Incidence of Acute Rejection
Most of the reviews from large transplant centers published in the Cyclosporine era reported an incidence of acute rejection in the range of 70% [18]. Of course, the incidence of acute rejection at any individual center will be influenced by the mix of transplanted patients (e.g., age and disease indication), biochemical monitoring protocol, biopsy protocol, the minimal pathologic criteria utilized for the diagnosis of ACR, the immunosuppressive drug regime utilized, among other factors.

When tacrolimus became widely utilized as a first line agent for routine immunosuppression the incidence of acute rejection fell into the range of 60% [19]. At this point acute rejection was no longer a cause of graft failure or of patient mortality. In fact, as additional newer immunosuppressive agents became widely available even chronic rejection was no longer a significant cause of graft failure [20]. Very recent data from ongoing trials of new immunosuppressive agents indicate that ACR rates at one-year may fall below 25% [21, 22] . Episodes of ACR are likely to be seen only when levels of immunosuppressive drugs are subtherapeutic (poor compliance, serious infection, malignancy, drug toxicity etc).

There are a limited number of defined clinical parameters that appear to influence the rate of ACR in a clinical cohort. In the immediate post-transplant period (< 6 weeks), a multivariate analysis revealed that young recipient age, lack of renal impairment, fewer HLA-DR matches, cold ischemia time > 15 hrs, higher pre-transplant AST, lack of edema, and older donor age increased the risk of ACR [23]. Several studies have suggested that the risk of ACR is higher for patients transplanted for autoimmune hepatitis and fulminant hepatic failure, and lower for alcoholic liver disease [23, 24] .

Grading of Acute Cellular Rejection
A large number of grading schemes have been published over the years, most based largely on qualitative assessment of the degree of portal inflammation, bile duct damage and endotheliitis [14, 25, 26, 27, 28, 29, 30] . The NIDDK system was demonstrated to be reproducible by pathologists and to be clinically relevant in terms of predicting patient outcome [29]. It therefore was used as the core of the consensus system developed by a panel of pathologists, hepatologists and transplant surgeons from North America, Europe and Asia. A numerical scoring system utilized in Europe was modified and incorporated into the system as well [27, 28] . The resulting Banff schema [31], as it was designated, has been prospectively validated [32] and is currently used in most multicenter trials of new immunosuppressive drug regimes. [See Appendix for Banff Schema]

In daily diagnostic work the precise grade of rejection is not critical, since ACR is usually treated according to protocols that do not change depending on grade. More important, actually, is whether the minimal criteria for the diagnosis of ACR rejection have been met. The histologic threshold at which treatment for ACR (usually a steroid bolus) is instituted has never been adequately assessed, particularly in a prospective manner. This problem was nicely illustrated by the results of the separate United States and European trials that compared the efficacy of cyclosporine A to tacrolimus on the incidence of acute rejection and on patient survival [19, 27] . The two trials were conducted with a very similar study design and survival data were essentially the same. However, the cumulative one year incidence of ACR was reported to be 65% for CyA treated and 59% for FK-506 treated patients in the U.S. centers compared to only 50% and 40% respectively in the European centers [19, 27] . Although there are a number of possible contributing factors, it is likely that the disparity was in part due to a lower threshold for the diagnosis of ACR among U.S. surgical pathologists. A meta-analysis of published reports from 1984 to 1993 in U.S. and European transplant centers revealed a reported incidence of ACR ranging from 24% to 80% [33]. On the other hand, a study of 762 adult transplant patients enrolled between 4/15/1990 and 6/30/1994 into the NIDDK sponsored Liver Transplantation Database revealed a one-year cumulative incidence of ACR of 64% and no significant difference in the rate between the three centers. The pathologists in this study were experienced at evaluating transplant biopsies and were using a grading system they themselves had developed and validated [23].

On rare occasion protocol biopsies from patients with entirely normal biochemical profiles will exhibit histologic features that would be diagnostic of "treatable ACR". The histologic changes may in fact resolve without additional immunosuppressive therapy, indicating that histologic rejection may not equate perfectly with biologically significant rejection in every instance [34, 35] . A recent meta-analysis of 15 studies reporting on histologic findings in early post-transplant protocol biopsies revealed 131 instances in which histologic ACR was evident but there was no biochemical evidence of graft dysfunction. Without additional immunosuppression only 14% of these patients subsequently developed graft dysfunction requiring medical therapy [36]. In fact, emerging data from animal models indicate that some degree of host immune response in the allograft may be beneficial, leading to a state of tolerance. Specifically, the elimination of steroids from the immunosuppressive drug regime may be important in promoting tolerance of the allograft and the ability to completely withdraw or sharply reduce the need for other immunosuppressive agents later. This is obviously of particular interest in the setting of transplantation for HCV cirrhosis.

Treatment of mild focal portal inflammatory cell infiltrates that fall short of the diagnostic criteria for ACR has been shown to be unnecessary in human allograft recipients [37].

