SPDL1 and neoplasm: In one study, PD-L1 cell surface expression paralleled sPD-L1 secretion in response to cytokines in melanoma cells.8 Some cancer cells secrete a majority of their PD-L1 on exosomes, even with only minimal amounts of cellular PD-L1 found.9 11 The leading hypothesis is that the tumour microenvironment, including non-neoplastic cells, may also be contributing to the production of sPD-L1 and evPD-L1.