Invited lecture: New enzymes producing antigenic peptides presented by HLA class I molecules

We studied a proteasome-independent peptide derived form tumor protein MAGE-A3, and identified insulin-degrading enzyme as the protease producing both the C-terminus and the N-terminus of this peptide. Insulin-degrading enzyme is a cytosolic metallopeptidase not previously known to play a role in the class I processing pathway. The parental protein MAGE-A3 appears to be degraded along two parallel pathways involving either insulin-degrading enzyme or the proteasome, each pathway producing a distinct set of antigenic peptides presented by class I molecules. Using a series of novel antibodies recognizing catalytic subunits of the human proteasome in their native conformation, we also identified proteasomes that are intermediate between the standard proteasome and the immunoproteasome. They contain only one (ß5i) or two (ß1i and ß5i) of the three inducible catalytic subunits of the immunoproteasome. These intermediate proteasomes represent 30-54% of the proteasome content of human liver, colon, small intestine and kidney. They are also present in human tumor cells and dendritic cells. We studied the processing of a series of antigenic peptides by these intermediate proteasomes, and identified two tumor antigens that are processed exclusively either by intermediate proteasomes ß5i or by intermediate proteasomes ß1i-ß5i