On infiltrating inflamed tissue, macrophages respond to the local microenvironment and develop one of two broad phenotypes: classically activated (M1) macrophages that cause tissue injury and alternatively activated macrophages that promote repair. Understanding how this polarization occurs in vivo is far from complete, and in this study, using a Th1-mediated macrophage-dependent model of acute glomerulonephritis, nephrotoxic nephritis, we examine the role of suppressor of cytokine signaling (SOCS)1 and SOCS3. Macrophages in normal kidneys did not express detectable SOCS proteins but those infiltrating inflamed glomeruli were rapidly polarized to express either SOCS1 (27 ± 6%) or SOCS3 (54 ± 12%) but rarely both (10 ± 3%). Rat bone marrow-derived macrophages incubated with IFN-γ or LPS expressed SOCS1 and SOCS3, whereas IL-4 stimulated macrophages expressed SOCS1 exclusively. By contrast, incubation with IFN-γ and LPS together suppressed SOCS1 while uniquely polarizing macrophages to SOCS3 expressing cells. Macrophages in which SOCS3 was knocked down by short interfering RNA responded to IFN-γ and LPS very differently: they had enhanced STAT3 activity; induction of macrophage mannose receptor, arginase and SOCS1; restoration of IL-4 responsiveness that is inhibited in M1 macrophages; and decreased synthesis of inflammatory mediators (NO and IL-6) and costimulatory molecule CD86, demonstrating that SOCS3 is essential for M1 activation. Without it, macrophages develop characteristic alternatively activated markers when exposed to classical activating stimuli. Lastly, increased glomerular IL-4 in nephrotoxic nephritis inhibits infiltrating macrophages from expressing SOCS3 and was associated with attenuated glomerular injury. Consequently, we propose that SOCS3 is essential for development of M1 macrophages in vitro and in vivo.

The role of resident renal mononuclear phagocytes in acute kidney injury is controversial with experimental data suggesting both deleterious and protective functions. To help resolve this, we used mice transgenic for the human diphtheria toxin receptor under the control of the CD11b promoter and treated them with diphtheria toxin, or liposomal clodronate, or both to deplete monocyte/mononuclear phagocytes prior to renal ischemia/reperfusion injury. Although either system effectively depleted circulating monocytes and resident mononuclear phagocytes, depletion was most marked in diphtheria toxin-treated mice. Despite this, diphtheria toxin treatment did not protect from renal ischemia. In contrast, mice treated with clodronate exhibited reduced renal failure and acute tubular necrosis, suggesting key differences between these depletion strategies. Clodronate did not deplete CD206-positive renal macrophages and, unlike diphtheria toxin, left resident CD11c-positive cells unscathed while inducing dramatic apoptosis in hepatic and splenic mononuclear phagocyte populations. Abolition of the protected phenotype by administration of diphtheria toxin to clodronate-treated mice suggested that the protective effect of clodronate resulted from the presence of a cytoprotective intrarenal population of mononuclear phagocytes sensitive to diphtheria toxin-mediated ablation.

Macrophage infiltration is a common feature of renal disease and their presence has been synonymous with tissue damage and progressive renal failure. More recently work has focused on the heterogeneity of macrophage activation and in particular their ability to curtail inflammation and restore normal function. This has led to the view that it is macrophage function rather than their number that is important in determining the outcome of inflammatory disease. This review will focus on the pathways that regulate macrophage infiltration and activation and how these could be manipulated to control renal inflammatory disease. In particular, the ability of specific cell surface receptors and intracellular signaling pathways to control macrophage activation and how macrophages can be genetically manipulated to develop properties that favor resolution over ongoing injury.

Ischemia/reperfusion injury is a leading cause of acute renal failure triggering an inflammatory response associated with infiltrating macrophages, which determine disease outcome. To repair the inflammation we designed a procedure whereby macrophages that overexpress the anti-inflammatory agent interleukin (IL)-10 were adoptively transferred. These bone marrow-derived macrophages were able to increase their intracellular iron pool that, in turn, augmented the expression of lipocalin-2 and its receptors. Infusion of these macrophages into rats after 1 h of reperfusion resulted in localization of the cells to injured kidney tissue, caused increases in regenerative markers, and a notable reduction in both blood urea nitrogen and creatinine. Furthermore, IL-10 therapy decreased the local inflammatory profile and upregulated the expression of pro-regenerative lipocalin-2 and its receptors. IL-10-mediated protection and subsequent renal repair were dependent on the presence of iron and lipocalin-2, since the administration of a neutralizing antibody for lipocalin-2 or administration of IL-10 macrophages pretreated with the iron chelating agent deferoxamine abrogated IL-10-mediated protective effects. Thus, adoptive transfer of IL-10 macrophages to ischemic kidneys blunts acute kidney injury. These effects are mediated through the action of intracellular iron to induce lipocalin-2.

