The pathologist plays a primary role in the diagnosis of native diseases, intraoperative assessment of donor tissue where indicated, and in the postoperative management of transplant complications (Lerut et al. 1988; Demetris et al. 1987). Other than assessment of surgical margins in cases of malignancy of the native liver, as mentioned above, frozen sections at the time of transplant procedure are usually done for donor organ evaluation, when the surgical team is not sure of the donor disease course prior to his/her demise in situations of cadaveric transplant (Kakizoe et al. 1990b). The main concern for intraoperative assessment of the donor liver is for steatosis and, while routinely performed in adult centers, this indication is much less frequent in the pediatric age group. There is data to show that steatosis in the donor liver, especially the microvesicular type, is associated with an adverse graft survival and increased complications, especially acute cellular rejection (Todo et al. 1989). Determination of the percentage of liver parenchyma involved by steatosis is the main role for the pathologist in this situation. Other findings such as evidence of hepatitis, portal inflammation, or even incidental cirrhosis due to undetected primary liver disease may alert the surgeon to not use the donor organ for transplant. In most instances, at least in pediatric transplants, the gross appearance of the liver is sufficient to decide on the suitability for transplant, and this decision is made by the surgeon.

Posttransplant handling of biopsy or tissue specimens from an allograft liver is a critical step for diagnosis of complications. Early posttransplantation, the biopsy can be entirely fixed in formalin if the suspicion is mainly rejection. As infections frequently occur in the first month posttransplant, or whenever there is a suspicion of infection, we recommend getting two cores, submitting one in formalin and the other fresh for microbiological cultures. Electron microscopy is very rarely needed in the early postoperative evaluation of liver allograft biopsies. In major transplant centers such as the Children’s Hospital of Pittsburgh, rapid processing of transplant biopsies is available as an option, wherein a biopsy done in the morning is processed on a two and a half to 3 h automated processing cycle and is available for a final read the same day. This facilitates management of patients in whom sudden variations in liver functions may require immediate changes in therapy. While routine hematoxylin and eosin (H&E) staining may be sufficient, it is helpful in many instances to do a PAS stain with diastase digestion, a trichrome stain, and possibly a reticulin stain to study the overall architecture of the liver biopsy. Immunohistochemical markers for bile duct proliferation, such as cytokeratin AE1/AE3 or cytokeratin 7, may be indicated in cases where biliary complications are suspected or in late biopsies for assessment of chronic rejection. While the AE1/AE3 stain is a good marker to study bile duct injury in rejection and in other complications, it does not highlight to the same extent the degree of ductular proliferation seen in biliary obstruction as the CK7 immunostain. Specific stains for infectious organisms may be performed where indicated, and these include bacterial and fungal stains in biopsies that show the presence of abscesses or neutrophil collections and viral stains such as cytomegalovirus (CMV), herpes, and adenovirus immunostains in suspected cases of opportunistic viral hepatitis. Epstein-Barr virus (EBV) in situ hybridization for EBV-encoded RNA (EBER-1) is done in cases where serology and/or EBV PCR turns positive or is high and when the histological pattern is that of hepatitis. It cannot be adequately stressed that allograft liver biopsies need to be interpreted in the light of liver function tests as well as in consultations with the transplant team to arrive at the right diagnosis. It is imperative to have at least an ALT, AST, GGTP, and bilirubin value in hand while evaluating the biopsies. Other significant data such as analyses for viral infections and levels of immunosuppressive agents also aid in the interpretation. Correlation with imaging studies will also help evaluate the biliary tree and vascular structures. Information on any immediate treatment given prior to the biopsy will also help assess the biopsy specimen. As a routine, every biopsy at CHP is reviewed with the clinician usually prior to sign-out so as to arrive at the right diagnosis.

There are several factors that determine the occurrence of immediate postoperative complications following transplant, the type of donor organ, the time taken for organ retrieval and transport, the time to transplant, and in some instances the duration of vascular clamp time, all of which play a role in the postoperative function of the allograft. In our center, a baseline liver biopsy is frequently performed immediately postperfusion of the organ. The changes seen in this biopsy may be entirely nonspecific, showing some minor hemorrhages in the subcapsular region and some inflammatory cells, or may show more dramatic changes of parenchymal necrosis and steatosis (Fig. 15.2a). In rare instances, incidental donor diseases such as Von Meyenburg complexes that may have no clinical significance may be encountered (Fig. 15.2b, c). This baseline biopsy serves as a guide when comparing subsequent biopsies in the postoperative period for evaluation of complications.

