Case 7 -

Acute Cellular Rejection Julia C. Iezzoni

Clinical History The patient, a 52-year-old African-American male, underwent orthotopic liver transplantation for end-stage liver disease due to chronic hepatitis B. Two weeks after transplantation, he developed a low-grade fever, malaise, and reduced bile output. Liver function tests demonstrated mildly elevated aminotransferases and alkaline phosphastase, and a markedly elevated total bilirubin. A liver biopsy was performed. (Figures 22-25 - H&E) Diagnosis: Acute Cellular Rejection Case 7 - Figure 1 - H&E Case 7 - Figure 2 - H&E Case 7 - Figure 3 - H&E Case 7 - Figure 4 - H&E Pathologic findings: Sections of the liver allograft biopsy show an inflammatory infiltrate that is confined mainly to the portal tracts. It is composed mostly of lymphocytes, with occasional plasma cells and eosinphils. The interlobular bile ducts demonstrate lymphocyte-mediated damage, with lymphocytic infiltration of the bile duct epithelium. Additionally, there is endothelialitis, characterized by subendothelial infiltration by lymphocytes of the terminal hepatic venules, with disruption and focal discohesion of the endothelial cells from the underlying connective tissue. Discussion Definitions and terminology Rejection is broadly defined as an immune-mediated recipient reaction against allogenic antigens of the transplanted organ that is elicited by the genetic disparity between the donor and the recipient. This reaction manifests as varying degrees of allograft damage and dysfunction, and in the most severe form, allograft loss. Three distinct clinicopathologic types of hepatic allograft rejection have been identified: hyperacute rejection, acute cellular rejection, and chronic rejection (Table 1). While all are immune-mediated processes, they differ in specific pathogenetic mechanism, histologic features, prognosis, and response to therapy. Acute cellular rejection (ACR) will be the focus of this discussion. Table 1: Classification of Hepatic Allograft Rejection Terminology Timing Mechanism Outcome Response to therapy Hyperacute Hours to first week Preformed antibodies against donor antigens Fulminant allograft failure None; fatal without re-transplantation Acute cellular rejection First week to years Alloimmune T-cell-mediated Eventual allograft failure if untreated Excellent Chronic rejection First month to years Alloimmune T-cell-mediated; arterial ischemia of intrahepatic bile ducts Eventual allograft failure Variable, but frequently poor necessitating re-transplantation Clinical features Acute cellular rejection is common and is the most frequent cause of graft dysfunction during the first few weeks after transplantation. The reported incidence of ACR varies from 50% to 80%, and this variability probably reflects the differences in the minimal criteria used to make the diagnosis, the type of immunosuppressive given, and the number of biopsies performed. While the histologic features of ACR occur in up to 80% of patients, it may be clinically significant in only approximately 50% of these patients. Acute cellular rejection may occur as early as one week after transplantation, and the vast majority of ACR episodes occur in the first 1 to 2 months post-transplantation. Acute cellular rejection also may occur at any point later in the post-transplantation period. These later episodes of ACR usually reflect inadequate immunosuppression, sometimes due to patient non-compliance. Alternatively, they may be the result of a primary nonrejection process (e.g. viral infection) that causes increased expression of the immune-stimulatory alloantigens, which in turn, elicits a rejection response. Most cases of ACR are asymptomatic, with ACR detected on protocol biopsies or biopsies performed when biochemical abnormalities are found. When present, the clinical findings of ACR are nonspecific. They include fever, malaise, and allograft swelling or tenderness. In severe cases, there may be pale-colored bile or decreased bile flow. Liver dysfunction, when present, usually manifests biochemically as a predominantly cholestatic pattern of liver function tests. The clinical and laboratory findings of ACR, however, lack both diagnostic sensitivity and specificity. Accordingly, liver histology is considered the "gold standard" for the diagnosis of ACR. Pathologic features The hallmark morphologic features of ACR are: 1) Mixed portal inflammation; 2) Bile duct inflammation and damage; and 3) Endothelialitis. These three features comprise the "diagnostic triad" of ACR. Mixed portal inflammation: The inflammatory infiltrate is mixed, and is composed predominantly of lymphocytes, some of which may be blastic, admixed with lesser numbers of plasma cells, eosinophils, and neutrophils. Eosinophils are highly characteristic of, though not specific for ACR. In some cases, the inflammatory infiltrate may be so dense as to obscure the bile ducts and portal vein branches. The inflammatory infiltrate usually is confined to the portal tract. In more severe cases, the infiltrate causes portal expansion, which may be associated with