Practical Renal Allograft Pathology - Springer Link

Practical Renal Allograft Pathology

31

Cinthia B. Drachenberg and John C. Papadimitriou

Abbreviations AIN AMR ATN C4d CD3 CD68 CMR CMV CNI DGF DSA EBV EM FSGS GBM H&E HUS IF IF/TA KTxBx MAPI MPGN MT PAMS

Allergic interstitial nephritis Antibody-mediated rejection Acute tubular necrosis C4d complement fragment Stain for T-cells Stain for macrophages T-cell-mediated rejection Cytomegalovirus Calcineurin inhibitor Delayed graft function Donor-specific antibody Epstein–Barr virus Electron microscopy Focal segmental glomerulosclerosis Glomerular basement membranes Hematoxylin and eosin Hemolytic uremic syndrome Immunofluorescence stains Interstitial fibrosis/tubular atrophy of no specific etiology Kidney transplant biopsies Maryland aggregate pathology index for donor biopsies Membranoproliferative glomerulonephritis Masson’s trichrome stain Periodic acid methenamine silver stain (Jones stain)

C.B. Drachenberg, M.D. (*) • J.C. Papadimitriou, M.D., Ph.D. Department of Pathology, University of Maryland School of Medicine, 22 South Greene Street, NBW43, Baltimore, MD 20201, USA e-mail: [email protected]; [email protected]

PAS PTC PTLD PV PVN SV40 TG TMA

Periodic acid stain Peritubular capillaries Post-transplant lymphoproliferative disorder Polyomavirus Polyomavirus allograft nephropathy Simian virus 40 stain for polyomavirus Transplant glomerulopathy Thrombotic microangiopathy

Renal Allograft Biopsy: Generalities Routine evaluation of kidney transplant biopsies (KTxBx) has been essential for the continuous clinical, surgical, and pharmacological advances in renal transplantation. Most therapeutic decisions, including the selection of immunosuppression protocols to be tailored to the individual patient, are primarily based on KTxBx findings [1]. The current chapter has been prepared for the nonpathologists involved in the treatment of renal transplant patients, to help them become familiar with the main histopathological concepts in this field.

Clinicopathological Integration A few histological findings are pathognomonic of specific pathological processes leading to unambiguous morphological diagnoses (e.g., intimal arteritis for rejection, CMV inclusions). In practice, however, most KTxBx can and need to be interpreted only in the context of the clinical presentation and other laboratory data. Close communication between the pathologist and the treating clinical team results in the best diagnostic yield from KTxBx examination. Pertinent clinical information should therefore be made available to the pathologist at the time of biopsy interpretation (see Table 31.1).

M.R. Weir and E.V. Lerma (eds.), Kidney Transplantation: Practical Guide to Management, DOI 10.1007/978-1-4939-0342-9_31, © Springer Science+Business Media New York 2014

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C.B. Drachenberg and J.C. Papadimitriou

356 Table 31.1 Clinical information Time post-transplant Type of transplant: living donor or cadaveric Donor history if pertinent Infection including history of HIV, hepatitis C, etc. Peritransplantation surgical complications Indication for biopsy Increase in serum creatinine (rapid, slow) Type of immunosuppression Changes in immunosuppression Use of nephrotoxic drugs Urine culture results Proteinuria Dehydration Renal artery stenosis Abnormal imaging studies (i.e., hydronephrosis)

Prevention, diagnosis, and treatment of antibodymediated rejection (AMR) require monitoring for the presence of circulating donor-specific antibodies (DSAs) at regular intervals after transplantation, as well as at the time of biopsy and whenever rejection is suspected [2–4]. As discussed in detail below, accurate classification of rejection according to the Banff schema requires correlation between the biopsy findings and DSA studies [2].

Biopsy Adequacy According to the Banff schema a specimen must contain ten or more glomeruli and at least two arteries to be considered adequate [5]. Due to the fact that many pathological processes involve the allograft in a patchy, random manner, the sensitivity of the biopsy depends on the amount of tissue available for evaluation. To diagnose or rule out acute allograft rejection with some degree of certainty, it is necessary to evaluate a sample of renal cortex containing sufficient number of glomeruli and arteries. Evaluation of two tissue needle cores is desirable to achieve a sensitivity of 99 % for the diagnosis of acute rejection. This is in contrast to a sensitivity of 90 % for a single core [6, 7]. Biopsies containing only medulla are in general considered inadequate because in those samples, acute allograft rejection will be missed or underestimated in half of the cases [8]. On the other hand, examination of renal medulla may be sufficient to diagnose other pathological entities (i.e., acute pyelonephritis, polyomavirus nephropathy) (PVN) [9]. The amount of tissue required also depends on the biopsy indication. There are clinical situations in which enough tissue material must be obtained to allow for the performance of ancillary studies such as immunofluorescence and electron microscopy (EM). This is particularly important if the biopsy is performed for a possible glomerular disease.

Fig. 31.1 H&E-stained biopsy from a graft with excellent function (protocol biopsy at 12 months). There is no inflammation. The tubules are “back to back” with no significant intervening connective tissue. A normal arteriole. Note the thin wall and the widely open lumen

Routine Histological Studies Light microscopic evaluation of renal biopsies includes the performance of hematoxylin and eosin (H&E)-stained sections (Fig. 31.1), as well as a set of routinely used special stains (periodic acid stain [PAS], periodic acid methenamine silver stain (Jones stain) [PAMS], Masson’s trichrome stain [MT], and C4d complement fragment [C4d] stain) [4, 5]. Table 31.2 summarizes the main morphological features and diagnostic associations identified with each of these stains (Fig. 31.2). The C4d stain is required for the identification of an important subset of AMRs [4, 10, 11]. C4d is a complement split product that remains covalently bound to target structures after complement activation has occurred, allowing for its recognition in tissue sections. This is in contrast to most products of complement activation that are immediately degraded and cannot be detected on standard tissue processing. Deposition of C4d in peritubular capillaries (PTC) identifies ongoing humoral rejection that may otherwise escape identification. The C4d stain can be performed in fresh-frozen as well as formalin-fixed tissue with comparable results [4]. Although not considered part of the routine biopsy work-up, immunohistochemical stains for macrophages (i.e., CD68) and T-cells (i.e., CD3) can highlight the subtle forms of microvascular inflammation that characterize early AMR [12, 13].

