HC6: Mechanisms of adaptive immunity

T-cell activation

There are several types of lymphocytes:

• CD8 T-cell
  • Mass destructors → kill cells
• B-cell
  • Production of antibodies
• Regulatory T-cell
  • Dampen everything
• CD4 T-cell
  • Mostly helper-cells → aid other cells in fighting pathogens

There are 2 types of MHC molecules, which are distinguished by their peptides being produced in 2 different cellular compartments:

• MHC class I: peptides derived from protein synthesis by the presenting cell → the antigen is produced by the cell itself
  • An exception is cross-presentation of endocytosed proteins on MHC class I molecules
  • Can be expressed by all nucleated cells
  • CD8 T-cells bind to the a3domain of MHC class I
    • The cell is infected and needs to be cleared
• MHC class II: peptides derived from endocytosed proteins → the antigen is taken up from the surroundings
  • These proteins are extracellular
  • Can be expressed by all antigen-presenting cells
    • Dendritic cells always present MHC class II
  • CD4 T-cells bind to the b2domain of MHC class II
    • The cell itself isn't infected and doesn't need to be cleared

How do T-cells get activated?

Activation of the adaptive immune system starts when T-cells are activated. T-cells get activated by antigen presenting cells. Antigen presenting cells are part of the innate immune system:

1. Dendritic cells, macrophages or neutrophils engulf pathogens such as bacteria and viruses and digest these to obtain antigens
   • The bacteria is engulfed by a phagosome, which fuses with a lysosome that degrades the bacteria into smaller particles
     • Lysosomes have enzymes and a very low pH
2. The cells present the foreign antigens with MHC molecules on their surface
3. A naive T-cell binds transiently to the antigen presenting cell it meets
   • The TCR screens the peptide through the MHC-complex
4. The TCR delivers an antigen-specific signal
   • T-cell-antigen presenting cell interaction is stabilized for days
5. A second signal is required to trigger activation of the naive T-cell → co-stimulation
   • Co-stimulation plays an important role → the combination of an antigen-specific signal and a co-stimulatory signal is required to activate a naive T-cell
   • The co-signal is derived from an alarm signal from the dendritic cell
     • A B7 unit is expressed → IL-2 production
       • If there isn't any B7 expression, the T-cell won't be activated
       • A B7 unit has a CD80 or CD86 receptor, which will connect to CD28 on the naive T-cell
6. Activated T-cells switch on the expression of various genes, including interleukin-2 (IL-2)
   • IL-2 drives proliferation and differentiation of activated naive T-cells
     • Naive T-cells express the low affinity IL-2 receptor, activated cells express the high-affinity receptor
     • Immunosuppressive drugs act by suppressing IL-2 production or action → T-cell responses are dampened
       • For example cyclosporin, tacrolimus and rapamycin

In conclusion:

• T-cells can only be activated when the antigen is presented → an antigen alone drifting near a T-cell doesn't evoke a response
• Signal 1 is the connection between the MHC molecule and the T-cell receptor
  • This strongly locked by the CD4 connection
• Signal 2 is the danger signal from the dendritic cell → the B7 unit connecting to the C28

If there isn't any danger signal, the T-cell normally lets go. If it doesn't let go, there's something wrong with the T-cell because it responds to things that aren't important → the T-cell becomes anergic. It's an inactivated but alive T-cell that is unresponsive.

The presence of co-stimulatory molecules (like B7) is upregulated by microbial or pathogenic substances. They increase the expression of co-stimulatory molecules on the antigen presenting cell (APC) → they play an import role in creating danger signal in T- and B-cell activation.

Where does activation take place?

There are multiple sites involved in activation of adaptive immunity:

1. Dendritic cells pick up the antigen
2. The dendritic cells transport it to the draining lymph node
3. In the lymph node, they stimulate adaptive immunity by presenting the antigen on their MHC molecules
4. Once the right T-cell has found its antigen, the adaptive immune response kicks in

There are several pathways used by dendritic cells to process and present antigens:

• Receptor mediated endocytosis of bacteria
  • MHC class II
  • CD4 T-cell
• Macropinocytosis of bacteria or viruses
  • MHC class II
  • CD4 T-cell
• Viral infection
  • MHC class I
  • CD8 T-cell
• Cross-presentation of exogenous viral antigens
  • MHC class I
  • CD8 T-cell
• Transfer of viral antigens from infected dendritic cell to resident dendritic cell
  • MHC class I
  • CD8 T-cell

When a T-cell has entered the lymph node, the following happens:

1. T-cells enter a lymph node across high endothelial venules in the cortex
2. T-cells monitor antigens presented by macrophages and dendritic cells
   • Endothelial cells start expressing receptors which T-cells can use to enter the lymph node
3. T-cells that do not encounter a specific antigen leave the node in the efferent lymph
4. T-cells that encounter a specific antigen proliferate and differentiate to effector cells

This whole process takes about 4 days for naive T-cells → adaptive immunity takes some time to kick in. Memory cells can get activated quicker.

What happens when T-cells get activated?

