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 invo lved in the folding and assembly of major histocompatibility complex (MHC) class I molecules. In the present study, expression of calreticulin was ana lyzed in human peripheral blood T lymphocytes. Pulse-chase experiments in [ S-35]methionine-labeled T cell blasts showed that calreticulin was associat ed with several proteins in the endoplasmic reticulum and suggested that it was expressed at the cell surface. Indeed, the 60-kDa calreticulin was lab eled by cell surface biotinylation and precipitated from the surface of act ivated T cells together with a protein with an apparent molecular mass of 4 6 kDa. Cell surface expression of calreticulin by activated T lymphocytes w as further confirmed by immunofluorescence and flow cytometry, studies that showed that both CD8+ and CD4+ T cells expressed calreticulin in the plasm a membrane. Low amounts of cell surface calreticulin were detected in resti ng T lymphocytes. By sequential immunoprecipitation using the conformation independent monoclonal antibody HC-10, we provided evidence that the cell s urface 46-kDa protein co-precipitated with calreticulin is unfolded MHC I. These results show for the first time that after T cell activation, signifi cant amounts of calreticulin are expressed on the T cell surface, where the y are found in physical association with a pool of beta(2)-free MHC class I molecules.