T cell receptors and MHC proteins Flashcards
what are the types of T cell receptors?
T helper cells – CD4+ve: Augment immune responses
T cytotoxic cells – CD8+ve: Specifically kill infected host cells
how are TCRs expressed?
only on membranes, not as soluble proteins
what is the structure of TCRs?
- C-terminus has hydrophobic amino acids to embed into the membrane
- Alpha chain and beta chain linked by flexible disulphide bond hinge
- Broadly Fab-like structure
- Extracellular domains of the T cell receptor are homologous to the variable and constant regions of immunoglobulins.
- Each V region contains 3 CDRs
o Vα and Vβ domains each have 3 CDRs (1 – 3) - CDR3 regions of a and b chains are the most variable.
what are the two subsets of TCRs?
alpha-beta TCRs - 95%, more diverse
gamma-delta TCRs - 5%, less diverse
what does expression of TCRs on the cell surface require?
Expression of TCR on the cell surface requires association with additional proteins, called CD3
what is the TCR complex?
alpha and beta subunits, CD3 subunits (epsilon, delta, gamma) and zeta
- this complex is required for optimal cell surface expression and signalling
CD3 subunit associates closely with the alpha-beta chains of TCR
what is the CD3 subunit comprised of?
epsilon, delta, gamma subunits and zeta subunit
ITAMs in its cytoplasmic region which can be phosphorylated to lead to downstream signalling
what makes up the TCR genes?
2 gene loci, each for alpha and beta chains
- alpha: chromosome 14, made of 2 exons for V region and lots of J regions (like light chain)
- beta: chromosome 7, made of multiple V exons, J exons and D exons (like heavy chain)
these gene recombine in the thymus
how do TCR genes recombine?
Somatic recombination of TCR V region genes:
- Same recombination machinery as that used by developing B lymphocytes.
- Occurs in thymus
- J exon recombined to lie next to a V exon and then the C region
-Rag1 and Rag2 proteins facilitate this in the thymus
what processes drive TCR diversity?
- Multiple copies of V region gene segment [Vn x Jn/Vn x Dn x Jn]
- alpha x beta chain combination [Va x Ja] x [Vb x Db x Jb] = ~ 6 x 10^6
- Junctional diversity = ~2 x 10^11
- Concentrated in the CDR3s of TCR alpha and beta chains
Total diversity = ~ 10^18
- Greater than B cell diversity as they have more gene segments than B cells
how are the CDRs encoded for in TCRs?
CDR1 and CDR2 encoded in germline, whereas CDR3 is generated via VDJ recombination
why do TCRs not undergo somatic hypermutation?
The V regions of TCRs do NOT undergo somatic mutation
- Possibly too dangerous – high likelihood of TCRs that recognise the body’s own tissues
- Doesn’t need to bind to antigen in free solution like antibody, so may not need to bind with such high affinity
- needs to still recognise MHC proteins to function - mutation may disrupt this
what are B cells important for? what kind of antigen do B cells recognise?
B cell immunity is particularly important in defence against extracellular pathogens
- B cells recognise free, native antigens on the surface of pathogens
what are T cells important for? what kind of antigen do T cells recognise?
T cells are important in defence against intracellular pathogens e.g. viral infections
- T cells recognise cell-associated, processed antigen
how do T cells recognise intracellular pathogens?
Major histocompatibility proteins (MHC):
- protein → peptide → MHC → cell surface → T cell recognition
- Protein is degraded into peptide, which is bound to MHC and transported to the cell surface for recognition
- T cells recognise cell-associated, processed antigen
- Samples of antigenic material inside the infected cell are displayed on the surface to be recognised by the T cell
what do T cells require to recognise antigen?
T cells require antigen presentation by cells expressing MHCs
what are MHC proteins?
- Discovered during research on graft rejection.
- Encoded by the genes of the Major Histocompatibility Complex Chromosome 6 (in humans)
- Also known as HLA molecules in humans (human leukocyte antigen).
- e.g. HLA-A, HLA-B, HLA-C – 3 gene loci encoding 3 different MHCs
- very polymorphic
- e.g. >1400 alleles of HLA-B locus. Alleles may differ by up to 20 a.a. substitutions
what is the major role of MHC proteins?
