Immunology Chapter 9

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  • Sarah, nicely done.
    However things change quickly. Recently the receptor TOSO/FAIM3/FcmuR was identified as an Fc receptor for IgM. It occurs on post-GC B-cells and a number of blood cancers.
    http://www.ncbi.nlm.nih.gov/pubmed/21908732
    Best, Colin
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Immunology Chapter 9

  1. 1. Chapter 9 Immunity Mediated by B Cells and Antibodies
  2. 2. Focus of Chapter 9 How Antibodies Clear Infection <ul><li>Antibodies recruit “destructive, nonspecific” immune system components to the infecting pathogen </li></ul><ul><ul><li>How? </li></ul></ul><ul><ul><ul><li>Antibodies bind and link the pathogen to effector molecules or cells that will destroy the pathogen </li></ul></ul></ul>
  3. 3. RECALL Pathogens extracellular Antibodies secreted in 2 ND LT & Bone Marrow B cell function extracellular spaces Other Pathogens fluids B cell PM cell virus ~~~~~~ ~~~ Next cell bacteria Y Y Y intracellular virus
  4. 4. Ab  toxic  destructive to pathogens <ul><li>How do Ab reduce infection? </li></ul><ul><li>What happens? </li></ul>Molecular adaptor (opsonize) PHAGOCYTOSIS Neutralize = pathogen surface covered Growth/replication  Y 2 pathogen Y Y Y Y Y phagocyte ? ? 3 Role of AB = Y  reduces infection Y Y Y 1 Y Y Y Y pathogen Y Y Y Y
  5. 5. COMPLEMENT activation <ul><li>Recall: Chapter 1, Figure 1.5 </li></ul><ul><li>Opsonization is enhanced by the actions of complement </li></ul><ul><li>Complement Ag-binding function of Ab </li></ul>Complement = set of proteins that do not discriminate between Ags 3
  6. 6. Antibody production by B lymphocytes
  7. 7. The Development Of B Cells Can Be Divided Into six Broad Phases
  8. 8. <ul><li>Stem cell in bone marrow to the mature naïve B cell </li></ul><ul><li>Location of B cells at the different stages </li></ul><ul><li>State of the Ig H- and L- chain genes </li></ul><ul><ul><li>Form of Ig expressed </li></ul></ul>Peripheral circulation Bone Marrow
  9. 9. B cells need activated T cell help to mature into Ab-secreting Plasma Cells <ul><li>“ Generally” need T cell help </li></ul><ul><ul><li>This delays onset of Ab production until a week after infection begins </li></ul></ul><ul><li>In addition, B cells take time to switch isotype and undergo and affinity maturation … </li></ul>
  10. 10. Last Two Main Phases of B-cell Development <ul><li>Plasma cells can differentiate directly from: </li></ul><ul><ul><li>activated B cells </li></ul></ul><ul><ul><li>isotype switched, somatically hypermutated centrocytes </li></ul></ul><ul><ul><li>memory B cells </li></ul></ul>
  11. 11. T cell help, Isotype Switching & Affinity Maturation <ul><li>Why? </li></ul><ul><ul><li>Production of high-affinity antibodies that are MOST effective at dealing with pathogens </li></ul></ul><ul><ul><li>During the course of an infection the effectiveness of the Abs steadily increases </li></ul></ul><ul><ul><li>Experience retained in the form of memory B cells and high affinity Abs to provide long-term immunity to reinfection </li></ul></ul><ul><li>What’s the alternative to waiting? </li></ul>
  12. 12. B cell Activation without T-cell Help <ul><li>Faster primary response to activate B cells without the need for T-cell help </li></ul><ul><ul><li>Provides early defense </li></ul></ul><ul><ul><li>Abs  IgM isotype and of low affinity </li></ul></ul><ul><ul><li>Keeps infection at relatively low level until better antibody response can develop </li></ul></ul><ul><li>How do B cells become activated? </li></ul>
  13. 13. 7-1 B-cell activation requires cross-linking of surface immunoglobulin
  14. 14. Protein or carbohydrate epitopes Bacterial cell Naïve Mature B cell IgM’s  X-linked by repetitive Ag epitopes Ig  Ig  IgM Ig  Ig  Note that BCR signal transduction resembles that of TCR Signal transduction extracellular  intracellular
  15. 15. How does BCR signal transduction resemble TCR signal transduction <ul><li>BCR </li></ul><ul><li>Associated with cytoplasmic protein tyrosine kinases </li></ul><ul><ul><li>PTK’s activated by receptor clustering </li></ul></ul><ul><li>Ig  & Ig  associate with IgM to form functional BCR </li></ul><ul><ul><li>Cytoplasmic tails with 2 ITAMs </li></ul></ul><ul><ul><li>Activates intracellular signaling pathways </li></ul></ul><ul><li>TCR </li></ul><ul><li>Associated with cytoplasmic protein tyrosine kinases </li></ul><ul><ul><li>PTK’s activated by receptor clustering </li></ul></ul><ul><li>CD3 associate with TCR to form functional TCR </li></ul><ul><ul><li>Cytoplasmic tails with 2 ITAMs </li></ul></ul><ul><ul><li>Activates intracellular signaling pathways </li></ul></ul>Similar intracellular signaling pathways!
