Lecture 18 Specialized Immunity at the GI Epithelial Barriers Flashcards
What organ/tissue has the largest number of lymphocytes?
The Intestines: 50 x 10^9
The GI immune system is the largest and most complex
*reflects large surface area which must also resist invasion by bacteria in the lumen
The oral cavity and vagina has what type of epithelial barrier?
Multilayer
At the epithelial layer, how are Ags captured?
DCs congregate immediately under epithelia, migrate into the epithelial layer, and even extend dendrites into the lumen to capture Ags
The DCs that capture Ags at the epithelia travel to where?
The nearest draining LN to present Ag to naive T cells
At sites of organized mucosal lymphoid tissues, how are Ags captured?
Specialized microfold (M) cells deliver Ags across the epithelial barrier directly to subepithelial DCs that then present Ag locally in adjacent mucosal T-cell areas
What are sites where some adaptive immune responses specialized for the particular mucosa are initiated?
These collections of immune cells are called Gut-associated lymphoid tissue (GALT)
The effector lymphocytes that are generated in the draining lymph nodes or GALT of a particular regional immune system (small bowel) will enter the blood and the travel where?
Preferentially home back to the same organ (e.g. dermis, lamina propria)
There are specialized cells which are restricted to one or more regional immune systems but are not present throughout the immune system including:
M cells in the gut
γδ T cells in epithelia
Subsets of IgA producing B cells
Plasma cells
Regional immune systems have what important regulatory functions?
Functions that serve to prevent unwanted responses to nonpathogenic microbes and foreign substances that are likely to be present at different barriers
What layer is underlying the epithelium?
A layer of loose connective tissue in the gut called the lamina propria that contains blood vessels, lymphatic vessels, and mucosa-associated lymphoid tissues
The GI tract has what two remarkable properties?
First, total surface area is more than 200m^2
Second, the lumen of the gut is teeming with more than 500 different species of bacteria, amounting to approximately 10^14 bacteria
What functions do commensal organisms have?
- degradation of components of our diet that our own cells cant digest
- compete with potentially pathogenic microbes in the gut and prevent harmful infections
When do commensal organisms become potentially lethal?
If they cross the mucosal barrier
How can non-commensal pathogenic organisms possibly become part of GI organisms?
If they are ingested in contaminated food or water
*these pathogenic organisms include bacteria, viruses, protazoa, and helminthic parasites
Non-commensal pathogenic organisms often cause disease without doing what?
Invading the epithelial cells
What trick must the mucosal immune system pull off?
Must be able to recognize and eliminate pathogens while maintaining the symbiotic relationship with normal microflora
The innate immune protection in the gut is mediated in part by what barrier?
Physical and chemical barrier provided by the mucosal epithelial cells and their mucus secretions
How are adjacent intestinal epithelial cells held together?
By proteins that form tight junctions preventing the movement of microbes between the cells into the lamina propria
Epithelial cells produce what type of defensins?
anti-microbial peptides defensins
What immune cells are in the lamina propria that can induce inflammation?
DCs, Mo, and neutrophils
Most of the responses are induced by ______ but, instead, these responses can be ___-______ actions in the gut
PRRS
Anti-inflammatory
The physical/chemical barrier is formed by several viscous proteins called what?
mucins
What do mucins prevent?
Prevent microbes from contacting epithelial cells
What type of glycoproteins do mucins include?
Both secreted and cell surface glycoproteins
Secreted mucins form a two layer gel:
Outer less-dense layer that is normally colonized by bacteria
Inner more-dense layer that is bacteria-free
There are anti-microbial substances in the mucus layers produced by what cells?
epithelial cells
The apical surface of GI epithelial cells is coated with what?
Membrane-bound mucin proteins called the glycocalyx
What does the glycocalyx serve as?
Like the secreted mucus, it serves as a physical barrier to prevent microbial contact
Mucins are constitutively produced by what?
Epithelial cells and submucosal glands
Mucins are replaced by newly synthesized molecules how often?
every 6 to 12 hours
What can induce dramatic increases in mucin production during inflammation?
cytokines:
IL-1, 4, 6, 9, TNF, IFNα
What proteins can also increase mucin production?
Elastase from neutrophils and microbial adhesive proteins
Inflammatory molecules not only increase mucin production but also alter what?
Their glycosylation which are thought to increase barrier function against pathogens
What are defensins constitutively produced by?
Epithelial and Paneth cells to provide innate immune protection against luminal bacteria
How do defensins exert lethal toxic effects on microbes?
By inserting into and causing loss of integrity of their outer phospholipid membranes
What microbes are more resistant to defensins?
Encapsulated bacteria
In the small bowel, what are the major defensins?
