CD274 and melanoma: While some in vitro studies reported that COX-2 inhibition reduced the expression of PD-L1 [24,49,51] and may potentially reduce exhaustion of T-cells in the tumor microenvironment [41], others highlighted that treatment with celecoxib did not affect PD-L1 levels in melanoma and NSCLC cells [22,52], and PD-L1 staining was also shown to be dramatically greater in mPGES1 knock out derived tumor tissues compared to controls [22].