Nursing APA Assignment

REVIEW

Crescents in primary glomerulonephritis: a pattern of injury with dissimilar actors. A pathophysiologic perspective

Hernán Trimarchi1

Received: 4 May 2021 /Revised: 2 June 2021 /Accepted: 18 June 2021 # IPNA 2021

Abstract Cellular crescents are defined as two or more layers of proliferating cells in Bowman’s space and are a hallmark of inflammatory active glomerulonephritis and a histologic marker of severe glomerular injury. In general, the percentage of glomeruli that exhibit crescents correlates with the severity of kidney failure and other clinical manifestations of nephritic syndrome. In general, a predominance of active crescents is associated with rapidly progressive glomerulonephritis and a poor outcome. The duration and potential reversibility of the underlying disease correspond with the relative predominance of cellular or fibrous components in the crescents, the initial location of the immunologic insult inside the glomerulus, and the sort of involved cells and inflammatory mediators. However, the presence of active crescents may not have the same degree of significance in the different types of glomerulopathies. The pathophysiology of parietal cell proliferation may have dissimilar origins, underscoring the fact that the resultant crescents are a non-specific morphological pattern of glomerular injury with different implications in clinical prognosis in the scope of glomerular diseases.

Keywords Crescents . Proteinuria . Glomerulonephritis . Microhematuria . Glomerulus

Introduction

Cellular crescents are defined as two or more layers of prolif- erating cells in Bowman’s space, affecting 10% or more of the glomerular circumference. In general, the presence of cres- cents in a kidney biopsy represents a matter of concern. Many aspects are to be taken into consideration when they are diagnosed: the number of crescents with respect to the number of obtained glomeruli in a kidney biopsy, the subtypes of crescents according to the degree of proliferation and fibro- sis, the relationship between crescents and the underlying glo- merulopathy, the pathophysiological implications of crescents with other concomitant primary glomerular lesions such as endothelial and/or mesangial proliferation, fibrinoid necrosis, vascular infiltration by inflammatory cells, glomerular base- ment membrane (GBM) insults, and podocyte damage.

Cellular crescents can be reversible, but when the growth of parietal epithelial cells (PECs) associate with an epithelial–

mesenchymal transition-like change in cell phenotype, fibrous crescents form, and crescents become irreversible. Different molecular pathways trigger the activation of PECs. Crescent formation requires vascular injury causing ruptures in the GBM that trigger plasmatic coagulation within Bowman’s space (Fig. 1). This vascular necrosis can be triggered by different mechanisms, as small vessel vasculitis, immune complex glomerulonephritis, anti-GBM disease, and C3 glo- merulonephritis that all share complement activation but com- prise different upstream immunologic pathways outside the kidney [1].

Moreover, the burden crescents may be playing in the clin- ical scene are of utmost importance. As proliferative lesions, the presence of hematuria is a hallmark [2]. The decrease in the glomerular filtration area is accompanied by a concomitant fall in the glomerular filtration rate (GFR) that, if not solved, will render the compromised glomeruli to obliteration and a permanent decrease in kidney function. Varying degrees of proteinuria are always present due to the damage caused to the glomerular filtration barrier. The functional consequence of crescent formation is a decline in single-nephron GFR for two reasons. First, the increasing intra-glomerular mass in- creases the counter-pressure that together with oncotic pres- sure counterbalances arterial filtration pressure and may lead

* Hernán Trimarchi [email protected]

1 Nephrology Service, Hospital Británico de Buenos Aires, Perdriel 74 (1280), Buenos Aires, Argentina

https://doi.org/10.1007/s00467-021-05199-1

/ Published online: 27 July 2021

Pediatric Nephrology (2022) 37:1205–1214

to the collapse of the glomerular tuft [3]. Second, once a glo- merular crescent involves and obstructs the urinary pole, that single nephron will not contribute to the total GFR, rendering atubular glomeruli [1, 4].

In this review, the pathophysiology of crescents is discussed, as well as the impact crescents may play in the main primary glomerular diseases.

