Antigen (Immunogenic and Immune Response)

Dr Akande J.O

DefinitionsAntigens:

• Any foreign substance that elicits an immune response whenintroduced into the tissues of a susceptible animal and capable ofcombining with the specific antibodies formed.

• Generally high molecular weight

• Typically, proteins or polysaccharides. Polypeptides, lipids, nucleicacids and many other materials also can also function as antigens

• Microbes are antigenic and they contain and produce manyantigens

• Generates a specific immune response and induces the formationof a specific antibody or T cells response or both.

• reacts with the products of a specific immune response

Cont’d• Epitope (antigenic determinant): Active regions of an immunogen

(or antigen) that binds to antigen-specific receptors on

lymphocytes or to secreted antibodies.

• Antigenicity: The ability of antigen to react specifically with a free

Ab or membrane coupled antibody (BCR).

• Immunogen: Molecule that stimulate a specific immune response.

• NB: Immunogens are antigens but all antigens are notimmunogens

• Immunogenicity: The ability to induce a humoral or cell mediated

immune response.

• Antibody (Ab): A specific protein that is produced in response toan immunogen and reacts with an antigen

Cont’d

• Paratope: “The site in the variable (V) domain of anantibody or T-cell receptor that binds to an epitope on anantigen

• Hapten: Small foreign molecule that is not antigenic.• Must be coupled to a carrier molecule to be antigenic.• Once antibodies are formed they will recognize hapten.• Small molecules with a low Molecular Weight (Less than

10,000) that could never induce an immune responsewhen administered by themselves unless it coupled to acarrier molecule.

• Has the property of antigenicity but not immunogenicity.

ExamplesInfectious materials

• a. microbial structures : cell walls, capsules, flagella, pili, viral capsids, envelope-associated glycoproteins, etc

• b. microbial toxins

Noninfectious materials

• a. allergens (pollen, hair, foods, dander, bee venom, drugs, and other agents causing allergic reactions)

• b. foreign tissues and cells (from transplants and transfusions)

• c. the body's own cells that the body fails too recognize a as "normal self“ (cancer cells, infected cells, cells involved autoimmune diseases)..

Classification of antigens According to chemical nature

• 1.Proteins-virtually all

• 2.Polysaccharides ––potentially but not always

• 3.Nucleic acids ––poor antigens

• 4.Lipids-may act as haptens

According to mode of action

• 1. Thymus dependent ––Protein antigens

• 2. Thymus independent --Polysaccharides

According to epitope

• 1.Unideterminant univalent

• 2.Unideterminant multivalent

• 3.multdeterminant multvalent

According to Source

• 1.Exogenous

• 2.Endogenous

Types of Antigens

T-independent antigens• ––Complex carbohydrates (generally polysaccharides )• –Do not require processing • ––Can directly interact with B cells • ––No memory

– E.g. Pneumococcal polysaccharide, lipopolysaccharide, Flagella

T-dependent antigens ––Require macrophages or other APC ––Require T—helper cells ––Require major histocompatibility––Mostly proteinse.g. Microbial proteins

Exogenous: • External antigens e.g. bacterial infection Endogenous:• Typically peptides derived from any protein; e.g. viral infections-infected cell

Superantigens

• Superantigens (SAg) are microbial proteins that can bind to T cells and then activate T cell.

• The activation of T cells is non-specific polyclonal.

• Bind Vβ of TCR to α of MHC II.

2 Types:

1. Endogenous SAg e.g. virally encoded membrane proteins infected mammalian cells (integrated viruses – retroviruses, EBV)

2. Exogenous SAg - exotoxins produced by bacteria such as Staphylococcus eneterotoxin

Actions of superantigens1. This activation of very large numbers of T4—lymphocytes results in the secretion of

excessive amounts of a cytokine called interleukin-2(IL-2) as well as the activation of self-reactive T--lymphocytes

2. Production of high level of IL—2 can result in circulation of IL--2 in the blood leading to symptoms such as fever, nausea, vomiting, diarrhea, and malaise.

3. Excess stimulation of IL-2 secretion can also lead to production of other cytokines such as

tumor necrosis factor-alpha (TNF-alpha),

interleukin-1(IL-1),

inflammatory chemokines such as IL—8

platelet -activating factor (PAF)

Can lead to the same endothelial damage ,acute respiratory distress syndrome, disseminated intravascular coagulation, shock, and multiple organ system failure seen with Endotoxin..

