Antigen Capture and Presentation to Lymphocytes

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Chapter 3

• Antigen Capture and Presentation to Lymphocytes
• (What Lymphocytes See)

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Nature of Antigens

• B- and T-cells recognize different types of Ag
• B-cells have a membrane bound Ab that recognize a
  wide variety of macromolecules (sugars, lipids,
  proteins, nucleic acid), small chemicals in
  soluble or cell membrane associated forms
• T-cells see only peptide fragments and only when
  displayed in special display molecules on the
  host cell
• Chapter 4 Receptors

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Barriers to Adaptive Immunity

• Very low frequency of naïve lymphocytes with
  specificity for any 1 Ag, less than 1 in 100,000,
  must recognize and activate when Ag is present
• Different microbes need different adaptive immune
  responses
• virus in the blood is B cell response
• virus in a cell is a T cell response

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T-cell Ag Recognition

• Majority of T-cells recognize peptides bound to
  and displayed by MHC on APC
• MHC (major histocompatibility complex) is a
  genetic locus responsible for the display
  molecules of the immune system
• T-cell receptor must recognize Ag and MHC
  molecule,
• small subset may recognize lipid and non-protein
  Ag on uncommon class I MHC-like molecules
  (non-polymorphic) poorly understood
• Display of Ag is done by the APC that help naïve
  T-cells form the immune response
• professional APC will display Ag and can also
  provide extra signals to activate T-cells
• Differentiated Effector T cells must see Ag on APC

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MHC and TRC Interaction

• MHC restriction different T cells can see
  peptides on MHC
• recognizes peptides and MHC

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Capture of Ag

• Ag is concentrated by APC in lymphoid tissue
  where the immune response is initiated
• Microbes enter by contact (skin), ingestion (GI
  tract) and inhalation (respiratory) occasionally
  injected by insects into the blood stream
• all are lined with epithelium physical barrier
  that contains professional APC

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Capture and Display

• Soluble proteins are picked up by dendritic cells
  in the lymph node
• Blood-borne Ag by dendritic cells in the spleen
• Concentrate Ag in regions of lymph tissue most
  likely to contact T-cell
• happened 12-18 hours after Ag gets to lymph
  tissue
• Naïve T cell circulates at least once/day through
  the LN

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APC Dendritic Cells

• Present in the epithelial barrier and in lymphoid
  tissue
• Langerhans cells in skin are immature at
  activating lymphocytes, movement to the lymph
  node causes the maturation of dendritic cells
• Capture by phagocytosis (particulate) or
  pinocytosis (soluble) using receptors for microbe
  Ag such as terminal mannose residues

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Maturation

• Macrophages and epithelial cells secrete TNF and
  IL-1 in response to microbes
• help dendritic cell round up, lose adhesion to
  epithelium and move to lymphoid tissue
• express surface receptors for chemoattractant
  cytokines chemokines in the T cell zone and
  help to move to lymphatics
• During the migration to lymph node the dendritic
  cell matures into APC that can stimulate T-cells
• increase MHC expression and co-stimulatory
  molecules required for full T-cell response

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APCs Functions

• Dendritic cells are most potent APC
• influence response of T-cell and nature or
  response subsets of dendritic cells can
  activate CD4 T-cell into distinct populations
  (Chapter 5)
• Macrophage are APC and prevalent in all tissues
• phagocytose microbes and display Ag to effector
  T-cell activate macrophage to kill microbes
  (Chapter 6)
• B-cells ingest protein Ag and display them to
  helper T-cell
• important for humoral immunity (Chapter 7)

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CD8 T-cells Activation

• All nucleated cells can present Ag to CTLs
• Professional APC may initiate response to CD8
  T-cells to Ag of intracellular microbes
• viruses can rapidly infect cells and only be
  eradicated by CTLs CD8 T-cells respond to Ag
• may infect other cells that may not produce all
  the signals to activate T-cells how do we clear
  infection??

