July 2024/Target Knowledge three-minute read.
This target knowledge section validates the epithelium and endothelium structure/function as disease modifying pharmacologic targets.
The information below has been organized for the reader to appreciate that the epithelium and endothelium are more than physical barriers. These mosaic like cells are fluid, responsive cell signal orchestrators. Additionally, as multifunctional bio-sensors the epithelium and endothelium have substantial capacity to limit untoward downstream cellular events.
It is beneficial to scroll to the end of this page to ascertain the epi/endo biological properties as they pertain to various medical conditions. Note, the specific molecular pathways of this biologic response modifier are not shared in this information.
The ability of anu bio-pharma’s biologic response modifier to modulate tight junction proteins, and innate/adaptive immune cells uniquely expands the therapeutic landscape. anu bio-pharma’s breakthrough technology is analogous to development and market relevance of anti-TNF therapy.
The information below demonstrates the basic science (known target knowledge) for development of this innovative biologic response modifier. This information substantiates the clinical impact this first in class biologic will provide to the medical management of multiple disease conditions (respiratory, gastrointestinal, urothelial); immuno-oncology (as an immuno-adjuvant). The cardio-vascular relevance of this biologic response modifier is presented via separate PDF materials.
Currently, the medical community does not have a therapeutic to address a loss of epithelial – endothelial integrity. In addition to the Home page, the navigation bar has a heading labeled Market Relevance. The section provides an overview of the market impact (human/financial merits) regarding this disease modifying treatment.
Noteworthy, in drug development molecular modeling and docking based studies are not interchangeable with patient usage/efficacy studies.
● The key elements of adaptive immunity (specificity, memory, diversity, self/non-self discrimination), antigens have to be processed and presented to immune cells. Antigen presentation is mediated by MHC class I molecules, and the class II molecules found on the surface of antigen-presenting cells (APCs) and certain other cells. MHC class I and class II molecules are similar in function: they deliver short peptides to the cell surface allowing these peptides to be recognized by CD8+ (cytotoxic) and CD4+ (helper) T cells, respectively. MHC class II molecules are expressed by APCs, such as dendritic cells, macrophages and B cells (and, under IFNγ stimuli, by mesenchymal stromal cells, fibroblasts and endothelial cells, as well as by epithelial cells and enteric glial cells).
● Until a few years ago, lymphatic vessels and lymphatic endothelial cells (LEC) were viewed as part of a passive conduit for lymph and immune cells to reach lymph nodes. However, recent work has shown that LEC are active immunological players whose interaction with dendritic cells and T cells is of important immunomodulatory relevance. While the immunological interaction between LEC and other immune cells has taken a center stage, molecular analysis of LEC antigen processing and presentation machinery is still lagging. Herein we review the current knowledge of LEC MHC I and MHC II antigen processing and presentation pathways, Including the role of LEC in antigen phagocytosis, classical, and non-classical MHC II presentation, proteasome processing and MHC I presentation, and cross-presentation. The ultimate goal is to provide an overview of the LEC antigen processing and presentation machinery that constitutes the molecular basis for their role in MHC I and MHC II-restricted immune responses.
Front. Immunol., 07 May 2019
The Antigen Processing and Presentation Machinery in Lymphatic Endothelial Cells
Laura Santambrogio, Stella J. Berendam and Victor H. Engelhard
● The epithelium maintains its selective barrier function through the formation of complex protein-protein networks that mechanically link adjacent cells and seal the intercellular space. The protein networks connecting epithelial cells form three adhesive complexes: desmosomes, adherens junctions and tight junctions. These complexes consist of transmembrane proteins that interact extracellularly with adjacent cells and intracellularly with adaptor proteins that link to the cytoskeleton.
● The ability of epithelial cells to organize through cell-cell adhesion into a functioning epithelium serves the purpose of a tight epithelial protective barrier. Contacts between adjacent cells are made up of tight junctions (TJ), adherens junctions (AJ), and desmosomes with unique cellular functions and a complex molecular composition. These proteins mediate firm mechanical stability, serves as a gatekeeper for the paracellular pathway, and helps in preserving tissue homeostasis. TJ proteins are involved in maintaining cell polarity, in establishing organ-specific apical domains and also in recruiting signaling proteins involved in the regulation of various important cellular functions including proliferation, differentiation, and migration. As a vital component of the epithelial barrier, TJs are under a constant threat from proinflammatory mediators, pathogenic viruses and bacteria, aiding inflammation and the development of disease.
Front Physiol. 2019 Jan 23;9:1942.
Tight Junction Proteins and Signaling Pathways in Cancer and Inflammation: A Functional Crosstalk.
Ajaz A Bhat, Srijayaprakash Uppada
Endothelial tight junctions are similar to adherens junctions, but are composed of interactions of tight junction proteins: occludin, claudins (3/5), and JAM-A. Occludin and claudins are integral membrane proteins, each with four transmembrane domains and two extracellular loop domains. The extracellular loop domains of occludin or claudins form homotypic binding with the extracellular domains of like molecules on neighboring endothelial cells. JAM-A, a member of the immunoglobin superfamily of proteins, is also present in tight junctions, though the role of JAM-A in tight junctions is not understood. Occludin, claudins, and JAM-A are connected to the actin cytoskeleton via zona occludens proteins (ZO-1, ZO-2) and α-catenin. In addition to connecting junction proteins to the cytoskeleton, ZO proteins serve as signaling molecules (guanylate kinases) or scaffolding proteins that recruit other signaling molecules via PDZ and Src homology 3 (SH3) binding domains. Hence ZO proteins play both structural and signaling roles in tight junctions. The connection between tight junctions and the actin cytoskeleton is further stabilized by actin cross-linking proteins (e.g., spectrin or filamen) and accessory proteins (e.g., cingulin or AF-6).