Main Ancillary Studies Infectious Processes In addition to standard serological studies for infectious processes (i.e., CMV, hepatitis C, HIV), immunohistochemical

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Table 31.2 Routinely* used special stains: main diagnostic value PAS* Tubules: Tubulitis, destruction of tubular basement membranes, thickening of basement membranes (atrophy) Glomeruli: Glomerulitis (mononuclear cells in capillaries), reduplication of basement membranes (transplant glomerulopathy), increase in mesangial matrix, hyalinosis Vessels: Hyalinosis Other: Identification of fungal organisms Silver stain (PAMS, Jones stain)* Tubules: Tubulitis, destruction of tubular basement membranes, thickening of basement membranes (atrophy) Glomeruli: Glomerulitis (mononuclear cells in capillaries), reduplication of basement membranes (transplant glomerulopathy), increase in mesangial matrix Trichrome stain* Collagen deposition: Interstitial fibrosis, arterial sclerosis, glomerular sclerosis, fibrin thrombi, fibrinoid necrosis C4d stain* Staining of peritubular capillaries in AMR (also see ref. [10]) Immunohistochemical stains for viral infections Cytomegalovirus SV40 (cross reacts with BK virus and JC virus) EBV latent membrane protein (LMP-1) EBER—Epstein–Barr-encoded RNA (in situ Hybridization) Adenovirus

Fig. 31.2 Masson’s trichrome-stained biopsy from a well functioning graft. Stain for collagen highlights minimal connective tissue between tubules. There is no evidence of tubular atrophy or interstitial fibrosis

stains and in situ hybridization are used for the identification of a variety of infectious organisms in tissue: SV40 (simian virus 40 stain for BK and JC polyomaviruses), CMV (for cytomegalovirus), EBV (for Epstein–Barr virus/post-transplant lymphoproliferative disorder [PTLD]), immunostains for adenovirus, etc. Stains for acid-fast bacilli and fungi are performed in biopsies with granulomatous inflammation or if these infections are suspected [1]. Molecular studies, particularly PCR, are also useful to demonstrate the presence of infectious organisms in tissue samples although these studies are not routinely used for diagnostic purposes. Routine determination of polyomavirus BK viral loads in plasma and urine is essential for prevention and monitoring

of polyomavirus allograft nephropathy (PVN). Evaluation of routine urine cytology with identification of the PV-infected “decoy cells” is an inexpensive and simple tool to determine if a patient has significant PV replication in the urinary tract. Persistent PV BK urinary shedding is associated with PVN and has markedly increased risk of graft loss [14, 15]. Electron microscopy studies and immunofluorescence stains (IF; for immunoglobulins and complement components) are required for the evaluation of post-transplant glomerular processes in the same manner as it is done in native kidney biopsies. Furthermore, ultrastructural studies of glomeruli and PTC are essential for the diagnosis of early AMR in order to demonstrate the characteristic basement membrane abnormalities. It is important to remember that the immunofluorescence technique can be applied only to tissues that are fresh-frozen (i.e., not fixed in formaldehyde).

Pretransplant Biopsy Deceased Donor Evaluation (Harvest Biopsies) Routine use of kidneys from less than ideal donors requires pretransplant histological evaluation for determination of organ quality. Pretransplant assessment is indicated for kidneys from older donors, or from those with history of hypertension, diabetes, or renal dysfunction (extended criteria donors) [16].

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are now routinely utilized for transplantation with excellent outcomes [20]. Frozen section evaluation is required for rapid histological characterization diagnosis of any donor mass. In this setting, the two most common lesions are small renal cell carcinomas and angiomyolipomas. The latter are benign connective tissue tumors that do not pose any risk to the recipient irrespective of their size [21].

Other Donor-Related Issues

Fig. 31.3 Masson’s trichrome-stained biopsy from a donor with chronic hypertensive disease (extended criteria donor). The tubules are separated by an abnormal amount of connective tissue which is highlighted by the collagen stain. An arteriole with a thick wall and narrowed rounded lumen is noted in the top center. The H&E-stained insert demonstrates an interlobular artery with fibrointimal sclerosis (arteriosclerosis)

A variety of other processes can be identified in pretransplant donor biopsies, one of the most common being mesangial IgA deposition and membranous nephropathy, both of which appear not to have a negative post-transplantation impact. Similarly, other forms of mild glomerulonephritis can resolve after transplantation [22]. In addition, we have seen successful outcomes after transplantation of kidneys with mild sarcoidosis and eclampsia-related glomerular injury. The decision about transplanting organs with known pathological processes is made after careful consideration, on a case-by-case basis.

Living Donor Evaluation Hypertensive Disease The most common donor-related histological abnormalities are secondary to long-standing arterial hypertension [17]. The large and small arteries show sclerosis and sclerosis with hyalinosis, respectively, leading to various degrees of luminal narrowing (Fig. 31.3). The interstitium shows increased fibrosis and there is corresponding atrophy of the tubular components. Subcapsular ischemic scars as well as a significant number of scattered obsolescent glomeruli are also indicative of chronic hypertension (nephrosclerosis). The Maryland aggregate pathology index (MAPI) is a histological scoring system based on the presence or absence of five morphological features associated with hypertensive/chronic ischemic renal parenchymal injury (hyalinosis of glomerularsize arterioles, >15 % global glomerulosclerosis, significant arterial sclerosis, periglomerular fibrosis, and cortical scar) which are assigned scores according to their relative significance. The MAPI has been shown to be predictive of posttransplant graft survival or failure [16]. Donor Diabetes

Living donors may present with potentially significant renal disease requiring careful evaluation before donation. The risk factors for renal disease associated with living-donation have been characterized [23]. Kidney biopsies are more often performed for hematuria in otherwise healthy living donors. In this context the identification of thin basement membrane disease is common; in general terms this process is not considered per se a contraindication for donation.

Post-implantation Biopsies: Time 0 Biopsy Similar to donor harvest biopsies, baseline graft biopsies performed before or after reperfusion provide essential information about the status of the organ at the time of transplantation, in particular regarding the presence of underlying chronic vascular disease (donor hypertensive disease) [22]. Microvascular and/or glomerular thrombosis in implantation biopsies correlates with ischemic injury and was associated with higher risk of rejection in one study [24].

Approximately 4 % of all kidney transplants in the United States are derived from diabetic donors [17, 18]. Identification of fully developed nodular diabetic glomerulosclerosis in a donor kidney biopsy is a contraindication for transplantation; however, milder diabetic glomerular involvement is compatible with acceptable outcomes [18, 19].

