Studies of normothermic machine perfusion in human kidney transplantation
Normothermic machine perfusion (NMP) of human kidneys is a new preservation technique that allows for the assessment and treatment of kidneys prior to transplantation. However, there is currently no standard protocol for NMP of kidneys and limited understanding of the underlying mechanisms that occur during the perfusion period. My PhD set out to determine the effects of NMP and to identify areas of improvement of NMP for future protocols.
Firstly, I used the Cambridge cohort of the NMP clinical trial to examine the effect of NMP on graft outcomes. NMP for one-hour was deemed to be safe and feasible in renal transplant. Kidneys with a prolonged cold ischaemic time prior to NMP, and extended second period of cold ischaemia after NMP resulted in prolonged delayed graft function and poorer early graft function. However, there was no effect on long term graft function and no significant benefit compared to those kidneys that received static cold storage (SCS). In the same cohort I assessed the relationship between urinary biomarkers and existing quality assessment scores (QAS) to evaluate the role of NMP as an assessment tool. Kidneys from older donors with longer cold ischaemia had poorer QAS and this resulted in worse 12 month follow-up outcomes compared to kidneys with better QAS score. Kidneys with higher QAS had increased urine concentration of biomarkers, but these biomarkers did not correlate with graft outcomes.
I then sought to evaluate the role of heme in the NMP perfusate. I found that older units of red blood cells had high levels of heme and that levels increased significantly during one-hour of NMP. There was also an increase in the levels of inflammatory cytokines in the perfusate and expression of genes associated with apoptosis, oxidative stress and inflammation. However there appeared to be no association between the levels of heme and these inflammatory markers along with graft outcomes in the clinical series. High levels of interleukin-6 (IL-6) were consistently found in the perfusate after one-hour NMP in both clinical and discard series. Therefore I used a human organ culture model to investigate the potential of inhibiting IL-6. I found a significant decrease in the expression of inflammatory genes and markers of oxidative stress with IL-6 inhibition using an anti-IL-6 antibody.
In summary, an end period of one-hour NMP is safe and feasible in clinical kidney transplantation. However, the negative effects of cold ischaemia are not ameliorated by one- hour NMP. Furthermore, it cannot protect against additional cold ischaemic injury after NMP. NMP can be used as a platform for assessment and urinary biomarkers are a potential marker of kidney quality and outcomes that could be used clinically Packed red blood cells from the blood bank are convenient for use during NMP. However, older units of red cells contain higher levels of heme which increases during NMP. Although this appeared to have no detrimental effect during one-hour NMP it may have consequences for longer durations of perfusion. Inhibition of IL-6 is a potential therapeutic target to reduce inflammation that warrants further investigation.