Regulation of alloreactivity in the popliteal lymph node assay by the new immunosuppressants: malononitrilamides

Malononitrilamides (MNAs) represent a new class of low molecular weight immunosuppressants and have been shown to prevent and reverse ongoing acute allograft rejection and effectively prolong xenograft survival in rodents. MNAs were also found to be potent inhibitors of B and T cell-mediated autoimmune processes and mediate their effects by binding specifically to dihydro-orotate dehydrogenase (DHODH), inhibiting de novo pyrimidine biosynthesis, thereby blocking T and B cell proliferation and strongly suppressing the IgM and IgG antibody production. Here we evaluated the effects of the MNAs (HMR 1279 and HMR 1715) on the in vivo lymphoproliferation that occurs after challenge with allogeneic cells in a local graft-versus-host reaction in Lewis x Brown Norway F1 hybrid rats by measuring the enlargement of the popliteal lymph nodes (PLN) draining the site of allogeneic cell injection. Oral administration of the MNAs dose-dependently prevented the localized lymphoproliferative response in the PLN assay and suppressed the lymph node hyperplasia. The MNAs even acted therapeutically when they were given during an ongoing alloreactivity as late as days 4 or 5 after challenge. Consistent with the mode of action, a complete reversal of the immunosuppression on the lymphoproliferation in vivo was attempted in this protocol by the addition of exogenous uridine during days 0-5. These data suggest the HMR 1279 and HMR 1715 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.