There are several late medical complications that may jeopardize long-term survival after pediatric liver transplantation (LT). A significant number of recipients develop subclinical but progressive graft fibrosis, as increasingly evidenced in protocol liver biopsies performed 1 to 10 years post-LT, presumably related to chronic autoimmune/alloimmune injury of the allograft, possibly as a result of under-immunosuppression. On the other hand, chronic over-immunosuppression exposes the patient to the toxicities of immunosuppressants (IS), including chronic renal dysfunction, infections, de novo malignancies, and cardiovascular (CV) risks. These considerations are well corroborated by a recent study of 101 LT recipients who have survived 20 years after transplant. Thirty-four percent of them had stage 2 to 5 chronic kidney disease (CKD), and 30% had abnormalities in the liver tests related to chronic rejection or biliary problems. Ten of 22 (45%) survivors with abnormalities in the liver tests had signs of chronic rejection on the liver biopsy, including 6 with signs of plasma cell–rich rejection, 4 of whom progressed to cirrhosis; 9 of 13 (69%) with normal liver tests had developed a quiescent graft fibrosis at 20 years post-LT. This illustrates the need for clinical trials to focus on the mechanisms of immune dysregulation post-LT and on optimal immunosuppressive strategies to minimize these long-term posttransplant complications.
Autoimmunity or Alloimmunity
Autoimmune or alloimmune features are being increasingly recognized in LT recipients, associated or not with the liver graft injury. These include the findings of elevated immunoglobulin levels, positive titers of serum non–organ-specific autoantibodies, and the specific alloimmune response exemplified by the presence of donor-specific antibodies (DSA) directed against class II antigens, especially DQ antigens. Their clinical significance still remains questionable, and further studies are required to clarify the mechanisms behind this post-transplant immune dysregulation (see Chapter 44 ).
Clinical Relevance of Serum Non–organ-specific Autoantibodies and de novo Autoimmune Hepatitis
Positive titers of anti-nuclear, anti–smooth muscle, or anti–liver-kidney-microsomal antibodies are reported in up to 70% of recipients by 5 to 10 years after LT, regardless of the primary indication. They have been reported in LT recipients with normal liver biochemistry, with chronic hepatitis, or isolated allograft fibrosis progressing toward cirrhosis.
The occurrence of chronic hepatitis with liver dysfunction (namely, elevation of aminotransferases and/or gamma-glutamyl-transferase and/or bilirubin levels), newly recognized serum non–organ-specific autoantibodies, raised serum immunoglobulin levels (particularly immunoglobulin [Ig]G), and dense plasma cell infiltrates in liver biopsy was described more than two decades ago in children undergoing LT for conditions other than autoimmune disorders, and it was termed de novo autoimmune hepatitis (AIH). Since the initial pediatric report in 1998, several series of de novo AIH were reported in children, with prevalence up to 22% of LT recipients in some series, as well as in adults. The patients usually do not respond well to increases of calcineurin inhibitor (CNI) doses, but rather to predniso(lo)ne, often combined with azathioprine (or mycophenolate mofetil), using the same schedule as for classical AIH. Lifelong maintenance therapy with steroids is recommended and reported to provide excellent graft and patient survival.
Alternative names, such as alloimmune hepatitis, de novo immune hepatitis, plasma cell hepatitis, and graft dysfunction mimicking AIH, have been used mostly in adult series, reflecting the incompletely understood pathogenic mechanisms. Several overlapping pathogenic mechanisms have been proposed. The overall consensus is that autoantibody positivity reflects rather than causes the graft injury, and routine annual monitoring is recommended.
In its latest update in 2016, the Banff Working Group on Liver Allograft Pathology proposed replacing the term de novo AIH with plasma cell–rich rejection and included de novo anti–human leukocyte antigen (HLA) DSAs and antibodies to anti–glutathione S-transferase T1 (GSTT1) in null recipients of GSTT1-positive donors as contributory but not mandatory features for the diagnosis of plasma cell–rich rejection ( Table 27.1 ). In this update, non–organ-specific autoantibodies are no longer included as diagnostic criteria, whereas C4d immunostains for detection of complement components are recommended on all biopsies diagnosed as plasma cell–rich rejection. A recent review argues against the use of this nomenclature in pediatric patients who fulfill the criteria for de novo AIH, including increased serum IgG levels and circulating non–organ-specific autoantibodies, as they respond well to the immunosuppressive regimen used for classical AIH ( Table 27.2 ). The authors suggest that pediatric de novo AIH should be categorized separately from adult de novo AIH/plasma cell hepatitis.
