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Masking of a cathepsin G cleavage site in vivo contributes to the proteolytic resistance of major histocompatibility complex class II molecules
Author(s) -
Burster Timo,
Macmillan Henriette,
Hou Tieying,
Schilling James,
Truong Phi,
Boehm Bernhard O.,
Zou Fang,
Lau Kenneth,
Strohman Michael,
Schaffert Steven,
Busch Robert,
Mellins Elizabeth D.
Publication year - 2010
Publication title -
immunology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.297
H-Index - 133
eISSN - 1365-2567
pISSN - 0019-2805
DOI - 10.1111/j.1365-2567.2010.03247.x
Subject(s) - biology , major histocompatibility complex , proteases , microbiology and biotechnology , cleavage (geology) , cathepsin g , endocytic cycle , mhc class i , biochemistry , gene , enzyme , cell , endocytosis , paleontology , fracture (geology)
Summary The expression of major histocompatibility complex class II (MHC II) molecules is post‐translationally regulated by endocytic protein turnover. Here, we identified the serine protease cathepsin G (CatG) as an MHC II‐degrading protease by in vitro screening and examined its role in MHC II turnover in vivo . CatG, uniquely among endocytic proteases tested, initiated cleavage of detergent‐solubilized native and recombinant soluble MHC II molecules. CatG cleaved human leukocyte antigen (HLA)‐DR isolated from both HLA‐DM‐expressing and DM‐null cells. Even following CatG cleavage, peptide binding was retained by pre‐loaded, soluble recombinant HLA‐DR. MHC II cleavage occurred on the loop between fx1 and fx2 of the membrane‐proximal β2 domain. All allelic variants of HLA‐DR tested and murine I‐A g7 class II molecules were susceptible, whereas murine I‐E k and HLA‐DM were not, consistent with their altered sequence at the P1’ position of the CatG cleavage site. CatG effects were reduced on HLA‐DR molecules with DRB mutations in the region implicated in interaction with HLA‐DM. In contrast, addition of CatG to intact B‐lymphoblastoid cell lines (B‐LCLs) did not cause degradation of membrane‐bound MHC II. Moreover, inhibition or genetic ablation of CatG in primary antigen‐presenting cells did not cause accumulation of MHC II molecules. Thus, in vivo , the CatG cleavage site is sterically inaccessible or masked by associated molecules. A combination of intrinsic and context‐dependent proteolytic resistance may allow peptide capture by MHC II molecules in harshly proteolytic endocytic compartments, as well as persistent antigen presentation in acute inflammatory settings with extracellular proteolysis.