A three-dimensional tissue culture model for the study of attach and efface lesion formation by enteropathogenic and enterohaemorrhagic Escherichia coli

Biomanufacturing of 3D Tissue Constructs in Microgravity and their Applications in Human Pathophysiological Studies

The growing interest in bioengineering in-vivo-like 3D functional tissues has led to novel approaches to the biomanufacturing process as well as expanded applications for these unique tissue constructs. Microgravity, as seen in spaceflight, is a unique environment that may be beneficial to the tissue-engineering process but cannot be completely replicated on Earth. Additionally, the expense and practical challenges of conducting human and animal research in space make bioengineered microphysiological systems an attractive research model. In this review, published research that exploits real and simulated microgravity to improve the biomanufacturing of a wide range of tissue types as well as those studies that use microphysiological systems, such as organ/tissue chips and multicellular organoids, for modeling human diseases in space are summarized. This review discusses real and simulated microgravity platforms and applications in tissue-engineered microphysiological systems across three topics: 1) application of microgravity to improve the biomanufacturing of tissue constructs, 2) use of tissue constructs fabricated in microgravity as models for human diseases on Earth, and 3) investigating the effects of microgravity on human tissues using biofabricated in vitro models. These current achievements represent important progress in understanding the physiological effects of microgravity and exploiting their advantages for tissue biomanufacturing.

Spaceflight Analogue Culture Enhances the Host-Pathogen Interaction Between Salmonella and a 3-D Biomimetic Intestinal Co-Culture Model

Physical forces associated with spaceflight and spaceflight analogue culture regulate a wide range of physiological responses by both bacterial and mammalian cells that can impact infection. However, our mechanistic understanding of how these environments regulate host-pathogen interactions in humans is poorly understood. Using a spaceflight analogue low fluid shear culture system, we investigated the effect of Low Shear Modeled Microgravity (LSMMG) culture on the colonization of Salmonella Typhimurium in a 3-D biomimetic model of human colonic epithelium containing macrophages. RNA-seq profiling of stationary phase wild type and Δhfq mutant bacteria alone indicated that LSMMG culture induced global changes in gene expression in both strains and that the RNA binding protein Hfq played a significant role in regulating the transcriptional response of the pathogen to LSMMG culture. However, a core set of genes important for adhesion, invasion, and motility were commonly induced in both strains. LSMMG culture enhanced the colonization (adherence, invasion and intracellular survival) of Salmonella in this advanced model of intestinal epithelium using a mechanism that was independent of Hfq. Although S. Typhimurium Δhfq mutants are normally defective for invasion when grown as conventional shaking cultures, LSMMG conditions unexpectedly enabled high levels of colonization by an isogenic Δhfq mutant. In response to infection with either the wild type or mutant, host cells upregulated transcripts involved in inflammation, tissue remodeling, and wound healing during intracellular survival. Interestingly, infection by the Δhfq mutant led to fewer transcriptional differences between LSMMG- and control-infected host cells relative to infection with the wild type strain. This is the first study to investigate the effect of LSMMG culture on the interaction between S. Typhimurium and a 3-D model of human intestinal tissue. These findings advance our understanding of how physical forces can impact the early stages of human enteric salmonellosis.

Imaging host-pathogen interactions using epithelial and bacterial cell infection models

The age-old saying, seeing is believing, could not be truer when we think about the value of imaging interactions between epithelial cells and bacterial pathogens. Imaging and culturing techniques have vastly improved over the years, and the breadth and depth of these methods is ever increasing. These technical advances have benefited researchers greatly; however, due to the large number of potential model systems and microscopy techniques to choose from, it can be overwhelming to select the most appropriate tools for your research question. This Review discusses a variety of available epithelial culturing methods and quality control experiments that can be performed, and outlines various options commonly used to fluorescently label bacterial and mammalian cell components. Both light- and electron-microscopy techniques are reviewed, with descriptions of both technical aspects and common applications. Several examples of imaging bacterial pathogens and their interactions with epithelial cells are discussed to provide researchers with an idea of the types of biological questions that can be successfully answered by using microscopy.

Advanced 3D Cell Culture Techniques in Micro-Bioreactors, Part II: Systems and Applications

In this second part of our systematic review on the research area of 3D cell culture in micro-bioreactors we give a detailed description of the published work with regard to the existing micro-bioreactor types and their applications, and highlight important results gathered with the respective systems. As an interesting detail, we found that micro-bioreactors have already been used in SARS-CoV research prior to the SARS-CoV2 pandemic. As our literature research revealed a variety of 3D cell culture configurations in the examined bioreactor systems, we defined in review part one “complexity levels” by means of the corresponding 3D cell culture techniques applied in the systems. The definition of the complexity is thereby based on the knowledge that the spatial distribution of cell-extracellular matrix interactions and the spatial distribution of homologous and heterologous cell–cell contacts play an important role in modulating cell functions. Because at least one of these parameters can be assigned to the 3D cell culture techniques discussed in the present review, we structured the studies according to the complexity levels applied in the MBR systems.

Advanced 3D Cell Culture Techniques in Micro-Bioreactors, Part I: A Systematic Analysis of the Literature Published between 2000 and 2020

Bioreactors have proven useful for a vast amount of applications. Besides classical large-scale bioreactors and fermenters for prokaryotic and eukaryotic organisms, micro-bioreactors, as specialized bioreactor systems, have become an invaluable tool for mammalian 3D cell cultures. In this systematic review we analyze the literature in the field of eukaryotic 3D cell culture in micro-bioreactors within the last 20 years. For this, we define complexity levels with regard to the cellular 3D microenvironment concerning cell–matrix-contact, cell–cell-contact and the number of different cell types present at the same time. Moreover, we examine the data with regard to the micro-bioreactor design including mode of cell stimulation/nutrient supply and materials used for the micro-bioreactors, the corresponding 3D cell culture techniques and the related cellular microenvironment, the cell types and in vitro models used. As a data source we used the National Library of Medicine and analyzed the studies published from 2000 to 2020.