- major role in antigen presentation and initiation of T cell responses
what is MHC restriction?
T lymphocytes can only recognise antigen in the context of self-MHC molecules
how was MHC restriction discovered?
Experiments with inbred mouse strains and virally infected cells
- Had the same MHC proteins on their surface
- Strains A and B were immunised with a virus and their T cells were isolated and cultured with cells infected with the same virus
- If T cells are taken from strain A and mixed them with cells from mouse A, then the T cells kill the infected cells
- If T cells are taken from strain B, the T cells were unable to kill infected cells from mouse A
- Cannot kill cells from a different strain
- T cells will only recognise antigen presented by self-MHC proteins
what were the two theories for MHC restriction?
2 receptors on T cells – one (TCR) for antigen, one for MHC?
1 receptor on T cells (TCR) – recognises antigen + MHC?
why does MHC restriction occur?
determined by x-ray crystallography:
- Saw unknown peptide antigen bound as part of the structure
- Proved that T cells recognised self-MHC associated with foreign peptide (antigen)
what did crystallographic studies on TCR and MHC demonstrate?
- MHC binds peptide
- TCR recognises complex of foreign peptide + self-MHC
- CDR1 and CDR2 bind self-MHC (germline encoded, which is why T cells don’t undergo somatic mutation)
- CDR3 binds peptide - variation induced by junctional diversity
what are the two types of T cells?
T helper cells - CD4+ve
Cytotoxic T cells - CD8+ve
which MHC class interacts with CD8+ve cells?
MHC class I interacts with cytotoxic T cells
- Expressed by all nucleated cells
- Present peptides derived from endogenous proteins to cytotoxic CD8+ve T cells
- Endogenous – protein is synthesised by the infected cell
which MHC class interacts with CD4+ve cells?
MHC class II interacts with helper T cells
- More restrictive expression pattern
- Expressed by certain leukocytes: dendritic cells, B cells, macrophages – antigen-presenting cells
- Present peptides derived from exogenous proteins to helper (CD4) T cells
- Exogenous – protein taken up from outside the cell
what is the structure of MHCI protein?
2 chains: alpha chain and beta chain
- polymorphic transmembrane alpha chain
- invariant b2-microglobulin to provide stability
- polymorphisms are clustered in domains furthest from the membrane which bind to antigen
what is the structure of MHCII protein?
2 polymorphic transmembrane alpha and beta chains
- Domains closest to membrane are immunoglobulin-like, and furthest are most polymorphic
what are the functional domains of MHC proteins?
- Membrane-proximal domains are Ig-like
- Membrane-distal domains are bind peptide
- Membrane distal domains contain polymorphism
how do MHCI proteins bind peptide?
MHCI bind peptides 8-10 amino acids long:
- Bottom of groove made of beta-pleated sheets, and alpha-helices curl up the sides of the domain
- N and C-termini of peptides bind to invariant sites at ends of the groove.
- Two or three “anchor residues” on the peptides bind to “specificity pockets” formed by polymorphic residues at the base of the groove
- Fairly closed to restrict length of peptide that can bind
how do MHCII proteins bind to peptide?
MHCII bind peptides 13-18 amino acids long:
- Peptide backbone interacts with conserved residues that line the base groove
- “Anchor residues” on the peptide bind to “specificity pockets” formed by polymorphic residues.
what is a proteosome?
multi-subunit complex that breaks down misfolded proteins
what is an immunoproteosome?
proteosome induced by interferon
how does MHCI present antigen to CD8+ve cells?
e.g virus-infected cell presenting to cytotoxic T cell
-Cell that is infected with virus will convert the proteosome to an immunoproteasome to process peptide to the right length (8-10 aa) to interact with MHCI
- Peptides transported to ER by ATP-hydrolysis driven transporter, TAP (transporter associated with antigen presentation)
- Peptides loaded onto MHCI in ER
- MHCI-peptide transported to cell surface for recognition by cytotoxic T cell
how does MHCII present antigen to CD4+ve cells?
e.g. macrophage/dendritic cell/B cell taken up antigen and needs help from T helper cell (CD4+ve)
- antigen taken up by phagocytosis or endocytosis
- phagolysosome breaks down the bacteria - acidification in vesicles promotes unfolding & proteolysis
- longer peptides (13-18 aa) associate with MHCII in the endocytic compartment
- MHCII-peptide transported to cell surface for recognition by helper T cell
what is cross-presentation by MHCs?
