NATURAL PROCESSING SITES FOR HUMAN CATHEPSIN-E AND CATHEPSIN-D IN TETANUS TOXIN - IMPLICATIONS FOR T-CELL EPITOPE GENERATION

Cathepsin E is an aspartic proteinase that has been implicated frequen tly in Ag processing for presentation on class II MHC molecules, but n o information exists on its cleavage specificity within Ags in relatio n to known T cell epitopes, We have analyzed the processing by catheps in E of a large C-terminal domain of tetanus toxin (residues 872-1315) , and we have compared the processing products with those liberated by cathepsin D, a related aspartic proteinase also thought to be involve d in class II MHC-restricted Ag processing, Processing products were a nalyzed by N-terminal Edman degradation and mass spectrometry followin g reverse-phase HPLC separation of peptides, A total of 28 cleavage si tes was identified, 11 of which were recognized by both cathepsins E a nd D. Most, although not all, sites were between pairs of hydrophobic residues and were located within the 200-amino-acid C terminal region known to contain several human T cell epitopes, Previously described T cell epitopes, for example, between residues 1273 and 1284, were flan ked by cathepsin E and D cleavage sites, These data are consistent wit h an important role for cathepsins E and/or D in Ag processing in the human immune system.