Novel Antibiotic Testing Approaches Reveal Reduced Antibiotic Efficacy Against Shiga Toxin-Producing Escherichia coli O157:H7 Under Simulated Microgravity

As a foodborne and environmental pathogen, Shiga toxin-producing Escherichia coli O157:H7 could pose a health threat to immunocompromised astronauts during a space mission. In this study, novel approaches, including real-time testing and direct evaluation of resistance mechanisms, were used to evaluate antibiotic efficacy against E. coli O157:H7 under low-shear modeled microgravity (LSMMG) produced using a rotary cell culture system. When compared with normal gravity (NG), bacterial growth was increased under LSMMG in the presence of sub-inhibitory nalidixic acid concentrations and there was an accompanying up-regulation of stress-related genes. LSMMG also induced transcriptional changes of the virulence genes stx1 and stx2, highlighting the potential risk of inappropriate antibiotic use during a spaceflight. The degree of bacterial cell damage induced by the antibiotics was reduced under LSMMG, suggesting low induction of reactive oxygen species. Efflux pumps were also shown to play an important role in these responses. Increased cell filamentation was observed under LSMMG upon ampicillin treatment, possibly reflecting a protective mechanism against exposure to antibiotics. These observations indicate that, in the presence of antibiotics, the survival of E. coli O157:H7 is greater under LSMMG than under NG, indicating that antibiotic therapies may need to be adjusted during space missions.

Modeling Host-Pathogen Interactions in the Context of the Microenvironment: Three-Dimensional Cell Culture Comes of Age

Tissues and organs provide the structural and biochemical landscapes upon which microbial pathogens and commensals function to regulate health and disease. While flat two-dimensional (2-D) monolayers composed of a single cell type have provided important insight into understanding host-pathogen interactions and infectious disease mechanisms, these reductionist models lack many essential features present in the native host microenvironment that are known to regulate infection, including three-dimensional (3-D) architecture, multicellular complexity, commensal microbiota, gas exchange and nutrient gradients, and physiologically relevant biomechanical forces (e.g., fluid shear, stretch, compression). A major challenge in tissue engineering for infectious disease research is recreating this dynamic 3-D microenvironment (biological, chemical, and physical/mechanical) to more accurately model the initiation and progression of host-pathogen interactions in the laboratory. Here we review selected 3-D models of human intestinal mucosa, which represent a major portal of entry for infectious pathogens and an important niche for commensal microbiota. We highlight seminal studies that have used these models to interrogate host-pathogen interactions and infectious disease mechanisms, and we present this literature in the appropriate historical context. Models discussed include 3-D organotypic cultures engineered in the rotating wall vessel (RWV) bioreactor, extracellular matrix (ECM)-embedded/organoid models, and organ-on-a-chip (OAC) models. Collectively, these technologies provide a more physiologically relevant and predictive framework for investigating infectious disease mechanisms and antimicrobial therapies at the intersection of the host, microbe, and their local microenvironments.

A primary human macrophage-enteroid co-culture model to investigate mucosal gut physiology and host-pathogen interactions

Integration of the intestinal epithelium and the mucosal immune system is critical for gut homeostasis. The intestinal epithelium is a functional barrier that secludes luminal content, senses changes in the gut microenvironment, and releases immune regulators that signal underlying immune cells. However, interactions between epithelial and innate immune cells to maintain barrier integrity and prevent infection are complex and poorly understood. We developed and characterized a primary human macrophage-enteroid co-culture model for in-depth studies of epithelial and macrophage interactions. Human intestinal stem cell-derived enteroid monolayers co-cultured with human monocyte-derived macrophages were used to evaluate barrier function, cytokine secretion, and protein expression under basal conditions and following bacterial infection. Macrophages enhanced barrier function and maturity of enteroid monolayers as indicated by increased transepithelial electrical resistance and cell height. Communication between the epithelium and macrophages was demonstrated through morphological changes and cytokine production. Intraepithelial macrophage projections, efficient phagocytosis, and stabilized enteroid barrier function revealed a coordinated response to enterotoxigenic and enteropathogenic E. coli infections. In summary, we have established the first primary human macrophage-enteroid co-culture system, defined conditions that allow for a practical and reproducible culture model, and demonstrated its suitability to study gut physiology and host responses to enteric pathogens.

Three-dimensional organotypic co-culture model of intestinal epithelial cells and macrophages to study Salmonella enterica colonization patterns

Three-dimensional models of human intestinal epithelium mimic the differentiated form and function of parental tissues often not exhibited by two-dimensional monolayers and respond to Salmonella in key ways that reflect in vivo infections. To further enhance the physiological relevance of three-dimensional models to more closely approximate in vivo intestinal microenvironments encountered by Salmonella, we developed and validated a novel three-dimensional co-culture infection model of colonic epithelial cells and macrophages using the NASA Rotating Wall Vessel bioreactor. First, U937 cells were activated upon collagen-coated scaffolds. HT-29 epithelial cells were then added and the three-dimensional model was cultured in the bioreactor until optimal differentiation was reached, as assessed by immunohistochemical profiling and bead uptake assays. The new co-culture model exhibited in vivo-like structural and phenotypic characteristics, including three-dimensional architecture, apical-basolateral polarity, well-formed tight/adherens junctions, mucin, multiple epithelial cell types, and functional macrophages. Phagocytic activity of macrophages was confirmed by uptake of inert, bacteria-sized beads. Contribution of macrophages to infection was assessed by colonization studies of Salmonella pathovars with different host adaptations and disease phenotypes (Typhimurium ST19 strain SL1344 and ST313 strain D23580; Typhi Ty2). In addition, Salmonella were cultured aerobically or microaerobically, recapitulating environments encountered prior to and during intestinal infection, respectively. All Salmonella strains exhibited decreased colonization in co-culture (HT-29-U937) relative to epithelial (HT-29) models, indicating antimicrobial function of macrophages. Interestingly, D23580 exhibited enhanced replication/survival in both models following invasion. Pathovar-specific differences in colonization and intracellular co-localization patterns were observed. These findings emphasize the power of incorporating a series of related three-dimensional models within a study to identify microenvironmental factors important for regulating infection.