Similar to many other cell types, the endothelial cytoskeleton is composed of microtubules, intermediate filaments and actin filaments. These structures are important for endothelial cell morphology, adhesion, and barrier function. While the structural support provided by all cytoskeletal components is important for barrier integrity, the actin cytoskeleton is most centrally important for regulation of endothelial permeability.
Regulation of Endothelial Barrier Function.
Yuan SY, Rigor RR.
San Rafael (CA): Morgan & Claypool Life Sciences; 2010.
The remarkable plasticity of the endothelial cells could be seen as its principal property. One can make the assumption that each one of the trillion endothelial cells included in our body is phenotypically distinct since, like a chameleon, each one has to sense and adapt to the needs of the various neighboring cells and to many different microenvironments.
Phenotypic Heterogeneity of the Endothelium
I. Structure, Function, and Mechanisms
William C. Aird
Circulation Research. 2007;100:158–173
Epithelial and endothelial barrier integrity, essential for homeostasis, is maintained by cellular boarder structures known as tight junctions (TJs). In critical illness, TJs may become disrupted, resulting in barrier dysfunction manifesting as capillary leak, pulmonary edema, gut bacterial translocation, and multiple organ failure.
Tight junctions (TJs) are protein complexes that form the semi-permeable connections between cells lining corporeal compartments. In endothelial and epithelial cell layers, TJs are responsible for the selective barriers that permit specialized organ function. Epithelial TJs regulate alveolar air-fluid balance in the lungs, the production of appropriately concentrated urine in the kidney, as well as the absorption of nutrients and containment of bacteria throughout the gastrointestinal tract. TJs in endothelia maintain intravascular volume and regulate the flux of fluid and solutes between blood vessels and organ parenchyma. Endothelial and epithelial barrier dysfunction in the setting of critical illness can result in malabsorption of nutrients, translocation of gut bacteria, capillary leak, interstitial edema, tissue dysoxia, and organ failure.
Zonula occluden-1 (ZO-1) is ubiquitously expressed in epithelial and endothelial cells and has multiple domains to facilitate cellular signaling. ZO-1 has three PDZ domains that bind claudins and JAMs, a GUK domain that binds occludin, a SH3 domain that binds the transcription factor ZONAB, and a carboxy-terminus that interacts with cytoskeletal F-actin. These multiple protein interactions couple the extra- and intracellular signaling that allows the intricacy and plasticity of TJ function.
The density and identity of claudins species dictate the permselectivity of organ-specific vascular segments. Claudin-5 is expressed by all endothelial cells. Claudins-1, 3, and 12 are also present in vascular endothelial TJs albeit at low levels. The blood-brain barrier is established by numerous TJs containing large amounts of claudins-3, 5, and 12. Occludin and zonulin (also known as prehaptoglobin-2) are expressed throughout the epithelium and endothelium while JAM-A and JAM-C are restricted to endothelium.
Intensive Care Med Exp. 2018 Sep 26;6(1):37.
Tight junction structure, function, and assessment in the critically ill: a systematic review
David Vermette, Pamela Hu, Michael F Canarie, Melissa Funaro, Janis Glover, Richard W Pierce
Junctional adhesion molecules are a family of glycoproteins characterized by two immunoglobulin folds (VH- and C2-type) in the extracellular domain. Junctional adhesion molecule proteins localize to intercellular junctions of polarized endothelial and epithelial cells but can also be expressed on circulating leukocytes and platelets. In addition, they bind several ligands, in both a homophilic and heterophilic manner, and associate with several cytoplasmic partners. All these features represent the likely determinants for the role of junctional adhesion molecule proteins in processes as diverse as junction assembly, leukocyte transmigration and platelet activation.
Curr Opin Cell Biol. 2003 Oct;15(5):525-30.
The JAM family of junctional adhesion molecules
Gianfranco Bazzoni
Junctional adhesion molecule (JAM) is a member of the immunoglobulin superfamily localized at the tight junction of polarized cells and on the cell surface of leukocytes. Several members of the family mediate cell polarity, endothelium permeability, and leukocytes migration through a multitude of homophilic and heterophilic interactions with intrafamily and extra family partners. Several members of the JAM family interact with PDZ domain-containing scaffolding proteins such as ZO-1, claudin, and afadin, regulating cell–cell contact maturation and the generation of junctional complexes such as TJs and adherens junctions.
Front Cell Dev Biol. 2022; 10: 843671.
The Roles of Junctional Adhesion Molecules (JAMs) in Cell Migration
Junqi Wang and Han Liu
The endothelium forms the inner cell lining of all blood vessels and lymphatics in the body. The endothelium is involved in most if not all disease states, either as a primary determinant of pathophysiology or as a victim of collateral damage.
Given that endothelial cells are among the first cells to come into contact with circulating pathogens and are the first cells that immune cells interact with when invading tissue parenchyma, they are strategically ideally positioned as a first-line defense system to participate in immune responses.
Immunomodulation by endothelial cells — partnering up with the immune system?