The origin of a crescent

The formation of a crescent appears to represent a non-specific response to a severe injury against the glomerular capillary wall (Fig. 1) [5]. The initiating event is the development of

physical gaps (also called rents or holes) in the glomerular capillary wall, then in the GBM, and finally in Bowman’s capsule, first in the visceral layer where podocytes lie and then the parietal layer [5]. This histological direction of events pre- sents relevant considerations to be made. Due to a primary insult to the endothelial compartment of the filtration barrier and the subsequent proliferation of cells, dysmorphic microhematuria is a frequent finding in the urinary sediment. As podocytes are both focally and secondarily damaged, the damage at the filtration barrier is lesser, and consequently proteinuria is not as severe as that found in podocytopathies or in subepithelial deposition of immune complexes. The pres- ence of gaps in the glomerular filtration barrier allows the leak of coagulation factors and leukocytes to begin the repair

Activated fibroblast

Quiescent fibroblast

Macrophages

CD11b+ dendritic cell

Lymphocyte

Healthy

podocyte

Quiescent PEC

Proliferative PECs

Endothelial cell

Stressed

podocyte

Fig. 1 Leading cells and molecules involved in the development of a crescent. Inside the glomerulus, the development of a crescent is due to many factors: An initial vascular injury causing ruptures (rents ) in the GBM causes the leakage of plasmatic pro-coagulant molecules (tissue factor, fibrinogen, and fibrin ) secreted by podocytes, the endothelium, and macrophages, within Bowman’s space. Involved stressed podocytes start to efface their processes, can migrate to the forming crescent, or

finally detach, and endothelial cells tend to duplicate. PECs start to pro- liferate, stimulated by several actors: influx of macrophages and lympho- cytes from interstitium and circulation and fibroblasts from neighboring parenchyma, infiltrating CD11b+ dendritic and T cells. Involved glomer- ular filtration barrier is in bold; activated cells (macrophages, lympho- cytes, PECs, podocytes, fibroblasts) present a deeper hue than non- activated ones

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mechanisms. Fibrinogen is converted to fibrin, while macro- phages and lymphocytes promote crescent formation [6]. In addition, crescent formation is initially mediated by T helper 1 CD4+ T cells, while macrophages and fibrin may act as effec- tors of cellular immunity [7]. Nephritogenic Th17 CD4 effec- tor cells, controlled by STAT3-dependent Treg17 cells ex- pressing the chemokine receptor CCR6, also play a role [8, 9].

The leakage of these molecules and cells to Bowman’s space through the capillary wall rents represents the first step that drives the formation of cellular crescents. The detachment of podocytes and podocyturia, and the loss of parietal cells due to the inflammatory process render both the GBM and Bowman’s capsule basement membrane denuded, promoting the adherence of both basement membranes. Neighboring pa- rietal cells respond to this insult with proliferation, favored by the subsequent participation of coagulation factors, particular- ly fibrin; tissue factor, which promotes fibrin deposition; and several different cell types, including macrophages, dendritic cells, renal progenitor cells, and interstitial fibroblasts. The contents in Bowman’s space pass to the interstitium and con- tribute to the periglomerular inflammation and the migration of fibroblasts. Stimulation of toll-like receptor 4 (TLR4) or 9 (TLR9) also takes part in the development of crescents [10] (Fig. 1).

As mentioned above, thrombogenesis is critical in the for- mation of crescents. Coagulation factors lead to the cross- linking of fibrin [11]. The primary stimulus to fibrin deposi- tion in crescent formation appears to be tissue factor, derived from endothelial cells, podocytes, and macrophages [12]. Finally, injured glomerular endothelial cells secrete tissue fac- tor due to macrophage-derived interleukin-1 and tumor necro- sis factor stimulation [13].

