Transplantation Pathology
Roger K. Moreira, MD
Douglas A. Simonetto, MD
20.1 OVERVIEW
Chronic liver disease is a major cause of morbidity and mortality worldwide. In the United States alone, nearly 40,000 patients progress to liver failure and death annually. Liver transplantation is currently the standard treatment for various forms of severe liver disease, including acute liver failure or end-stage liver disease due to any etiology, as well as selected metabolic and neoplastic conditions.
Clinical findings
Chronic liver diseases are often silent until cirrhosis with clinical decompensation ensues. Events that define decompensation include ascites, hepatic encephalopathy, variceal bleeding, and nonobstructive jaundice, which are often triggered by precipitating factors such as bacterial infections, portal vein thrombosis, surgery, or hepatocellular carcinoma. Acute liver failure, on the other hand, is characterized by jaundice, coagulopathy, and the development of hepatic encephalopathy within 8 weeks of the onset of symptoms. Subacute liver failure has a similar presentation, except hepatic encephalopathy develops later, between 8 and 24 weeks of the onset of jaundice.
For both decompensated cirrhosis and acute/subacute liver failure, liver transplantation represents a lifesaving treatment. Unfortunately, however, liver allografts remain a scarce resource and, despite great improvements in the organ allocation system, approximately 2,500 to 3,000 patients are removed from the waiting list every year in the United States due to death or poor overall clinical status (Organ Procurement and Transplantation Network [OPTN] data, 2008 to 2015).
The organ transplant system in the United States is currently managed by a nonprofit organization—the United Network for Organ Sharing (UNOS). In 2002, the UNOS adopted the model of end-stage liver disease (MELD) score as the organ allocation system. The MELD score is an objective, laboratory-based measurement which has been validated in predicting 3-month waiting list mortality.1 For children 12 years of age and under, the pediatric end-stage liver disease (PELD) score replaces MELD. In addition, some conditions are listed for liver transplant as UNOS status 1A, which takes priority over the MELD allocation score: acute liver failure, primary nonfunction of transplanted livers, hepatic artery thrombosis, and Wilson disease. The benefit of transplantation should be weighed against posttransplant morbidity
and mortality, being reserved for patients with considerable decline in their quality of life and/or high mortality without transplant.
and mortality, being reserved for patients with considerable decline in their quality of life and/or high mortality without transplant.
 Figure 20.1 Etiology of liver disease in patients undergoing liver transplantation in the United States From UNOS database [2015]; 7,127 transplanted patients. |
Etiology
According to the UNOS database, 7,127 liver transplants were performed in the United States in 2015 and hepatocellular carcinoma was the most common indication for liver transplantation (often in the setting of cirrhosis due to various etiologies), followed by hepatitis C-related cirrhosis, alcoholic liver disease, and nonalcoholic fatty liver disease (Fig. 20.1). Acute liver failure accounts for less than 10% of all transplants performed in the United States annually, with acetaminophen overdose being responsible for almost half of these cases.
20.2 EVALUATION OF DONOR LIVER BIOPSY
Liver biopsy is an important adjunct tool in the evaluation of potential allografts. Its main role is to confirm the quality/viability of the donor organ and exclude features that would either contraindicate transplan-tation or increase the likelihood of various adverse short and long-term outcomes. As a result of organ shortage, increasing utilization of allografts from suboptimal donors has occurred in recent years, leading to a higher proportion of donor livers requiring pretransplantation frozen section evaluation. The so-called “extended donor criteria” includes several established risk factors for posttransplantation organ dysfunction and failure, such as old age (>60 years), hypernatremia (>155 mEq/L), macrovesicular steatosis >40%, cold ischemia time >12 hours, partial liver allografts, and donation after cardiac death (DCD) donors.2 Suboptimal livers also include allografts from donors with prolonged hemodynamic instability, use of vasopressors, chronic viral
hepatitis (B or C), as well as the presence of mass lesions or significant fibrosis. Although most risk factors are defined clinically, the main role of pathologists in the pretransplantation frozen section evaluation is to assess steatosis and exclude any miscellaneous findings (significant inflammation, necrosis, granulomatous processes, fibrosis, neoplasms, etc.) which may not have been detected on clinical evaluation.
hepatitis (B or C), as well as the presence of mass lesions or significant fibrosis. Although most risk factors are defined clinically, the main role of pathologists in the pretransplantation frozen section evaluation is to assess steatosis and exclude any miscellaneous findings (significant inflammation, necrosis, granulomatous processes, fibrosis, neoplasms, etc.) which may not have been detected on clinical evaluation.
In order to avoid diagnostic errors, the liver sample being submitted for frozen section by the transplant surgeon must be obtained and handled properly. Although a single biopsy is generally adequate (either needle core or wedge biopsy), additional samples should be obtained if the appearance of the liver is grossly heterogeneous. A large-core biopsy (i.e., away from the liver capsule) is preferred if there is concern for fibrosis, because subcapsular wedges can show nonspecific subcapsular fibrosis that is suboptimal for staging purposes.
The fresh tissue should ideally be carefully wrapped in a paper towel or gauze soaked in preservation solution and transported to the pathology lab immediately. Prolonged exposure to normal saline can cause significant histologic artifact, characterized by cellular discohesiveness and pyknosis-like chances of hepatocytes, mimicking necrosis (Fig. 20.2). Dry absorbent materials and sample compression can result in tissue dehydration and absorption of fat, potentially resulting in significant histologic changes and underestimation of steatosis. Tissue freezing itself frequently causes small cytoplasmic vacuoles in hepatocytes that are essentially indistinguishable from microvesicular steatosis on hematoxylin and eosin (H&E)-stained sections. Oil red O or other fat stains may aid in this distinction, but require experience for proper interpretation.
The importance of donor steatosis as a risk factor in the setting of liver transplantation is well established and consistently observed in several clinical and experimental studies.3, 4, 5, 6 Macrovesicular steatosis, defined histologically as one or a few large fat droplets that displace the hepatocyte nucleus to the periphery of the cell, represents the main finding in this context and has been associated with various adverse outcomes during the early transplantation period, including primary graft dysfunction and nonfunction, as well as prolonged intensive care unit and hospital stay.7, 8, 9 This is thought to be related in part to the
displacement of fat droplets from the cytoplasm of hepatocytes into sinusoids (because of mechanical factors and/or ischemia), leading to sinusoidal obstruction, lipid peroxidation and formation of free radicals upon reperfusion, and endothelial injury.7 Examination of failed allografts owing to primary nonfunction and early posttransplant biopsies of steatotic allografts often show large, coalescent fat droplets within hepatic sinusoids—referred to as lipopeliosis (Fig. 20.3)—which illustrates one of the primary pathogenic mechanisms in this scenario.
displacement of fat droplets from the cytoplasm of hepatocytes into sinusoids (because of mechanical factors and/or ischemia), leading to sinusoidal obstruction, lipid peroxidation and formation of free radicals upon reperfusion, and endothelial injury.7 Examination of failed allografts owing to primary nonfunction and early posttransplant biopsies of steatotic allografts often show large, coalescent fat droplets within hepatic sinusoids—referred to as lipopeliosis (Fig. 20.3)—which illustrates one of the primary pathogenic mechanisms in this scenario.
