Fig. 5: Binding of Peptide Epitopes from Endogenous Antigens to MHC-I Molecules
by a Dendritic Cell

Endogenous antigens are those located within the cytosol of the cells of the body. Examples include:

a. Viral proteins produced during viral replication,
b. Proteins produced by intracellular bacteria such as Rickettsias and Chlamydias during their replication,
c. Proteins that have escaped into the cytosol from the phagosome of phagocytes such as antigen-presenting cells
d. Tumor antigens produced by cancer cells, and
e. Self peptides from human cell proteins.

Dendritic cells bind epitopes from endogenous antigens to MHC-I molecules and present them to naive T8-lymphocytes in order to activate these naive T8-lymphocytes.

1. Antigens are engulfed by dendritic cells and placed in a phagosome. Some of the proteins escape from the phagosome into the cytosol of the dendritic cell where they become endogenous antigens.
2. These endogenous antigens pass through proteasomes where they are degraded into a series of peptides.

3. The peptides are transported into the rough endoplasmic reticulum (ER) by a transporter protein called TAP.
4. The peptides then bind to the grooves of newly synthesized MHC-I molecules.
5. The endoplasmic reticulum transports the MHC-I molecules with bound peptides to the Golgi complex.
6. The Golgi complex, in turn, transports the MHC-I/peptide complexes by way of an exocytic vesicle to the cytoplasmic membrane where they become anchored. Here, the peptide and MHC-I/peptide complexes can be recognized by naive T8-lymphocytes by way of TCRs and CD8 molecules having a complementary shape.

(Through the process of cross-presentation, some antigen-presenting dendritic cells can cross-present epitopes of exogenous antigens to MHC-I molecules for eventual presentation to naive T8-lymphocytes.)

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