Autologous killing by a population of intermediate T-cell receptor cells and its NK1.1⁺ and NK1.1^- subsets, using Fas ligand/Fas molecules

Self-reactive clones, estimated by anti-Vβ monoclonal antibodies (mAb) in conjunction with the MIs system, are confined to a population of intermediate (int) T-cell receptor (TCR) (or CD3) cells (i.e. TCR(int) cells), but are not found among TCR(high) cells. The next questions to be answered are whether autologous killing is confined to TCR(int) cells and how such killing is mediated. In this study, ⁵¹Cr-labelled thymocytes of syngeneic or allogeneic origin were used as target cells (4-hr assay). When liver and splenic mononuclear cells (MNC) obtained from B6 mice were used as effector cells, prominent autologous killing was seen in liver MNC, but not splenic MNC. Such killing was not seen when thymocytes from B6-lpr/lpr mice (i.e. Fas^-) were used as target cells, nor when liver MNC from MRL-gld/gld mice (i.e. Fas ligand^-) were used as effector cells (target thymocytes of MRL-+/+ mice). Cell separation experiments using a cell sorter revealed that autologous killing was mediated for the most part by CD3(int) cells, while allogeneic killing was mediated entirely by natural killer (NK) cells. TCR(int) cells and TCR(high) cells. Among CD3(int) cells, the NK1.1⁺ subset (i.e. NK1.1⁺ T cells) manifested a higher level of autologous killing than did the NK1.1^- subset. Consistent with the results of a functional assay, it was found by reverse-transcription polymerase chain reaction (RT-PCR) assay that CD3(int) cells among liver MNC showed the expression of Fas ligand mRNA, while thymocytes expressed Fas mRNA, when class I major histocompatibility complex (MHC)⁺ thymocytes (from β₂-microglobulin-deficient mice) were used as target cells, NK cells, but not CD3(int) cells, showed potent cytotoxicity. These results suggest that autologous killing is a major function of TCR(int) cells with self-reactivity, and that such killing is mediated by means of Fas ligand/Fas molecules.