Overexpression of IL-15 promotes tumor destruction via NK1.1+ cells in a spontaneous breast cancer model.

BACKGROUND: Natural Killer (NK) cells play an important role in tumor prevention, but once tumors form, the numbers as well as the cytotoxic functions of NK cells are reduced. IL-15 is a cytokine that increases and activates NK cells. Here we will examine the anti-tumor role of IL-15 in a spontaneous breast cancer model. METHODS: To achieve this, Polyoma Middle T (MT) mice that form spontaneous breast cancer were crossed with mice that either overexpress IL-15 (IL-15 transgenic (TG)) or mice that lack IL-15 (IL-15 knockout (KO)). We compared survival curves and tumor formation in IL-15 KO/MT, MT and IL-15 TG/MT groups. In addition, the phenotype, activation and contribution of NK cells and CD8 T cells to tumor formation were examined in each of these mouse strains via flow cytometry, ELISA, adoptive transfer and antibody depletion experiments. RESULTS: IL-15KO/MT tumors formed and progressed to endpoint more quickly than MT tumors. These tumors displayed little apoptosis and poor CD8 T cell infiltration. In contrast, IL-15 TG/MT mice had increased survival and the tumors displayed extensive cell death, high proportions of activated NK cells and a higher infiltration of CD8 T cells than MT tumors. CD8 T cells in IL-15 TG/MT tumors were capable of secreting IFNγ, possessed markers of memory, did not display an exhausted phenotype and were frequently NK1.1+. Long-term antibody depletion studies in IL-15 TG/MT mice revealed that NK1.1+, but not CD8 T cells, were critical for tumor destruction. Lastly, human NK cells, when exposed to a similar cytokine environment as that found in IL-15TG/MT tumors, were capable of killing human breast cancer cells. CONCLUSIONS: This study reveals that high levels of IL-15 can promote tumor destruction and reduce metastasis in breast cancer via effects on NK1.1+ cells. Our results suggest that strategies aimed at increasing NK cell activation may be effective against solid epithelial cancers.

M2-polarized and tumor-associated macrophages alter NK cell phenotype and function in a contact-dependent manner.

The crosstalk between NK cells and M1 macrophages has a vital role in the protection against infections and tumor development. However, macrophages in the tumor resemble an M2 phenotype, and, at present, their effect on NK cells is less clear. This study investigated whether tumor-associated macrophages (TAMs) have a role in altering NK cell function and phenotype using in vitro cocultures of murine NK cells with peritoneal or bone marrow-derived, M2-polarized macrophages or TAMs isolated from spontaneous mouse breast tumors. We report here that both peritoneal and bone marrow-derived M2 macrophages, as well as TAMs, substantially inhibit NK cell activation and concordant cytotoxicity against tumor cells. The mechanism for this inhibition was found to require contact between the respective cell types. Both M2 macrophages and TAMs are producers of the immunosuppressive cytokine TGF-ß. The inhibition of TGF-ß restored the cytotoxicity of NK cells in contact with M2 macrophages, implicating TGF-ß in the mechanism for NK cell inhibition. In addition to affecting NK cell function, TAMs also induced a CD27lowCD11bhigh-exhausted NK cell phenotype, which corresponds with the reduced activation and cytotoxicity observed. This study reveals a novel implication of TAMs in the tumor-associated inhibition of NK cell function by demonstrating their capacity to directly alter NK cell cytotoxicity and phenotype in a contact-dependent mechanism involving TGF-ß. These findings identify the interaction between NK cells and TAMs as a prospective therapeutic target to enhance NK cell effector function for effective NK cell cancer therapies.

The breast tumor microenvironment alters the phenotype and function of natural killer cells.

Natural killer (NK) cells are innate immune cells with the ability to identify and eliminate transformed cells. However, within tumors, many studies have described NK cells as non-functional. The developmental stage of tumor-associated NK cells and how this may relate to functionality has not been explored. We examined the developmental state of NK cells from polyoma middle T antigen (pyMT) transgenic mouse (MMTV-pMT) breast tumors. In pyMT tumors, NK cells were immature as evidenced by their decreased expression of DX5 and their CD27(low)CD11b(low) phenotype. These immature NK cells also had increased expression of NKG2A and expressed low levels of NKp46, perforin, and granzyme B. In contrast, splenic NK cells isolated from the same mice maintained their maturity and their expression of activation markers. To delineate whether the tumor microenvironment directly alters NK cells, we adoptively transferred labeled NK cells and followed their activation status in both the spleen and the tumor. NK cells that arrived at the tumor had half the expression of NKp46 within three days of transfer in comparison to those which arrived at the spleen. In an effort to modify the tumor microenvironment and assess the plasticity of intratumoral NK cells, we treated pyMT tumors with IL-12 and anti-TGF-ß. After one week of treatment, the maturity of tumor-associated NK cells was increased; thus, indicating that these cells possess the ability to mature and become activated. A better understanding of how NK cells are modified by the tumor microenvironment will help to develop strategies aimed at bolstering immune responses against tumors.