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Myeloid-derived suppressor cells (MDSCs) accumulate in tumor-bearing hosts and play a key role in the suppression of the innate and adaptive immunity. Chemotherapeutic strategies have been developed to deplete or deactivate MDSCs in different tumor models. The pyrimidine analog, 5-fluorouracil (5-FU) is found to reduce the tumor size by depleting MDSCs. Here, we asked whether the purine analog, fludarabine (Flu), could exert similar effects. Employing a lymphoma model, we demonstrated that in mice with advanced tumors (where MDSC-induced suppression was present), treatment with a single low-dose Flu (25, 50, 100 mg/kg) elevated the numbers of splenic MDSCs and serum arginase activity, and simultaneously, increased the tumor growth (only the highest dose). On the other hand, in mice with palpable tumors (where the MDSC-induced suppression was in progress), treatment with Flu had no significant effects on the tumor growth or the number of splenic MDSCs. In contrast to Flu, treatment with low-dose 5-FU, irrespective of tumor stage, caused tumor regression which coincided with significant reductions in the numbers of splenic MDSCs and blood neutrophils, but increases in the ratios of splenic CD4 + T and CD8 + T cells to suppressive MDSCs. Finally, in healthy mice (where MDSC-induced immuosuppression did not exist), 5-FU, but not Flu induced significant decreases in the number of myeloid cells in the bone marrow, naturally occurring splenic MDSCs and thymocytes. In conclusion, Flu exacerbates MDSC-induced immunosuppression in a tumor stage-dependent manner, whereas 5-FU alleviates the suppressive effects of MDSC at all stages of tumor development. [ABSTRACT FROM AUTHOR]

As reported previously, large numbers of neutrophils appear in the circulation during tumor development. However, the relationship between these cells and myeloid-derived suppressor cells (MDSCs), as well as their susceptibility to myelosuppressive drugs have not been yet investigated. Here, we employed a lymphoma model to characterize tumor-associated circulating neutrophils, including their sensitivity to 5-fluorouracil (5-FU), busulfan (Bu) or treosulfan (Treo). Tumor-bearing mice exhibited pronounced elevations in the numbers of splenic MDSCs and circulating neutrophils, MDSCs, and granulocytic-MDSCs (G-MDSCs). Although these cells were not affected by 5-FU at a dose of 17 mg/kg, 50 and 100 mg/kg were equally effective in causing dramatic tumor regression, normalizing neutrophil number, and significantly reducing the numbers of splenic MDSCs, B cells, and bone marrow myeloid cells. Treatment with Bu (10, 30 or 60 mg/kg) only reduced the number of circulating neutrophils, with no effects on these other parameters. At a concentration of 500 mg/kg, Treo was ineffective, whereas, doses of 1500 and 3000 mg/kg comparably reduced the tumor size as well as the numbers of circulatory neutrophils and bone marrow myeloid cells. Finally, in comparison to 5-FU alone, a combined treatment with low-dose 5-FU and Treo resulted in a more pronounced reduction in the numbers of circulatory neutrophils and bone marrow myeloid cells, together with longer survival time. We conclude that tumor-associated circulatory neutrophils represent blood MDSCs/G-MDSCs that are highly sensitive to 5-FU and Treo, but not Bu. Moreover, the efficacy of 5-FU can be potentiated by Treo. [ABSTRACT FROM AUTHOR]