Combined GM-CSF treatment and M-CSF inhibition of tumor-associated macrophages induces dendritic cell-like signaling in vitro

Macrophages demonstrate plasticity, and tumor-associated macrophages (TAM) can function as immunosuppressive cells in the tumor microenvironment. Therefore, in this study, we aimed to reprogram TAM in vitro with cytokine signal alteration. Granulocyte macrophage colony stimulating factor (GM-CSF) treatment alone did not lead to changes in the expression of M1 (including IL-1β, TNFα and CXCL-10) or M2 (including CD36, CD206 and CCL17) molecules by TAM in vitro, although they adopted a round morphology and were less adhesive to the culture dish. When macrophage colony stimulating factor (M-CSF) signals were suppressed by siRNA against the M-CSF receptor (M-CSFR) in conjunction with GM-CSF treatment, the signal transduction pathway of TAM was altered, and the expression of STAT1, STAT5 and STAT6, which are usually expressed by dendritic cells, was increased. However, the same treatment did not alter the TAM expression pattern of M1/M2 marker molecules. With respect to the NF-κB pathway, GM-CSF and M-CSFR siRNA combination treatment significantly induced the expression of p65, which is usually not expressed by TAM, while p50 and p105 expression by TAM was not affected by the treatment. These findings indicate that our model could not redirect TAM to a monocyte-derived dendritic cell-like phenotype based on the analysis of M1/M2 marker expression, but it was able to modify cell signaling pathways toward a dendritic cell-like pattern. Therefore, the present data suggest that TAM demonstrate plasticity toward dendritic cell-like signal transduction patterns, and that the alteration of the tumor microenvironment has the potential to reverse the immunosuppressive properties of TAM.