Adaptive Immunity I Innate and Adaptive Immunity Humoral

Adaptive Immunity I

Innate and Adaptive Immunity Humoral Cellular

Characteristics of the IR Innate Adaptive • Broad specificity • Recognizes molecular patterns • Rapid response • No memory • Narrow specificity • Recognizes specific antigens • Delayed response (why? ) • Memory

Functions of the (innate) immune system • • • Barriers: Physical, chemical, microbial Recognition: PAMPs, other receptors Remove and destroy: Phagocytes Distinguish self and non-self: Phagocytes, NK cells Memory?

Functions of the adaptive IR • Barriers: • Recognition: • Remove and destroy: Distinguish self and non-self: • Memory?

The adaptive immune response A brief overview

Zing!! Y Y Y Y Y T M Y B Zing!! Th Zing!! Tc Zing!! P Y Y Y M Y Y Zing!! Y Y Y Y Th 1 Th 2 Treg Th 17 Y Y

Topics for today • • • Antigen receptors Antibody structure and function Antigen presentation and MHC restriction Antibody receptor rearrangement MHC diversity

Antigens and antigen receptors • Antigen: A macromolecule (usually protein) that is specifically recognized by an antibody. • “Generates an antibody” • Antibody: A molecule that is produced in response to and is specific for a particular antigen. • Antigen = Anything that induces an adaptive immune response.

Antigen receptors come in two flavors Antibodies T cell receptors • • Made by B cells Cell-associated and soluble Bivalent Recognize antigen directly Expressed by T cells Cell associated Monovalent Recognize antigen in conjunction with MHC Both: • Recognize specific antigens • Each B or T cell expresses a single antigen specificity One cell, one antibody or T cell receptor, one antigen

Antibody structure: “lock and key” Bivalent: Two identical antigen binding sites

Epitopes

Figure 3 -1 part 3 of 3 Antigenbinding

Figure 3 -1 part 1 of 3

Anatomy of an antibody • Fab : – Antigen binding – Variable region – Confers antigen specificity – Different for every antibody • Fc: – Constant regions – Confers Fc receptor binding on immune cells – Crystalized

Classification of antibodies: • Idiotype: antigen binding specificity: – – Variable region B cells only produce a single idiotype A single idiotype may recognize closely related antigens Defined by Fab • Isotype: heavy chain structure (antibody “class”) – Defined by their Fc domain – Named for popular letters of the alphabet: Ig(immunoglobulin)G, Ig. A, Ig. M – Different isotypes have different functions

Idiotype vs Isotype • Antigen specificity • Conferred by variable region • Independent of isotype • Antibody class • Ig. G, Ig. M, Ig. A, Ig. E, etc • Conferred by constant region • Antibodies of different isotypes may have the same idotype (antigen specificity)

Isotype structure Monomeric Ig. M Ig. G Pentameric Ig. M Ig. E Ig. A http: //www. microbiologybook. org/mayer/Ig. Struct 2000. htm

Isotype structure • Valency: – Monomers = bivalent – Multimers = multivalent Ig. G Pentameric Ig. M • Fc fragment domains: – Two or three domains – Determine receptor binding: • Multimers • Immune cell activation – Fc receptors present on various types of immune cells

Isotype function • Fc fragment determines receptor binding • Ig. M: – Surface receptor: cell associated antibody – Primary response: initial antigen recognition • Ig. G: – Most abundant isotype – Antigens in serum – Bacteria, viruses, toxins • Ig. E: Allergens, parasitic antigens • Ig. A: Mucosal immunity

Antibody function: Antigen removal Y Y Y Y Y – B cell activation – Antibody-dependent cellular cytotoxicity P Y Agglutination Complement fixation Opsonization Neutralization Activation of immune cells: Y • • •

Agglutination Ig. G Ig. M • Pathogen/toxin inactivation • Enhanced phagocytosis

Complement fixation (activation)

Opsonization and phagocytosis

Neutralization Toxin Neutralization C ^ Virus neutralization

http: //www. colorado. edu/intphys/Class/IPHY 3430 -200/014 immune. htm

The T cell receptor • Surface receptor • Structure: – a and b chains – Variable and constant regions – Antigen-specific • Recognizes antigen presented by the major histocompatibility complex (MHC)

Antibody: • Soluble or membrane bound • Heavy and light chains • Variable and constant regions • Bivalent T cell receptor: • Membrane bound • a and b chains • Variable and constant regions • Monovalent

Antigen recognition • B cell receptors (antibody) recognize antigen directly • T cell receptors recognize antigen only in conjunction with MHC molecules on the surface of antigen presenting cells

Antibody-antigen interaction T cell receptor antigen interaction

MHC restriction Th • T cells recognize antigen bound to MHC • MHC molecules come in 2 flavors: – MHCI: Present on all nucleated cells – MHCII: Present on antigen presenting cells • T cells are MHC restricted – Helper T cells recognize antigen bound to MHCII – Cytotoxic T cells recognize antigen bound to MHCI Tc

Helper T cells are MHC II restricted Cytokines Cytotoxic T cells are MHC I restricted

Antigen presentation

MHC structure: MHC I • Two chains: a, b 2 microglobulin • Peptide-binding cleft • Three globular domains • a 1 and a 2 peptide binding • a 3 constant

MHC structure: MHCII • a and b chains • Peptide-binding cleft • Peptide binding and constant domains

How the peptide gets into the binding cleft: Antigen processing • MHCI – Intracellular antigens – Cytoplasmic processing • MHCII – Extracellular antigens – Endosome processing

Antigen Processing Thought question: Why is processing different for MHCI vs MHCII?

