survival by preventing macrophage reprogramming from phagocytosis to macropinocytosis by negatively regulating IFN-γ production upon macrophage infection.

Mechanisms for the Invasion and Dissemination of Salmonella

Salmonella enterica is a gastroenteric Gram-negative bacterium that can infect both humans and animals and causes millions of illnesses per year around the world. Salmonella infections usually occur after the consumption of contaminated food or water. Infections with Salmonella species can cause diseases ranging from enterocolitis to typhoid fever. Salmonella has developed multiple strategies to invade and establish a systemic infection in the host. Different cell types, including epithelial cells, macrophages, dendritic cells, and M cells, are important in the infection process of Salmonella. Dissemination throughout the body and colonization of remote organs are hallmarks of Salmonella infection. There are several routes for the dissemination of Salmonella typhimurium. This review summarizes the current understanding of the infection mechanisms of Salmonella. Additionally, different routes of Salmonella infection will be discussed. In this review, the strategies used by Salmonella enterica to establish persistent infection will be discussed. Understanding both the bacterial and host factors leading to the successful colonization of Salmonella enterica may enable the rational design of effective therapeutic strategies.

Transcriptome Analyses of Chicken Primary Macrophages Infected With Attenuated Salmonella Typhimurium Mutants

Salmonella enterica is one of the most common foodborne illnesses in the United States and worldwide, with nearly one-third of the cases attributed to contaminated eggs and poultry products. Vaccination has proven to be an effective strategy to reduce Salmonella load in poultry. The Salmonella Typhimurium Δcrp-cya (MeganVac1) strain is the most commonly used vaccine in the United States; however, the mechanisms of virulence attenuation and host response to this vaccine strain are poorly understood. Here, we profiled the invasion and intracellular survival phenotypes of Δcrp-cya and its derivatives (lacking key genes required for intra-macrophage survival) in HD11 macrophages and the transcriptome response in primary chicken macrophages using RNA-seq. Compared to the parent strain UK1, all the mutant strains were highly defective in metabolizing carbon sources related to the TCA cycle and had greater doubling times in macrophage-simulating conditions. Compared to UK1, the majority of the mutants were attenuated for invasion and intra-macrophage survival. Compared to Δcrp-cya, while derivatives lacking phoPQ, ompR-envZ, feoABC and sifA were highly attenuated for invasion and intracellular survival within macrophages, derivatives lacking ssrAB, SPI13, SPI2, mgtRBC, sitABCD, sopF, sseJ and sspH2 showed increased ability to invade and survive within macrophages. Transcriptome analyses of macrophages infected with UK1, Δcrp-cya and its derivatives lacking phoPQ, sifA and sopF demonstrated that, compared to uninfected macrophages, 138, 148, 153, 155 and 142 genes were differentially expressed in these strains, respectively. Similar changes in gene expression were observed in macrophages infected with these strains; the upregulated genes belonged to innate immune response and host defense and the downregulated genes belonged to various metabolic pathways. Together, these data provide novel insights on the relative phenotypes and early response of macrophages to the vaccine strain and its derivatives. The Δcrp-cya derivatives could facilitate development of next-generation vaccines with improved safety.

Endomembrane remodeling and dynamics in Salmonella infection

Salmonellae are bacteria that cause moderate to severe infections in humans, depending on the strain and the immune status of the infected host. These pathogens have the particularity of residing in the cells of the infected host. They are usually found in a vacuolar compartment that the bacteria shape with the help of effector proteins. Following invasion of a eukaryotic cell, the bacterial vacuole undergoes maturation characterized by changes in localization, composition and morphology. In particular, membrane tubules stretching over the microtubule cytoskeleton are formed from the bacterial vacuole. Although these tubules do not occur in all infected cells, they are functionally important and promote intracellular replication. This review focuses on the role and significance of membrane compartment remodeling observed in infected cells and the bacterial and host cell pathways involved.

Perturbation of ATG16L1 function impairs the biogenesis of Salmonella and Coxiella replication vacuoles

Anti-bacterial autophagy, known as xenophagy, is a host innate immune response that targets invading pathogens for degradation. Some intracellular bacteria, such as the enteric pathogen Salmonella enterica serovar Typhimurium (S. Typhimurium), utilize effector proteins to interfere with autophagy. One such S. Typhimurium effector, SopF, inhibits recruitment of ATG16L1 to damaged Salmonella-containing vacuoles (SCVs), thereby inhibiting the host xenophagic response. SopF is also required to maintain the integrity of the SCV during the early stages of infection. Here we show disruption of the SopF-ATG16L1 interaction leads to an increased proportion of cytosolic S. Typhimurium. Furthermore, SopF was utilized as a molecular tool to examine the requirement for ATG16L1 in the intracellular lifestyle of Coxiella burnetii, a bacterium that requires a functional autophagy pathway to replicate efficiently and form a single, spacious vacuole called the Coxiella-containing vacuole (CCV). ATG16L1 is required for CCV expansion and fusion but does not influence C. burnetii replication. In contrast, SopF did not affect CCV formation or replication, demonstrating that the contribution of ATG16L1 to CCV biogenesis is via its role in autophagy, not xenophagy. This study highlights the diverse capabilities of bacterial effector proteins to dissect the molecular details of host-pathogen interactions.

Development of a Genomics-Based Approach To Identify Putative Hypervirulent Nontyphoidal Salmonella Isolates: Salmonella enterica Serovar Saintpaul as a Model

