The pathogenesis of human membranous nephropathy: we are (almost) there.

Primary membranous nephropathy is an autoimmune disease usually associated with antibody to phospholipase A2 receptor (anti-PLA2R). The study by Meyer-Schwesinger et al. describes the first mouse model induced using a PLA2R system to study the pathogenicity of anti-PLA2R. Hyperimmune rabbit anti-PLA2R IgG can induce a primary membranous nephropathy-like glomerulopathy with proteinuria in mice. However, to conclusively establish the pathogenicity of human anti-PLA2R will require additional studies using PLA2R and anti-PLA2R of human origin.

What is myeloperoxidase doing in ANCA-associated glomerulonephritis?

O'Sullivan et al. describe glomerular localization of myeloperoxidase (MPO) in anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) and correlate the amount of deposition with severity of injury. MPO is the antigen against which anti-MPO ANCAs are directed, the antigen to which pathogenic T cells that can induce antibody-independent AAV are directed, and MPO can induce glomerular injury directly by interacting with H2O2 and a halide to halogenate glomerular structures. All three of these roles are likely involved in human disease.

Update on endocarditis-associated glomerulonephritis.

Glomerulonephritis (GN) due to infective endocarditis (IE) is well documented, but most available data are based on old autopsy series. To update information, we now present the largest biopsy-based clinicopathologic series on IE-associated GN. The study group included 49 patients (male-to-female ratio of 3.5:1) with a mean age of 48 years. The most common presenting feature was acute kidney injury. Over half of the patients had no known prior cardiac abnormality. However, the most common comorbidities were cardiac valve disease (30%), intravenous drug use (29%), hepatitis C (20%), and diabetes (18%). The cardiac valve infected was tricuspid in 43%, mitral in 33%, and aortic in 29% of patients. The two most common infective bacteria were Staphylococcus (53%) and Streptococcus (23%). Hypocomplementemia was found in 56% of patients tested and ANCA antibody in 28%. The most common biopsy finding was necrotizing and crescentic GN (53%), followed by endocapillary proliferative GN (37%). C3 deposition was prominent in all cases, whereas IgG deposition was seen in <30% of cases. Most patients had immune deposits detectable by electron microscopy. Thus, IE-associated GN most commonly presents with AKI and complicates staphylococcal tricuspid valve infection. Contrary to infection-associated glomerulonephritis in general, the most common pattern of glomerular injury in IE-associated glomerulonephritis was necrotizing and crescentic glomerulonephritis.

The etiology of glomerulonephritis: roles of infection and autoimmunity.

Despite major advances in understanding genetic predispositions ('first hits'), pathogenic immune responses, and the mediators of tissue injury in glomerulonephritis (GN), there remains a dearth of knowledge about the etiologic events, or 'second hits', which trigger these diseases. This paper reviews evidence that infections initiate most forms of GN through numerous simultaneous and/or sequential pathways that begin with activation of the innate immune response and lead to autoimmunity. These pathways include immune dysregulation, adjuvant or bystander effects, epitope spreading, molecular mimicry, epitope conformational changes, and antigen complementarity that, in genetically susceptible individuals, result in the nephritogenic autoimmune responses that underlie GN. Infections may also have direct effects on glomerular cells. Rapid expansion in knowledge of the microbiome and its role in health and disease, as well as systems biology approaches to glomerular disease offer the potential to develop preventive approaches to GNs that can now be treated only with immunosuppression.

Basic and translational concepts of immune-mediated glomerular diseases.

Genetically modified immune responses to infections and self-antigens initiate most forms of GN by generating pathogen- and danger-associated molecular patterns that stimulate Toll-like receptors and complement. These innate immune responses activate circulating monocytes and resident glomerular cells to release inflammatory mediators and initiate adaptive, antigen-specific immune responses that collectively damage glomerular structures. CD4 T cells are needed for B cell-driven antibody production that leads to immune complex formation in glomeruli, complement activation, and injury induced by both circulating inflammatory and resident glomerular effector cells. Th17 cells can also induce glomerular injury directly. In this review, information derived from studies in vitro, well characterized experimental models, and humans summarize and update likely pathogenic mechanisms involved in human diseases presenting as nephritis (postinfectious GN, IgA nephropathy, antiglomerular basement membrane and antineutrophil cytoplasmic antibody-mediated crescentic GN, lupus nephritis, type I membranoproliferative GN), and nephrotic syndrome (minimal change/FSGS, membranous nephropathy, and C3 glomerulopathies). Advances in understanding the immunopathogenesis of each of these entities offer many opportunities for future therapeutic interventions.

The contribution of chronic kidney disease to the global burden of major noncommunicable diseases.

Noncommunicable diseases (NCDs) are the most common causes of premature death and morbidity and have a major impact on health-care costs, productivity, and growth. Cardiovascular disease, cancer, diabetes, and chronic respiratory disease have been prioritized in the Global NCD Action Plan endorsed by the World Health Assembly, because they share behavioral risk factors amenable to public-health action and represent a major portion of the global NCD burden. Chronic kidney disease (CKD) is a key determinant of the poor health outcomes of major NCDs. CKD is associated with an eight- to tenfold increase in cardiovascular mortality and is a risk multiplier in patients with diabetes and hypertension. Milder CKD (often due to diabetes and hypertension) affects 5-7% of the world population and is more common in developing countries and disadvantaged and minority populations. Early detection and treatment of CKD using readily available, inexpensive therapies can slow or prevent progression to end-stage renal disease (ESRD). Interventions targeting CKD, particularly to reduce urine protein excretion, are efficacious, cost-effective methods of improving cardiovascular and renal outcomes, especially when applied to high-risk groups. Integration of these approaches within NCD programs could minimize the need for renal replacement therapy. Early detection and treatment of CKD can be implemented at minimal cost and will reduce the burden of ESRD, improve outcomes of diabetes and cardiovascular disease (including hypertension), and substantially reduce morbidity and mortality from NCDs. Prevention of CKD should be considered in planning and implementation of national NCD policy in the developed and developing world.

