CD4 and neoplasm: Mitchell et al. [52] demonstrated that after Toca 511/5-FC treatment, myeloid-derived suppressor cells (MDSC) were significantly decreased in subcutaneous tumors, while CD4+ and CD8+ T cells were both significantly increased, suggesting that “bystander effects” due to intratumoral production of 5-FU resulted in local myelotoxicity within the tumor microenvironment, while maintaining systemic immune function.