The chemokine receptors CCR2 and CX3CR1 mediate monocyte/macrophage trafficking in kidney ischemia-reperfusion injury

The retinal environment induces microglia-like properties in recruited myeloid cells

Background

Microglia are essential to the development of the CNS and its homeostasis. Our prior findings suggested a niche model to describe the behaviors of retinal microglia. Here, we ask whether new myeloid cells recruited to the retina are constrained to resemble endogenous microglia morphologically and functionally.

Methods

Use of CD11c^DTR/GFP transgenic mouse allowed identification of two niches of retinal microglia distinguished by being GFP^lo or GFP^hi. We also used transgenic mice in which CX3CR1⁺ cells expressed YFP and were depletable following tamoxifen-induced expression of diphtheria toxin subunit A. We employed several ablation and injury stimulation protocols to examine the origin and fate of myeloid cells repopulating the retina. Analysis of retinal myeloid cells was done by microscopy, flow cytometry, and qRT-PCR.

Results

We found that the origin of new GFP^hi and GFP^lo myeloid cells in the retina of CD11c^DTR/GFP mice, whether recruited or local, depended on the ablation and stimulation protocols. Regardless of origin, new GFP^lo and GFP^hi retinal myeloid cells were CD45^medCD11b⁺Ly6G⁻Ly6C^loIba1⁺F4/80⁺, similar to endogenous microglia. Following tamoxifen-induced diphtheria toxin ablation, myeloid cell repopulation differed in the retina compared to the brain and optic nerve. Stimulation of replacement GFP^hi cells was substantially attenuated in repopulating retinas after tamoxifen-induced diphtheria toxin ablation compared to control or radiation-ablated mice. In radiation bone marrow chimeric mice, replacement GFP^hi myeloid cells from the circulation were slow to repopulate the retina unless stimulated by an optic nerve crush injury. However, once stimulated, recruited GFP^hi cells were found to concentrate on injured retinal ganglion cells and were morphologically similar to GFP^hi cells in non-ablated control CD11c^DTR/GFP mice.

Conclusions

The results support the idea that GFP^hi cells in the CD11c^DTR/GFP mouse, whether recruited or from resident microglia, mark a unique niche of activated retinal myeloid cells. We conclude that the retinal environment has a potent influence on the function, morphology, and proliferative capacity of new myeloid cells regardless of their origin, compelling them to be equivalent to the endogenous microglia.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1546-9) contains supplementary material, which is available to authorized users.

Tubular GM-CSF Promotes Late MCP-1/CCR2-Mediated Fibrosis and Inflammation after Ischemia/Reperfusion Injury

BACKGROUND

After bilateral kidney ischemia/reperfusion injury (IRI), monocytes infiltrate the kidney and differentiate into proinflammatory macrophages in response to the initial kidney damage, and then transition to a form that promotes kidney repair. In the setting of unilateral IRI (U-IRI), however, we have previously shown that macrophages persist beyond the time of repair and may promote fibrosis.

METHODS

Macrophage homing/survival signals were determined at 14 days after injury in mice subjected to U-IRI and in vitro using coculture of macrophages and tubular cells. Mice genetically engineered to lack Ccr2 and wild-type mice were treated ±CCR2 antagonist RS102895 and subjected to U-IRI to quantify macrophage accumulation, kidney fibrosis, and inflammation 14 and 30 days after the injury.

RESULTS

Failure to resolve tubular injury after U-IRI results in sustained expression of granulocyte-macrophage colony-stimulating factor by renal tubular cells, which directly stimulates expression of monocyte chemoattractant protein-1 (Mcp-1) by macrophages. Analysis of CD45⁺ immune cells isolated from wild-type kidneys 14 days after U-IRI reveals high-level expression of the MCP-1 receptor Ccr2. In mice lacking Ccr2 and wild-type mice treated with RS102895, the numbers of macrophages, dendritic cells, and T cell decreased following U-IRI, as did the expression of profibrotic growth factors and proimflammatory cytokines. This results in a reduction in extracellular matrix and kidney injury markers.

CONCLUSIONS

GM-CSF-induced MCP-1/CCR2 signaling plays an important role in the cross-talk between injured tubular cells and infiltrating immune cells and myofibroblasts, and promotes sustained inflammation and tubular injury with progressive interstitial fibrosis in the late stages of U-IRI.

Nonclassical Monocytes in Health and Disease

Monocytes are innate blood cells that maintain vascular homeostasis and are early responders to pathogens in acute infections. There are three well-characterized classes of monocytes: classical (CD14+CD16−in humans and Ly6Chiin mice), intermediate (CD14+CD16+in humans and Ly6C+Treml4+in mice), and nonclassical (CD14−CD16+in humans and Ly6Cloin mice). Classical monocytes are critical for the initial inflammatory response. Classical monocytes can differentiate into macrophages in tissue and can contribute to chronic disease. Nonclassical monocytes have been widely viewed as anti-inflammatory, as they maintain vascular homeostasis. They are a first line of defense in recognition and clearance of pathogens. However, their roles in chronic disease are less clear. They have been shown to be protective as well as positively associated with disease burden. This review focuses on the state of the monocyte biology field and the functions of monocytes, particularly nonclassical monocytes, in health and disease.

The Evolving Roles of Macrophages in Organ Transplantation

Organ transplantation is a life-saving strategy for patients with end-stage organ failure. Over the past few decades, organ transplantation has achieved an excellent success in short-term survival but only a marginal improvement in long-term graft outcomes. The pathophysiology of graft loss is multifactorial and remains incompletely defined. However, emerging evidence suggests macrophages as crucial mediators of acute and chronic allograft immunopathology. In this process, macrophage-mediated mobilization of first-line defenses, particularly phagocytosis and the release of acute inflammatory mediators, is important, but macrophages also launch adaptive alloimmune reactions against grafts through antigen processing and presentation, as well as providing costimulation. Additionally, crosstalk with other immune cells and graft endothelial cells causes tissue damage or fibrosis in transplanted organs, contributing to graft loss or tolerance resistance. However, some macrophages function as regulatory cells that are capable of suppressing allogeneic T cells, inhibiting DC maturation, inducing the differentiation of Tregs, and subsequently promoting transplant tolerance. This functional diversity of macrophages in organ transplantation is consistent with their heterogeneity. Although our knowledge of the detrimental or beneficial effects of macrophages on transplants has exponentially increased, the exact mechanisms controlling macrophage functions are not yet completely understood. Here, we review recent advances in our understanding of the multifaceted nature of macrophages, focusing on their evolving roles in organ transplantation and the mechanisms involved in their activation and function in allograft transplantation. We also discuss potential therapeutic options and opportunities to target macrophage to improve the outcomes of transplant recipients.

Enhancing regeneration after acute kidney injury by promoting cellular dedifferentiation in zebrafish

Acute kidney injury (AKI) is a serious disorder for which there are limited treatment options. Following injury, native nephrons display limited regenerative capabilities, relying on the dedifferentiation and proliferation of renal tubular epithelial cells (RTECs) that survive the insult. Previously, we identified 4-(phenylthio)butanoic acid (PTBA), a histone deacetylase inhibitor (HDI), as an enhancer of renal recovery, and showed that PTBA treatment increased RTEC proliferation and reduced renal fibrosis. Here, we investigated the regenerative mechanisms of PTBA in zebrafish models of larval renal injury and adult cardiac injury. With respect to renal injury, we showed that delivery of PTBA using an esterified prodrug (UPHD25) increases the reactivation of the renal progenitor gene Pax2a, enhances dedifferentiation of RTECs, reduces Kidney injury molecule-1 (Kim-1) expression, and lowers the number of infiltrating macrophages. Further, we found that the effects of PTBA on RTEC proliferation depend upon retinoic acid signaling and demonstrate that the therapeutic properties of PTBA are not restricted to the kidney but also increase cardiomyocyte proliferation and decrease fibrosis following cardiac injury in adult zebrafish. These studies provide key mechanistic insights into how PTBA enhances tissue repair in models of acute injury and lay the groundwork for translating this novel HDI into the clinic.

This article has an associated First Person interview with the joint first authors of the paper.

Summary: Mortality associated with AKI is in part due to limited treatments available to ameliorate injury. The authors identify a compound that accelerates AKI recovery and promotes cellular dedifferentiation.

[Research progress of exosomes in epithelial-mesenchymal transition]

Exosome, a membranous vesicle with biological activity, not only transmits active substances between cells but also transfers information between cells to participate in cell communication. Epithelial-mesenchymal transition (EMT) is a process by which epithelial cells acquire migratory and invasive properties to become mesenchymal stem cells. EMT is essential for the development of a spectrum of diseases. Studies have shown that exosomes have dual effects on EMT to, on the one hand, promote EMT and tumor cell invasion and metastasis and accelerate angiogenesis and tumor growth; on the other hand, exosomes can suppress tumor cell invasion, inhibit fibrosis of the heart, liver and kidney, and improve myocardial infarction by inhibiting EMT. Exosomes modulate EMT-related signaling pathways by carrying EMT-related proteins or miRNA to exert their bi-directional regulatory effects.

