Mucosal Immunity Flashcards
Describe the basic features of the mucosal immune system
• Thin layer of mucosal epithelium lining internal body surfaces is the only physical barrier against invasion of the underlying tissues by pathogens
• These surfaces therefore require continual protection → barrier can be easily penetrated
• Mucosal immune system comprises the body surfaces lined by mucosa-secreting epithelium
− GI Tract
− Upper and lower respiratory tract
− Urogenital tract
• Important in gas exchange, food exchange, sensory activities and reproduction → important to life
• They are permeable due to their function – gives vulnerability to pathogens
• Also sites of entry for non-pathogenic foreign antigens:
− 30-35kg food proteins per year
− commensal bacteria living in symbiosis → 1012 organisms per ml in the colon
− Have to be tolerant against these, lack of tolerance leads to:
− Crohns (response to commensals)
− Coeliac (response to wheat)
The mucosal immune system may be the original vertebrate immune system:
• In traditional immunology, mucosal system is considered a sub-compartment of the immune system
• In terms of size and function, this isn’t true
• Forms the largest part of the body’s immune tissues → ¾ of all lymphocytes, and producing most of the Ig’s
• The mucosal system, in particular the gut, may have been the first part of the adaptive immune system to evolve → linked to the need to deal with the commensals that co-evolved with vertebrates.
− Organised lymphoid and Ig antibodies are first found in the gut of primitive fishes
− The thymus and avian Bursa derive from the embryonic intestine
Describe the effector lymphocyte population of the intestine
Mucosal system contains large numbers of effector lymphocytes even in the absence of disease:
• Most of the scattered lymphocytes have the appearance of lymphocytes that have been activated by antigen (eg, they have received signal 1).
• In the intestine, the effector cells are found in 2 main compartments:
− Epithelium → intra-epithelial lymphocytes, which are nearly all CD8+ T cells
− Lamina propria → more heterogenous, large numbers of CD4+ and CD8+ T cells, plasma cells, macrophages, DCs, eosinophils and mast cells. Neutrophils are rare, but increase in inflammatory conditions.
The intraepithelial lymphocytes;
• 10-15 lymphocytes for every 100 epithelial cells
• Given the enormous surface area of the mucosa, this makes the IELs one of the single largest populations of lymphocytes in the body
• More than 90% of the IELs are T cells → around 80% of these CD8+
• In young people, contains an unusually large amount of y:d T cells. In adults, y:d T cells are more equal in the mucosa and bloodstream
• CD8+ can be divided into two subets:
− Type A → convential a:b TCR and conventional a:b CD8+. Derived from naïve CD8+ T cells in the peyers patch. Function as conventional CD8+ T cells, killing virus infected cells and secrete IFNy
− Type B → express the a:a CD8+, and either an a:b or y:d TCR.
• IELs express high levels of the activating NK receptor NKG2D → binds MIC-A and MIC-B on intestinal epithelial cells in response to cellular injury, stress or ligation of TLRs.
➢ In this sense, can be seen as a bridge between innate and adaptive immunity
• Their role in the gut may be rapid recognition and elimination of epithelial cells expressing an abnormal phenotype as a result of stress or infection.
• Gut IELs also stimulate release of anti-microbial peptides, removing the source of inflammation.
• They release cytokines such as keratinocyte growth factor → improving barrier function
• Suppress inflammation by producing TGF-B
→ Healthy intestinal mucosa therefore displays many characteristics of a chronic inflammatory immune response
− numerous effector lymphocytes
− Presence not dependent on infection by a pathogen
− it is the result of the local responses continually being made to the myriad of antigens bombarding the mucosal surface.
Describe the lymphoid tissue of the gut
Palatine tonsils, adenoids and lingual tonsils:
• Form Waldeyers ring at the back of the mouth at the entrance of the gut and airways
• Often enlarged in childhood because of recurrent infections
Peyers Patches
• In the small intestine
• Present in the fetal gut
• Domelike aggregates of lymphoid cells that project into the lumen
− contains B cell follicles with T cell areas around these
− sub-epithelial dome contains DCs, T cells and B cells
• 100-200 PPs in the small intestine
• Much richer in B cells than the systemic peripheral lymphoid organs
• Separating the lymphoid tissue from the gut is a layer of lymphoid associated epithelium
− Contains conventional enterocytes anda smaller number of specialized epithelial cells called microfold (M) cells
− M cells do not secrete digestive enzymes or mucous, and lack a surface glycocalyx → they are directly exposed to pathogens and are the route by which antigen enters the PP
− Follicle associated epithelium also contains lymphocytes and DCs
Isolated lymphoid follicles
• Found throughout the small and large intestine
• Composed of an epithelium containg M cells overlying organized lymphoid tissue, but contain mainly B cells
• Develop only after birth in response to antigen stimulation due to colonization of the gut by commensals.
• Arise from small aggregates in the intestinal wall called cryptopatches → contain DCs
→ PPs and ILFs are connected by the lymphatics to draining MLNs
How are T cells activated in response to intestinal antigen?
