Antigen Processing and T-cell Ontogeny

Question 1

Target Cells

vs

APC’s

Answer 1

Professional antigen-presenting cells (APCs) display peptides on class II MHC molecules to CD4⁺ helper T-cells.

Target cells present peptides on class I MHC to CD8⁺ cytotoxic T-cells.

Question 2

Cytosolic Pathway

Characteristics

Answer 2

• Class I MHC presents endogenous antigens
• Peptides can be any protein which have been synthesized in the cytoplasm of the cell:
  
  • viral components
  • intracelluar bacteria
  • altered self proteins
  • normal self proteins
    
    • typically does not trigger an immune response
• Recognized by CD8⁺ cytotoxic T-cells
• Site of activity is the target cell

Question 3

Cytosolic Pathway

Mechanism

Answer 3

1. Endogenous Ag degraded within the cytoplasm by a proteasome.
2. Cytosolic peptides trasported by TAP (transporter associated with atigen processing) into the RER.
3. Class I MHC α-chains synthesized and associated with β₂-microglobulin through chaperone-mediated interactions in RER.
4. Class I MHC loaded with peptide and moves from RER to Golgi to plasma membrane.

Question 4

Endocytic Pathway

Characteristics

Answer 4

• Class II MHC binds peptides which have been phagocytized or endocytosed from the extracellular environment.
• Any protein which was synthesized elsewhere and subsequently taken in by APC could be expressed on MHC II.
• If protein seen as “non-self” it can be recognized by CD4⁺ helper T-cells.
• Once activated T_h cells will synthesize cytokines that regulate cellular & humoral immunity.
• Site of activity is the professional APC.

Question 5

Endocytic Pathway

Mechanism

Answer 5

1. Exogenous Ag taken up by phagocytosis, endocytosis, or pinocytosis depending on the APC.
   
   • Waits for MHC II molecules
2. Class II MHC α- and β-chains synthesized in the RER and associated with the invariant (Ii)-chain
   
   • guides MHC where it needs to go
   • blocks the binding grooves so peptides cannot be loaded
3. αβIi complexes transported through Golgi to endosomes and lysosomes
4. Invariant chain progressively degraded ultimately leaving only CLIP (class II-associated invariant chain peptide) in the peptide binding groove
5. CLIP removed and exogenous peptide from endosome or lysosome added
6. Peptide-loaded class II MHC transported to plasma membrane

Question 6

Antigen-processing Pathways

Summary

Answer 6

If a peptide is recognized as foreign on class I MHC by CD8 T-cells it signals that there is a problem within this cell ⇒ kill the cell.

If a peptide is recognized as foreign on class II MHC by CD4 T-cell it signals that there is a problem within the body ⇒ produce cytokines to help activate/modulate the immune system.

All antigens must eventually go on MHC II to activate a proper immune response.

Question 7

Vesicular Pathogens

Answer 7

• Some microbes have evolved to resist the microbicidal activites of phagocytes.
  
  • Ex. Tuberculosis, protozoa
• Can survive and replicate within the phagolysosomes.
• Since pathogens exist in the source compartment for class II MHC, peptides are located onto MHC II despite being produced within the cell.
  
  • Stimulates IFN-γ production by CD4 T cells.
• IFN-γ promotes macrophage activation and killing.
  
  • Stimulates granuloma formation if cannot kill.
• Some of these microbes may enter the cytoplasm of infected cells and divide there so loaded onto MHC I.
  
  • When cytotoxic cells try to kill them may just distribute pathogen upon lysis.

Question 8

Issues with MHC

Answer 8

MHC unable to discriminate between foreign and self antigens.

• Concentration of self-proteins will outnumber microbial or “altered self” antigens.
  
  • Virally infected cells make IFN α/β within hours of infection.
  • Stimulates MHC I production
  • High concentration of viral Ag allows Ag loading onto newly synthesized MHC I ⇒ Activates CD8 T-cells.
• MHC molecules are constantly displaying self-antigens.
  
  • TCR has induced tolerance which prevents autoimmunity.

Question 9

T-cell Development

Overview

Answer 9

1. Bone marrow progenitor cells (Pro-T cell expressing CD2) migrate to the thymus gland
2. Immature cells enter the cortex and mature as they travel towards the medulla.
3. Mature naive T cells leave thymus through peripheral blood circulation and travel to 2° lymphoid organs
4. Cells populate 2° lymphoid organs where they can encounter Ag and exert effector functions

Question 10

Thymus

Answer 10

• Primary lymphoid organ for T-cells
• Shrinks after puberty or corticoid exposure
• As thymocytes (developing T-cells) move from cortex to medulla they become more mature and express different surface markers
• Direct contact with:
  
  • non-lymphoid thymic stromal cells
    
    • soluble and membrane-bound factors
  • bone marrow derived interdigitating dentritic cells
    
    • aids thymic education (selection)
• Thymic microenvironment directs T cell development through direct cell-cell contact and cytokines
  
  • Which cytokine they encounter determines the type of T cell they become

Question 11

T-cell Differentiation

Answer 11

1. Pro-T cells (CD2⁺, CD7⁺) migrate from bone marrow to thymic cortex.
2. TCR rearrangement forms pre-T cells.
   
   • Express low levels of TCR and CD3.
   • No CD4 or CD8 yet = double negative cells (TCR^+low, CD3⁺, CD4^-, CD8^-)
3. Following TCR expression, both CD4 and CD8 are coexpressed to produce double positive cells (TCR^+low, CD3⁺, CD4⁺, CD8⁺)
   
   • Thymic education occurs
     
     1. Positive selection
        
        • Ensures that T cell can bind peptides on MHC molecules
        • Bindings results in a “to live” signal
        • Can only recognize Ag when bound to the same MHC encountered in the thymus = MHC restriction
     2. Negative selection
        
        • Removes cells that recognize self-peptides bound to MHC
        • Autoreactive cells get a “to die” signal
        • Failure results in potentially auto-reactive T cells
4. Cells which survive selection down regulate either CD4 or CD8 to become single positive cells.
5. These cells upregulate expression of αβ TCR molecules.
   
   • Become either mature Th cells or mature Tc cells.
6. Mature naïve T cells enters circulation and travel to 2° lymphoid organs or back to bone marrow.

Question 12

MHC Polymorphisms

Answer 12

• Allelic forms of MHC I & II expressed determines which peptides can be presented.
  
  • Also determines which T cells survive positive and negative selection.
• Suggests that there is a genetically predetermined capacity to react immunologically against a given immunogen.
• Plays a role in individual immune responses leading to resistance or susceptibility to infection & autoimmunity.
• Usually only a few epitopes from any antigen or pathogen can be presented effectively to a T cell.
  
  • Expression of multiple MHC genes increases likelihood of success.
• Polymorphism provides protection for species at large.

Question 13

Autoimmunity

Answer 13

MHC alleles have been linked to autoimmune disease.

No HLA allele associated with 100% of cases so probably only one of the factors involved.