Clinical Significance of Serum Anti–Human Leukocyte Antigen Donor-Specific Antibodies and Antibody-Mediated Rejection
Although the liver allografts have long been considered less susceptible to antibody-mediated rejection (AMR) caused by anti-HLA DSAs compared with cardiac or kidney allografts, an increasing number of publications suggest a pathogenic role for anti-HLA DSAs also in the LT. As reviewed by Del Bello et al., any kind of aggression, such as T-cell–mediated rejection or viral and bacterial infection, may induce production of HLA class I and II antigens by hepatocytes, bile duct epithelium, or endothelial cells within the liver transplants. Donor Kupffer cells and interstitial dendritic cells are gradually replaced during the post-transplant period by recipient accessory cells that express self-major histocompatibility complex molecules, which are then capable of presenting antigens to T-lymphocytes. All these changes could then facilitate the development of acute and chronic rejection and potentially also idiopathic graft hepatitis, graft fibrosis, and unexplained biliary complications.
Studies of serial biopsies in LT recipients have suggested that the increased prevalence of graft fibrosis with time may be related to low-grade AMR. Anti-HLA class 2 DSA (mostly anti-DQ) are found in 50% – 60% of children after LT, most of which are formed de novo . Histopathological lesions most strongly associated with persistent DSA presence include low-grade portal, periportal, and perivenular lymphoplasmacytic inflammation with low-grade interface and perivenular necro-inflammatory activity and noninflammatory fibrosis. Putative chronic AMR lesions are associated with microvascular endothelial C4d deposition. In the study of Miyagawa-Hayashino et al., which included 67 LT recipients who had undergone a protocol liver biopsy 5 to 20 years post-transplantation (median, 11 years), a higher degree of fibrosis associated with diffuse C4d-positive endothelial staining was demonstrated in patients with positive DSAs compared with DSA-negative patients.
Low levels of CNI and noncompliance have been identified as predictive factors for de novo DSA formation. In studies examining children who were weaned off all immunosuppression after parental living donor LT, the prevalence of DSAs was higher in those who failed successful weaning (i.e., nontolerant patients), but persistent de novo DSAs were also found in the tolerant patients. Consequently, the absence of DSAs did not appear as a reliable indicator of expected tolerance when planning weaning of immune suppression.
The currently most sensitive Luminex single-antigen bead assay detects anti-HLA DSAs without the ability to specify potential cytotoxic effects. This makes it difficult to assess which antibodies have an influence on the graft outcome and which are innocuous. Anti-DQ DSAs have been associated with worse allograft outcomes, but severe graft injury may also occur with negative or low DSAs. There are data suggesting that DSA of IgG3 subclass and/or those that bind the first subcomponent of complement complex C1 (C1q) are particularly damaging. One recent study has shown that the ability of DSAs to bind complement components (C3d) correlated with the risk of graft failure in LT recipients. These findings may help to identify patients at the higher risk for allograft loss, but they need to be validated in other cohorts of LT recipients.
In standard clinical practice, routine DSA monitoring may help IS management (e.g., to avoid minimization of immune suppression), but further work is needed to better define the following : (1) DSA-associated tissue injury patterns; (2) antibody characteristics (C1q, median fluorescence intensity [MFI], titer, IgG subclass, etc.); and (3) appropriate immune suppressant levels. Stringent chronic AMR criteria ( Table 27.3 ) will help to avoid overdiagnosis until the entire spectrum of pathomorphological manifestations is appreciated. Treatment of AMR, however, is not well defined; plasmapheresis, rituximab, polyclonal antibodies, or bortezomib have been used with variable results.
|Probable chronic active AMR (all four criteria are required):|
|Possible chronic active AMR:|
One increasingly described effect of chronic immunosuppression is the development of allergic conditions in up to 40% of LT recipients, including allergic rhinitis, asthma, atopic dermatitis, food allergies, eosinophilic disease of the gastrointestinal tract, and anaphylaxis. CNIs are implicated as a potential mediator of the post-transplant allergy through a few proposed mechanisms: (1) induction of autoreactivity through abnormal thymic T-cell maturation; (2) abnormal T-regulatory cell numbers and function; and (3) Th1/Th2 imbalance with predominance of Th2 responses leading to allergic tendency. These conditions are more frequent in infants than in older children, with eczema, food allergy and eosinophilic esophagitis, gastritis, and/or enteritis being the most common and occurring closer to the time of transplantation, usually within 1 year, than asthma and allergic rhinitis, which tend to occur later. In a recent Canadian retrospective study of 273 children after transplantation including 111 LT recipients, Marcus et al. confirmed a striking disparity in the prevalence of de novo allergy, which occurred in 40.5% of LT recipients and 40% of heart recipients, compared with only 4% of kidney recipients. A greater prevalence of food allergies was identified in LT (15.3%) than in cardiac graft recipients (4.9%). The majority of patients who developed de novo allergy were codiagnosed with autoimmunity/immune-mediated disorders.
