Differential effects of low-dose fludarabine or 5-fluorouracil on the tumor growth and myeloid derived immunosuppression status of tumor-bearing mice.

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, 100mg/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.