Jacob Amersfoort, Guy Eelen & Peter Carmeliet
Nature Reviews Immunology Published: 14 March 2022
Oropharyngeal, airway, intestinal, and genital mucosal epithelia are the main portals of entry for the majority of human pathogenic viruses. To initiate systemic infection, viruses must first be transmitted across the mucosal epithelium and then spread across the body. However, mucosal epithelia have well-developed tight junctions, which have a strong barrier function that plays a critical role in preventing the spread and dissemination of viral pathogens. Viruses can overcome these barriers by disrupting the tight junctions of mucosal epithelia, which facilitate paracellular viral penetration and initiate systemic disease. Disruption of tight and adherens junctions may also release the sequestered viral receptors within the junctional areas, and liberation of hidden receptors may facilitate viral infection of mucosal epithelia. This review focuses on possible molecular mechanisms of virus-associated disruption of mucosal epithelial junctions and its role in transmucosal viral transmission and spread.
Tissue Barriers. 2021 Oct 2;9(4):1943274.
Virus-associated disruption of mucosal epithelial tight junctions and its role in viral transmission and spread.
Sharof Tugizov
Department of Medicine, School of Medicine, University of California, San Francisco, 513 Parnassus Ave, San Francisco, CA 94143, USA
HIV-1 infection occurs primarily through mucosal transmission.
Vascular permeability differs among organs, adapts to physiological needs, and reflects the underlying biology of each organ. Vascular permeability also contributes to the pathophysiology of many diseases. Increased permeability is a prominent feature of asthma and other inflammatory airway diseases, arthritis, chronic bowel disease, cancer, infections, trauma, ischemic stroke, and many other conditions where leakage can result in edema, impaired function, and morbidity.
The overlapping border of endothelial cells is separated by a narrow cleft bridged by two types of junctions. Tight junctions form the barrier near the luminal (apical) surface. Adherens junctions, typically located more basally, join adjacent endothelial cells. Focal changes in junctions result in endothelial gaps and leakage. Gap formation requires detachment of complexes involving the adherens junction protein VE-cadherin that joins adjacent endothelial cells through homophilic interactions.
Adjacent endothelial cells are connected by two types of intercellular junctions: adherens junctions and tight junctions. The number and organization of these junctions underlie permeability differences in the vasculature to accommodate organ- and tissue-specific needs. Mediators that increase permeability activate kinases, phosphatases, and other enzymatic activities that control phosphorylation of junctional proteins and focal gap formation between endothelial cells.
Trends Mol Med. 2021 Apr;27(4):314-331.
Permeability of the Endothelial Barrier: Identifying and Reconciling Controversies
Lena Claesson-Welsh, Elisabetta Dejana, Donald M. McDonald
Endothelial cells lining blood vessels regulate vascular barrier function, which controls the passage of plasma proteins and circulating cells across the endothelium. In most normal adult tissues, endothelial cells preserve basal vascular permeability at a low level, while they increase permeability in response to inflammation. Therefore, vascular permeability is tightly controlled by a number of extracellular stimuli and mediators to maintain tissue homeostasis. Accordingly, impaired regulation of endothelial permeability causes various diseases, including chronic inflammation, asthma, edema, sepsis, acute respiratory distress syndrome, anaphylaxis, tumor angiogenesis, and diabetic retinopathy.
Vascular endothelial (VE)-cadherin, a member of the classical cadherin superfamily, is a component of cell-to-cell adherens junctions in endothelial cells and plays an important role in regulating vascular permeability. VE-cadherin mediates intercellular adhesion through trans-interactions formed by its extracellular domain, while its cytoplasmic domain is anchored to the actin cytoskeleton via α- and β-catenins, leading to stabilization of VE-cadherin at cell-cell junctions. VE-cadherin-mediated cell adhesions are dynamically, but tightly, controlled by mechanisms that involve protein phosphorylation and reorganization of the actomyosin cytoskeleton. Phosphorylation of VE-cadherin, and its associated-catenins, results in dissociation of the VE-cadherin/catenin complex and internalization of VE-cadherin, leading to increased vascular permeability.
J Nippon Med Sch. 2017;84(4):148-159.
Dynamic Regulation of Vascular Permeability by Vascular Endothelial Cadherin-Mediated Endothelial Cell-Cell Junctions
Seung-Sik Rho, Koji Ando, Shigetomo Fukuhara
The epithelial lining of luminal organs such as the gastrointestinal and respiratory tract forms a regulated, selectively permeable barrier between luminal contents and the underlying tissue compartments. Paracellular permeability across epithelial and endothelial cells is in large part regulated by an apical intercellular junction also referred to as the tight junction (TJ). The tight junction and its subjacent adherens junction (AJ) constitute the apical junctional complex (AJC). The AJC is composed of a multiprotein complex, which affiliates with the underlying apical perijunctional F-actin ring. Such AJC association with the perijunctional F-actin ring is vital for maintaining its structure and function in health. Stimuli such as nutrients, internal signaling molecules and cytokines influence the apical F-actin organization and also modulate the AJC structure and paracellular permeability. Here we review some of the key stimuli that influence F-actin organization, AJC structure and paracellular permeability.
Regulation of Paracellular Transport across Tight Junctions by the Actin Cytoskeleton
Matthias Bruewer and Asma Nusrat.
