The use of a three-dimensional cell culture model to investigate host-pathogen interactions of Francisella tularensis in human lung epithelial cells

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.

Three-dimensional models of the cervicovaginal epithelia to study host-microbiome interactions and sexually transmitted infections

2D cell culture systems have historically provided controlled, reproducible means to analyze host-pathogen interactions observed in the human reproductive tract. Although inexpensive, straightforward, and requiring a very short time commitment, these models recapitulate neither the functionality of multilayered cell types nor the associated microbiome that occurs in a human. Animal models have commonly been used to recreate the complexity of human infections. However, extensive modifications of animal models are required to recreate interactions that resemble those in the human reproductive tract. 3D cell culture models have emerged as alternative means of reproducing vital elements of human infections at a fraction of the cost of animal models and on a scale that allows for replicative experiments. Here, we describe a new 3D model that utilizes transwells with epithelial cells seeded apically and a basolateral extracellular matrix (ECM)-like layer. The model produced tissues with morphologic and physiological resemblance to human cervical and vaginal epithelia, including mucus levels produced by cervical cells. Infection by Chlamydia trachomatis and Neisseria gonorrhoeae was demonstrated, as well as the growth of bacterial species observed in the human vaginal microbiota. This enabled controlled mechanistic analyses of the interactions between host cells, the vaginal microbiota, and STI pathogens. Affordable and semi high-throughput 3D models of the cervicovaginal epithelia that are physiologically relevant by sustaining vaginal bacterial colonization, and facilitate studies of chlamydial and gonococcal infections.

A Three-Dimensional Lung Cell Model to Leptospira Virulence Investigations

Leptospirosis is a worldwide zoonosis and a serious public health threat in tropical and subtropical areas. The etiologic agents of leptospirosis are pathogenic spirochetes from the genus Leptospira. In severe cases, patients develop a pulmonary hemorrhage that is associated with high fatality rates. Several animal models were established for leptospirosis studies, such as rodents, dogs, and monkeys. Although useful to study the relationship among Leptospira and its hosts, the animal models still exhibit economic and ethical limitation reasons and do not fully represent the human infection. As an attempt to bridge the gap between animal studies and clinical information from patients, we established a three-dimensional (3-D) human lung cell culture for Leptospira infection. We show that Leptospira is able to efficiently infect the cell lung spheroids and also to infiltrate in deeper areas of the cell aggregates. The ability to infect the 3-D lung cell aggregates was time-dependent. The 3-D spheroids infection occurred up to 120 h in studies with two serovars, Canicola and Copenhageni. We standardized the number of bacteria in the initial inoculum for infection of the spheroids and we also propose two alternative culture media conditions. This new approach was validated by assessing the expression of three genes of Leptospira related to virulence and motility. The transcripts of these genes increased in both culture conditions, however, in higher rates and earlier times in the 3-D culture. We also assessed the production of chemokines by the 3-D spheroids before and after Leptospira infection, confirming induction of two of them, mainly in the 3-D spheroids. Chemokine CCL2 was expressed only in the 3-D cell culture. Increasing of this chemokine was observed previously in infected animal models. This new approach provides an opportunity to study the interaction of Leptospira with the human lung epithelium in vitro.

Three-dimensional low shear culture of Mycobacterium bovis BCG induces biofilm formation and antimicrobial drug tolerance

Mycobacteria naturally grow as corded biofilms in liquid media without detergent. Such detergent-free biofilm phenotypes may reflect the growth pattern of bacilli in tuberculous lung lesions. New strategies are required to treat tuberculosis, which is responsible for more deaths each year than any other bacterial disease. The lengthy 6-month regimen for drug-sensitive tuberculosis is necessary to remove antimicrobial drug tolerant populations of bacilli that persist through drug therapy. The role of biofilm-like growth in the generation of these sub-populations remains poorly understood despite the hypothesised clinical significance and mounting evidence of biofilms in pathogenesis. We adapt a three-dimensional Rotary Cell Culture System to model M. bovis BCG biofilm growth in low-shear detergent-free liquid suspension. Importantly, biofilms form without attachment to artificial surfaces and without severe nutrient starvation or environmental stress. Biofilm-derived planktonic bacilli are tolerant to isoniazid and streptomycin, but not rifampicin. This phenotypic drug tolerance is lost after passage in drug-free media. Transcriptional profiling reveals induction of cell surface regulators, sigE and BCG_0559c alongside the ESX-5 secretion apparatus in these low-shear liquid-suspension biofilms. This study engineers and characterises mycobacteria grown as a suspended biofilm, illuminating new drug discovery pathways for this deadly disease.