  16. 16. B cell Signal Cascade <ul><li>Receptor clustering </li></ul><ul><li>Receptor-associated tyrosine kinases phosphorylate the ITAMs on the Ig  & Ig  cytoplasmic tails </li></ul><ul><li>Syk binds to the phosphorylated ITAMs of the Ig  cytoplasmic tail </li></ul>Tyrosine kinases Ig  cytoplasmic tail (mature naïve B cell) ITAMs phosphate Lyn
  17. 17. mature naïve Ig  Lyn <ul><li>What does Syk bind to? </li></ul><ul><ul><li>Phosphorylated ITAMS of  chain!!! </li></ul></ul><ul><li>Recall: clustering of BCRs </li></ul><ul><ul><li>minimum of two receptor complexes </li></ul></ul><ul><ul><li>Syk are close together </li></ul></ul><ul><li>What is the result or function of this “closeness”? </li></ul><ul><ul><li>Transphosphorylation </li></ul></ul>Transphosphorylation
  18. 18. What is the function of the previous intracellular pathway? <ul><li>Extracellular  Intracellular signals! </li></ul><ul><li>What is the purpose/functio n of the B cell signal cascade? </li></ul><ul><ul><li>Pathway that relays signals produced to the B-cell nucleus </li></ul></ul><ul><li>What are the results of the B-cell nucleus receiving signals? </li></ul><ul><ul><li>Gene expression modulation </li></ul></ul><ul><ul><li>Why does the IS want to modulate the gene expression in a B-cell </li></ul></ul><ul><ul><ul><li>“ B-cell Activation” </li></ul></ul></ul><ul><li>Is X-linking of the BCR by Ag sufficient to activate a mature naïve B cell? </li></ul>
  19. 19. NO! Additional signals are required to “activate” a mature naïve B cell <ul><li>Requirement for the association of BCR with its co-receptor </li></ul><ul><ul><li>B-cell co-receptor  3 proteins </li></ul></ul><ul><ul><ul><li>CR2 = complement receptor 2 </li></ul></ul></ul><ul><ul><ul><li>CD19 </li></ul></ul></ul><ul><ul><ul><li>CD81 = TAPA-1 </li></ul></ul></ul><ul><li>Functions of co-receptor proteins </li></ul><ul><ul><li>CR2 = binds to complement deposited on pathogen </li></ul></ul><ul><ul><li>CD19 = receptor signaling chain </li></ul></ul><ul><ul><li>CD81 = unknown B-cell co-receptor function????? </li></ul></ul>
  20. 20. How are signals delivered? <ul><li>Binding of CR1 (on the B-cell) to C3b (on the pathogen) makes it susceptible to cleavage to C3d </li></ul><ul><li>CR2 (part of the B-cell co-receptor) can then bind to the C3d </li></ul><ul><ul><li>Signal is sent through CD19 </li></ul></ul>
  21. 21. Synergetic cooperation between B-cell receptor & B-cell co-receptor <ul><li>CR2  C3d X-links BCR to its co-receptor </li></ul><ul><li>Results in clustering together </li></ul><ul><li>CD19 phosphorylation by BCR-associated tyrosine kinases </li></ul><ul><li>Phosphorylated CD19 binds intracellular signaling molecules </li></ul><ul><li>BCR + BCR co-receptor signals, synergize to  signals by 1,000- 10,000-fold </li></ul><ul><li>Is this “combined effect” of the BCR signal with its co-receptor signal (signal 1) and CD19  BCR intracellular signaling molecules (Lyn, etc) (signal 2) sufficient to activate a mature naïve B cell? </li></ul>Signal 2 Signal 1 Ig  Ig  : Lyn
  22. 22. Additional signals from helper T cells are required <ul><li>“ Generally”  helper CD4 T cell signal requirement </li></ul><ul><ul><li>Which CD4 T cells help? </li></ul></ul><ul><ul><ul><li>The “effector CD4 T cells” produced when naïve CD4 T cells encounter antigen and became activated </li></ul></ul></ul><ul><ul><ul><ul><li>Which “effector CD4 T cells”? </li></ul></ul></ul></ul><ul><ul><ul><ul><li>TH1 or TH2 ????? </li></ul></ul></ul></ul><ul><li>Wait a minute…do all mature naïve B cells even require help? </li></ul>
  23. 23. <ul><li>9-3: The antibody response to antigens does not (always) require T-cell help </li></ul>
  24. 24. The Ab response to certain Ags DOES NOT require T cell Help <ul><li>Chemical and antigenical “distinctions” in mammalian versus bacteria </li></ul><ul><ul><li>polysaccharides, lipopolysaccharides and peptidoglycans </li></ul></ul><ul><li>One has repetitive epitopes … </li></ul><ul><li>Is it bacteria or mammalian? </li></ul><ul><ul><ul><li>Repetitive epitopes are a major target of Ab response to extracellular pathogens </li></ul></ul></ul><ul><li>Some repetitive epitope Ags can activate mature naïve B cells without CD4 T cell help ! </li></ul>
  25. 25. Whether a B cell needs T-cell help or not depends on the nature of the Ag <ul><li>Two classifications of Ags </li></ul><ul><ul><li>Thymus-dependent Ags ( TD Ag ) </li></ul></ul><ul><ul><li>Thymus-independent Ags ( TI Ag ) </li></ul></ul><ul><ul><ul><li>Immunodeficient pts without thymus can make Ab against TI Ags </li></ul></ul></ul><ul><li>For TI Ags the need for CD4 T cell help can be overcome in 2 different ways </li></ul><ul><li>TI-1 Ag </li></ul><ul><ul><ul><li>Bind to BCR and other receptors on B cells – Ex. ( LPS  TLR’s) </li></ul></ul></ul><ul><ul><ul><ul><li>Combination = B cell induction to proliferate & differentiate </li></ul></ul></ul></ul><ul><li>TI-2 Ag </li></ul><ul><ul><ul><li>Bind to repetitive Ag’s and cause extensive crosslinking of the BCR’s that no additional signal is needed to activate the B-cell. </li></ul></ul></ul>
  26. 26. What is LPS? <ul><li>Lipopolysaccharide (surface of pathogens) </li></ul><ul><ul><li>Ex: LPS=Gram-negative bacteria </li></ul></ul><ul><li>What is LBP? </li></ul><ul><ul><li>Soluble LPS binding protein </li></ul></ul>
  27. 27. LPS of gram-negative bacteria can activate B cells to become Ab-producing plasma cells <ul><li>BCR is specific for LPS epitope </li></ul><ul><li>LPS forms complex with soluble LPS binding protein (LBP) </li></ul><ul><li>Signals - CD14/TLR-4 + BCR + B-cell co-receptor sufficient to activate B cell  plasma cells </li></ul>
  28. 28. LPS of gram-negative bacteria can activate B cells to become Ab-producing plasma cells <ul><li>LPS binding to CD14/TLR-4 = a co-activating signal </li></ul><ul><li>Co-activating signal for another Ag on the bacterium to bind to its specific BCR </li></ul><ul><li>This B cell goes on to produce Ab specific for the bacterial antigen, not LPS </li></ul>
