**3.1 Cell-mediated rejection of allografts**

After liver transplantation, antibody-mediated, hyperacute vasculitic rejection can occur in individuals with preformed antibodies against the donor's MHC class I–encoded antigens. Under most other circumstances, acute allograft rejection is initiated by the large number of recipient T cells that recognise donor alloantigens (Stefanova I et al., 2003). Thus, the transplantation of MHC histoincompatible tissues elicits a strong, cytopathic, T celldependent immune response to donor tissues. By the T cell-dependent pathway to rejection, graft alloantigens are processed by specialised antigen presenting cells (APCs). Graft MHC molecules are internalised by donor and recipient APCs (Figure 3), following intracellular processing, and MHC peptide fragments are presented to the recipient's T cells (Watschinger B, 1995; Afzali B et al., 2008). Antigen presentation involves the engagement of these peptide antigenic fragments within a groove on the MHC molecules of the APC

alloantigens. In addition, infiltrating leucocytes also launch the process, and it exhibits specificity and memory and is prevented by lymphocyte depletion (Gowans J L, 1962). The major histocompatibility complex (MHC) was identified as encoding the dominant transplantation antigens, and these were shown to be identical to serologically defined human leucocyte antigens (HLA), and subsequently to the elements responsible for the selfrestriction of immunological responses to conventional antigens. The molecular and cellular

> Liver failure

Fig. 2. The evolution of the immune response after liver transplantation. MHC, major histocompatibility complex; TCR, T cell receptor; APC, antigen presenting cell; IFN,

After liver transplantation, antibody-mediated, hyperacute vasculitic rejection can occur in individuals with preformed antibodies against the donor's MHC class I–encoded antigens. Under most other circumstances, acute allograft rejection is initiated by the large number of recipient T cells that recognise donor alloantigens (Stefanova I et al., 2003). Thus, the transplantation of MHC histoincompatible tissues elicits a strong, cytopathic, T celldependent immune response to donor tissues. By the T cell-dependent pathway to rejection, graft alloantigens are processed by specialised antigen presenting cells (APCs). Graft MHC molecules are internalised by donor and recipient APCs (Figure 3), following intracellular processing, and MHC peptide fragments are presented to the recipient's T cells (Watschinger B, 1995; Afzali B et al., 2008). Antigen presentation involves the engagement of these peptide antigenic fragments within a groove on the MHC molecules of the APC

interferon; TNF, tumour necrosis factor

**3.1 Cell-mediated rejection of allografts** 

rejection

Cell activation

Cytokines

APC

T cell, lymphocytes

Donor liver

basis of graft rejection will be described in the next section (Figure 2).

surface. Acute cellular rejection is the best-characterised graft-specific form of immune rejection. Clinically apparent acute cellular rejection is defined by an often-sudden deterioration in allograft function; biopsy analysis of the transplanted tissue shows infiltration by host T cells and other mononuclear leucocytes and signs that these infiltrating cells have damaged the graft. Despite the routine use of immunosuppressive therapy, acute rejection is not rare. Studies show that CD4 and CD8 T cells both participate in acute rejection, although the rejection response is mediated primarily by CD4 T cells. CD4 T cells are activated by the above direct and indirect pathways, and primarily mediate the rejection response (Watschinger B., 1995). Although CD4 T cells are important in rejection, many activated CD8 T cells infiltrate the transplant tissue at the time of rejection, along with other mononuclear leucocytes (Strom TB et al., 1975). The cells of the innate immune system, such as natural killer (NK) cells, are also present in allografts during rejection. NK cells can recognise alloantigens because they constitutively express inhibitory receptors that are specific for self-MHC class I antigens; in addition, cytokines secreted by activated CD4 or CD8 T cells can promote the activation of NK cells, which can initiate and aggravate the rejection response (Dollinger MM et al., 1998).

