Human proximal tubular epithelial cell-derived small extracellular vesicles mediate synchronized tubular ferroptosis in hypoxic kidney injury.

Hypoxia is the key pathobiological trigger of tubular oxidative stress and cell death that drives the transition of acute kidney injury (AKI) to chronic kidney disease (CKD). The mitochondrial-rich proximal tubular epithelial cells (PTEC) are uniquely sensitive to hypoxia and thus, are pivotal in propagating the sustained tubular loss of AKI-to-CKD transition. Here, we examined the role of PTEC-derived small extracellular vesicles (sEV) in propagating the 'wave of tubular death'. Ex vivo patient-derived PTEC were cultured under normoxia (21 % O2) and hypoxia (1 % O2) on Transwell inserts for isolation and analysis of sEV secreted from apical versus basolateral PTEC surfaces. Increased numbers of sEV were secreted from the apical surface of hypoxic PTEC compared with normoxic PTEC. No differences in basolateral sEV numbers were observed between culture conditions. Biological pathway analysis of hypoxic-apical sEV cargo identified distinct miRNAs linked with cellular injury pathways. In functional assays, hypoxic-apical sEV selectively induced ferroptotic cell death (↓glutathione peroxidase-4, ↑lipid peroxidation) in autologous PTEC compared with normoxic-apical sEV. The addition of ferroptosis inhibitors, ferrostatin-1 and baicalein, attenuated PTEC ferroptosis. RNAse A pretreatment of hypoxic-apical sEV also abrogated PTEC ferroptosis, demonstrating a role for sEV RNA in ferroptotic 'wave of death' signalling. In line with these in vitro findings, in situ immunolabelling of diagnostic kidney biopsies from AKI patients with clinical progression to CKD (AKI-to-CKD transition) showed evidence of ferroptosis propagation (increased numbers of ACSL4+ PTEC), while urine-derived sEV (usEV) from these 'AKI-to-CKD transition' patients triggered PTEC ferroptosis (↑lipid peroxidation) in functional studies. Our data establish PTEC-derived apical sEV and their intravesicular RNA as mediators of tubular lipid peroxidation and ferroptosis in hypoxic kidney injury. This concept of how tubular pathology is propagated from the initiating insult into a 'wave of death' provides novel therapeutic check-points for targeting AKI-to-CKD transition.

High risk clear cell renal cell carcinoma microenvironments contain protumour immunophenotypes lacking specific immune checkpoints.

Perioperative immune checkpoint inhibitor (ICI) trials for intermediate high-risk clear cell renal cell carcinoma (ccRCC) have failed to consistently demonstrate improved patient outcomes. These unsuccessful ICI trials suggest that the tumour infiltrating immunophenotypes, termed here as the immune cell types, states and their spatial location within the tumour microenvironment (TME), were unfavourable for ICI treatment. Defining the tumour infiltrating immune cells may assist with the identification of predictive immunophenotypes within the TME that are favourable for ICI treatment. To define the immunophenotypes within the ccRCC TME, fresh para-tumour (pTME, n = 2), low-grade (LG, n = 4, G1-G2) and high-grade (HG, n = 4, G3-G4) tissue samples from six patients with ccRCC presenting at a tertiary referral hospital underwent spatial transcriptomics sequencing (ST-seq). Within the generated ST-seq datasets, immune cell types and states, termed here as exhausted/pro-tumour state or non-exhausted/anti-tumour state, were identified using multiple publicly available single-cell RNA and T-cell receptor sequencing datasets as references. HG TMEs revealed abundant exhausted/pro-tumour immune cells with no consistent increase in expression of PD-1, PD-L1 and CTLA4 checkpoints and angiogenic genes. Additional HG TME immunophenotype characteristics included: pro-tumour tissue-resident monocytes with consistently increased expression of HAVCR2 and LAG3 checkpoints; an exhausted CD8+ T cells sub-population with stem-like progenitor gene expression; and pro-tumour tumour-associated macrophages and monocytes within the recurrent TME with the expression of TREM2. Whilst limited by a modest sample size, this study represents the largest ST-seq dataset on human ccRCC. Our study reveals that high-risk ccRCC TMEs are infiltrated by exhausted/pro-tumour immunophenotypes lacking specific checkpoint gene expression confirming that HG ccRCC TME are immunogenic but not ICI favourable.