Antigen receptors: Specificity and diversity • • 1015+ different antigen specificities Virtually every antigen ever invented But not self antigens Where does the diversity come from?

The origins of antibody diversity • Germ line theory: – One gene in the germ line for every variable region – 1015 antibody genes? – Nope • Somatic mutation theory: – Only a few variable region genes – Contact with antigen induces mutations resulting in (with luck) a matching sequence • Both are true • Neither is true

Combined theory of antibody diversity • Antibody genes contain many different coding sequences • Extensive recombination occurs during B cell maturation • Diversity is generated by: – – Random combinations Junctional diversity Sloppy recombination Pairing of heavy and light chains • Each B or Tcell clone expresses a unique antibody sequence • Relevant clones are selected

Antibody genes: Germline • Heavy and light chain loci • Each has multiple coding sequences: – Variable region: multiple sequences – Constant region: multiple sequences • Random recombination

Variable region genes • Each cell has two light chain loci (k and l) and one heavy chain locus. • Each light chain variable locus has: – V (variable) regions – J (joining) regions • Each heavy chain variable locus has: – V regions – J regions – D (diversity) regions • Multiple alternative sequences for each region • Random rearrangement during maturation results in one VL, one JL, one VH, one JH, and one DH

Antibody genes: Germ-line structure L H H L • Two light chain loci • V, D, and J regions • Up to 40 regions in each category • Randomly recombine

T cell receptor rearrangement

Factors that contribute to antigen receptor diversity • V, J, D gene variability: Large number of genes • V, J, D gene joining: – Selection – Order – Orientation • Junctional diversity: – Sloppy recombination – N region insertion • Heavy and light gene selection

Recombination activating genes (RAG) • • RAG 1 and RAG 2 Enzyme complex Catalyze recombination and joining Essential for development of mature B and T cells

Somatic hypermutation • Enhanced diversity and specificity after antigen contact • Occurs in peripheral tissues in mature B cells only • Rapidly accumulating point mutations in V regions • Selection for best fit • Increases strength of binding

Somatic hypermutation

MHC diversity • B and T cells are clonal: – Each cell expresses a single receptor type – Each cell is specific for one antigen type • Antigen-presenting cells have multiple specificities – Each cell may express several different MHC types – Each MHC molecule may associate with several different antigens

Y Y B Y Y T Y APC Y Y B Y Y Y B T

Genetics of the MHC • • Germ-line expression: no rearrangement Multiple genes present in the germ line Co-dominant expression Multiple alleles present in the population

MHC locus • Three regions: – MHCI and MHCII: Encode antigenpresentation molecules – MHCIII: Complement genes, some cytokine genes, others • Each MHC region has several genes – MHCI: A, B, C – MHCII: DP, DQ, DR – Each MHCII gene encodes a and b chains

MHC diversity • MHC presents all possible epitopes • No somatic rearrangement • Each cell expresses multiple MHC molecules • Genetic polymorphism • High mutation rate • Degenerate peptide binding

Genetic polymorphism • • • Multiple MHC genes Many alleles for each gene Multiple combinations Inherited independently Expression is co-dominant Population-wide diversity > individual diversity

Polymorphism of MHCII genes Locus HLA-DPA HLA-DPB HLA-DQA HLA-DQB HLA-DRA HLA-DRB 1 HLA-DRB 3 HLA-DRB 4 Number of alleles 12 HLA-DRB 5 http: //pathmicro. med. sc. edu/bowers/mhc. htm 88 17 42 2 269 30 7 12

MHC Peptide binding • MHC-antigen binding differs from B and T cell antigen receptor binding: • “Degenerate” binding: Binding is not antigen-specific. A single MHC molecule can associate with many different peptides • Each antigen-presenting cell expresses several different MHC specificities • Different MHC on one cell can bind different peptides • Each cell can present many different peptides

Diversity: antigen receptor vs MHC • Antigen receptor : – Each cell encodes a single polypeptide sequence – One receptor specificity per cell – V region selection – Junctional diversity – Somatic hypermutation • MHC : – Each cell encodes multiple genes and multiple alleles – multiple MHC proteins per cell – Degenerate peptide binding – No gene rearrangement – Polymorphic: up to 600 alleles for each locus

The role of diversity Antigen receptor diversity MHC diversity • Recognition of all possible • Limited diversity of individuals antigens by the individual • Extensive diversity of the • Protects the organism population • Mediates population-wide disease resistance – Large gene pool: more resistant population – Limited gene pool: higher frequency of disease susceptibility and genetic disease

Definitions: The language of MHC • MHC = – Genetic region MHCI, III – Gene products of MHCI and II • HLA = Human Leukocyte Antigen – Gene products of MHCI and II – Human MHC molecules • Allele names: Popular numbers – DR 5, DQ 2, DP 1, etc – MHC 2 with a and b • Haplotype = An individual’s combination of alleles

MHCII association with autoimmune diseases Haplotype MHCII