Proteases in MHC class I presentation and cross-presentation.

Cells that have mutated their genes or are virally infected are a potential threat to a host. Consequently, the immune system has evolved mechanisms for CD8 T lymphocytes to identify such cells and eliminate them. The generation of CD8 T cell responses occurs in two phases, both of which critically involve the process of Ag presentation. In the first phase, sentinel cells gather Ags present in tissues and then present them to naive CD8 T cells in ways that stimulate their maturation into effectors. In the second phase, these effector cells seek out and eliminate the pathological cells. The abnormal cells are identified through their presentation of immunogenic Ags that they are producing. The Ag presentation mechanisms used by the sentinel cells can be different from those in other cells. This article will review these mechanisms with a focus in each case on how antigenic peptides are generated for presentation.