  29. 29. There is a second type of TI Ag: TI-2 <ul><li>Repetitive carbohydrates or protein epitopes at a high density on a pathogen’s surface </li></ul><ul><ul><li>Stimulate B cells specific for the Ag </li></ul></ul><ul><ul><li>Extensive X-linking of BCR to B-cell co-receptor </li></ul></ul><ul><li>What’s the result of this “Extensive X-linking ” </li></ul><ul><ul><ul><li>May over-ride need for additional signals </li></ul></ul></ul><ul><li>How long does this take? </li></ul><ul><ul><ul><ul><li>48 hrs after Ag encounter </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Ex: bacterial cell wall polysaccharides with B-1 cell as the IS responder </li></ul></ul></ul></ul>
  30. 30. Ab responses induced by TI-2 Ags <ul><li>Induce early Ab response to contain an infection – typically B-1 cells </li></ul><ul><ul><li>Limitations </li></ul></ul><ul><ul><ul><li>Little isotype switching >> IgM (some IgG) </li></ul></ul></ul><ul><ul><ul><li>No hypermutation  </li></ul></ul></ul><ul><ul><ul><ul><li>What is the result of no hypermutation? </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>no  affinity for Ag </li></ul></ul></ul></ul></ul><ul><ul><ul><li>No long-term immunological memory  no long lasting immunity for 2nd encounter </li></ul></ul></ul>
  31. 31. 9-4: Activation of naïve B cells by most antigens requires help from CD4 T cells
  32. 32. B cells needing T-cell help and Thymus-dependent Ags (TD Ags) <ul><li>Bulk of pathogen-specific Ab are produced by TD Ags </li></ul><ul><li>What does TD antigen do? </li></ul><ul><ul><li>TD Ags activate B cells in 2nd LT </li></ul></ul><ul><ul><li>Does this make sense? </li></ul></ul><ul><ul><ul><li>Well…..the 2nd LT is where B cells , specific Ag and helper CD4 T cells are brought together </li></ul></ul></ul>
  33. 33. B-cell Meets it’s Antigen <ul><li>If a B-cell meets it’s antigen  </li></ul><ul><ul><li>signals (BCR + co-receptor) are sent to the nucleus and induce changes in the expression of adhesion molecules and chemokine receptors at the surface </li></ul></ul><ul><li>These changes trap the B-cell in the T-cell area close to the B-cell zone. </li></ul><ul><ul><li>This allows for effector T-cells to test their TCR’s against the Ag-MHC II on the B-cell  cognate interactions and conjugate pairs </li></ul></ul>
  34. 34. Mature naïve B cells need T-cell help with Thymus-dependent Ags (TD Ags) Ag delivery dendritic cells <ul><li>Who delivers the Ag </li></ul><ul><li>from the afferent lymphatic vessel? </li></ul><ul><ul><li>P-APC, MHCII: dendritic cell </li></ul></ul>CCL19 and 21 CCL13 <ul><li>B cell migrate in blood or afferent lymph to LN </li></ul><ul><li>B cells leave blood  HEV  LN cortex  meets Ag </li></ul><ul><li>B-cells are drawn to T-cell areas by CCL21 and CCL 19 just like T-cells </li></ul><ul><li>B-cell doesn’t meet it’s antigen  drawn to follicle by CCL13 </li></ul><ul><li>Ags are trapped in the T-cell areas of LN </li></ul><ul><li>“ Ag specific CD4 T cell helpers” activated in presence of IL-4  Th2 help activate the B-cells </li></ul>Infected tissue 
  35. 35. Mature naïve B cells become trapped in the T-cell zone of 2nd LT if they encounter their “cognate” helper T cell <ul><li>Recirculating naïve B cells enter the LN T-cell zone from the blood  HEV </li></ul><ul><li>B cells encounter helper TH2 cells specific for the same Ag </li></ul><ul><li>B cells interact with TH2 to form a “1° focus” of proliferating activated B cells and TH2 cells in the medullary cords </li></ul><ul><li>Mostly IgM </li></ul><ul><li>Under influence of IL-5 & 6 </li></ul>
  36. 36. Does the BCR have a role in B cell activation? <ul><li>Does the BCR have a role in B cell activation? </li></ul><ul><ul><li>Two distinct BCR roles in B cell activation: </li></ul></ul><ul><ul><ul><li>Binding antigen  sends a signal to the B cells’ nucleus to change gene expression </li></ul></ul></ul><ul><ul><ul><li>Internalizing Ag by receptor mediated endocytosis  processing and presenting it to helper T-cells </li></ul></ul></ul>
  37. 37. Two signals  B cell proliferation & differentiation into plasma cell B-cell activation in response to TD-Ag requires T cell help
  38. 38. What happens when TH2 cell’s TCR binds peptide Ag:MHC class II molecules on the B cell surface? <ul><li>B cell CD40 :CD40 T cell ligand </li></ul><ul><li>Why ? </li></ul><ul><ul><li>a signal for the B cell to activate the transcription factor, NF  B  which then up regulates intercellular adhesion molecule 1 ( ICAM-1 ) expression which can bind to LFA-1 on T-cell </li></ul></ul><ul><ul><ul><li>What’s the functional result of the up-regulation of ICAM-1on the B cell’s surface? </li></ul></ul></ul><ul><ul><li>Strengthen the cognate interactions between the B and T-cell </li></ul></ul><ul><ul><li>Reorganization of cytoskeleton and golgi allow focused secretion of cytokines onto the B-cell </li></ul></ul><ul><ul><li>IL-4 being one of the most significant cytokines to drive B-cell proliferation and differentiation </li></ul></ul><ul><li>Signal sent to T-cell nucleus to make of IL-4 ? </li></ul><ul><ul><li>Essential for B cell proliferation and development to plasma cells </li></ul></ul><ul><ul><li>Characteristic of Th2 cells </li></ul></ul>
  39. 40. What happens to these dividing B cells? <ul><li>Recall: B cells activated by interaction with cognate helper T cells in the LN T-cell areas form a 1° focus of dividing B and T cells in the medullary cords of the LN </li></ul><ul><li>Result is dividing B lymphoblasts and some will secrete IgM </li></ul><ul><li>How long does a 1° focus of dividing B cells last? </li></ul>A few days
  40. 41. How long does it take for a GC to appear? What is the physiological symptom of GC formation? <ul><li>Division rate  1/6hr </li></ul><ul><li>Large metabolically active cells </li></ul><ul><ul><li>Centroblast </li></ul></ul><ul><li>Morphology of follicle changes  secondary follicle </li></ul><ul><li>Domination by the germinal center (lots of new B cells) </li></ul>Medullary cords PM cells IL5 & IL6 TH2 Y Y Y IgM 1° focus  1° follicles Still attached to TH2 1 2 Can isotype switching happen in a primary follicle? Several days 1 week Lymph node swelling some Primary Focus B cells
