T-cell Receptor

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T-cell Receptor
Feb 9, 2004
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T-cell Receptor

• The biochemical signals that are triggered in T
  cells by antigen recognition are transduced not
  by the T cell receptor itself but by invariant
  proteins called CD3 an z (zeta), which are
  noncovalenlty linked to the antigen receptor to
  form the TCR complex.

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TCR

• Mature T cells express one of two types of TCR a
  heterodimer composed either of a and b chains or
  of g and d chains.
• Because T cells expressing ab receptors account
  for 90 of T-cell helper function and cytotoxic
  activity, the major focus of this discussion will
  be on this type of TCR.
• The gd T cells, whose physiologic role is still
  unclear, will be reviewed later on.

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T-cell Receptor

• T cells also express other membrane receptors
  that do not recognize antigen but participate in
  responses to antigens these are collectively
  called accessory molecules.

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T-cell

• Therefore, we will first focus on the TCR
  followed by a discussion on accessory molecules

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TCR

• The antigen receptor of MHC-restricted CD4
  helper T cells and CD8 cytotoxic lymphocytes is
  a heterodimer
• As mentioned before the TCR consists of two
  transmembrane polypeptide chains, designated a
  and b, covalently linked to each other by
  disulfide bonds.

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TCR

• Each a and b chain consists of one Ig-like
  N-terminal variable region (V), one Ig-like
  constant (C) domain, a hydrophobic transmembrane
  region, and a short cytoplasmic region.
• Thus the extracellular portion of the ab
  heterodimer is structurally similar to the
  antigen-binding fragment (Fab) of an Ig, which is
  made up of the V and C regions of a light chain
  and the V region and one C region of a heavy
  chain.

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TCR
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TCR

• The V region of the TCR a and b chains contain
  short stretches of amino acids where the
  variability between different TCRs is
  concentrated, and these form the hypervariable or
  complementarity-determining regions (CDRs).
• Three CDRs in the a chain are juxtaposed to three
  similar regions in the b chain to form the
  peptide recognizing complex.

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TCR

• An analysis of TCR sequence diversity has shown
  that the vast majority of amino acid variation
  resides in the region between the V- and J-region
  gene segments, which corresponds to the CDR3
  regions of antibodies.
• This has led to models in which the CDR3 loops of
  Va and Vb make the principal contacts with the
  antigenic peptide bound to the MHC

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TCR-MHC Interactions

• The CDR3 loops of Va and Vb make the principal
  contacts with the antigenic peptide bound to the
  MHC.

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TCR a ß GENES THE GENERATION OF TCR DIVERSITY

• To generate the diversity of TCRs required to
  recognize a wide spectrum of antigenic
  determinants, the TCR a and b genes use a
  strategy of recombination similar to that of the
  immunoglobulin genes.
• The germline TCR b-gene locus contains 20-30 V
  (variable), 2 D (diversity), and 13 J (joining)
  gene segments

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Rearrangement of the TCR a and ß genes.

• The TCR a-gene locus contains multiple V and J
  segments, only several of which are shown here.
  Similarly, the TCR b-gene locus contains multiple
  V, D, and J segments.
• During T-cell ontogeny, the TCR genes rearrange
  (arrows), so that one of the Va segments pairs
  with the Ja segment and a Vb segment pairs with a
  Db and Jb segment. The two C (constant) segments
  in the b gene are very similar, and differential
  use of Cb1 and Cb2 does not contribute to TCR
  diversity.

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T-CELL ONTOGENY
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CD3 z chain
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CD3

• TCRs occur as either of two distinct
  heterodimers, ab or gd, both of which are
  expressed with the non-polymorphic CD3
  polypeptides g, d, e, and z.
• The CD3 polypeptides, especially z and its
  variants, are critical for intracellular
  signaling.

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T-cell Receptor
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SIGNAL TRANSDUCTION BY THE TCR

• Key to the ability of the TCR to deliver
  intracellular signals is its interactions with
  protein tyrosine kinases (PTKs).
• In unstimulated T cells, Fyn, a member of the Src
  family of PTKs, associates with the cytoplasmic
  domains of CD3 chains.
• A second Src-like PTK, called Lck, binds to the
  cytoplasmic domains of CD4 and CD8 and thus can
  be brought into proximity with the TCR through
  the interactions of these coreceptors with the
  MHC.

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SIGNAL TRANSDUCTION BY THE TCR

• Stimulation of the TCR by antigen-MHC triggers
  the phosphorylation of tyrosine residues in the
  cytoplasmic domains of the CD3 chains of the
  receptor complex.
• According to a widely accepted model of TCR
  signaling, Lck and Fyn are responsible for these
  initial phosphorylation events.

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IL-2GeneTranscription
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gd TCR

• The gd TCR are a second type of TCR.
• Their function remains largely unresolved.
• They do not recognize MHC-associated peptides and
  are not MHC restricted.
• In mice and chickens they are found in the small
  bowel mucosa and termed intraepithelial
  lymphocytes.
• In humans they are found in the tongue, uterus
  and vagina.

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gd TCR

• In mice many gd TCR T-cells develop in neonatal
  life and express one particular TCR with
  essentially no variability in the V region.
• Therefore it is not known whether these subsets
  perform different T-cell function.

