MRL LPR CD4(-)CD8(-) AND CD8(+) T-CELLS, RESPECTIVELY, MEDIATE FAS-DEPENDENT AND PERFORIN CYTOTOXIC PATHWAYS/

Autoimmune-prone MRL/lpr mice, homozygous for the lpr mutation, exhibi t defective apoptosis and develop generalized lymphoproliferation with the accumulation of a double-negative (DN: CD4(-)CD8(-))T cell popula tion. The capacity of lpr T lymphocytes to effectuate Fas- and perfori n-mediated cytotoxicity was investigated. Spleen and lymph nodes cells spontaneously lyse Fas(+) targets (thymocytes) through a Fas-mediated mechanism as a consequence of their overexpression of Fas ligand (Fas L) confirmed by semiquantitative reverse transcription (RT)-PCR and im munoprecipitation analysis. This cytotoxicity was greatly increased af ter stimulation of the effecters by phorbol myristate acetate (PMA) ionomycin. Under these conditions, MRL/lpr spleen and LN cells exhibit ed strong Fas-mediated Ca2+-independent cytotoxic activity against wil d-type Fas(+) (H-2 compatible or incompatible) thymocytes or lipopolys accharide (LPS)-transformed blast cells. Such Fas-mediated cytotoxic a ctivity was also observed with C57BL/6-lpr, but never with wild-type C 57BL/6 or MLR+/+ effecters. Depletion experiments showed that the effe ctor cells of this Fas-mediated cytotoxicity were DN T cells. This sub set, which represent in vivo activated T cells, can spontaneously lyse Fas(+) targets by a mechanism that does not need the interaction of t he T cell receptor (TCR) with major histocompatibility complex molecul e plus antigen. This lytic potential is increased by PMA + ionomycin, which sends a second activation signal to these primed T cells. Theref ore, the small amounts of Fas receptor expressed on MRL/lpr tissues ma y account for their nonspecific autoimmune attack by DN cells. In Con A-containing medium, which allows detection of the perforin-mediated p athway against Fas(-) targets, cytotoxic CD8(+) effecters were detecte d that are able to kill lpr thymocytes via a Ca2+-dependent pathway. T hus, in MRL/lpr mice, these CD8(+) cells could constitute potent cytot oxic effectors against cells presenting antigen to their TCR.