Download Lecture 11: Mucosal Immunity

Lecture 11: Mucosal Immunity
(based on lecture by Dr. Betsy Herold, Einstein)
Questions to Consider
 How is the mucusal immune system different from the
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systemic immune system?
How does the immune system prevent overreaction to
antigenic loads?
How does the mucosal immune system protect itself
from infection?
How do pathogens bypass mucosal immunity?
What Th subtypes are preferentially activated in the
mucosal immune system?
Overview of Mucosal Immune
System
 Major components
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GI tract
Respiratory tract
Genital tract
 Unique attributes
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First line of defense
Constantly exposed to
Ag
Worldwide Mortality From
Mucosal Infections
Components of the Mucosal
Immune System
GALT: Gut Associated Lymphoid
Tissue: Anatomic Sites
 Peyer’s patches
 Aggregates of lymphoid cells with
B cell follicles & smaller T cell
areas
 Lymphoid follicles
 Smaller; mainly B cells
 Also found in respiratory tract
(BALT) , lining of nose (NALT)together referred to as MALTmucosa associated
 Appendix (Tonsils, adenoids)
 Mesenteric nodes
 Peyer’s patches & lymphoid
follicles connected by lymphatics
to draining nodes
 Peyer’s patches, mesenteric
nodes differentiate independently
of systemic immune system
during fetal development (control
chemokines)
M cells: Microfold Cells:
Specialized Epithelial Cells
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Epithelial cells covering Peyers Patches
Differ from epithelial cells (enterocytes)
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No microvilli; broader microfolds
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Do not secrete enzymes, mucus, and no thick surface glycocalyx
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Transport organisms from gut lumen to immune cells across epithelial barrier
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Endocytose or phagocytose Ag at AP surface & deliver it to DCs or T cells via
transcytosis at BL surface
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Note: some pathogens (Shigella, Salmonella, Yersinia) exploit M cells as a way to
penetrate the intestinal epithelium
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CXCR4 HIV strains bind to M cells and may get transported across the epithelium
to infect immune cells
Uptake and Transcytosis of
Antigen Across M Cells
Pocket in basal membrane of M cell encloses T cells and DCs
Dendritic Cells
 DCs recruited to the mucosa in response to chemokines
constitutively expressed by epithelial cells
 Extend processes across epithelium to capture Ag in lumen
 DCs also prevalent within lamina propria
• CCL20 (MIP-3α) & CCL9 (MIP-1γ) bind to receptors on DCs
(CCR6 and CCR1, respectively)
• Interference with CCL20 signaling blocks recruitment & may
prevent HIV in macaques; Nature 2009 Apr 23;458(7241):1034-8.
• Ag loaded DC migrate from dome region of Peyer’s patch to T
cell area or to draining lymphatics to mesenteric nodes
Effector T Cells
 Resident T cells found in epithelium and lamina propria
 Epithelium contains mostly CD8 T cells, whereas lamina propria is more
heterogenous (CD4, CD8, plasma cells, macrophages, DCs, eosinophils
and mast cells)
In intestine and respiratory tract, plasma cells predominately IgA
In genital tract: IgG> IgA
 Neutrophils are found usually only in response to inflammation/infection
 T cells in lamina propria of small intestine express integrinα4β7 and
CCR9, which attracts them into the tissue from bloodstream;
 Epithelial cell T cells express integrin αEβ7, which binds to E-cahedrin on
epithelial cells.
Mucosal Lymphocyte Life Cycle and
Gut-specific Homing Receptors
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Naïve T & B cells emanate from thymus & bone marrow &
circulate in bloodstream
Enter Peyer’s patches (or nodes) through endothelial
venules directed by homing receptors, CCR7 & L-selectin
If no Ag is encountered, exit via efferent lymphatics & return
to bloodstream
If Ag is encountered, cells become activated, exit via
lymph nodes to thoracic duct & recruited back to gut
T cells that first encounter Ag in GALT express gut-specific
homing receptors (α4β7 and CCR9)
Homing Receptors
 Expression of homing receptors triggered by GALT DC
 α4β7 binds to the mucosal vascular addressin (MAdCAM-1) expressed on
gut endothelial cells
 CCR9 binds to CCL25(TECK) on gut epithelium (small intestine)
 Priming explains why vaccination by mucosal route against intestinal
infections (e.g. Rotavirus) ensures imprinting to the gut.
