Transplantation; A force to be reckon with or is it not?

Transplantation is the replacement of defective organs/tissues and is one of the biggest achievements of modern day medicine involving the use of donor organs and tissues such as kidneys, lungs, intestines, liver, pancreas, blood, foetal tissue, bone marrow and retina. MHC or major histocompatibility complex are clusters of genes localised within chromosome 6p21.31 and varies in length depending on their haplotype. MHC genes are known to have a high polymorphic rate and are determined within the germ line. Human MHC is known as HLA or human leukocyte antigen and consists of 4 million DNA base pairs, 98 genes and 96 pseudogenes (Fig.1). MHC is classified as class I, II (Fig.2) and III and was initially proposed by Jan Klein in 1977. Both class I and II MHC antigens play a pivotal role in antigen presentation [AP] thus essential in the process of transplantation which has increased since the 1950s. Majority of class I molecules are found amongst nucleated cells and platelets while class II molecules are of lymphoid origin thus found in thymic stromal cells, antigen presenting cells [APCs] such as dendritic cells [DC] and B-cells. The hallmark of HLA alleles is their diversity thus; the genes within the complex play an important role in the immune system and control a number of antigens that influence graft rejection. All immuno-competent hosts recongnize and initiate an immune response to foreighn antigens on grafted cells or tissues resulting in rejection while immuno-compromised hosts identify the foreighn antigens on grafted cells or tissues of immuno-competent nature such as lymphoid cells, leading to damage of host tissues and cell lines due to the activation of T cells by MHC molecules. Therefore, MHC molecules are essential in mechanisms invovling the maintainence of a successful transplant (Goodsell, 2005).

 Grafts are classified as; xenografts, allografts and isografts (Fig.3). Xenografts are defined as grafts between various species such as heterologous while allografts are grafts between members of the same species. Members of identicle genetic composition of the same species such as twins are subjected to isografts. The law of transplantation states allografts that lack histocompatability gene expression are accepted into a primate. Transportation of any MHC antigen that is absent in the recipeint and is present in the donor results in graft rejection. The laws of transplantation also states grafts between sygenic animals consisting of an inbred strain are accepted while grafts between allogenic animals are rejected. Although hybrids between two inbred strains such as grafts from either parents will be accepted, grafts to eaither parents may be rejected(Fig.4). However, there are some exceptions in the case of allogenic grafts of solid oragns such as kidneys and the liver that may survive (MSKCC, 2010).

A major obstacle for transplantation of organs is rejection of a graft due to the differences in MHC/HLA antigens between the donor and recepient. It is almost impossible to have identicle organs between donars and recepients and complications arise during the crossing of the HLA barrier. Thus transplantation requires suitable mechanisms for suppression of allograft rejection. Mechanisms involved in allograft rejection conspire to host defence responses to infection, antigen-specific reactions, allo-antigen recognition (Fig.5), adhesion of cells and accessory signals. Lymphocyte responses to allo-antigens are higher than those to conventional antigens. T cells identify allogenic MHC molecules thus an elevated level of allo-reactive T-cells may be observed resulting from a high quantity of peptides within the peptide groove of a single allogenic MHC molecule. Allogenic T cells are unlikely to identify allogenic MHCs with an empty cleft, hence, the allo-reactive T cells recongize allogenic MHC peptides.  AP is affected by graft derived cells and APCs of the recepient. The term direct-recognition[DR] is used when AP is goverend by graft derived APCs (Fig.6). DR lacks processing of alloantigens by host APCs and their associtiation with self-MHC molecules of the cell. Thus, the classical pathway of AP is not apart of DR. However, graft MHC antigens are recognized via host APCs thus known as indirect-recognition[IR] (Schurman et al 1994).  DR associated allo-recognition is followed by lymphokine secretion of allo-reactive T cells within the graft. Expression of class II molecules on endothelial and epithelial cells are induced by IFN-γ presenting CD4 + T helper cells. Upon post-transplantation, leukocytes exit the graft thus the epithelial and endothelial cells remain as donor MHC class II cells with consequences such as naive T cell deletion, memory cell anergy with inhibition of IL-2 secretion and transition of antigen-specific T cell clones to Th2 phenotype silencing or switching. Other studies have showed addition of B7 may lead to proliferation of resting T cells prompting lack of effects of co-stimulation by non-professional APCs (Womer et al 2001). T regs such as CD4+CD25+Foxp3 specific for the DR pathway have shown to prevent acute rejection while T regs spceific for both IR and DR prevented chronic and acute rejection establishing a chimeric hematopoietic state hence demonstrating potential theraputic approaches to induce permenant immunological tolerance amongst allogenic transplantation patients (Joffre et al 2008).