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Accessory Molecules
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CD45
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CD45

• CD45 is a large (180-220 kd) transmembrane cell
  surface molecule that is expressed by all
  leukocytes, including all T lymphocytes.
• The cytoplasmic domain of CD45 has tyrosine
  phosphatase activity.
• CD45 activity is at the very early steps of TCR
  signaling, indicating that CD45 is required for
  the functional coupling of the TCR and its PTKs.

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CD45

• Memory and naive T cells also differ in their
  surface phenotypes, most notably in their
  expression of CD45 isoforms.
• Alternative splicing of CD45 mRNA gives rise to a
  number of different isoforms of CD45 that differ
  in the size and composition of their
  extracellular domains.
• Naive T cells express 205- to 220-kd isoforms
  designated CD45RA, whereas memory T cells express
  a 180-kd isoform called CD45RO.

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CD28
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COSTIMULATION BY CD28

• Despite their complexity, the signals delivered
  by the TCR are insufficient to fully activate T
  cells.
• Rather, T-cell activation requires the delivery
  of both the TCR signals and a second set of
  signals generated by costimulatory molecules.
• In the absence of the proper costimulus,
  stimulation of the TCR alone can induce a T cell
  to enter a state in which it remains viable but
  is refractory to stimulation by antigen. This
  state, which is known as anergy, can be
  long-lived, persisting for weeks to months in
  vitro.

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COSTIMULATION BY CD28

• The best characterized (and probably the most
  important) costimulatory molecule is CD28, a
  44-kd glycoprotein that is expressed as a
  homodimer on the surfaces of virtually all CD4 T
  cells and approximately 50 of CD8 T cells.
• CD28 binds two distinct cell surface molecules,
  B7.1 and B7.2, found on dendritic cells,
  macrophages, and activated B cells.
• The combination of TCR stimulation and the
  interaction of CD28 with its B7 ligands fully
  activates T cells and results in substantially
  greater lymphokine production than can be induced
  by TCR signals alone

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COSTIMULATION BY CD28

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CTLA-4

• The number of antigen-specific T cells falls
  dramatically when an immune response terminates.
• Following successful clearance of virus, the
  number of virus-specific CTLs in a mouse can drop
  from 108 to 106a decrease of 99.
• The decline reflects apoptosis, perhaps triggered
  by cytokine withdrawal or by engagement of Fas or
  other members of the tumor necrosis factor (TNF)
  receptor family.

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CTLA-4

• One important negative regulator of T-cell
  activation is, a T-cell surface molecule induced
  on activation and not found on resting cells.
• CTLA-4 shares considerable sequence homology with
  CD28 and, like CD28, binds B7.1 and B7.2 on the
  APC.
• Unlike CD28, however, CTLA-4 delivers inhibitory
  signals to T cells, so that engagement of CTLA-4
  tends to strongly diminish T-cell responses.
• Mice genetically engineered to lack CTLA-4 die
  with massive polyclonal expansion of T
  lymphoblasts.

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CD2
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CD2

• CD2 is a glycoprotien present on more than 90 of
  mature T-cells and 50-70 of thymocytes.
• This molecule contains two extracellular Ig
  domains.
• The principle ligand for CD2 is LFA-3 (CD58).

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CD2

• CD2 functions both as an adhesion molecule and
  signal transducer.
• The association of CD2 with the TCR complex
  helps to aggregate the TCR in the regions of
  cellcell contact, allowing the stabilization of
  low-affinity TCR/MHC interactions.
• Finally, CD2 is involved in the regulation of
  cytokine production by T cells.
• Stimulation via the CD2 pathway can skew the
  cytokine profile toward a TH2-like phenotype.

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Integrins

• We have already discussed integrins in the
  context of neutrophils.
• The major functions of T-cell integrins are to
  mediate adhesion to APCs, endothelial cells, and
  extracellular matrix proteins.
• The avidity of integrins for their ligands is
  increased rapidly on exposure of the T-cells to
  cytokines called chemokines and after stimulation
  of T-cells through the TCR.

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Integrins
Figure 6-11
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Integrins

• Integrins will be discussed on Wednesday in more
  detail.

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CD44
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CD44

• CD44 is expressed by activated and memory cells
  in comparison to naïve cells.
• This molecule is responsible for retension of T
  cells in extravascular tissues at sites of
  infection and for the binding to endothelial
  cells at sites of infection and in mucosal
  tissues.

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Effector Molecules
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CD40L

• The CD40L on T-cells binds to the CD40 on B-cells
  thus an important mediator of stimulation of B
  cells.
• We have covered the CD40L related to our PBL.

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CD95 (Fas receptor)

• Activated T cells also express a ligand for death
  receptor Fas (CD95). Engagement of Fas by Fas
  ligand on T-cells results in apoptosis and is
  important for eliminating T-cells.
• FasL also provides one o the mechanisms by which
  CTLs kill targets.

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CD95 (Fas receptor)
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T-cell Subtypes

• T helper Th1 cells secrete pro-inflammatory
  cytokines (IFN-g, TNF, and IL-2.
• Whereas T helper Th2 cells produce cytokines that
  generally stimulate Ig responses (IL-4, -5, -6,
  -9, and -10).
• These biases tend to be self-reinforcing IL-10
  represses Th1 cell activity and IFN-g inhibits
  Th2 cells.

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T cells
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T-cells

• It is not clear whether the Th1/Th2 distinction
  corresponds to a simple dichotomy or rather to
  two extreme poles, between which intermediate
  patterns of cytokine production can be found.
• In addition, there is mounting evidence for other
  helper classes.

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Three Steps to Activation
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T-cell Receptor