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Pseudomonas aeruginosa is a major cause of severe infections that lead to bacteremia and high patient mortality. P. aeruginosa has evolved numerous evasion and subversion mechanisms that work in concert to overcome immune recognition and effector functions in hospitalized and immunosuppressed individuals. Here, we have used multilaser spinning-disk intravital microscopy to monitor the blood-borne stage in a murine bacteremic model of P. aeruginosa infection. P. aeruginosa adhered avidly to lung vasculature, where patrolling neutrophils and other immune cells were virtually blind to the pathogen's presence. This cloaking phenomenon was attributed to expression of Psl exopolysaccharide. Although an antiPsl mAb activated complement and enhanced neutrophil recognition of P. aeruginosa, neutrophil-mediated clearance of the pathogen was suboptimal owing to a second subversion mechanism, namely the type 3 secretion (T3S) injectisome. Indeed, T3S prevented phagosome acidification and resisted killing inside these compartments. Antibody-mediated inhibition of the T3S protein PcrV did not enhance bacterial phagocytosis but did enhance killing of the few bacteria ingested by neutrophils. A bispecific mAb targeting both Psl and PcrV enhanced neutrophil uptake of P. aeruginosa and also greatly increased inhibition of T3S function, allowing for phagosome acidification and bacterial killing. These data highlight the need to block multiple evasion and subversion mechanisms in tandem to kill P. aeruginosa., Introduction Pseudomonas aeruginosa is an important opportunistic bacterial pathogen that causes life-threatening infections in critically ill and immunocompromised individuals. It is one of the most common causes of Gram-negative pneumonia [...]

Resident macrophages play a unique role in the maintenance of tissue function. As phagocytes, they are an essential first line defenders against pathogens and much of the initial characterization of these cells was focused on their interaction with viral and bacterial pathogens. However, these cells are increasingly recognized as contributing to more than just host defense. Through cytokine production, receptor engagement and gap junction communication resident macrophages tune tissue inflammatory tone, influence adaptive immune cell phenotype and regulate tissue structure and function. This review highlights resident macrophages in the liver and lung as they hold unique roles in the maintenance of the interface between the circulatory system and the external environment. As such, we detail the developmental origin of these cells, their contribution to host defense and the array of tools these cells use to regulate tissue homeostasis. [ABSTRACT FROM AUTHOR]

The article presents a study that explores the programmed death ligand 1 (PD-L1) neutrophils contribute to injury-induced infection susceptibility. It describes a rapid increase in neutrophil cell numbers in the lungs following induction of thermal injury. It provides evidence that injury promotes changes to neutrophils that are important for wound healing but contribute to infection susceptibility.

It has long been appreciated that understanding the interactions between the host and the pathogens that make us sick is critical for the prevention and treatment of disease. As antibiotics become increasingly ineffective, targeting the host and specific bacterial evasion mechanisms are becoming novel therapeutic approaches. The technology used to understand host‐pathogen interactions has dramatically advanced over the last century. We have moved away from using simple in vitro assays focused on single‐cell events to technologies that allow us to observe complex multicellular interactions in real time in live animals. Specifically, intravital microscopy (IVM) has improved our understanding of infection, from viral to bacterial to parasitic, and how the host immune system responds to these infections. Yet, at the same time it has allowed us to appreciate just how complex these interactions are and that current experimental models still have a number of limitations. In this review, we will discuss the advances in vivo IVM has brought to the study of host‐pathogen interactions, focusing primarily on bacterial infections and innate immunity. [ABSTRACT FROM AUTHOR]

As babies leave their mothers' wombs, they are colonized by commensal bacteria. A new study shows that these microbes in a newborn's gut can promote an increase in circulating neutrophils that can guard the body from infections. Altering a neonate's gut microbiota with perinatal antibiotic treatment can impair neutrophils, leaving the newborn vulnerable to attack by harmful bacteria (pages 524-530)., Globally, around 4 million newborn babies die yearly due to sepsis (1). Prolonged use of antibiotics is currently under clinical practice in the interest of preventing early-onset sepsis in newborns [...]

