T cells – specificity, education, maturation and function

Question 1

How important are T cells

Answer 1

T cells are vital for acquired immunity (diversity), memory and self/non-self-recognition

Arise from a common myeloid progenitor

Question 2

Where do they originate from

Answer 2

They originate from the bone marrow from HSC and mature In the thymus

Question 3

Where do T cells migrate to after leaving the thymus

Answer 3

From here they migrate to secondary lymphoid tissues – sites of lymphocyte activation

Question 4

Where do T cells encounter antigens for the first time

Answer 4

They circulate between peripheral blood and these sites until they encounter an antigen

T cells re-circulate from the blood to tissues (patrols) - other immune cells (like B cells do not do this)

Question 5

What do T cells have that is different to each other

Answer 5

Each T cell has an individual type of receptor which is complementary to a specific epitope

Question 6

What happens upon activation

Answer 6

When activated they undergo clonal selection which creates numerous T cells with the same receptor

They have the property of memory – this allows for faster immune response with a greater magnitude to future infections from the same antigen

Question 7

What is the structure TCR

Answer 7

Has a constant and variable regions (epitope specific)

Is a heterodimer of alpha-beta subunits which belongs to the Ig superfamily

Has 3 domains

Variable extracellular domain

Constant extracellular domain

Constant trans-membrane domain

Question 8

What are the characteristics of the TCR

Answer 8

The TCR alpha chain has a molecular weight of 40-50 kDa and the beta chain has a weight of 40-45 kDa

The TCR N-terminal (extracellular) domain contains a hypervariable region synonymous with the complementary determining regions (CDR’s) of the immunoglobulins

Each TCR chain has 3 hypervariable regions

TCR beta-chain has a 4th hypervariable – this does not appear to interact with antigens

At the extracellular C domain, each chain has a cysteine residue (consensus sequence) - links to the alpha-beta via a disulphides

Question 9

What are the interactions of the TCR

Answer 9

T cells must be shown the Ag complexes with MHC – they cannot interact directly with the Ag

Multiple levels of protein-protein interactions occur at the TCR/MHC immunological synapse

Each TCR is associated with ancillary proteins – these form the TCR-complex

CD3 is a heteromeric complex with extended cytoplasmic regions (phosphorylation sites)

CD4/CD8 are co-receptors with specificity for the MHC-II (CD4) or MHC-I (CD8)

Question 10

What is the structure of CD3

Answer 10

CD3 is a heterohexamer consisting of

One epsilon-delta heterodimer

One epsilon-gamma heterodimer

One zeta-zeta homodimer

Question 11

Characteristics of CD3

Answer 11

+ve charged polar residues inside the TCR transmembrane domain

This attracts –ve residues in the CD3 subunit transmembrane domains

Immunoreceptor tyrosine activation motifs (ITAM’s) are involved in signal transduction

Question 12

What interactions occur between the TCR complex and MHC-peptides

Answer 12

MHC-peptide complex associates with complementary TCR hypervariable regions

These intermolecular interactions initiate local (microenvironmental) conformational changes

Conformational are observed through the length of the alpha and beta chains of the TCR

Changes translate to CD3 complex –> ITAM phosphorylation

Question 13

What are the genetics of the TCR complex

Answer 13

The TCR has hypervariable regions and constant regions like B cells

Variation of the TCR is achieved through somatic recombination

TCR loci contain numerous gene segments which are inherited through the germ line - this accounts for the diversity of TCRs

Question 14

What is the loci of the TCR gene

Answer 14

Alpha gene locus (12q11.2) contains 70-80 variable gene segments and 61 joining gene segments with a single constant gene segment

Beta-chain (7q34) contains 52 variable gene segments and 13 joining gene segments with 2 constant gene segment

Question 15

How does somatic recombination effect TCR gene expression

Answer 15

Random V(D)J recombination and genetic splicing occurs for both the alpha and beta chain genes in immature T-cells

For beta-chain, the J gene segments do not seem to be important for Ag recognition

However D(beta)1 and D(beta)2

Question 16

How is TCR expression controlled

Answer 16

by allelic exclusion

Successful “functional” somatic rearrangement of the TCR genes at one allele results in the switching off of the other allele (maternal or paternal)

During TCR recombination the alpha-chain may sustain many recombination attempts before the functional TCR is established (bone marrow)

This occurs during T cell education

Question 17

What does T cell development ensure

Answer 17

only self-restricted/self-tolerant T cells enter the periphery (patrol for Ag)

Question 18

What are the stages of T cell education

Answer 18

CD4-CD8- double negative (DN) develop TCR

CD4+CD8 – double positive (DP) mature to CD4 or CD8 single positive (SP) MHC-II/MHC-I restricted

Question 19

What occurs during early thymocyte maturation (DN phases)

Answer 19

cortical interactions

Initially thymocytes entering the thymus medulla encounter NOTCH ligands – causes proliferation of thymocytes

DN1 – plastic thymocytes, up regulate CD25 once in the cortex – proves thymocytes are responding IL-2 signalling

DN2 – commit to T cell lineage and beta chain rearranges

DN3 – completion of beta chain rearrangement and alpha/beta TCR development

DN4 – immature single positive (ISP) before DP stage

Question 20

What occurs during positive selection

Answer 20

By the end of early maturation CD4+CD8+ DP thymocytes are generated and must be selected to ensure MHC self-recognition

DP thymocytes are small and non-proliferative

DP thymocytes interact with the cortex epithelial cells which express high levels of self MHC-I/MHC-II

Moderate to strong binding creates survival and proliferation

Weak or no binding causes death by neglect (apoptosis) - positive selection

Question 21

What occurs during negative selection

Answer 21

DP thymocytes that make it through positive selection progress to the negative selection stage

