Transcript T cell receptors

T Cell Receptor
W. Robert Fleischmann, Ph.D.
Department of Urologic Surgery
University of Minnesota Medical School
[email protected]
(612) 626-5034
Objectives
• To understand the central role of T cells
• To understand how T cell receptors are
generated
• To understand how T cell receptors function
• To understand the T cell receptor complex
and its function
• To understand CD4 and CD8 co-receptors
and their functions
• To understand the alloreactivity of T cells
Central Role of T Cells
• CD4+ helper T cells respond to antigen
presentation to initiate the adaptive
immune response.
– Th1 cells that produce IFN- and IL-2
stimulate CD8+ cytotoxic T cells.
– Th2 cells that produce IL-10 and IL-4
• Turn down production of Th1 cells
• Stimulate mature B cells to divide and
differentiate to become Ig-producing plasma
cells.
Helper T Cells
• Helper T cells recognize antigenic peptides that
are presented by antigen-presenting cells.
• Helper and cytotoxic T cells bind and recognize
antigenic peptides via their T cell receptors.
• Each helper and cytotoxic T cell bears a T cell
receptor that recognizes one unique antigenic
peptide.
• In total, there exist helper T cells that have the
capability of reacting with essentially every
possible antigenic peptide.
• The sum total of all antigenic peptides that a
person’s helper T cells can recognize is called
the person’s antigenic repertoire.
T Cell Receptors
• T cell receptors (TCRs) are heterodimers
  dimer
  dimer
  T cell receptors are highly specific for
antigen.
– In this way,  T cell receptors are very analogous
to Ig molecules on B cells.
 cell receptors appear to recognize
classes of antigens that are present on
groups of pathogens
– In this way,  T cell receptors appear to be more
analogous to Toll-like receptors of innate immunity
(that are involved with pattern recognition) than
they are with adaptive immunity.
Recognition Specificity of TCR
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Recognizes self MHC
Recognizes antigen
Recognizes self MHC
Fails to recognize antigen
Fails to recognize MHC
Cannot recognize antigen
Stylized Structure of T Cell Receptor
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Roles of  and  T cells
  T cells
– Recognize antigen that is processed and presented by
an MHC molecule.
– Activate the adaptive immune response.
– Can kill by granulysin and perforin.
Roles of  and  T cells
  T cells
– Require neither antigen processing nor presentation for antigen
recognition.
– Have very few  variants.
– May be more involved in the innate immune response than in the
adaptive immune response (pattern recognition).
• Recognize the microbial phospholipid antigen, 3-formyl-1-butyl
pyrophosphate found on Mycobacterium tuberculosis, other bacteria,
and parasites.
• Persistent infections with Mycobacterium tuberculosis, Hemophilus
influenzae, Plasmodium (malaria) and Leishmania parasites cause
enhanced production of  T cells that may kill by granulysin and
perforin.
  T cells are also prevalent in chronic autoimmune conditions, such as
lupus myositis, and multiple sclerosis.
  T cells produce a broad spectrum of cytokines and appear to recruit
 T cells to the site of pathogen invasion.
  T cells may also act as antigen-presenting cells.
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TCR Region of the Genome
The TCR region of the genome encodes a number of sections of the
TCR receptor that can be mixed and matched by recombination,
analogous to Ig recombination.
TCR Gene Rearrangement
• The  chain, like the Ig light chain, is encoded
by V, J, and C gene segments.
– Rearrangement leads to a selection of a VJ
combination attached to a single C gene segment.
– The  chain has a transmembrane cytoplasmic
tail, so the TCR  receptor is expressed as a
membrane-bound chain.
• The  chain, like the Ig heavy chain, is
encoded by V, D, J, and C gene segments.
– Rearrangement leads to a selection of a VDJ
combination attached to one of two C gene
segments.
– The  chain has a transmembrane cytoplasmic
tail, so the TCR  receptor is expressed as a
membrane-bound chain.
Mechanism of TCR Rearrangement
•
•
•
•
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The pre-T cell expresses the recombination-activating
genes, RAG-1 and RAG-2 recombinases.
The RAG-1/2 recombinase enzyme recognizes
conserved recombination signal sequences flanking
regions of introns between various V, D, and J coding
sequences.
As for Ig rearrangement, the RAG-1/2 recombinase
nicks one strand of the TCR DNA between the coding
sequence and the recombination signal sequence.
The RAG-1/2 excises DNA loops. (“throw-away” loops
of DNA have been found in thymocytes)
Each rearranged DNA sequence will encode a single
type of TCR.
Allelic Exclusion
• Each T cell will produce a single  chain from just one
of the chromosomal loci (allelic exclusion).
• Two  chains can be produced by a single cell.
• Therefore two  TCR sets may be expressed on a
given T cell.
