T Cell Activation & Generation of Effector T Cells

Question 1

What are the two types of adaptive immune responses?

Answer 1

There are two types of adaptive immune responses.

The first is mediated by B lymphocytes which produce antibodies. They are part of humoral immunity and deal with extracellular microbes.

The second is mediated by T lymphocytes (of which there are multiple types). They are part of cellular immunity and deal with intracellular microbes.

Question 2

Describe the life stages of T lymphocytes.

Answer 2

• they’re generated in bone marrow and undergo maturation in thymus
• mature naïve T cells are released from the thymus into the blood
• they recirculate between the blood and peripheral lymphoid organs (lymph nodes, spleen, MALT)
• if they encounter antigens that they recognise, leads to lymphocyte activation, proliferation & differentiation into effector/memory cells

Effector T cells: specialised functions
Memory T cells: memory responses (faster, more efficient)

Question 3

What types of intracellular microbes, etc. do T cells respond to?

Answer 3

T cells are designed to fight intracellular microbes:

• intracellular bacteria in phagosomes of phagocytes
• viruses: free in cytoplasm of cells (phagocytes or non-phagocytes e.g. epithelial cells)
• cancer cells (mutated proteins from cancer cells)

Question 4

What do TCRs recognise?

Answer 4

T cells do not recognise antigens directly, they only recognise them after processing and presentation.

Most T cells (αβ TCR T cells) recognise peptides.
=> T cells recognise cell-bound Ags (peptides)
=> peptides from foreign Ags only when bound to major histocompatibility complex (MHC) molecules

Question 5

Describe the structure of the TCR.

Answer 5

T cells recognise antigens via their T cell receptor (TCR).

• it has a similar structure to B cell antigen receptor (BCR)
• it’s made up of 2 chains: α and β (most common TCR type) - the other is γ and δ (the TCR in γδ T cells)
• each chain has 1 variable (V) domain and 1 constant (C) domain (the antigen binding site formed by the two variable domains Vα + Vβ)
• the V and C domains of TCRs and BCRs are homologous

Question 6

Describe MHCs.

Answer 6

MHC (major histocompatibility complex) molecules display peptides from processed Ag. There are two types: MHC I and MHC II.

MHC I: presentation of peptides to CD8+ T cells composed of α chain + β2-microglobulin

MHC II: presentation of peptides to CD4+ T cells composed of α chain + β chain

MHC I presents on all nucleated cells, and MHC II presents on APCs.

Question 7

Describe APCs.

Answer 7

APCs are antigen presenting cells.
They are cells that specialise in the capture, processing and presentation of antigens (Ag) to CD4+ T cells.

Professional APCs:

• dendritic cells => the only APCs capable to present to naïve T cells
• macrophages => present to previously activated effector T cells

Question 8

Describe how and to whom the dendritic cells and macrophages present antigens to.

Answer 8

Dendritic cells will uptake the antigen, process and present it to naïve T cells. This activates them and induces them to become effector T cells.

Macrophages uptake the antigen (via phagocytosis) and present them to previously activated T effector cells to warrant an increased immune response.

Question 9

Describe dendritic cells.

Answer 9

They are located in nearly all tissues that we have. Together, they form a mesh or network so that they can scan for pathogens, detect the, take them up, process them and present them to T cells to activate them (if needed).
Dendritic cells in the skin are called Langerhans cells.

Question 10

Describe the three signals needed for naïve T cell activation.

Answer 10

The first signal is TCR recognition of the antigen presented by the MHC. However, this alone is not enough to activate a naïve T cell.

It needs a second signal called co-stimulation, which is delivered by the ligation of C28 with the co-stimulatory molecules on the APC, CD80/CD86.

The third signal is by the cytokines produced by the dendritic cells. These cytokines will specifically determine which type of helper cell the T cell differentiated into, based on the type of antigen.

Question 11

Describe Signal 1 in T cell activation.

Answer 11

Naïve T cells need signals in addition to Ag to get activated

SIGNAL 1: recognition of Ag (peptide:MHC complex) on APC

• not sufficient to induce T cell activation
• without signal 2 => no response or anergy of T cell

Question 12

Describe Signal 2 in T cell activation.

Answer 12

SIGNAL 2: co-stimulation:

• it is the binding of co-stimulatory molecules (B7 family, e.g. CD80/CD86) on APC by co-stimulatory receptor (CD28) on the T cell
• together with signal 1, it leads to activation of naïve T cells
• this is more important for naïve T cells than for restimulation of previously activated effector or memory T cells
• APCs exposed to infection increase the expression of co-stimulatory molecules (B7) and of MHC

Question 13

Describe Signal 3 in T cell activation.

Answer 13

SIGNAL 3: cytokines produced by APCs (after infection):

=> regulate the differentiation of activated T cells into different types of effector T cells

• e.g. IL-12 and IFN-γ from APC => differentiation into Th1
• e.g. IL-4 from APC => differentiation into Th2)

=> ensure the right type of effector T cell is generated

• e.g. effector T cell type that is most suited to respond to the infection that triggered the response
• Th1 <=> macrophage co-operation
• Th2 <=> B cell and eosinophils / mast cell co-operation

Question 14

List some functions of macrophages.

