The alloreactivity in the popliteal lymph node (PLN) assay is regulated by malononitrilamides (MNAs)

Malononitrilamides (MNA 279 and MNA 715) represent a new class of low molecular weight immunosuppressants and belong to the derivatives of the primary metabolite of leflunomide A7771726. They have been shown to prevent and reverse established acute allograft rejection and effectively prolong xenograft survival, and have also been found to be potent inhibitors of B- and T-cell mediated autoimmune processes. The MNAs mediate their effects by binding specifically to dehydro orotate-dehydrogenase (DHODH) and inhibiting de novo pyrimidine biosynthesis, thereby blocking T- and B-cell proliferation and strongly suppressing the IgM and IgG antibody production. In this study we evaluated the effects of MNA 279 and MNA 715 on the in vivo lymphoproliferation that occurs after challenge with allogeneic cells in a local graft-versus-host (GvH) reaction in Lewis x Brown-Norway (LBN) F1-hybrid rats by measuring the enlargement of the PLN draining the site of allogeneic cell injection. Oral administration of one of the two MNAs (7.5 to 50 mg/kg) on day 0, dose-dependently prevented the localized lymphoproliferative response and suppressed the lymph node hyperplasia. The MNAs even acted therapeutically when they were given during an ongoing alloreactivity as late as day 4 or 5 after challenge. Consistent with the mode of action that MNAs inhibit de novo pyrimidine biosynthesis, a complete reversal of the immunosuppression on the lymphoproliferation in vivo was attempted in this protocol by addition of exogenous uridine during days 0 to 5. These data suggest that MNA 279 and MNA 715 mediate their antiproliferative and immunosuppressive effects in the PLN-assay in vivo by decreasing the activity of DHODH in the lymph node cells and thereby inhibiting pyrimidine biosynthesis.