 Figure 20.2 Saline preservation artifact. Hepatocytes appear discohesive, with glassy cytoplasm and piknotic-appearing nuclei, resembling early necrosis. |
 Figure 20.3 Lipopeliosis. Large coalescent fat globules extruded from hepatocytes are seen within congested sinusoids. |
Some confusion exists with regard to the nomenclature for the types of steatosis, making interpretation of the literature and communication of pathological findings quite challenging in certain situations. For example, the term microvesicular steatosis is often inappropriately used in the literature to refer to small-droplet steatosis, which virtually always coexist with typical large-droplet steatosis (as seen in nonalcoholic fatty liver disease, for example). In contrast, true microvesicular steatosis is characterized by very small lipid droplets which impart a “bubbly” appearance to the hepatocyte cytoplasm and that are often difficult to recognize on H&E stains (Fig. 20.4). Both small- and large-droplet fat are currently considered part of the spectrum of macrovesicular steatosis. Microvesicular steatosis is typically seen in specific clinical contexts that includes Reye syndrome, acute fatty liver of pregnancy, and toxicity by certain medications, and is thought to be related to mitochondrial toxicity. Unfortunately, in the context of liver transplantation, we must keep in mind that the terms microvesicular steatosis and small-droplet steatosis have been used interchangeably by some authors. See Chapter 16 for further discussion.
Although the specific method of interpretation differs slightly among pathologists, macrovesicular steatosis should be evaluated quantitatively (as a percentage). Macrovesicular steatosis is estimated as the percentage area of the hepatic lobules occupied by large-droplet steatosis (Fig. 20.5). Although no specific guidelines exist, livers with 30% large-droplet steatosis or less are considered adequate, without significant increased risk of adverse outcome, whereas allografts showing >60% are generally considered inadequate and, therefore, are not used for transplantation. Allografts with moderate (30% to 60%) large-droplet steatosis may or may not be utilized, depending on local
preferences and additional risk factors related to both donor and recipient. Finally, digital/computerized tools are available to assist in this interpretation but yield values that are significantly different (approximately half) compared to visual (i.e., “eyeball”) estimates by pathologists.10 Therefore, validation of values yielded by digital analysis and correlation with outcomes is necessary before this technique is applied during liver allograft procurement.
preferences and additional risk factors related to both donor and recipient. Finally, digital/computerized tools are available to assist in this interpretation but yield values that are significantly different (approximately half) compared to visual (i.e., “eyeball”) estimates by pathologists.10 Therefore, validation of values yielded by digital analysis and correlation with outcomes is necessary before this technique is applied during liver allograft procurement.
 Figure 20.4 Microvesicular steatosis versus small-droplet steatosis. The left side of the panel shows true microvesicular steatosis, with very small fat droplets that impart a “bubbly” appearance to the cytoplasm. In contrast, small-droplet steatosis (seen on the right side of the panel in conjunction with large-droplet steatosis) is characterized by larger droplets than microvesicular steatosis, with greater variation in droplet size. |
 Figure 20.5 Semi-quantitative assessment of steatosis. Mild (5% to 33%), moderate (33% to 66%), and severe (>66%) steatosis are illustrated in the left, middle, and right parts of the panel, respectively. |
20.3 THE LIVER TRANSPLANTATION PROCEDURE
Basic understanding of different surgical procedures utilized in liver transplantation is useful for pathologists, because certain posttransplant complications are associated with specific techniques. Orthotopic liver transplantation represents the most common procedure. A whole cadaveric liver is utilized to replace the diseased liver in its original location within the abdominal cavity. In orthotopic liver transplants, an end-to-end anastomosis is performed between donor and recipient vena cava, hepatic veins, hepatic arteries, and portal veins, whereas the bile duct anastomosis may be duct-to-duct or donor duct to recipient Roux-en-Y jejunostomy. Donor cholecystectomy is also routinely performed. Several technically more complex variations of the liver transplant procedure exist which require either surgical reconstruction of vascular structures, which is associated with increased risk of hepatic artery thrombosis, or surgical manipulation of the liver itself (e.g., “split” liver, reduced-size liver, and right or left lobe living donor procedure), which result in increased risk for both vascular and biliary complications.
20.4 TIMING OF POSTTRANSPLANT COMPLICATIONS
One of the most fundamental pieces of clinical information to take into account when assessing a liver transplant biopsy is the posttransplant time, because different categories of complications are strongly associated with specific posttransplant periods (Table 20.1). Although some variability and exceptions are recognized for most entities, posttransplant complications can broadly be classified into early (first week), delayed (second week to 3 to 6 months), and late (after 3 to 6 months). This classification is useful in narrowing our differential diagnosis and should be used routinely, in conjunction with other clinical data. Comparison with the donor (postperfusion) biopsy, if available, can also be helpful, excluding the possibility of preexisting (i.e., donor) and periprocedure changes, particularly when evaluating biopsies in the first few weeks posttransplantation.
20.5 PRESERVATION/REPERFUSION INJURY
Definition
Also known as harvesting injury, ischemia/reperfusion injury, and functional cholestasis, this diagnosis refers to a constellation of functional and histologic
abnormalities related to harvesting, preservation, and reperfusion of allografts.
abnormalities related to harvesting, preservation, and reperfusion of allografts.
Table 20.1 Timing of Posttransplantation Complications | ||||||||||
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Clinical findings
Clinically, the terms primary graft dysfunction, initial poor function, and primary nonfunction are utilized to describe liver function abnormalities in the early posttransplantation period that are thought to at least in part be related to preservation/reperfusion injury. The liver dysfunction ranges from slightly delayed function and mild transaminase and bilirubin elevations to early allograft failure. Preservation/reperfusion injury represents a diagnosis of exclusion; therefore, vascular thrombosis, drug-induced injury, infections, and other causes of early graft dysfunction need to be reasonably excluded.