Bone marrow-derived Ly6C- macrophages promote ischemia-induced chronic kidney disease

Macrophages play an important role in renal injury and repair after acute kidney injury (AKI) and the subsequent chronic kidney disease (CKD) that often results. However, as macrophages have a high degree of plasticity and heterogeneity, the function(s) of macrophage subtypes in AKI-to-CKD progression are not fully understood. Here, we focused on Ly6C⁻ macrophages, which are derived from the embryonic yolk sac and post-development become resident in the kidneys. We found that C–C chemokine receptor type 2 (CCR2) deficiency, which blocks the migration of Ly6C⁺ macrophages from the bone marrow to the sites of injury, alleviated ischemia-induced AKI in mice. Unexpectedly, though, CCR2 deficiency worsened the subsequent renal fibrosis, which was marked by notable intra-renal infiltration of Ly6C⁻ macrophages. These Ly6C⁻ macrophages were greater in number in both the acute and chronic phases after ischemia reperfusion (I/R) in kidneys of wild type (WT) mice, and we showed them to be derived from the bone marrow by bone marrow chimerism. Clodronate Liposomes (CLs)-mediated depletion of renal Ly6C⁻ macrophages in CCR2^−/− mice or in WT mice after I/R alleviated the renal injury and fibrosis. On the contrary, adoptive transfer of Ly6C⁻ macrophages from injured kidneys of WT mice into immune-deficient mice was sufficient to induce renal injury and fibrosis. Transcriptome sequencing of Ly6C⁻ macrophages from injured kidneys revealed that they secreted various cytokines and growth factors, which were associated with the transdifferentiation of fibroblasts into myofibroblasts. This transdifferentiation effect was further supported by in vitro studies showing that Ly6C⁻ macrophages induced the secretion of extracellular matrix proteins from co-cultured fibroblasts. In conclusion, the presence of bone marrow-derived Ly6C⁻ macrophages after ischemia induces AKI and worsens subsequent CKD.

M2 macrophages in kidney disease: biology, therapies, and perspectives

Tissue macrophages are crucial players in homeostasis, inflammation, and immunity. They are characterized by heterogeneity and plasticity, due to which they display a continuum of phenotypes with M1/M2 presenting 2 extremes of this continuum. M2 macrophages are usually termed in the literature as anti-inflammatory and wound healing. Substantial progress has been made in elucidating the biology of M2 macrophages and their potential for clinical translation. In this review we discuss the current state of knowledge in M2 macrophage research with an emphasis on kidney disease. We explore their therapeutic potential and the challenges in using them as cellular therapies. Some new regulators that shape macrophage polarization and potential areas for future research are also examined.

Angiopoietin 1 influences ischemic reperfusion renal injury via modulating endothelium survival and regeneration

Background

Damage to the endothelium due to ischemia reperfusion injury (IRI) leads to a disruption of the microvasculature, which could be influenced by angiopoietin 1 via its effects on endothelium. We investigated the physiological and therapeutic roles of angiopoietin 1 in renal IRI using angiopoietin 1 knockout and over-expression mice.

Methods

Renal IRI was induced by clamping the right renal artery seven days after left uninephrectomy for 25 min followed by reperfusion. A whole body angiopoietin 1 knockout was achieved by induction with tamoxifen. The renal tubule over-expression of angiopoietin 1 was induced by doxycycline.

Results

In the normal mice, the renal expression of angiopoietin 1 increased 7 days to 14 days after IRI. The angiopoietin 1 knockout caused a delay in the recovery of renal function, less tubular regeneration and more residual tubular necrosis. The endothelial density was lower and the VE-cadherin protein loss was greater in the knockout mice. The over-expression of angiopoietin 1 attenuated the tubular necrosis and renal function impairment 1 and 3 days after IRI. The loss of the endothelium was ameliorated in the over-expression mice. This protective effect was associated with the up-regulation of the gene expression of epidermal growth factor, hepatocyte growth factor, and insulin like growth factor-1 and less tubular apoptosis. The over-expression of angiopoietin 1 stimulated tumor necrosis factor-α, C-C chemokine receptor type 2 and CX3C chemokine receptor 1 inflammatory gene expression, but did not influence macrophage infiltration.

Conclusions

Altogether, the augmentation and downregulation of angiopoietin 1 attenuated renal damage and impaired renal recovery, respectively, by influencing the survival/regeneration of the endothelium. The manipulation of angiopoietin 1 represents a novel therapeutic approach for the treatment of ischemic kidney injury.

Electronic supplementary material

The online version of this article (10.1186/s10020-019-0072-7) contains supplementary material, which is available to authorized users.

Macrophages: versatile players in renal inflammation and fibrosis

Macrophages have important roles in immune surveillance and in the maintenance of kidney homeostasis; their response to renal injury varies enormously depending on the nature and duration of the insult. Macrophages can adopt a variety of phenotypes: at one extreme, M1 pro-inflammatory cells contribute to infection clearance but can also promote renal injury; at the other extreme, M2 anti-inflammatory cells have a reparative phenotype and can contribute to the resolution phase of the response to injury. In addition, bone marrow monocytes can differentiate into myeloid-derived suppressor cells that can regulate T cell immunity in the kidney. However, macrophages can also promote renal fibrosis, a major driver of progression to end-stage renal disease, and the CD206⁺ subset of M2 macrophages is strongly associated with renal fibrosis in both human and experimental diseases. Myofibroblasts are important contributors to renal fibrosis and recent studies provide evidence that macrophages recruited from the bone marrow can transition directly into myofibroblasts within the injured kidney. This process is termed macrophage-to-myofibroblast transition (MMT) and is driven by transforming growth factor-β1 (TGFβ1)-Smad3 signalling via a Src-centric regulatory network. MMT may serve as a key checkpoint for the progression of chronic inflammation into pathogenic fibrosis.

Resident macrophages reprogram toward a developmental state after acute kidney injury

Acute kidney injury (AKI) is a devastating clinical condition affecting at least two-thirds of critically ill patients, and, among these patients, it is associated with a greater than 60% risk of mortality. Kidney mononuclear phagocytes (MPs) are implicated in pathogenesis and healing in mouse models of AKI and, thus, have been the subject of investigation as potential targets for clinical intervention. We have determined that, after injury, F4/80hi-expressing kidney-resident macrophages (KRMs) are a distinct cellular subpopulation that does not differentiate from nonresident infiltrating MPs. However, if KRMs are depleted using polyinosinic/polycytidylic acid (poly I:C), they can be reconstituted from bone marrow-derived precursors. Further, KRMs lack major histocompatibility complex class II (MHCII) expression before P7 but upregulate it over the next 14 days. This MHCII- KRM phenotype reappears after injury. RNA sequencing shows that injury causes transcriptional reprogramming of KRMs such that they more closely resemble that found at P7. KRMs after injury are also enriched in Wingless-type MMTV integration site family (Wnt) signaling, indicating that a pathway vital for mouse and human kidney development is active. These data indicate that mechanisms involved in kidney development may be functioning after injury in KRMs.

Valproic acid exerts specific cellular and molecular anti-inflammatory effects in post-operative conjunctiva

Valproic acid (VPA) is a histone deacetylase inhibitor used clinically for neurological disorders. It is also potentially useful as anti-fibrotic therapy as it reduced collagen deposition in the post-operative conjunctiva. In this study, we further evaluated the effects of VPA on post-operative inflammation using the mouse model of conjunctival scarring. VPA, injected into the subconjunctiva immediately after surgery, did not cause any adverse tissue response when examined by live microscopy and produced an apparent reduction of proinflammatory and proangiogenic markers in immunohistological examinations. In-depth analyses of the treated operated tissues revealed that VPA selectively inhibited the CD45^highF4/80^low macrophage subset as well as the production of specific proinflammatory cytokines/ chemokines, including CXCL1, IL-5, IL-6, and IL-10 which were reduced by ≥ 2.0-fold. VPA also specifically reduced tissue NF-кB2 p100 protein by mean 3.87-fold. On conjunctival fibroblasts, VPA treatment resulted in decreased secretion of specific cytokines, including CCL2, VEGF-A, and IL-15. In the presence of TNF-α, VPA inhibited the induction of specific cytokines/chemokines, notably CCL5 and VEGF-A, as well as NF-кB2 p100. In corroboration, VPA suppressed TNF-α stimulation of NF-кB reporter transcription by 1.51-fold. These data indicate that VPA can modulate both proinflammatory cellular and molecular targets in a selective manner and may therefore attenuate surgery-induced conjunctival inflammation. These and previous findings suggest that, by suppressing key mediators of both inflammation and fibrosis, VPA is a useful therapeutic for improving surgical outcome involving the conjunctiva. KEY MESSAGES: VPA inhibited recruitment of a CD45^highF4/80^low macrophage subset. VPA reduced chemokine and cytokine levels in treated tissues. VPA selectively suppressed tissue NF-кB2 p100 levels. VPA suppressed TNF-α induction of chemokines, cytokines and NF-кB2 p100 expression. VPA suppressed TNF-α stimulation of NF-кB reporter.

