DEFINITION AND TYPES OF ANTIGENS Any chemical structure Soluble or corpuscle Simple or complex

1. DEFINITION AND TYPES OF ANTIGENS Any chemical structure Soluble or corpuscle Simple or complex Originated from the body or comes from outside Genetically self or non-self Natural or artificial… …that binds specifically to an immunoglobulin or that can be presented on MHC complex molecules to TCR

2. Antigenicity– capability of an antigen to bind specifically with components of the adaptive immune system; TCR or BCR/antibody. Immunogenicity– the capability of an antigen to induce an adaptive immune response. Tolerogenicity– the capability to induce immunological tolerance;specific immune non-responsiveness.

3. COMPLEX ANTIGENS CONSIST OF THE CARRIER AND MULTIPLE ANTIGENIC DETERMINANTS (EPITOPES) Antigenic determinants - those parts of the antigen, which are recognized by a defined immunoglobulin (B-cell receptor or antibody) orby T-cell receptor Carrier -not directly involved in the binding of immunoglobulin or T- cell receptor These terns can only be used to describe the interaction of particular antigenic determinant and single immunoglobulin or T-cell receptor

4. Antigenic determinant / Epitope The part of the antigenic molecule that is recognized by a defined immunoglobulin (B cell receptor or antibody) or by a T cell receptor.

5. Factors influencing immunogenicity • Genetics • Species • Individual • Responders vs.non-responders • Foreignness • Size • Age • Dose • Route • subcutaneous > intravenous > oral > intranasal • Adjuvant • substances that enhance an immune response to an antigen • (aluminium salts, LPS, Freund’s adjuvant, TLR ligands) COMPLEX EFFECTS depot effect – prolonged antigen uptake by APCs Activation of innate immunity

6. Factors influencing immunogenicity II. • Physical form • - particle (cell, colloid)or • soluble • - denaturated or native • Possibility of further degradation and processing • - antigenpresentation by APC

7. Factors influencing immunogenicity III. Chemical structure: Proteins: proteins, lipoproteins, glycoproteins. Usually they are highly immunogenic antigens. Polysaccharids: lipopolysaccharids, polysaccharids are immunogenic. B cells are able to recognise them, but B cells do not process or present any polysaccharids to T cells. Nucleic acids: have a very weak immunogenicity. However, simple strand DNA or protein+nucleic acid can be very immunogenic! Lipids: usually are not immunogenic, but can be haptens

8. The nature of antigenic determinants

9. B cell epitope T cell epitope Recognized by T cells proteins mainly (8-23 amino acids) Requires processing by APC Recognized by B cells proteins polysaccharides Lipids DNA Steroids etc. (many artificial molecules) cell- or matrix associated vs. solute

10. B cell activation with the help of T cells MHC II + peptide cytokines A polysaccharids are not involved in antigen presentation!

11. B-cell activation without the help of T-cells T-INDEPENDENT ANTIGEN TI-1 T-INDEPENDENT ANTIGEN TI-2 B-Cell Simultaneous activation of BCR and other receptors on B cells (i.e. LPS binding protein /CD14/) induces the B cells to proliferate and differentiate Strong crosslinking of BCRby repetitive polysaccharide or protein epitopes

12. T-independent B cell activation

13. Ig (antibody) BCR (mIg)

14. Complex macromolecules usually contain several epitopes

15. ! The antibodies that bind to exposed and accessible molecules on the surface of a pathogen are generally more effective. ! The binding of an antigen to an antibody is based solely on non-covalent forces: Van der Waals forces and hydrophobic interactions accompanied by electrostatic forces (salt bridges) and hydrogen bonds between the chemical group on the antigen and particular amino acid residues on the antibody. ANTIGEN RECOGNITION = CELL ACTIVATION

16. Superantigen Conventional Antigen Polyclonal T cell response 1:4 - 1:10 Superantigens • Proteins which can bind to several T-cells with adequate TCR and activate them. • Definition Monoclonal/Oligoclonal T cell response 1:104 - 1:105 (107/1011 activated T cell 1010 / 1011)

17. SUPERANTIGENS Simultaneous binding toTCR and MHC complex molecules mimicking a specific antigen (without a peptide being actually presented). These sites are shared by many TCRs causing their activation and differentiation, resulting in an excessive polyclonal response that can involve 2-20% of the CD4 T cells! Consequently,  massive cytokine production and release IL-1, IL-2, TNF-α  SYSTEMIC SHOCK + a useful adaptive immune response is suppressed!