Parenchymal Changes in Acute Rejection
Parenchymal changes, particularly centered in the perivenular zones, have been described as a feature (or consequence] of acute rejection for some time [26]. However, there are a number of other etiologies for these changes as well, and confusion existed in the early literature regarding their specificity for ACR. In the early post-transplant period (<30 days) centrilobular hepatocyte ballooning degeneration, necrosis, or drop-out was most often attributed to perfusion (harvesting/re-perfusion) injury, although in some cases a precise etiology was not clear [38]. Many authors did not accept these changes as a feature of ACR, except perhaps in very severe cases. However, others concluded that perivenular hepatocyte necrosis, especially if associated with hepatic venular subendothelial inflammation, was a rejection phenomenon, even if typical portal tract changes were not prominent [39]. Two recent studies have concluded that if centrilobular necro-inflammatory changes due to rejection persist, perivenular fibrosis often develops and there is a substantial risk of chronic rejection [40, 41] .

In the late post-transplant period (> 3 months) perivenular fibrosis, hepatocyte necrosis/drop-out, and mononuclear cell infiltrates are not uncommon, but the precise etiology is often not clear. Two systematic studies (one in children and the other in adults) identified a variety of situations leading to these changes. In the majority of cases chronic rejection was thought to be responsible, following uncontrollable ACR (as described above). Rarer causes included ACR, vascular thrombosis, recurrent HCV hepatitis, and perhaps an early stage of de novo autoimmune hepatitis [42, 43] . In a few patients in both studies the cause of the centrilobular changes remained unknown. In earlier reports azothioprine was implicated as an important cause of centrilobular degenerative changes, stenosis of hepatic venules, and perivenular fibrosis [44, 45] . In some patients a veno-occlusive like pattern developed due to a combination of azothioprine toxicity and ACR with prominent central vein endotheliitis [46]. However, azothioprine is no longer routinely used as part of the immunosuppressive regime in most U.S. centers.

Centrilobular Rejection
In the mid-1990s a histological pattern of injury was noted in post-transplant biopsies consisting of subendothelial inflammation involving the hepatic venules, as usually seen in ACR, but without the typical portal tract changes of ACR. Perivenular hepatocyte necrosis and/or dropout associated with a mononuclear cell infiltrate were also present. There was some controversy regarding whether this lesion represented tacrolimus (FK-506) toxicity or an unusual form of acute ACR. Part of the difficulty lay in institutional differences in the histological criteria of this lesion. In the face of perivenular hepatocyte dropout and mononuclear cell infiltrates it can be difficult to appreciate subendothelial inflammation consistently. In centers where the definition of the lesion required the presence of subendothelial inflammation (so-called "central venulitis") studies showed that there was no relationship with tacrolimus use, and that the lesion generally responded to increased immunosuppression, supporting a rejection phenomenon [47, 48, 49] . This unusual form of rejection appears to occur later than typical ACR [49].

If subendothelial inflammation is not required for the diagnosis of this lesion, then it more closely resembles the centrilobular parenchymal changes described in the section before this one. Again, injury as a consequence of rejection would be most likely, but tacrolimus toxicity would also be a diagnostic consideration [50].

References

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APPENDIX

BANFF SCHEMA FOR ACUTE CELLULAR REJECTION

I. GLOBAL ASSESSMENT

Indeterminate Portal inflammatory cell infiltrate that fails to meet the criteria for the diagnosis of acute rejection 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

[Note: A rejection infiltrate is defined as a mixed inflammatory cell infiltrate with bile duct damage and/or subendothelial inflammation]

II. REJECTION ACTIVITY INDEX (RAI)

III. PORTAL INFLAMMATION

1. Mostly lymphocytic inflammation involving, but not noticeably expanding a minority of the portal tracts 2. Expansion of most or all of the triads by a mixed infiltrate containing lymphocytes with occasional blasts, neutrophils and eosinophils 3. Marked expansion of most or all of the triads by a mixed infiltrate containing numerous blasts and eosinophils with inflammatory spillover into the periportal parenchyma

IV. BILE DUCT INFLAMMATION/DAMAGE

1. A minority of the ducts are cuffed and infiltrated by inflammatory cells and show only mild reactive changes, such as increased N:C ratio of the epithelial cells 2. Most of the ducts are infiltrated by inflammatory cells. More than an occasional duct shows degenerative changes, such as nuclear pleomorphism, disordered polarity and cytoplasmic vacuolization 3. As above for 2, with most or all of the ducts showing degenerative changes or focal lumenal disruption

V. VENOUS ENDOTHELIAL INFLAMMATION

1. Subendothelial lymphocytic infiltration involving some, but not a majority of the portal and/or hepatic venules 2. Subendothelial inflammation involving most or all of the portal and/or hepatic venules 3. As above for 2, with moderate or severe perivenular inflammation that extends into the perivenular parenchyma and is associated with perivenular hepatocyte necrosis