Spatial Transcriptomics in Kidney Tissue.

Unlike bulk and single-cell/single-nuclei RNA sequencing methods, spatial transcriptome sequencing (ST-seq) resolves transcriptome expression within the spatial context of intact tissue. This is achieved by integrating histology with RNA sequencing. These methodologies are completed sequentially on the same tissue section placed on a glass slide with printed oligo-dT spots, termed ST-spots. Transcriptomes within the tissue section are captured by the underlying ST-spots and receive a spatial barcode in the process. The sequenced ST-spot transcriptomes are subsequently aligned with the hematoxylin and eosin (H&E) image, giving morphological context to the gene expression signatures within intact tissue. We have successfully employed ST-seq to characterize mouse and human kidney tissue. Here, we describe in detail the application of Visium Spatial Tissue Optimization (TO) and Visium Spatial Gene Expression (GEx) protocols for ST-seq in fresh frozen kidney tissue.

Cellular milieu in clear cell renal cell carcinoma.

Clear cell renal cell carcinoma (ccRCC) is globally the most prevalent renal cancer. The cells of origin in ccRCC have been identified as proximal tubular epithelial cells (PTEC); however, the transcriptomic pathways resulting in the transition from normal to malignant PTEC state have remained unclear. Immunotherapy targeting checkpoints have revolutionized the management of ccRCC, but a sustained clinical response is achieved in only a minority of ccRCC patients. This indicates that our understanding of the mechanisms involved in the malignant transition and resistance to immune checkpoint therapy in ccRCC is unclear. This review examines recent single-cell transcriptomics studies of ccRCC to clarify the transition of PTEC in ccRCC development, and the immune cell types, states, and interactions that may limit the response to targeted immune therapy, and finally suggests stromal cells as key drivers in recurrent and locally invasive ccRCC. These and future single-cell transcriptomics studies will continue to clarify the cellular milieu in the ccRCC microenvironment, thus defining actional clinical, therapeutic, and prognostic characteristics of ccRCC.

Hypoxic human proximal tubular epithelial cells undergo ferroptosis and elicit an NLRP3 inflammasome response in CD1c+ dendritic cells.

Inflammasomes are multiprotein platforms responsible for the release of pro-inflammatory cytokines interleukin (IL)-1ß and IL-18. Mouse studies have identified inflammasome activation within dendritic cells (DC) as pivotal for driving tubulointerstitial fibrosis and inflammation, the hallmarks of chronic kidney disease (CKD). However, translation of this work to human CKD remains limited. Here, we examined the complex tubular cell death pathways mediating inflammasome activation in human kidney DC and, thus, CKD progression. Ex vivo patient-derived proximal tubular epithelial cells (PTEC) cultured under hypoxic (1% O2) conditions modelling the CKD microenvironment showed characteristics of ferroptotic cell death, including mitochondrial dysfunction, reductions in the lipid repair enzyme glutathione peroxidase 4 (GPX4) and increases in lipid peroxidation by-product 4-hydroxynonenal (4-HNE) compared with normoxic PTEC. The addition of ferroptosis inhibitor, ferrostatin-1, significantly reduced hypoxic PTEC death. Human CD1c+ DC activated in the presence of hypoxic PTEC displayed significantly increased production of inflammasome-dependent cytokines IL-1ß and IL-18. Treatment of co-cultures with VX-765 (caspase-1/4 inhibitor) and MCC950 (NLRP3 inflammasome inhibitor) significantly attenuated IL-1ß/IL-18 levels, supporting an NLRP3 inflammasome-dependent DC response. In line with these in vitro findings, in situ immunolabelling of human fibrotic kidney tissue revealed a significant accumulation of tubulointerstitial CD1c+ DC containing active inflammasome (ASC) specks adjacent to ferroptotic PTEC. These data establish ferroptosis as the primary pattern of PTEC necrosis under the hypoxic conditions of CKD. Moreover, this study identifies NLRP3 inflammasome signalling driven by complex tubulointerstitial PTEC-DC interactions as a key checkpoint for therapeutic targeting in human CKD.

Spatially Resolved Transcriptomes of Mammalian Kidneys Illustrate the Molecular Complexity and Interactions of Functional Nephron Segments.

