α-Galactosylceramide ameliorates autoimmune diabetes in non-obese diabetic mice through a suppressive effect mediated by CD8+ T cells

Type 1 diabetes is an autoimmune disorder resulting from lymphocyte-mediated destruction of insulin-producing β cells in pancreas. Natural killer T cells are regulatory immune components controlling autoreactivity and immune homeostasis. Although early studies supported that amelioration of autoimmune diabetes by natural killer T cells was associated with Th1/2 shift, other Th2-independent regulatory mechanisms were also suggested. Since natural killer T cells are critical for the generation of CD8+ regulatory T cells controlling anterior chamber-associated immune deviation and CD8+ regulatory T cells also participate in suppression of immune responses like experimental autoimmune encephalomyelitis, we investigate whether the similar suppressive effects are involved in α-galactosylceramide-induced immune tolerance in non-obese diabetic mice. We demonstrate that repeated exposure of α-galactosylceramide reveals a hyporesponsiveness of total or antigen-presenting cells-depleted splenocytes upon anti-CD3/28 antibodies stimulation. The dispensability of dendritic cells in the hyporesponsiveness is consistent with the comparable expression of costimulatory molecules on CD11c+ subsets between α-galactosylceramide- and vehicle-treated mice. α-Galactosylceramide treatment not only affects the effector T cell subsets and their cytokine production but also increases the secretion of transforming growth factor-β by splenocytes, implying the suppressive regulation. The adoptive transfer experiments demonstrate the suppressive effect of T cells from α-galactosylceramide-treated non-obese diabetic mice when co-transferred with vehicle-treated littermates. Finally, it reveals that CD8+ subset among antigen-presenting cells-depleted splenocytes tends to confer the suppression since the protective ability vanishes upon withdrawal of CD8+ subset. These results suggest that repeated exposure of α-galactosylceramide ameliorates autoimmune diabetes in non-obese diabetic mice mediated by CD8+ T cell-associated suppression.