ADAPTIVE IMMUNE RESPONSE

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ADAPTIVE IMMUNE RESPONSE

• Immune response to infections
• 1. Innate response
• Neutrophils, macrophages, NK cells at site of
  infection
• 2. Initiation of adaptive response
• iDCs phagocytose Ag and take to nodes
• DCs present Ag to T cells
• Get specific helper (CD4) T cells
• help B cell activation humoral immunity
• Activate macrophages CMI
• Get specific cytotoxic (CD8) T cells
• Kill virally infected cells CMI

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ADAPTIVE IMMUNE RESPONSET CELLS

• Development in thymus
• Immunogenetic events produce multiple TCRs
• Similar to B cells
• TCR gene with multiple V, D, and J segments
• Recognize 1015 epitopes
• T cells with TCRs that react with self antigens
  deleted
• Start with double negative cells no TCR, CD4 or
  CD8
• Next get double positive cells TCR, CD4, CD8
• Positive selection weak recognition of MHCI or
  MHCII
• Negative selection if reaction is strong
  apoptosis
• 2 of cells survive

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ADAPTIVE IMMUNE RESPONSET CELLS
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ADAPTIVE IMMUNE RESPONSET CELLS

• TCR complex
• T cell receptor similar to Fab of antibodies
• Alpha and beta Ig-like chains with variable and
  constant domains
• Hydrophobic transmembrane regions
• CD3 proteins
• Gamma epsilon delta epsilon, each with one
  Ig-like domain
• Transmembrane segment binds to TCR
• Cytoplasmic domain of each has one ITAM
• ITAM immunoreceptor tyrosine-based activation
  notif for signaling
• Zeta chains disulfide-liked homodimer, each
  protein has 3 ITAM
• Transmembrane region with aspartate

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ADAPTIVE IMMUNE RESPONSET CELLS
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ADAPTIVE IMMUNE RESPONSET CELLS

• Co-receptors
• CD4 protein binds to MHC-II on APC
• CD4 T cells helper T cells
• MHC-II only on APCs
• CD8 protein binds to MHC-I on target cells
• CD8 T cells cytotoxic T cells
• MHC-I on all nucleated cells

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ADAPTIVE IMMUNE RESPONSET CELLS

• Accessory molecules lymphocyte surface molecule
  distinct from Ag receptor complex adhesion or
  signaling
• CD45R, CD28, CD40L
• Adhesion molecules tighten interaction with APC
• LFA (leukocyte function-associated antigen)-1,
  CD2
• Cytokine receptors IL-1R, IL-2R, others

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ADAPTIVE IMMUNE RESPONSEantigen presenting cells

• Antigen Presentation to T cells
• MHC (major histocompatibility complex) 1
• all nucleated cells
• MHC-encoded alpha (hvy) chain and beta 2 (lt)
  microglobulin
• Polymorphic alpha 1 and alpha 2 domains for
  closed binding cleft
• agretope
• Conserved alpha 3 domains binding site for CD8
• Beta 2 interacts noncovalently with alpha 3

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ADAPTIVE IMMUNE RESPONSEantigen presenting cells

• Abbas Fig 4-4

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ADAPTIVE IMMUNE RESPONSEantigen presenting cells

• MHC-II dimer of alpha and beta subunits
• APCs
• Both chains MHC encoded
• Alpha 1 and beta 1 domains variable and form open
  binding cleft
• Most of variability on beta chain
• Agretope
• Alpha 2 and beta 2 folded into Ig domains
• B2 Ig domain binds to CD4

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ADAPTIVE IMMUNE RESPONSEantigen presenting cells

• Abbas Fig 4-6

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ADAPTIVE IMMUNE RESPONSEantigen presenting cells

• Peptide presentation by MHC I molecules
• MHC I molecule synthesized in endoplasmic
  reticulum (ER)
• Cytoplasmic protein (including microbial)
  degraded by proteosome
• Usually dealing with degraded (misfolded etc)
  normal proteins
• Degraded proteins marked by ubiquitin
• To ER via TAP (transporter assoc w Ag processing)
• Ag peptide binds to cleft in hvy chain of MHC-I
• Peptide of 8 or 9 amino acids
• MHC-I with peptide to cell membrane via golgi and
  exocytic vesicle
• Uninfected cells usually coated with MHC-I with
  self proteins

