Tumor cell-exogenous stimuli that can lead to the overexpression of PD-L1 are cytokines (e.g., interferon (IFN)-γ, tumor necrosis factor (TNF)-α) [21,22] and various other stimuli from the TME like hypoxia or pseudohypoxia [18,23], antitumor drugs (chemotherapeutics, targeted therapeutics) [24] or metabolites (e.g., lactate) [25]. Here, TNF is linked to neoplasm.