Antigen Presentation and the MHC

Question 1

Define antigen presentation.

Answer 1

The process of displaying antigen by MHC molecules is called antigen presentation.

Question 2

When we refer to antigen presenting cells, what are we specifically talking about?

Answer 2

Specialized cells displaying antigen and class II MHC molecules are referred to as antigen
presenting cells (APCs), even though all nucleated cells express MHC class I molecules and
can present antigen via these molecules.

Question 3

Which are the major professional antigen presenting cells (APCs)?

Describe their action. How can they take up an antigen?

Answer 3

APCs take up antigen, either by surface receptors or by phagocytosis and then present it to immunologically competent lymphocytes. MHC class II expression is to a large extent confined
to APCs, which are:
- Mononuclear Phagocytes (macrophages)
- Dendritic Cells
- B Lymphocytes.

Question 4

For the following cell types, provide the location, whether or not it is phagocytic, and whether or not it is Class II.

Mononuclear phagocytes (macrophages)
dendritic cells
B lymphocytes

Answer 4

macrophages- blood, liver, spleen tissues, it is phagocytic and class II

dendritic cells- skin, lymphoid tissues, phagocytic, Class II

B lymphocytes- lymphoid tissues, sites of immune reactions, NOT phagocytic, Class II

Question 5

What are mononuclear phagocytes localized in tissues referred to as? Mononuclear phagocytes that line the sinusoids of the liver? Brain?

Answer 5

tissues-macrophages
sinusoids of liver- Kupffer cells
brain-microglial cells

Question 6

What are Langerhans’ cells? What do they form/where? What do they do?

Answer 6

Skin APCs are Langerhans’ cells, which form a continuous cellular sheet at the junction of
the dermis and epidermis. These cells are capable of migrating via the afferent lymphatics into the paracortex of the draining lymph node. Within the paracortex, they interdigitate with T lymphocytes, presenting antigen carried from the skin to immune responsive cells.

Question 7

Where are follicular dendritic cells found? What do they do?

Answer 7

Follicular dendritic cells are found in B lymphocyte areas of lymph nodes and spleen. These
cells present antigen to B lymphocytes.

Question 8

There are two distinct lineages of dendritic cells. Describe.

Answer 8

conventional
and plasmacytoid

Plasmacytoid dendritic cells produce large quantities of interferon in response to viral infections.

Question 9

Describe dendritic cell maturation in Figure 3.

Answer 9

Scanning electron micrographs are in the right panels and show
maturation from immature at the top to mature dendritic cells at the bottom. In the left panels, green fluorescence depicts MHC class
II molecules and red fluorescence depicts lysosomal protein. When the two colors occupy the same cellular locale they appear as yellow. The top panel depicts highly phagocytic immature
dendritic cells and the bottom panel depicts non-phagocytic
mature dendritic cells that present large quantities of peptide in the context of MHC class II molecules.

Question 10

Describe B lymphocytes. When is B lymphocyte antigen presentation most important?

Answer 10

B lymphocytes are also rich in surface class II molecules and have been shown to process
and present antigen, particularly when the B lymphocyte is immunologically specific for the antigen. 

B lymphocyte antigen presentation is most important during secondary antibody
responses.

Question 11

Name some cells that express Class II MHC molecules.

Answer 11

Macrophages, Langerhans’ cells, interdigitating dendritic cells, and B lymphocytes all
express Class II MHC

Question 12

Describe antigen processing (the steps).

What must happen to complex antigens before they can be recognized by T lymphocytes?

When does the actual association of the antigen take place for Class I vs Class II?

Answer 12

• Complex antigens (e.g. cells, proteins) are degraded or processed into small antigenic
  fragments that are recognizable by T lymphocytes.
• These fragments are peptides that associate with either MHC class I (see Figure 5) or class II molecules (see Figure 6).
• The actual association of the antigenic fragments takes place following cytoplasmic
  production (Class I) of the antigen or alternatively following phagocytosis or endocytosis (Class II) of the antigen.

Question 13

Describe MHC Class I antigen presenting pathway.

Answer 13

virus infects cell

viral proteins synthesized in cytosol

peptide fragments of viral proteins bound MHC class I in ER

Bound peptides transported by MHC Class I to cell surface

Figure 5 p 5 or slide 8.

Question 14

Describe MHC Class II antigen presenting pathway. (cytosol and B-cell)

Answer 14

antigen is taken up from the extracellular space into intracellular vesicles

in early endosomes of neutral pH, endosomal proteases are inactive

acidification of vesicles activates proteases to degrade antigen into peptide fragments

Vesicles containing peptides fuse with vesicles containing MHC Class II molecules

B cell:
antigen specific B cell binds antigen

specific antigen efficiently internalized by receptor-mediated endocytosis

high density of specific antigen fragments presented

Slide 9

Question 15

What are MHC class I molecules typically derived from?