Department of Pathology and Laboratory Medicine, Emory University, Whitehead Research Building, Room 105E, 615 Michael Street, Atlanta, Georgia 30322
Tight junctions of both epithelial and endothelial cells are critical in regulating the permeability across the epithelia and the TJ complex is a hub for signaling pathways which governs the metastatic potential in several cancers.
Front. Physiol., 23 January 2019
Tight Junction Proteins and Signaling Pathways in Cancer and Inflammation: A Functional Crosstalk
Ajaz A. Bhat, Srijayaprakash Uppada, Iman W. Achkar
A key step in metastasis is the interaction and penetration of the vascular endothelium by cancer cells. Tight Junctions (TJ) are located between the cancer epithelial cells and between the endothelial cells functioning in an adhesive manner. They represent a critical barrier which the cancer cells must overcome in order to penetrate and initiate metastasis.
Biological Response Modifiers may also be used as anticarcinogens, with the following goals:
1) to stop, control, or suppress processes that permit cancer growth.
2) to make cancer cells more recognizable, and therefore more susceptible, to destruction by the immune system.
3) to boost the killing power of immune system cells, such as T cells, natural killer cells, and macrophages.
4) to alter growth patterns in cancer cells to promote behavior like that of healthy cells.
5) to block or reverse the processes that change a normal cell or a precancerous cell into a cancerous cell.
6) to enhance the ability of the body to repair or replace normal cells damaged or destroyed by other forms of cancer treatment, such as chemotherapy or radiation.
7) to prevent cancer cells from spreading to other parts of the body.
Tumor cells exist in sophisticated three-dimensional (3D) microenvironment in vivo commonly referred to as tumor microenvironment (TME), which consists of various types of cells, biochemical cues and biophysical cues. TME has been found to play an important role in various cancer pathological behaviors and treatment response, while biophysical cues in TME were recognized as new characteristics of cancer. The authors of previous studies mainly focused mainly focused on the effect of biochemical cues in TME (e.g., chemokines, metabolic factors) on immunotherapy resistance. However, recent advances in functional biomaterials and micro/nano technologies have enabled researchers to engineer the biophysical microenvironment of cancer cells and stromal cells (including endothelial cells, epithelial cells, stromal cells and immune cells) in vitro mimicking the native situation. With these powerful tools, biophysical cues play crucial roles in the immunologic microenvironment and immune cell biological behaviors as evidenced by mounting data. In particular, biological cues interfere with the integrity of the cascade of anti-tumor immunity by affecting the immune cells biological behaviors, which synergistically contribute to immunotherapy resistance. For example, the increased matrix stiffness in TME inhibits the ability of dendritic cells (DCs) to recognize and internalize tumor antigens, and as well as reducing their DCs’ migration ability will also be reduced. When this occurs, DCs cannot smoothly present antigens to, or complete the activation of T cells, which leads to the failure of the first step of anti-tumor immunity. The key to cancer immunotherapy rests on the ability of a large number of activated T cells and therapeutic antibodies to enter the tumor. However, if a TME features an abnormally dense ECM structure, as with dense desmoplasia in pancreatic cancer and prostate cancer, T cells become blocked in the dense matrix area surrounding the tumor, resulting in an immune-excluded TME phenotype and thus a very low response to immunotherapy. Further, the dense ECM structure will also form a physical barrier to hinder the delivery of anti-PD-1/PD-L1 therapeutic antibodies with high molecular weight. Therefore, understanding the functions of biophysical cues in immunotherapy resistance is vital. Advanced Drug Delivery Reviews. Volume 186, July 2022
Targeting the tumor biophysical microenvironment to reduce resistance to immunotherapy. Tian Zhang, Yuanbo Jia, Yang Yu,
● Adoptive cell therapy (ACT), based on treatment with autologous tumor infiltrating lymphocyte (TIL)-derived or genetically modified chimeric antigen receptor (CAR) T cells, has become a potentially curative therapy for subgroups of patients with melanoma and hematological malignancies. To further improve response rates, and to broaden the applicability of ACT to more types of solid malignancies, it is necessary to explore and define strategies that can be used as adjuvant treatments to ACT. Stimulation of endogenous dendritic cells (DCs) alongside ACT can be used to promote epitope spreading and thereby decrease the risk of tumor escape due to target antigen downregulation, which is a common cause of disease relapse in initially responsive ACT treated patients. Addition of checkpoint blockade to ACT and DC stimulation might further enhance response rates by counteracting an eventual inactivation of infused and endogenously primed tumor-reactive T cells. This review will outline and discuss therapeutic strategies that can be utilized to engage endogenous DCs alongside ACT and checkpoint blockade, to strengthen the anti-tumor immune response.
Front Immunol. 2020 Sep 25;11:578349.
Leveraging Endogenous Dendritic Cells to Enhance the Therapeutic Efficacy of Adoptive T-Cell Therapy and Checkpoint Blockade
Mie Linder Hübbe, Ditte Elisabeth Jæhger, Thomas Lars Andresen, Mads Hald Andersen
Endothelial barrier function changes after tissue injury by allergens, pathogens, toxins, trauma, burns, and other stimuli. Cytokine storm in coronavirus disease 2019 (COVID-19) is an example. Mechanisms that regulate endothelial permeability under these conditions have multiple features in common: endothelial junction organization is altered, gaps form, and the transendothelial hydrostatic pressure gradient drives leakage.
