T Cell Receptor

1
T Cell Receptor

• Reading Chapter 9

2
Key Concepts

• TCR
• Membrane bound, no soluble form
• Heterodimer of ? and ? chains or ? and ? chains
• Expression levels of TCR on T cells
• Expression and function requires CD3 (signal
  transducing complex)

3

• T cells activation is specific for antigen plus
  MHC and not antigen alone
• T cells recognize antigen only when it is
  presented by a self-MHC molecule
• Self-restriction is specific to T cells (not
  exhibited in B cells)
• How? Dual-receptor model vs. Altered-self model

4

• Isolation and identification of the TCR
• Generation of clonotypic antibodies isolated ??
  and ?? heterodimers
• Hedrick and Davis use subtractive hybridization
  to isolate TCR genes
• Identified clone
• Specifically expressed in T cell clones
• Gene sequence that rearranges
• Expressed as a membrane bound protein

5

• TCR Structure
• Resembles the Fab fragment of Ig
• Two domains with an intrachain disulfide bond
• NH2 domain contains sequence variation (3 CDRs in
  V? and V?)
• C region with the short connecting sequence
• Forms a disulfide bond between the two chains
• Transmembrane region
• Site of interaction with CD3
• Cytoplasmic tail
• 5-12 aa at COOH end

6

• Specific characteristics of TCRs
• Expression levels
• Function
• ?? TCR interacts with peptide-class II MHC on APC
  cells and target cells
• Activated cells proliferate T cells with high
  affinity receptors are selected for the
  cell-mediated immune response
• ?? TCR recognizes antigen that is not presented
  by MHC
• Ex. phospholipid antigens on bacteria and
  parasites
• Direct recognition of antigen innate immunity
• Secrete cytokines recruitment of ?? T cells

7

• ?? TCR and ?? TCR have distinct shape
• ?? TCR and ?? TCR exhibit different elbow angles
  between the long axis of the variable and
  constant regions
• Influences the interaction with co-receptors
  results in intracellular signaling differences
• ?? TCR forms a deep cleft on the surface of the
  molecule where phospholipid can bind directly (no
  MHC presentation is required)

8

• Organization and rearrangement of TCR Genes
• ?? and ?? TCR are expressed only in T lymphocytes
• Functional TCR genes are produced by
  rearrangement similar to Ig genes
• V and J segments undergo rearrangement in ? and ?
  chains
• V,D,J, segments undergo rearrangement in the ?
  and ? chains

9
(No Transcript)
10

• Rearrangement of TCR variable regions
• ? chain (homologous to L chain of Ig) is encoded
  by V,J, C gene segments, rearrangement involves
  VJ joining
• ? chain (homologous to H chain of Ig) is encoded
  by V,D,J and C segments, rearrangement involves
  VDJ joining (see notes)

11
Mechanism for TCR DNA Rearrangement

• Conserved RSS with 12 bp (1 turn) or 23 bp (2
  turn) spacer sequences
• All rearrangements follow the 12/23 joining rule
• Pre T cells express recombination-activating
  genes RAG1 and RAG2
• RAG1/2 recombinase enzymes recognize RSS
  sequences and promote V-J and V-D-J joining
• Consequence each cellular rearrangement event
  generates a unique DNA sequence for TCR
  expression in that cell

12

• Rearranged TCR genes
• Functional rearrangement generates 3 CDRs in
  variable region
• CDR1 and CDR2 are generated by combinatorial
  joining
• CDR3 is generated by junctional diversity and N
  region nucleotide addition

13
(No Transcript)
14
See supplemental notes
15
See supplemental notes
16

• TCR complex is TCR-CD3 membrane complex
• CD3 function
• CD3 associates with ?? and ?? TCR
• Complex of 5 invariant polypeptide chains that
  associate and form 3 dimers
• Most common dimer is ?? homodomer, importance of
  CD3 dimers and ITAM

17

• T cell Accessory membrane molecules
• Function strengthen the interaction between T
  cells and APCs or target cells
• CD4 and CD8 are important co-receptors for the
  recognition of peptide-MHC

18

• Structural features of CD4 and CD8
• CD4 55 kDa,monomeric membrane glycoprotein with
  4 extracellular Ig like domains, hydrophobic
  transmembrane region, long cytoplasmic tail with
  phosphorylatable serines
• CD8 disulfide linked ?? heterodimer or ??
  homodimer
• Small glycoprotein, 30-38 kDa, 1 extracellular Ig
  region, hyrdrophobic transmembrane region,
  cytosplasmic tail with 25-27 aa (many can be
  phosphorylated)

19

• Contact of class I MHC with HLA-A2-peptide
• CD8 binds by contacting ?2, ?3, ?2 microglobulin
  domains in MHC I

20

• Membrane distal domain of CD4 interacts with the
  hydophobic pocket formed by ?2 and ?2 domains of
  MHC II

21

• Affinity of TCR for Peptide-MHC complexes
• TCR binding affinity
• Importance of cell adhesion molecules and
  strengthening the trimolecular complex
• Sequence of events regulating the activation of T
  cells

22
(No Transcript)
23

• 3D structure of a TCR-peptide -MHC complex
• Interaction between human TCR, HLA-A2 class I
  MHC-peptide complex

24

• Hypervariable loops (CDRs) of human ? and ?
  chains of TCR and ?1, ?2 chains of HLA-A2
• Recognition of peptide-MHC occurs as variable
  regions CDR1 and CDR3 contact peptide and large
  area of MHC
• Peptide is buried deep within class I MHC
• TCR fits across the peptide and interacts with
  high points of MHC

25

• Class I and Class II MHC-peptide complexes
  interact with T cells differently
• Different angle at which TCR sits on the MHC
  complex
• Greater number of contact aas between TCR and
  class II MHC than that seen in TCR and class I
  MHC interaction

26
(No Transcript)