Future of Solid Organ Transplantation: Organ-Specific Tolerance

A transplant between two people who are not genetically identical is called an allotransplant and the process is called allotransplantation. Donor organs and tissues can be from people who are living, or people who have died because of a significant brain injury or lack of circulation. Allotransplantation can create a rejection process where the immune system of the recipient attacks the foreign donor organ or tissue and destroys it. The recipient may need to take immunosuppressive medication for the rest of their life to reduce the risk of rejection of the donated organ. In general, deliberately induced immunosuppression is performed to prevent the body from rejecting an organ transplant. The adverse effects associated with these agents and the risks of long-term immunosuppression present a number of challenges for the clinician. Immune tolerance, or immunological tolerance, or immunotolerance, is a state of unresponsiveness of the immune system to substances or tissue that have the capacity to elicit an immune response in a given organism.

In clinical practice, operational tolerance is defined as "a well-functionning graft lacking histological signs of rejection, in the absence of any immunosuppressive drugs in an immunucompetent host" [7,8].
An animal is formally proven to be tolerant when in the absence of immunosuppression, a second graft from the same donor is accepted, while a graft from a third-party donor is rejected.
In general, operationally tolerant transplant recipient cannot be identified prospectively. Due to the lack of biomarkers to guide weaning or cessation of immunsuppressive drugs, the majority of recipients will rely on life-long immunsuppressive therapy. This situation is especially problematic in kidney transplantation where tolerance is a very rare event [9].
In general there are two kinds of tolerance; central (intrathymic) and peripheral (non-thymic).
Positive selection, also called thymic education, ensures that only clones with TCRs and moderate affinity for self-MHC are allowed to develop.
Negative selection by means of apoptosis occurs when T cells have extremely high affinity for the MHC-self-peptide complex.
Many potentially reactive T cells escape thymic selection; this reflects that many antigens are absent intrathymi- Запрошені статті / Guest Articles cally or present at insufficient levels to induce tolerance in the thymus; so several non-thymic mechanisms prevent autoimmunity and are also capable of rendering peripheral T cell repertoires tolerant. These mechanisms are: -sequestration of antigens into privileged sites; -apoptosis of T cells caused by persistent activation or neglect; -clonal anergy (lack of costimulation) (CD28-CD80/86, CD40-CD40L); -regulatory T cells (Tregs, CD4+CD25+FoxP3+ T cells).
Clinical research to induce full or partial tolerance in transplant patients has been induced in allograft transplantation in many centers. A state of indefinite survival of a well-functionning allograft without the need for maintenence immunsuppression was the main target of the researchers. Rare cases of operational tolerance after transplantation with complete cessation of immunosuppressive therapy have been reported [10,11].
Full tolerance was achieved with myeloablative therapy before organ transplantation in combination with induced donor chimerism in hematologic malignancies treated with bone marrow transplantation [12].
At present partial tolerance or minimal immunsuppression is possible. This partial or incomplete, donor-specific tolerance has been termed prope tolerance or minimal immunsuppression tolerance [13,14].
Stable graft function for 1 year or more referred as functional or operational tolerance [15,16].
The reasons for graft loss can be broadly classified into three categories: 1) inflammation induced reactions against graft tissues, specifically ischemia-reperfusion (I-R) injury; 2) immun-initiated reactions against graft tissues; 3) direct organ toxicity by immunsuppressive drugs. When an alloantigen is recognized, the innate and adaptive immun systems respond synergistically to reject the allograft through non-exclusive pathways, including contactdependent T cell cytotoxicity, granulocyte activation by either Th1-or Th2-derived cytokines, NK cell activation, alloantibody production and complement activation [17].
Improvements in the short term success of renal and extra-renal transplantation have had a minimal impact on long term success and the rate of late graft loss is essentially unchanged [18,19]. The advantages associated with the avoidance of chronic immunsuppression continue to drive the enthusiasm for implementing approaches to induce tolerance to transplanted organ allografts as the term chronic rejection is mainly characterized by antibody-mediated rejection and a score to reflect insterstitial fibrosis and tubular atrophy [20].

Strategies for inducing transplantation tolerance
There are two obligatory components to achieving transplantation tolerance: depletion of alloreactive Tconv and upregulation of alloreactive Treg cells. The balance between graft destruction and regulation can be shifted using strategies to inhibit the activity of Tconv cells and/or increase the relative frequency or functional activity of alloantigenreactive Treg cells.
Mixed chimeric and cellular tolerogenic therapies are being trialed where drug-based therapies have failed [21,22].

