Spot urinary sodium in CKD patients: correlation with 24h-excretion and evaluation of commonly used prediction equations.

BACKGROUND: Salt intake in CKD patients can affect cardiovascular risk and kidney disease progression. Twenty-four hour (24h) urine collections are often used to investigate salt metabolism but are cumbersome to perform. We assessed urinary sodium (U-Na) concentration in spot urine samples and investigated the correlation with 24h U-Na excretion and concentration in CKD patients under nephrological care. Further, we studied the role of CKD stage and diuretics and evaluated the performance of commonly used formulas for the prediction of 24h U-Na excretion from spot urine samples. METHODS: One hundred eight patients of the German Chronic Kidney Disease (GCKD) study were included. Each participant collected a 24h urine and two spot urine samples within the same period. The first spot urine sample (AM) was part of the second morning urine. The second urine sample was collected before dinner (PM). Patients were advised to take their medication as usual without changing dietary habits. U-Na concentrations in the two spot urine samples and their average ((AM + PM)/2) were correlated with U-Na concentration and total Na excretion in the 24h urine collections. Correlations were subsequently studied after stratification by CKD stage and diuretic intake. The usefulness of three commonly applied equations to estimate 24h U-Na excretion from spot urine samples (Kawasaki, Tanaka and Intersalt) was determined using Bland-Altman plots, analyses of sensitivity, specificity, as well as positive (PPV) and negative predictive values (NPV). RESULTS: Participants (42 women, 66 men) were on average (± SD) 62.2 (± 11.9) years old, with a mean serum creatinine of 1.6 (± 0.5) mg/dl. 95% had arterial hypertension, 37% diabetes mellitus and 55% were on diuretics. The best correlation with 24h U-Na total excretion was found for the PM spot U-Na sample. We also found strong correlations when comparing spot and 24h urine U-Na concentration. Correction of spot U-Na for U-creatinine did not improve strength of correlations. Neither CKD stage, nor intake of diuretics had significant impact on these correlations. All examined formulas revealed a significant mean bias. The lowest mean bias and the strongest correlation between estimated and measured U-Na excretion in 24h were obtained using the Tanaka-formula. Also, application of the Tanaka-formula with PM U-Na provided best sensitivity, specificity, PPV and NPV to estimate U-Na excretion > 4g/d corresponding to a salt consumption > 10g/d. CONCLUSION: U-Na concentration of spot urine samples correlated with 24h U-Na excretion especially when PM spot U-Na was used. However, correlation coefficients were relatively low. Neither CKD stage nor intake of diuretics appeared to have an influence on these correlations. There was a significant bias for all tested formulas with the Tanaka-formula providing the strongest correlation with measured 24h U-Na excretion. In summary, using spot urine samples together with the Tanaka-formula in epidemiological studies appears feasible to determine associations between approximate salt intake and outcomes in CKD patients. However, the usefulness of spot-urine samples to guide and monitor salt consumption in individual patients remains limited.

Double-negative T cells have a reparative role after experimental severe ischemic acute kidney injury.