IR involves the same pathway as the conventional pathway (Fig.6). Hence, IR involved with allo-reactivity does not completley explain the presence of allo-reactive cells in higher quantities in the peripheral T-cell pool. Other arguments favouring DR is due to observations involving rat transplants where graft rejection was compramised by antibodies of the recepient antigens of MHC class II. Majority of APC populations within a graft is formed by passenger leukocytes of the intestital tissue and have been observed within recepient spleens. This population dissapears upon transplantation and is replaced by recipient leukocytes followed by a gradual shift from DR to IR after transplantation. Therefore, DR is considered to be important in the initial stages of transplantation. IR associated with acute rejection results in progression into chronic rejection. This theory has been supported by blood lymphocyte studies amongst kidney transplant patients (Schurman et al 1994). IR associated with skin grafts lacking MHC class II antigens result in rapid rejection although organ rejection in some patients may be due to both IR and DR. Deletion of donor APC results in prolonged survival of the graft and is caused by transportation of donor antigens to recepient lymph nodes by donor APCs. The direct stimulation of effector cells involved in rejection of grafts with the sensitization reaction occuring via the IR pathway by class I antigens of the donor APC or APCs consisting of allogenic determinants lead to elevations in expression of MHC class II causing reduced antigenecity of graft via the reduction of the amounts of potential allogenic determinants for the IR pathway (Gokman et al 2008). There are 2 important possibilities for the production of an IR by peptides of MHC; the repertoire hypothesis where bias selection of T-cells result in the identification of peptides of fragments formed by MHC or the membrane expression hypothesis involving the availability of endocytosed MHC as peptides for APCs following proteolytic and endocytotic activity. MHC peptides are considered to be better for the stimulation of an IR comparatively to minor histocompatability antigen peptides thus unfavouring any importance for the pathway involving DR pathways (Game et al 2002) . However, if IR plays the centre stage in graft survival so does antigen matching to prevent rejection of grafts although this has not completely been established as yet. The amino acid seuqence of MHC antigens will have the key to unlocking the mystery of their immunogenicity while antigen compatability may also depend on the potential of peptide representation by the recepient’s MHC molecules. Currently efforts are being made to gain a better understanding into the amino acid sequcences of donor MHCs (Gould et al 1999).

 Graft rejection is classified as hyperacute [HAR], accelerated and chronic (Yang et al 2007). Although HAR and humoral xenograft rejection may be prevented, xenografts may be subjected to cell-mediated rejection although some transgenic pigs are able to overcome this type of rejection. An advantage of xenotransplantation over allotransplantation is the ability to pre-transplant donor organs to indice rejection responses by recepients. Transplantaion of such organs into baboons survived for 8 days without any HAR (Rojers et al 1998). Another approach to prevent rejection of transgenic organs may be the use of CD46 with genetic modifications in a non-segregating manner such as large vector like chromosomes and transgene expression of inducible nature of pig cell lines have been developed and have become promising organ donors (Klymiuk et al 2009).