Available transcriptomes of the mammalian kidney provide limited information on the spatial interplay between different functional nephron structures due to the required dissociation of tissue with traditional transcriptome-based methodologies. A deeper understanding of the complexity of functional nephron structures requires a non-dissociative transcriptomics approach, such as spatial transcriptomics sequencing (ST-seq). We hypothesize that the application of ST-seq in normal mammalian kidneys will give transcriptomic insights within and across species of physiology at the functional structure level and cellular communication at the cell level. Here, we applied ST-seq in six mice and four human kidneys that were histologically absent of any overt pathology. We defined the location of specific nephron structures in the captured ST-seq datasets using three lines of evidence: pathologist's annotation, marker gene expression, and integration with public single-cell and/or single-nucleus RNA-sequencing datasets. We compared the mouse and human cortical kidney regions. In the human ST-seq datasets, we further investigated the cellular communication within glomeruli and regions of proximal tubules-peritubular capillaries by screening for co-expression of ligand-receptor gene pairs. Gene expression signatures of distinct nephron structures and microvascular regions were spatially resolved within the mouse and human ST-seq datasets. We identified 7,370 differentially expressed genes (p adj < 0.05) distinguishing species, suggesting changes in energy production and metabolism in mouse cortical regions relative to human kidneys. Hundreds of potential ligand-receptor interactions were identified within glomeruli and regions of proximal tubules-peritubular capillaries, including known and novel interactions relevant to kidney physiology. Our application of ST-seq to normal human and murine kidneys confirms current knowledge and localization of transcripts within the kidney. Furthermore, the generated ST-seq datasets provide a valuable resource for the kidney community that can be used to inform future research into this complex organ.

Promoting Plant-Based Therapies for Chronic Kidney Disease.

Chronic kidney disease (CKD) is debilitating, increasing in incidence worldwide, and a financial and social burden on health systems. Kidney failure, the final stage of CKD, is life-threatening if untreated with kidney replacement therapies. Current therapies using commercially-available drugs, such as angiotensin-converting enzyme inhibitors, angiotensin II receptor blockers and calcium channel blockers, generally only delay the progression of CKD. This review article focuses on effective alternative therapies to improve the prevention and treatment of CKD, using plants or plant extracts. Three mechanistic processes that are well-documented in CKD pathogenesis are inflammation, fibrosis, and oxidative stress. Many plants and their extracts are already known to ameliorate kidney dysfunction through antioxidant action, with subsequent benefits on inflammation and fibrosis. In vitro and in vivo experiments using plant-based therapies for pre-clinical research demonstrate some robust therapeutic benefits. In the CKD clinic, combination treatments of plant extracts with conventional therapies that are seen as relatively successful currently may confer additive or synergistic renoprotective effects. Therefore, the aim of recent research is to identify, rigorously test pre-clinically and clinically, and avoid any toxic outcomes to obtain optimal therapeutic benefit from medicinal plants. This review may prove to be a filtering tool to researchers into complementary and alternative medicines to find out the current trends of using plant-based therapies for the treatment of kidney diseases, including CKD.

Adenine overload induces ferroptosis in human primary proximal tubular epithelial cells.

The pathogenesis of crystal nephropathy involves deposition of intratubular crystals, tubular obstruction and cell death. The deposition of 8-dihydroxyadenine (DHA) crystals within kidney tubules, for instance, is caused by a hereditary deficiency of adenine phosphoribosyl transferase in humans or adenine overload in preclinical models. However, the downstream pathobiological patterns of tubular cell attrition in adenine/DHA-induced nephropathy remain poorly understood. In this study, we investigated: (i) the modes of adenine-induced tubular cell death in an experimental rat model and in human primary proximal tubular epithelial cells (PTEC); and (ii) the therapeutic effect of the flavonoid baicalein as a novel cell death inhibitor. In a rat model of adenine diet-induced crystal nephropathy, significantly elevated levels of tubular iron deposition and lipid peroxidation (4-hydroxynonenal; 4-HNE) were detected. This phenotype is indicative of ferroptosis, a novel form of regulated necrosis. In cultures of human primary PTEC, adenine overload-induced significantly increased mitochondrial superoxide levels, mitochondrial depolarisation, DNA damage and necrotic cell death compared with untreated PTEC. Molecular interrogation of adenine-stimulated PTEC revealed a significant reduction in the lipid repair enzyme glutathione peroxidase 4 (GPX4) and the significant increase in 4-HNE compared with untreated PTEC, supporting the concept of ferroptotic cell death. Moreover, baicalein treatment inhibited ferroptosis in adenine-stimulated PTEC by selectively modulating the mitochondrial antioxidant enzyme superoxide dismutase 2 (SOD2) and thus, suppressing mitochondrial superoxide production and DNA damage. These data identify ferroptosis as the primary pattern of PTEC necrosis in adenine-induced nephropathy and establish baicalein as a potential therapeutic tool for the clinical management of ferroptosis-associated crystal nephropathies (e.g., DHA nephropathy, oxalate nephropathy).

