Tripeptidyl Peptidase II Is the Major Peptidase Needed to Trim Long Antigenic Precursors, but Is Not Required for Most MHC Class I Antigen Presentation

Abstract

Recent reports concluded that tripeptidyl peptidase (TPPII) is essential for MHC class I Ag presentation and that the proteasome in vivo mainly releases peptides 16 residues or longer that require processing by TPPII. However, we find that eliminating TPPII from human cells using small interfering RNA did not decrease the overall supply of peptides to MHC class I molecules and reduced only modestly the presentation of SIINFEKL from OVA, while treatment with proteasome inhibitors reduced these processes dramatically. Purified TPPII digests peptides from 6 to 30 residues long at similar rates, but eliminating TPPII in cells reduced the processing of long antigenic precursors (14–17 residues) more than short ones (9–12 residues). Therefore, TPPII appears to be the major peptidase capable of processing proteasome products longer than 14 residues. However, proteasomes in vivo (like purified proteasomes) release relatively few such peptides, and these peptides processed by TPPII require further trimming in the endoplasmic reticulum (ER) by ER aminopeptidase 1 for presentation. Taken together, these observations demonstrate that TPPII plays a specialized role in Ag processing and one that is not essential for the generation of most presented peptides. Moreover, these findings reveal that three sequential proteolytic steps (by proteasomes, TPPII, and then ER aminopepsidase 1) are required for the generation of a subset of epitopes.

ISCOMATRIX Adjuvant Induces Efficient Cross-Presentation of Tumor Antigen by Dendritic Cells via Rapid Cytosolic Antigen Delivery and Processing via Tripeptidyl Peptidase II

“…A broader role for TPPII in cross-presentation was suggested by the finding that TPPII mediates trimming of proteasomal products 15 aa or longer for further processing and MHC class I binding (25). However, whether TPPII is indispensable for the generation of most MHC class I epitopes was challenged by another group who found that inhibition of TPPII by siRNA does not reduce the presentation of MHC class I epitopes in the presence of functional proteasomes (26). Analysis of epitopes generated by APCs from TPPII Ϫ/Ϫ mice suggests that TPPII has a moderately destructive role for certain epitopes (27).…”

Tripeptidyl Peptidase II Mediates Levels of Nuclear Phosphorylated ERK1 and ERK2

“…That TPP2 is an essential protein is supported by the finding that homozygous knock down in mice and Caenorhabditis elegans is lethal (12). TPP2 is involved in antigen processing (13)(14)(15)(16)(17)(18), cell growth, DNA damage repair and carcinogenesis (19 -22), fat metabolism, feeding behavior, and obesity (12,23). While the majority of those processes has been linked to TPP2 by describing individual phenotype and expressional changes of TPP2 as well as connected individual proteins (24 -26), only antigen processing and feeding behavior have been directly related to TPP2 peptidase activity by identifying involved peptide substrates (6,9,10,23).…”

“…While the majority of those processes has been linked to TPP2 by describing individual phenotype and expressional changes of TPP2 as well as connected individual proteins (24 -26), only antigen processing and feeding behavior have been directly related to TPP2 peptidase activity by identifying involved peptide substrates (6,9,10,23). TPP2 has been shown to generate a few specific epitopes via its endopeptidase activity, for instance, for specific HLA allele mediated peptide presentation (10,11,17,27). It has been suggested a role of TPP2 in obesity relies on exopeptidase cleavage of the satiety signaling neuropeptide cholecystokinin-8, which subsequently regulates feeding behavior (12,17,28).…”

Role of Metalloproteases in Vaccinia Virus Epitope Processing for Transporter Associated with Antigen Processing (TAP)-independent Human Leukocyte Antigen (HLA)-B7 Class I Antigen Presentation

“…Several proteases have been implicated in the processing of endogenously synthesized antigens independent of the classical proteasome pathway as follows: signal peptidase (43,44); furin (45,46); tripeptidyl peptidase II (47)(48)(49); lysosomal chloroquine-sensitive enzymes (50,51); and caspases (52,53). In this study, diverse proteolytic activities are required to generate the two HLA-B7-restricted epitopes studied.…”