While differences in human virulence have been reported across nontyphoidal Salmonella (NTS) serovars and associated subtypes, a rational and scalable approach to identify Salmonella subtypes with differential ability to cause human diseases is not available. Here, we used NTS serovar Saintpaul (S. Saintpaul) as a model to determine if metadata and associated whole-genome sequence (WGS) data in the NCBI Pathogen Detection (PD) database can be used to identify (i) subtypes with differential likelihoods of causing human diseases and (ii) genes and single nucleotide polymorphisms (SNPs) potentially responsible for such differences. S. Saintpaul SNP clusters (n = 211) were assigned different epidemiology types (epi-types) based on statistically significant over- or underrepresentation of human clinical isolates, including human associated (HA; n = 29), non-human associated (NHA; n = 23), and other (n = 159). Comparative genomic analyses identified 384 and 619 genes overrepresented among isolates in 5 HA and 4 NHA SNP clusters most significantly associated with the respective isolation source. These genes included 5 HA-associated virulence genes previously reported to be present on Gifsy-1/Gifsy-2 prophages. Additionally, premature stop codons in 3 and 7 genes were overrepresented among the selected HA and NHA SNP clusters, respectively. Tissue culture experiments with strains representing 4 HA and 3 NHA SNP clusters did not reveal evidence for enhanced invasion or intracellular survival for HA strains. However, the presence of sodCI (encoding a superoxide dismutase), found in 4 HA and 1 NHA SNP clusters, was positively correlated with intracellular survival in macrophage-like cells. Post hoc analyses also suggested a possible difference in intracellular survival among S. Saintpaul lineages. IMPORTANCE Not all Salmonella isolates are equally likely to cause human disease, and Salmonella control strategies may unintentionally focus on serovars and subtypes with high prevalence in source populations but are rarely associated with human clinical illness. We describe a framework leveraging WGS data in the NCBI PD database to identify Salmonella subtypes over- and underrepresented among human clinical cases. While we identified genomic signatures associated with HA/NHA SNP clusters, tissue culture experiments failed to identify consistent phenotypic characteristics indicative of enhanced human virulence of HA strains. Our findings illustrate the challenges of defining hypo- and hypervirulent S. Saintpaul and potential limitations of phenotypic assays when evaluating human virulence, for which in vivo experiments are essential. Identification of sodCI, an HA-associated virulence gene associated with enhanced intracellular survival, however, illustrates the potential of the framework and is consistent with prior work identifying specific genomic features responsible for enhanced or reduced virulence of nontyphoidal Salmonella.

Stabilization but no functional influence of HIF-1α expression in the intestinal epithelium during Salmonella Typhimurium infection

The hypoxia-inducible transcription factor 1 (HIF-1) has been shown to enhance microbial killing and to ameliorate the course of bacterial infections. While the impact of HIF-1 on inflammatory diseases of the gut has been studied intensively, its function in bacterial infections of the gastrointestinal tract remains largely elusive. With the help of a publicly available gene expression data set, we could infer significant activation of HIF-1 after oral infection of mice with Salmonella Typhimurium. Immunohistochemistry and western blot analysis confirmed marked HIF-1α protein stabilization, especially in the intestinal epithelium. This prompted us to analyze conditional Hif1a-deficient mice to examine cell type-specific functions of HIF-1 in this model. Our results demonstrate enhanced non-canonical induction of HIF-1 activity upon Salmonella infection in the intestinal epithelium as well as in macrophages. Surprisingly, Hif1a deletion in intestinal epithelial cells did not impact on inflammatory gene expression, bacterial spread or disease outcome. In contrast, Hif1a deletion in myeloid cells enhanced intestinal Cxcl2 expression and reduced the cecal Salmonella load. In vitro, HIF-1α-deficient macrophages showed an overall impaired transcription of mRNA encoding pro-inflammatory factors, however, intracellular survival of Salmonella was not impacted by HIF-1α deficiency.

IFN-alpha receptor deficiency enhances host resistance to oral Salmonella enterica serovar Typhimurium infection during murine pregnancy

PROBLEM

Maternal tolerance during pregnancy increases the risk of infection with certain intracellular pathogens. Systemic Salmonella enterica serovar Typhimurium (S.Tm) infection during pregnancy in normally resistant 129X1/SvJ mice leads to severe placental infection, as well as fetal and maternal deaths. However, the effect of oral infection with S.Tm in pregnant mice and the roles of infection-induced inflammation and cell death pathways in contributing to susceptibility to infection are unclear.

METHOD OF STUDY

Non-pregnant and pregnant C57BL/6J wild-type (WT) and cell death pathway-altered mice (IFNAR1^-/- , Caspase-1, 11^-/- , RIP3^-/- ) were infected orally with S.Tm. Host survival and fetal resorption were determined. Bacterial burden in mesenteric lymph nodes (MLNs), spleen, liver, and placentas was enumerated at various time points post-infection. Serum cytokine expression was measured through cytometric bead array.

RESULTS

Oral infection of WT mice with S.Tm on days 9-10 of gestation resulted in systemic dissemination of the bacteria, substantial placental colonization, and fetal loss 5 days post-infection. Histopathological examination of the placentas indicated that infection-induced widespread focal necrosis and neutrophil infiltration throughout the spongiotrophoblast (SpT) layer. In the non-pregnant state, IFNAR1^-/- mice exhibited increased survival following oral S.Tm infection relative to Caspase-1, 11^-/- , RIP3^-/- , and WT mice. The increased resistance to S.Tm infection in IFNAR1^-/- mice was seen during pregnancy as well, with decreased bacterial burden within MLNs, spleen, and placenta, which correlated with the decreased resorptions relative to WT and Caspase-1, 11^-/- mice.

CONCLUSION

Oral S.Tm exposure leads to placental infection, inflammation, and resorption, whereas IFNAR1 deficiency enhances host resistance both in the non-pregnant and pregnant states.

SteC and the intracellular Salmonella-induced F-actin meshwork

Salmonella enterica serovars infect a broad range of mammalian hosts including humans, causing both gastrointestinal and systemic diseases. Following uptake into host cells, bacteria replicate within vacuoles (Salmonella-containing vacuoles; SCVs). Clusters of SCVs are frequently associated with a meshwork of F-actin. This meshwork is dependent on the Salmonella pathogenicity island 2 encoded type III secretion system and its effector SteC. SteC contains a region with weak similarity to conserved subdomains of eukaryotic kinases and has kinase activity that is required for the formation of the F-actin meshwork. Several substrates of SteC have been identified. In this mini-review, we attempt to integrate these findings and propose a more unified model to explain SCV-associated F-actin: SteC (i) phosphorylates the actin sequestering protein Hsp27, which increases the local G-actin concentration (ii) binds to and phosphorylates formin family FMNL proteins, which enables actin polymerisation and (iii) phosphorylates MEK, resulting in activation of the MEK/ERK/MLCK/Myosin II pathway, leading to F-actin bundling. We also consider the possible physiological functions of SCV-associated F-actin and similar structures produced by other intracellular bacterial pathogens.

