PDCD1 and breast cancer: Furthermore, consistent with our results during in mouse mammary tissue during involution we did not observe significant co-expression of CD274 with PDGFA, an established fibroblast marker, or with PDGFB the heterodimeric partner of PDGFA. Therefore, we predict that this mechanism of immune suppression is mediated, in part, by cells of the TME may be at play in breast cancer patients and that analysis of LEC and monocyte/macrophage content could be utilized to predict whether a breast cancer patient is likely to respond to PD-1-targeted therapy.