Asparagine Endopeptidase Is Not Essential for Class II MHC Antigen Presentation but Is Required for Processing of Cathepsin L in Mice

Abstract

Class II MHC molecules survey the endocytic compartments of APCs and present antigenic peptides to CD4 T cells. In this context, lysosomal proteases are essential not only for the generation of antigenic peptides but also for proteolysis of the invariant chain to allow the maturation of class II MHC molecules. Recent studies with protease inhibitors have implicated the asparagine endopeptidase (AEP) in class II MHC-restricted Ag presentation. We now report that AEP-deficient mice show no differences in processing of the invariant chain or maturation of class II MHC products compared with wild-type mice. In the absence of AEP, presentation to primary T cells of OVA and myelin oligodendrocyte glycoprotein, two Ags that contain asparagine residues within or in proximity to the relevant epitopes was unimpaired. Cathepsin (Cat) L, a lysosomal cysteine protease essential for the development to CD4 and NK T cells, fails to be processed into its mature two-chain form in AEP-deficient cells. Despite this, the numbers of CD4 and NK T cells are normal, showing that the single-chain form of Cat L is sufficient for its function in vivo. We conclude that AEP is essential for processing of Cat L but not for class II MHC-restricted Ag presentation.

Design of cell-permeable, fluorescent activity-based probes for the lysosomal cysteine protease asparaginyl endopeptidase (AEP)/legumain

“…The mammalian legumain homologue is a lysosomal cysteine protease that is a member of the clan CD protease family which includes the caspases, separase and the gingipains [2]. Mammalian legumain has been ascribed a role in the initiation of invariant chain processing during MHC class II mediated antigen presentation [3,4]. Although the nature of this activity remains controversial, legumain is undoubtedly a key player in lysosomal proteolysis, contributing to the processing of antigenic peptides as well as the processing of the papain family cathepsins [5].…”

Lysosomal Cysteine and Aspartic Proteases Are Heterogeneously Expressed and Act Redundantly to Initiate Human Invariant Chain Degradation

“…Expression of CatL in the Priess line could be an unusual artifact of transformation. It is of interest that AEP and CatL activities are both high in the Priess B cell line, in light of evidence that AEP is required for CatL processing (19). Nevertheless, these data demonstrate that cathepsin activity is not necessarily equivalent among EBV-transformed B cells.…”

“…AEP has been implicated both in the productive generation of TT peptides and in the destructive processing of an immunodominant epitope from MBP (21,23). Processing of the CatL precursor likewise completely depends on the presence of AEP (19). The role of AEP in MBP processing, however, is a subject of debate, as destruction of MBP peptide 85-99 has also been attributed to cathepsin G, a serine protease present in the lysosomal compartment of human B lymphocytes (25).…”

“…The role of AEP in MBP processing, however, is a subject of debate, as destruction of MBP peptide 85-99 has also been attributed to cathepsin G, a serine protease present in the lysosomal compartment of human B lymphocytes (25). AEP is dispensable for Ag presentation of epitopes from OVA and myelin oligodendrocyte glycoprotein in the AEP knockout mouse (19). The role of AEP in MBP processing, as well as the dispensability of AEP in the processing of other epitopes containing asparagine, necessitates a reappraisal of the role of AEP in Ag presentation.…”

“…The most abundant aspartyl protease cathepsin D (CatD) (17) is dispensable for class II MHC presentation and degradation of Ii in CatD knockout mice (18). Likewise, AEP, which is required to activate CatL and has been proposed as a candidate for Ii cleavage, plays no immediate role in Ii degradation in AEP knockout mice of the H-2 b haplotype (19).…”