Since IR is predominanlty favoured, it is more likely to cause graft rejection thus DR may prevent graft rejection upon the completion of the initial immune response stage as evident by some observations made during a study in slow skin grafts involving lack of MHC antigen expression in donors where some recepients under-went rapid graft rejection while others had prolonged graft rejection. Thus, the results of the study are coherent with the hypothesis involving MHC peptides and IR. However, recent skin graft studies conducted with mismatched minor or class I antigens using cells with and withiout class II antigens from donors alongside recepients with identical class II resulted in the rejection of class II deficient grafts than the ones with class II. Experimental data also showed, recepient CD4+ cells stimulate rapid graft rejection amongst the class II deficient while they prevent rejection in the presence of class II antigens. However, DR may initiate rapid graft rejection during the initial stages and is evident from experimental studies conducted using mutant mice deficient of class II antigens that acutely reject allografts using CD4+ cell dependent mechanisms. Thus, this may suggest the exsistence of an inhibitory effect by DR upon transplantatation. The tendency of DR to inhibit IR with time is another hypothesis suggesting an explanation governing the importance of MHC antigen matching during graft survival and has been explained in greater detail in Fig.7. The conventional concepts rely mainly on the clarity of the MHC antigen disparities in the promotion of graft rejection while there have been new found evidence shifting this view to the importance of sharing of antigens during the promotion of graft survival (Gould et al 1999).

Class II matching is more important than class I as transplant studies indicated class II matched pigs had long term survival than class I mismatched pigs. Class II molecules are in line with the repertoire hypothesis as they are more likely to be invovled with IR. However, current technical flaws have prevented researchers from obtaining the percentage of peptides bound to class II and class I thus the claims are inconclusive (Gould et al 1999). Genes of class II have been implicated in regulatory mechanisms of  regulatory T cell [T-reg] responses. Class II matching between donor and recepient allograft subjects favour  T-reg tolerance to transplants providing an area for future investigations. Another study invovling the regulation of class II included the use of class II genes of recepient cardiac allografts that have indicated donor specific tolerance following long term survival of B6. However, thrid party allografts were subjected to immunosuppression while accepted cardiac transplants lacked allograft vasculopathy. Furthermore, initiation of tolerogenic responses by intracellular antibodies were mediated by Tregs polarizing anti-graft responses to Th2 cytokine producers thus implicating the role of class II is likely to  be Treg induction and potentiates a mechanism invovling specific T cell inactivation during antibody induces allograft rejection (Leguern et al 2010).

However, investigative studies have established both MHC class I and II molecules are expressed with various retinal transplant tissue. Rabbits have been used to observe responses to allogenic sub-retinal neuro-retinal transplants in the form of either embryonic cell fragments or embryonic retina of full thickness. Scleral deposition of both classes of MHCs on the side of the graft with defects in pigmented epithelium of the retina were observed, suggesting different host immune responses against intact neuroretinal and fragmented grafts. Immune reponses were absent in full thickness grafts that has enabled scientists to embark in new stragtegies in therapeutics and transplants for retinal degenrative diseases (Ghosh et al 2000).

Engraftment of hematopoietic stem cells from unrelated donors has been influenced via disparities between the donor and recepient for HLA-A, B and C alleles. Recent studies have indicated disparity between donor and the recepient at the HLA-A, B and C loci is a risk factor for graft failure after bone marrow transplantation amongst leukemia patients  where the donor was unrelated. The risk factor also increases with the number of mis-matched loci. Tranplant donor and recepient pairs with different HLA antigens consist of different alleles and is referred to as antigen mismatched. These HLA mismatches are subjected to amino acid substitutions within the peptide binding region. Other reasons for graft failure increase include; multiple mismatch amongst HLA class I alleles or multiple antigens, mismatch in HLA class II alleles, the stage of the disease during the process of transplantation, homozygous donor and totle body radiation dose. These characteristics are mainly used to assess the risks invovled in the failure of a graft upon stem cell transplantation amongst chronic myeloid leukemia with myeloablative preconditioning patients (Peterdorf et al 2001).

Other research relating to non-marrow ablative hematopoeitic cell transplant in correlation with MHC moleculs has been invovled in identifying the role of MHC in preventing type I diabetes [TID]. Destruction of islets amongst TID patients is a result of aberrant T-cell activation within a permisive genetic background (Serreze et al 2001). Both non-obese diabetic mice [NOD] and humans with TID have shown a genetic link may be associated with MHC class II alleles. It has also been observed NOD mice express a specific single MHC class II molecule at I-Ag7, a primary gene in the suceptability of the disease. However, the mechanisms invovling the susceptability of the disease by MHC class II molecules is yet to be determined although it has been hypothesized that impairment of MHC class II molecule/peptide interactions have altered mechanisms involved in mediating T cell tolerance. Although, the pattern of inheritance amongst NOD disorder is complex, the main allele, I-Ag7, has been observed amongst marrow derived cells suggesting donor cells play a role in the exertion of influence via class II molecules. This has been confirmed via investigative studies. MHC class II molecules are said to play an important role in the modulation of T cell resposes and the selection of T cells (Beilhack et al 2005).  