The Role of Macrophages in Staphylococcus aureus Infection

Staphylococcus aureus is a member of the human commensal microflora that exists, apparently benignly, at multiple sites on the host. However, as an opportunist pathogen it can also cause a range of serious diseases. This requires an ability to circumvent the innate immune system to establish an infection. Professional phagocytes, primarily macrophages and neutrophils, are key innate immune cells which interact with S. aureus, acting as gatekeepers to contain and resolve infection. Recent studies have highlighted the important roles of macrophages during S. aureus infections, using a wide array of killing mechanisms. In defense, S. aureus has evolved multiple strategies to survive within, manipulate and escape from macrophages, allowing them to not only subvert but also exploit this key element of our immune system. Macrophage-S. aureus interactions are multifaceted and have direct roles in infection outcome. In depth understanding of these host-pathogen interactions may be useful for future therapeutic developments. This review examines macrophage interactions with S. aureus throughout all stages of infection, with special emphasis on mechanisms that determine infection outcome.

Cathepsins in Bacteria-Macrophage Interaction: Defenders or Victims of Circumstance?

Macrophages are the first encounters of invading bacteria and are responsible for engulfing and digesting pathogens through phagocytosis leading to initiation of the innate inflammatory response. Intracellular digestion occurs through a close relationship between phagocytic/endocytic and lysosomal pathways, in which proteolytic enzymes, such as cathepsins, are involved. The presence of cathepsins in the endo-lysosomal compartment permits direct interaction with and killing of bacteria, and may contribute to processing of bacterial antigens for presentation, an event necessary for the induction of antibacterial adaptive immune response. Therefore, it is not surprising that bacteria can control the expression and proteolytic activity of cathepsins, including their inhibitors – cystatins, to favor their own intracellular survival in macrophages. In this review, we summarize recent developments in defining the role of cathepsins in bacteria-macrophage interaction and describe important strategies engaged by bacteria to manipulate cathepsin expression and activity in macrophages. Particularly, we focus on specific bacterial species due to their clinical relevance to humans and animal health, i.e., Mycobacterium, Mycoplasma, Staphylococcus, Streptococcus, Salmonella, Shigella, Francisella, Chlamydia, Listeria, Brucella, Helicobacter, Neisseria, and other genera.

Phagolysosomal Survival Enables Non-lytic Hyphal Escape and Ramification Through Lung Epithelium During Aspergillus fumigatus Infection

Aspergillus fumigatus is the most important mould pathogen in immunosuppressed patients. Suboptimal clearance of inhaled spores results in the colonisation of the lung airways by invasive hyphae. The first point of contact between A. fumigatus and the host is the lung epithelium. In vitro and ex vivo studies have characterised critical aspects of the interaction of invasive hyphae on the surface of epithelial cells. However, the cellular interplay between internalised A. fumigatus and the lung epithelium remains largely unexplored. Here, we use high-resolution live-cell confocal microscopy, 3D rendered imaging and transmission electron microscopy to define the development of A. fumigatus after lung epithelium internalisation in vitro. Germination, morphology and growth of A. fumigatus were significantly impaired upon internalisation by alveolar (A549) and bronchial (16HBE) lung epithelial cells compared to those growing on the host surface. Internalised spores and germlings were surrounded by the host phagolysosome membrane. Sixty per cent of the phagosomes containing germlings were not acidified at 24 h post infection allowing hyphal development. During escape, the phagolysosomal membrane was not ruptured but likely fused to host plasma membrane allowing hyphal exit from the intact host cell in an non-lytic Manner. Subsequently, escaping hyphae elongated between or through adjacent epithelial lung cells without penetration of the host cytoplasm. Hyphal tips penetrating new epithelial cells were surrounded by the recipient cell plasma membrane. Altogether, our results suggest cells of lung epithelium survive fungal penetration because the phagolysosomal and plasma membranes are never breached and that conversely, fungal spores survive due to phagosome maturation failure. Consequently, fungal hyphae can grow through the epithelial cell layer without directly damaging the host. These processes likely prevent the activation of downstream immune responses alongside limiting the access of professional phagocytes to the invading fungal hypha. Further research is needed to investigate if these events also occur during penetration of fungi in endothelial cells, fibroblasts and other cell types.

Salmonella Virulence and Immune Escape

Salmonella genus represents the most common foodborne pathogens causing morbidity, mortality, and burden of disease in all regions of the world. The introduction of antimicrobial agents and Salmonella-specific phages has been considered as an effective intervention strategy to reduce Salmonella contamination. However, data from the United States, European countries, and low- and middle-income countries indicate that Salmonella cases are still a commonly encountered cause of bacterial foodborne diseases globally. The control programs have not been successful and even led to the emergence of some multidrug-resistant Salmonella strains. It is known that the host immune system is able to effectively prevent microbial invasion and eliminate microorganisms. However, Salmonella has evolved mechanisms of resisting host physical barriers and inhibiting subsequent activation of immune response through their virulence factors. There has been a high interest in understanding how Salmonella interacts with the host. Therefore, in the present review, we characterize the functions of Salmonella virulence genes and particularly focus on the mechanisms of immune escape in light of evidence from the emerging mainstream literature.

Influenza A virus interactions with macrophages: Lessons from epithelial cells

Influenza viruses are an important cause of respiratory infection worldwide. In humans, infection with seasonal influenza A virus (IAV) is generally restricted to the respiratory tract where productive infection of airway epithelial cells promotes viral amplification, dissemination, and disease. Alveolar macrophages (MΦ) are also among the first cells to detect and respond to IAV, where they play a pivotal role in mounting effective innate immune responses. In contrast to epithelial cells, IAV infection of MΦ is a "dead end" for most seasonal strains, where replication is abortive and newly synthesised virions are not released. Although the key replicative stages leading to productive IAV infection in epithelial cells are defined, there is limited knowledge about the abortive IAV life cycle in MΦ. In this review, we will explore host factors and viral elements that support the early stages (entry) through to the late stages (viral egress) of IAV replication in epithelial cells. Similarities, differences, and unknowns for each key stage of the IAV replicative cycle in MΦ will then be highlighted. Herein, we provide mechanistic insights into MΦ-specific control of seasonal IAV replication through abortive infection, which may in turn, contribute to effective host defence.

