Calreticulin is expressed on the cell surface of activated human peripheral blood T lymphocytes in association with major histocompatibility complex class I molecules.
Calreticulin is an endoplasmic reticulum resident molecule known to be involved in the folding and assembly of major histocompatibility complex (MHC) class I molecules. In the present study, expression of calreticulin was analyzed in human peripheral blood T lymphocytes. Pulse-chase experiments in [35S]methionine-labeled T cell blasts showed that calreticulin was associated with several proteins in the endoplasmic reticulum and suggested that it was expressed at the cell surface. Indeed, the 60-kDa calreticulin was labeled by cell surface biotinylation and precipitated from the surface of activated T cells together with a protein with an apparent molecular mass of 46 kDa. Cell surface expression of calreticulin by activated T lymphocytes was further confirmed by immunofluorescence and flow cytometry, studies that showed that both CD8+ and CD4+ T cells expressed calreticulin in the plasma membrane. Low amounts of cell surface calreticulin were detected in resting T lymphocytes. By sequential immunoprecipitation using the conformation independent monoclonal antibody HC-10, we provided evidence that the cell surface 46-kDa protein co-precipitated with calreticulin is unfolded MHC I. These results show for the first time that after T cell activation, significant amounts of calreticulin are expressed on the T cell surface, where they are found in physical association with a pool of beta2-free MHC class I molecules.
Arosa
F A
FA
Laboratory of Molecular Immunology, Institute for Molecular and Cell Biology, University of Porto, 4150 Porto, Portugal. farosa@ibmc.up.pt
de Jesus
O
O
Porto
G
G
Carmo
A M
AM
de Sousa
M
M
D000918:Antibody Specificity; D002135:Calcium-Binding Proteins / Q000276:immunology / Q000302:isolation & purification / Q000378:metabolism*; D037282:Calreticulin; D002352:Carrier Proteins; D002462:Cell Membrane / Q000378:metabolism*; D015395:Histocompatibility Antigens Class I / Q000302:isolation & purification / Q000378:metabolism*; D006801:Humans; D018960:Hyaluronan Receptors; D008213:Lymphocyte Activation; D008562:Membrane Glycoproteins; D008565:Membrane Proteins / Q000302:isolation & purification / Q000378:metabolism; D024101:Mitochondrial Proteins; D018832:Molecular Chaperones / Q000276:immunology / Q000302:isolation & purification / Q000378:metabolism*; D011233:Precipitin Tests; D011485:Protein Binding; D011951:Receptors, Complement / Q000276:immunology; D012261:Ribonucleoproteins / Q000276:immunology / Q000302:isolation & purification / Q000378:metabolism*; D013601:T-Lymphocytes / Q000276:immunology*
C087299:C1QBP protein, human; D002135:Calcium-Binding Proteins; D037282:Calreticulin; D002352:Carrier Proteins; D015395:Histocompatibility Antigens Class I; D018960:Hyaluronan Receptors; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D024101:Mitochondrial Proteins; D018832:Molecular Chaperones; D011951:Receptors, Complement; D012261:Ribonucleoproteins; C032402:complement 1q receptor