Clonal deletion of major histocompatibility complex class I‐restricted CD4+CD8+ thymocytes in vitro is independent of the CD95 (APO‐1/Fas) ligand

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Abstract
The CD95 (APO‐1/Fas) ligand (CD95L) mediates apoptosis in sensitive target cells, Ca2+‐independent cytotoxicity of cells from perforin knock‐out mice, and peripheral deletion of activated T cells through engagement of its cognate receptor CD95. Double‐positive thymocytes show a high constitutive expression of CD95. Therefore, we used a model system and investigated whether negative selection through apoptosis might involve CD95/CD95L. We analyzed whether CD95L may induce antigen‐specific deletion of double‐positive thymocytes from mice transgenic for a lymphocytic choriomeningitis virus (LCMV)/H2b‐specific T cell receptor (TCR). These cells are deleted in vitro upon addition of the LCMV‐peptide 33–41 in a major histocompatibility complex‐class I‐restricted fashion. Deletion was not blocked by soluble mouse and human CD95‐Fc receptor decoys. CD95‐Fc receptor decoys, however, were effective in blocking apoptosis induced by mouse CD95L‐transfected L929 cells in sensitive CD95+ target cells and in thymocytes. These results suggest that TCR‐induced deletion of immature thymocytes in vitro is independent of CD95L. Thus, our data argue against a role of CD95L in negative selection of MHC‐class I‐restricted autoreactive thymocytes.