Control of antigen presentation by a single protease cleavage site

Protein antigens require limited proteolytic processing to generate peptide s for binding to class II MHC molecules, but the proteases and processing s ites involved are largely unknown. Here we analyze the effect of eliminatin g the th ree major asparagine endopeptidase (AEP)-processing sites in the m icrobial antigen tetanus toxin C fragment. The mutant antigen is highly res istant to proteolysis by AEP and crude lysosomal extracts and is dramatical ly impaired in its ability to be processed and presented to T cells. Remark ably, processing at a single asparagine residue (1219) is obligatory for op timal presentation of many T cell epitopes in this antigen. These studies d emonstrate that cleavage at a single processing site can be crucial for eff ective antigen presentation.