T-cell receptor

billion different possible—>huge diversity

heterodimer of α-chain and ß-chain

consists of variable & constant region, a transmembrane region and a cytoplasmic tail with a positive charge to associate with signaling proteins (too small to directly transfer signals)

connected by a disulfide bond

similarities with antibodies: with Fab (fragment antigen binding) from Ab but instead of bivalent Ab monovalent TCR and on cell surface

germline DNA consists of different gene segments for variable V, joining J & diversity D (only on ß-chain) and also constant region C

gene segments recombine to rearrange DNA, in α-chain first rearrangement VJ

then transcription, splicing and translation to T-cell receptor (protein)

for ß-chain sequential rearrangement: first VD and then VDJ

constant gene segment depends on which J gene segment is used as everything in between is regarded as a exon

α-chain = light chain, ß-chain = heavy chain

many different gene segments present for V, J & D

each V gene segment has a leader peptide L to go to cell s

urface

identical to the TCR

recombination signal sequence RSS with 23-base pair spacer or 12 bp spacer that space heptamer and nonamer

RSS of 23 bp spacer can only rearrange with RSS of 12 bp spacer (based on locus different spacer on gene segments)

12/23 rule: no V to J joining in Hß-chain

V segment can either be forward or reverse transcriptional orientated relative to the downstream gene segment

forward: during recombination formation of a loop (alignment of the 2 RSS), after recombination loop excised from the chromosome (together with both RSS)

reverse: alignment of RSS leads to a coiled configuration, after recombination coiled region retained & chromosome in an inverted orientation

for heavy chain gene two separate recombination events required

mediates recombination (rearrangement activating gene)

has endonuclease activity inducing single-strand breaks

heterodimer of RAG-1 &-2

explanation 12/23 rule: RAG1 binds RSS with 23 bp spacer & RAG2 with 12 bp spacer —>prevents recombination of the same spacer length

endonuclease induces single-strand break after RSS are brought together

generated free OH reacts with the phosphate bond on the second strand (first enzymatic and then chemical cleavage)

—>formation of a DNA hairpin

N and P nucleotides bring the most diversity

N nucleotides are extra nucleotides that are not present in the germline DNA

P = palindromic because of hairpin (part of germline DNA)

Artemis nuclease complex opens DNA hairpin and generates palindromic P-nucleotide

random N-nucleotide addition by TdT (terminal Deoxynucleotide Transferase (polymerase + exonuclease)

matches found between the N-nucleotides & pairing of the strands

unpaired nucleotides are removed by exonuclease

gaps are filled by DNA synthesis and ligation to form coding joint

around 5x10^13 for immunoglobulin (BCR) and around 10^18 for TCRs

less diversity of Ig because less N nucleotides in kappa —>but somatic hypermutation

gene rearrangement is not present in all species

chicken use instead gene conversion

genes take up variable sequences from pseudogenes to ensure some diversity

multiple rounds of gene conversion can alter affinities of Ab or Ag

rearrangement limited due to small number of gene segments

interaction of TCR with peptide & MHC

distal part of the TCR has hypervariability loops (HV1-4) pointing all distal from the cell surface

HV = CDR (complementarity determining region)

CDR1&2 coded by V region

CDR 3 from the association of 3’ end V & J —>much diverse

TCR/MHC interaction: 6 CDR interact with peptide & MHC molecule /HV4 not involved)

CDR3 mostly with peptide, CDR1 &2 mostly on MHC

mostly from bacteria (staphylococcal enterotoxin, toxic shock syndrome toxin)

induce massive proliferation of added to T-cell culture as they bind MHC (mostly MHCII) & ß-chain

no interaction with CDR3—>activates many different T-cells

for efficient activation more stimuli than NHC and antigen needed

not the same as co-stimulator receptors

CD4 & CD8

interact with MHC to increase binding affinity between MHC & TCR (otherwise weak)

has 4 Ig-like domains

interacts with ß2 of MHCII

heterodimer of α and ß Ig-like structure

n(unconventional express homodimer αα)

needs a long domain between the functional domain & cell membrane to reach α3 of MHCI

interacts with α3 of MHCI

non-conventional TCR with DNA rearrangement

δ chain encoded in alpha locus —>security if alpha chain produced, no δ-chain made —>only γδ or αß

for γ chain separate locus, much less diversity (gene segments) than for conventional TCRs

not restricted by MHC

repertoire so large that we can respond to any peptide

limitation is ability of MHC molecules to bind peptides (less diverse than TCR)

recognition of only peptidic compounds (no sugars, lipids etc in conventional TCR)