It has been reported that in crescentic glomerulonephritis, there exists a decreased fibrinolytic activity due to both a reduction in tissue-type plasminogen activator and an increase in plasminogen activator inhibitor-1 (PAI-1) [14]. This results in extraglomerular fibrin cross-linking in Bowman’s space. As a potent chemoattractant, fibrin contributes to the recruitment of macrophages into the glomeruli [15]. Protease-activated receptor-2 (PAR-2), a cellular receptor in glomerular cells and macrophages, worsens crescentic glomerulonephritis in- creasing PAI-1 expression and inhibit ing matrix metalloproteinase-9 activity [16]. By contrast, mice lacking PAR-2 have reductions in PAI-1 activity, fibrin deposition, and crescent formation [16].

Macrophages and crescents

Circulating macrophages or those close to the periglomerular area are critical in the formation of crescents as tissue factor expression and fibrin deposition derive from macrophages and may comprise from 40 to 70 percent of the cellular com- ponent of early crescents in some sorts of glomerulonephritis

[17] (Fig. 1). The migration of macrophages is a result of the action taken by macrophage chemoattractant protein-1 (MCP- 1), macrophage migration inhibitory factor (MIF), macro- phage inflammatory protein-1-alpha (MIP-1-alpha), osteo- pontin, and chemokine receptor 2B (CCR2B), the receptor for MCP-1, which are all essential actors [18, 19]. Moreover, vascular cell adhesion molecule (VCAM)-1, inter- cellular adhesion molecule (ICAM)-1, and CD44 (a marker of PECs) may also participate in the migration of macrophages [20]. Finally, granulocyte–macrophage colony-stimulating factor (GM-CSF) may increase expression of VCAM-1, MCP-1, and IL-1 beta and play a role in the genesis of cres- cents [21]. Among the macrophage-secreted molecules, IL-1, TNF, and TGF-beta are the most relevant. TGF-beta appears to play an important role in the progression of crescentic glo- merulonephritis and a lower response to immunosuppression [22]. Macrophages contribute to kidney dysfunction and tis- sue damage in established crescentic glomerulonephritis as it progresses from the acute inflammatory to a chronic fibrotic phase [23].

Several studies in patients with crescentic glomerulone- phritis found increased glomerular expression of matrix me- talloproteinases (MMP) -2, -3, -9, and -11 and tissue inhibitor of metalloproteinases (TIMP)-1, which correlated with cellu- lar crescents and disease activity [24]. In experimental studies, MMP-9 protects against experimental crescentic glomerulo- nephritis through its fibrinolytic activity [16].

Dendritic cells, T cells, and crescents

Dendritic cell depletion at early stages appears to flare glo- merulonephritis, while their depletion at a later stage leads to attenuation [25]. The CD11b+ subset of dendritic cells pro- motes crescentic glomerulonephritis, whereas the smaller pop- ulation of CD103+ dendritic cells protects from glomerulone- phritis by promoting Treg accumulation [26] (Fig. 1). T cells are found in Bowman’s space as well as in crescents [27]. T helper cells in the glomeruli are involved in the secretion of chemoattractants such asMCP andMIP, cytokines such as IL- 12 and IL-18, mast cells, and costimulatory ligands on mac- rophages (CD80 and CD86) [28]. The role of T cells in glo- merular injury may be related to antigen recognition and mac- rophage recruitment [7].

Glomerular parietal epithelial cells and crescents

Glomerular PECs are significant constituents of crescents [29] (Fig. 1). PECs have a high capacity to proliferate, presumably in response to growth factors, such as platelet-derived growth factor and fibroblast growth factor-2 [15]. Mice deficient in CD44 are partly protected from crescentic glomerulonephritis [2]. Since PECs are not major sources of procoagulant mole- cules or growth factors, it is unlikely that they are as important

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as macrophages and interstitial fibroblasts in determining the course and consequences of crescent formation. However, glomerular parietal epithelial cells can undergo dedifferentia- tion and become macrophage-like inflammatory effector cells and may be the primary cells producing type I collagen [30]. Macrophages and the coagulation system modulate PECs and are the main driving molecules in this respect. PECs may also proliferate and/or trans-differentiate into myofibroblasts pos- sibly by action of TGF-ß and PDGF, participating in glomer- ular crescents [31].

