Intestinal Ecosystem & Immunology Flashcards

Question

What are features of the BARRIER of innate defense of the mucosal immune system?

Answer 1

1. Glycocalyx → Goblet cells produce mucous to create a thick barrier that covers the GI epithelium and prevents easy access - Pathogens become trapped in the mucous and are expelled via peristalsis. - Mucous also acts as a reservoir for secretory IgA. 2. Epithelial Cell Tight Junctions → prevent the passages of macromolecules through the single epithelial layer

Answer 2

Protein that regulates intestinal permeability by modulating tight junctions/inducing tight junction disassembly - Impacted in various diseases such as celiac disease - Increased zonulin levels → loosening of tight junctions (more permeable tight junctions) - Structural and functional homology with toxin secreted by Vibrio cholerae → loosens tight junctions

Answer 3

- Enzymes in the stomach (pepsin) and small bowel (trypsin, chymotrypsin, pancreatic proteases). - Break down large polypeptides into di-peptides and tri-peptides. - Peptides < 8-10 AA are poor immunogens. - Enzymes cytotoxic to pathogens. **T cell epitopes need specific lengths of peptides to be presented on MHC. If the nutrients are broken down fast enough and short enough, they are not antigenic/immunogenic anymore (too short)

Answer 4

1. Lactoferrin → binds iron and inhibits bacterial growth 2. Lysozyme → cleaves cell wall of gram positive bacteria 3. Defensins → 30-40 aa peptides that disrupts the cell membranes of bacteria and fungi causing lysis (punch wholes into bacteria → causes them to burst/cytosolic release

Answer 5

- >400 species of commensal bacteria - Provide enzymatic break down of food - Competes with pathogenic bacteria for space and nutrients - Prevents colonization of the gut - Antibiotics disrupt homeostasis **When taking antibiotics, helps to clear the pathogen, but also clears commensal bacteria which are good for gut health (not great)

Answer 6

1. Unique epithelium for antigen uptake 2. Unique lymphocyte repertoire 3. IgA dominated humoral response 4. A need to minimize injury to the mucosal tissue while providing protection

Answer 7

- Transport antigens across the mucosal surface → via vesicles « Trancytosis » - M cells take up antigen by endocytosis and phagocytosis - Extensively folded cell membrane - large surface area - Can be exploited by pathogens to gain entry into Creates a « pocket » for DCs, lymphocytes Ag uptake on basolateral side → DC’s migrate to T cell areas in PP or mesenteric LNs

Answer 8

Luminal antigens are taken up by M cells and presented to T cells by macrophages - DCs can reach (extend processes across epithelaial layer) through and direclty sample Ag from gut lumen **Unique mechanism

Answer 9

The dominant class of Ab in the mucosal immune system - Can prevent pathogen adherence to epithelium → block the adhesion sites before they bind to the receptors on epithelial cells to get internalized Can exist in multiple forms: - Serum-IgA is mostly monomer - Mucosa → IgA is dimer linked by J chain (joining peptide chain) - After transcytosis, associated with secretory component B cells (in ILF and PP) have to undergo class switching to be able to produce IgA, B cells in different regions release dimers/monomers → mediated by TGF-b !!! Certain pathogens can cleave IgA as a defense mechanism → cleave in the hinge region (ex: influenza, pneumonia, meningitidis, gonorrhoeae, etc.)

Answer 10

Mucosal tissue → 80% IgA, 13% IgM, 6% IgG Systemic tissues (BM, LN, spleen) → 52% IgG, 30% IgA, 17% IgM IgG daily total production rate → 3000 mg (up to 5 grams) IgA daily total production rate → 4800 - 9000 mg

Answer 11

1. IgA released as dimer in Lamina Propria binds to the Poly-Ig receptor on the basolateral surface of epithelial cells 2. Internalized → vesicular transport (trancytosis) 3. Release of IgA dimer at the apical face of epithelial cell (IgA dimer bound to secretory component) - When tethered to « secretory component » = sIgA

Answer 12

IgA-secreting B cells express a4b7, which binds to MAdCAM-1 and directs them to mucosal tissues

Answer 13

Chemokine - Receptor (on lymphocytes) CCL25 - CCR9 → small intestinal epithelium and endothelium CCL28 - CCR10 → Salivary glands, trachea, bronchi, mammary glands, stomach, large intestine, SI, BM CCL20 - CCR6 → Peyer’s patch subepithelial some and inflamed epithelium