[Part of Abstract] In conclusion, the presence of SARS-CoV-2 virus within endothelial cells suggests that direct viral effects, as well as perivascular inflammation, may contribute to endothelial cell injury. It is likely that endothelitis, endothelial injury, endothelial cell dysfunction and impaired microcirculatory function in different vascular beds contributes markedly to life-threatening complications of COVID-19, such as venous thromboembolic disease and multiple organ involvement.
European Respiratory Journal 2020 56: 2001634;
Endothelial cell dysfunction: a major player in SARS-CoV-2 infection (COVID-19)?
Alice Huertas, David Montani, Laurent Savale, Jérémie Pichon, Ly Tu, Florence Parent
The disruption of tight junction (TJ) complexes at the lateral contact of epithelial cells, the loss of contact between epithelial cells and extracellular matrix (ECM), and relevant changes in the communication between epithelial and immune cells, are deleterious alterations that mediate the disruption of the alveolar epithelial barrier and thereby the formation of lung edema in acute respiratory distress syndrome (ARDS).
The TJ complexes include transmembrane proteins such as occludin, claudins, tricellulin, and other junction adhesion molecules (JAM), and intracellular adaptor proteins like cingulin and zonula occludens (ZO) that ultimately bind to actin fibers of the cytoskeleton. Occludin, ZO-1, and claudin-4 have been shown to be important components of TJs in the alveolar epithelium. Occludin is required for maintaining the integrity of the alveolar epithelial barrier. Claudin-4 improves the barrier function of the pulmonary epithelial barrier by promoting AFC function. ZO-1 is a scaffold protein that serves as a link between transmembrane TJ proteins (occludin, claudin) and the actin cytoskeleton, being an important element that influences the structure and function of the alveolar epithelial barrier. Actin and myosin, the two main components of the anchored cytoskeleton, interact to regulate cell tension and contraction, which also influence epithelial permeability. Alterations in the expression, localization and assembly of these proteins within the TJ complexes and in their interactions with the actin fibers of the cytoskeleton result in the dysfunction of TJs with the consequent increase in paracellular permeability.
Ann Transl Med. 2018 Jan; 6(2): 32.
New insights into the mechanisms of pulmonary edema in acute lung injury
Raquel Herrero, Gema Sanchez, and Jose Angel Lorente
Airway inflammation can disrupt lung protection and regeneration by inducing swelling of mucous membranes lining the bronchi, increasing bronchial mucous production, and decreasing movement of the thick mucus by ciliated cells, thereby limiting microbial clearance. Respiratory disorders such as COPD, asthma, and silicosis, compounded by smoking and corticosteroid use, are independent risk factors for M.tb infection because they cause airway inflammation driving airway obstruction, making airways smaller which increases air velocity and displaces air to unobstructed areas. Thus, airway inflammation can result in prolonging M.tb infection and deposition/movement of M.tb to healthy areas of the lung.
Trends Microbiol. 2017 Aug; 25(8): 688–697.
Integrating Lung Physiology, Immunology and Tuberculosis
Jordi B. Torrelles and Larry S. Schlesinger
Though the primary responsibility of the respiratory system is to carry out efficient gas exchange between inhaled air and the bloodstream, it also plays a pivotal role in maintaining respiratory homeostasis, and when dysregulated, can contribute to disease.
While innate and adaptive immune cells are fundamental for protection, the respiratory epithelium also plays an indispensable role in host defense.
Historically, the respiratory epithelium was thought to function as a first line of defense by constituting a passive barrier against pathogen invasion. Indeed, while it does serve this purpose, the airway epithelium also engages in constant immunological activity, with each cell subtype actively contributing to the maintenance of respiratory health in the face of viral and bacterial attack, or allergen exposure.
Virus-associated molecular patterns are recognized by a variety of intracellular pattern recognition receptors including toll-like receptors and rig-like helicases e.g. MDA5 and RIG-I, which stimulates release of type 1 and 3 interferons. Collectively, interferons mediate antiviral immunity by inhibiting viral replication, recruiting immune cells (e.g., T cells, B cells, natural killer cells), and upregulating MHC class I expression, leading to increased CD8+ T cell activity. Activated airway epithelial cells also secrete proinflammatory cytokines IL-1α, IL-1β, and TNF-α.
Bacteria can be recognized by multiple TLRs stimulating the release of antimicrobial peptides including human cathelicidin LL-37, defensins, lactoferrin, lysozyme and SLPI. LL-37 mediates recruitment of monocytes, neutrophils, and CD4+ T cells, while defensins, lactoferrin, and lysozyme can destroy bacteria through different mechanisms. AECs also secrete SLPI, which acts in an autocrine/paracrine manner to protect the epithelium against serine proteases and proteolytic enzymes. In response to bacterial infection airway epithelial cells also release a plethora of chemokines, for example, the neutrophilic chemoattractant CXCL8, as well as CCL2, CCL3, CC4, and CCL5, which mediate recruitment of several leukocytes (e.g., monocytes/macrophages, T cells, DCs, neutrophils and natural killer cells). In response to viral and bacterial co-infection, the respiratory epithelium produces IL-17C, which is thought to stimulate neutrophil activity.
Review Article
Cellular and functional heterogeneity of the airway epithelium
Jordan D. Davis & Tomasz P. Wypych
Mucosal Immunology volume 14, pages 978–990 (19 February 2021)
The pulmonary alveolar epithelium is mainly composed of two types of epithelial cells (pneumocytes).
