Daclizumab (Dac), an Ab against the IL-2R α-chain, inhibits brain inflammation in patients with multiple sclerosis, while expanding CD56 bright immunoregulatory NK cells in vivo. We hypothesized that this unexpected expansion is paradoxically IL-2 driven; caused by the increased availability of T cell-derived IL-2 for NK cell signaling. To this end, we performed ex vivo functional analyses of CD56 bright NK cells and T cells from patients in clinical trials with Dac. We developed in vitro models to investigate mechanisms for ex vivo observations. We observed that Dac treatment caused decreased numbers and proliferation of FoxP3+ T regulatory cells (Tregs), a model T cell population known to be dependent on IL-2 for proliferation and survival. As anticipated, Dac therapy inhibited IL-2 signaling in all T cells; however, we also observed functional adaptation of T cells to low IL-2 signal in vivo, characterized by the concomitant enhancement of IL-7 signaling on all T cells and parallel increase of CD127 expression by Tregs. In contrast, IL-2 signaling on CD56 bright NK cells was not inhibited by Dac and their in vivo proliferation and cytotoxicity actually increased. Mechanistic studies indicated that the activation of CD56 bright NK cells was likely IL-2 driven, as low doses of IL-2, but not IL-15, mimicked this activation in vitro. Our study provides insight into the role that IL-2 and CD25 play in functional regulation of two important immunoregulatory cell populations in humans: FoxP3+ Tregs and CD56 bright NK cells.