  41. 42. Germinal Center <ul><li>Specialized B-cell microenvironment where: </li></ul><ul><ul><li>proliferation </li></ul></ul><ul><ul><li>Somatic hypermutation </li></ul></ul><ul><ul><li>Selection of antigen binding </li></ul></ul><ul><li>What are dark zones? </li></ul><ul><ul><li>Close packed centroblasts </li></ul></ul><ul><li>What are light zones? </li></ul><ul><ul><li>Non-dividing centrocytes that will interact with FDC’s </li></ul></ul>
  42. 43. 9-7 Activated B cells undergo somatic hypermutation and isotype switching in the specialized microenvironment of the B-cell zone
  43. 44. Common theme of lymphocyte development B cell maturation in germinal center Activation proliferation selection Activation proliferation selection
  44. 45. Proliferation in the GC, Okay What about hypermutation and affinity maturation in the GC? centroblasts dividing in germinal center somatic hypermutation & isotype switching T-cell cytokines Nondividing centrocytes Mutated surface Ig Post hypermutation Surface Ig of centrocyte Affinity for a specific antigen higher lower equal
  45. 46. What’s the upshot of this hypermutation & affinity maturation <ul><li>GC centrocytes express Igs with a </li></ul><ul><ul><li>range of affinities for the specific Ag </li></ul></ul><ul><ul><li>The B-cells have to compete again for access to antigen on the FDC’s and then for access to antigen specific T-cells. </li></ul></ul><ul><li>What happens to centrocytes that fail to bind Ag:T-cell CD40 ligand? </li></ul><ul><ul><li>Centrocytes that fail to bind Ag/CD40 ligand on helper T cell interaction </li></ul></ul><ul><ul><ul><li>Apoptosis </li></ul></ul></ul>
  46. 47. Mutated centrocytes compete! To engage a helper T cell Centrocyte  bind Ag  process antigen  surface = MHCII + Ag Mutated centrocytes now compete! Access of Ag on FDC’s Ag-specific helper T cells MHCII 1 2 Follicular dendritic cells provide a source of intact Ag Mutated centrocyte
  47. 48. B cells recognize Ag as FDC surface “immune complexes” <ul><li>Localized in GC’s </li></ul><ul><li>FDC’s bind Ag in form of immune complexes (Ag:complement or Ag:Ab:complement) </li></ul><ul><ul><li>Bind to FDC Fc receptors </li></ul></ul><ul><ul><li>Fc and complement receptors on FDCs </li></ul></ul><ul><li>Immune complexes are not internalized </li></ul><ul><li>Persist on FDC’s for long periods (years) </li></ul>
  48. 49. Iccosomes are immune-complex coated bodies <ul><li>Bundles of membrane coated with immune complexes also bud off from the surface of FDCs  iccosomes </li></ul><ul><ul><li>FDCs have a prominent cell body and dendritic processes </li></ul></ul><ul><ul><li>Immune complexes are bound on the FDC surface  become clustered  prominent beads are formed along the dendrites </li></ul></ul><ul><ul><li>Beads shed from the cells  iccosomes </li></ul></ul>Iccosomes  taken up by Ag specific B cells in the GC  bound  B cells process & present Ag
  49. 50. Newly formed centrocytes move from the dark zone of the gc to contact FDCs in the light zone <ul><li>Newly formed centrocytes move from GC dark zone  captures Ag from FDC or iccosomes  moves to GC light zone outer regions to helper T cells </li></ul><ul><li>Engagement of peptide:MHCII by TCR complex & CD40 ligand  induces the centrocyte to express Bcl-xL proteins </li></ul><ul><ul><li>Bcl-xL functions to prevent death by apoptosis </li></ul></ul>
  50. 51. <ul><li>Hypermutated B cells interact with FDCs displaying surface immune complexes </li></ul><ul><li>B cells  do not bind Ag or poor Ag binding receptors due to mutated beyond recognition, therefore they cannot compete for access to the FDCs  apoptosis </li></ul><ul><li>B cells  receptors that bind Ag well  receive signals from the T cell  are induced to express Bcl-xL </li></ul><ul><ul><li>Results in preventing apoptosis </li></ul></ul><ul><ul><li>Therefore, these B cells survive </li></ul></ul>
  51. 52. <ul><li>After somatic hypermutation… B cells with high-affinity receptors for Ag are rescued from apoptosis </li></ul><ul><li>I.e. highest-affinity Ag receptors are selected for survival  differentiation into Ab-producing plasma cells and into long lived memory cells </li></ul><ul><li>Affinity of Abs for specific Ag increase during immune response is called </li></ul><ul><ul><li>Affinity maturation </li></ul></ul>
  52. 53. Survivors interact with Ag-specific T cells…Why? <ul><li>Mutual engagement of ligands and receptors on surviving centrocyte with T helper cell leads to further proliferation of both B and T cells , serves to: </li></ul><ul><ul><li>increases population of selected high-affinity , isotype-switch B cell </li></ul></ul><ul><ul><li>Some B cells are directed down the path of plasma or memory cells </li></ul></ul><ul><li>Height of adaptive immune response = need large # of Abs to fight infection  selected centrocytes leave GC  differentiate into Ab-producing plasma cells </li></ul>
  53. 54. What happens to centrocytes that fail to obtain, internalize & present Ag? <ul><li>What happens when centrocyte fails  does not obtain, internalize and present Ag  Apoptosis  Macrophage engulfment in the gc </li></ul><ul><li>Tingible body macrophages are macrophages that have recently engulfed the apoptotic centrocytes are a characteristic feature of gc, </li></ul><ul><ul><li>because of their contents they are called “tingible body macrophages” </li></ul></ul><ul><li>What happens when somatic hypermutation produces centrocytes bearing Ig reactive to self-Ag? </li></ul><ul><ul><li>Contact with helper T cells in the GC  render these centrocytes anergic </li></ul></ul>
  54. 55. Chapter 9 – Lecture Notes Immunity Mediated by B Cells and Antibodies
  55. 56. Chapter 9 continued
  56. 57. 9-9: Interactions with T cells are required for isotype switching in B cells
  57. 58. Interactions with T cells are required for isotype switching in B cells, Why? <ul><li>Cytokine effects on the switching of Ig synthesis to a particular isotype </li></ul><ul><ul><ul><li>Induce </li></ul></ul></ul><ul><ul><ul><li>augment </li></ul></ul></ul><ul><ul><ul><li>inhibit </li></ul></ul></ul><ul><li>1st Igs = IgM and IgD </li></ul><ul><ul><li>B cells in GC activated by Ag  switch H-chain isotype  IgG, IgA or IgE </li></ul></ul><ul><li>The isotype to which an individual B cells switches is determined by? </li></ul>
