CD274 and neoplasm: PD-L1 expression in tumor cells is regulated by several mechanisms: It is often associated with the presence of inflammatory mediators, such as Interferon gamma (IFN-γ, a cytokine critical to both innate and adaptive immunity, and functions as the primary activator of macrophages and in stimulates natural killer cells and neutrophils) and the loss of PTEN with consequent PI3K activation.