Bacteria That Cause Enteric Diseases Stimulate Distinct Humoral Immune Responses

Bacterial enteric pathogens individually and collectively represent a serious global health burden. Humoral immune responses following natural or experimentally-induced infections are broadly appreciated to contribute to pathogen clearance and prevention of disease recurrence. Herein, we have compared observations on humoral immune mechanisms following infection with Citrobacter rodentium, the model for enteropathogenic Escherichia coli, Vibrio cholerae, Shigella species, Salmonella enterica species, and Clostridioides difficile. A comparison of what is known about the humoral immune responses to these pathogens reveals considerable variance in specific features of humoral immunity including establishment of high affinity, IgG class-switched memory B cell and long-lived plasma cell compartments. This article suggests that such variance could be contributory to persistent and recurrent disease.

Listeria monocytogenes cancer vaccines: bridging innate and adaptive immunity

Purpose of the Review

Immunotherapy has emerged as a promising cancer treatment, however success in only select clinical indications underscores the need for novel approaches. Recently Listeria monocytogenes-based vaccines have been developed to drive tumor specific T-cell responses. Here, we discuss recent preclinical studies using L. monocytogenes vaccines, innate immune pathways that influence T-cell priming, and new vaccine strategies in clinical trials.

Recent Findings

Recent studies indicate that in addition to inducing antigen specific T-cell responses, L. monocytogenes vaccines remodel the TME. In addition, several innate immune pathways influence adaptive immune responses to L. monocytogenes and modulating these pathways holds promise to enhance anti-tumor T-cell responses.

Summary

The interplay between innate and adaptive immune responses to L. monocytogenes is poorly understood. Understanding these interactions will facilitate the design of better anti-cancer vaccines and improved use of combination therapies.

Restriction of intracellular Salmonella replication by restoring TFEB-mediated xenophagy

Macroautophagy/autophagy functions as a part of the innate immune system in clearing intracellular pathogens. Although this process is well known, the mechanisms that control antibacterial autophagy are not clear. In this study we show that during intracellular Salmonella typhimurium infection, the activity of TFEB (transcription factor EB), a master regulator of autophagy and lysosome biogenesis, is suppressed by maintaining it in a phosphorylated state on the lysosomes. Furthermore, we have identified a novel, antibacterial small molecule autophagy (xenophagy) modulator, acacetin. The xenophagy effect exerted by acacetin occurs in an MTOR (mechanistic target of rapamycin kinase)-independent, TFEB-dependent manner. Acacetin treatment results in persistently maintaining active TFEB in the nucleus and also in TFEB mediated induction of functional lysosomes that target Salmonella-containing vacuoles (SCVs). The enhanced proteolytic activity due to deployment of lysosomes results in clamping down Salmonella replication in SCVs. Acacetin is effective as a xenophagy compound in an in vivo mouse model of infection and reduces intracellular Salmonella burden.

ABBREVIATIONS

3-MA: 3-methyladenine; BafA1: bafilomycin A₁; CFU: colony-forming units; DQ-BSA: dye quenched-bovine serum albumin; EEA1: early endosome antigen 1; FITC: fluorescein isothiocyanate; FM 4-64: pyridinium,4-(6-[4-{diethylamino}phenyl]-1,3,5-hexatrienyl)-1-(3[triethylammonio] propyl)-dibromide; GFP: green fluorescent protein; LAMP1: lysosomal associated membrane protein 1; MAPILC3/LC3: microtubule associated protein 1 light chain 3; MOI: multiplicity of infection; MTOR: mechanistic target of rapamycin kinase; RFP: red fluorescent protein; SCVs: Salmonella-containing vacuoles; SD: standard deviation; SDS: sodium dodecyl sulfate; SEM: standard mean error; SQSTM1: sequestosome 1; TBK1: TANK binding kinase 1; TFEB: transcription factor EB.

The cellular microbiology of Salmonellae interactions with macrophages

Salmonellae are important enteric pathogens that cause gastroenteritis and systemic illnesses. Macrophages are important components of both the innate and acquired immune system, acting as phagocytes with significant antimicrobial killing activities that present antigen to the adaptive immune system. Macrophages can also be cultured from a variety of sites as primary cells, and the study of the survival and interactions of Salmonellae with these cells is a very early model of infection and cellular microbiology. This review traces the history of discoveries made using Salmonellae infection of macrophages and addresses the possibility of future research in this area, in particular with regards to understanding the complexity of individual bacteria and macrophage cell variability and how such heterogeneity may alter the outcome of infection.

Salmonella Typhimurium Invalidated for the Three Currently Known Invasion Factors Keeps Its Ability to Invade Several Cell Models

To establish an infection, Salmonella has to interact with eukaryotic cells. Invasion of non-phagocytic cells (i.e., epithelial, fibroblast and endothelial cells) involves either a trigger or a zipper mechanism mediated by the T3SS-1 or the invasin Rck, respectively. Another outer membrane protein, PagN, was also implicated in the invasion. However, other unknown invasion factors have been previously suggested. Our goal was to evaluate the invasion capability of a Salmonella Typhimurium strain invalidated for the three known invasion factors. Non-phagocytic cell lines of several animal origins were tested in a gentamicin protection assay. In most cells, we observed a drastic decrease in the invasion rate between the wild-type and the triple mutant. However, in five cell lines, the triple mutant invaded cells at a similarly high level to the wild-type, suggesting the existence of unidentified invasion factors. For the wild-type and the triple mutant, scanning-electron microscopy, confocal imaging and use of biochemical inhibitors confirmed their cellular uptake and showed a zipper-like mechanism of internalization involving both clathrin- and non-clathrin-dependent pathways. Despite a functional T3SS-1, the wild-type bacteria seemed to use the same entry route as the mutant in our cell model. All together, these results demonstrate the existence of unknown Salmonella invasion factors, which require further characterization.