  58. 59. … Cognate interactions with helper T cell <ul><li>What does “cognate” mean? </li></ul><ul><li>Cognate interactions are cell-cell interactions between B and T lymphocytes specific for the same antigen </li></ul><ul><li>How does the helper T cell control the particular isotype to which a switch is made? </li></ul><ul><ul><li>controlled by the cytokines secreted by the helper T cell </li></ul></ul><ul><li>Roles of specific cytokines in switching is area of hot research in immunology </li></ul>
  59. 60. Interactions with T cells are required for isotype switching in B cells <ul><li>“Based on mouse B cell experiments” </li></ul><ul><li>Differences in humans (from mouse) are not worked out </li></ul><ul><ul><li>Example, switching to IgA in humans involve TGF-  and IL-10 , not IL-5 </li></ul></ul>
  60. 61. Which cytokines are most involved? <ul><li>Prominent players include cytokines </li></ul><ul><ul><li>secreted by TH2 cells </li></ul></ul><ul><ul><ul><li>IL-4, IL-5 and TGF  </li></ul></ul></ul><ul><li>What do these cytokines do? </li></ul><ul><ul><li>Activate naïve B cells to differentiate into plasma cells </li></ul></ul><ul><ul><ul><li>Secreting IgM </li></ul></ul></ul><ul><ul><li>Induce the production of other antibody isotypes </li></ul></ul><ul><ul><ul><li>IgG2, IgG4 ( weak opsonizing Abs) </li></ul></ul></ul><ul><ul><ul><li>IgA, IgE </li></ul></ul></ul><ul><li>IFN-  is a characteristic TH1 cytokine </li></ul><ul><ul><li>Switches B cells to make IgG1 </li></ul></ul><ul><ul><li>Strong opsonizing Ab in humans </li></ul></ul>
  61. 62. How do T-cell cytokines induce isotype switching? <ul><li>Stimulate transcription from the switch regions that lie 5’ to each H-chain C gene </li></ul><ul><li>Example, activated B cells exposed to IL-4 </li></ul><ul><ul><li>Transcription from a site upstream of the switch regions of C  1 and C  is detected prior to switching </li></ul></ul><ul><li>What’s the molecular mechanism for isotype switching? </li></ul><ul><ul><li>Transcript opens up the chromatin </li></ul></ul><ul><ul><li>Makes the switch regions accessible to somatic recombination machinery </li></ul></ul><ul><ul><ul><li>Somatic recombination machinery places a new C gene in juxtaposition to the V-region </li></ul></ul></ul>
  62. 63. What else does isotype switching by cognate helper T cells require? <ul><li>B cell surface CD40 binding to T-cell CD40 ligand </li></ul><ul><ul><li>This is very important… </li></ul></ul><ul><li>How do we know this is important? </li></ul><ul><li>Example, pts who lack CD40 ligand ( Hyper-IgM syndrome ) </li></ul><ul><ul><li>Immunodeficient </li></ul></ul><ul><ul><li>Abnormally high levels of IgM in blood serum </li></ul></ul><ul><ul><li>Almost no IgG and IgA </li></ul></ul><ul><ul><li>B cells are unable to switch isotype </li></ul></ul><ul><ul><li>Cannot make Ab responses to TD Ags </li></ul></ul><ul><ul><ul><li>Males </li></ul></ul></ul><ul><ul><ul><ul><li>gene for CD40 ligand is on the X-Chr </li></ul></ul></ul></ul><ul><ul><ul><ul><li>X-linked </li></ul></ul></ul></ul>
  63. 64. Hyper-IgM syndrome No GC in Lymph Nodes GC in Lymph nodes
  64. 65. Isotype switched, affinity matured B-cells can differentiate into plasma cells or memory cells <ul><li>When the infection is bad, and secreted Ab’s are needed then the centrocytes will be told to become plasma cells – IL-10 </li></ul><ul><li>As the infection begins to subside, the centrocytes will be told to become memory B-cells – IL-4 </li></ul>
  65. 67. Antibody effector functions <ul><li>What does isotype switching do? </li></ul><ul><li>Isotype switching diversifies Ab Fc region functional properties </li></ul><ul><li>Two distinct functions of Fc regions </li></ul><ul><ul><li>Deliver Ab to anatomical sites otherwise inaccessible </li></ul></ul><ul><ul><li>Link bound Ag to molecules or cells that will effect it’s destruction </li></ul></ul><ul><ul><ul><li>These cells carry Fc receptors (Examples: macrophages and neutrophils). </li></ul></ul></ul>
  66. 68. 9-11 IgM, IgG and IgA Abs protect the blood and extracellular fluids
  67. 69. IgM, IgG and IgA Ab functions <ul><li>Protect the blood and extracellular fluids </li></ul><ul><li>IgM = 1st Ab, pentamer, enters the blood carried to site of infection </li></ul><ul><li>Pentameric nature = strong binding to microorganisms or particulate Ags </li></ul><ul><li>Large size limits penetration to infected tissue </li></ul><ul><ul><li>Reduced ability to passively leave the blood to penetrate infected tissue </li></ul></ul><ul><ul><li>No IgM Fc receptors on phagocytic cells or leukocytes </li></ul></ul><ul><ul><ul><li>IgM cannot directly recruit these cells </li></ul></ul></ul><ul><ul><ul><li>IgM Fc regions does, however, bind complement and can activate the complement system </li></ul></ul></ul>
  68. 70. Which blood-borne Ab is dominant later in an immune response? <ul><li>IgG </li></ul><ul><ul><li>Smaller </li></ul></ul><ul><ul><li>Together IgM and IgG function to prevent blood-borne infection </li></ul></ul><ul><ul><ul><li>septicemia </li></ul></ul></ul>
  69. 71. What about IgA? <ul><li>Synthesized by plasma cells in 2nd LT </li></ul><ul><li>Two forms </li></ul><ul><ul><li>Monomeric </li></ul></ul><ul><ul><li>Dimeric </li></ul></ul><ul><li>Monomeric IgA </li></ul><ul><ul><li>Secreted from plasma cells derived from B cells that switched their Ab isotype in the LN or spleen </li></ul></ul><ul><ul><li>Enters extracellular spaces and helps IgG protect against bacteria and virus particles </li></ul></ul><ul><li>Dimeric IgA </li></ul><ul><ul><li>Made in 2nd LT underlying mucosal surfaces </li></ul></ul>
  70. 72. IgA & IgG are transported across epithelial barriers by specific receptor proteins <ul><li>Recall: IgM, IgG and monomeric IgA provide Ag-binding within fluids and tissues of the body </li></ul><ul><li>Dimeric IgA protects epithelia surfaces that communicate with the external environment (mucosal membranes susceptible to infection) </li></ul><ul><ul><li>Linings of the GI tract, eyes, nose, throat, mammary glands, respiratory, urinary and genital tracts </li></ul></ul><ul><ul><li>Dimeric IgA made in the lamina propria </li></ul></ul><ul><ul><ul><li>Connective tissue underlying basement membrane of mucosal epithelium </li></ul></ul></ul><ul><ul><ul><li>IgA-secreting plasma cells are on one side and the target pathogens are on the other side </li></ul></ul></ul><ul><ul><ul><li>The dimeric IgA molecules are transported across the epithelium by receptors on the basolateral surface of the epithelial cells </li></ul></ul></ul>
