Biomarkers for chronic rejection: In angiotensin II proteins we trust?

Despite much improved immunosuppression protocols in the clinic and excellent graft survival in the short term, the long-term outcome of kidney transplants remains not ideal, and most of the kidney allografts are eventually lost to chronic rejection, for which we currently don’t have effective means of interventions. The pathological changes in kidney allografts that undergo chronic rejection are very different from those in acute rejection, and the imposing features in the grafts have been described in specific terms as interstitial fibrosis and tubular atrophy (IFTA). This form of rejection is progressive and irreversible, and inevitably leads to permanent loss of kidney functions.¹ However, IFTA is not a single disease but a common pathology of a constellation of kidney diseases developed through a multiplicity of mechanisms. For example, ischemia reperfusion injury, early rejection episodes, anti-rejection drugs, especially the calcineurin inhibitors, and donor specific antibodies, as well as post-transplant infections, are all known to result in chronic kidney allograft rejection with characteristic features of IFTA.²

Apparently, there is a great need in the clinic to identity those who are at high risk of developing IFTA, ideally before presence of irreversible damage to the grafts, so that therapeutic measures could be instituted to delay or event prevent graft loss. This will lead to major improvements in the long-term transplant outcomes. In this issue of Transplantation,³ Mohammed-Ali and colleagues reported a urine protein signature that appears to satisfy this need. The authors demonstrated in a small cohort of kidney transplant patients that the urine levels of six angiotensin II-regulated proteins provide a distinctive marker that discriminates kidney transplant patients with or without IFTA.³ This study is built upon prior animal models of kidney fibrosis and human polycystic kidney diseases^4,5 and represents a major step forward from their transcriptional profiling studies of kidney biopsies.⁶ They presented intriguing data that in kidney transplant patients who underwent protocol biopsies, those with IFTA consistently excreted high levels of angiotensin II-regulated proteins in the urine, as opposed to patients with stable renal allograft functions without IFTA. Of note, this angiotensin II protein signature has certain interesting features. It seems that this signature is unique and doesn’t overlap with other biomarkers of chronic graft dysfunction, which makes this signature particularly attractive. Also, the urine angiotensin II protein signature is correlated strongly with the severity and progression of IFTA and is reasonably informative in segregating IFTA patients from stable controls (AUC = 0.86). Moreover, urine excretion of the six angiotensin II-regulated proteins is positively associated with one another, and all together, they are highly responsive to treatment with renin-angiotensin blockers in IFTA patients. Additionally, this urine protein signature has no obvious correlations with how long the graft has been transplanted (graft age). Thus, this protein signature may have additional prognostic values in assessing responses to potential therapies.

So, why are the angiotensin II proteins associated with, and seemingly predictive of, chronic renal allograft rejection? Under normal conditions, the angiotensin I protein is converted from its precursor, the angiotensinogen which is produced in the liver, by renin, and the resulting angiotensin I protein is further converted to angiotensin II by the angiotensin-converting enzyme (ACE). The final product angiotensin II is a powerful vasoconstrictor in the body, and plays a critical role in regulating blood volume and blood pressure. Importantly, renin is released by the kidney juxtaglomerular cells, primary due to reduced blood flow to the kidney to drive the renin-angiotensin system.⁷ Thus, it is likely that the urine angiotensin II protein signature is a result of persistent stress in the kidney transplant in response to a variety of insults, including immune attacks, and the increased urine angiotensin II proteins is probably a consequence of such ongoing stress rather than the trigger of IFTA. In fact, the authors provided evidence that in patients with elevated urine angiotensin II proteins, treatment with rennin-angiotensin blockers led to a sharp reduction in the levels of angiotensin II proteins in the urine. What remains unknown but certainly requires further investigations is the potential impact of such blockers on the incidence and progression of IFTA in those patients. This is an important issue that awaits further clarification.

The key question remains, is the angiotensin II protein signature identified in the current study ready for the prime time? As presented, this study is clearly underpowered and underperformed. There are only 15 patients included in the IFTA group and 20 patients as controls, and all assessments are performed based on protocol biopsies. For a complex clinical problem like chronic rejection, findings based on this type of study design are far from definitive. There are remarkable variations in the urine levels of individual angiotensin II proteins even in the control groups, which makes it challenging to set the exact thresholds or cutoff values for multi-variable comparisons, and importantly, for clinical decision-making. Furthermore, how this protein signature fits in, or complements other biomarkers from kidney transplant patients using different platforms is unclear.⁸ Like many other biomarker studies, the angiotensin II protein signature reported in this paper is a step forward in addressing an unmet clinical need, but its true values await further clinical validations.