Hoodwinking the Big-Eater to Prosper: The Salmonella-Macrophage Paradigm

Salmonella is a major cause of morbidity and mortality in the developing and underdeveloped nations. Being a foodborne disease, Salmonella infection is primarily contracted through the ingestion of contaminated food or water, or due to close contact with infected/carrier individuals. It is an intracellular pathogen, which can survive and replicate in various cells including macrophages, dendritic cells, epithelial cells, and other white blood cells. Once Salmonella crosses the intestinal barrier, it disseminates to various systemic sites by circulation via immune cells. One of the major cell types which are involved in Salmonella infection are host macrophages. They are the niche for intracellular survival and proliferation of Salmonella and a mode of dissemination to distal systemic sites. These cells are very crucial as they mediate the mounting of an appropriate innate and adaptive anti-Salmonella immune response. In this review, we have tried to concise the current knowledge of complex interactions that occur between Salmonella and macrophages.

Ultrastructural Imaging of Salmonella-Host Interactions Using Super-resolution Correlative Light-Electron Microscopy of Bioorthogonal Pathogens

The imaging of intracellular pathogens inside host cells is complicated by the low resolution and sensitivity of fluorescence microscopy and by the lack of ultrastructural information to visualize the pathogens. Herein, we present a new method to visualize these pathogens during infection that circumvents these problems: by using a metabolic hijacking approach to bioorthogonally label the intracellular pathogen Salmonella Typhimurium and by using these bioorthogonal groups to introduce fluorophores compatible with stochastic optical reconstruction microscopy (STORM) and placing this in a correlative light electron microscopy (CLEM) workflow, the pathogen can be imaged within its host cell context Typhimurium with a resolution of 20 nm. This STORM-CLEM approach thus presents a new approach to understand these pathogens during infection.

Defective Phagocytic Properties of HIV-Infected Macrophages: How Might They Be Implicated in the Development of Invasive Salmonella Typhimurium?

Human immunodeficiency virus type 1 (HIV-1) infects and kills T cells, profoundly damaging the host-specific immune response. The virus also integrates into memory T cells and long-lived macrophages, establishing chronic infections. HIV-1 infection impairs the functions of macrophages both in vivo and in vitro, which contributes to the development of opportunistic diseases. Non-typhoidal Salmonella enterica serovar Typhimurium has been identified as the most common cause of bacterial bloodstream infections in HIV-infected adults. In this review, we report how the functions of macrophages are impaired post HIV infection; introduce what makes invasive Salmonella Typhimurium specific for its pathogenesis; and finally, we discuss why these bacteria may be particularly adapted to the HIV-infected host.

Renitence vacuoles facilitate protection against phagolysosomal damage in activated macrophages

As professional phagocytes, macrophages are susceptible to endolysosomal membrane damage inflicted by the pathogens and noxious particles they ingest. Whether macrophages have mechanisms for limiting such damage is not well understood. Previously, we reported a phenomenon, termed "inducible renitence," in which lipopolysaccharide (LPS) activation of macrophages protected their endolysosomes against damage initiated by the phagocytosis of silica beads. To gain mechanistic insight into the process, we analyzed the kinetics of renitence and morphological features of LPS-activated versus resting macrophages following silica bead-mediated injury. We discovered novel vacuolar structures that form in LPS-activated but not resting macrophages following silica bead phagocytosis. Because of their correlation with renitence and damage-resistant nature, we termed these structures "renitence vacuoles" (RVs). RVs formed coincident with silica bead uptake in a process associated with membrane ruffling and macropinocytosis. However, unlike normal macropinosomes (MPs), which shrink within 20 min of formation, RVs persisted around bead-containing phagosomes. RVs fused with lysosomes, whereas associated phagosomes typically did not. These findings are consistent with a model in which RVs, as persistent MPs, prevent fusion between damaged phagosomes and intact lysosomes and thereby preserve endolysosomal integrity.

Intracellular Growth of Bacterial Pathogens: The Role of Secreted Effector Proteins in the Control of Phagocytosed Microorganisms

The ability of intracellular pathogens to subvert the host response, to facilitate invasion and subsequent infection, is the hallmark of microbial pathogenesis. Bacterial pathogens produce and secrete a variety of effector proteins, which are the primary means by which they exert control over the host cell. Secreted effectors work independently, yet in concert with each other, to facilitate microbial invasion, replication, and intracellular survival in host cells. In this review we focus on defined host cell processes targeted by bacterial pathogens. These include phagosome maturation and its subprocesses: phagosome-endosome and phagosome-lysosome fusion events, as well as phagosomal acidification, cytoskeleton remodeling, and lysis of the phagosomal membrane. We further describe the mode of action for selected effectors from six pathogens: the Gram-negative Legionella, Salmonella, Shigella, and Yersinia, the Gram-positive Listeria, and the acid-fast actinomycete Mycobacterium.

Macrophage-Bacteria Interactions-A Lipid-Centric Relationship

Macrophages are professional phagocytes at the front line of immune defenses against foreign bodies and microbial pathogens. Various bacteria, which are responsible for deadly diseases including tuberculosis and salmonellosis, are capable of hijacking this important immune cell type and thrive intracellularly, either in the cytoplasm or in specialized vacuoles. Tight regulation of cellular metabolism is critical in shaping the macrophage polarization states and immune functions. Lipids, besides being the bulk component of biological membranes, serve as energy sources as well as signaling molecules during infection and inflammation. With the advent of systems-scale analyses of genes, transcripts, proteins, and metabolites, in combination with classical biology, it is increasingly evident that macrophages undergo extensive lipid remodeling during activation and infection. Each bacterium species has evolved its own tactics to manipulate host metabolism toward its own advantage. Furthermore, modulation of host lipid metabolism affects disease susceptibility and outcome of infections, highlighting the critical roles of lipids in infectious diseases. Here, we will review the emerging roles of lipids in the complex host-pathogen relationship and discuss recent methodologies employed to probe these versatile metabolites during the infection process. An improved understanding of the lipid-centric nature of infections can lead to the identification of the Achilles' heel of the pathogens and host-directed targets for therapeutic interventions. Currently, lipid-moderating drugs are clinically available for a range of non-communicable diseases, which we anticipate can potentially be tapped into for various infections.

