We have examined the malignant tumor microenvironment (TME) from a perspective that is often noted but overlooked: tissue repair. It has long been observed that tumors resemble ‘wounds that do not heal,’ but the reasons remain obscured. We found that macrophages in the tumor microenvironment orchestrate wound healing and immune tolerance in response to tumor cell death, a feature common to all solid tumors. We used genetic and pharmacological methods to block macrophage-mediated engulfment of dying tumor cells, a process referred to as efferocytosis. We found that MerTK-mediated efferocytosis of dying tumor cells induces macrophage-mediated production of Th2-like cytokines, including IL-4, IL-13, IL-10, and TGF-beta. Blockade of the receptor tyrosine kinase MerTK through pharmacological tyrosine kinase inhibition or through genetic MerTK abalation blocked macrophage-mediated efferocytosis in the tumor microenvironment, impairing M2-like macrophage polarization and expression of Th2-like cytokines in the tumor microenvironment. Because cell death is a major histological feature of the breast during post-partum involution of milk-producing breast epithelia, we studied the impact of MerTK-mediated efferocytosis in a model of post-partum breast cancer (ppBC). Currently, ppBC accounts for nearly 25% of all breast cancers in young (pre-menopausal) women. In contrast to breast cancers diagnosed during pregnancy, which correlate with a favorable prognosis, ppBCs are highly aggressive, metastatic, and life-threatening, even when corrected for molecular breast cancer subtype and for the age of the woman at diagnosis. We used genetic and pharmacological methods to block MerTK-mediated efferocytosis in ppBCs, demonstrating that efferocytosis increased ppBC metastasis by nearly 10-fold, but that genetic or pharmacological MerTK inhibition markedly decreased metastasis in ppBCS. Similarly, targeted inhibition of efferocytosis-induced cytokines, including TGF-beta, substantially reduced metastasis of efferocytosis-competent ppBCs. Collagen deposition, TReg expansion, and Th2 cytokine upregulation were found to be dependent on efferocytosis-induced TGF-beta signaling. These findings in ppBC, in which widespread tumor cell death was initiated by physiological post-partum involution, prompted our interest in the impact of therapeutically-induced cell death and subsequent efferocytosis on pro-malignant leukocyte behaviors in the tumor microenvironment. As many as 70-80% of primary breast tumors treated with cytotoxic chemotherapy show a partial response in the pre-surgical (neoadjuvant) setting, and lack of a pathological complete response (pCR) is a strong predictor of tumor recurrence following surgical tumor excision. We tested the hypothesis that efferocytosis, the immune system's inherent response to cell death, results in tumor repair (increased ‘wound healing’) in response to cytotoxic treatment. In support of this hypothesis, a single dose of doxorubicin or lapatinib induced a transient wave tumor cell death within 24 hours, followed at post-treatment day 4 with increased M2 macrophages and TRegs within the tumor microenvironment, and increased expression of Th2-like cytokines. Importantly, increased metastases were seen in tumor-bearing mice treated once with doxorubicin over what was seen in untreated mice. However, targeted inhibition of efferocytosis using a MerTK kinase inhibitor (TKIs) in combination with doxorubicin blocked expression of Th2-like cytokines, limited the number of M2-like macrophages in the tumor microenvironment, and prevented therapy-enhanced tumor metastasis. These findings support continued investigation into the role of MerTK and efferocytosis in therapeutic tumor responses.
Note:This abstract was not presented at the conference.
Citation Format: Rebecca S. Cook. Taking out the trash: Efferocytosis in the tumor microenvironment. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A126.
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