L9 - Immunology of the gut mucosa 1 - Dr Jamie Mann

Question 1

What is the primary function of the mucosal immune system?

Answer 1

The mucosal immune system protects internal epithelial surfaces (e.g., gastrointestinal, respiratory, and urogenital tracts) from infection by pathogens such as bacteria, viruses, fungi, and parasites.

Question 2

Which anatomical sites are considered part of the mucosal immune system?

Answer 2

The gastrointestinal tract, respiratory tract, urogenital tract, associated lymph nodes, and mammary glands.

Question 3

Why is the mucosal immune system crucial for survival?

Answer 3

These surfaces are constantly exposed to pathogens. Without an effective mucosal immune response, harmful microbes could establish infections, replicate, and spread throughout the body.

Question 4

How does the skin compare to mucosal surfaces in immune protection?

Answer 4

The skin is a strong physical barrier preventing pathogen entry. In contrast, mucosal surfaces are more vulnerable as they are exposed to external environments and require specialized immune protection.

Question 5

What is the historical evolutionary link between mammalian skin and mucosal surfaces?

Answer 5

Millions of years ago, mammalian skin was originally mucosal tissue that secreted mucus. This adaptation was crucial for early marine organisms in protecting against pathogens in the ocean.

Question 6

What is the role of mucus in immune defense?

Answer 6

Mucus acts as a physical barrier, trapping pathogens and preventing them from reaching epithelial cells. It also contains antimicrobial molecules that neutralize threats.

Question 7

How do hagfish use mucus as a defense mechanism?

Answer 7

Hagfish secrete large amounts of mucus when threatened. This mucus clogs the gills of predators, effectively suffocating them and preventing predation.

Question 8

What challenges do mucosal sites in the respiratory tract protect us from

Answer 8

• Particulates
• Pollutants
• Allergens
• Airborne pathogens

Question 9

What challenges do mucosal sites in the gastrointestinal tract protect us from

Answer 9

• Food
• Commensals
• Ingested pathogens

Question 10

What challenges do mucosal sites in the urogenital tract protect us from

Answer 10

Introduced pathogens
- Commensals
- Sperm `

Question 11

What are the key functions of the mucosal immune system beyond pathogen defense?

Answer 11

It selectively responds to harmful pathogens but tolerates commensals, food antigens, and sperm to prevent unnecessary immune activation and inflammation.

Question 12

How does the mucosal immune system regulate commensal microbes?

Answer 12

It maintains a symbiotic relationship with commensals, allowing them to exist without triggering inflammation, while still being capable of responding to pathogenic microbes.

Question 13

Do food proteins always get broken down into amino acids before absorption?

Answer 13

No, while most dietary proteins are broken into amino acids, small amounts of intact proteins can enter the bloodstream.

Question 14

Why doesn’t the immune system usually attack dietary proteins?

Answer 14

The mucosal immune system induces oral tolerance, a process that prevents an immune response against harmless food antigens, reducing the risk of food allergies.

Question 15

What happens when oral tolerance fails?

Answer 15

Failure in oral tolerance can lead to food allergies, where the immune system mistakenly recognizes food proteins as harmful and triggers an inappropriate immune response (e.g., peanut allergy).

Question 16

What do nutritionists and physiologists typically say about dietary protein absorption? ( and what is the however)

Answer 16

They often state that dietary proteins are fully broken down into polypeptides, then amino acids, which are absorbed by the gastrointestinal tract—meaning no intact proteins enter the bloodstream.

🔹 However, this is not entirely true!

✅ Small amounts of intact dietary proteins can cross the gut barrier and enter the bloodstream, but in most people, they do not trigger an immune response due to oral tolerance mechanisms.

Question 17

What key experiment demonstrates oral tolerance?

Answer 17

Mice are fed an antigen (ovalbumin, OVA) and later immunized with the same antigen.
🔹 Group 1 (fed OVA) → Weakened immune response to OVA after immunization.
🔹 Group 2 (not fed OVA, control) → Strong immune response after immunization.