Allograft rejection has been defined as “an immunological reaction to the presence of a foreign tissue or organ which has the potential to result in graft dysfunction and failure” (Demetris et al. 1997a). There are several mechanisms by which a recipient can reject the donor allograft. The main reason is usually a genetic disparity between the donor and recipient and may be related to either blood group or major histocompatibility antigens (MHC antigens). Sindhi et al. have shown that the HLA-DOA gene and the B lymphocyte, in which it is exclusively expressed, are plausible candidates contributing to pediatric liver transplant rejection (Sindhi et al. 2008). The exact mechanism, however, is unclear. Starzl et al. have shown that tolerance is frequently observed in long-term liver allograft survivors (Starzl 2004; Starzl et al. 1992). They have also shown that as tolerance of the graft develops, recipients progressively require less maintenance immunosuppression and can sometimes be completely withdrawn off medications (Starzl and Zinkernagel 2001). It has also been shown that recipients of living-related donors can be more successfully weaned than mismatched cadaveric donors.

Rejection can be divided into antibody-mediated rejection, acute cellular rejection (ACR), and chronic rejection. Antibody-mediated rejection usually occurs within the first few weeks of transplantation, while ACR can occur at any time posttransplant. Chronic rejection is usually the result of persistent or progressive acute cellular rejection and may develop in months to years.

Biliary tract complications such as anastomotic adhesions, bile leakage, ascending cholangitis, biliary strictures, and obstruction can occur following pediatric liver transplant in up to 35 % of cases (Starzl and Demetris 1990a; Demetris et al. 1987). If not detected early, these can lead to biliary cirrhosis and graft loss. Anastomotic strictures appear in the early months following transplantation and are treated by surgical intervention, which decreases the risk of graft loss (Lerut et al. 1988; Anderson et al. 2010). Other biliary strictures could arise as a result of biliary ischemia resulting from any of the above vascular causes or as a result of preformed anti-donor antibodies or reperfusion injury. Living donor grafts, which have become progressively more frequent in children, also have increased biliary complications (Kling et al. 2004). Routine laboratory testing for liver enzymes helps the diagnosis with findings of elevated GGTP and alkaline phosphatase, and subsequently bilirubin. Cholangiography helps confirm the diagnosis in many cases (Meersschaut et al. 2000).

The next major group of posttransplant complications is infections. They may be due to any organism and can occur at any time in the postoperative period. While bacterial infections and sepsis are seen early, fungal infections and viral complications usually occur in the first few weeks or beyond. It is always useful to consider infections as a possibility while reviewing any allograft biopsy (Table 15.5).

The most common diseases that recur posttransplant include viral hepatitis B and C and autoimmune hepatitis. Since the indication for pediatric liver transplant is only occasionally hepatitis B or hepatitis C infection, or related complications, the few instances where hep B or hep C is found posttransplant may represent either primary disease or a recurrence. The clinical and histological features are similar to that seen in viral hepatitis, and serological and PCR viral loads help in confirming the diagnosis.

Autoimmune hepatitis presents a difficult challenge in that it has a high incidence of recurrence posttransplant and can recur at any time during the posttransplant course (Balan et al. 1999; Demetris and Sebagh 2008; Detre et al. 1989; Wright et al. 1992). Serological confirmation may be difficult in these cases due to persistence of positive pre-transplantation serological tests. The histological features are also difficult to recognize due to the overlap with acute cellular rejection and viral infections. A plasma cell-rich infiltrate with interface activity, biliary injury with peribiliary edema and fibrosis, central perivenulitis with single cell hepatocyte necrosis, and absence of evidence of EBV or other viral infection, together would raise this possibility. Recurrent AIH invariably results in rapid graft failure and is one of the indications for re-transplantation. AIH also has the potential to recur in re-transplanted livers.

