Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright © 2008 Saunders, An Imprint of Elsevier
SECTION IV – Transplant...
encounter of the APC and the T lymphocyte is generally considered to be the first point of possible immunosuppressive atta...
our understanding of the immune responses has evolved, more specific targeting of immune system activation has become
  po...
Figure 27-5 Principal mechanisms of innate and adaptive immunity. The mechanisms
of innate immunity provide the initial de...
Table 27-2 -- Summary of Cell Surface CD Markers
MARKER MAIN CELLULAR EXPRESSION                                        FU...
MARKER MAIN CELLULAR EXPRESSION                                         FUNCTION


ADCC, antibody-dependent cellular cytot...
must bind to the antigen presented on the MHC on an APC (signal 1), be stabilized by co-stimulatory molecules (signal 2), ...
response to antigens ( Fig. 27-8 ). To balance the enhancing response, another T-cell surface molecule is present that inh...
The two distinct TH populations (TH1 and TH2 subsets)
                                                                    ...
In humans there are nine different immunoglobulin
                                                                        ...
digestion, that are bound to MHC molecules. There are two types of cell surface MHC molecules: class I and class II ( Fig....
include tumor necrosis factor-α (TNF-α) and TNF-β, complement components, heat shock protein, and nuclear transcription
fa...
with foreign antigens. For the most part, class I molecules contain peptides that originate inside the cell, whereas class...
Studies have been conducted to compare HLA-DR typing with the traditional serologic method versus PCR methods. Serologic
 ...
The mechanism of chronic rejection is less clearly defined and is an area of intense study. Chronic rejection appears as f...
complexes
    10.   Inducing donor-specific transplantation tolerance[5]

  Potential sites for regulation are discussed i...
Although outcomes have significantly improved, infections remain a major problem in transplantation despite prophylaxis.[1...
Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright ...
Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright ...
Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright ...
Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright ...
Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright ...
Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright ...
Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright ...
Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright ...
Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright ...
Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright ...
Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright ...
Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright ...
Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright ...
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  1. 1. Townsend: Sabiston Textbook of Surgery, 18th ed. Copyright © 2008 Saunders, An Imprint of Elsevier SECTION IV – Transplantation and Immunology CHAPTER 27 – Transplantation Immunology and Immunosuppression Darla K. Granger, MD Suzanne T. Ildstad, MD Transplantation of solid organs has become the treatment of choice for end-stage renal, hepatic, cardiac, and pulmonary disease. The field has progressed rapidly in the past 5 decades, primarily because of the development of safer and more effective immunosuppressive agents. After Carrel described a reliable technique for vascular anastomoses in the early 1900s, the technical problems confronting surgeons seeking to replace diseased kidneys and other solid organs were largely resolved. However, the crucial advance that made clinical organ transplantation feasible between unrelated individuals was the development of immunosuppressive drugs to prevent or control rejection. The combination of azathioprine with corticosteroids, introduced in 1962, was the first effective clinical immunosuppressive regimen. The introduction of cyclosporine in 1978, a specific and nonmyelotoxic immunosuppressant, changed heart and liver transplantation from research to service procedures and dramatically increased the success rates of renal transplantation. Continued improvements in the control of rejection at both the cellular and molecular level have been possible as a result of increased understanding of the complexity of the immune system and the events that constitute the rejection process. Because outcomes may vary with the type of graft and the patient's clinical history, the choice of immuno-suppression depends on complete understanding of the interrelationship between the host and graft. In the past decade a diverse armamentarium of immunosuppressive agents targeting various aspects of the immune system has emerged and allowed a significant reduction in the toxicity of immunosuppression. CONCEPTUAL APPROACHES TO IMMUNOSUPPRESSIVE THERAPY The key components of the immune system are lymphocytes, antigen-presenting cells (APCs), and effector cells ( Fig. 27-1 ). Each plays a specific role in generating an immune response to foreign invaders, most notably pathogens. Unfortunately, the immune system cannot discriminate good invaders (organ transplants) from bad invaders (pathogens). Lymphocytes have an essential, central role in the immune response and mediate its specificity.