The generation of T cell receptor ligands

1. Antigen Presentation to T cells The generation of T cell receptor ligands MHC complex and its functions

2. Antigen Processing Antigen Presentation Peptide binding completes the folding and assembly of newly synthesized MHC class I molecules. In the ER, Class II molecules are prevented from binding peptides. They are escorted to endosomal compartment where they are charged with vesicular peptides.

3. Two major intracellular Compartments- Cytosol Vesicular system- ER, golgi, Endosome, lysosome etc.

4. Infectious agents • A. Certain bacteria and viruses replicate inside the cytosol or nucleus. Viruses and plasmodium in RBC. CD8+ T cells • B Some in endosomes and lysosomes • CD4+ T cells • TH1 • TH2 • Class I • Class II

5. Listeria, some protozoa Mycobacterium Salmonella, Yersinia

6. MHC molecules are synthesized in cytosol and then translocated into the lumen of the ER. How do cytosolic peptides reach Class I molecules? Class I molecules are unstable without the peptide.

7. There are transporter proteins in the ER which belong to the ABC family of proteins. Transporter Associated with Antigen Processing-1 Transporter associated with antigen processing-2 TAP1/TAP2 heterodimer. Mutations in TAP1 or TAP2 can result in loss of antigen presentation by Class I molecules. TAP1:TAP2 transport peptides-ATP hydrolysis. 8-10 amino acid peptides with hydrophobic or basic amino acid residues at the carboxy terminus.

9. Proteasome (multicatalytic protease complex) Cytosolic protein degradation 28 subunits 4 stacked rings of 7 subunits each with a hollow core. The protease active sites lined up in the core. As the protein goes through it is degraded. LMP2 and LMP7, two subunits are encoded within the MHC locus and are near TAP1 and TAP2 genes. Interferons induce their expression. MECL1, another subunit not present in MHC locus also induced by interferons. Change in specificity of proteasome. Generates more calss I binding peptides.

10. 6 proteolytic sites 3 in each of the center rings

11. MHC class I are held in ER till they bind peptide. Newly synthesized molecules bind to chaperone protein-Calnexin Once b2-microglobulin binds to MHC class I a chain, calnexin dissociates and replaced by calreticulin and TAP1/TAP2 proteins. ERp57 is another chaperone. Cytosolic peptides transported by TAP1/2 then bind to correctly folded MHC class I molecules. The complex is stable and transported to surface.

12. Virus evading mechanisms: HSV produces a protein that binds and inhibits TAP. Adeno- produce a protein that binds to class I and retains it in ER. CMV makes a molecule that accelerates retrograde translocation.

14. Peptides presented by MHC class II are generated in the acidified endocytic vesicles. Pathogens such as Leishmania and mycibacteria replicate in the intracellular vesicles in macrophages. Protens are degraded by acid proteases and the peptides bind to class II molecules.

16. The invariant chain and MHC class II Invariant (Ii) chain binds to the newly formed MHC class II molecules in the ER and prevents peptide loading. Literally blocks the peptide binding groove. As the endosome becomes more acidic, the acid protease cathepsin cleaves the Ii leaving behind CLIP. CLIP is then replaced by the peptide.

18. MHC class II compartment (MIIC) Specialized in transporting Class II molecules to the surface.

19. A specialized HLA molecule known as HLA DM helps to load the peptide to MHC class II molecule. Stabilized class II, removes clip, removes unstably bound peptides. HLA DO is a negative regulator of HLA DM. Thymic epithelium.

21. Summary Stable MHC/peptide complexes are necessary for effective antigen presentation to T cells. Class I: cytosolic peptides transported to ER and then bind to Class I molecules. Class II. The endosomal peptides bind to class II molecules.