Lc5: Mucosal immunity

Question 1

Malt

Answer 1

• Largest immune system of body
• Differs from systemic immunity
• Exposed to very large amounts of antigens
• MALT is guarded by a thin selectively permeable epithelial layer
• Mucosal tolerance
  o Respond to pathogens
  o Tolerate commensals

o GI
o Respiratory
o Urogenital
o Eye
o Exocrine glands

Question 2

intestinal epithelium

Answer 2

enterocytes
goblet cells
paneth cells
entroendrocrine cells
M-cells

Question 3

entrocytes

Answer 3

• Enterocytes most common cell in the gut
  o Absorb nutrients
   Passive diffusion
   Carrier-mediated
   Paracellular diffusion
  o Maintain barrier function
   Tight junctions
   Desmosomes
   Adherens junctions
  o Secrete AMPs (beta-defensins)

Question 4

goblet cells

Answer 4

• Goblet cell –> mucus secreting
  o More in colon than small intestine
  o 2 layers of mucus in colon
   Deep layer is impermeable to bacteria
   In crypts of SI the mucus is also more dense to protect stem cells

o Mucus traps AMP to kill bacteria
o MUC2 most common mucin
 Mucins have many O-linked oligosaccharides
 Mucins can be secreted or membrane bound
 Mucin expression and quality is controlled by cytokines (etc.)

o GC-associated antigen passages  allows for antigen delivery to DCs (tolerance antigens)

Question 5

paneth cells

Answer 5

deep crypt secretory cells of the SI
o To protect stem cells
o Secrete AMPs
 Lysozyme
 Alfa-defensins (HD5 and HD6)  make pores on bacterial membranes
 RegIII (lectins)

o Secrete factors for stem cell function (Wnt3, Egf)
o Not found in colon (counterpart of Paneth cells are the deep crypt secretory cells)

Question 6

entroendocrine cells

Answer 6

detect nutrients and release hormones for digestion
o 1% of epithelium of GI
o Found in SI and colon
o 12 types
o 20 hormones (serotonin, etc.)
o Role in immunity  sensors of microbial metabolites
o Release cytokines and peptide hormones in response to PAMPs
 Influence barrier function
 Increase IgA production
 Stimulate crypt cell proliferation
o GLP-1 secretion (anti-inflammatory)

Question 7

M-cells

Answer 7

o Only present above the dome regions of Peyer’s patches  follicle associated epithelium
 The regions need to be in contact with antigens
 Low IgA
 Few Paneth cells
 Few goblet cells
o Capture antigens (or whole bacteria and viruses) and deliver them to APCs (DCs and B-cells) found in their basolateral pockets
 Phagocytosis
 Receptor-mediated endocytosis
 Fluid-phase pinocytosis
o Have short-irregular microvilli
o Large fenestrated membranes
o Have glycoprotein 2 (GP2) which binds type 1 pili on gram(-) bacteria

Question 8

antigen/nutrient delivery

Answer 8

• Enterocyte absorption of nutrients
• M-cells
• GAPs in SI (GC-associated antigen passages)
• Trans-epithelial dendrites
• Paracellular transport
• Barrier breach

Question 9

peyer’s patches

Answer 9

• Aggregates of secondary lymphoid tissue in the submucosa
• Covered by FAE
• Lots of DCs
• Large B cell follicles and adjacent T cell areas (mainly CD4+ but there are also cytotoxic T cells)

Question 10

mesentric lymph nodes

Answer 10

• Collect lymph-borne antigens from SI and colon
• Site of differentiation of effector and regulatory lymphocytes that home back to the lamina propria
• Similar functions as GALT (including Peyer’s):
  o B cells  IgA plasma cells
  o Naïve T cells  effector
  o Naïve T cells  Tregs

Question 11

DC types

Answer 11

• CD103(+)CD11c(+) are the classical DCs in the gut mucosa
• CD103 and sirpalfa define subsets of CD11c(+) DCs
  o Sirpalfa(+)  support induction of Tregs
  o Sirpalfa(-)  can cross present to allow for induction of cytotoxic T cells
   Both induce Th17
  o CD103(-)Sirpalfa(+) induce Th1

Question 12

macrophages

Answer 12

• Resident in lamina propria and unlikely to play a major role in priming naive cells
• They engulf and clear bacteria, secreting cytokines, and maintaining intestinal homeostasis
• Some phagocytose and kill microbes while secreting anti-inflammatory cytokine IL-10 (occurs within local TGF-β environment)!!!!!!