Summary iNKT cells are a subset of innate T cells that recognize glycolipids presented on CD1d molecules and protect against bacterial infections, including S. pneumoniae . Using lung intravital imaging, we examined the behavior and mechanism of pulmonary iNKT cell activation in response to the specific iNKT cell ligand α-galactosylceramide or S. pneumoniae infection. In untreated mice, the major fraction of iNKT cells resided in the vasculature, but a small critical population resided in the extravascular space in proximity to monocyte-derived DCs. Administration of either α-GalCer or S. pneumoniae induced CD1d-dependent rapid recruitment of neutrophils out of the vasculature. The neutrophils guided iNKT cells from the lung vasculature via CCL17. Depletion of monocyte-derived DCs abrogated both the neutrophil and subsequent iNKT cell extravasation. Moreover, impairing iNKT cell recruitment by blocking CCL17 increased susceptibility to S. pneumoniae infection, suggesting a critical role for the influx of iNKT cells in host defense. [ABSTRACT FROM AUTHOR]

This study identified and characterized enteropathogenic Escherichia coli (EPEC) in the Canadian food supply. Eighteen of 450 E. coli isolates from food animal sources were identified as atypical EPEC (aEPEC). Several of the aEPEC isolates identified in this study possessed multiple virulence genes, exhibited adherence and attaching and effacing (ME) lesion formation, disrupted tight junctions, and were coclassified with the extraintestinal pathogenic E. coli (EXPEC) and enterotoxigenic E. coli (ETEC) pathotypes. [ABSTRACT FROM AUTHOR]

Enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC and EHEC) are food-borne pathogens that cause severe diarrhoeal disease in humans. Citrobacter rodentium is a related mouse pathogen that serves as a small animal model for EPEC and EHEC infections. EPEC, EHEC and C. rodentium translocate bacterial virulence proteins directly into host cells via a type III secretion system (T3SS). Non-LEE-encoded effector A (NleA) is a T3SS effector that is common to EPEC, EHEC and C. rodentium and is required for bacterial virulence. NleA localizes to the host cell secretory pathway and inhibits vesicle trafficking by interacting with the Sec24 subunit of mammalian coatamer protein II complex (COPII). Mammalian cells express four paralogues of Sec24 (Sec24A-D), which mediate selection of cargo proteins for transport and possess distinct, but overlapping cargo specificities. Here, we show that NleA binds Sec24A-D with two distinct mechanisms. An NleA protein variant with greatly diminished interaction with all Sec24 paralogues does not properly localize, does not inhibit COPII-mediated vesicle budding, and does not confer virulence in the mouse infection model. Together, this work provides strong evidence that the interaction and inhibition of COPII by NleA is an important aspect of EPEC- and EHEC-mediated disease. [ABSTRACT FROM AUTHOR]

Enteropathogenic Escherichia coli (EPEC) is a diarrhoeal pathogen that adheres to epithelial cells of the small intestine and uses a type III secretion system to inject effector proteins into host cells. EPEC infection leads to disruption of host intestinal tight junctions that are important for maintaining intestinal barrier function. This disruption is dependent on the bacterial type III secretion system, as well as the translocated effectors EspF and Map. Here we show that a third type III translocated bacterial effector protein, NleA, is also involved in tight junction disruption during EPEC infection. Using the drug Brefeldin A, we demonstrate that the effect of NleA on tight junction integrity is related to its inhibition of host cell protein trafficking through COPII-dependent pathways. These results suggest that NleA's striking effect on virulence is mediated, at least in part, via its role in disruption of intestinal barrier function. [ABSTRACT FROM AUTHOR]

Enterohemorrhagic Escherichia coli (EHEC) is a water- and food-borne pathogen that causes hemorrhagic colitis. EHEC uses a type III secretion system (T3SS) to translocate effector proteins that subvert host cell function. T3SS-substrates encoded outside of the locus of enterocyte effacement are important to E. coli pathogenesis. We discovered an EHEC secreted protein, NleF, encoded by z6020 in O-island 71 of E. coli EDL933 that we hypothesized to be a T3SS substrate. Experiments are presented that probe the function of NleF and its role in virulence. Immunoblotting of secreted and translocated proteins suggest that NleF is secreted by the T3SS and is translocated into host cells in vitro where it localizes to the host cytoplasm. Infection of HeLa cells with E. coli possessing or lacking nleF and transient expression of NleF-GFP via transfection did not reveal a significant role for NleF in several assays of bacterial adherence, host cytoskeletal remodeling, or host protein secretion. However, competitive coinfection of mice with Citrobacter rodentium strains possessing or lacking nleF suggested a contribution of NleF to bacterial colonization. Challenge of gnotobiotic piglets also revealed a role for NleF in colonization of the piglet colon and rectoanal junction. [ABSTRACT FROM AUTHOR]