α-defensins: produced by paneth cells
What are the major defensins in the colon?
β-defensins produced by epithelial cells
Defects in defensin production by epithelial cells is linked to what disease?
Crohn’s disease - a chronic inflammatory disease that can involve the entire GI tract
TLRs and NOD-like receptors expressed by intestinal epithelial cells promote what?
Immune responses to invasive pathogens and also limit inflammatory responses to commensal bacteria
What may commensals express?
Lipopolysaccharaide, peptidoglycans, CpG DNA, and flagellin
What has evolved to limit TLR-induced proinflammatory responses to commensal bacteria?
Stringent control mechanisms
Functional responses to TLR signaling increases barrier function but not what?
inflammation
TLRs only show what kind of expression in certain areas?
compartmentalized expression
What TLR recognizes bacterial flagellins?
TLR5
TLR5 is exclusively expressed on what surface in the GI tract?
On the basolateral surface and thus it will be accessible only to bacteria that have invaded through the barrier
NLR family receptors for flagellins are expressed where?
GI
In the cytosol of intestinal epithelial cells
What is the function of NLR family receptors in the GI tract?
They will activate inflammatory responses only when pathogenic bacteria or their products gain acess to the cytosol
Adaptive immune response in the GI tract is initiated by APCs closely associated with what?
The mucosal epithelial cells and lymphocytes
What is a major pathway of Ag delivery from the lumen to the GALT?
Through specialized cells within the gut epithelium called microfold (M) cells
The major form of adaptive immunity in the gut is ______ immunity
humoral immunity
*directed at microbes in the lumen
The dominant protective cell-mediated immune response consists of what cells?
Th17 cells
A major mechanism for controlling responses in the gut is the activation of what cells?
Treg
What are specialized intestinal epithelial cells found int he small bowel epithelium overlying Peyer’s patches and lamina propria lymphoid follicles?
M cells
M cells have (shorter/longer) villi and engage in transport of (processed/intact) microbes or molecules across the mucosal barrier into the gut-associated lymphoid tissues, where they are handed off to (B cells/DCs)
shorter
intact
DCs
DCs are present in the intestinal mucosa and sample Ags for presentation to T cells where?
In GALT and mesenteric LNs
Some DCs extend what between intestinal epithelial cells into the lumen to sample Ags? What other cells may also do so in the same manner?
Dendritic processes
Mo may also sample luminal Ags in this manner
Other DCs present in the lamina propria sample Ags that derived from lumina contents and have done what?
Gotten through the epithelial barrier
The gut-homing properties of effector lymphocytes are imprinted where?
in the lymphoid tissues, where they have undergone differentiation from naive precursors
DCs in GALT, including Peyer’s patches and mesenteric lymph nodes, are induce by what to convert dietary vitamin A into retinoic acid?
Thymic stromal lymphopoieten
When niave B or T cells are activate by Ag in GALT, they are exposed to what that induces the expression of the chemokine receptor CCR9 and the integrin α4β7 on the plasma cells and effector T cells?
Retinoic acid
Why do the effector lymphocytes enter the circulation and home back into the gut lamina propria?
Because the chemokine CCL25 (the ligand for CCR9) and the adhesion molecule MadCAM (the ligand for α4β7) are displayed on lamina propria endothelial cells
In T-dep IgA class switching, DCs in the subepithelial dome of Peyer’s patches, captures bacterial Ags delieved by M cells and migrate to where?
To the interfollicular zone, where they present Ag to naive CD4+ T cells
What do activated T cells in T-dep IgA class switching differentiate into?
Th cells with a T follicular helper phenotype
What do Tfh cells engage with in cognate interactions
With Ag-presenting IgM+IgD+ B cells that have also taken up and processed the bacterial antigen
B cell class switching to IgA is stimulated through T cell CD40L binding to B cell CD40, together with the action of what?
TGF-β
*This T cell-dep pathway yields high-affinity IgA Abs
T-Ind IgA class switching involves what?
DC activation of IgM+IgD+ B cells, including B-1 cells
TLR ligand-activated DCs secrete what to induce T-ind IgA class switching?
BAFF, APRIL, TGF-β
Which T cell pathway yields relatively low-affinity IgA Ab to intestinal bacteria?
independent
IgA produced in the lamina propria bind to what receptor at the base of an epithelial cell?
poly-Ig receptor
How does IgA produced in the lamina propria reach the lumen?
poly-Ig receptor - IgA complex is transported across the epithelial cell, and the bound IgA is released into the lumen by proteolytic cleavage
Lamina propria of the small bowel is rich in what cytokine producing cells?
IL-17-producing cells
What subtype of T cells are not in the colon?