[1] The rejection reaction begins when T lymphocytes recognize foreign histocompatibility antigens on cells of the transplanted tissue. The foreign antigen is thought to be presented directly to host lymphocytes by APCs, most notably dendritic cells and macrophages, which phagocytose and then display the processed antigenic epitope on their surface. The ability to differentiate self from nonself resides with lymphocytes.[1] Early in the development of the body's immune system, groups or clones of lymphocytes are formed that have discrete target specificity. A lymphocyte can therefore recognize only one or a few closely related antigens. The range of possible antigen configurations is matched by a panoply of lymphocyte clones arrayed against them. Immune specificity is acquired during early development, and it is postulated that fully competent clones of small resting lymphocytes await immunologic stimulation by foreign tissue antigens ( Fig. 27-2 ). Among the vast variety of antigens that can be recognized are foreign antigens, which are governed by the major histocompatibility complex (MHC).[1] Figure 27-1 Principal cells of the immune system. The major cell types involved in immune responses and their Stimulation of a resting lymphocyte by the antigen for functions are shown. Micrographs in the left panels which it is specific causes it to transform into a large active illustrate the morphology of some of the cells of each type. cell that secretes chemical communicators called cytokines. (From Introduction to the immune system) These soluble proteins or glycoproteins ( Table 27-1 ) are effective across short distances and, in turn, amplify the response and activate other cells.[1] Before the antigen is disposed of, however, a series of cellular and subcellular events ensue. Interference with this complex series of events at one or more stages offers many opportunities for therapeutic intervention to suppress the rejection response.[2] For transplant patients,
  2. 2. encounter of the APC and the T lymphocyte is generally considered to be the first point of possible immunosuppressive attack. Once the lymphocyte has responded to a foreign antigen and becomes activated ( Fig. 27-3 ), immunosuppressive therapy is less effective. Many cells and molecules are involved. Specific effectors, such as preformed antibodies and activated killer (cytotoxic) lymphocytes, as well as nonspecific agents such as platelets, neutrophils, complement, and coagulation factors, are difficult to suppress. Suppression of only one or two effectors is ineffective.[3] Table 27-1 -- Summary of Cytokines and Their Associated Functions[*] CYTOKINE CELL SOURCE FUNCTIONS Interleukin-1 IL-1 Mononuclear phagocytes, T and B cells, NK, Proliferation of T and B cells; fever, inflammation; endothelial cell activation; increases cells fibroblasts, neutrophils, smooth muscle liver protein synthesis. Binds to CD121 cells Interleukin-2 IL-2 Activated T cells T-cell growth factor, cytotoxic T-cell generation; B-cell proliferation/differentiation; growth/activation of NK cells. Binds to CD122 Interleukin-4 IL-4 CD4+ T cells, mast cells B-cell activation/differentiation, T- and mast cell growth factor. Binds to CDw124 Interleukin-5 IL-5 T cell Eosinophil proliferation/activation. Binds to CD125 Interleukin-6 IL-6 Mononuclear phagocyte, T cell, endothelial B-cell proliferation/differentiation; T-cell activation; increases liver acute phase cells reactants; fever, inflammation. Binds to CD126 Interleukin-7 IL-7 Bone marrow, thymic stromal cells, spleen cells Stimulates growth of progenitor B cells and T cells and mature T cells Interleukin-8 IL-8 Lymphocytes, monocytes, multiple other cell Stimulates granulocyte activity, chemotactic activity; potent angiogenic factor types Interleukin-9 IL-9 Activated TH2 lymphocytes Enhances proliferation of T cells, mast cell lines, erythroid precursors, and megakaryoblastic cell lines Interleukin-10 IL-10 Mononuclear phagocyte, T cells B-cell activation/differentiation, inhibition, mononuclear phagocyte Interleukin-11 IL-11 Fibroblasts, bone marrow stromal cell lines Stimulates growth of hematopoietic multipotential and committed megakaryocytic and macrophage progenitors, stimulates growth of plasmacytomas, inhibits adipogenesis Interleukin-12 IL-12 Mononuclear phagocyte, dendritic cell IFN-γ synthesis, T-cell cytolytic function, CD4+ T-cell differentiation Interleukin-13 IL-13 Activated T cells Inhibits cytokine and nitric oxide production by activated macrophages, induces B-cell proliferation, stimulates IgE and IgG isotype switching Interleukin-14 IL-14 T cells and some B-cell tumors Enhances proliferation of activated B cells, inhibits