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Unformatted text preview: (What we and other vertebrates have in addition to an innate immune system) Adaptive immune system • Makes specific immune response to a pathogen • (1790s): Jenner showed that exposure to cowpox protected against infection by a related virus: smallpox • Adaptive immune response has “memory” that can confer life-long immunity to infection – 1781: Swedish sailors bring measles virus to Faeroe Islands – 1846: Another ship with measles-infected sailors visits Faeroe Islands. People older than 64 did not get measles because they had life-long immunity to measles virus. Comparison of innate and adaptive immunity Two arms of the adaptive immune response
• Humoral: antibodies (proteins produced by B cells) that circulate in the blood. Antibodies can recognize virally-infected cells and free viruses. • Cellular: Lymphocytes (T cells) that do not produce a soluble product. T cells can recognize virallyinfected cells, but not free virus. Adaptive immune system I: Antibodies
• Antibodies normally work very well to get rid of pathogens. • Antibodies don’t work well to combat HIV. The power of the humoral immune response
• Typical mammal (e.g., you, a mouse) can make >1016 different types of antibodies. • Antibodies can bind to many types of antigens. (protein, carbohydrate, nucleic acid, lipid, small molecules). • Mice can raise antibodies against synthetic compounds that don’t exist in nature (e.g., buckyballs). • Antibodies can be exquisitely specific: e.g., can distinguish between ortho-, meta-, and paraAminobenzoic acid. Vocabulary
• Antibody (Ab) or Immunoglobulin (Ig) a Y-shaped membrane-bound or soluble protein that binds specifically to an antigen (Ag). The hypervariable regions (HV) or complementarity determining regions (CDR) of the antibody contact the antigen.
– IgG (Immunoglobulin G) is the most abundant form of antibody in the blood. • Antigen (Ag) - The target of an antibody or a T cell (can be protein or non-protein). An antibody binds to a region of the antigen called the epitope. • Epitope - The region of an antigen that is recognized by an antibody or a T cell receptor. Generating antibody diversity Clonal selection Each cycle of cell growth and proliferation takes ~12 hours, so takes ~1 week to make clone of ~20,000 identical B cells • Niels Jerne proposed the “Clonal Selection Theory” in 1954 -- see Bi1 website link to his 1984 Nobel Prize lecture. Clonal selection theory accounts for:
• Diversity -- information coding for all Abs is in DNA • Self/non-self discrimination -- eliminate cell clones bearing all self-reactive receptors on their surface or else inactivate them • Memory -- increase in number of cells the second time around Antibodies Antibodies (Immunoglobulins)
Fab Fab Fab = Fragment antigen binding) •
Fc Note two identical Fabs, so two antigen binding sites 2 heavy chains, 2 light chains Variable (V) domains (VH and VL) Constant (C) domains (CH1 and CL) Fc = Fragment “constant” • Fc region (nearly constant in sequence) of Abs has effector functions (bind Fc receptors, complement, etc.). Having two identical antigen binding sites allows antibodies to bind tightly to surfaces with repeating epitopes Fc Note -- Fc regions of antibodies are exposed when Fabs bind to surface antigens. Affinity versus avidity
IgG IgM Affinity is a measure of the strength of a binding interaction: A + B <--> AB Equilibrium dissociation constant (KD) = [AB]/[A][B] Strength of binding between a multivalent protein and an antigen or ligand containing multiple binding sites is the AVIDITY. High avidity can compensate for low affinity. Fab IgG Moderate affinity when only one Fab is bound Higher “apparent” affinity due to avidity effects Fab IgG No effects of tethering to a surface if only one Fab is bound No avidity effects if antigen is not tethered to a surface Hypervariable regions fall in loops of V domain structure Crystal structures show that loops contact antigen Rename them CDRs (Complementarity Determining Regions) CDR1 CDR3 CDR2 Figure 3.7 Clicker question
We need ~108 different kinds of antibodies to ensure immunity against most/all pathogens. How can an individual make 108 different types of antibodies?
1) 2) 3) 4) Encode each antibody gene in the genome. Separate antibodies into different segments, then mix and match gene segments. Start with one antibody gene, then mutate it to create different specificities. Make antibodies that have no defined structure, then they can fold around an antigen. How can we make enough different antibodies to protect us from all possible pathogens?
need 108 different antibodies --> 104 heavy chains and 104 light chains • Human cells have ~30,000 genes • If every heavy chain and every light chain encoded by a different gene, would use up half the cell’s genes just making antibodies!
