Induction of nonspecific x-irradiation-resistant suppressor cell activity in vivo and prolongation of vascularized allograft survival by SK&F 105685, a novel immunomodulatory azaspirane.

SK&F 105685 is a novel azaspirane with immunosuppressive activity in animal models of autoimmune disease. This study evaluates the efficacy and mechanism of action of the compound in rat recipients of cardiac allografts. Short-term SK&F 105685 therapy (20 mg/kg/day by gavage) proved effective both in the pretreatment (days -14 to -8 or -7 to -1; allograft at day 0) and treatment (days 0 to 6) protocols, with cardiac allograft survival prolonged to 14-17 days (acute rejection = 7 days; P < 0.001). SK&F 105685 pretreatment exerted at least additive effects with subtherapeutic CsA (1.5 mg/kg/day x 7 days i.m.) given after transplantation, with 50% of allografts surviving > 50 days. SK&F 105685 therapy diminished the immunohistological features of acute rejection, with the cellular infiltrate suppressed and the induction of IL-2/transferrin receptors, and elaboration of IL-2/IFN-gamma essentially abolished, as compared with the grafts in untreated hosts. These correlated with normal frequency of CD4, CD5, CD8 phenotype subsets and B cells in recipient lymphoid organs, as shown by flow microfluorimetry. Adoptive transfer of untreated or x-irradiated (2000 rads) spleen cells from SK&F 105685-modulated hosts significantly prolonged the survival of donor-specific or third-party test cardiac allografts to 10-15 days, suggesting the presence of nonspecific x-irradiation-resistant suppressor cells in the transferred inoculum. Their activity could be enriched by Percoll density centrifugation and screened by the ability to inhibit Con A-driven proliferation of normal cells in the coculture assay. The light-density x-irradiation-resistant spleen cell fraction (1.07 g/ml) was consistently and significantly more suppressive than the heavy-density (1.09 g/ml) interface, or the corresponding unseparated cells. Thus SK&F 105685 therapy abrogates rejection response and significantly prolongs the survival of vascularized cardiac allografts in rats. This effect is associated with selective depression of host alloreactivity/immune activation at the graft site, and simultaneous induction of suppressor cells in recipient spleen, comparable to natural or nonspecific suppressor cells generated by TLI. This unique activity profile is consistent with the concept that SK&F 105685 should be considered as a critical chemical adjunct in novel therapeutic strategies representing TLI-equivalent.