T cells mediate organ injury and repair. A proportion of unconventional kidney T cells called double-negative (DN) T cells (TCR+ CD4- CD8-), with anti-inflammatory properties, were previously demonstrated to protect from early injury in moderate experimental acute kidney injury (AKI). However, their role in repair after AKI has not been studied. We hypothesized that DN T cells mediate repair after severe AKI. C57B6 mice underwent severe (40 min) unilateral ischemia-reperfusion injury (IRI). Kidney DN T cells were studied by flow cytometry and compared with gold-standard anti-inflammatory CD4+ regulatory T cells (Tregs). In vitro effects of DN T cells and Tregs on renal tubular epithelial cell (RTEC) repair after injury were quantified with live-cell analysis. DN T cells, Tregs, CD4, or vehicle were adoptively transferred after severe AKI. Glomerular filtration rate (GFR) was measured using fluorescein isothiocyanate (FITC)-sinistrin. Fibrosis was assessed with Masson's trichrome staining. Profibrotic genes were measured with qRT-PCR. Percentages and the numbers of DN T cells substantially decreased during repair phase after severe AKI, as well as their activation and proliferation. Both DN T cells and Tregs accelerated RTEC cell repair in vitro. Post-AKI transfer of DN T cells reduced kidney fibrosis and improved GFR, as did Treg transfer. DN T cell transfer lowered transforming growth factor (TGF)ß1 and α-smooth muscle actin (αSMA) expression. DN T cells reduced effector-memory CD4+ T cells and IL-17 expression. DN T cells undergo quantitative and phenotypical changes after severe AKI, accelerate RTEC repair in vitro as well as improve GFR and renal fibrosis in vivo. DN T cells have potential as immunotherapy to accelerate repair after AKI.NEW & NOTEWORTHY Double-negative (DN) T cells (CD4- CD8-) are unconventional kidney T cells with regulatory abilities. Their role in repair from acute kidney injury (AKI) is unknown. Kidney DN T cell population decreased during repair after ischemic AKI, in contrast to regulatory T cells (Tregs) which increased. DN T cell administration accelerated tubular repair in vitro, while after severe in vivo ischemic injury reduced kidney fibrosis and increased glomerular filtration rate (GFR). DN T cell infusion is a potential therapeutic agent to improve outcome from severe AKI.

Kidney Immune Cell Characterization of Humanized Mouse Models.

Experimental studies often fail to translate to clinical practice. Humanized mouse models are an important tool to close this gap. We immunophenotyped the kidneys of NOG (EXL) and NSG mouse strains engrafted with human CD34 + hematopoietic stem cells or PBMCs and compared with immune cell composition of normal human kidney. Human CD34 + hematopoietic stem cell engraftment results in steady renal immune cell populations in mouse kidney with key similarities in composition compared with human kidney. Successful translation of experimental mouse data to human diseases is limited because of biological differences and imperfect disease models. Humanized mouse models are being used to bring murine models closer to humans. However, data for application in renal immune cell-mediated diseases are rare. We therefore studied immune cell composition of three different humanized mouse kidneys and compared them with human kidney. NOG and NOGEXL mice engrafted with human CD34 + hematopoietic stem cells were compared with NSG mice engrafted with human PBMCs. Engraftment was confirmed with flow cytometry, and immune cell composition in kidney, blood, spleen, and bone marrow was analyzed in different models. The results from immunophenotyping of kidneys from different humanized mouse strains were compared with normal portions of human kidneys. We found significant engraftment of human immune cells in blood and kidney of all tested models. huNSG mice showed highest frequencies of hTCR + cells compared with huNOG and huNOGEXL in blood. huNOGEXL was found to have the highest hCD4 + frequency among all tested models. Non-T cells such as hCD20 + and hCD11c + cells were decreased in huNSG mice compared with huNOG and huNOGEXL. Compared with normal human kidney, huNOG and huNOGEXL mice showed representative immune cell composition, rather than huNSG mice. In summary, humanization results in immune cell infiltration in the kidney with variable immune cell composition of tested humanized mouse models and partially reflects normal human kidneys, suggesting potential use for translational studies.

Single cell and spatial transcriptomics analysis of kidney double negative T lymphocytes in normal and ischemic mouse kidneys.

T cells are important in the pathogenesis of acute kidney injury (AKI), and TCR+CD4-CD8- (double negative-DN) are T cells that have regulatory properties. However, there is limited information on DN T cells compared to traditional CD4+ and CD8+ cells. To elucidate the molecular signature and spatial dynamics of DN T cells during AKI, we performed single-cell RNA sequencing (scRNA-seq) on sorted murine DN, CD4+, and CD8+ cells combined with spatial transcriptomic profiling of normal and post AKI mouse kidneys. scRNA-seq revealed distinct transcriptional profiles for DN, CD4+, and CD8+ T cells of mouse kidneys with enrichment of Kcnq5, Klrb1c, Fcer1g, and Klre1 expression in DN T cells compared to CD4+ and CD8+ T cells in normal kidney tissue. We validated the expression of these four genes in mouse kidney DN, CD4+ and CD8+ T cells using RT-PCR and Kcnq5, Klrb1, and Fcer1g genes with the NIH human kidney precision medicine project (KPMP). Spatial transcriptomics in normal and ischemic mouse kidney tissue showed a localized cluster of T cells in the outer medulla expressing DN T cell genes including Fcer1g. These results provide a template for future studies in DN T as well as CD4+ and CD8+ cells in normal and diseased kidneys.