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Cross-Presentation or Priming

• Mechanism unknown but likely
• professional APC ingest infected cells and
  display Ag on MHC class I and MHC class II so can
  activate CD8 and CD4 T-cells, respectively, in
  close proximity
• may be why CD4 T-helper cells can activate CD8
  cells (Chapter 6)
• Primed CTL can kill infected cells without APC
  present

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MHC Molecules

• Membrane proteins on APC that present Ag to
  T-cells
• major determinant of tissue graft acceptance or
  rejection
• MHC locus is a collection of genes found in all
  mammals
• Human leukocyte antigens (HLA) found on WBC
• 2 sets of highly polymorphic genes that create
  MCH class I and II to display Ag to T-cell
• polymorphic 2 or more alternative forms, stable
  frequency, 1 allele from each parent
• also have non-polymorphic genes that function in
  Ag presentation and some with unknown function

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MHC I / MHC II

• Contain peptide binding site at the amino
  terminus different subunits but overall
  structure is similar

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MHC I CD8 T-cell

• ? chain is non-covalently linked to ?2
  microglobulin (gene outside the MHC locus)
• ?1 and ?2 form the peptide binding cleft or
  groove to hold a peptide 8-11 AA long
• AA in the bottom of the cleft and arms/sides of
  MHC contact the T-cell receptor
• polymorphic genes encode different residues in
  cleft so can bind different peptides, additional
  changes made so MHC can recognize different
  T-cells
• ?3 region is invariant and has the binding site
  for the co-receptor CD8
• T-cell activation MHC associated peptide Ag by
  T-cell receptor and simultaneous recognition of
  MHC by the co-receptor (Chapter 5)

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MHC II CD4 T-cell

• 2 chains ? and ?
• Amino terminus is made up of ?1 and ?1 domains
• polymorphic and form a cleft that is large enough
  for 10-30 AA residues
• ?2 domain binds co-receptor for CD4
• non-polymorphic

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Must Remember

• CD4 recognizes Ag on MHC class II T helper
  cell
• CD8 recognizes Ag on MHC class I CTL
• Also must have the appropriate co-receptor

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MHC Properties and Functions
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MHC Inheritance

• Genes are co-dominant inherited alleles from
  both parents and are expressed equally
• 3 genes for MHC class I
• HLA-A, HLA-B and HLA-C are one set of genes so
  cell can express 6 different MHC I molecules
• 3 genes for MHC class II
• HLA-DR, HLA-DQ and HLA-DP make up the ? and ?
  chain and both are polymorphic
• ? chain from 1 allele may pair with a ? chain of
  another allele 10-20 different MHC class II
  molecules
• MHC genes are highly polymorphic many different
  alleles present among different individuals in
  the population
• no 2 individuals are alike (except twins and
  inbreds)
• existence of multiple alleles allows for any MHC
  molecule to present any peptide helps prevent
  new or mutated microbes from causing problems
• Not gene recombination as seen in B cell
  receptors (Chapter 4)

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Location of MHC

• MHC class I is on all nucleated cells
• MHC class II on professional APC (dendritic
  cells, macrophages and B cells)

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Peptide Binding Clefts

• Side chains of the peptide AA fit in the pockets
  formed in the cleft and anchor the peptide there
  by anchor residues
• Some residues are pushed upward and are
  recognized by the T-cell receptor

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Features of Peptide Binding
Must know these things
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Peptide Binding

• 1 MHC molecule can display 1 peptide at a time
• APC can present many different MHC/peptides with
  each peptide being different
• dependent on anchoring residues fitting cleft
• broad specificity bind many but not all
  peptides
• few MHC to do many peptides
• rarely binds anything but protein Ag
• peptide to MHC is a low affinity interaction
  (stay bound in decrease concentration of Ag)
• slow off rate few structural constraints on
  binding
• display a long time until find the right T-cell
  receptor

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Acquiring Peptides

• Acquire peptide during biosynthesis inside the
  cell microbe Ag from inside the host cell and
  T-cell recognize cell associated microbes
• MHC class I cytosolic proteins
• MHC class II proteins in intracellular vesicles
• Only loaded MHC molecules are stably expressed on
  surface empty molecules are destroyed before
  the leaving the cell
• display only useful MHC molecules

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Self Recognition

• MHC molecules can display foreign and self Ag so
  why dont we develop self Ag activated immunity?
• T-cell may need to see only 0.1 to 1 of its Ag
  on the 105 MHC molecules on APC
• T-cells that recognize self-Ag are either killed
  or inactivated (Chapter 9)
• MHC cant display whole Ag but only peptides Ag
  processing

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Processing Pathways

• MHC II extracellular proteins that come into
  the APC and are in vesicles
• MHC I proteins in cytosol of nucleated cells
  such as viral proteins, microbial proteins

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Different Outcomes

• Different cellular organelles and proteins
  require
• Sample all extra- and intra-cellular proteins
• Activate different classes of T-cell by different
  pathways due to separation of processign