Type I cells are the larger of the two cell types; they are thin, flat epithelial lining cells (membranous pneumocytes), that form the structure of the alveoli. They are squamous (giving more surface area to each cell) and have long cytoplasmic extensions that cover more than 95% of the alveolar surface. These pneumocytes joined one another and other alveolar cells by tight junctions, forming an impermeable barrier to limit the infiltration of fluid into the alveoli.
The cuboidal alveolar type II cells line the remainder of the alveolus. Alveolar type II cells have a key role in the innate immune response and as self-renewing progenitors to replace alveolar type I cells during regeneration of the alveolar epithelium. Their secretion of surfactant protein helps to maintain homeostasis in the distal lung and exert protective, antimicrobial properties.
Type I pneumocytes have three main functions.
Facilitate gas exchange
Maintain ion and fluid balance within the alveoli
Communicate with type II pneumocytes to secrete surfactant in response to stretch.
Type II pneumocytes have four main functions.
Produce and secrete pulmonary surfactant – surfactant is a vital substance that reduces surface tension, preventing alveoli from collapsing.
Expression of immunomodulatory proteins that are necessary for host defense
Transepithelial movement of water
Regeneration of alveolar epithelium after injury
Alveolar macrophages play an essential role in the immune system. They collect inhaled particles from the environment, such as coal, silica, and asbestos, and microbes, including viruses, bacteria, and fungi. Alveolar macrophages have a receptor named toll-like receptor, which binds to another receptor on the surface of microbial cells, the pathogen-associated molecular receptor. This interaction facilitates the phagocytosis of the pathogen and the secretion of pro-inflammatory cytokines to enhance the local immune response. Within the alveolar macrophage, engulfed microbes become fused with their lysosome to destroy the pathogen.
Histology, Alveolar Cells April 21, 2022.
Josiah P. Brandt; Pujyitha Mandiga.
The lung interfaces with the environment across a continuous epithelium composed of various cell types along the proximal and distal airways. At the alveolar structure level, the epithelium, which is composed of type I and type II alveolar epithelial cells, represents a critical component of lung homeostasis. Indeed, its fundamental role is to provide an extensive surface for gas exchange. Additional functions that act to preserve the capacity for such unique gas transfer have been progressively identified. The alveolar epithelium represents a physical barrier that protects from environmental insults by segregating inhaled foreign agents and regulating water and ions transport, thereby contributing to the maintenance of alveolar surface fluid balance. The homeostatic role of alveolar epithelium relies on the regulated/controlled production of the pulmonary surfactant, which is not only a key determinant of alveolar mechanical stability but also a complex structure that participates in the cross-talk between local cells and the lung immune and inflammatory response. In regard to these critical functions, a major point is the maintenance of alveolar surface integrity, which relies on the renewal capacity of type II alveolar epithelial cells, and the contribution of progenitor populations within the lung.
Int J Biochem Cell Biol. 2013 Nov;45(11):2568-73.
Alveolar epithelial cells: master regulators of lung homeostasis
Loïc Guillot, Nadia Nathan, Olivier Tabary
The lung must maintain a proper barrier between airspaces and fluid filled tissues in order to maintain lung fluid balance. Central to maintaining lung fluid balance are epithelial cells which create a barrier to water and solutes. The barrier function of these cells is mainly provided by tight junction proteins known as claudins. Epithelial barrier function varies depending on the different needs within the segments of the respiratory tree. In the lower airways, fluid is required to maintain mucociliary clearance, whereas in the terminal alveolar airspaces a thin layer of surfactant enriched fluid lowers surface tension to prevent airspace collapse and is critical for gas exchange. As the epithelial cells within the segments of the respiratory tree differ, the composition of claudins found in these epithelial cells is also different. Among these differences is claudin-18 which is uniquely expressed by the alveolar epithelial cells. Other claudins, notably claudin-4 and claudin-7, are more ubiquitously expressed throughout the respiratory epithelium. Claudin-5 is expressed by both pulmonary epithelial and endothelial cells.
Review Semin Cell Dev Biol. 2015 Jun; 42:47-57.
Claudins: Gatekeepers of lung epithelial function
Barbara Schlingmann, Samuel A Molina, Michael Koval
Tight junctions (TJs) are essential for normal function of epithelia, restricting paracellular diffusion and contributing to the maintenance of cell surface polarity. Superficial cells of the urothelium develop TJs, the basis for the paracellular permeability barrier of the bladder against diffusion of urinary solutes.
Host-directed therapy (HDT) is an emerging approach in the field of anti-infectives. The strategy behind HDT is to interfere with host cell factors that are required by a pathogen for replication or persistence, to enhance protective immune responses against a pathogen, to reduce exacerbated inflammation and to balance immune reactivity at sites of pathology. In this Review, we compare different HDT approaches for viral and bacterial infections and describe the progress made in their development, focusing mostly on tuberculosis (TB) and sepsis for bacterial infections and on chronic viral hepatitis and AIDS for viral infections.