T-Cell Expression and Release of Kidney Injury Molecule-1 in Response to Glucose Variations Initiates Kidney Injury in Early Diabetes.

Half of the mortality in diabetes is seen in individuals <50 years of age and commonly predicted by the early onset of diabetic kidney disease (DKD). In type 1 diabetes, increased urinary albumin-to-creatinine ratio (uACR) during adolescence defines this risk, but the pathological factors responsible remain unknown. We postulated that early in diabetes, glucose variations contribute to kidney injury molecule-1 (KIM-1) release from circulating T cells, elevating uACR and DKD risk. DKD risk was assigned in youth with type 1 diabetes (n = 100; 20.0 ± 2.8 years; males/females, 54:46; HbA1c 66.1 [12.3] mmol/mol; diabetes duration 10.7 ± 5.2 years; and BMI 24.5 [5.3] kg/m2) and 10-year historical uACR, HbA1c, and random blood glucose concentrations collected retrospectively. Glucose fluctuations in the absence of diabetes were also compared with streptozotocin diabetes in apolipoprotein E -/- mice. Kidney biopsies were used to examine infiltration of KIM-1-expressing T cells in DKD and compared with other chronic kidney disease. Individuals at high risk for DKD had persistent elevations in uACR defined by area under the curve (AUC; uACRAUC0-10yrs, 29.7 ± 8.8 vs. 4.5 ± 0.5; P < 0.01 vs. low risk) and early kidney dysfunction, including ∼8.3 mL/min/1.73 m2 higher estimated glomerular filtration rates (modified Schwartz equation; Padj < 0.031 vs. low risk) and plasma KIM-1 concentrations (∼15% higher vs. low risk; P < 0.034). High-risk individuals had greater glycemic variability and increased peripheral blood T-cell KIM-1 expression, particularly on CD8+ T cells. These findings were confirmed in a murine model of glycemic variability both in the presence and absence of diabetes. KIM-1+ T cells were also infiltrating kidney biopsies from individuals with DKD. Healthy primary human proximal tubule epithelial cells exposed to plasma from high-risk youth with diabetes showed elevated collagen IV and sodium-glucose cotransporter 2 expression, alleviated with KIM-1 blockade. Taken together, these studies suggest that glycemic variations confer risk for DKD in diabetes via increased CD8+ T-cell production of KIM-1.

Molecular and functional profiling of apical versus basolateral small extracellular vesicles derived from primary human proximal tubular epithelial cells under inflammatory conditions.

Proximal tubular epithelial cells (PTEC) are central players in inflammatory kidney diseases. However, the complex signalling mechanism/s via which polarized PTEC mediate disease progression are poorly understood. Small extracellular vesicles (sEV), including exosomes, are recognized as fundamental components of cellular communication and signalling courtesy of their molecular cargo (lipids, microRNA, proteins). In this study, we examined the molecular content and function of sEV secreted from the apical versus basolateral surfaces of polarized human primary PTEC under inflammatory diseased conditions. PTEC were cultured under normal and inflammatory conditions on Transwell inserts to enable separate collection and isolation of apical/basolateral sEV. Significantly increased numbers of apical and basolateral sEV were secreted under inflammatory conditions compared with equivalent normal conditions. Multi-omics analysis revealed distinct molecular profiles (lipids, microRNA, proteins) between inflammatory and normal conditions for both apical and basolateral sEV. Biological pathway analyses of significantly differentially expressed molecules associated apical inflammatory sEV with processes of cell survival and immunological disease, while basolateral inflammatory sEV were linked to pathways of immune cell trafficking and cell-to-cell signalling. In line with this mechanistic concept, functional assays demonstrated significantly increased production of chemokines (monocyte chemoattractant protein-1, interleukin-8) and immuno-regulatory cytokine interleukin-10 by peripheral blood mononuclear cells activated with basolateral sEV derived from inflammatory PTEC. We propose that the distinct molecular composition of sEV released from the apical versus basolateral membranes of human inflammatory PTEC may reflect specialized functional roles, with basolateral-derived sEV pivotal in modulating tubulointerstitial inflammatory responses observed in many immune-mediated kidney diseases. These findings provide a rationale to further evaluate these sEV-mediated inflammatory pathways as targets for biomarker and therapeutic development.