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.

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 Three-Dimensional Human Tissue-Engineered Lung Model to Study Influenza A Infection

Influenza A virus (IAV) claims ∼250,000-500,000 lives annually worldwide. Currently, there are a few in vitro models available to study IAV immunopathology. Monolayer cultures of cell lines and primary lung cells (two-dimensional [2D] cell culture) is the most commonly used tool, however, this system does not have the in vivo-like structure of the lung and immune responses to IAV as it lacks the three-dimensional (3D) tissue structure. To recapitulate the lung physiology in vitro, a system that contains multiple cell types within a 3D environment that allows cell movement and interaction would provide a critical tool. In this study, as a first step in designing a 3D-Human Tissue-Engineered Lung Model (3D-HTLM), we describe the 3D culture of primary human small airway epithelial cells (HSAEpCs) and determined the immunophenotype of this system in response to IAV infections. We constructed a 3D chitosan-collagen scaffold and cultured HSAEpCs on these scaffolds at air-liquid interface (ALI). These 3D cultures were compared with 2D-cultured HSAEpCs for viability, morphology, marker protein expression, and cell differentiation. Results showed that the 3D-cultured HSAEpCs at ALI yielded maximum viable cells and morphologically resembled the in vivo lower airway epithelium. There were also significant increases in aquaporin-5 and cytokeratin-14 expression for HSAEpCs cultured in 3D compared to 2D. The 3D culture system was used to study the infection of HSAEpCs with two major IAV strains, H1N1 and H3N2. The HSAEpCs showed distinct changes in marker protein expression, both at mRNA and protein levels, and the release of proinflammatory cytokines. This study is the first step in the development of the 3D-HTLM, which will have wide applicability in studying pulmonary pathophysiology and therapeutics development.

Antimicrobial efficacy against Pseudomonas aeruginosa biofilm formation in a three-dimensional lung epithelial model and the influence of fetal bovine serum

In vitro models that mimic in vivo host-pathogen interactions are needed to evaluate candidate drugs that inhibit bacterial virulence traits. We established a new approach to study Pseudomonas aeruginosa biofilm susceptibility on biotic surfaces, using a three-dimensional (3-D) lung epithelial cell model. P. aeruginosa formed antibiotic resistant biofilms on 3-D cells without affecting cell viability. The biofilm-inhibitory activity of antibiotics and/or the anti-biofilm peptide DJK-5 were evaluated on 3-D cells compared to a plastic surface, in medium with and without fetal bovine serum (FBS). In both media, aminoglycosides were more efficacious in the 3-D cell model. In serum-free medium, most antibiotics (except polymyxins) showed enhanced efficacy when 3-D cells were present. In medium with FBS, colistin was less efficacious in the 3-D cell model. DJK-5 exerted potent inhibition of P. aeruginosa association with both substrates, only in serum-free medium. DJK-5 showed stronger inhibitory activity against P. aeruginosa associated with plastic compared to 3-D cells. The combined addition of tobramycin and DJK-5 exhibited more potent ability to inhibit P. aeruginosa association with both substrates. In conclusion, lung epithelial cells influence the efficacy of most antimicrobials against P. aeruginosa biofilm formation, which in turn depends on the presence or absence of FBS.