How are these antigens degraded?

Answer 15

Antigenic peptides that bind to MHC class I molecules are typically derived from viruses that take over the biosynthetic machinery of the cell, resulting in the production of viral proteins (foreign antigens).

These viral proteins are degraded by the host cell’s proteasomes [long
cylindrical structures, comprised of subunits LMP2 and LMP7 that contain multicatalytic proteases] into small peptide fragments.

Question 16

How are peptides generated in the cytoplasm transported into the endoplasmic reticulum?

Answer 16

Peptides generated in the cytoplasm are transported into the endoplasmic reticulum by TAP-1
and TAP-2 (Transporters Associated with Antigen Processing-1, -2).

See figure 7 p 6, or slide 11

Question 17

Where do newly synthesized MHC class I molecules assemble?

What do they associate with? Where do they go next… Describe this process of peptide loading and transport of MHC Class I molecules to the cell surface.

Answer 17

Newly synthesized MHC lass I molecules assemble in the endoplasmic reticulum with the
help of several chaperones (calnexin, Erp57, calreticulin).

These MHC class I molecules
associate with TAP-1,-2 and when peptides are transported into the endplasmic reticulum
they are trimmed by ERAAP (endoplasmic reticulum aminopeptidase associated with antigen processing) and the peptides bind to the MHC molecule, the peptide-MHC complex leaves
the endoplasmic reticulum and is transported through the Golgi apparatus to the cell surface.
See Figure 8a and 8b.

Question 18

How are antigens captured by class II MHC degraded?

Answer 18

MHC class II associated peptides are derived from antigens captured and internalized by
specialized APCs. These antigens are degraded enzymatically, in endosomes and lysosomes,
into peptides that bind MHC class II molecules.

Figure 9

p 8 
Peptides that bind to MHC class II molecules are generated in acidified endocytic vesicles.

Question 19

Where are Class II MHC molecules synthesized? Describe the peptide loading and transport of MHC Class II molecules to the cell surface.

Answer 19

Class II MHC molecules are synthesized in the endoplasmic reticulum and are transported to
endosomes with an associated protein, invariant chain (Ii), which occupies the peptide binding cleft of the newly synthesized MHC class II molecule. Within the endosome, acidification cleaves Ii leaving a short peptide fragment, CLIP (class II-associated invariant
chain peptide), bound to the peptide binding groove of the MHC class II molecule. 

Once such endosomes fuse with a vesicle containing foreign antigen, CLIP is removed by a peptide
unloader/loader, DM, which then places foreign peptides in the groove of the MHC class II
molecule. (Degradation of Ii increases the mobility of these antigen presenting cells.) The
peptide MHC complex then transits to the cell surface See Figure 10.

p 8

Question 20

Describe what MHC Class I and II molecules contain under normal conditions (in the absence of foreign antigen).

Answer 20

MHC class I molecules under normal conditions (in the absence of foreign
antigen) are loaded with self peptides derived from the normal degradation of self cellular proteins. 

MHC class II molecules, under normal conditions, are thought to contain only CLIP in
their peptide binding groove.

Question 21

Describe MHC Class II restricted presentation of antigen to T-lymphocytes.

Answer 21

• CD4+T lymphocytes do not recognize free or soluble antigens.
• Rather, they recognize antigen on the surface of APCs in the context of class II molecules.
• The interaction between T lymphocytes and antigen and the class II molecule is highly
  specific and will result in specific T cell proliferation and differentiation.

Question 22

Describe MHC restriction.

Answer 22

These lymphocytes are said to be genetically restricted by the class II molecule on which the
antigenic determinants was first recognized. This is called MHC restriction.

Question 23

Describe the action of CD4+T lymphocytes.

How are the different subsets designated?

Answer 23

CD4+ T lymphocytes can either mediate macrophage activation or act as helper cells in antibody responses (by secreting cytokines).

See Figure 12. p 10

The subsets of CD4+ T lymphocytes that:

• Activate macrophages are designated Th1.
• Induce antibody synthesis are designated Th2.

Question 24

What do Th1 and Th2 do?

Answer 24

(These are subsets of C4+) 
Th1 cell recognizes complex of bacterial peptide with MHC class II and activates macrophage

Helper T cell recognizes complex of antigenic peptide with MHC class II and activates B cell

Complex of antigen fragment and class II MHC molecule forms the ligand for
the TCR (T cell receptor for antigen).