Summary: Enterohemorrhagic and enteropathogenic Escherichia coli (EHEC and EPEC) maintain an extracellular lifestyle and use a type III secretion system to translocate effector proteins into the host cytosol. These effectors manipulate host pathways to favor bacterial replication and survival. NleA is an EHEC/EPEC- and related species-specific translocated effector protein that is essential for bacterial virulence. However, the mechanism by which NleA impacts virulence remains undetermined. Here we demonstrate that NleA compromises the Sec23/24 complex, a component of the mammalian COPII protein coat that shapes intracellular protein transport vesicles, by directly binding Sec24. Expression of an NleA-GFP fusion protein reduces the efficiency of cellular secretion by 50%, and secretion is inhibited in EPEC-infected cells. Direct biochemical experiments show that NleA inhibits COPII-dependent protein export from the endoplasmic reticulum. Collectively, these findings indicate that disruption of COPII function in host cells contributes to the virulence of EPEC and EHEC. [Copyright &y& Elsevier]

• Alveolar macrophages are the first line of defense against external insults and maintain lung homeostasis. • Proteomics applications have been used to study the multifaceted roles of AMs in different respiratory diseases. • Advanced proteomics technologies help identify potential candidates in AMs that can be targeted for diagnosis and therapy. As we learn more about chronic lung diseases, we are seeing that an unbalanced immune system plays a key role in disease pathogenesis. Innate immune cells, particularly tissue-resident macrophages, are important navigators of immunity, both during infection and in non-communicable lung disease. In the lung, alveolar macrophages are considered some of the most critical and diverse immune cells, yet despite an array of studies over the years, alveolar macrophages remain poorly understood. In this review, we highlight the importance of alveolar macrophages in health and disease, and discuss how proteomics can be used to elucidate mechanistic information and identify potential targets for therapy development. [ABSTRACT FROM AUTHOR]

Summary Bacterial biofilm infections are difficult to eradicate because of antibiotic insusceptibility and high recurrence rates. Biofilm formation by Pseudomonas aeruginosa , a leading cause of bacterial keratitis, is facilitated by the bacterial Psl exopolysaccharide and associated with heightened virulence. Using intravital microscopy, we observed that neutrophilic recruitment to corneal infections limits P. aeruginosa biofilms to the outer eye surface, preventing bacterial dissemination. Neutrophils moved to the base of forming biofilms, where they underwent neutrophil extracellular trap formation (NETosis) in response to high expression of the bacterial type-3 secretion system (T3SS). NETs formed a barrier "dead zone," confining bacteria to the external corneal environment and inhibiting bacterial dissemination into the brain. Once formed, ocular biofilms were resistant to antibiotics and neutrophil killing, advancing eye pathology. However, blocking both Psl and T3SS together with antibiotic treatment broke down the biofilm and reversed keratitis, suggesting future therapeutic strategies for this intractable infection. Graphical Abstract Highlights • P. aeruginosa keratitis infections result in biofilm formation on the cornea • NETs form at the base of the biofilm, triggered by the type-3 secretion system (T3SS) • NETs stop bacterial dissemination into the brain but promote antibiotic resistance • Blocking exopolysaccharide Psl and the T3SS allowed neutrophils to break down the biofilm Thanabalasuriar et al. characterize the interplay between neutrophils and Pseudomonas biofilms on the cornea. At the base of the biofilm, neutrophils encountered expression of the type-3 secretion system, leading to an expansive deposition of neutrophil extracellular traps (NETs). NET formation impeded bacterial dissemination into the brain yet promoted antibiotic resistance. [ABSTRACT FROM AUTHOR]

Summary During sepsis, small blood vessels can become occluded by large platelet aggregates of poorly understood etiology. During Staphylococcal aureus infection, sepsis severity is linked to the bacterial α-toxin (α-hemolysin, AT) through unclear mechanisms. In this study, we visualized intravascular events in the microcirculation and found that intravenous AT injection induces rapid platelet aggregation, forming dynamic micro-thrombi in the microcirculation. These aggregates are retained in the liver sinusoids and kidney glomeruli, causing multi-organ dysfunction. Acute staphylococcal infection results in sequestration of most bacteria by liver macrophages. Platelets are initially recruited to these macrophages and help eradicate S . aureus . However, at later time points, AT causes aberrant and damaging thrombosis throughout the liver. Treatment with an AT neutralizing antibody (MEDI4893 ∗ ) prevents platelet aggregation and subsequent liver damage, without affecting the initial and beneficial platelet recruitment. Thus, AT neutralization may represent a promising approach to combat staphylococcal-induced intravascular coagulation and organ dysfunction. [ABSTRACT FROM AUTHOR]

