Immunology Chapter 9
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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 Immunology Chapter 9 Presentation Transcript

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