PTGS2 and cancer: It is currently unclear how far the COX2/PGE2 system is involved in the induction and maintenance of PD-L1 and PD-L2 expression on cancer cells and cancer-associated myeloid and stromal cells, but these two inhibitory pathways appear to be at least partially independent, resulting in a synergistic effectiveness of their blockade in preclinical mouse models [134,135,136,137,138].