Etiology and pathogenesis
The pathogenesis of preservation/reperfusion injury is primarily related to damage of different cell types in the liver during warm ischemia, cold ischemia, and reperfusion. Warm ischemia refers to the period during which the liver is subjected to suboptimal perfusion at body temperature (before or during organ harvesting). During this phase, injury predominantly affects the hepatocytes, particularly if warm ischemia lasts more than 120 minutes.11, 12, 13 Warm ischemia is particularly problematic in liver donation after cardiac death, because no blood flow to the liver occurs for a variable period of time before organ procurement. Cold ischemia refers to the period in which the allograft is perfused with preservation solution and kept in ice. This period is thought to primarily cause endothelial injury14 and should ideally be less than 12 hours, because longer cold ischemia periods have been associated with early adverse graft outcomes.15,16 Finally, during the reperfusion period, in which the allograft is reimplanted and perfused with blood at body temperature, there is further damage to both hepatocytes and endothelial cells, much of which is related to the release of reactive oxygen species and various inflammatory mediators from activated Kupffer cells.17
Histologic findings
Histologically, the most characteristic preservation/reperfusion injury-related changes are centrilobular hepatocyte swelling (Fig. 20.6) with associated hepatocanalicular cholestatasis. Scattered acidophil bodies and neutrophilic infiltration are also commonly seen. With more prominent injury, various degrees of parenchymal necrosis develop, sometimes associated with a ductular reaction. If significant steatosis is present, preservation/reperfusion injury-related hepatocyte injury can cause extrusion of fat droplets into sinusoids (i.e., “lipopeliosis”) (Fig. 20.3). Some degree of microvesicular steatosis is also thought to develop during organ ischemia, presumably owing to
mitochondrial dysfunction and abnormal fatty acid metabolism. Preservation/reperfusion injury-related histologic changes can persist for up to several weeks after transplantation in severe cases.
mitochondrial dysfunction and abnormal fatty acid metabolism. Preservation/reperfusion injury-related histologic changes can persist for up to several weeks after transplantation in severe cases.
 Figure 20.6 Preservation/reperfusion injury. Prominent zone 3 hepatocyte swelling. Zone 3 cholestasis was also present in this case (not shown). |
Differential diagnosis
The differential diagnosis for preservation/reperfusion injury primarily includes other forms of liver injury, which occur in the early posttransplantation period. Early biliary obstruction, hepatic artery thrombosis, drug-induced liver injury, and humoral rejection represent the main entities to be excluded. Although ductular reaction can be seen in preservation/reperfusion injury, portal/periductal edema, and portal/ductal neutrophilic infiltrate are usually absent. Endoscopic retrograde cholangiopancreatography (ERCP) or magnetic resonance retrograde cholangiopancreatography (MRCP) are often performed in the early transplantation period and provide useful information to evaluate bile duct obstruction or stricture. Arterial thrombosis can also closely mimic preservation/reperfusion injury, because changes are often centrilobular (swelling and necrosis) and a ductular reaction can be present (often associated with true biliary obstructive findings when there also is ischemic bile duct injury). Correlation with Doppler studies is essential to exclude this possibility. There also can be significant histologic overlap between preservation/reperfusion injury and humoral rejection. The presence of fibrin thrombi (in sinusoids, portal veins, and/or central veins), although rarely found, suggests humoral rejection. A C4d immunostain and correlation with donor-specific antibodies can also be helpful.
20.6 SMALL-FOR-SIZE SYNDROME
Definition
Portal hyperperfusion/small-for-size syndrome is an uncommon, pathologically difficult to recognize complication that occurs most commonly in allograft livers that are less than 30% of the ideal recipient liver volume or less than 0.8% of the recipient body weight.18
Clinical findings
The typical clinical scenario in portal hyperperfusion/small-for-size syndrome is unexplained cholestasis, coagulopathy, and ascites in the early posttransplant period, most commonly occurring in the setting of reduced-size or living donor allografts. The clinical features are not specific for this entity, mimicking several other posttransplant complications, but liver transplant surgeons are often aware of this possibility, being alerted when there is splanchnic congestion and portal hypertension upon revascularization of the allograft.
Etiology and pathogenesis
Portal hyperperfusion/small-for-size syndrome is thought to be related primarily to portal hyperperfusion, which leads to decreased levels of adenosine (a vasodilator) from the hepatic circulation (i.e., “adenosine washout”), which in turn causes hepatic artery vasoconstriction, with subsequent low arterial flow, thrombosis, and ischemic cholangitis.
Histologic findings
The histologic changes in portal hyperperfusion/small-for-size syndrome are rather subtle and nonspecific and a high degree of suspicion is required for this diagnosis. In early phases (starting minutes after transplantation), there is denudation of the endothelium of portal veins and sinusoids as well as hemorrhage into portal connective tissue. Subsequently, there is regeneration of the endothelium, reactive endothelial cells, subendothelial edema, and, eventually, fibrointimal hyperplasia, luminal obliteration, and recanalization. Nodular regenerative hyperplasia is also seen, presumably owing to vascular flow abnormalities. The hepatic lobules show a constellation of nonspecific abnormalities, including centrilobular cholestasis, microvesicular steatosis, parenchymal atrophy, and necrosis. In severe cases, ischemic cholangitis may be seen.18
Differential diagnosis
Portal hyperperfusion/small-for-size syndrome is essentially a diagnosis of exclusion. The possibility of arterial thrombosis/stricture, sepsis, biliary obstruction, ischemic cholangitis unrelated to portal hyperperfusion/small-for-size syndrome, and preservation injury should be carefully excluded. Vascular studies and ERCP/MRCP are helpful in excluding mechanical and/or anastomotic complications. Arteriography has been reported to show segmental narrowing, poor peripheral filling, and even reversal of flow in cases of portal hyperperfusion/small-for-size syndrome.18
Prognosis
The prognosis of portal hyperperfusion/small-for-size syndrome will depend on the severity of the disease. Some cases are effectively treated by decreasing portal venous flow using octreotide, splenic artery ligation, or mesocaval shunts.