• However, a single T cell will express a single antigenbinding specificity. How does this occur?
– There are many MHC molecules that could possibly be
expressed (and be recognized by TCRs).
– Most TCRs will fail to recognize self and they will not
survive passage through the thymus.
– Only those T cells with a TCR that recognizes self will
survive passage through the thymus. These T cells are
said to be self-restricted T cells.
– Thus, it is unlikely that two  TCRs expressed on the
same T cell will both be self-MHC restricted and, therefore,
functional.
3294
27
3752
140
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 germ-line repertoire = (3,294) x (3,752) = 12,592,228;  germ-line repertoire = (27) x (140) = 3,780
Generation of the TCR Repertoire
• Combinatorial joining of V, D, and J region generates
TCRs with many different antigen-binding specificities.
• Increased diversity can be attained by alternative
joining of D gene sequences to give VJ, VDJ, or VDDJ
chains.
• As is seen with Ig gene rearrangements, 0-6
nucleotides can be added at the junctions between V
and D and between D and J.
– P-region nucleotide addition: endonuclease cleavage and
repair can lead to the addition of nucleotides that are
palindromic.
– N-region nucleotide addition: additional non-encoded
nucleotides can be added by a terminal deoxynucleotide
transferase.
• Different from Ig gene rearrangements, somatic cell
mutations do not appear to play a role in generating
TCR diversity.
Parham
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Note: RSS = recombination signal sequence
Importance of Lack of Somatic Mutation
in Generation of Diversity
• B cells generate additional diversity by somatic cell
mutations that occur in the Ig region as the B cell is
dividing and differentiating. This permits the generation
and selection of Ig with enhanced antigen-binding
affinity.
• T cells lack the ability to generate somatic cell mutations.
– This is good because the T cell rearrangements are selected in
the thymus.
– Lack of somatic cell mutations means that T cell specificity for
antigen doesn’t change after the T cell leaves the thymus.
– This reduces the chance of generating self-reactive TCRs when
the T cell is activated (causing autoimmunity).
The TCR Complex
• The TCR is associated with CD3, an antigen
found on all T cells.
• CD3 is composed of three dimers that
aggregate around the TCR.
 
 
  or  where  and  are encoded from the
same gene. Thus, they are products of the same
mRNA that is differentially spliced at the 3’ end
(COOH end of protein).
• CD3 dimers all have a cytoplasmic signal
transduction domain called ITAM
(immunoreceptor tyrosine-based
activation motif).
T cell Receptor Complexed with CD3
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ITAM = immunoreceptor tyrosine-based
activation motif
T cell
Receptor
Complexed
with CD3
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ITAM = immunoreceptor tyrosine-based
activation motif
CD4 and CD8 Co-Receptors
• Both CD4 and CD8 T cell co-receptors contain
immunoglobulin-like domains.
• CD4 molecule
– A 55 kDa monomer, with 4 Ig-like domains
– Binds to a conserved region on Class II antigen
– Binds to signal transduction molecule p56lck and forms
a bridge that also binds to the  chains of CD3
• CD8 molecule
  heterodimer or  homodimer of 30-38 kDa
monomers, with 1 Ig-like domain, held together by a
disulfide bond
– Binds to a conserved region on Class I antigen
Affinity of TCR for MHC/Peptide
Increases with Binding of Co-receptors
• The affinity of the TCR for the MHC/peptide
complex is relatively modest.
• CD4 or CD8 adds to binding affinity.
• Other co-receptors add to binding affinity.
– CD2 binds to B7 (co-stimulatory molecule)
– LFA-1 (leukocyte functional antigen; aka, integrin)
binds to ICAM-1 (intercellular adhesion molecule)
– CD28 binds to B7
– CD45R binds to CD22
Binding Strength of T Cell Receptor
•Binding strength is initially relatively weak.
•Can be increased by co-receptor binding.
•Lends additional specificity (CD4/CD8 binding).
•Sends co-activation signals (binds to B7).
•Stabilizes binding (CD45, LFA-1)
T Cell: APC Contacts
Note: B7:CD28 interaction required for activation of CD8+ T cell (through activation
of CD4+ T cell), but not for killing by CD8+ T cell
Alloreactivity of T Cells
• CD4+ and CD8+ T cells are alloreactive to
MHCs other than their cognate MHC molecule.
– Alloreactive means that they immunologically react
to MHCs other than their own because they are
allogenic or genetically different.
– Thus, MHCs are alloantigens and are the reason
why allographs (transplants) are rejected.
– MHCs may be directly recognized as foreign
antigens (direct allorecognition).
– MHCs bound to foreign proteins may be directly
recognized as foreign antigens (direct
allorecognition).
– MHCs may be internalized, processed, and
presented to the immune system bound to self-MHC
(indirect allorecognition).