Mucosal immunity

• Describe the gut microbiota

  The human intestine is home to around 10^14 bacteria, which are separated from the immune system by a mucus and epithelial layer. The gut microbiota is comprised of mostly Commensal bacteria, which are important in supporting digestion, providing colonisation resistance, and Regulation of the intestinal immune system. However, the microbiota is also linked in the pathogenesis of many diseases including; colon cancer, type I diabetes and obesity.
• Why is there a balanced needed between immunity and tolerance in the gut?

  There is a huge antigenic load in the GI tract; therefore, it is important that there is balance between immunity to protect from infection, but also tolerance to protect the microbiota. Too much tolerance can result in disease, such as cancer, and too much immunity results in autoimmune disease, such as inflammatory bowel disease.
• What is ulcerative colitis?

  This is a type of IBD, which is characterised by inflammation and ulcerations of the large bowel.
• What is Crohn’s disease?

  This is a type of IBD that can affect any part of the digestive tract from the mouth to the anus and is characterised by cobblestoning of colon.
• Describe the action of T-regs

  • Suppress CD4+ T cells and inhibit self-harmful T cell reactions
  • They are made either in the thymus during T cell development (natural Tregs) or in the periphery from CD4+ T cells (induced Tregs)
• Describe the action of Tregs in the intestine

  There are many Tregs in the intestine, which are generated by the gut-draining lymph nodes. Exposure to Commensal bacteria, particularly clostridium, following birth drives proliferation of Tregs.
• Describe how dendritic cells stimulate primary T cell responses

  Intestinal DCs can generate a primary T cell response by surveilling the environment for antigen and presenting it to T cells in the mesenteric lymph nodes. This results in priming and expansion of T cells, which can drain into the gut. This can promote both protection against pathogens and immune tolerance by presenting microbiota antigens and promoting Treg expansion.
• Describe the action of macrophages in the intestinal immune system

  Macrophages can sense bacteria via PRRs but they are hypo-responsive to bacterial stimulation. They play a key role in regulatory functions in the gut by; producing and sensing of IL-10, reducing local inflammation, promoting renewal of epithelial cells, modulating T-cell differentiation in situ, and maintaining Tregs in the intestinal lamina.
• Describe how intestinal microbiota regulate immune cells

  • SCFAs, e.g., acetate, are generated by Commensal bacteria, which digest dietary fibre
  • SCFAS act on dendritic cells to enhance their ability to generate Tregs
  • Also act on macrophages to induce hypo-responsiveness to bacteria
• What factors play into development of IBD?

  • Genetic susceptibility: <35% concordance of IBD in identical twins
  • Environmental triggers: antibiotics, stress, infections, diet, and lifestyle
  • Immune response: dysregulated CD4+ T cell responses against Commensals and dysregulated DC and macrophage function.
  • Luminal microbial antigens and adjuvants: gut microbiota essential for animal models of colitis and dysbiosis occurs in human IBD
• What causes IBD?

  Breakdown in immune homeostasis in the intestine results in barrier defects and immune regulation defects. This results in enhanced T cell responses against Commensal bacteria and inflammation and damage to surrounding tissue.
• Describe the role of cytokines in IBD

  Macrophages produce the inflammatory cytokine TNF-alpha and IL-12, which induces T-helper 1 cells. Th1 cells produce IFN-gamma and TNF-alpha. Dendritic cells also secrete TNF-alpha, as well as IL-6 and TGF-beta, which induce T-helper 17 cells that are stabilised by IL-23. Th17 cells contribute to tissue inflammation by secreting IL-17, IL-21, and IL-22.
• How do T cell responses become dysregulated in IBD?

  DCs stimulate primary T cell responses and sensing bacteria, but inflammatory macrophages also shape T cell responses and accumulate in the intestine in IBD. These cells secrete inflammatory cytokines, which cause tissue damage but also stimulate T cells to create a knock-on effect of more cytokine production.

  
• How do inflammatory macrophages drive T cell responses in IBD?

  Macrophages are hyper-responsive to bacterial stimulation in IBD and produce excess inflammatory cytokines. However, it is not clear how macrophage dysfunction drives T cell pathology in IBD.
• Describe the genetic links associated with IBD

  Genome wide association studies showed that:

  • Crohn’s disease: NOD2 gene mutation, this is a bacterial sensing gene for macrophages and DCs
  • Ulcerative colitis: MHC gene mutation
  • IL-10 gene mutation always results in paediatric IBD

  Earlier stages of IBD are thought to be due to abnormal responses of DCs and macrophages to bacterial stimulation, which then triggers more immune responses.
• Describe the dual action of immune cells in the lungs

  The lungs are heavily exposed to allergens and pathogens, so immune cells need to respond quickly to pathogens and respiratory infections. But these cells also need to be able to inhibit inflammation quickly, in order to prevent tissue damage.
• Describe the role of alveolar macrophages in innate immunity within the lungs

  • Alveolar macrophages sit on the airway side of the epithelial barrier and are directly exposed to inhaled particles, allergens and pathogens.
  • Express CD200R: downregulates immune cells
  • Express TGF-beta receptor: maintains tissue homeostasis
  • Express TREM2: regulates the immune system
• Describe the origin of alveolar macrophages

  Alveolar macrophages usually self renew in childhood but in adulthood they are replenished from blood monocytes, which are recruited to the lungs and differentiate to form macrophages.
• Describe how monocytes play a role in respiratory infections

  During infection, monocytes are recruited to the lungs and differentiate to form alveolar macrophages. Both macrophages and monocytes produce inflammatory cytokines that promote further mobilisation of monocytes from the bone marrow, which creates a knock-on effect.
• Describe how the uterus is a specialised environment

  • Massive amount of tissue remodelling in uterus during the menstrual cycle
  • Repeated cycles of proliferation, differentiation, shedding and repair
  • Extensive requirement of angiogenesis
  • Dynamic changes in the number of immune cells during endometrial breakdown alongside vascular remodelling
• Describe M1 macrophages

  • Classical activation: LPS and IFN-gamma
  • Mediate type I inflammation
  • Highly responsive to pathogens
  • Secrete proinflammatory cytokines
  • Kill intracellular parasites and mediate tumour resistance
• Describe M2 macrophages

  • Alternative activation: IL-4, glucocorticoids, TGF-beta
  • Immunosuppressive
  • Contribute to type II inflammation and fibrosis when dysregulated
  • Involved in tissue remodelling and angiogenesis
  • Parasite encapsulation and tumour promotion
• Describe how M2 macrophages are involved in tissue remodelling

  M2 macrophages exhibit regulatory activity at all stages of repair and regeneration:

  • phagocytose cellular debris
  • produce mediators that stimulate fibroblasts to differentiate into myofibroblasts
  • tissue remodelling and angiogenesis
  • synthesise and breakdown ECM components
  • anti-inflammatory
• Describe the role of macrophages in repair and remodelling of the uterus

  Prior to menstruation: macrophages scattered throughout the tissue

  Endometrial breakdown: massive influx of macrophages and neutrophils in areas of tissue detachment and breakdown

  Endometrial repair and re-epithelialisation: neutrophil numbers decrease during tissue remodelling but macrophages are still present and localised to areas surrounding newly repaired luminal epithelium
• Describe uterine macrophages

  • Expression markers of alternative activation, e.g., CD206
  • High leaves of TGF-beta receptor
  • Express genes involved in tissue remodelling and repair pathways, e.g., collagen deposition