The Idd3 genetic interval confers protection against multiple autoimmune diseases, including type 1 diabetes and experimental autoimmune encephalomyelitis. The favored candidate gene in this interval is Il2, which is polymorphic between susceptible and resistant strains of mice. IL-2 regulates the growth/death of effector T cells as well as the generation/maintenance of regulatory T cells (Tregs), and recent studies have shown that NOD. Idd3 Tregs are more suppressive than their NOD counterparts. We have further dissected the mechanisms underlying the differential suppression by NOD and NOD. Idd3 Tregs and find that it is determined by CD11b+ CD11c− APCs. Thus, contrary to what might be expected, our data suggest that the differential suppressive activity of NOD and NOD. Idd3 Tregs is not due to an effect of the Idd3 genetic interval on T cells but rather is due to differences in the APC compartment.