Antigens

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Antigens

• Hugh B. Fackrell

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• Assigned Reading
• Content Outline
• Performance Objectives
• Key terms
• Key Concepts
• Short Answer Questions

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ASSIGNED READING

• Chapter 4 pp 87-106

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OUTLINE

• Immunogenicity vs antigenicity
• Factors that influence immunogenicity
• immunogen contributions
• biological system contributions

• Epitopes
• B cell epitopes
• T cell epitopes
• Haptens
• Mitogens

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Definitions

• Antigen - a substance recognized by T and B cell
  receptors
• Immunogen - a substance capable of eliciting
  immune response
• Hapten - a substance of low molecular weight,
  that can bind antibodies but induce response only
  if covalently attached to large carrier
  Klein, 1992

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Definitions (cont)

• Antigenic Determinant Specific sequence of
  amino acids that combine with receptor
• Epitope an antigenic determinant on the surface
  of a molecule
• Cryotope An antigenic determinant that is buried
  within a molecule and thus is unavailable in the
  intact molecule

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Immunogen vs Antigen

• Immunogen
• Any compound that triggers the immune response
• Antigen
• Any compound that combines with the products of
  an immune response

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Factors Influencing Immunogenicity

• immunogen contributions
• biological system contributions

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Immunogen Contributions

• Chemical nature
• Chemical complexity
• Molecular size
• Conformation
• Foreigness

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Effects of Biological System on Immunogenicity

• Genotype
• Dose of Immunogen
• Route of Injection
• Adjuvants

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Genotypic Influences

• Immune Responsiveness
• Some strains of inbred mice respond to specific
  immunogen (high responders)
• Other strains do not (low responders)
• Hybrids of above have intermediate response
• Structure of MHC receptors on cell membrane

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MHC Receptors
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Dose of Immunogen

• Tolerance Specific non response to an immunogen
• Dose dependent
• Too low- no effect
• Low dose Tolerize T cell
• High dose Tolerize B cells

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Mitcheson Tolerance Exp
Immunity
Low Zone Tolerance
High Zone Tolerance
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Tolerogenic vs Immunogenic

• Immunogenicity
• antigens first processed by APC
• APC reacts with lymphocyte
• response
• Tolerogenicity
• antigens react directly with lymphocyte
• NO response

Immunogenicity requires co stimulation
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Route of Injection

• Parenteral injection
• other than digestive tract
• intravenous, intradermal, sucutaneous,
  intramuscular, intraperitioneal
• Exposure to different lymphocyte populations
• intravenous - spleen
• subcutaneous- lymph nodes

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Adjuvants

• Substances that enhance immunogenicity of an
  antigen
• alum
• Freunds ( contains Mycobacterium)
• Mechanisms
• prolong life of antigen
• enhance co-stimulatory signals
• nonspecific multiplication of lymphocyte
• granuloma formation

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Haptens

• Antigenic but not immunogenic
• small molecules eg DiNitroPhenol
• Flexible molecules eg gelatin
• When linked to a carrier molecule becomes
  immunogenic
• Hapten- Carrier conjugates
• Hapten immunodominant determinant

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Epitopes

• Size
• Specificity
• Affinity

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Construction of Epitope

• Continuous
• stretch of contiguous residues endowed with
  distinctive conformational features
• Discontinous
• group of residues that are non contiguous in the
  sequence but are brought together by the folding
  of the polypeptide chain
• 
  Klein 1992

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B cell Epitopes
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Immunoglobulin B cell receptor
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Antigen - Antibody binding

• Hydrogen bonds
• Electrostatic forces
• Van der Walls interactions
• Hydrophobic interactions

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(No Transcript)
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T cell Epitopes
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Mitogens
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The End
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Molecular Dissection ofProteinaceous Antigens

• Created Maria Sawicki
• Updated Hugh B. Fackrell
• Dec 1997

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X - Ray Crystallography

• Crystallized Antigen or Antigen - Antibody
  Complex
• Electron Density Map of the Molecule
• Three Dimensional Images

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Construction of Epitope

• Continuous
• stretch of contiguous residues endowed with
  distinctive conformational features
• Discontinous
• group of residues that are non contiguous in the
  sequence but are brought together by the folding
  of the polypeptide chain
• 
  Klein 1992

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ANTIGEN RECOGNITION SYSTEMS

• Immunoglobulins
• intact antigen is presented with several
  conformational epitopes
• T cell receptors
• processed antigen is presented and can be
  recognized only in complex with Major
  HIstocompability Complex .

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Synthetic Peptides

• Overlapping Epitopes
• Immunodominance
• 
  Benjamini ,1984

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Prediction of antigenicity

• Region accesibility
• Novotny 1988
• Segmental mobility
• Westhof et al 1984, Tainer et al 1985
• Relative hydrophilicity
• Hoop and Woods1983, Hoop 1986
• Chain termination
• Thorton and Sibanda 1983
• Segmental variability
• Jemmerson and Parerson 1985

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Epitope MappingCross-reactivity studies

• Native Protein and Peptide
  Fragments
• Synthetic Peptides
• Closely Related Proteins
• Van Renmortel, 1988

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Studies of Native Proteins and Peptide Fragments

• Conformational influences
• Native protein derivatives
• Overlapping peptide sequences
• Derivatives of active peptides
• Synthetic peptides
• Atassi, 1978

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The End
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Performance Objectives

• Key terms, concepts
• short answers

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KEY TERMS

• Accessibility, antigen, antigenic determinants,
  antigenic site mobility,
• antigenicity, continous (linear) determinants),
  discontinous (conformational) determinants
• epitopes, agreotopes, haptens, immunodominance,
  immunogen, immunogenicity

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KEY CONCEPTS

• Differentiate between and antigen's inductive and
  reactive abilities.
• Explain the factors that bestow immunogenicity on
  molecules
• Describe the discrete and distinctive sites on
  and antigen that are recognized by a B cell
  Immunoglobulin

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KEY CONCEPTS

• Describe the discrete and distinctive sites on
  and antigen that are recognized by a T cell
  receptor
• Describe the discrete and distinctive sites on
  and antigen that are recognized by the Major
  Histocompatability Complex
• Discuss some important characteristics of protein
  antigenic determinants

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SHORT ANSWER QUESTIONS
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• Differentiate between immunogenicity and
  antigenicity.
• All immunogens are antigens but all antigens are
  not immunogens. Explain with the use of specific
  examples.
• An entire microbe or molecule does not induce and
  immune response. Explain.
• Irrespective of the number of epitopes that a
  molecule has, the immune response is biased and
  sees what it wants to see. Explain.

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• Haptens are usually coupled to a carrier molecule
  before they are immunogenic. How can you
  demonstrate that the selected immunoglobulins are
  specific for the hapten and not for the carrier?
• As a corollory, would you expect ALL of the
  antibodies to be specific for the hapten? explain.

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• Why is the definition of an antigen considered to
  be an operational definition.
• Differentiate between epitopes and agreotopes.
• Differentiate between epitopes recognized by B
  cell receptor and epitopes recognized by T cell
  receptors.