Multicenter Validation of a Urine CXCL10 Assay for Noninvasive Monitoring of Renal Transplants

Pathophysiology of rejection in kidney transplantation

INTRODUCTION

Rejection remains a major obstacle to successful kidney transplantation. The complex pathophysiology of rejection depends on a fine-tuned interplay between the innate and adaptive immune systems.

AREAS COVERED

This review provides a comprehensive analysis of the pathophysiology of rejection of kidney grafts, performed through careful selection of most relevant papers available on the topic in the PubMed database. The two types of rejection usually observed at the kidney biopsy, i.e. cellular and humoral rejection, are described with an accurate outline of the biological processes that lead to their development.

EXPERT OPINION

The incidence of T-cell-mediated rejection is decreasing, and most cases promptly respond to appropriate immunosuppression. However, late diagnosis or incomplete response to treatment may have deleterious consequences in the long term. The main issue is represented by antibody-mediated rejection, which unsatisfactorily responds to aggressive immunosuppression, especially when diagnosed late. Prevention of acute ABMR rests on HLA-specific antibody detection prior to transplantation, adequate immunosuppression, and optimal patients' compliance. Late diagnosis and poor response to treatment inevitably lead to chronic ABMR, for which no therapies are currently available.

Biomarkers of Rejection in Kidney Transplantation

Alloimmune injury is a major cause of long-term kidney allograft failure whether due to functionally stable (subclinical) or overt clinical rejection. These episodes may be mediated by immune cells (cellular rejection) or alloantibody (antibody-mediated rejection). Early recognition of immune injury is needed for timely appropriate intervention to maintain graft functional viability. However, the conventional measure of kidney function (ie, serum creatinine) is insufficient for immune monitoring due to limited sensitivity and specificity for rejection. As a result, there is need for biomarkers that more sensitively detect the immune response to the kidney allograft. Recently, several biomarkers have been clinically implemented into the care of kidney transplant recipients. These biomarkers attempt to achieve multiple goals including (1) more sensitive detection of clinical and subclinical rejection, (2) predicting impending rejection, (3) monitoring for the adequacy of treatment response, and (4) facilitating personalized immunosuppression. In this review, we summarize the findings to date in commercially available biomarkers, along with biomarkers approaching clinical implementation. While we discuss the analytical and clinical validity of these biomarkers, we identify the challenges and limitations to widespread biomarker use, including the need for biomarker-guided prospective studies to establish evidence of clinical utility of these new assays.

Immune Checkpoint Inhibitor Therapy for Kidney Transplant Recipients - A Review of Potential Complications and Management Strategies

Immune checkpoint inhibitor (ICI) therapy has enabled a paradigm shift in Oncology, with the treatment of metastatic cancer in certain tumor types becoming akin to the treatment of chronic disease. Kidney transplant recipients (KTR) are at increased risk of developing cancer compared to the general population. Historically, KTR were excluded from ICI clinical trials due to concern for allograft rejection and decreased anti-tumor efficacy. While early post-marketing data revealed an allograft rejection risk of 40%-50%, 2 recent small prospective trials have demonstrated lower rates of rejection of 0%-12%, suggesting that maintenance immunosuppression modification prior to ICI start modulates rejection risk. Moreover, objective response rates induced by ICI for the treatment of advanced or metastatic skin cancer, the most common malignancy in KTR, have been comparable to those achieved by immune intact patients. Non-invasive biomarkers may have a role in risk-stratifying patients before starting ICI, and monitoring for rejection, though allograft biopsy is required to confirm diagnosis. This clinically focused review summarizes current knowledge on complications of ICI use in KTR, including their mechanism, risk mitigation strategies, non-invasive biomarker use, approaches to treatment of rejection, and suggestions for future directions in research.

