1. Transplantation
The Ags expressed on tissues and organs as well as leukocytes "MHC Ags" are highly polymorphic in the population, so matching donor and recipient is extremely difficult.
Our understanding of cellular and molecular mechanisms associated with graft rejection and the use of immunosuppressive therapies have made transplantation of various cells, tissues or organs very commonplace.

2. Relationship between donor and recipient

3. (1) An Autograft It is a graft or transplant from one area to another on the same individual. e.g. transplantation of normal skin from one area of an individual to a burned area of the same individual. The graft is recognized as autochthonous or autologous "self", no immune response is induced against it. (2) Isograft or syngraft It is a graft or transplant of cells, tissues or organ from one individual to another who is syngeneic "genetically identical" to the donor. e.g. the transplantation of kidney from one identical "homozygotic" twin to the other. The two individuals "donor & recipient" are described as histocompatible.

4. (3) An Allograft It is a graft or transplant from one individual to an MHC-disparate individual of the same species. Because of the high degree of MHC polymorphism within a given outbred species, this allogenic transplant will result in rejection of the grafted foreign tissue. The two individuals are histoincompatible. (4) A xenograft It is a graft between a donor and a recipient from different species. The transplant is recognized as foreign, the immune response will reject the graft. Donor and recipient are histoincompatible.

5. Allografts are rejected after 2 weeks "firs-set rejection".
Second allografts from the same donor to the same recipient, the graft is rejected after 6-8 days "2nd-set-rejection" due to specific immunologic memory for Ags expressed by the graft.
CD4+ T cells play crucial role in rejection. This can be proved by transferring these cells from sensitized individual to an allograft into a normal syngeneic recipient. If the second recipient is transplanted with the same allograft used in the first transplantation, second-set rejection ensues.

6. Abs can also contribute to the destruction of grafted tissue in second-set-rejection.
Histological examination of the site of rejection reveals lymphocytic, monocytic cellular infiltration "DTH" both CD4+ and CD8+ cells are also present.
Animals that lack T lymphocytes do not reject allografts or xenografts.
The process of rejection slows down considerably or does not occur at all in immunosuppressed individuals.

7. Categories of Allograft Rejection
(1) Hyperacute rejection
(2) Acute rejection.
(3) Chronic rejection
It occurs within few minutes to few hours of transplantation as a result of destruction of the transplant by preformed Abs to incompatible MHC Ags and in some cases to carbohydrates expressed on transplanted tissues "e.g. on endothelial cells".

8. Preformed Abs are generated as a result of previous transplantation, blood transfusions or multiple pregnancies.
These cytotoxic Abs activate complement,platelet activation, interstitial hemorrhage in the transplanted tissue which decrease the flow of blood in it.
CMI is not involved in hyperacute rejection.
(2) Acute rejection.
Occurs in a recipient who has not previously been sensitized to the transplant.
It is mediated by T cells as a direct recognition of alloantigens expressed by the donor cells.

9. The transplanted tissue is a mismatch and insufficient immunosuppressive treatment to prevent rejection.
Cell mediated immunity is manifested by intense infiltration of lymphocytes and M.
Acute rejection reaction may be reduced by immuno-suppressive therapy.

10. (3) Chronic rejection
Caused by both Ab & CMI.
It occurs months or years after the transplanted tissue has assumed its normal function.
It is characterized by slow, progressive loss of function.
Bone marrow and skin grafts are very sensitive to rejection compared to heart, kidney and liver grafts.

11. Role of MHC Molecules in Allograft Rejection
Transplantation Ags or MHC Ags are associated with graft rejection as they evoke an immune response.
MHC molecules are cell surface proteins.
All nucleated cells express MHC class I molecules whereas MHC class II are expressed by APCS.
Immune response will be directed predominantly against MHC class I Ags expressed on the cells in the grafted tissue.

12. Foreign MHC molecules activate an enormous number of T cell clones in the recipient. It is estimated that up to 5% of all T cell clones in the body may be activated in response to alloantigen activation.
The combination of non-self MHC molecule with their bound peptides cross reacts with TCRs expressed on many different T cell clones.
Cytokines secreted by activated CD4+ T cell clones activate alloreactive cytotoxic T cells.