Mechanisms of Salmonella pathogenesis in animal models

Animal models play an important role in understanding the mechanisms of bacterial pathogenesis. Here we review recent studies of Salmonella infection in various animal models. Although mice are a classic animal model for Salmonella, mice do not normally get diarrhea, raising the question of how well the model represents normal human infection. However, pretreatment of mice with oral streptomycin, which apparently reduces the normal microbiota, leads to an inflammatory diarrheal response upon oral infection with Salmonella. This has led to a re-evaluation of the role of various Salmonella virulence factors in colonization of the intestine and induction of diarrhea. Indeed, it is now clear that Salmonella purposefully induces inflammation, which leads to the production of both carbon sources and terminal electron acceptors by the host that allow Salmonella to outgrow the normal intestinal microbiota. Overall use of this modified mouse model provides a more nuanced understanding of Salmonella intestinal infection in the context of the microbiota with implications for the ability to predict human risk.

Role of the Cytoskeleton in Myeloid Cell Function

During an innate immune response, myeloid cells undergo complex morphological adaptations in response to inflammatory cues, which allow them to exit the vasculature, enter the tissues, and destroy invading pathogens. The actin and microtubule cytoskeletons are central to many of the most essential cellular functions including cell division, cell morphology, migration, intracellular trafficking, and signaling. Cytoskeletal structure and regulation are crucial for many myeloid cell functions, which require rapid and dynamic responses to extracellular signals. In this chapter, we review the roles of the actin and microtubule cytoskeletons in myeloid cells, focusing primarily on their roles in chemotaxis and phagocytosis. The role of myeloid cell cytoskeletal defects in hematological disorders is highlighted throughout.

G-quadruplex oligonucleotide AS1411 as a cancer-targeting agent: Uses and mechanisms

BACKGROUND

AS1411 is a 26-mer G-rich DNA oligonucleotide that forms a variety of G-quadruplex structures. It was identified based on its cancer-selective antiproliferative activity and subsequently determined to be an aptamer to nucleolin, a multifunctional protein that preferentially binds quadruplex nucleic acids and which is present at high levels on the surface of cancer cells. AS1411 has exceptionally efficient cellular internalization compared to non-quadruplex DNA sequences.

SCOPE OF REVIEW

Recent developments related to AS1411 will be examined, with a focus on its use for targeted delivery of therapeutic and imaging agents.

MAJOR CONCLUSIONS

Numerous research groups have used AS1411 as a targeting agent to deliver nanoparticles, oligonucleotides, and small molecules into cancer cells. Studies in animal models have demonstrated that AS1411-linked materials can accumulate selectively in tumors following systemic administration. The mechanism underlying the cancer-targeting ability of AS1411 is not completely understood, but recent studies suggest a model that involves: (1) initial uptake by macropinocytosis, a form of endocytosis prevalent in cancer cells; (2) stimulation of macropinocytosis by a nucleolin-dependent mechanism resulting in further uptake; and (3) disruption of nucleolin-mediated trafficking and efflux leading to cargoes becoming trapped inside cancer cells.

SIGNIFICANCE

Human trials have indicated that AS1411 is safe and can induce durable remissions in a few patients, but new strategies are needed to maximize its clinical impact. A better understanding of the mechanisms by which AS1411 targets and kills cancer cells may hasten the development of promising technologies using AS1411-linked nanoparticles or conjugates for cancer-targeted therapy and imaging. This article is part of a Special Issue entitled "G-quadruplex" Guest Editor: Dr. Concetta Giancola and Dr. Daniela Montesarchio.

Macropinosomes are Key Players in Early Shigella Invasion and Vacuolar Escape in Epithelial Cells

Intracellular pathogens include all viruses, many bacteria and parasites capable of invading and surviving within host cells. Key to survival is the subversion of host cell pathways by the pathogen for the purpose of propagation and evading the immune system. The intracellular bacterium Shigella flexneri, the causative agent of bacillary dysentery, invades host cells in a vacuole that is subsequently ruptured to allow growth of the pathogen within the host cytoplasm. S. flexneri invasion has been classically described as a macropinocytosis-like process, however the underlying details and the role of macropinosomes in the intracellular bacterial lifestyle have remained elusive. We applied dynamic imaging and advanced large volume correlative light electron microscopy (CLEM) to study the highly transient events of S. flexneri's early invasion into host epithelial cells and elucidate some of its fundamental features. First, we demonstrate a clear distinction between two compartments formed during the first step of invasion: the bacterial containing vacuole and surrounding macropinosomes, often considered identical. Next, we report a functional link between macropinosomes and the process of vacuolar rupture, demonstrating that rupture timing is dependent on the availability of macropinosomes as well as the activity of the small GTPase Rab11 recruited directly to macropinosomes. We go on to reveal that the bacterial containing vacuole and macropinosomes come into direct contact at the onset of vacuolar rupture. Finally, we demonstrate that S. flexneri does not subvert pre-existing host endocytic vesicles during the invasion steps leading to vacuolar rupture, and propose that macropinosomes are the major compartment involved in these events. These results provide the basis for a new model of the early steps of S. flexneri epithelial cell invasion, establishing a different view of the enigmatic process of cytoplasmic access by invasive bacterial pathogens.

Space: A Final Frontier for Vacuolar Pathogens

There is a fundamental gap in our understanding of how a eukaryotic cell apportions the limited space within its cell membrane. Upon infection, a cell competes with intracellular pathogens for control of this same precious resource. The struggle between pathogen and host provides us with an opportunity to uncover the mechanisms regulating subcellular space by understanding how pathogens modulate vesicular traffic and membrane fusion events to create a specialized compartment for replication. By comparing several important intracellular pathogens, we review the molecular mechanisms and trafficking pathways that drive two space allocation strategies, the formation of tight and spacious pathogen-containing vacuoles. Additionally, we discuss the potential advantages of each pathogenic lifestyle, the broader implications these lifestyles might have for cellular biology and outline exciting opportunities for future investigation.

The frequency and duration of Salmonella-macrophage adhesion events determines infection efficiency

Salmonella enterica causes a range of important diseases in humans and a in a variety of animal species. The ability of bacteria to adhere to, invade and survive within host cells plays an important role in the pathogenesis of Salmonella infections. In systemic salmonellosis, macrophages constitute a niche for the proliferation of bacteria within the host organism. Salmonella enterica serovar Typhimurium is flagellated and the frequency with which this bacterium collides with a cell is important for infection efficiency. We investigated how bacterial motility affects infection efficiency, using a combination of population-level macrophage infection experiments and direct imaging of single-cell infection events, comparing wild-type and motility mutants. Non-motile and aflagellate bacterial strains, in contrast to wild-type bacteria, collide less frequently with macrophages, are in contact with the cell for less time and infect less frequently. Run-biased Salmonella also collide less frequently with macrophages but maintain contact with macrophages for a longer period of time than wild-type strains and infect the cells more readily. Our results suggest that uptake of S. Typhimurium by macrophages is dependent upon the duration of contact time of the bacterium with the cell, in addition to the frequency with which the bacteria collide with the cell.

