Suppression of NK Cell-Mediated Bone Marrow Cell Rejection by CD4⁺CD25⁺ Regulatory T Cells: Linkage of Adaptive to Innate Responses.

While a link between the innate to adaptive immune system has been established, studies demonstrating direct effects of T cells in regulating Natural Killer (NK) cell function have been lacking. Naturally occurring CD4⁺CD25⁺ regulatory T cells (Tregs) have been shown to potently inhibit adaptive responses by T cells. We therefore investigated whether Tregs could affect NK cell function in vivo. Using a bone marrow transplantation (BMT) model of hybrid resistance, in which parental (H2^d) marrow grafts are rejected by the NK cells of the F1 recipients (H2^bxd), we demonstrate that the in vivo removal of host Tregs significantly enhances NK-cell mediated BM rejection. This heightened rejection was mediated by the specific NK cell Ly-49+ subset previously demonstrated to reject the BMC in this donor/host pairing. The depletion of Tregs could also further increase rejection already enhanced by treating recipients with the NK cell activator, poly I:C. Although splenic NK cell numbers were not significantly altered, increased splenic NK in vitro cytotoxic activity was observed from the recovered cells. The regulatory role of Tregs was confirmed in adoptive transfer studies in which transferred CD4⁺CD25⁺ Tregs resulted in abrogation of NK cell-mediated hybrid resistance. Thus, Tregs can potently inhibit NK cell function in vivo and their depletion may have therapeutic ramifications with NK cell function in BMT and cancer therapy.