13. Mechanism of Alloantigen Recognition by T Cells
Fig.18.2 shows two mechanisms; direct and indirect.
The direct activation of T cells by alloantigens is due to recognition of donor-derived MHC Ags expressed by donor cells serving as APC.
Indirect activation of T cells occurs via recognition of donor-derived cellular peptides(mostly minor H Ags) bound to MHC Ags expressed by host APC.
Role of T-cell Lineage and Cytokines in Allograft Rejection
Alloactivation of T cells generates both Allospecific CD4+ and CD8+ cells.

14. The cytokines produced are synthesized mainly by activated CD4+ T-cell clones.
The most important cytokines generated during these responses are IL-2,IFN-α, IFN-β, IFN-γ, TNF-α and TNF-β.
IL-2 is important for T-cell proliferation and for differentiation of CTL and TH1 cells participating in the DTH reactions associated with allograft rejection.
IFN-γ is important for the activation of macrophages, which migrate to the graft area and cause tissue damage.
TNF-β is cytotoxic to the cells present in the graft. .

15. IFN-α, IFN-β, TNF-β and TNF-α increase the expression of class I molecules, while IFN-γ increases the expression of class II molecules on both host and allograft cells, thus increasing the effectiveness of Ag recognition and enhancing graft rejection.
Depending on the microenvironment in peripheral lymphoid organs, helper CD4+ T cells differentiate in different types of effector cells that mediate different types of responses.
In the presence of IL-12, TH1 cells dominate and mediate IFN-γ-dependent macrophage activation and DTH,.

16. In the presence of IL-4 TH2 cells mediate IL-5 dependent eosinophilic rejection.
In the presence of TGF-β, IL-6 and IL-23, TH17 cells emerge and are thought to mediate neutrophilic rejection.
In the presence of TGF-β alone, Treg cells dominate and promote allograft acceptance by multiple mechanisms,

19. Laboratory Tests in Tissue typing
Phenotyping of MHC of both donor and recipient "cross-matching" to minimize risks of graft rejection.
Tissue typing consists of the analysis of HLA and allow to determine the degree of foreignness between the two individuals.
HLA analyses in bone marrow transplantation is critical to minimize graft rejection.
1- Serologic Detection of MHC Ags:
A panel of HLA-specific antisera or monoclonal Abs is used to determine the phenotype of MHC Ags expressed on cells.

20. Antisera may be obtained from people who had multiple transplantations or transfusions and from multiparous women.
Immunofluorescence or fluorescence activated cell sorting (FACs) analysis are used for this purpose.
Lymphocytotoxicity test is also used, in this test, MHC-specific Abs in the presence of complement, are used as cytotoxic, complement-fixing Abs to determine their ability to damage the target cells, which are lymphocytes.
Lymphocytes of the donor and recipient are reacted with a panel of Abs with specificity for a wide range of MHC class I and class II alleles. e.g. HLA-A, HLA-DRB.
Complement-mediated killing of such cells indicates that the test Ab has reacted with the cell surface MHC. Thus the cells are positive for that MHC allele.

21. 2- Genotyping of MHC using PCR
3- Mixed Leukocyte Reactions "MLR"
Leukocytes from donor and recipient.
Culture together for several days.
Donor T cell respond to allo-MHC Ags expressed on the recipient cells and are stimulated to produce cytokines and proliferate in the presence of these Ags.
The proliferation is measured by introducing radiolabeled thymidine into the culture.

23. Prolongation of Allograft Survival
Immunosuppressive Therapy Induction Therapy
It is used to suppress the immune system two weeks pretransplantation to reduce the incidence of immediate rejection of the graft.
 Maintenance Therapy
Combinations of synergistic immunosuppressive drugs are used to interfere with specific immune mechanisms(such as T cell activation), drug doses are lower than the induction therapy to allow the immune system to function at low level and to minimize the incidence of opportunistic infections.
Specific Treatments

24. In some cases, episodes of acute rejection may occur months or years after transplantation. Immunosuppressive drugs are used at levels similar to those used in induction therapy regimens.
(1) Anti-inflammatory Agents
e.g. corticosteroids "prednisolone, methylprednisolone"
Corticosteroids down-regulate gene expression of several genes that code for inflammatory cytokines. These include IL-1, IL-2, IL-3, IL-4, IL-5, IL-8, TNF-α, and GM-CSF.

