Species differences in [125I]interleukin-1 binding in brain, endocrine and immune tissues

There were dramatic species differences in the level of [125I]recombinant human interleukin-1 alpha ([125I]hIL-1 alpha) binding with high levels of binding present in mouse and rabbit tissues, while no specific binding was present in rat and guinea pig tissues. Utilizing [125I]hIL-1 alpha, moderate to high levels of specific binding were observed in EL-4 6.1 cells (representative of Type I IL-1 receptors) and in mouse hippocampus, spleen and testis; however, no specific [125I]hIL-1 alpha binding was present in Raji cells (representative of Type II IL-1 receptors) and in rat tissues. On the other hand, utilizing [125I]hIL-1 beta, high specific IL-1 binding was present in EL-4 6.1 and Raji cells and moderate binding was evident in mouse tissues, whereas specific [125I]hIL-1 beta binding to rat tissues was not detectable. Moreover, no IL-1 binding in rat tissues was observed using [125I]hIL-1 receptor antagonist, [125I]mouse IL-1 beta or the homologous radioligands [125I]rat IL-1 beta or [125I]rat IL-1 receptor antagonist. These data demonstrate that under optimal conditions for labeling Type I or Type II IL-1 receptors, no specific binding is observed in rat tissues suggesting the presence of novel IL-1 receptor(s) in rat tissues.