After a T-cell is activated, the following happens:

1. Proliferation
   • Cell division and clonal expansion
2. Differentiation: depending on the pathogen, the necessary function is different → different kinds of T-cells are made
   • Secretion of cytokines
3. Death
   • Important for down-regulation of immune response

Mature dendritic cells can activate CD4 T-cells:

1. A mature dendritic cell presents its antigen via the MHC class II molecule to a naive CD4 T-cell
2. The naive CD4 T-cell is activated and proliferates
   • This is all dependent of the IL-2 cytokine
3. Depending on the cytokines present in the micro-environment, the T-cell differentiates to a type of T-cell
   • Th1-cell: a T-helper cell important for fighting intracellular pathogens
   • Th2-cell: a T-helper cell for B-cell responses against extracellular pathogens
     • Promotes production of antibodies by B-cells
   • Th17-cell: a T-helper cell for combatting infections of skin and mucosa and autoinflammation
     • For example fungi
   • iTreg-cell: a regulatory T-cell that dampens everything

Mature dendritic cells can also activate CD8 T-cells, cytolytic or cytotoxic T-cells. These connect to MHC class I molecules. CD8 T-cells require a stronger co-stimulatory activity than needed to activate CD4 T-cells. In case of a weak co-stimulatory activity, CD4 T-cells can help:

• CD4-derived cytokines can increase co-stimulation by for example increasing B7 on APC
• CD4-derived cytokines such as IL-2 can activate CD8-cells directly

Cytotoxic T-cells can kill several infected cells in succession:

1. A cytotoxic CD8 T-cell recognizes a virus-infected cell
2. The CD8 T-cell programs the first target cell to die
3. The CD8 T-cell moves on to the second target cell
4. The first target cell dies, the second is dying and the third is being attacked

In conclusion, activation of memory T-cells by an antigen presenting cell is less demanding than activation of naive T-cells.

B-cell activation

How do B-cells get activated?

In the first place, B-cells are activated by the crosslinking of a B-cell receptor with antigens → an antigen-specific signal. However, additional signals are required for B-cell activation → the B-cell co-receptor complex:

• CD21/CR2: actually binds to the bacteria
• CD19: a signaling chain that, once CR2 is connected, signals to the B-cell
• CD18: unknown function but is very necessary

The B-cell receptor and B-cell co-receptor cooperate in B-cell activation. For most antigens, even more signals are required → they are T-cell dependent. There are 2 types of antigens:

• T-cell independent antigens → quick, but not very high affinity
  • Do not require help of T-cells for activation of B-cells
  • No induction of B-cell memory
  • No class switching → only produces IgM
  • For example bacterial polysaccharides with repeating epitopes
  • Problematic for vaccine development
• T-cell dependent antigens
  • Require CD4 cells
  • Induction of B-cell memory
  • Class switching is possible

In case of T-cell-independent B-cell activation, B-cells become activated when their receptors are cross-linked by antigens → antibody production. This response isn't of a very high quality.

In case of T-cell-dependent B-cell activation, specialized Th2 cells stimulate the proliferation and differentiation of naive B-cells:

1. A B-cell binds to an antigen and presents it on their MHC class II to the T-cells
2. The TFH-cell recognizes a peptide derived from the B-cells antigen
   • A TFH-cell is a follicular helper cell
     • This is basically a Th2-cell
3. The naive B-cell and TFH-cell exchange signals that begin the process of B-cell activation
   • For example IL-2
4. The B-cell starts to activate → becomes a plasma cell

The T-cell can also encounter the same antigen presented by a dendritic cell, which also will lead to it presenting IL-2 → this will eventually also activate the B-cell. In conclusion, B-cells can be activated by T-cells responding to B-cells or by T-cells responding to dendritic cells.

How is unwanted activation regulated?

During activation, multiple measures are taken to prevent unwanted responses:

• Selection during maturation in the bone marrow or thymus
  • Binding to self-antigens can lead to deletion or inactivation of immature B-cells in the bone marrow
  • T-cells that show affinity for MHC-molecules presented by cortex thymic epithelial cells are positively selected in the thymus
  • T-cells specific for and binding too strongly to self-antigens are removed in the thymus by negative selection
• Regulation of B- and T-cell responses after the cells have left the primary lymphoid organs
  • Anergy
  • Regulatory T-cells that dampen the immune response

What is the function of antibodies?

Antibodies have many different effector functions:

• Neutralization of toxins → ensure that they're not recognized anymore by the immune system
• Inhibition of adherence to the epithelium
• Complementing activation
• Fc receptors and complement receptors provide opsonization

There are many immunoglobulin isotypes:

• IgG
• IgE
• IgD
• IgM
• IgA

Clinical examples

Asthma:

If a Th2 response goes wrong, it can cause an immune response against an allergen that isn't dangerous. This happens in case of asthma:

1. Peptides of a degraded allergen are presented by an antigen-presenting cell
2. A naive T-cell starts to promote a Th2 response
3. The Th2 response promotes the development of B-cells that promote the recruitment of innate immune cells
4. A cascade of inflammatory responses is activated

Immunotherapy:

In case of adoptive T-cell transfer, the adaptive immune system is used as a base for cancer immunotherapy:

1. Cytotoxic T-cells are taken out of the patient's body
2. They're in-vitro selected for their tumor-antigen
3. They're expanded in-vitro
4. They're put back into the body

This creates a high amount of CD8 T-cells that have a high affinity for the tumor and starts to destroy it.

The danger signal is important to activate T-cells, but it also initiates the expression of PD-L1 on the antigen-presenting cell → it connects PD1 to PD-L1. This complex has a higher affinity for CD28 than the B7-receptor → it inhibits the activation through negative feedback. It prevents over-responsion. Tumors can take advantage of this system by starting to express PD-L1 in order to deactivate T-cells instead of killing them. By inhibiting this system, the T-cell can be reactivated again to target the tumor.