The Atypical Chemokine Receptor 2 Limits Progressive Fibrosis after Acute Ischemic Kidney Injury

Following renal ischemia-reperfusion injury (IRI), resolution of inflammation allows tubular regeneration, whereas ongoing inflammatory injury mediated by infiltrating leukocytes leads to nephron loss and renal fibrosis, typical hallmarks of chronic kidney disease. Atypical chemokine receptor 2 (ACKR2) is a chemokine decoy receptor that binds and scavenges inflammatory CC chemokines and reduces local leukocyte accumulation. We hypothesized that ACKR2 limits leukocyte infiltration, inflammation, and fibrotic tissue remodeling after renal IRI, thus preventing progression to chronic kidney disease. Compared with wild type, Ackr2 deficiency increases CC chemokine ligand 2 levels in tumor necrosis factor-stimulated tubulointerstitial tissue in vitro. In Ackr2-deficient mice with early IRI 1 or 5 days after transient renal pedicle clamping, tubular injury was similar to wild type, although accumulation of mononuclear phagocytes increased in postischemic Ackr2^-/- kidneys. Regarding long-term outcomes, Ackr2^-/- kidneys displayed more tubular injury 5 weeks after IRI, which was associated with persistently increased renal infiltrates of mononuclear phagocytes, T cells, Ly6C^high inflammatory macrophages, and inflammation. Moreover, Ackr2 deficiency caused substantially aggravated renal fibrosis in Ackr2^-/- kidneys 5 weeks after IRI, shown by increased expression of matrix molecules, renal accumulation of α-smooth muscle actin-positive myofibroblasts, and bone marrow-derived fibrocytes. ACKR2 is important in limiting persistent inflammation, tubular loss, and renal fibrosis after ischemic acute kidney injury and, thus, can prevent progression to chronic renal disease.

The innate immune receptor TREM-1 promotes liver injury and fibrosis

Inflammation occurs in all tissues in response to injury or stress and is the key process underlying hepatic fibrogenesis. Targeting chronic and uncontrolled inflammation is one strategy to prevent liver injury and fibrosis progression. Here, we demonstrate that triggering receptor expressed on myeloid cells 1 (TREM-1), an amplifier of inflammation, promotes liver disease by intensifying hepatic inflammation and fibrosis. In the liver, TREM-1 expression was limited to liver macrophages and monocytes and was highly upregulated on Kupffer cells, circulating monocytes, and monocyte-derived macrophages in a mouse model of chronic liver injury and fibrosis induced by carbon tetrachloride (CCl4) administration. TREM-1 signaling promoted proinflammatory cytokine production and mobilization of inflammatory cells to the site of injury. Deletion of Trem1 reduced liver injury, inflammatory cell infiltration, and fibrogenesis. Reconstitution of Trem1-deficient mice with Trem1-sufficient Kupffer cells restored the recruitment of inflammatory monocytes and the severity of liver injury. Markedly increased infiltration of liver fibrotic areas with TREM-1-positive Kupffer cells and monocytes/macrophages was found in patients with hepatic fibrosis. Our data support a role of TREM-1 in liver injury and hepatic fibrogenesis and suggest that TREM-1 is a master regulator of Kupffer cell activation, which escalates chronic liver inflammatory responses, activates hepatic stellate cells, and reveals a mechanism of promotion of liver fibrosis.

A review of the role of immune cells in acute kidney injury

Acute kidney injury (AKI) is a systemic disease occurring commonly in patients who are critically ill. Etiologies of AKI can be septic or aseptic (nephrotoxic, or ischemia-reperfusion injury). Recent evidence reveals that innate and adaptive immune responses are involved in mediating damage to renal tubular cells and in recovery from AKI. Dendritic cells, monocytes/macrophages, neutrophils, T lymphocytes, and B lymphocytes all contribute to kidney injury. Conversely, M2 macrophages and regulatory T cells are essential in suppressing inflammation, tissue remodeling and repair following kidney injury. AKI itself confers an increased risk for developing infection owing to increased production and decreased clearance of cytokines, in addition to dysfunction of immune cells themselves. Neutrophils are the predominant cell type rendered dysfunctional by AKI. In this review, we describe the bi-directional interplay between the immune system and AKI and summarize recent developments in this field of research.

Mcp1 Promotes Macrophage-Dependent Cyst Expansion in Autosomal Dominant Polycystic Kidney Disease

BACKGROUND

In patients with autosomal dominant polycystic kidney disease (ADPKD), most of whom have a mutation in PKD1 or PKD2, abnormally large numbers of macrophages accumulate around kidney cysts and promote their growth. Research by us and others has suggested that monocyte chemoattractant protein-1 (Mcp1) may be a signal for macrophage-mediated cyst growth.

METHODS

To define the role of Mcp1 and macrophages in promoting cyst growth, we used mice with inducible knockout of Pkd1 alone (single knockout) or knockout of both Pkd1 and Mcp1 (double knockout) in the murine renal tubule. Levels of Mcp1 RNA expression were measured in single-knockout mice and controls.

RESULTS

In single-knockout mice, upregulation of Mcp1 precedes macrophage infiltration. Macrophages accumulating around nascent cysts (0-2 weeks after induction) are initially proinflammatory and induce tubular cell injury with morphologic flattening, oxidative DNA damage, and proliferation-independent cystic dilation. At 2-6 weeks after induction, macrophages switch to an alternative activation phenotype and promote further cyst growth because of an additional three-fold increase in tubular cell proliferative rates. In double-knockout mice, there is a marked reduction in Mcp1 expression and macrophage numbers, resulting in less initial tubular cell injury, slower cyst growth, and improved renal function. Treatment of single-knockout mice with an inhibitor to the Mcp1 receptor Ccr2 partially reproduced the morphologic and functional improvement seen with Mcp1 knockout.

CONCLUSIONS

Mcp1 is upregulated after knockout of Pkd1 and promotes macrophage accumulation and cyst growth via both proliferation-independent and proliferation-dependent mechanisms in this orthologous mouse model of ADPKD.

Rictor deficiency in dendritic cells exacerbates acute kidney injury

Dendritic cells (DCs) are critical initiators of innate immunity in the kidney and orchestrate inflammation following ischemia-reperfusion injury. The role of the mammalian/mechanistic target of rapamycin (mTOR) in the pathophysiology of renal ischemia-reperfusion injury has been characterized. However, the influence of DC-based alterations in mTOR signaling is unknown. To address this, bone marrow-derived mTORC2-deficient (Rictor^-/-) DCs underwent hypoxia-reoxygenation and then analysis by flow cytometry. Adoptive transfer of wild-type or Rictor^-/- DC to C57BL/6 mice followed by unilateral or bilateral renal ischemia-reperfusion injury (20 min ischemia) was used to assess their in vivo migratory capacity and influence on tissue injury. Age-matched male DC-specific Rictor^-/- mice or littermate controls underwent bilateral renal ischemia-reperfusion, followed by assessment of renal function, histopathology, and biomolecular and cell infiltration analysis. Rictor^-/- DCs expressed more costimulatory CD80/CD86 but less coinhibitory programmed death ligand 1 (PDL1), a pattern that was enhanced by hypoxia-reoxygenation. They also demonstrated enhanced migration to the injured kidney and induced greater tissue damage. Following ischemia-reperfusion, Rictor^-/- DC mice developed higher serum creatinine levels, more severe histological damage, and greater proinflammatory cytokine production compared to littermate controls. Additionally, a greater influx of both neutrophils and T cells was seen in Rictor^-/- DC mice, along with CD11c⁺MHCII⁺CD11b^hiF4/80⁺ renal DC, that expressed more CD86 but less PDL1. Thus, DC-targeted elimination of Rictor enhances inflammation and migratory responses to the injured kidney, highlighting the regulatory roles of both DCs and Rictor in the pathophysiology of acute kidney injury.

Divergent effects of AKI to CKD models on inflammation and fibrosis

Chronic kidney disease (CKD) is a condition with significant morbidity and mortality that affects 15% of adults in the United States. One cause of CKD is acute kidney injury (AKI), which commonly occurs secondary to sepsis, ischemic events, and drug-induced nephrotoxicity. Unilateral ischemia-reperfusion injury (UIRI) without contralateral nephrectomy (CLN) and repeated low-dose cisplatin (RLDC) models of AKI to CKD demonstrate responses characteristic of the transition; however, previous studies have not effectively compared the pathogenesis. We demonstrate both models instigate renal dysfunction, inflammatory cytokine responses, and fibrosis. However, the models exhibit differences in urinary excretory function, inflammatory cell infiltration, and degree of fibrotic response. UIRI without CLN demonstrated worsening perfusion and function, measured with ^99mTc-mercaptoacetyltriglycine-3 imaging, and physiologic compensation in the contralateral kidney. Furthermore, UIRI without CLN elicited a robust inflammatory response that was characterized by a prolonged polymorphonuclear cell and natural killer cell infiltrate and an early expansion of kidney resident macrophages, followed by T-cell infiltration. Symmetrical diminished function occurred in RLDC kidneys and progressively worsened until day 17 of the study. Surprisingly, RLDC mice demonstrated a decrease in inflammatory cell numbers relative to controls. However, RLDC kidneys expressed increased levels of kidney injury molecule-1 (KIM-1), high mobility group box-1 ( HMGB1), and colony stimulating factor-1 ( CSF-1), which likely recruits inflammatory cells in response to injury. These data emphasize how the divergent etiologies of AKI to CKD models affect the kidney microenvironment and outcomes. This study provides support for subtyping AKI by etiology in human studies, aiding in the elucidation of injury-specific pathophysiologic mechanisms of the AKI to CKD transition.

Characterization of kidney CD45intCD11bintF4/80+MHCII+CX3CR1+Ly6C- "intermediate mononuclear phagocytic cells"

Kidney immune cells play important roles in pathogenesis of many diseases, including ischemia-reperfusion injury (IRI) and transplant rejection. While studying murine kidney T cells, we serendipitously identified a kidney mononuclear phagocytic cell (MPC) subset characterized by intermediate surface expression of CD45 and CD11b. These CD45^intCD11b^int MPCs were further identified as F4/80⁺MHCII⁺CX3CR1⁺Ly6C^- cells, comprising ~17% of total CD45⁺ cells in normal mouse kidney (P < 0.01) and virtually absent from all other organs examined except the heart. Systemic clodronate treatment had more significant depletive effect on the CD45^intCD11b^int population (77.3%±5.9%, P = 0.03) than on CD45^highCD11b⁺ population (14.8%±16.6%, P = 0.49). In addition, CD45^intCD11b^int MPCs had higher phagocytic function in the normal kidney (35.6%±3.3% vs. 24.1%±2.2%, P = 0.04), but lower phagocytic capacity in post-ischemic kidney (54.9%±1.0% vs. 67.8%±1.9%, P < 0.01) compared to the CD45^highCD11b⁺ population. Moreover, the CD45^intCD11b^int population had higher intracellular production of the pro-inflammatory tumor necrosis factor (TNF)-α (58.4%±5.2% vs. 27.3%±0.9%, P < 0.001) after lipopolysaccharide (LPS) stimulation and lower production of the anti-inflammatory interleukin (IL)-10 (7.2%±1.3% vs. 14.9%±2.2%, P = 0.02) following kidney IRI, suggesting a functional role under inflammatory conditions. The CD45^intCD11b^int cells increased early after IRI, and then abruptly decreased 48h later, whereas CD45^highCD11b⁺ cells steadily increased after IRI before declining at 72h (P = 0.03). We also identified the CD45^intCD11b^int MPC subtype in human kidney. We conclude that CD45^intCD11b^int F4/80⁺MHCII⁺CX3CR1⁺Ly6C^-population represent a unique subset of MPCs found in both mouse and human kidneys. Future studies will further characterize their role in kidney health and disease.

Tim-3 exacerbates kidney ischaemia/reperfusion injury through the TLR-4/NF-κB signalling pathway and an NLR-C4 inflammasome activation

T cell immunoglobulin domain and mucin domain-containing molecule-3 (Tim-3), a member of the immunoglobulin superfamily, has been shown to play a crucial role in host adaptive immunity and tolerance. However, its role in kidney ischaemia-reperfusion injury (IRI) remains unknown. In this study, we investigated the role and mechanism of Tim-3 signalling after kidney IRI. In an established murine model of kidney IRI, we found that Tim-3 expression is enhanced on monocytes/macrophages. Anti-Tim-3 antibody RMT3-23 ameliorates biochemical and histological kidney injury, reduces apoptosis and decreases macrophage infiltration and cytokine production in ischaemic kidneys. Cell culture experiments also demonstrated that the role of Tim-3 in IRI-induced macrophage activation leads to the secretion of proinflammatory cytokines and chemokines. In addition, Toll-like receptor (TLR)-4 and Nod-like receptor (NLR) family CARD domain-containing protein 4 (NLR-C4) expression were enhanced after kidney IRI and decreased significantly by RMT3-23. Tim-3 not only promotes TLR-mediated nuclear factor kappa B (NF-κB) activation and cytokine and chemokine release, but also participates in NLR-C4 inflammasome activation. Taken together, our data confirm that Tim-3 signalling enhances injury after kidney IRI and demonstrated that Tim-3 is involved in regulating TLR-4/NF-κB signalling and NLR-C4 inflammasome activation, which provide evidence that Tim-3 signalling is critical for kidney IRI and may provide a new means to ameliorate kidney tissue immune responses in the clinics.

Parabiosis reveals leukocyte dynamics in the kidney

The immune cellular compartment of the kidney is involved in organ development and homeostasis, as well as in many pathological conditions. Little is known about the mechanisms that drive intrarenal immune responses in the presence of renal tubular and interstitial cell death. However, it is known that tissue-resident leukocytes have the potential to have distinct roles compared with circulating cells. We used a parabiosis model in C57BL/6 CD45 congenic and green fluorescent protein transgenic mice to better understand the dynamics of immune cells in the kidney. We found F4/80^Hi intrarenal macrophages exhibit minimal exchange with the peripheral circulation in two models of parabiosis, whether mice were attached for 4 or 16 weeks. Other intrarenal inflammatory cells demonstrate near total exchange with the circulating immune cell pool in healthy kidneys, indicating that innate and adaptive immune cells extensively traffic through the kidney interstitium during normal physiology. Neutrophils, dendritic cells, F4/80^Low macrophages, T cells, B cells, and NK cells are renewed from the circulating immune cell pool. However, a fraction of double-negative T (CD4^- CD8^-) and NKT cells are long-lived or tissue resident. This study provides direct evidence of leukocyte sub-populations that are resident in the renal tissue, cells which demonstrate minimal to no exchange with the peripheral blood. In addition, the data demonstrate continual exchange of other sub-populations through uninflamed tissue.

The atypical chemokine receptor 2 limits renal inflammation and fibrosis in murine progressive immune complex glomerulonephritis

The atypical chemokine receptor 2 (ACKR2), also named D6, regulates local levels of inflammatory chemokines by internalization and degradation. To explore potential anti-inflammatory functions of ACKR2 in glomerulonephritis, we induced autologous nephrotoxic nephritis in C57/BL6 wild-type and Ackr2-deficient mice. Renal ACKR2 expression increased and localized to interstitial lymphatic endothelium during nephritis. At two weeks Ackr2^-/-mice developed increased albuminuria and urea levels compared to wild-type mice. Histological analysis revealed increased structural damage in the glomerular and tubulointerstitial compartments within Ackr2^-/- kidneys. This correlated with excessive renal leukocyte infiltration of CD4⁺ T cells and mononuclear phagocytes with increased numbers in the tubulointerstitium but not glomeruli in knockout mice. Expression of inflammatory mediators and especially markers of fibrotic tissue remodeling were increased along with higher levels of ACKR2 inflammatory chemokine ligands like CCL2 in nephritic Ackr2^-/- kidneys. In vitro, Ackr2 deficiency in TNF-stimulated tubulointerstitial tissue but not glomeruli increased chemokine levels. These results are in line with ACKR2 expression in interstitial lymphatic endothelial cells, which also assures efflux of activated leukocytes into regional lymph nodes. Consistently, nephritic Ackr2^-/- mice showed reduced adaptive cellular immune responses indicated by decreased regional T-cell activation. However, this did not prevent aggravated injury in the kidneys of Ackr2^-/- mice with nephrotoxic nephritis due to simultaneously increased tubulointerstitial chemokine levels, leukocyte infiltration and fibrosis. Thus, ACKR2 is important in limiting renal inflammation and fibrotic remodeling in progressive nephrotoxic nephritis. Hence, ACKR2 may be a potential target for therapeutic interventions in immune complex glomerulonephritis.

Neutrophil depletion protects against zomepirac-induced acute kidney injury in mice

Acyl glucuronide (AG) metabolites of carboxylic acid-containing drugs have been implicated in drug toxicity. Zomepirac (ZP) is a non-steroidal anti-inflammatory drug that was withdrawn from the market because of anaphylactic reactions and renal injury. We recently established a novel mouse model of ZP-induced kidney injury by increasing zomepirac acyl-glucuronide (ZP-AG) concentration via pretreatment with tri-O-tolyl phosphate, a nonselective esterase inhibitor, and l-buthionine-(S,R)-sulfoximine, a glutathione synthesis inhibitor. Although we have shown that ZP-AG is responsible for ZP-induced kidney injury in mice, the exact pathogenic mechanisms of ZP-induced kidney injury have not been investigated yet. In this study, we aimed to investigate the role of immune cells in the pathogenesis of ZP-induced kidney injury, as a representative of AG toxicity. We found that the counts of neutrophils and inflammatory monocytes increased in the blood of mice with ZP-induced kidney injury. However, clodronate liposome- or GdCl₃-induced monocyte and/or macrophage depletion did not affect blood urea nitrogen and plasma creatinine levels in mice with ZP-induced kidney injury. Neutrophil infiltration into the kidneys was observed in mice with ZP-induced kidney injury, whereas anti-lymphocyte antigen 6 complex, locus G (Ly6G) antibody pretreatment prevented the renal neutrophil infiltration and partially protected against ZP-induced kidney injury. The mRNA expression of neutrophil-infiltrating cytokines and chemokines, interleukin-1α and macrophage inflammatory protein-2α, increased in mice with ZP-induced kidney injury, whereas pretreatment with anti-Ly6G antibody resulted in a marked reduction of their expression. These results suggest that ZP-AG might be involved in kidney injury, partly via induction of neutrophil infiltration. Therefore, this study may provide an important understanding on toxicological role of ZP-AG in vivo that helps to understand toxicity of AG metabolites.

