Human monocytes display remarkable plasticity and can differentiate into macrophages or dendritic cells (DCs) depending on environmental cues. In vitro, granulocyte-macrophage colony-stimulating factor (GM-CSF) promotes macrophage differentiation, whereas GM-CSF plus interleukin-4 (IL-4) is the classical model for generating monocyte-derived DCs (Mo-DCs). We previously found that combined inhibition of mechanistic target of rapamycin complex 1 (mTORC1) and peroxisome proliferator-activated receptor gamma (PPARγ) redirects GM-CSF-treated monocytes toward a DC phenotype. These cells display a highly immunogenic activation profile, including significantly higher interleukin-12 (IL-12) production than conventional GM-CSF/IL-4-derived Mo-DCs. We found that this differentiation program is critically dependent on IRF4, yet proceeds without evidence of canonical IL-4/STAT6 signaling, suggesting that distinct upstream pathways converge on IRF4 to regulate monocyte fate. Here, we show that this pharmacological combination can rewire intratumoral monocyte differentiation in vivo using humanized mouse models of cancer.
To directly assess monocyte fate within tumors, human monocytes were injected intratumorally into MDA-MB-231 tumors established in immunodeficient, non-humanized NSG-SGM3 mice. Mice received systemic temsirolimus plus GW9662 every other day for 5 days. Combined mTORC1/PPARγ inhibition redirected transferred monocytes away from macrophage differentiation and toward CD1c⁺CD16⁻ Mo-DC-like cells. Moreover, one-hour ex vivo preconditioning of monocytes with both inhibitors before intratumoral transfer biased their subsequent differentiation toward a DC-like phenotype, supporting a cell-intrinsic effect on monocyte fate.
In a separate experiment, CD34⁺ humanized NSG-SGM3 mice bearing MDA-MB-231 tumors received daily systemic temsirolimus plus GW9662 for 20 days. Treatment restrained tumor growth and remodeled the human tumor myeloid compartment, reducing CD16⁺CD1c⁻ tumor-associated macrophages while enriching CD1c⁺ myeloid populations.
These findings demonstrate that combined mTORC1 and PPARγ inhibition can pharmacologically rewire human monocyte differentiation within tumors and remodel the tumor myeloid compartment in vivo. Targeting monocyte fate may therefore provide a strategy to enhance cancer immunotherapy.