Mucosal protection in gut Flashcards
Provide an overview of protective mechanisms in gut
- Non-immunological defence mechanisms represent an important line of intestinal defence in addition to humoral and cellular immunity, and include:
- Microbiological defences (i.e., normal gut flora)
- Physical defences
- Chemical defences
Describe physical defence mechanisms
A single layer of intestinal epithelial cells (IECs) provides a physical barrier between the lamina propria (internal milieu) and the intestinal lumen, which contains normal gut flora and pathogens.
Physical Defences: Tight Junctions
- Intestinal epithelial cells are held together (cell-to-cell adhesion) by tight junctions, which form a seal against the external environment.
- Primary barrier to the diffusion of solutes and traversal of pathogens through the intercellular space, creating a boundary between the apical and the basolateral plasma membrane domains.
- Infection occurs only when a pathogen can colonize or cross through these barriers.
- Tight junction:
- ‘Kissing points’.
- No intracellular space.
- Adherens junction & Desmosomes:
- Opposing membranes are 15-20 nm apart.
Physical Defences: Epithelial Cell Turnover and Peristalsis
- Intestinal epithelial cell (IEC) turnover is constant (every 4-5 days) and involves:
1. Shedding of cells damaged by microbial infection or stresses.
2. Replenishment by intestinal stem cells.
- IEC turnover and peristalsis contractions help expel colonized pathogens and prevent overgrowth of normal gut flora.
Describe chemical defence
- Often the first line of defence against infection.
- Certain chemicals are produced by the host to protect against infections by GIT pathogens:
- Bile: produced in liver, helps with digestion, antibacterial.
- Enzymes:
- Pepsin: produced in stomach, helps with digestion, antibacterial.
- Trypsin/chymotrypsin/lipase: produced in pancreas, help with digestion, antibacterial.
- Lysozyme: produced in upper intestinal tract, antibacterial.
- Gastric acid: produced in stomach, helps with digestion antibacterial.
- Mucus; viscous, antibacterial
Chemical Defences: Mucus
- Highly viscous, hydrophobic gel that covers mucosal surfaces and protects epithelial cells against chemical and microbial insult. For example:
- Microbes coated in mucus may be prevented from adhering to the epithelium.
- Mucus retains dimeric IgA to maximize exclusion of pathogens from gut epithelium
- Mucin glycoproteins are a major component of mucus and responsible for the viscosity of the mucus layer.
- Mucus layers range in thickness: 10 μm in the eye and trachea, 300 μm in the stomach, 700 μm in the intestine.
- Mucus layer is not static but moves to clear trapped material. For example:
- In GIT, the outer mucus layer is continually removed by peristalsis.
Chemical Defences: Mucus-Secreting Cells in the GIT
In the stomach:
- Surface mucus cells (within gastric pit).
- Neck mucus cells (within gastric gland).
In the small and large intestine:
- Brunner’s glands (within submucosa):
- Localized to the duodenum.
- Also produce alkaline fluids.
- Goblet cells (within epithelium).
Chemical Defences: Gastric Mucus Layer
- HCl (acid) produced by parietal cells in the human stomach is concentrated enough to digest the stomach itself, yet gastric epithelium remains undamaged because it is acid-resistant.
- Gastric mucus forms a protective layer over the gastric epithelium and acts as a diffusion barrier by secreting bicarbonate ions that remain trapped in the mucus gel, establishing a gradient: from pH 1-2 at the lumen, to pH 6-7 at the cell surface.
Chemical Defences: Intestinal Mucus Layer
- In the small intestine, the mucus forms a diffusion barrier containing antibacterial products that limit penetration by bacteria.
- In the colon, bacteria are compartmentalized to the outer loose mucus layer; the inner mucus layer, which is attached to the epithelium, is almost free of bacteria and protects the epithelium (because outer layer has trapped microbes).
How Can Pathogens Overcome the Mucus Barrier?
- Several pathogens have developed mechanisms to subvert mucosal defensive measures, colonize the GIT, and cause infection. Examples include:
- Helicobacter pylori (H. pylori) can swim through gastric mucus in the stomach and attach to epithelial cells beneath, where it can cause inflammation over the course of a lifelong infection. for more see [[Microbiology B5 - Lecture 2]] eg vacA (exotoxin) and secretory enzymes (mucinase, lipase, protease); flagella
- Enterohaemorrhagic Escherichia coli (EHEC) produce proteins that specifically degrade mucin to gain access to the intestinal epithelium.
- Defective mucus release, resulting in defective mucous layers and stagnation.
Provide an overview of the mucosal immune system
- Immunological defence mechanisms represent an important line of intestinal defence in addition to non-immunological defence mechanisms that can be breached relatively easily.
- These defence mechanisms are referred to as the mucosal immune system and are specifically adapted to generate a response to antigens encountered in:
- Upper and lower respiratory tract.
- Urogenital tract.
- GIT.
- And exocrine glands associated with these organs, e.g., pancreas, salivary glands, etc.
- As such, the mucosal immune system forms the largest part of the human body’s immune tissue.
Where are immune cells in the GIT?
- Lymphocytes (Bs, Ts) and other immune-system cells (e.g., macrophages, dendritic cells) are found throughout the GIT, in organized tissues, and scattered throughout the surface epithelium of the mucosa and lamina propria.