The Emerging Role of Renal Tubular Epithelial Cells in the Immunological Pathophysiology of Lupus Nephritis.

Systemic lupus erythematosus (SLE) is a systemic, autoimmune disease that can involve virtually any organ of the body. Lupus nephritis (LN), the clinical manifestation of this disease in the kidney, is one of the most common and severe outcomes of SLE. Although a key pathological hallmark of LN is glomerular inflammation and damage, tubulointerstitial lesions have been recognized as an important component in the pathology of LN. Renal tubular epithelial cells are resident cells in the tubulointerstitium that have been shown to play crucial roles in various acute and chronic kidney diseases. In this context, recent progress has been made in examining the functional role of tubular epithelial cells in LN pathogenesis. This review summarizes recent advances in our understanding of renal tubular epithelial cells in LN, the potential role of tubular epithelial cells as biomarkers in the diagnosis, prognosis, and treatment of LN, and the future therapeutic potential of targeting the tubulointerstitium for the treatment of patients with LN.

Oxidative stress and inflammasome activation in human rhabdomyolysis-induced acute kidney injury.

Acute kidney injury (AKI) is a life-threatening complication of rhabdomyolysis. The pathophysiological mechanisms of rhabdomyolysis-induced AKI (RIAKI) have been extensively studied in the murine system, yet clinical translation of this knowledge to humans is lacking. In this study, we investigated the cellular and molecular pathways of human RIAKI. Renal biopsy tissue from a RIAKI patient was examined by quantitative immunohistochemistry (Q-IHC) and compared to healthy kidney cortical tissue. We identified myoglobin casts and uric acid localised to sites of histological tubular injury, consistent with the diagnosis of RIAKI. These pathological features were associated with tubular oxidative stress (4-hydroxynonenal staining), regulated necrosis/necroptosis (phosphorylated mixed-lineage kinase domain-like protein staining) and inflammation (tumour necrosis factor (TNF)-α staining). Expression of these markers was significantly elevated in the RIAKI tissue compared to the healthy control. A tubulointerstitial inflammatory infiltrate accumulated adjacent to these sites of RIAKI oxidative injury, consisting of macrophages (CD68), dendritic cells (CD1c) and T lymphocytes (CD3). Foci of inflammasome activation were co-localised with these immune cell infiltrate, with significantly increased staining for adaptor protein ASC (apoptosis-associated speck-like protein containing a caspase activation and recruitment domain) and active caspase-1 in the RIAKI tissue compared to the healthy control. Our clinical findings identify multiple pathophysiological pathways previously only reported in murine RIAKI, providing first evidence in humans linking deposition of myoglobin and presence of uric acid to tubular oxidative stress/necroptosis, inflammasome activation and necroinflammation.

Role of inflammation and inflammasome activation in human bile cast nephropathy.

Bile cast nephropathy (BCN) is an underdiagnosed cause of acute kidney injury (AKI). The precise pathogenesis of bilirubin tubular toxicity remains unknown. The aim of this study is to explore the cellular and molecular pathophysiology of human BCN. Paraffin-embedded sections of renal biopsy tissue from a BCN patient were stained by immunohistochemistry (IHC) for oxidative stress (4-hydroxynonenal), immune cell subpopulations, including dendritic cells (CD1c), macrophages (CD68) and T cells (CD3), and inflammasome activation by staining for active-caspase-1 and the inflammasome adaptor protein, ASC (apoptosis-associated speck-like protein containing a caspase activation and recruitment domain). Quantitative analyses of IHC staining were compared to healthy renal cortical tissue. We identified yellow to brown granular casts within the BCN case, consistent with the presence of bile pigment. The presence of bile pigment was associated with strong tubular 4-hydroxynonenal staining intensity, a marker of oxidative stress. Diffuse tubulointerstitial inflammatory cell infiltrate was detected, with elevated CD1c, CD68 and CD3 staining. Foci of inflammasome activity were co-localized with this intense immune cell infiltration, with increased active-caspase-1 and ASC staining. Our findings are the first to suggest that bile casts may lead to oxidative stress and trigger the inflammasome signalling cascade, leading to interstitial inflammation and driving AKI pathobiology. SUMMARY AT A GLANCE The report suggests that bile casts may lead to oxidative stress and trigger the inflammasome signalling cascade, leading to interstitial inflammation and driving bile cast nephropathy pathobiology.

Underlying Histopathology Determines Response to Oxidative Stress in Cultured Human Primary Proximal Tubular Epithelial Cells.

Proximal tubular epithelial cells (PTEC) are key players in the progression of kidney diseases. PTEC studies to date have primarily used mouse models and transformed human PTEC lines. However, the translatability of these models to human kidney disease has been questioned. In this study, we investigated the phenotypic and functional response of human primary PTEC to oxidative stress, an established driver of kidney disease. Furthermore, we examined the functional contribution of the underlying histopathology of the cortical tissue used to generate our PTEC. We demonstrated that human primary PTEC from both histologically 'normal' and 'diseased' cortical tissue responded to H2O2-induced oxidative stress with significantly elevated mitochondrial superoxide levels, DNA damage, and significantly decreased proliferation. The functional response of 'normal' PTEC to oxidative stress mirrored the reported pathogenesis of human kidney disease, with significantly attenuated mitochondrial function and increased cell death. In contrast, 'diseased' PTEC were functionally resistant to oxidative stress, with maintenance of mitochondrial function and cell viability. This selective survival of 'diseased' PTEC under oxidizing conditions is reminiscent of the in vivo persistence of maladaptive PTEC following kidney injury. We are now exploring the impact that these differential PTEC responses have in the therapeutic targeting of oxidative stress pathways.

Specialized Roles of Human Natural Killer Cell Subsets in Kidney Transplant Rejection.

Background: Human natural killer (NK) cells are key functional players in kidney transplant rejection. However, the respective contributions of the two functionally distinct human NK cell subsets (CD56bright cytokine-producing vs. CD56dim cytotoxic effector) in episodes of allograft rejection remain uncertain, with current immunohistochemical methods unable to differentiate these discrete populations. We report the outcomes of an innovative multi-color flow cytometric-based approach to unequivocally define and evaluate NK cell subsets in human kidney allograft rejection. Methods: We extracted renal lymphocytes from human kidney transplant biopsies. NK cell subsets were identified, enumerated, and phenotyped by multi-color flow cytometry. Dissociation supernatants were harvested and levels of soluble proteins were determined using a multiplex bead-based assay. Results were correlated with the histopathological patterns in biopsies-no rejection, borderline cellular rejection, T cell-mediated rejection (TCMR), and antibody-mediated rejection (AMR). Results: Absolute numbers of only CD56bright NK cells were significantly elevated in TCMR biopsies. In contrast, both CD56bright and CD56dim NK cell numbers were significantly increased in biopsies with histopathological evidence of AMR. Notably, expression of the activation marker CD69 was only significantly elevated on CD56dim NK cells in AMR biopsies compared with no rejection biopsies, indicative of a pathogenic phenotype for this cytotoxic NK cell subset. In line with this, we detected significantly elevated levels of cytotoxic effector molecules (perforin, granzyme A, and granulysin) in the dissociation supernatants of biopsies with a histopathological pattern of AMR. Conclusions: Our results indicate that human NK cell subsets are differentially recruited and activated during distinct types of rejection, suggestive of specialized functional roles.

Biomarkers and the role of mast cells as facilitators of inflammation and fibrosis in chronic kidney disease.

