l-arginine and docetaxel synergistically enhance anti-tumor immunity by modifying the immune status of tumor-bearing mice

• L-arginine and docetaxel combined therapy enhances the anti-tumor effects in breast cancer mice by improving the immune status.
• The anti-tumor effects were associated with improved systemic host immune response and local tumor immunity.
• The combined therapy leads to the proliferation of CD8+ and CD4+ Th1 effector T cells and IFN-γ, as well as decreased frequency of MDSCs.

l-arginine (l-Arg) supplementation has been reported to enhance the function of immune cells, including dendritic cells (DCs) and T lymphocytes, in cancer models thereby countering the suppressive effects of myeloid-derived suppressor cells (MDSCs). The balance of the active immune cells is one factor that determines the progression of cancers in vivo. Docetaxel (DTX), an immunomodulatory chemotherapeutic agent, is now widely used in several types of malignancies including breast cancer. We hypothesized that the combination of DTX and l-Arg would elicit a more robust antitumor response than either molecule alone. To test this hypothesis we utilized BALB/c mice inoculated with 4T1 mammary carcinoma cells. DTX and l-Arg synergistically limited tumor growth in vivo and moderately increased the life span of tumor bearing mice. The anti-tumor effects were associated with the proliferation of splenic CD8+ CTL and CD4+ Th1 effector cells, as well as increased serum levels of interferon gamma. More importantly, DTX + l-Arg effectively increased anti-tumor immunity within the tumor microenvironment. Furthermore, the combined therapy increased the number of myeloid (mDCs) and plasmacytoid (pDCs) dendritic cells, potent activators of the T cell response, and enhanced expression of the maturation markers CD86 and MHC II (required for antigen presentation). The combination therapy also reduced the proliferation of MDSCs. These data suggest that DTX + l-Arg may be a novel therapeutic strategy for breast cancer patients.