Lymphatic vessels found to undergo dramatic changes during kidney transplant rejection

Scientists have uncovered how lymphatic vessels – the kidney's 'plumbing system' – undergo dramatic changes during chronic transplant rejection, becoming structurally disorganised and spreading to unusual parts of the kidney.

Researchers at the Wellcome Sanger Institute, University College London (UCL) and the University of Cambridge used single-cell sequencing combined with powerful 3D imaging to look at small lymphatic vessels in kidney tissue, comparing healthy kidneys with transplanted kidneys that had been rejected.

Published today (16 September) in the Journal of Clinical Investigation, the research sheds new light on a major unsolved challenge in kidney transplantation and could open the door to new treatments that help transplants last longer.

Kidney transplantation is the most common form of solid organ transplant worldwide. Although the short-term outcomes of kidney transplantation – within a year after surgery – are very good, the long-term outcomes are poorer. Within 10 years, and depending on what country patients are treated in, roughly 50 per cent of kidney grafts will fail.

Researchers know that a big component of why kidney transplant failure occurs is that the patient's immune system attacks parts of the new kidney – such as the blood vessels within it. However, the role of the lymphatic vessels is far less understood. In healthy kidneys, lymphatic vessels act as the organ's plumbing system – playing a vital role in draining excess fluid and helping to regulate immune activity. Therefore, the researchers sought to gain a deeper understanding of the lymphatic system during transplant rejection.

The team from the Sanger Institute, UCL and their collaborators used two different and powerful methods - single cell RNA sequencing and advanced 3D imaging. They studied samples from both healthy and transplant rejection patients. Single-cell sequencing allows scientists to study the activity of genes in individual cells, one at a time. The researchers did this on a very large scale to generate a huge amount of data. Then the team stained large chunks of kidney tissue whilst still intact and used a procedure to make it transparent. This 3D imaging helped validate the predictions from the single-cell genetic analysis.

The researchers found that during kidney transplant rejection, the lymphatic vessels within the transplant change their shape and organization. The vessels spread into deeper parts of the kidney known as the medulla, which normally has no lymphatic vessels within it. At the same time the cell junctions, which are protein anchors that connect cells, go from looking like loose buttons to tightening up like zippers. This is a change that in other contexts is associated with immune cells getting trapped and unable to escape.

Additionally, the researchers found that the balance of T cells inside and around the vessels was disrupted. These T cells released signals that made the vessels switch on molecules acting like "brakes" for the immune system, in an attempt to calm inflammation. However, this protective response was not enough, as other immune cells and antibodies were seen to be directly attacking the kidney. Strikingly, the vessels themselves were also carrying signs that they too were being targeted by the same harmful antibodies.

These findings challenge the view that lymphatic vessels are simply good or bad in transplant rejection. This study suggests that the lymphatic system is normally protective but impaired in transplant rejection as the findings show the vessels change in ways that could encourage rejection by altering their structure and fuelling immune responses. The results pave the way for research to focus on regenerating or protecting the lymphatic system in chronic kidney rejection.

You can think of lymphatic vessels as the kidney's plumbing system - clearing away excess fluid, immune cells and inflammation. Until now, we have struggled to really understand what these vessels do in kidney transplantation because they are so difficult to study. Using new imaging techniques, we've shown that these vessels undergo dramatic changes during rejection and are themselves a target of the immune system."

Dr. Daniyal Jafree, first author at the Wellcome Sanger Institute and clinician-scientist at UCL Great Ormond Street Institute of Child Health

Professor David Long, co-senior author, Professor of Paediatric Nephrology at UCL Great Ormond Street Institute of Child Health and Deputy Theme Lead of the NIHR GOSH Biomedical Research Centre, said: "Our innovative methods have allowed us to clearly demonstrate the important role of lymphatic vessels in transplant rejection. By combining single-cell sequencing with advanced 3D imaging, we've made a significant step forward in kidney transplant research."

Professor Menna Clatworthy, co-senior author and Professor of Translational Immunology at the University of Cambridge and the Wellcome Sanger Institute, said: "By uncovering how lymphatic vessels change at both the structural and molecular level, we now have a much clearer picture of the immune environment during transplant rejection. This could help us identify new therapeutic targets to preserve kidney transplants for longer."