Innate recognition

• Describe the primary innate defences

  Commensal bacteria: provide colonisation immunity by filling available niches + depleting available metabolites.

  Mucus: embedded with antibacterial factors and makes it difficult for pathogens to access the epithelium

  Epithelial layer: contains specialised cells that produce antimicrobial factors
• Describe some examples of specialised innate defence mechanisms at barriers

  • Chemical barriers: bile and pH
  • Antimicrobial proteins: lysozymes and phospholipase A2
  • Antimicrobial peptides: defensins and beta defensins packaged in lamellar bodies in the lungs and skin
  • Lectins: produced by paneth cells in the gut and help recognise pathogens
• Describe the action of lysozymes in primary innate immunity

  Lysozymes digest the cell wall of gram positive and gram negative (more difficult) bacteria.

  • Interacts with and degrades peptidoglycan structures
  • Exposes lipid bilayer, making bacteria more susceptible to extracellular antimicrobial factors
  • Gram negative bacteria: LPS creates barrier to peptidoglycan
• Describe the action of defensins in primary innate immunity

  Defensins utilise electrostatic attraction and the transmembrane electric field to embed within the lipid bilayer. The defensin peptides form a pore within the pathogen cell membrane to make it easier for antimicrobial factors to get inside the cell.
• What is the function of tissue-resident macrophages?

  Tissue-resident macrophages respond to and can locally control pathogens, but if they fail they will recruit circulating innate cells, such as monocytes and neutrophils. Upon entering tissue monocytes will differentiate into inflammatory macrophages.
• Describe the process of phagocytosis

  This is initiated when cell surface receptors are engaged, such as scavenger receptors. The bound material is internalised within a large membrane enclosed endocytic vesicle known as the phagosome. This phagosome becomes acidified to kill the pathogen, before fusing with one or more lysosomes to form the phagolysosome.
• Describe the antimicrobial factors produced by phagocytes

  • Toxic oxygen derived products, e.g., superoxide ion, hydrogen peroxide
  • Toxic nitrogen species, NO
  • Antimicrobial peptides
  • macrophages: cathelicidin
  • neutrophils: alpha and beta defensins, cathelicidin and lactoferricin
  • Lysozymes and acid hydrolases
  • Lactoferrin- neutrophils
• Describe the production of ROS

  These include superoxide anion and hydrogen peroxide.

  • Superoxide generated by membrane associated NADPH oxidase
  • 1 set of subunits found in neutrophil granules/ macrophage lysosome and the other components are within the cytosol
  • NADP oxidase reaction results in transient oxygen consumption, known as the respiratory burst
  • Produces superoxide anion in lumen of phagolysosome
  • Further chemical and enzymatic reactions produce hydrogen peroxide
• Describe the action of fMLP receptor

  FMLP receptor recognises N-formylmethionine, which is an amino acid present in prokaryotes but not eukaryotes. FMLP receptor induces Rac2, which is present in the cytosol and part of the NADPH oxidase complex. NADPH oxidase forms part of phagolysosome and produces ROS.
• Describe the action of interferons

  Interferons are responsible for activating natural killer cells. IFN-alpha and IFN-beta can be synthesised by many cell types and is usually produced by infected cells.
• What are the events that occur during early inflammation?

  As inflammation begins, neutrophils are attracted and then monocytes, which differentiate into macrophages. Then endothelial cells become less tightly joined, which allows the exit of fluid and proteins from the blood. This creates oedema. Clotting occurs around the pathogens and macrophages and neutrophils secrete prostaglandins and leukotrienes.
• Describe the change in blood vessels during inflammation

  • Cytokines produced by macrophages cause dilation of local small blood vessels, which increases local blood flow
  • Leukocytes move to periphery of blood vessel as a result of increased expression of adhesion molecules by the endothelium
  • Leukocytes extravasate at site of infection and blood clotting occurs in the microvessels
• What are perivascular macrophages?

  These are macrophages that are localised next to the vasculature and allows hotspot recruitment of neutrophils in tissue.
• Describe some of the major chemokines

  CCL2- attracts monocytes from blood stream

  CXCL8- migration of neutrophils
• Describe the long-range effects of macrophage activation

  Macrophages can have long range effects, such as increasing body temperature via endogenous pyrogens, TNF-alpha, IL-6 and IL-1-beta. PGE2 targets the hypothalamus and increases heat from brown fat.
• Describe C-reactive protein

  Member of the pentraxin family and binds to phosphocholine on bacterial and fungal cell walls. It acts as an opsonin and can activate the complement cascade.