Podocytes and crescents

Podocytes are considered in general to be terminally dif- ferentiated cells and have not been regarded as main par- ticipants in crescent formation. However, it has been dem- onstrated that new podocytes could be recruited from glo- merular parietal epithelial cells through differentiation and proliferation [32]. In murine models and in patients with anti-GBM antibody disease, podocytes already adhered to both the GBM and the parietal basement membrane can build up podocyte bridges between the glomerular tuft and Bowman’s capsule [33]. Podocyte bridging could be an important early event in the development of crescents [33] (Fig. 1). Podocytes also populate crescents and may undergo epithelial–mesenchymal transformation also at early stages of crescentic glomerular diseases [34]. Nestin, a podocyte marker, has been encountered in cres- cents [35]. Finally, CD133- and CD24-positive renal pro- genitor cells localized in Bowman’s capsule at the vascular and urinary poles of the glomerulus are capable of regenerating podocytes [36]. The formation of tight junc- tions between podocytes is an early ultrastructural abnor- mality in crescentic glomerulonephritis, preceding foot process effacement and podocyte bridging after inflamma- tory injury. Podocyte-to-podocyte tight junction formation may be a compensatory mechanism to maintain the integ- rity of the glomerular filtration barrier following severe endocapillary injury and to avoid podocyte depletion and podocyturia [37].

Fibroblasts and crescents

Interstitial fibroblasts are the second most frequent cell type after macrophages [5] (Fig. 1). They enter Bowman’s space from the periglomerular interstitium through gaps in Bowman’s capsule. In the crescent itself, fibroblasts are a major source of interstitial collagen, which characterizes the transition from cellular to fibrous crescents [15]. As men- tioned previously, PECs can undergo mesenchymal differen- tiation and can also contribute to the collagen content of cres- cents [30].

The fate of crescents

The presence of crescents does not necessarily predict ir- reversible glomerular damage. Whether crescents progress or resolve may depend upon the integrity of Bowman’s capsule and the cellular composition of the crescent. Interstitial collagen synthesis and progression to fibrous crescents are more frequent when Bowman’s capsule rup- tures and fibroblasts and macrophages act in Bowman’s space [15]. Albeit fibrous crescents generally correlate with glomerulosclerosis, there is no evidence that an initial event in crescents can cause injury to the glomerular cap- illaries. Thus, crescent formation appears to be a conse- quence and a response, and not a cause, of severe active glomerular injury. However, there is increasing evidence that aggressive crescents may occlude the outflow from Bowman’s capsule to the proximal tubule, producing atubular glomeruli with secondary degeneration of glomer- uli and tubules and the disappearance of the involved neph- ron [38, 39].

The clinical significance of crescents in the different glomerulopathies

The impact of crescents on the different glomerulopathies varies widely. It mainly depends on the underlying glomer- ular disease and on the number and stage of the crescents. As mentioned previously, the initial step that leads to cres- cent formation is the damage to the glomerular capillary wall. Thus, crescents are not frequently observed in prima- ry podocyte damage, as may occur in minimal change dis- ease, primary focal and segmental glomerulosclerosis, membranous nephropathy, or genetic podocytopathies. Similarly, crescents are rarely observed in primary GBM disorders, such as Alport’s syndrome or thin basement membrane disease. However, when the GBM is immuno- logically damaged by antibodies against one of its compo- nents, as occurs in Goodpasture’s disease, the situation is completely different. Thence, crescents are more frequent- ly observed in those situations in which the endothelium is immunologically damaged, which results in endothelial proliferation (Table 1). This phenomenon is observed in immune complex and complement activation on the subendothelial space, as in membrano-proliferative glo- merulopathies, and in neutrophil-involved processes, such as anti-neutrophil cytoplasm antibody-associated (ANCA) vasculitis and post-infectious glomerulonephritis. Finally, endothelial proliferation can also lead to varying degrees of crescent formation, as is reported in IgA nephropathy (IgAN, see below) (Table 1). Macrophages, lymphocytes, and dendritic cells all play critical roles in these settings.

