Figure 2.
MDSCs promote tumor development. Schematic depicting treatment paradigm (A), in which MDSCs were targeted using a low-dose 5-fluorouracil (5-FU) treatment strategy during tumor engraftment and growth in the GL261 syngeneic glioma model. Kaplan–Meier survival curves (B) demonstrate that tumor-bearing mouse survival was increased with low-dose 5-FU treatment (10 mg/kg, red) compared with higher-dose (50 mg/kg, blue) and vehicle control (black) groups. Examples of tumor burden as measured by bioluminescence (C) show differences in low-dose 5-FU compared with the control treatment group. Analysis of peripheral blood MDSC levels (based on CD244.2 and GR-1 positivity) in tumor-bearing mice using flow cytometry (D) demonstrates that 5-FU treatment reduces MDSCs. Analysis of brain tissue in tumor-bearing mice using flow cytometry (E) reveals that 10 mg/kg 5-FU increased intratumoral CD8-positive T cells (CD3+/CD8+) and reduced T regulatory (TReg) cells (CD3+/CD4+/CD25+/Foxp3+). Micrographs of tumor-bearing mice (F) confirm a reduction in MDSCs (GR-1+/ARG-1+, yellow arrows in top panels), neutrophils (GR-1+/ARG-1−, yellow arrows in top panels), TRegs (FOXP3+/CD4+, yellow arrows in bottom panels), and CD4+ activated T cells (FOXP3−/CD4+, brown arrow bottom panel) with a concomitant increase in CD8+ T cells with 10 mg/kg 5-FU treatment compared with vehicle control. These immune cell populations were not detected after treatment with 50 mg/kg 5-FU. Values shown are means ±standard deviation, scale bar = 20 μm, and nuclei were counterstained with DAPI. *, p <.05, **, p <.01, and ***, p <.001 by one-way ANOVA, and statistical significance for in vivo studies was calculated using log-rank analysis. Abbreviations: IC Tx, intracranial transplantation; IP, intraperitoneal; MDSC, Myeloid-derived suppressor cell.