A novel ex vivo set-up for dynamic long-term characterization of processes on mucosal interfaces by confocal imaging and simultaneous cytokine measurements

Matrix stiffness regulates endosomal escape of uropathogenic E. coli

Uropathogenic E. coli (UPEC) infection in vivo is characterized by invasion of bladder umbrella epithelial cells followed by endosomal escape and proliferation in the cytoplasm to form intracellular bacterial communities. By contrast, UPEC infection in tissue culture models results in bacteria being trapped within Lamp1-positive endosomes where proliferation is limited. Pharmacological disruption of the actin cytoskeleton has been shown to facilitate UPEC endosomal escape in vitro and extracellular matrix stiffness is a well-characterized physiological regulator of actin dynamics; therefore, we hypothesized that substrate stiffness may play a role in UPEC endosomal escape. Using functionalized polyacrylamide substrates, we found that at physiological stiffness, UPEC escaped the endosome and proliferated rapidly in the cytoplasm of bladder epithelial cells. Dissection of the cytoskeletal signaling pathway demonstrated that inhibition of the Rho GTPase RhoB or its effector PRK1 was sufficient to increase cytoplasmic bacterial growth and that RhoB protein level was significantly reduced at physiological stiffness. Our data suggest that tissue stiffness is a critical regulator of intracellular bacterial growth. Due to the ease of doing genetic and pharmacological manipulations in cell culture, this model system may provide a useful tool for performing mechanistic studies on the intracellular life cycle of uropathogens.

Ubiquitination of Innate Immune Regulator TRAF3 Orchestrates Expulsion of Intracellular Bacteria by Exocyst Complex

Although the intracellular trafficking system is integral to most physiologic activities, its role in mediating immune responses to infection has remained elusive. Here, we report that infected bladder epithelial cells (BECs) mobilized the exocyst complex, a powerful exporter of subcellular vesicles, to rapidly expel intracellular bacteria back for clearance. Toll-like receptor (TLR) 4 signals emanating from bacteria-containing vesicles (BCVs) were found to trigger K33-linked polyubiquitination of TRAF3 at Lys168, which was then detected by RalGDS, a guanine nucleotide exchange factor (GEF) that precipitated the assembly of the exocyst complex. Although this distinct modification of TRAF3 served to connect innate immune signaling to the cellular trafficking apparatus, it crucially ensured temporal and spatial accuracy in determining which among the many subcellular vesicles was recognized and selected for expulsion in response to innate immune signaling.

Role of Eosinophils and Tumor Necrosis Factor Alpha in Interleukin-25-Mediated Protection from Amebic Colitis

The parasite Entamoeba histolytica is a cause of diarrhea in infants in low-income countries. Previously, it was shown that tumor necrosis factor alpha (TNF-α) production was associated with increased risk of E. histolytica diarrhea in children. Interleukin-25 (IL-25) is a cytokine that is produced by intestinal epithelial cells that has a role in maintenance of gut barrier function and inhibition of TNF-α production. IL-25 expression was decreased in humans and in the mouse model of amebic colitis. Repletion of IL-25 blocked E. histolytica infection and barrier disruption in mice, increased gut eosinophils, and suppressed colonic TNF-α. Depletion of eosinophils with anti-Siglec-F antibody prevented IL-25-mediated protection. In contrast, depletion of TNF-α resulted in resistance to amebic infection. We concluded that IL-25 provides protection from amebiasis, which is dependent upon intestinal eosinophils and suppression of TNF-α.

The intestinal epithelial barrier is important for protection from intestinal amebiasis. We discovered that the intestinal epithelial cytokine IL-25 was suppressed during amebic colitis in humans and that protection could be restored in the mouse model by IL-25 administration. IL-25 acted via eosinophils and suppressed TNF-α. This work illustrates a previously unrecognized pathway of innate mucosal immune response.

Drug and Vaccine Development for the Treatment and Prevention of Urinary Tract Infections

Urinary tract infections (UTI) are among the most common bacterial infections in humans, affecting millions of people every year. UTI cause significant morbidity in women throughout their lifespan, in infant boys, in older men, in individuals with underlying urinary tract abnormalities, and in those that require long-term urethral catheterization, such as patients with spinal cord injuries or incapacitated individuals living in nursing homes. Serious sequelae include frequent recurrences, pyelonephritis with sepsis, renal damage in young children, pre-term birth, and complications of frequent antimicrobial use including high-level antibiotic resistance and Clostridium difficile colitis. Uropathogenic E. coli (UPEC) cause the vast majority of UTI, but less common pathogens such as Enterococcus faecalis and other enterococci frequently take advantage of an abnormal or catheterized urinary tract to cause opportunistic infections. While antibiotic therapy has historically been very successful in controlling UTI, the high rate of recurrence remains a major problem, and many individuals suffer from chronically recurring UTI, requiring long-term prophylactic antibiotic regimens to prevent recurrent UTI. Furthermore, the global emergence of multi-drug resistant UPEC in the past ten years spotlights the need for alternative therapeutic and preventative strategies to combat UTI, including anti-infective drug therapies and vaccines. In this chapter, we review recent advances in the field of UTI pathogenesis, with an emphasis on the identification of promising drug and vaccine targets. We then discuss the development of new UTI drugs and vaccines, highlighting the challenges these approaches face and the need for a greater understanding of urinary tract mucosal immunity.