Biopsies for Graft Dysfunction in the Early Post-transplant Period

Renal Cell Carcinoma With the increased utilization of organs from older donors there has been an increase in the identification of renal cell carcinomas both in deceased and living donors (≈1 %). Organs with small, completely resected, low grade tumors

Acute Tubular Necrosis Biopsies from patients with delayed graft function (DGF) in the early post-transplant period usually show acute tubular necrosis (ATN). This injury is presumed to be ischemic in nature as a result of the organ procurement procedures and

Delayed Graft Function


Fig. 31.4 Acute tubular necrosis. Marked dilatation of the tubular lumina and flattening (simplification) of the tubular epithelium are noted on the left image. Compare with Fig. 31.1 where the normal tubular cells are tall and fill most of the luminal space. The image on the right shows features of tubular injury and scattered tubular microcalcifications, which tend to develop in areas of tubular necrosis

reperfusion injury. Tubular injury can be subtle, consisting only of cytoplasmic vacuolization, loss of brush border, and isolated cell necrosis or apoptosis. In more pronounced cases there is frank disintegration and sloughing of the tubular cells that can accumulate as debris in the tubular lumina. When there is pronounced tubular cell loss there is flattening of the remaining tubular lining and partial denudation of the basement membranes. Tubular calcifications are common in areas of previous tubular injury (Fig. 31.4). Interstitial edema and mild interstitial inflammation, often including some eosinophils, can be present in ATN. If the inflammation is significant and there is tubulitis, a concurrent acute cellular rejection should be suspected. The presence of multifocal neutrophilic capillaritis and/or microscopic thrombi requires correlation with the C4d stain and DSA studies to rule out ATN associated with early acute AMR.

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Fig. 31.5 Cortical infarct. Ischemic necrosis of the renal cortex is appreciated on the left side of the image (loss of nuclear profiles). On the right a band of neutrophilic infiltrates demarcates the edge of the infarct

Fig. 31.6 Thrombotic microangiopathy (TMA) leading to glomerular necrosis in a pre-sensitized patient with early acute AMR (a), and similar changes due to ischemic endothelial injury in a patient with prolonged cold ischemia time (b)

Coagulation Necrosis: Infarction Severe ischemic injury often secondary to hemodynamic issues or surgical complications may result in focal of diffuse parenchymal necrosis. Localized infarcts are often unsuspected clinically and may become evident only in a needle biopsy for evaluation of DGF. Identification of infarction in core biopsies is usually associated with significant ischemic injury and always leads to subsequent fibrous scarring of the infarcted areas (Fig. 31.5). Therefore, depending on the extent of parenchyma involved, infarcts may be associated with poor graft function or primary non-function.

Ischemic Endothelial Injury In patients with DGF, glomerular endothelial cell swelling and microthrombi that may mimic early acute AMR may result from ischemic injury to endothelial cells [25, 26]

(Fig. 31.6). These changes are usually reversible unless the ischemic process has also led to multifocal areas of infarction. In patients with microvascular changes suspected to represent ischemic endothelial injury, examination of a C4d stain and serological studies to document the absence of DSA are necessary to confirm the purely ischemic nature of the injury.

Preexisting Donor Hypertensive Vascular Disease Prolonged DGF or poor renal function may be the result of suboptimal perfusion of the graft due to preexisting vascular sclerosis (narrowing) [16]. In addition to the donor-related vascular sclerosis these biopsies may show frank ATN or milder forms of tubular injury (Fig. 31.4).

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Fig. 31.7 Isometric tubular vacuolization. The tubular cells have a “foamy,” clear cytoplasm with accumulation of multiple vacuoles. Tubular injury secondary to multiple etiologies (i.e., calcineurin inhibitor [CNI] toxicity, ischemia, osmotic nephrosis) can manifest with this change

Suboptimal Graft Function Suboptimal or slow graft function is a common indication for early KTxBx in patients that do not require dialysis but that continue to have poorer than expected renal function. The most common pathological findings in these patients are tubular injury (i.e., tubular cell vacuolization, ATN-like changes, etc.), and donor-related issues, particularly related to chronic hypertensive donor disease (e.g., patches of cortical scarring, vascular sclerosis, and arteriolar hyalinosis) or less commonly related to preexisting conditions such as donor diabetes. In patients with persistent tubular injury, ongoing ischemia (i.e., stenosis of the renal artery anastomosis), urinary obstruction, and drug toxicity should be ruled out clinically. Acute calcineurin inhibitor (CNI) toxicity can be the cause of suboptimal graft function in the early post-transplant period. Isometric tubular vacuolization (finely vacuolated cytoplasmic droplets), a feature typically associated with acute CNI toxicity, is not specific and can be observed in most other forms of tubular injury including ischemic injury and in osmotic nephrosis (Fig. 31.7). A more specific lesion associated with high levels of CNI consists of ballooning, apoptosis or necrosis, and drop-out of arteriolar myocytes. These uncommon changes are dose related and are typically reversible with dose adjustments [27] (Fig. 31.8). Thrombotic microangiopathy (TMA), most commonly idiopathic, or considered to be secondary to CNI or rapamycin toxicity, may be associated with early graft dysfunction. TMA is discussed in detail in section “Thrombotic microangiopathy” [28, 29].

Fig. 31.8 Myocyte injury due to CNI toxicity. High power view of a pre-glomerular arteriole shows marked vacuolization of some myocytes (arrows). Some of the clear spaces represent myocyte cell necrosis (drop-out); note scattered granular structures consistent with nuclear fragments/apoptotic bodies

Acute Allograft Rejection Early Acute AMR The most characteristic lesions of early acute AMR result from lytic endothelial cell graft injury due to damage by alloantibodies (i.e., anti-HLA DSA). Specifically, circulating DSA in conjunction with complement activation leads to endothelial cell swelling and necrosis with superimposed formation of microthrombi. Accumulation of inflammatory cells (neutrophils and macrophages) in glomerular and PTC appearing as glomerulitis and capillaritis is also a common feature of acute AMR [4]. Early acute AMR develops in a proportion of presensitized patients despite conditioning protocols and may also occur in the early post-transplantation period in patients with no apparent risk for AMR [30]. In contrast to milder forms of early acute AMR which are characterized by attenuated endothelial injury that may even resemble bland ATN, in hyperacute rejection, the most severe form of AMR, there are abundant preformed circulating antibodies leading to diffuse arterial and venous necrosis with secondary thrombosis and parenchymal infarction [4]. A diagnosis of pure acute AMR requires exclusion of features of T-cell-mediated rejection (CMR). The current Banff classification of acute AMR is based on the combination of (1) C4d+, (2) the presence of circulating

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31 Practical Renal Allograft Pathology Table 31.3 Banff 97 diagnostic categories for renal allograft biopsies—Banff ‘09 update

