Download The T Cell Receptor: Structure and Genetic Basis

The T Cell Receptor:
Structure and Genetic Basis
Jeffrey K. Actor, Ph.D.
MSB 2.214, 500-5344
Lecture Objectives:
•
Present an overview of T cell receptor (TCR)
structure and organization of the genes encoding the
TCR chains.
•
Understand underlying mechanisms involved in
generation of TCR diversity.
•
Compare/contrast the B cell receptor with that of the
T cell receptor.
The T Lymphocyte
Dealing with intracellular pathogens
• While antibodies are critical for response to antigens
present outside of cells, there is need for effective
response to antigens inside cells.
• T cells evolved to handle intracellular pathogens:
viruses, bacteria and parasites.
• Whereas B cells recognize physical conformations and
large molecular shapes, T cells recognize small
antigenic determinants expressed on the surface of host
cells associated with major histocompatibility complex
molecules.
The T Lymphocyte
The “Ringleaders” of the Immune Response
Every mature T cell expresses a TCR with specificity for
an antigenic determinant.
• Regulates immune responses.
• Integral in cell mediated immunity.
• Critical in B cell-antibody production.
The T Cell Receptor (TCR)
•
Each T cell has a TCR: a transmembrane heterodimer
composed of two disulfide-linked polypeptide chains.
• alpha [] and beta [] chains, or
• gamma [] and delta []
•
The TCR is expressed on the cell surface in association
with co-receptor (accessory) molecules.
•
The TCR is not secreted, and remains membrane bound
throughout the activation process.
The T Cell Receptor
•
Each chain (, , , or ) represents a distinct protein
with molecular weight between 40 and 60 kDa.
•
•
Most T lymphocytes express alpha [] and beta []
chains on their surface.
Cells that express gamma [] and delta [] chains
comprise only 5% of normal circulating T cells in
healthy adults.
•
An individual T cell can express either an  or a 
heterodimer as its receptor, but never both.
The T Cell Receptor
• Two polypeptides which span the
cell membrane.
•
Each peptide comprised of a
constant and a variable region.
•
Intra- and inter- disulfide links.
•
Antigen binding site is farthest
from cell membrane.
•
Ag binding site is comprised of
physical structure contributed by
both peptides.
The T Cell Receptor
The T Cell Receptor Complex
The structure of the
TCR complex:
- the antigen-binding
chains,  and 
- the associated signal
CD3 transduction
complex
CD3 comprised of , , and 
chains;
 (zeta) or  (eta) or  (theta);
 (-) and (+) are electrostatic
interactions.
TCR Antigen Binding
The interaction of
TCR, MHC, and
linear peptide. The
complementarity
determining
regions (CDRs) of
the TCR Variable
regions and
peptide bound in
the peptide-binding
groove of an MHC
class I molecule
are depicted.
Genes Coding for TCR
• T cell receptor genes are closely related members of the
immunoglobulin gene superfamily.
• Each chain consists of a constant (C) and a variable (V)
region, formed by a gene-sorting mechanism similar to
antibody formation.
• The repertoire is generated by combinatorial joining of
variable (V), joining (J), and diversity (D) genes, and by N
region diversification (nucleotides inserted by the enzyme
deoxynucleotidyl-transferase).
Genes Coding for TCR
• V, D, and J genes are mixed together in a more
complicated manner than for the Immunoglobulin
genes.
•  and  use only V and J segments.
•  and  use V, D, and J segments.
• There are more V and V genes (50-100) than V
and V genes (5-10) in the germ line.
Germline Genes Coding for TCR

Germline Genes Coding for TCR

Location of  and  TCR Genes
• The  and  chain genes are mixed together in one locus.
The genes encoding the  chain are entirely located
between the cluster of V and J gene segments.
• The organization of the  chain locus is extremely complex.
Order of TCR Gene Rearrangement
• The earliest cell entering the thymus has its TCR genes in
the germ line configuration (unrearranged*).
*( Some rearrangement can occur in bone marrow)
• Both  and  chain genes then begin to rearrange, more or
less simultaneously.
• If the  chain genes rearrange successfully, then  chain
genes also start to rearrange. If both  and  genes
rearrange functionally, no further gene rearrangement
takes place and the cell remains a  T cell.
• If  and/or  rearrangements are not functional, then  gene
rearrangement continues followed by  gene
rearrangement. In this manner, a  product appears, and
the cell becomes an  T cell.
Allelic Exclusion
• If gene rearrangements are successful, no
further rearrangements occur. This is a process
called allelic exclusion.
• If all the gene rearrangements are unsuccessful,
the second copy of genes on respective
chromosomes will begin the rearrangement
process.
• If this recombination is not successful, then the
cell undergoes apoptosis.