4. Activation of self- reactiveT- lymphocytes can also lead too autoimmune attack.

Examples of superantigens

1. Toxic shock syndrome toxin-1 (TSST-1 )

2. Streptococcal pyrogenic exotoxin (Spe), produced by rare invasive strains and scarlet fever strains of Streptococcus pyogenes (group A beta streptococci).

3. Staphylococcal enterotoxins (SE) ,

4. Superantigens associated with Streptococcus pyogenes are also thought to be responsible for psoriasis.

5. Antigens associated with Mycobacterium tuberculosis, rabies virus, and possibly HIV may also function as superantigens

The basis of immunogenicity

1. Foreignness

2. Size

3. Chemical Composition

4. Physical Form: Particulate > Soluble, Denatured > Native

5. Degradability

6. Genetics

Genotype

– Species

– Individual

• Responders vs Non-responders

7. Age

8. Dose of antigen

9. Route of administration: Subcutaneous, Intravenous

10. Adjuvants:

Degree of Foreignness

• The body must be able to distinguish “self” from “non-self”

• The greater the phylogenetic distance between two organisms, the greater the structural differences, hence foreignness (e.g. chicks vs goats)

• Some macromolecules show conservancy of structure across phyla (e.g., collagen and cytochrome C)

• Other macromolecules “outside” an organism’s system can be immunogenic! (e.g., cornea and sperm cells)

Molecular size

• Most immunogens are 100,000 daltons (Da)

• Most molecules < 5-10,000 are poor ones

Chemical structure/heterogeneity

• All four levels of protein structure contribute to structural complexity…1°, 2°, 3°, 4°

• Lipids can induce immune response if presented properly– Lipids are typically ‘haptens’ carried by proteins (Ab’s are

produced through the lipid portion)

– Ab’s can form through steroids & fatty acid derivatives

– Several clinical assays use Ab’s to check for these substances• Ab’s through leukotrienes for evaluation of asthma

• Ab’s through steroids -> to measure amts in patient’s circulation

• TCR recognise lipid Ag associate with CD1(resembles MHC I)

– T cells recognize through lipids of Mycobacterium

Ability to be processed/presented

• Development of both Humoral and Cell-mediatedImmune response requires T cell recognition of processed/ presented Ag

• large, insoluble macromolecules and polymers are better immunogens than small and soluble

• Those molecules resistant to enzyme degradation (esp. D-amino acids) are poor immunogens

The bio system contributes to immunogenicity

1) Genotype of recipient– genetic makeup of person is important

-there is a strong genetic link to immune response

-e.g., MHC gene products, genes encoding B/T receptors

2) Dosage and route of Ag adminisration – experementallyevidence indicates a dose-response curve to every immunogen

-insufficient doses nonresponse or tolerance

-single doses insignificant response (excessive too!)

-“booster” shots are required for many immunizations

-route affects which immune organ/cells involved…

Adjuvants (L. adjuvare = “to help”)

• Substance which, when added to Ag, enhances its immunogenicity; used for immunizations

how this works is not entirely understood, but they appear to help by:

• Persistence of Ag in tissue

• Enhancement of co-stimulatory triggers (B7 molecules)

• Increased local inflammation

• Nonspecific increase of lymphocytes

Cont’d• It is important to distinguish between a carrier for a hapten and

an adjuvant.

• A hapten will become immunogenic when conjugated covalently to a carrier.

• It will not become immunogenic if mixed with an adjuvant. Thus, an adjuvant enhances the immune response to immunogens but does not confer immunogenicity to haptens.

• The most widely used adjuvant in humans is alum precipitate, a suspension of aluminum hydroxide on which the antigen is absorbed.

• This adjuvant causes aggregation of a soluble antigen and allows continuous slow release of antigen.

• In addition, it has a slight irritant effect that enhances the ingestion and processing of an antigen by macrophages which present the antigen to T cells, leading to T-cell activation.

Humoral Immune Response to Antigen

Humoral Immune Response to Antigen

• First exposure to antigen "A”:

– begin to make low levels of antibody in about a week

• Second exposure to antigen "A”:

– produces a much faster response, and

– several orders of magnitude higher levels of antibody.

– Ability of antibody to bind antigen also increases dramatically in the secondary response.

• Injecting a new antigen "B” with "A"

– Elicits only a primary response

– Shows that a memory or prior exposure is required for the accelerated response.

Humoral or B-Cell Mediated Immune Response

Produces secreted antibodies (proteins)• Bind to antigens and identify the antigen complex

for destruction. • Antibodies act on antigens in the serum and

lymph • B-cell produced antibodies may be

– attached to B-cell membranes or – Free in the serum and lymph.