1. Normal 2. Antibody-mediated changes (may coincide with categories 3, 4 and 5 and 6) Due to documentation of circulating antidonor antibody, C4da, and allograft pathology C4d deposition without morphologic evidence of active rejection C4d+, the presence of circulating antidonor antibodies, no signs of acute or chronic CMR or AMR (i.e., g0, cg0, ptc0, no ptc lamination (0) and/or thromboses III. Arterial—v3 Chronic active antibody-mediated rejectionb C4d+, the presence of circulating antidonor antibodies, morphologic evidence of chronic tissue injury, such as glomerular double contours and/or peritubular capillary basement membrane multilayering and/or interstitial fibrosis/tubular atrophy and/or fibrous intimal thickening in arteries 3. Borderline changes: “Suspicious” for acute T-cell-mediated rejection (may coincide with categories 2 and 5, and 6) This category is used when no intimal arteritis is present, but there are foci of tubulitis (t1, t2, or t3) with minor interstitial infiltration (i0 or i1) or interstitial infiltration (i2, i3) with mild (t1) tubulitis 4. T-cell-mediated rejection (CMR, may coincide with categories 2 and 5 and 6) Acute T-cell-mediated rejection (type/grade): IA. Cases with significant interstitial infiltration (>25 % of parenchyma affected, i2 or i3) and foci of moderate tubulitis (t2) IB. Cases with significant interstitial infiltration (>25 % of parenchyma affected, i2 or i3) and foci of severe tubulitis (t3) IIA. Cases with mild to moderate intimal arteritis (v1) IIB. Cases with severe intimal arteritis comprising >25 % of the luminal area (v2) III. Cases with “transmural” arteritis and/or arterial fibrinoid change and necrosis of medial smooth muscle cells with accompanying lymphocytic inflammation (v3) Chronic active T-cell-mediated rejection “Chronic allograft arteriopathy” (arterial intimal fibrosis with mononuclear cell infiltration in fibrosis, formation of neo-intima) 5. Interstitial fibrosis and tubular atrophy, no evidence of any specific etiology (may include nonspecific vascular and glomerular sclerosis, but severity graded by tubulointerstitial features) Grade I. Mild interstitial fibrosis and tubular atrophy (50 % of cortical area) 6. Other: Changes not considered to be due to rejection—acute and/or chronic (for diagnoses see table 14 in [49]; may include isolated g, cg, or cv lesions and coincide with categories 2, 3, 4, and 5) Modified from ref. [4] The 2009 updates are underlined. All existing scoring categories (g glomerulitis, t tubulitis, v intimal arteritis, i mononuclear interstitial inflammation, ptc peritubular capillaritis, cg allograft glomerulopathy/double contours, ct tubular atrophy, ci interstitial fibrosis, cv vascular fibrointimal thickening) remain unchanged [5, 31] ATN acute tubular necrosis a Please refer to Banff 2007 classification paper [31] b Suspicious for antibody-mediated rejection if C4d (in the presence of antibody) or alloantibody (C4d+) not demonstrated in the presence of morphologic evidence of tissue injury

antidonor antibodies, and (3) morphologic evidence of acute tissue injury (see Table 31.3 [4, 31]). Acute AMR is categorized in types or grades I, II, or III (I. ATN-like minimal inflammation, II. capillary and or glomerular inflammation and/or thromboses, and III. arterial necrosis). In essence, the Banff Category I (C4d+ and ATN-like changes only) represents a milder form of Category II (characterized additionally by microvascular inflammation and microthrombi). Category III is defined by arterial fibrinoid necrosis resembling lesions seen in hyperacute rejection and therefore represents a more severe form of AMR in comparison to Categories I and II.

Differential diagnosis of acute AMR: The most important differential diagnosis of early acute AMR is ischemic injury leading to microvascular thrombosis [25, 26] (Fig. 31.6). History of prolonged cold ischemia time and/or surgical complications may support the impression of ischemic graft injury; however, evaluation of C4d staining and DSA studies are essential to rule out a concurrent component of acute AMR. In our experience some degree of C4d staining may be present in early biopsies with ischemic injury, but this typically resolves in subsequent biopsies. Renal function recovers in most patients with ischemic endothelial injury, unless

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Fig. 31.9 (a) Acute CMR (Type I). The interstitium contains dense, active inflammatory infiltrates. The tubule in the center is partially destroyed by severe tubulitis. (b) Acute CMR with severe tubulitis


(Type 1B). CD3 demonstrates numerous T-lymphocytes that have permeated the tubular basement membranes

there is very extensive microvascular thrombosis or infarcts which would eventually lead to diffuse glomerular sclerosis and formation of scars. TMA of other etiologies—including CNI toxicity—may also require differentiation from acute AMR (see section “Thrombotic Microangiopathy”).

Acute CMR Acute CMR is characterized by interstitial T-cell and macrophage infiltrates, tubular inflammation (tubulitis), and inflammation of the arterial endothelium (intimal arteritis). A diagnosis of pure acute CMR requires exclusion of AMR (evaluation of C4d stain and negative DSA studies). The current Banff classification of acute CMR uses the following categories: Borderline—suspicious, Type I (A and B), Type II (A and B), and Type III (see Table 31.3). Borderline changes: “Suspicious” for acute CMR is used when no intimal arteritis is present, but there are foci of tubulitis with minor interstitial infiltration or interstitial infiltration with only mild tubulitis. On the other hand, Types I–III are considered diagnostic of acute CMR. In essence, Type I rejection is defined by tubulitis (Fig. 31.9), Type II is defined by arterial inflammation without necrosis (intimal arteritis, endarteritis) (Fig. 31.10), and Type III is characterized by arterial necrosis and accompanying lymphocytic inflammation (Fig. 31.11). Whereas Type II CMR (intimal arteritis) is the most diagnostic form, Type I requires differentiation from other causes of graft inflammation (i.e., PVN, infections, nonspecific inflammation) and Type III requires differentiation from AMR due to overlap of necrotizing vascular lesions in the severe forms of CMR and AMR. Differential diagnosis of CMR: In the early posttransplantation period when most transplants lack significant nonspecific changes acute rejection reactions can be diagnosed with relative ease. In the late post-transplant period,

Fig. 31.10 Acute CMR (Type II). Interlobular artery with intimal arteritis characterized by subintimal accumulation of lymphocytes and associated endothelial cell damage. Inset: CD3 stain highlights T-lymphocytes in the intimal arteritis. Interstitial perivascular inflammation is also prominent

Fig. 31.11 Acute CMR with arterial necrosis (Type III). In addition to prominent interstitial infiltrates and tubulitis, an artery shows transmural necrosis (arrow). Note scattered lymphoid cells in the vascular wall and an area of intimal arteritis


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Fig. 31.12 Cortical scarring. A focal scar with prominent chronic inflammation is noted in the right area of the top image. Extensive, diffuse scarring is noted in the lower image, also associated with marked chronic inflammation

nonspecific features of chronicity secondary to multiple immunological and non-immunological insults greatly complicate pathological interpretations. In particular, scarred areas often show interstitial inflammation with some degree of tubulitis in atrophying tubules. Intimal arteritis is indicative of alloimmune injury and is considered one of the most important features of acute CMR (see section “Spectrum of Arterial Changes”). Biopsies with atypical features should always raise the suspicion for a combined process (i.e., mixed AMR and CMR). In the absence of arterial involvement, tubulointerstitial inflammation alone may also be secondary to infections (section “Infections”), obstruction, scarring from previous injury, etc. Inflammation identified in scars is typically considered nonspecific and unresponsive to treatment, therefore of little diagnostic value (Fig. 31.12). Two issues, however, should be considered in biopsies with extensive scarring: (1) it is important to assess if the specimen is representative of the clinical presentation and of the kidney in general (i.e., a repeat biopsy is indicated if only inflamed scars are sampled in a kidney with ideal function because the specimen is likely not representative) and (2) inflamed scars may hold the clue to a specific diagnosis (i.e., evolving PV allograft nephropathy, evolving obstructive nephropathy); evaluation of SV40 stain, correlation with ultrasound findings, etc. will provide important information in these cases.