Process of Recombination
• Recombination of V, D, and J gene segments is
coordinated by recombinase-activating genes RAG-1
and RAG-2.
• The enzymes recognize specific DNA signal sequences
consisting of a heptamer, followed a spacer of 12 or 23
bases, and then a nonamer (Recombination Signal
Sequences).
• If either RAG gene is impaired or missing the
homologous recombination events are abolished,
giving rise to severe combined immunodeficiency
(SCID).
Rearrangement of
the T-cell receptor
genes.
Recombination of V, D,
and J gene segments is
coordinated by
recombinase-activating
genes RAG-1 and RAG2.
If either RAG gene is
impaired or missing the
homologous
recombination events
are abolished, giving rise
to severe combined
immunodeficiency
(SCID).
Generation of Diversity
Ig
Number of V gene pairs
Junctional Diversity
Total Diversity
TCR
3.4 x 106 5.8 x 106
~3 x 107 ~2 x 1011
~1014
~1018
•Greater diversity in TCR compared to B cell Ig receptor.
No Somatic Mutation in TCR
• Unlike immunoglobulin genes, genes encoding TCR do not
undergo somatic mutation. There is no change in TCR
affinity during activation, differentiation, and expansion.
More on T Lymphocyte Development
• T lymphocytes develop in the thymus.
• In the thymus, the cells develop both a CD4
and a CD8 marker. They lose one marker
prior to leaving the thymus.
– The cells with a CD4 marker are called helper T
cells (Th cells).
– The CD8 positive cells that develop are cytotoxic T
cells (Tc cells).
Genesis of Mature T Lymphocytes
(single positive)
(double positive)
5% exported to periphery
95% apoptosis
Thymic Selection
++, CD4+CD8+ cell
interacts with Thymic
epithelial cell
Interaction = Positive Selection
No interaction =
MHC + self
CD4+CD8+ cell dies
or
MHC + non-self
++, CD4+CD8+ cell
interacts with interdigitating cell
High affinity interaction =
Deletion
Low affinity interaction =
Survival
Commitment CD4+ or
CD8+
Double positive in thymus, leave as committed single positive cells
T Helper Cells
• Different phenotypic populations exist.
– TH1, TH2, TH17, Treg ……and more…..
• All express the CD4 molecule.
• Aid effector T lymphocytes in cell-mediated
immunity.
• Aid antigen-stimulated subsets of B cells to
proliferate and differentiate toward
antibody-producing cells.
• Regulatory role for tolerization events.
T Regulatory Cells
• T regs are a population of T Helper cells
(CD4+CD25+), characterized by TGF-β
secretion, that also serve as regulators of
response.
T Cytotoxic Cells
• T cytotoxic cells (CTLs) express CD8 and
are cytotoxic against tumor cells and host
cells infected with intracellular pathogens.
Natural Killer T Cells
• Natural killer T cells (NKT) share
properties of both T cells and natural killer
(NK) cells.
– NKT cells are different from NK cells.
• These cells recognize lipid and glycolipid
antigen.
• Have a limited TCR repertoire.
“Unconventional” Antigen
Presentation to T Cells
• Lipids/glycolipids
• Superantigens
Non-Classical Ag Presentation
Lipids and Glycolipids
CD1 is a Surface glycoprotein
which can present
lipids/glycolipids to T cells.
•
•
•
•
Non-MHC encoded,
Non-polymorphic
Expressed in association with 2microglobulin
Binds hydrophobic region of
lipid, exposing polar region for T
cell interaction
Can present to  or  T cells,
and NKT cells.
“Unconventional” Antigen
Presentation to T Cells (cont’d)
• Lipids/glycolipids
• Superantigens
Superantigens
• Superantigens bind directly to
T-cell receptors and MHC,
without processing.
• “Presented” by MHC II, but
not in peptide groove
• Involves direct interaction to
V region of TCR; activates
any T cell expressing specific
V TCR segment
• Non-specific activation of
large number of T cells
• Various organisms have
superantigens in makeup
– Staphylococcus, rabies
V VD
V VD
J
J
J
J
C
C
C
C
Fig 47.1 Garland Science
Superantigens
Case #47, Geha and
Notarangelo:
“Toxic Shock Syndrome”
V VD
V VD
J
J
J
J
C
C
C
C
Fig 47.1 Garland Science
Comparison of
B Cell and T Cell
Receptors
B Cell vs T Cell Receptors
BCRs and TCRs SHARE these properties:
 integral membrane proteins, present in thousands of identical
copies exposed at the cell surface
 made before the cell ever encounters an antigen
 encoded by genes assembled by the recombination of DNA
 allelic exclusion ensures only one receptor with a single
antigenic specificity
 demonstrate N region addition during gene rearrangement
 have a unique binding site to recognize antigenic determinant
(epitope)
 binding depends on complementarity of the receptor with the
epitope
 binding of antigen occurs by non-covalent forces
B Cell vs T Cell Receptors
BCRs and TCRs DIFFER in these properties:
 structure
 genes that encode them
 type of epitope to which they bind
 TCRs do not somatically mutate
 TCRs do not undergo isotype switching
 TCR gene recombination exhibits far greater junctional
diversity than Ig genes
 TCRs are never secreted from the T cell
Comparison: BCR vs TCR
Clinical Vignette:
Dysfunction in Gene Rearrangement
• Omenn syndrome is characterized by generalized
erythematous skin rash, lymph node enlargement,
hepatosplenomegaly, shift in immunoglobulin
isotypes, and evidence of combined immune
deficiency.