- Some dendritic cells present exogenous peptide associated with MHCI to cytotoxic T cells
- Triggers naïve cytotoxic T cell to be active
- Dendritic cell isn’t actually infected
- Allows antigen presentation to cytotoxic T cells without the dendritic cells themselves being infected
- Important in cytotoxic T cell responses to tumours
what is required for T cell activation?
TCR complex (alpha-beta, CD3, zeta-chain), MHC and co-receptor
what is the role of co-receptors on the T cell?
to stabilise the interaction between TCR and MHC to facilitate signalling
what is the co-receptor of cytotoxic T cells?
CD8
what is the co-receptor of T helper cells?
CD4
which co-receptor does MHCII + antigen interact with?
CD4 co-receptor
which co-receptor does MHCI + antigen interact with?
CD8 co-receptor
how do T cell co-receptors function?
CD4/CD8 interact with invariant regions on MHCII/MHCI - not polymorphic
CD4 and CD8 act as co-receptors for the TCR complex. Both contain Ig-like domains
Engagement of the CD4/CD8 co-receptors with the TCR complex enhances phosphorylation of the ITAMs, promoting T cell activation
- CD4/CD8 C-terminus associates with Lck tyrosine kinase which phosphorylates ITAM motifs on the CD3 complex which is associated with the TCR
what is thymic selection?
T cell enters thymus and undergoes somatic recombination of TCR genes
- Rearrangement of T cell receptor genes (αβ γδ)
- TCR must be able to recognise self-MHC but doesn’t react against self
- MHC selection (only in αβ chain T cells) (gamma-delta doesn’t require antigen presentation)
what is positive MHC selection?
TCR must bind to self-MHC
- T cells which fail this undergo apoptosis
what is negative MHC selection?
any TCRs which bind to self-peptide undergo apoptosis
how are T cells exposed to non-thymus proteins during thymic selection?
AIRE allows expression of non-thymus proteins in the thymus
- e.g. insulin, so that the TCR doesn’t recognise self-proteins
- prevents autoimmune disease
how are MHC proteins encoded?
Encodes by the Major Histocompatibility gene Complex (MHC), expressed on chromosome 6
- MHCI – 3 gene loci: HLA-A,B,C encode for polymorphic alpha chain
- MHCII: 3 gene loci: HLA-DP,DQ,DR encode for polymorphic alpha and beta chains
- very polymorphic e.g. HLA-B has 5000 alleles
how are MHC proteins diverse?
Co-dominant expression (i.e. alleles inherited from each parent expressed on cells): increases no. MHC proteins on surface of cells to allow binding of a wide range of peptides
- however MHC diversity is inherited and small compared to that of B and T cell receptors
which part of the MHC protein facilitates diversity?
Allelic variation occurs predominantly in the peptide-binding groove
- Diversity is in region of molecule that recognises antigen peptide
what are the consequences of MHC polymorphism?
- graft rejection
- ensures wide recognition of foreign peptides BUT variability of MHC molecules is small compared to that of TCR (all inherited in genome)
- T cell responses determined by an individual’s MHC type
- each MHC allele can bind a restricted range of related peptides - Responders and Non-responders
- e.g. Inbred mice have MHC proteins which don’t respond to antigens
- MHC polymorphism evolved in response to pathogen - Black death? Flu? HIV?
what are the functions of MHC proteins?
- antigen presentation to T cells, T cell activation
- development of T cell repertoire/tolerance in thymus
- self/non-self-recognition (NK cells “detect” alterations MHCI)
- when they detect self-MHC, NK cells are inhibited from killing
- association with certain autoimmune diseases
- Choice of mate? – attracted to someone with different MHC proteins from self
summarise antigen presentation to T cells:
- CD8 and CD4 act as co-receptors for the TCR, interacting with MHCI and MHCII, respectively, and contribute to T cell activation.
- MHC proteins play a role in thymic selection
- MHC proteins are very polymorphic and co-dominantly expressed, allowing binding of a wide range of peptides
- An individual’s T cells can only recognise antigen presented by self-MHC molecules (MHC restriction).