How are T cells activated in response to intestinal antigen?
• DCs are found in the PP in two main areas:
1. Subepithelial dome region, acquiring antigen from M cells → these tend to produce IL-10 in response to antigen uptake, which prevents induction of pro-inflammatory T cells
➢ However, during an infection, these DCs are rapidly recruited into the epithelial layer of the PP
➢ Bacterial products also activate DCs to express co-stimulatory molecules, allowing them to activate pathogen-specific naïve T cells
2. Also abundant in the wall of the intestine outside the PP, in the lamina propria
How do DCs acquire antigen through the epithelial barrier?
1. Soluble antigens such as food proteins might be transported directly across or between enterocyts
2. M cells in the surface epithelium near PPs (as just explained)
• PPs and ILFs highly adapted for uptake of antigen from intestinal lumen
• M cells in the LAE are continually taking up antigen by endocytosis or phagocytosis
• Transported across the M cell by transcytosis
• Because M cells are more accessible than enterocytes, a number of pathogens target M cells to gain access to the subepithelial space, even though this puts them in contact with the immune system
• Basal membrane of the M cell is extensively folded → forms a pocket that encloses DCs and lymphocytes.
− DCs take up the transported antigen and process it for presentation to T cells
− DCs are recruited to the LAE in response to chemokines CCL20 and CCL9
− Antigen loaded DCs then migrate to the T cell area of the PP to prime T cells
− Primed T cells then activate B cells and initiate class switching to IgA
3. Enterocytes may capture and internalize antigen:antibody complexes by means of FcRn on their surface, and transport them by transcytosis. Lamina propria DCs expressing FcRn can pick these up
4. An enterocyte infected with a pathogen may undergo apoptosis and its remains are phagocytosed by the DC
5. They may extend their dendrites between the cells of the epithelium to sample antigen in the lumen → controversial due to thick mucous layer
What are the outcomes of antigen presentation?
- Tissue DCs migrate to the MLN to activate naïve T cells
- PP DCs present antigen to naïve T cells in the PP
- Tissue DCs present antigen to already activated effector T cells
How does the immune system ignore the commensal antigens and food, but fight harmful pathogens?
• Barrier function
• Tregs
What is the role of the gut commensals?
Human intestine home to 1014 bacteria:
• x10 number of human cells
• x100 more bacterial genes versus human
• weigh around 1kg!
Important function in regulating human health:
• food digestion (energy/nutrient release)
• protection from intestinal infection
− illustrated by broad-spectrum antibiotics
− kill large numbers of commensals, creating a niche for bacteria that normally wouldn’t be able to compete successfully
− eg) C.difficile grows in antibiotic treated guts and causes diarrhea
• regulation of the intestinal immune system
− in germ-free animals, they are reduced size of lymphoid organs, low serum Ig levels, fewer mature T cells, reduced immune response of Th1 and Th17 types
− PPs do not develop mormal
− ILFs absent
Describe the barrier function of the intestine
Epithelial cells are an important barrier:
• Not greatest physical barrier – only 1 cell thick
• Cant make it thicker because of the food absorption function
• But they are more than just a physical barrier:
− Secrete antimicrobial peptides
− Produce cytokines and chemokines
Mucous is an important barrier in the intestine:
• Composed of mucins made by goblet cells
• Stomach → outer loose and inner stratified layer. Stops stomach digesting itself
• Small intestine → discontinuous layer, allows for nutrient absorption
• Colon → sterile stratified layer. Outer loose layer with microbiota living within
Commensals don’t normally enter the inner mucous layer
• TLR signaling in the epithelial cells is important in protecting
• Makes the epithelial cells more resistance to inflammation induced damage
As well as being a barrier, recent evidence suggests potential active properties of mucins:
• Regulate the immune system → bind to cells to regulate their function. Dampen pro-inflammatory DC responses
• Anti-bacterial
How does IgA contribute to intestinal immunity?
- Commensal bacteria stimulate production of IgA
- Produced locally by plasma cells in the mucosal wall
- IgA in the blood is mainly in the form of a monomer → produced in the bone marrow by plasma cells derived from B cells activated in LNs
- IgA in mucosal tissues is a polymer (dimer) – the two Igs linked by a J chain
- The naïve B cells are in the PPs and MLNs
- Class switching of activated B cells to IgA is controlled by TGF-b → T cell dependent and occurs only in the organized lymohoid tissues.
