Expression of sPD-L1 levels in an ex vivo liver perfusion model

The programmed cell death protein 1 (PD-1) acts as a central inhibitory immune checkpoint receptor. The soluble form of its primary ligand, sPD-L1, was found to be elevated in serum of patients with cancer, infectious diseases and chronic inflammation. So far, the hepatic origin of sPD-L1 has received relatively little attention and is therefore subject of this study in the context of normothermic machine perfusion (NMP) of liver grafts. sPD-L1 concentrations as well as several well-established clinically relevant laboratory parameters were determined in the perfusate of 16 donor liver grafts undergoing normothermic machine perfusion (NMP) up to 30 hours (h). sPD-L1 levels continuously increased during NMP and significantly correlated with markers of hepatic synthesis (cholinesterase), acute-phase proteins (von Willebrand factor, procalcitonin, antithrombin, interleukin-6, fibrinogen) and liver decay markers (gamma-glutamyltransferase, alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase). Perfusate leukocytes were in the lower reference range, and decreased after 12 h. Mean sPD-L1 levels in the perfusate correlated with donor levels of gamma-glutamyltransferase, alanine aminotransferase, creatinine and blood urea nitrogen. Our study reveals a significant increase of concentration of sPD-L1 following ischemia-reperfusion injury in a hepatic ex vivo model. sPD-L1 concentrations during NMP correlate with established acute-phase proteins and liver cell decay markers suggesting that hepatic sPD-L1 synthesis or shedding increases during the acute phase and cell decay. Furthermore, sPD-L1 correlates with established liver function and synthesis parameters as well as with donor laboratory values and might therefore be a potential biomarker for hepatic function of liver grafts.