Lecture 17: Immunity at Mucosal Surfaces

Characteristics of the mucosal immune system

Covers ~ 400 m² of surface area (vs. 2

m² of skin)

•Constantly exposed to food ags,

commensals, pathogens, and

environmental particles

•Requires tight epithelial barrier function

•Must balance tolerance (food/microbiota)

and immunity (pathogens)

Core challenges for the mucosal immune system

Enormous ag load

•Risk of inflammation → tissue damage →

loss of barrier

Key mucosal sights

•Gastrointestinal tract (the largest immune organ)

•Respiratory tract

•Urogenital tract

•Lacrimal, salivary, mammary glands

Different Types of Epithelium Line Barrier Tissue

The type of epithelium a tissue uses is the first clue to the kind of immune defenses it needs–

different types depending on the organ

All epithelia face enormous ag load – pathogens, food ags, commensal bacteria (friendly ags)

Intestine muscosae

- microvilli ↑

absorption

- Goblet cells

produce mucus

Respiratory tract mucosae

- ciliated

epithelium +

mucus clearance

Oral cavity,

esophagus mucosae

protects against

friction, non-

drying surfaces

Skin

ough, water-

resistant outer layer

Anatomic features of the mucosal immune system

Effector mechanisms of the mucosal immune system

Immunoregulatory environment of the mucosal immune system

Mucosal Infections are One of the Biggest Global Health Problems

Many major pathogens enter

through mucosal surfaces:

- respiratory tract (influenza, SARS-

CoV-2, RSV, TB)

- gastrointestinal tract (cholera,

rotavirus, norovirus)

- urogenital tract (HIV, HPV,

gonorrhea, chlamydia)

Highlights why effective mucosal

immunity is critical for global health

Most infectious diseases that kill humans do so because they breach a mucosal barrier first.

What is MALT (mucosa-associated lymphoid tissue)?

• umbrella term

• Also present in urogenital tract (HIV entry & STI immunity), mammary gland, conjunctiva

NALT

-tonsils, adenoids

-strong Th1/Th17

responses to

respiratory viruses

BALT

-inducible in adults

-site of TB priming in

lung

GALT

-Peyer’s patches,

isolated lymphoid

follicles, and

appendix

-oral tolerance, IgA,

defense against

enteropathogens

What is an induction site?

Where the mucosal immune response is initiated

GALT

BALT

NALT

Urogenital tract

Lacrimal glands

Salivary glands

Mammary glands

What is the effector sight?

Where the mucosal immune response is carried out

Lamina propria (large intestine) (plasma cells, T cells, ILCs)

Epithelium (intraepithelial lymphocytes, goblet cells, Paneth

cells)

IgA-producing plasma cells

Mucus secretion, AMP production, barrier fortification

What is Peyer’s patch?

Specialized Epithelium Overlying Intestinal Lymphoid Tissues Takes Up Particulate Antigens

M cell: microfold cell – transport ag from lumen

FAE lack thick mucus → easier access for ag sampling

Subepithelial dome rich in DCs and lymphocytes

Leads to activation of B and T cells in GCs

What is an isolated lymphoid follicle (ILF)?

Specialized Epithelium Overlying Intestinal Lymphoid Tissues Takes Up Particulate Antigens

Smaller inducible lymphoid structures distributed along intestine

Also contain M cells overlying lymphoid follicles

Sample ag and deliver it to underlying DCs

Connect to mesenteric lymph nodes via lymphatics

What is transcytosis?

M cells take up commensal bacteria, pathogens, and particles from the gut lumen

Transport (transcytose) them across the epithelium

Deliver ags directly to B cells, T cells, and DCs in the subepithelial dome

Enable rapid initiation of mucosal immune responses

What are the developmental stages of GALT?

Cryptopatches (present at birth)

- Small clusters of DCs and lymphoid tissue-inducer (LTi) cells

ILFs

- Develop after birth; expand in response to microbial colonization

Peyer’s Patches

- Present at birth but grow and mature postnatally

- Require microbial signals for full development

What are germ free mice?

1. pretty much have no good or bad bacteria

2. Poorly developed Peyer’s patches

   x ILFs

   ↓ IELs

   ↓ IgA-secreting plasma cells

   ↓ AMPs

   ↓ immune mediators (cytokines)

Maturation of GALT is Driven by

Acquisition of Commensal Microbiota

Early-life antibiotics may impair GALT development

Cesarean delivery alters initial microbial colonization

Dysbiosis can drive IBD, food allergies, asthma (systemic effects)

Small intestine effector sites

Intestinal

epithelium (single-

cell layer)

IELs (or IETs) –

mostly CD8+

No B cells

ILCs

Produce AMP,

cytokines

Interacts directly

with commensals,

sIgA, nutrients

large intestine effector site

Lamina propria

Rich in Innate cells

(macrophages,

DCs, ILCs)

CD4+ and CD8+ T

cells

B cells and IgA-

secreting plasma

cells

Tregs

Site of most IgA

production and

immune effector

activity

Innate Immune Defenses of Intestinal Immune System

1. absorptive subsets 

2. secretary subsets 

3. Intestinal epithelium is not just a barrier but an active immune organ composed

   of specialized innate cell types.

Absorptive subsets

Enterocytes (nutrient absorption,

sense microbes via TLRs, secrete

cytokines)

b. M cells

What makes up the Secretory subsets

Goblet cells – b. Paneth cells – AMPs

c. Enteroendocrine cells – hormones

and neuropeptides

d. Tuft cells – Cytokines and lipid

mediators – imp. in parasite

response and Type 2 immunity

Characteristics of the secretory subsets?

