Shifting the balance away from tumor-mediated immune suppression toward tumor immune rejection is the conceptual foundation for a variety of immunotherapy efforts currently being tested. These efforts largely focus on activating antitumor immune responses but are confounded by multiple immune cell populations, including myeloid-derived suppressor cells (MDSCs), which serve to suppress immune system function. We have identified immune-suppressive MDSCs in the brains of GBM patients and found that they were in close proximity to self-renewing cancer stem cells (CSCs). MDSCs were selectively depleted using 5-flurouracil (5-FU) in a low-dose administration paradigm, which resulted in prolonged survival in a syngeneic mouse model of glioma. In coculture studies, patient-derived CSCs but not nonstem tumor cells selectively drove MDSC-mediated immune suppression. A cytokine screen revealed that CSCs secreted multiple factors that promoted this activity, including macrophage migration inhibitory factor (MIF), which was produced at high levels by CSCs. Addition of MIF increased production of the immune-suppressive enzyme arginase-1 in MDSCs in a CXCR2-dependent manner, whereas blocking MIF reduced arginase-1 production. Similarly to 5-FU, targeting tumor-derived MIF conferred a survival advantage to tumor-bearing animals and increased the cytotoxic T cell response within the tumor. Importantly, tumor cell proliferation, survival, and self-renewal were not impacted by MIF reduction, demonstrating that MIF is primarily an indirect promoter of GBM progression, working to suppress immune rejection by activating and protecting immune suppressive MDSCs within the GBM tumor microenvironment.

Glioblastoma (GBM) contains a self-renewing, tumorigenic cancer stem cell (CSC) population which contributes to tumor propagation and therapeutic resistance. While the tumor microenvironment is essential to CSC self-renewal, the mechanisms by which CSCs sense and respond to microenvironmental conditions are poorly understood. Scavenger receptors are a broad class of membrane receptors that are well characterized on immune cells and instrumental in sensing apoptotic cellular debris and modified lipids. Here we provide evidence that CSCs selectively utilize the scavenger receptor CD36 to promote their maintenance using patient-derived CSCs and in vivo xenograft models. We detected CD36 expression in GBM cells in addition to previously described cell types including endothelial cells, macrophages and microglia. CD36 was enriched in CSCs and was able to functionally distinguish self-renewing cells. CD36 was co-expressed with integrin alpha 6 and CD133, previously described CSC markers, and CD36 reduction resulted in concomitant loss of integrin alpha 6 expression, self-renewal and tumor initiation capacity. We confirmed that oxidized phospholipids, ligands of CD36, were present in GBM and found that the proliferation of CSCs, but not non-CSCs, increased with exposure to oxidized low-density lipoprotein. CD36 was an informative biomarker of malignancy and negatively correlated to patient prognosis. These results provide a paradigm for CSCs to thrive by the selective enhanced expression of scavenger receptors, providing survival and metabolic advantages.

No abstract

With the goal to remove the roots of cancer, eliminate metastatic seeds, and overcome therapy resistance, the 2014 inaugural International Cancer Stem Cell (CSC) Conference at Cleveland, OH, convened together over 320 investigators, including 55 invited world-class speakers, 25 short oral presenters, and 100 poster presenters, to gain an in-depth understanding of CSCs and explore therapeutic opportunities targeting CSCs. The meeting enabled intriguing discussions on several topics including: genetics and epigenetics; cancer origin and evolution; microenvironment and exosomes; metabolism and inflammation; metastasis and therapy resistance; single cell and heterogeneity; plasticity and reprogramming; as well as other new concepts. Reports of clinical trials targeting CSCs emphasized the urgent need for strategically designing combinational CSC-targeting therapies against cancer.