Lecture 18 - Mucosal Immunity II Flashcards
Outline the gross anatomy and function of the GIT
Stomach:
• Digestion of food
• Neutralisation
Small intestine:
• Duodenum, jejunum, ileum
• Digestion
• Absorption of nutrients
Large intestine:
• Absorption of water and vitamins, storage
Describe the tissue structure of the intestine
1. Mucosa: • Epithelium Form villi, have microvilli on apical surface • Lamina propria • Muscularis mucosae
- Submucosa
- Adventitia
• Most external
• Adjacent to perineal cavity
How does the small intestine have such a great surface area?
Valves of Kerking (folds)
Villi
Microvilli
→ 200 square metres
What sort of antigens is the gut mucosa exposed to?
Gut mucosa is exposed to innocuous antigens, as well as pathogenic ones
The gut immune system has to differentiate between these things
Describe the presence of immune cells in the gut
‘Physiological inflammation’
There are numerous immune cells present in the steady state:
Innate immunity: • Epithelial cells • Mast cells • Eosinophils • Macrophages • DCs • Innate lymphoid cells • Unconventional T cells
Adaptive immunity:
• Plasma cells
• T cells
Describe the role of epithelial cells in mucosal immunity
Epithelial cells in the GIT mucosa play an important role:
- Physical barrier
• Tight junctions
2. Mucous: • From goblet cells Functions: • Physical barrier against bacteria • Matrix for IgA • Contains decoy molecules that further interfere with bacterial invasion
- Antimicrobial peptides
• Paneth cells - Defensins
• Cathelicidins, lectins
4. Microbial sensing and inflammation • Epithelial have PRRs that detect bacteria and trigger inflammation through NFKB signalling • TLRs (luminal, basolateral) • NODs • Ligation of PRRs results in the production of pro-inflammatory cytokines, chemokines and defensins: - IL-1 - IL-6 - CXCL8 - CCL1 etc. • Also important for gut homeostasis
What mediates the immune function of epithelial cells in the gastric mucosa?
Immune mediators
• IL-22
• Bacterial recognition (PRR-PAMPs)
What effect do bacteria have on the immune function of the epithelial cells?
Microbial products (from commensal microbiota) stimulate TLRs and NOD2
This results in a ‘tolerising’ response in the cells
What observation has been made in Crohn’s disease?
NOD2 receptor mutations have been associated with Crohn’s disease
What effect can antibiotics have on the gut?
Antibiotics can wipe out commensal bacteria, allowing for the overgrowth of pathogenic bacteria (e.g. C. difficile)
Can lead to colitis (pseudomembranous colitis)
Describe the compartmentalisation of the adaptive immune system in the gut
- Inductive sites
- where Ags are sampled and responses are induced
GALT: gastric associated lymphoid tissue: • Organised tissues • Peyer's patches • Isolated lymphoid follicles (ILFs) • Mesenteric lymph nodes
- Effector sites
- where effect cells perform their action after activation and differentiation
• Lamina propria
• Surface epithelium
Describe the regional differences in lymphoid tissue along the GIT
Isolated lymphoid follicles:
• Found along in both small and large intestines
PPs:
• Only found in small intestine (usually distally)
Duodenum:
• Intra-epithelial lymphocytes
• LP lymphocytes
Large intestine:
• ILFs
Compare the numbers of PP and ILFs in the average human
PPs: 100-200
(Each contains 5-200 follicles)
ILFs: Thousands
Compare the development of PPs and ILFs
PP Anlagen develop before birth
ILFs: develop after birth
Shows that the development of PP is not dependent on exposure to commensal bacteria, whereas the development of ILFs is dependent on exposure to commensal bacteria after birth
Describe the structure of PPs
Resemble LNs, but there are more B cells
- FAE: follicle associated epithelium
• M cell at apex
• Enterocytes - Sub-epithelial dome (SED)
• DCs
• T cells
• B cells - Follicles
• Germinal centres
• B cells, plasma cells - T cell area
• Under the follicles
What are M cells?