Viscous, slippery and sticky

• Negatively charged → retains sIgA and AMPs

• Substrate for commensals

• Major component - mucins (MUC2)

Intestinal Epithelium Contains T Cells

IELs provide rapid, front-line defense at the mucosal barrier

Include conventional CD8⁺ T cells and unconventional (innate-like) T cells

What is Type A IEL?

Function like CTL (TCR

activation → perforin,

granzyme, FasL)

Type B IEL?

Function like NK cells (act

independently of TCR

activation through NKG2D)

Loss or dysfunction of IELs

contributes to:

Celiac disease

Chronic infections

Barrier breakdown in IBD

What do ILCs do in the epithelium?

• ILCs in GALT respond to Microbes that breach Epithelium

• Abundant in mucosal tissues (especially intestine)

  Mirror T-helper cell subsets in cytokine profiles

  Activate without antigen-specific TCRs (respond

  to cytokines + epithelial signals)

Excess ILC1

chronic inflammation, Crohn’s disease association

Excess ILC2

allergies, asthma, eosinophilic GI disease

Excess ILC3

IL-17-driven inflammation, IBD

Reduced ILC3

loss of IL-22 → weak barrier → dysbiosis

Mucosal Immune System Establishes & Maintains Tolerance to Harmless Ags

Immune Priming and Tolerance are Different Outcomes of

Intestinal Exposure to Ag

Immunity or tolerance example

The same ag delivered orally can generate immunity or tolerance depending on danger signals and

context

Pathogens → PAMPs → danger → immunity

Food + commensals → no PAMPs → steady-state DC signals → tolerance

Why commensals don’t trigger inflammation even though they

have PAMPs

1. Spatial segregation: mucus, sIgA, and AMPs keep commensals away

2. Epithelial TLRs are compartmentalized

3. Specialized tolerogenic phagocytes

4. Microbial metabolites instruct tolerance

5. Absence of contextual danger

-Danger is not just pathogen or PAMP

-True danger : PAMP/DAMP + barrier breach + inflammatory cytokines

Intestinal epithelial cells express TLRs in

strategic, non-reactive ways:

•TLR5 on basolateral surface only

•TLR9 can produce tolerogenic signals (IL-10)

when signaling from apical side

•TLR4 is often low or absent on apical

membrane

Thus, commensal PAMPs hitting apical TLRs

→ tolerance, not inflammation.

Routes of Antigen Uptake in the Intestine

The gut does NOT rely on just one

mechanism to sample antigens; it uses

multiple, highly specialized routes

depending on ag type

What do M Cells do?

Bulk and receptor-mediated

transcytosis of microbes & particles.

Deliver ags to CD103⁺ DCs beneath

FAE

What does paracellular transport do?

Limited ag diffusion between

epithelial cells

•Occurs when tight junctions are

physiologically “open” or mildly

loosened

•Controlled, not pathological

What does FcRn: neonatal Fc receptor do?

Transports IgG-ag complexes across

enterocytes

Allows sampling of luminal antigens

bound to maternal IgG (neonates) or

endogenous IgG

What do goblet cells do?

Goblet cells can deliver soluble

proteins to underlying tolerogenic DCs.

Important for oral tolerance to food ags

What is Apoptosis-Dependent Transfer?

•IECs undergoing apoptosis release

antigenic material

•Phagocytosed by local DCs &

macrophages

What do TEDS do?

•CX3CR1⁺ macrophages extend

dendrites between epithelial cells into

the lumen

•Capture bacteria or particles directly

•Important for commensal sampling

and early pathogen sensing

Tregs During Pathogenic Exposure

Pathogens trigger innate and

adaptive immune activation

•Effector T cells, ILCs,

macrophages, and antibodies

clear infection

•Tregs prevent excessive tissue

damage during the response

Tregs During Exposure to Innocuous Ags

•Tregs enforce tolerance and

suppress unnecessary

inflammation

•Prevent local and systemic

immune reactivity

•Protect epithelial barrier integrit

Loss of mucosal Treg function

contributes to:

•IBD

•Celiac disease

•Food allergy

•Autoimmune disorders

•Chronic infections

•HIV mucosal dysfunction

Tregs suppress effector

responses through:

•IL-10 (suppresses DCs, Th1,

Th17, macrophages)

•TGF-β (promotes IgA class

switching + epithelial repair)

•CTLA-4 (removes co-stimulatory

molecules from DCs)

•IL-2 consumption (starves

effector T cells)

•Amphiregulin (creates tissue

repair microenvironment)

Transcytosis of IgA Across Epithelia is Mediated by

Polymeric Ig Receptor (pIgR)

1. Binding (Basolateral Surface)

Dimeric IgA (with J chain) binds to pIgR

on the basolateral side of epithelial

cells.

2. Endocytosis

The pIgR–IgA complex is internalized

into the epithelial cell.

3. Transcytosis

Vesicles move the complex across the

cell toward the apical surface.

4. Release (Apical Surface)

pIgR is cleaved.

IgA is released into the lumen as

secretory IgA (sIgA).

•sIgA = IgA dimer + secretory

component (cleaved part of pIgR),

which protects IgA from proteolysis.

How is IgA secreted?

IgA

dimer + pIgR

secretory

component

(soluble in lumen)

Mucosal IgA has Many Functions