PDCD1LG2 and lung cancer: It is currently accepted for lung cancer patients8,23 but the threshold level or the applicability to other tumour types remains debated.24,25 PD-L2 is a second PD-1 ligand, described to be expressed by tumour cells and components of tumour microenvironment.26 Similar to PD-L1, PD-L2 is also mainly regulated by interferons and endowed with the ability to inhibit T cell activity and proliferation.