CD274 and breast cancer: Additionally, as a significant percentage of mammary tumors exhibit PD-L1 expression also in their stroma,12 in the next step, we evaluated the PD-L1 protein expression in macrophages, which are known as key contributors of tumor stroma formation.13 14 We have studied the PD-L1 expression using immunofluorescent microscopy in the M0, interferon γ (IFNγ)-stimulated M0, and M2 phenotypes; additionally, CD206 as a characteristic marker for the M2 phenotype was used.