Using spaceflight analog bioreactor technology, Cheryl Nickerson at Arizona State University and collaborators developed and validated a new three-dimensional (3-D) intestinal co-culture model containing multiple differentiated epithelial cell types and phagocytic macrophages with antibacterial function to study infection by multiple pathovars of Salmonella. This study is the first to show that these pathovars (known to possess different host adaptations, antibiotic resistance profiles and disease phenotypes), display markedly different colonization and intracellular co-localization patterns using this physiologically relevant new 3-D intestinal co-culture model. This advanced model, that integrates a key immune cell type important for Salmonella infection, offers a powerful new tool in understanding enteric pathogenesis and may lead to unexpected pathogenesis mechanisms and therapeutic targets that have been previously unobserved or unappreciated using other intestinal cell culture models.

Microgravity as a biological tool to examine host-pathogen interactions and to guide development of therapeutics and preventatives that target pathogenic bacteria

Space exploration programs have long been interested in the effects of spaceflight on biology. This research is important not only in its relevance to future deep space exploration, but also because it has allowed investigators to ask questions about how gravity impacts cell behavior here on Earth. In the 1980s, scientists designed and built the first rotating wall vessel, capable of mimicking the low shear environment found in space. This vessel has since been used to investigate growth of both microorganisms and human tissue cells in low shear modeled microgravity conditions. Bacterial behavior has been shown to be altered both in space and under simulated microgravity conditions. In some cases, bacteria appear attenuated, whereas in others virulence is enhanced. This has consequences not only for manned spaceflight, but poses larger questions about the ability of bacteria to sense the world around them. By using the microgravity environment as a tool, we can exploit this phenomenon in the search for new therapeutics and preventatives against pathogenic bacteria for use both in space and on Earth.

Development of a Multicellular Three-dimensional Organotypic Model of the Human Intestinal Mucosa Grown Under Microgravity

Because cells growing in a three-dimensional (3-D) environment have the potential to bridge many gaps of cell cultivation in 2-D environments (e.g., flasks or dishes). In fact, it is widely recognized that cells grown in flasks or dishes tend to de-differentiate and lose specialized features of the tissues from which they were derived. Currently, there are mainly two types of 3-D culture systems where the cells are seeded into scaffolds mimicking the native extracellular matrix (ECM): (a) static models and (b) models using bioreactors. The first breakthrough was the static 3-D models. 3-D models using bioreactors such as the rotating-wall-vessel (RWV) bioreactors are a more recent development. The original concept of the RWV bioreactors was developed at NASA's Johnson Space Center in the early 1990s and is believed to overcome the limitations of static models such as the development of hypoxic, necrotic cores. The RWV bioreactors might circumvent this problem by providing fluid dynamics that allow the efficient diffusion of nutrients and oxygen. These bioreactors consist of a rotator base that serves to support and rotate two different formats of culture vessels that differ by their aeration source type: (1) Slow Turning Lateral Vessels (STLVs) with a co-axial oxygenator in the center, or (2) High Aspect Ratio Vessels (HARVs) with oxygenation via a flat, silicone rubber gas transfer membrane. These vessels allow efficient gas transfer while avoiding bubble formation and consequent turbulence. These conditions result in laminar flow and minimal shear force that models reduced gravity (microgravity) inside the culture vessel. Here we describe the development of a multicellular 3-D organotypic model of the human intestinal mucosa composed of an intestinal epithelial cell line and primary human lymphocytes, endothelial cells and fibroblasts cultured under microgravity provided by the RWV bioreactor.

Continuous culture of Cryptosporidium parvum using hollow fiber technology

Diarrheal disease is a leading cause of pediatric death in economically low resource countries. Cryptosporidium spp. are the second largest member of this group and the only member for which no treatment exists. One of the handicaps to developing chemotherapy is the lack of a reproducible long-term culture method permitting in vitro drug screening beyond 48 h. We have adapted the well-established hollow fiber technology to provide an environment that mimics the gut by delivering nutrients and oxygen from the basal layer upwards while allowing separate redox and nutrient control of the lumen for parasite development. Using this technique, oocyst production was maintained for >6 months, producing approximately 1×10(8)oocysts ml(-1)day(-1), compared with 48 h with a yield of 1×10(6)oocysts ml(-1) in two-dimensional cultures. Oocysts, after 4 and 20 weeks in culture, produced a chronic infection in a TCR-α-deficient mouse model. In vivo infectivity of oocysts was confirmed using oocysts from a 6 week culture in a dexamethasone immunosuppressed mouse model.

From Single Cells to Engineered and Explanted Tissues: New Perspectives in Bacterial Infection Biology

Cell culture techniques are essential for studying host-pathogen interactions. In addition to the broad range of single cell type-based two-dimensional cell culture models, an enormous amount of coculture systems, combining two or more different cell types, has been developed. These systems enable microscopic visualization and molecular analyses of bacterial adherence and internalization mechanisms and also provide a suitable setup for various biochemical, immunological, and pharmacological applications. The implementation of natural or synthetical scaffolds elevated the model complexity to the level of three-dimensional cell culture. Additionally, several transwell-based cell culture techniques are applied to study bacterial interaction with physiological tissue barriers. For keeping highly differentiated phenotype of eukaryotic cells in ex vivo culture conditions, different kinds of microgravity-simulating rotary-wall vessel systems are employed. Furthermore, the implementation of microfluidic pumps enables constant nutrient and gas exchange during cell cultivation and allows the investigation of long-term infection processes. The highest level of cell culture complexity is reached by engineered and explanted tissues which currently pave the way for a more comprehensive view on microbial pathogenicity mechanisms.

