Signal two is lymphocyte activation, also known as costimulation, a process in which ligands on the APC cell bind to T-lymphocyte receptors. CD80 and CD86 markers on dendritic cells engage CD28 receptors on T lymphocytes (T-lymphocyte receptor, TCR); the receptor complex is then internalized and activates signal transduction pathways (Wiesner and Fung 2011). The TCR complex binds immunophilin; this stimulates calcineurin which activates nuclear factor of T-lymphocyte activation (NFAT). NFAT then translocates to the cell nucleus and signals IL-2 transcription (Fig. 1). Two immunosuppressive drugs target immunophilin, and cyclophilin and FK-binding protein (FKBP) are targets of cyclosporine and tacrolimus.

Signal three is clonal expansion; transduction pathways from signal two activate numerous molecules, including IL-2 and other cytokines. Newly synthesized IL-2 is produced by T lymphocytes and binds to IL-2 receptors on the T-lymphocyte surface in a paracrine and autocrine fashion. This stimulates the downstream pathway of the mammalian target of rapamycin (mTOR) which is the trigger for cell proliferation (Fig. 1) (Martinez and Rosen 2005). Immunosuppressive drugs target multiple steps at this process, both basiliximab and daclizumab are monoclonal antibodies against the IL-2 receptor, and sirolimus and everolimus are mTOR inhibitors that block cell proliferation.

The last step in the immunological rejection response is graft inflammation. Activated T lymphocytes precipitate differentiation in their ability to mediate effector functions (i.e., secrete cytokines and kill) and by modifying an array of cell surface molecules, including L-selectin (lymph node homing receptor) and chemokine receptors. This allows cells to migrate from lymphoid organs and circulate throughout the periphery. Activated T lymphocytes express a multitude of chemokine receptors which attract various immune cells to the site of inflammation, the allograft. The result is an inflammatory milieu with toxic and vasoactive mediators. Effector T lymphocytes reencounter their specific alloantigens on the graft and initiate effector mechanisms leading to cytolysis (Heeger and Dinavahi 2012).

Liver transplant immunosuppression can be classified into two distinct phases: induction and maintenance. Induction immunosuppression usually refers to a period of intense immunosuppression given in the first 30 days after transplant when the risk of organ rejection is highest (Kirk 2006). Maintenance immunosuppression refers to the immunosuppressive regimen an organ transplant recipient is continued on after the initial 30 days; in liver transplant recipients, there is usually a decrease in the number of drugs and dosages needed to prevent rejection (Wiesner and Fung 2011). With the exception of autoimmune liver disease and hepatitis C virus, most LTRs may receive similar regimens with steroid induction with or without adjunctive antibody therapy, followed by two maintenance agents and eventual steroid withdrawal (Fig. 2).

Induction regimens in the USA have typically referred to the use of antilymphocyte-depleting agents; this is only used in a small percentage of liver transplant recipients, but in the past decade, the use of induction has increased. In 2003, the overall use of induction immunosuppression regimens was approximately 20 %; in 2013, nearly 30 % of liver transplant recipients were given induction therapy (Fig. 3). More commonly peri- and postoperatively, these patients are started on a common drug regimen that includes high-dose corticosteroids with a rapid taper, a calcineurin inhibitor, and an antimetabolite agent such as mycophenolate (Wiesner and Fung 2011).

The role of antilymphocyte-depleting agents has been clearly defined in other organ transplants such as kidney transplant; however, in liver transplant recipients, the data is not as clear. Traditionally, induction therapy has been given as an adjunct to conventional immunosuppression to prevent the risk of rejection, to delay or minimize calcineurin exposure to prevent renal dysfunction, or to facilitate steroid avoidance and decrease the risk of HCV recurrence.