Biomarkers in Kidney Transplantation: A Rapidly Evolving Landscape

The last decade has seen an explosion in clinical research focusing on the use of noninvasive biomarkers in kidney transplantation. Much of the published literature focuses on donor-derived cell-free DNA (dd-cfDNA). Although initially studied as a noninvasive means of identifying acute rejection, it is now clear that dd-cfDNA is more appropriately described as a marker of severe injury and irrespective of the etiology, elevated dd-cfDNA ≥0.5% portends worse graft outcomes. Blood gene expression profiling is also commercially available and has mostly been studied in the context of early identification of subclinical rejection, although additional data is needed to validate these findings. Torque teno virus, a ubiquitous DNA virus, has emerged as a biomarker of immunosuppression exposure as peripheral blood Torque teno virus copy numbers might mirror the intensity of host immunosuppression. Urinary chemokine tests including C-X-C motif chemokine ligand 9 and C-X-C motif chemokine ligand 10 have recently been assessed in large clinical trials and hold promising potential for early diagnosis of both subclinical and acute rejection, as well as, for long-term prognosis. Urinary cellular messenger RNA and exosome vesicular RNA based studies require additional validation. Although current data does not lend itself to conclusion, future studies on multimodality testing may reveal the utility of serial surveillance for individualization of immunosuppression and identify windows of opportunity to intervene early and before the irreversible allograft injury sets in.

Urine CXCL10 as a biomarker in kidney transplantation

PURPOSE OF REVIEW

Urine CXCL10 is a promising biomarker for posttransplant renal allograft monitoring but is currently not widely used for clinical management.

RECENT FINDINGS

Large retrospective studies and data from a prospective randomized trial as well as a prospective cohort study demonstrate that low urine CXCL10 levels are associated with a low risk of rejection and can exclude BK polyomavirus replication with high certainty. Urine CXCL10 can either be used as part of a multiparameter based risk assessment tool, or as an individual biomarker taking relevant confounders into account. A novel Luminex-based CXCL10 assay has been validated in a multicenter study, and proved to be robust, reproducible, and accurate.

SUMMARY

Urine CXCL10 is a well characterized inflammation biomarker, which can be used to guide performance of surveillance biopsies. Wide implementation into clinical practice depends on the availability of inexpensive, thoroughly validated assays with approval from regulatory authorities.

Urine CXCL10 to Assess BK Polyomavirus Replication After Kidney Transplantation

BACKGROUND

Urine CXCL10 is a biomarker for renal allograft inflammation induced by rejection, urinary tract infection, or BK polyomavirus (BKPyV) replication. This study aimed to compare urine CXCL10 levels in different stages of BKPyV reactivation and to investigate urine CXCL10 as a biomarker for BKPyV replication.

METHODS

We included 763 urine samples (235 patients) from an interventional, randomized trial obtained in the context of regular screening for urine CXCL10 levels. All urine samples had a complete urine sediment analysis, no rejection episode noted within 30 d before urine collection, and a urine decoy cell analysis was conducted within ±3 d.

RESULTS

Urine CXCL10 levels were 2.31 ng/mmol in samples without BKPyV viruria, slightly rose to 4.35 ng/mmol with BKPyV viruria, and then markedly increased to 16.42 ng/mmol when decoy cells were detectable, but still in the absence of BKPyV DNAemia ( P  < 0.001). The highest urine CXCL10 values were observed in samples with BKPyV DNAemia (median 42.59 ng/mmol). The area under the curve of urine CXCL10 levels to detect ≥3 decoy cells was 0.816. At a CXCL10 cutoff of 3 ng/mmol, the negative predictive value was 97%. The area under the curve of urine CXCL10 levels to detect BKPyV DNAemia was 0.882, with a negative predictive value of 99% at a CXCL10 cutoff of 3 ng/mmol.

CONCLUSIONS

Urine CXCL10 levels are already significantly elevated in BKPyV viruria (especially with decoy cell shedding) and further increase with BKPyV DNAemia. Low urine CXCL10 values can rule out the presence of ≥3 decoy cells and BKPyV DNAemia with high certainty.