20.7 ANTIBODY-MEDIATED REJECTION
Definition
Also known as humoral or hyperacute rejection, antibody-mediated rejection represents a form of graft injury/dysfunction, which occurs in the early posttransplantation period (hours to initial weeks) owing to preformed recipient antibodies or antidonor antibodies developed after transplantation. There is increasing evidence that antibody-mediated rejection also plays a role in complications occurring later, including acute cellular rejection and chronic rejection.19
Clinical findings
The typical pattern of hyperacute rejection (as described for other solid organs) may occur in unconditioned ABO-incompatible livers, but is very rarely seen in practice. This pattern of injury presents intraoperatively as uneven perfusion of the liver after vascular anastomosis, associated with swelling and dusky discoloration of the liver, and subsequent development of decreased bile production, coagulopathy and other signs of very early organ failure. A relatively more common scenario in liver transplantation is a slightly more protracted (acute) course, with varying degrees of liver dysfunction in the initial hours to several days posttransplant. This occurs in patients with positive donor-specific antibodies—often mimicking the presentation of acute cellular rejection. The liver dysfunction ranges from mild (delayed graft function) to complete loss of graft function with resulting patient death or need for retransplantation.20,21
Etiology and pathogenesis
Liver allografts are significantly less susceptible to antibody-mediated injury compared to other solid organ allografts. Nonetheless, antibody-mediated rejection seems to play an important role in a minority of cases of early liver allograft dysfunction, and there is mounting evidence that a humoral/antibody-mediated component also exists in cases of T-cell-mediated cellular rejection.19,22 Endothelial cells are the main target of the antibody-mediated injury, with various antibodies binding to antigens expressed in hepatic endothelial cells (sinusoids and hepatic vessels), with resulting endothelial cell injury, inflammatory cell adhesion, thrombosis, and hemorrhage.23 Antilymphocyte antibodies and, in rare cases, antiendothelial antibodies have been implicated in the pathogenesis of antibody-mediated rejection.
Histologic findings
The histologic findings in antibody-mediated rejection largely reflect its pathogenesis. In its early stages (first week posttransplantation), antibody-mediated rejection can very closely mimic preservation/reperfusion injury, with centrilobular hepatocyte injury and hepatocanalicular cholestasis, whereas the portal tract findings can be minimal to absent. As the process becomes more established (typically weeks 2 to 4 posttransplant), portal findings resembling biliary obstruction also become apparent, including portal edema, neutrophilic infiltrates, and ductular reaction (Fig. 20.7), often accompanied by a cholestatic pattern of liver enzyme elevations.19,24 These “biliary-type” features in antibody-mediated rejection are thought to be owing to immune-mediated injury of the peribiliary vascular plexus, leading to biliary injury without obstruction. Central vein inflammation (usually neutrophilic and/or eosinophilic, rather than lymphocytic, as in acute cellular rejection) can also be seen in some cases. In a recent interinstitutional study by O’Leary et al.,25 the histologic features most predictive of antibody-mediated rejection were portal eosinophilia, portal vein endothelial hypertrophy, and eosinophilic central venulitis. The overall histologic picture in antibody-mediated rejection, therefore, is not specific, and can mimic other early posttransplant complications. A high index of suspicion is needed for this diagnosis and the recognition of compatible morphologic findings should trigger further testing to exclude this possibility, including testing for donor-specific antibodoes and C4d immunostaining (see further discussion in “Special stains and immunostains”).
Special stains and immunostains
Widely regarded as a reliable maker of antibody-mediated rejection in renal and cardiac allografts,26,27 the precise significance of the various patterns of C4d staining by immunohistochemistry in liver allografts is not
as well established. In our liver pathology service, we only occasionally use this stain. Nonetheless, this technique has gained acceptance in recent years in the context of compatible histology and laboratory testing and should be used as an adjunct tool in cases showing features resembling either preservation/reperfusion injury or biliary-type features when there is no supporting clinical evidence or risk factors for these processes. C4d is deposited along the endothelial lining of hepatic vessels, being strongest in portal veins, venules, peribiliary plexus, and portal “stroma” (likely representing either C4d deposition within portal microvessels or complement spillage form injured vessels), then with progressively weaker staining within the lobular sinusoids (strongest in the periportal areas and weaker toward the central vein), in keeping with the direction of the blood supply to the liver (Fig. 20.8).22,28,29 A “positive” interpretation for C4d immunostaining is typically reserved for cases showing expression in >50% of portal tracts. This approach represents the main criteria in most studies and is often accompanied by variable staining of portal vein endothelium and sinusoids.30, 31, 32 The Banff Working Group has recently proposed a scoring system for C4d immunohistochemistry interpretation in liver allografts which is as follows: 0, no C4d deposition in portal microvasculature; 1, minimal (<10% of portal tracts) C4d deposition in >50% of the circumference of portal microvascular endothelia (portal veins and capillaries); 2, focal (10-50% of portal tracts) C4d deposition in >50% of the circumference of portal microvasculature endothelia; and 3, Diffuse (>50% of portal tracts) C4d deposition in >50% of the circumference of portal microvascular endothelia, often with extension into inlet venules or periportal sinusoids. It is worth emphasizing that variable staining (most often weak, but sometimes strong) can be seen in a wide range of liver diseases,33 and C4d should only be considered a helpful adjunct study to be used in the presence of compatible/suspicious histologic changes and in correlation with recipient donor-specific antibody assays.
as well established. In our liver pathology service, we only occasionally use this stain. Nonetheless, this technique has gained acceptance in recent years in the context of compatible histology and laboratory testing and should be used as an adjunct tool in cases showing features resembling either preservation/reperfusion injury or biliary-type features when there is no supporting clinical evidence or risk factors for these processes. C4d is deposited along the endothelial lining of hepatic vessels, being strongest in portal veins, venules, peribiliary plexus, and portal “stroma” (likely representing either C4d deposition within portal microvessels or complement spillage form injured vessels), then with progressively weaker staining within the lobular sinusoids (strongest in the periportal areas and weaker toward the central vein), in keeping with the direction of the blood supply to the liver (Fig. 20.8).22,28,29 A “positive” interpretation for C4d immunostaining is typically reserved for cases showing expression in >50% of portal tracts. This approach represents the main criteria in most studies and is often accompanied by variable staining of portal vein endothelium and sinusoids.30, 31, 32 The Banff Working Group has recently proposed a scoring system for C4d immunohistochemistry interpretation in liver allografts which is as follows: 0, no C4d deposition in portal microvasculature; 1, minimal (<10% of portal tracts) C4d deposition in >50% of the circumference of portal microvascular endothelia (portal veins and capillaries); 2, focal (10-50% of portal tracts) C4d deposition in >50% of the circumference of portal microvasculature endothelia; and 3, Diffuse (>50% of portal tracts) C4d deposition in >50% of the circumference of portal microvascular endothelia, often with extension into inlet venules or periportal sinusoids. It is worth emphasizing that variable staining (most often weak, but sometimes strong) can be seen in a wide range of liver diseases,33 and C4d should only be considered a helpful adjunct study to be used in the presence of compatible/suspicious histologic changes and in correlation with recipient donor-specific antibody assays.