Induction therapy as an adjunct to conventional immunosuppression refers to adding a lymphocyte-depleting or a nonlymphocyte-depleting agent as an adjunct to a conventional calcineurin-based regimen. The most commonly used lymphocyte-depleting agent in the USA is antithymocyte globulin (Thymoglobulin); this is a polyclonal rabbit, derived antibody preparation which targets multiple epitopes on T lymphocytes. The result is nonspecific T-lymphocyte depletion with a dose-dependent lymphopenia that suppresses T lymphocytes for up to 90 days post administration (Boillot et al. 2009). (A more in–depth explanation of pharmacology and monitoring will be explained in the section “ Immunosuppressive Drugs ”.)

Antithymocyte globulin induction to prevent rejection in liver transplantation has been evaluated in two small, randomized controlled trials. Boillot et al. evaluated antithymocyte globulin induction added to conventional maintenance therapy with tacrolimus/mycophenolate/prednisone vs. conventional therapy without induction, in 93 patients (Boillot et al. 2009). The primary outcome of rejection at 5 years was not different between groups. Bogetti et al. evaluated antithymocyte globulin induction added to maintenance of tacrolimus/prednisone in 22 patients (Bogetti et al. 2005). Compared to tacrolimus/prednisone alone, induction did not improve rejection rates at 3 months. Neither study showed differences in safety outcomes, including infection and malignancy. This limited data suggests that adding antithymocyte globulin induction therapy does not reduce rejection risk in liver transplant recipients.

Alemtuzumab (Campath) is a monoclonal antibody that targets the CD52 T-lymphocyte receptor that depletes mature lymphocytes for up to 1 year post administration. Experience using alemtuzumab in liver transplantation is limited. An NIH-sponsored study showed that alemtuzumab induction permits weaning to spaced tacrolimus monotherapy with comparable efficacy outcomes as conventional immunosuppression in de novo liver transplant recipients (Marcos et al. 2004). However, despite comparable outcomes between groups in the subgroup with hepatitis C virus, a large number of those exposed to alemtuzumab experienced complications or graft loss related to hepatitis C virus reactivation. Similar trials evaluating alemtuzumab induction followed by tacrolimus monotherapy in liver transplant yielded similar efficacy results, but excluded patients with hepatitis C virus (Tzakis et al. 2004; Tryphonopoulos et al. 2005). A recent retrospective case-controlled study of 140 patients without hepatitis C virus found that alemtuzumab followed by tacrolimus monotherapy, compared to conventional therapy, was associated with comparable efficacy outcomes, less hypertension and fewer cases of rejection, but a 43.6 % relative risk increase for infections, particularly viral infections (Levitsky et al. 2011). Overall, while alemtuzumab appears to permit less intense maintenance immunosuppression in liver transplantation, its increased risk of infection and hepatitis C reactivation remains concerning.

The lymphocyte-nondepleting agents, basiliximab (Simulect) and daclizumab (Zenapax), selectively target the activated T-lymphocyte IL-2 receptor, CD25, which stops downstream signaling of T-lymphocyte proliferation and activation, and effects are sustained for 1–2 months post administration. Several studies have evaluated interleukin-2 receptor antagonists (IL2RAs) as induction agents to prevent acute rejection in liver transplantation. A meta-analysis including 12 randomized controlled trials and 3,251 patients showed that IL2RA induction was associated with significant reduction in rejection at 1 year (23 % vs. 28 %) (Wang et al. 2010). The difference was significant for the subgroup of eight studies of daclizumab, but not for the four studies of basiliximab. Regardless of IL2RA, the overall improvement in rejection rates did not translate to an improvement in graft or patient survival. A recent Cochrane review comparing any antibody induction therapy to no induction found induction to be associated with statistically significantly less acute rejection (RR 0.85, CI 0.75–0.96), but nonsignificant differences in death or graft loss (Penninga et al. 2012).

Induction therapy has been used to facilitate calcineurin inhibitor minimization. It is known that calcineurin inhibitors cause renal dysfunction. Studies have shown that there is an 8 % incidence of chronic renal failure at 1 year post-OLT and 18 % at 5 years posttransplant. Also, an elevated creatinine 1 month posttransplant carries an increased risk of developing chronic renal failure (Gonwa et al. 2001). The presence of renal dysfunction is associated with significant morbidity and mortality after liver transplant.