Describe GALT
- Organized lymphoid tissues in the gut are known as the gut-associated lymphoid tissue (GALT), for example:
- Peyer’s patches.
- Solitary lymphoid follicles of the intestine.
- Mesenteric lymph nodes.
- Appendix.
- Tonsils/adenoids
Describe Peyer’s patches
- Found in the small intestine (rich within the ileum), see also [[Anatomy B5 - Lecture 2]].
- Consists of many B-cell follicles with germinal centers, and smaller T-cell areas.
- Subepithelial dome (SED) is rich in dendritic cells, T cells, and B cells.
- Overlying the lymphoid tissue and separating them from the gut lumen is a layer of follicle-associated epithelium containing:
- Conventional intestinal epithelial cells (enterocytes).
- Specialized epithelial cells (microfold or M cells).
Briefly describe mechanisms of antigen uptake
- Antigens present at mucosal surfaces must be transported across the epithelial barrier before they can stimulate the mucosal immune system.
- The intestine has distinct routes and mechanisms of antigen uptake:
- Uptake by Peyer’s patches, mediated by M cells.
- Direct uptake by dendritic cells.
- Both routes lead to T cell activation.
Describe uptake by M cells and DCs
- Microfold cells (M cells) mediate the transport of luminal antigens and bacteria across the epithelial barrier (transcytosis).
- M cells are localized to Peyer’s Patch lymphoid tissue.
- Antigens that are transported across the epithelial barrier are taken up by dendritic cells within Peyer’s Patches to facilitate antigen presentation to naïve T cells.
Antigen Uptake by Dendritic Cells
- Dendritic cells can extend processes across the epithelial layer to capture antigen from the lumen of the gut.
- This allows dendritic cells to acquire antigens across the intact epithelial barrier without the need for M cells.
- After antigen uptake, dendritic cells transport them to T cell areas eg Peyers, patches, or mesenteric lymph nodes via lymphatics that drain the intestinal wall.
Describe ither immune cells that generate an immune response in addition to GALT
- In addition to GALT, the intestinal mucosa contains many immune-system cells scattered throughout the surface epithelium of the mucosa and in the lamina propria:
- Lamina propria contains CD4+ and CD8+ T cells, as well as IgA-producing plasma cells, macrophages, dendritic cells, and mast cells.
- The epithelium contains mainly lymphocytes, the vast majority of which are CD8+ T cells.
- CD4+ T cells predominate in the lamina propria, whereas CD8+ T cells predominate in the epithelium.
What is the Role of Epithelial Cells in Microbial Recognition ?
- Epithelial cells provide more than a physical barrier.
- Upregulation of particular receptors on the basal surface allows them to recognize bacteria that have invaded the epithelial barrier:
- Basolateral surface/vacuoles: Toll-like receptors (TLRs).
- Cytoplasm: nucleotide-binding oligomerization proteins (NODs).
- This recognition triggers an influx of inflammatory cells/lymphocytes into the mucosa from the bloodstream, assisting in the induction of a specific immune response to antigens of infectious agents.
Describe the IgA response
- The dominant class of antibody in the mucosal immune system is (dimeric) IgA.
- To generate an IgA-mediated response to antigen, naïve B cells are activated by antigen as IgM-producing B cells in Peyer’s Patches and mesenteric lymph nodes, undergo isotype switching to IgA-producing B cells, and are then redistributed in the intestinal immune system.
- Once in the lamina propria, plasma cells synthesize and secrete IgA into the subepithelial space.
- To reach its target antigens in the gut lumen, IgA has to be transported across the epithelium, in a process known as transcytosis.
Translocation/Transcytosis of IgA
- Translocation of IgA across the IEC barrier is mediated through a process called transcytosis.
- IgA dimers are secreted by plasma cells in the lamina propria and bind to the polymeric immunoglobulin receptor (pIgR) on the IEC basolateral surface.
- The IgA-pIgR complex is endocytosed and transported to the apical surface for release to the intestinal mucus layer and intestinal lumen.
- Secreted IgA, together with the secretory component (SC; a proteolytic cleavage product of pIgR that remains associated with dimeric IgA), is known as secretory IgA (sIgA), which is important for neutralizing extracellular pathogens.
What are the functions fo IgA?
- secreted IgA on gut surface can bind and neutralise pathogens and toxins
- IgA is able to bind and neutralise antigens internalised in endosomes
- IgA can export toxins and pathogens from the lamina propria while being secreted i.e. opsonisation
Descrue hte CD4a nd CD8 respnes
- To generate a T cell response, naïve T cells must be activated and redistributed in the intestinal immune system.
- Pathogens that penetrate the epithelium activate dendritic cells, to give strong co-stimulatory signals so that when they present antigen to naïve CD4 T cells in the lamina propria, effector TH1 and TH2 cells are generated to stimulate an active immune response.
- CD4 TH1 cells produce macrophage-activating cytokines.
- CD4 TH2 cells produce cytokines that stimulate B cells to produce antibodies.
CD8+ T Cell Response
- Peptides from invasive organisms bound to major histocompatibility complex (MHC) class I molecules on infected epithelial cells are recognized by intraepithelial lymphocytes (mainly CD8+ T cells loaded with antimicrobial proteins).
- CD8+ T cells then release antimicrobial proteins/cytotoxic signs, inducing death of infected cells eg by perforin/granzyme, Fas/FasL pathways. ^[recall also against tumour cells]