Antigen: Ovalbumin (OVA) is a protein found in egg white and is being used as a model antigen in this experiment.
Feeding OVA (Group 1): This step introduces the antigen to the immune system via the gut. The gut-associated lymphoid tissue (GALT) plays a crucial role in processing orally administered antigens. Under normal circumstances, the GALT promotes tolerance to these antigens rather than initiating a strong immune response.
Immunization with OVA (Both Groups): Immunization is a process designed to elicit a strong, protective immune response against a specific antigen. It typically involves injecting the antigen along with an adjuvant (a substance that enhances the immune response).
Group 1 Result (Weakened Immune Response): Because Group 1 was previously exposed to OVA through feeding, their immune system has, to some extent, become tolerant to it. When they are later immunized with OVA (a process that usually triggers a strong response), their response is weaker compared to the control group.
Group 2 Result (Strong Immune Response): Group 2, which was not previously exposed to OVA, has a naive immune system with respect to this antigen. Therefore, the immunization effectively triggers a strong immune response, as expected.

Question 18

What is oral tolerance

Answer 18

a type of peripheral tolerance where the immune system becomes less responsive to antigens e.g. food proteins ( the body then recognises that these ingested substances are harmless –> no strong immune response against them is mounted)

Question 19

Step-by-Step Breakdown of the oral tolerance mouse experiment :

Answer 19

1️⃣ Feeding Phase (Day 0):
Experimental Group: Mice are fed ovalbumin (OVA), a model antigen.
Control Group: Mice receive a control solution (PBS) instead of OVA.

2️⃣ Immunization Phase (~Day 14):
All mice are injected with OVA + adjuvant to induce an immune response.
The adjuvant enhances immune activation, mimicking a real infection.

3️⃣ Measurement of Immune Response:
Researchers assess T-cell-mediated immunity using a delayed-type hypersensitivity (DTH) test (footpad swelling).
Serum antibody levels (IgA) are measured to evaluate humoral immune responses.

Question 20

What does this experiment prove?

Answer 20

Prior exposure to an antigen via the gut can induce immune tolerance, preventing a strong immune response upon later exposure.

Question 21

What happens if an animal is fed one antigen but immunized with a different antigen?

Answer 21

The immune system still responds to the new antigen, meaning tolerance is antigen-specific.

Question 22

What immune responses are measured in this experiment

Answer 22

• T-cell response (delayed-type hypersensitivity, through a footpad swelling test)
• Serum antibody levels

Question 23

Why does prior feeding of an antigen reduce immune responses?

Answer 23

The mucosal immune system actively suppresses immune responses to prevent unnecessary inflammation against harmless food proteins.

Question 24

What was the purpose of the mouse tolerance induction experiment?

Answer 24

To demonstrate that prior oral exposure to an antigen (OVA) reduces the immune response upon later systemic immunization.

Question 25

Why were different strains of mice (CBA, C3H, A) used in the study?

Answer 25

To show that oral tolerance is not strain-specific and occurs across different genetic backgrounds.

Question 26

What were the two groups of mice in the tolerance induction experiment?

Answer 26

🔹PBS-fed group (control): Received sham feeding (PBS) before immunization. 🔹 OVA-fed group (experimental): Fed ovalbumin (OVA) before immunization.

Question 27

What was measured after immunization in the tolerance induction experiment?

Answer 27

Serum anti-OVA antibody titers to assess the strength of the immune response.

Question 28

What were the key results of the tolerance induction experiment?

Answer 28

🔹PBS-fed mice: High anti-OVA antibody levels (strong immune response). 🔹 OVA-fed mice: Lower anti-OVA antibody levels (indicating tolerance induction). 🔹 All three mouse strains showed oral tolerance, but with variation in response levels.

Question 29

What does this tolerance induction experiment prove about oral tolerance?

Answer 29

Oral exposure to an antigen before systemic exposure can suppress the immune response, preventing excessive immune activation.

Question 30

How does this tolerance induction experiment relate to food allergies?

Answer 30

Failures in oral tolerance mechanisms may contribute to food allergies, where the immune system incorrectly mounts a response to dietary antigens.

Question 31

Is oral tolerance only observed in mice?

Answer 31

No, it has been observed in other species, including pigs, where prior feeding of an antigen (e.g., soya protein) leads to a reduced antibody response upon later immunization.

