Antigens & Antigen Receptors Lecture B PDF

Antigens & Antigen Receptors Lecture B Dr Kikki Bodman-Smith and Dr Natalie Riddell [email protected] or [email protected] AY03 or 12AY02 Theme 3 Discussion Board Lecture content Lecture A (Tuesday): Part 1: General Principals, Antigens and immunogens Part 2: The B cell receptor and A...

Antigens & Antigen Receptors Lecture B Dr Kikki Bodman-Smith and Dr Natalie Riddell [email protected] or [email protected] AY03 or 12AY02 Theme 3 Discussion Board Lecture content Lecture A (Tuesday): Part 1: General Principals, Antigens and immunogens Part 2: The B cell receptor and Antibodies Part 3: The T cell receptor Lecture B (Today): Part 1: MHC class I & II, Antigen processing and presentation Part 2: Genetics of antigen recognition Lecture B, Part 1 Structure of MHC class I and II Dendritic cells Antigen processing/presentation Learning profile items: Illustrate the structure of MHC class I Describe the process of MHC class I antigen presentation Illustrate the structure of MHC class II Describe the process of MHC class II antigen presentation Compare antigen presentation between MHC class I and class II List the different antigen presenting cells Explain the role of dendritic cells (DCs) in the capture and presentation of antigen Antigen receptors B cells T cells T cell receptor Paratopes (antigen binding sites) Epitopes Mechanisms of antigen presentation MHC class I MHC class II Antigen presenting molecules T cell receptor complex Refresher T cells bind linear arrays of approximately 9 amino acids via the TCR i.e. NOT native Ag; it is broken down to 1o structure (PROCESSED) and PRESENTED to T cell by molecules on surface of Ag presenting cells (e.g. macrophages & dendritic cells) Fig. 3-17 What do we mean by antigen processing and presentation? This is how complex antigens/proteins are broken down into peptides and loaded into molecules for ‘presentation’ to T cells Depends on type of antigen and type of cell: All cells capable of some presentation and antigen presenting cells (APCs) use 2 mechanisms through 2 classes of presenting molecules: The major histocompatibility complex (MHC) class I and II What do we mean by antigen processing and presentation? Endogenous antigens Exogenous antigens MHC class I MHC class II CD8 T cell (Cytotoxic T cell or Tc) CD4 T cell (Helper T cell or Th) The structure of MHC class I and II and features of the peptides they present Class I MHC Found on all nucleated cells & platelets Consists of a single polypeptide chain, bound to β2-microglobulin a1 and a2 domains consist of 4 b strands and an a helix These form a groove or cleft which is the Ag-binding site of the molecule Class I MHC peptide From side β2m From side Class II MHC On professional Antigen presenting cells : Dendritic cells Macrophages B cells Formed by two chains a and b a1 and b1 domains form peptide binding site similarly to Class I Class II MHC Class I vs II MHC 8-10 amino acid peptides Looking down from top towards cell surface Fig. 7-1 and 7-3 13-18 amino acid peptides Features of peptides binding to MHC molecules Features of peptides binding to MHC molecules Dendritic Cells Dendritic cells (DCs) Location: Skin GI and respiratory tracts Parenchyma The capture and display of microbial antigens The capture and presentation of protein antigens by DCs 1. Antigen is processed at the site of infection by dendritic cells (DCs). 2. The DCs become migratory; they detach and enter the lymphatic vessels (soluble antigen can also enter lymphatics) 3. These vessels empty into lymph nodes in the parcortex area bringing the DCs (and soluble antigens) 4. The DCs then become positioned on the fibroblastic reticular cell (FRC) network (soluble antigens can be processed by other APCs here) 5. Here they are scanned by naïve CD4 or CD8 T cells (discussed in detail in Theme 4) Fig. 14-10 Mechanisms of Antigen Presentation for MHC class I and II Overview of endogenous (MHC I) and exogenous (MHC II) MHC I – CD8 - Tc MHC II – CD4 - Th Fig. 2-8 Fig. 2-8 Fig. 7-12 Overview of endogenous (MHC I) and exogenous (MHC II) MHC I – CD8 - Tc In the endogenous pathway the peptide antigens come from within the cell Proteins are processed by the proteasome into small peptides and imported into the ER Here they are loaded into MHC class I The antigen:MHC I complexes are transported to the cell membrane Fig. 7-12 Overview of endogenous (MHC I) and exogenous (MHC II) MHC II – CD4 - Th In the exogenous pathway the peptide antigens come from outside the cell Antigens are engulfed into endocytic compartments and degraded into peptides The peptides are loaded into MHC class II in cytoplasmic vesicles The antigen:MHC II complexes are transported to the cell membrane Fig. 7-12 Class I presentation – antigen processing and transport Fig. 7-14 In the endogenous pathway the peptide antigens come are produced by processing of proteins by the proteasome Proteins are usually marked for degradation by a post-translational modification called ubiquitination The peptides are translocated into the ER lumen by heterodimeric TAP (transporter associated with antigen processing) Class I presentation – assembly and stabilisation Fig. 7-15 MHC class I α chain associates with the chaperone protein Calnexin and ERp57 Β2 macroglobulin binding displaces Calnexin. Calreticulin and Tapasin (TAP associated protein) also bind, creating a “peptide loading complex” Peptides (that may have been further processed by ERAP so that they are 8-10 amino acids in length) can then bind, stabilising the complex so that it can then be transported to the cell surface Class II presentation – generation and assembly Fig. 7-16 MHC II and invariant chain (Ii) made in ER, then transported through Golgi to a vacuole that fuses