Most pathology laboratories prefer light samples to be collected and stored in 10 % aqueous formaldehyde solution (formalin). Formalin is safe and readily disposed of and preserves morphologic features for review. It also allows for additional tissue processing using immunohistochemical techniques or molecular studies to be used if necessary. Alternative solutions include Bouin’s, Duboscq-Brazil, Zenker’s, or 4 % paraformaldehyde fixatives, each with varying uses [9].

Slide preparation also varies according to pathology laboratory, but typically includes sectioning of the preserved tissue into 2 μm slices and serial staining with hematoxylin and eosin (H&E) stain , periodic acid-Schiff reaction (PAS), silver stain, or trichrome stain [9].

Tissue fixation for EM requires fixation in either 2–3 % glutaraldehyde or 1–4 % paraformaldehyde. This provides the greatest degree of structural preservation. It is occasionally necessary to reprocess samples stored in either the frozen section or the paraffin-embedded section if glomeruli are not available on the EM processed sections. This can lead to cellular artifact, but generally is considered acceptable for review of the GBM for both structural integrity and for immune deposition [9].

Some pathology laboratories have gone to immunoperoxidase (IP) staining for review of immune deposition due to relative ease of processing. However, most renal pathologists agree that dark field imaging with IF is the preferred modality to identify immune deposition in pathology samples. The processing of these materials is laboratory dependent, though formalin fixation is typically adequate with freezing and subsequent sectioning used to label the immune complexes [9].

Clinically, this collection of diseases share certain features, namely, the presence of a nephritic sediment with red blood cells either in red cell casts or with dysmorphic characteristics, some amount of proteinuria, and rapid loss of renal function. Renal failure with oliguria, severe azotemia, and hypertension are the hallmark of the initial presentation and typically lead to progression to ESRD within 3 months of diagnosis if left untreated [16]. Patients may also have evidence of hypertension—owing to disrupted sodium regulation and possibly renin-mediated mechanisms—and edema. There are of course more distant or systemic effects that can be seen as well, though these are generally a characteristic of the underlying disease. An example would be the pulmonary hemorrhage seen in anti-GBM disease (Goodpasture’s disease) [17].

The number of disease conditions that are known to cause RPGN is limited. Each has a specific pathogenic mechanism by which it leads to glomerular capillary injury and disruption, outlined separately, but they can be organized according to their pattern of injury. The first and most straightforward is anti-GBM disease, the second group being a far more heterogeneous array of disease wherein immune complexes in the glomerular capillary tuft lead to local injury, and the last being the pauci-immune vasculitides wherein no immune depositions can be detected, but pathogenic antineutrophil cytoplasmic antibodies (ANCA) lead to notable inflammatory activation which can be observed histologically. The latter is the most common form of crescentic glomerulonephritis , accounting for approximately 60–80 % of all cases [16].

While the underlying pathogenesis of the specific disease entities is reviewed below, their progression to an RPGN is typical and includes necrotizing inflammation which leads to GBM disruption and leakage of serum proteins including fibrin and fibronectin into the urinary space which generates a robust activation of both the visceral and parietal epithelial layers in Bowman’s space. In turn, these cells dedifferentiate and proliferate and lead to the formation of the prototypical crescent (Fig. 12.1) [16].

The crescents themselves are a histologic signal of injury; the association of this pattern of injury with the clinical manifestations of the RPGNs has led to these terms being considered near synonymous [18].

