Week 10 Part 3 - Mucosal Immunity

Question 1

Mucosal Surfaces - Overview

Answer 1

Sites where material passes between the body and the environment
Mucosal surfaces are thin permeable barriers to body’s interior
Three quarters of all lymphocytes and plasma cells are in secondary lymphoid tissues servicing mucosal surfaces
Layer of epithelial cells joined by tight junctions & covered by layer of mucus
Mucosal tissues are dynamic -> cell layer turns over every 2 days, & mucus is continuously expelled from the body

Question 2

Secretory Cells of Intestinal Epithelila

Answer 2

Goblet cells (mucins)
Paneth cells (antimicrobial peptides, lectins & cytokines)
Mucosal leukocytes (secretory IgA)
Epithelial cells (complement components)

Question 3

Mucins

Answer 3

Protects epithelial surfaces
Very large proteins comprising repeated amino acid motifs rich in serine & threonine which carry short negatively charged glycans
Secreted by goblet cells
Heavily hydrated polyanionic glycans
- bind defensins & antimicrobial peptides
- bind secretory IgA
- trap and kill microbes

Question 4

What makes Mucus Viscous?

Answer 4

Intertwining of mucins
-> Impedes movement of microbes & particles

Question 5

How do Mucosal Surfaces Limit Excessive Inflammation?

Answer 5

Proactive immune response: constantly responding against gut microorganisms → activated effector T and B cells ready to respond → infections stopped early, damage limited
Mucosal immune system limits activation of inflammation to decrease damage
CD4 Treg turn off inflammation → secrete IL-10 to suppress inflammatory cytokines

Question 6

Example of how Mucosal Surfaces Limit Excessive Inflammation

Answer 6

1. Healthy tissue protected by mucosal immunity
2. Bacteria gain access to LP by endocytosis, activate macrophages but don’t cause inflammation
3. Local effector cells respond to limit infection, dendritic cells travel to mesenteric lymph node to activate adaptive immunity
4. Effector B and T cells that are highly specific for invading bacteria colonise are of infection
5. Infection is terminated with either minor tissue damage or no need for repair

Question 7

Toll-Like Receptors and NOD Receptors

Answer 7

Intestinal epithelial cells have toll-like receptors
- senses bacteria that circumvent mucus (apical surface)
- senses bacteria that invade epithelium (basal lateral surface)
Cytoplasmic NOD receptors: detect bacterial cell walls

Question 8

Signals from TLRs and NOD Receptors

Answer 8

Leads to:
- secretion of antimicrobial peptides and cytokines (IL-1,IL-6)
- killing of bacteria, recruitment and activation of neutrophils, monocytes, eosinophils, T cells & immature DCs
Epithelial cells respond with a quick, local response that usually eliminates infection without causing damage

Question 9

Intestinal Macrophages - Inflammation-Anergic

Answer 9

Intestinal macrophages are proficient at phagocytosis to eliminate bacteria and dead or dying cells
Cannot perform functions associated with initiation and maintenance of inflammation
Express MHC class II but lack co-stimulatory molecules
- not a professional APCs
- inflammation anergic
Blood monocytes decrease their inflammatory capacity when differentiated (under influence of TGF-β) into intestinal macrophages

Question 10

Monitoring in Gut Epithelium

Answer 10

Sentinel cells detecting foreign antigen and contribute to immune hypo-responsiveness.
Two-way interaction: epithelial cells instruct the immune cells and vice versa

Question 11

Pathways for Antigen Uptake in Gut

Answer 11

1. M cells present antigen to APCs under epithelial layer
2. DCs extend dendrites between epithelial cells
3. Transcytosis - transporting Ig receptors in epithelial cells which can retrieve and deliver IgG-antigen complexes to APCs

Question 12

M Cells Location

Answer 12

M cells (microfold cells): positioned over Peyer’s patches & lymphoid follicles
Microbes are funnelled intact towards the M cells of follicle-associated epithelium
- follicle-associated epithelium overlies lymphoid tissue

