Understanding MHC: Generation and Binding of Peptides for TCR Recognition

1. GENERATION OF LIGANDS FOR THE TCR March 24, 2009 10:00-11:00

2. MHC MOLECULES BIND PEPTIDES MHC molecules can bind many different peptides MHC class I and class II bind different types of peptide The shape of the peptide binding groove determines the type of peptide that can be bound

3. THE MHC CLASS I BINDING SITE IS A CLOSED GROOVE Closed groove = peptides are restricted in size

4. PEPTIDES BIND MHC CLASS I BY THEIR ENDS Peptides that bind Class I are 8-10 amino acids long

5. MHC CLASS II BINDING GROOVE IS OPEN Open groove = binding of peptides of variable length

6. PEPTIDES MAKE MULTIPLE CONTACTS WITH MHC CLASS II MHC class II can accommodate peptides between 13-25 amino acids long

7. AT THE DIAMOND: MHC Class I = Big bun, little hot dog MHC Class II = Little bun, big hot dog

8. THE SOURCE OF THE PEPTIDE DETERMINES IF IT ASSOCIATES WITH MHC CLASS I OR CLASS II

9. PATHOGENS END UP IN DIFFERENT CELLULAR COMPARTMENTS A cell can be divided into 2 compartments: cytosol vesicular system Pathogens that live inside a cell, e.g.,viruses and bacteria that replicate intracellularly, are in the cytosol Some pathogens (bacteria) or their toxins get taken up by cells and are found in vessicles.

10. Processing and Presentation of Cytosolic Proteins

11. MHC CLASS I MOLECULE Note: MHC Class I is a single chain associated on membrane with b2 microglobulin

12. ASSOCIATION OF PEPTIDES WITH CLASS I: GENERATION OF PEPTIDES The cytosol contains a large, multi- subunit protein complex called the PROTEASOME The proteasome degrades proteins into 8-10 amino acid peptides TAP, a.k.a, transporter associated with antigen processing, carries peptides into the ER

13. Figure 3-18 part 1 of 2 ASSOCIATION OF PEPTIDE WITH MHC CLASS I Association of MHC Class I with TAP Chaperones (calnexin or calreticulin) stabilize MHC class I Peptide ready complex: MHC class I, b2 microglobulin, tapasin, TAP

14. Figure 3-18 part 2 of 2 ASSOCIATION OF PEPTIDE WITH MHC CLASS I MHC Binds Peptide and Is Expressed MHC Class I must associate with a peptide to be expressed Most MHC Class I molecules contain self peptides

15. Processing and Presentation of Extracellular Proteins

16. MHC CLASS II MHC class II is an ab heterodimer Each chain has a peptide binding site associated with an Ig-like domain

17. ASSOCIATION OF PEPTIDES WITH CLASS II: GENERATION OF PEPTIDES Extracellular antigen is taken up by vesicles called endosomes The pH of the endosome drops Reduced pH activates proteinases that generate peptides

18. MHC CLASS II a AND b CHAINS DIMERIZE IN THE ER In the ER, MHC Class II molecules are associated with Ii Ii sits in the peptide binding groove MHC Class II + Ii leave the ER in a vesicle In the vesicle, Ii is cleaved. But the CLIP fragment remains in the groove

19. MHC CLASS II ASSOCIATES WITH PEPTIDE The vesicle containing MHC Class II fuses with the endosome Peptide can’t bind…CLIP is in the groove HLA-DM causes the release of CLIP Peptide gets in the groove and the complex gets expressed

20. CO-RECEPTOR ASSOCIATION WITH MHC MHC class I binds CD8 MHC class II binds CD4

21. TCR RECOGNIZES BOTH PEPTIDE AND MHC Both chains of the TCR contact both peptide and MHC TCR only binds peptide presented by the correct (self) MHC

22. CD8 cells: Kill Recognize Class I CD4 cells: Help Recognize Class II MHC PATTERNS OF EXPRESSION REFLECT FUNCTION Cells that might need to be killed express Class I Cells that can activate CD4s express Class II MHC Class I is on all nucleated cells MHC Class II is only on “professional” APC

23. A STRATEGY TO EFFECTIVELY FIGHT PATHOGENS Peptides associate with the class of MHC that will stimulate the subset of T cells that is best fitted to eliminate the pathogen

24. A WELL CO-ORDINATED ATTACK The promoters of genes involved in antigen processing and presentation are activated by a common factor (CIITA) During an inflammatory response interferon g is secreted Interferon g induces expression of CIITA

25. SUPERANTIGENS BYPASS THE NORMAL RECOGNITION MECHANISM Superantigens: Staphylococcal enteroxin Toxic shock syndrome toxin-1 Activate large numbers of T cells in an antigen non-specific leads to excess production of mediators Bridge MHC and TCR