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ADAPTIVE IMMUNE RESPONSEantigen presenting cells

• Peptide presentation by MHC II molecules
• Phagocytized proteins degraded in phagolysosomes
  by APCs
• MHC-II molecule synthesized in ER
• Invariant chain (Ii) placed in cleft
• Ii provides stability and prevents MHC-II from
  binding with ER peptides
• Lysosomal and exocytic vesicles merge
• Invariant chain replaced with degraded peptide
• cleft open at ends longer peptides (30 aa)
  presented
• Fused vesicle migrates to cell membrane

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ADAPTIVE IMMUNE RESPONSEantigen presenting cells

• Antigen processing and presentation Abbas Fig
  5-8

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ADAPTIVE IMMUNE RESPONSEantigen presenting cells

• Cross-presentation of Ag
• iDCs phagocytise virally infected cells
• Some peptides migrate to cytoplasm and processed
  to MHC I
• Also, some cytoplasmic peptides from phagocytised
  cells processed to MHC II

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ADAPTIVE IMMUNE RESPONSEantigen presenting cells

• Cross-presentation of Ag to CD8 T cells. Abbas
  Fig 5-7

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ADAPTIVE IMMUNE RESPONSEantigen presenting cells

• Antigen Presenting Cells (APCs) Dendritic cells
  (DCs), B cells, and Macs
• Dendritic cells only APC that can initiate Ag
  specific response
• Presumably from myeloid and lineage
• Precursor DCs in blood differentiate into
  immature DCs (iDC)
• iDC recognized antigen with TLR and other
  receptors activated
• Pinocytosis/phagocytosis and cytokine production,
  now DC
• DCs can no longer phagocytose go to T-cell area
  of lymph nodes where they present Ag to T cells
• Langerhans cells are skin DC

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ADAPTIVE IMMUNE RESPONSEantigen presentation to
T cells

• T Lymphocytes
• Recognize antigens (on dendritic cells) in
  peripheral lymphoid organs, resulting in
  expansion of Ag-specific lymphocyte pool
• Differentiate into effector cells and memory
  cells
• Effector lymphocytes become activated when they
  recognize Ag
• Activation of T cells requires recognition of Ag
  displayed on APCs, costimulators, and cytokines
  produced by both APC and T cell

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ADAPTIVE IMMUNE RESPONSE antigen presentation to
T cells

• Activation of CD4 T Cells
• MHC II on APC binds with TCR
• TCR recognizes foreign protein MHC self
  proteins
• Interaction strengthened by binding of CD4 to MHC
  II molecule
• Signal transmitted via TCR/CD3 complex zeta
  proteins
• Adhesion molecule interactions
• LFA-3 on APC with CD2 on T cell
• ICAM-1 on APC with LFA-1 on T cell
• Costimulatory signals also required
• B7 on APC to CD28 on T cell
• cytokines
• Partial activation without adequate costimulatory
  signal Anergy or apoptosis method to eliminate
  self-reacting cells or induce tolerance

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ADAPTIVE IMMUNE RESPONSEHelper T cell Function

• TH1 and TH2 CD4 (helper) T cells
• TH0 cells mature into TH1 or TH 2 depending on
• Nature and concentration of antigen
• How antigen presented
• Type of APC
• Cytokines
• TH1 IgM, IgG, activated Macs
• TH2 humoral response IgA, IgE

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ADAPTIVE IMMUNE RESPONSECytotoxic T cells

• Activation of CD8 T cells cytotoxic T cells
  (CTLs)
• Precursors in nodes bind TCR CD8 to MHC-1 of
  APC costim
• TCR recognizes foreign protein in self MHC
  molecule
• Specific clone expands by 100,000
• Activated CTLs bind with target cell
• Granulysin, granzymes and perforin released from
  granules apoptosis
• Also interaction of FasL on CTL with Fas on
  target apoptosis
• Apoptosis
• Cell DNA and internal membranes fragment
• Shrink to apoptotic bodies which are easily
  phagocytosed
• Clean cell death as apposed to necrosis