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Pathogenesis of crescent formation in the different glomerulopathies

The pathophysiology of crescent formation could be divided into two main steps. The first one pertains to each of the different glomerular entities and the capability to damage the endothelial side of the glomerular filtration barrier, from where podocytes and parietal cells interact between them- selves and with the surrounding extraglomerular cells to re- spond to this insult; the second step is the development of the crescent itself, which appears to involve a stereotypical path- way, with certain distinguishing features [1].

In the first step, either loss of tolerance (crescentic glomer- ulonephritis such as ANCA-positive or anti-GBM); an anti- body response to autologous molecules such as galactose- deficient IgA (IgAN) or to external epitopes (post-infectious glomerulonephritis); or a combined loss of tolerance plus identification of own molecules as antigens (immune complex crescentic glomerulonephritis, immune complex membrano- proliferative glomerulonephritis, lupus nephritis) leads to the priming of B and T cell clones and the synthesis of circulating autoantibodies that will eventually damage the endothelial glomerular cells [1].

In the second step, the varying degrees of activation of one or more of the three different complement pathways depend- ing on the glomerulopathy under concern will contribute to the vascular damage. In this regard, vascular necrosis, throm- bosis, or endothelial proliferation will follow, with rupturing of the GBM, plasma leakage to Bowman’s space, and podocyte stress. Podocytes may respond in different ways: cellular hypertrophy, detachment, expression of an antigen- presenting cell phenotype [1], or rarely proliferation, as in collapsing FSGS (see below). Extraglomerular cells migrate to the scene through a weakened or ruptured Bowman’s cap- sule, while effector T cells then recognize nephritogenic

antigens presented by podocytes or parietal cells within the urinary space. Coagulation factors, and the already-mentioned mitogenic signals, induce parietal cell hypertrophy, tuft en- croachment, and eventually glomerular obliteration [1, 40].

Distinguishing features among certain glomerulopathies

As mentioned, the impact of crescents in the different glomer- ular diseases depends mainly on the number of crescents and on the crescent cellularity, but also the initial location of the glomerular insult and the immunogenic capability of the anti- gen. In general, the higher the number of crescents and the proliferation of cells, the worse the prognosis will be (Table 1). Endothelial and basement membrane immune de- posits tend to carry a bad prognosis due to the capability to evoke an inflammatory response that will lead to podocyte damage and parietal cell activation and proliferation. Mesangial deposits present dissimilar behavior. In general, a low number of crescents is not associated with a bad outcome; according to the Oxford classification, a C1 score is associated with a good response to immunosuppression, while over 25% of crescents is associated with a grim prognosis despite the employment of immunosuppressants [41]. It has also been proposed that the development of endothelial proliferation in IgAN is associated with the generation of crescents [42]. Finally, a primary podocyte damage, as occurs in minimal change disease or FSGS due to circulating permeability fac- tors or to genetic mutations, is rarely accompanied by cres- cents, probably due to a primary lack of damage to the endo- thelial side of the glomerular filtration barrier. Subepithelial immune complex deposition as shown in primary and idio- pathic membranous nephropathy is rarely complicated by a high number of crescents. Occasional crescents can be

Table 1 Primary glomerulopathies and crescents

Glomerulopathy Presence of endothelial proliferation

Presence of podocyte proliferation

Presence of crescents Clinical course Progression to CKD 5

Anti-GBM disease 100% No >75% Acute 50%a

ANCA+ 100% No >75% Acute 30% a

IC extra-capillary GN 100% No >50% Acute 30% a

IgA <20% No 36% Chronic 20–30%*

Collapsing FSGS 0% Yes 24% Acute 50% a

MN 0% No 0.25% Chronic 30%**

IC MPGN 70% No 30% Chronic 12%***

C3 glomerulopathy 60% No 30% Chronic 25%***

Abbreviations: IC immune complex; GN glomerulonephritis; FSGS focal and segmental glomerulosclerosis; MN membranous nephropathy; MPGN membrano-proliferative glomerulonephritis; CKD 5 stage 5 chronic kidney disease

Symbols: a days to weeks; *at 20 years; **at 30 years; ***at 5 years

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observed and may not play a role in the outcome of membra- nous nephropathy (Table 1).

