The T cell arm of the immune system of higher vertebrates is specific
for antigenic peptides bound to cell surface major histocompatibility
complex (MHC) molecules. These peptides are derived from two distinct
pathways of antigen processing. The class I, or endogenous pathway, ut
ilizes proteasomes and the ubiquitin system for protein degradation, w
ith subsequent transport of the resulting peptides into the lumen of t
he endoplasmic reticulum by a specific peptide transporter, called TAP
. The expression of distinct proteasome subsets is regulated by the cy
tokine gamma interferon (IFN-gamma). The class II, or exogenous pathwa
y, utilizes the endosomal and lysosomal pathways for protein degradati
on, and a number of immune-specific accessory molecules including the
class-II associated Invariant chain (I-i) and MHC-encoded HLA-DM (H2-D
M in mouse) molecules.