Augmentation of natural killer cell-mediated anti-tumor effects by molecular targeting and regulatory T cell depletion

Accumulating evidence indicates that the innate and adaptive immune systems make a crucial contribution to the anti-tumour effects of immunotherapy-based cancer treatments. We sought means to augment immune anti-tumor effector mechanisms through agents that target the cancer as well as means to promote immune effects. Bortezomib is a proteasome inhibitor that has direct anti-tumor effects. We and others have previously demonstrated that bortezomib could also sensitize tumor cells to killing via the death ligand, TRAIL. Natural killer (NK) cells represent a potent anti-tumor effector cell. Therefore we investigated whether bortezomib could sensitize tumor cells to NK cell-mediated killing. Pre-incubation of tumor cells with bortezomib had no effect on short-term NK cell killing or purified granule killing assays. Using a 24 hour lysis assay, increases in tumor killing was only observed using perforin-deficient NK cells. This increased killing was found to be dependent on both TRAIL and FasL, correlating with an increase in tumor expression of Fas and DR5. Long-term tumor outgrowth assays allowed for the detection of this increased tumor killing by activated NK cells following bortezomib treatment of the tumor. In a tumor purging assay, in which tumor:bone marrow cell mixtures were placed into lethally irradiated mice, only treatment of these mixtures with a combination of NK cells with bortezomib resulted in significant tumor-free survival of the recipients. These results demonstrate that bortezomib treatment can sensitize tumor cells to cellular effector pathways. Therefore, the combination of proteasome inhibition with immune therapy may result in increased anti-tumor efficacy. We are also interested in activating endogenous NK cells with IL-2. IL-2 has been extensively examined to promote clinical T and NK cell responses. Regulatory T cells (Tregs) have been shown to suppress many aspects of the immune system, including natural killer (NK) cell-mediated responses. In our study, we demonstrated that in vivo administration of IL-2 led to an activation and expansion of both NK cells and immunosuppressive Tregs. Therefore, we attempted to augment NK cell anti-tumor effects by concurrently depleting Tregs using anti-CD25. Increased NK cell activation by IL-2 correlated with an increase in classical, short-term NK cell killing in vitro regardless of depletion of Tregs. However, when splenocytes from treated mice were used in long-term tumor outgrowth assays, we observed that the combination of Treg depletion with IL-2 administration led to improved anti-tumor effects compared to either treatment alone. Importantly, these in vitro data correlated with in vivo survival of leukemia-bearing mice in which co-treatment of IL-2 with anti-CD25 led to significantly improved survival compared to mice treated with either IL-2 alone or Treg depletion. Prior depletion of NK1.1+ cells, but not CD8+ cells, completely abrogated all anti-tumor effects mediated by IL-2 and anti-CD25 combination therapy. These findings demonstrate that superior NK cell-mediated anti-leukemic effects can be achieved with IL-2 administration and concurrent depletion of CD25+ cells. Thus the combined therapies of not only sensitizing tumors by molecular targeting, but also stimulating immune responses as well as removing negative regulators may lead to powerful anti-tumor therapies.