Answer 14

1. T cells enter Peyer’s patches from blood vessels, directed by homing receptors CCR7 and L-selectin 2. T cells in the Peyer’s patch encounter antigen tranposrted acress m cells and become activated by DCs 3. Activated T cells drain via mesenteric lymph nodes to the thoracic duct and return to the gut via the bloodstream 4. Activated T cells expressing a4b7 integrin and CCR9 home to the lamina propria and intestinal epithelium of small intestine

Answer 15

Antigen presentation in Peyer’s patches → Mesenteric LN → (Thoracic Duct) → Blood Stream → Other mucosal organs (Resp. tract, Breast, Intestinal mucosa, Urinary tract, Salivary/Lacrimal gland) Inductive Site = Afferent Limb, Peyer’s Patch (Antigen entry) Effector Site = Efferent Limb, Lamina Propria Lymphocytes and Plasma Cells (Other immunized areas)

Answer 16

Innate T lymphocytes: - Intraepithelial → limited TCR repertoire, Ag-specific - Innate lymphoid cells → no TCR, lineage negative, no Ag-specificity Adaptative T lymphocytes: (MHC-restricted) - Mostly CD4+ T helper cells; few CD8+ T cells - Th1, Th2, Th17/22, Th9, Treg (LOTS), Tfh - Ag-specific

Answer 17

1. Strategically located to respond to antigenic stimulation 2. Mostly CD8+ T cells 3. Mainly ab TCR in humans // yg TCR in mice 4. Intercalated in between epithelial cells above peyer's patches Features of CD8+ T cells: - Limited repertoire of TCR → marked difference compared to peripheral T cells - Recognize a limited # of antigens - Prevents indiscriminate inflammation - Recognition of self-stress antigens (MIC-A,MIC-B) → T-cells induce apoptosis of injured epithelial cells *Hard to study because it's a rare population

Answer 18

Cell next to intraepithelial lymphocytes is not functional anymore → induces expression of MIC-A/MIC-B → recognized by IEL CD8+ T cells which induce apoptosis of the non-functional neighbouring cell (via perforin/granzyme pathway) NKG2D on IELs bind to MIC-A/-B and activate IEL

Answer 19

Marker of memory phenotype of mucosal T cells/IEL Gut homing markers → CCR6, CCR9, a4:b7 integrins

Answer 20

IEL → mostly CD8+ T cells Adaptative T lymphocyte → CD4+ T cells

Answer 21

- T-cells are predominantely CD4+ (95% CD45RO+) - Limited capacity to proliferate - Weak proliferative responses to mitogens or specific antigens - Still act as helpers for B-cells

Answer 22

Type 1 - Intracellular pathogens (bacteria, parasites, viruses) Th1 → secrete IFNy, TNFa Type 2 - Worms, allergens Th2 → secrete IL-5, IL-9, IL-13, IL-4 (humans) Type 3 - Extracellular pathogens (bacteria, parasites, fungi) Th17/22 → IL-17, IL-22 Follicular helper T cells (Tfh) Tregs

Answer 23

IL-12, Type1 IFN, TNFa → T-bet → Th1 TSLP, IL-25, IL33 → GATA3 → Th2 IL-1, IL-6, IL-23, TGFb → RORyt → Th17/22 The cytokines that induce TF are produced DCs

Answer 24

Severe enteropathy caused by mutations in the Foxp3 gene that result in the defective development of CD4+CD25+ regulatory T cells *CD25 = alpah chain of IL-2 receptor

Answer 25

TGF-b *Tregs respond to TGF-b and can produce it Transfer of Tregs into mice with IBD can lead to resolution of colitis (lack of Tregs → colitis/inflammation in the gut)

Answer 26

IL-10 - Increased production of IgA - Decreased cytokine production by DC, T-cells, macrophages - Promotes TH2 response - IL-10 knockout mice: severe enterocolitis TGF-beta - Increased IgA by promoting class switching - Maintain functional CD4+CD25+cells in the periphery. IL-10 and TGF-beta (TGFb) are involved in regulating B cell class switching and IgA production, with IL-10 inducing TGFb secretion and TGFb receptor upregulation, ultimately leading to increased IgA secretion

Answer 27

Innate Lymphoid cells (ILC) Visualized with reporter mice (fluorescent protein fused to specific signature cytokine promotors to visualize the cells that produce them)

Answer 28

General Features: - Population induced at steady state & inflammation/infection states - Lymphocyte morphology - Common developmental origin - common lymphoid progenitor (CLP) ? - Lack of rearranged antigen receptors - Important sources of Th cell signature cytokines (IFNg, IL-5/IL-9/IL-13, IL-17A/IL22) *Specific to type of immune response* Function: - Innate immunity against microbes, immune surveillance (tumors) - Lymphoid tissue formation, remodeling and repair - Epithelia/Stroma function, homeostasis, remodeling and repair - Strong immunopathology - Many aspects unclear!! - Often in close contact with epithelial layer - Dysregulation leads to chronic inflammatory diseases Regulation: - Induced by the same/overlapping cytokines as T helper cells - No MHC restriction (?) Co-stimulation (?)