Question 13

ILCs

Answer 13

• Enriched at barrier surfaces (PP or lamina propria in gut)
• Lack antigen-specific T-cell receptors
• Rapidly respond to host- or microbial-derived stimuli
• Three subsets producing different cytokines
  o (IL12/15/18) –> ILC1 –> IFN-y
  o (IL25/33) –> ILC2 –> IL-5 and IL-13
  o (IL1b/23) –> ILC3 –> IL17 and IL22 (and IL-2 which can induce Tregs)
• Modulate adaptive immunity by producing soluble mediators with (in)direct effects
• Communication between ILC3s, macrophages and Treg cells in the intestine, which reinforces tolerance and tissue homeostasis

Question 14

gut homing

Answer 14

• Effector lymphocytes in GALT or MLN are imprinted with integrin and chemokinereceptor dependent gut-homing properties
  o α4β7 binds MadCAM (on lamina propria venules)
  o CCR9 binds CCL25 (intestinal epithelial cells), produced by IECs
• DCs in GALT and MLNs provide signals for increased expression of α4β7 and CCR9 on effector cells
  o This requires retinoic acid (RA) secretion by DCs
• DCs in GALT and MLNs express retinaldehyde dehydrogenase (RALDH)
  o IECs also express RALDH and can synthesize retinoic acid
   

Question 15

intrepithelial lymphocytes

Answer 15

• Specialized lymphocytes in paracellular space between IEC
• Participate in tissue surveillance and barrier function
• Predominantly CD8+ cytotoxic cells
• Limited diversity of antigen receptors; may be evolved in this way to recognize microbes that are commonly encountered at the epithelial surface
• Ready to kill IEC that are damaged, infected or stressed, by potent expression and secretion of granzymes

Question 16

Th differentiation factors

Answer 16

• Microbial antigen type and APC type
• Strength of TCR stimulation and co-stimulation
• Cytokine gradients
• Induction of lineage-specific transcription factors
  o Th1 > T-bet
  o Th2 > GATA3
  o Th17 > RORγt
  o Treg > Forkhead box P3 (FoxP3)
• Th17 and Treg cells are the two largest population of CD4+ T cells in the gut in homeostasis
  
  • The commensals affect the mucosal immune system in such way that homeostasis is achieved by enforcing an equilibrium between both

Question 17

Th17

Answer 17

• SFB binding to gut epithelium causes production of serum amyloid A which induces IL-1, IL6 and TGFbeta release by macrophages which causes Th17 differentiation
• Citrobacter cause release of Il23 and IL1beta which causes the homeostatic Th17 to become inflammatory  they switch produce IFN-γ and GM-CSF when stimulated by IL-23 and IL-1β
• Homeostatic Th17 produce cytokines IL-17A, IL-17F, and IL-22
• Receptors for IL-17 and IL-22 are expressed on IECs
• IL-17A and IL-22 induce expression of proteins for barrier function > protect host from bacterial invasion
  o mucins and AMPs by IECs
  o tight junction formation between IECs

Question 18

Treg

Answer 18

• Suppress immune responses and induce a tolerant response to commensal antigens and food
• Express high levels of IL-2 receptor α chain (CD25) and FOXP3, required for development and function
• FOXP3+ Tregs among CD4+ T cells is about twofold greater in the intestine than in other tissues
• Clostridium bacteria produce SCFAs which cause Treg differentiation
• CD103+ DCs and macrophages produce RA and TGFbeta which induce Treg differentiation and inhibit Th1 and Th2 differentiation

Question 19

humoral immune response at mucosa

Answer 19

• Primarily directed at neutralizing luminal microbes
• Mediated mainly by IgA produced in lamina propria and transported into lumen by poly-Ig receptor
• Selective induction of IgA isotype switching in B cells is partly responsible for the abundance of IgA-producing plasma cells, IgG and IgM are also secreted into the lumen but in smaller quantities
• 2 g IgA per day, 60-70% of total Ab production

Question 20

IgA isotype switching

Answer 20

• T-cell dependent (TD) and T-cell independent (TI) mechanisms to induce activation -induced deaminase, enzyme mediating switch recombination
• TGF -β / RA plays important role (DC derived)
• TD: high -affinity IgA
• TI: low -affinity IgA

Question 21

IgA and sIgA

Answer 21

• IgA is a dimer molecule
• The J chain connects the 2 IgA molecules and is ligand for the polymeric Ig receptor (pIgR) expressed on the basolateral surfaces of IECs
• The most common IgA is secretory IgA (sIgA)
• sIgA is found in the secretions of the intestines, lungs, tear glands, the urogenital system, and the mammae

Question 22

sIgA effector functions

Answer 22

• SIgA can recognize and neutralize gut bacteria
  o Specific interaction with antigen-binding Fab segment of SIgA
  o Non-specific interaction with Fc and secretory component segments of SIgA
• Thereby preventing commensals and pathogens from invading through the mucosal epithelial barrier
• Fab-specific binding
  o anchoring of intraluminal bacteria to mucus
  o attenuation of bacterial motility, growth, and adhesion to gut epithelium
• Fab-independent glycan-mediated binding
  o glycans in the secretory component bind to bacteria
  o reduce their motility, thereby preventing them to reach the epithelium
• SIgA promotes the growth of beneficial bacterial, such as Bacteroides species  glycans on Fc regions and secretory component serve as carbon sources for the microbes

Question 23

neuroimmune interaction

Answer 23

a. IL-6 (enteric neuron) –> reduce Treg differentiation
b. VIP (EN) –> binds to VPAC2 on ILC can promote or reduce barrier function
c. CSF1 (EN) –> increased differentiation and survival (these macrophages then produce BMP2 which causes colonic motility)
d. Environmental stress –> IL33 –> serotonin (EC cells) –> increased motility via enteric neurons
e. Bacterial products –> TLRs on enteric glial cells –> neurotrophic factors –> ILC3 –> IL22 production (inflammation)
f. Microbiota –> increase AHR –> altered neuronal expression profile and colonic motility