Peritubular capillary basement membrane changes in chronic renal allograft rejection: Comparison of light microscopic and ultrastructural observations

An unusual pattern of peritubular capillary injury involving apoptosis in a renal transplant patient

Microvascular injury is an important factor in renal allograft survival. Repeated episodes of endothelial injury from chronic antibody-mediated rejection typically manifest at the ultrastructural level as circumferential multilayering of remodeled glomerular basement membrane material and peritubular capillary basal lamina. In contrast to this typical pattern of microvascular injury, a renal transplantation case is presented in which focally dilated and multilayered segments of peritubular capillary basal lamina bearing lipid droplets were interspersed with ultrastructurally normal unilayered segments of basal lamina devoid of lipid droplets. Glomerular basement membranes were not affected by this process. The peak incidence of lipid droplets within the peritubular capillary walls coincided with a peak in apoptotic activity within the allograft. Lesser amounts of the same lipidic material were identified in the mesangial matrix and an arteriolar wall. Mesangial electron-dense deposits were detected at two weeks posttransplantation and their appearance coincided with elevated immunological activity in the glomeruli, as determined by immunofluorescence microscopy. The unusual ultrastructure and immunological activity observed in this case may reflect a process of impaired apoptotic clearance within the allograft. The six biopsies from a single patient are discussed in the setting of a highly sensitized renal transplant recipient who received prophylactic terminal complement blockade by eculizumab. The findings may be relevant to the study of apoptosis, efferocytosis, microvascular injury, eculizumab, rejection, lupus, and drug-related disease.

Refinement of the criteria for ultrastructural peritubular capillary basement membrane multilayering in the diagnosis of chronic active/acute antibody-mediated rejection

Chronic active/acute antibody-mediated rejection (cABMR) is the main cause of late renal allograft loss. Severe peritubular capillary basement membrane multilayering (PTCML) assessed on electron microscopy is one diagnostic feature of cABMR according to the Banff 2013 classification. We aimed to refine the PTCML criteria for an earlier diagnosis of cABMR. We retrospectively investigated ultrastructural features of 159 consecutive renal allografts and 44 nonallografts. The presence of serum donor-specific antibodies at the time of biopsy of allografts was also examined. Forty-three patients (27.0%) fulfilled the criteria of cABMR, regardless of PTCML, and comprised the cABMR group. Forty-one patients (25.8%) did not exhibit cABMR features and comprised the non-cABMR allograft control group. In addition, 15 zero-day wedge resections and 29 native kidney biopsies comprised the nonallograft control group. When the diagnostic accuracies of various PTCML features were assessed using the cABMR and non-cABMR allograft control groups, ≥4 PTCML, either circumferential or partial, in ≥2 peritubular capillaries of the three most affected capillaries exhibited the highest AUC value (0.885), greater than the Banff 2013 classification (0.640). None of the nonallograft control groups exhibited PTCML features. We suggest that ≥4 PTCML in ≥2 peritubular capillaries of the three most affected cortical capillaries represents the proper cutoff for cABMR.

Regardless of etiology, progressive renal disease causes ultrastructural and functional alterations of peritubular capillaries

Progressive renal diseases are associated with rarefaction of peritubular capillaries, but the ultrastructural and functional alterations of the microvasculature are not well described. To study this, we analyzed different time points during progressive kidney damage and fibrosis in 3 murine models of different disease etiologies. These models were unilateral ureteral obstruction, unilateral ischemia-reperfusion injury, and Col4a3-deficient mice, we analyzed ultrastructural alterations in patient biopsy specimens. Compared with kidneys of healthy mice, we found a significant and progressive reduction of peritubular capillaries in all models analyzed. Ultrastructurally, compared with the kidneys of control mice, focal widening of the subendothelial space and higher numbers of endothelial vacuoles and caveolae were found in fibrotic kidneys. Quantitative analysis showed that peritubular capillary endothelial cells in fibrotic kidneys had significantly and progressively reduced numbers of fenestrations and increased thickness of the cell soma and lamina densa of the capillary basement membrane. Similar ultrastructural changes were also observed in patient's kidney biopsy specimens. Compared with healthy murine kidneys, fibrotic kidneys had significantly increased extravasation of Evans blue dye in all 3 models. The extravasation could be visualized using 2-photon microscopy in real time in living animals and was mainly localized to capillary branching points. Finally, fibrotic kidneys in all models exhibited a significantly greater degree of interstitial deposition of fibrinogen. Thus, peritubular capillaries undergo significant ultrastructural and functional alterations during experimental progressive renal diseases, independent of the underlying injury. Analyses of these alterations could provide read-outs for the evaluation of therapeutic approaches targeting the renal microvasculature.