T cell metabolic reprogramming in acute kidney injury and protection by glutamine blockade.

T cells play an important role in acute kidney injury (AKI). Metabolic programming of T cells regulates their function, is a rapidly emerging field, and is unknown in AKI. We induced ischemic AKI in C57BL/6J mice and collected kidneys and spleens at multiple time points. T cells were isolated and analyzed by an immune-metabolic assay. Unbiased machine learning analyses identified a distinct T cell subset with reduced voltage-dependent anion channel 1 and mTOR expression in post-AKI kidneys. Ischemic kidneys showed higher expression of trimethylation of histone H3 lysine 27 and glutaminase. Splenic T cells from post-AKI mice had higher expression of glucose transporter 1, hexokinase II, and carnitine palmitoyltransferase 1a. Human nonischemic and ischemic kidney tissue displayed similar findings to mouse kidneys. Given a convergent role for glutamine in T cell metabolic pathways and the availability of a relatively safe glutamine antagonist, JHU083, effects on AKI were evaluated. JHU083 attenuated renal injury and reduced T cell activation and proliferation in ischemic and nephrotoxic AKI, whereas T cell-deficient mice were not protected by glutamine blockade. In vitro hypoxia demonstrated upregulation of glycolysis-related enzymes. T cells undergo metabolic reprogramming during AKI, and reconstitution of metabolism by targeting T cell glutamine pathway could be a promising novel therapeutic approach.

T Cell Nrf2/Keap1 Gene Editing Using CRISPR/Cas9 and Experimental Kidney Ischemia-Reperfusion Injury.

Aims: T cells play pathophysiologic roles in kidney ischemia-reperfusion injury (IRI), and the nuclear factor erythroid 2-related factor 2/kelch-like ECH-associated protein 1 (Nrf2/Keap1) pathway regulates T cell responses. We hypothesized that clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)-mediated Keap1-knockout (KO) augments Nrf2 antioxidant potential of CD4+ T cells, and that Keap1-KO CD4+ T cell immunotherapy protects from kidney IRI. Results: CD4+ T cell Keap1-KO resulted in significant increase of Nrf2 target genes NAD(P)H quinone dehydrogenase 1, heme oxygenase 1, glutamate-cysteine ligase catalytic subunit, and glutamate-cysteine ligase modifier subunit. Keap1-KO cells displayed no signs of exhaustion, and had significantly lower levels of interleukin 2 (IL2) and IL6 in normoxic conditions, but increased interferon gamma in hypoxic conditions in vitro. In vivo, adoptive transfer of Keap1-KO CD4+ T cells before IRI improved kidney function in T cell-deficient nu/nu mice compared with mice receiving unedited control CD4+ T cells. Keap1-KO CD4+ T cells isolated from recipient kidneys 24 h post IR were less activated compared with unedited CD4+ T cells, isolated from control kidneys. Innovation: Editing Nrf2/Keap1 pathway in murine T cells using CRISPR/Cas9 is an innovative and promising immunotherapy approach for kidney IRI and possibly other solid organ IRI. Conclusion: CRISPR/Cas9-mediated Keap1-KO increased Nrf2-regulated antioxidant gene expression in murine CD4+ T cells, modified responses to in vitro hypoxia and in vivo kidney IRI. Gene editing targeting the Nrf2/Keap1 pathway in T cells is a promising approach for immune-mediated kidney diseases.

Immune Checkpoint Molecule TIGIT Regulates Kidney T Cell Functions and Contributes to AKI.

