Chapter 6. Antigen Presentation to T lymphocytes

Transcription

1 Chapter 6 Antigen Presentation to T lymphocytes

2 Generation of T-cell Receptor Ligands T cells only recognize Ags displayed on cell surfaces These Ags may be derived from pathogens that replicate within cells, or from pathogens or their products that cells internalize by endocytosis T cells can detect the presence of intracellular pathogens because infected cells display on their surface peptide fragments derived from pathogen s proteins These foreign peptides are delivered to cell surface by host MHC molecules

3 Generation of T-cell Receptor Ligands Antigen processing: generation of peptides from an intact Ag involves modification of native protein Antigen presentation: display of peptide at cell surface by MHC molecule

4 Generation of T-cell Receptor Ligands MHC class I and II molecules deliver peptides to cell surface from two distinct intracellular compartments Viruses and certain bacteria replicate in cytosol, whereas many pathogenic bacteria and parasites replicate in endosomes and lysosomes Immune system has different strategies for eliminating pathogens from these two sites Cells infected with viruses or with bacteria that live in cytosol are eliminated by cytotoxic (CD8) T cells

5 Generation of T-cell Receptor Ligands Pathogens and their products in vesicular compartments of cells are detected by CD4 T cells 2 functional classes of CD4 T cells - Th1 cells - activate macrophages to kill intravesicular pathogens - Th2 cells activate B cells to make Ab Microbial Ags may enter vesicular compartment in either of 2 ways Some bacteria including mycobacteria invade macrophages and flourish in intracellular vesicles Other bacteria and their products can be internalized by phagocytosis, endocytosis into intracellular vesicles of cells that then present Ag to T cells

6 Generation of T-cell Receptor Ligands MHC class I and II molecules deliver peptides to cell surface from two distinct intracellular compartments MHC class I molecules deliver peptides originating in cytosol to cell surface, where they are recognized by CD8 T cells MHC class II molecules deliver peptides originating in vesicular system to cell surface, where they are recognized by CD4 T cells

7 MHC class I & class II molecules acquire peptides generated in 2 distinct intracellular compartments: cytosol & endolysosomal system

8 MHC molecules must present peptides from pathogens that replicate in different sites

9 Peptides that bind to MHC class I molecules are actively transported from cytosol to endoplasmic reticulum (ER) Polypeptide chains of proteins destined for cell surface, which include MHC molecules are translocated during synthesis into lumen of ER where the chains must fold correctly and assemble with each other, before complete protein can be transported to cell surface Thus, the peptide-binding site of MHC class I molecule is formed in lumen of ER When bound peptide is absent in MHC molecules, they are unstable and thus MHC class I proteins are present at abnormally low levels on cell surface

10 Generation of T-cell receptor ligands Peptides that bind to MHC class I molecules are actively transported from cytosol to ER Transporters associated with antigen processing-1/2 (TAP1 and TAP2) mediate ATP-dependent transport of ions, sugars, amino acids and peptides across membranes in many types of cells Two types of proteins form a heterodimer Tap complex has some specificity for peptides it will transport It prefers peptides of between 8 and 16 aa with hydrophobic or basic residues at C-terminus

11 MHC class I presents peptides that are generated in the cytosol and that are transported from the cytosol to the endoplasmic reticulum (ER) by a peptide transporter (TAP) that resides in the ER membrane

12 Generation of T-cell receptor ligands Peptides for transport into ER are generated in cytosol Much cytosolic proteins are degraded by proteasome Proteasome can exist in two forms: constitutive proteasome and immunoproteasome found in cells stimulated with interferons When cells are stimulated by IFN, constitutive components are replaced by immunoproteasome That may change specificity of proteasome such that there is increased cleavage of polypeptides after hydrophobic residues, and reduced cleavage after acidic residues

13 Generation of T-cell receptor ligands Peptides for transport into ER are generated in cytosol This produces peptides with C-terminal residues that preferred anchor residues for binding to most MHC class I and are also preferred structures for transport by TAP PA28 proteasome-activator molecule can increase rate at which peptides can be released from proteasome by opening up ends of cylinder This increased efflux of peptides that might be cleaved from proteasome will allow potentially antigenic peptides that might be cleaved further by proteasome to escape this additional processing

14 The structure of the proteasome, a large, cytosolic, catalytic protease complex Horizontal cross-section Longitudinal section