Question 32

What experiment demonstrated oral tolerance in pigs?

Answer 32

Pigs were fed soya protein (soya based diets at 3 weeks old) before being immunized with soya protein - 🔹 PBS-fed pigs (control) → Developed a strong anti-soya antibody response. 🔹 Soya-fed pigs → Showed a reduced immune response, indicating tolerance induction.

Question 33

Study: Weaning Piglets onto Soya-Based Diets at 3 Weeks Old ✅ What were the key findings and what does this indicate ?

Answer 33

1️⃣ Antibody responses were generated to food antigens (soya) after initial (prime) feeding. 2️⃣ Antibody response at weaning (green) was comparable to an injected protein antigen. 3️⃣ The antibody response in the weaning group was not boosted upon re-exposure, unlike the naïve or injected groups. 🔹 Oral exposure to soya at weaning induces a regulated immune response. 🔹 Oral tolerance mechanisms prevent excessive immune activation upon re-exposure

Question 34

How does the historical example of Mithridates relate to oral tolerance?

Answer 34

Mithridates of Pontus (100 BC) reportedly consumed small doses of poison daily to build resistance. 🔹 While not true oral tolerance, this concept resembles the principle of gradual immune adaptation to antigens.

Question 35

Why is the historical example relevant to immunology?

Answer 35

It illustrates early human attempts to induce tolerance, similar to how the immune system adapts to harmless dietary antigens.

Question 36

What is the mucosal immune system vs the systemic immune system

Answer 36

the mucosal immune system is a specialised branch of the immune system that protects mucosal surfaces exposed to the external environment whilst the systemic immune system involves a network of organ tissues and cells that protect internal, non mucosal tissues from pathogens

Question 37

Which antibody plays a dominant role in mucosal immunity, and what is its function?

Answer 37

Secretory IgA (sIgA) provides the first line of defense by neutralizing pathogens and toxins while preventing an overactive immune response to harmless antigens.

Question 38

what does systemic immunity rely on

Answer 38

IgG, circulating immune cells, cytokines and antibodies : primarily IgG to detect and eliminate infections throughout the body's internal compartments

Question 39

List three key differences between mucosal and systemic immunity.

Answer 39

1️⃣ Mucosal immunity protects barrier surfaces, while systemic immunity protects internal organs and tissues. 2️⃣ Secretory IgA (sIgA) dominates mucosal immunity, whereas IgG dominates systemic immunity. 3️⃣ Mucosal immunity is tolerogenic to non-harmful antigens (e.g., food, commensals), whereas systemic immunity responds aggressively to eliminate pathogens.

Question 40

which organs, tissues and cells make up the systemic immune system

Answer 40

primary lymphoid organs like bone marrow and the thymus and secondary lymphoid organs such as the sleen and peripheral lymph nodes

Question 41

what is the lactating breast considered to be a part of

Answer 41

the mucosal immune system

Question 42

besides the gastrointestinal, respiratory and urogential ( as well as lactating breast) what other parts of the body are considered to be apart of the mucosal immune system?

Answer 42

The oral cavity, pharynx, middle ear, and the glands associated with these tissues, such as the salivary glands and lacrimal glands.

Question 43

What are the four main types of barriers in the mucosal immune system?

Answer 43

Physical, chemical, innate immune cell and microbial barriers that work together to protect mucosal surfaces from infections.

Question 44

What makes up the mucus layer ( physical barrier - innate immunity) and what is their function

Answer 44

glycoporteins aka mucins which form a gel-like layer that traps pathogens, prevents adhesion, and facilitates their removal.

Question 45

What is the function of epithelial tight junctions in mucosal immunity ( physical barrier - innate immunity)

Answer 45

They form a physical seal between epithelial cells, preventing microbial entry into tissues.

Question 46

What is the function of cilia in the respiratory tract (physical barrier - innate immunity) ?

Answer 46

Cilia propel mucus and trapped microbes outward, preventing them from reaching the lungs.

Question 47

How Does pH Act as a Chemical Barrier? (innate immunity)

Answer 47

The acidic environment of the stomach and vagina inhibits microbial growth, preventing infections.

