Lecture 2ANTIGENS AND THEIR

PROCESSING2013/2014

ANTIGENS

1. The structure of antigens2. Antigen processing and presentation3. Peptide-MHC molecule: structure and

assembly4. Antigen recognition by B and T cells5. Superantigens 6. Stress proteins (heat shock proteins)

TYPES OF ANTIGENS INVOLVED IN PATHOGENESIS OF DISEASES

Microbial antigens (bacterial, viral, fungal, parasitic ones)

Blood group antigens Transplantation alloantigens (MHC and minor

ones) Allergens Autoantigens (organ and/or tissue specific) Tumor antigens (tumor specific and tum.

associated) Superantigens: eg.SEB-staphylococcal

enterotoxin B Heat shock proteins

ANTIGENS FOR B CELLS

• Antigens contain epitopes that can bind to the antigen-binding sites of antibodies

• Antigens (haptens) may have almost any chemical nature

• Antigens on native proteins are usually discontinous segments of aminoacids at cell surface

• Antigens as immunogens must contain carrier epitopes for activating helper T cells

Antigen presenting cells (APC)

• Professional APC: dendritic cells – the only ones which can stimulate naive T cells,

• Activated macrophages,• B lymphocytes – ingest protein antigens

and display them to helper T cells• All nucleated cells can function as APC,

if possess foreign, usually microbial antigens in the cytoplasm following infection

ANTIGEN PROCESSING AND PRESENTATION FOR CLASS I MHC

• Peptides presented on most cell types are synthetized endogenously,

• Peptides presented by professional APC can be acquired through endocytosis,

• Peptides are processed by proteasomes and enter endoplasmic reticulum through TAP transporter,

• Peptides are bound at both termini within the binding cleft,

• Antigens presented by class I MHC molecules are recognized by CD8 T cells

ANTIGEN PROCESSING AND PRESENTATION FOR CLASS II MHC

• Only professional APC (dendritic cells, macrophages and B cells) express class II MHC constitutively

• Peptides presented by professional APC are acquired mostly by receptor-mediated endocytosis, or pinocytosis

• Peptides presented by class II molecules often have terminal extensions

• Antigen presented by class II MHC molecules are recognized by CD4 T cells

Cross-presentation (cross-priming)

• If infected cells and their viral antigens are ingested by APCs and broken down in APC cytoplasm,

• If APC itself is infected

• APC then acts as any nucleated cell, using proteasome pathway and presents peptides via MHC class I to CD8+ CTL

• The same APC may display peptides via MHC class II to CD4+ T helper cells

Why and what for is the cross-presentation?

• About 25 % of class I molecules present antigens of exogenous origin,

• Up to 20% of MHC class II molecules present peptides derived from either cytoplasmic or nuclear antigens,

• Naive cytotoxic T cells require dendritic cells for their activation,but most viruses are not tropic for DC and thus are not usually present in the cytosol of APC.

• This is solved by sneaking out of the vacuole containing ingested external antigens to the cytosol ,

• Similarly, proteasome-derived peptides are taken up by so-called autophagosomes, by the mechanism of autophagy. The fusion with MHC class containing MIIC, where proteolytic cleavage of any intact proteins may also take place.

MAJOR HISTOCOMPATIBILITY (MHC) ANTIGENS

• Histocompatibility antigens are cell surface expressed on all cells (class I) and on APC, B cells, monocytes/macrophages (class II)

• Their physiologic function is to display peptides derived from protein antigens to antigen-specific T lymphocytes

MAJOR HISTOCOMPATIBILITY (MHC) ANTIGENS (2)

• They are targets for graft rejection• They are inherited from both parents as

MHC haplotypes and are co-dominantly expressed

• They exist in multiple alleles (variants) distinct in particular individuals

Features of peptide binding to MHC molecules

• Each MHC molecule displays one peptide – self if not infected

• Many different peptides can bind to the same MHC molecules

• Peptides have very slow off-rate

• Stable expression requires bound peptide

• MHC molecules bind only peptides

Nonclassical MHC molecules• They include HLA-E, HLA-F, HLA-G, MICA,

MICB, HFE• They may be precursors to classical MHC ones• They are expressed on various cells but most

often in gastro-intestical tract• Some have well- defined function: HLA-G

expressed on placental-maternal trophoblast interface, protect conceptus from destruction by NK cells. The latter possess ILT2 inhibitory receptor that recognize HLA-G.

The family of CD1 non-MHC molecules• Encoded by a set of genes on chromosome 1• CD1 is also involved in the presentation of Ag

to T cells,l but it Ag-binding groove contains mainly hydrophobic aminoacids and its entrance is narrow

• CD1 molecules present lipids or glycolipids but not proteins

• There are four CD1 molecules on human cells – CD1a, b, c (on cortical thymocytes, dendritic cells) CD1d – on G-I tract, hepatocytes, lymphoid and myeloid cells.

ABO BLOOD GROUP ANTIGENS• ABO locus – encodes a glycosyl transferase and

has three alleles: A, B (alloenzymes), O – functionally silent

• O-individuals make anti-A and anti-B antibodies due to exposure to common bacteria

• O-individuals are universal donors because their anti-A, B Abs bind to so many different cells in the recipient that they are effectively diluted

• A and B-individuals may not donate blood to an O recip. because their erythrocytes will be lysed by

anti-A and anti- B Ab

PROPERTIES OF SUPERANTIGENS

• Presented and recognized as an unprocessed, native protein

• Contact TCR and MHC molecules in less variable regions (V) outside the traditional antigen – binding groove,

• TCR recognition is not MHC restricted

PROPERTIES OF SUPERANTIGENS - 2

• Stimulate both CD4 and CD8 T cells binding MHC class II or class I, in atypical site, without MHC restriction

• Stimulated T cells proliferate, secreted cytokines, later become anergic and die

• Many superantigens are microbial toxins (eg. TSST-toxic shock syndrome toxin)

B-CELL SUPERANTIGENS (SAgs)

• spA (staphylococcal protein A) contains five Ig-binding domains

• Two or more of them can bind to VH3 Fabs

• PFV – sialoprotein from human liver and gut has at least six Fab-binding sites

• HIV-1 gp 120 also reacts with VH3 IgsThe latter appear the most frequent ligand for SAgs

SUPERANTIGENS ARE PRODUCED BY:

• Bacteria: S. Aureus. S.pyogenes, Mycobacterium tuberculosis, Yersinia pseudotuberculosa

• Mycoplasma arthritidis

• Viruses: Herpes ssp., EBV, HIV,

• Parasites: Toxoplasma gondi

• Some plants

SUPERANTIGENS ARE ETIOLOGIC AND/OR PATHOGENIC AGENTS IN THE FOLLOWING:

• Staphylococcal food poisoning

• Staphylococcal toxic shock syndrome

• Staphylococcal scalded skin syndrome

• Streptococcal toxic shock syndrome

• Scarlet fever rash

• Guttate psoriasis (probable)

HEAT SHOCK PROTEINS (STRESS PROTEINS)

• Found in all prokaryotic and eukaryotic cells

• Are essential in the assembly, folding and transport of other molecules (chaperone function)

• Cells exposed to various stresses express higher levels of these proteins

HEAT SHOCK PROTEINS (STRESS PROTEINS) - 2

• Their aminoacid sequences are highly conserved, so bacterial heat shock proteins

• Paradoxically, they seem to be target antigens in the protective response against many infectious organisms

• Antibodies against HSP are found in several diseases, such as rheumatoid arthritis

Thank you for your attention!