Properties and Overview of Immune Responses

با نام و ياد خدا Antigens E. Salehi Tel: 66419536 Email: eislahi@sina.tums.ac.ir

1- تعاريف 2- ويژگي هاي آنتي ژنها 3- انواع آنتي ژنها 4- اپي تپ و انواع آن 5- انواع آنتي ژنها از نظر منشاء 6- عوامل مواثر بر ايمونوژنسيته 7- الگوهاي مولكولي و پذيرنده هاي آن

Definitions Immunogen Antigen (Ag) Tolergen Allergen Epitope or Antigenic Determinant Hapten Carrier Super-Antigen Adjuant

Characters of Antigen Degree of “foreignness”-Based on genetic relatedness Molecular Size-Usually MW >100,000; immunogenic MW <10,000 non immunogenic MW 10,000-100,000 is imunogenically variable

Characters of Antigen Chemical composition-Proteins; Primary, secondary, tertiary and quaternary structure all contribute. Ability to be processed by APCs- Function of size

Chemical Nature of Immunogens Proteins Polysaccharides Nucleic Acids Lipids Some glycolipids and phosopholipids can be immunogenic for T cells and illicit a cell mediated immune response

Epitopes In protein antigens epitopes can be defined in terms of: Amino acid composition Protein location Length (5-15 amino acids) epitopes: Immunodominant Epitopes bound by a greater proportion of antibodies than others in a normal in vivo immune response Also known as Major Antigenic Sites Sequestered Epitopes can be divided into 2 classes: Discontinuous epitopes Continuous (linear) epitopes 15

Antigenic valence: Total number of determinant which can be combined with Ab.

17

Discontinuous Epitopes Constitutive residues are non-sequential in the primary sequence. Highly conformational dependant. Account for approx. 90% of epitopes on a given antigenic (globular) protein. 18

Linear (continuous) Epitopes Constitutive residues are sequential in the primary sequence of the protein. Fewer conformational constraints on Ab recognition. Often contain residues that are not implicated in antibody interaction. 19

Epitopes Sequential Conformational Ab-binding sites

Types of Epitopes Conformational / Discontinuous epitopes: recognized by B cells non-linear discrete amino acid sequences, come together due to folding. Sequential / Continuous epitopes: recognized by T cells & B cells linear peptide fragments 21

Types of Peptide Epitope Conformational Antibody or “B cell” Epitope Epitope LinearB cell Epitope Non-Conformational T cell Epitope Class I MHCs all cells Foreign and self proteins 8-10 amino acids Class II MHCs Professional Antigen Presenting cells Foreign proteins 8-20 amino acids 22

T cells and B cells use Distinct Antigen Receptors to Recognize Fundamentally Different Forms of Antigen B cells can recognize linear or conformational epitopes on cell surfaces, of proteins, of carbohydrates or of lipids. The B cell antigen receptor is a form of membrane Ig. T cells recognize linear peptide fragments bound to MHC class I or class II molecules. 23

Sperm whale myoglobin (1vxg) contains five sequential epitopes (red, green, magenta, blue, orange) and two conformational epitopes (yellow, pink). 24

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Properties of Epitopes They occur on the surface of the protein and are more flexible than the rest of the protein. They have high degree of exposure to the solvent. The amino acids making the epitope are usually charged and hydrophilic. 26

Antigenic Determinants Recognized by B cells and Ab Composition Proteins, polysaccharides, nucleic acids Sequence (linear) determinants Conformational determinants Size 4-8 residues

Antigenic Determinants Recognized by B cells and Ab Composition Size Number Limited (immunodominant epitopes) Located on the external surfaces of the Ag Fe

Antigenic Determinants Recognized by T cells Composition Proteins (some lipids) Sequestered determinants Processed MHC presentation (lipid presentation by MHC-like CD1) Size 8 -15 residues Number Limited to those that can bind to MHC

Types of Antigens T-cell independent antigens-Does not require T cell involvement; polysaccharides T-cell dependent antigens-Requires T cell involvement; proteins

Types of Antigens T-independent Polysaccharides Properties Polymeric structure Polyclonal B cell activation Yes -Type 1 (TI-1) No - Type 2 (TI-2) Resistance to degradation Examples Pneumococcal polysaccharide, lipopolysaccharide Flagella

Types of Antigens T-dependent Proteins Structure Examples Microbial proteins Non-self or Altered-self proteins

