Kidney transplantation is considered the best option to treat end-stage kidney disease, but the recipient's immune system may respond with rejection to the transplanted organ, leading to permanent kidney damage and failure.

The current standard for rejection monitoring in transplanted recipients is regular blood creatinine testing and kidney biopsies. Creatinine doesn't detect rejection until damages had occurred, causing some amount of kidney failure, and kidney biopsies are only done at set timepoints.

A new test called CXCL10 is shown to be more effective in detecting rejection from previous research and can be done as often as needed. Therefore, The investigators are doing this randomized trial to test CXCL10 as part of clinical care and to help design a larger national clinical trial in the future.

Kidney transplant offers hope to thousands of people living with chronic kidney disease, but the average five-year-old who gets a kidney transplant should expect to need a second transplant before they reach adulthood. In adults, transplantation extends patient survival by 30-40 years compared to dialysis. In children, almost 30% lose the transplant within 5 years, with acute rejection as the most common cause. Despite the urgent need, approaches for monitoring and early acute rejection detection are largely unchanged for the past 20 years. The goal of this study is to introduce innovative monitoring that will permit timely acute rejection diagnosis and effective treatment, preserve kidney function, delay graft failure and spare the lives of children with chronic kidney failure.

In pediatric kidney transplants, a single episode of acute rejection confers up to a 19-fold increased risk of histologically visible scarring, the hallmark sign of progressive graft damage. Acute acute rejection is associated with subsequent development of anti-donor HLA antibodies, and chronic types of antibody-mediated and T cell-mediated acute rejection that are refractory to treatment. Acute rejection is readily treated with IV corticosteroids or depleting anti-thymocyte antibody treatments. These major adverse outcomes from acute rejection can only be mitigated or avoided if acute rejection is 1) identified and treated early before chronic inflammation is established; and 2) treated fully with confirmation that the acute inflammation has resolved.

The reported adverse outcomes relate predominantly to "clinical" presentations of acute rejection, i.e. those cases presenting with already worsening kidney failure. Subclinical acute rejection refers to the earlier phase of acute rejection where inflammation is onset but has not yet led to clinical graft dysfunction. This matters enormously because although acute rejection may be invisible to standard clinical monitoring, early identification and treatment of subclinical acute rejection improves long-term function and allograft survival. Acute rejection is a process, not an event. It starts with acute inflammation, and in the absence of immediate treatment it initiates a cascade of immune cytokine signals that causes memory types of effector cells to mature and leads to the suppression of regulatory types of immune cells that would otherwise promote tolerance to the allograft. This immune cell profile within the kidney is characteristic of chronic rejection and often co-exists in late-detected episodes of acute rejection.

Subclinical acute rejection is typically NOT associated with concurrent chronic inflammation or the presence of donor specific antibodies. With timely treatment of acute rejection, there is better preservation of histology, kidney function and survival . The pivotal trial by Rush et al., demonstrated that treatment of subclinical acute rejection detected by protocol surveillance biopsies improved long term outcome compared to clinical acute rejection monitoring and treatment. Natural history studies in adults and in children confirm the progressive nature of untreated subclinical acute rejection and the superior outcome when it is treated. Whether clinically detected or detected on biopsy surveillance (subclinical), both are considered as "acute rejection" and are treated similarly, based on the histological severity.

Unfortunately, damage related to delayed diagnosis and treatment is compounded by the lack of tools to evaluate treatment response. Although it is not standard, follow-up biopsies after acute rejection treatment reveal that 50-61% of children have persistent acute rejection subclinical 6-8 weeks after treatment - undetected by kidney function monitoring . Earlier diagnosis and treatment are therefore critical to prevent damage and preserve long-term function, as are interventions to ensure that reversal of subclinical inflammation is complete.

Acute rejection is much more common in children than in adults. This risk is compounded by the greater inefficacy of standard kidney function monitoring used for adults (serum creatinine) making it even harder to detect acute rejection in children or to monitor recovery after acute rejection treatment. This has led to a greater reliance on invasive kidney biopsies in children to surveil for subclinical acute rejection. Rates of surveillance biopsy-confirmed subclinical acute rejection reported in children range from 44-58% in the first year after transplant, much higher than in adults (4.2%).

Late acute rejection (>1 year post-transplant) in adolescence has an even worse prognosis, due to the extended periods of time with indolent and progressive inflammation before kidney failure is detected. Late acute rejection is beyond the post-transplant phase where routine biopsy surveillance is practical. This more entrenched and refractory inflammation is associated with 2 to 10-fold higher risk of chronic damage and graft failure compared to acute rejection in the first post-transplant year. Risk factors include non-adherence, which is also notoriously difficult to detect. Non-adherence is associated with higher rates late acute and chronic rejection, and is directly implicated in more than a third of pediatric renal allograft losses. Additional age-related and sex-related immune changes that peak in adolescence place this group at particularly high-risk. Timely detection of subclinical late acute rejection is currently impossible, since diagnosis requires already progressive functional deterioration to indicate a biopsy. Early acute rejection detection with new non-invasive tools could dramatically improve the outlook in these high-risk young people.