 Figure 20.7 Antibody-mediated rejection. Portal tract with edema and ductular reaction, mimicking biliary tract obstruction. |
 Figure 20.8 C4d immunostain. Strong staining of portal vein endothelium, portal capillaries, and some staining of portal stromal cells in this example of antibody-mediated rejection. |
Differential diagnosis
Being essentially a complication of the early posttransplantation period, the differential diagnosis of antibody-mediated rejection mostly includes other processes that typically occur during this same time frame. The two primary differential diagnoses of antibody-mediated rejection are preservation/reperfusion injury and biliary obstruction. As discussed above, early changes of antibody-mediated rejection can closely mimic preservation/reperfusion injury, including changes of centrilobular hepatocyte swelling and cholestasis. Although some degree of ductular reaction can be seen in preservation/reperfusion injury, other biliary obstruction-type features such as portal edema and neutrophilic infiltrate (which can be seen in antibody-mediated rejection) are typically absent. Correlation with high-risk clinical features for preservation injury can also be helpful. The differential diagnosis with early biliary obstruction cannot be made reliably based on histologic features alone and the latter is best excluded clinically by ERCP/MRCP. Hepatic artery thrombosis should also be considered in the differential diagnosis, because it often presents with centrilobular-predominant findings (hepatocyte swelling or necrosis), sometimes with associated cholestasis and nonspecific portal changes. Correlation with Doppler studies is essential to exclude this possibility.
Prognosis
The prognosis of antibody-mediated rejection is difficult to establish because of various factors related to diagnostic criteria and definition of cases. Bona fide cases of antibody-mediated rejection can cause a wide range of graft injury, from subclinical to rapid early graft loss. The recognition and appropriate diagnosis of this entity is important in practice owing to the availability of potentially useful therapeutic interventions, including plasmapheresis, intravenous immunoglobulin (IVIG), splenectomy,34 anti-CD-20 agents (e.g., rituximab), and lymphocyte depleting agents (e.g., bortezomib).35
20.8 ACUTE CELLULAR REJECTION
Definition
Acute cellular rejection is defined by a host immune response to foreign (donor) antigens present in the allograft. Acute cellular rejection is mainly driven by cytotoxic T lymphocytes. The liver shows characteristic histologic patterns of injury, which can result in allograft damage and dysfunction.
Clinical findings
In spite of significant improvements in immunosuppressive therapy and lower rates of acute cellular rejection after the introduction of current immunosuppressive agents, rejection remains an important cause of liver allograft dysfunction and graft loss.36, 37, 38 Compared to other solid organ allografts, however, the liver is significantly more resistant to the deleterious effects of cellular rejection, with most episodes being either subclinical (i.e., seen in protocol biopsies without laboratory evidence of liver dysfunction) or mild, without associated long-term organ damage following treatment.39,40
Clinical factors associated with an increased risk for acute cellular rejection include older allograft donor age (≥30 years), prolonged cold ischemic time (≥15 hours), “healthy” recipients (young age, low Child-Pugh score, normal creatinine, etc.), HLA-DR mismatch, and baseline autoimmune conditions (autoimmune hepatitis, primary biliary cirrhosis, primary sclerosing cholangitis, etc.). Lower rates of rejection have been associated with pretransplant alcoholic liver disease.41, 42, 43, 44, 45, 46
Mild acute cellular rejection is often clinically asymptomatic, typically presenting with isolated mild elevations of the liver enzymes. In moderate to severe rejection, fever, enlargement and tenderness of the allograft, and decreased bile output may be noted, along with leukocytosis and eosinophilia in some cases. Although allograft rejection characteristically presents with a predominantly “cholestatic” pattern of liver enzyme elevations (i.e., predominant elevation of bilirubin, alkaline phosphatase, and γ-glutamyl transpeptidase, with relatively minor elevation of transaminases), nonselective enzyme elevation is also rather common. Clinical and laboratory findings overall lack both sensitivity and specificity for the diagnosis of acute cellular rejection and histopathologic examination of a biopsy sample is required to confirm the diagnosis.47 In practice, most cases of acute cellular rejection occur early in the posttranplantation period, most commonly in the first month. Late rejection (3-6 months posttransplantation or later) is less frequent and generally associated with suboptimal immunosuppression.
Etiology and pathogenesis
Although complex and still not entirely understood, the basic pathogenesis of acute cellular rejection is related to the recognition of alloantigens by antigen-presenting cells, with subsequent cytotoxic T-lymphocyte activation and cell-mediated injury to the allograft, primarily targeting interlobular bile ducts and hepatic microvasculature.
Histologic findings
Histopathologic examination represents the gold standard for the diagnosis of acute cellular rejection and is based on a triad of features which include: (1) portal inflammation, (2) lymphocytic cholangitis/bile duct injury, and (3) endotheliitis (Table 20.2). At least two of the components of this triad must be present for the diagnosis of acute cellular rejection according to the widely adopted Banff criteria for liver allograft rejection (Table 20.3).47 Although the composition and intensity of portal inflammation varies significantly from case to case, depending on various factors (mainly posttransplantation period, as discussed below), typical “rejection-type” infiltrates are characterized by a mixture of lymphocytes that often display an “activated” or “blast-like” morphology—with slightly larger nuclei and with more cytoplasm. Scattered eosinophils (which tend to be more prominent in patients treated with steroid-sparing or lymphocyte-depleting immunosuppressive regimens), neutrophils, macrophages, and plasma cells can also be seen (Fig. 20.9). Interface activity is absent in mild cases but is seen in moderate and severe acute cellular rejection (Fig. 20.10).