Th17
The steady-state of Th17 cells is required for protection against what?
pathogenic species of bacteria
Th17 cells appear to play a special role in maintaining what?
mucosal epithelial barrier function
What are two cytokines produced by Th17 cells in the GI
IL-17, -22
What do IL-17 and IL-22 act on?
Intestinal epithelial cells and induce the expression of mucins and β-defensins
IL-17 and IL-22 protect the epithelial cells against what?
Microbe-induced injury
Treg cells are abundant in what?
GALT
What do Treg cells prevent?
inflammatory reactions against intestinal commensal microbes
It is estimated that the proportion of FoxP3+ Treg among CD4+ cells is about 2-fold greater where?
in the lamina propria than in other peripheral LNs
What factors contribute to the generation of Treg cells in the GI?
DCs
Local production of retinoic acid (promotes FoxP3 exp)
Local production of TGF-β (also promotes FoxP3 exp and inhibits the generation of Th1 and Th2 cells)
Treg are thought to suppress immune responses primarily by production of what?
immunosuppressive cytokine IL-10
What cytokines appear to play crucial roles in maintaining homeostasis in the gut immune system?
TGF-β, IL-10 and IL-2
Deficiencies in TGF-β, IL-10 and IL-2 or their receptors result in what?
pathologic bowel inflammation
Mutations in the IL-10 and IL-10R genes are associated with what disease?
Severe colitis in children, confirming the importance of IL-10 in preventing patholgoic intestinal inflammation in humans
Why is it thought that IL-10 deficiencies provoke innate and adaptive immune responses to commensals?
Because the inflammation does not occur in mice raised in germ-free conditions
In the case of TGF-β and IL-10-dependent regulation of gut inflammation, evidence indicates that what cells are important sources of these cytokines?
Treg
Selective deletion of the IL10 gene in FoxP3+ cells leads to what?
Severe colitis but no other manifestations of inflammatory disease
Treg-produced IL-10 plays a critical role in what?
maintaining homeostasis in the GI tract
What is another important source of IL-10 besides Treg cells?
Mo
Defect in the production of IL-2 or IL2R also leads to inflammatory bowl disease which is thought to be the consequence of defects in what?
The development and function of Treg cells, which require IL-2
What is oral tolerance?
Systemic adaptive immune tolerance to Ags that are ingested orally
What is the physiologic role of oral tolerance?
To prevent potentially harmful immune responses to food proteins and commensal bacteria
The underlying mechanisms of oral tolerance are not well understood but likely include the mechanisms:
Anergy
Deletion
Treg-mediated suppression
How can the propensity of the immune system in the gut to suppress local immune responses to Ags in the intestinal lumen could be manifested in other parts of the body
Because of the circulation of Treg to other tissues
Attempts to treat autoimmune disease or allergies by oral or nasal administration of relevent self antigens or allergens have been (successful/unsuccessful)
unsuccessful
Limited expression of pro-inflammatory cytokines by APCs and an excess of TGF-β results in what?
The differentiation of naive T cells into T reg cells which suppress Th1, Th2, and Th17
Up to 10% of the T cells in GALT are what type of T cells?
Treg
Commensal organisms in the intestines are required for and regulate innate immune responses in the gut, and also influence what?
Systemic innate immunity
Commensal bacterial (PAMPs) and the TLRs they bind to on the epithelial cells are needed for what?
proliferation and repair of the intestinal epithelial barrier after injury
What stimulates the expression of muscins and anti-microbial peptides that preven Gram-positive bacterial colonization?
Microflora
How does gut bacteria dampen nuetrophil infalmmatory responses ?
Sort-chain fatty acids from gut bacteria
How do commensals enhance the ability of circulating nuetrophils to kill gram-positive bacteria?
Peptidoglycan
Microflora is require for systemic anti-viral functions of what cells?
Mo, DCs, and NKs
Intestinal commensal organisms influence what types of adaptive immune responses?
Local and systemic
How is the production of IgA dependent on the presence of luminal flora?
Commensal organisms induce the expression of IgA switch factors, including BAFF, APRIL, and retinoic acid, which are required for T-dep and T-indep B cell class switching to IgA
Serum IgE levels, blood basophil numbers, and mast-cell dependent allergic reactions outside the gut are all elevated in ______ mice
germ-free
IgA in the gut reduces innate responses to commensals and also limits what?
Activation of B cells and Ab responses of other isotypes, both locally and systemically
Certain species of commensals in the gut are also required for accumulation of what types of cells in the gut?
