Changes at the glomerulo-tubular junction in renal transplants

Non-glomerular Tip Lesion Focal Segmental Glomerulosclerosis as a Negative Predictor in Idiopathic Membranous Nephropathy

Objective

To assess the significance of focal segmental glomerulosclerosis (FSGS) variants on clinicopathological characteristics and short-term outcomes in idiopathic membranous nephropathy (IMN) patients.

Methods

The clinicopathological data of 146 IMN patients diagnosed between December 2016 and March 2019 in our center were collected and analyzed. These patients were divided into the pure IMN group, IMN with glomerular tip lesion (GTL) group, and IMN with non-GTL FSGS group.

Results

The IMN with non-GTL FSGS and IMN with GTL groups both had higher proportions of patients with hypertension, lower serum albumin, and severe proteinuria, while the IMN with non-GTL FSGS group additionally showed higher blood pressure and serum cholesterol, and lower serum IgG than the IMN group (all P<0.05). As for pathology, the IMN with non-GTL FSGS group had higher proportions of patients with acute tubular injury and moderate to severe chronic injuries than the IMN group (all P<0.05). In the IMN, IMN with GTL, and IMN with non-GTL FSGS groups, the overall one-year remission rates were 81.6%, 76%, and 58.8%, respectively. Furthermore, the IMN with non-GTL FSGS group showed the lowest cumulative incidence to reach remission within one year. Multivariate Cox logistic analysis demonstrated that higher level of serum anti-M-type phospholipase A2 receptor antibody and the existence of non-GTL FSGS lesion were independent predictors for no remission in IMN patients.

Conclusion

The non-GTL FSGS lesion was a novel negative predictor in IMN and should be taken into account in the management of IMN.

Problems with 'focal segmental glomerulosclerosis'

The term 'focal segmental glomerulosclerosis (FSGS)' has been applied to many different conditions. All classifications of 'FSGS', including those describing 'variants', perpetuate the misconceptions that the entities included have something in common and that the term 'FSGS' has some value. With a rigorous approach to renal biopsies showing segmental lesions, especially with knowledge of clinical circumstances and with detailed analysis of features such as the appearance of lesions and their position within glomeruli, a pathologist can provide information that is clinically more useful than merely the bald diagnosis 'FSGS'. More precise terms should be used. 'Overload changes' can be used to describe the changes seen in reduced renal mass. 'Tip changes' can be seen in many conditions and are not a disease in themselves. 'The glomerular tip lesion as originally defined' means the occurrence of tip changes in otherwise normal glomeruli, in the nephrotic syndrome. 'Early classical segmental sclerosing glomerulopathy' is the combination of tip changes and otherwise abnormal glomeruli, in the nephrotic syndrome. 'Late classical segmental sclerosing glomerulopathy' means segmental lesions at various sites within glomeruli, in the nephrotic syndrome. 'Collapsing glomerulopathy' is distinctive, and its inclusion in classifications emphasises the lack of specificity of 'FSGS'.

Extent of Glomerular Tubularization Is an Indicator of the Severity of Experimental Acute Kidney Injury in Mice

BACKGROUND

Acute kidney injury (AKI) secondary to ischemia continues to be a major clinical problem due to its high morbidity and mortality, and limited treatment options. Animal models are critical to both the study of the pathophysiology of AKI and the development of new interventions. Although histological changes at the glomerulotubular junction have been described in AKI, we examined here whether the extent of glomerular tubularization correlates with the degree of renal insufficiency in this condition.

METHODS

Groups of mice with ischemia/reperfusion AKI were utilized in which the severity of renal insufficiency was defined. The resulting level of glomerular tubularization was analyzed, and the involved cell type was identified by immunohistochemistry and electron microscopy.

RESULTS

The extent of glomerular tubularization increased significantly with the degree of renal insufficiency. Low level glomerular tubularization was present in normal mouse kidneys, while it was more common and increasingly circumferential in mice with more severe loss of kidney function. The parietal monolayer of cuboidal cells in glomeruli was contiguous with proximal tubular cells, showing a well-developed luminal brush border and positive staining for proliferating cell nuclear antigen and Lotus tetragonolobus, a proximal tubular cell-specific lectin.

CONCLUSION

Increased levels of glomerular tubularization represent a poorly understood response to ischemic AKI in mice. As such, this glomerular 'tubularization score' may be useful to complement standard injury scores in experimental and, if detected, in clinical AKI.

