IMMUNOSUPPRESSIVE PROPERTIES OF METHOTREXATE - APOPTOSIS AND CLONAL DELETION OF ACTIVATED PERIPHERAL T-CELLS

The folate antagonist methotrexate (MTX) is extensively used in graft- versus-host disease, rheumatoid arthritis, and other chronic inflammat ory disorders. In addition to its antiinflammatory activity associated with increased release of adenosine, MTX exerts antiproliferative pro perties by inhibition of dihydrofolate reductase and other folate-depe ndent enzymes. However, the mechanisms of immunosuppressive properties associated with low-dose MTX treatments are still elusive, We report here that MTX (0.1-10 mu M) induces apoptosis of in vitro activated T cells from human peripheral blood, PBL exposed to MTX for 8 h, then ac tivated in drug-free medium, underwent apoptosis, which was completely abrogated by addition of folinic acid or thymidine, Apoptosis of acti vated T cells did not require interaction between CD95 (Fas, APO-1) an d its ligand, and adenosine release accounted for only a small part of this MTX activity, Apoptosis required progression of activated T cell s to the S phase of the cell cycle, as it was prevented by drugs or an tibodies that interfere with IL-2 synthesis or signaling pathways, MTX achieved clonal deletion of activated T cells in mixed lymphocyte rea ctions, Finally, in vitro activation of PBL taken from rheumatoid arth ritis patients after MTX injection resulted in apoptosis, Altogether, the data demonstrate that MTX can selectively delete activated periphe ral blood T cells by a CD95-independent pathway, This property could b e used as a new pharmacological end point to optimize dosage and timin g of MTX administration. It may account for the immunosuppressive effe cts of low-dose MTX treatments.