Antigen Presentation PDF

Antigen Presentation Adaptive Immunity Adaptive Immunity Ag Capture & Presentation to Lymphocytes Ag Capture & Presentation to Lymphocytes 1. Adaptive immune responses are activated with the binding of Ags to Ag receptors on lymphocytes. 2. B cell mediated humoral immune responses involves...

Antigen Presentation Adaptive Immunity Adaptive Immunity Ag Capture & Presentation to Lymphocytes Ag Capture & Presentation to Lymphocytes 1. Adaptive immune responses are activated with the binding of Ags to Ag receptors on lymphocytes. 2. B cell mediated humoral immune responses involves the recognition between membranebound antibody molecules and various Ags including proteins, polysaccharides, lipids and DNA/RNAs. 3. T cells recognize only protein Ags that are processed and presented to TcRs (TCRs). NB: T-cell Receptor or T Cell Receptor Two Important Questions in Adaptive Immune Responses 1. How do the rare B & T cells specific for a microbial Ag encounter the same microbe given that the microbe can enter anywhere? 2. How does the immune system produce the effector cells and molecules most effective for eliminating a particular type of infection? TcR & Ag Recognition TcR & Ag Recognition TcR & Ag Recognition Ags recognized by T cells 1. Ags must be bound to and displayed by MHC molecules of the antigen presenting cells (APCs). 2. Ags to be presented to T cells are anchored in the MHC molecules via “anchor residues”. 3. TcR recognizes both MHC residues (polymorphic) and some residues of the Ags. Ags recognized by T cells 4. MHC molecules are a class of proteins whose functions are to display peptide molecules in the immune system. 5. T cells can only “see” Ags presented by MHC molecules, hence the term “MHC restriction”. 6. Each T cell has a dual specificity for Ags and MHC. 7. Cells capturing and presenting Ags are APCs. Naïve T cells need to encounter Ags displayed by (e.g.) dendritic cells to start clonal expansion. Capture & Display of Microbial Ags 1. Protein Ags are captured by dendritic cells and concentrated in the peripheral lymphoid organs to initiate immune responses. 2. Microbes enter the host via (1) skin; (2) gastrointestinal tract; (3) respiratory tract. All are covered with a layer of epithelial cells. 3. Microbial Ags are captured either by dendritic cells lining the epithelium or by APCs in spleen if microbes travel in blood stream. Capturing & Displaying Microbial Ags Dendritic cells 1. Epithelia and sub-epithelial tissues have a net work of dendritic cells functioning as APCs. 2. The skin dendritic cells are referred to as Langerhans cells that are immature since they are not yet able to stimulate T cell activation. 3. Those skin dendritic cells have surface receptors capable of catching and internalizing microbial Ags. Dendritic cells They were first described by Paul Langerhans (thus the name) in 1869. Paul Langerhans 1847 - 1888 Dendritic cells The name dendritic cell was coined by Ralph M. Steinman and Zanvil A. Cohn in 1973. Ralph M. Steinman 1943 - 2011 Nobel 2011 For "his discovery of the dendritic cell and its role in adaptive immunity". Dendritic cells Messengers between the innate and adaptive immune systems. Dendritic Cells Two Types of Dendritic Cells The Immune System Nobel Prize 2018 James P. Allison, 70 Tasuku Honjo, 76 TcR PD-1, IL4 & 5 Ag capturing & display by dendritic cells 1. Microbes stimulate innate immune responses by binding to TLRs on dendritic cells, resulting in TNF and IL-1 production leading to dendritic cell activation. 2. Activated dendritic cells express CCR7 that functions as a chemoattracting receptor for moving to T cell zone in lymph nodes. 3. During this migration process, dendrictic cells become mature from Ag capturing cells to APCs capable of stimulating T cells. Ags presented by dendritic cells Major Ag Presenting Cells Dendritic cells in cytotoxic T cell activation (cross-presentation) 1. Professional Ag-presenting cells (e.g. dendritic cells) ingest infected cells, break down microbial Ags and present them in association with MHC molecules. 2. With the help of 2nd signal, T cells are activated. 3. Cross-presentation of Ags is for CD8+ T cells (cytotoxic)/class I MHC. Dendritic cells in cytotoxic T cell activation (cross-presentation) 4. Some microbes especially viruses get into the host cells fast and CD8+ CTLs will have to be able to recognize and respond to the internalized microbial Ags. How to achieve this? 5. One way is dendritic cells ingest the infected cells and display the microbial Ags for recognition by CD8+ CTLs (cross-presentation). 6. Dendritic cells express both MHC class I & II and thus are capable of activating both CD4+ & CD8+ T cells. Cross-presentation of microbial Ags 1. To activate naïve T cells to becoming effector T cells, APCs are needed. 2. Once Effector cells are produced, e.g. CD8+ CTL, the effectors T cells will be able to perform the killing of infected cells without the need of co-stimulator or signal #2. MHC molecules 1. MHC molecules are membrane proteins on APCs involved in presenting peptide Ags for recognition by T cells. 