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An asparaginyl endopeptidase processes a microbial antigen for class II MHC presentation
B Manoury1, E W Hewitt, N Morrice
1Department of Biochemistry, University of Dundee, UK.
Nature
|January 1, 1999
Summary
A newly identified enzyme, asparaginyl endopeptidase (AEP), is crucial for processing foreign antigens like tetanus toxin, unlike previously assumed cathepsins. This finding impacts understanding of immune system antigen presentation.
Area of Science:
- Immunology
- Proteolysis
- Molecular Biology
Background:
- Antigen processing requires breakdown of foreign proteins into peptides within endosomes/lysosomes for presentation by MHC class II molecules.
- The specific proteases responsible for antigen processing, particularly for MHC class II presentation, remain largely unidentified.
- Lysosomal cathepsins are generally considered the primary enzymes for antigen processing.
Purpose of the Study:
- To identify the key proteases involved in the processing of foreign protein antigens for T cell presentation.
- To investigate the role of asparagine-specific cysteine endopeptidases in antigen processing.
- To determine the impact of N-glycosylation on antigen processing by specific proteases.
Main Methods:
- Purified lysosomes from a human B-cell line were used to process a domain of the tetanus toxin antigen (TTCF).
- Competitive peptide inhibitors targeting B-cell asparaginyl endopeptidase (AEP) were designed and used to block its activity in vitro and in vivo.
- The effect of N-glycosylation on AEP activity was assessed in vitro.
Main Results:
- The dominant proteolytic activity in lysosomal processing of TTCF was identified as an asparagine-specific cysteine endopeptidase, similar to mammalian asparaginyl endopeptidases (AEP), not cathepsins.
- Inhibitors of AEP specifically blocked TTCF processing in vitro and slowed its presentation to T cells in vivo.
- Preprocessing TTCF with AEP accelerated its presentation, confirming AEP's key role.
- N-glycosylation of asparagine residues was found to block AEP action in vitro.
Conclusions:
- Asparagine-specific cysteine endopeptidase (AEP) plays a critical role in processing foreign antigens for T cell presentation, challenging the established role of cathepsins.
- AEP inhibitors can modulate antigen presentation in vivo.
- N-glycosylation may regulate antigen processing by preventing AEP cleavage, potentially allowing preferential processing of microbial antigens over self-antigens.