The investigators currently lack a reliable, noninvasive clinical test for subclinical acute rejection, which means that acute rejection may fester undetected until substantial kidney damage has occurred. The introduction of surveillance biopsies has led to earlier detection of subclinical acute rejection, improved allograft function and increased survival. However, concerns about procedure morbidity and cost have hindered widespread adoption of surveillance biopsies, in use at only 34% of pediatric transplant centers. These same issues practically limit their use for serial monitoring, especially after the first transplant year, highlighting the need for a noninvasive alternative.

There is ample evidence that urinary CXCL10 testing is very effective at both identifying acute rejection AND confirming response to treatment. Since 2004, there have been 19 reports that support its use for detection of subclinical acute rejection, including 4 reports in children. Other cytokines that have not been fully validated in children include CXCL9 and IL-6. Levels of urinary CXCL10 increase in advance of clinical acute rejection diagnosis and decreases after its treatment and resolution . Urinary CXCL10 is also effective at identifying inflammation from other important causes of graft injury, such as CMV interstitial nephritis, subclinical pyelonephritis and BK polyomavirus nephritis. Persistent elevation of urinary CXCL10 signals unresolved acute inflammation, such as from untreated or persistent subclinical acute rejection and is associated with chronic scarring and progressive kidney failure. Despite the wealth of evidence in support, CXCL10 testing has not yet been translated into routine practice because of the lack of studies that demonstrate clinical effectiveness to improve outcome.

This proposal will test the feasibility of a definitive clinical trial in transplanted children. The goal of the definitive clinical trial is to determine whether serial urinary CXCL10 monitoring will reduce the incidence of and severity of allograft failure. It will be a prospective, multicenter, randomized, controlled trial where the intervention is serial urinary monitoring of CXCL10 alongside routine clinical testing at 1-3-month intervals, with test results used to guide subsequent care decisions. Patients randomized to control will have standard of care monitoring, with biobanking of urine samples for later analysis. The primary outcome will be the difference in relative decline (from baseline) of kidney function after 2 years of monitoring. The will also collect preliminary data on validity of cytokines CXCL9 and IL-6, for potential inclusion in the definitive study.

This pilot study design is a prospective, multicenter, randomized, controlled study of the feasibility and acceptability of a definitive clinical trial. It will follow the same procedures as the definitive trial; however, the primary aims will be to demonstrate that logistical and recruitment requirements can be met as well as to collect economic data to inform the optimal testing procedure (local vs. central). The pilot will enroll 60 prevalent kidney transplant recipients who are >6 months post-transplant.

The objectives of this pilot trial are to evaluate the feasibility of a definitive trial:

1. Demonstrate capacity to recruit, randomize and retain 60 participants
2. Identify and resolve challenges in executing the study protocol regarding sample acquisition, CXCL10 testing, and adherence to the mandated trial protocols
3. Comparison of cost and logistical supports needed for local vs. central CXCL10 testing options

Inclusion Criteria:

• Prevalent pediatric kidney transplant recipients (<19 years at transplantation) who are more than 6 months after transplant, with informed consent and assent.
• Must be available to follow-up for two years after initiation of urinary CXCL10 monitoring

Exclusion Criteria:

• Expected transfer to adult care in the next 2 years.
• In center routine follow-up interval >3 months between visits
• Non-adherence to routine transplant clinic visits.
• Recent acute rejection episode in the last 3 months prior to enrolment. Patients may be enrolled if rejection was more than 3 months prior to enrolment. No exclusion for donor-specific antibody in the absence of acute cellular or antibody-mediated rejection.
• Estimated glomerular filtration rate <30 ml/min/1.73m2 (stage IV/V CKD).
• Inability to reliably obtain urinary samples for monitoring purposes.
• Contraindication to kidney biopsy.

The study includes prevalent pediatric kidney transplant recipients who are below 19 years of page at the time of transplant and who are more than 6 months after their transplant.

Deidentified data and results will be retained for use in the definitive clinical trial that is anticipated to follow the pilot trial. They may be further shared with any qualified investigator upon request, with approval of the principal investigator. The purpose of the data retention is to make data/samples available for ancillary studies related to the primary study, in collaboration with the PI after the primary study is complete.

Deidentified participant data will be retained for at least 5 years after primary study reporting to the sponsor has been completed.

• Relevant Research Ethics Board Approval
• Approval from the principal investigator