Lecture 11-Activation of naïve T cells

• Naïve T cells are activated in lymph nodes and spleen.• Dendritic cells are key antigen presenting cells for naïve T

cells.• Naïve T cells require MHC/peptide plus antigen-presenting

cell "costimulation" in the form of B7 molecules.• Only "professional" antigen presenting cells express B7

molecules. And these only express B7 when activated.• Antigen stimulation without costimulation can lead to loss

of T cell responsiveness and immune tolerance.

Reading Chapter 6

How naïve, mature T cells find antigen

• Antigen transport. Naïve cells are mainly in lymph nodes and spleen. Dendritic cells transport antigen to draining lymph nodes, blood borne antigens lodge in spleen.

• T cell entry to lymph nodes. T cells recirculate through lymph nodes using homing receptors, integrins and responding to chemokines constitutively produced by the lymph node stroma. (Parham 6.2-6.3)

• T cell scan of APCs. T cells scan antigen presenting cells. Interaction is initiated by adhesion molecules. (Parham 6.4)

• Antigen recognition.

Figure 6-3

T cell entry into lymph nodes from capillaries

Figure 6-3

Figure 6-7

Recognition by T cells of peptide/MHC on dendritic cell leads to stronger adhesion and a more prolonged interaction.

CD8killer

CD8 CD4

CD4Th1

CD4Th2

naive

effectors

Activation, proliferation, differentiation

Diverse outcomes of immune activation of mature T cells

Th1 vs Th2 is defined by distinctive pattern of cytokine expression

1 1

2

inappropriateactivation

leads to cell inactivation

inappropriateactivation

leads to cell inactivation

CD8 CD4

How are these cell fate decisions made?

T cells encountering antigen on non-activated professional antigen presenting cells become inactivated

inactivated

Figure 6-10

Major professional antigen presenting cells.

Appropriate activation of naïve T cells requires in addition to a ligand for the T cell receptor a second interaction with CD28. This second signal is called "costimulation." Under many conditions, dendritic cells provide this signal during a primary response.

Costimulation by signaling through CD28

• It is currently believed that naïve T cells require activation by dendritic cells, macrophages or other "professional" antigen presenting cells.

• Dendritic cells are most efficient because they not only take up and present antigen, but also because they move efficiently to lymph nodes to encounter naïve T cells.

• These professional antigen presenting cells provide peptide MHC complexes along with a critical "costimulation."

• Costimulation is believed to be a crucial interaction between the innate and adaptive immune systems that regulates T cell responses.

Special importance of dendritic cells in activating naïve T cells

Figure 6-1

Figure 6-2Immature

dendritic cells in tissues actively capture, take

up, and process antigens.

Upon maturation,

they migrate to draining lymph

nodes and present

antigens to T cells.

Green, MHC

Red, Lysosomes

Dendritic cells make multiple contacts with T

cells

Figure 6-12

Activated

Activated Dendritic cells express B7

molecules

Only activated antigen presenting cells

express B7 molecules which are among the

most important costimulatory signals.

In macrophages and dendritic cells, innate immune signals stimulate B7 expression.

Figure 6-15Inert proteins do

not stimulate B7

expression, even if they are foreign.

Adjuvant: important component of vaccines

Ovalbumin

Ovalbumin+ Adjuvant

No detectable antibody response

Strong antibody response

Adjuvants work by slowing the release of antigen, promoting phagocytosis, and providing inflammatory signals that promote costimulation

Figure 12-4

***

Active ingredientN-actylmuramyl-L-alanine-D-isoglutamine (MDP)

**

**

MDP is derived from mycobacterial proteoglycan

Figure 6-15 Inert proteins

given with adjuvants

can stimulate a response.

Diptheria/ tetanus/ pertussis vaccine

Mixture of diptheria toxin (a soluble protein) tetanus toxin (another soluble protein) and killed Bordetella pertussis bacteria. The presence of the bacteria stimulates an improved response to the toxins (presumably by upregulating B7 expression by antigen presenting cells). It also leads to more inflammation and discomfort at the site of injection.

Current adjuvant research is trying to identify ways of improving the efficacy of vaccine adjuvants while at the same time reducing unwanted side effects.

B cells upregulate B7 upon antigen receptor crosslinking

Cross presentation for CD8 priming

•Problem: how can naïve CD8 cells get activated if activation requires peptide/MHC class I plus costimulation?

This would require that professional antigen presenting cells get infected with virus, for example.

•A second problem is that viruses that manage to suppress MHC class I would evade detection.

•Cross presentation of exogenous proteins on MHC class I can be specifically carried out by dendritic cells.

Figure 3-19

There is an important exception to the notion that class I MHC molecules present peptides generated within the cytoplasm.

What if the virus doesn’t infect macrophages or dendritic cells? How do viral peptides get presented by professional antigen presenting cells so that an immune response can be induced?

The answer is that dendritic cells, but not other cells, have a specialized mechanism for shuttling peptides taken up by endocytosis or phagocytosis into the MHC class I pathway.

Dendritic cell "cross presentation" of ingested antigens on MHC class I

CD36 and

v5

Take up apoptotic cells for processing within the dendritic cell

Cross presentation = Dendritic cell, but not other cells, can take up antigens and shuttle them into the class I pathway.

This is most efficient when the antigens are particulate

Provides a mechanism to get viral and bacterial antigens into profession antigen presenting cells.

This induces an immune response to the offending pathogens.

Trends in Immunology

22:141-148 (2001)

In some cases, CD8 T cells require T cell help for activation

Note that in almost all cases, two (or more) distinct cell types must be independently activated in order for a productive immune response to occur. In all cases, both innate and

antigen-specific stimulus is required.

CD4

CD8

CD8

B

1) Antigen specific2) Innate signals

1) Antigen specific2) Innate signals

1) Antigen specific2) Antigen specific

1) Antigen specific2) Antigen specific

1) Ag+MHC Class II2) B7 coexpressed by APC

1) Ag+MHC Class I2) B7 coexpressed by APC

1) Ag+MHC Class I2) CD4 effector Ag+MHC Class II

1) Ag:sIg crosslinking2) CD4 effector Ag+MHC Class II

Failsafe

Figure 6-18IL-2 is an important T cell cytokine

In the absence of appropriate costimulation, T cells can become inactivated (anergic).

Manipulation of costimulation for therapy

Tumor immunityTumors typically express MHC class I molecules, but fail to provoke an immune response because of CD4 or CD8 T cell tolerance. Some tumors do express unique antigens (such as mutated oncogenes) that could be used to direct killer T cells, but they fail to express costimulatory molecules.

Approaches•Force expression of B7 molecules in antigen tumor cells by gene therapy.•Immunize patients with their own tumors in the presence of powerful adjuvants.

B7

Concepts

• Naïve T cells encounter antigen in the draining lymph nodes and spleen, not at the site of infection.

• Antigen encounter involves transport of antigen, often by uptake of dendritic cells followed by their migration.

• Dendritic cells are key antigen presenting cells for naïve T cells. Other important professional antigen presenting cells are macrophages and B cells.

• Naïve T cells require peptide/MHC plus costimulation.• A major source of costimulation is B7 expressed by activated

professional antigen presenting cells.• B7 expression by dendritic cells and macrophages is often

stimulated by innate immune ligands.• Encounter of antigen in the absence of costimulation leads to

tolerance. And this is a major pathway of self-tolerance to peripheral antigens.

• Crosspresentation of exogenous antigens on MHC class I molecules is carried out by dendritic cells, but no other cells.

• Inducing costimulation is an important aspect of vaccine adjuvants.