Lectures 5

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• Lectures 5 6
• Antigen Recognition
• by T Lymphocytes

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Lecture objectives

• Big questions?
• How TCR recognizes antigens?
• Why many autoimmune diseases are associated with
  particular types of MHC genotypes?
• Why MHC molecules are the major antigens
  responsible for transplantation rejections?
• Structure of TCRs
• CDRs
• Differences of recognized antigen between TCR and
  immunoglobulins (Ig)
• TCR germ line configuration and rearrangements
• TCR specificity
• Clonality of T cells
• Differences between TCR a and b chains
• Names of human MHC I and II genes.
• How antigens are processed for presentation on
  MHC I and II?
• MHC class I and II molecules sample and present
  antigens of different origins. How?
• MHC polymorphism
• How many different MHCs a person can express?
  Why?
• Structures of MHC I and II

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Core content
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A big pictureHow do T cells recognize antigens?
MHC molecule
TCR
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Similarity between TCR and Ig

• Both
• Bind antigen
• Have variable region
• Constant region
• Each binding site is a heterodimer (composed of 2
  different chains)
• TCRs act only as receptors
• Igs act as receptors and effector molecules
  (soluble antigen-binding molecules)

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Figure 3-2
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TCR genes undergo DNA rearrangement in thymus
No Ds in Va gene DJ first then VDJ in b gene
rearrangement
Un-rearranged
rearranged
expression
rearranged
Un-rearranged
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Figure 3-4
If you can not make antigen receptors as in the
case of SCID patients,
you can not make T and B cells, and are
susceptible even to opportunistic pathogens (e.g.
C. albicans)
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Figure 3-6
TCR complex
CD3 chains transmit signals
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Figure 3-7
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Figure 3-8 part 1 of 2
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Figure 3-8 part 2 of 2

• gd T cells recognize a limited set of antigens
• Host MHC class 1b T10/22, MICA/B
• Nonproetin alkylamines
• Bacterial products Mycobacterial Hsp,
  superantigens (SEA)
• Host heat shock proteins
• Transformed or stressed host cells

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Figure 3-9
Antigen processing is required to present antigen
peptides to TCR.TCRs bind short antigen peptides
but not whole antigen proteins
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Figure 3-10
The structures of CD4 and CD8
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Figure 3-12
MHC class I molecules present antigens to CD8 T
cells, and MHC class II molecules present
antigens to CD4 T cells
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Figure 3-13 part 1 of 2
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Figure 3-13 part 2 of 2
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Figure 3-14
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Figure 3-15
The peptide-binding groove of MHC molecules
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Figure 3-16
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Two different types of antigensExtracellular
for MHC II and intracellular for MHC IThey are
generated in different compartments
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Figure 3-17
Transport of Cytosolic Peptides into ERRole of
transporter associated with antigen processing,
TAP
Bare lymphocyte syndrome No TAP expression ? no
MHC I expression ? no CD8 T cells ? defective
cytotoxic activity against virus-infected cells?
chronic respiratory infection TAPTAP1TAP2 Prot
eosome a barrel shaped protein complex composed
of 28 subunits 8 or more AA-long polypeptides
are transported MHC I cannot leave ER without
loaded peptides
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Figure 3-18
Assembly of antigen peptide/MHC class I
complex.Molecular chaperons (calnexin,
calreticulin and tapasin)aid the folding of MHC
I and loading of peptides
In the absence of infection, self peptides are
presented on MHC, but do not activate T cells. A
HSV protein inhibits TAP function, and an Adeno
virus protein inhibits MHC I expression.
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Figure 3-20
Assembly of antigen peptide/MHC class II
complex-Extracellular microorganisms are taken
up by macrophages via phagocytosis, and by B
cells by cell surface Ig-mediated
endocytosis-MHC II molecules bind peptides in
the fused vesicles, not in ER-Invariant chain,
CLIP and HLA-DM guide the peptide loading-After
losing CLIP, MHC II must bind peptides or gets
degraded.-Certain pathogens (e.g. mycobacteria),
when engulfed, prevent the fusion of phagosomes
and lysosomes, and persist in phagosomes.
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Figure 3-21
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Figure 3-11
TCR recognition of antigens induce T cell
activation, functional maturation, and
killing/activation of target cells
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Figure 3-22
Different cell distribution of MHC class I and II