 MAdCAM-1 also expressed in other mucosal sites: T cells primed in GALT
can recirculate as effector cells to respiratory, genital or lactating breast
tissue: “common mucosal immune system”
 Vaccines: Mucosal route can be used to protect multiple mucosal sites
Homing ReceptorsRole of MAdCAM-1 and Chemokines
Secretory IgA
Dominant class in gut & respiratory tract (not genital
tract)
In blood, IgA mostly monomer (IgA1:IgA2=10:1)
In mucosa, dimer linked by J chain (IgA1: IgA2=3:2)
Class switching from IgM to IgA producing cells
occurs in response to TGFβ
Common intestinal pathogens can cleave IgA1
IgA2 more resistant
IgA in Gut
 Activated B cells (like T cells) express
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homing integrin (α4β7) & CCR9/10,
which localizes them to gut
IgA producing plasma cells secrete IgA
dimers, bind to poly-Ig receptor
expressed on BL surface of immature
epithelial cells at base of intestinal
crypts
Bound complex taken up by cells;
traversed to AP surface by transcytosis
Poly-Ig receptor cleaved releasing IgA
dimer & secretory component at
luminal surface; secretory IgA
IgA binds to mucins at epithelial
surface via carbohydrates on secretory
component;
• Retention of IgA at epithelial
surface prevents adherence of
microbes & neutralizes toxins, etc.
Intracellular IgA Neutralizes Ags (e.g. LPS)
 IgA does not activate complement pathway
 Does not trigger inflammatory response
 Restricts commensal flora to the lumen
IgA Deficiency
 Common: 1:500-1:700 in Caucasian population
 Most individuals have no clinical problems
 Associated with IgG2 subclass deficiency→↑ risk
infections
 IgM may replace IgA in secretions; IgM also is J-chain
linked & binds poly-Ig receptor
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IgM-producing plasma cells are ↑ in IgA deficiency
 KO mouse model: KO poly-Ig receptor susceptible to
mucosal infections; KO IgA, no ↑ susceptibility
Mucosal T cells
 Most T cells in lamina propria
 CD45RO+ (similar to effector/memory T cells)
 Express gut homing markers
 Express receptors for inflammatory chemokines e.g
CCL5 (RANTES)
 Proliferate poorly in response to Ag or mitogens
 Secrete large amounts of cytokines (IL-10, IL-5 and
IFN-γ constitutively
 Function in healthy gut uncertain
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? regulatory role
IEL: Intraepithelial Lymphocytes
 10-15 lymphocytes/100 epithelial cells
 90% are T cells; 80% CD8+
 Express homing markers CCR9 &αEβ7; binds E-
cadherein on epithelial cells
 Activated
 + perforin and granzyme in intracellular granules
 Relatively restricted use of V(D)J gene segments;
responsive to limited Ag repertoire
Functions of IEL
Type A: conventional CD8 cytotoxic
effectors
MHC-restricted
express CD8α:β
Type B; Express CD8α:α
Express NKG2D(activating C-type
lectin NK receptor) which binds to 2
MHC-like-molecules; MIC-A, MIC-B
that are expressed on epithelial
cells in response to stress/damage
& killed via perforin/granzyme
pathway
Activation of these IEL cells
mediated by
IL-15
↑in celiac disease
IEL Kill Infected Epithelial Cells
IEL Kill Stressed Epithelial Cells
Mucosal Response to
Infection
 Mucosal surfaces are not sterile
 Mucosal immune system must differentiate
harmless (endogenous flora) from pathogenic
microbes and respond differently
 Gut is most frequent site of infection
Epithelial Cells are Immune Cells
 Mucosal epithelial cells are polarized
 Apical surface faces the intestinal lumen; BL surface
faces the adjacent epithelial cells and underlying
basement membrane
 Polarized expression of different
receptors/proteins/channels
 EXPRESS Toll-like Receptors (TLRs) at both
membranes, but responses differ
 Activation by commensal bacteria has an essential
role in maintaining colonic homeostasis
Pathogen-related Specificity of TLR Molecules
Nat Cell Biol. 2006 Dec;8(12):1327-36
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Polarity of TLR Responses
 Interaction of TLR with microbes activates signaling complex (NFkB, MAPK,
IFNs)→ transcription of inflammatory and immunoregulatory genes
(chemokines, cytokines and costimulatory molecules, defensins)
 Human IECs express a spectrum of TLRs, including TLR2, TLR4, TLR5,
and TLR9
• Genital tract epithelial cells express full array of TLRs
 Polarized responses differ & may explain differential response to microbes:
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BL TLR9 signals IkB degradation & activation of NF- kB
AP TLR9 stimulation invokes a unique response in which
ubiquitinated IkB accumulates in the cytoplasm preventing NFkB activation.
AP TLR9 stimulation confers intracellular tolerance to
subsequent TLR challenges
TLR9-deficient mice display a lower NF- kB activation threshold
& are highly susceptible to experimental colitis.
