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An acid protease secreted by transformed cells interferes with antigen processing
1Laboratory of Cellular and Molecular Immunology, National Institute of Allergy and Infectious Diseases, Bethesda, Maryland 20892.
Abstract:
The major excreted protein of malignantly transformed mouse fibroblasts (MEP), which is the precursor to lysosomal cathepsin L, was used to study the effect of exogenous acid proteases on antigen processing. When MEP and native pigeon cytochrome c were added to Chinese hamster ovary (CHO) cells expressing transfected major histocompatability complex class II gene products, the antigen-specific T-cell hybridoma 2B4 did not respond to the antigen. MEP appears to destroy the antigen in an acid compartment of the presenting cell because: (a) MEP is only active as a protease under acid conditions; (b) mannose 6-phosphate inhibited the internalization of MEP and blocked its effect on antigen processing; (c) the destruction required the simultaneous entry of the antigen and MEP into the cells; and (d) cytochrome c fragment 66-104 which does not need to be processed stimulated 2B4 in the presence of MEP. These results support the hypothesis that antigen processing requires internalization of the antigen into an acidic compartment, and they provide a new model for the investigation of the contribution of acid proteases to the reduced immunocompetence of tumor-bearing animals.
Insights
Exogenous acid proteases, like major excreted protein (MEP), can impair antigen processing by degrading antigens within acidic cellular compartments. This suggests a role for acid proteases in the reduced immune function observed in tumor-bearing animals.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Antigen processing is crucial for T-cell recognition and immune responses.
- Lysosomal cathepsin L, a key enzyme in antigen processing, is involved in breaking down proteins into peptides for presentation.
- The role of exogenous acid proteases in modulating antigen processing remains incompletely understood.
Purpose of the Study:
- To investigate the impact of exogenous acid proteases on antigen processing by antigen-presenting cells.
- To elucidate the mechanism by which major excreted protein (MEP) affects antigen presentation.
- To explore the potential contribution of acid proteases to impaired immunocompetence in tumor-bearing hosts.
Main Methods:
- Utilized Chinese hamster ovary (CHO) cells expressing major histocompatability complex class II molecules.
- Introduced exogenous major excreted protein (MEP) and pigeon cytochrome c into CHO cells.
- Assessed T-cell hybridoma 2B4 response to antigen presentation.
- Investigated the effect of mannose 6-phosphate on MEP internalization and function.
Main Results:
- MEP inhibited the antigen-specific T-cell response, indicating impaired antigen processing.
- MEP's protease activity is optimal under acidic conditions, consistent with lysosomal environments.
- Mannose 6-phosphate blocked MEP internalization, preventing its effect on antigen processing.
- Simultaneous entry of antigen and MEP was required for antigen destruction.
- A processed fragment of cytochrome c stimulated T-cell response in the presence of MEP.
Conclusions:
- Antigen processing necessitates internalization into an acidic cellular compartment.
- Exogenous acid proteases, such as MEP, can interfere with antigen processing.
- These findings provide a novel model to study the role of acid proteases in tumor-associated immunodeficiency.