• All cells express MHC I for comprehensive
  surveillance by CD8 T cells
• Only some cells express high levels of MHC II
  and MHC I
• These are B cells, macrophages, dendritic cells
  and thymic epithelial cells.
• B cells, macrophages and dendritic cells are
  called professional antigen- presenting cells
  (APC).
• IFN-g increases the expression of MHC II in APC
  and induces the expression in non-APC cells at
  sites of infection

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Figure 3-23
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Figure 3-24 part 1 of 2
CD8/NK
NK
Remains intracellular
NK
MHC CLASS I molecules form ligands to activate
CD8 cells and inhibit NK cells
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Polymorphism presence of multiple alternative
forms (alleles) of a gene.
Help peptide loading
With antigen peptides activate CD4 T cells
Polymorphism allows the population can handle a
variety of pathogens.
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Major Histocompatibility Complex (HLA locus)

• MHC molecules in human is also called HLA (human
  leukocyte antigen)
• Class I and II locus.
• HLA-DR alpha chain is monomorphic
• HLA-DRB1 is most polymorphic in MHC II genes
• HLA-DRB1 is always present in any individual
• HLA-DRB3/4/5 is present in some but not all
  people.
• A heterozygote person expresses two haplotypes.
• A person can express 3-6 class I and 3-8 class II
  isoforms.
• 2198 possible class II isoforms in the human
  population.
• 440 MHC I isoforms in the human population.
• MHC isoforms ? presentable antigen peptides ?
  TCR repertoire

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Figure 3-25
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Figure 3-27
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Genetics of MHC gene expressionboth alleles are
expressed (co-dominant)

• In any mating, four possible combinations of
  haplotypes can be found in the offspring thus
  siblings are also likely to differ in the MHC
  allele they express.
• Halplotype The particular combination of MHC
  alleles found on a given chromosom 6.

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Figure 3-26
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Figure 3-28
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Figure 3-29
Each MHC isoform binds a characteristic set of
peptides-Anchor residues in peptides are
important for binding to MHC-Not all residues
are important
Degenerate binding allows each MHC molecule
handles many peptides.
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Figure 3-30
T cell receptor recognition of antigens is
MHC-restricted
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Figure 3-31
Being heterozygous for MHC is advantageous in
presenting antigens
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Figure 3-32
Pathogens can select MHC polymorphism in human
population
A scenario
Selection 2 as the result of epidemic of a new
pathogen
Selection 1 as the result of multiple successive
infections
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MHC Heterozygocity makes AIDS people more
resistant to HIV
Complete Heterozygocity
Homozygocity
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HLA typing how?Microcytotoxicity assay for
detection of HLA antigensAnti-HLA serum, or
monoclonal antibody, is mixed with live
lymphocytes. Specific Ig binds to the polymorphic
protein moiety of the HLA molecule expressed on
the cell surface. Exogenous complement is added
to the well which will result in lysis of cells
to which antibody has been bound. Cell death is
determined by ethidium bromide vital stain
exclusion. Flow cytometry or ELISAMonoclonal
antibodies to different MHC alleles have been
generated.Using panels of these antibodies, HLA
typing before transplantation is possible.RFLP
Restriction Fragment Length PolymorphismDigestion
of genomic DNA with certain restriction enzymes
followed by hybridization with radio-labeled MHC
gene probes gives MHC isotype-specific digestion
patterns.PCR Polymerase Chain ReactionPCR
using MHC gene-specific primers