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: Recognition: Remove and destroy: Distinguish self and non-self: Memory? Physical, chemical, microbial PAMPs, other receptors Phagocytes Phagocytes, NK cells Ask for examples

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

The adaptive immune response A brief overview

Zing!! Zing!! T B M Th Tc Zing!! Zing!! Zing!! P M Zing!! Y Y Y Y Y Y Treg Th17

Humor = fluid (latin)

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

Antigen-binding Figure 3-1 part 3 of 3

Figure 3-1 part 1 of 3

Anatomy of an antibody Fab : Fc: 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, IgA, IgM Different isotypes have different functions

Idiotype vs Isotype Antibody class Antigen specificity IgG, IgM, IgA, IgE, etc Conferred by constant region Antibodies of different isotypes may have the same idotype (antigen specificity) Antigen specificity Conferred by variable region Independent of isotype

Isotype structure Monomeric IgM IgG IgE IgA Pentameric IgM http://www.microbiologybook.org/mayer/IgStruct2000.htm

Isotype structure Valency: Fc fragment domains: Monomers = bivalent Multimers = multivalent Fc fragment domains: Two or three domains Determine receptor binding: Multimers Immune cell activation Fc receptors present on various types of immune cells IgG Pentameric IgM

Isotype function Fc fragment determines receptor binding IgM: IgG: Surface receptor: cell associated antibody Primary response: initial antigen recognition IgG: Most abundant isotype Antigens in serum Bacteria, viruses, toxins IgE: Allergens, parasitic antigens IgA: Mucosal immunity

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

Agglutination Pathogen/toxin inactivation Enhanced phagocytosis IgG IgM Pathogen/toxin inactivation Enhanced phagocytosis

Complement fixation (activation)

Opsonization and phagocytosis

Neutralization Toxin Neutralization C ^ Virus neutralization

http://www.colorado.edu/intphys/Class/IPHY3430-200/014immune.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)

T cell receptor: 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 T cells recognize antigen bound to MHC 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 Cytotoxic T cells are MHC I restricted Cytokines

Antigen presentation

MHC structure: MHC I Two chains: a, b2 micro-globulin Peptide-binding cleft Three globular domains a1 and a2 peptide binding a3 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 Two light chain loci V, D, and J regions Up to 40 regions in each category Randomly recombine H L

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) RAG1 and RAG2 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

B Y T B Y T APC B Y 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: Each MHC region has several genes MHCI and MHCII: Encode antigen-presentation 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 Number of alleles HLA-DPA 12 HLA-DPB 88 HLA-DQA 17 HLA-DQB 42 HLA-DRA 2 HLA-DRB1 269 HLA-DRB3 30 HLA-DRB4 7 HLA-DRB5 http://pathmicro.med.sc.edu/bowers/mhc.htm

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 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 Antigen receptor : Each cell encodes a single polypeptide sequence One receptor specificity per cell V region selection Junctional diversity Somatic hypermutation

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

Definitions: The language of MHC Genetic region MHCI, II, III Gene products of MHCI and II HLA = Human Leukocyte Antigen Human MHC molecules Allele names: Popular numbers DR5, DQ2, DP1, etc MHC2 with a and b Haplotype = An individual’s combination of alleles

MHCII association with autoimmune diseases Haplotype MHCII