Glomerular tip lesion: a distinct entity within the minimal change disease/focal segmental glomerulosclerosis spectrum

BACKGROUND

The glomerular tip lesion (GTL) is a distinctive but controversial histopathologic lesion occurring in patients with idiopathic nephrotic syndrome. The relationship of GTL to minimal change disease (MCD) and idiopathic focal segmental glomerulosclerosis (FSGS) has been disputed.

METHODS

In order to define the clinical features and natural history of GTL, we retrospectively reviewed the presenting clinical features, biopsy findings and outcome of 47 cases. Presenting clinical features of GTL were compared to those of controls with MCD (N= 61) or idiopathic FSGS (N= 50).

RESULTS

The cohort of GTL consisted of 45 adults and two children (mean age 47.5 years; range 12 to 79 years), including 76.6% Caucasians and 53% males. At presentation, 93.6% of patients had edema, 89.1% had nephrotic syndrome (mean urine protein 8.31 g, mean serum albumin 2.27 g/dL, and mean cholesterol 340.6 mg/dL), and 34.8% had renal insufficiency. Mean time from onset of renal disease to biopsy was 2.4 months. At biopsy, glomerular segmental lesions included GTL alone in 26%, GTL and peripheral lesions in 6%, GTL and indeterminate lesions in 36%, and GTL with peripheral and indeterminate lesions in 32%. No initial biopsy contained perihilar sclerosis and most (81%) segmental lesions were cellular. Follow-up data were available in 29 patients, of whom 21 received steroids alone and eight received sequential therapy with steroids and a cytotoxic agent. At a mean follow-up of 21.5 months, 58.6% of patients achieved complete remission of nephrotic syndrome, 13.8% had partial remission, and 27.6% had persistent nephrotic proteinuria. Only one patient progressed to end-stage renal disease (ESRD). Predictors of nonremission included severity of proteinuria at presentation and % peripheral lesions. When compared to controls with MCD and idiopathic FSGS, GTL more closely resembled MCD with respect to high incidence of nephrotic syndrome (P < 0.001), severity of proteinuria (P < 0.05), short duration from onset to biopsy (P < 0.001), and absence of chronic tubulointerstitial disease (P < 0.0054).

CONCLUSION

Within the MCD/FSGS spectrum, GTL is a distinctive and prognostically favorable clinical-pathologic entity whose presenting features and outcome more closely approximate those of MCD.

The glomerulo-tubular junction: a target in renal diseases

Both global and segmental glomerulopathies may damage specific areas of the renal glomerulus. Diseases associated with glomerular hyperperfusion cause lesions at the vascular pole, while diseases associated with proteinuria often damage the tubular pole. Atubular glomeruli are now known to be plentiful in a variety of common renal diseases. These glomeruli are disconnected from their tubule at the tubular pole and therefore cannot participate in the production of urine. It is widely believed that the disconnection is a result of external compression by periglomerular fibrosis. However, the variable anatomy and cell populations within both the glomerulus and the beginning of the proximal tubule at the glomerulo-tubular junction may also have important roles to play in the response to damage at this sensitive site of the nephron.

Glomerular prolapse as precursor of one type of segmental sclerosing lesions

A distinctive segmental glomerular abnormality is confined to the region of the tubular opening. The hypothesis was that this followed prolapse of the tuft into the tubule. Analysis was made of 39 renal biopsy specimens with acute postinfective glomerulonephritis, later material from ten cases, four specimens from three women with pre-eclampsia, and 21 control specimens, with morphometry of glomeruli and immunohistological examination for immunoproteins and monocytes/macrophages. Prolapse was found in 14 specimens with acute postinfective glomerulonephritis, associated in eight with adhesion to Bowman's capsule and local alterations in the tuft, which together constitute early tip changes. Another three had early tip changes only and eight others had thin adhesions between the tuft and capsule next to the tubular opening. Later material confirmed this order of development and showed another late change, with sclerosed and hyaline material in the tuft and adhesion at the tubular origin. Findings in pre-eclampsia were comparable. Glomeruli were significantly larger in acute postinfective glomerulonephritis than in controls and were shown by others to be larger in pre-eclampsia than in normal pregnancy. Immunohistology showed IgM and a few foamy monocytes/macrophages in early tip changes but not in prolapsed loops. Glomerular prolapse appears to be a temporary consequence of acute enlargement of the tuft, probably causes mechanical damage to epithelial cells, and is a precursor of permanent structural changes near the tubular origin. This gives a unifying hypothesis to explain how these changes can be seen in acute postinfective glomerulonephritis, pre-eclampsia, and many other human and experimental renal disorders.