Exploiting the power of OMICS approaches to produce E. coli O157 vaccines

Enterohemorrhagic Escherichia coli (EHEC) strains are well-documented human pathogens and causative agents of diarrheal episodes and hemorrhagic colitis. The serotype O157:H7 is highly virulent and responsible for both outbreaks and sporadic cases of diarrhea. Because antibiotic treatment is contraindicated against this pathogen, development of a human vaccine could be an effective intervention in public health. In our recent Infection and Immunity paper, we applied integrated approaches of in silico genome wide search combined with bioinformatics tools to identify and test O157 vaccine candidates for their protective effect on a murine model of gastrointestinal infection. Using genomic/immunoinformatic approaches that are further described here, we categorized vaccine candidates as high, medium, and low priorities, and demonstrate that some high priority candidates were able to significantly induce Th2 cytokines and reduce EHEC colonization. Using the STRING database, we have recently evaluated the vaccine candidates and predict functional protein interactions, determining whether correlations exist for the development of a multi-subunit vaccine, targeting different pathways against EHEC O157:H7. The overall approach is designed to screen potential vaccine candidates against EHEC; however, the methodology can be quickly applied to many other intestinal pathogens.

Microbial monitoring of crewed habitats in space-current status and future perspectives

Previous space research conducted during short-term flight experiments and long-term environmental monitoring on board orbiting space stations suggests that the relationship between humans and microbes is altered in the crewed habitat in space. Both human physiology and microbial communities adapt to spaceflight. Microbial monitoring is critical to crew safety in long-duration space habitation and the sustained operation of life support systems on space transit vehicles, space stations, and surface habitats. To address this critical need, space agencies including NASA (National Aeronautics and Space Administration), ESA (European Space Agency), and JAXA (Japan Aerospace Exploration Agency) are working together to develop and implement specific measures to monitor, control, and counteract biological contamination in closed-environment systems. In this review, the current status of microbial monitoring conducted in the International Space Station (ISS) as well as the results of recent microbial spaceflight experiments have been summarized and future perspectives are discussed.

3-D intestinal scaffolds for evaluating the therapeutic potential of probiotics

Biomimetic in vitro intestinal models are becoming useful tools for studying host-microbial interactions. In the past, these models have typically been limited to simple cultures on 2-D scaffolds or Transwell inserts, but it is widely understood that epithelial cells cultured in 3-D environments exhibit different phenotypes that are more reflective of native tissue, and that different microbial species will preferentially adhere to select locations along the intestinal villi. We used a synthetic 3-D tissue scaffold with villous features that could support the coculture of epithelial cell types with select bacterial populations. Our end goal was to establish microbial niches along the crypt-villus axis in order to mimic the natural microenvironment of the small intestine, which could potentially provide new insights into microbe-induced intestinal disorders, as well as enabling targeted probiotic therapies. We recreated the surface topography of the small intestine by fabricating a biodegradable and biocompatible villous scaffold using poly lactic-glycolic acid to enable the culture of Caco-2 with differentiation along the crypt-villus axis in a similar manner to native intestines. This was then used as a platform to mimic the adhesion and invasion profiles of both Salmonella and Pseudomonas, and assess the therapeutic potential of Lactobacillus and commensal Escherichia coli in a 3-D setting. We found that, in a 3-D environment, Lactobacillus is more successful at displacing pathogens, whereas Nissle is more effective at inhibiting pathogen adhesion.

Pathogenesis of human enterovirulent bacteria: lessons from cultured, fully differentiated human colon cancer cell lines

Hosts are protected from attack by potentially harmful enteric microorganisms, viruses, and parasites by the polarized fully differentiated epithelial cells that make up the epithelium, providing a physical and functional barrier. Enterovirulent bacteria interact with the epithelial polarized cells lining the intestinal barrier, and some invade the cells. A better understanding of the cross talk between enterovirulent bacteria and the polarized intestinal cells has resulted in the identification of essential enterovirulent bacterial structures and virulence gene products playing pivotal roles in pathogenesis. Cultured animal cell lines and cultured human nonintestinal, undifferentiated epithelial cells have been extensively used for understanding the mechanisms by which some human enterovirulent bacteria induce intestinal disorders. Human colon carcinoma cell lines which are able to express in culture the functional and structural characteristics of mature enterocytes and goblet cells have been established, mimicking structurally and functionally an intestinal epithelial barrier. Moreover, Caco-2-derived M-like cells have been established, mimicking the bacterial capture property of M cells of Peyer's patches. This review intends to analyze the cellular and molecular mechanisms of pathogenesis of human enterovirulent bacteria observed in infected cultured human colon carcinoma enterocyte-like HT-29 subpopulations, enterocyte-like Caco-2 and clone cells, the colonic T84 cell line, HT-29 mucus-secreting cell subpopulations, and Caco-2-derived M-like cells, including cell association, cell entry, intracellular lifestyle, structural lesions at the brush border, functional lesions in enterocytes and goblet cells, functional and structural lesions at the junctional domain, and host cellular defense responses.

Critical analysis of 3-D organoid in vitro cell culture models for high-throughput drug candidate toxicity assessments

Drug failure due to toxicity indicators remains among the primary reasons for staggering drug attrition rates during clinical studies and post-marketing surveillance. Broader validation and use of next-generation 3-D improved cell culture models are expected to improve predictive power and effectiveness of drug toxicological predictions. However, after decades of promising research significant gaps remain in our collective ability to extract quality human toxicity information from in vitro data using 3-D cell and tissue models. Issues, challenges and future directions for the field to improve drug assay predictive power and reliability of 3-D models are reviewed.

Ricin crosses polarized human intestinal cells and intestines of ricin-gavaged mice without evident damage and then disseminates to mouse kidneys

Ricin is a potent toxin found in the beans of Ricinus communis and is often lethal for animals and humans when aerosolized or injected and causes significant morbidity and occasional death when ingested. Ricin has been proposed as a bioweapon because of its lethal properties, environmental stability, and accessibility. In oral intoxication, the process by which the toxin transits across intestinal mucosa is not completely understood. To address this question, we assessed the impact of ricin on the gastrointestinal tract and organs of mice after dissemination of toxin from the gut. We first showed that ricin adhered in a specific pattern to human small bowel intestinal sections, the site within the mouse gut in which a variable degree of damage has been reported by others. We then monitored the movement of ricin across polarized human HCT-8 intestinal monolayers grown in transwell inserts and in HCT-8 cell organoids. We observed that, in both systems, ricin trafficked through the cells without apparent damage until 24 hours post intoxication. We delivered a lethal dose of purified fluorescently-labeled ricin to mice by oral gavage and followed transit of the toxin from the gastrointestinal tracts to the internal organs by in vivo imaging of whole animals over time and ex vivo imaging of organs at various time points. In addition, we harvested organs from unlabeled ricin-gavaged mice and assessed them for the presence of ricin and for histological damage. Finally, we compared serum chemistry values from buffer-treated versus ricin-intoxicated animals. We conclude that ricin transverses human intestinal cells and mouse intestinal cells in situ prior to any indication of enterocyte damage and that ricin rapidly reaches the kidneys of intoxicated mice. We also propose that mice intoxicated orally with ricin likely die from distributive shock.

