Antigens

March 4, 2003

Part I Introduction

Chapter 1. Overview of the Immune System

Chapter 2. Cells and Organs of the Immune System

Part II Generation of B-cell and T-cell Responses

Chapter 3. Antigens

本章大綱 :

1. Immunogenicity Versus Antigenicity

2. Factors That Influence Immunogenicity

3. Epitopes

4. Haptens and the Study of Antigenicity

Immunogenicity vs Antigenicity

Immunogenicity:

the ability to induce a humoral and/or cell-mediated immune response

Antigenicity:

the ability to combine specifically with Ab and/or cell-surface Ig/TCR

- Although a substance that induces a specific immune response is usually called an antigen, it is more appropriately called an immunogen.

- Although all molecules that have the property of immunogenicity also have the property of antigenicity, the reverse is not true.

- Haptens are antigenic but incapable, by themselves, of inducing a specific immune response.

Factors That Influence Immunogenicity

Contribution of the immunogen to immunogenicity:

- Foreighness- Molecular size- Chemical composition and heterogenicity- Susceptibility to antigen processing and presentation

Contribution of the biological system to immunogenicity:

- Genotype of the recipient animal- Immunogen dosage and route of administration- Adjuvants

Foreignness

- Generally, the greater the phylogenetic distance between two species, the greater the structural (and therefore the antigenic) disparity between them.

- Some macromolecules (e.g., collagen and cytochrome c) were highly conserved throughout evolution and therefore display very little immunogenicity across diverse species lines.

Molecular size

- There is a correlation between the size of a macromolecule and its mmunogenicity.

- The best immunogens tend to have a molecular mass approaching 100,000 Da.

- Generally, substances with a molecular mass less than 5,000 – 10,000 Da are poor immunogens.

Chemical composition and complexity

- Synthetic homopolymers tend to lack immunogenicity regardless of their size.

- All 4 levels of protein organization – primary, secondary, tertiary and quaternary – contribute to the structural complexity of a protein and hence affect its immunogenicity.

- polypeptides, polysaccharides, (nucleic acids, lipid)

4 levels of protein organizational structure

Susceptibility to antigen processing and presentation

- Ability to be processed and presented with an MHC molecules on the surface of an antigen- presenting cell or altered self-cell

Processing and presentation of exogenous antigens

Processing and presentation of endogenous antigens

Contribution of the biological system to immunogenicity:

- Genotype of the recipient animal (strains)

- Immunogen dosage and route of administration

- Adjuvants

Immunogen dosage and route of Administration

Doses: too low no response too high tolerance

Routes: orally (從口入的 )

parenterally (非從口入的 )

- intravenous (iv) : into a vein - intradermal (id) : into the skin - subcutaneous (sc) : beneath the skin - intramuscular (im) : into a muscle - intraperitoneal (ip) : into the peritoneal cavity

Adjuvants

- Latin adjuvare, to help

- Substances that, when mixed with an antigen

and injected with it, enhance the immunogenicity

of that antigen.

Effects of adjuvants

1. Prolong antigen persistence - slower release of antigen at the injection site

2. Enhance co-stimulatory signals - increased expression of MHC & B7 molecules - secretion of cytokines increased antigen-presenting ability maximal activation of TH cells

3. Increase local inflammation - formation of a dense, macrophage-rich mass of cells

4. Stimulate lymphocyte proliferation nonspecifically

Contribution of the immunogen to immunogenicity:

- Foreighness- Molecular size- Chemical composition and heterogenicity- Susceptibility to antigen processing and presentation

Contribution of the biological system to immunogenicity:

- Genotype of the recipient animal- Immunogen dosage and route of administration- Adjuvants

Epitopes

- Immune cells do not interact with, or recognize, an entire immunogen molecules; instead, lymphocytes recognize discrete sites on the macromolecule called epitopes, or antigenic determinants.

- The recognition of antigens by T cells and B cells is fundamentally different.

T-cell and B-cell epitopes

- Because B cells bind antigen that is free in solution, the epitopes they recognize tend to be highly accessible sites on the exposed surface of the immunogen.