Despite their differences, the initial treatment of these diseases is similar, largely because of the incredibly high risk of irreversible renal injury and risk of other morbidities and mortality [19]. Typically the treatment of any of these diseases is divided into induction and maintenance phases. Induction therapy is often initiated in advance of definitive diagnosis. Sometimes this is with serologic evidence, but often, even that is not yet available. The cornerstones of treatment have been steroid therapy and plasma exchange for nearly 40 years [20]. Recent meta-analysis on the topic has shown that the use of adjunct plasma exchange can reduce the rate of progression to ESRD in these patients, noting that ESRD rates are reduced at both 3 and 12 months [21]. Pulse glucocorticoid therapy is also typically started at the time of clinical presentation. While subject to varying patterns of administration, typically 500–1000 mg of methylprednisolone is given over 2–3 days as initial therapy to help calm the inflammatory cascades active in the tuft. Lastly the use of immunosuppression therapy is used to reign in the production of the injurious autoantibodies. The classical treatment for RPGN has been oral or intravenous cyclophosphamide (CYC) administered according to a variety of protocols either as continuous oral therapy or as pulse intravenous therapy [21]. With the advent of newer immunosuppressive therapies, the treatments used in practice have increased, though the details of their selection are outside the scope of this publication.

Typically this disease presents as a pulmonary renal syndrome with acute onset of both lower respiratory complaints and changes in urinary character [17, 19]; however, more subtle cases can be seen. Because of its tendency to rapidly progress, it should be considered in all cases of acute pulmonary renal disease, and coordination with a nephrologist is recommended for urgent initiation of plasmapheresis if needed in the setting of diffuse alveolar hemorrhage [21, 22].

The GBM is formed through the fusion of the endothelial and podocyte basement membranes. The normal GBM is a tight meshwork of collagen fibers, with alpha 3, alpha 4, and alpha 5 subtypes of type IV collagen fibers forming hexamers that then crosslink through their non-collagenous (NC1) domains. In patients with anti-GBM disease, immunoglobulin G (IgG) can be identified on immunofluorescence overlying all of the glomerular basement membrane, and the causative antibodies have been found to be directed against those NC1 domains of alpha 3 or alpha 5 type IV collagen [23].

Recent work has demonstrated that the disease is one of quaternary conformational change—the specific cause of which is unknown, though it has been linked to smoking, viral infections, and solvent exposure—which leads to molecular conformational shifting and exposure of the normally sheltered NC1 domains with subsequent autoantibody formation. Those autoantibodies then bind to the NC1 domains and lead complement fixation and to a type II hypersensitivity reaction with robust inflammatory recruitment and consequent local injury [22, 23].

This disease manifests as the nephritic syndrome due to the ongoing glomerular injury, and patients have pulmonary symptoms as well given that the alpha-3 subunit is also expressed in the alveoli of the lungs. This can lead to a very dramatic clinical presentation of the disease with diffuse pulmonary hemorrhage as the initial presenting symptom. Prior to the use of the above-described therapies, anti-GBM disease was nearly universally fatal, though with the use of modern treatments, the 5-year survival now exceeds 80 % [22].

The challenges present in diagnosing this disease combined with its rarity make this a challenging disease on which to do randomized clinical trials. However, given that it is understood to be an immune-mediated disease, immunosuppressive therapies are considered necessary as part of its treatment. If diagnosed during its fulminant phase, plasmapheresis is often indicated. At the same time, induction is achieved, as described above for all of the RPGNs, classically done with CYC and high-dose glucocorticoid therapy [16, 19, 22]. The introduction of anti-CD20 monoclonal antibody (rituximab) therapies has been used as well in small patient cohorts, the largest a series of eight patients who had received steroids and CYC as well as plasmapheresis. All were started on rituximab within 2 months of diagnosis. At approximately 2 years follow-up, patient and renal survival were 100 and 75 %, respectively [24].

The prognosis for patients affected by anti-GBM disease is largely dependent on the promptness of the diagnosis and initiation of effective treatment. While historically the disease was considered fatal [25], the above therapies have led to a 5-year survival rate in excess of 80 %, and only about a third of patients require long-term dialysis [22].