Crescentic glomerulonephritis

Glomerular crescents are the result of the proliferation of Bowman’s capsule parietal cells that encroach into the capil- lary tuft (Fig. 1). When crescents predominate, the clinical course is that of a rapidly progressive glomerulonephritis and a nephritic profile, with acute loss of kidney function, hypertension, hematuria, and varying degrees of proteinuria [43]. Three types of crescentic or extra-capillary glomerulo- nephritis are observed: anti-GBM glomerulonephritis, im- mune complex–mediated glomerulonephritis, and ANCA vasculitis, the most common form of crescentic glomerulone- phritis, accounting for >60% of all cases [44] (Table 1).

It has been proposed that the immune-related chemo- kine stromal cell–derived factor-1 (SDF-1)/C-X-C chemo- kine receptor (CXCR) 4 axis, known to be crucial for cell migration and proliferation [45], might play a role in all three types of primary glomerulonephritis. In human and rat crescentic glomerulonephritis, marked SDF-1 and CXCR4 upregulation in podocytes and PECs, respectively, suggests that SDF-1 produced by podocytes might trigger PEC/progenitor cell activation via CXCR4, leading to the formation of crescentic lesions [45]. Angiotensin II can promote cell proliferation and migration through the angio- tensin II type-1 (AT1) receptor [46]. Macrophages, possi- bly recruited by neutrophils [44], accumulate in the inter- stitial compartment, in the periglomerular area, and in the tuft itself (Fig. 1). This infiltration progressively increases inside the glomeruli, paralleling the progression of cellular hyperplasia. Interstitial macrophages produce MMP-12 elastase, a proteolytic enzyme that contributes to focal rup- tures of Bowman’s capsule, typically encountered in pa- tients with extra-capillary glomerulonephritis [47], sug- gesting a possible relationship between blood leakage from the glomerular capillaries and crescent formation [48]. Thus, in patients with extra-capillary glomerulonephritis, macrophage-driven lysis of glomerular membranes could induce PEC activation by promoting blood spillage through the GBM (Fig. 1) and producing angiotensin II (ang II), which via AT1 receptors stimulates podocytes to express SDF-1. On the other hand, excessive production of ang II stimulates podocytes to produce SDF-1 as well, pro- moting parietal progenitor activation via CXCR4 and CXCR7 receptors [44]. Finally, in a murine anti-GBM glo- merulonephritis model, crescent formation was preceded by the already-mentioned formation of podocyte bridges with PECs, considered the initiating event for cell prolif- eration on the capsular side and the formation of cellular crescents [49].

IgA nephropathy

In IgAN, it appears that fibrinogen and fibrin-related antigens but not fibrin are persistently positive in the crescents of this nephropathy. In addition, components of the GBM, such as type IV and V collagens, laminin, fibronectin, and cytokeratin, were consistently positive at all stages of cres- cents. Vimentin, usually distributed in podocytes and parietal and interstitial cells, was also found at all stages of the cres- cents. These findings may suggest that in the early stages of crescent formation in IgAN, podocytes play an important role, and that the accumulation of intrinsic basement membrane constituents is associated with the formation and progression of the crescents (Fig. 1). It appears that monocytes/ macrophages do not play a key role in the development of crescents in IgAN [50]. This finding may explain why the appearance of some crescents in kidney biopsies may not be regarded as severe lesions when compared to extra-capillary glomerulonephritis. As mentioned above, in IgAN, endothe- lial proliferation—a histological pattern frequently overlooked in this nephropathy—is associated with the devel- opment of crescents [41, 42].