Question 48

Name Two Antimicrobial Peptides (AMPs) that act as chemical barriers in Mucosal Immunity (innate immunity)

Answer 48

Defensins and cathelicidins which disrupt microbial membranes, killing bacteria and viruses.

Question 49

Defensins and cathelicidins **** (produced by, cationic nature and immune modulation) (innate immunity)

Answer 49

These peptides are primarily produced by epithelial cells lining various body surfaces like the skin, respiratory tract, gastrointestinal tract, and urogenital tract, creating a protective barrier at these entry points. Cationic nature: They are positively charged, which allows them to interact with the negatively charged bacterial membranes, disrupting their structure and leading to cell death. Immune modulation: Beyond direct antimicrobial activity, defensins and cathelicidins can also modulate the immune response by attracting immune cells to the site of infection and promoting wound healing.

Question 50

What Is the Role of Lysozyme in Mucosal chemical Defense?

Answer 50

Lysozyme, found in tears, saliva, and mucus, breaks down bacterial cell walls, preventing infection.

Question 51

What is the function of lactoferrin in mucosal chemical immunity? (innate immunity)

Answer 51

It sequesters iron, depriving bacteria of an essential nutrient and inhibiting their growth.

Question 52

What are two ways that resident microbiota help prevent infections? (innate immunity)

Answer 52

1️⃣ Compete with pathogens for nutrients and attachment sites. 2️⃣ Produce metabolites (e.g., short-chain fatty acids) that enhance epithelial integrity.

Question 53

What are four key innate immune cells involved in mucosal immunity?

Answer 53

1️⃣ Macrophages – Engulf pathogens and regulate inflammation. 2️⃣ Dendritic Cells – Sample antigens and initiate adaptive immune responses. 3️⃣ Neutrophils – Rapidly recruited to infection sites to clear pathogens. 4️⃣ Innat lymphoid cells (ILCs) - Early cytokine responses for immunity and tissue repair

Question 54

What is MALT, and what is its role in mucosal immunity?

Answer 54

MALT (Mucosa-Associated Lymphoid Tissue) is a collection of immune tissues in mucosal surfaces that contain B cells, T cells, and antigen-presenting cells to help initiate immune responses and maintain tolerance.

Question 55

What are the different subtypes of MALT based on location?

Answer 55

1️⃣ NALT – Nasal-Associated Lymphoid Tissue (upper respiratory tract) e.g. tonsils 2️⃣ BALT – Bronchial-Associated Lymphoid Tissue (lungs) 3️⃣ GALT – Gastrointestinal-Associated Lymphoid Tissue (intestines) e.g. peyers patches ( Mesenteric Lymph Nodes – Drain and support immune responses in the gut)

Question 56

what are examples of regional lymph nodes

Answer 56

Retropharyngeal, submaxillary and parotid LN Mediastinal LN Mesenteric LN

Question 57

What is a Peyer’s patch, and why is it important?

Answer 57

Peyer’s patches are large lymphoid aggregates found in the small intestine. They contain B cells and T cells and play a key role in initiating immune responses to gut antigens.

Question 58

How does HIV affect mucosal immunity, particularly in the gut?

Answer 58

HIV depletes CD4 T cells, leading to destruction of lymphoid structures like Peyer’s patches, leaving the host vulnerable to infections.

Question 59

What are isolated lymphoid follicles, and how do they differ from Peyer’s patches?

Answer 59

Isolated lymphoid follicles are smaller, dispersed lymphoid aggregates in the intestine that contain B and T cells, but they are generally smaller than Peyer’s patches. Exist in a range of sizes, with the larger displaying germinal center characteristics

Question 60

What are cryptopatches, and how do they function in mucosal immunity?

Answer 60

Cryptopatches are tiny lymphoid aggregates in the gut that serve as sites for early immune cell development and may contribute to immune responses. Considered to develop into single B-cell follicles. Contain mainly innate lymphoid cells (ILCs) – see slide 14.

Question 61

What Is the Role of Mesenteric Lymph Nodes in mucosal immunity ?

Answer 61

Mesenteric lymph nodes are large immune structures that help drain lymph from the intestines and coordinate immune responses to gut antigens.