Answer 14

=> they phagocytose microbes (e.g. Mycobacteria tuberculosis)

=> they are involved in Ag presentation to effector CD4+ T cells (Th1)

=> they induce the activation of Th1 cells (see later slides)

=> Th1 cells activate macrophage to kill ingested microbes

Question 15

What are CD8+ T cells specialised to do?

Answer 15

All nucleated cells can present peptides derived from proteins from antigens present in the cytosol to CD8+ T cells.

• all nucleated cells can get infected by viruses
• all nucleated cells can get cancer-causing mutations

Thus, CD8+ cells are specialised to:

• recognise viral antigens and mutated proteins
• eliminate cells infected by viruses/malignant cells

Question 16

Describe antigen processing & presentation to CD4+ T cells.

Answer 16

Exogenous Ags (e.g. bacteria) are taken up in cells, processed and presented by MHC II to CD4+ T cells.

Exogenous pathogens (bacteria that grow outside cells):

• are taken up by phagocytes
• eliminated via killing in phagocytes
• or eliminated by antibodies (via neutralisation, opsonisation and complement activation)

CD4+ T cell effectors help macrophages (Th1) and B cells (Th2) to eliminate extracellular bacteria.

Question 17

Describe the mechanism of antigen processing and presentation to CD4+ T cells.

Answer 17

A pathogen/bacteria is endocytosed into an endosome/phagosome, where it is broken down. They will end up in endo-lysosomes, where they will generate peptides.

MHC molecules are proteins that are produced in the ER. In the ER, the newly-generated MHC molecules are coupled an the invariant chain; it has a fragment that fills and stabilises the groove and thus the MHC molecules.
The complex of MHC and invariant chain leaves the ER, via the Golgi, into an exocytic vesicle. While the vesicle travels towards the surface of the cell, it will end up in the late-endosomal compartment, where the invariant chain is broken down. Only a small fragment called CLIP remains in the groove of the MHC.

The compartment also expressed another molecule that also happens to be an MHC molecule It is not involved in antigen presentation, it is instead involved in trying to free the groove of the MHC II molecules to allow it to eventually load peptides. It steals away CLIP from the MHC II molecule, which is now ready with a free groove, and can try the peptides that came from the degradation of the proteins present in the bacteria.

If it doesn’t find any proteins that fit into the groove, the molecule will be degraded. If it does, it’ll be stabilised and will leave via an endocytic vesicle, fuse with the plasma membrane. It can now be scanned by CD4+ T cells that make contact with it, and eventually get activated if they recognise the antigen.

Question 18

Describe antigen processing & presentation to CD8+ T cells.

Answer 18

Cytosolic Ags (e.g. viruses, mutated proteins in cancer cells) are processed and presented by MHC I to CD8+ T cells.

Pathogens that grow free in the cytosol (viruses) or pathogens (bacteria, viruses) that are taken up in phagosomes but are then released into the cytosol.
They are efficiently eliminated via killing by CD8+ T cells.

CD8+ T cells are specialised to eliminate cells infected by viruses and cancer cells.

Question 19

List the types of effector T cells.

Answer 19

Th (helper) cells: express CD4 (CD4+ T cells)

• Th1: help phagocytes to kill ingested microbes
• Th2: help eosinophils/mast cells to kill helminths
• Th17: role in defense against bacteria & fungi
• Tfh (T follicular helper); help B cells (class switch and affinity maturation)

Cytotoxic T lymphocytes (CTL): they express CD8 and kill cells infected by microbes that grow free in cytosol.

Regulatory T cells (CD4+CD25+FOXP3+): they are involved in immune tolerance & inhibition of immune responses.

Question 20

Describe the generation of Th1 effector cells & its main roles.

Answer 20

Cytokines that induce differentiation into Th1: IL-12 and IFN-γ from APCs infected with bacteria (e.g. Mycobacteria, Listeria).

The main cytokine produced by Th1 is IFN-γ.

The main role of Th1 is to activate phagocytes (macrophages), which increases destruction of intracellular pathogens.
It’s other role is to stimulate production of IgG Abs, which increases phagocytises of microbes.

Question 21

Describe the generation of Th2 effector cells & its main roles.

Answer 21

Cytokines that induce differentiation into Th2: they are less well defined (it is believed to be IL-4, IL-25, IL-33).

The main cytokines produced by Th2 are IL-4, IL-5, IL-13.

The main role of Th2 cells is to help B cells produce IgE:

• IgE opsonises helminths
• it activates eosinophils & mast cells
• induces eosinophil & mast degranulation and the killing of helminths

Question 22

Describe the main roles of CD8+ effector T cells.

Answer 22

CD8+ T cells (cytotoxic T cells, CTLs) specialised to:

• recognise viral antigens and mutated proteins
• eliminate cells infected by viruses/malignant cells

Their main mechanism is to kill infected cells. The killing is antigen-specific and contact-dependent.
This eliminates reservoirs of infection in the body.

Question 23

What mechanisms do CD8+ T cells use to kill?

Answer 23

There is killing mediated by cytolytic molecules:

• Perforin: forms pores for the delivery of granzymes
• Granzymes A, B, C: initiate apoptosis - they are delivered at the site of contact between CTL and the target; this prevents the killing of neighbouring healthy cells

There is also killing mediated by death receptor pathway (Fas/FasL).