inflammatory spillover into the periportal parenchyma. Lobular inflammation is not characteristic of ACR. Bile duct inflammation and damage: There is lymphocytic infiltration of the interlobular bile ducts, which results in variable degrees of bile duct damage. This damage manifests as nuclear disarray, pyknosis, cytoplasmic vacuolization, or cell dropout of the bile duct epithelium. Endothelialitis: There is lymphocytic subendothelial infiltration of the portal vein branches and/or terminal hepatic venules with evidence of endothelial cell damage. This may result in disruption and focal lifting of the endothelial cells from the underlying connective tissue ("tenting"). In severe cases of rejection, the entire circumference of the venule is involved and may be associated with perivenular hepatocyte necrosis. In the early transplant period, endothelialitis is the histologic feature that is the most specific for ACR; later in the transplant course, its diagnostic specificity is less as other causes of endothelialitis may manifest (e.g. recurrent hepatitis C). The characteristic inflammation and at least one of the two other features of the diagnostic triad are required to make the diagnosis of ACR. In addition, the diagnosis of ACR is strengthened if greater than 50% of the bile ducts are damaged or if unequivocal endothelialitis of portal vein branches or terminal hepatic venules can be identified. The distribution within the hepatic lobule of the histologic features of ACR directly reflects the basis for this immune response. Specifically, the main alloantigens that elicit ACR are expressed principally on bile duct epithelial cells and vascular endothelial cells. Accordingly, these cells are the primary targets of immune attack. Since hepatocytes express lower levels of these alloantigens, they are relatively spared. Accordingly, the histologic features of ACR are centered on the portal tract and/or hepatic terminal venule, with relatively little involvement of the lobular hepatocytes. An uncommon morphologic pattern of ACR that predominantly affects the centrilobular region (zone 3) has been described. Sometimes referred to as "parenchymal rejection", this pattern consists of a perivenular mononuclear inflammatory infiltrate, associated with variable degrees of terminal hepatic venule endothelialitis and centrilobular hepatocyte necrosis. Such cases may be associated with only mild and focal features of ACR within the portal tracts. Re-biopsy after treatment for ACR sometimes is performed to evaluate the histologic response to treatment or to investigation liver enzymes that remain persistently elevated despite anti-rejection therapy. Following treatment for ACR, endothelialitis is the first of the histologic features to dissipate, followed by the portal inflammation, both of which usually resolve within 5-7 days. Bile duct damage, however, may persist for up to several weeks after ACR has been treated. These damaged bile ducts, which often are associated with a neutrophilic periductular inflammatory infiltrate, histologically may mimic other lesions, such as large duct problems. Accordingly, awareness that immunosuppression therapy has been administered for a recent episode of ACR is crucial for correct biopsy interpretation. Banff grading A uniform, reliable and reproducible system to grade the histologic severity of ACR is important for patient management and multi-institutional research and communication. The Banff grading scheme for ACR was devised for to serve this purpose. It provides a common nomenclature and set of histologic criteria for grading the severity of ACR. The Banff scheme was designed to be scientifically correct, clinically useful, simple to apply, and reproducible. The Banff scheme is composed of two parts, the global assessment and the rejection activity index (RAI). The global assessment is a verbal grading based on the overall appearance of the biopsy specimen and is particularly weighted by the severity of portal tract inflammation (Table 1). The RAI is a semiquantitative system that evenly weights the three histologic features that characterize ACR, specifically portal inflammation, bile duct inflammation/damage, and venular endothelialitis. Each of these three features is critically evaluated and semiquantitatively scored on a 0 to 3 scale according to specific histologic criteria (Table 2). The three scores then are added together to arrive at a final Rejection Activity Index (RAI), similar to the histologic activity index (HAI) scoring of necroinflammatory activity in chronic hepatitis. The RAI, like other semiquantitative assessments of histologic activity, is particularly useful for research purposes and the evaluation of new therapeutic regimens. For routine patient care, either method may be used. Importantly, the grading of ACR presupposes that the diagnosis of ACR has been established. The grading schemes should not to be used to determine whether the histologic features in a given case are due to ACR or some other condition. The use of the "indeterminate" category of ACR should be restricted to cases that