Tamm-Horsfall Protein Regulates Mononuclear Phagocytes in the Kidney

Tamm-Horsfall protein (THP), also known as uromodulin, is a kidney-specific protein produced by cells of the thick ascending limb of the loop of Henle. Although predominantly secreted apically into the urine, where it becomes highly polymerized, THP is also released basolaterally, toward the interstitium and circulation, to inhibit tubular inflammatory signaling. Whether, through this latter route, THP can also regulate the function of renal interstitial mononuclear phagocytes (MPCs) remains unclear, however. Here, we show that THP is primarily in a monomeric form in human serum. Compared with wild-type mice, THP^-/- mice had markedly fewer MPCs in the kidney. A nonpolymerizing, truncated form of THP stimulated the proliferation of human macrophage cells in culture and partially restored the number of kidney MPCs when administered to THP^-/- mice. Furthermore, resident renal MPCs had impaired phagocytic activity in the absence of THP. After ischemia-reperfusion injury, THP^-/- mice, compared with wild-type mice, exhibited aggravated injury and an impaired transition of renal macrophages toward an M2 healing phenotype. However, treatment of THP^-/- mice with truncated THP after ischemia-reperfusion injury mitigated the worsening of AKI. Taken together, our data suggest that interstitial THP positively regulates mononuclear phagocyte number, plasticity, and phagocytic activity. In addition to the effect of THP on the epithelium and granulopoiesis, this new immunomodulatory role could explain the protection conferred by THP during AKI.

Macrophages in Renal Injury and Repair

Acute kidney injury (AKI) is a growing global health concern, yet no treatment is currently available to prevent it or to promote kidney repair after injury. Animal models demonstrate that the macrophage is a major contributor to the inflammatory response to AKI. Emerging data from human biopsies also corroborate the presence of macrophages in AKI and their persistence in progressive chronic kidney disease. Macrophages are phagocytic innate immune cells that are important mediators of tissue homeostasis and host defense. In response to tissue injury, macrophages become activated based on specific signals from the damaged microenvironment. The activation and functional state of the macrophage depends on the stage of tissue injury and repair, reflecting a dynamic and diverse spectrum of macrophage phenotypes. In this review, we highlight our current understanding of the mechanisms by which macrophages contribute to injury and repair after AKI.

Activated Renal Dendritic Cells Cross Present Intrarenal Antigens After Ischemia-Reperfusion Injury

BACKGROUND

The healthy kidney contains an extensive population of renal mononuclear phagocytes (RMPs), with substantial phenotypic and functional diversity. However, how this diverse population is affected by ischemia-reperfusion injury (IRI), an obligate part of renal transplantation, is not yet well understood. The aim of this study was to characterize the phenotypic and functional alterations in RMPs induced by IRI.

METHODS

Renal mononuclear phagocytes were studied 24 and 72 hours after 30 minutes of renal ischemia or sham operation. Kidneys were digested and the phenotypes of renal leukocyte populations were analyzed via flow cytometry. Multiphoton microscopy was used to image renal dendritic cells (DCs) in vivo using CD11c reporter mice. The capacity of renal DCs to present antigen was examined by assessment of proliferation of ovalbumin-specific T cells in rat insulin promoter-membrane-bound ovalbumin transgenic mice after sham or IRI procedures.

RESULTS

Ischemia-reperfusion injury induced influx of monocytes, DCs, macrophages, and neutrophils into the kidney. Classification of RMP subpopulations based on CD11b/CD11c expression demonstrated that the RMPs that increased in response to IRI were predominantly newly recruited monocyte-derived inflammatory DCs. In vivo multiphoton imaging of CD11c-EYFP mice revealed that intrarenal DCs exhibited increased number and activity of dendrites in the postischemic period. Ischemia-reperfusion injury also promoted DC-dependent cross-presentation of renal antigens to CD8 T cells in the draining lymph node.

CONCLUSIONS

In response to renal IRI, RMP populations are skewed toward those derived from inflammatory monocyte precursors. In addition, renal DCs undergo functional activation, increasing their capacity to activate antigen-specific CD8 T cells in renal draining lymph nodes.

Inflammation and Fibrosis in Polycystic Kidney Disease

Polycystic kidney disease (PKD) is a commonly inherited disorder characterized by cyst formation and fibrosis (Wilson, N Engl J Med 350:151-164, 2004) and is caused by mutations in cilia or cilia-related proteins, such as polycystin 1 or 2 (Oh and Katsanis, Development 139:443-448, 2012; Kotsis et al., Nephrol Dial Transplant 28:518-526, 2013). A major pathological feature of PKD is the development of interstitial inflammation and fibrosis with an associated accumulation of inflammatory cells (Grantham, N Engl J Med 359:1477-1485, 2008; Zeier et al., Kidney Int 42:1259-1265, 1992; Ibrahim, Sci World J 7:1757-1767, 2007). It is unclear whether inflammation is a driving force for cyst formation or a consequence of the pathology (Ta et al., Nephrology 18:317-330, 2013) as in some murine models cysts are present prior to the increase in inflammatory cells (Phillips et al., Kidney Blood Press Res 30:129-144, 2007; Takahashi et al., J Am Soc Nephrol JASN 1:980-989, 1991), while in other models the increase in inflammatory cells is present prior to or coincident with cyst initiation (Cowley et al., Kidney Int 43:522-534, 1993, Kidney Int 60:2087-2096, 2001). Additional support for inflammation as an important contributor to cystic kidney disease is the increased expression of many pro-inflammatory cytokines in murine models and human patients with cystic kidney disease (Karihaloo et al., J Am Soc Nephrol JASN 22:1809-1814, 2011; Swenson-Fields et al., Kidney Int, 2013; Li et al., Nat Med 14:863-868, 2008a). Based on these data, an emerging model in the field is that disruption of primary cilia on tubule epithelial cells leads to abnormal cytokine cross talk between the epithelium and the inflammatory cells contributing to cyst growth and fibrosis (Ta et al., Nephrology 18:317-330, 2013). These cytokines are produced by interstitial fibroblasts, inflammatory cells, and tubule epithelial cells and activate multiple pathways including the JAK-STAT and NF-κB signaling (Qin et al., J Am Soc Nephrol JASN 23:1309-1318, 2012; Park et al., Am J Nephrol 32:169-178, 2010; Bhunia et al., Cell 109:157-168, 2002). Indeed, inflammatory cells are responsible for producing several of the pro-fibrotic growth factors observed in PKD patients with fibrosis (Nakamura et al., Am J Nephrol 20:32-36, 2000; Wilson et al., J Cell Physiol 150:360-369, 1992; Song et al., Hum Mol Genet 18:2328-2343, 2009; Schieren et al., Nephrol Dial Transplant 21:1816-1824, 2006). These growth factors trigger epithelial cell proliferation and myofibroblast activation that stimulate the production of extracellular matrix (ECM) genes including collagen types 1 and 3 and fibronectin, leading to reduced glomerular function with approximately 50% of ADPKD patients progressing to end-stage renal disease (ESRD). Therefore, treatments designed to reduce inflammation and slow the rate of fibrosis are becoming important targets that hold promise to improve patient life span and quality of life. In fact, recent studies in several PKD mouse models indicate that depletion of macrophages reduces cyst severity. In this chapter, we review the potential mechanisms of interstitial inflammation in PKD with a focus on ADPKD and discuss the role of interstitial inflammation in progression to fibrosis and ESRD.

Breast Regression Protein-39/Chitinase 3-Like 1 Promotes Renal Fibrosis after Kidney Injury via Activation of Myofibroblasts

The normal response to kidney injury includes a robust inflammatory infiltrate of PMNs and macrophages. We previously showed that the small secreted protein breast regression protein-39 (BRP-39), also known as chitinase 3-like 1 (CHI3L1) and encoded by the Chi3l1 gene, is expressed at high levels by macrophages during the early stages of kidney repair and promotes tubular cell survival via IL-13 receptor α2 (IL13Rα2)-mediated signaling. Here, we investigated the role of BRP-39 in profibrotic responses after AKI. In wild-type mice, failure to resolve tubular injury after unilateral ischemia-reperfusion injury (U-IRI) led to sustained low-level Chi3l1 mRNA expression by renal cells and promoted macrophage persistence and severe interstitial fibrosis. Analysis of macrophages isolated from wild-type kidneys 14 days after U-IRI revealed high-level expression of the profibrotic BRP-39 receptor Ptgdr2/Crth2 and expression of the profibrotic markers Lgals3, Pdgfb, Egf, and Tgfb In comparison, injured kidneys from mice lacking BRP-39 had significantly fewer macrophages, reduced expression of profibrotic growth factors, and decreased accumulation of extracellular matrix. BRP-39 depletion did not affect myofibroblast accumulation but did attenuate myofibroblast expression of Col1a1, Col3a1, and Fn1 Together, these results identify BRP-39 as an important activator of macrophage-myofibroblast crosstalk and profibrotic signaling in the setting of maladaptive kidney repair.

A MicroRNA93-Interferon Regulatory Factor-9-Immunoresponsive Gene-1-Itaconic Acid Pathway Modulates M2-Like Macrophage Polarization to Revascularize Ischemic Muscle

BACKGROUND

Currently, no therapies exist for treating and improving outcomes in patients with severe peripheral artery disease (PAD). MicroRNA93 (miR93) has been shown to favorably modulate angiogenesis and to reduce tissue loss in genetic PAD models. However, the cell-specific function, downstream mechanisms, or signaling involved in miR93-mediated ischemic muscle neovascularization is not clear. Macrophages were best known to modulate arteriogenic response in PAD, and the extent of arteriogenic response induced by macrophages is dependent on greater M2 to M1 activation/polarization state. In the present study, we identified a novel mechanism by which miR93 regulates macrophage polarization to promote angiogenesis and arteriogenesis to revascularize ischemic muscle in experimental PAD.