Evaluation of combination therapy for Burkholderia cenocepacia lung infection in different in vitro and in vivo models

Burkholderia cenocepacia is an opportunistic pathogen responsible for life-threatening infections in cystic fibrosis patients. B. cenocepacia is extremely resistant towards antibiotics and therapy is complicated by its ability to form biofilms. We investigated the efficacy of an alternative antimicrobial strategy for B. cenocepacia lung infections using in vitro and in vivo models. A screening of the NIH Clinical Collection 1&2 was performed against B. cenocepacia biofilms formed in 96-well microtiter plates in the presence of tobramycin to identify repurposing candidates with potentiator activity. The efficacy of selected hits was evaluated in a three-dimensional (3D) organotypic human lung epithelial cell culture model. The in vivo effect was evaluated in the invertebrate Galleria mellonella and in a murine B. cenocepacia lung infection model. The screening resulted in 60 hits that potentiated the activity of tobramycin against B. cenocepacia biofilms, including four imidazoles of which econazole and miconazole were selected for further investigation. However, a potentiator effect was not observed in the 3D organotypic human lung epithelial cell culture model. Combination treatment was also not able to increase survival of infected G. mellonella. Also in mice, there was no added value for the combination treatment. Although potentiators of tobramycin with activity against biofilms of B. cenocepacia were identified in a repurposing screen, the in vitro activity could not be confirmed nor in a more sophisticated in vitro model, neither in vivo. This stresses the importance of validating hits resulting from in vitro studies in physiologically relevant model systems.

Three-Dimensional Rotating Wall Vessel-Derived Cell Culture Models for Studying Virus-Host Interactions

The key to better understanding complex virus-host interactions is the utilization of robust three-dimensional (3D) human cell cultures that effectively recapitulate native tissue architecture and model the microenvironment. A lack of physiologically-relevant animal models for many viruses has limited the elucidation of factors that influence viral pathogenesis and of complex host immune mechanisms. Conventional monolayer cell cultures may support viral infection, but are unable to form the tissue structures and complex microenvironments that mimic host physiology and, therefore, limiting their translational utility. The rotating wall vessel (RWV) bioreactor was designed by the National Aeronautics and Space Administration (NASA) to model microgravity and was later found to more accurately reproduce features of human tissue in vivo. Cells grown in RWV bioreactors develop in a low fluid-shear environment, which enables cells to form complex 3D tissue-like aggregates. A wide variety of human tissues (from neuronal to vaginal tissue) have been grown in RWV bioreactors and have been shown to support productive viral infection and physiological meaningful host responses. The in vivo-like characteristics and cellular features of the human 3D RWV-derived aggregates make them ideal model systems to effectively recapitulate pathophysiology and host responses necessary to conduct rigorous basic science, preclinical and translational studies.

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.

Cells and Culture Systems Used to Model the Small Airway Epithelium

The pulmonary epithelium is divided into upper, lower, and alveolar (or small) airway epithelia and acts as the mechanical and immunological barrier between the external environment and the underlying submucosa. Of these, the small airway epithelium is the principal area of gas exchange and has high immunological activity, making it a major area of cell biology, immunology, and pharmaceutical research. As animal models do not faithfully represent the human pulmonary system and ex vivo human lung samples have reliability and availability issues, cell lines, and primary cells are widely used as small airway epithelial models. In vitro, these cells are mostly cultured as monolayers (2-dimensional cultures), either media submerged or at air-liquid interface. However, these 2-dimensional cultures lack a three dimension-a scaffolding extracellular matrix, which establishes the intercellular network in the in vivo airway epithelium. Therefore, 3-dimensional cell culture is currently a major area of development, where cells are cultured in a matrix or are cultured in a manner that they develop ECM-like scaffolds between them, thus mimicking the in vivo phenotype more faithfully. This review focuses on the commonly used small airway epithelial cells, their 2-dimensional and 3-dimensional culture techniques, and their comparative phenotype when cultured under these systems.