Host-directed therapies for bacterial and viral infections
Stefan H. E. Kaufmann, Anca Dorhoi, Richard S. Hotchkiss & Ralf Bartenschlager
Nature Reviews Drug Discovery volume 17, pages 35–56 (2018)
The large amount of mucosa-associated lymphoid tissue and its specialization for the production of IgA make IgA the major immunoglobulin in humans. Given that it is synthesized and secreted by plasma cells located in the lamina propria of the digestive, respiratory, and urogenital tracts yet functions in external secretions, IgA must be delivered across an epithelial barrier. This requirement is accomplished by the polymeric IgA-receptor (pIgA-R), a single transmembrane protein synthesized by the epithelial cells. As discussed in detail in section iv, this receptor has a long (∼100 amino acid) cytoplasmic tail that contains most of the signals necessary to direct it through its cellular itinerary. However, unlike most other endocytic receptors that perform repeated rounds of cargo uptake, delivery and recycling, the extracellular domain of pIgA-R is cleaved upon delivery to the apical surface and released into the lumen with its ligand. The presence of the added “secretory component” stabilizes IgA in the gut lumen. This unique transcytotic system is expressed in many epithelia throughout the body, including kidney, trachea, and the digestive tract, including the liver.
Physiol Rev 83: 871–932, 01 Jul 2003
Transcytosis: Crossing Cellular Barriers
Tuma, Pamela L., and Ann L. Hubbard
Transcytosis occurs in the upper regions of the respiratory tract and involves two receptor systems, pIgA-R and FcRn. Secretory IgA is a known constituent of the lung’s immune defense system, with bronchial epithelial cells carrying out basolateral-to-apical transport of dIgA, which is secreted by local plasma cells in underlying lymphoid tissue.
Although second in serum behind IgG, IgA is the predominant Ig at mucosal surfaces and acts as a crucial actor in mucosal immunity. IgA possesses multifaceted functions that encompass immune exclusion, neutralization of antigens and pathogens plugged in mucus, regulation of microbiota, as well as regulation of immune cells in the submucosa. At mucosal sites, IgA is produced by dedicated plasma B cells in its dimeric form (d-IgA). D-IgA is then transported across the epithelium by the pIgR to reach mucosal secretions where it is released as secretory (S)-IgA. Two subclasses of IgA coexist in humans, with IgA1 predominating in serum as monomers, while IgA2 is enriched in external secretions (mainly as dimers), representing up to 50% of total IgA. To produce IgA, naive B cells mature through IgA class switching. Class switch recombination toward IgA depends on multiple mediators such as TGF-β, B-cell activation factor (BAFF), A proliferation-inducing ligand (APRIL), TSLP, and IL-6, as well as by external factors such as microbial stimuli acting on lung dendritic cells, through MyD88-dependent Toll-like receptor (TLR) activation.
Front Physiol. 2021; 12: 691227.
Epithelial Barrier Dysfunction in Chronic Respiratory Diseases
François M. Carlier, Charlotte de Fays, and Charles Pilette
IgA, the most abundantly produced antibody isotype in the body, has the dual roles of maintaining homeostasis with the microbiome and protecting from intestinal infection. Plasma cells located in the lamina propria secrete IgA, but the early stages of IgA production take place mainly in Peyer’s patches (PPs). PPs are lymphoid organs that are organized into B cell-rich follicles, T cell-rich interfollicular zones and a subepithelial dome (SED) rich in CD11c+ dendritic cells (DCs) that separates the epithelium from the follicles. Gut-derived antigens delivered across specialized epithelial cells continually stimulate PPs and PP follicles harbor chronic T cell-dependent germinal centers (GCs). PP GCs contain a high frequency of IgA+ cells and these give rise to IgA plasma cells. While a number of factors have been implicated in PP B cell switching to IgA, the strongest requirement established in vivo is for transforming growth factor β receptor (TGFβR) signaling.
Science. 2016 May 13; 352(6287): aaf4822.
IgA production requires B cell interaction with subepithelial dendritic cells in Peyer’s patches
Andrea Reboldi, Tal I. Arnon
● The mucosal immune system can be divided into inductive and effector sites based on their anatomical and functional properties. The mucosal inductive sites are collectively called mucosa-associated lymphoid tissue (MALT) and include gut-associated lymphoid tissues (GALT), nasopharyngeal-associated lymphoid tissue (NALT) and lymphoid sites. The MALT provides a continuous source of memory B and T cells that then move into effector sites. Mucosal effector sites include the lamina propria regions of the gastrointestinal, upper respiratory and reproductive tracts as well as secretory glandular tissues. These sites contain antigen-specific mucosal effector cells such as IgA-producing plasma cells, and memory B and T cells.
Inside the Mucosal Immune System. McGhee and Fujihashi 2012.
● Mucosal defense mechanisms are critical in preventing colonization of the respiratory tract by pathogens and penetration of antigens through the epithelial barrier. Recent research has now illustrated the active contribution of the respiratory epithelium to the exclusion of microbes and particles, but also to the control of the inflammatory and immune responses in the airways and in the alveoli. Epithelial cells also mediate the active transport of polymeric immunoglobulin-A from the lamina propria to the airway lumen through the polymeric immunoglobulin receptor. The role of IgA in the defense of mucosal surfaces has now expanded from a limited role of scavenger of exogenous material to a broader protective function with potential applications in immunotherapy. In addition, the recent identification of receptors for IgA on the surface of blood leukocytes and alveolar macrophages provides an additional mechanism of interaction between the cellular and humoral immune systems at the level of the respiratory tract.
European Respiratory Journal 2001 18: 571-588. Lung mucosal immunity: immunoglobulin-A.
C. Pilette, Y. Ouadrhiri, V. Godding, J-P. Vaerman, Y. Sibille
A variety of subpopulations of dendritic cells (DCs) are present in the organized lymphoid structures of the intestinal immune system, including the Peyer’s patches and mesenteric lymph nodes (MLNs), and also throughout the small intestinal and colonic lamina propria. DCs exist in all of these sites in the steady state, but can also develop from precursors in response to microbial and inflammatory stimuli.
