ANTIGEN PRESENTATION T – CELL RECOGNITION T – CELL ACTIVATION T – CELL EFFECTOR FUNCTIONS

1. ANTIGEN PRESENTATION T – CELL RECOGNITION T – CELL ACTIVATION T – CELL EFFECTOR FUNCTIONS

2. CLP T CELLS B CELLS Common lymphoid precursor T B Th CTL PC Activate B cells and macrophages T HELPER CELLS Kill virus- infected cells CYTOTOXIC T LYMPHOCYTES Produce antibodies PLASMA CELLS Lymphocyte subsets

3. Plasma cell B-lymphocyte cytokines BCR + antigen Antibody production Cytotoxic T-limfocyte (Tc) Cell killing TCR + peptide + MHC-I Effector cell retains specific receptor Effector cells secrete cytokines cytokines Helper T-lymphocyte (Th) Macrophage activation Lymphocyte activation Inflammation TCR + peptide + MHC-II RECOGNITION EFFECTOR CELL

4. Tc Th Exogenous Ag Endogenous Ag Peptides of exogenous proteins (toxin, bacteria, allergen) bind to class II MHC molecules Peptides of endogenous proteins (virus, tumor) bind to class I MHC molecules RECOGNITION OF EXOGENOUS AND ENDOGENOUS ANTIGENES BY T-LYMPHOCYTES

5. The number of different T cell antigen receptors is estimated to be 1,000,000,000,000,000 (1015 - 17) How can 6 invariant molecules have the capacity to bind to 1,000,000,000,000,000 different peptides?

6. A flexible binding site? A binding site that is flexible enough to bind any peptide? At the cell surface, such a binding site would be unable to • allow a high enough binding affinity to form a trimolecular complex with the T cell antigen receptor • prevent exchange of the peptide with others in the extracellular milieu

7. Venus fly trap Floppy Compact A flexible binding site? A binding site that is flexible at an early, intracellular stage of maturation formed by folding the MHC molecules around the peptide. Allows a single type of MHC molecule to • bind many different peptides • bind peptides with high affinity • form stable complexes at the cell surface • Export only molecules that have captured a peptide to the cell surface

8. MHC molecules • Adopt a flexible “floppy” conformation until a peptide binds • Fold around the peptide to increase stability of the complex • The captured peptides contribute to the stabilization of the complex • Use a small number of anchor residues to tether the peptide • - this allows different sequences between anchors • and different lengths of peptides WHERE PEPTIDE BINDING OCCURS?

9. THE ENDOGENOUS ANTIGEN PROCESSING PATHWAY Tc-cell α-chain+β2m MHC+peptide SELF ANTIGEN PROTEIN MHC-I + self peptide MHC-I + Ag peptide cytoplasm FLEXIBLE CLOSED MHC-I, LMP2/7, TAP IFN induced coordinated expression α-chain TAP1/2 gp96 calnexin Proteasome LMP2/LMP7

10. Hydrophobic transmembrane domain Lumen of ER Lumen of ER Peptide Peptide Peptide Peptide Peptide Peptide Peptide Peptide Peptide Peptide Peptide ER membrane ER membrane TAP-1 TAP-1 TAP-1 TAP-1 TAP-1 TAP-1 TAP-1 TAP-1 TAP-1 TAP-1 TAP-1 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 Cytosol Cytosol ATP-binding cassette (ABC) domain Peptide antigens from proteasome Transporters associated with antigen processing (TAP1 & 2) Transporter has preference for longer than 8 amino acid peptides with hydrophobic C termini.

11. Peptide Peptide Peptide Peptide Peptide Peptide Peptide Peptide Peptide Peptide Peptide TAP-1 TAP-1 TAP-1 TAP-1 TAP-1 TAP-1 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 TAP-2 TAP-1 TAP-1 TAP-1 TAP-1 TAP-1 Endoplasmic reticulum Maturation and loading of MHC class I B2-M binds and stabilises floppy MHC Tapasin, calreticulin, TAP 1 & 2 form a complex with the floppy MHC Calnexin binds to nascent class I chain until 2-M binds Cytoplasmic peptides are loaded onto the MHC molecule and the structure becomes compact

12. THE EXOGENOUS ANTIGEN PROCESSING PATHWAY Th-cell MHC-II + Ag peptide MHC-II + self peptide CLIP DMA/B Ii+αβ FLEXIBLE CLOSED • INVARIANT CHAIN (Ii) • Chaperone – conformation • Inhibition of peptide binding • Transport/retention DMA/DMB 1. Support the peptide receptive conformation 2. Exchange of CLIP for exogenous peptides

14. GENERATION OF MHC – I EPITOPES GENERATION OF MHC – II EPITOPES HLA-DR1/HLA-DR4 Viral protein B35 A2 C42 B27 HLA-DQ2/HLA-DQ7 HLA-A,B,C binding Overlapping peptides The Tc response is focused to few epitopes ENSURE RECOGNITION OF ANY PATHOGENIC PROTEIN The Th response is directed to overlapping epitopes ENSURE RECOGNITION OF ALL PROTEINS

15. ++ ++ ++ + ± (?) + ± W W W W W W W W W W W W N N h e h e h h e t e t t t C H F Q U P O M S L E Z R K B G D T X V I A N C H F Q U P O M S L E Z R K B G D T X V I A N W W W W W Y W W W W W Y ++ ++ ++ ++ TARGETS OF EPSTEIN-BARR VIRUS-SPECIFIC Tc (CTL) RESPONSES LATENT ANTIGENS - EBNA3 EBNA4 EBNA6 LMP2 EBNA5 EBNA2 EBNA1 LMP1 LYTIC ANTIGENS BHRF1 BMLF1 BMRF1 BZLF1 BARF0 • A poliklonális CTL válasz elsősorban a litikus antigének és az • EBNA3,4,6 nukleáris fehérjék ellen irányul • Erősen fókuszált egy adott MHC - peptid kombinációra • Az endogén EBNA1 nem processzálódik és így nem ismerhető fel

17. REGULATION OF CLASS I AND II MHC MOLECULES IFNγ IFNγR Type II immune IFNγ increases MHC expression Inflammatory cytokines and IFNγ induces MHC class II expression in certain tissue cells (endothelial, astrocyte, microglia) Co-ordinated upregulation of MHC-I, TAP, LMP and MHC-II, DM, Ii