Peritubular capillary basement membrane multilayering in early and advanced transplant glomerulopathy: quantitative parameters and diagnostic aspects

The ultrastructural quantitative aspects of peritubular capillary basement membrane multilayering (PTCBML) were examined in 57 kidney transplant biopsies with transplant glomerulopathy (TG). The measurements included three cutoffs [permissive: 1 PTC with 5 basement membrane (BM) layers, intermediate: 3 PTCs with 5 layers or 1 PTC with 7 layers, strict: 1 PTC with 7 layers and 2 PTCs with 5 layers] and the mean number of BM layers (PTC_circ). Two groups were assigned, namely patients with mild TG (Banff cg1a and cg1b) and those with moderate-to-severe TG (cg2 and cg3). Their respective clinical, serological, and morphological characteristics were then compared. The clinical data revealed that mild TG corresponded to early chronic antibody-mediated rejection (cABMR), while moderate-to-severe TG corresponded to the advanced stage of the disease. The permissive threshold displayed the lowest specificity (73 %) and the highest sensitivity (83 %) for moderate-to-severe TG, and its corresponding PTC_circ value was 3 layers. In contrast, the strict threshold-adopted by the Banff 2013 classification-displayed a specificity and sensitivity of 93 and 52 %, respectively, and the corresponding PTC_circ was 4 layers. In mild TG, 26 % of the cases met the permissive cutoff and 6 % the strict cutoff. Mild TG was associated with a lower PTC_circ (2.6 layers vs 4.5 layers in moderate-to-severe TG; p < 0.0001). Amongst the various criteria, the permissive criterion was associated most frequently with mild TG, and had prognostic relevance. Because of this, we propose its usage as a marker of early cABMR-induced PTCBML if non-alloimmune causes of PTCBML can be ruled out.

Peritubular Capillary Basement Membrane Multilayering in Renal Allograft Biopsies of Patients With De Novo Donor-Specific Antibodies

BACKGROUND

Severe peritubular capillary basement membrane multilayering (PTCBML) is part of the Banff definition of chronic antibody-mediated rejection. We retrospectively investigated whether assessment of the mean number of layers of basement membrane (BM) around peritubular capillaries (PTC) can be used in a cohort of patients with de novo donor-specific antibodies (dnDSA) as an early marker to predict long-term antibody-mediated injury.

METHODS

This is a retrospective cohort study with 151 electron microscopy samples from 54 patients with dnDSA, assessed at around 1 year after transplantation, for a mean number of BM layers around PTC and in serial biopsies. Graft survival and time to transplant glomerulopathy (TG) development were estimated in survival analyses.

RESULTS

We found that a mean PTCBML count greater than 2.5 layers assessed in a sample of 25 PTCs around 1 year after transplantation is indicative of the development of TG in patients with dnDSA (P = 0.001). In addition, in patients with serial biopsies available for electron microscopy analysis, we could distinguish 2 groups: patients with a mean PTCBML count of 2.5 or less on all biopsies, and patients who developed greater than 2.5 layers at any time after transplantation. The latter group reflected dnDSA patients at risk for TG development (P < 0.001). In patients with dnDSA, PTCBML score added significantly to the sensitivity and specificity of prediction of TG compared with microcirculation injury score alone.

CONCLUSIONS

Our results highlight the potential value of assessing the mean number of BM in PTC for early prediction of progression to chronic antibody-mediated injury.

Better understanding of transplant glomerulopathy secondary to chronic antibody-mediated rejection

Transplant glomerulopathy (TG) is generally accepted to result from repeated episodes of endothelial activation, injury and repair, leading to pathological abnormalities of double contouring or multi-layering of the glomerular basement membrane. TG is a major sequel of chronic active antibody-mediated rejection (cABMR), from pre-existing or de novo anti-HLA antibodies. Hepatitis C infection, thrombotic microangiopathy or other factors may also contribute to TG development. TG prevalence is 5-20% in most series, reaching 55%, in some high-risk cohorts, and is associated with worse allograft outcomes. Despite its prevalence and clinical significance, few well-studied treatment options have been proposed. Similar to desensitization protocols, plasmapheresis with or without immunoabsorption, high-dose intravenous immunoglobulin, rituximab, bortezomib and eculizumab have been proposed in the treatment of TG due to cABMR individually or in various combinations. Robust clinical trials are urgently needed to address this major cause of allograft loss. This review summarizes the current knowledge of the epidemiology, etiology, pathology, and the preventive and treatment options for TG secondary to cABMR.