Xenografts and allografts avoiding donor origin professional APCs induce T cell ignorance leading to organ acceptance and has been analysed currenlty via investigative studies using rat pancreatic islets in culture within mice to remove APCs. The technique is successfull amongst rodents but has been difficult to achieve using solid organs such as kidneys. However, there are mechanisms via total irradiation and cyclophosphamide pre-treatment of donors by which DC may be depleted from kidneys leading to prolonged graft survival amongst RT1[rat loci] incompatible hosts. As a shortage of human kidneys persist in transplantation, pig kidneys have been suggested for allotransplantation. However, this has been problamatic due to the presence of natural antibodies to MHC-antigens of vascular endothelium of pigs. Induction of T cell ignorance occur upon pig-human metanephroi transplantation, as this is after   PVG         PVG-RT1 rat transplantaion thus use of xenograft metanephroi in transplantation processes prove to be advantages in relation to the use of developmental  kidneys (Rogers et al 2001) (Fig.8).

The very latest in post-transplantation studies of lungs using mice indicate antibody development is due to mismatched donor-MHC class I. The mice indicated elevated expression of growth factors, chemokines and their receptors inducing IL-17, K-α 1 tubulin and collagen-V while IL-17 neutralisation via anti-IL-17 lead to the decrease of auto-antibodies and lesions caused by anti-MHC class I antibodies. Hence, the results indicated donor-MHC antibodies induce autoimmunity mediated by IL-17 that is an essential player of chronic rejection post-lung transplantation therefore; autoimmunity prevention is a factor to be considered in the prevention of transplant rejection and treatment (Fukami et al 2010).

Donor reactive T cells promote graft rejection although their mechanisms are unknown. Within 24-72 hours post transplantation within allografts IFN-γ and CXCL9/Mig are produced. Recipient CD8 memory T cells produce IFN-γ in response to MHC class I while donor cells produced CXCL9 due to IFNγ. Studies have also shown activated CD8 cells were detected in allografts 3 days prior to effecter cells produce IFN-γ within recipient spleens while the CD8 memory cells enhance the infiltration of effecter  T cells suggesting neutralization of CD 8 infiltrating memory T cells result in optimal inhibition of alloimmunity (Schenk et al 2008).

CD28/B7 is a co-stimulatory pathway associated with CD28 and B7 ligands CD86 and CD80 on APCs and is non-antigen-specific pathway. T cells activated via TCR in the absence of CD28 co-stimulation acquire an antigen-specific unresponsiveness or apoptosis resulting in an abortive immune response. Thus CD28/B7 may be used in selective immunosuppressant with the reagent CTLA4-Ig which binds to B7 with high affinity. Studies conducted using CTLA4-Ig in allograft rejection indicate its immunosuppressive effects lacking major toxicity and therapeutic potential and has since developed analogues which are currently in clinical trials for kidney transplantation (Dumont, 2004).

FK506 or Tacrolimus is a baseline immunosuppressant used in solid organ transplant such as liver allograft rejection, kidney, bone marrow, primary heart and primary adult pulmonary transplantation. FK506 has been compared to cyclosporine another potent immunosuppressive agent used in primary liver transplant. Studies have implicated FK506 associated graft rejection is low compared to cyclosporine and is said to be an efficient dose adjustable agent (AHN et al 2003).

The facts collectively suggest MHC class II is more important in graft response compared to class I. Thus, the survival of a transplant is dependent upon the behavioural patterns of MHC molecules and the complications involving the rejection of grafts suggest further research of MHC antigens and their correlations with the immune system and transplantation is essential for successful procedures in the future.