Chronic kidney disease (CKD) is a clinical syndrome with many adverse sequelae and is currently a major global health and economic burden. Regardless of aetiology, inflammation and fibrosis are common manifestations of CKD. Unfortunately, the underlying pathophysiological mechanisms are poorly understood, and robust prognostic and early diagnostic biomarkers of CKD are lacking. One immune cell population that has received little attention in the context of CKD is mast cells (MCs). This mini review will examine the role of MCs as facilitators of kidney inflammation and fibrosis, propose a mechanistic structure for MCs in CKD, and give consideration to biomarkers specific for MC activation that can be deployed clinically. MCs are derived from haematopoietic stem cells. They are characterised by electron-dense granules in the cytoplasm, filled with preformed mediators. MCs can synthesise a range of bio-active compounds. Activation of MCs modulates an innate immune and adaptive effector response. Increased MC counts have been observed in animal models of kidney disease and a range of kidney diseases in humans where MC presence has been linked to biomarkers of kidney function and tissue damage. To further implicate MCs in CKD, several chemokines, cytokines and proteases released by MCs have been observed in their own right in various kidney diseases and linked to progressive CKD. One compound released by MCs that is of particular interest is the MC-specific protease tryptase. This protease is capable of activating the G-protein coupled receptor (GPCR) protease activated receptor-2 (PAR-2). PAR-2 is widely expressed throughout the kidney and highly expressed in the tubular epithelial cells where its activation induces robust inflammatory and fibrotic responses. Novel prognostic and diagnostic biomarkers of CKD are needed. MC-specific proteases [tryptase, chymase and carboxypeptidase A3 (CPA3)] are easily detectable in the blood but questionably in the urine. This review aims to promote these as prognostic and diagnostic biomarkers in the context of CKD.

Human Tissue-Resident Mucosal-Associated Invariant T (MAIT) Cells in Renal Fibrosis and CKD.

BACKGROUND: Mucosal-associated invariant T (MAIT) cells represent a specialized lymphocyte population associated with chronic inflammatory disorders. Little is known, however, about MAIT cells in diseases of the kidney, including CKD. METHODS: To evaluate MAIT cells in human native kidneys with tubulointerstitial fibrosis, the hallmark of CKD, we used multicolor flow cytometry to identify, enumerate, and phenotype such cells from human kidney tissue biopsy samples, and immunofluorescence microscopy to localize these cells. We cocultured MAIT cells and human primary proximal tubular epithelial cells (PTECs) under hypoxic (1% oxygen) conditions to enable examination of mechanistic tubulointerstitial interactions. RESULTS: We identified MAIT cells (CD3+ TCR Vα7.2+ CD161hi) in healthy and diseased kidney tissues, detecting expression of tissue-resident markers (CD103/CD69) on MAIT cells in both states. Tissue samples from kidneys with tubulointerstitial fibrosis had significantly elevated numbers of MAIT cells compared with either nonfibrotic samples from diseased kidneys or tissue samples from healthy kidneys. Furthermore, CD69 expression levels, also an established marker of lymphocyte activation, were significantly increased on MAIT cells from fibrotic tissue samples. Immunofluorescent analyses of fibrotic kidney tissue identified MAIT cells accumulating adjacent to PTECs. Notably, MAIT cells activated in the presence of human PTECs under hypoxic conditions (modeling the fibrotic microenvironment) displayed significantly upregulated expression of CD69 and cytotoxic molecules perforin and granzyme B; we also observed a corresponding significant increase in PTEC necrosis in these cocultures. CONCLUSIONS: Our findings indicate that human tissue-resident MAIT cells in the kidney may contribute to the fibrotic process of CKD via complex interactions with PTECs.

Natural Killer Cells in Kidney Health and Disease.

Natural killer (NK) cells are a specialized population of innate lymphocytes that have a major effector function in local immune responses. While their immunological functions in many inflammatory diseases are well established, comparatively little is still known about their roles in kidney homeostasis and disease. Our understanding of kidney NK cells is rapidly evolving, with murine studies highlighting the functional significance of NK cells in acute and chronic forms of renal disease. Recent progress has been made in translating these murine findings to human kidneys, with indications of NK cell subset-specific roles in disease progression in both native and allograft kidneys. Clearly, a better understanding of the molecular mechanisms driving NK cell activation and importantly, their downstream interactions with intrinsic renal cells and infiltrating immune cells is necessary for the development of targeted therapeutics to halt disease progression. In this review, we discuss the properties and potential functions of kidney NK cells.