De novo Food Allergy
The development of de novo food allergies (most often to multiple foods) is more common after LT than after other solid organ transplants and much more frequent in children, especially in infants, compared with adult transplant recipients. The reported incidence of food allergies after liver transplant has been shown to vary from 5.6% to 38% in different series, which is considerably higher than the incidence of food allergy in the general population. The incidence has gone up with the increasing use of tacrolimus as the first-line immunosuppressive agent in children. Food allergy most frequently occurs within the first 2 years from the time of transplant. The symptoms could include diarrhea, failure to thrive, flushing, angioedema attacks, wheezing/chronic cough, vomiting, and life-threatening anaphylactic reactions.
Currently, the pathogenesis is not completely understood and is likely a combination of multiple pathways. Food allergy can develop either by passive transfer of donor IgE or allergen-specific donor lymphocytes from a sensitized donor, as initially reported in an adult recipient who presented with a skin rash and laryngeal dyspnea after eating peanuts 3 months after a combined liver and kidney transplantation from an organ donor who died from an anaphylactic reaction to peanut ingestion, or, more commonly, by a loss of tolerance to food antigens on re-exposure to the antigen. The proposed mechanisms for the loss of tolerance to food proteins include an effect of immune suppression toward a shift to Th2 response and IgE production, with deficient suppression of allergen-specific responses via T-reg cells, induced by IS, most notably tacrolimus. Another (or additional) mechanism where tacrolimus might contribute to the food allergy development is through changes in the gastrointestinal barrier resulting in increased mucosal permeability. An impaired secretory immunity based on elevated circulating mucosal IgA was also linked to the development of de novo food allergy in the liver transplant recipients receiving tacrolimus.
The reasons why new food allergies rarely develop following other non-hepatic transplants, even when tacrolimus is used, remain unclear. Young age at transplant appears a significant risk factor for post-transplant allergies. Because children with chronic cholestasis, such as biliary atresia, receive special feeding with an artificial formula and poorly diversified diet until LT, they are likely to have highly altered microbial ecology that can affect the function of the mucosal immune system. It has also been hypothesized that Epstein-Barr virus (EBV) infection, especially in pre-transplant-naive patients, may influence the host immune system to create predominant T-helper type 2 (Th2) cytokine profiles with less cytotoxic T-lymphocyte control, and increase the risks of atopy and allergy.
The identification of LT recipients who are at risk of developing food allergies is an important research goal, because life-threatening anaphylactic reactions may occur in these patients. Allergic conditions in children should be identified before transplantation, along with careful family history and regular assessment for their post-operative emergence. The parents should be educated about the possible development of new food allergies. Those who develop post-transplant food allergies should be managed as other food allergies with avoidance of known allergens and a prescription for auto-injectable epinephrine. Allergic reactions have been reported to either resolve spontaneously with increasing age or after switching tacrolimus to cyclosporine.
Post-transplant Immune Cytopenias
Post-LT, several different immune-mediated disorders have been reported, including autoimmune hemolytic anemia, idiopathic thrombocytopenic purpura, neutropenia, and pancytopenia. Alterations in B-lymphocyte regulation caused by calcineurin inhibition may be a predisposing factor to the development of post-transplant cytopenias. Reports on the resolution of post-transplant autoimmune cytopenias following the conversion of tacrolimus to the mTOR inhibitor sirolimus provide indirect evidence to the potential importance of CNI in the pathogenesis of these post-transplant phenomena. Rituximab, a chimeric murine/human monoclonal antibody to CD20, an antigen present on pre-B and mature B lymphocytes, has also been reported to be an effective therapy of autoimmune cytopenias post–solid organ transplant.
Bone marrow failure may occur in children who underwent LT for acute liver failure (ALF) of unknown etiology. Its severity ranges from mild pancytopenia to severe aplastic anemia, possibly related to immunologically-mediated injury of hematopoietic stem cells. Pancytopenia most often occurs after a median interval of around 1 month following the onset of ALF. Management includes immunoablation with anti-thymocyte/anti-lymphocyte globulin, CNIs and steroids, or hematopoietic stem cell transplantation if a matched related donor is available.
Nonimmune-Mediated Complications of Chronic Immunosuppression
Chronic Kidney Disease
CKD is likely underdiagnosed by standard screening methods. The true prevalence of CKD remains elusive because studies have used a variety of different diagnostic modalities, including estimated glomerular filtration rate (eGFR) formulas, renal inulin or radioisotope clearance for measuring or estimating the GFR, as well as different definitions of CKD. The definition of CKD used by the Kidney Disease Improving Global Outcomes of the US National Kidney Foundation includes either an eGFR lower than 60 mL/min/1.73 m 2 or the presence of structural kidney damage, with or without a decreased GFR ( Table 27.4 ).