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Complement component C1r mediated cleavage of the heavy chain of the major histocompatibility class I antigens
1Department of Biotechnology, Lund University, Sweden.
Insights
Activated complement components C1r and C1s cleave MHC class I heavy chains in native configurations. This proteolysis occurs between alpha 2 and alpha 3 domains, with fragments retaining beta 2-microglobulin interaction.
Area of Science:
- Immunology
- Proteomics
- Molecular Biology
Background:
- The complement system plays a crucial role in innate and adaptive immunity.
- Major Histocompatibility Complex (MHC) class I antigens present peptides to cytotoxic T lymphocytes.
- The interaction between complement and MHC molecules is not fully understood.
Purpose of the Study:
- To investigate the novel cleavage of MHC class I heavy chains by activated complement components C1r and C1s.
- To determine the specific domains involved in this proteolysis.
- To assess the impact of MHC class I antigen configuration on complement-mediated cleavage.
Main Methods:
- Immune precipitation using anti-MHC class I and anti-beta 2-microglobulin antibodies.
- Concanavalin A (Con A)-Sepharose precipitation followed by SDS-PAGE to characterize fragments.
- Analysis of cleavage on native versus heat-denatured MHC class I antigens.
Main Results:
- Activated C1r and C1s, at serum concentrations, cleave the heavy chain of MHC class I antigens.
- Cleavage is dependent on the native configuration of MHC class I; heat denaturation reduces cleavage.
- Proteolysis occurs between the alpha 2 and alpha 3 domains of the MHC class I alpha-chain.
- The resulting alpha 1/alpha 2 fragment remains associated with beta 2-microglobulin.
Conclusions:
- C1r and C1s possess a previously unrecognized enzymatic activity against MHC class I heavy chains.
- The native structure of MHC class I is essential for susceptibility to C1r/C1s-mediated cleavage.
- This interaction suggests a novel regulatory mechanism involving complement and antigen presentation.
Abstract:
Apart from cleaving C1s, we demonstrate for the first time that: 1) at concentrations found in serum, the activated forms of the complement components C1r in addition to C1s can cleave the heavy chain of MHC class I antigens, 2) the cleavage by C1r and C1s is seemingly dependent upon a native configuration of the MHC class I antigen, since heat denaturation of the HLA antigens reduce the cleavage. The proteolytic fragments following C1 cleavage were characterized by precipitation with Con A-Sepharose, anti-MHC class I and anti-beta 2-microglobulin antibodies. The proteolysis of the alpha-chain of MHC class I was shown to take place between the alpha 2- and alpha 3- domains as estimated by the Con A-Sepharose precipitation pattern on SDS-PAGE. The alpha 1/alpha 2 fragment was still shown to interact with beta 2-microglobulin as shown by immunoprecipitation.