Dysfunctions at human intestinal barrier by water-borne protozoan parasites: lessons from cultured human fully differentiated colon cancer cell lines

Some water-borne protozoan parasites induce diseases through their membrane-associated functional structures and virulence factors that hijack the host cellular molecules and signalling pathways leading to structural and functional lesions in the intestinal barrier. In this Microreview we analyse the insights on the mechanisms of pathogenesis of Entamoeba intestinalis, Giardia and Cryptosporidium observed in the human colon carcinoma fully differentiated colon cancer cell lines, cell subpopulations and clones expressing the structural and functional characteristics of highly specialized fully differentiated epithelial cells lining the intestinal epithelium and mimicking structurally and functionally an intestinal barrier.

Ex vivo systems to study host-microbiota interactions in the gastrointestinal tract

It is increasingly apparent that the microbial ecosystems in the mammalian gastrointestinal tract play an intricate role in health and disease. There is a growing interest in the development of targeted strategies for modulating health through the modification of these microbiota. Ecologists are faced with the challenge of understanding the structure and function of ecosystems, the component parts of which interact with each other in complex and diffuse ways. The human gut microbiota, with its high species richness and diversity (up to 1000 bacterial species per individual) including members of all three domains of life, situated in the dynamic environment of the gastrointestinal tract, is probably among the most complex ecosystems on this planet. In order to elucidate the mechanistic foundations, and physiological significance, of beneficial or pathogenic relationships between the gut microbiota and their hosts, researchers require tractable model ecosystems that allow to recapitulate and investigate host-microbe and microbe-microbe interactions. This review discusses ex vivo gastrointestinal models systems that can be used to gain mechanistic insights into the emergent properties of the host-microbial superorganism.

Culturing and applications of rotating wall vessel bioreactor derived 3D epithelial cell models

Cells and tissues in the body experience environmental conditions that influence their architecture, intercellular communications, and overall functions. For in vitro cell culture models to accurately mimic the tissue of interest, the growth environment of the culture is a critical aspect to consider. Commonly used conventional cell culture systems propagate epithelial cells on flat two-dimensional (2-D) impermeable surfaces. Although much has been learned from conventional cell culture systems, many findings are not reproducible in human clinical trials or tissue explants, potentially as a result of the lack of a physiologically relevant microenvironment. Here, we describe a culture system that overcomes many of the culture condition boundaries of 2-D cell cultures, by using the innovative rotating wall vessel (RWV) bioreactor technology. We and others have shown that organotypic RWV-derived models can recapitulate structure, function, and authentic human responses to external stimuli similarly to human explant tissues (1-6). The RWV bioreactor is a suspension culture system that allows for the growth of epithelial cells under low physiological fluid shear conditions. The bioreactors come in two different formats, a high-aspect rotating vessel (HARV) or a slow-turning lateral vessel (STLV), in which they differ by their aeration source. Epithelial cells are added to the bioreactor of choice in combination with porous, collagen-coated microcarrier beads (Figure 1A). The cells utilize the beads as a growth scaffold during the constant free fall in the bioreactor (Figure 1B). The microenvironment provided by the bioreactor allows the cells to form three-dimensional (3-D) aggregates displaying in vivo-like characteristics often not observed under standard 2-D culture conditions (Figure 1D). These characteristics include tight junctions, mucus production, apical/basal orientation, in vivo protein localization, and additional epithelial cell-type specific properties. The progression from a monolayer of epithelial cells to a fully differentiated 3-D aggregate varies based on cell type(1, 7-13). Periodic sampling from the bioreactor allows for monitoring of epithelial aggregate formation, cellular differentiation markers and viability (Figure 1D). Once cellular differentiation and aggregate formation is established, the cells are harvested from the bioreactor, and similar assays performed on 2-D cells can be applied to the 3-D aggregates with a few considerations (Figure 1E-G). In this work, we describe detailed steps of how to culture 3-D epithelial cell aggregates in the RWV bioreactor system and a variety of potential assays and analyses that can be executed with the 3-D aggregates. These analyses include, but are not limited to, structural/morphological analysis (confocal, scanning and transmission electron microscopy), cytokine/chemokine secretion and cell signaling (cytometric bead array and Western blot analysis), gene expression analysis (real-time PCR), toxicological/drug analysis and host-pathogen interactions. The utilization of these assays set the foundation for more in-depth and expansive studies such as metabolomics, transcriptomics, proteomics and other array-based applications. Our goal is to present a non-conventional means of culturing human epithelial cells to produce organotypic 3-D models that recapitulate the human in vivo tissue, in a facile and robust system to be used by researchers with diverse scientific interests.