In rare transplant patients biopsied for acute graft dysfunction there are clinical features suggesting the possibility of allergic interstitial nephritis (AIN) (e.g., exposure to allergenic drug(s), peripheral eosinophilia, rash). Because AIN and acute CMR Type I (tubulointerstitial inflammation) cannot be differentiated on morphological grounds, a diagnosis of AIN rests mainly on the clinicopathological correlations and subsequent follow-up [32].

Early Recurrence of Primary Glomerular Disease (Also See Section “Biopsies for Graft Dysfunction in the Late Post-transplant Period”) In the early post-transplantation period, recurrent focal segmental glomerulosclerosis (FSGS) may manifest within hours or days with increasing proteinuria. Graft dysfunction, including anuria, may also be present in the most severe cases. The earliest morphological evidence of recurrent FSGS requires electron microscopic studies in order to demonstrate effacement of the foot processes of podocytes. Progression of the FSGS is associated with appearance of fully developed segmental sclerotic lesions (Fig. 31.13). Recurrence of hemolytic uremic syndrome (HUS) may also manifest in early biopsies with the typical thrombotic microangiopathic features [33].

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Fig. 31.13 Recurrent focal segmental glomerulosclerosis (FSGS). Periodic acid stain (PAS)-stained section of a biopsy performed 3 weeks post-transplantation shows development of segmental sclerotic lesions (glomerulus on the right) in a patient who presented with recurrence of FSGS within days of transplantation


Fig. 31.14 Protocol biopsy with interstitial infiltrates and tubulitis: subclinical acute CMR. PAS highlights the outline of the tubular basement membranes. Several lymphocytes have gone through the basement membrane and are located amidst the tubular epithelial cells. Lymphocytes are differentiated from the epithelial cells by their darkly stained nuclei with condensed chromatin

Protocol Biopsies Protocol or surveillance biopsies are performed electively in normally functioning grafts for the purpose of early recognition and treatment of pathological processes. Subclinical acute allograft rejection, either CMR or AMR, can be found in a significant proportion of protocol biopsies [34, 35] (Figs. 31.14 and 31.15). The findings in subclinical AMR have been emphasized in a recent study that demonstrated a wide spectrum of lesions ranging from pure microvascular inflammation (glomerulitis, capillaritis) to well-developed chronic lesions (transplant glomerulopathy [TG]) [35].

Biopsies for Graft Dysfunction in the Late Post-transplant Period Chronic Active AMR Two broad categories of late-chronic AMR can be identified: (1) chronic microvascular injury developing in approximately 40 % of highly sensitized patients with or without history of early acute AMR and (2) chronic microvascular injury developing in approximately 20 % of non-sensitized patients who develop de novo DSA. In both settings most patients experience eventual progression to graft failure, but the rate of progression and the histological manifestations vary considerably from one patient to another and in serial biopsies from the same patient [11, 26, 30, 35–37].

Fig. 31.15 Subclinical AMR. Protocol biopsy at 12 months posttransplantation in a 62-year-old woman found to have multiple antiHLA donor-specific antibody (DSA) at the time of the biopsy. There is glomerulitis characterized by narrowing of the glomerular capillary lumina due to endocapillary hypercellularity and swelling of endothelial cells. Also note development of transplant glomerulopathy (duplication of glomerular basement membranes [GBM] at 11 o’clock)

The current Banff schema (Tables 31.3 and 31.4) defines chronic active AMR as follows: (1) C4d+, (2) the presence of circulating antidonor antibodies, and (3) morphologic evidence of chronic tissue injury, such as glomerular double contours and/or PTC basement membrane multilayering and/or interstitial fibrosis/tubular atrophy (IF/TA) and/or fibrous


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Table 31.4 Revised [62] classification of antibody-mediated rejection (ABMR) in renal allografts Acute/Active ABMR; all three features must be present for diagnosisa,b 1. Histologic evidence of acute tissue injury, including one or more of the following – Microvascular inflammation (g > 0 and/or ptc > 0) – Intimal or transmural arteritis (v > 0) – Acute thrombotic microangiopathy, in the absence of any other cause – Acute tubular injury, in the absence of any other apparent cause 2. Evidence of current/recent antibody interaction with vascular endothelium, including at least one of the following – Linear C4d staining in peritubular capillaries – At least moderate microvascular inflammation ([g + ptc] ≥ 2) – Increased expression of gene transcripts in the biopsy tissue indicative of endothelial injury 3. Serologic evidence of donor-specific antibodies (HLA or other antigens) Chronic, Active ABMR; all three features must be present for diagnosisa,c 1. Morphologic evidence of chronic tissue injury, including one or more of the following – Transplant glomerulopathy (cg > 0), if no evidence of chronic TMA – Severe peritubular capillary basement membrane multilayering (requires EM) – Arterial intimal fibrosis of new onset, excluding other causesj 2. Evidence of current/recurrent antibody interaction with vascular endothelium, including at least one of the following – Linear C4d staining in peritubular capillaries (C4d2 or C4d3 by IF on frozen sections or C4d > 0 by IHC on paraffin sections) – At least moderate microvascular inflammation ([g + ptc] ≥ 2) – Increased expression of gene transcripts in the biopsy tissue indicative of endothelial injury, if thoroughly validated 3. Serologic evidence of donor-specific antibodies (HLA or other antigens) C4d Staining without Evidence of Rejection; all three features must be present for diagnosisd 1. Linear C4d staining in peritubular capillaries (C4d 2 or C4d3 by IF on frozen sections, or C4d > 0 by IHC on paraffin sections) 2. g = 0, ptc = 0, cg = 0 (by LM and by EM if available), v = 0, no TMA, no peritubular capillary basement membrane multilayering, no acute tubular injury (in the absence of another apparent cause for this) 3. No acute cell-mediated rejection (Banff 97 type 1A or greater) or borderline changes Adapted from Hass M., et al. Am J Transpl 2014 (in press) g, cg, ptc, v, C4d correspond to Banff scores described on the Banff 97 schema (Racusen, LC et al. Kidney Int 1999 Feb;55(2):713–23) LM light microscopy, EM electron microscopy, TMA thrombotic microangiopathy, IF immunofluorescence, IHC Immunohistochemistry a For all ABMR diagnosis, it should be specified in the report whether the lesion is C4d-positive (C4d2 or C4d3 by IF on frozen sections; C4d> by IHC on paraffin sections) or without evident C4d deposition (C4d0 or C4d1 by IF on frozen sections; C4d0 by IHC on paraffin sections b These lesions may be clinically acute, smoldering or subclinical. Biopsies showing two of the three features, except those with DSA and C4d without histologic abnormalities potentially related to ABMR or TCMR (C4d staining without evidence of rejection may be designated as “suspicious” for acute active ABMR c In the absence of evidence of current/recent antibody interaction with the endothelium (those features in section 2), the term active should be omitted d The clinical significance of these findings may be quite different in grafts exposed to anti-blood-group antibodies (ABO-incompatible allografts)

