Critically, treatment with hIL-15 significantly up-regulated the FcRIV expression on both NK cells and macrophages and increased the expression of NKG2D and granzyme B on NK cells (P< 0.01) and increased the killing activity of NK cells toward the EL4-hCD20 and rituximab-coated EL4-hCD20 target cells (P< 0.001). interleukin-15, NK cells, macrophages == Abstract == The goal of cancer immunotherapy is to stimulate the host immune system to attack malignant cells. Antibody-dependent cellular cytotoxicity (ADCC) is a pivotal mechanism of antitumor action of clinically employed antitumor antibodies. IL-15 administered to patients with metastatic malignancy by continuous i.v. infusion at 2 g/kg/d for 10 days was associated with a 38-fold increase in the number and activation status of circulating natural killer (NK) cells and activation of macrophages which together are ADCC effectors. We investigated combination therapy of IL-15 with rituximab in a syngeneic mouse model of lymphoma transfected with human CD20 and with alemtuzumab (Campath-1H) in a xenograft model of human adult T cell leukemia (ATL). IL-15 greatly enhanced the therapeutic efficacy of both rituximab and alemtuzumab in tumor models. The additivity/synergy was shown to be associated with augmented ADCC. Both NK cells and macrophages were critical elements in the chain of interacting effectors involved in optimal therapeutic responses mediated by rituximab with IL-15. We provide evidence supporting the hypothesis that NK cells interact with macrophages to augment the NK-cell activation and expression of FcRIV and the capacity of these cells to become effectors of ADCC. The present study supports clinical trials of IL-15 combined with tumor-directed monoclonal antibodies. The goal of cancer immunotherapy is to R1530 stimulate the host immune system to attack cancer cells. Recombinant interleukin-2 (IL-2) is a prototypic immunotherapeutic treatment for patients with metastatic malignancies (1). Despite its accepted role, IL-2 has limitations. IL-2 has a dual role first as an immunomodulator stimulating the proliferation of effector cells that kill cancer cells but also a second role to prevent autoimmunity by acting as a checkpoint or suppressor of the immune system by the maintenance of inhibitory CD25+Foxp3+T-regulatory cells (Tregs) and by the activation-induced cell death (AICD) (2,3). Furthermore, IL-2 has been associated with capillary leak syndrome. These issues prompted a search for other immunotherapeutics with R1530 the benefits of IL-2 but with fewer negative adverse events. IL-15, a 14- to 15-kDa member of the 4-helix bundle family of cytokines that Rabbit polyclonal to PLCXD1 was described nearly simultaneously by the Waldmann Laboratory (4) and Grabstein et al. (5) signals through a heterotrimeric receptor. IL-2 and IL-15 in their heterotrimeric receptors use cytokine-specific receptor -chains, IL-2 receptor (IL-2R/CD25) for IL-2 and IL-15R (CD215) for IL-15. They share the IL-2/IL-15R chain (CD122) and with IL-4, IL-7, IL-9, and IL-21 the common -chain (CD132) (3). Both cytokines stimulate proliferation of T cells, induce generation of cytotoxic lymphocytes (CTLs), and stimulate the expansion of natural killer (NK) cells (2,3). However, in many adaptive immune responses IL-2 and IL-15 have distinct roles. In contrast to IL-2, IL-15 inhibits IL-2mediated AICD and does not activate functional Tregs nor does it cause a major capillary leak syndrome in mice or nonhuman primates (68). IL-15 predominately acts as a cell-surface molecule as part of an immunological synapse with IL-15R on antigen-presenting cells providing IL-15in transto mononuclear cells such as NK and CD8 memory cells (9,10). In preclinical toxicology studies, IL-15 induced prolonged expansion and activation of NK cells and CD8 memory T cells (11,12). On the basis of these distinctions, we suggested that IL-15 might be better than IL-2 as a cancer immunotherapeutic. In a number of murine models, IL-15 proved to be of value in the therapy of neoplasia (1318). A study of IL-15 safety was performed in rhesus macaques and the only toxicity was neutropenia due to a transient redistribution of neutrophils from the circulation to the tissues (11). A 12-d bolus i.v. infusion of 20 g/kg/d of IL-15 to rhesus macaques was associated with a 4- to 8-fold increase in the number of circulating NK cells (11,19). When administered by continuous i.v. infusion R1530 at 20 g/kg/d for 10 d, IL-15 led to a 10-fold increase in the number of circulating NK cells, a 15-fold increase in the number of circulating monocytes, and a massive 80- to 100-fold increase in the number of circulating effector memory CD8 T cells (12). Our s.c. infusions to the nonhuman primates at 20 R1530 g/kg/d for 10 d led to a 10-fold expansion in the number of circulating effector memory T cells. On the basis of murine models of cancer, great interest has been generated in bringing IL-15 to the clinic for patients with metastatic malignancies. We have completed patient accrual in three clinical trials of IL-15 with 1822 patients each, with different dosing strategies: one by bolus infusion, one by s.c. administration, and the third by continuous i.v. infusion ofEscherichia coli-produced rhIL-15 to patients with metastatic malignancies. With bolus infusions at a dose of 3 g/kg/d, there was a greater than 10-fold expansion of NK cells (20). When IL-15.