Efficient MHC class I-independent amino-terminal trimming of epitope precursor peptides in the endoplasmic reticulum

MHC class I ligands are produced mainly by proteasomal proteolysis, in conjunction with an unknown extent of trimming
by peptidases. Trimming of precursor peptides in the endoplasmic reticulum, a process postulated to be class I dependent,
may substantially enhance the efficiency of antigen presentation. However, monitoring of luminal peptide processing has not
so far been possible. Here we show that several precursor peptides with amino-terminal extensions are rapidly converted to
HLA-A2 ligands by one or several highly efficient metallo-peptidases found on the outer surface of, but also within,
microsomes. Surprisingly, luminal trimming is fully active in HLA class I- or TAP-deficient microsomes and precedes
peptide association with HLA class I molecules. Trimmed peptides are rapidly depleted from, and become undetectable in,
microsomes lacking the restricting class I molecules.