Treatment-Induced Tumor Cell Apoptosis and Secondary Necrosis Drive Tumor Progression in the Residual Tumor Microenvironment through MerTK and IDO1

Abstract

Efferocytosis is the process by which apoptotic cells are cleared from tissue by phagocytic cells. The removal of apoptotic cells prevents them from undergoing secondary necrosis and releasing their inflammation-inducing intracellular contents. Efferocytosis also limits tissue damage by increasing immunosuppressive cytokines and leukocytes and maintains tissue homeostasis by promoting tolerance to antigens derived from apoptotic cells. Thus, tumor cell efferocytosis following cytotoxic cancer treatment could impart tolerance to tumor cells evading treatment-induced apoptosis with deleterious consequences in tumor residual disease. We report here that efferocytosis cleared apoptotic tumor cells in residual disease of lapatinib-treated HER2 þ mammary tumors in MMTV-Neu mice, increased immunosuppressive cytokines, myeloid-derived suppressor cells (MDSC), and regulatory T cells (Treg). Blockade of efferocytosis induced secondary necrosis of apoptotic cells, but failed to prevent increased tumor MDSCs, Treg, and immunosuppressive cytokines. We found that efferocytosis stimulated expression of IFN-g, which stimulated the expression of indoleamine-2,3-dioxegenase (IDO) 1, an immune regulator known for driving maternal-fetal antigen tolerance. Combined inhibition of efferocytosis and IDO1 in tumor residual disease decreased apoptotic cell-and necrotic cellinduced immunosuppressive phenotypes, blocked tumor metastasis, and caused tumor regression in 60% of MMTV-Neu mice. This suggests that apoptotic and necrotic tumor cells, via efferocytosis and IDO1, respectively, promote tumor 'homeostasis' and progression.Significance: These findings show in a model of HER2 þ breast cancer that necrosis secondary to impaired efferocytosis activates IDO1 to drive immunosuppression and tumor progression. Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis):

A Potent and Selective Dual Inhibitor of AXL and MERTK Possesses Both Immunomodulatory and Tumor-Targeted Activity

“…It is believed that MERTK plays a predominant role in efferocytosis of M2-like macrophages, which produces immunosuppressive signals (6,22,23). Studies have previously shown that simultaneously inhibiting efferocytosis by blocking TAMs and inhibiting indoleamine-2,3-dioxegenase 1 led to decreased immunosuppressive phenotypes and induced tumor regression (34). INCB081776 inhibited phagocytosis of human monocytes, consistent with anti-phagocytic properties of other MERTK inhibitors (35).…”

The immunological consequences of radiation‐induced DNA damage

“…Historically, apoptosis has been considered to have predominantly anti‐inflammatory consequences. More recently, the phagocytic clearance of apoptotic cells by efferocytosis has been shown to increase immunosuppressive cytokines and leukocytes . However, these anti‐inflammatory effects are probably counterbalanced by the highly proinflammatory pre‐apoptotic fragmentation and sensing of dsDNA .…”

Regulation of efferocytosis as a novel cancer therapy

“…The results indicate that apoptotic and necrotic cancer cells promote tumor progression through efferocytosis and IDO1 respectively. Combined inhibition of these two processes showed a better result in tumor regression and inhibition of metastasis [52].…”

“…Taken together, efferocytosis hinders the inflammatory response and modulates the immune environment, which facilitates immune escape and promotes tumorigenesis and progression ( Fig. 1) [52]. Conversely, inhibition of efferocytosis in the context of cancer results in the release of intracellular components of dying cells.…”

“…A recent study [52] described two distinct mechanisms for cell death: apoptosis and secondary necrosis, both of which affect the tumor microenvironment in different ways. In the study, inhibition of efferocytosis did not suppress the production of tumor myeloid-derived suppressor cells, Treg cells, and some immunosuppressive mediators.…”