  71. 73. How are IgA & IgG transported across epithelial barriers? <ul><li>Dimeric IgA bound by a J chain diffuses across the basement membrane </li></ul><ul><li>Dimeric IgA bound by the poly-Ig receptor on the basolateral surface of epithelial cell </li></ul><ul><li>Transcytosis – receptor mediated transport from one side of a cell to the other of a macromolecule </li></ul><ul><li>Complex  transcytosis  vesicles  released  apical surface </li></ul><ul><li>Poly-Ig receptor  cleaved  releases IgA from epithelial cell membrane </li></ul><ul><ul><li>IgA still bound to a fragment of the receptor called the “secretory component” (holds IgA to mucus) </li></ul></ul><ul><li>Residual membrane-bound fragment of the poly-Ig receptor is nonfunctional  degraded </li></ul>
  72. 74. How are IgA & IgG transported across epithelial barriers? Apical surface Basolateral surface Receptor mediated endocytosis Transcytosis
  73. 75. Brambell receptor (FcRB) <ul><li>IgG actively transported from blood into extracellular spaces by an Fc receptor present on endothelial cells </li></ul><ul><ul><li>Receptor called FcRB </li></ul></ul><ul><ul><li>Similar structure to MHC class I (  1 &  2 domains to bind the Fc regions of the Ab) </li></ul></ul><ul><ul><li>Ab:receptor complex </li></ul></ul><ul><ul><ul><li>2 molecules of FcRB bind to the Fc region of one IgG </li></ul></ul></ul><ul><ul><ul><li>IgG delivery to extracellular spaces in connective tissue  protects IgG from degradation pathways of serum proteins </li></ul></ul></ul><ul><ul><ul><li>IgG relatively long lived half-life </li></ul></ul></ul>
  74. 76. Brambell receptor (FcRB)
  75. 77. Passive transfer of immunity <ul><li>Fetus physically protected by the mother from extracellular microorganisms </li></ul><ul><li>What about after birth ? </li></ul><ul><ul><li>Lack actively acquired immunity </li></ul></ul><ul><ul><li>Vulnerable to infections (microbial colonization of epithelia) </li></ul></ul><ul><ul><li>Receives IgA from its mother - secreted into breast milk </li></ul></ul><ul><ul><ul><li>Transferred to baby’s gut </li></ul></ul></ul><ul><ul><ul><ul><li>Bind to microorganism preventing attachment to the gut epithelium </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Pass in the feces </li></ul></ul></ul></ul></ul>
  76. 78. What about IgG during pregnancy? <ul><li>IgG from maternal circulation transported across placenta </li></ul><ul><ul><li>Directly into the babies bloodstream </li></ul></ul><ul><ul><li>Therefore babies have a level of IgG protection in their plasma equal to the mother’s level of IgG protection </li></ul></ul><ul><ul><li>Transport of the IgG antibodies across the placenta is also mediated by the FcRB. </li></ul></ul>
  77. 79. Antibody production is deficient in very young infants <ul><li>1st year = vulnerable to infection, deficient in Abs </li></ul><ul><li>During pregnancy maternal IgG Ab transported across the placenta </li></ul><ul><li>What happens to this IgG Ab ? </li></ul><ul><ul><li>Maternally derived IgG is catabolized </li></ul></ul><ul><ul><li>Gradually decreases until infant’s immune system produces its own Ab ( 6 months ) </li></ul></ul><ul><ul><li>Therefore, IgG levels are lowest in infants aged 3-12 months (time the infant is most vulnerable to infection) </li></ul></ul><ul><ul><ul><li>Premature babies are even more vulnerable </li></ul></ul></ul><ul><ul><ul><li>Take longer to attain immunocompetence after birth than full term babies </li></ul></ul></ul>
  78. 80. High-Affinity IgG and IgA are used to neutralize microbial toxins and animal venoms <ul><li>Some bacteria secrete toxins that disrupt the normal function of cells. </li></ul><ul><li>Diphtheria and tetanus toxins have a binding part and a toxic part. </li></ul><ul><li>Ab’s that bind to the binding part of the toxic molecule will block the toxins ability to enter the cell thus blocking its toxic effects. </li></ul><ul><li>For venoms , passive immunization is used. (antibodies created using a large animal inoculated with the venom) </li></ul><ul><ul><li>These antibodies are gathered from the animal and given to a snake bite victim to neutralize the toxins (no long term immunity) </li></ul></ul>
  79. 81. High-affinity neutralizing Ab’s prevent viruses and bacteria from infecting cells
  80. 82. Ab’s link effector cells to the antigen <ul><li>Abs = only effector molecules produced by B cells </li></ul><ul><li>Abs = 1  function = “adaptor molecules” </li></ul><ul><ul><li>neutralize the pathogen ( doesn’t destroy pathogen ) </li></ul></ul><ul><ul><li>bring “pathogens” and “effector” cells together </li></ul></ul><ul><ul><ul><li>Why? </li></ul></ul></ul><ul><ul><ul><li>So effector cells clear/destroy the pathogen </li></ul></ul></ul>
  81. 83. Fc receptors <ul><li>Abs bind through their Fc regions </li></ul><ul><li>Bind to what? </li></ul><ul><ul><li>Fc receptors on effector cells </li></ul></ul><ul><ul><ul><li>Fc receptors are specific for Ig isotype </li></ul></ul></ul><ul><ul><ul><li>Fc receptors have 2 main purposes </li></ul></ul></ul><ul><ul><ul><ul><li>Deliver Abs to sites where they would not be carried by blood and lymph circulation </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Attach Ag:Ab complexes to effector cells allowing effector cells to destroy the pathogens </li></ul></ul></ul></ul>
  82. 84. Fc of Ab and Fc  receptor of effector cell
  83. 85. IgE binds to high-affinity Fc receptors on mast cells, basophils and activated eosinophils. <ul><li>Fc  RI has high affinity for IgE. </li></ul><ul><li>Crosslinking of receptors by antigen causes release of histamine from mast cells  inflammation  recruits cells and proteins needed for host defense. </li></ul>