METHODS

In vitro (macrophages, endothelial cells, skeletal muscle cells under normal and hypoxia serum starvation conditions) and in vivo experiments in preclinical PAD models (unilateral femoral artery ligation and resection) were conducted to examine the role of miR93-interferon regulatory factor-9-immunoresponsive gene-1 (IRG1)-itaconic acid pathway in macrophage polarization, angiogenesis, arteriogenesis, and perfusion recovery.

RESULTS

In vivo, compared with wild-type controls, miR106b-93-25 cluster-deficient mice (miR106b-93-25-/-) showed decreased angiogenesis and arteriogenesis correlating with increased M1-like macrophages after experimental PAD. Intramuscular delivery of miR93 in miR106b-93-25-/- PAD mice increased angiogenesis, arteriogenesis, and the extent of perfusion, which correlated with more M2-like macrophages in the proximal and distal hind-limb muscles. In vitro, miR93 promotes and sustains M2-like polarization even under M1-like polarizing conditions (hypoxia serum starvation). Delivery of bone marrow-derived macrophages from miR106b-93-25-/- to wild-type ischemic muscle decreased angiogenesis, arteriogenesis, and perfusion, whereas transfer of wild-type macrophages to miR106b-93-25-/- had the opposite effect. Systematic analysis of top differentially upregulated genes from RNA sequencing between miR106b-93-25-/- and wild-type ischemic muscle showed that miR93 regulates IRG1 function to modulate itaconic acid production and macrophage polarization. The 3' untranslated region luciferase assays performed to determine whether IRG1 is a direct target of miR93 revealed that IRG1 is not an miR93 target but that interferon regulatory factor-9, which can regulate IRG1 expression, is an miR93 target. In vitro, increased expression of interferon regulatory factor-9 and IRG1 and itaconic acid treatment significantly decreased endothelial angiogenic potential.

CONCLUSIONS

miR93 inhibits interferon regulatory factor-9 to decrease IRG1-itaconic acid production to induce M2-like polarization in ischemic muscle to enhance angiogenesis, arteriogenesis, and perfusion recovery in experimental PAD.

Innate Immune Response in Kidney Ischemia/Reperfusion Injury: Potential Target for Therapy

Acute kidney injury caused by ischemia and subsequent reperfusion is associated with a high rate of mortality and morbidity. Ischemia/reperfusion injury in kidney transplantation causes delayed graft function and is associated with more frequent episodes of acute rejection and progression to chronic allograft nephropathy. Alloantigen-independent inflammation is an important process, participating in pathogenesis of injurious response, caused by ischemia and reperfusion. This innate immune response is characterized by the activity of classical cells belonging to the immune system, such as neutrophils, macrophages, dendritic cells, lymphocytes, and also tubular epithelial cells and endothelial cells. These immune cells not only participate in inflammation after ischemia exerting detrimental influence but also play a protective role in the healing response from ischemia/reperfusion injury. Delineating of complex mechanisms of their actions could be fruitful in future prevention and treatment of ischemia/reperfusion injury. Among numerous so far conducted experiments, observed immunomodulatory role of adenosine and adenosine receptor agonists in complex interactions of dendritic cells, natural killer T cells, and T regulatory cells is emphasized as promising in the treatment of kidney ischemia/reperfusion injury. Potential pharmacological approaches which decrease NF-κB activity and antagonize mechanisms downstream of activated Toll-like receptors are discussed.

The P2X7 receptor antagonist, oxidized adenosine triphosphate, ameliorates renal ischemia-reperfusion injury by expansion of regulatory T cells

Extracellular adenosine triphosphate (ATP) binds to purinergic receptors and, as a danger molecule, promotes inflammatory responses. Here we tested whether periodate-oxidized ATP (oATP), a P2X7 receptor (P2X7R) antagonist can attenuate renal ischemia-reperfusion injury and clarify the related cellular mechanisms. Treatment with oATP prior to ischemia-reperfusion injury decreased blood urea nitrogen, serum creatinine, the tubular injury score, and tubular epithelial cell apoptosis after injury. The infiltration of dendritic cells, neutrophils, macrophages, CD69⁺CD4⁺, and CD44⁺CD4⁺ T cells was attenuated, but renal Foxp3⁺CD4⁺ Treg infiltration was increased by oATP. The levels of IL-6 and CCL2 were reduced in the oATP group. Additionally, oATP treatment following injury improved renal function, decreased the infiltration of innate and adaptive effector cells, and increased the renal infiltration of Foxp3⁺CD4⁺ Tregs. Post-ischemia-reperfusion injury oATP treatment increased tubular cell proliferation and reduced renal fibrosis. oATP treatment attenuated renal functional deterioration after ischemia-reperfusion injury in RAG-1 knockout mice; however, Treg depletion using PC61 abrogated the beneficial effects of oATP in wild-type mice. Furthermore, oATP treatment after transfer of Tregs from wild-type mice improved the beneficial effects of Tregs on ischemia-reperfusion injury, but treatment after transfer of Tregs from P2X7R knockout mice did not. Renal ischemia-reperfusion injury was also attenuated in P2X7R knockout mice. Experiments using bone marrow chimeras established that P2X7R expression on hematopoietic cells rather than non-hematopoietic cells, such as tubular epithelial cells, plays a major role in ischemia-reperfusion injury. Thus, oATP attenuated acute renal damage and facilitated renal recovery in ischemia-reperfusion injury by expansion of Tregs.

Monocytes, Macrophages, and Osteoclasts in Osteosarcoma

Macrophages appear to have a fundamental role in the pathogenesis of osteosarcoma. These highly diverse plastic cells are subdivided into classical or inflammatory macrophages known as M1 and alternative macrophages, which decrease inflammation and are reparative, called M2. Although primary and metastatic osteosarcomas are infiltrated with M2 macrophages, targeting the M1 macrophages with the immune adjuvant muramyl tripeptide phosphatidyl ethanolamine (MTP-PE) has been the greatest recent therapeutic advance in osteosarcoma. This discrepancy between the presence of M2 and activation of M1 macrophages is intriguing and is likely explained either by the plasticity of M1 and M2 macrophages or nonclassical patrolling monocytes (PMos). To date, MTP-PE has been approved in combination with chemotherapy for nonmetastatic osteosarcoma, but its use in metastatic tumors has not been investigated. In this review, we focus on macrophages, monocytes, and osteoclasts, their role in osteosarcoma, and the potential for targeting these cells in this disease.

Proximal Tubule CD73 Is Critical in Renal Ischemia-Reperfusion Injury Protection

CD73-derived adenosine plays an anti-inflammatory role in various organs. However, its role in renal ischemia-reperfusion injury (IRI) is controversial. We targeted CD73 mutant mice to determine the function of CD73 expressed by various renal cell types under mild IRI conditions. Mice with CD73 deletion in proximal tubules exhibited exacerbated IRI, comparable with that of CD73-/- mice compared with WT mice. Mice with CD73 deletions in other cell types, including cortical type 1 fibroblast-like cells, mesangial cells, macrophages, and dendritic cells, showed small or no increases in injury above control mice when subjected to threshold levels of ischemia. Results from adoptive transfer experiments between WT and CD73-/- mice and pharmacologic studies modulating enzymatic activity of CD73 and extracellular adenosine levels supported a critical role of adenosine generated by proximal tubule CD73 expression in abrogating IRI. Renal adenosine levels were lower before and after ischemia in CD73-deficient mice. However, reduction in total acid-extractable renal adenosine levels was inadequate to explain the marked difference in kidney injury in these CD73-deficient mice. Furthermore, CD73 inhibition and enzyme replacement studies showed no change in total kidney adenosine levels in treated mice compared with vehicle-treated controls. Protection from IRI in neutrophil-depleted WT recipients was sustained by repopulation with bone marrow neutrophils from WT mice but not by those lacking adenosine 2a receptors (from Adora2a-/- mice). These data support the thesis that local adenosine generated by cells at the injury site is critical for protection from IRI through bone marrow-derived adenosine 2a receptors.

Human Alpha-1-Antitrypsin (hAAT) therapy reduces renal dysfunction and acute tubular necrosis in a murine model of bilateral kidney ischemia-reperfusion injury

Several lines of evidence have demonstrated the anti-inflammatory and cytoprotective effects of alpha-1-antitrypsin (AAT), the major serum serine protease inhibitor. The aim of the present study was to investigate the effects of human AAT (hAAT) monotherapy during the early and recovery phase of ischemia-induced acute kidney injury. Mild renal ischemia-reperfusion (I/R) injury was induced in male C57Bl/6 mice by bilateral clamping of the renal artery and vein for 20 min. hAAT (80 mg/kg, Prolastin^®) was administered daily intraperitoneally (i.p.) from day -1 until day 7 after surgery. Control animals received the same amount of human serum albumin (hAlb). Plasma, urine and kidneys were collected at 2h, 1, 2, 3, 8 and 15 days after reperfusion for histological and biochemical analysis. hAAT partially preserved renal function and tubular integrity after induction of bilateral kidney I/R injury, which was accompanied with reduced renal influx of macrophages and a significant decrease of neutrophil gelatinase-associated lipocalin (NGAL) protein levels in urine and plasma. During the recovery phase, hAAT significantly decreased kidney injury molecule-1 (KIM-1) protein levels in urine but showed no significant effect on renal fibrosis. Although the observed effect size of hAAT administration was limited and therefore the clinical relevance of our findings should be evaluated carefully, these data support the potential of this natural protein to ameliorate ischemic and inflammatory conditions.