Question 62

What are the two main cells involved in the adaptive mucosal immunity

Answer 62

1️⃣T-cells All the usual suspects – Th1, Th2, Th17, Treg 2️⃣B-cells Lots of IgA plasma cells, some IgM, and some IgG secreting cells.

Question 63

What Is Mucus?

Answer 63

Mucus is a gel-like secretion on mucosal surfaces made up of glycoproteins, water, lipids, and antimicrobial peptides. It is released by goblet cells. Its main function is to protect epithelial surfaces by trapping microbes and preventing direct contact with the epithelium.

Question 64

What are goblet cells, and what do they secrete?

Answer 64

Goblet cells are specialized epithelial cells that secrete mucin glycoproteins, water, lipids, DNA, and antimicrobial peptides to maintain the mucus layer and protect mucosal surfaces.

Question 65

How many types of mucin glycoproteins does the body produce?

Answer 65

The body produces 21 different types of mucin glycoproteins.

Question 66

How does mucus act as a protective barrier in the gastrointestinal tract?

Answer 66

Mucus forms a physical barrier that prevents microbes from directly contacting epithelial cells. It also traps bacteria, preventing them from invading the gut lining.

Question 67

What happens when the mucus barrier is compromised, such as in colitis

Answer 67

In conditions like colitis, the mucus layer becomes thinner, allowing bacteria to come into direct contact with the epithelial surface, leading to inflammation and damage.

Question 68

Which antibody predominates in mucosal secretions, and which predominates in the systemic compartment?

Answer 68

IgA predominates in mucosal secretions, while IgG is the dominant antibody in the systemic (blood) compartment.

Question 69

% proportion of immunoglobulins (IgM, IgG, IgD and IgA) in the systemic compartment

Answer 69

IgM = 17% IgG = 52% IgD = 1% IgA = 30%

Question 70

% proportion of immunoglobulins (IgM, IgG, IgD and IgA) in mucosal secretions

Answer 70

IgM = 13% IgG = 6% igD = 1% IgA = 80%

Question 71

How does IgA differ in the systemic compartment versus mucosal surfaces?

Answer 71

- Systemic IgA is mostly monomeric, with only 1-5% in polymeric form. - Mucosal IgA is predominantly in their polymeric form, with a smaller fraction as monomeric IgA.

Question 72

What are the key functions of IgA in mucosal immunity?

Answer 72

- Neutralization – Binds to pathogens and toxins to prevent infection. - Secretion into the Lumen – Actively transported into mucosal secretions where -pathogens reside. - Pathogen Removal – Can bind to invading pathogens inside the body and transport them back out via receptor-mediated translocation.

Question 73

what are IgA antibodies produced in response to in mucosal tissues

Answer 73

IgA antibodies are produced in response to the specific commensal bacteria present in a particular mucosal environment. This selective response helps shape the composition of the microbiota and maintain a balanced relationship between the host and commensals.

Question 74

what does IgA prevent

Answer 74

prevents commensal bacteria from entering mucosal tissues by binding to these microorganisms in the mucosal lumen.

Question 75

how can IgA neutralise potential threats

Answer 75

IgA can neutralize potential threats by binding to surface structures or toxins of commensal bacteria

Question 76

Why is IgA used as a marker for mucosal vaccine efficacy?

Answer 76

Since IgA is the primary mucosal antibody, its induction indicates a strong mucosal immune response, making it an important marker in mucosal vaccine development.

Question 77

What are the major cytokines involved in the differentiation of naïve T cells (Th0) into specific T cell subsets (Th2, Th1, Th17 and Treg)

Answer 77

IL-4 → Th2 IL-12 → Th1 TGF-β & IL-6 → Th1`7 TGF-β → Treg

Question 78

What is the role of Th1 cells in mucosal immunity?

Answer 78

Transcription factor: T-bet Cytokines secreted: IFN-γ Target: Intracellular pathogens (e.g., viruses, intracellular bacteria)

Question 79

What is the function of Th2 cells?

Answer 79

Transcription factor: GATA-3 Cytokines secreted: IL-4, IL-5, IL-13 Target: Parasites (e.g., helminths)

Question 80

What do Th17 cells do in mucosal immunity?