DCs have also been implicated in class switching to IgA, the predominant isotype at mucosal surfaces. Mechanisms of class switching to IgA are complex and likely to vary depending on the site at which it occurs, the type of B cell, dependence on T cells, and the presence of commensal versus pathogenic species. Nevertheless, a clear role for Peyer’s Patch DC in class switching to IgA has been demonstrated. Consistent with this, populations of DCs in the intestine produce a variety of different cytokines and other mediators implicated in class switching to IgA, including IL-10, TGFβ, IL-6 and APRIL (a proliferation-inducing ligand).
Nat Rev Immunol. 2008 Jun; 8(6): 435–446.
Dendritic cells in intestinal immune regulation.
Janine L Coombes and Fiona Powrie
The gut epithelium maintains both a barrier to and tolerance to the multitude of bacteria, viruses, fungi, and parasites that reside in and pass through the gastrointestinal tract. The intestinal epithelium is composed of absorptive enterocytes, mucus- and peptide-producing goblet cells, Paneth cells, which secrete antimicrobial peptides, microfold (M) cells, which specialize in antigen sampling and enteroendocrine cells which produce various hormones. Directly below the layer of epithelial cells are innate and adaptive immune cells, including B cells, T cells, macrophages, dendritic cells, and innate lymphoid cells. Epithelial and immune cells interact closely together using bidirectional communication to maintain immune tolerance to commensal microbes while retaining the ability to mount an inflammatory response to pathogens or invading organisms.
Immunologic Response in the Host
K. Madsen, H. Park, in The Microbiota in Gastrointestinal Pathophysiology, 2017
● The vascular system governs guidance to traveling immune cells and thereby supports protective immune functions that keep our body free of pathogens, cancer and foreign material. In case of inflammation or immune surveillance the cells lining the luminal site of blood vessels, known as endothelial cells (ECs), attract and direct traveling immune cells to suitable exit sites in the vasculature allowing cells to enter underlying tissue. ECs therefore fulfill an important supportive role in guidance and directional migration of trafficking immune cells. During inflammation ECs expose a variety of adhesion molecules at their surface that slow down and arrest traveling immune cells in the blood circulation. These adhesive molecules are thought to provide guidance cues to immune cells where to breech the blood vessel wall through a multi-step process known as transendothelial migration (TEM) or diapedesis.
Small GTPases. 2017; 8(1): 1–15.
Leukocyte transendothelial migration: A local affair
Lilian Schimmel, Niels Heemskerk, and Jaap D. van Buul
● The endothelium, a monolayer of endothelial cells, constitutes the inner cellular lining of the blood vessels (arteries, veins and capillaries) and the lymphatic system, and therefore is in direct contact with the blood/lymph and the circulating cells. The endothelium is a major player in the control of blood fluidity, platelet aggregation and vascular tone, a major actor in the regulation of immunology, inflammation and angiogenesis, and an important metabolizing and an endocrine organ. Endothelial cells controls vascular tone, and thereby blood flow, by synthesizing and releasing relaxing and contracting factors such as nitric oxide, metabolites of arachidonic acid via the cyclooxygenases, lipoxygenases and cytochrome P450 pathways, various peptides (endothelin, urotensin, CNP, adrenomedullin, etc.), adenosine, purines, reactive oxygen species and so on. Additionally, endothelial ectoenzymes are required steps in the generation of vasoactive hormones such as angiotensin II. An endothelial dysfunction linked to an imbalance in the synthesis and/or the release of these various endothelial factors may explain the initiation of cardiovascular pathologies (from hypertension to atherosclerosis) or their development and perpetuation.
Part 1: Multiple Functions of the Endothelial Cells—Focus on Endothelium-Derived Vasoactive Mediators
Michel Félétou.
San Rafael (CA): Morgan & Claypool Life Sciences; 2011.
● In order to be capable of engaging the key elements of adaptive immunity (specificity, memory, diversity, self/non-self discrimination), antigens have to be processed and presented to immune cells. Antigen presentation is mediated by MHC class I molecules, and the class II molecules found on the surface of antigen-presenting cells (APCs) and certain other cells. MHC class I and class II molecules are similar in function: they deliver short peptides to the cell surface allowing these peptides to be recognized by CD8+ (cytotoxic) and CD4+ (helper) T cells, respectively. MHC class II molecules are expressed by APCs, such as dendritic cells, macrophages and B cells (and, under IFNγ stimuli, by mesenchymal stromal cells, fibroblasts and endothelial cells, as well as by epithelial cells and enteric glial cells).
Antigen Processing and Presentation
Pavel Nesmiyanov, Volgograd State Medical University, Volgograd, Russia
British Society for Immunology
● The brain gut axis is a bi-directional pathway between the brain and the GI tract. Along with its role in digestion, the gut harbors about 80% of the body’s lymphocytes within the gut associated lymphoid tissue.
● At least two different repair mechanisms are thought to participate in full repair of the damaged gastric mucosa: the initial rapid process of mucosal restitution begins by migration of viable epithelial cells from gastric pits and glands; the subsequent slower process is replacement of lost cells by cell division. Intracellular events regulate these mucosal reparative processes. Aging appears to diminish its reparative capacity.
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