intimal thickening in arteries. Although all the morphological elements listed are known to be associated with chronic AMR, only the ones pertaining to the microvasculature are specific enough of AMR. Transplant glomerulopathy, characterized by extensive remodeling of the glomerular microvasculature with duplication of the glomerular basement membranes (GBM), is the most characteristic, albeit advanced, feature of chronic AMR (Figs. 31.16 and 31.17). Microvascular inflammation (glomerulitis, capillaritis) typically precedes the advanced chronic changes and has been demonstrated to correlate with graft outcome in several studies [12, 13, 35, 38].

Early remodeling of the glomerular microvasculature (development of double contours in the GBM) can only be appreciated on electron microscopic studies. More advanced duplication is appreciated by light microcopy with special stains for basement membranes (PAS, PAMS). Multilamellation of the basal lamina of PTC correlates with transplant glomerulopathy and also indicates chronic microvascular injury and remodeling. This hallmark feature of chronic AMR can only be appreciated on electron microscopy (Fig. 31.18). The 2013 Banff schema update (Table 31.4) provides detailed guidelines for the diagnosis of AMR, including acute/active and chronic forms of the disease.

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Fig. 31.16 Transplant glomerulopathy. PAMS (silver stain) demonstrates duplication of GBM in multiple glomerular loops. On the right, an electron micrograph demonstrates transplant glomerulopathy, char-


acterized by GBM duplication with interposition of cellular components and matrix, between the two layers of basement membrane. The capillary lumen is occluded by a markedly swollen endothelial cell

Fig. 31.17 Global involvement by glomerulitis, leading to occlusion of most capillary lumina. On the right, the same glomerulus stained with CD68 stain demonstrates large numbers of macrophages which typically accumulate in the microvasculature in patients with chronic AMR

Chronic CMR The 2013 Banff schema update (Table 31.4) provides detailed guidelines for the diagnosis of AMR, including acute/active and chronic forms of the disease. Typical acute CMR promptly responds to treatment [5] and leaves significant sequelae only in cases with very extensive or protracted tubulitis (results in tubular atrophy) or severe multifocal intimal arteritis (leads to arterial sclerosis). The current Banff schema defines the category of chronic active CMR as “chronic allograft arteriopathy” if there is identification of arterial intimal fibrosis with mononuclear cell infiltration in fibrosis or formation of neo-intima [39, 40] (Fig. 31.19). This category, which is likely to be modified in the near future as allograft arteriopathy, can also be associated with AMR.

IF/TA and arteriosclerosis can result from other etiologies and are not specific for chronic CMR (Fig. 31.20) (see sections “Chronic Changes: Graft Sclerosis” and “Spectrum of Arterial Changes”).

Biopsy Findings in the Setting of Noncompliance Acute CMR in the late transplant period is most commonly associated with decrease in immunosuppression, either medically indicated (i.e., severe infections, tumors) or due to patient noncompliance. In many cases the typical features of acute CMR are partially obscured by evolving chronic injury (parenchymal scarring, dense inflammatory infiltrates in scarred areas,


Fig. 31.18 Electron micrographs of peritubular capillaries (PTC). On the left, a normal PTC with a delicate basal lamina, closely apposed to the corresponding tubular basement membranes. On the right, PTC with exten-

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sive remodeling (multilamellation of the basal lamina), typical of biopsies with chronic AMR. There is increase in interstitial connective tissue leading to an abnormal separation of the PTC from its corresponding tubule

Glomerular Diseases

Fig. 31.19 Active transplant arteriopathy. Artery with severe fibrointimal thickening and transmural accumulation of inflammatory cells

nonspecific tubulitis in atrophying tubules, etc.). Clinicopathological correlations, comparison with previous biopsies, etc. are necessary in order to arrive at the best diagnosis in these cases. In patients that have interrupted immunosuppression (noncompliance), it is common to see combined features of acute CMR and acute AMR including C4d positivity in PTC due to appearance of circulating DSA [41] (Fig. 31.21).

In addition to transplant glomerulopathy, there are numerous glomerular diseases that affect the allografts at various time points after transplantation. Specifically, FSGS, membranoproliferative glomerulonephritis (MPGN) type 1, dense deposit disease, membranous glomerulopathy, and HUS are all known to recur after transplantation and in most cases their identification portends poor prognosis for the graft. Hepatitis C infection negatively impacts patient and graft outcomes overall; however, hepatitis C-related GN is a rare cause of graft failure per se [ 42 ]. It is important nevertheless to emphasize the significant morphological similarities between hepatitis C GN and transplant glomerulopathy, the latter being most commonly secondary to chronic AMR in patients with documented DSA [ 43 ]. Recurrent or de novo diabetic glomerulosclerosis is a common finding in long-term KTxBx and less commonly can also be seen within the first years after transplantation [44]. De novo FSGS is common in long-term transplants, appearing in approximately a third of patients with allograft scarring (interstitial fibrosis and tubular atrophy). FSGS in this context is typically secondary (adaptive) to nephron loss and glomerular hyperfiltration [ 45 , 46 ].

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Fig. 31.20 Masson’s trichromestained biopsy from a graft with moderate IF/TA. Note patches of preserved cortical parenchyma in which clusters of proximal tubules can be recognized in contrast to the areas with extensive tubular atrophy/fibrosis

Fig. 31.21 Mixed acute CMR and AMR. Biopsy from a patient who voluntarily interrupted all immunosuppression medications 4 weeks earlier. There is tubulitis indicating acute CMR, as well as capillaritis (arrow) and diffuse C4d positivity in PTC (inset)