15 The PA28 proteasome binds to either end of the proteasome

16 Retrograde transport from the ER to the cytosol enables exogenous proteins to be processed for cross-presentation by MHC class I molecules It seems that ER proteins can be returned to cytosol by same translocation system retrograde translocation This may be normal mechanism by which proteins in ER are turned over, and by which misfolded proteins in ER are removed and degraded The presentation of exogenous Ags such as proteins from virus-infected cells or from a tissue transplant by MHC class I molecules to CD8 T cells is called crosspresentation

17 Generation of T-cell receptor ligands Retrograde transport from the ER to the cytosol enables exogenous proteins to be processed for cross-presentation by MHC class I molecules Cross-presentation occurs particularly well in a subset of DCs that express CD8 on their surface This pathway is important in activating naïve CD8 T cells against viruses that do not infect APCs such as DCs

18 Generation of T-cell receptor ligands Newly synthesized MHC class I molecules are retained in ER until they bind peptide Folding and assembly of complete MHC class I molecule depends on association of MHC class I -chains first with 2-microglobulin and then with peptide and this process involves a number of accessory proteins Only after peptide has bound is MHC class I molecule released from ER and allowed to reach cell surface Accessory proteins - Calnexin - MHC class I loading complex: calreticulin, tapasin, Erp57

19 MHC class I molecules must bind peptide in order to leave the ER

20 Generation of T-cell receptor ligands Newly synthesized MHC class I molecules are retained in ER until they bind peptide In normal cells, MHC class I molecules are retained in ER for some time This is very important for function of MHC class I molecules because they must be immediately available to transport viral peptides to cell surface if cell becomes infected

21 Generation of T-cell receptor ligands Many viruses produce immunoevasins that interfere with antigen presentation by MHC class I molecules Some viruses have evolved means of evading recognition by preventing appearance of peptide:mhc class I complex at cell surface - immunoevasins

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24 Generation of T-cell receptor ligands Peptides presented by MHC class II molecules are generated in acidified endocytic vesicles Leishmania and mycobacteria replicate inside intracellular vesicles in macrophages After activation of macrophage, proteins in vesicles are degraded by proteases within vesicles into peptide fragments that bind to MHC class II molecules Extracellular pathogens and proteins that are internalized into endocytic vesicles are also processed Proteins that enter cells by endocytosis are delivered to endosomes, which become increasingly acidic and then acidic protease is activated, which cleave proteins into peptides

25 The peptides that are bound by MHC class II molecules are generated by proteases resident in acidic endocytic compartments

26 Generation of T-cell receptor ligands Invariant chain directs newly synthesized MHC class II molecules to acidified intracellular vesicles MHC class II molecules must be prevented from binding prematurely to peptides transported into ER Binding is prevented by assembly of newly synthesized MHC class II molecules with MHC class II-associated invariant chain Invariant chain can target delivery of MHC class II molecules to low-ph endosomal compartment where invariant chain is cleaved by acid proteases leaving class IIassociated invariant chain peptide (CLIP)

27 The invariant chain delivers newly synthesized MHC class II molecules to acidic intracellular compartments

28 Electron microscopy studies suggest that MIIC is a specialized endolysosomal compartment in which MHC class II molecules acquire antigenic peptides MHC II small gold particle Invariant chain large gold

29 Generation of T-cell receptor ligands A specialized MHC class II-like molecule catalyzes loading of MHC class II molecules with peptides MHC class II-like molecule called HLA-DM is not expressed at cell surface but is found predominantly in MIIC where it catalyze release of CLIP fragment from MHC class II:CLIP complexes and binding of other peptides to empty MHC class II molecule HLA-DM also catalyze release of unstably bound peptides from MHC molecules and stabilize empty MHC class II molecules

30 Invariant chain blocks the MHC class II peptide binding site in the ER

31 Generation of T-cell receptor ligands A specialized MHC class II-like molecule catalyzes loading of MHC class II molecules with peptides A second atypical MHC class II called HLA-DO is produced in thymic epithelial cells and B cells HLA-DO acts as a negative regulator of HLA-DM, binding to it and inhibiting both HLA-DM-catalyzed release of CLIP from and binding of other peptide to, MHC class II molecules

32 Generation of T-cell receptor ligands Stable binding of peptides by MHC molecules provides effective Ag presentation at cell surface For MHC molecules to perform their essential function of signaling intracellular infection, peptide:mhc complex must be stable at cell surface If complex were to dissociate too readily, pathogen in infected cell could escape detection In addition, MHC molecules on uninfected cells could pick up peptides released by MHC molecules on infected cells, falsely signal to cytotoxic T cells that a healthy cell is infected, triggering its unwarranted destruction