Answer 80

Transcription factor: RORγt Cytokines secreted: IL-17A/F Target: Extracellular bacteria (e.g., PMNs, neutrophils)

Question 81

What is the function of regulatory T cells (Tregs)?

Answer 81

Transcription factor: FoxP3 Cytokines secreted: IL-10, TGF-β Function: Suppress excessive immune responses to maintain tolerance

Question 82

What are antimicrobial peptides, and how do they function?

Answer 82

small cationic peptides less than 100 aa in size produce by epithelial cells and immune cells e.g. Paneth cells in crypts. They play a crucial role in mucosal immunity by punching holes in pathogen membranes, leading to pathogen destruction

Question 83

what are two examples of antimicrobial peptides

Answer 83

Defensins & Cathelicidins

Question 84

Defensins :

Answer 84

Small cysteine-rich cationic peptides that disrupt microbial membranes and modulate immune responses, classified into α- and β-defensins based on structure and expression patterns. Produced by Paneth cells.

Question 85

Cathelicidins

Answer 85

Linear antimicrobial peptides, exemplified by LL-37, that disrupt microbial membranes and regulate inflammation and immune signaling. Produced by enterocytes.

Question 86

What is the function of M cells in mucosal immunity?

Answer 86

M cells, located in Peyer's patches of the intestine, sample antigens from the external environment and transport them to immune cells like dendritic cells and T cells. This process is vital for initiating and modulating immune responses, helping the body recognize pathogens while maintaining tolerance to harmless substances.

Question 87

How do M cells help transport antigens?

Answer 87

`The minimal gap between the M cell pocket and the external environment facilitates efficient transport of antigens, organisms, and proteins into the body. This process aids dendritic cells and T cells in recognizing pathogens and initiating immune responses.

Question 88

How are M cells distinguished from neighbouring enterocytes in the FAE

Answer 88

M cells can be distinguished from neighboring enterocytes in the FAE by architectural features (short and blunted microvilli and an intraepithelial pocket housing mononuclear phagocytes and lymphocytes),

Question 89

M cell structure

Answer 89

- Apical microfolds These are poorly organized villi that lack the uniform microvilli of other intestinal epithelial cells. - Thin glycocalyx This allows for adhesion but also allows molecules to pass through. - Intraepithelial pockets These invaginations on the basolateral surface of M cells form pockets that harbor lymphocytes. - Basolateral membrane This membrane is invaginated, forming pockets that reduce the distance antigens have to travel to reach the basolateral membrane.

Question 90

What role do dendritic cells play in antigen sampling?

Answer 90

Professional antigen-presenting cells (APCs) extend dendritic arms between epithelial cells into the gut lumen to sample antigens. They detect foreign material and present it to T cells for immune activation.

Question 91

What are CD11c+ dendritic cells, and what is their unique function?

Answer 91

CD11c+ dendritic cells are specialized immune cells that extend projections into the gut lumen to sample pathogens and antigens. Their unique function is to present these antigens to T cells, thereby initiating adaptive immune responses and maintaining immune balance in the gut.

Question 92

where are CD11+ dendritic cells located

Answer 92

closly associated with epithelial cells in the gut lining

Question 93

How do CD11c+ dendritic cells sample antigens in the gut?

Answer 93

Extend dendrites through junctions of epithelial cells. Directly capture bacteria and antigens from the lumen. This process aids in immune surveillance and the initiation of immune responses.

Question 94

How are CD11c+ dendritic cells visualized in research studies?

Answer 94

Transgenic mice expressing GFP under the CD11c promoter demonstrate green fluorescence. Fluorescent markers are as follows: Green: CD11c+ dendritic cells Red: Cytokeratin (an epithelial marker) Blue: Nuclei This shows dendritic cells extending dendrites between epithelial cells into the gut lumen.

Question 95

Why is dendritic cell antigen sampling important for immune defense?

Answer 95

Captures pathogens & antigens from the gut lumen Presents antigens to T cells to trigger adaptive immune responses Helps maintain immune tolerance to commensal microbiota while defending against pathogens

Question 96

What is immune tolerance?