Rotating cell culture systems for human cell culture: human trophoblast cells as a model

The field of human trophoblast research aids in understanding the complex environment established during placentation. Due to the nature of these studies, human in vivo experimentation is impossible. A combination of primary cultures, explant cultures and trophoblast cell lines support our understanding of invasion of the uterine wall and remodeling of uterine spiral arteries by extravillous trophoblast cells (EVTs), which is required for successful establishment of pregnancy. Despite the wealth of knowledge gleaned from such models, it is accepted that in vitro cell culture models using EVT-like cell lines display altered cellular properties when compared to their in vivo counterparts. Cells cultured in the rotating cell culture system (RCCS) display morphological, phenotypic, and functional properties of EVT-like cell lines that more closely mimic differentiating in utero EVTs, with increased expression of genes mediating invasion (e.g. matrix metalloproteinases (MMPs)) and trophoblast differentiation. The Saint Georges Hospital Placental cell Line-4 (SGHPL-4) (kindly donated by Dr. Guy Whitley and Dr. Judith Cartwright) is an EVT-like cell line that was used for testing in the RCCS. The design of the RCCS culture vessel is based on the principle that organs and tissues function in a three-dimensional (3-D) environment. Due to the dynamic culture conditions in the vessel, including conditions of physiologically relevant shear, cells grown in three dimensions form aggregates based on natural cellular affinities and differentiate into organotypic tissue-like assemblies. The maintenance of a fluid orbit provides a low-shear, low-turbulence environment similar to conditions found in vivo. Sedimentation of the cultured cells is countered by adjusting the rotation speed of the RCCS to ensure a constant free-fall of cells. Gas exchange occurs through a permeable hydrophobic membrane located on the back of the bioreactor. Like their parental tissue in vivo, RCCS-grown cells are able to respond to chemical and molecular gradients in three dimensions (i.e. at their apical, basal, and lateral surfaces) because they are cultured on the surface of porous microcarrier beads. When grown as two-dimensional monolayers on impermeable surfaces like plastic, cells are deprived of this important communication at their basal surface. Consequently, the spatial constraints imposed by the environment profoundly affect how cells sense and decode signals from the surrounding microenvironment, thus implying an important role for the 3-D milieu. We have used the RCCS to engineer biologically meaningful 3-D models of various human epithelial tissues. Indeed, many previous reports have demonstrated that cells cultured in the RCCS can assume physiologically relevant phenotypes that have not been possible with other models. In summary, culture in the RCCS represents an easy, reproducible, high-throughput platform that provides large numbers of differentiated cells that are amenable to a variety of experimental manipulations. In the following protocol, using EVTs as an example, we clearly describe the steps required to three-dimensionally culture adherent cells in the RCCS.

Mouse models of Escherichia coli O157:H7 infection and shiga toxin injection

Escherichia coli O157:H7 has been responsible for multiple food- and waterborne outbreaks of diarrhea and/or hemorrhagic colitis (HC) worldwide. More importantly, a portion of E. coli O157:H7-infected individuals, particularly young children, develop a life-threatening sequela of infection called hemolytic uremic syndrome (HUS). Shiga toxin (Stx), a potent cytotoxin, is the major virulence factor linked to the presentation of both HC and HUS. Currently, treatment of E. coli O157:H7 and other Stx-producing E. coli (STEC) infections is limited to supportive care. To facilitate development of therapeutic strategies and vaccines for humans against these agents, animal models that mimic one or more aspect of STEC infection and disease are needed. In this paper, we focus on the characteristics of various mouse models that have been developed and that can be used to monitor STEC colonization, disease, pathology, or combinations of these features as well as the impact of Stx alone.

Organotypic 3D cell culture models: using the rotating wall vessel to study host-pathogen interactions

Appropriately simulating the three-dimensional (3D) environment in which tissues normally develop and function is crucial for engineering in vitro models that can be used for the meaningful dissection of host-pathogen interactions. This Review highlights how the rotating wall vessel bioreactor has been used to establish 3D hierarchical models that range in complexity from a single cell type to multicellular co-culture models that recapitulate the 3D architecture of tissues observed in vivo. The application of these models to the study of infectious diseases is discussed.

Alveolar epithelium protects macrophages from quorum sensing-induced cytotoxicity in a three-dimensional co-culture model

The quorum sensing signal N-(3-oxododecanoyl)-l-homoserine lactone (3-oxo-C(12) HSL), produced by Pseudomonas aeruginosa, exerts cytotoxic effects in macrophages in vitro, which is believed to affect host innate immunity in vivo. However, the medical significance of this finding to pulmonary disease remains unclear since the multicellular complexity of the lung was not considered in the assessment of macrophage responses to 3-oxo-C(12) HSL. We developed a novel three-dimensional co-culture model of alveolar epithelium and macrophages using the rotating wall vessel (RWV) bioreactor, by adding undifferentiated monocytes to RWV-derived alveolar epithelium. Our three-dimensional model expressed important architectural/phenotypic hallmarks of the parental tissue, as evidenced by highly differentiated epithelium, spontaneous differentiation of monocytes to functional macrophage-like cells, localization of these cells on the alveolar surface and a macrophage-to-epithelial cell ratio relevant to the in vivo situation. Co-cultivation of macrophages with alveolar epithelium counteracted 3-oxo-C(12) HSL-induced cytotoxicity via removal of quorum sensing molecules by alveolar cells. Furthermore, 3-oxo-C(12) HSL induced the intercellular adhesion molecule ICAM-1 in both alveolar epithelium and macrophages. These data stress the importance of multicellular organotypic models to integrate the role of different cell types in overall lung homeostasis and disease development in response to external factors.

Purified galactooligosaccharide, derived from a mixture produced by the enzymic activity of Bifidobacterium bifidum, reduces Salmonella enterica serovar Typhimurium adhesion and invasion in vitro and in vivo