• Geha & Notarangelo, 6th ed: Clinical Companion
Case #7
Omenn Syndrome
Diffuse, scaly rash on the
face and shoulders of an
infant with Omenn
syndrome. Conjunctivitis
present.
The skin is bright red and
wrinkled from edema and
infiltration of inflammatory
cells.
Case 7 Garland Science
Clinical Vignette: Omenn Syndrome
Q: In Omenn Syndrome there is fault in assembly of gene
segments that encode the variable regions of  and  chains of
the TCR. What is the mechanism underlying the defect?
A: Recombination of V, D, and J gene segments is coordinated
by recombinase-activating genes RAG-1 and RAG-2. Many
Omenn Syndrome patients have etiology of missense mutations
in RAG-1 or RAG-2 gene, with only partial recombinase activity.
•If either RAG gene is impaired or missing the homologous
recombination events are abolished, giving rise to severe
combined immunodeficiency (SCID, T-, B-).
TCR Summary
T lymphocytes are involved in regulation of immune response and
in cell mediated immunity.
Mature T cells express antigen-specific TCR in a complex with CD3
molecules. The TCR is a disulfide-linked heterodimer composed
of either  or  chains. T cells express either  or  chain
heterodimers, but never both.
T cell receptor genes are closely related members of the
immunoglobulin gene superfamily and derive part of their
structural diversity form recombination of different V, D, and J
gene segments.
During differentiation in the thymus, immature T cells undergo
rearrangement of their TCR, and commit to lineage of CD4+ or
CD8+ phenotype.
A child with Severe Combined Immunodeficiency (SCID) has a deficiency in
her lymphocyte populations, which include B and T lymphocytes. Regarding
normal B and and T lymphocytes, which property is not shared by both the B
and T cell receptors?
A.
B.
C.
D.
E.
they are made prior to encounters with antigen
they undergo somatic mutation after antigenic stimulation
they are encoded by recombined segments of DNA
they show allelic exclusion for single antigen receptor expression
they demonstrate N region addition during gene rearrangement
Option B (they undergo somatic mutation after antigenic stimulation) is correct. The B
and T cell receptors are integral membrane proteins present in thousands of identical
copies exposed on the cell surface, that are available to specifically react with antigen
prior to encountering antigen. The receptors are encoded for by a genes comprised of
recombined DNA segments. The tremendous binding potential for binding antigens
(>1015 different receptors) is in part due gene rearrangement mechanisms that bring
together Variable (V), Diversity (D), and Junction (J) gene sequences. However, only
the B cell receptor (the immunoglobulin molecule) undergoes somatic mutation after
antigenic exposure, thus driving increases in functional affinity of the antibody-antigen
reaction.
Element Name: WOMAN; Symbol: WO
Atomic Weight: (don't even go there!)
Element Name: MAN; Symbol: XY
Atomic Weight: (180 +/- 50)
Physical properties: Generally round in
form. Boils at nothing and may freeze any
time. Melts whenever treated properly.
Very bitter if not used well.
Physical properties: Solid at room
temperature; gets bent out of shape easily.
Fairly dense and sometimes flaky. Difficult
to find a pure sample. Aging samples
unable to conduct electricity as easily as
young samples.
Chemical properties: Very active. Highly
unstable. Possesses strong affinity to gold,
silver, platinum, and precious stones.
Violent when left alone. Able to absorb
great amounts of exotic food. Turns
slightly green when placed next to a better
specimen.
Usage: Highly ornamental. An extremely
good catalyst for dispersion of wealth.
Probably the most powerful income
reducing agent known.
Caution: Highly explosive in
inexperienced hands.
Chemical properties: Attempts to bond
with WO any chance it can get; tends to
form strong bonds with itself. Becomes
explosive when mixed with Kd (Element:
Child) for prolonged period of time.
Neutralize by saturating with alcohol.
Usage: None known. Possibly good
methane source.
Caution: In the absence of WO, this
element rapidly decomposes and begins to
smell.
Comparison: BCR vs TCR