• Estimate: Generating antibody diversity: Modular design of antibodies
• Heavy chain variable region encoded in 3 gene segments (modules): V, D, and J
– 100s of different V gene segments – Tens of different D gene segments – >5 different Js • Light chain variable region encoded in 2 gene segments: V and J
– Hundreds of different V gene segments – >5 different Js Generating antibody diversity • • • Permanent rearrangement of the DNA in a B cell Same thing happens in T cells to generate T cell receptors Mature B and T cells do NOT have the same DNA as other cells in the body Generation of diversity (G.O.D.)
• • • • • Multiple V, D, J gene segments Junctional diversity -- random pairing of a V and a J gene segment (light chain) or a V, D, and a J gene segment (heavy chain). Addition or deletion of bases during joining of V to D and/or V to J and/or D to J creates even more diversity within CDR3 Combinatorial pairing of H and L chains Somatic hypermutation -- high rate of mutation in antibody variable region genes during clonal expansion of a B cell. Result: some B cell descendents produce antibodies that bind more tightly to an antigen (these are stimulated to divide further); others produce antibodies that don’t bind as tightly (these are not stimulated to divide further). Clicker question
Differential RNA splicing creates antibody diversity. 1) True 2) False Clicker question
Differential RNA splicing creates antibody diversity. 1) True 2) False
Antibody diversity is created by differential recombination at the DNA level, NOT the RNA level. Clicker question
1) 2) 3) 4) Why is differential RNA splicing not a good method for the immune system to create a diverse set of antibodies? Differentially-spliced RNA transcripts would not be inherited by clonal descendents of a stimulated B cell. RNA splicing is inefficient, thus not enough of the correctly spliced antibody would be produced by each cell. Differential RNA splicing would not provide an adequate level of potential diversity. Differential RNA splicing only occurs in different cell types. A single cell type (e.g., a B cell) always splices RNAs in the same way. B cell development
• Choice to become: Plasma cell -- produce secreted form of antibody; secrete ~2000 antibodies/sec; short lifetime (days) Memory cell -- have already somatic hypermutation. Can confer life-long immunity to infection. T lymphocytes can also become long-lived memory cells. CD4 memory T cells are an important reservoir for HIV. Extra slides for reference The diversity of antigen receptors in both B and T cells is generated by rearrangements of gene segments
Antibodies (and T cell receptors) are encoded by sets of gene segments. During development of a B (or T) cell, gene segments are joined randomly by DNA recombination (irreversible). Juxtaposed gene segments encode variable part of the antibody (or T cell receptor). Different cells join gene segments differently, so receptors are unique. Each B cell bears many copies of its unique receptor (membrane-bound antibody). Each T cell bears many copies of its unique receptor (T cell receptor; TCR). Figure 1-18 Arrangement of CDRs in Ab combining sites: CDR3s always in center, CDR1 and CDR2 always on sides CDR2 (L) CDR3 (H) CDR1 (H) CDR2 (H) CDR1 (L) CDR3 (L)
Branden and Tooze, Fig. 15.13 Variability within antibody V domains clusters in three regions
Wu and Kabat index of variability: # aa that occur at that position / frequency of most common aa at that position 3 hypervariable (HV) regions (CDRs) Light Heavy 24-34 31-35 50-56 50-65 89-97 95-102 Janeway et al. Immunobiology Figure 3.6 V regions encoded by >1 gene segment (light chains) Important point: Rearrangement for antibody genes is at the DNA level -- different from RNA splicing, which occurs in many genes Kuby Immunology textbook V regions encoded by >1 gene segment (heavy chains) Kuby Immunology textbook Complement activation - classical pathway animation
Don’t need to memorize the details. Play movie: Complement System.swf (included with this PowerPoint file. Anaphylatoxin: peptides produced during complement fixation that mediate inflammation. Induce anaphylactic shock when injected into animals. Gene segment recombination Animation Play movie: Gene Recombination.swf (included with this PowerPoint file. Opsinization of invader by antibodies enhances phagocytosis (Animation) Play movie: Phagocytosis.swf (included with this PowerPoint file. ...
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- Spring '09