Question 25

Compare resting APC/immature APC to mature.

Answer 25

Resting (immature) APCs (e.g. dendritic cells) are highly phagocytic but do not present
antigen particularly well. During the innate immune response and after phagocytosis of an
antigen (e.g. a microorganism), dendritic cells mature and present antigen very well to T
lymphocytes.

(Microorganisms and their products are particularly good at inducing the maturation of dendritic cells.) In addition, mature dendritic cells express on their cell surfaces
large amounts of co-stimulatory molecules, e.g. B7 (also known as CD80 and CD86), and can also produce large quantities of cytokines required for T lymphocyte proliferation and differentiation.

Immature dendritic cells do not express large amounts of co-stimulatory
molecules or cytokines.

Question 26

Activation of a naive T cell by antigen requires two signals. Describe both.

What ensures that naive T lymphocytes are activated only at the correct time and place?

Answer 26

Activation of a naïve T cell by antigen requires two signals.

The first signal is the
presentation of peptides by MHC and the second signal is the interaction between B7 on the APC and CD28 on the membrane of the T cell.

These two signals lead to T cell activation (See Figure 13.)

The regulated expression of co-stimulatory molecules ensures that naive T
lymphocytes are activated only at the correct time and place.

Question 27

APCs that have taken up an antigen (which is not a microorganism) do not necessarily express B7. What happens if it doesn’t express B7?

Answer 27

T cells that recognize peptides expressed by MHC class II on the surface of the APC are stimulated to express CD40 ligand, CD40L (also known as CD154). CD40L engages CD40 on the surface of the APC and this signal induces the expression of B7 by the APC.

Question 28

What effect will CD28 ligation of B7 induce?

Answer 28

CD28 ligation of B7 induces T cell proliferation and differentiation.

See p 12 (Figure 14)

Question 29

Describe the cellular reactions required for production of antibodies.

Production of antibody to most antigens requires which cells?

What do B cells do? What is the result?

Answer 29

Production of antibody to most antigens requires not just B cells but also T cells.

B cells take up antigen, then B cells process antigen and display processed peptides on MHC class II molecules. This process activates the B cell to make B7. T cells recognize MHC presented antigen and B7 co-stimulates CD28 on the surface of the T cell to activate the naive T cell.

This activation induces the expression of CD40L. CD40L engages CD40 on the
surface of the B cell, activating the T cell to produce cytokines, allowing the B cell to
proliferate and differentiate into plasma cells that secrete antibody. See Figure 15.

Question 30

What mediates the activation of T lymphocytes?

Answer 30

The activation of T lymphocytes is mediated by;

the interaction of T cell antigen receptors (TCRs) with their ligands (major histocompatibility molecule-peptide complexes, MHCpeptide),
and by a specific co-stimulatory signal like CD28 and B7 or CD40L and CD40.

Question 31

What happens within seconds of MHC-peptide engagement?

Answer 31

Within seconds of MHC-peptide engagement, the TCR initiates a phosphorylation cascade that triggers multiple branching signaling pathways.

These early signals may be sufficient to trigger some effector functions, such as killer T cell execution of target cells.

Question 32

What does T cell proliferation require? How long might this take?

Answer 32

more complex functions, such as T cell proliferation, require TCR engagement
and signaling for many minutes or hours. The mechanisms of sustained TCR engagement is
the formation of a specialized contact, termed the immunological synapse.

Question 33

What defines the mature immunological synapse?

Answer 33

The mature immunological synapse is defined by a specific pattern of receptor segregation with a central cluster of TCRs surrounded by a ring of adhesion molecules (like LFA-1).

Question 34

What does the formation of the immunological synapse provide?

What comprises the immunological synapse?

Answer 34

The formation of the immunological synapse provides a mechanism for sustained TCR
engagement and signaling. An immunological synapse is shown in Figure 16 and is
comprised of a central cluster within concentric rings.

is on the T cell surface and I-CAM-1 is on the APC surface.

Question 35

What is on the intermediate ring, outer ring and inner circle?

Answer 35

The intermediate ring is enriched in adhesion molecules (like LFA-1-ICAM-1 complexes) that promote efficient TCR-MHC-peptide interaction leading to biological response. LFA-1

The inner circle contains TCR, CD4, and co-stimulatory molecules (like CD28). The
immunological synapse provides a higher-order molecular mechanism of junction formation,
MHC-peptide transport, and cluster stabilization.

p 14/15

Question 36

Co-stimulatory molecule expressed on the surface of CD4+ T lymphocytes:
A. CD4.
B. CD8.
C. CD28.
D. CD95.
E. Non-polymorphic regions of class I MHC molecules.

Answer 36

C