Answer 96

A mechanism employed by the immune system to dampen immune responses against harmless antigens, such as food proteins or commensal bacteria.

Question 97

What role do dendritic cells (DCs) play in immune tolerance?

Answer 97

DCs are antigen-presenting cells that sample the environment. They are crucial in determining whether an immune response should be activated or suppressed to maintain tolerance.

Question 98

How did researchers study the role of DCs in tolerance?

Answer 98

They depleted DCs in a mouse model using the CD11c-DTR system, where DCs express the diphtheria toxin receptor (DTR) and can be eliminated by diphtheria toxin.

Question 99

What is the purpose of the CD11c-DTR system in immunological research?

Answer 99

The CD11c-DTR system allows researchers to selectively deplete dendritic cells (DCs) in mice by expressing the diphtheria toxin receptor (DTR) on DCs. When diphtheria toxin (DT) is administered, only DCs are eliminated, helping scientists study their role in immune responses and tolerance.

Question 100

Why don’t normal mice get affected by diphtheria toxin?

Answer 100

Mice are naturally resistant to diphtheria toxin because they do not express the diphtheria toxin receptor (DTR).

Question 101

How did researchers deplete DCs in their experiment?

Answer 101

They engineered transgenic mice to express the diphtheria toxin receptor (DTR) under the CD11c promoter (a marker for dendritic cells). When diphtheria toxin (DT) was administered, it killed DCs via apoptosis.

Question 102

What happens when dendritic cells are depleted?

Answer 102

No antigen presentation occurs, preventing T cell activation, and no immune response is generated.

Question 103

What are OT-I and OT-II mice, and how do they help in T cell research?

Answer 103

These mice have T cells engineered to recognize a specific OVA (Ovalbumin) peptide. OT-I mice: Have CD8+ T cells recognizing OVA via MHC Class I (H-2Kb). OT-II mice: Have CD4+ T cells recognizing OVA via MHC Class II (I-Ab). Used to study cytotoxic (CD8+) vs. helper (CD4+) T cell responses.

Question 104

What happens when OT-II T cells are transferred into CD11c-DTR mice?

Answer 104

If DCs are present: OT-II T cells proliferate upon encountering OVA. If DCs are depleted (DT treatment): No proliferation occurs.

Question 105

How was T cell proliferation assessed?

Answer 105

Using CFSE staining, which allows tracking of T cell division. Proliferating cells lose CFSE intensity over time as the dye gets diluted.

Question 106

What do the CFSE flow cytometry graphs in Peyer’s Patch (PP) and Mesenteric Lymph Node (MLN) show?

Answer 106

- Without DT (-DTx): 20% of OT-II T cells proliferate in Peyer’s Patch. 49% of OT-II T cells proliferate in the Mesenteric Lymph Node. - With DT (+DTx) (DC depletion): Only 3% of OT-II T cells proliferate (significantly reduced)

Question 107

What conclusion can be drawn from the CFSE proliferation data?

Answer 107

- Dendritic cells (DCs) are essential for priming naïve CD4+ T cells after mucosal immunization with OVA. - Without DCs, T cells fail to proliferate, showing that DCs are crucial for initiating immune responses.

Question 108

Why is the response in the Mesenteric Lymph Node (MLN) stronger than in Peyer’s Patch (PP)?

Answer 108

The MLN is a major site for antigen presentation, leading to higher T cell activation.

Question 109

The MLN is a major site for antigen presentation, leading to higher T cell activation.

Answer 109

It demonstrates that conventional dendritic cells (cDCs) are critical for mucosal immune responses. Deleting CD11c+ DCs prevents immune activation, which is crucial for understanding tolerance mechanisms.

Question 110

What are the major subsets of CD11c+ dendritic cells (DCs) involved in immune responses?

Answer 110

CD103+ DCs – Found in mucosal tissues, responsible for activating T cells and inducing gut-homing properties. CX3CR1+ DCs – More associated with maintaining immune homeostasis and tolerance rather than activating T cells. (CD11c: general marker)

Question 111

Which dendritic cell population is responsible for triggering T cell proliferation?

