Antigens and “foreignness” Antigens (or, more properly, immunogens) have a series of features which confer immunogenicity. One of these features is “foreignness.” So, we can infer that – most often – antigens – ultimately – originate externally. (There are exceptions, of course. Some cells become transformed by disease [e. g., cancer] or by aging. In such instances, the antigens have an internal origin.)

What is the significance of these premises? They indicate that antigens are processed and presented.

There are two presentation systems… Are there two processing systems? CYTOSOLIC ENDOCYTOTICYes: CYTOSOLIC and ENDOCYTOTIC Does this imply that there are different degrees of foreignness? And that there are different cells for different degrees of foreignnness?

Cytosolic: Uses MHC-I Processes ENDOGENOUS proteins (e. g., viral proteins, proteins from bacteria which invade cells, parasites, and normal cellular constituents [!])

Cytosolic PROCESSING : Proteins are marked by adding ubiquitin ( a 76 amino acid polypeptide ) in an ATP dependent manner. Fed into a “proteasome”; a “big ball of degradative enzymes.” Proteasomes of immune system may become specialized by addition of products from LMP loci (embedded in MHC-II “region”.) Phenotype of LMP is production of “Low Molecular- mass Peptides.” LMP’s are polymorphic.

Cytosolic PROCESSING : Proteins are marked by adding ubiquitin (a 76 amino acid poly- peptide) in an ATP dependent manner. Fed into a “proteasome”; a “big ball of degradative enzymes.” Proteasomes of immune system may become specialized by addition of products from LMP loci (embedded in MHC-II “region”.) Phenotype of LMP is production of “Low Molecular-mass Peptides.”

Cytosolic PRESENTATION: The peptides ( produced by proteasomes ) are introduced into the rough endoplasmic reticulum through an ATP dependent process mediated by TAP (transporter associated with antigen processing.) TAP are specified by loci embedded in MHC-II “region.” A chaperone, calnexin, associates with alpha chain; induces conformational change that allows microglobulin to form quaternary complex. Two additional proteins, calreticulin and tapasin, allow association of the MHC-I with the TAP-complex and the loading of processed Ag. The MHC-I  processed Ag is then delivered to golgi (for glycosylation) and then to cell surface.

Cytosolic PRESENTATION: The peptides produced by proteasomes are intro- duced into the rough endoplasmic reticulum through an ATP dependent process mediated by TAP A chaperone, calnexin, associates with alpha chain. The asspciation induces conformational change that allows microglobulin to form quaternary complex. Two additional proteins, calreticulin and tapasin, allow association of the MHC-I with the TAP The MHC-I/processed Ag is then delivered to golgi (for glycosylation) and then to cell surface.

Endocytotic: Uses MHC-II Processes EXOGENOUS proteins (viz., foreign antigens.) Uses APC’s (esp., “professional” antigen presenting cells) How do the APC’s acquire antigens to process?

Endocytotic PROCESSING: The antigen is introduced into cell by phagocytosis or endocytosis (receptor mediated or pinocytotic). The Ag passes through a series of vesicles that have increasing acidity ( numerically decreasing pH ): early endosomes ( pH ) ( aka phagosome ); late endosomes ( pH ) ( aka endolysosome ); and lysosome ( pH ). Lysosomes contain ~140 acid dependent hydrolases including proteases, nucleases, lipases, glycosidases, phospholipases, phosphatases...

Endocytotic PROCESSING: KEY is the presence of the INVARIANT PROTEIN (Ii) synthesized with the alpha and beta chains of MHC-II. The invariant chain exists as a trimer and has attached to it three sets of alpha/beta chains. The invariant chain has a series of signals that contribute to the delivery of the complex from the lumen of the rough endoplasmic reticulum, through the golgi, and through the microbodies ( early endosome, late endosome, lysosome ). In the lysosome, the bulk of the invariant chain is degraded; a portion, CLIP (Class II Invariant chain Peptide) remains in the cleft until displaced by processed antigen.

Endocytotic PROCESSING: INVARIANT PROTEIN (Ii) is synthesized with the alpha and beta chains of MHC-II. The invariant chain exists as a trimer and has attached to it three sets of alpha/beta chains.

Endocytotic PROCESSING: INVARIANT PROTEIN (Ii) with the alpha and beta chains of MHC-II. The invariant chain exists as a trimer and has attached to it three sets of alpha/beta chains. The invariant chain signals the delivery of the complex from the lumen of the rough endoplasmic reticulum, through the golgi, and through the microbodies In the lysosome, the bulk of the invariant chain is degraded; a portion, CLIP (Class II Invariant chain Peptide) remains in the cleft until displaced by processed antigen.

Antigen presenting cells… Dendritic cells: Langerhans cells found in epidermis and mucous membranes. Interstitial dendritic cells which populate most organs. Interdigitating dendritic cells present in T-cell areas of secondary lymphoid tissue and the thymic medulla Circulating dendritic cells… in the blood… and lymph (known as veiled cells.)

Cross-talk… What is the consequence?

But not all immunogens are proteins… There may be specialized systems for presentation of non-peptide antigens. There are non-protein antigens which do not require presentation. Instead they stimulate B-cells directly without T-cell intervention.