High accuracy 4D cell tracking into explanted skin using two-photon excitation microscopy

Two-photon excitation microscopy (2PEM) analysis of large explanted organs is still laborious, principally because of tissue movements inducing lateral and axial drifts during extended imaging sessions. Here, we describe a two-step approach to track motile T cells in murine dorsal explanted skin with the best accuracy. First, we compared various explanted skin mounting methods for 2PEM analysis to define the setup allowing for minimal sample drift over time. Second, we developed two algorithms with the ImageJ software (National Institute of Health, Bethesda, MD) to correct the residual drift using lateral and axial registration of the collagen network. Finally, we applied the macro we developed to track fluorescent T cells in explanted skin. We found that our newly developed macro is more efficient than freely or commercially available software for shift correction, leading to more accurate velocity calculations. Our work provides a practical guide for investigators interested to employ skin-imaging approaches and offers a free alternative to commercial software for correcting lateral and axial drifts.

Host-pathogen checkpoints and population bottlenecks in persistent and intracellular uropathogenic Escherichia coli bladder infection

Bladder infections affect millions of people yearly, and recurrent symptomatic infections (cystitis) are very common. The rapid increase in infections caused by multidrug-resistant uropathogens threatens to make recurrent cystitis an increasingly troubling public health concern. Uropathogenic Escherichia coli (UPEC) cause the vast majority of bladder infections. Upon entry into the lower urinary tract, UPEC face obstacles to colonization that constitute population bottlenecks, reducing diversity, and selecting for fit clones. A critical mucosal barrier to bladder infection is the epithelium (urothelium). UPEC bypass this barrier when they invade urothelial cells and form intracellular bacterial communities (IBCs), a process which requires type 1 pili. IBCs are transient in nature, occurring primarily during acute infection. Chronic bladder infection is common and can be either latent, in the form of the quiescent intracellular reservoir (QIR), or active, in the form of asymptomatic bacteriuria (ASB/ABU) or chronic cystitis. In mice, the fate of bladder infection, QIR, ASB, or chronic cystitis, is determined within the first 24 h of infection and constitutes a putative host-pathogen mucosal checkpoint that contributes to susceptibility to recurrent cystitis. Knowledge of these checkpoints and bottlenecks is critical for our understanding of bladder infection and efforts to devise novel therapeutic strategies.

Salmonella transiently reside in luminal neutrophils in the inflamed gut

BACKGROUND

Enteric pathogens need to grow efficiently in the gut lumen in order to cause disease and ensure transmission. The interior of the gut forms a complex environment comprising the mucosal surface area and the inner gut lumen with epithelial cell debris and food particles. Recruitment of neutrophils to the intestinal lumen is a hallmark of non-typhoidal Salmonella enterica infections in humans. Here, we analyzed the interaction of gut luminal neutrophils with S. enterica serovar Typhimurium (S. Tm) in a mouse colitis model.

RESULTS

Upon S. Tm(wt) infection, neutrophils transmigrate across the mucosa into the intestinal lumen. We detected a majority of pathogens associated with luminal neutrophils 20 hours after infection. Neutrophils are viable and actively engulf S. Tm, as demonstrated by live microscopy. Using S. Tm mutant strains defective in tissue invasion we show that pathogens are mostly taken up in the gut lumen at the epithelial barrier by luminal neutrophils. In these luminal neutrophils, S. Tm induces expression of genes typically required for its intracellular lifestyle such as siderophore production iroBCDE and the Salmonella pathogenicity island 2 encoded type three secretion system (TTSS-2). This shows that S. Tm at least transiently survives and responds to engulfment by gut luminal neutrophils. Gentamicin protection experiments suggest that the life-span of luminal neutrophils is limited and that S. Tm is subsequently released into the gut lumen. This "fast cycling" through the intracellular compartment of gut luminal neutrophils would explain the high fraction of TTSS-2 and iroBCDE expressing intra- and extracellular bacteria in the lumen of the infected gut.

CONCLUSION

In conclusion, live neutrophils recruited during acute S. Tm colitis engulf pathogens in the gut lumen and may thus actively engage in shaping the environment of pathogens and commensals in the inflamed gut.