Transcript T-Cell Receptor PP - University of Arizona

T-cell Receptor
2/9/04
Feb 9, 2004
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
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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 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 Receptor
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 complementaritydetermining 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 Vand 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 α & β 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 α
and β 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 nonpolymorphic 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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•
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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-2
Gene
Transcription
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gd TCR
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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
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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
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•
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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
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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
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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 106—a 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
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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
cell–cell 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
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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
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Figure 6-11
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
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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
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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 Tcells 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 IFNg 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
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