The prebiotic Bimuno(®) is a mixture containing galactooligosaccharides (GOSs), produced by the galactosyltransferase activity of Bifidobacterium bifidum NCIMB 41171 using lactose as the substrate. Previous in vivo and in vitro studies demonstrating the efficacy of Bimuno(®) in reducing Salmonella enterica serovar Typhimurium (S. Typhimurium) colonization did not ascertain whether or not the protective effects could be attributed to the prebiotic component GOS. Here we wished to test the hypothesis that GOS, derived from Bimuno(®), may confer the direct anti-invasive and protective effects of Bimuno(®). In this study the efficacy of Bimuno(®), a basal solution of Bimuno(®) without GOS [which contained glucose, galactose, lactose, maltodextrin and gum arabic in the same relative proportions (w/w) as they are found in Bimuno(®)] and purified GOS to reduce S. Typhimurium adhesion and invasion was assessed using a series of in vitro and in vivo models. The novel use of three dimensionally cultured HT-29-16E cells to study prebiotics in vitro demonstrated that the presence of ∼ 5 mg Bimuno(®) ml(-1) or ∼ 2.5 mg GOS ml(-1) significantly reduced the invasion of S. Typhimurium (SL1344nal(r)) (P<0.0001). Furthermore, ∼ 2.5 mg GOS ml(-1) significantly reduced the adherence of S. Typhimurium (SL1344nal(r)) (P<0.0001). It was demonstrated that cells produced using this system formed multi-layered aggregates of cells that displayed excellent formation of brush borders and tight junctions. In the murine ligated ileal gut loops, the presence of Bimuno(®) or GOS prevented the adherence or invasion of S. Typhimurium to enterocytes, and thus reduced its associated pathology. This protection appeared to correlate with significant reductions in the neutral and acidic mucins detected in goblet cells, possibly as a consequence of stimulating the cells to secrete the mucin into the lumen. In all assays, Bimuno(®) without GOS conferred no such protection, indicating that the basal solution confers no protective effects against S. Typhimurium. Collectively, the studies presented here clearly indicate that the protective effects conferred by Bimuno(®) can be attributed to GOS.

The generation of 3-D tissue models based on hyaluronan hydrogel-coated microcarriers within a rotating wall vessel bioreactor

With the increasing necessity for functional tissue- and organ equivalents in the clinic, the optimization of techniques for the in vitro generation of organotypic structures that closely resemble the native tissue is of paramount importance. The engineering of a variety of highly differentiated tissues has been achieved using the rotating wall vessel (RWV) bioreactor technology, which is an optimized suspension culture allowing cells to grow in three-dimensions (3-D). However, certain cell types require the use of scaffolds, such as collagen-coated microcarrier beads, for optimal growth and differentiation in the RWV. Removal of the 3-D structures from the microcarriers involves enzymatic treatment, which disrupts the delicate 3-D architecture and makes it inapplicable for potential implantation. Therefore, we designed a microcarrier bead coated with a synthetic extracellular matrix (ECM) composed of a disulfide-crosslinked hyaluronan and gelatin hydrogel for 3-D tissue engineering, that allows for enzyme-free cell detachment under mild reductive conditions (i.e. by a thiol-disulfide exchange reaction). The ECM-coated beads (ECB) served as scaffold to culture human intestinal epithelial cells (Int-407) in the RWV, which formed viable multi-layered cell aggregates and expressed epithelial differentiation markers. The cell aggregates remained viable following dissociation from the microcarriers, and could be returned to the RWV bioreactor for further culturing into bead-free tissue assemblies. The developed ECBs thus offer the potential to generate scaffold-free 3-D tissue assemblies, which could further be explored for tissue replacement and remodeling.

Detection of epithelial-cell injury, and quantification of infection, in the HCT-8 organoid model of cryptosporidiosis

BACKGROUND

Intestinal cells grown in microgravity produce a three-dimensional tissue assembly, or "organoid," similar to the human intestinal mucosa, making it an ideal model for enteric infections such as cryptosporidiosis.

METHODS

HCT-8 cells were grown in a reduced-gravity, low-shear, rotating-wall vessel (RWV) and were infected with Cryptosporidium parvum oocysts. Routine and electron microscopy (EM), immunolabeling with fluorescein-labeled Vicia villosa lectin and phycoerythrin-labeled monoclonal antibody to a 15-kD surface-membrane protein, and quantitative polymerase chain reaction (qPCR) using probes for 18s rRNA of C. parvum and HCT-8 cells were performed.

RESULTS

The RWV allowed development of columnar epithelium-like structures. Higher magnification revealed well-developed brush borders at the apical side of the tissue. Incubation with C. parvum resulted in patchy disruption of the epithelium and, at the surface of several epithelial cells, in localized infection with the organism. EM revealed irregular stunting of microvilli, foci of indistinct tight junctions, and areas of loose paracellular spaces. qPCR showed a 1.85-log (i.e., 70-fold) progression of infection from 6 h to 48 h of incubation.

CONCLUSION

The HCT-8 organoid displayed morphologic changes indicative of successful and quantifiable infection with C. parvum. The HCT-8 organoid-culture system may have application in interventional in vitro studies of cryptosporidiosis.

The effects of low-shear stress on Adherent-invasive Escherichia coli

The impact of low-shear stress (LSS) was evaluated on an Adherent-invasive Escherichia coli clinical isolate (AIEC strain O83:H1) from a Crohn's disease patient. High-aspect ratio vessels (HARVs) were used to model LSS conditions to characterize changes in environmental stress resistance and adhesion/invasive properties. Low-shear stress-grown cultures exhibited enhanced thermal and oxidative stress resistance as well as increased adherence to Caco-2 cells, but no changes in invasion were observed. An AIEC rpoS mutant was constructed to examine the impact of this global stress regulator. The absence of RpoS under LSS conditions resulted in increased sensitivity to oxidative stress while adherence levels were elevated in comparison with the wild-type strain. TnphoA mutagenesis and rpoS complementation were carried out on the rpoS mutant to identify those factors involved in the LSS-induced adherence phenotype. Mutagenesis results revealed that one insertion disrupted the tnaB gene (encoding tryptophan permease) and the rpoS tnaB double mutant exhibited decreased adherence under LSS. Complementation of the tnaB gene, or medium supplemented with exogenous indole, restored adhesion of the rpoS tnaB mutant under LSS conditions. Overall, our study demonstrated how mechanical stresses such as LSS altered AIEC phenotypic characteristics and identified novel functions for some RpoS-regulated proteins.