2. MHCs were discovered as genetic determinants for accepting or rejecting tissue grafts between individuals. But grafting is NOT a naturally occurring process. So what is the natural function of MHC? 3. Human MHCs are called human leukocyte antigens (HLAs). MHC molecules 3. The physiological function of MHC molecules is to present peptides derived from protein Ags to Ag specific T cells (hence the MHC restriction). 4. The cluster of genes making up the locus for MHC includes genes encoding MHC and other proteins. 5. Human MHCs are called human leukocyte antigens (HLAs) since they were found on leukocytes by Abs. 6. Typically, MHC locus has 2 sets of highly polymorphic genes, class I & II MHC genes. Genes of MHC locus Structure of MHC class I & II - Peptide-binding cleft - Invariant portions for binding to CD8 (aa3 in class I) & CD4 (bb2 in class II). MHC molecules 1. Class I & II MHC have a peptide-binding cleft at the amino terminus. 2. Class I & II share overall structural features. 3. The aa1 & aa2 domains form the peptide binding cleft (groove), sufficient for 8-11 aa. The floor of the cleft binds to the peptide to be displayed. The aa3 domain is invariant and serves as the binding site for CD8 (TcR coreceptor). Structure of MHC class I CD8 binding Domain - a 3 MHC molecules 1. Class II MHC has 2 chains, aa & bb. The aminoterminal region, aa11 & bb11 form the peptide binding cleft. 2. The bb22 domain has the nonpolymorphic binding site for CD4 co-receptor of the TcR complex. 3. Since there are 3 polymorphic class I genes and each person inherits one set from each parent, a cell can have 6 different class I molecules. Class II is more complex and each person can have many more than 6 different class II molecules. Structure of MHC class II CD4 binding Domain - b 2 Some Genetics Round vs Wrinkled Peas Alleles: R (round) & r (wrinkled) Three genotypes: RR, Rr, & rr Two phenotypes: Round (RR & Rr) Wrinkled (rr) Expression of MHC molecules 1. Class I molecules are widely expressed in almost all nucleated cells. 2. Class II MHC molecules are mainly found in dendritic cells, macrophages and B cells. 3. MHC genes are co-dominantly expressed, i.e. alleles from both parents are expressed equally. 4. As a result, MHC genes are extremely polymorphic, ~5k, an important feature to ensure that some individuals in the population will be able to present a particular Ag, given the diversity is inherited, not via gene recombination like Ag receptors. Properties of MHC Peptide-binding cleft of MHC molecules 1. There are pockets on the floor of the cleft. Side chains of the peptide fit the pocket and anchor the peptide to be displayed. 2. Each MHC can present one peptide at a time, while each MHC can present many different peptides. 3. MHC molecules bind only peptides, not other type of Ags. Binding of peptides to MHC Don Wiley 1944 - 2001 Albert Lasker, 1995 Basic Medical Research Award Features of peptide binding to MHC molecules 1. MHC molecules have broad specificity to accommodate the need of binding to different Ags, since fitting to MHC binding cleft needs only 1 or 2 residues. 2. MHC binds to proteins/peptides only. 3. Peptides to be presented are recruited during intracellular assembly. 4. MHC stability requires Ags binding. 5. MHC/Ag binding affinity is high. 1. Peptide binding to MHC MHC is incapable of distinguishing self from non-self. 2. A single T cell can see a peptide displayed by only 0.1% - 1% of the 105 MHC molecules on APCs. 3. To avoid autoimmunity, T cells reacting to self Ags will be eliminated or inactivated. Pathways of intracellular Ag processing 1. Extracellular proteins are internalized by APCs and processed in vesicles and displayed by Class II MHC. 2. Class I MHC molecules display Ags processed in the cytoplasm of any nucleated cells. 3. Such diverse Ag processing ensures different T cells recognize Ags from different compartments. Intracellular Ag processing TA P Pathways of Ag processing Class II – Newly formed Class II has a bound invariant chain Ii that has a CLIP sequence. A Class II like molecule DM functions to remove CLIP so that Ags can bind. Ag processing for Class II MHC 1. Protein Ags are taken up by APCs into vesicles and degraded into peptides. 2. Newly made Class II MHC molecules binds to CLIP (class II invariant chain peptide). 3. DM (MHC class II–like) helps to remove CLIP so that Ags gain access to Class II MHC. Class II MHC & Ag processing Ag processing for Class I MHC 1. Proteins enter host cells via phagocytosed microbes or synthesized by invading viruses. 2. Cytoplasmic proteins are unfolded, ubiquitinated and degraded in proteasomes. 3. Processed peptides are moved by transporter associated w/ Ag presentation (TAP). Class I MHC & Ag processing Ag cross-presentation Some cells lack the ability to present Ags to CD8+ CTLs. Certain DCs are capable of ingesting infected cells and cross-present the Ags. Same can occur to presenting to Class II MHC. Ag processing in separate compartments To mount the best immune responses to extracellular and intracellular microbes, the immune system chooses to separate class I and class II pathways. Ag Processing & Presentation h