Antigen Processing and Presentation

1
Antigen Processing and Presentation

• Reading
• Chapter 8

2
Important Concepts

• Antigen processing
• Events leading to the degradation of antigen into
  peptides that form the peptide-MHC complex
• Antigen presentation
• Degraded peptides associate with MHC
  intracellularly
• Peptide- MHC display on an APC
• Peptide-MHC complex is transported to the
  membrane for display
• MHC molecules and binding of peptides
• Class I and binding of peptides derived from
  endogenous antigens
• Class II and binding of peptides derived from
  exogenous antigen

3

• Rosenthal and Shevach
• Demonstration of self-MHC restriction use of
  inbred and congenic strains
• T helper cells respond only to macrophages of the
  same MHC haplotype (same Class II alleles)
• Antigen recognition by CD4 T helper cells is
  class II restricted

4

• Zinkernagel and Doherty
• Demonstration of self-MHC restriction in CD8 T
  cells
• Immunization of mice with lymphocytic
  choriomeningitis (LCM) virus
• Activated T cytotoxic cells (CTLs) attacked only
  syngeneic virus-infected cells
• Congenic mice studies Antigen recognition by
  CD8 T cells is class I MHC restricted

5

• Evidence for the Role of Antigen Presenting Cells
  (APCs)
• Early dogma antigen recognition by B and T cells
  is the same (recognition of the entire native
  antigen)
• Contradicting dogma
• experiments by Gell and Benacerraf demonstrating
  T cell recognition of denatured protein
• Additional data from Ziegler and Unanue
  Shimonkevitz Townsend et al.

6
(No Transcript)
7

• Target cells
• cells that display peptides associated with class
  I MHC to CD8 T cells
• Antigen Presenting Cells
• Cells that display peptides associated with class
  II MHC to CD4 T cells
• Differ in antigen uptake, expression of class II
  MHC and co-stimulatory activity
• Dendritic cells
• Macrophages
• B Cells
• Non professional APCs induced to express class
  II MHC molecules or co-stimulatory proteins

8

• Elimination of intracellular and extracellular
  antigens
• Cytosolic pathway processing pathway for
  endogenous antigens
• Endogenous antigen is degraded within the cytosol
  by proteasomes and assembled with class I MHC in
  RER
• Endocytic pathway processing pathway for
  exogenous antigens taken up by endocytosis
• Exogenous antigen are internalized and degraded
  within acidic endocytic compartments and are
  bound by class II MHC

9

• Cytostolic proteolytic system
• pathway used in normal turnover of intracellular
  proteins
• Ubiquitin-protein complex is degraded by
  proteasome
• Large, cylindrical particle of 4 rings of protein
  subunits
• Subunits are encoded by 2 MHC gene clusters and
  1 non-MHC gene that are induced by T cell
  cytokine (INF-?)
• Degradation reaction
• Requires ATP
• Occurs within the central channel of the
  proteasome
• Generates a variety of peptides

10

• Transport of peptides to RER
• TAP (transporter associated with antigen
  processing)
• Identified in mutant mice, RMA-S decreased levels
  of class I MHC on membrane
• Members of ATP-binding cassette proteins
• Membrane-spanning heterodimer of TAP1 and TAP2
• Possess domain projecting to lumen of RER and ATP
  binding domain

11

• Peptides generated by proteasome plus LMP2, LMP7,
  LMP10 are 8-10 aa long
• TAP has high affinity for proteins 8-10 aa long
  and hydrophobic or basic carboxyl-terminal aa
  (preferred anchor residues)
• In RER class I ? chain associates with peptide
  with the help of molecular chaperones
• Calnexin
• Calreticulin
• Tapasin

12

• Molecular Chaperones and peptide-class I MHC
  assembly
• class I MHC ? chain associates with calnexin
  until ?2 microglobulin binds
• class I MHC ? chain/ ?2 microglobulin
  heterodimer bind to calreticulin and the
  TAP-associated protein tapasin
• Chaperones help fold the class I MHC protein for
  proper antigen binding and export from RER

13

• TAP1 and TAP2 genes map within class II MHC
  region adjacent to LMP2 and LMP7 genes
• TAP1 and TAP2 genes are polymorphic
• Differences in allelic forms of TAP may
  contribute to variation in the response to
  antigen between individuals
• TAP deficiencies lead to diseases and
  autoimmunity, ex. bare lymphocyte syndrome,
  TAP-deficiency syndrome

14

• Exogenous Pathway
• Antigen is internalized by phagocytosis,
  endocytosis or both
• Internalized peptides are degraded into peptides
  within endocytic compartments and presented on
  class II MHC
• Process takes 1-3 hours
• Involves early (pH 6.0-6.5), late endosomes
  (endolysosomes, pH 5.0-6.0) and lysosomes (pH
  4.5-5.0)
• Antigens are degraded by lytic enzymes into 13-18
  amino acids
• Involves 6 major steps

15

• Invariant chain restricts class II
  peptide-binding and guides transport of class II
  MHC to endosomes
• Class II bind invariant chain in RER
• blocks binding to endogenous antigen or premature
  binding to exogenous antigen
• Sorting signals in its cytoplasmic tail direct
  the transport of class II MHC from trans-golgi
  network to endocytic compartments
• Peptide assembly with class II MHC
• Invariant chain is degraded while moving through
  endocytic compartments, digestion leaves CLIP

16

• Generation of antigenic peptides in endocytic
  processing pathway
• Internalized exogenous antigen moves through
  early endosome, late endosome, and lysosomes by
  mechanisms not clearly understood
• Antigen is digested into peptides
• Associates with class II MHC transported in
  vesicles to late endosomes

17

• CLIP (class II-associated invariant chain
  peptide) remains in the binding groove of class
  II MHC molecule
• HLA-DM dissociates CLIP from class II MHC in
  endosomal compartment to allow antigen binding
• Non-classical HLA-DO may act as a negative
  regulator of class II antigen by binding to
  HLA-DM
• Expressed only in B cells and in cells of the
  thymus expression is not induced by IFN-?

• Peptide-class II MHC complex is transported
  to membrane

18

• Presentation of bacterial nonpeptide antigens
  (lipids and glycolipids)
• Presented by CD1 family proteins associated with
  ?2-microglobulin
• 5 genes encode human CD1 proteins
• Genes are located on Chromosome 1 not within MHC
• Expression of specific genes varies within cell
  types
• CD1D1 genes are expressed on nonprofessional
  APCs and certain subsets of B cells
• CD1A, B ,and C are expressed on professional
  APCs and immature thymocytes