Recognition of interlobular bile duct inflammation and injury is critical in the diagnosis of acute cellular rejection. In its early/mild form, there is lymphocytic infiltration through the epithelial basement membrane and lymphocytes are seen among bile duct epithelial cells (Fig. 20.11). Morphologic evidence of bile duct
epithelial injury subsequently appears in the form of nuclear enlargement, nuclear overlapping, occasional mitotic figures, and cytoplasmic vacuolization (Fig. 20.12). In severe cases, luminal obliteration occurs and the small bile ducts may be difficult to recognize within the inflammatory infiltrate (in which case a keratin immunostain may help). With persistent or recurring bile duct injury, nuclear pleomorphism, hyperchromasia, dyspolarity, cytoplasmic eosinophilia, and atrophic appearance of ducts (collectively referred to as bile duct senescence) can be seen and are generally thought to represent features of early chronic rejection.47,48 In cases with significant bile duct injury and destruction, hepatocanalicular cholestasis can be present (see “Differential diagnosis” below for further discussion).
epithelial injury subsequently appears in the form of nuclear enlargement, nuclear overlapping, occasional mitotic figures, and cytoplasmic vacuolization (Fig. 20.12). In severe cases, luminal obliteration occurs and the small bile ducts may be difficult to recognize within the inflammatory infiltrate (in which case a keratin immunostain may help). With persistent or recurring bile duct injury, nuclear pleomorphism, hyperchromasia, dyspolarity, cytoplasmic eosinophilia, and atrophic appearance of ducts (collectively referred to as bile duct senescence) can be seen and are generally thought to represent features of early chronic rejection.47,48 In cases with significant bile duct injury and destruction, hepatocanalicular cholestasis can be present (see “Differential diagnosis” below for further discussion).
Table 20.2 Banff Grading Criteria for Acute Cellular Rejection (Global Assessment) | ||||||||||||
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Table 20.3 Banff Grading Criteria for Acute Cellular Rejection (Rejection Activity Index, RAI) | ||||||||||||||||||||||||||||
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 Figure 20.9 Acute cellular rejection, inflammation. Mixed portal inflammation with relatively large, “activated” lymphocytes and eosinophils. |
 Figure 20.10 Acute cellular rejection, interface activity. A case of moderate to severe rejection showing interface activity. |
Finally, endotheliitis—the third component of the acute cellular rejection triad—is characterized
by lymphocytic inflammation targeting the vascular endothelium of portal veins, central veins or, less commonly, hepatic arteries. Endotheliitis is histologically characterized by lymphocyte attachment to endothelial cells, generally associated with endothelial injury/reactive changes, or by the presence of subendothelial lymphocytes (subendotheliitis) causing detachment, or “lifting,” of endothelial cells from the vascular basement membrane (Fig. 20.13). Portal vein branches are the most commonly affected structures, whereas central vein endotheliitis (often accompanied by centrilobular inflammation and injury—i.e., central perivenulitis) is more common in moderate to severe rejection or in late-occurring rejection (Fig. 20.14). Inflammatory or necrotizing arteritis is identified in rare instances of severe rejection, typically involving larger arteries near the hepatic hilum (therefore, not usually present in biopsy samples).47,49, 50, 51
by lymphocytic inflammation targeting the vascular endothelium of portal veins, central veins or, less commonly, hepatic arteries. Endotheliitis is histologically characterized by lymphocyte attachment to endothelial cells, generally associated with endothelial injury/reactive changes, or by the presence of subendothelial lymphocytes (subendotheliitis) causing detachment, or “lifting,” of endothelial cells from the vascular basement membrane (Fig. 20.13). Portal vein branches are the most commonly affected structures, whereas central vein endotheliitis (often accompanied by centrilobular inflammation and injury—i.e., central perivenulitis) is more common in moderate to severe rejection or in late-occurring rejection (Fig. 20.14). Inflammatory or necrotizing arteritis is identified in rare instances of severe rejection, typically involving larger arteries near the hepatic hilum (therefore, not usually present in biopsy samples).47,49, 50, 51
 Figure 20.11 Acute cellular rejection, bile duct injury. Mild bile duct injury with subtle epithelial changes and focal lymphocytic infiltration. |
 Figure 20.12 Acute cellular rejection, bile duct injury. More significant bile duct injury is seen in this case, which shows prominent epithelial injury and partial bile duct destruction. |
 Figure 20.13 Acute cellular rejection, portal vein endotheliitis. |
 Figure 20.14 Acute cellular rejection, central vein endotheliitis. |
In the most common form of acute cellular rejection, occurring in the initial 1 to 3 months posttransplantation, the histologic findings tend to be “typical,” as described above, with lymphocytic portal inflammatory infiltrates, portal vein endotheliitis, and readily recognizable bile duct injury, with interface activity and central vein endothelialitis/central perivenulitis developing as rejection progresses toward the moderate to severe end of the spectrum. The diagnosis of acute cellular rejection during this period is usually obvious. In the late posttransplantation period (3 to 6 months or later), however, variant histologic patterns
of rejection become more prevalent and may cause diagnostic difficulties52 (described in detail below).
of rejection become more prevalent and may cause diagnostic difficulties52 (described in detail below).
Finally, moderate to excellent interobserver agreement has been reported51 for the various histologic criteria of acute cellular rejection: portal inflammation (κ= 0.86 to 0.88), subendothelial inflammation (κ = 0.39 to 0.63), and bile duct damage (κ = 0.42 to 0.49). The interobserver reproducibility for the overall diagnosis of acute cellular rejection in this study was good (κ = 0.50 to 0.62 in 2 separate interpretations), whereas the intraobserver reproducibility was good to excellent among five participating pathologists (κ = 0.53 to 0.89).
Histologic variants
Several variant histologic patterns are recognized in liver allografts and are thought to represent forms of alloimmune attack to the liver (i.e., rejection), all occurring predominantly in the late posttransplantation period.
Plasma cell-rich hepatitis
This term refers to a chronic hepatitis with markedly increased plasma cells, typically with prominent interface and lobular activity, resembling autoimmune hepatitis in native livers (Fig. 20.15). When occurring in patients with underlying autoimmune liver diseases (autoimmune hepatitis, primary biliary cirrhosis, primary sclerosing cholangitis, etc.) in their native livers, this is often considered to represent either recurrent or de novo autoimmune/alloimmune hepatitis. One study also identified an IgG4-rich subpopulation within cases of plasma cell hepatitis, which was associated with more aggressive histology and high rates of response to increased immunosuppression.53
 Figure 20.15 Acute cellular rejection, plasma cell-rich variant. Clusters of plasma cells are seen within this portal tract. Also notice the significant amount of bile duct injury and occasional eosinophils. |
The nomenclature (autoimmune-like hepatitis54 and posttransplant plasma cell hepatitis55) and the precise etiology in patients with concurrent recurrent hepatitis C has been a subject of debate. The prevailing evidence is that this pattern represents an alloimmune/rejection phenomenon, rather than a variant pattern of recurrent hepatitis C, as evidenced by an increased frequency of antinuclear antibodies and other autoantibodies, association with subtherapeutic levels of immunosuppression, and a trend toward a more favorable prognosis in response to increased immunosuppression. This pattern has been associated with progressive disease and poor prognosis.54,55
At Mayo clinic, our general approach in these posttransplant plasma cell-rich cases is to use the term plasma cell hepatitis, followed by grading and staging, favoring this pattern to represent an alloimmune phenomenon/rejection variant, noting that this pattern can sometimes be associated with an aggressive course.