Fig. 3. Pathways of alloantigen presentation. (A) In the direct pathway, recipient T cells recognise intact allogeneic MHC molecules on the surface of donor APCs. The direct pathway is responsible for the large proportion of T cells that have reactivity against alloantigens due to the cross-reactivity of the T cell receptor (TCR) with self and foreign MHC molecules. (B) In the indirect pathway, recipient APCs trafficking through the allograft phagocytose allogeneic material are shed by donor cells (mostly peptides derived from allogeneic MHC molecules) and presented to the T cells on recipient MHC molecules

#### **3.2 Humoral-mediated rejection of allografts**

The humoral immune response is also important in the mediation of allograft rejection. The production of anti-donor MHC antibodies is associated with acute and chronic graft damage, usually in the form of graft vasculopathy. These antibodies can damage the graft by activating complement and mononuclear cells with Fc receptors that recognise the heavy chain of antibodies. Thus, Fc receptor–expressing leucocytes can be activated by antibodycoated donor cells. Anti-donor antibodies can also directly inhibit signalling cascades within endothelial cells (Li F et al., 2009). Humoral-mediated rejection of allografts is often observed following kidney, heart and lung transplantation, but liver allografts appear to recover in relation to the development of humoral-mediated rejection. Most transplant organs manifest insidious and inexorable dysfunction as time passes. Although this process was formerly called 'chronic rejection', it is not clear that donor-specific immune rejection is the sole or even the primary cause in many conditions (Seetharam A et al., 2010). Pathology analysis often reveals fibrosis and atrophy in the absence of infiltration by T cells and other mononuclear leucocytes. Potential additional causes for chronic allograft failure include viral infection, recurrence of the original disease and drug toxicity. In general, humoralmediated rejection of allografts is relatively uncommon in liver transplantation.

#### **3.3 Memory T cell mediated rejection of allografts**

Following T cell activation and proliferation, homeostasis of the adaptive immune system is restored by cell death – via "neglect" – of most antigen-specific T cells. A small number of T cells, however, survive and become long-lasting memory cells that ensure protective immunity against pathogens. Memory T cells can be divided into central memory and effector memory subsets, based on their circulation pattern and functional responsiveness. With regard to organ transplantation, upon re-exposure to donor antigens donor-reactive memory T cells are more sensitive to antigens, function more rapidly, produce effector cytokines, survive longer than naïve T cells and directly or indirectly produce cytolytic effects on the transplanted tissue (Ku C C et al., 2000; Sallusto F et al., 2000; Garcia S et al., 1999 & Barber DL et., 1999). Central memory T cells are responsible for recall antigen responses, and effector memory T cells survey peripheral tissues and immediately respond to invading pathogens (Sallusto F et al., 2004). As a consequence of continuous exposure to foreign antigens, memory T cells accumulate with time and represent approximately 50% of the total T cell pool in adults. Recipients who have not received a transplanted graft can still generate donor-reactive T cells, which can appear through immunisation by direct exposure to alloantigens via pregnancy or blood transfusion (Bingaman A W et., 2002). Furthermore, donor-reactive memory T cells can be generated in the absence of alloantigen exposure through heterologous immunity. Some memory T cells are therefore primed by an antigenic pathogen-derived peptides and cross-react with allogeneic peptides presented by the self or the donor MHC molecules. Alloreactive naïve T cells can acquire a memory phenotype and generate a substantial pool of donor-reactive memory T cells after transplantation, even when a recipient is under immunosuppressive therapy. Furthermore, the use of antibodies that deplete host T cells can amplify this phenomenon by inducing homeostatic T cell proliferation in response to lymphopenia (Wu Z et al., 2004). Because of their capacity to rapidly generate effector immune responses upon rechallenge, memory T cells appear to be particularly efficient at mediating allograft rejection (Zheng X X et al., 1999 & Schenk A D et al., 2008). In addition, memory T cells are less sensitive than naïve T cells to many immunosuppressive strategies. Compared with conventional T cells, memory T cells are less sensitive to T cell-depleting antibodies and therapeutics that block the CD28 and CD154 costimulatory signallers which inhibit the mammalian target of rapamycin (Pearl J P et al., 2005; Vu M D et al., 2006; Adams A B et al., 2003 & Araki K et al., 2009). The effects of memory T cells on the allograft response have been well delineated in animal models of allograft tolerance, wherein the generation of memory T cells by pre-sensitisation, heterologous immunity or homeostatic proliferation prevents the graft-protecting effects of most tolerising therapeutic strategies (Koyama I et al., 2007 & Valujskikh A et al., 2002). In contrast to human recipients, animals live in the protected environments of transplantation

recover in relation to the development of humoral-mediated rejection. Most transplant organs manifest insidious and inexorable dysfunction as time passes. Although this process was formerly called 'chronic rejection', it is not clear that donor-specific immune rejection is the sole or even the primary cause in many conditions (Seetharam A et al., 2010). Pathology analysis often reveals fibrosis and atrophy in the absence of infiltration by T cells and other mononuclear leucocytes. Potential additional causes for chronic allograft failure include viral infection, recurrence of the original disease and drug toxicity. In general, humoral-