Infections PV-associated allograft nephropathy (PVN) is caused by the BK virus in most instances and is characterized by the presence of viral cytopathic changes in tubular cells. The nuclei of the infected cells are large, hyperchromatic, and contain a large basophilic, “gelatinous” inclusion (Fig. 31.22). Immunohistochemical stains or in situ hybridization for SV40 are necessary to confirm the presence of polyoma

virus in the intranuclear inclusions. In the early stages of PVN the viral cytopathic changes are associated with minimal or no inflammation. PVN is a multifocal process that can be missed in the early stages due to sampling. As the disease progresses, patches of dense mononuclear inflammation, sometimes granulomatoid in nature, develop associated with ongoing virally induced tubular damage and evolving tubular atrophy/loss. Viral inclusions are very sparse or may be absent in the late stages of PVN [9]. Concurrent evaluation of urine cytology for the presence of “decoy cells” or determination of BK viruria is a useful adjuvant in the diagnosis but cannot replace viremia as the most significant marker of parenchymal involvement in PVN. Persistent BK viremia of >10,000 viral copies/mL (for several weeks) is indicative of PVN, even if an allograft biopsy is negative for SV40-infected cells (presumptive PVN). Patients with uncontrolled PVN have increasing amounts of BK viral DNA in blood. Serial quantitation of BK viral load in plasma is critical for diagnosis, and is used to monitor the response to treatment [47]. Other manifestations of BK virus infections in kidney transplant patients include symptomatic PV cystitis, ureteritis leading to ureteral obstruction, and polyomavirusassociated neoplasms [48]. JC PVN, a rare complication of immunosuppression, has similar morphological and clinicopathological features as BK PVN. Although the same diagnostic algorithm applies to JC PVN viremia is typically low, in comparison [49]. Patients with long-standing polyomavirus replication are at higher risk of developing carcinomas of the urinary tract [50] (Fig. 31.22).


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Fig. 31.22 (a) Polyomavirus allograft nephropathy (PVN). On the left, H&E stain demonstrates a tubule lined by cells with dark, enlarged nuclei strongly suggestive of viral cytopathic changes. On the right, SV40 stain demonstrates numerous nuclei of PV-infected cells. (b) Polyomavirus-associated bladder tumor in a patient with long-standing BK infection. (a) H&E stain; (b) SV40 stain with diffuse staining of the tumor cells; and (c) Papanicolaou stain of urine cytology with tumor fragment. The abnormal urine cytology findings lead to the diagnosis of the tumor

CMV CMV infection is currently very rare. The diagnosis is made by the identification of enlarged cells with the typical cytopathic changes. In these cells the large nucleus contains a prominent eosinophilic inclusion often separated by the nuclear membrane by a halo (“owl’s eye” appearance) and the abundant enlarged cytoplasm contains smaller granular reddish inclusions. The cytopathic effects of CMV infection can be observed most often in the glomerular endothelial cells leading to CMV glomerulitis in which the enlarged

infected endothelial cells together with reactive changes in adjacent endocapillary cells lead to occlusion of the glomerular capillary lumina (Fig. 31.23). In contrast, in CMV tubulointerstitial nephritis there are enlarged infected tubular cells with associated tubulointerstitial mononuclear inflammation. Immunohistochemical stains for CMV confirm the presence of the virus in the cells with cytopathic changes [51] (Fig. 31.23). Adenovirus allograft infection should also be considered in the morphological differential diagnosis of viral cytopathic changes in KTxBx. Renal manifestations of this infection

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Fig. 31.23 Cytomegalovirus (CMV) nephritis. H&E-stained glomerulus on the left contains a CMV-infected cell (arrow) and prominent glomerulitis. Inset shows tubular cells with typical nuclear and cytoplasmic CMV inclusions. On the left, immunostain for CMV in a glomerulus with minimal glomerulitis highlights a cluster of CMV-infected cells

Fig. 31.24 Post-transplant lymphoproliferative disorder is characterized by extensive, confluent inflammatory infiltrates that show more cytological atypia and more destructive growth in comparison to the infiltrates in acute rejection

are very rare and are typically associated with systemic symptoms. Adenovirus graft nephropathy presents with prominent tubulointerstitial inflammation which may be granulomatous and is often destructive. The diagnosis is confirmed with adenovirus immunohistochemical stains or in situ hybridization [52].

atypia are very helpful for the diagnosis [53] (Fig. 31.24). Immunohistochemical stains for EBV-related proteins to identify expression on the B-lymphocytes composing the PTLD are routinely done to confirm the diagnosis. Monomorphic PTLD is evaluated with the tools used to characterize lymphomas.

EBV

Bacterial and Fungal Pyelonephritis

EBV-related PTLD is characterized by polymorphic (mixed cellularity) or monomorphic predominantly B-cell inflammatory infiltrates. PTLD involves the renal parenchyma forming nodular, expansile, and sometimes destructive lesions. This is in contrast to the predominantly T-cell infiltrates in typical acute rejection. The presence of necrosis and cytological

Acute pyelonephritis. In bacterial infections the tubulointerstitial inflammation may be associated with necrosis and neutrophils are the predominant cell type [54] (Fig. 31.25). Clusters of neutrophils in the tubular lumina (purulent casts) are typical of acute pyelonephritis. Fungal infections have overlapping features with bacterial infections but may show


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Fig. 31.25 Acute bacterial pyelonephritis is characterized by a predominance of neutrophilic infiltrates in the interstitium and in the tubules. A large purulent cast is noted in this image

Fig. 31.27 De novo TMA in a patient with CNI toxicity. The hilum of this glomerulus is distended and occluded by a thrombus

Fig. 31.26 Fungal pyelonephritis is characterized by prominent lymphocytic and macrophage infiltrates, sometimes forming granulomas. Inset, silver stain demonstrates clusters of fungal spores within the inflamed area

Fig. 31.28 Chronic TMA is characterized by thickening of the arterial walls, mostly secondary to edematous (myxoid) fibrointimal proliferation

a predominance of mononuclear inflammation and granulomas (Fig. 31.26). Although sometimes special stains (e.g., PAS for fungi) can identify the organism in question, microbiology studies and clinical correlation are essential for the diagnosis.

related to endothelial injury with swelling, associated with dissolution of the mesangium (mesangiolysis) and fragmented endothelial cells. The arteriolar walls may show fibrinoid necrosis. Chronic TMA (late findings) results from organization of acute lesions and appears with areas of segmental glomerular sclerosis, mesangial expansion, and duplication of the capillary basement membranes (double contours). Chronic TMA manifests in interlobular size arteries with mucoid intimal hyperplasia (accumulation of edematous paucicellular amorphous material in the intima) which leads to marked narrowing of the lumen (Fig. 31.28). Generalized chronic ischemic parenchymal changes are often present.

