Lecture 8 Flashcards
Celiac disease intro
- chronic inflammatory disease primarily affecting SI
- dysregulated inflammatory response initiated by gluten (NOT an allergic response)
- inflammation leads to damage and atrophy of intestinal villae
Result:
- lose absorptive capability
- breach in the epithelial barrier can lead to increased risk of infection
- due to loss of intestinal structure, Cd is associated with nutrient deficiencies
- increased risk of cancer from constant cell division
Iceberg disease
Symptomatic CD (tip of iceberg):
- manifest mucosal legion
- damaged intestinal villi
Silent CD (middle):
- manifest mucosal lesion
- damaged intestinal villi
- BUT no symptoms
Latent CD: (bottom):
- no mucosal lesion
- normal intestinal villi
- patients with the potential to develop symptoms
Genetic susceptibility:
- HLA-DQ2
- DQ8
- positive serology (TTG)
- must have to develop Cd BUT not everyone with them will develop CD
Problem of celiac disease
In CD, patients protein digestion (enzyme function) and amino acid absorption is NORMAL
Gluten proteins are resistant to enzymatic digestion (with or without CD, everyone will have issues digesting gluten)
Problem: damage of the SI epithelial barrier, laminal propria, and intestine tissue structure all cause LOSS OF VILLI and open wounds in the SI
Result: secondary complications for nutrient absorption and risk of infection
PART 1
How is gluten passing intestine if its resistant to digestion: epithelial barrier integrity
Integrity of apical junction all complexes in epithelial barrier
Tight Junctions:
- seals neighbouring cells together and prevents leakage between epithelial cells
- Claudins, JAMs, Occludins
- anchor the tight junction proteins to zona occludins (ZO) which hold structure together
Adherens Junctions
- join the actin bundle in one cell to the next
- ex: Cadherins and catenins
Lamina propria is below epithelial barrier: this is where CD immune response starts
- aggregation of immmune cells in lamina propria, can access bloodstream from lamina propria
PART 2: How is gluten passsing if its resistant to digestion (epithelial barrier integrity)
RhoA: central role in regulating cell shape and structure
Cdc42: regulates cell polarity and morphology
Both function as adaptor proteins and bind to a Zo: key role in cell migration to fill gaps in epithelial barrier
the LOSS of apical junctional complexes decreases epithelial membrane barrier integrity
- healthy barrier:
— mucus secretion (protective barrier)
— high expression of apical junctional complex components
— no space for lumen contents such as gluten to pass
- leaky barrier:
— more permeable
— reduced mucus secretion
— decreased junctional complex protein expression: contributes to CD severity BUT AFTER epithelial barrier is damaged
— lumen contents (gluten) can pass between epithelial bells and blood stream or lamina propria
Absorption of whole/large proteins in the small intestine
- intact protein absorption is limited in adults (not a major CD mechanism)
IN ENTEROCYTE
- majority (90%) of enterocyte intact protein uptake is degraded in lysosomes and processed protein fragments are released (10% of intact protein released across base lateral membrane)
IN M CELLS
M cells: embedded within the epithelial membrane and overlay Peyers patches, phagocytose a small amount of luminal protein
- 50% of those proteins degraded into smaller fragments and secreted across basolateral membrane
- 50% are released across basolateral membrane as intact
- immune cells (antigen presenting cells, APCs) will take up the protein and present it to a T cell (IMMUNE RESPONSE)
**Gut associated lymphoid tissue and immune cell populations
Antigen presenting cell (APC)
- two types of APCs are important in the first step of the CD response:
— dendritic cell (DC)
— macrophage (AND B CELLS? Slide 18)
Lymphocytes:
- T cells
- B cells
In peyers patch and the lamina propria
- enriched immune cell types (APCs, T cells, B cells) in close association to facilitate rapid activation of the immune response
M cells dispersed throughout the epithelial barrier that covers a Peyers Patch
STEP 1
Mechanism of antigen capture: How APCs capture luminal antigens (gluten) which will start immune response in CD
Step 1: antigen uptake by APC (PHAGOCYTOSIS/ induction of immune response)
M cell options (M cell mediates transcytosis):
1. Pass gluten directly to an APC
2. Gluten passes freely through the M cell and an APC in the lamina propria picks it up
Luminal capture by periscoping:
- APC goes between epithelial cells, grabs something from lumen and decides if its foreign or normal
Luminal capture through M-cell trans cellular pores
- normal antigenic surveillance
Translocation of small antigen via goblet cell
- lesser mechanism
STEP 2 part A
Interaction between T cell and APC
Presentation to T cell and T cell activation
- APC can be found in main entry sites of the body (GI tract, airway, skin)
- presentation of antigen to T cell is done through MHC expressed on the APC cell surface
-in the CD response, MHC Class II used to present the antigen (gluten) to a CD4+ T cell - form immunological synapse
Step 2 Part B
CD4+ T cell subsets
TH0: naive T cell, never encountered antigen before
Once activated, naive T cell differentiates into T helper subset (TH1) and proliferate to form MANY T cell clones (clinal expansion)
- has signature cytokines and transcription factors
Cytokines already secreted in the micro environment where a naive T cell is activated will direct which T helper subset is formed (IL-12 helps promote differentiation into TH1)
Activation of specific transcription factors directs development of each T cell subset
(Tbet directs naive T cells to develop into TH1 cells)
Each T cell subset secretes a signature cytokines (TH1 cells secrete interferon-y, IFN-y)
