Lecture 3a: Adaptive immunity Antigen recognition by B and T cell receptors

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1 Lecture 3a: Adaptive immunity Antigen recognition by B and T cell receptors HLS 4806 Lisa Kalischuk-Tymensen Summary: Innate immunity 6 7 5b8 9 Bacterium Pathogen Host Immune Response Innate Immunity Humoral omplement ellular Macrophage Neutrophil NK cells

2 Overview of immune response Pathogen Host Immune Response Innate Immunity Adaptive Immunity Humoral ellular Humoral ellular omplement Macrophage Neutrophil NK cells Antibodies (B cells) T cells (helper T cells, cytotoxic T cells) Adaptive Immunity: the 3 rd line of defense The main function of the adaptive immune system: to specifically recognize and attack pathogens (especially intracellular pathogens such as viruses)

3 Adaptive immune responses Immunity is acquired by contact with pathogen ( immunization ) Leads to activation of B and T cells (lymphocytes) Specific antigen recognition Memory (allows host to withstand repeated attack) Humoral effectors ellular effectors B and T lymphocytes Lectures 3 & 4a Antigen Recognition Lectures 4b & 5 Activation Lecture 6 Effector Activity

4 Specificity of adaptive immune responses Innate immune system large numbers of cellular and humoral effectors to defend against invaders that continuously attack Non-specific recognition: (PAMPs via TLRs, opsonized pathogens via complement receptors) In contrast: an important function of adaptive immune system is to specifically recognize and react to foreign antigen (such as those associated with pathogens) and eliminate them. Antigens Definition: Any substance (usually foreign) that binds specifically to an antibody or a T- cell receptor Determining which antigens that the adaptive immune system must respond to (i.e., make antibodies to) in order to provide protection is a major goal of vaccine design. Pathogens express MANY unique proteins, some of which are specifically recognized by antibodies produced by the adaptive immune system λ λ λ λ

5 Antigen recognition: specificity of B and T lymphocytes Each B cell or T cell has receptors that only recognize 1 antigen This means that we need >1 million different B cells or T cells to provide immunity!!! This means that only 1 out of 1 million B or T cells can recognize a specific pathogen antigen via specific receptors. Problem: cannot stock-pile B and T cells This means we do not have enough pathogen-specific B or T cells handy to launch a successful assault. lonal selection and expansion: adaptive immunity on demand Once a B cell recognizes its antigen the B cell is activated When activated by antigen, proliferate and differentiate into plasma cells and memory cells Plasma cells produce antibodies Memory cells are responsible for long term memory...quicker response next time the host is infected with the same pathogen Antigen recognition Proliferation & differentiation (clonal expansion) Figure 1-12 KUBY

6 lonal expansion of T cells Each T cell only recognizes 1 antigen. When a T cell is activated by its antigen, proliferate AND differentiate into effector T cells (3 main types): Helper T lymphocytes produce cytokines and chemokines that direct immune response ytotoxic T lymphocytes Kill cells infected with intracellular pathogens Regulatory T cells (lecture 11) Why do you get sick before you get better? Because it takes time to clonally expand B and T lymphocytes! Lag phase

7 Specific antigen recognition Specific recognition of antigen via receptors associated with B cells and T cells Each B and T cell has receptors that recognize and bind only ONE specific antigen Only ~ 1/1,000, 000 B or T cells recognizes a specific antigen Repertoire is large enough to ensure that most antigens are recognized BR: Membrane-bound TR Antibody: Secreted B cell receptors (BRs) B cell receptor has 2 forms BR membrane-bound Antibody secreted Antibodies often referred to as immunoglobulin (Ig) There are 5 classes of Ig s IgM, IgD, IgG, IgA, IgE Antibodies have several functions including: Opsonization: antibody binds specifically to pathogens and promotes phagocytosis omplement fixation: antibody activates the complement cascade (which pathway?) to destroy antibody-bound pathogens MORE FUNTIONS IN LETURE 6b! Promote phagocytosis omplement fixation

8 Antibodies overall structure 2 light-chains 2 types: Κ orλ V V 2 heavy-chains V D D V Also see Figure 4-6 KUBY Variable region: Antigen binding (Fab) onstant region of the heavy chain (Fc) : Effector activity i.e., opsonization, complement activation Each class (isotype) has a different Fc IgM, IgD, IgG, IgA, IgE Therefore each class has a slightly different function Membrane-bound BR: 2 parts region of the BR remains inserted into plasma membrane of B cell terminus has a very short cytoplamic tail NOT capable of transducing an activation signal...requires 2 accessory proteins Transmits signal that causes B-cell activation leading to clonal expansion and antibody production Membrane-bound Ig: recognition Accessory proteins: signaling BR: 2 parts Figure 4-22 KUBY

