ENDOSOMAL PROTEASES AND ANTIGEN-PROCESSING

T cells are activated by fragments of antigenic proteins bound to majo r histocompatibility compatibility complex (MHC) molecules and display ed on the cell surface. MHC class II proteins scavenge processed prote in antigens from within endosomal compartments. The antigenic peptides are generated within these and other intracellular compartments using the array of proteolytic enzymes normally involved in terminal protei n degradation. Antigen-presenting cells use different mechanisms to ex ploit and control the activity of these enzymes so as to ensure the ge neration of a wide variety of peptides, while preventing the destructi on of antigenic epitopes by excessive proteolysis.