  84. 86. High points of 9.11 - 9.16 & 9.21 - 9.25 <ul><li>High-affinity IgG and IgA Abs are used to </li></ul><ul><ul><li>neutralize microbial toxins and animal venoms </li></ul></ul><ul><li>High-affinity neutralizing Abs prevent </li></ul><ul><ul><li>viruses and bacteria from infecting cells </li></ul></ul><ul><li>Fc receptors of hematopoietic cells are signaling receptors that </li></ul><ul><ul><li>bind the Fc regions of Abs </li></ul></ul><ul><li>Phagocyte Fc receptors facilitate Ab-coated pathogen </li></ul><ul><ul><ul><li>recognition </li></ul></ul></ul><ul><ul><ul><li>uptake </li></ul></ul></ul><ul><ul><ul><li>destruction </li></ul></ul></ul><ul><li>IgE binds to high-affinity Fc receptors on </li></ul><ul><ul><ul><li>mast cells </li></ul></ul></ul><ul><ul><ul><li>basophils </li></ul></ul></ul><ul><ul><ul><li>activated eosinophils </li></ul></ul></ul><ul><li>Fc receptors activate NK cells to destroy Ab-coated human cells </li></ul><ul><ul><li>Antibody-dependent cell-mediated cytotoxicity (ADCC) (look up) </li></ul></ul>
  85. 87. Immune complexes and the complement system <ul><li>Immune complexes of Ag:Ab are often </li></ul><ul><ul><li>ingested by phagocytic cells and destroyed intracellularly </li></ul></ul><ul><ul><li>fate of most foreign materials that have Abs produced against them </li></ul></ul><ul><li>But the fate of immune complexes also depends heavily upon their ability to activate the complement system </li></ul>
  86. 88. Immune Complex Clearance <ul><li>C3b can coat particulate antigen (immune complexes) that can then be bound by erythrocytes and cleared in the liver or spleen. </li></ul>
  87. 90. Classical Complement Cascade <ul><li>Need one IgM or at least two IgG. </li></ul><ul><li>C1 binds to the Fc regions and needs at least two binding sites to become stable on the pathogens surface. </li></ul><ul><li>The rest of the cascade proceeds the same way as you learned previously. </li></ul><ul><li>Don’t forget that the alternative pathway can amplify the cascade that was started by the Ab’s (classical). </li></ul>
  88. 91. The complement system – A review of what you have already learned. Slides 91 – 119 are to help you in remembering complement <ul><li>~ 30 serum proteins </li></ul><ul><ul><li>Interact in a complex reaction sequences </li></ul></ul><ul><ul><ul><li>“ complement cascade” </li></ul></ul></ul><ul><li>Results in the manifestation of </li></ul><ul><ul><ul><li>inflammation </li></ul></ul></ul><ul><ul><ul><li>phagocytosis </li></ul></ul></ul><ul><ul><ul><li>lysis of foreign cells </li></ul></ul></ul><ul><li>Why call it COMPLEMENT ? </li></ul><ul><ul><li>The action of these proteins </li></ul></ul><ul><ul><ul><li>“ complements” antibody-mediated reactions </li></ul></ul></ul>
  89. 92. Complement components circulate in blood and body tissues <ul><li>What is the complement system & how does it function? </li></ul><ul><li>Enzymes that work in a cascade  one becomes active  cleaves another to activate it  etc. </li></ul><ul><li>Center stage = C3  cleaved ( C3b and C3a )  becomes activated by </li></ul><ul><ul><li>Abs aggregated in immune complexes </li></ul></ul><ul><ul><li>bacterial compounds that 1st bind and activate other complement proteins </li></ul></ul>
  90. 93. Complement components C3b and C3a <ul><li>C3b and C3a degradation products have direct </li></ul><ul><ul><li>opsonizing effects </li></ul></ul><ul><ul><li>chemotactic effects </li></ul></ul><ul><ul><li>inflammatory effects </li></ul></ul><ul><li>One of the functions is to activate the lytic pathway involving C5-C9 </li></ul>
  91. 95. Several complement pathways <ul><li>Classical pathway </li></ul><ul><li>Alternative pathway </li></ul><ul><li>Lectin pathway </li></ul><ul><li>Proteins of the complement system (CS) </li></ul><ul><ul><li>5% of serum proteins in vertebrates </li></ul></ul><ul><li>Classical pathway proteins are designated by number following C for complement </li></ul><ul><ul><ul><li>Range C1  C9 </li></ul></ul></ul><ul><li>Proteins of the alternative pathway </li></ul><ul><ul><li>C3, C5  C9 proteins </li></ul></ul><ul><ul><li>factor B </li></ul></ul><ul><ul><li>factor D </li></ul></ul>
  92. 96. Cascade <ul><li>Complement system proteins act in a sequence (cascade) </li></ul><ul><li>Each protein activates the next one in the series </li></ul><ul><ul><li>cleaves the next protein </li></ul></ul><ul><ul><ul><li>Resulting components have new functions </li></ul></ul></ul><ul><li>What jump starts the classical pathway? </li></ul><ul><ul><li>Ag:Ab </li></ul></ul><ul><li>Is it the same for the alternative pathway? </li></ul><ul><ul><li>No…initiated by bacteria and fungi cell-wall polysaccharides </li></ul></ul>
  93. 97. C3 <ul><li>C3 has major role in initiation of mechanisms that lead to microbial destruction </li></ul><ul><ul><li>In fact, both classical and alternative pathways lead to the cleavage of C3 into fragments </li></ul></ul><ul><ul><ul><li>C3a </li></ul></ul></ul><ul><ul><ul><li>C3b </li></ul></ul></ul><ul><li>C3a and C3b initiate </li></ul><ul><ul><li>cytolysis </li></ul></ul><ul><ul><li>inflammation </li></ul></ul><ul><ul><li>opsonization </li></ul></ul>
  94. 98. The complement component C1
  95. 100. Cleavage of C4 exposes a reactive thioester bond that covalently attaches the C4b fragment to the pathogen surface
  96. 101. Activated C1s cleaves C4 and C2 to produce C4b and C2a, which associate to form the classical C3 Convertase
  97. 102. Formation of the alternative C3 convertase
  98. 103. The two types of C3 convertase have similar structures and functions
  99. 104. C3 activation by the alternative C3 convertase is a process analogous to C3 activation by the classical C3 convertase
  100. 105. View from above of complement deposition <ul><li>Keep in mind that classical activation is also amplified by the alternative pathway C3 convertase. </li></ul>
  101. 106. Cytolysis <ul><li>Most important function of the complement cascade is… </li></ul><ul><ul><li>destroy foreign cells (microbial/pathogens) </li></ul></ul><ul><li>How does the IS do this? </li></ul><ul><ul><li>damages the cell membrane to the point that the cellular contents leak out </li></ul></ul><ul><ul><ul><li>cytolysis </li></ul></ul></ul><ul><li>C5 - C9 = membrane attack complex </li></ul><ul><ul><ul><ul><li>MAC </li></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>produces transmembrane channels through the cell membrane </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>leads to cell lysis or cytolysis </li></ul></ul></ul></ul></ul>