immunoglobulin synthesis Interleukin-15 IL-15 Mononuclear phagocyte, others NK and T-cell proliferation Interferon-γ IFN-γ NK and T cells Increased expression of class I and class II MHC, activates macrophages and endothelial cells, augments NK activity, antiviral. Binds to CDw119 Interferon-α, β IFN-α, Mononuclear phagocyte—α Mononuclear phagocyte increases class I MHC expression, antiviral, NK-cell activation. β Binds to CD118 Fibroblast—β Tumor necrosis TNF-α, NK and T cells, mononuclear phagocyte B-cell growth/differentiation, enhances T-cell function, macrophage activator, factor-α, β β neutrophil activator. Binds to CD120 Transforming TGF-β T cells, mononuclear phagocyte T-cell inhibition growth factor-β T cell Neutrophil activator, endothelial activation Lymphotoxin Adapted from Abbas AK, Lichtman AH, Pober JS: Cellular and Molecular Immunology, 4th ed. Philadelphia, WB Saunders, 2000. MHC, major histocompatibility complex; NK, natural killer. * Cytokines are secreted polypeptides that mediate autocrine (act on self) and paracrine (nearby) cellular communication but do not bind antigen. They include compounds previously termed interleukins and lymphokines. In the early days of organ transplantation, the major problem was suppression of allograft rejection. Even though such suppression can be achieved, its consequences and potential dangers are apparent. Immunosuppressive agents act largely in a broad, nonspecific manner to suppress the entire immune response. As a result, there is increased risk for opportunistic infections and malignancy. Effective general immunosuppression can cripple the host's response to infections or suppress other proliferating cells (e.g., bone marrow and intestinal mucosal cells). Infections with agents such as cytomegalovirus (CMV) and Pneumocystis carinii, which are not life threatening to normal individuals, frequently become lethal to a transplant recipient. As
  3. 3. our understanding of the immune responses has evolved, more specific targeting of immune system activation has become possible. At present, clinical immunosuppression relies on three general approaches. The first is to simply deplete circulating lymphocytes by destroying them. The second is to use an inhibitor of lymphocyte activation (cyclosporine or tacrolimus [formerly FK-506]) to interrupt the early events of antigen-induced T-lymphocyte activation and cytokine production crucial for the subsequent cascade of immunologic events leading to graft rejection. The third is to use various metabolic inhibitors (e.g., azathioprine, mycophenolate mofetil [MMF]) to interfere with the lymphocyte proliferation essential for amplification of the response. These agents are biochemically specific but do not distinguish between dividing lymphocytes and other proliferating cells. [7] [8] Future progress in immunosuppressive therapy involves the successful implementation of an antigen-specific approach in which the goal is to induce long-lasting donor-specific unresponsiveness (immunologic tolerance) in the host while preserving general immunocompetence.[5] The full promise of transplantation will not be fulfilled until graft rejection can be specifically and safely prevented while the integrity of the immune system as a whole is maintained. Such tolerance of the recipient to allografted organs without the requirement for nonspecific immunosuppression is the ultimate goal in clinical transplantation.[5] Approaches to achieve tolerance are discussed later. Finally, because the number of individuals who can benefit from a transplant far exceeds the number of donors available, xenotransplantation and stem cell–mediated regeneration of damaged tissues are considered by some to hold promise for the future. THE CELLS INVOLVED Key Components of the Immune System The immune system is composed of innate and adaptive immune responses ( Fig. 27-4 ). Innate, or natural, immunity is a powerful early defense mechanism that is immediate and precedes the adaptive immune response. Phagocytes, natural killer (NK) cells, and complement are the critical components of innate immunity. Adaptive immune responses entail sequential, genetically programmed phases involving recognition of antigen by lymphocytes; activation, differentiation, and proliferation of the lymphocytes; and an effector phase to eliminate the antigen. T and B lymphocytes are the only cells with specific receptors for recognizing antigen. However, innate immune responses also influence the development of adaptive immune responses. The two components cross-regulate each other by bidirectional cellular crosstalk ( Fig. 27-5 ). Figure 27-4 Specificity of innate immunity and adaptive immunity. The important features of the specificity and receptors of innate and adaptive immunity are summarized, with selected examples, some of which are illustrated in the boxed panels. (From Introduction to the immune system. In Abbas A, Lichtman AH: Basic Immunology: Functions and Disorders of the Immune System, Updated Edition 2006-2007, 2nd ed. Philadelphia, Elsevier, 2006.)