18. Superantigens • Examples • Staphylococcal enterotoxins • Staphylococcal toxic shock toxin • Staphylococcal exfoliating toxin • Streptococcal pyrogenic exotoxins In patients undergoing immune suppressive treatment, suffering from staphylococcal food toxicity, or in patients with severe burn injuries, the S. aureus can be life-threatinng, causing toxic shock and Disseminated Intravascular Coagulation (DIC). Therefore, it is vey important to know the local and actual resistency profile! (The early and successful empirical antibiotic treatment together with correct resistency profiling can be life-saver for these patients!

19. HAPTEN Small chemical structures cannot induce B-cell response on their own - Hapten (i.e. DNP) 1. + Carrier + Hapten 2. + Hapten primed

20. Antibody response generated against a hapten-carrier conjugate Carrier + hapten Antibodies Carrier specific Hapten specific Carrier +Hapten specific

21. Drugs as haptens The immune responses induced by drugs are dose independent responses. Drug induced hemolytic anaemia • The drug is a hapten, and attached to the red blood cells, it acts as full antigen (pl. high dose penicillin, cephalosporins, tetracyclin – Pay attention! The RBC alone is not an antigen!) Drug induced thrombocytopenia • Chinin type: the drug is hapten, or the membrane components of the thrombocytes (usually GPIIb/IIIa) change their structure, and become neoantigen. • Heparin induced thrombocytopenia: heparin alone, or with thrombocyte factor 4 (Pf4) can induce immune respone.

22. Antigenic determinants recognized by the innate immune system • PAMP - PRR (the basis of the immune response) • Components in the microbes’ cell wall - complement system - opsonisation, complement activation • Mannose containing-polysaccharids - mannose-binding proteins –opsonisation, complement activation • Polyanions - scavenger receptors – Phagocytosis • Lipoproteins, cell wall components (Gram + bacteria) - TLR-2 - Macrophag activation, secretion of inflammatory cytokines • dsRNS - TLR-3 – antiviral interferon secretion • LPS (Gram - bacteria) - TLR-4 - Macrophag activation, secretion of inflammatory cytokines • Flagellin (bacterium) - TLR-5 - Macrophag activation, secretion of inflammatory cytokines • simple strand viral RNA - TLR-7 – antiviral interferon secretion • CpG containing DNA - TLR-9 - Macrophag activation, secretion of inflammatory cytokines

23. Example (eg. Prevenar – pneumococcus vaccine) In case of immunisation against bacteria it is a problem that purified bacterial polysaccharide components of the vaccine cannot activate T cells, consequently, there will be no memory B cell response. Therefore in several polysaccharide vaccines, polysaccharide chains are conjugated to proteins to induce T cell dependent immune response. A protein used for this purpose is called CRM197  modified diphteria toxin (toxoid): one amino acid of the original protein is changed (Glu  Gly) thus the new protein is not toxic (toxoid). (Although it keeps its immunogenicity and induces a neutralizing antitoxin antibody response.) Glu  Gly toxin toxoid toxoid toxoid polysaccharides

24. B cells after the recognition of polysaccharide epitopes are able to present peptides derived from the carrier protein to the T cells Peptides from the toxoid toxoid polysaccharide MHCII TCR BCR T cell B cell cytokines (SLE: bakterial DNA-protein complexes  anti-DNA antibodies)