Citrobacter rodentium is a natural mouse pathogen widely used as a model for enteropathogenic and enterohemorrhagic Escherichia coli infections in humans. While C. rodentium causes self-limiting colitis in most inbred mouse strains, it induces fatal diarrhoea in susceptible strains. The physiological pathways as well as the genetic determinants leading to susceptibility have remained largely uncharacterized. Here we use a forward genetic approach to identify the R-spondin2 gene as a major determinant of susceptibility to C. rodentium infection. Robust induction of R-spondin2 expression during infection in susceptible mouse strains causes a potent Wnt-mediated proliferative response of colonic crypt cells, leading to the generation of an immature and poorly differentiated colonic epithelium with deficiencies in ion-transport components. Our data demonstrate a previously unknown role of R-spondins and Wnt signalling in susceptibility to infectious diarrhoea and identify R-spondin2 as a key molecular link between infection and intestinal homoeostasis. [ABSTRACT FROM AUTHOR]

We have analyzed the means by which the Nck family of adaptor proteins couples adhesion proteins to actin reorganization. The nephrin adhesion protein is essential for the formation of actin-based foot processes in glomerular podocytes. The clustering of nephrin induces its tyrosine phosphorylation, Nck recruitment, and sustained localized actin polymerization. Any one of three phosphorylated (p)YDXV motifs on nephrin is sufficient to recruit Nck through its Src homology 2 (SH2) domain and induce localized actin polymerization at these clusters. Similarly, Nck SH3 mutants in which only the second or third SH3 domain is functional can mediate nephrin-induced actin polymerization. However, combining such nephrin and Nck mutants attenuates actin polymerization at nephrin-Nck clusters. We propose that the multiple Nck SH2-binding motifs on nephrin and the multiple SH3 domains of Nck act cooperatively to recruit the high local concentration of effectors at sites of nephrin activation that is required to initiate and maintain actin polymerization in vivo. We also find that YDXV motifs in the Tir protein of enteropathogenic Escherichia coli and nephrin are functionally interchangeable, indicating that Tir reorganizes the actin cytoskeleton by molecular mimicry of nephrin-like signaling. Together, these data identify pYDXV/Nck signaling as a potent and portable mechanism for physiological and pathological actin regulation. [ABSTRACT FROM AUTHOR]

During respiration, humans breathe in more than 10,000 liters of non-sterile air daily, allowing some pathogens access to alveoli. Interestingly, alveoli outnumber alveolar macrophages (AMs), which favors alveoli devoid of AMs. If AMs, like most tissue macrophages, are sessile, then this numerical advantage would be exploited by pathogens unless neutrophils from the blood stream intervened. However, this would translate to omnipresent persistent inflammation. Developing in vivo real-time intravital imaging of alveoli revealed AMs crawling in and between alveoli using the pores of Kohn. Importantly, these macrophages sensed, chemotaxed, and, with high efficiency, phagocytosed inhaled bacterial pathogens such as P. aeruginosa and S. aureus , cloaking the bacteria from neutrophils. Impairing AM chemotaxis toward bacteria induced superfluous neutrophil recruitment, leading to inappropriate inflammation and injury. In a disease context, influenza A virus infection impaired AM crawling via the type II interferon signaling pathway, and this greatly increased secondary bacterial co-infection. • AMs patrol and clean the alveolar spaces • AMs have directed movement toward inhaled bacteria • AM migration is crucial for bacterial clearance • AM migration is impaired during viral infection Neupane et al. develop an intravital method to image alveolar macrophages in the lung in real time, overcoming the challenge presented by the air-liquid-air barrier of the alveoli. They use this in vivo approach to show that alveolar macrophages move between alveoli to provide efficient immune surveillance of the airway and phagocytosis of inhaled bacteria before they can induce harmful lung inflammation. Furthermore, they show that respiratory virus infections can interfere with alveolar macrophage surveillance, leading to bacterial superinfection. [ABSTRACT FROM AUTHOR]