  4. 4. Figure 27-5 Principal mechanisms of innate and adaptive immunity. The mechanisms of innate immunity provide the initial defense against infections. Some of the mechanisms prevent infections (e.g., epithelial barriers), whereas others eliminate microbes (e.g., phagocytes, natural killer [NK] cells, and the complement system). Adaptive immune responses develop later and are mediated by lymphocytes and their products. Antibodies block infections and eliminate microbes, and T lymphocytes eradicate intracellular microbes. The kinetics of the innate and adaptive immune responses are approximations and may vary in different infections. (From Introduction to the immune system. In Abbas A, Lichtman AH: Basic Immunology: Functions and Disorders of the Immune System, Updated Edition 2006-2007, 2nd ed. Philadelphia, Elsevier, 2006.) Development of the lymphoid system begins with pluripotential stem cells in the liver of the fetus. As the fetus matures, the bone marrow becomes the primary site for lymphopoiesis. The pre–T cells migrate to the thymus, which becomes the primary lymphoid organ wherein CD3+ T lymphocytes mature and become educated to self. Mature T cells are then released to populate peripheral lymphoid tissues, including the lymph nodes, spleen, and gut. In the thymus, T cells acquire their cell surface antigen- specific receptors (T-cell receptors [TCRs]) ( Table 27-2 ), which in turn confer specificity to the immune system and immune responses.[1] Another lymphocyte subpopulation produced by the hematopoietic stem cell is the B cell. B cells derive their name from the primary lymphoid organ that produces B cells in birds, the bursa of Fabricius. In humans and other mammals, bone marrow is the primary site of B-cell development.[1] T cells, B cells, NK cells, and APCs have unique roles in orchestrating the immune response. It is a very tightly controlled network, with most communication mediated by cytokines. B cells have the unique capacity to synthesize antibody. A behavioral difference between B and T cells reflects their functional abilities. B cells are specialized to respond to whole antigen by synthesizing and secreting antibody that can interact with antigen at distant sites. The T cells that are responsible for cell- mediated immunity must migrate to the periphery to neutralize or eliminate foreign antigens. From the peripheral blood, T cells enter the lymph nodes or spleen through highly specialized regions in the postcapillary venules. After exiting the lymphoid tissue through the efferent lymph, they percolate through the thoracic duct and return to the blood to begin recirculation in quest of antigen. When an organ is transplanted, responsive clones of T cells are activated in the organ itself. In addition, donor dendritic cells leave the graft, home to host lymph nodes, and stimulate both host T cells and B cells therein. Activated T cells leave the lymph nodes and can augment the cellular response in the graft. B cells send out antibody molecules that bind to antigens in the graft within a few days, thereby mediating destructive reactions.[1] Considerable progress has been made in dissecting the mechanisms of T-cell maturation in the thymus. Precursor T cells migrate to the thymus, where they undergo a series of preprogrammed maturational changes. All T cells express on their surface an antigen-specific TCR that is the site for antigen binding. The majority of T cells are αβ-TCR+. A smaller subpopulation, which primarily resides in the gut, is γδ-TCR+. There are also transmembrane proteins (CD3) with the TCR. Collectively, these complexes compose the TCR complex and provide the signaling molecules needed to respond to foreign antigens. As the components and mechanism of T-cell activation have been defined, new, more specific immunosuppressive agents have been developed to selectively suppress the rejection response. Thymic stromal cells produce two types of molecules that are important for T-cell maturation. The first type consists of thymic hormones (e.g., thymopoietin, thymosin) and the cytokine interleukin-7 (IL-7), which regulate functional differentiation of the peripheral T-cell system. The second type consists of MHC molecules, which are important for selection of the T-cell repertoire. Fundamental properties of a mature T-cell repertoire include (1) restriction to self-MHC and (2) tolerance to self-antigens.[1]
