Abstract
While macrophage enrichment and lymphocyte depletion have been described in glioblastoma, intratumoral neutrophils and their effect on glioblastoma have been under-characterized. While tumor-associated neutrophils (TANs) were initially regarded as passive bystanders due to their short-lived nature, investigation of TANs in other cancer types revealed pro-tumoral roles. Therefore, we sought to characterize TANs in the glioblastoma microenvironment using transcriptomic analysis and define their oncologic effects. Flow cytometric analysis of patient samples for neutrophils (CD11b+/CD15+/CD66b+) revealed higher percentages of TANs in glioblastoma compared to low-grade gliomas (1.76% [n=13] vs. 0.33% [n=6], p=0.03). Using the Transwell migration assay with glioblastoma tumor conditioned-media (CM), we found that recruitment of circulating neutrophils to tumor sites is mediated by leukotriene-B4 chemoattraction and that this interaction can be blocked with the addition of LtB4 receptor antagonist, LY293111. TANs were morphologically activated, unlike circulating neutrophils from GBM patients (P< 0.05) and, while not intravascular, were close to blood vessels. We performed single-cell RNA sequencing of isolated TANs and found a distinct transcriptomic profile relative to circulating neutrophils from these patients, particularly upregulated osteopontin. Osteopontin concentration was significantly higher in TAN CM than in patient-matched peripheral blood neutrophil CM (3.2ng/mL [n=3] vs. 0.02ng/mL [n=3], p< 0.05). Because osteopontin is linked to GBM stem cell-like phenotype maintenance and TANs localized to the perivascular niche where GBM stem cells reside, we investigated TAN-GBM stem cell interactions and osteopontin as a potential mediator. We found TAN CM increased proliferation and stem cell markers (Nanog, Oct4, Sox2) of stem cell-containing GBM neurospheres (p< 0.01). These effects were blocked by osteopontin-neutralizing antibodies (p< 0.01). Our work defines neutrophil-mediated pro-tumoral effects and their mechanisms and identifies a novel approach to target GBM stem cells—by disrupting the immune cell mediators that create their supportive microenvironment in the perivascular niche.
Flow cytometric evaluation of cancer stem cell markers following sorafenib treatment in HepG2 cells