In the medulla, antigen presenting cells such as dendritic cells present MHC coupled self-antigen

Cells which bind moderately survive

Cells which bind with a high affinity undergo apoptosis

Surviving cells by default are reactive to foreign Ag presented by self-MHC

Most DP thymocytes do not make it through thymic selection

Question 22

How long does each stage take

Answer 22

The cortex stage – early maturation and proliferation of DN, DP generation and positive selection takes 13-15 days

The medulla stage – negative selection and CD4/CD8 lineage commitment take 4-5 days to complete

Roughly a 3 week process

Question 23

What does thymic selection do

Answer 23

ensures that T cells recognise foreign Ag bound to self-MHC

Question 24

What does a lineage commitment refer to

Answer 24

generation of single positive (SP) CD4+ or CD8+ T cells

The thymic selection process “screens” TCR reactivity, however T cells are still CD4 / CD8 double positive

The mechanisms of T helper or T cytotoxic lineage commitment is not fully defined

Question 25

What is the stochastic model

Answer 25

MHC class restriction is a random process CD4 or CD8 on immature DP T cell is downregulated Binding of remaining CD4 or CD8 to MHC-II or MHC-I commits T cell class and MHC restriction

Question 26

What is the instructive model

Answer 26

DP immature T cells bind to either MHC-I or MHC-II Signal results in the retention of CD8 or CD4 respectively

Question 27

What is the kinetic model

Answer 27

Duration of the signal If a DP cell is persistently engaging with MHC-I – results CD4 down-regulation and CD8 retention

Question 28

What happens to the thymus as we age

Answer 28

T cell education and maturation occurs early on in life We have all our T cell repertoire by adolescence (T cells long lived) Neonatal thymectomy causes a drop in T cell number but no compromise in immune function

Question 29

What happens if thymic selection fails

Answer 29

self-reactive T cells may enter the periphery and cause autoimmune diseases

Question 30

How many types of T-helper cell are there and what do they do

Answer 30

4 types TH1 cells produce pro-inflammatory cytokines - TH2 cells produce cytokines involved with B cell antibody development (TH1 and TH2 mutually antagonistic) - TH17 cells important for neutrophil activation and cancer immunity - Treg cells (regulatory T cells) supress immune response

Question 31

What is the function of T-cytotoxic cells

Answer 31

kill cells bearing appropriate stimulating Ag – viral Ag or altered-self (tumour antigen)

Question 32

What does SCID refer to

Answer 32

Stands for severe combined immunodeficiency disorder – a condition characterised by a lack of function T helper cells

Question 33

What effect does SCID have on a person

Answer 33

T cells (particularly helper cells) are central for immunological processes SCID affects the humoral response (Ig production from B cells) - decreased opsonisation and complement activation --> fatal a year after birth Genetic x-linked most cases involve mutations that encode the common gamma-chain of cytokine receptors for IL-2, 4, 7, 15 and 21

Question 34

How is SCID treated

Answer 34

HSC transplant

Question 35

What are T cells that progress through thymic selection called and what are their characteristics

Answer 35

naïve T cells Reside in the G0 and require proper activation for maturation Condensed chromatin, small, re-circulate blood/lymph Naïve T cells (CD4 and CD8) require co-stimulation for full activation and proliferation

Question 36

What happens at the immunogenic synpase

Answer 36

TCR + CD4 or CD8 engages with the MHC-II or MHC-I respectively – this interaction alone does not possess sufficient avidity to activate T cells Interaction with co-stimulatory molecules CD80 (APC) and CD28 (T cell) provides sufficient immunosynaptic signal to induce T cell activation TCR/MHC engagement without CD28 co-stimulation results in the generation or proliferation of an anergic T cell – a T cell that is not immune active to a particular antigen – tolerant) CD28 alone has no effect on a T cell Cytokines also have a role in T cell co-stimulation and will drive polarisation Therefore T cell activation requires TCR stimulation CD28 co-stimulation Cytokine stimulation

Question 37

What does T cell polarisation refer to

Answer 37

(differentiation) APC also secrete cytokines --> T cell polarisation T cells activated in the presence of APC derived IFN(gamma) and IL-12 will differentiate into Th1 cells (STAT1 driven) T cells activated in the presence of IL-4 and IL-33 will differentiate into Th2 cells (STAT6 driven) T cells activated in the presence of TGF(beta) and retinoic acid differentiate into Treg cells (Foxp3 driven)

Question 38

How are T cells regulated

Answer 38

Activated T cells down regulate CD28 and up-regulate co-inhibitory molecules CTLA-4 and programmed death 1 (PD-1) 90% of T cells die by apoptosis post-antigen exposure – the rest remain as memory cells G0 like naïve - are distinguished by immunophenotype Central memory T cells (Tcm) are CCR7* and CD62L+ (home to lymphatics) Effector memory T cells (Tem) are CCR7- and CD62L+ (tissues)

Question 39

What are gamm-delta T cells

Answer 39

is a rare population of T cells Arise during TCR gene arrangement from DN T cell in the thymus and express TCR's with a gamma-delta chains Many gamma-delta cells do not require peptide bound MHC presentation for activation

Question 40

What are the characteristics of gamma-delta T cells

Answer 40

The gamma-delta receptor is more like a TLR and functions innately Gamma-delta T cells rapidly activate in 1-2 days Are found predominantly within intra-epithelial region of the gut mucosa They recognise and destroy invading bacteria – induce apoptosis of infected cells and aid with tissue repair The V(gamma)9 / V(delta)2 version is important for Mycobacterium tuberculosis recognition (causes tuberculosis) Also secrete cytokines like proinflammatory IFN(gamma) and activate NK cells – early response links innate and adaptive systems Secretes IL-10 in its later response