Thrombotic Microangiopathy Morphological Definition Acute TMA (early findings) is characterized by fibrin thrombi in glomerular capillaries and/or arterioles (Fig. 31.27). The glomerular tufts show various changes

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In KTxBx TMA Is Associated with Various Clinical Settings TMA in the context of DGF may have the following etiologies: (a) early acute AMR which is typically associated with neutrophilic peritubular capillaritis, diffuse C4d positivity, and DSA (Fig. 31.6); (b) ischemic endothelial injury which often presents with heterogeneous parenchymal involvement (normal appearing glomeruli alternating with extensive thrombotic glomerular capillary injury) (Fig. 31.6); (c) CNI toxicity often associated with high drug levels; and (d) recurrence of primary HUS [33]. TMA in patients with DSA is important at all times posttransplantation, often leading to accelerated graft failure. Acute TMA lesions predominate in early acute AMR; however, acute and chronic lesions typically coexist in patients with late-chronic AMR. C4d positivity is variable in the latter setting [13, 26, 30]. “Idiopathic” or de novo TMA is often attributed to CNI or rapamycin drug toxicity. This diagnosis can be made with relative certainty in patients with toxic levels of CNI agents or if the lesions disappear after discontinuation of the drug; however, all other etiologies (in particular AMR) need to be excluded [27, 28, 55]. Recurrence of primary HUS is high in patients with genetic abnormalities of the complement system. Genetic abnormalities of complement factors H and I have been associated with de novo TMA. It has been suggested that genetic complement abnormalities may represent an important risk factor for drug toxicity-related TMA [33].

Fig. 31.29 Severe arteriolar hyalinosis. There is very marked thickening of the wall due to extensive accumulation of amorphous extracellular material (hyalin). The lumen is markedly narrowed (arrow)

considered features of chronic CNI toxicity [57] (Fig. 31.29). In part because these pathological features are nonspecific, it has been suggested that the prevalence of CNI toxicity has been overestimated [58, 59]. In the appropriate clinical setting, newly developing nodular arteriolar hyalinosis and striped fibrosis are often considered an indication of CNI toxicity (Fig. 31.30).

Spectrum of Arterial Changes Chronic Changes: Graft Sclerosis Variable degrees of parenchymal fibrous scarring (interstitial fibrosis, tubular atrophy, arterial sclerosis, arteriolar hyalinosis, etc.) are universally observed in long-term renal allografts, even in patients that never had clinically evident episodes of acute rejection or acute graft dysfunction. With careful clinicopathological correlations, however, allograft failure can be attributed to a specific etiology in the vast majority of patients [56]. The Banff schema provides the descriptive diagnosis of IF/TA to be applied when there is no evidence of any specific etiology. IF/TA is graded as follows: I. mild interstitial fibrosis and tubular atrophy (50 % of cortical area).

Chronic CNI Toxicity Interstitial fibrosis, tubular atrophy, glomerulosclerosis (focal segmental and global), and arteriolar hyalinosis are

The arterial branches in KTxBx provide important information about the status of the graft and their lesions correlate with a variety of diagnostic entities. Inflammation of the arterial endothelium (intimal arteritis, endarteritis) used to be considered the most specific diagnostic feature of acute T-cell-mediated allograft rejection. More recently it is widely accepted that this lesion is also associated with the presence of DSA and AMR in which case the presence of arterial inflammation denotes a worse prognosis. From the practical point of view, the dual implication of intimal arteritis (for both CMR and AMR) highlights the importance of correlating the KTxBx findings with concurrent DSA studies [39, 40]. When arteriosclerosis is identified in very early KTxBx it most likely represents hypertensive or age-related donor vascular changes (Fig. 31.3). Arteriosclerosis is a common finding in long-term KTxBx and it is related to both nonimmune injury (e.g., recipient’s hypertensive disease) and alloimmune injury, including chronic CMR and chronic exposure to DSA (Fig. 31.31). In its active phase, vascular rejection is easily recognizable by the presence of cellular inflammatory lesions involving the


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Fig. 31.30 Striped fibrosis. Masson’s trichrome-stained biopsy shows areas of interstitial fibrosis/tubular atrophy, alternating with areas of better preserved (less fibrotic) cortical parenchyma

It has been proposed that accelerated arteriosclerosis that occurs in the presence of circulating DSA (particularly Class II HLA) should be considered another variant of transplant arteriopathy [39].

Clinicopathological Discrepancies Polyomavirus Allograft Nephropathy

Fig. 31.31 Severe arteriosclerosis in a 10-year-old transplant associated with generalized ischemic changes including interstitial fibrosis, tubular atrophy, and glomerular sclerosis. The etiology of arteriosclerosis in allografts is often multifactorial

intimal layer (intimal arteritis, endarteritis) predominantly composed of T-cells and macrophages, typically in association with endothelial injury and activation. The term transplant arteriopathy applies to the chronic arterial lesions resulting from acute or ongoing alloimmune injury (previous endarteritic/vaculitis lesions), but identification of chronic lesions is complicated by the fact that these lesions are paucicellular or acellular (fibrotic, sclerotic lesions), strongly resembling other forms of arteriosclerosis (Fig. 31.31). Since old, inactive, transplant arteriopathy may be morphologically indistinguishable from simple hypertensive arteriosclerosis, evaluation of previous KTxBx from the same patient and performance of immunostains for T-lymphocytes in the paucicellular lesions can help support a diagnosis of transplant arteriopathy. Further complicating the issue, chronic exposure to DSA appears to be associated with rapid development of vascular sclerosis evolving without clearly evident cellular/endarteritic lesions, therefore resembling non-alloimmune arteriosclerosis.

A diagnosis of “presumptive PVN” is considered in a patient with a biopsy without significant pathology and negative SV40 stain, but sustained BK viremia (>10,000 copies/mL for 3 weeks) [47]. In rare instances, false-negative BK viremia has been reported in patients with BK PVN due to infection with viral genotypes for which the currently used diagnostic tests have low sensitivity [60]. A discrepancy of this type, or the possibility of JC PVN, should be considered in patients with histological evidence of PVN—SV40-positive-infected cells, but negative BK viremia.

AMR The absence of C4d staining does not preclude a diagnosis of AMR, particularly in late-chronic AMR if the other diagnostic elements are present. Specifically, microvascular inflammation is considered to be strongly suggestive of AMR and has important prognostic implications [11, 61]. Negative DSA with histological evidence of AMR may be related to fluctuating DSA levels, technical inability to demonstrate anti-HLA DSA that is actually present, and nonanti-HLA antibodies (i.e., anti-endothelial antibodies, etc.). Lack of correlation between transplant glomerulopathy and proteinuria is not uncommon. Although extensive transplant glomerulopathy in patients with chronic active AMR is typically associated with proteinuria, there is no linear correlation between proteinuria and the extent of transplant glomerulopathy, particularly in earlier stages.

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Other Diffuse parenchymal scarring in protocol KTxBx is clinically problematic and requires careful clinicopathological correlations. The main processes leading to this situation are multifocal infarcts in the peritransplantation period leading to cortical scars or ongoing ischemic injury, ongoing or repeated infections (chronic pyelonephritis), evolving PVN, and history of previous CMR episodes. A small number of patients have prominent scarring that cannot be explained. Evaluation of previous biopsies (pretransplant and post-transplant if available) may help explain the fibrosing process.

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