Biochemical analysis of plasma-soluble invariant chains and their complex formation with soluble HLA-DR
V Rebmann1, K Dornmair, H Grosse-Wilde
1Institute of Immunology, University Hospital of Essen, Germany.
Insights
Soluble CD74 (sCD74) and its complexes with soluble HLA-DR (sHLA-DR) are present in human plasma. These findings reveal novel insights into the circulating forms of CD74 and their association with HLA-DR in healthy individuals.
Area of Science:
- Immunology
- Molecular Biology
Background:
- The invariant chain (CD74) is crucial for peptide loading into HLA class II molecules within the cytoplasm.
- A small fraction of CD74:class II complexes are found on the cell surface.
Purpose of the Study:
- To identify and quantify soluble CD74 (sCD74) and sCD74:sHLA-DR complexes in human plasma.
- To characterize the isoforms and molecular weights of sCD74 in plasma.
Main Methods:
- ELISA was used to quantify sCD74 and sCD74:sHLA-DR complexes in EDTA plasma from 86 healthy individuals.
- SDS-PAGE was employed to analyze the molecular weights of sCD74 isoforms in samples with high sCD74 concentrations.
Main Results:
- sCD74 was detected in all plasma samples, with approximately 10% showing elevated levels.
- sCD74:sHLA-DR complexes were universally present and positively correlated with sCD74 concentrations (r=0.83, p<0.001).
- SDS-PAGE identified four sCD74 isoforms (45, 43, 35, 31 kDa); only 45 and 43 kDa isoforms were complexed with sHLA-DR.
Conclusions:
- CD74 exists in a soluble form in the plasma of healthy individuals.
- Soluble CD74 forms complexes with soluble HLA-DR in circulation.
- These findings highlight the presence and interaction of soluble CD74 and HLA-DR in human plasma.
Abstract:
The invariant chain (CD74) is preferentially localized in the cytoplasm and regulates the loading of exogenous derived peptides into HLA class II heterodimers. In addition, a small proportion of CD74:class II complexes is also expressed on the cell surface. We identified and quantified soluble CD74 (sCD74) molecules in the plasma and sCD74:sHLA-DR complexes by ELISA. EDTA plasma samples from 86 healthy probands were analyzed. sCD74 could be detected in all samples with a mean concentration of 1.14 relative units +/- 1.04 SD (range 0.17-4.31). Approximately 10% of the samples had increased amounts of sCD74 (>3.0 relative units). Complexes of sCD74 and sHLA-DR were detected in all samples and their quantities were positively correlated (r=0.83, p<0.001) with the sCD74 concentrations. SDS-PAGE analysis of plasma samples with high sCD74 concentrations (>3.0 relative units) revealed four isoforms of sCD74 with molecular weights of 45, 43, 35, 31 kDa corresponding to known sizes of intracellular CD74. However, only molecular weights of the 45 and 43 kDa isoforms of sCD74 are found complexed with sHLA-DR. Our data demonstrate, that CD74 molecules are present in their soluble form in the plasma of healthy probands and form complexes with soluble HLA-DR molecules.
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