Answer 29

Type 1 → ILC1 (includes NK) Type 2 → ILC2 Type 3 → ILC3 (also includes LTi cells) *Other subsets as well inside these 3 groups

Answer 30

Activated ILC2s, a type of innate lymphoid cell, produce cytokines like IL-4, IL-5 and IL-13, which promote type 2 inflammation, eosinophilia, mucus hypersecretion, and tissue remodeling, contributing to conditions like asthma and allergic diseases. - Muscle contraction (IL-13) - IL-13 → Goblet cell activation → mucus hypersecretion - IL-4 → Th2 cell differentiation - IL-5 → eosinophil chemoattractant

Answer 31

Response to extracellular pathogens (bacterial infection) → ex: Helicobacter, Citrobacter, E. coli → trapped by DCs → secrete IL-23 to activate ILC3s NCR- ILC3s → secrete IL-17 NCR+ iLC3s → secrete IL-22 Associated with neutrophil chemoattraction → professional phagocytes IL-22 → production of antimicrobial peptides and proteins by epithelial cells

Answer 32

A subtype of Innate Lymphoid cells LTi (lymphoid tissue inducer) cells → essential for the development of lymph nodes and Peyer's patches during embryonic development, and they are also involved in maintaining lymphoid tissue architecture and promoting innate immunity. LTi cells constitutively produce lymphotoxin (LTα1β1) and tumor necrosis factor (TNF) → binds to LTbR on stromal cells which → stimulate mesenchymal cells to produce chemokines and adhesion molecules essential for lymphoid organogenesis Due to chemokine production, they also attract naive B and T cells

Answer 33

Type I → covered by simple epithelium - expresses a simple polymeric Ig receptor (pIgR) that allows dimeric IgA to access the lumen - Intestine, lungs, uterus Type II → covered by stratified squamous epithelium which provides physical protective barriers for activities that are important for the host species - Oral cavity, Vaginal cavity

Answer 34

1. Mechanism = Antigen Sampling 2. PP locations (ILF, cryptopatches) 3. Gut is the organ with the most lymphocytes in the whole body 4. IgA is the dominant Ab (requires class switching of B cells by TGF-b + transport through epithelial layer by PIgR) 5. Signature cytokines (Th, ILC)

Answer 35

*More bacteria/microbes than cells in our body Commensal = microbes in our body, but do not treaten us 1. Prevent colonization by more pathogenic species → compete for space and nutrients 2. Produce metabolites (ex: vitamins) that are used by the host 3. Colonization of the gut begins immediately after birth – 10^13-10^14 microorganisms – 400 to 500 different species – Majority are obligate anaerobes → work in anaerobic conditions 4. Negative effects on normal bacterial flora may explain the rise of immune disorders (allergies and IBD) → antibiotics may help these patients *We still need protection/barrier because if we put them in the blood, we would dye from sepsis due to huge inflammatory cascade

Answer 36

*The gut has adapted to the presence of these commensal microbes → evolved to utilize them 1. Formation of anatomical structures (Peyer's patches) 2. Expansion of germinla center reactions involving B and T cells 3. Increased IgA production by intestinal B cells 4. Expansion of IEL populations 5. Bacterial antagonism 6. Maintain GI tract peristalsis and intestinal mucosal integrity 7. Convert dietary precarcinogens and carcinogens to noncarcinogens 8. Synthesis of vitamin K and vitamin B complexes **However, translocating bacteria can cause infections in debilitated patients

Answer 37

They often relate to the metabolites produced by the microbiota more than the actual composition of the microbiota → which comes down to what we eat Ex: Cancer, obesity and metabolic syndrom, IBD, cardiovascular disease

Answer 38

Germ-free: - Tinner/longer finger-like villi - Lower number of cells - Less IgA - Decreased mucus thickness / altered mucus properties - Less vascularized