Answer 111

CD103+ CD11c+ dendritic cells are responsible for T cell activation and proliferation. These DCs uptake antigens, migrate to mesenteric lymph nodes (MLN) or Peyer’s patches, and activate naïve T cells.

Question 112

What molecules are essential for gut-homing of T cells, and how are they induced?

Answer 112

- CCR9 – Binds to CCL25, guiding T cells to the gut. - α4β7 integrin – Binds to MAdCAM-1, which is expressed in gut vasculature. * These molecules are upregulated by mucosal DCs but not by blood-derived DCs

Question 113

How do blood-derived and mucosal dendritic cells differ in T cell activation?

Answer 113

- Mucosal DCs induce gut-homing markers (CCR9, α4β7) on T cells, directing them to mucosal tissues. - Blood-derived DCs can activate T cells but do not induce gut-homing properties, meaning those T cells will not migrate to the gut.

Question 114

Why is the ability of dendritic cells to program T cell homing important for vaccine development?

Answer 114

To protect mucosal surfaces (e.g., gastrointestinal or respiratory tract), vaccines must stimulate mucosal DCs to ensure that T cells acquire the correct homing markers (CCR9, α4β7) and migrate to the appropriate site. Using blood-derived DCs may activate T cells but not direct them to mucosal tissues.

Question 115

What molecules are involved in lymphocyte homing to mucosal tissues?

Answer 115

Lymphocytes express Alpha4Beta7 and CCR9, while endothelial cells express MAdCAM-1 and CCL25. These interactions help lymphocytes locate the correct vasculature and enter tissues.

Question 116

What is the main mechanism behind oral tolerance?

Answer 116

Induction of regulatory T cells (Tregs), not deletion or anergy.

Question 117

How does CCL25 guide lymphocytes?

Answer 117

CCL25 creates a gradient that lymphocytes follow, allowing them to home to specific tissues

Question 118

How do Tregs contribute to oral tolerance?

Answer 118

Tregs suppress immune responses and prevent inflammation in the intestines by producing immunosuppressive cytokines like IL-10.

Question 119

What happens when CD4 T cells are introduced into SCID mice?

Answer 119

The mice develop colitis due to T cell activation by dendritic cells, leading to inflammation in the intestines.

Question 120

What did researchers discover about IL-10 producing Tregs in SCID mice?

Answer 120

When SCID mice were fed OVA, IL-10 producing Tregs proliferated and prevented colitis, demonstrating that Tregs protect against inflammation.

Question 121

What role do CD103+ dendritic cells play in Treg activation?

Answer 121

They are crucial for activating FoxP3+ Tregs, which suppress immune responses.

Question 122

How do researchers identify Tregs?

Answer 122

By their high FoxP3 expression, which is a hallmark of regulatory T cells.

Question 123

What happens if Tregs are deleted in mice?

Answer 123

Severe inflammation occurs, proving Tregs are essential for immune regulation.

Question 124

How do Treg populations change after antigen exposure?

Answer 124

They expand significantly over days, reaching up to 56% by later stages.

Question 125

How can understanding oral tolerance help in medicine?

Answer 125

It can be applied to vaccines, autoimmunity, food allergies, and intolerances.

Question 126

What molecules help lymphocytes home to mucosal tissues?

Answer 126

Lymphocytes express Alpha4Beta7 integrin and CCR9, which bind to MAdCAM-1 and CCL25 on endothelial cells to facilitate tissue entry.

Question 127

How does CCL25 guide lymphocyte migration?

Answer 127

CCL25 forms a gradient that lymphocytes follow, helping them locate and enter the correct tissue.

Question 128

What happens when MAdCAM-1 binds to Alpha4Beta7?

Answer 128

The interaction helps anchor lymphocytes to the vasculature, allowing them to exit blood vessels into tissues.

Question 129

How do Tregs contribute to immune regulation?

Answer 129

Tregs suppress immune responses by secreting IL-10, which prevents excessive inflammation.

Question 130

What is a SCID mouse, and why is it used in immune studies?

Answer 130

A SCID (Severe Combined Immunodeficiency) mouse lacks T cells, B cells, and NK cells, making it highly susceptible to infections. It is used to study immune cell interactions.