9 BR: antigen recognition Recognize MANY types of organic molecules (proteins, polysaccharide, lipid, nucleic acids, soluble antigens, toxins) Do not recognize entire antigen molecule, only recognize discrete sites (epitopes or antigenic determinants) 4-12 residues that are brought together by protein folding Native antigens (often recognize conformational structure rather than linear structure) Important for vaccine development F e Antigen recognition: a problem of diversity Each B cell receptor (antibody) binds ONE specific antigen Require >1 million different antibodies for protection!!! V V D V D V Antibody Problem: the human genome contains only ~ genes BR B cell

10 V V D Genetic basis for receptor generation V D V Generation of a vast array of BRs is accomplished by recombination of various gene segments encoded in the germline V 1 V 2 V n 1 2 n Antibodies (secreted B cell receptors) are made in a modular design V 2 1 Series of cut and paste events Gene organization: Κ light-chain (V segments) ~41(Κ) and ~34 (λ) ~5(Κ) and ~5(λ) V 1 V 2 V n 1 2 n Gene organization: heavy chain (VD segments) ~ 48 ~ 23 ~ 6 L V 1 L V 2 L D1 D2 D3 V n Dn n P P µ δ γ3 γ1 γ2 γ4 ε α1 α2

11 Gene recombination is required to generate a light-chain protein Figure 5-4 KUBY Mechanism of variable-region DNA rearrangements Recombination signal sequences (RSS) direct where recombination occurs. During DNA rearrangement, gene segments adjacent to oneturn RSS can only join with segments adjacent to twoturn RSS. This prevents 2 V or 2 segments from joining one another. Figure 5-6 KUBY Heptamer AAGTG 23 bp GTGTA 23 bp Two turn RSS Nonamer AAAAAA TGTTTTTGG Lambda light chains Nonamer GGTTTTTGT AAAAAA Heptamer ATGTG GTGAA L V L V L V 2 Heavy chains 1 D Kappa light chains 1 12 bp 12 bp One turn RSS 2

12 Gene segments are re-joined by V(D) recombinase V V V RAG 1/2 Recombination Activating Genes (RAG-1 and RAG-2) V Figure 5-7 KUBY ombinatorial V(D) joining of various germ-encoded segments contributes to BR/antibody diversity Let s do the math! ~41(Κ) and ~34 (λ) ~5(Κ) and ~5(λ) V 1 V 2 V n 1 2 n Table 5-2 KUBY

13 unctional flexibility contributes to BR/antibody diversity Recombination of V(D) segments is often imprecise (i.e., flexible ). This flexibility may result in either a productive or nonproductive rearrangement of the V(D) segments. Figure 5-9 KUBY Introduction of P- and N-nucleotides at the joins between V(D) gene segments during rearrangement contributes to BR/antibody diversity Figure 5-13 KUBY P = produces Palindromic sequences (indicated by brackets) Addition of N-nucleotides (red)

14 Somatic hypermutation also contributes to BR/antibody diversity The DNA coding for the variable region (Fab) of the BR or antibody (i.e., V(D) genes), has an mutation rate of > H the normal rate (= hypermutation) Resulting mutated BR s that have the highest affinity (i.e., increased ability to bind antigen) are more likely to be undergo clonal proliferation and survive to produce more antibodies. The rest of the B cells that are of lower affinity will not proliferate and will die due to neglect (via apoptosis). The result is a collection of B cells whose receptors have a higher average affinity for their cognate antigen. This process is called affinity maturation. At least 5 processes contribute to BR/antibody diversity 1. ombinatorial V(D) joining of various germ-encoded DNA segments 2. ombinations of different heavy and light chains 3. unctional flexibility 4. P- and N-nucleotide addition 5. Somatic hypermutation

15 Allelic exclusion The Ig heavy-chain and light-chain genes of ONLY ONE parental chromosome are expresses per cell. This ensures that B cells possess a single antigenic specificity. Figure 5-10 KUBY Story of the successful BR. Synthesize the heavy-chain first... Progenitor B cell Apoptosis 2nd 1st Heavy chain gene + VD recombination Productive rearrangement? No - Yes Transcribe/translate µ(δ) Heavy chain Maternal µ or µ Paternal Pre B cell Note: heavy chain = µ (IgM) Allele#1: µ Allele#2: µ Allelic exclusion : Each B cell only expresses the heavychain from one chromosome

16 ...then synthesize the light chain Maternal Pre B cell µ or µ Paternal µ Apoptosis 4 th + No Light chain gene V recombination Productive rearrangement? - Yes Transcribe/translate Light chain µ+ λ Mature B cell Allele#1: Κ Allele#2: Κ Allele#1: λ Allele#2: λ Allelic exclusion : Each B cell only expresses the lightchain from one chromosome lass switching of antibodies P lass switching: Each class of antibody (IgM, IgD, IgG, IgE, IgA) has a slightly different effector function (opsonization, complement fixation, neutralization, AD). Antibodies can switch class from IgM (always the 1 st antibody made) to another class by changing their constant region (Fc) of the heavy-chain. This makes it possible to tailor the adaptive immune response with a specialized antibody for each different kind of pathogen. The class of antibody produced is controlled by the cytokines that are present when class switching takes place. ertain cytokines or combinations of cytokines (usually produced by helper T cells) influence B cells to switch to one class or another. L V 1 L V 2 P L V n D1 D2 D3 Dn n µ δ γ3 γ1 γ2 γ4 ε α1 α2 IgM IgD IgG3, IgG1... IgE IgA Heavy-chain Figure 5-16 KUBY