Nat Cell Biol. 2006 Dec;8(12):1327-36
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Epithelial Cells Also Have Intracellular
Sensors for Infection
 TLRs within intracellular vesicles
 NOD1/NOD2 (nucleotide-binding oligomerization
domain)
 NOD1 recognizes muramyl tripeptide on
GNR
 NOD2 recognizes muramyl dipeptide in
peptidoglycan of most bacteria
 Signaling activates NFkB pathways
 Chemokines, cytokines, defensins
 Activation of signaling pathways: doubleedged sword
 Facilitate further invasion (e.g. IL-1β and
TNFα disrupt tight junctions
 Inflammation causes symptoms, but also
recruits immune cells & initiates adaptive
immune response to eliminate microbe
Salmonella Invade Epithelium by Three Routes
 Adhere to M cells, cause apoptosis of M cell, infect
macrophages and epithelial cells; trigger TLR5(flagellin) at BL
membrane and trigger NFkB inflammatory pathways
 Invade by direct adherence of fimbriae to luminal epithelial
surface
 Enter DCs that sample gut luminal contents
Balance: Tolerance vs. Immune Response
Mechanisms of oral tolerance
Deletion of Ag-specific T cells?
Regulatory T cells (TH3)?
Produce TGFβ; immunosuppressive
Consequences of
Mucosal Tolerance Breakdown
 Celiac disease
Genetically susceptible (HLADQ2)
 Generate IFNγ-CD4 T cell response to protein
gluten (gliaden) leading to inflammation
 ? Food allergies
 Crohn’s disease
 Overresponsiveness to commensal gut flora
 NOD2 mutations and uncommon polymorphic
variants
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Commensal Bacteria Prevent Disease
 Normal gut flora maintains
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health
Compete with pathogenic
bacteria prevent them from
colonizing & invading
Directly inhibit proinflammatory
signaling pathways
TLR response to commensals
controls inflammation
Loss of normal gut flora (i.e. in
response to antibiotics) allows
other bacteria to grow: C difficile
Integrity of intestinal epithelium
disrupted (trauma, infection,
vascular disease)
 Nonpathogenic commensal
invade blood
stream→disease
Immune Response to Endogenous Flora
 Recognized by adaptive immune system
 sIgA and T cells recognize commensals
 Effect responses not typically elicited
 Do not typically invade: compartmentalized response
 Animals raised in germfree (gnotobiotic) environment
 Reduced size of lymphoid organs
 Low Ig levels
 Reduced immune responses
DC Response to Pathogens and Commensals
 DCs loaded with commensal
 Activate B cells into IgA producers
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redistributed to lamina propria
Epith cells produce TGFβ, TSLP,
PGE2- maintain DCs in quiescent
state
When present Ag to naive T cells,
generate Treg response (antiinflammatory)
Commensals do not penetrate intact
epithelium, do not activate NFkB,
lack virulence factors
If regulatory mechanisms fail,
systemic immune responses
generated (TH1)→disease
Protective and Pathological
Responses to Intestinal Helminths
 TH2 responses protective
 TH1 responses produce
inflammatory reaction that
damages mucosa
 IL3 and IL9 recruit mucosal
mast cells-produce PGS,
leukotrienes and proteasesremodel intestinal mucosa,
create hostile environment to
parasite.
 Host response to parasites
involves turnover of epithelial
cells which helps eliminate
parasite: double edged
sword as compromises
intestinal function as newly
produced epithelial cells
defective in absorption
 Parasites evolved
mechanisms to modulate
immune response
Response to Invasive
Pathogens
 The predominantly tolerant microenvironment “changed” in
response to pathogens
 DCs now become fully activated and present Ag to T cells
to generate effector T cell response
 Both DC populations – inflammatory and regulatory may
exist simultaneously: state of physiological inflammation
 Hygiene hypothesis: absence of exposure to helminths
and other Ags results in hypersensitivity responses to
harmless environmental Ags and increased autoantigen
responses
Summary
 Mucusal immune system avoids making active responses to
majority of Ags encountered but recognizes both pathogenic and
non-pathogenic Ags.
 Disruption of this balance leads to disease
 Local DCs play key role
• DCs in Peyer’s patches in almina propria produce IL-10, rather
than pro-inflammatory IL-12
• Response to Ag is local IgA and induction of tolerance
• This tolerant response maintained by TSLP, TGFβ, PGE2
produced by local epithelial & stromal cells
• Thus DCs migrate to mesenteric node but lack co-stimulatory
molecules to activate naïve T cells into effectors
• Induce gut homing molecules on T cells, to restrict any
response to mucosa
Questions to Consider
 How is the mucusal immune system different from the




systemic immune system?
How does the immune system prevent overreaction to
antigenic loads?
How does the mucosal immune system protect itself
from infection?
How do pathogens bypass mucosal immunity?
What Th subtypes are preferentially activated in the
mucosal immune system?