  5. 5. Table 27-2 -- Summary of Cell Surface CD Markers MARKER MAIN CELLULAR EXPRESSION FUNCTION T Cell Associated CD3 T cells, thymocytes Cell surface expression and signal transduction with TCR; ε is required for both expression and signal transduction CD4 Class II–restricted T cells, thymocyte subsets, Adhesion molecule, binds to class II MHC; signal transduction; thymocyte development; primary monocytes, macrophages receptor for HIV retroviruses CD5 T cells, B-cell subset Ligand for CD72 CD8 Class I–restricted T cells, thymocyte subsets Adhesion molecule, binds to class I MHC; signal transduction, thymocyte development CD28 T cells (most CD4+, some CD8+) T-cell receptor for the co-stimulatory molecules CD80 (B7-1) and CD86 (B7-2) CD152 Activated T lymphocytes Inhibitory signaling in T cells, binds CD80 (B7-1) and CD86 (B7-2) on antigen-presenting cells + CD154 Activated CD4 T cells Activates B cells, macrophages, and endothelial cells; ligand for CD40 B Cell Associated CD10 Immature and some mature B cells, granulocytes Cell surface metallopeptidase CD19 Most B cells B-cell activation, forms coreceptor with CD21 and CD81 to synergize with signals from B-cell antigen receptor complexes CD20 Most or all B cells ? B-cell activation or regulation, calcium ion channel CD21 Mature B cells, follicular dendritic cells B-cell activation; receptor for C3d, forms a coreceptor with CD19 and CD81 to deliver activated signals in B cells; EBV receptor CD40 B cells, macrophages, dendritic cells, endothelial cells, Role in B-cell activation by T-cell contact; receptor for CD154 (CD40 ligand); macrophage, epithelial cells dendritic cell, and endothelial cell activation CD80 Dendritic cells, activated B cells, macrophages Co-stimulator for T-cell activation, ligand for CD28 and CD152 (CTLA-4) CD86 B cells, monocytes Co-stimulator for T-cell activation, ligand for CD28 and CD152 (CTLA-4) Myeloid Cell Associated CD11a Leukocytes Adhesion, binds to CD54 (ICAM-1), CD102 (ICAM-2), CD50 (ICAM-3) CD11b Granulocytes, monocytes, NK cells Adhesion, phagocytosis of iC3b-coated particles CD11c Granulocytes, monocytes, NK cells, dendritic cells Similar to CD11b; major CD11, CD18 integrin on macrophages and dendritic cells NK Cell Associated CD16a Macrophages, NK cells Low-affinity Fc receptor; activation of NK cells, ADCC CD16b Neutrophils Immune complex–mediated neutrophil activation CD57 NK cells, subset of T cells ? Adhesion Platelet Associated CD31 Platelets, monocytes, granulocytes, B cells, endothelial Adhesion molecule in leukocyte diapedesis cells, T cells CD41 Platelets, megakaryocytes Platelet aggregation and activation, binds to fibrinogen Miscellaneous CD25 Activated T cells and B cells Complexes with IL-2R, high-affinity IL-2 receptor CD34 Precursors of hematopoietic cells Ligand for L-selectin, cell-to-cell adhesion CD55 Broad Regulation of complement activation; binds C3b, C4b CD58 Broad Adhesion, ligand for CD2 CD59 Broad Inhibits formation of complement MAC CDw70 Activated T and B cells, macrophages Binds CD27, co-stimulatory signals CD95 Multiple cell types Binds Fas ligand, mediates activation-induced cell death CD102 Endothelial cells, monocytes, other leukocytes Ligand for CD11a CD18 (LFA-1), cell-cell adhesion CD105 Endothelial cells, activated macrophages Binds TGF-β, modulates cell response to TGF-β
  6. 6. MARKER MAIN CELLULAR EXPRESSION FUNCTION ADCC, antibody-dependent cellular cytotoxicity; ICAM, intracellular adhesion molecule; IL, interleukin; LFA, leukocyte function associated; MAC, membrane attack complex; NK, natural killer; TCR, T-cell receptor; TGF, transforming growth factor. The development of self-tolerance occurs through both central and peripheral mechanisms. Each of these mechanisms is vital for discrimination of self from nonself. Central tolerance is achieved through clonal deletion occurring in the thymus.[3] Acquisition of the TCR complex is mediated through a series of genetically programmed maturational steps. Pre–T cells, not yet expressing CD4 or CD8 molecules, enter the thymus and proliferate to an intermediate stage of development, where they become double- positive (CD4+ and CD8+) cells. These cells are educated by self-MHC class I or class II complexes (present on host stromal cells). T cells expressing TCR molecules that interact at an intermediate affinity with self-MHC are selected to survive, whereas those with too low or too high affinity for MHC do not. This phenomenon is termed positive selection. Cells that do not bind to class I or class II undergo programmed cell death or death by neglect. After positive selection occurs, the developing T cells are exposed to self-antigens. If they react too strongly to self-antigen–MHC complexes, they are deleted from the immune repertoire, a phenomenon termed negative selection ( Fig. 27-6 ).