Accounting for just over half of all diagnosed RPGN syndromes and sometimes recognized in more indolent circumstances, this group of diseases is characterized by necrotizing changes on biopsy in the absence of immune deposits [19]. This group of diseases includes granulomatosis with polyangiitis (GPA), eosinophilic granulomatosis with polyangiitis (EGPA , formerly Churg-Strauss syndrome), and microscopic polyangiitis (MPA). The bulk of these patients have circulating ANCA. Clinically they present in a variety of different ways, each of which is typically associated with a collection of extrarenal manifestations that can suggest which disease is causative, though it does require tissue biopsy to establish the definitive diagnosis. The varying extrarenal characteristics, as well as the frequency of specific ANCA subtype s with those diseases, are summarized in Table 12.2. These diseases do occasionally present with less fulminant disease. So-called early systemic disease is defined as serologic positivity, but with sCr below 1.7 mg/dL and no critical extrarenal organ dysfunction [26]. This distinction has implications for treatment.

The association of ANCA and necrotizing glomerular disease was first recognized by Davies in 1982 [27]; subsequent work has gone on to support a pathogenic mechanism for these autoantibodies. There are two identified culpable antibody subtypes, the anti-myeloperoxidase (MPO) subtype and the anti-proteinase 3 subtype (PR-3). These antigens are typically sequestered into neutrophilic granules and are exposed only during periods of inflammatory activation [17, 19]. Directly linking this antibody activity to the disease, however, has been proven difficult. Renal biopsies of affected patients are rife with activated neutrophils, and indeed, the number of activated neutrophils seen on biopsy has been correlated with the degree of renal insufficiency [28], but they do not have observable immune complexes attributable to ANCA, hence the name “pauci-immune.” Additional evidence for a pathogenic role of ANCA has grown with ANCA being elutable from affected kidney tissue, and in the case of anti-MPO antibodies, necrotizing glomerulonephritis can be induced with infusion of ANCA [29]. Interestingly, anti-LAMP 2 (antibodies directed against lysosomal membrane protein-2) have been shown to be present in upward of 80 % of all active ANCA-associated vasculitides (AAV) [30, 31]. While the understanding of the disease is improving, there are many aspects of the pathogenesis that remain to be understood.

Again, the induction therapy for this disease has included treatment as described for an RPGN with cyclophosphamide as the core of the induction immunosuppressive regimen since the 1970s. Typically, maintenance therapy is necessary due to high rates of relapse and has historically been accomplished with azathioprine. Recent studies have also set out to explore the utility of B cell-depleting agents like rituximab both as an alternative induction agent and a maintenance therapy.

In situations where patients may present with less threatening disease, so-called early systemic AAV, several studies have shown non-inferiority of methotrexate (MTX) as an induction agent. In one randomized controlled trial patients with confirmed ANCA positivity, sCr <1.7 mg/dL and no evidence of critical organ dysfunction were randomized to receive either oral cyclophosphamide or oral MTX for 12 months. There were similar rates of remission at 18-month follow-up, though the time to remission was longer in the MTX patients, particularly those with more extensive disease [26].

Cyclophosphamide is the historical gold standard for induction therapy. Oral or intravenous (IV) administration of CYC has also been extensively studied [32–34]. Indeed similar response rates were seen in patients receiving either oral or IV CYC, though the cumulative dose of those patients receiving IV therapy was less. Given that the complication profile of CYC is largely directed by its cumulative dose, there may be some long-term benefit to limiting the absolute amount of CYC administered. However, long-term follow-up of patients previously enrolled in the largest of these trials has failed to show a difference in treatment-related adverse events. It did, however, show a higher relapse rate among patients receiving the IV therapy, though this ultimately was not reflected in differences in the overall patient morbidity or mortality at just over 4 years [35].

Rituximab has also been used for both induction and maintenance in these patients as well with high efficacy. Two large, well-conducted trials have evaluated the use of rituximab for induction therapy with similar efficacy [36, 37]. Follow-up trials have gone on to show non-inferiority for maintenance therapy with rituximab as well [38]. The current treatment recommendations for AAV are summarized in Table 12.3.