Crescent formation in IgAN is associated with activation of the lectin and alternative pathways [51]. Some studies have reported that IgAN patients with increased glomerular mannose-binding lectin (MBL)–associated serine protease type 1 (MASP-1) deposition had a higher level of proteinuria and an increased rate of extra-capillary proliferation, glomer- ular sclerosis, and kidney dysfunction [52]. Hashimoto et al. observed increased intensity of properdin and factor B stain- ing in murine IgAN with more severe glomerular injury in- cluding crescent formation, suggesting the involvement of an activated alternative pathway [53]. Complement components and factors related to complement activation are partly pro- duced by intrinsic glomerular cells including mesangial and endothelial cells [54]. Glomerular C4d-positive IgAN patients are associated with an increased rate of stage 5 chronic kidney disease. Espinosa et al. showed that kidney survival at 10 years was significantly lower in C4d-positive IgAN patients than in C4d-negative IgAN patients and reported that IgAN with crescents was associated with higher levels of tissular C5b-9, MASP 1/3, MASP2, properdin, and factor B than those without crescents [51]. Therefore, crescent formation in IgAN may be associated with activation of both alternative and lectin pathways.

Collapsing FSGS

A recent study in experimental crescentic glomerulonephritis found that CD44, a cell surface glycoprotein that plays a key role in various cellular processes, is expressed in activated PECs and that its deficiency was associated with a reduced number of PECs in Bowman’s space [1]. In addition, CD44

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deficiency reduced glomerular cell proliferation and reduced albuminuria, indicating a link among CD44-expresing activat- ed PECs, the formation of crescents, and the development of albuminuria. In association with CD44 expression, CD9, a tetraspanin involved in cell proliferation, migration, adhesion, and survival, was found in PECs of a collapsing glomerulosclerosis rodent model [55]. Blocking CD9 attenu- ated the ability of PECs to proliferate and migrate, and atten- uated glomerulosclerosis. One possible mechanism of PEC activation via CD9 relates to the activation of epidermal growth factor receptor, a key driver of kidney damage in early stages of glomerulonephritis [54]. Thus, the local expression of CD9 is related to crescent formation in collapsing glomer- ulonephritis [1].

One interesting observation is that the anti-Thy1.1 model for collapsing FSGS is induced by targeting podocytes with anti-Thy1.1 antibody, which results in podocyte injury and activation of PECs. The subsequent formation of adhesions between the glomerular capillary tuft and Bowman’s capsule provides an entry site for activated PECs [2]. Activated PECs contribute to deposition of extracellular matrix material, which ultimately results in segmental and global glomerulosclerosis. The number of glomerular proliferating cells is reduced in the absence of CD44, which most likely can be explained by a reduction in proliferating PECs, because the anti-Thy1.1 model is not characterized bymassive glomer- ular infiltration of immune cells. Despite a decreased number of proliferating glomerular cells and reduced proteinuria, the absence of CD44 did not result in fewer histologically affected glomeruli. In summary, it appears that acquired glomerular CD44 expression by activated PECs is required for the path- ogenesis of experimental crescent formation in collapsing FSGS [2]. Finally, it is worthy of mention that in collapsing FSGS, the expression “pseudocrescents” is frequently employed to describe the occupation of Bowman’s space by hypertrophic and stressed podocytes. This term must not be confused with the cellular proliferation and intense inflamma- tion encountered in crescents as described in this manuscript.

The dynamics of crescents: from proliferation to fibrosis

As already mentioned, an initial microvascular injury leads to rupture of the GBM, which leads to the leakage of plasma proteins into Bowman’s space, driving hyperplasia of PECs as the key cellular component of the crescent and encroaching upon the involved glomerulus. Single-nephronGFR decreases because tuft collapse, rupture of the Bowman’s capsule, and influx of inflammatory cells and fibroblasts are secondary events (Fig. 1). Periglomerular immune cell infiltrates or fi- brosis due to local intense inflammation are subsequent events that may affect the dynamics and prognosis of the disease [1].

In this regard, the development of fibrosis in a proliferative crescent will eventually lead the clinical picture from an acute to a chronic setting. However, although the time it takes for such transformation in clinical practice would be of utmost relevance, there are no clinical studies addressing this topic, except in the case of rapidly progressive glomerulonephritis.