Tuft-to-capsule adhesions and their precursors: differences between the vascular and tubular poles of the human glomerulus

Human glomerular capillary tufts were removed by microdissection and scanning electron microscopy was used to examine the surface of the capillary tuft and the interior of its Bowman's capsule in order to identify connections between the tuft and capsule. Glomeruli were examined in histologically normal renal cortex from 12 kidneys removed for tumour and 12 renal allografts removed for end-stage rejection. In normal kidney, the glomerular tuft was connected to Bowman's capsule by single podocytes and their processes. At the vascular pole, these were predominantly associated with parietal podocytes which lined Bowman's capsule. At the tubular pole, occasional podocytic processes derived from the capillary tuft bridged Bowman's space and connected to Bowman's capsule where there were no parietal podocytes. These podocytic connections were also found in all rejected transplants, but in addition adhesions were identified which consisted of thicker connections between the tuft and capsule. At the vascular pole, tuft-to-capsule adhesions were found in all 12 kidneys; these were always associated with parietal podocytes. Tubular pole adhesions were identified in ten of the 12 transplants. They were associated with abnormal squamous cells, but not with parietal podocytes. When the capillary tuft herniated into the proximal tubule, the tuft sometimes formed an adhesion with the origin of the proximal tubule. These observations suggest that podocyte connections between the glomerular tuft and Bowman's capsule may be precursors of glomerular adhesions at the vascular pole. Since tuft-to-capsule adhesions at the vascular pole differ morphologically from those at the tubular pole, this may reflect different pathogenetic mechanisms at the opposite poles of the glomerulus.

Stenosis of the tubular neck: a possible mechanism for progressive renal failure

Fibrosis in chronic renal failure is closely associated with declining function. Its role in affecting function is less well defined. The radiation model of chronic renal failure was used to examine the tissue distribution of fibrosis and scarring and its role in influencing the loss of function in chronic renal disease. A striking and progressive pattern of fibrosis and narrowing of the glomerulotubular neck was found in irradiated pig kidneys. These narrowed necks increased in prevalence with time after irradiation. At 20 weeks after irradiation, the average neck diameter reduction was 60%, as compared with nonirradiated controls, a percentage that is consistent with a reduction in flow and pressure at this critical point of the nephron. Glomerulotubular neck narrowing may thus directly reduce the glomerular filtration rate of an individual nephron. Fibrotic neck stenoses may be a factor in progressive chronic renal failure.

Pathogenesis of segmental glomerular changes at the tubular origin, as in the glomerular tip lesion

Segmental abnormalities at the glomerulo-tubular junction, or tip changes, are a common and early feature in several experimental and human renal disorders. Scanning and transmission electron microscopic study of an experimental model, Lewis rats given anti-glomerular basement membrane antibodies, showed that at 5 days there were monocytes/macrophages in glomerular capillary loops next to the tubular origin, with abnormalities of visceral epithelium, including prolapse of groups of these cells into the tubular origin. At 12 days, there was contact and adhesion between glomerular capillary loops and Bowman's capsule. Transmission electron microscopic study of human renal biopsies showed that the human tip changes resembled the later lesions seen in rats, with intracapillary foam cells confirmed by immunohistological study to be monocytes/macrophages. These findings show that abnormalities of the glomerular tuft precede adhesion to Bowman's capsule and that the earliest changes recognized by light microscopy in human renal biopsies are relatively late in their natural history, meaning at least several days old. Prolapse of visceral epithelial cells into the tubular origin may explain the characteristic site of tip changes.

Segmental sclerosing glomerular lesions

Segmental sclerosing glomerular lesions are usually all grouped together and called focal segmental glomerulosclerosis. This has meant that the term that was originally used for a defined clinical entity is now applied to a variety of conditions in man and experimental animals, with the assumption that the morphological changes are the same in all conditions. Studies of the position of segmental lesions within glomeruli, the size of glomeruli and the proportion of glomeruli affected have shown that this assumption is wrong. Such studies have identified a disease that corresponds to the original clinical concept of focal segmental glomerulosclerosis. This begins with the nephrotic syndrome in patients whose renal biopsies show large glomeruli with mesangial hypercellularity and segmental lesions at every tubular origin. Later the biopsies have segmental lesions throughout the glomerular tuft. This disease differs clinically and pathologically from other conditions that have segmental sclerosing lesions, such as in patients with reduced renal mass or patients with hypertension and proteinuria. The term focal segmental glomerulosclerosis is now too ambiguous and unsatisfactory to be used without qualification.