Primary sclerosing cholangitis also recurs to a similar extent as autoimmune hepatitis and can be difficult to differentiate from biliary obstructive causes such as postoperative ischemic biliary strictures or strictures related to vascular thrombosis (Abu-Elmagd et al. 1993a; Balan et al. 1999; Dvorchik et al. 2002; Lerut et al. 1988). The histology is similar to that of the primary disease (Fig. 15.7g, h). There have been rare instances of recurrent giant cell hepatitis or a recurrent familial intrahepatic cholestasis-BSEP disease, and tumors have recurred in the allograft liver, especially in cases with extrahepatic extension or extensive vascular invasion (Pappo et al. 1994; Siebold et al. 2010). We have also encountered recurrence of Budd-Chiari syndrome in allografts.

While there has been a steady increase in the expectancy in survival of liver allografts in the tacrolimus era as opposed to the cyclosporine A era, due to better control of episodes of rejection, there appears to have been an increased incidence of EBV-related PTLD in these patients (Demetris et al. 1992a; Fung et al. 1996; Jain et al. 2000). In order to decrease complications of prolonged immune suppression, attempts at withdrawal of immune suppression for sustained periods of time have been initiated, especially after pediatric liver transplant (Ramos et al. 1995; Mazariegos 2011; Mazariegos et al. 1997a, 2007). Recent advances in identification of specific gene transcripts after adult liver transplant that predict the ability to achieve tolerance raises the possibility of identification of similar mechanisms in children. It has been shown that tolerant patients exhibit a higher incidence of circulating plasmacytoid dendritic cells relative to conventional myeloid dendritic cells (Thomson et al. 2001; Jain et al. 2011). The 20-year actuarial patient and graft survival is 35.8 and 32.6 %, respectively, in a large adult and pediatric cohort of liver transplant recipients under tacrolimus immunosuppression. In that series and others, a significantly better survival was seen among children (Jain et al. 2000, 2011). The 10-year graft survival for children with liver transplants is about 62 % according to the Organ Procurement and Transplantation Network (OPTN) data (OPTN data as of 12/30/2011). The causes of death after pediatric liver transplantation are usually related to liver failure caused either by thrombosis of hepatic artery or primary nonfunctional graft or severe rejection (Jain et al. 2000, 2011). Sepsis and opportunistic infections are other major reasons for posttransplant death (Kashyap et al. 2001; Kahn et al. 1988; Yandza et al. 1992; Soltys et al. 2007). Rare instances of posttransplant neoplasia have also resulted in graft failure and death.

Small intestinal transplant was first attempted in the 1960s with limited success even in the cyclosporine era. It was not until the 1980s with the advent of tacrolimus that it gained momentum, and the introduction of induction protocols with antilymphocyte antibodies appears to have improved the outcome of these grafts (Starzl 2000, 2001). Small bowel transplants are now being done with increasing frequency in children suffering from intestinal failure for a variety of diseases (Kocoshis 1994). The biggest factor in determining the need for intestinal transplantation appears to be the degree of dependency on total parenteral nutrition (TPN), which has its associated complications including infections and liver failure. In most situations, the transplantation is usually for a short gut syndrome as a result of prior surgeries for various congenital defects and conditions. The complications following small bowel transplant (SBT) are to some extent similar to those of liver transplant but have unique features without much of the surgical difficulties associated with a liver transplant. The survival of patients with SBT has also improved in the tacrolimus era, as compared to the cyclosporin era. Two types of intestinal transplant procedures are performed: one consisting of isolated small bowel and the second combining small bowel and liver transplantation. A third type of surgery is one in which the small bowel is a component of a multivisceral transplant, consisting of a combination of small bowel, large bowel, portions of duodenum, and stomach, including pancreas and liver (Jaffe et al. 1989). While overall improvement in postoperative care and pretransplant preparation have contributed to increased survival of grafts, the type of surgical procedure plays a role in that there appears to be a consistently better outcome for liver/SBT patients compared to SBT alone. This is thought to be related to relatively easier vascular anastomoses and the availability of collaterals in the combined versus the isolated SBT patients. Overall, rejection seems to be the main postoperative complication following SBT. Close monitoring in the immediate postoperative period along with regular endoscopic examination and protocol biopsies has been recommended to improve the overall outcome. Several recent reviews and chapters highlight the pathology of SBT in adults and children (Husain et al 2010; Ruiz 2009; White and Ranganathan 2011).