Eckardt, Kai-Uwe

Abstract Introduction Acute interstitial nephritis is an important differential diagnosis in patients with deteriorating kidney function. Diagnosis currently requires kidney biopsy, an invasive procedure associated with risks. We hypothesized that urinary T cells may serve as a non-invasive biomarker for acute interstitial nephritis. Methods A total of 320 patients undergoing clinically indicated kidney biopsy were enrolled in a discovery cohort at Charité Berlin (n = 80), an internal validation cohort at Charité (n = 100), and an external validation cohort at The First Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou (n = 140). Urinary immune cells were assessed by flow cytometry. Renal T cell infiltration was evaluated by immunofluorescence in kidney biopsy specimens from the discovery and internal validation cohorts, including 16 patients with acute interstitial nephritis and 9 patients without acute interstitial nephritis. Additionally, CXCL9 was measured by ELISA in 102 urine samples from these cohorts. Results Across all cohorts, 27 patients (8.4%) were diagnosed with acute interstitial nephritis. In the discovery cohort, multiple urinary T cell subsets were increased in acute interstitial nephritis, with activated CD4 + effector memory T cells expressing CD38 and HLA-DR showing the strongest diagnostic performance. This marker outperformed urinary monocytes, eosinophils, and CXCL9 and was validated in both independent cohorts. Across all cohorts, the area under the receiver operating characteristic curve was 0.84 and increased to 0.91 after exclusion of 8 patients receiving corticosteroids. A cutoff of 211 activated CD4 + effector memory T cells per 100 mL urine yielded a sensitivity of 78% and a specificity of 81%. Urinary activated CD4 + effector memory T cell counts correlated with renal CD4 + and CD4 + CD38 + T cell infiltration in acute interstitial nephritis. Conclusions Urinary activated CD4 + effector memory T cells expressing CD38 and HLA-DR represent a promising non-invasive biomarker for the diagnosis of acute interstitial nephritis. Lay Summary Acute interstitial nephritis is a frequent cause of acute kidney injury and is usually diagnosed by kidney biopsy, an invasive procedure that carries risks. We investigated whether immune cells in urine could help identify AIN in a less invasive manner. In this multicenter study, urine samples from 320 patients undergoing kidney biopsy in Germany and China were analyzed. We found that a specific population of activated T cells was markedly increased in the urine of patients with AIN compared with patients with other kidney diseases. This urinary marker outperformed other immune cell types and the inflammatory cytokine CXCL9 and was successfully validated in independent cohorts from both Germany and China. Importantly, urinary activated T cell counts correlated with immune cell infiltration within the kidney. These findings suggest that urinary activated T cells may serve as a non-invasive biomarker to support the diagnosis of AIN and facilitate earlier clinical decision-making.

Abstract Aims Cardiovascular disease (CVD) is the leading cause of mortality in chronic kidney disease (CKD). While CKD is known to give rise to systemic inflammation, its inciting factors remain poorly defined. Oxalate, long implicated in rare genetic kidney disorders, accumulates with decreased kidney function and has emerged as a driver of inflammation and independent risk factor for CVD. Here, we investigate the immunological mechanisms linking oxalate nephropathy to systemic inflammation, cardiac damage and kidney injury. Methods and results Oxalate nephropathy was induced in C57BL6/N mice through an oxalate-enriched diet. Oxalate induced systemic immune activation, renal fibrosis, and adverse cardiac remodelling, including pulmonary congestion with systolic and diastolic dysfunction. Flow cytometry analysis identified interleukin (IL)-17A as a dominant inflammatory effector, with expansion of Th17 and Th17-like Treg in the kidney, intestine, and spleen. Bulk mRNA sequencing confirmed these findings in kidney and heart. In line, plasma IL-17A was increased in oxalate-fed mice. Confirming the oxalate-IL-17A relationship, plasma IL-17A was elevated in patients with primary hyperoxaluria. Gut microbiome analysis by 16S amplicon sequencing showed only mild oxalate-induced alterations in mice. However, soluble oxalate directly enhanced Th17 polarization and disrupted mitochondrial respiration in vitro. In vivo, antibody-mediated IL-17A blockade improved kidney function, cardiac fibrosis, reduced neutrophil infiltration, and partially restored cardiac function in oxalate-fed mice. Conclusion Our study identifies oxalate as a systemic immunometabolic stressor and IL-17A as a central mediator of oxalate-induced cardiorenal injury. These findings establish the oxalate-IL-17A axis as a mechanistic link between CKD and CVD and suggest IL-17A inhibition as a potential therapeutic strategy to reduce cardiovascular damage in CKD.