Antigen Presentation to T lymphocytes

Transcription

1 Antigen Presentation to T lymphocytes Immunology 441 Lectures 6 & 7 Chapter 6 October 10 & 12, 2016 Jessica Hamerman jhamerman@benaroyaresearch.org Office hours by arrangement

2 Antibodies and T cell receptors (TCR) use similar structures to bind antigens

3 Antibodies bind intact proteins T cell receptors bind peptide+mhc

4 Antigen processing: How are the peptides bound by MHC molecules generated? Antigen presentation: How do the peptides bind to MHC molecules and get delivered to the cell surface to be recognized by T cells? MHC genes and polymorphism: How does this ensure our T cells can respond to a wide variety of pathogens?

5 What is the goal of antigen processing and presentation? 1. To present peptides derived from all classes of pathogens to T cells for recognition by the T cell receptor. 2. To activate T cells with the appropriate effector function for clearance of the pathogen.

6 MHC molecules come in 2 flavors: MHC Class I MHC Class II expressed on most cells binds CD8 coreceptor expressed on professional antigen presenting cells binds CD4 coreceptor Fig

7 MHC molecules come in 2 flavors: MHC Class I MHC Class II

8 MHC molecules come in 2 flavors: MHC Class I MHC Class II Binds 8-10 aa peptides N and C-termini of peptide contribute to binding Closed binding groove Binds 13-? aa peptides N and C-termini of peptide do not contribute to binding Open binding groove The peptide is an integral part of MHC structure, thus MHC molecules are unstable in the absence of bound peptide

9 MHC molecules activate T cells with effector function appropriate to the type of infection MHC Class I + peptide activates CD8 Cytotoxic T cells MHC Class II + peptide activates CD4 Helper T cells Causes killing of cell Activates macrophage to kill intracellular pathogen Induces B cell class switching and somatic hypermutation

10 Pathogens can be found in different intracellular compartments vesicular system intracellular bacteria extracellular bacteria extracellular proteins (helminth) Cytoplasm viruses some bacteria

11 MHC molecules sample different intracellular compartments

12 MHC Class I processing and presentation MHC Class I binds peptides derived from cytoplasmic proteins but the peptide binding groove of MHC Class I never faces the cytoplasm

13 MHC Class I processing and presentation TAP=Transporters associated with Antigen Processing The TAP proteins form a peptide transporter in the ER membrane that translocates peptides from the cytoplasm to the ER in an ATP-dependent manner TAP1 and TAP2 genes are found in the MHC TAP1/2 expression is induced by interferons, which signal the presence of viral infection

14 MHC Class I processing and presentation Discovered by the analysis of mutant cells with low levels of MHC Class I on their surface Normal MHC Class I levels could be achieved by the addition of peptide to the medium in which the cells were growing, suggesting a defect in the delivery of peptides to the MHC at the surface Genetic analysis identified TAP1/2 genes as mutated in these cells These cells also showed that MHC Class I molecules are unstable in the absence of peptide

15 MHC Class I processing and presentation Where do the peptides come from that get transported by the TAP transporter?

16 MHC Class I processing and presentation The proteasome is a cytoplasmic multiprotein complex that degrades proteins to peptides in the cytoplasm both for normal protein turnover and for MHC Class I binding Ubiquitinated proteins are substrates for the proteasome The proteasome degrades properly folded proteins in the normal process of protein turnover and degrades misfolded proteins or mistranslated proteins

17 MHC Class I processing and presentation The expression of some subunits of the proteasome are induced by interferons the immunoproteasome is modified for efficient peptide generation: Cleaves proteins into peptides that are more likely to bind to MHC Class I and be transported by TAP Increases the rate of protein flow through the proteasome to ensure proteins aren t over-digested into too short peptides Peptides produced by the proteasome are protected by chaperones from further proteolysis before transport by TAP into the ER

18 MHC Class I processing and presentation MHC Class I molecules are unstable until complexed with β2m and peptide and therefore require chaperones for stabilization in the ER. These chaperones also hold MHC Class I molecules in an open conformation until the binding of high affinity peptides delivered by TAP.

19 MHC Class I processing and presentation The importance of MHC Class I molecules in presenting viral antigens is underscored by large numbers of viral-encoded immunoevasins that block steps in the MHC Class I processing and presentation pathway HSV-1 HCMV Adenovirus HCMV

20 MHC Class I processing and presentation Immunoevasins (CMV) (Adenovirus) (HSV) (γ herpes virus)

21 MHC Class II processing and presentation MHC Class II binds peptides derived from proteins in the vesicular system Where do MHC Class II molecules meet peptides generated in the endosyomal/ lysosomal network?

22 MHC Class II processing and presentation MHC class II molecules travel to the endosomal network to meet internalized proteins Proteins are degraded by acid proteases called cathepsins in the acidic endosomal system. bacteria

23 MHC Class II processing and presentation How do MHC Class II molecules get from the ER to the endosomal network? Why don t they bind peptides in the ER? bacteria

24 MHC Class II processing and presentation How do MHC Class II molecules get from the ER to the endosomal network? Why don t they bind peptides in the ER? The Invariant Chain (Ii): Associates with MHC class II in the ER (3 Ii: 3 MHC) Blocks the peptide binding groove Has a sequence in its cytoplasmic tail that targets the complex to the endosomal system

25 MHC Class II processing and presentation In the acidic endosome, cathepsins sequentially digest Ii leaving a small peptide called CLIP (class II-associated invariant chain peptide) blocking the peptide binding groove CLIP must be removed for antigenic peptides to bind CLIP CLIP

26 MHC Class II processing and presentation CLIP

27 MHC Class II processing and presentation HLA-DM is an MHC Class II-like protein encoded in the MHC HLA-DM is only found in the peptide loading compartment with MHC Class II Cells lacking HLA-DM have MHC Class II with CLIP bound at their cell surface HLA-DM catalyzes the removal of CLIP and the binding of antigenic peptides, though it does not bind peptides itself CLIP HLA-DM also removes weakly binding peptides from MHC class II to allow for higher affinity peptides to bind in a process called peptide editing

28 Genetic deficiencies in MHC processing: Bare Lymphocyte Syndromes Family with 2 of 5 siblings chronically ill with viral respiratory infections Normal vaccine responses Normal blood antibody titers Very few CD8 T cells No MHC class I on the surface of cells Mutations in TAP1 or 2 ER Cell number cytoplasm MHC Class I (fluorescence) Deficiencies in MHC Class II are caused by mutations in transcription factors required for the expression of MHC Class II molecules

29 An important exception cross-presentation: This is an exception to the rule that MHC class I presents peptides generated in the cytoplasm Dendritic cells, but not other cells, have a specialized mechanism for shuttling peptides taken up by endocytosis or phagocytosis into the MHC class I pathway.

30 An important exception cross-presentation: Most cells are not professional APC and will not be present in LN where naïve T cells are found (and are not good at activating these naïve T cells anyway)

31 An important exception cross-presentation: Even if a professional APC, such as a DC, is infected, many viruses shut down MHC Class I antigen presentation pathway

32 An important exception cross-presentation: If DC phagocytose dying infected cells (or bits of infected cells), they have a specialized machinery to bring peptides from these endocytic compartments out of the endosomal system and into the MHC I presentation pathway.

33 What is the goal of antigen processing and presentation? 1. To present peptides derived from all classes of pathogens to T cells for recognition by the T cell receptor. 2. To activate T cells with the appropriate effector function for clearance of the pathogen.

34 MHC molecules sample different intracellular compartments and activate different types of T cells: