Leflunomide is a new immunosuppressive drug whose active metabolite, A77 1726, impairs cellular nucleotide metabolism by inhibiting the dihydroorotate dehydrogenase (DHODH), a rate-limiting enzyme of de novo pyrimidine synthesis. Furthermore, A77 1726 suppresses tyrosine kinases involved in signal transduction pathways. We investigated the immunosuppressive effects of leflunomide in experimental autoimmune neuritis (EAN) in rats, which is a model of immune-mediated neuropathies. In EAN that was actively induced by subcutaneous injection of peripheral nerve myelin, leflunomide completely prevented paraparesis if applied orally from the day of immunization. Leflunomide was much more effective than azathioprine, which did not mitigate EAN at all. Even when leflunomide was administered therapeutically after the appearance of the first neuropathical signs, it halted the progression and markedly reduced the severity and duration of EAN. Inflammatory infiltrates, demyelination and axonal degeneration in sciatic nerve sections of leflunomide-treated EAN rats were strongly reduced. Leflunomide-treated rats did not mount autoantibodies as specified by ELISA (enzyme-linked immunosorbent assay) with a mixture of peripheral myelin proteins, including P2 and myelin basic protein. In EAN that was adoptively transferred by injection of neuritogenic cells of a P2-specific T-helper line, application of leflunomide also clearly reduced signs of disease. Additional injection of uridine did not neutralize the effect of leflunomide. Similarly, transfer of neuritogenic P2-specific T cells, which were activated in the presence of A77 1726 plus uridine in vitro, still resulted in reduced severity of adoptive transfer EAN in vivo, although proliferation of these cells in vitro was identical to that of control cells. The T-cell receptor-mediated in vitro activatability of a P2-specific T-cell hybridoma was diminished by high concentrations of A77 1726, as evidenced by reduced Ca(2+) flux into the cytosol. Together with the findings in adoptive transfer EAN, this indicates that the antiproliferative effect is probably not the only mechanism of immunosuppressive action by leflunomide. In summary, leflunomide suppresses EAN efficiently and may constitute a promising therapy for immune-mediated neuropathies.