Mononuclear phagocyte subpopulations in the mouse kidney

Mononuclear phagocytes are the most common cells in the kidney associated with immunity and inflammation. Although the presence of these cells in the kidney has been known for decades, the study of mononuclear phagocytes in the context of kidney function and dysfunction is still at an early stage. The purpose of this review is to summarize the present knowledge regarding classification of these cells in the mouse kidney and to identify relevant questions that would further advance the field and potentially lead to new opportunities for treatment of acute kidney injury and other kidney diseases.

Macrophages and RhoA Pathway in Transplanted Organs

RhoA is a small GTPase that, via its downstream effectors, regulates a variety of cell functions such as cytokinesis, cell migration, vesicular trafficking, and phagocytosis. As such the RhoA pathway is also pivotal for proper functioning of immune cells including macrophages. By controlling actin cytoskeleton organization, RhoA pathway modulates macrophage's polarity and basic functions: phagocytosis, migration, and extracellular matrix degradation. Numerous studies indicate that macrophages are very important effectors contributing to acute and chronic rejection of transplanted organs. In this review we discuss the role of RhoA pathway in governance of macrophage's functions in terms of transplanted organs.

Macrophages as Effectors of Acute and Chronic Allograft Injury

Organ transplants give a second chance of life to patients with end-stage organ failure. However, the immuno-logical barriers prove to be very challenging to overcome and graft rejection remains a major hurdle to long-term transplant survival. For decades, adaptive immunity has been the focus of studies, primarily based on the belief that T cells are necessary and sufficient for rejection. With better-developed immunosuppressive drugs and protocols that effectively control adaptive cells, innate immune cells have emerged as key effector cells in triggering graft injury and have therefore attracted much recent attention. In this review, we discuss current understanding of macrophages and their role in transplant rejection, their dynamics, distinct phenotypes, locations, and functions. We also discuss novel therapeutic approaches under development to target macrophages in transplant recipients.

CCR2 Positive Exosome Released by Mesenchymal Stem Cells Suppresses Macrophage Functions and Alleviates Ischemia/Reperfusion-Induced Renal Injury

Mesenchymal stem cells (MSCs) derived exosomes have been shown to have protective effects on the kidney in ischemia/reperfusion-induced renal injury. However, the key components in the exosomes and their potential mechanisms for the kidney protective effects are not well understood. In our current study, we focused on the abundant proteins in exosomes derived from MSCs (MSC-exo) and found that the C-C motif chemokine receptor-2 (CCR2) was expressed on MSC-exo with a high ability to bind to its ligand CCL2. We also proved that CCR2 high-expressed MSC-exo could reduce the concentration of free CCL2 and suppress its functions to recruit or activate macrophage. Further, knockdown of CCR2 expression on the MSC-exo greatly abolished these effects. Finally, we also found that CCR2 knockdown impaired the protective effects of MSC-exo for the renal ischemia/reperfusion injury in mouse. The results indicate that CCR2 expressed on MSC-exo may play a key role in inflammation regulation and renal injury repair by acting as a decoy to suppress CCL2 activity. Our study may cast new light on understanding the functions of the MSC-exo and these receptor proteins expressed on exosomes.

Identification of potential candidate genes for hypertensive nephropathy based on gene expression profile

Background

This study was aimed to explore the molecular mechanisms of hypertensive nephropathy (HTN).

Methods

Gene expression profile of GSE37460, which based on 27 healthy living donor samples (HTN group) and 15 hypertensive nephropathy samples (control group), were downloaded from Gene Expression Omnibus (GEO) database. The differentially expressed genes (DEGs) between two groups were identified. STRING database was used to reveal protein-protein interaction (PPI) network of DEGs, followed by the functional enrichment analysis of the PPI network. Additionally, miRNA-DEG regulatory network was constructed to reveal the validated miRNAs targeting the DEGs.

Results

In total, 51 up-regulated genes and 140 down-regulated genes were obtained. In the PPI network, cytochrome P450 3A4 (CYP3A4) and angiotensin II receptor type 1 (AGTR1) had a higher degree, and CYP3A4 interacted with CYP4A11. The DEGs in the network were significantly enriched in drug metabolism, focal adhesion and arachidonic acid metabolism. Furthermore, in the miRNA-DEG regulatory network, hsa-miR-335-5p and hsa-miR-26b-5p were the two most outstanding miRNAs. AGTR1, CYP3A4 and CYP4A11 were predicted to be regulated by hsa-miR-26b-5p.

Conclusion

The DEGs, such as AGTR1, CYP3A4 and CYP4A11 may play critical roles in the development of HTN likely via the regulation by hsa-miR-26b-5p and taking part in some pathways.

Noninvasive Imaging of CCR2+ Cells in Ischemia-Reperfusion Injury After Lung Transplantation

Ischemia-reperfusion injury-mediated primary graft dysfunction substantially hampers short- and long-term outcomes after lung transplantation. This condition continues to be diagnosed based on oxygen exchange parameters as well as radiological appearance, and therapeutic strategies are mostly supportive in nature. Identifying patients who may benefit from targeted therapy would therefore be highly desirable. Here, we show that C-C chemokine receptor type 2 (CCR2) expression in murine lung transplant recipients promotes monocyte infiltration into pulmonary grafts and mediates graft dysfunction. We have developed new positron emission tomography imaging agents using a CCR2 binding peptide, ECLi1, that can be used to monitor inflammatory responses after organ transplantation. Both ⁶⁴ Cu-radiolabeled ECL1i peptide radiotracer (⁶⁴ Cu-DOTA-ECL1i) and ECL1i-conjugated gold nanoclusters doped with ⁶⁴ Cu (⁶⁴ CuAuNCs-ECL1i) showed specific detection of CCR2, which is upregulated during ischemia-reperfusion injury after lung transplantation. Due to its fast pharmacokinetics, ⁶⁴ Cu-DOTA-ECL1i functioned efficiently for rapid and serial imaging of CCR2. The multivalent ⁶⁴ CuAuNCs-ECL1i with extended pharmacokinetics is favored for long-term CCR2 detection and potential targeted theranostics. This imaging may be applicable for diagnostic and therapeutic purposes for many immune-mediated diseases.

ISN Forefronts Symposium 2015: The Diverse Function of Macrophages in Renal Disease

Experimental and human studies indicate that macrophages play a key role within the diseased kidney and represent a target for novel therapies. This brief review outlines the involvement and nature of macrophages in renal disease and highlights the phenotypic plasticity of these cells and their responsiveness to the renal microenvironment.

A systems approach to renal inflammation in SLE

Lupus disease and its complications including lupus nephritis (LN) are very disabling and significantly impact the quality of life and longevity of patients. Broadly immunosuppressive treatments do not always provide the expected clinical benefits and have significant side effects that contribute to patient morbidity. In the era of systems biology, new strategies are being deployed integrating diverse sources of information (molecular and clinical) so as to identify individual disease specificities and select less aggressive treatments. In this review, we summarize integrative approaches linking molecular disease profiles (mainly tissue transcriptomics) and clinical phenotypes. The main goals are to better understand the pathogenesis of lupus nephritis, to identify the risk factors for renal flare and to find the predictors of both short and long-term clinical outcome. Identification of common key drivers and additional patient-specific key drivers can open the door to improved and individualized therapy to prevent and treat LN.

Toll‑like receptor 4 contributes to acute kidney injury after cardiopulmonary resuscitation in mice

Toll-like receptor 4 (TLR4) activation mediates renal injury in regional ischemia and reperfusion (I/R) models generated by clamping renal pedicles. However, it remains unclear whether TLR4 is causal in the kidney injury following global I/R induced by cardiac arrest (CA) and cardiopulmonary resuscitation (CPR). The present study used wild-type (C3H/HeN) and TLR4-mutant (C3H/HeJ) mice to produce the CA/CPR model. CA was induced by injection of cold KCl and left untreated for different time periods. After resuscitation (72 h), the level of blood urea nitrogen (BUN) and serum creatinine (Scr), as well as histological changes in renal tissue were assessed to evaluate the severity of acute kidney injury (AKI). The expression of TLR4, intercellular adhesion molecule-1 (ICAM-1), myeloperoxidase (MPO) and growth-regulated oncogene-β (GRO-β) in kidney tissues was detected. The results demonstrated that the levels of Scr and BUN increased significantly in C3H/HeN and C3H/HeJ mice after CPR. CPR also resulted in increased expression of TLR4, ICAM-1, GRO-β and MPO in a CA-duration dependent manner. However, there was decreased expression of ICAM-1, GRO-β and MPO in C3H/HeJ mice compared with that in C3H/HeN mice. C3H/HeJ mice were resistant to AKI as demonstrated by the minor changes in renal histology and function following CPR. In conclusion, mice suffered from AKI after successful CPR and severe AKI occurred in mice with prolonged CA duration. TLR4 and its downstream signaling events that promote neutrophil infiltration via ICAM-1 and GRO-β may be important in mediating inflammatory responses to renal injury after CPR.

