Innate and adaptive immunity 2

• Describe the complement proteins

  These are proteins produced by the liver and immune cells that become activated by cleavage, leading to an activation cascade. These proteins are present in both the blood and the tissues, with low concentrations in the CSF, and generate antimicrobial effects. These proteins can be activated following three pathways.
• Describe the different anti-microbial roles of complement proteins

  1. Form the membrane attack complex, which makes holes in bacterial membrane.
  2. C3b proteins coat microbial surface and tags them for phagocytosis (opsonisation).
  3. C5a induces a chemotaxis of neutrophils and monocytes.
• Describe the classical pathway of complement activation

  This pathway acts as part of the adaptive immune system as it is activated by the binding of antigens to the Fc portion of antibodies. This reveals the complement binding site which results in activation of C1 and consequently C4 and C2 proteins that form the C4b2b complex. This forms C3 convertase, which cleaves C3 into C3a and C3b. Then C4b2a3b complex forms, which is the c5 convertase that cleaves C5 to C5a and C5b.
• Describe the lectin complement pathway

  This pathway is activated when mannose-binding lectin binds to mannose on bacterial cell surface, which activates MBL-associated serine proteases that cleave C2 and C4 to form C4b2a, the C3 convertase. This pathway then follows the classical pathway, with the same c5 convertase.
• Describe the alternative complement pathway

  This pathway is activated by the binding of C3b protein to bacterial surface, which leads to the formation of the alternative C3 convertase, C3bBb. This binds with C3b to form the C5 convertase, (C3)2Bb.
• Describe how the complement cascade leads to opsonisation

  C3b bind to surfaces not coated with sialic acid residues (bacterial surfaces), which is proteolytically cleaved to form iC3b. This acts as an opsonin by binding to complement receptors CR2 and CR3 (binding to CR1 is not sufficient for opsonisation).
• Describe the formation and action of the membrane attach complex

  This forms from C5b, C6, C7, C8 and C9, which insert into the bacterial cell membrane forming transmembrane channels, which lead to osmotic lysis of gram negative bacteria.
• Describe the complement proteins that act as inflammatory mediators

  • C5a, c3a and C4a can act as anaphylatoxins (activate mast cells)
  • C5a is important for chemotaxis
  • These proteins can also cause vascular permeability
• Describe T-cells

  There are two main types of T cells known as CD8 and CD4 T cells, which are characterised by glycoprotein markers on their surface.

  • CD8 T cells: form cytotoxic T cells that participate in cell-mediated immunity
  • CD4 T cells: these differentiate into effector T helper cells, which help B cells become plasma cells and help CD8 T cells become cytotoxic T cells, as well as activate macrophages via the secretion of IFN-gamma

  T cell receptors can recognise peptide antigens presented on the surface of antigen presenting cells, such as macrophages, via MHC molecules.
• Describe MHC molecules

  Major Histocompatibility Complexes consist of two major classes that present peptide antigens to the 2 main types of T cells.

  • MHC I : present antigens of cytoplasmic origin to CD8 T cells
  • MHC II: present phagocytosed antigens of extracellular origin to CD4 T cells
• Describe antibody-mediated immunity

  Antibodies can kill microbes via:

  • agglutination: causes microbes or pathogen to aggregate and therefre blocks binding to target
  • Faciitate phagocytosis via opsonisation
  • Activation of complement proteins
  • Block adhesion to host cells
• What mechanisms do bacteria deploy to avoid recognition by phagocyte?

  Bacteria capsules prevents bacteria from contacting phagocytes. As well as this, the capsule can prevent binding of C3b receptors found on phagocytes from binding to C3b on the bacterial cell wall. Furthermore, the capsule shields the bacteria from antibody binding to antigens, and therefore prevent complement activation via the classical pathway.
• Describe the structure of bacterial capsules

  Capsules are loose and relatively unstructured polymer coats that are mainly composed of polysaccharides.
• Describe the different mechanisms of bacteria to resist phagocyte killing

  • Resistance to nitric oxide and reactive oxygen species: catalase, cell surface polysaccharides interact with reactive oxygen species and they can reduce the strength of oxidative burst.
  • Resistance to defensins
  • Tough cell walls that refract the action of lysosomal proteases and lysosomes
  • Resistance to low pH by producing urease and proton pumps
• Describe how bacteria prevent MAC mediated lysis

  Bacteria can have a long O-antigen, which means that MAC is deposited away from the cell membrane.

  (C5b in the MAC has a high affinity for O-antigen)
• How do bacteria prevent binding of complement proteins?

  • Produce surface protein that prevents opsonisation: M protein of Streptococci acts as a receptor for factor H that potentiates C3bBb dissociation
  • Produce a protease to destroy complement: C5a and C3b proteases
  • Shed outer membrane vesicles so that complement binds to the shedded membrane
• Describe the strategies of bacteria to avoid antibody response

  • sIgA protease
  • Molecular mimicry: express antigen similar to host structures, e.g., sialic acid capsule
  • Coating with host proteins, e.g., fibronectin binding protein
  • Phase variation: switching on and off of surface proteins
• What is antigenic variation?

  Changes in gene sequence leading to changes in the amino acid sequence of an antigen
• What is phase variation?

  This is reversible on/off switching resulting in variation in the level of expression of one or more antigens
• Describe the four mechanisms of bacterial phase and antigen variation

  1. DNA inversion: Cre-LoxP system, LoxP sites facing one another
  2. DNA methylation: turn off and on promoters
  3. Complex recombination systems: e.g., gene transfer by homologous recombination
  4. Slipped strand mispairing: involves repetitive DNA that results in frame shifts that affects translation or promoter activity
• How can bacteria avoid phagocytosis?

  • Produce molecules that depolymerise actin and therefore block phagocytic processes
  • They release toxins that kill phagocytes by inducing apoptosis
  • Produce antioxidants, e.g., catalase, and urease (raises pH) that allow them to resist killing