Splenocytes were removed from both CatG-deficient mice and C57BL6 control mice, and cell surface and total expression of MHC II (I-Ab) was analysed by flow cytometry. Levels of surface (Fig. 6d) or total (not shown) I-Ab in B cells, DCs, and resting or activated Belnacasan macrophages did not differ between CatG-deficient and control mice. Analysis of peritoneal macrophages also revealed no differences in
I-Ab expression between CatG−/− and C57BL6 control mice (data not shown). We concluded that, by several criteria, CatG lacks the ability to modulate steady-state MHC II levels in vivo and in live, cultured APCs. Our findings provide information on the mechanisms by which MHC II molecules resist endosomal proteolysis, a key biochemical requirement for their function in presentation of peptides captured in endocytic compartments. In their native conformation, purified, detergent-solubilized MHC II molecules failed to be degraded by most lysosomal proteases tested (cathepsins D, L, S, H, and B). The resistance of MHC II molecules to these
proteases thus is an inherent property selleck products of the folded MHC II ectodomains. In contrast, purified MHC II molecules were susceptible to proteolytic attack by CatG at a single cleavage site, which is broadly, but not universally, conserved amongst MHC II molecules. However, using several independent approaches, we were unable to detect any involvement of CatG in the turnover of MHC II molecules embedded in membranes of live APCs. These results
show, on the one hand, that proteolytic resistance of MHC II molecules is not absolute, allowing some scope for regulated turnover; on the other hand, they suggest that the CatG cleavage site is inaccessible in the membrane-embedded native MHC II protein, in vivo. The resistance of MHC II molecules to many endosomal proteases is structurally plausible: the immunoglobulin superfamily domain fold, which is adopted by the membrane-proximal domains, is well known to be highly protease-resistant, and the peptide-loaded antigen-binding groove is highly compact. Initiation of HLA-DR proteolysis by CatG in vitro involved site-specific cleavage between leucine (L) and glutamine (Q) within fx1 and fx2 of the lower loop of the β domain, isothipendyl which may be one of very few sites with sufficient flexibility to allow proteolytic attack. These findings are reminiscent of previous studies, in which CatG was demonstrated to initiate cleavage within the flexible hinge regions of immunoglobulins.39 The membrane-proximal location of the cleavage site, away from the antigen binding groove, is consistent with our observation that CatG cleaves peptide-loaded MHC II molecules, and that peptide binding is retained by CatG-cleaved DR molecules. The fact that peptide-loaded molecules are substrates for CatG supports the notion that CatG is capable of initiating proteolysis of MHC II molecules in their native conformation.