Describe their function and structural features
Microfold cells
Cells that are specialised in Ag uptake and handover:
• Do not process and present Ag
• Continually transcytose Ag from the lumen of the gut (take up by endocytosis, then exocytose from basal membrane)
• Major pathway of Ag sampling in GALT
• Intimate contact with DCs, T and B cells on basal side
Structure:
• Folded luminal surface
• No microvilli
• No glycocalyx underlying the cells → greater access for lumen Ags
What are LP-DCs?
What is their function?
How do they access antigen?
Lamina propria DCs
• Take up Ag (through a number of routes), process, drain to mesenteric LNs for activation of naïve T cells
• ? Like M cells outside of the PPs
Antigen capture:
• ? Transepithelial processes for capturing luminal Ag
• Endocytosis of apoptotic epithelial cells
• Antibody mediated (FcRn-dep.) transepithelial transport: Ab bound to FcR on luminal side that have bound antigen are transcytosed
Describe how DCs ‘condition’ for gut-specific responses
CD103+ DCs (of gastric mucosa) produce certain molecules: • TGF-β • IL-10 • Retinoic acid (RA) • TSLP (thymic stromal lymphopoietin)
This results in the DCs having a largely tolerising effect on lymphocytes:
• FoxP3 up-regulation in T cells → Tregs
Where does retinoic acid come from?
What does it bring about in the gut?
DCs produce RA from dietary vit A:
Dietary vitamin A + Retinol → RA
Functions: 1. IgA class switching
- i/Treg induction
- Gut homing of lymphocytes:
• Imprinting of gut-specific homing receptors:
- α4β7
- CCR9 - Inhibition of Th1 and Th17 phenotypes
What is oral tolerance?
How is it induced?
Non-responsiveness to the antigens present in food
Food Ags are foreign, yet the immune system does not respond to them
Originates from mesenteric LNs
Induction of oral tolerance:
- Food Ag in gut
- Tregs are induced
- Tregs secrete TGF-β and IL-10
- Suppression of effector T cells and IgE production
Compare the immune response in the gut to commensal and pathogenic bacteria
Commensals:
• Generally do not invade past the mucosal epithelium
• No detection of foreign Ags by DCs (no PRR ligation)
• No inflammation
• DCs have tolerising effect on naïve T cells in mesenteric LNs (TGF-beta, RA, TSLP)
→ Tregs
Pathogenic bacteria:
• Invade past mucosa to LP, where they are sensed by DCs
• DCs express co-stimulatory molecules and cytokines
• DCs activate naïve T cells in mesenteric LNs
→
Th1 (viruses)
Th2 (parasites)
Th17 (bacteria)
Describe how activated lymphocytes ‘home’ to effector sites in the gut
Lymphocytes are stimulated to express certain surface molecules:
• CCR9
• α4β7
CCR9:
• Chemokine receptor for chemokine expressed in the gut
• Receptor for CCL25
• CCL25 expression by small intestine epithelial cells
α4β7:
• Binds MAdCAM-1 on mucosal vascular endothelium
Describe the compartmentalisation of CD4 and CD8 T cells in the gut mucosa
CD4 T cells: LP
CD8 T cells: epithelial layer, (intraepithelial)
Compare the immune cells in the LP and the mucosal epithelium
LP and mucosal epithelium have distinct populations of immune cells (despite being so close)
LP: • Plasma cells • CD4 T cells • Innate cells • (much more heterogeneous than epithelium)
Epithelial layer:
• CD8 T cells
• γδ T cells
What are intra-epithelial cells?
Describe which cells are IELs
Which molecules do they express?
Cell types:
• >90% T cells, 80% of those are CD8+:
- type a: α:β TCR, CD8α:β T cells
- type b: γ:δ TCR, CD8α:α T cells
• T(RM)
These cells are lodged in between the epithelial cells of the intestinal mucosa
Express:
• αEβ7 (ligand for E-cadherin, expressed on epithelium)
• CCR9
Describe Ab protection in the gut mucosa
How is the isotype selected?
How does this Ab help against infection?
IgA is the predominant isotype
Activated B cells home to gut and differentiate into plasma cells
IgA class switching:
• The microenvironment in the GALT stimulates class switching to the IgA isotype:
- TGF-β
- RA
- NO
( CD40-CD40L interactions with ‘helper’ T cells)
sIgA protection:
• Inhibition of microbial adherence
• Neutralisation of toxins / enzymes