with late endosomes → MHC class II enriched compartment (MIIC) Antigen enters via receptor-mediated endocytosis, via early endosome until it forms the MIIC late endosome Ii degrades leaving CLIP, then DM facilitates the replacement of CLIP with peptide 13-18 amino acids in length Finally complex is transported to the cell surface Features of the pathways of antigen processing Peptide length 8-10aa 13-18aa What is the purpose of MHC class I vs. class II presentation? Class I – endogenous Virally infected cells/Tumour cells/Intracellular pathogens Need to kill the infected/abnormal cells Cytotoxic T cells Class II – exogenous External pathogens Need to be destroyed by antibodies, most of which are made with T cell help T helper cells Summary Major Histocompatibilty Complex Class I MHC formed of one chain (a1, a2, a3 domains) and interacts with cytotoxic/CD8+ T cells. Also has b2-microglobulin Class II MHC formed of two chains (a1, a2 domains on one b1, b2 on the other) and interacts with helper/CD4+ T cells Both MHC I and MHC II present peptide antigens that must be loaded inside cells Antigen presentation Antigen (peptide) presentation by MHC class I or II is an absolute requirement for αβ T cell activation Discrete mechanisms of presentation The method of presentation controls the activation of the right T cells to get the right immune response Professional APCs, like dendritic cells, play an important role here Lecture B, Part 2 Genetics of antigen recognition: Overview of the genetics of MHC B/T cell development & overview of the genetics of diversity Learning profile items: Explain the different factors that influence an antigen’s immunogenicity Describe the differences between T and B cell antigens An overview of the genetics of the MHC (HLA complex) The HLA complex The Human leukocyte antigens (HLA) complex is a single stretch found on Chromosome 6 in humans that contains genes for the MHC and other immune-related genes Fig. 7-7 HLA-A, HLA-B & HLA-C genes encode MHC CLASS I HLA-DP, HLA-DQ and HLA-DR encode MHC CLASS II (each has an a and b gene) Some cytokine genes are found here, as well as other non-classical MHC genes such as HLA-DM and HLA-G Key features of genes encoding MHC class I and II MHC class I and II genes are highly polymorphic, especially amongst amino acids in the peptide binding groove, but their close proximity means they usually are inherited as a haplotype Most humans are heterozygous for haplotype (ie have inherited completely different polymorphisms from their mother and father 15,000 alleles identified! They are also polygenic (more than one gene with the same function) The genes are expressed codominantly (all the genes are expressed at the same time) Fig. 7-9 Diversity of MHCs within an outbred population Because MHCs are polymorphic, polygenic and co-dominant, the diversity of MHC molecules found in any given person is huge It also means that even children express very different MHC to their parents PROS: provides resilience in a population to emerging pathogens CONS: makes tissue typing in organ transplantation essential to avoid rejection of the organ as “non-self” (especially for MHC class I, as it is found on all nucleated cells) B cell and T cell development and an overview of the genetics of diversity Refresher: BCR and TCR structure Both the BCR and TCR have protein chains that have variable regions, based on the Ig fold, at their amino terminus Diversity of Ag receptors Each B or T cell expresses ONE receptor that is specific for ONE antigen But the number of possible receptors is HUGE B cells -- BCR (Ig) -- ~5x1013 T cells – TCR -- ~1x1018 In fact the number of possible BCRs/Igs is higher because of a process call somatic hypermutation/ affinity maturation THE ME 4 Diversity of Ag receptors How do vaccines work? Immunoglobulin gene rearrangement The immunoglobulin genes contain segments known as VDJ segments – V = Variable Segments – D = Diversity Segments – J = Joining Segments Both the heavy and light chains have V and J segments. The heavy chain (shown to the right) also has D segments There are many copies of each of these segments, and by selecting one of each, diversity is achieved The TCR loci utilise a similar mechanism Video tutorial Loads of detail in Chapter 6 of Kuby 8th Edition on mechanisms, but not required learning for this module B cell development in the bone marrow Heavy chains rearranged Light chains rearranged B cells develop from progenitors in the bone marrow Each B cell’s expression of the BCR begins as the genes encoding the heavy and light chains are rearranged giving each cell a unique BCR to recognise an epitope The resulting cells which cannot make functional heavy or light chains die by apoptosis (positive selection), as do those which bind to self molecules whilst arranging their BCRs (negative selection) T cell development in the Thymus Fig. 8-1 DN = double negative ie not expressing CD4 or CD8 Thymocytes enter the thymus not expressing the TCR or coreceptors (CD4 or CD8). Each thymocyte’s expression of TCR begins after rearrangement of the DVJ segments genes encoding α and β chains giving each cell a unique TCR to recognise an epitope If this does not result in a functional TCR (non-productive rearrangement), the T cells die by apoptosis Summary The MHC is polymorphic, polygenic and co-dominantly expressed to present a wide diversity of antigen to T cells Both BCR and TCR have variable regions that are randomly generated to recognise a huge diversity of antigen The processing of presentation of antigens on antigen presenting cells is critical to driving strong adaptive immune response, as it is essential to the activation of T cells. Thank you for your attention Next Event: Hands on Small-group Tutorial