SIGNIFICANCE STATEMENT: T cells mediate pathogenic and reparative processes during AKI, but the exact mechanisms regulating kidney T cell functions are unclear. This study identified upregulation of the novel immune checkpoint molecule, TIGIT, on mouse and human kidney T cells after AKI. TIGIT-expressing kidney T cells produced proinflammatory cytokines and had effector (EM) and central memory (CM) phenotypes. TIGIT-deficient mice had protection from both ischemic and nephrotoxic AKI. Single-cell RNA sequencing led to the discovery of possible downstream targets of TIGIT. TIGIT mediates AKI pathophysiology, is a promising novel target for AKI therapy, and is being increasingly studied in human cancer therapy trials. BACKGROUND: T cells play pathogenic and reparative roles during AKI. However, mechanisms regulating T cell responses are relatively unknown. We investigated the roles of the novel immune checkpoint molecule T cell immunoreceptor with Ig and immunoreceptor tyrosine-based inhibitory motif domains (TIGIT) in kidney T cells and AKI outcomes. METHODS: TIGIT expression and functional effects were evaluated in mouse kidney T cells using RNA sequencing (RNA-Seq) and flow cytometry. TIGIT effect on AKI outcomes was studied with TIGIT knockout (TIGIT-KO) mice in ischemia reperfusion (IR) and cisplatin AKI models. Human kidney T cells from nephrectomy samples and single cell RNA sequencing (scRNA-Seq) data from the Kidney Precision Medicine Project were used to assess TIGIT's role in humans. RESULTS: RNA-Seq and flow cytometry analysis of mouse kidney CD4+ T cells revealed increased expression of TIGIT after IR injury. Ischemic injury also increased TIGIT expression in human kidney T cells, and TIGIT expression was restricted to T/natural killer cell subsets in patients with AKI. TIGIT-expressing kidney T cells in wild type (WT) mice had an effector/central memory phenotype and proinflammatory profile at baseline and post-IR. Kidney regulatory T cells were predominantly TIGIT+ and significantly reduced post-IR. TIGIT-KO mice had significantly reduced kidney injury after IR and nephrotoxic injury compared with WT mice. scRNA-Seq analysis showed enrichment of genes related to oxidative phosphorylation and mTORC1 signaling in Th17 cells from TIGIT-KO mice. CONCLUSIONS: TIGIT expression increases in mouse and human kidney T cells during AKI, worsens AKI outcomes, and is a novel therapeutic target for AKI.

TCRαß+ CD4-/CD8- "double negative" T cells in health and disease-implications for the kidney.

Double negative (DN) T cells, one of the least studied T lymphocyte subgroups, express T cell receptor αß but lack CD4 and CD8 coreceptors. DN T cells are found in multiple organs including kidney, lung, heart, gastrointestinal tract, liver, genital tract, and central nervous system. DN T cells suppress inflammatory responses in different disease models including experimental acute kidney injury, and significant evidence supports an important role in the pathogenesis of systemic lupus erythematosus. However, little is known about these cells in other kidney diseases. Therefore, it is important to better understand different functions of DN T cells and their signaling pathways as promising therapeutic targets, particularly with the increasing application of T cell-directed therapy in humans. In this review, we aim to summarize studies performed on DN T cells in normal and diseased organs in the setting of different disease models with a focus on kidney.

T Lymphocytes in Acute Kidney Injury and Repair.

Innate and adaptive immune systems participate in the pathogenesis of acute kidney injury (AKI). Considerable data from different research teams have shown the importance of T lymphocytes in the pathophysiology of AKI and, more recently, prevention and repair. T cells can generate or resolve inflammation by secreting specific cytokines and growth factors as well as interact with other immune and stromal cells to induce kidney injury or promote tissue repair. There also are emerging data on the role of T cells in the progression of AKI to chronic kidney disease and organ cross-talk in AKI. These data set the stage for immunomodulatory therapies for AKI. This review focuses on the major populations of T lymphocytes and their roles as mediators for AKI and repair.

TCR+CD4-CD8- (double negative) T cells protect from cisplatin-induced renal epithelial cell apoptosis and acute kidney injury.