Following T cell activation and proliferation, homeostasis of the adaptive immune system is restored by cell death – via "neglect" – of most antigen-specific T cells. A small number of T cells, however, survive and become long-lasting memory cells that ensure protective immunity against pathogens. Memory T cells can be divided into central memory and effector memory subsets, based on their circulation pattern and functional responsiveness. With regard to organ transplantation, upon re-exposure to donor antigens donor-reactive memory T cells are more sensitive to antigens, function more rapidly, produce effector cytokines, survive longer than naïve T cells and directly or indirectly produce cytolytic effects on the transplanted tissue (Ku C C et al., 2000; Sallusto F et al., 2000; Garcia S et al., 1999 & Barber DL et., 1999). Central memory T cells are responsible for recall antigen responses, and effector memory T cells survey peripheral tissues and immediately respond to invading pathogens (Sallusto F et al., 2004). As a consequence of continuous exposure to foreign antigens, memory T cells accumulate with time and represent approximately 50% of the total T cell pool in adults. Recipients who have not received a transplanted graft can still generate donor-reactive T cells, which can appear through immunisation by direct exposure to alloantigens via pregnancy or blood transfusion (Bingaman A W et., 2002). Furthermore, donor-reactive memory T cells can be generated in the absence of alloantigen exposure through heterologous immunity. Some memory T cells are therefore primed by an antigenic pathogen-derived peptides and cross-react with allogeneic peptides presented by the self or the donor MHC molecules. Alloreactive naïve T cells can acquire a memory phenotype and generate a substantial pool of donor-reactive memory T cells after transplantation, even when a recipient is under immunosuppressive therapy. Furthermore, the use of antibodies that deplete host T cells can amplify this phenomenon by inducing homeostatic T cell proliferation in response to lymphopenia (Wu Z et al., 2004). Because of their capacity to rapidly generate effector immune responses upon rechallenge, memory T cells appear to be particularly efficient at mediating allograft rejection (Zheng X X et al., 1999 & Schenk A D et al., 2008). In addition, memory T cells are less sensitive than naïve T cells to many immunosuppressive strategies. Compared with conventional T cells, memory T cells are less sensitive to T cell-depleting antibodies and therapeutics that block the CD28 and CD154 costimulatory signallers which inhibit the mammalian target of rapamycin (Pearl J P et al., 2005; Vu M D et al., 2006; Adams A B et al., 2003 & Araki K et al., 2009). The effects of memory T cells on the allograft response have been well delineated in animal models of allograft tolerance, wherein the generation of memory T cells by pre-sensitisation, heterologous immunity or homeostatic proliferation prevents the graft-protecting effects of most tolerising therapeutic strategies (Koyama I et al., 2007 & Valujskikh A et al., 2002). In contrast to human recipients, animals live in the protected environments of transplantation

mediated rejection of allografts is relatively uncommon in liver transplantation.

**3.3 Memory T cell mediated rejection of allografts** 

laboratories and do not usually contain substantial numbers of memory T cells. This is one of the reasons that may explain the difficulties of translating into the clinic the results of protocols capable of creating allograft tolerance in rodent models. But the results cannot be applied in clinical conditions. Given the lower efficacy of conventional immunosuppressive drugs on activated memory lymphocytes, it is not surprising that memory T cells also exert harmful effects in clinical transplantation. In transplant studies, it is clearly understood that memory T cells – however they are generated – pose a significant barrier to inducing tolerance to allografts (Chalasani G et al., 2002; Zhai Y et al., 2002 & Adams AB et al., 2003). Thus, a better understanding of how to target this cell population and the designing novel of therapies that inhibit these cells would be beneficial.