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Mechanisms for Ag Internalization

• Binding of microbes to surface receptors
• Receptors that recognize Ab or products of
  complement fixation on a microbe
• B-cells internalize proteins on their Ag
  receptors (Chapter 7)
• Phagocytosis of microbes
• Phagocytosis of proteins
• 4 and 5 are without specific recognition events

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Loading of MHC II

• Endosomal peptides
• proteins enter intracellular vesicles and may
  fuse with lysosome, proteins broken down by
  proteolytic enzymes to peptides
• APC synthesize MHC II in ER and each has a
  sequence called class II invariant chain peptide
  (CLIP) which binds tightly to the cleft
• MHC II/CLIP complex begins way to cell surface in
  an exocytic vesicle which fuses with endosomal
  vesicle of peptides, also contains DM protein
  that removes CLIP so peptide can enter into cleft
• if no peptide picked up, MHC II is degraded in
  endosome
• MHC II/peptide complex is now stable and can move
  to the surface
• Not all peptides end up on MHC II only the
  immunodominant epitopes of Ag maybe 1-2 peptides

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Loading of MHC I

• Cytosolic peptides
• proteins from cytoplasmic viruses and from
  phagocytosed microbes that may break thru the
  vesicle and escape to the cytoplasm
• proteolysis ubiquitin binds to protein and
  leads to proteosomes for digestion
• some cleaved peptides are small enough to fit
  into MHC I
• MHC I made in the ER while peptides are in
  cytoplasm requires special transport proteins
  called TAP (transporters associated with Ag
  presentation)
• TAP pumps peptides into ER so can get on MHC I
  loose association between TAP and MHC on inner
  surface of ER
• right fit of peptide and MHC will stabilize the
  complex and move it to the cytoplasm
• no peptide leads to MHC degradation

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No MCH I Peptides on MHC II

• Even though MHC I and MHC II are in ER, wont
  cross load peptides because CLIP is in MHC II and
  not vacated until reaches the endosome

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Viral Avoidance

• Some viruses can avoid the Ag presentation by
  preventing MHC I pathway
• remove MHC from the ER
• inhibit transcription of MHC
• block peptide transport function of TAP
• Decrease CD8 T-cell activation
• Counterbalanced by NK cells to recognize virally
  infected cells innate response, recognizes
  decrease in MHC I on surface (Chapter 6)

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Physiological Significance

• T-cell recognition to MHC-association peptide is
  that T-cells will see and respond only to
  cell-associated Ag
• MHC can be loaded inside cells so can only
  recognize the Ag of phagocytosed and
  intracellular microbes ? combated by T-cell
  mediated immunity
• MHC I and MHC II pathways of Ag presentation so
  can handle Ag in best possible way
• extracellular microbes are captured by APC
  including B-cells and macrophages presented by
  MHC II CD4 T cells (help produce Ab by B
  cells)
• cytosolic Ag displayed by MHC I on nucleated
  cells associated peptides are recognized by
  CD8 cells that differentiate into CTLs to kill
  infected cells and eradicate infection

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Other APC Functions

• Present 2nd signal for T-cell activation
• insure adaptive response is to microbe and not
  self proteins (Chapters 5 and 7)
• Different types of microbial products and innate
  immune response may activate APC to produce 2nd
  signal response to LPS from bacteria
• express co-stimulators recognized by T-cell
  receptors
• APC will secrete cytokines recognized by T cell
  receptor
• activates T-cell proliferation and differentiation

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Ag Recognition by B-cells

• B-cells use membrane bound antibodies to
  recognize many Ag, soluble or microbial surface
  DOES NOT REQUIRE PROCESSING
• B-cells differentiate in response to Ag and other
  signals to secrete Ab (Chapter 7)
• secreted Ab bind to Ag and lead to neutralization
  and elimination
• Not understood how Ag finds that appropriate
  B-cell
• Lymphoid follicles of LN and spleen contain a
  population of cells called follicular dendritic
  cells (FDC) that use Fc receptor to bind Ag that
  are coated Ab and receptor for C3d complement
  protein to bind Ag with attached complement
  (Chapter 7)

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Summary

• MHC class I on all nucleated cells and recognize
  cytosolic proteins CD8 T cell - CTL
• MHC class II expressed mainly on professional
  APCs (dendritic cells, macrophages and B-cells)
  and recognize vesicular proteins CD4 T cell
  T helper cell