17 Summary: BR Every mature B cell produces ONE and only ONE kind of BR or antibody, made up of ONE and only ONE kind of heavy-chain plus ONE kind of light chain. However, because of: 1. ombinatorial V(D) joining of various germ-encoded DNA segments 2. ombinations of different heavy and light chains 3. unctional flexibility 4. P-region and N-region nucleotide addition, and 5. Somatic hypermutation the receptors on different B cells are so diverse, that collectively, our B cells can probably recognize any organic molecule that could exist. Summary: what processes contribute to adaptive immunity? lass switching: Each class of antibody (IgM, IgG, IgD, IgA, IgE) has a slightly different effector function (opsonization, neutralization, complement fixation, AD). Antibodies can switch class from IgM (always the 1 st antibody made) to another class by changing their constant region (Fc). This makes it possible to tailor the adaptive immune response with a specialized antibody for each different kind of pathogen. Somatic hypermutation: By mutating the variable region (Fab) of the BR or antibody, and by using binding and proliferation to select those mutations that have increased the BR s ability to bind antigen, B cell receptors can adapt. The result is a collection of B cells whose receptors have a higher average affinity for their cognate antigen. This process is called affinity maturation.

18 T cell Receptors (TRs) TR is similar in structure to an antibody, however, the TR always remains bound to surface of T cell (NO secreted form) Each T cell has TRs that recognize and bind only ONE specific antigen TRs generally recognize protein antigens (i.e., peptides) V V D V V D T cell V D V TR Antibody T cell Receptors (TRs) 2 Types of T cell receptors: αβ TR (~95%) γδ TR (~5%) αβ TR recognizes a MH-antigen complex on surface of antigen presenting cells (can not recognize antigen by itself; lecture 3b) αβ TR recognizes BOTH the antigen and the MH molecule T cell receptor (TR) recognizes BOTH the antigen and MH. These are presented by antigen presenting cells (AP). AP MH-antigen complex T cell TR

19 γδ TR Small population (~5%) of T cells express a TR that contain γ and δ chains instead of α and β chains The γδ T cells predominate in the mucosal epithelia and have a repertoire biased toward certain bacterial and viral antigens Repertoire is limited γδ T cells can recognize antigen in an MH-independent manner γδ T cells play a role in responses to certain viral and bacterial pathogens Structure of the αβ T cell Receptor Heterodimer with one α and one β chain of roughly equal length Both α and β chains have a variable (V) and constant () region V regions of the α and β chains contain hypervariable regions that determine the specificity for antigen α chain β chain Variable region: antigen recognition V V D onstant region T cell

20 Structure of the αβ T cell Receptor A transmembrane region with hydrophobic amino acids A very short cytoplamic tail NOT capable of transducing an activation signal V V D T cell Short transmembrane region NO activation signal TR and D3 omplex TR is closely associated with a group of proteins collectively called the D3 complex γ chain δ chain 2 ε chains 2 ξ chains D3 proteins are invariant D3 complex transduces signals to the interior of the cells following interaction of Ag with the TR D3 complex necessary for cell surface expression of TR during T cell development Figure 9-9 KUBY

21 Genetic Basis for TR diversity Generation of a vast array of BRs is accomplished by recombination of various V, D and gene segments encoded in the germline Generation of a vast array of TRs is accomplished by similar mechanisms to BRs TR β chain genes have V, D and gene segments TR α chain genes have V and gene segments V V D D3 T cell ombinatorial V(D) joining of various germ-encoded segments contributes to TR diversity (just like BR/antibodies!) RAG 1/2 Figure 9-6 KUBY

22 At least 3 processes contribute to TR diversity 1. ombinatorial V(D) joining of various germ-encoded DNA segments 2. ombinations of different α and β chains 3. unctional flexibility Immunogenicity Immunogenicity refers to the ability to induce a humoral or cell-mediated immune response Antigenicity refers to the ability to recognize TRs, BRs, and antibodies. All immunogens are antigenic but not all antigens are immunogenic. What influences immunogenicity? Foreignness Molecular size (larger molecules are more readily phagocytosed and processed) hemical composition (i.e., insoluble molecules are more readily phagocytosed and processed) Ability to be processed and presented (which is necessary for T cell responses, and helper T cells are necessary for antibody production)...see Lecture 3b

23 Summary: omparison of BR and TR Property BR TR Genes Variable regions generate antigen-binding site Yes Yes Many V(D)s, Few s Yes Yes V(D) recombination Yes Yes Allelic exclusion Yes Yes Somatic hypermutation Yes No lass switching (antibodies only) Yes No Proteins Membrane-bound form Yes Yes Secreted form Yes (antibodies) No Isotypes with different functions Yes No

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