Regulation of Macropinocytosis by Diacylglycerol Kinase ζ

Macropinosomes arise from the closure of plasma membrane ruffles to bring about the non-selective uptake of nutrients and solutes into cells. The morphological changes underlying ruffle formation and macropinosome biogenesis are driven by actin cytoskeleton rearrangements under the control of the Rho GTPase Rac1. We showed previously that Rac1 is activated by diacylglycerol kinase ζ (DGKζ), which phosphorylates diacylglycerol to yield phosphatidic acid. Here, we show DGKζ is required for optimal macropinocytosis induced by growth factor stimulation of mouse embryonic fibroblasts. Time-lapse imaging of live cells and quantitative analysis revealed DGKζ was associated with membrane ruffles and nascent macropinosomes. Macropinocytosis was attenuated in DGKζ-null cells, as determined by live imaging and vaccinia virus uptake experiments. Moreover, macropinosomes that did form in DGKζ-null cells were smaller than those found in wild type cells. Rescue of this defect required DGKζ catalytic activity, consistent with it also being required for Rac1 activation. A constitutively membrane bound DGKζ mutant substantially increased the size of macropinosomes and potentiated the effect of a constitutively active Rac1 mutant on macropinocytosis. Collectively, our results suggest DGKζ functions in concert with Rac1 to regulate macropinocytosis.

Active macropinocytosis induction by stimulation of epidermal growth factor receptor and oncogenic Ras expression potentiates cellular uptake efficacy of exosomes

Exosomes are approximately 100-nm vesicles that consist of a lipid bilayer of cellular membranes secreted in large quantities from various types of normal and disease-related cells. Endocytosis has been reported as a major pathway for the cellular uptake of exosomes; however, the detailed mechanisms of their cellular uptake are still unknown. Here, we demonstrate the active induction of macropinocytosis (accompanied by actin reorganisation, ruffling of plasma membrane, and engulfment of large volumes of extracellular fluid) by stimulation of cancer-related receptors and show that the epidermal growth factor (EGF) receptor significantly enhances the cellular uptake of exosomes. We also demonstrate that oncogenic K-Ras-expressing MIA PaCa-2 cells exhibit intensive macropinocytosis that actively transports extracellular exosomes into the cells compared with wild-type K-Ras-expressing BxPC-3 cells. Furthermore, encapsulation of the ribosome-inactivating protein saporin with EGF in exosomes using our simple electroporation method produces superior cytotoxicity via the enhanced cellular uptake of exosomes. Our findings contribute to the biological, pharmaceutical, and medical research fields in terms of understanding the macropinocytosis-mediated cellular uptake of exosomes with applications for exosomal delivery systems.

Regulation of bacterial virulence by Csr (Rsm) systems

Most bacterial pathogens have the remarkable ability to flourish in the external environment and in specialized host niches. This ability requires their metabolism, physiology, and virulence factors to be responsive to changes in their surroundings. It is no surprise that the underlying genetic circuitry that supports this adaptability is multilayered and exceedingly complex. Studies over the past 2 decades have established that the CsrA/RsmA proteins, global regulators of posttranscriptional gene expression, play important roles in the expression of virulence factors of numerous proteobacterial pathogens. To accomplish these tasks, CsrA binds to the 5' untranslated and/or early coding regions of mRNAs and alters translation, mRNA turnover, and/or transcript elongation. CsrA activity is regulated by noncoding small RNAs (sRNAs) that contain multiple CsrA binding sites, which permit them to sequester multiple CsrA homodimers away from mRNA targets. Environmental cues sensed by two-component signal transduction systems and other regulatory factors govern the expression of the CsrA-binding sRNAs and, ultimately, the effects of CsrA on secretion systems, surface molecules and biofilm formation, quorum sensing, motility, pigmentation, siderophore production, and phagocytic avoidance. This review presents the workings of the Csr system, the paradigm shift that it generated for understanding posttranscriptional regulation, and its roles in virulence networks of animal and plant pathogens.

Salmonellae interactions with host processes

Salmonellae invasion and intracellular replication within host cells result in a range of diseases, including gastroenteritis, bacteraemia, enteric fever and focal infections. In recent years, considerable progress has been made in our understanding of the molecular mechanisms that salmonellae use to alter host cell physiology; through the delivery of effector proteins with specific activities and through the modulation of defence and stress response pathways. In this Review, we summarize our current knowledge of the complex interplay between bacterial and host factors that leads to inflammation, disease and, in most cases, control of the infection by its animal hosts, with a particular focus on Salmonella enterica subsp. enterica serovar Typhimurium. We also highlight gaps in our knowledge of the contributions of salmonellae and the host to disease pathogenesis, and we suggest future avenues for further study.

Invasive Salmonella Typhimurium ST313 with naturally attenuated flagellin elicits reduced inflammation and replicates within macrophages

Invasive non-typhoidal Salmonella (iNTS) are an important cause of septicemia in children under the age of five years in sub-Saharan Africa. A novel genotype of Salmonella enterica subsp. enterica serovar Typhimurium (multi-locus sequence type [ST] 313) circulating in this geographic region is genetically different to from S. Typhimurium ST19 strains that are common throughout the rest of the world. S. Typhimurium ST313 strains have acquired pseudogenes and genetic deletions and appear to be evolving to become more like the typhoidal serovars S. Typhi and S. Paratyphi A. Epidemiological and clinical data show that S. Typhimurium ST313 strains are clinically associated with invasive systemic disease (bacteremia, septicemia, meningitis) rather than with gastroenteritis. The current work summarizes investigations of the broad hypothesis that S. Typhimurium ST313 isolates from Mali, West Africa, will behave differently from ST19 isolates in various in vitro assays. Here, we show that strains of the ST313 genotype are phagocytosed more efficiently and are highly resistant to killing by macrophage cell lines and primary mouse and human macrophages compared to ST19 strains. S. Typhimurium ST313 strains survived and replicated within different macrophages. Infection of macrophages with S. Typhimurium ST19 strains resulted in increased apoptosis and higher production of proinflammatory cytokines, as measured by gene expression and protein production, compared to S. Typhimurium ST313 strains. This difference in proinflammatory cytokine production and cell death between S. Typhimurium ST19 and ST313 strains could be explained, in part, by an increased production of flagellin by ST19 strains. These observations provide further evidence that S. Typhimurium ST313 strains are phenotypically different to ST19 strains and instead share similar pathogenic characteristics with typhoidal Salmonella serovars.