IgA transport across the epithelium:
• Immature epithelial cells located at the base of crypts express polymeric Ig receptor (pIgR) on their basolateral surface
• Transports IgA by transcytosis to the luminal surface
• Released by proteolytic cleavage of the extracellular domain of the receptor
• Part of the cleaved pIgR remains associated with IgA → secretory pieces
• The resulting IgA is referred to as secretory IgA
In some animals (but not humans) there is another route – the hepatobilliary route
• IgA not bound by pIgR taken up into the portal veins which drain into the liver
• IgA taken up by hepatocytes and transported into the bile duct
• The bile duct drains into the upper small intestine
Function of IgA
• In the gut lumen, it binds to the layer of mucus coating the epithelium
• Prevents invasion by pathogenic organisms and maintains homeostatic balance between the host and the commensals
− Inhibits microbial adherence to the epithelium
− Can neutralize microbial toxins and enzymes
− Can neutralize LPS and viruses it encounters inside epithelial cells → then re-exported into the gut lumen and excreted from the body.
• Can enhance the uptake and trancytosis of luminal antigen through M cells
• Secretory IgA has little capacity to activate the classical pathway of complement and act as an opsonin, so it does not induce inflammation.
How do DCs prevent T cells being activated by commensals?
In general:
• In the presence of commenals, PGE2, TGFB and TSLP inhibits DC maturation
• During infection, microorganisms penetrate the epithelium and activate DCs
But it is not this simple, as there are different subsets of DCs.
CD11b+
• In the supepithelial dome region of the PP
• In resting conditions, produce IL-10 in response to antigen uptake, which prevents priming of T cells to become pro-inflammatory.
• However, during an infection, these DCs are rapidly recruited into the epithelial layer of the PP
• Bacterial products also activate DCs to express co-stimulatory molecules, allowing them to activate pathogen-specific naïve T cells
CD103+
• In the lamina propria
• Once loaded with antigen, leave the mucosa to migrate to T cell areas of the MLNs
• In the MLNs, interact with naïve T cells and induce the gut-homing properties that enable T cells to return to the intestinal wall as effector T cells.
• Migration of CD103+ DCs depends on the expression of CCR7 → this is constitutive, does not need the presence of pathogens or inflammatory stimuli, although these do enhance it.
• 5-10% of the mucosal DCs migrate to the MLN every day in the resting intestine
• CD103+ DCs also produce retinoic acid
− Derived from metabolism of dietary vitamin A
− RA endows DCs with the capacity to induce gut-homing molecules on B cells
• CD103+ DCs respond poorly to inflammatory stimuli such as TLR ligands, and they produce IL-10, so when they arrive in the MLN under resting conditions, they induce generation of Foxp3+ Tregs → assisted by TGF-B.
− Anti-inflammatory behavior of the CD103+ DCs is actively promoted by factors constitutively expressed in the mucosa
− PGE2, TSLP, RA and TGFB=-B
How are Tregs induced in the intestine?
• Crucial suppressive CD4+ T cell subset
• Inhibits self-harmful T cell reactions
• Made in either the thymus (nTregs) or periphery (iTregs)
• A study in mice found that Treg numbers are highest in the intestine
− Induced by the commensals
− Could be important therapeutically → if you need to promote tolerance, give back some more commensals.
In general:
• In response to commensals → immature DCs give weak co-stimulation and induce CD4 to differentiate into Tregs
• In response to pathogens → activated DCs express strong co-stimulatory molecules, and induce CD4 differentiation into effector cells.
But again, this is not that simple. Different DC subsets have different capacities to activate Tregs.
• CD103+ T regs very effective → express avb8 → activates TGFb → activates T regs
Commensals can also induce Tregs:
• Ligation of TLRs important
• Different bacteria species have specific functions:
− PSA drives differentiation of IL-10 producing Tr1 Tregs
− SFB induces appearance of Th1, Th17 and Foxp3+ T regs
Describe the features of IBD
- Chronic inflammation of the intestine
- 2.4 million affected in Europe
- 20-30 age of onset for Crohns
- 30-40 age of onset for ulcerative colitis
- Not disease associated with frailty → often effect young, healthy adults
What causes IBD?
• Breakdown of immune homeostasis, leading to inflammation
• Can be due to barrier defects, or immune regulation defects (eg, enhanced T cell responses against commensals)
What causes the inflammation?
• In IBD, macorphages and DCs secrete more IL-12 → stimulates Th1
• Also produce more IL6, TGF-B, IL-23 → stimulate Th17
• Th1 and Th17 cells secrete many inflammatory cytokines
• Macrophages and DCs can also directly secrete the inflammatory cytokine TNFa
Current treatments for IBD:
• Steroids to dampen the immune system
• If unresponsive, Infliximab (blocks TNFa function)
• If unresponsive, surgery (remove affected area of intestine)
What causes the breakdown in immune homeostasis of the intestine in IBD?
Enviromental factors:
• Diet → people put on FODMAP diet
• Smoking → interestingly, this is potentially protective
• Antibiotics
• Infection
• Stress
→ Environmental factors not well understood, but likely to be important as only 75,000 IBD patients and healthy controls
− 161 IBD susceptibility genetic loci identified
• Many of these genes associated with the immune system eg)
− NOD2 involved in recognition of microbial antigens
− STAT3 involved in Th17 biology
− IL-10 associated genes
− TGF-B associated genes