  102. 107. Inflammation <ul><li>C3a and C5a contribute to acute inflammation </li></ul><ul><li>What exactly do they do? </li></ul><ul><ul><li>bind mast cells, basophils and blood platelets </li></ul></ul><ul><ul><li> release of histamine </li></ul></ul><ul><ul><ul><li>Histamine increases blood vessel permeability </li></ul></ul></ul><ul><li>C5a acts as a powerful chemotactic factor </li></ul><ul><li>What’s this mean? </li></ul><ul><ul><li>C5a is able to attract phagocytes to areas where complement has been fixed </li></ul></ul>
  103. 108. The Classical Pathway- Step 1: C1 component <ul><li>C1 component is comprised of three parts </li></ul><ul><ul><li>C1q (hexamer) </li></ul></ul><ul><ul><li>C1r (dimer) </li></ul></ul><ul><ul><li>C1s (dimer) </li></ul></ul><ul><ul><ul><li>Held together by calcium ions </li></ul></ul></ul>
  104. 109. The Classical Pathway- Step 1: Activation of C1 <ul><li>Initiated by the binding of C1q to C1q-specific receptors on the Fc portions of adjacent Abs (minimum 2 IgG Abs or one IgM) </li></ul><ul><ul><li>Humans, all Abs have these receptors </li></ul></ul><ul><ul><ul><li>exceptions of IgG4 , IgA and IgE </li></ul></ul></ul><ul><li>Abs possessing these receptors bind or “fix” complement </li></ul><ul><ul><li>Abs without these receptors, cannot fix complement </li></ul></ul><ul><li>Once C1q has bound the C1q specific receptors of two adjacent antibodies, it activates C1r , which in turn activates C1s </li></ul><ul><li>Activated C1s subcomponent then activates C4 </li></ul>
  105. 110. The Classical Pathway- Step 2: Activation of C4 <ul><li>C4 is the 2nd complement component of the cascade but was the 4th complement component identified, “C4” </li></ul><ul><li>Activated C1s activates C4 by cleaving it into two fragments </li></ul><ul><ul><li>C4a </li></ul></ul><ul><ul><li>C4b </li></ul></ul><ul><li>C4b binds to the surface of the cell membrane near the site of the Ag:Ab complex </li></ul><ul><li>Once bound to the surface, C4b then binds the C2a complement component </li></ul>
  106. 111. The Classical Pathway- Step 3: Activation of C2 <ul><li>C2 component is cleaved by the combined activities of C4b and C1s </li></ul><ul><ul><li>C2a </li></ul></ul><ul><ul><li>C2b </li></ul></ul><ul><li>C2a portion remains attached to the C4b component on the cell membrane </li></ul><ul><li>C4b-C2a complex is referred to C3 convertase </li></ul><ul><li>C3 convertase activates the C3 component of complement </li></ul>
  107. 112. The Classical Pathway- Step 4: Cleaving C3 and amplification <ul><li>C3 is activated by the activity of C3 convertase </li></ul><ul><ul><li>This cleaves C3 </li></ul></ul><ul><ul><ul><li>C3a </li></ul></ul></ul><ul><ul><ul><li>C3b </li></ul></ul></ul><ul><ul><li>A single molecule of C3 convertase is capable of activating hundreds of molecules of C3 </li></ul></ul><ul><ul><ul><li>amplifying the cascade by providing large amounts of C3a and particularly C3b </li></ul></ul></ul>
  108. 113. The Classical Pathway- Step 4: Activation of C3 <ul><li>Both C3a and C3b have biological activity </li></ul><ul><ul><li>C3a functions as an anaphylatoxin </li></ul></ul><ul><ul><li>C3b attaches to the cell membrane near the site of activation </li></ul></ul><ul><ul><ul><li>where it is capable of acting as an opsonin </li></ul></ul></ul><ul><li>Some C3b combines with the C3 convertase to form active classical C5 convertase (C4b2a3b) </li></ul><ul><li>Active classical C5 convertase activates complement component C5 </li></ul><ul><li>Alternative C5 convertase (C3b 2 Bb) </li></ul>
  109. 114. Wait a minute…What about MAC? <ul><li>What about it’s formation? </li></ul><ul><li>Who is MAC? </li></ul><ul><ul><li>The membrane attack complex (MAC) </li></ul></ul><ul><ul><ul><li>formed as the result of the assembly and activation of complement components C5 through C9 </li></ul></ul></ul><ul><ul><ul><li>involved in cytolysis of the target cell </li></ul></ul></ul>
  110. 115. The Classical Pathway- Step 5: Activation of C5, C6 and C7 <ul><li>C5 is cleaved by C5 convertase </li></ul><ul><ul><li>C5a </li></ul></ul><ul><ul><li>C5b </li></ul></ul><ul><li>Both of which have biological activity </li></ul><ul><li>C5a functions as an anaphylatoxin and a chemotaxin </li></ul><ul><li>C5b binds C6 and C7 </li></ul><ul><ul><li>forming a complex that attaches to the surface of the target cell membrane </li></ul></ul>
  111. 116. The Classical Pathway- Step 6: Activation of C8 and C9 <ul><li>C8 component of complement then binds the C5b-C6-C7 complex on the cell membrane </li></ul><ul><li>This complex is capable of forming small pores in the membrane of the target cell </li></ul><ul><ul><li>= compromising its integrity </li></ul></ul><ul><li>This is enhanced by the polymerization of the C9 complement component </li></ul><ul><li>The polymerization of C9 leads to the formation of </li></ul><ul><ul><li>transmembrane channels </li></ul></ul><ul><ul><li> target cell lysis </li></ul></ul>
  112. 117. MAC
  113. 119. Complement System = Major defense mechanism against microbial infection <ul><li>Especially extracellular bacteria </li></ul><ul><li>Complement components are plasma proteins of several functional groups </li></ul><ul><li>Become activated by infections in 3 different ways </li></ul><ul><ul><li>classical pathways </li></ul></ul><ul><ul><li>alternative pathways </li></ul></ul><ul><ul><li>lectin pathways </li></ul></ul><ul><li>The activation of phagocytes and inflammatory reactions by complement provides protection </li></ul><ul><ul><li>before the antibody response develops </li></ul></ul><ul><ul><li>after the antibody response develops </li></ul></ul><ul><li>The defense mechanism of complement-mediated phagocytosis of pathogens evolved long before the existence of Ab </li></ul><ul><li>Molecular Hx = Ab that actually provided the “complementary function” rather than the complement </li></ul>

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