• Each B lymphocyte makes a unique antibody molecule (immunoglobulin or Ig)

• Over a million different B lymphocytes are produced in each individual– So, each individual can recognize more than a

million different antigens

Immunity and the Immune Response System

Cell-Mediated Immunity and T Cells

• T cell receptors are cell surface receptors that bind nonself substances on the surface of other cells

• Major histocompatibility complex (MHC) proteins protrude from the surfaces of most cells in mammals

– They help to distinguish self from nonself

– They coordinate interactions among lymphocytes and macrophages

• Cytokines are soluble signal proteins released by T cells

– They bind and alter the behavior of their target cells

Cell Mediated Immune System: T lymphocytes• T-cells mature in the thymus (thus the name T-cell)

• Act on antigens appearing on the surface of individual cells.

• Over a million different kinds of T-cells

– Each produces a different receptor in the cell membrane

– Each receptor is composed of 1 molecule each of two different proteins

– Each receptor binds a specific antigen

– Receptor only recognizes antigens which are "presented" to it within another membrane protein of the MHC type

• Recognizes specific antigens bound to the antigen-presenting structures on the surface of the presenting cell.

• Recognizes antigens presented by B-cells, macrophages, or any other cell type

T Cells and their Functions

• Have a specific receptor for a fragment of antigen

• Cytotoxic T-cells:

– Contain a surface protein called CD8

– Destroy pathogen infected cells, cancer cells, and foreign cells (transplanted organs)

• Helper T-cells:

– Contain a surface protein called CD4

– Regulate both cellular and humoral immune systems

– This regulation reduces autoimmunity.

Autoimmune disease

• Self immunity

• Some examples:

– rheumatic fever

– rheumatoid arthritis

– ulcerative colitis

– myasthenia gravis

– Lyme disease (microbial etiology)

– Guillan-Barre syndrome (microbial etiology)

– Reiter’s syndrome or reactive arthritis (microbial etiology)

– Insulin dependent diabetes mellitus (IDDM) (microbial etiology?)

Interactions of the Components of The Immune Response

• T-cells, B-cells, and macrophages use MHC-II receptors for presentation;

• All other cells use MCH-I

– (responsible for most of tissue graft rejection)

• When a T-cell is presented with an antigen:

– its receptor binds to the antigen and

– it is stimulated to divide and produce helper T-cells

• activate B-cells with bound antigen

• suppressor T-cells

– regulate the overall response

• Cytotoxic "killer" T-cells

– kill cells with antigen bound in MHC-I

Role of Immunity in InfectionsLocalized Infections

• Immunity to infection is usually short-term and transient– Mucosal (secretory or IgA) immunity in the gut or respiratory

tract wanes over time• Proof of concept: live, oral rotavirus vaccine:

– immunity declines over time and reinfection with “wild” type rotaviruses occurs

• Repeated localized (e.g., gastrointestinal) re-infection is possible. Examples:– Viruses: rotaviruses, noroviruses, adenoviruses and some

enteroviruses.– Salmonella spp, Shigella spp., Campylobacter spp, and E. coli

spp. cause localized infections– Giardia lamblia and Cryptosporidium parvum

Role of Immunity in Infections:Generalized/Systemic/Disseminated Infections

• Immunity against generalized/systemic/disseminated infection is usually lifelong, unless immune system is severely compromised

• Hepatitis A and E and many enteroviruses are viruses causing systemic/generalized/disseminated infections

• Salmonella typhi is a bacterium causing systemic infection

• Typically, immunity against severe illness is long-term and probably lifelong

– Proof of concept: live, oral poliovirus vaccine and poliomyelitis eradication; susceptibles are newborns and infants

• Antigenic changes in microbes may overcome long-term immunity and increase risks of re-infection or illness

Role of Selection of New Microbial Strains in Susceptibility to Infection and Illness

• Antigenic changes in microbes overcome immunity, increasing risks of re-infection or illness– Antigenically different strains of microbes appear and are

selected for over time and space– Constant selection of new strains (by antigenic shift and drift)– Partly driven by “herd” immunity and genetic recombination,

reassortment , bacterial conjugation, bacteriophage infection and point mutations

• Antigenic Shift:– Major change in virus genetic composition by gene substitution

or replacement (e.g., reassortment)• Antigenic Drift:

– Minor changes in virus genetic composition, often by mutation involving specific codons in existing genes (point mutations)

• A single point mutation can greatly alter microbial virulence