In a rat model of crescentic glomerulonephritis reproducing anti-GBM disease, the time elapsed from the development of cellular crescents to scarring and fibrosis was 4 to 6 weeks with almost 100% of glomeruli affected by global and diffuse glomerulosclerosis and severe interstitial medullary damage [39].

In clinical practice, when the number of cellular crescents exceeds 50% of the kidney sample, the prompt immunosup- pressive intervention may delay, decrease, or even be unable to stop the evolution of cellular to fibrotic crescents. Without intervention, in ANCA glomerulopathies, the time it takes to develop fibrosis may progress from an acute to a chronic phase within 1 to 2 weeks [56]. A similar extrapolation can be made with anti-GBM disease and immune complex extra- capillary glomerulonephritis: all entities with a rapidly pro- gressive course. In general, the number of crescents evolving to fibrosis parallels the degree of GFR decline. In the case of IgAN, if the number of crescents exceeds 25% in a kidney biopsy, the evolution to fibrosis and a bad prognosis appears to be the rule despite immunosuppression [41, 42].

Novel and potential therapeutic approaches

Based on the molecular mechanisms of crescent formation detailed in the manuscript, some of the intervening molecules may be plausible for pharmacologic assessment. Most of the already-tested antagonists have been explored in other fields outside nephrology. The blockade of specific molecules at different stages of crescent formation may be useful to im- prove the prognosis of glomerulopathies in which extra- capillary lesions are usually associated with poor outcomes. At the initial steps, Th17 CD4 effector lymphocytes express chemokine receptor CCR6 [8, 9]. Besides oncology, chemo- kine receptor CCR6 antagonist CO339589 has been success- fully assessed in autoimmune diseases such as psoriasis, rheu- matoid arthritis, and multiple sclerosis [57]. Already-available inhibitors of TLR-4 and/or TLR-9, whose activity has been proven in the development of crescents, could be useful to ascertain in the glomerular setting. Ibudilast, a TLR4 antago- nist, has been tested in autoimmune asthma and NI-101 in rheumatoid arthritis [58, 59]. In the same line, TLR 9 ODN 2088 has been shown to modulate macrophage chemotaxis in spinal cord injury [60]. A natural antagonist of CCR2, 747, s e l e c t i v e l y a t t enua t e s mac rophage ac t i v i t y i n hepatocarcinoma [61]. In a clinical trial, a novel selective MCP-1 receptor antagonist CCX140-B was given in addition to standard care in a randomized, double-blind study [62].

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Patients were randomized to placebo, 5 mg, or 10 mg of CCX140-B daily. Reduction in albuminuria was greatest in patients receiving low-dose CCX140-B, indicating MCP-1 inhibition on top of ACE inhibitors or ARBs conferred further renoprotection in diabetic nephropathy [62]. Finally, PAR-2, the already-mentioned receptor that intervenes in crescent de- velopment [16], has been antagonized by AZ3451 in patients with osteoarthritis [63]. As previously commented, the ex- pression of the tetraspanin CD9 increases markedly in PECs in mouse models of crescentic glomerulonephritis and FSGS. CD9 gene targeting in PECs prevents glomerular damage, and CD9 deficiency prevents the oriented migration of PECs into the glomerular tuft and their acquisition of CD44 and β1 integrin expression, offering another potential therapeutic pathway to target formation of crescents [55].

Conclusion

Crescents constitute the hallmark of inflammation in active glomerulonephritis and are a marker of glomerular injury. However, the presence of cellular crescents may not lead to the same bad outcome in the different types of primary glo- merulopathies. The pathophysiology of parietal cell prolifera- tion varies among these entities. Crescents are a non-specific morphological pattern of glomerular injury with different im- plications in the clinical outcome of the different glomerular diseases. Unraveling the diverse actors that play in each of the glomerulopathies may lead to a better understanding of the pathophysiology of these entities, as well as to tailored and more specific therapies.

Author contribution The author entirely contributed to the development of the manuscript.

Declarations

Conflict of interest The author declares no competing interests.

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