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HUMORAL IMMUNE RESPONSE

• HIR production of antibodies by plasma (B)
  cells
• Some Definitions
• Albumin, globulin serum proteins
• Gamma-globulin immunoglobulin
• Immunogen, antigen
• Epitope (linear, conformational)
• Hapten carrier
• Adjuvant (alum, Freunds, liposomes, CW
  components)
• Immune tolerance (dose, co-stimulators,
  transplants )
• T (cell) independent antigen T dependent Ag
• Route of Ag administration
• Immunoglobulin superfamily on next slide

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HUMORAL IMMUNE RESPONSE

• Immunoglobulin (antibody) structure

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HUMORAL IMMUNE RESPONSE

• Immunoglobulin structure
• Heavy chain determines class/subclass IgM, G, A,
  D, E
• Mu, gamma, alpha, delta, epsilon
• Light chain (kappa, lambda)
• Domains constant and variable
• One variable domain on each lt chain and one
  variable domain on each hvy chain
• Three constant domains on hvy chain of IgG and
  IgA four on IgM and IgE
• One constant domain on lt chains
• Variable domains on hvy/lt chains have 3
  complementary determining regions (CDRs)
  complements epitope structure

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HUMORAL IMMUNE RESPONSE

• Immunoglobulin structure
• Fab portion variable region
• Fc portion
• C fixatin
• bind to Fc receptors on effector cells
• Fc of IgG and IgA involved with transfer across
  placenta and mucosa
• Hinge region
• Papain 2 Fab Fc Pepsin F(ab)2 Fc
• Membrane-spanning portion
• Interchain disulfide bonds
• Isotype IgD, IgM, IgG1, IgG2, IgG3, IgG4, IgA1,
  IgA2, IgE
• Allotype variation of isotype in different
  individuals
• Idiotype protein sequences in variable region
• Interacts with epitope

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HUMORAL IMMUNE RESPONSE

• Immunoglobulin Classes

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HUMORAL IMMUNE RESPONSE

• Membrane-associated antibodies
• All isotypes may be membrane-associated on B
  cells
• All membrane-associated AAs monomeric
• Last CH region has 26 AAs with hydrophobic side
  chains
• Membrane spanning region
• Secreted antibodies
• Found in blood and other extracellular fluids
• IgG and IgE are monomeric
• IgD negligible
• IgM and IgA are multimeric
• Have tail pieces on end of hvy chains which are
  bound to J chains

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HUMORAL IMMUNE RESPONSE

• Immunoglobulin Isotypes
• IgD lt1 of serum Ig
• membrane Ig on early B-cells
• IgM 5-10 of serum Ig
• membrane Ig on early B-cells
• IgD and IgM are only isotypes that can be
  expressed together on same cell
• 5 day half-life in serum
• Serum IgM pentameric 10 Ag-binding sites tail
  piece and j chain
• Complement fixation opsonin bacteriolysis
• First isotype after immunization then wanes

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HUMORAL IMMUNE RESPONSE

• Immunoglobulin Isotypes
• IgG 85 of serum Ig
• 4 subclasses all monomeric
• 23 day half-life
• T cells required
• Complement fixation opsonin
• Maternal IgG transported across placenta and
  neonatal intestinal epithelium
• Second isotype after immunization persists
• Also anamnestic (booster) response

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HUMORAL IMMUNE RESPONSE

• Immunoglobulin Isotypes
• IgA 5-15 of serum Ig
• 2 subclasses both dimer, trimer? with tail piece
  and J chain
• IgA transported across epithelial cells has
  additional secretory component
• IgE lt1 of serum Ig
• Bind to Fc receptors on Mast cells, basophils and
  eosinophils
• Immediate hypersensitivity
• Parasite infections

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Transport of IgA through Epithelial Cells
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HUMORAL IMMUNE RESPONSE