25. Corticosteroids also inhibit expression of adhesion molecules, which in turn inhibit leukocyte migration to sites of inflammation, corticosteroids therefore inhibit the activity of inflammatory cells.
Corticosteroids promote the release of cellular endonucleases, leading to the induction of apoptosis in lymphocytes and eosinophils. In addition, they reduce phagocytosis and killing by neutrophils and macrophages and reduce expression of MHC class II molecules. In this way, corticosteroids inhibit T cell activation and T cell function.

26. (2) Cytotoxic drugs
Antimetabolites(azathioprine, mercaptopurine, mycophenolate, they are purine antagonists). They interfere with the synthesis of RNA and DNA by inhibiting inosinic acid which acts as precursor of the purines.
Chlorambucil and Cyclophosphamide Alkylate DNA and interfere with its metabolism. They were originally developed to treat cancer, as they are also cytotoxic to lymphocytes, they have been used as immunosuppressive agents.

27. (3) Agents That Interfere With Cytokine Production and Signaling
Cyclosporine and FK-506(tacrolimus). They interfere with calcineurin by binding to immunophilins B, a family of intracellular proteins involved with lymphocyte signaling pathways, thus signal transduction pathways needed for clonal expansion of lymphocytes are inhibited. In addition, Cyclosporine induces the synthesis of TGF-β, a cytokine with immunosuppressive capability. Cyclosporine is effective when administered before transplantation but is ineffective in suppressing ongoing rejection.
Cyclosporine is nephrotoxic and associated with an increased risk of cancer when administered longterm.

28. (4) Immunosuppressive Antibody Therapy
Antilymphocyte Ab preparations, such as horse and rabbit antilymphocyte globulin(ATG) have been used as adjuncts to standard immunosuppressive therapy. They may induce serum sickness.
Monoclonal Abs and engineered mouse-human chimeric Abs or humanized Abs are being used. OKT3 is directed against CD3 expressed on T cells. Daclizamab and Basiliximab are specific for IL-2 receptor α-chain(CD25) have been used. They down-regulate expression of IL-2 receptor on activated T cells.

29. N.B It has been suggested that skewing of responses toward TH17 or TH1 and away from Treg cells may be responsible for acute rejection of organ transplants. Blocking key cytokines in vivo, most notably IL-6, may result in a shift from TH17 toward a regulatory phenotype(Treg) to help prevent transplant rejection.

30. Hematopoietic Stem Cell Transplantation(HCT)
Transplantation of hematopoietic stem cells constitutes a special transplantation situation because it is performed mostly between an immunocompetent donor and an immunocompromised recipient.
Historically, the procedure involved the use of bone marrow as a source; today, peripheral blood is more commonly used.
HCT is used to treat a variety of conditions, including SCID, Wiskott-Aldrich syndrome* and advanced leukemia.

31. HCT is also used to treat blood cell diseases such as thalassemia and sickle cell diseases, in which a mutant gene is inherited. The mutant gene expresses itself only in the blood-forming hematopoietic cells. In these patients, transplantation is a form of gene therapy: the genetically abnormal blood-forming stem cells are replaced with normally functioning cells.
The ultimate goal of HCT is to restore or reconstitute normal hematopoiesis of the recipient. Hematopoietic stem cells give rise to all blood cell lineages.

32. Small number of these pluripotent cells also circulate in the blood.
G-CSF is used to increase the number of hematopoietic stem cells in both the marrow and the blood. The quantitation of these cells is facilitated by their unique expression of CD34.
Syngeneic stem cell transplantation is associated with low risk because of the genetic similarity between donor and recipient.

33. Autologous HCT is a technique of obtaining stem cells from blood or marrow and returning them to the same individual. It is commonly used to treat patients with hematologic malignancies such as leukemia, lymphoma or myeloma.
Allogenic HCT involves the use of donor cells obtained from blood, bone marrow or umbilical cord sources and placental blood in which the concentration of blood cell-forming stem cells is even greater than in the blood of adults. Two rejection outcomes may result:

34. The donor stem cells and the hematopoietic cells to which they give rise may be rejected by the recipient(host-versus-graft effect) OR
An immune reaction to host MHC Ags may occur(graft-versus-host disease).
When the recipient is immunocompetent, immune rejection by host T cells is usually prevented by intensive immunosuppressive therapy of the recipient prior to the transplant.
Patients with immunodeficiency diseases(such as SCID) do not require induction therapy since there is no risk of rejection by the host.
To reduce the risk of graft-versus-host disease, T cells are eliminated from the donor cell population e.g. by monoclonal anti-T-cell Abs and complement.

