Processing and Presentation of T-Cell

by Geoff Day

Review of T-cell Function

Recognize antigens displayed on cell surfaces

Antigens are displayed on cells that have:

1. Harboring pathogens

- viruses

- intracellular bacteria

2. Internalized pathogens within cell's vesicles

- endocytosis of pathogens or their products

• Effects of T-cell recognition

1. cell death
2. activation to kill intravesicular bacteria and parasites
3. activation of B-cells to secrete Ig to eliminate extracellular bacteria/toxins

Important Components of Antigen Presentation

• The Major Histocompatibility Complex (MHC)

- peptide-binding glycoproteins

- displays foreign peptides from antigen degradation on the cell surface

- 2 Major Classes

1. MHC class I molecules

- picks up foreign peptides degraded in the cytosol (Cytosolic Pathogens)

- presents antigen to CD8 T-cells

- causes cell death freeing host of infection

2.  MHC class II molecules

- picks up peptides degraded in Endocytic Vesicles

- presents antigen to CD4 T-cells

- results:

a. Activation to kill intravesicular bacteria and parasites

b. Activation of B-cells to secrete Ig to eliminate extracellular bacteria/toxins

•  CD8 T-cells

1. Cytotoxic T-cells
2. Kills cells infected with viruses
3. Distinguished by CD8 molecule on cell
4. Important defense against cytosolic pathogens

• CD4 T-cells

1. detect pathogens and/or their products in vesicular components
2. distinguished by CD4 molecule on cell
3. specialized to activate other cells

1. TH1 Cells

• activate macrophages to kill intravesicular pathogens
• "inflammatory T cells"
• produce cytokines (IFN-g , TNF)

2. TH2 Cells or Helper T-cells

• activate B-cells to make antibody

    4. 2 ways antigens enter vesicular components

• invade macroiphages ex. Mycobacterium of tuberculosis and leprosy
• internalize by endocytosis ex. Extracellular bacteria and their toxins

• To Sum It Up

1. MHC I and MHC II class molecules are distinguishing markers telling T-cells where the pathogens are derived
2. CD8 T-cells or CD4 T-cells respond accordingly
3.  

Structural Comparisons of MHC I and MHC II Molecules

• Similarities

1. cell surface glycoproteins
2. peptide-binding cleft
3. formed from two domains folded together (similar to Ig domains)
4. bound peptides are sandwiched into peptide-binding cleft
5. T-cell receptor interacts with both:
6. MHC molecule
7. Peptide fragment of molecule
8. bond peptide with high affinity, while retaining ability to bind to a wide variety of peptides

• Differences

MHC I                                                                                       MHC II

1. - only a -chain spans entire membrane                                    - a - and b - chains span entire membrane
2. - 4 domains (3 a , and 1 b 2- microglobulin)                            - 4 domains (2 a and 2 b )
3. - ends of peptide binding-cleft are more closed                       - ends of peptide cleft are more open (looser bound) (tighter bound)
4. - bind short amino acids (8-10) by both ends                      - length has no constraint as ends are free            (backbone is bound)

Characteristics of MHC I and MHC II Molecules

• Peptides are stable bound

1. high affinity
2. bind a wide variety

• MHC class I molecules bind short peptides

1. 8-10 amino acid peptides
2. bound at both ends by hydrogen bonds and ionic interactions
3. some variation in length is accommodated by kinking
4. Anchor residues
5. amino acid side chains give specificity to peptide binding
6. each MHC I binds different anchor residue patterns ( ex. Hydrophobic or basic)

• MHC class II molecules bind with no length constraint

1. 13 or greater amino acids
2. ends of peptides are not bound
3. binding occurs with interaction between peptide backbone and side chains of MHC II residues
4. longer chains are often trimmed by peptidases to 13-17 amino acids

• Expression of MHC molecules differs between tissues

1. MHC I are expressed on all nucleated cells
2. Non-nucleated cells (RBCs) express little or no MHCs
3. MHC II are normally found on B lymphocytes and Macrophages
4. Expression of both is regulated by cytokines

Antigen Processing and Presentation by MHC I Molecules

• Peptides of cytosolic proteins are made in the cytosol

Proteasome

1. plays a major role in cytosolic protein degradation
2. multicatalytic protease complex
3. proteins are introduced into core and broken down into smaller peptide fragments

Retrograde Translocation

1. misfolded proteins in ER are sent back into cytosol by same mechanism in which they came in
2. in cytosol, proteins are degraded and transported back into ER

• Peptides binding MHC Class I molecules are transported from Cytosol to ER

1. peptides have to enter ER membrane in order to bind to MHC molecules
2. MHC molecules are unstable in absence of bound peptides

• Transporters associated with Antigen Processing -1 and -2

1. Tap-1 or Tap-2
2. ATP binding cassette proteins involved in transporting short peptides from cytosol into lumen of ER
3. Requires ATP hydrolysis
4. Specificity

• Prefers peptides of 8 or more amino acids
• Hydrophobic or basic residues of carboxy terminus

• MHC Class I molecules do not leave ER unless they bind peptides

• MHC class I is not fully assembled until peptide bound (remains partially folded)

1. Calnexin, a chaperon-like protein, binds to MHC class I until b 2-Microglobulin binds
2. Calnexin is released and the new complex binds to two more chaperone proteins (calreticulin and tapasin)
3. Tapasin forms bridge with TAP transporter in which degraded peptides travel through                                        - degraded in proteasome in cytosol
4. Peptide fragment is delivered and binds to MHC I
5. MHC complex is released and transported through Golgi Complex to the cell surface

 

Antigen Processing and Presentation of MHC II Molecules

• Membrane enclosed vesicles prevent contact between proteasomes and pathogens
• Proteins are degraded by endosomal or lysosomal proteases inside activated macrophages

1. endosomes become acidic activating acid proteases
2. acid proteases degrade antigen into peptide fragments

• Invariant Chain (Li) delays peptide binding

1. (Li) is needed to prevent binding of intracellular peptides and partially folded proteins
2. blocks peptide bonding and facilitates export into appropriate endosomal compartment with low pH

• Invariant Chain is cleaved in two stages while outside ER

1. second stage presents the CLIP (Class II-associated Invariant-chain peptide)
2. once CLIP dissociates or is displaced, peptides may bond

• Once bound, the peptide:MHC class II complex moves to the cell surface

 

Special Case for MHC Class II-like Molecule (HLA-DM)

• Often through mutant cell lines MHC class II molecules move to cell surface with CLIP still bound
• HLA-DM

1. closely resembles MHC class II molecule
2. does not appear to require peptide for stabilization
3. not expressed at cell surface, but predominantly found in MHC II compartment
4. function:

• Stabilizes empty MHC class II molecules
• Catalyzes release of CLIP
• Catalyzes binding of peptide
• Removes and replaces unstable peptides (peptide editing) ® insures survival of MHC II

    5. role of HLA-DM parallels role of TAP molecules in MHC class I molecules

• both facilitate peptide bonding
• displays possible coevolution

 

Criteria for Effective Antigen Presentation

• the peptide:MHC complex must be stable at the cell surface

1. if dissociates too readily, may escape detection
2. prevents uninfected cells from picking up released antigen peptides
3. stability allows for long term display

• if dissociation of peptide:MHC complex should occur, peptides form surrounding extracellular fluid would not bind in empty groove

1. removal of peptides from purified MHC class I ® denaturation
2. dissociation of MHC class I of live cell ® conformation change

• B2-microglobulin dissociates
• a chain is internalized

            3. MHC class II aggregate and rapidly degrade with no bound peptide