Non-redundant role of the chemokine receptor CX3CR1 in the anti-inflammatory function of gut macrophages

Mucosal immunity at the intestinal level is constantly challenged by the presence of external food and microbial antigens and must be kept under strict control to avoid the rise of aberrant inflammation. Among cells of the innate immunity, macrophages expressing the chemokine receptor CX3CR1 are strategically located near the gut epithelial barrier. These cells contribute to the maintenance of homeostasis by producing the anti-inflammatory cytokine IL-10; however, their role in the control of full blown inflammation and tissue injury is controversial. In this study we investigated mice proficient or deficient for the expression of the CX3CR1 receptor in a model of dextran sulphate sodium (DSS) induced acute colitis. We found that KO mice (CX3CR1^GFP/GFP) had a more severe disease compared to WT mice (CX3CR1^GFP/+), both in terms of histological examination of colonic tissues and leukocyte infiltration, with an expansion of macrophages and CD4-Th17 lymphocytes. The expression of several inflammatory mediators (IL-1β, IL-6, IFNγ, iNOS) was also significantly upregulated in KO mice, despite higher IL-10 production. Overall, our study demonstrates that macrophages expressing a functional CX3CR1 receptor have an important and non-redundant role in controlling the abnormal intestinal inflammation that may lead to tissue damage.

Deficiency of cold-inducible ribonucleic acid-binding protein reduces renal injury after ischemia-reperfusion

BACKGROUND

Renal ischemia-reperfusion injury, commonly caused by major operation and shock, leads to acute kidney injury and is associated with high morbidity and mortality. Cold-inducible ribonucleic acid-binding protein, a cold shock protein, has recently been identified as a damage-associated molecular pattern. We hypothesized that cold-inducible ribonucleic acid-binding protein exacerbates severity of injury in renal ischemia-reperfusion.

METHODS

Renal ischemia was induced in an 8-week-old male C57BL/6 wild-type mice and Cirp(-/-) mice via bilateral clamping of renal pedicles for 30 minutes, followed by reperfusion for 5 or 24 hours and harvest of blood and renal tissue for analysis. Anti-cold-inducible ribonucleic acid-binding protein antibody or non-immunized immunoglobulin G (IgG) was injected intravenously (10 mg/kg body weight) at time of reperfusion.

RESULTS

After renal ischemia-reperfusion, Cirp(-/-) mice demonstrated a reduction of blood urea nitrogen and creatinine of 53% and 60%, respectively, compared with wild-type mice. Serum IL-6 levels were reduced significantly: 70% in Cirp(-/-) mice compared with wild-type mice after renal ischemia-reperfusion. Levels of nitrotyrosine, an oxidatively modified protein marker, and cyclooxygenase-2, an inflammatory mediator, also were significantly decreased in the kidneys of the Cirp(-/-) mice compared with wild-type mice after renal ischemia-reperfusion. Renal caspase-3 activity was decreased in Cirp(-/-) mice compared with wild-type mice after renal ischemia-reperfusion, which corresponded to the reduction of apoptotic cells determined by terminal deoxynucleotidyl transferase dUTP nick-end labeling assay. Injection of neutralizing anti-cold-inducible ribonucleic acid-binding protein antibody into wild-type mice led to an 82% reduction in blood urea nitrogen compared with the vehicle after renal ischemia-reperfusion.

CONCLUSION

Deficiency of cold-inducible ribonucleic acid-binding protein results in less renal injury after renal ischemia-reperfusion by attenuating inflammation and oxidative stress. Furthermore, blockade of cold-inducible ribonucleic acid-binding protein shows a protective effect, indicating cold-inducible ribonucleic acid-binding protein as a target in the treatment of renal ischemia-reperfusion.

A Critical Role for Monocytes/Macrophages During Intestinal Inflammation-associated Lymphangiogenesis

BACKGROUND

Inflammation-associated lymphangiogenesis (IAL) is frequently observed in inflammatory bowel diseases. IAL is believed to limit inflammation by enhancing fluid and immune cell clearance. Although monocytes/macrophages (MΦ) are known to contribute to intestinal pathology in inflammatory bowel disease, their role in intestinal IAL has never been studied mechanistically. We investigated contributions of monocytes/MΦ to the development of intestinal inflammation and IAL.

METHODS

Because inflammatory monocytes express CC chemokine receptor 2 (CCR2), we used CCR2 diphtheria toxin receptor transgenic (CCR2.DTR) mice, in which monocytes can be depleted by diphtheria toxin injection, and CCR2 mice, which have reduced circulating monocytes. Acute or chronic colitis was induced by dextran sodium sulfate or adoptive transfer of CD4CD45RB T cells, respectively. Intestinal inflammation was assessed by flow cytometry, immunofluorescence, disease activity, and histopathology, whereas IAL was assessed by lymphatic vessel morphology and density.

RESULTS

We demonstrated that intestinal MΦ expressed vascular endothelial growth factor-C/D. In acute colitis, monocyte-depleted mice were protected from intestinal injury and showed reduced IAL, which was reversed after transfer of wild-type monocytes into CCR2 mice. In chronic colitis, CCR2 deficiency did not attenuate inflammation but reduced IAL.

CONCLUSIONS

We propose a dual role of MΦ in (1) promoting acute inflammation and (2) contributing to IAL. Our data suggest that intestinal inflammation and IAL could occur independently, because IAL was reduced in the absence of monocytes/MΦ, even when inflammation was present. Future inflammatory bowel disease therapies might exploit promotion of IAL and suppression of MΦ independently, to restore lymphatic clearance and reduce inflammation.

Postischemic microvasculopathy and endothelial progenitor cell-based therapy in ischemic AKI: update and perspectives

Acute kidney injury (AKI) dramatically increases mortality of hospitalized patients. Incidences have been increased in recent years. The most frequent cause is transient renal hypoperfusion or ischemia which induces significant tubular cell dysfunction/damage. In addition, two further events take place: interstitial inflammation and microvasculopathy (MV). The latter evolves within minutes to hours postischemia and may result in permanent deterioration of the peritubular capillary network, ultimately increasing the risk for chronic kidney disease (CKD) in the long term. In recent years, our understanding of the molecular/cellular processes responsible for acute and sustained microvasculopathy has increasingly been expanded. The methodical approaches for visualizing impaired peritubular blood flow and increased vascular permeability have been optimized, even allowing the depiction of tissue abnormalities in a three-dimensional manner. In addition, endothelial dysfunction, a hallmark of MV, has increasingly been recognized as an inductor of both vascular malfunction and interstitial inflammation. In this regard, so-called regulated necrosis of the endothelium could potentially play a role in postischemic inflammation. Endothelial progenitor cells (EPCs), represented by at least two major subpopulations, have been shown to promote vascular repair in experimental AKI, not only in the short but also in the long term. The discussion about the true biology of the cells continues. It has been proposed that early EPCs are most likely myelomonocytic in nature, and thus they may simply be termed proangiogenic cells (PACs). Nevertheless, they reliably protect certain types of tissues/organs from ischemia-induced damage, mostly by modulating the perivascular microenvironment in an indirect manner. The aim of the present review is to summarize the current knowledge on postischemic MV and EPC-mediated renal repair.

New insights into the multidimensional concept of macrophage ontogeny, activation and function

Macrophages have protective roles in immunity to pathogens, tissue development, homeostasis and repair following damage. Maladaptive immunity and inflammation provoke changes in macrophage function that are causative of disease. Despite a historical wealth of knowledge about macrophages, recent advances have revealed unknown aspects of their development and function. Following development, macrophages are activated by diverse signals. Such tissue microenvironmental signals together with epigenetic changes influence macrophage development, activation and functional diversity, with consequences in disease and homeostasis. We discuss here how recent discoveries in these areas have led to a multidimensional concept of macrophage ontogeny, activation and function. In connection with this, we also discuss how technical advances facilitate a new roadmap for the isolation and analysis of macrophages at high resolution.

Macrophages in kidney injury, inflammation, and fibrosis

Macrophages are found in normal kidney and in increased numbers in diseased kidney, where they act as key players in renal injury, inflammation, and fibrosis. Macrophages are highly heterogeneous cells and exhibit distinct phenotypic and functional characteristics in response to various stimuli in the local microenvironment in different types of kidney disease. In kidney tissue necrosis and/or infection, damage- and/or pathogen-associated molecular patterns induce pro-inflammatory macrophages, which contribute to further tissue injury, inflammation, and subsequent fibrosis. Apoptotic cells and anti-inflammatory factors in post-inflammatory tissues induced anti-inflammatory macrophages, which can mediate kidney repair and regeneration. This review summarizes the role of macrophages with different phenotypes in kidney injury, inflammation, and fibrosis in various acute and chronic kidney diseases. Understanding alterations of kidney microenvironment and the factors that control the phenotype and functions of macrophages may offer an avenue for the development of new cellular and cytokine/growth factor-based therapies as alternative treatment options for patients with kidney disease.

The Mononuclear Phagocyte System in Organ Transplantation

The mononuclear phagocyte system (MPS) comprises monocytes, macrophages and dendritic cells (DCs). Over the past few decades, classification of the cells of the MPS has generated considerable controversy. Recent studies into the origin, developmental requirements and function of MPS cells are beginning to solve this problem in an objective manner. Using high-resolution genetic analyses and fate-mapping studies, three main mononuclear phagocyte lineages have been defined, namely, macrophage populations established during embryogenesis, monocyte-derived cells that develop during adult life and DCs. These subsets and their diverse subsets have specialized functions that are largely conserved between species, justifying the introduction of a new, universal scheme of nomenclature and providing the framework for therapeutic manipulation of immune responses in the clinic. In this review, we have commented on the implications of this novel MPS classification in solid organ transplantation.