The Basics of Renal Allograft Pathology

Renal allograft biopsy provides critical information in the management of renal transplant patients, and must be analyzed in close collaboration with the clinical team. The histologic correlates of acute T-cell mediated rejection are interstitial inflammation, tubulitis, and endothelialitis; polyomavirus nephropathy is a potential mimic. Evidence of antibody-mediated rejection includes C4d deposition; morphologic acute tissue injury; and donor specific antibodies. Acute tubular injury/necrosis is a reversible cause of impaired graft function, especially in the immediate post-transplant period. Drug toxicity, recurrent disease, chronic injury, and other entities affecting both native and transplant kidneys must also be evaluated.

The specificity of acute and chronic microvascular alterations in renal allografts

The diagnosis of an antibody-mediated rejection (AMR) is made when there is evident histologic injury in the presence of detectable donor-specific alloantibodies (DSA) and diffuse peritubular capillary C4d staining (C4d-pos). In the presence of only detectable DSA or C4d-pos, the tissue injury is currently considered "presumptive" for antibody causation. In acute antibody-mediated rejection (AAMR), diagnostic morphologic features include microvascular inflammation (MVI), specifically glomerulitis and peritubular capillaritis. In the case of chronic active AMR (CAAMR), these inflammatory lesions have progressed to chronic microvascular injury, transplant glomerulopathy (TG) and peritubular capillary basement membrane multilayering (PTCBMML). Either TG or PTCBMML is sufficient morphological evidence for a diagnosis of CAAMR. Unfortunately, these lesions are not specific. MVI, TG, and PTCBMML are found in the setting of cell-mediated immunity, as well as in association with non-alloimmune mechanisms. The available treatments for AMR and CMR are different, and it is important to ascertain the dominant mechanism when approaching an individual patient. At present, no gold standard exists to establish the specific pathogenesis in the more ambiguous cases. We detail here the differential diagnosis of MVI, TG, and PTCBMML.

Diagnostic significance of peritubular capillary basement membrane multilaminations in kidney allografts: old concepts revisited

BACKGROUND

Injury to peritubular capillaries and capillary basement membrane multilamination (PTCL) is a hallmark of antibody-mediated chronic renal allograft rejection. However, the predictive diagnostic value of PTCL is incompletely studied.

METHODS

We analyzed the diagnostic significance of PTCL and propose diagnostic strategies. We evaluated 360 diagnostic native and 187 transplant kidney specimens by electron microscopy (terminology: PTCL-C, severe; PTCL subgroup C3, very severe multilamination; see Materials and Methods for definitions).

RESULTS

PTCL was not pathognomonic for any specific disease. PTCL-C/C3 was rare in native kidneys (C, 6%; C3, 1%), associated mainly with late thrombotic microangiopathy (C: 78%; C3: 11% of cases). In allografts, PTCL-C/C3 was significantly more common, especially in specimens more than 24 months after transplantation (C, 47%; C3, 31%). PTCL-C/C3 was found in acute (C, 20%; C3, 7%) and chronic T-cell rejection (C, 67%; C3, 29%), calcineurin inhibitor toxicity (C, 36%; C3, 18%), or C4d(+) specimens (C, 61%; C3, 50%) with odds ratios between 4 and 36. PTCL-C3 was more predominant in cases with antibody-mediated injury. Highest odds ratios (81-117) for PTCL-C/C3 were noted in combined injuries, that is, mixed chronic T-cell and concurrent chronic antibody-mediated rejection. Positive predictive values of PTCL-C and C3 are the following: all rejection types, 89% and 93%; all Banff chronic rejection types, 69% and 71%; and chronic presumptive antibody rejection, 37% and 49%, respectively. Corresponding negative predictive values of C and C3 for different Banff rejection categories are between 50% and 94%.

CONCLUSIONS

The presence of PTCL-C3 is a helpful adjunct finding to diagnose rejection-induced tissue injury but cannot precisely predict the Banff rejection category. Conversely, the absence of PTCL-C3 is helpful in excluding chronic, Banff category II antibody-mediated rejection.

Chronic allograft nephropathy: An update

Chronic allograft nephropathy is the most prevalent cause of renal transplant failure in the first post-transplant decade, but its pathogenesis has remained elusive. Clinically, it is characterized by a slow but variable loss of function, often in combination with proteinuria and hypertension. The histopathology is also not specific, but transplant glomerulopathy and multilayering of the peritubular capillaries are highly characteristic. Several risk factors have been identified, such as advanced donor age, delayed graft function, repeated acute rejection episodes, vascular rejection episodes, and rejections that occur late after transplantation. A common feature of chronic allograft nephropathy is that it develops in grafts that have undergone previous damage, although the mechanism(s) responsible for the progressive fibrosis and tissue remodeling has not yet been defined. Hypotheses to explain chronic allograft nephropathy include the immunolymphatic theory, the cytokine excess theory, the loss of supporting architecture theory, and the premature senescence theory. The most effective option to prevent chronic allograft nephropathy is to avoid graft injury from both immune and nonimmune mechanisms.