Treg (regulatory cell): a type of this subset that stops CD immune response
- suppress over reactive T cell response (TH1 and IFNy production)
- identified by expression of transcription factor (FOXP3) and secrete anti inflammatory cytokine (IL-10)
- Tregs function suppressed in CD
KNOW BRACKETS
Environmental component of CD
Gluten makes up proteins in wheat, barley, rye
Wheat: gliadins, glutenins
Barley: hordeins
Rye: secalins
All also referred to as prolamins and have high glutamine (Q) and proline (P) content
- the high proline content makes these proteins resistant to digestion
- the high glutamine content make these proteins excellent substrates for deamidation reactions by the enzyme tissue transglutaminase (TTG) (glutamine has an amide group in its side chain)
- result: presence of larger peptides with high glutamine and proline content in the small intestine which cant be absorbed by intestinal epithelial cells and INSTEAD:
— these larger peptides remain in the intestine where they stimulate an immune response and cause tissue damage
— CD patients have normal functioning protein digestion, they DO NOT digest gluten any differently
BONUS QUESTION :
What sub family do all the gluten proteins belong to
Pooideae
Pathophysiology of CD (genes)
Most important gene associated with CD is the human leukocyte antigen (HLA) cluster of genes which encode alpha and beta receptor chains of MHC class II receptors
- The DQ loci is most important within the HLA cluster; specifically HLA-DQ2 and HLA-DQ8 forms
- not everyone with these have CD but everyone with CD has these
Pathophysiology: role of tissue transglutaminase (TTG)
- TTG does post translational deamination on glutamine residues in gluten = negative charge
— negative charge creates higher affinity for the two HLA clusters: - HLA-DQ2 has high affinity for negatively charged residues at position 4,6, and 7 of its antigen binding groove
- HLA-DQ8 has high affinity for negatively charged residues at position 1 and 9 of antigen binding groove
- antigen binding groove has positive charges at each position where the AA of the antigen fits into the MHC molecule
- deamidation of glutamine residues in gluten proteins (gliadin) also increases T-cell activation (due to TTG)
Pathophysiology of CD
Phase One: uptake and processing of gluten peptides
- Gluten is incompletely digested into large peptides with high proline and glutamine content in small intestine
- TTG converts glutamine to glutamate
— it deamidates glutamine residues, increasing gluten peptides binding affinity to HLA-DQ2 and DQ8 - Large peptides leak across epithelial barrier via multiple mechanisms:
- APC mediated luminal capture of gluten peptides (periscoping or through M cell trans cellular pores)
- leaky or damaged membrane (after damage to epithelial barrier)
- M cell transcytosis
- epithelial cell luminal uptake of antigen and presentation at basolateral membrane
Pathophysiology of CD
Phase Two: Presentationn of gluten peptides to CD4+ cells
- Naive CD4+ T cells (T0) encounter gluten antigen for the first time (presented by APC)
- T cell differentiates into a TH1 cell (T helper 1) which increases expression of the transcription factor Tbet
- the T cell will then proliferate producing MANY TH1 clones (clinal expansion) - All activated TH1 clones produce and secrete interferon-y (IFNy)
- The cascade of cytokine production that follows induces tissue damage..
Pathophysiology of CD
Phase 2/3: T cell activation and tissue damage
(After colonal expansion)
Part 1
- IFNy triggers MPP activation (MMP-12, MMP-13)
- promotes epithelial cell apoptosis, ECM degradation, lamina propria degradation, loss of epithelial barrier integrity, villi atrophy - IFNy activates APCs in the lamina propria and caused them to secrete TNFa
- TNFa stimulates MMP-1 and MMP-3 which cause further tissue damage and epithelial apoptosis - IL-21 produced by TH1 cell (and by APCs, damaged epithelial cells) further promote the TH1 response and activates IELs:
- IEL (intraepithelial lymphocyte, typically CD8+ T cells and natural killer cells (NK cells)) further promotes epithelial cell apoptosis and loss of barrier integrity
- (Treg would normally stop this but its not working) - IL-21 amplifies the effects of IL-15
- IL-15 is produced by APCs and promotes epithelial cell apoptosis by stimulating epithelial cell expression of MIC-A - IL-21 also activates B cells
- produce antibodies that support further tissue damage
Pathophysiology of CD
Phase 2/3: T cell activation and tissue damage
(After colonal expansion)
Part 2: MIC proteins
IL-15 induced epithelial cells to express MIC proteins (MIC-A)
MIC-A is the ligand for the receptor NKG2D on the surface of the IEL (NK or CD8+ T cell) embedded in the epithelial barrier
- binding leads to IEL activation and proliferation
Activated IELs mediate epithelial cell apoptosis by two mechanisms that cause tissue damage and barrier permeability:
- Binding of FasLigand (IEL surface) to FAs (epithelial cell surface) cell contact dependent
- Releasing granules containing perforin and granzymes produced by the IELs cell contact independent
- perforin pokes holes in epithelial cell plasma membrane
- granzymes activate caspase-3 in epithelial cell
Pathophysiology of CD
Phase 4 (end result): tissue damage
And review of the entire process
wrote most of this myself
- T cell activated
- TH1 clones all secreting IFNy
- IFNy activates MMPs (12,13) which degrade barrier
- APCs secrete TNF-a which activates more MMPs (1,3)
- IL-21 amplifies IL-15 effects which stimulates intestinal epithelial cell to express MIC-A
- MIC-A ligates with NKG2D on IEL surface
- IEL activated and proliferates (100s more created)
- IELs cause epithelial cell apoptosis via the 2 mechanisms discussed
- IL-21 activates B-cells which make antibodies that attack small intestine (TO BE CONTINUED IN NEXT DECK)