Isolated central perivenulitis
Central perivenulitis is a term recognized by the Banff Working Party, which refers to centrilobular changes that include central vein endotheliitis, perivenular inflammation, and perivenular hepatocyte injury/dropout (Fig. 20.16). In allografts, central venulitis is most often associated with portal-based acute cellular rejection and commonly seen in the context of moderate to severe rejection. However, central perivenulitis occasionally can be seen as an isolated finding, with portal tracts that are either normal or
show only mild nonspecific inflammation. This entity is called isolated central perivenulitis and is generally thought to be a low-grade smoldering rejection, one that can often be treated with optimization of immunosuppressants.
show only mild nonspecific inflammation. This entity is called isolated central perivenulitis and is generally thought to be a low-grade smoldering rejection, one that can often be treated with optimization of immunosuppressants.
 Figure 20.16 Acute cellular rejection, central perivenulitis. Prominent inflammation and hepatocyte dropout are seen adjacent to a central vein. |
One study identified this pattern of injury in 28% of liver transplant patients with long-term follow-up.56 It was seen primarily in the late posttransplantation period, with many patients having histories of previous episodes of central perivenulitis in the early posttransplant period (usually with portal-based features of acute cellular rejection). Clinically, this pattern is most often associated with mild liver enzyme abnormalities. When untreated, a small minority of patients develop complications included zone 3 fibrosis, ductopenia, and de novo autoimmune/alloimmune hepatitis.
Chronic hepatitis pattern
When presenting in the late posttransplantation period, acute cellular rejection-related features are often more difficult to recognize, as bile duct injury, “activated” lymphocytes, and endotheliitis, can be sparse and some features can be absent. Thus the biopsy has to be carefully searched for features of acute cellular rejection and other causes of chronic hepatitis (such as recurrent disease, chronic hepatitis E, drug effect, etc.) have to be carefully excluded. When no cause is identified and no definite features or rejection are seen, the pattern of injury is called idiopathic posttransplantation hepatitis (Fig. 20.17). A significant proportion of allografts evolving to cirrhosis without a clear etiology persistently show this histologic pattern.57, 58, 59, 60, 61, 62 Chronic hepatitis E virus in particular should be carefully excluded, because chronic hepatitis E is a treatable cause of allograft dysfunction and failure63,64 (see detailed discussion in the “Viral hepatitis” in Chapter 8).
 Figure 20.17 Idiopathic posttransplant hepatitis. Chronic portal inflammation without diagnostic features of acute cellular rejection. |
Grading of acute cellular rejection
The grading system proposed by the Banff group47,48—an international consensus document devised by a panel of transplant pathologists, clinicians, and surgeons in 1997—remains the preferred method of grading rejection in transplant centers around the world. An important concept to keep in mind is that this system was designed exclusively as a grading tool. Therefore, the scoring system associated with this method cannot be used to assess a sample for the diagnosis of rejection per se—only to grade it once the diagnosis has independently been established.
The Banff system endorses two forms of rejection assessment: the “global assessment,” whereby acute cellular rejection is classified as mild, moderate, or severe (or grade I-III), according to a specified set of criteria, and the “rejection activity index” (RAI), in which all three components of acute cellular rejection (portal inflammation, bile duct injury, and endothelialitis) are evaluated separately and ascribed a score from 0 to 3, with the overall rejection activity index being the sum of all three individual scores. This method has been validated by a number of subsequent studies, which have shown the Banff schema to be an accurate, reproducible, and clinically useful method for grading acute cellular rejection40,65,66 (Tables 20.2 and 20.3).
One of the difficulties in grading rejection relates to the presence of histologic variants. Isolated central perivenulitis, for example, cannot be adequately graded by standard acute cellular rejection Banff criteria. According to the Banff group recommendations for interpretation of late liver allograft dysfuction,67 these cases should be interpreted as minimal/indeterminate for rejection when perivenular inflammation involves a minority of central veins with patchy perivenular hepatocyte loss, mild rejection when the changes involve the majority of central veins, moderate rejection when there is at least focal confluent perivenular hepatocyte dropout, but without bridging necrosis, and severe rejection when confluent central-to-central necrosis is present (Table 20.4).
Differential diagnosis
The differential diagnosis for acute cellular rejection will depend on the posttransplant period and the specific histologic pattern. For “typical” cases of early acute cellular rejection, drug reactions are one of the closest mimics, because drug reactions can be associated with bile duct inflammation/duct injury and increased eosinophils. Endotheliitis, however,
represents a more specific rejection feature and is very helpful in this distinction. Drug reactions are also commonly associated with more pronounced lobular inflammation and sometimes with granulomas, both relatively uncommon in acute cellular rejection, particularly in the mild end of the spectrum. Hepatocanalicular cholestasis is also common in drug-related injury but only seen in moderate to severe rejection, typically with pronounced bile duct injury. Cholestasis that is out of proportion to the degree of bile duct injury, therefore, represents a useful feature to favor drug effect. Biliary obstruction may occasionally mimic rejection. However, biliary obstruction is typically associated with neutrophilic (rather than lymphocytic) cholangitis, significant ductular reaction, and portal edema—none of which is typical for rejection. The degree of portal infiltrate is also generally more pronounced in rejection. Again, endotheliitis, when present, favors rejection.
represents a more specific rejection feature and is very helpful in this distinction. Drug reactions are also commonly associated with more pronounced lobular inflammation and sometimes with granulomas, both relatively uncommon in acute cellular rejection, particularly in the mild end of the spectrum. Hepatocanalicular cholestasis is also common in drug-related injury but only seen in moderate to severe rejection, typically with pronounced bile duct injury. Cholestasis that is out of proportion to the degree of bile duct injury, therefore, represents a useful feature to favor drug effect. Biliary obstruction may occasionally mimic rejection. However, biliary obstruction is typically associated with neutrophilic (rather than lymphocytic) cholangitis, significant ductular reaction, and portal edema—none of which is typical for rejection. The degree of portal infiltrate is also generally more pronounced in rejection. Again, endotheliitis, when present, favors rejection.
