Cathepsin G, and Not the Asparagine-Specific Endoprotease, Controls the Processing of Myelin Basic Protein in Lysosomes from Human B Lymphocytes

Abstract

The asparagine-specific endoprotease (AEP) controls lysosomal processing of the potential autoantigen myelin basic protein (MBP) by human B lymphoblastoid cells, a feature implicated in the immunopathogenesis of multiple sclerosis. In this study, we demonstrate that freshly isolated human B lymphocytes lack significant AEP activity and that cleavage by AEP is dispensable for proteolytic processing of MBP in this type of cell. Instead, cathepsin (Cat) G, a serine protease that is not endogenously synthesized by B lymphocytes, is internalized from the plasma membrane and present in lysosomes from human B cells where it represents a major functional constituent of the proteolytic machinery. CatG initialized and dominated the destruction of intact MBP by B cell-derived lysosomal extracts, degrading the immunodominant MBP epitope and eliminating both its binding to MHC class II and a MBP-specific T cell response. Degradation of intact MBP by CatG was not restricted to a lysosomal environment, but was also performed by soluble CatG. Thus, the abundant protease CatG might participate in eliminating the immunodominant determinant of MBP. Internalization of exogenous CatG represents a novel mechanism of professional APC to acquire functionally dominant proteolytic activity that complements the panel of endogenous lysosomal enzymes.

Diverse regulatory roles for lysosomal proteases in the immune response

“…For example, a substantial accumulation of cathepsin L is observed in lysosomes of AEP-deficient mice and this may partially compensate for the predicted shortfall in tetanus toxin processing [7], Matthews et al (submitted for publication). In addition, there is strong evidence that the protease content of primary cells and immortalised cell lines may be very different [35,36], Matthews et al (submitted for publication). In fact, even different immortalised human B cell lines may differ in protease content [16].…”

“…For example, cathepsin G, an enzyme more commonly associated with the granules of neutrophils, is accumulated from exogenous sources by primary human B cells and contributes to destructive processing of the autoantigen myelin basic protein [35]. In contrast, in EBVtransformed human B cells similar destructive processing of MBP is mediated by endogenous AEP [35,41].…”

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

“…AEP has been implicated in many aspects of the class II MHC presentation pathway: activation of other lysosomal proteases (19,38), destruction of Ii (16), and both destruction and creation of peptide epitopes (21,23,39). The results obtained so far have been contradictory, and no publication has reconciled findings that suggest an important role for AEP in Ag processing and presentation with works that suggest a nonessential role for AEP or that attribute the proposed role of AEP to other enzymes (19,25).…”

“…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). AEP is dispensable for Ag presentation of epitopes from OVA and myelin oligodendrocyte glycoprotein in the AEP knockout mouse (19).…”

“…MBP peptide 85-99 has been implicated in multiple sclerosis (MS), and CD4 ϩ T cells reactive to this peptide have been cloned from individuals with MS (24). It has been claimed, however, that cathepsin G, and not AEP, is responsible for destructive processing of MBP peptide 85-99 (25).…”