have minor degrees of a mixed inflammatory infiltrate that possibly represent mild or early ACR but fail to meet the minimal diagnostic criteria for ACR. "Indeterminate" should not be used as a "catchall" term for cases in which one is unsure whether the histologic features are due to ACR or some other etiology, such as chronic hepatitis C. Table 2: Banff global assessment grading of acute cellular rejection Global Assessment* Criteria Indeterminate Portal inflammatory infiltrate that fails to meet the criteria for the diagnosis of acute rejection (see text) Mild Rejection infiltrate in a minority of the triads, that is generally mild, and confined within the portal spaces Moderate Rejection infiltrate, expanding most or all of the triads Severe As above for moderate, with spillover into periportal areas and moderate to severe perivenular inflammation that extends into the hepatic parenchyma and is associated with perivenular hepatocyte necrosis *Verbal description of mild, moderate or severe acute rejection could also be labeled as Grade I, II, and III respectively. Table 3: Banff rejection activity index (RAI) for acute cellular rejection Category Criteria Score Portal inflammation Mostly lymphocytic inflammation involving, but not noticeably expanding,, a minority of the portal triads 1 Expansion of most or all of the triads by a mixed infiltrate containing lymphocytes with occasional blasts, neutrophils and eosinophils 2 Marked expansion of most or all of the triads by a mixed infiltrate containing numerous blasts and eosinophils with inflammatory spillover into periportal parenchyma 3 Bile duct inflammation / damage A minority of the ducts are cuffed and infiltrated by inflammatory cells and show only mild reactive changes such as increased nuclear:cytoplasmic ratio of the epithelial cells 1 Most or all of the ducts infiltrated by inflammatory cells. More than an occasional duct shows degenerative changes such as nuclear pleomorphism, disordered polarity and cytoplasmic vacuolization of the epithelium 2 As above for 2, with most or all of the ducts showing degenerative changes or focal luminal disruption 3 Venous endothelial inflammation Subendothelial lymphocytic infiltration involving some, but not a majority of the portal and/or hepatic venules 1 Subendothelial infiltration involving most or all of the portal and/or hepatic venules 2 As above for 2, with moderate or severe perivenular inflammation that extends into the perivenular parenchyma and is associated with perivenular hepatocyte necrosis 3 Note: Total score = sum of components Differential diagnosis While exceptions arise, the majority of causes of liver allograft dysfunction tend to occur at characteristic time intervals after transplantation. As such, liver allograft biopsy interpretation is facilitated considerably by the knowledge of the post-transplantation time interval. In reference to the differential diagnosis of ACR, most cases of ACR occur within the first 2 months post-transplantation. As such, the diagnosis of ACR later than 2 months post-transplantation should be made with caution. Recurrent hepatitis C is the most common differential diagnosis problem for ACR. This will be discussed in detail in Case 9. Recurrent primary biliary cirrhosis (PBC) may be difficult to distinguish from ACR because both process are characterized by a mixed portal inflammatory infiltrate and lymphocyte-mediated bile duct damage. The presence of endothelialitis is helpful to indicate ACR. Granulomatous inflammation centered on bile ducts is very specific for PBC. Additionally, recurrent PBC tends to occur later in the post-transplantation period. Post-transplantation lymphoproliferative disorder (PTLD) may present as a monomorphous or polymorphous infiltrate, and it can involve the portal tracts. The polymorphous low grade lesions are most problematic to distinguish from ACR because of the mixed composition of the infiltrate. Bile duct damage and endothelialitis are not characteristic of PTLD, and if present, favor ACR. Significant cytologic atypia, necrosis (even single cell necrosis), and frank plasmacytoid features of the lymphoid cells, as well as architectural effacement by the infiltrate, favor PTLD. Immunohistochemistry is helpful, as ACR is composed of a mixed population of lymphocytes, usually with T-cells predominating, while PTLDs are almost always of B-cell phenotype. Also, demonstration of certain Epstein-Barr antigen favors PTLD over ACR. References Demetris AJ, Batts KP, Dhillon AP, et al. Banff Schema for grading liver allograft rejection: An international consensus document. Hepatol 1997;25:658-663. Snover DC, Sibley RK, Freese DK. Orthotopic liver transplantation: A pathologic study of 63 serial liver biopsies from 17 patients with specific reference to the diagnostic features and natural history of rejection. Hepatol 1984;4:1212-1222. Batts KP. Acute and chronic hepatic allograft rejection: Pathology and classification. Liver Transpl Surg 1999;5:S21-S29. Vierling JM. Immunology of acute and chronic hepatic allograft rejection. Liver Transpl Surg 1999;5:S1-S20.