• Neonatal Immunity Neonates lack ability to
  mount effective immune response. Therefore, use
  maternal IgG and IgA
• IgG transported across placenta into fetal
  circulation
• IgG and IgA in breast milk ingested
• IgG and IgA neutralize organisms in gut
• IgG transported across gut epithelium into
  circulation
• Neonatal Fc receptor for IgG on placta and gut
  epithelium cells

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HUMORAL IMMUNE RESPONSEB cell development
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HUMORAL IMMUNE RESPONSE

• Immunogenetics
• Chromosomes 2, 22, 14 have genes for kappa,
  lambda, hvy chains
• Variable region genes upstream from constant
  region genes
• Genetic recombination events to form variable
  region of hvy and lt chains
• V (variable) and J (joining) segments for light
  chains
• 300 V gene segments and 5 J segments 1500
  possible
• V, D (diversity), and J segments for hvy chains
• 300 - 1000 V genes, 12 D genes, and 6 J genes
  lt100,000 possible
• Association of hvy and light chain gt 1,000,000
  possible

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HUMORAL IMMUNE RESPONSE

• Immunogenetics (continued)
• Class switching
• Genes for constant region of hvy chain in set
  order
• IgM, D, G3, G1, G2, G4, E, A1, A2
• Different constant region genes deleted in
  response to T cell cytokines

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HUMORAL IMMUNE RESPONSE

• Production of antibodies by B cell
• B cell receptor Ig molecule specific for
  non-self antigens
• Cells with receptors for self antigens removed
  during development in bone marrow
• Ig IgD and IgM on naive B cells
• B cell receptor complex
• IgM or IgD Ig alpha/Ig beta with immunoreceptor
  tyrosine-based activity motif (ITAM)
• Signal transducing receptor initiates activation
  signal via Ig ITAM
• Ultimately activates factors that induce
  transcription of genes

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B Cell Antigen Receptor Complex
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HUMORAL IMMUNE RESPONSE

• Antibody response to T (cell) independent
  antigens
• Surface Ig receptors recognize repeating sites on
  Ag
• Long polysaccharides
• Get cross-linking of many Ig receptors,
  activating signal cascade
• Clonal expansion and plasma cell development of
  specific B cells
• Disadvantages of T independent response
• No T cell cytokines for class switching IgM only
• Ab has lower affinity for Ag (limited affinity
  maturation?)
• No response in young (lt2yr) children

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HUMORAL IMMUNE RESPONSE

• Antibody response to T (cell) dependent antigens
• Ig receptors on B cell recognize Ag but
  cross-linking inadequate to activate cell
• Therefore need second signal from T helper cell
  thus
• 1) Ag binds to Ig receptor on B cell as above
• 2) Some bound Ag internalized, processed and
  presented in MHC-II surface molecule to T cell
  (which has receptor for this Ag) i.e., B cell
  functions as APC. T cell, so activated, secretes
  cytokines which are received by and stimulate the
  B cell
• 3) The two signals stimulate the B cell to
  replicate, clonal expansion
• A) Plasma cells which produce and secrete the Ig
  of the receptor
• In nodes and bone marrow
• B) Memory cells with the same receptor
• Anamnestic (booster) response

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HUMORAL IMMUNE RESPONSE

• Activation of B cells continued
• A second signal may be provided by C receptor
• Interaction of B cell with T cell via CD40 (B
  cell) CD40L (T cell)
• Class switching from influence of helper T cell
  cytokines
• Initial IgM
• INFg IgG
• IL-4 IgA, IgE
• IL-5 IgA (also induces eosinophil production)
• Somatic mutation increased affinity
• Primary and Secondary responses
• Primary immune response strong IgM
• Secondary immune response (second exposure to Ag)
  faster, more IgG

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C in B Cell Activation
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T Cell - Mediated B Cell Activation
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HUMORAL IMMUNE RESPONSE

• Antibodies inactivate microorganisms by
• Agglutination
• Neutralization
• Antibody to toxins
• Antibody to microbial surface molecules that bind
  to host cells
• Opsonization
• Natural killer cells have receptors for IgG
• Eosinophils have receptors for IgG, IgA, and IgE
• Complement fixation
• Neutrophils, macrophages have receptors for C3b
• Gram negative organisms susceptible to MAC