Download 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
Marcus Jackson, age 7 months, is
brought to the clinic for evaluation of
possible immunodeficiency. He has had a
history of recurrent upper respiratory
infections and urinary tract infections.
What tests would you employ to evaluate
Marcus and why?
Marcus’ total WBC count was normal.
However, his differential count showed a
decrease in lymphocytes. His RBCs and platelets
were normal.
Proliferation studies showed no proliferation of
T cells following mitogen stimulation. However,
there was proliferation of B cells following
mitogen stimulation.
There are low levels of IgG and IgM detected.
Why were only IgG and IgM detected?
What are your thoughts?
What would you do next?
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
Kuby
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
Kuby
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.
Kuby
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
•
•
•
•
•
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
Kuby
 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
Kuby
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
Kuby
ITAM = immunoreceptor tyrosine-based
activation motif
T cell
Receptor
Complexed
with CD3
Kuby
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).
Marcus Jackson
No T cells were detected in Marcus’
blood by flow cytometry. B cell levels
were normal.
CD4+ and CD8+ T cells were detected
but at very low levels.
What are your thoughts?
Marcus Jackson
Marcus’ T cells have CD4 and CD8.
However, they have no CD3. Without
CD3, the T cells cannot be activated.
Marcus’ is scheduled for a bone marrow
transplant.