- Because most T cells recognize antigen only when it is combined with an MHC molecule, T cell epitope, as a rule, cannot be considered apart from their associated MHC molecules.

Conformation of the epitope recognized by B cells

1. The ability to function as a B-cell epitope is determined by the nature of the antigen- binding site on the antibody molecules displayed by B cells.

3-D structure of anoctapeptide hormone

(angiotensin II) complexedwith a monoclonal Ab Fab

Fragment.

Red: angiotensin IIBlue: the heavy chainPurple: the light chain

Model of interaction between hen egg-white lysozyme (HEL) and Fab fragment of anti-HEL antibody

light chain (yellow) a glutamine resiue (red)

heavy chain (blue)

HEL

(green)

Ag binding sites for different antibody

- The interactions of Ab with Ag are through non-covalent bonds.

- 4 types of non-covalent bonds:

a. Ionic (or electrostatic) bond

Ionic bonds form between surfaces of opposite charge.

b. Hydrogen bond Hydrogen bonds form between hydrogen atoms and two other electronegative atoms such as oxygen and nitrogen.

c. Van der Waals’ force

Van der Waals’ forces occur at very close ranges between two atoms. Fluctuations in the electrical charge within electron clouds can lead to attractive or repulsive forces between atoms, dependent on the distance between them.

d. Hydrophobic bond

Hydrophobic bond is created by the behavior of hydrophobic subunits in aqueous environments. These tend to be pushed together to minimize the instability they cause in the network of hydrogen- bonded water molecules.

2. The B-cell epitope on native proteins generally are composed of hydrophilic amino acids on the protein surface that are topographically accessible to membrane-bound or free antibody.

Ab elicited by immunization with the (T,G)-A-L copolymer react largely with the exposed tyrosine

and glutamic acid residues. Anti-(T,G)-A-L Abs do not eact with the A-(T,G)-L copolymer

3. B-cell epitopes can contain sequential or nonsequential amino acids.

Sperm whale myoglobulin contains 5 sequential B-cell epitopes

6-8 aa

Hen egg-white lysozyme composes one nonsequential (conformational) epitope

HEL

Red: light chainBlue: heavy chain

Antibody to native HEL does not bind to reduced HEL

Inhibition of reaction between HEL loop and anti-loop antiserum by natural loop or closed synthetic loop only

4. B-cell epitopes tend to be located in flexible regions of an immunogen and display site mobility.

5. Complex proteins contain multiple overlapping B-cell epitopes, some of which are immunodominant.

Properties of T-cell epitopes

1. Antigenic peptides recognized by T cells form trimolecular complexes with a TCR and an MHC molecule.

2. - The antigen-binding cleft on an MHC molecule interacts with various oligomeric peptides (nonamer for class I & 12 –25 residues for class II) that function as T-cell epitopes.

- A given MHC molecule can bind a variety of different peptides (broad but selective interaction).

3. Antigen processing is required to generate peptides that interact specifically with MHC molecules.

Processing and presentation of exogenous and endogenous antigens

4. T-cell epitopes tend to be on the “inside” of the protein molecule

5. Immunodominant T-cell epitopes are determined in part by the set of MHC molecules expressed by an individual.

Correlation of MHC-binding ability and T-cell-activating ability of synthetic peptides

TCR and MHC-peptide

TCR

peptide

MHC

Haptens and the study of antigenicity

Hapten: Small organic molecules that are antigenic but not immunogenic.

Carrier: Large molecules that are chemically coupled to haptens yield immunogenic hapten-carrier conjugates.

(1)

TABLE 3-5 REACTIVITY OF ANTISERAWITH VARIOUS HAPTENS (2)

Drug allergies (藥物過敏 )

When medicines become immunogens, …………..

Penicillin allergy

本章大綱 :

1. Immunogenicity Versus Antigenicity

2. Factors That Influence Immunogenicity

3. Epitopes

4. Haptens and the Study of Antigenicity