35. Graft-Versus-Host Disease
HCT from a donor to an HLA-disparate recipient results in a reaction mounted by
The grafted T cells against the recipient MHC (and/or minor H) Ags. This response manifests as graft-versus-host disease(GVHD)
GVHD occurs when immunocompetent lymphoid cells are transplanted into individuals who are immunologically compromised (such as those who have undergone high-dose radiation therapy or chemotherapy).

36. Acute GVHD is responsible for 15-40% of HCT transplant mortality and is the major cause of morbidity after allogenic HCT.
Chronic GVHD occurs in up to 50% of patients who survive three months after HCT.
Acute GVHD represents an exaggerated inflammatory response against foreign Ags
(host allo-Ags), donor lymphocytes encounter tissues in the recipient that have been damaged due to an underlying disease of the host or pre-HCT therapy regimens such as radiation or chemotherapy.

37. In humans, GVHD may produce splenomegaly, hepatomegaly, lymphadenopathy, diarrhea, anemia, weight loss, and other disorders in which the underlying causes are inflammation and destruction of tissues.
GVHD is initiated by donor-derived T cells that recognize the recipient MHC Ags
(and minor H Ags) as foreign.
Standard procedures are used to eliminate all mature T cells from the donor hematopoietic stem cell source, but the pluripotent stem cells will give rise to donor- derived T cells overtime.

38. Most of the innate inflammatory cells that participate in the destruction of host cells recruited to the site of the reaction by cytokines mainly TNF-α and IL-1 released by activated donor participating in the GVH reaction.
Unless, the reaction is controlled, GVHD activates destructive immune defense mechanisms carried out by donor and host derived cells that may lead to death of the recipient.
GVHD can be modulated and controlled using a variety of immunosuppressive agents
Such as cyclosporine or tacrolimus along with short course methotrexate.

39. The Fetus: A Tolerated Allograft
The fetus Expresses paternal MHC Ags that are not expressed by the mother, It is not rejected by the mother as an allograft.
Mother can mount an Ab response against fetal Ags e.g Rh Abs, in addition, women who experienced multiple births have Abs to the father MHC Ags.
Abs are harmless to the fetus.
Mother does not produce Cytotoxic T cells against the fetus.

40. Fetal trophoblasts, which constitute the outer layer of the placenta and come in contact with maternal tissue, do not express polymorphic MHC class I or Class II molecules; they appear to express only the nonpolymorphic class Ib MHC molecule, HLA-G. thus, the fetal trophoblast does not prime for a cellular immune response associated with allograft rejection.
HLA-G may provide a ligand for the KIRs on maternal NK cells, thus preventing them from killing the fetal cells.
HLA-G is also expressed in thymic medullary epithelium, where it might ensure T-cell tolerance to this molecule.

41. No cells expressing large amounts of MHC class II molecules(e. g
No cells expressing large amounts of MHC class II molecules(e.g., dendritic cells) have been found in placenta.
α-fetoprotein, a protein synthesized in the yolk sac and fetal liver, also appears to be important in the survival of the fetus and has been demonstrated to have immunosuppressive properties.
As fetus is not affected by immune components, it is one of the Immunologically privileged sites.

42. Transplantation of Specific Organs and Tissues 
1-Skin
Transplanted in case of burns, chronic wounds, diabetic ulcers, and venous ulcers. Commonly autologous grafts, recently artificial skin has been used( consisting of sromal elements and cultured cells of allogenic or xenogenic origin).
2-Kidney
In case of end-stage renal failure.
Graft survival exceeds 85% at one year even with organs from unrelated donors.
3-Liver
In cases like hepatoma and biliary atrasia.
Successful in about two-thirds of recipients at 1 year.

43. 4-Heart
In cardiac failure.
Survival rates in excess of 80% at 1 year.
5-Lung
In advanced pulmonary or cardiopulmonary diseases.
Sometimes performed together with heart transplantation.
6-Bone marrow
In cases of incurable leukemias and lymphomas, congenital immunodeficiency diseases.
There is risk of GVH , transplantation of hematopoietic cells being used.

44. 7- Corneas
In case of blindness.
HLA matching not advantageous since this is a privileged site that normally lacks lymphatic drainage.
8-Pancreas
In case of diabetes mellitus.
Pancreas and kidney transplantation sometimes performed together, success rates approaching that seen wit kidney transplants.