Hapten-carrier conjugates Definition Structure native determinants haptenic determinants Native determinants Haptenic determinants

Superantigens Definition Conventional Antigen Superantigen Polyclonal T cell response 1:4 - 1:10 Monoclonal/Oligoclonal T cell response 1:104 - 1:105

Superantigens Definition Examples Staphylococcal enterotoxins Staphylococcal toxic shock toxin Staphylococcal exfoliating toxin Streptococcal pyrogenic exotoxins T cell SAg: exotoxin, protein of reverse translate virus B cell SAg: SPA(staphylococcal protein A) HIV:gp120

Types of Antigens regading the source of Antigen Alloantigens-”same species” ---- ABO blood type, HLA, et al. Heteroantigens-”different species” Xeno-substances ---- Various pathogens and their products,xeno- proteins, etc.

- common Ags shared by different species 4. Autoantigens- Self component Release of sequestered antigen- Such as lens protein,sperm etc. Change of molecular structure of auto-tissues Degeneration of protein Forbidden clone rejuvenate 5. Heterophile Ag (forssman Ag) - common Ags shared by different species - no specificity of species significance immunopathology & Diagnosis

Tumor antigen Tumor specific Ag, TSA --only express on the tumor cells but normal cells   Tumor associated Ag,TAA --Its express is high on tumor cells but low on normal cells,eg. AFP CEA

Factors influencing immune response of Ag Antigen Properties 1.Foreigness 2.Chemical properties of Ag Chemical nature Proteins>Polysaccharides >Nucleic Acids >Lipids

Factors influencing immune response of Ag Antigen Properties 3.Molecule weight (size) reasonable large molecule( >10.0 kd) has good immuogenecity. more stationary more surface structure for lymphocyte to recognize

Factors influencing immune response of Ag Antigen Properties 4. Complexity of Ag structure ring > linear aromatic ring

Factors influencing immune response of Ag Antigen Properties 5. Conformation and accessibility 6. Physical states : Polymer > monomer Particulate > Soluble Denatured > Native 7.Degradability Ag processing by Ag Presenting Cells (APC)

Factors influencing immune response of Ag Body Factors Genetics, Species:MHC Individual :Age, health, etc.

Factors influencing immune response of Ag Method of Administration Dose Route Subcutaneous>Intravenous>Intragastric times Adjuvant Substances that enhance an immune response to an Ag

Biological Influences on Immunogenicity Adjuvants-Enhance immunogenicity Potential mechanism Ag persistence Cell signaling or cytokine effect Induction of inflammation Lymphocyte stimulation Change the chemical and physical charactes of Ag Improves the Ag process and presentation ability of macrophages stimulates proliferation of lymphocytes Types Biological adjuvant:BCG,LPS Synthesized adjuvant Freund’s incomplete adjuvant Freund’s complete adjuvant Chemical adjuvant Alum 45

Cross reaction: Reaction between the same Ab and different Ag with same similar determiants.

Mechanism of cross reaction Common Ag determinant Similar structure of Ag determinant  Significance: Because there are some common antigen determinants between different microbes, so the antiserum against one kind of Ag can also react with another Ag and couse a cross reaction .

Determinants Recognized by the Innate Immune System Adaptive Immune System – Discrete Determinants Reacts with a specific pathogen Innate Immune System – Broad Molecular Patterns Reacts with a variety of pathogens

Determinants Recognized by the Innate Immune System PAMPs – Pathogen Associated Molecular Patterns PRRs – Pattern Recognition Receptors

PAMP PRR Biological Consequence of Interaction Microbial cell wall components Complement Opsonization; Complement activation Mannose-containing carbohydrates Mannose-binding protein Polyanions Scavenger receptors Phagocytosis Lipoproteins of Gram + bacteria Yeast cell wall components TLR-2 (Toll-like receptor 2) Macrophage activation; Secretion of inflammatory cytokines

PAMP PRR Biological Consequence of Interaction Double stranded RNA TLR-3 Production of interferon (antiviral) LPS (lipopolysaccharide of Gram – bacteria) TLR-4 Macrophage activation; Secretion of inflammatory cytokines Flagellin (bacterial flagella) TLR-5

PAMP PRR Biological Consequence of Interaction U-rich single stranded viral RNA TLR-7 Production of interferon (antiviral) CpG containing DNA TLR-9 Macrophage activation; Secretion of inflammatory cytokines