The use of antithymocyte globulin induction to delay the initiation of calcineurin inhibitors has been evaluated in two retrospective studies. Tchervenkov et al. showed that antithymocyte globulin induction and delayed CNI introduction were associated with a modest, yet significantly improved, serum creatinine at 6 months (1.39 vs. 1.56 mg/dL) (Tchervenkov et al. 2004). Similarly, Soliman et al. showed modest improvements in serum creatinine and estimated glomerular filtration rates at 1 year (1.26 vs. 1.37 mg/dL and 81 vs. 75 mL/min) with antithymocyte globulin induction (Soliman et al. 2007). Both studies found reduced rejection rates with antithymocyte globulin induction, suggesting that antithymocyte globulin may permit the safe delay of CNI introduction in an attempt to improve short- to midterm renal function.

The use of IL2RAs to delay CNI has also been evaluated. The ReSpECT study was a prospective, randomized controlled trial comparing three arms: daclizumab with delayed reduced-dose tacrolimus/mycophenolate/prednisone, delayed reduced-dose tacrolimus/mycophenolate/prednisone, or standard-dose tacrolimus/prednisone (n = 517) (Neuberger et al. 2009). At 26 weeks, the estimated GFR was significantly improved in the group that received daclizumab (difference of 14.5 mL/min) compared to standard-dose tacrolimus, without differences in other efficacy or safety endpoints. Furthermore, a meta-analysis subgroup including six randomized controlled trials, with durations ranging 3–24 months, showed that the addition of IL2RAs to delayed or reduced CNIs was associated with improved renal function (estimated GFR difference 6.29 mL/min, 3 studies, n = 641) and lower rates of renal dysfunction (RR 0.46; CI 0.27–0.78, 5 studies, n = 778) (Goralczyk et al. 2011). A recent Cochrane review showed that any antibody induction was associated with higher serum creatinine (0.04 mg/dL; CI 0.003–0.08 mg/dL), although GFR was not significantly affected (Penninga et al. 2012). These data suggest that IL2RAs can be used to safely delay the initiation of CNIs, and this strategy is associated with improved short- to midterm renal function.

Last, induction regimens have been employed as a strategy to facilitate steroid avoidance. Along with the numerous metabolic effects of steroids, they have been associated with recurrence of hepatitis C, a major cause of liver transplantation and graft failure. Antithymocyte globulin and IL2RAs have been successful in steroid-free induction with no difference in acute cellular rejection; however, a difference in hepatitis C recurrence or adverse metabolic effects of steroid use has not been shown (Eason et al. 2001; Turner and Knechtle 2013). The use of alemtuzumab as an induction agent to minimize steroid use is not recommended because it has been associated with an increased incidence of hepatitis C recurrence (Turner and Knechtle 2013).

Maintenance immunosuppression typically refers to the immunosuppressive regimen a liver transplant recipient (LTR) is started on after the first 30 days. However, one should think of a maintenance regimen in the immediate postoperative state. Early initiation of CNI regimens has shown to prevent acute rejection and prolong graft survival. The addition of antimetabolite agents confers additional immunosuppression while augmenting the long-term side effects of CNIs. The clinician must also take into account a recipient’s underlying liver disease. For instance, LTRs transplanted for autoimmune hepatitis will require higher doses of immunosuppression, often with the complement of low-dose steroids indefinitely. In contrast, patients transplanted for alcoholic cirrhosis and steatohepatitis will require lower doses of immunosuppression and can often be maintained low-dose dual-drug regimens or monotherapy. In rare instances, LTRs can maintain operational tolerance, whereas they have good graft function off all immunosuppression. Figure 3 shows the current immunosuppressive regimens in the USA. An in-depth review of the available immunosuppressive medications and their use in maintenance of the liver transplant recipient, as well as therapeutic monitoring and short- and long-term side effects, will be explored in the following section on “Immunosuppressive Drugs.”