Functionally active T cell receptor/CD3 complexes are present at the surface of cloned cytotoxic T cells without fluorescence-immunological detectability.

The cytotoxic T cell clone 10BK.1 is activated in response to the ovalbumin peptide OVA257-264 in a major histocompatibility complex class I-restricted manner. Following activation 10BK.1 cells proliferate, secrete lymphokines, and kill syn- and allogeneic target cells. Using immunofluorescence analysis we detected CD8, LFA-1, and ICAM-1 on the surface of 10BK.1 cells, but no CD3 or T cell receptor (TCR). In contrast, the proliferative response of 10BK.1 cells to antigen was efficiently blocked by soluble antibodies directed at CD3 epsilon or TCR alpha beta, but not by antibodies directed at TCR gamma delta. In addition, lysis of target cells was blocked by F(ab')2 fragments of antibodies directed at CD3 epsilon, and 10BK.1 cells proliferate in response to immobilized anti-CD3 epsilon or anti-TCR alpha beta antibodies. Furthermore, 10BK.1 cells lyse hybridoma cells that secrete antibodies directed at CD3 epsilon or TCR alpha beta, but not TCR gamma delta. These results demonstrate that functionally active molecules of the TCR/CD3 complex exist on the surface of 10BK.1 cells, obviously in low amounts, undetectable by immunofluorescence. In contrast, using permeabilized cells, we found high cytoplasmatic expression of TCR alpha beta, CD3 epsilon, and zeta-chain in 1OBK.1 cells, indicating that the low level of TCR/CD3 expression on the surface is not a consequence of a reduced synthesis of these molecules, but that the transport of these molecules to the surface is reduced. Our data demonstrate that the absence of TCR/CD3 complexes on the surface of cells, as detected by immunofluorescence, does not warrant the conclusion that these complexes are also functionally absent from the surface of these cells.