Lecture 6

T cells

Thymic selection of T cells

most thymocytes die by

neglect

T cell recognition of MHC/peptide (compared with antibody)

antibody must be able to recognise an almost infinite variety of shapes

TCR confined to recognising MHC plus peptide

therefore, following thymic selection TCR are less variable than antibody

TCR-alphabeta associates with

zeta, epsilon-gama and epsilon-delts CD3 chains

TCR ab transmembrane domains are negatively charged, while CD3 transmembrane regions are negatively charged which mediates association between the different TCR molecules

ITAMs

tyrosine signalling molecules

3 in the zeta CD3 chain

CDR sequences of TCR

TCR has 3 CDR

1 and 2 mainly contact MHC and are encoded in V genes

3 contacts mainly peptide and encoded by J (alpha chain) or D-J (beta chain)

TCR gene structure

CDR3 formed at the junction of the V gene and the J gene - so very variable because there is addition randomly of nucleotides in this gap when joining occurs

Co-evolution of these genes in humans with MHC

Joining is messy which causes diversity

Alpha and beta each have 3 CDR regions

CDR3 makes the most contact with pepitde

MAIT cells

use alpha beta chain

recognise products of bacterial metabolism - vitamin B

Receptors generally look the same between all MAIT cells

Recognise a non-classical MHC - MR1

MR1 is very unstable so generally doesn’t reach surface unless has vitamin B

gamma-delta T cells

Gamma deltas have different chains - don’t use alpha beta

But have similar role to alpha beta TCR

Different gene set

Recognise CD1d

iNKT cells

use alpha beta chain

not NK cells  bind glycolipid antigens (most famously alphagal - from sea sponge

T cell types and their ligands

CAR T cell components

tumour binding variable antibody fragment (scFv)

scFv is fused to T cell signalling components in the cytoplasm (e.g. CD28, CD3 zeta - this has three ITAMs so can just use it as the endogenous purpose)

binding of CAR to surface tumour antigen leads to T cell activation

CAR T cell immunotherapy

extracellular domain (mimics antibody) - the two antibody chains which have been connected to become a single subunit so can be expressed and folded together

then transmembrane domain

then costimulatory domain e.g. CD28 and signalling domain e.g. CD3-zeta

CAR T cell process of cell death

expression of CAR on T cells

splice antibody heavy an dlight chain variable domains of anti-cancer antibody onto cytoplasmic domains of CD3-zeta and CD28

transduce patient T cells with this chimeric construct

this hardwires T cells to attack the cancer

CAR T cell therapy overall

clinical process is very expensive

CAR permenantly integrated into the genome of the T cell

Then T cells put into the patient and they should product T cells

Concern with the CAR itself becoming a cancer because the retrovirus likes to insert into active areas of chromatin where genes are being expressed - concerning because could knock out tumour supressor genes or promote growth gene

Or could cause damage to DNA in other ways

So there are risks of deleterious effects - but this has been shown to be very low

Use less infectious viruses to deliver the gene

CAR T cells effictivenes

70-80% response rates in B cell lymphomas

effective against relpased/refractoey disease

Usually used when people have already tried and failed with conventional therapies

2 ways to insert the chimeric receptor into the T cell

lentiviral vector

or transposon

Transposons instead of viruses - inefficient but integrate randomly which means more likely to dinsert into a junk area rsther than an important one

Now trying to do this whole process in vivo - to make this a lot easier and cheaper

CAR signalling domains

2 on left have been approved, simple ones seem to work better but some more complex ones are being developed as well

one on right is designed so that the chimeric receptor is only active when the drug AP1903 is administered and the cells lose their efficacy when the drug has been removed from the system