The application of low shear modeled microgravity to 3-D cell biology and tissue engineering

The practice of cell culture has been virtually unchanged for 100 years. Until recently, life scientists have had to content themselves with two-dimensional cell culture technology. Clearly, living creatures are not constructed in two dimensions and thus it has become widely recognized that in vitro culture systems must become three dimensional to correctly model in vivo biology. Attempts to modify conventional 2-D culture technology to accommodate 3-D cell growth such as embedding cells in extracellular matrix have demonstrated the superiority of concept. Nevertheless, there are serious drawbacks to this approach including limited mass transport and lack of scalability. Recently, a new cell culture technology developed at NASA to study the effects of microgravity on cells has emerged to solve many of the problems of 3-D cell culture. The technology, the Rotating Wall Vessel (RWV) is a single axis clinostat consisting of a fluid-filled, cylindrical, horizontally rotating culture vessel. Cells placed in this environment are suspended by the resolution of the gravitational, centrifugal and Coriolis forces with extremely low mechanical shear. These conditions, which have been called "low shear modeled microgravity", enable cells to assemble into tissue-like aggregates with high mass transport of nutrients, oxygen and wastes. Examples of the use of the RWV for basic cell biology research and tissue engineering applications are discussed.

Morphological characteristics and proliferation of keratocytes cultured under simulated microgravity

This study probed the changes of keratocytes cultured under simulated microgravity. Keratocytes were isolated from rabbit corneas using collagenase digestion method. Cells were seeded in a 55-mL capacity high-aspect-ratio vessel (HARV) of rotary cell culture system (RCCS) at a density of 1 x 10(4) cells/mL. Dehydrated bovine acellular corneal stroma (5 x 5 x 1 mm, n = 30) was used as a carrier for keratocyte culture. Rotational speed was set at 15, 20, and 30 rpm in the first, second, and third week of culture, respectively. Histological evaluation showed that keratocytes in simulated microgravity culture grew into carriers, but those under conventional gravity grew on the surface of carriers. Scanning electron microscopic evaluation showed that after 19 days in culture, keratocytes on the carriers were spherical and spread in the spaces among the collagen fibers. Cells were dendritic or spindle shaped, and they developed many foot processes linked with surrounding cells. The absorbance values of the simulated microgravity group were significantly higher (P < 0.01) than that of the conventional group from 10 to 19 days of culture. The RCCS obviously enhanced the proliferation of rabbit keratocytes and facilitated the cells' growth into or on the dehydrated bovine acellular corneal stroma. Cells showed more natural morphology.

In vitro cell culture infectivity assay for human noroviruses

A 3-dimensional organoid human small intestinal epithelium model was used.

Human noroviruses cause severe, self-limiting gastroenteritis that typically lasts 24–48 hours. Because of the lack of suitable tissue culture or animal models, the true nature of norovirus pathogenesis remains unknown. We show that noroviruses can infect and replicate in a 3-dimensional (3-D), organoid model of human small intestinal epithelium. Cells grown on porous collage-coated beads under fluid shear conditions in rotating wall vessel bioreactors differentiate into 3-D architectures resembling both the morphologic and physiologic function of in vivo tissues. Microscopy, PCR, and fluorescent in situ hybridization provided evidence of norovirus infection. Cytopathic effect and norovirus RNA were detected at each of the 5 cell passages for genogroup I and II viruses. Our results demonstrate that the highly differentiated 3-D cell culture model can support the natural growth of human noroviruses, whereas previous attempts that used differentiated monolayer cultures failed.

Differences in Chlamydia trachomatis serovar E growth rate in polarized endometrial and endocervical epithelial cells grown in three-dimensional culture

In vitro studies of obligate intracellular chlamydia biology and pathogenesis are highly dependent on the use of experimental models and growth conditions that mimic the mucosal architecture and environment these pathogens encounter during natural infections. In this study, the growth of Chlamydia trachomatis genital serovar E was monitored in mouse fibroblast McCoy cells and compared to more relevant host human epithelial endometrium-derived HEC-1B and cervix-derived HeLa cells, seeded and polarized on collagen-coated microcarrier beads, using a three-dimensional culture system. Microscopy analysis of these cell lines prior to infection revealed morphological differences reminiscent of their in vivo architecture. Upon infection, early chlamydial inclusion distribution was uniform in McCoy cells but patchy in both epithelial cell lines. Although no difference in chlamydial attachment to or entry into the two genital epithelial cell lines was noted, active bacterial genome replication and transcription, as well as initial transformation of elementary bodies to reticulate bodies, were detected earlier in HEC-1B than in HeLa cells, suggesting a faster growth, which led to higher progeny counts and titers in HEC-1B cells upon completion of the developmental cycle. Chlamydial development in the less relevant McCoy cells was very similar to that in HeLa cells, although higher progeny counts were obtained. In conclusion, this three-dimensional bead culture system represents an improved model for harvesting large quantities of infectious chlamydia progeny from their more natural polarized epithelial host cells.

Three-dimensional organotypic models of human colonic epithelium to study the early stages of enteric salmonellosis

In vitro cell culture models used to study how Salmonella initiates disease at the intestinal epithelium would benefit from the recognition that organs and tissues function in a three-dimensional (3-D) environment and that this spatial context is necessary for development of cultures that more realistically resemble in vivo tissues/organs. Our aim was to establish and characterize biologically meaningful 3-D models of human colonic epithelium and apply them to study the early stages of enteric salmonellosis. The human colonic cell line HT-29 was cultured in 3-D and characterized by immunohistochemistry, histology, and scanning electron microscopy. Wild-type Salmonella typhimurium and an isogenic SPI-1 type three secretion system (TTSS) mutant derivative (invA) were used to compare the interactions with 3-D cells and monolayers in adherence/invasion, tissue pathology, and cytokine expression studies. The results showed that 3-D culture enhanced many characteristics normally associated with fully differentiated, functional intestinal epithelia in vivo, including better organization of junctional, extracellular matrix, and brush-border proteins, and highly localized mucin production. Wild-type Salmonella demonstrated increased adherence, but significantly lower invasion for 3-D cells. Interestingly, the SPI-I TTSS mutant showed wild-type ability to invade into the 3-D cells but did not cause significant structural changes to these cells. Moreover, 3-D cells produced less interleukin-8 before and after Salmonella infection. These results suggest that 3-D cultures of human colonic epithelium provide valuable alternative models to study human enteric salmonellosis with potential for novel insight into Salmonella pathogenesis.