Dipeptidyl Peptidase Inhibition Alters Myeloid Derived Suppressor Cell Phenotype and Induces Regression of Pediatric Sarcomas.

Tumor associated macrophages and myeloid derived suppressor cells (MDSCs) are increasingly recognized to play an important role in cancer immune escape, suppression of adaptive immunity, promotion or tumor progression, and are associated with poor prognosis in adult cancers. Recently, it has been reported that pediatric solid tumors are characterized by marked infiltration of macrophages. Little is known about the presence and the role of these cells in the pathobiology of pediatric cancers. We sought to characterize tumor infiltrating myeloid cells in a murine model of embryonal rhabdomyosarcoma (ERMS).

C57BL/6 mice were injected with 1×10⁶ RMS M3-9-M murine ERMS cells intramuscularly into the gastrocnemius muscle. Tumors and spleens were excised on days 7, 12, 16 and 19 post injection and myeloid cell were analyzed by Fluorescence Activated Cell Sorting (FACS) for CD11b, CD11c, Gr-1 and IL-4Ralpha expression. Cells were sorted and were analyzed for gene expression of iNOS, Arg1, IDO, S100A8 and S100A9 using quantitative RT-PCR. ERMS bearing mice were treated with the Dipeptidyl peptidase inhibitors PT-100 (20 mcg/day) and 4175 (100 mcg/day) by gavage for 10 days.

Murine ERMS show substantial numbers of MDSCs both in the tumor and spleen of tumor bearing mice, with increasing numbers as tumor burden increased (P=0.006). Substantial numbers of these cells express IL-4Ra, a marker that has been associated with a suppressive phenotype; with a frequency of 16% and 9% of total cells respectively in tumors and spleens from ERMS bearing mice when compared to 3% in naïve spleen. Comparisons of sort-purified myeloid cell subsets by relative quantitation of gene expression revealed the presence of an expanded population of cells that bear the hallmarks of MDSCs with high expression of IDO, iNOS, Arg1, S100A8 and A100A9. Treatment with DPP inhibitors induced tumor regression that is T cell dependent and occurs despite cessation of therapy when tumors are still measurable. No recurrences were observed during a 50 day follow-up and mice were protected in a tumor-specific manner from tumor rechallenge. DPP inhibitor treatment was associated with decrease MDSC infiltration, 17% in the control group vs. 7% in the treated group (P=0.02). Interestingly, when comparing MDSCs to non-MDSC myeloid cells, PT-100 altered MDSC gene expression by reducing IDO expression (The fold change was 17000 in controls vs. 1800 in the treated mice) and iNOS expression (73000 vs. 4400). Conversely, Arg1 expression increased in the treated group from 10 fold change to 700. Similarly, S100A8 and S100A9 expression increased with DPP inhibition (22 vs. 380 fold change and 15 vs. 1550) respectively.

Murine ERMS contain large numbers of MDSCs characterized by cell surface markers and gene expression profile that is associated with a suppressive phenotype. Tumor progression is associated with increasing numbers of MDSCs. Treatment with the DPP inhibitors induced tumor regression and was associated with decrease frequency of MDSCs with alteration of gene expression. Immunomodulation of MDSCs might be a strategy to enhance the effectiveness of immune-based therapies for pediatric solid tumors.

No relevant conflicts of interest to declare.