Mechanisms of Disease: Host-Pathogen Interactions between Burkholderia Species and Lung Epithelial Cells

Members of the Burkholderia species can cause a range of severe, often fatal, respiratory diseases. A variety of in vitro models of infection have been developed in an attempt to elucidate the mechanism by which Burkholderia spp. gain entry to and interact with the body. The majority of studies have tended to focus on the interaction of bacteria with phagocytic cells with a paucity of information available with regard to the lung epithelium. However, the lung epithelium is becoming more widely recognized as an important player in innate immunity and the early response to infections. Here we review the complex relationship between Burkholderia species and epithelial cells with an emphasis on the most pathogenic species, Burkholderia pseudomallei and Burkholderia mallei. The current gaps in knowledge in our understanding are highlighted along with the epithelial host-pathogen interactions that offer potential opportunities for therapeutic intervention.

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.

Interactions of Francisella tularensis with Alveolar Type II Epithelial Cells and the Murine Respiratory Epithelium

Francisella tularensis is classified as a Tier 1 select agent by the CDC due to its low infectious dose and the possibility that the organism can be used as a bioweapon. The low dose of infection suggests that Francisella is unusually efficient at evading host defenses. Although ~50 cfu are necessary to cause human respiratory infection, the early interactions of virulent Francisella with the lung environment are not well understood. To provide additional insights into these interactions during early Francisella infection of mice, we performed TEM analysis on mouse lungs infected with F. tularensis strains Schu S4, LVS and the O-antigen mutant Schu S4 waaY::TrgTn. For all three strains, the majority of the bacteria that we could detect were observed within alveolar type II epithelial cells at 16 hours post infection. Although there were no detectable differences in the amount of bacteria within an infected cell between the three strains, there was a significant increase in the amount of cellular debris observed in the air spaces of the lungs in the Schu S4 waaY::TrgTn mutant compared to either the Schu S4 or LVS strain. We also studied the interactions of Francisella strains with human AT-II cells in vitro by characterizing the ability of these three strains to invade and replicate within these cells. Gentamicin assay and confocal microscopy both confirmed that F. tularensis Schu S4 replicated robustly within these cells while F. tularensis LVS displayed significantly lower levels of growth over 24 hours, although the strain was able to enter these cells at about the same level as Schu S4 (1 organism per cell), as determined by confocal imaging. The Schu S4 waaY::TrgTn mutant that we have previously described as attenuated for growth in macrophages and mouse virulence displayed interesting properties as well. This mutant induced significant airway inflammation (cell debris) and had an attenuated growth phenotype in the human AT-II cells. These data extend our understanding of early Francisella infection by demonstrating that Francisella enter significant numbers of AT-II cells within the lung and that the capsule and LPS of wild type Schu S4 helps prevent murine lung damage during infection. Furthermore, our data identified that human AT-II cells allow growth of Schu S4, but these same cells supported poor growth of the attenuated LVS strain in vitro. Collectively, these data further our understanding of the role of AT-II cells in Francisella infections.

Recellularization of decellularized lung scaffolds is enhanced by dynamic suspension culture

Strategies are needed to improve repopulation of decellularized lung scaffolds with stromal and functional epithelial cells. We demonstrate that decellularized mouse lungs recellularized in a dynamic low fluid shear suspension bioreactor, termed the rotating wall vessel (RWV), contained more cells with decreased apoptosis, increased proliferation and enhanced levels of total RNA compared to static recellularization conditions. These results were observed with two relevant mouse cell types: bone marrow-derived mesenchymal stromal (stem) cells (MSCs) and alveolar type II cells (C10). In addition, MSCs cultured in decellularized lungs under static but not bioreactor conditions formed multilayered aggregates. Gene expression and immunohistochemical analyses suggested differentiation of MSCs into collagen I-producing fibroblast-like cells in the bioreactor, indicating enhanced potential for remodeling of the decellularized scaffold matrix. In conclusion, dynamic suspension culture is promising for enhancing repopulation of decellularized lungs, and could contribute to remodeling the extracellular matrix of the scaffolds with subsequent effects on differentiation and functionality of inoculated cells.