Manipulating innate immune system
TLRs drive innate immune responses as part of I-R (ischemia-reperfusion) injury and this leads to the subsequent initiation of adaptive alloimmune responses; so deficiency in the TLR adaptor protein MyD88 leads to donor antigen-specific tolerance. MyD88 deficiency is associated with an altered balance of Tregs over Tconv cells promoting tolerance instead of rejection.

Lymphodepletional strategies
Lymphodepletion in the form of "induction therapy" is an effective strategy for adressing the precursor frequency of alloreactive Tconv cells at the time of organ transplantation and preventing acute allograft rejection. However, ongoing maintenance therapy during post-deletional cell repopulation is necessary to prevent T memory cells from driving rejection and alloantibody formation (mAb, radiation and cytotoxic drugs are necessary) [23].

Cellular therapy
A. İn addition to CD4+ CD25+ FoxP3+ nTregs and iTregs; Tr1 cells produce large amounts of IL Using rabbit ATG and Rituximab (plus FK and Sirolimus) for tolerance induction in living-donor renal recipient [32].
Alemtuzumab (Campath-1H), mAb to CD 52, found densely distributed on T and B cells and NK cells [33]. Alemtuzumab in combination therapy with costimulation blockade, regulatory T cell infusion and donor stem cell transfusion are some of the novel approaches to tolerance induction currently in study [34][35][36][37][38].
В. B cells have also been shown to serve a regulatory role; unlike Tregs there are no validated molecular or phenotipic markers to define Bregs, so they are currently defined on the functional basis of their IL-10 production [39].
Particularly the role of transitional B cells is important; they represent a regulatory B cell population based on their increased IL-10 production; meanwhile it is noticed that no difference in B cell subsets (total, naive, transitional) or inhibitory cytokines (IL-10 and TGFb) was detected when compared to healthy controls The tolerogenic properties of DCs include the ability to acquire and present antigen, expand and respond to antigen-specific Tregs, constitutively express low levels of MHC and costimulatory molecules, produce high IL-10 and TGFb and low IL-12, resist activation by danger signals and CD40 ligation, resist killing by NK or T cells and promote apoptosis of effector T cells [57].
-Mesenchymal stromal cells (MSCs) have immunomodulatory properties, they inhibit T cell activation and proliferation possibly due to the production of nitric oxide and IDO (indoleamine-2,3-dioxygenase) [61]. MSCs harvested from term fetal membranes have been shown to significantly suppress allogeneic lymphocyte proliferation in mixed lymphocye reactions (MLR) by suppressing IFNg and IL-17 production and increasing IL-10 production [62,63].
Chimerism is the concept that cells of different donor origins can coexist in the same organism. It might be derived into "mixed" or "microchimerism" and "full" or "macrochimerism".
Mixed is defined as the presence of both donor and recipient cell lineages coexisting in the recipient bone marrow.
Full chimerism implies complete elimination of recipient hematopoietic lineages and population of the recipient bone marrow by 100 % donor cells [64].
The main aim should be that donor cells that could attack the host and cause GVHD need to be eliminated while at the same time preserving the recipient's ability to produce immune populations that can defend against infections [65]. This might be realised by partiall irradiation of the recipient bone marrow with peripheral deletion of recipient T cells allowed for the development of both donor and recipient hematopoietic cells and induction of tolerance to donor tissue without the need for full myoablation [66][67][68]. Lastly in kidney transplantation, as the tolerance has two components, central and peripheral, the induction strategy consists of thymic irradiation to allow for development of a donor T cell reservoir in these organ recipients [69][70][71]. 6. An increase proportion of central memory cells and a decreased proportion of effector cells [78].

Kidney Transplant Tolerance
7. Upregulation of many TGFb regulated genes, as well as downregulation of costimulatory and T cell activation genes [79].

Conclusions
Limited data exist on the capacity of the currently defined biomarkers of tolerance to identify patients in which immunosuppressive drugs can be withdrawn.
Induction of chimerism in combination with kidney transplantation might provide development of central tolerance by deletion [80].
The proteosome inhibitor Bortezomib in combination with donor specific transfusion (DST) might be suitable since Bortezomib induces apoptosis of highly activated lymphocyte including plasma cells, B cells and T cells [83,84]