DNA damage is a novel response to sublytic complement C5b-9-induced injury in podocytes.

In response to Ab-complement-mediated injury, podocytes can undergo lysis, apoptosis, or, when exposed to sublytic (<5% lysis) amounts of C5b-9, become activated. Following the insertion of sublytic quantities of C5b-9, there is an increase in signaling pathways and growth factor synthesis and release of proteases, oxidants, and other molecules. Despite an increase in DNA synthesis, however, sublytic C5b-9 is associated with a delay in G(2)/M phase progression in podocytes. Here we induced sublytic C5b-9 injury in vitro by exposing cultured rat podocytes or differentiated postmitotic mouse podocytes to Ab and a complement source; we also studied the passive Heymann nephritis model of experimental membranous nephropathy in rats. A major finding was that sublytic C5b-9-induced injury caused an increase in DNA damage in podocytes both in vitro and in vivo. This was associated with an increase in protein levels for p53, the CDK inhibitor p21, growth-arrest DNA damage-45 (GADD45), and the checkpoint kinases-1 and -2. Sublytic C5b-9 increased extracellular signal-regulated kinase-1 and -2 (ERK-1 and -2), and inhibiting ERK-1 and -2 reduced the increase in p21 and GADD45 and augmented the DNA damage response to sublytic C5b-9-induced injury. These results show that sublytic C5b-9 induces DNA damage in vitro and in vivo and may explain why podocyte proliferation is limited following immune-mediated injury.

Primary Membranous Nephropathy.

Membranous nephropathy (MN) is a unique glomerular lesion that is the most common cause of idiopathic nephrotic syndrome in nondiabetic white adults. About 80% of cases are renal limited (primary MN, PMN) and 20% are associated with other systemic diseases or exposures (secondary MN). This review focuses only on PMN. Most cases of PMN have circulating IgG4 autoantibody to the podocyte membrane antigen PLA2R (70%), biopsy evidence PLA2R staining indicating recent immunologic disease activity despite negative serum antibody levels (15%), or serum anti-THSD7A (3%-5%). The remaining 10% without demonstrable anti-PLA2R/THSd7A antibody or antigen likely have PMN probably secondary to a different, still unidentified, anti-podocyte antibody. Considerable clinical and experimental data now suggests these antibodies are pathogenic. Clinically, 80% of patients with PMN present with nephrotic syndrome and 20% with non-nephrotic proteinuria. Untreated, about one third undergo spontaneous remission, especially those with absent or low anti-PLA2R levels, one-third progress to ESRD over 10 years, and the remainder develop nonprogressive CKD. Proteinuria can persist for months after circulating anti-PLA2R/THSD7A antibody is no longer detectable (immunologic remission). All patients with PMN should be treated with supportive care from the time of diagnosis to minimize protein excretion. Patients with elevated anti-PLA2R/THSD7A levels and proteinuria >3.5 g/d at diagnosis, and those who fail to reduce proteinuria to <3.5 g after 6 months of supportive care or have complications of nephrotic syndrome, should be considered for immunosuppressive therapy. Accepted regimens include steroids/cyclophosphamide, calcineurin inhibitors, and B cell depletion. With proper management, only 10% or less will develop ESRD over the subsequent 10 years.

Cellular and molecular biology of membranous nephropathy.

There have been dramatic increases in the understanding of the molecular mechanisms involved in membranous nephropathy (MN) over the past 2 decades. Most of these have come directly from studies carried out in the Heymann nephritis models of MN in rats, which closely simulate the clinical and pathologic features of the human disease. Once considered a prototypical example of circulating immune complex trapping in glomeruli, we now recognize that MN develops, in both the rat and man, consequent to an autoimmune process involving antibodies directed to antigens on the foot processes of podocytes that form subepithelial immune deposits. Proteinuria is a consequence of sublytic complement C5b-9 attack on podocytes. The podocyte response to sublytic C5b-9 includes up-regulated expression of genes for production of oxidants, proteases, prostanoids, growth factors, CTGF, transforming growth factor (TGF) and TGF receptors leading to overproduction of extracellular matrix components that result in 'spike' formations. Other podocyte changes including detachment, apoptosis and alterations in cell cycle regulatory proteins favoring hypertrophy over proliferation also contribute to proteinuria and to development of glomerular sclerosis. Finally, progression of chronic proteinuric MN to chronic kidney disease (CKD) and renal failure likely results in part from additional effects of sublytic C5b-9 on proximal tubular epithelial cells, resulting in interstitial inflammation and fibrosis with a fall in glomerular filtration rate (GFR). Expanded understanding of the molecular pathophysiology of MN has important implications for monitoring disease activity, predicting disease course and designing new approaches to therapy for this common glomerular disease.