bacterial antigenic structure

8/2/2019 Bacterial Antigenic Structure

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Antigens & Immunogens


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Immunogen

* A foreign substance, when introducedinto

host body, stimulate formation of

specificantibodies or sensitized lymphocytes

Antigen

* Antigens have the ability to combinespecifically with antibodies produced

or

sensitized T-lymphocytes induced


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Haptens- Low molecular weight substances

- These substances not immunogenic by itself

- If couple to a larger carrier molecule (albumin,globulins), they become immunogenic

- Examples :

simple chemicals and drugs:

penicillin, sulphonamid, aspirin, cosmetic,

tranquillizers, neomycin skin ointment


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Epitope or antigenic determinant

Small part of an antigen that interacts with anantibody

Any given antigen may have several epitopes.

Each epitope is recognized by a differentantibody or TCR


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Factors influencingImmunogenicty

1-Foreigness :Foreign substances are immunogenic

2- Molecular size:

High molecular weight increaseimmunogenicty

3- Chemical structure complexity:

High complexity increase immunogenicty

` Protein - most effectivePolysaccharide simple , complex

Lipids - poor

Nucleic Acids - poor

4- Route of administration:


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Factors influencingImmunogenicty5- Method of administration:

a- Antigen dose:

Appropriate dose optimum

antigenictyLow dose low- zone tolerance

High dose high-zone tolerance

b- Adjuvant:

Substance when injected with an antigen

enhance immunogenicty


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YYY

T dependent Ag - T cell help to B cells

B

Signal 1

antigen & antigen

receptor

Th

1. T cell antigen receptor

2. Co-receptor (CD4)

3.CD40 Ligand

Th

Signal 2 - T cell help IL-2 and other cytokines


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T-independentantigenspolysaccharides and proteins with many

identical polypeptides

Many bacterial capsules & bacterial flagellarproteins

This stimulates the B cell to generate antibody-secreting plasma cells without the help of IL-2.

While this response is an important defense againstmany bacteria, it generates a weaker response thanT-dependent antigens and generates no memory

cells.


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Thymus independent antigens

Principal example:

LPS


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Principal example(s):

Bacterial capsules

Bacterial flagella

Mechanism:

extensive cross-linking of mIg


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T

Dependent

Antigens

TI-1

Antigens

TI-2

Antigens

Induce responses in athymics No Yes Yes

Polyclonally activate B cells No Yes No

Require repeating epitopes No No Yes

Affinity maturation Yes No No

Isotype switching Yes No Limited

Immunological memory Yes No No

T Dependent & Independent Antigens


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Innate Immunity

PAMPs: Pathogen Associated Molecular PatternsPRRs: Pattern Recognition Receptors


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Y

Inflammatoryand effector

cytokines

PRR

PAMP

Innate Immune cells

Direct Bactericidal ActivityPhagocytosisOxygen burstAnti-microbial peptides


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Bacterial Structures Flagella

Pili Capsule

Plasma Membrane

Cytoplasm Cell Wall

Lipopolysaccharides

Teichoic Acids

Inclusions

Spores

Chapter 4


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Macromolecule Primary Subunits Where found incell

Proteins amino acids Flagella, pili, cellwalls, cytoplasmicmembranes,ribosomes,cytoplasm

Polysaccharides sugars(carbohydrates)

capsules, inclusions(storage), cell walls

Phospholipids fatty acids membranes

Nucleic Acids(DNA/RNA)

nucleotides DNA: nucleoid(chromosome),

plasmidsrRNA: ribosomes;mRNA, tRNA:cytoplasm


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CytoplasmBacterial chromosome (DNA)

Ribosomes

various sorts ofinclusionsBacterial CpG DNA: (PuPuCpGPyPy) ISS-DNA

unmethylated, present in expected 1/16frequency,

CpG in vertebratesmethylated, frequency at around 1/60

TLR 9 Potent Immune response Macrophage activation

Cytoplasmic membrane Lipid rich


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Cell wall & its associated

structuresCapsule

Pili and FimbriaeFlagella


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Capsule

Polysaccharide

Except B. anthracis protein

polymer of D-glutamic acidPhagocytosis protection

Anticapsular antibodies

protect animal

K antigens


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Fimbriae and PiliFimbriae attachment & Pili conjugationF antigens

Short protein appendages

smaller than flagellaAdhere bacteria to surfaces

E. coli has numerous types K88, K99, F41, etc.

Antibodies to will block adheranceF-pilus; used in conjugation

Exchange of genetic information


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Flagella

Flagellin

H antigen

MotilityTLR 5


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Chapter 4

Cell WallPeptido-glycan Polymer (amino acids +

sugars)

Unique to bacteria

Sugars; NAG & NAMN-acetylglucosamine

N-acetymuramic acid

D form of Amino acids used

Hard to break down D form

Amino acids cross link NAG & NAM


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Teichoic AcidsGram + only

Glycerol, Phosphates, & Ribitol

Attachment for PhagesLiptoteichoic acid ion transport


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E d f


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Endotoxin ofGram-negatives

cytopl.mem.

peptidoglycan

outer mem.

Gram-negative

cell

Lipid A Corepolysaccharide

O sidechain

The toxic partHelps solubilise Lipid A Somatic antigen

Lipopolysaccharide(LPS)


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Lipopolysaccharide (LPS)G- bacteria onlyLipid A

Polysaccharide

O Antigen somatic antigenAppearance of Colonies

Mucoid = Smooth (lots of LPS or capsule)

Dry = Rough (little LPS or capsule)

O Ags differ in Sugars, not Lipid A

Configuration of Glycosidic linkage Oserotypes


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Proteins of Outer

membraneLipoproteinOuter membrane proteins

Outer membrane phospholipids


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LipopeptidesPeptidoglycan

Lipoteichoic Acid

Unmethylated DNA Oligomers


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a- Antigens related to bacterial cellsSomatic antigen (O) - cell wall Gm ve bacter.Capsular antigen (K) - usually polysaccharide

Flagellar Ag (H) - flagellinFimbrial Ag (F) Fimbrial protein

b- Antigen secreted by bacteria: - Exotoxins

- Enzymes


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Other antigensPorins

Heat shock proteins

Mycobacteria Trehalose dimycolate (cordfactor)

M protein ofStreptococci

Protein A Staph. Aureus Fc portion of IgG


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bacterial antigenic structure

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