Immunology of the gut mucosa (Pt. 1)

• LO:


  • Understand and define the concept of immune tolerance, particularly in the context of the immune system's response to food antigens.

  • Explain the basic components and functions of the mucosal immune system, including the mucosal structure and relevant immune cells.
• What does the mucosal immune system do?


  • The mucosal immune system is there to protect internal epithelial surfaces.

  • These include the gut, the respiratory tract, the urinogenital tract and exocrine secretory glands linked to these organs, such as the salivary, lachrymal, pancreas, and mammary glands.
• Challenges to mucosal surfaces

  • Respiratory tract:

  1. Particulates
  2. Pollutants
  3. Allergens
  4. Airborne pathogens

  • Gastrointestinal tract:

  1. Food Commensals
  2. Ingested pathogens

  • Urinogenital tract:

  Introduced pathogens

  (commensals)

  Sperm
• Induction of ‘oral tolerance’ (mucosally-induced systemic tolerance) – the classic mucosal immunology protocol

  
• Tolerance induction by prior feeding in mice

  
• IgG1 anti-soya in pig serum after feeding

  
• Innate immune defences


  Barriers: Mucus, cilia, pH, Tight-cell junctions

  Cells: epithelial cells, neutrophils, tissue macrophages, dendritic cells Receptors: TLRs, mannose, scavenger

  Mediators: lysozyme, defensins, cathelicidins
• Adaptive Immune defences


  Tissues: Mucosa Associated Lymphoid Tissue:

  • Nasal associated lymphoid tissue (NALT), e.g. tonsils
  • Bronchial associated lymphoid tissue (BALT)
  • Gastrointestinal associated lymphoid tissue (GALT) e.g. Peyers patches

  Regional lymph nodes:

  • Retropharyngeal, submaxillary and parotid LN
  • Mediastinal LN
  • Mesenteric LN

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

  
• Mucosal Associated Lymphoid Tissues

  
• Mucosal Associated Lymphoid Tissues


  Peyer’s patches: Multi-follicular.

  Isolated Lymphoid Follicles: Exist in a range of sizes, with the larger displaying germinal center characteristics

  Cryptopatches : Considered to develop into single B-cell follicles. Contain mainly innate lymphoid cells (ILCs)

  
• Antibody effector mechanisms in mucosal tissues: transport of dimeric IgA across epithelium by pIgR/SC in mammals 


  IgA antibodies found in mucosal secretions (saliva & mucus)

  • Monomeric IgA and Secretory IgA.
  • Production in response to specific commensal bacteria present in mucosal environment; helps shape the composition of the microbiota & maintain balanced rs w/ the host and commensals.
  • prevents commensal bacteria from entering mucosal tissues; binding to these microorganisms inmucosal lumen.
  • neutralize potential threats by binding to surface or toxins of commensal bacteria

  
• T cells in the mucosa

  
• Innate lymphoid cells (ILC) in the mucosa

  
• Peyers patch M cells facilitate uptake of particulate antigen.

  
• Mucosal CD11c +ve DC extend dendrites through epithelium to trap luminal bacteria.


  • The fluorescent images from the intestine of a transgenic mouse. These mice have GFP under the control of the CD11c promoter. So whichever cell expresses CD11c (DC marker) is green.

  • DC’s are closely opposed to the epithelial cells (bottom left). And if you look on the right, you can see that a DC is sticking a dendrite between the epithelial cells into the lumen.

  • Survey lots of antigens in this manner.

  
• CD11c KO mice


  • Diphtheria is not a disease that mice get.

  • If you modify the mouse so that only CD11c DC express the diphtheria toxin receptor (DTR), only those cells will be susceptible to the diphtheria toxin (DT).

  • If exposed to DT the CD11c DC will die by apoptosis.

  
• Conventional DCs (CD11c+) are essential for priming of naive CD4+T cells following mucosal immunization with OVA

  
• DC subtypes exist


  CD11c: general marker

  CD103: Integrin marker found in tissues with mucosal surfaces, such as the gut and respiratory tract

  CX3CR1: A chemokine receptor found in the lamina propria DC of the gut

  Culture OT1 and OTII cells with the various DC subsets:

  Only CD103+DCs activate antigen-specific T-cells:

  A)in vitro pulsed APC plus TCR-transgenic T-cells

  C)Adoptive transfer of OVA-pulsed DC triggers T-cell proliferation in vivo

  
• DC subtypes exist (Pt. 2)


  CD103+ DCs being transported around in lymphatics to local lymph nodes, and these cells being important for the induction of mucosal immune responses

  • This is dependent on a set of homing receptors

  • α4β7 and CCR9

  
• Mucosal (mesenteric LN or Peyers patch) but not blood derived cDCs induce gut-homing T cells

  
• Proposed oral tolerance mechanisms:


  • Deletion of specific T-cells
  • Induction of energy
  • Induction of regulatory T-cells
• Regulatory T-cells


  • The literature on regulatory T-cells is vast.

  • The main areas of research are in autoimmunity and in controlling mucosal immune responses.

  • They are a specialized subset of T lymphocytes (T cells) that play a critical role in immune regulation and maintaining immune tolerance. Their primary function is to suppress excessive or inappropriate immune responses, preventing autoimmune reactions and controlling inflammation.

  • cell surface markers: CD4 and CD25

  • Forkhead box protein P3 (FOXP3) is a critical transcription factor

  • Cells secrete: IL-10, IL-4, TGF-β
• The first demonstration of ‘Tr1 cells’

  
• Treg creation

  Thymus:

  learn to recognize self and non-self antigens, prevent autoimmune disease

  Body (MLN & Peyer's Patch):

  control immune responses to things we eat

  Recognise food antigens presented by DC

  Prevent overreaction to things eaten, cause allergies/intolerance
• Mesenteric lymph node CD103+ DC preferentially induce Tregs

  
• Treg KO mice: DEREG


  • the DTR is under the control of the FoxP3 promoter, allowing the KO of regulatory T cells when they are exposed to DT
  • Injection of DT induces transient depletion of Foxp3 expressing cells (Tregs)

  
• Demonstration that Tregs induced by antigen immunisation/feeding are involved in controlling immune responses to intestinal proteins:


  Group 1: injected OVA first and then fed the mice, generating an active immune response to OVA.

  Group 2: feed with OVA, then inject OVA, and then feed with OVA (this is the classical oral tolerance exp..)

  Group 3: a deviated form of the traditional oral tolerance protocol! First feed with OVA (generate OVA-specific Treg), then give DT (to deplete OVA-specific Tregs), then inject with OVA (generate OVA-specific immunity), then feed with OVA.

  
• Demonstration that Tregs induced by antigen immunisation/feeding are involved in controlling immune responses to intestinal proteins (Results):

  
• Summary


  •Oral tolerance is a fascinating immunological phenomenon with practical implications in managing immune responses to food antigens and maintaining gut homeostasis.

  •Dendritic cells, particularly CD103 DC, are crucial in inducing tolerance.

  •Treg cells play a pivotal role in regulating immune responses, and their expansion can be modulated by antigen exposure.

  •Understanding the mechanisms of oral tolerance has implications for autoimmune diseases and food allergies.