Properties and Overview of Immune Responses

Properties Properties and and

Overview of Immune Overview of Immune ResponsesResponses

E. SalehiE. Salehi

Tel: 66419536Tel: 66419536

Email: Email: [email protected]@sina.tums.ac.ir

AntigensAntigens

با نام و ياد خدا

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ويژگي هاي آنتي ژنها -2

انواع آنتي ژنها -3

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

• 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 AntigenCharacters of Antigen

Characters of AntigenCharacters 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 Chemical Nature of ImmunogensImmunogens

• Proteins• Polysaccharides• Nucleic Acids• Lipids

– Some glycolipids and phosopholipids can be immunogenic for T cells and illicit a cell mediated immune response

EpitopesEpitopes

• 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

Antigenic valenceAntigenic valence::

Total number of determinant which can be combined with Ab.

Epitopes could be -

contiguous (when Ab binds to a contiguous sequence of amino acids)

non-contiguous (when Ab binds to non-contiguous residues, brought together by folding).

Sequential epitopes are contiguous epitopes.

Conformational epitopes are non-contiguous antigenic determinants.

Discontinuous EpitopesDiscontinuous 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.

Linear (continuous) Linear (continuous) EpitopesEpitopes

• 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.

EpitopesEpitopes

Sequential Conformational

Ab-binding sites

Types of EpitopesTypes 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

•

Types of Peptide Epitope

LinearB cell Epitope

Antibody or “B cell” EpitopeConformational

Non-Conformational

Class II MHCsProfessional Antigen

Presenting cells

Foreign proteins

8-20 amino acids

Class I MHCs

all cells

Foreign and self proteins

8-10 amino acids

T cell Epitope

Epitope

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.

T cells and B cells use Distinct Antigen ReceptorsT cells and B cells use Distinct Antigen Receptorsto Recognize Fundamentally Different Forms of Antigento Recognize Fundamentally Different Forms of Antigen

Sperm whale myoglobin (1vxg) contains five sequential epitopes (red, green,

magenta, blue, orange) and two conformational epitopes (yellow, pink).).

ها تپ اپي شناسايي ها كاربرد تپ اپي شناسايي كاربرد

Properties of EpitopesProperties 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.

Antigenic DeterminantsAntigenic Determinants Recognized by B cells and Ab Recognized by B cells and Ab

• Composition– Proteins, polysaccharides, nucleic acids– Sequence (linear) determinants– Conformational determinants

• Size– 4-8 residues

Antigenic DeterminantsAntigenic Determinants Recognized by B cells and Ab Recognized by B cells and Ab

• Composition• Size• Number

– Limited (immunodominant epitopes)

– Located on the external surfaces of the Ag

Fe

Antigenic DeterminantsAntigenic DeterminantsRecognized by T cellsRecognized 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 AntigensTypes of Antigens

• T-cell independent antigens-Does not require T cell involvement;

polysaccharides

• T-cell dependent antigens-Requires T cell involvement;

proteins

Types of AntigensTypes of AntigensT-independentT-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 AntigensTypes of AntigensT-dependentT-dependent

• Proteins

• Structure

• Examples– Microbial proteins

– Non-self or Altered-self proteins

Hapten-carrier conjugatesHapten-carrier conjugates

• Definition

Native determinants

Haptenic determinants• Structure

– native determinants– haptenic determinants

SuperantigensSuperantigens

Conventional Antigen

Monoclonal/Oligoclonal T cell response

1:104 - 1:105

Superantigen

Polyclonal T cell response

1:4 - 1:10

• Definition

SuperantigensSuperantigens• 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 Types of Antigens regading the source of Antigenthe source of Antigen

1. Alloantigens-”same species” ---- ABO blood type, HLA, et al.2. Heteroantigens-”different

species”3. Xeno-substances ---- Various pathogens and their

products,xeno- proteins, etc.

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 rejuvenate5. Heterophile Ag (forssman Ag)- common Ags shared by different species- no specificity of species - significance immunopathology & Diagnosis

• 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

Tumor antigen

Factors influencing immune Factors influencing immune response of Agresponse of Ag

Antigen PropertiesAntigen Properties

1.Foreigness

2.Chemical properties of Ag Chemical nature Proteins>Polysaccharides >Nucleic

Acids >Lipids

3.Molecule weight (size)reasonable large molecule( >10.0 kd)

has good immuogenecity.– more stationary– more surface structure for

lymphocyte to recognize



4. Complexity of Ag structure ring > linear aromatic ring



5. Conformation and accessibility 6. Physical states : Polymer > monomer

Particulate > Soluble Denatured > Native

7.Degradability– Ag processing by Ag Presenting Cells (APC)



Genetics, Species:MHC

Individual :Age, health, etc.


Body FactorsBody Factors

Dose RouteSubcutaneous>Intravenous>Intragastric

timesAdjuvant

– Substances that enhance an immune response to an Ag


Method of AdministrationMethod of Administration

Biological Influences on Biological Influences on ImmunogenicityImmunogenicity

• 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

Cross reactionCross reaction::

• Reaction between the same Ab and different Ag with same similar determiants.

Mechanism of cross reactionMechanism 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 Determinants Recognized by the Innate Immune SystemInnate 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 Determinants Recognized by the Innate Immune SystemInnate Immune System

• PAMPs – Pathogen Associated Molecular Patterns

• PRRs – Pattern Recognition Receptors

PAMP PRRBiological Consequence of Interaction

Microbial cell wall components

Complement Opsonization; Complement activation

Mannose-containing carbohydrates

Mannose-binding protein

Opsonization; Complement activation

Polyanions Scavenger receptors

Phagocytosis

Lipoproteins of Gram + bacteriaYeast cell wall components

TLR-2 (Toll-like receptor 2)

Macrophage activation; Secretion of inflammatory cytokines


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)



U-rich single stranded viral RNA

TLR-7 Production of interferon (antiviral)

CpG containing DNA


Documents

Properties and Overview of Immune Responses