Question 13

Function of M Cells

Answer 13

Capture microbes and antigens on apical surface -> internalise in endocytic vesicles that fuse on basolateral surface with plasma membrane: transcytosis
Transported antigens and microbes collect in intraepithelial pocket -> encounter DCs, T and B cells
These DCs secrete IL-10 -> prevents production of inflammatory cytokines by activated T cells
Production of bacteria-specific effector cells, and plasma cells making bacteria-specific antibodies

Question 14

Treg Cells and IgA in Oral Tolerance - Healthy Gut

Answer 14

Antigens from food transported via M cells to DCs (subpopulation that express CD103) -> travel to mesenteric lymph nodes
CD103+ DCs present antigens to T cells -> differentiation into Treg cells
Treg cells suppress response to food antigens
CD103+ DCs cause B cells to switch to secreting IgA antibodies

Question 15

Oral Tolerance - Commensal Microbes

Answer 15

Commensal microbes are beneficial if in gut lumen, but if breach -> potential pathogen
Specific secreted IgA antibodies: recognise commensal microbes
- limits size of commensal microbe population
- IgA responses lack classical memory and can respond to a flux in commensal microbe composition

Question 16

Oral Tolerance - Presence of Infection

Answer 16

In presence of an infection DCs can capture antigen in the absence of M cells
Antigen-loaded CD103+ DCs migrate to T cell areas of mesenteric lymph nodes to stimulate antigen-specific T cells -> effector T cells
Activated Th cells activate B cells -> plasma cells -> secrete IgA specific for commensals and pathogens

Question 17

Secretory IgM and IgA

Answer 17

Perpetual sampling of contents of the gut lumen (M cells and DCs) → a constant population of effector T cells
Activated follicular helper T cells (and IL-4, IL-10 & TGF-β) help B cells in Peyer’s patches and mesenteric lymph nodes to develop into plasma cells
Switch from IgM to secrete IgA specific for pathogens and commensals
IgA has higher affinity for antigen than IgM
Transcytosis delivers IgA to gut lumen

Question 18

Secretory IgM and IgA in Mucous Layer

Answer 18

The antibodies form disulphide bonds with mucins → tethers antibodies to the mucus where they bind microbes, preventing them from reaching the epithelial surface
Coating of protective antibody needs constant replenishment: ~ 80% of the body’s plasma cells in the lamina propria of mucosal tissues

Question 19

Effector Cells in Mucosal Tissue - B and T Cells

Answer 19

Naïve lymphocytes activated in Peyer’s patches
Effector B and T cells leave Peyer’s patches and travel to lymph node -> enter blood -> travel to gut-associated lymphoid tissue
Enter by α4β7 integrin on T and B cells binding MAdCAM-1 on intestinal endothelium
CCL25 chemokine binds CCR9 on T and B cells
Effector B cells differentiate into plasma cells → secrete IgA or IgM
Effector T cells: killing and cytokine secretion

Question 20

Effector Cells in Mucosal Tissue - Mucosal DCs

Answer 20

Mucosal DCs induce T cells to express gut homing receptors
A diet derived factor retinoic acid (metabolite of vitamin-A) released by CD103+ DCs cause CCR9 expression and transient increase α4β7 integrin on T cells
On entry into the mucosa adhesion to E-cadherin retains the T cells in the epithelium

Question 21

Effector Cells in Mucosal Tissue - Intra-Epithelial Lymphocytes

Answer 21

One intra-epithelial lymphocyte (IELs) present for every 10 intestinal villous epithelial cells
IELs are already activated by antigen
Always in an activated state because of the constant stimulation caused by the continual sampling of the gut by M cells and DCs
Antigen-experienced IELs serve as gateway guards at the intestinal epithelial surface constantly monitoring the epithelial layer for infection or disruption

Question 22

Mucosal Immune System vs Systemic Immune System

Answer 22

Mucosal: avoids inflammation by being proactive and constantly making adaptive immune responses against possible pathogens before infection
Systemic: avoids making an adaptive immune response unless it is absolutely necessary and then relies on inflammation to orchestrate the response