Activated Eosinophil Subsets are an Integral Part of the Tumor Microenvironment in Lung Metastasis, Displaying Anti-tumorigenic Activities

Cancer-associated mortality is nearly exclusively a result of tumor metastasis, and the lung is a main metastatic site for multiple tumors. Eosinophils are granulocytes, which reside in mucosal tissues such as the lungs. Although eosinophils are highly equipped to shape the tumor microenvironment, their roles and phenotypes in lung metastasis are understudied.

Intravenous injection of PyMT (breast cancer), B16-F10 (malignant melanoma) and MC38 (colorectal cancer) cells resulted in experimental lung metastasis, which was associated with increased recruitment of eosinophils. At least two eosinophil subsets were identified within the tumor microenvironment as defined by the expression of Siglec-F and CD125. Immunohistochemical staining of the metastatic lungs revealed distinct location for eosinophils (i.e. within tumors and in the lung parenchyma). Recruitment of eosinophils into the metastatic lung was independent of the hallmark CCR3-eotaxin chemokine axis since eosinophils were readily detected in the lungs of tumor-colonized Ccr3^-/- mice. Consistent with our in vivo findings, PyMT, B16-F10 and MC38 cells did not express eotaxins, yet tumor-secreted factors, present in their culture media induced rapid and marked eosinophil migration in vitro and following intraperitoneal injections in vivo. Adoptive transfer of eosinophils to eosinophil-deficient mice (i.e. ΔdblGATA mice) undergoing breast cancer associated lung metastasis resulted in preferential and rapid homing of eosinophils displaying high Siglec-F into the metastatic lung compare to naïve lung. In addition, tumor-secreted factors prolonged eosinophil survival. Similarly, conditioned media of human colorectal, melanoma and breast cancer cells, prolonged human eosinophil survival in vitro. Intravenous injection of PyMT and B16-F10 cells to wild type and ΔdblGATA mice, resulted in markedly increased tumor burden in ΔdblGATA mice. In support of these findings, co-culture of eosinophils with PyMT or B16-F10 cells, resulted in significant eosinophil-driven cytotoxicity towards the tumor cells. Notably, the antitumorigenic activities of eosinophils towards PyMT cells were lung-specific since no difference was observed between wild type and ΔdblGATA mice, in a model of primary breast cancer.

We demonstrate potent recruitment and anti-tumorigenic activities for eosinophils in the lungs in response to several tumor cell types. These data highlight eosinophils as a novel cellular target for immunotherapy in lung metastasis.