Table 20.4 Grading of Late Rejection with Isolated Central Perivenulitis Pattern | ||||||||||||
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In the late posttransplantation period, atypical histologic features of rejection become more common (Table 20.5) and raise additional considerations in the differential diagnosis. Cases showing a plasma cell-rich chronic hepatitis morphology (discussed above) can closely mimic alloimmune or de novo autoimmune hepatitis, but this distinction cannot be accurately made on histologic grounds alone and a final diagnosis requires correlation with clinical, serological, and histological findings. Differentiating this pattern from that of recurrent hepatitis C, on the other hand, is very important for management purposes (increased immunosuppression for rejection and decreased immunosuppression, with or without antiviral therapy, for recurrent hepatitis C). Even in nontransplanted individuals with chronic hepatitis C, there can be some rejection-like features, including bile duct injury and endothelialitis.68 However, these features in chronic hepatitis C are focal and mild and not the predominant pattern of injury. Thus, in most cases the distinction between recurrent hepatitis C and rejection is readily made (for details, see section on recurrent diseases).
Table 20.5 Histologic Features of Early and Late Acute Cellular Rejection (ACR) | ||||||||||||||||||||||||
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Prognosis and treatment
Most cases of acute cellular rejection are mild (often subclinical, as seen in protocol biopsies), with no significant long-term adverse effect on graft outcome, when they are managed successfully with either optimization of immunosuppression alone or
in conjunction with steroid therapy.39,40 Currently, acute cellular rejection only uncommonly progresses to graft failure.69 However, a large recent retrospective study showed that the long-term graft survival of patients with late acute cellular rejection (>90 days posttransplantation) is significantly worse compared with patients with early acute cellular rejection only.70 Severe rejection and variant rejection patterns such as plasma cell hepatitis are more often associated with adverse graft outcomes. Finally, a minority of cases of acute cellular rejection are steroid resistant and management options for these patients include T-cell-depleting agents, thymoglobulin, mycophenolate mofetil, anti-IL-2, and OKT3.
in conjunction with steroid therapy.39,40 Currently, acute cellular rejection only uncommonly progresses to graft failure.69 However, a large recent retrospective study showed that the long-term graft survival of patients with late acute cellular rejection (>90 days posttransplantation) is significantly worse compared with patients with early acute cellular rejection only.70 Severe rejection and variant rejection patterns such as plasma cell hepatitis are more often associated with adverse graft outcomes. Finally, a minority of cases of acute cellular rejection are steroid resistant and management options for these patients include T-cell-depleting agents, thymoglobulin, mycophenolate mofetil, anti-IL-2, and OKT3.
20.9 CHRONIC (DUCTOPENIC) REJECTION
Definition
Chronic rejection is a form of immune-mediated liver allograft injury characterized by bile duct loss (ductopenia) and/or obliterative (foam cell) arteriopathy of medium- and large-sized vessels, often evolving to graft loss. Because most cases are diagnosed on liver biopsies (in which foam cell arteriopathy is rarely seen), the term chronic ductopenic rejection or posttransplant vanishing bile duct syndrome has been used.
Clinical findings
Chronic rejection historically accounted for up to 20% of all cases of allograft failure,71,72 but the current incidence has substantially decreased (to as low as 3% of failed allografts), likely owing to improvements in immunosuppression therapy, monitoring, and early histologic recognition.69 Chronic rejection is usually asymptomatic and most patients present with increasing liver enzymes showing a cholestatic pattern. Chronic rejection can eventually lead to jaundice as the disease progresses, typically in the setting of previous episodes of severe or steroid-resistant acute cellular rejection. The diagnosis of chronic rejection requires a liver biopsy that shows the presence of typical histologic features and the exclusion of other forms of biliary tract disease, particularly biliary obstruction. Other clinical risk factors for the development of chronic rejection include donor age >40, recipient age <30, and underlying primary biliary cirrhosis or autoimmune hepatitis.
Histologic findings
The histologic hallmarks of chronic rejection in liver allografts are: (1) bile duct loss (ductopenia) and/or (2) foam cell arteriopathy, along with several other less specific findings that are thought to be secondary to these two basic abnormalities. Of these two, ductopenia represents the main finding in the vast majority of chronic rejection cases (Fig. 20.18), because foam cell arteriopathy is relatively less common and primarily seen in medium- to large-sized arterial vessels that are not typically sampled in biopsies (Fig. 20.19).
 Figure 20.18 Ductopenia. No interlobular bile ducts are seen within this mildly inflamed portal tract. Notice the presence of several hepatic artery branch profiles, which should normally be accompanied by a similar-sized bile duct. Most portal tract lacked a bile duct in this case. |
 Figure 20.19 Foam cell arteriopathy. A medium-size arterial vessel close to the hepatic hilum showing prominent subintimal foamy macrophages in this failed allograft with chronic rejection. |
Early in the course of the chronic rejection, a constellation of reactive bile duct epithelial changes can be seen, including marked nuclear pleomorphism, hyperchromasia, epithelial dyspolarity, cytoplasmic eosinophilia, epithelial flattening, and atrophic
appearance of ducts (collectively referred to as bile duct senescence), without obvious bile duct loss (Fig. 20.20). Early chronic rejection, if untreated, can progress to ductopenic rejection.
appearance of ducts (collectively referred to as bile duct senescence), without obvious bile duct loss (Fig. 20.20). Early chronic rejection, if untreated, can progress to ductopenic rejection.
 Figure 20.20 Early chronic rejection. The bile ducts in this example show pronounced epithelial dysmorphic changes, including marked nuclar enlargement, irregularity, overlapping, as well as flattening of the epithelium. |
Ductopenia is typically defined as the absence of bile ducts in >50% of portal tracts. The “50% rule” was developed in part from early morphometric studies of large tissue sections (autopsy and wedge biopsies), mostly from patients with Alagille syndrome73 and primary biliary cirrhosis.74 The same 50% rule is used with needle biopsies, but its application has several practical difficulties, including a significant proportion of incomplete and/or tangentially sectioned portal tracts, dense inflammatory infiltrates obscuring the visualization of bile ducts, and sometimes a very small number of portal tracts.75 If a sufficient number of complete portal tracts are present within the sample, the absence of bile ducts in most portal tracts indicates ductopenia. In needle core biopsy samples, which typically have approximately 10 to 15 portal tracts per sample,75,76 a useful method of assessing bile duct loss is to identify branches of the hepatic artery. These small arteries run side by side with similar-sized interlobular bile ducts within the portal tracts and both structures should normally be identified in close proximity on histologic sections (bile duct-hepatic artery parallelism). Cytokeratin 7 or 19 imunostains are also very useful for evaluating bile duct loss.
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