Acute kidney injury (AKI) due to cisplatin is a significant problem that limits its use as an effective chemotherapeutic agent. T cell receptor+CD4-CD8- double negative (DN) T cells constitute the major T cell population in the human and mouse kidney, express programmed cell death protein (PD)-1, and protect from ischemic AKI. However, the pathophysiological roles of DN T cells in cisplatin-induced AKI is unknown. In this study, wild-type mice were treated with cisplatin (30 mg/kg) or vehicle, and the effects on kidney DN T cell numbers and function were measured. In vitro experiments evaluated effects of kidney DN T cells on cisplatin-induced apoptosis and PD ligand 1 (PD-L1) in renal epithelial cells. Adoptive transfer experiments assessed the therapeutic potential of DN T cells during cisplatin-induced AKI. Our results show that kidney DN T cell population increased at 24 h and declined by 72 h after cisplatin treatment. Cisplatin treatment increased kidney DN T cell proliferation, apoptosis, CD69, and IL-10 expression, whereas CD62L, CD44, IL-17A, interferon-γ, and TNF-α were downregulated. Cisplatin treatment decreased both PD-1 and natural killer 1.1 subsets of kidney DN T cells with a pronounced effect on the PD-1 subset. In vitro kidney DN T cell coculture decreased cisplatin-induced apoptosis in kidney proximal tubular epithelial cells, increased Bcl-2, and decreased cleaved caspase 3 expression. Cisplatin-induced expression of PD ligand 1 was reduced in proximal tubular epithelial cells cocultured with DN T cells. Adoptive transfer of DN T cells attenuated kidney dysfunction and structural damage from cisplatin-induced AKI. These results demonstrate that kidney DN T cells respond rapidly and play a protective role during cisplatin-induced AKI.

CD4+ T Cell-Derived NGAL Modifies the Outcome of Ischemic Acute Kidney Injury.

CD4+ T cells mediate the pathogenesis of ischemic and nephrotoxic acute kidney injury (AKI). However, the underlying mechanisms of CD4+ T cell-mediated pathogenesis are largely unknown. We therefore conducted unbiased RNA-sequencing to discover novel mechanistic pathways of kidney CD4+ T cells after ischemia compared with normal mouse kidney. Unexpectedly, the lipocalin-2 (Lcn2) gene, which encodes neutrophil gelatinase-associated lipocalin (NGAL) had the highest fold increase (∼60). The NGAL increase in CD4+ T cells during AKI was confirmed at the mRNA level with quantitative real-time PCR and at the protein level with ELISA. NGAL is a potential biomarker for the early detection of AKI and has multiple potential biological functions. However, the role of NGAL produced by CD4+ T cells is not known. We found that ischemic AKI in NGAL knockout (KO) mice had worse renal outcomes compared with wild-type (WT) mice. Adoptive transfer of NGAL-deficient CD4+ T cells from NGAL KO mice into CD4 KO or WT mice led to worse renal function than transfer of WT CD4+ T cells. In vitro-simulated ischemia/reperfusion showed that NGAL-deficient CD4+ T cells express higher levels of IFN-γ mRNA compared with WT CD4+ T cells. In vitro differentiation of naive CD4+ T cells to Th17, Th1, and Th2 cells led to significant increase in Lcn2 expression. Human kidney CD4+ T cell NGAL also increased significantly after ischemia. These results demonstrate an important role for CD4+ T cell NGAL as a mechanism by which CD4+ T cells mediate AKI and extend the importance of NGAL in AKI beyond diagnostics.

Reparative T lymphocytes in organ injury.

Acute organ injuries such as acute cerebrovascular accidents, myocardial infarction, acute kidney injury, acute lung injury, and others are among the leading causes of death worldwide. Dysregulated or insufficient organ repair mechanisms limit restoration of homeostasis and contribute to chronic organ failure. Studies reveal that both humans and mice harness potent non-stem cells that are capable of directly or indirectly promoting tissue repair. Specific populations of T lymphocytes have emerged as important reparative cells with context-specific actions. These T cells can resolve inflammation and secrete reparative cytokines and growth factors as well as interact with other immune and stromal cells to promote the complex and active process of tissue repair. This Review focuses on the major populations of T lymphocytes known to mediate tissue repair, their reparative mechanisms, and the diseases in which they have been implicated. Elucidating and harnessing the mechanisms that promote the reparative functions of these T cells could greatly improve organ dysfunction after acute injury.