Th17 while other commensal species contribute to the development of Treg cells
Diet and other environmental factors and host genetics have a major effect on gut _____
microbial composition
The balanced microbial composition results in _____ that maintains the homeostasis
symbiosis
_____ would lead to dysregulation of the immune system and to inflammation in susceptible host (genetics)
Dysbiosis
How may dysbiosis occur?
Due to various environmental factors
Undernutrition is associated with defects in what?
innate and adaptive immunity
Recurrent (enteric) infections predispose to _______, as well as impared intestinal mucosal barrier function.
nutrient deficiencies (macronutrient and micronutrient)
*these factors further susceptibility to infection and worsening nutritional status
What shapes gut microbiota (structure and function), and the microbiota adapts in ways that promote nutrient processing?
Diet
Since the microbiota and immune systems co-evolve, malnutrition affects what?
Both the immune system as well as microbiota
What does the microbiota act as?
A barrier to enteropathogen infection
Dietary products shape the microbial community, and the microbiota changes the nutritional value of the consumed food. Unmodified dietary components are absorbed in the intestine, where they do what?
interact with immune cells
Microbe (pathogen) - associated molecular patterns (MAMPs/PAMPs) signaling via TLRs can modify what?
local mucosal immunity
What is an example of a microbe-modified dietary component stimulating IL-10 production regulating metabolic activities in the lamina propria?
Acetate produced by the fermentation of polysaccharides can stimulate IL-10 production
What is IBD?
Inflammatory bowel disease: chronic remitting inflammation in the small or large bowel, likely due to poorly regulated responses to commensal bacteria
What is IBD a result of?
Defects in the regulation of immune responses to commensal organisms in the gut in genetically susceptible individuals
What are the two main types of IBD?
Crohn’s disease: affect the entire thickness of the bowel wall tissue; most frequently involves the terminal ileum
Ulcerative colitis: patchy damage; restricted to colonic mucosa
What are the symptoms of IBD?
abdominal pain, vomiting, diarrhea, and weight loss
What are the treatments for IBD?
various anti-inflammatory drugs, such as sulfasalazine, corticosteroids, TNF antagonists, and antimetabolites
In IBD what are the defects in innate immunity to gut commensals?
defective expression of defensins, leading to increased commensal bacterial invasion through epithelium
Inadequate negative regulation of innate immune responses to commensal organisms in Crohn’s disease is linked to polymorphisms in the gene encoding the NOD2 cytoplasmic innate immune receptor
What are abnormal Th17 and Th1 responses in IBD?
There is an active Th17 response in the affected parts of the bowel
Crohn’s disease is also characterized by granulomatous inflammation driven by IFN-y producing Th1 cells
It is possible IBD may be caused by inadequate ________ suppression of immune responses to commensal organisms
Treg-mediated
An absence or depletion of what cells leads to IBD?
Treg
Mice deficient of Treg because of deletion of what receptors also develop colitis?
IL2 or IL2R or knowckout of Foxp3
In humans, what does FOXP3 result in?
A failure to develop Treg and cause the disease called immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX), which includes severe gut inflammation
What is celiac disease?
Gluten-sensitive enteropathy: an inflammatory disease of the small bowel mucosa caused by immune responses against ingested gluten proteins present in wheat
What is celiac disease characterized by?
chronic inflammation in the small bowel mucosa
Patients with celiac disease produce what Abs?
IgA and IgG specific for gluten as well as autoantibodies specific for transglutaminase 2A, and enzyme that modifies the gluten protein gliadin
Abs are sensitive ______ marker for the celiac disease
diagnostic
There is strong evidence that what T cells are involved in the disease pathogenesis?
CD4
T cells specific for gliadin peptides are found in celiac patients, and the inflammatory process in the bowel includes what?
T cells and T cell cytokines
There is high relative risk for development of gluten enteropathy among people who carry what?
two class II HLA alleles HLA-DQ2 and HLA-DQ8, and gliadin peptides bind strongly to the MHC molecules encoded by these alleles
How is celiac disease treated?
by diets restricted to gluten-free foods
Gluten peptides that are highly resistant to intestinal proteases reach what?
The lamina propria
What creates potent immunostimulatory epitopes that are presented via HLA-DQ2 or HLA-DQ8 on APCs?
Cross-linking and deamidation of gluten peptides by transglutaminase 2
Activatded CD4 T cells secrete mainly what cytokines that induces the release of MMPs by myofibroblasts resulting in mucosal remodeling and villus atrophy (celiac)
Th1 cytokines such as IFN-y
What cytokines are produced driving the production of auto-Abs to gluten and TG2?
Th2 cytokines
What cytokines seem to play a role in polarizing and maintaining the Th1 response in celiac disease?
IL-18, IFN-y or IL-21
What cytokine links the adaptive immune system to innate immune responses?
IL-15