Characterizing chronic kidney disease (CKD) at all stages is an essential part of rational management and the renal biopsy plays a key role in defining the processes involved. There remain no global guidelines available to the renal community on indications for this important diagnostic, prognostic, and relatively safe test. Although most nephrologists recognize several clear indications for a renal biopsy, it is still underutilized. It not only helps the clinician to manage the patient with CKD, but it can also help clarify the epidemiology of CKD, and aid research into the pathobiology of disease with the aim of discovering new therapies. It may be useful for instance in elderly patients with CKD, those with diabetes and presumed 'hypertensive nephropathy', and in some patients with advanced CKD as part of the pretransplant work-up. In some populations (for example, immunoglobulin A nephropathy and ANCA vasculitis), renal biopsy allows disease classification that may predict CKD progression and response to therapy. For the individual, interval renal biopsy may be of use in providing ongoing therapeutic and prognostic information. Molecular advances will change the landscape of renal pathology and add a new dimension to the diagnostic precision of kidney biopsy. Organizing the multiplicity of information available in a renal biopsy to maximize benefits to the patient, as well as to the epidemiologist and researcher, is one of the challenges that face the nephrology community.

Acute kidney injury has a high mortality and lacks specific therapies, with ischemia/reperfusion injury (IRI) being the predominant cause. Macrophages (M phi) have been used successfully in cell therapy to deliver targeted therapeutic genes in models of inflammatory kidney disease. Heme oxygenase-1 (HO-1) catalyzes heme breakdown and has important cytoprotective functions. We hypothesized that administration of M phi modified to overexpress HO-1 would protect from renal IRI. Using an adenoviral construct (Ad-HO-1), HO-1 was overexpressed in primary bone marrow-derived M phi (BMDM). In vitro Ad-HO-1 M phi showed an anti-inflammatory phenotype with increased phagocytosis of apoptotic cells (ACs) and increased interleukin (IL)-10 but reduced TNF-alpha and nitric oxide (NO) following lipopolysaccharide/interferon-gamma (IFN gamma) stimulation compared to control transduced or unmodified M phi. In vivo, intravenously (IV) injected M phi homed preferentially to the post-IRI kidney compared to uninjured control following experimental IRI. At 24 hours postinjury, despite equivalent levels of tubular necrosis, apoptosis, and capillary density between groups, the injection of Ad-HO-1 M phi resulted in preserved renal function (serum creatinine reduced by 46%), and reduced microvascular platelet deposition. These data demonstrate that genetically modified M phi improve the outcomes in IRI when administered after the establishment of structural injury, raising the prospect of targeted cell therapy to support the function of the acutely injured kidney.

Aging is thought to be associated with a higher susceptibility to renal ischemia-reperfusion injury (IRI). To study whether defective induction of hemeoxygenase-1 (HO-1, a protective and anti-inflammatory enzyme) might contribute to this, we found that while 12-month-old mice had similar baseline renal function and HO-1 expression, the induction of HO-1 usually seen in ischemia-reperfusion was reduced. This was also associated with worsened renal function and acute tubular necrosis in the aged compared with young mice. In the older mice, heme arginate (HA) induced HO-1 in the cortex and medulla, significantly improved renal function, and reduced tissue injury. Cellular HO-1 induction in the medulla in response to injury or HA treatment was found to be interstitial rather than epithelial, as evidenced by its colocalization with macrophage markers. In vitro, HA treatment of primary macrophages resulted in marked HO-1 induction without impairment of classical activation pathways. Macrophage depletion, caused by diphtheria toxin treatment of 12-month-old CD11b-DTR transgenic animals, resulted in the loss of interstitial HO-1-positive cells and reversal of the protective phenotype of HA treatment. Thus, failure of HO-1 induction following renal IRI worsens structural and functional injury in older mice and represents a therapeutic target in the elderly. Hence, HO-1-positive renal macrophages mediate HA-induced protection in IRI.

Renal ischaemia reperfusion injury (IRI) is a common cause of acute kidney injury (AKI) in patients and occlusion of renal blood flow is unavoidable during renal transplantation. Experimental models that accurately and reproducibly recapitulate renal IRI are crucial in dissecting the pathophysiology of AKI and the development of novel therapeutic agents. Presented here is a mouse model of renal IRI that results in reproducible AKI. This is achieved by a midline laparotomy approach for the surgery with one incision allowing both a right nephrectomy that provides control tissue and clamping of the left renal pedicle to induce ischaemia of the left kidney. By careful monitoring of the clamp position and body temperature during the period of ischaemia this model achieves reproducible functional and structural injury. Mice sacrificed 24 hr following surgery demonstrate loss of renal function with elevation of the serum or plasma creatinine level as well as structural kidney damage with acute tubular necrosis evident. Renal function improves and the acute tissue injury resolves during the course of 7 days following renal IRI such that this model may be used to study renal regeneration. This model of renal IRI has been utilized to study the molecular and cellular pathophysiology of AKI as well as analysis of the subsequent renal regeneration.