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
ADAPTIVE IMMUNE RESPONSE
T 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
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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
ADAPTIVE IMMUNE RESPONSE
T CELLS
ADAPTIVE IMMUNE RESPONSE
T 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
ADAPTIVE IMMUNE RESPONSE
T CELLS
ADAPTIVE IMMUNE RESPONSE
T 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
ADAPTIVE IMMUNE RESPONSE
T 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
ADAPTIVE IMMUNE RESPONSE
antigen 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
ADAPTIVE IMMUNE RESPONSE
antigen presenting cells
• Abbas Fig 4-4
ADAPTIVE IMMUNE RESPONSE
antigen 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
ADAPTIVE IMMUNE RESPONSE
antigen presenting cells
• Abbas Fig 4-6
ADAPTIVE IMMUNE RESPONSE
antigen 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
ADAPTIVE IMMUNE RESPONSE
antigen 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
ADAPTIVE IMMUNE RESPONSE
antigen presenting cells
• Antigen processing and presentation – Abbas Fig 5-8
ADAPTIVE IMMUNE RESPONSE
antigen 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
ADAPTIVE IMMUNE RESPONSE
antigen presenting cells
• Cross-presentation of Ag to CD8 T cells. Abbas Fig 5-7
ADAPTIVE IMMUNE RESPONSE
antigen presenting cells
• Antigen Presenting Cells (APCs) = Dendritic cells (DCs),
B cells, and Macs
• Dendritic cells = “only APC that can initiate Ag specific
response”
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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
– Langerhan’s cells are skin DC
ADAPTIVE IMMUNE RESPONSE
antigen 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
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
ADAPTIVE IMMUNE RESPONSE
Helper T cell Function
• TH1 and TH2 CD4 (helper) T cells
– TH0 cells mature into TH1 or TH 2 depending
on
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Nature and concentration of antigen
How antigen presented
Type of APC
Cytokines
– TH1 = IgM, IgG, activated Macs
– TH2 = humoral response; IgA, IgE
ADAPTIVE IMMUNE RESPONSE
Cytotoxic 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
HUMORAL IMMUNE RESPONSE
• HIR = production of antibodies by plasma (B) cells
• Some Definitions
– Albumin, globulin = serum proteins
• Gamma-globulin = immunoglobulin
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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
HUMORAL IMMUNE RESPONSE
• Immunoglobulin (antibody) structure
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
HUMORAL IMMUNE RESPONSE
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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
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• 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
HUMORAL IMMUNE RESPONSE
• Immunoglobulin Classes
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
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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
HUMORAL IMMUNE RESPONSE
• Immunoglobulin Isotypes
– IgD = <1% 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
HUMORAL IMMUNE RESPONSE
• Immunoglobulin Isotypes
– IgG = 85% of serum Ig
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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
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 = <1% of serum Ig
• Bind to Fc receptors on Mast cells, basophils and eosinophils
• Immediate hypersensitivity
• Parasite infections
Transport of IgA through Epithelial Cells
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
HUMORAL IMMUNE RESPONSE
B cell development
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 = <100,000 possible
– Association of hvy and light chain = > 1,000,000 possible
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
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 tyrosinebased activity motif (ITAM)
• Signal transducing receptor initiates activation signal via Ig
ITAM
– Ultimately activates factors that induce transcription of genes
B Cell Antigen Receptor Complex
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 (<2yr) children
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
HUMORAL IMMUNE RESPONSE
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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
C’ in B Cell Activation
T Cell - Mediated B Cell Activation
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