Emerging Roles for RIPK1 and RIPK3 in Pathogen-Induced Cell Death and Host Immunity

Receptor-interacting protein kinases 1 and 3 (RIPK1 and RIPK3 ) are homologous serine-threonine kinases that were recognized for their roles in directing programmed necrotic cell death or necroptosis under a broad range of pathologic settings. Emerging evidence suggests new physiologic roles for RIPK1 and RIPK3 in mediating cell death of innate immune responses. Our review discusses current evidence on the mechanisms and the impact of RIPK1- and/or RIPK3-dependent cell death in responses to a variety of viral and bacterial pathogens. Furthermore, the discussion also summarizes emerging roles for RIPK1 and RIPK3 in other facets of host immunity, including the maintenance of epithelial barrier function and pro-inflammatory processes that may, in some cases, manifest independent of cell death. Finally, we briefly consider the therapeutic opportunities in targeting RIPK1- and RIPK3-dependent processes in infection and immunity.

Salmonella modulates B cell biology to evade CD8(+) T cell-mediated immune responses

Although B cells and antibodies are the central effectors of humoral immunity, B cells can also produce and secrete cytokines and present antigen to helper T cells. The uptake of antigen is mainly mediated by endocytosis; thus, antigens are often presented by MHC-II molecules. However, it is unclear if B cells can present these same antigens via MHC-I molecules. Recently, Salmonella bacteria were found to infect B cells, allowing possible antigen cross-processing that could generate bacterial peptides for antigen presentation via MHC-I molecules. Here, we will discuss available knowledge regarding Salmonella antigen presentation by infected B cell MHC-I molecules and subsequent inhibitory effects on CD8(+) T cells for bacterial evasion of cell-mediated immunity.

The role of macrophages in the innate immune response to Streptococcus pneumoniae and Staphylococcus aureus: mechanisms and contrasts

Macrophages are critical mediators of innate immune responses against bacteria. The Gram-positive bacteria Streptococcus pneumoniae and Staphylococcus aureus express a range of virulence factors, which challenge macrophages' immune competence. We review how macrophages respond to this challenge. Macrophages employ a range of strategies to phagocytose and kill each pathogen. When the macrophages capacity to clear bacteria is overwhelmed macrophages play important roles in orchestrating the inflammatory response through pattern recognition receptor-mediated responses. Macrophages also ensure the inflammatory response is tightly constrained, to avoid tissue damage, and play an important role in downregulating the inflammatory response once initial bacterial replication is controlled.

Live-attenuated bacteria as a cancer vaccine vector

In the emerging field of active and specific cancer immunotherapy, strategies using live-attenuated bacterial vectors have matured in terms of academic and industrial development. Different bacterial species can be genetically engineered to deliver antigen to APCs with strong adjuvant effects due to their microbial origin. Proteic or DNA-encoding antigen delivery routes and natural bacterial tropisms might differ among species, permitting different applications. After many academic efforts to resolve safety and efficacy issues, some firms have recently engaged clinical trials with live Listeria or Salmonella spp. We describe here the main technological advances that allowed bacteria to become one of the most promising vectors in cancer immunotherapy.

The SdiA-regulated gene srgE encodes a type III secreted effector

Salmonella enterica serovar Typhimurium is a food-borne pathogen that causes severe gastroenteritis. The ability of Salmonella to cause disease depends on two type III secretion systems (T3SSs) encoded in two distinct Salmonella pathogenicity islands, 1 and 2 (SPI1 and SPI2, respectively). S. Typhimurium encodes a solo LuxR homolog, SdiA, which can detect the acyl-homoserine lactones (AHLs) produced by other bacteria and upregulate the rck operon and the srgE gene. SrgE is predicted to encode a protein of 488 residues with a coiled-coil domain between residues 345 and 382. In silico studies have provided conflicting predictions as to whether SrgE is a T3SS substrate. Therefore, in this work, we tested the hypothesis that SrgE is a T3SS effector by two methods, a β-lactamase activity assay and a split green fluorescent protein (GFP) complementation assay. SrgE with β-lactamase fused to residue 40, 100, 150, or 300 was indeed expressed and translocated into host cells, but SrgE with β-lactamase fused to residue 400 or 488 was not expressed, suggesting interference by the coiled-coil domain. Similarly, SrgE with GFP S11 fused to residue 300, but not to residue 488, was expressed and translocated into host cells. With both systems, translocation into host cells was dependent upon SPI2. A phylogenetic analysis indicated that srgE is found only within Salmonella enterica subspecies. It is found sporadically within both typhoidal and nontyphoidal serovars, although the SrgE protein sequences found within typhoidal serovars tend to cluster separately from those found in nontyphoidal serovars, suggesting functional diversification.

Pathogenesis of adherent-invasive Escherichia coli

The etiology of Crohn's disease (CD) is complex and involves both host susceptibility factors (i.e., the presence of particular genetic alleles) and environmental factors, including bacteria. In this regard, adherent-invasive Escherichia coli (AIEC), have recently emerged as an exciting potential etiological agent of CD. AIEC are distinguished from commensal strains of E. coli through their ability to adhere to and invade epithelial cells and replicate in macrophages. Recent molecular analyses have identified genes required for both invasion of epithelial cells and replication in the macrophage. However, these genetic studies, in combination with recent genome sequencing projects, have revealed that the pathogenesis of this group of bacteria cannot be explained by the presence of AIEC-specific genes. In this article, we review the role of AIEC as a pathobiont in the pathology of CD. We also describe the emerging link between AIEC and autophagy, and we propose a model for AIEC pathogenesis.