[3] This occurs by a process called apoptosis. Programmed Cell Death Apoptosis is a form of regulated cell death whereby the nucleus of the cell condenses and becomes fragmented, the plasma membrane becomes vesiculated, and the dead cell is rapidly phagocytosed. There is subsequently no release of the cellular contents, and an inflammatory response does not occur. This programmed cell death is an important homeostatic mechanism that limits the lymphoid pool so that it remains relatively constant throughout a lifetime. Activation-induced cell death is an apoptotic pathway that is important for maintenance of self- tolerance in the periphery. The hallmark of this system is Fas (CD95)/FasL (CD95 ligand) interactions. The physiologic importance of this system is to prevent uncontrolled T-cell activation Figure 27-6 Steps in the maturation and selection of major histocompatibility complex (MHC)-restricted T lymphocytes. Maturation of T lymphocytes in the and resulting autoimmune disease. The importance thymus proceeds through sequential steps that are often defined by expression of the of Fas/FasL to peripheral tolerance was first CD4 and CD8 coreceptors. The T-cell receptor (TCR) b chain is first expressed at discovered in two mouse strains: lpr and gld. The the double-negative pre–T-cell stage, and the complete TCR is expressed in double- lpr mutation occurs in the gene that encodes Fas positive cells. Maturation culminates in the development of CD4+ and CD8+ single- positive T cells. As in B cells, failure to express antigen receptors at any stage leads and results in lack of Fas expression. The gld to death of the cells by apoptosis. (From Introduction to the immune system. In mutation results in a defective FasL protein that Abbas A, Lichtman AH: Basic Immunology: Functions and Disorders of the lacks the ability to bind to the Fas receptor. Either Immune System, Updated Edition 2006-2007, 2nd ed. Philadelphia, Elsevier, 2006.) of these mutations results in severe, accelerated autoimmune disease.[6] Fas is a surface receptor expressed on activated T cells. Expression of FasL occurs in response to increased levels of IL-2 secreted by activated T cells. This expression of Fas and FasL leads to cell death through apoptosis.[6] The Fas/FasL system is believed to be one mechanism for keeping immune responses from being too robust. Binding of FasL to Fas results in the activation of intracellular cysteine proteases, which ultimately results in the fragmentation of nucleoproteins and apoptotic cell death. CD4+ T cells appear to be more sensitive to the Fas/FasL interaction than CD8+ T cells are. CELL-TO-CELL INTERACTIONS Once confronted with an antigen, the response of lymphocytes is complex. Multiple cell-to-cell interactions are required to produce the immune response.[1] T cells, B cells, APCs, and cytokines all play a role. Critical to this response are professional APCs—dendritic cells and macrophages—which bind antigen and present it to T and B cells. Protein antigens need to be digested by phagocytic cells before the antigenic information can be presented to the lymphocyte for self and nonself recognition by the MHC. In addition, activated macrophages produce and secrete IL-1, a cytokine that further amplifies the response and stimulates T- and B-lymphocyte activation.[1] For a productive immune response to be generated, the TCR complex
  7. 7. must bind to the antigen presented on the MHC on an APC (signal 1), be stabilized by co-stimulatory molecules (signal 2), and result in intracellular signaling leading to activation of the lymphocyte and production of cytokines (signal 3). T-Lymphocyte Activation T-cell activation is an elegant series of events that are continuously being further delineated (see Fig. 27-6 ). Antigen recognition by T cells is the initiating stimulus for their activation and proliferation, cytokine production, and performance of regulatory or cytolytic effector functions. The TCR is composed of membrane proteins expressed only on T lymphocytes. The TCR does not recognize soluble antigens; rather, it must recognize antigen in the context of peptide (6-13 amino acids in length)-MHC complexes on the surface of APCs. Associated with the TCR is the CD3 molecule. Together they make up the TCR complex.[1] Most TCRs are heterodimers that consist of two transmembrane polypeptide chains designated α and β, which are bonded covalently as previously mentioned; other TCRs are composed of aa chains. All TCRs have a variable region that confers antigen specificity. The αβ-TCR is noncovalently associated with CD3. This highly conserved complex of proteins is responsible for providing the signaling components to the antigen-binding TCR heterodimer. Binding of a foreign antigen results in conformational change in the complex. The associated CD3 molecules transduce the intracellular signals after antigen binding occurs. The development of monoclonal antibodies directed against CD3, such as OKT3, which interfere with T-cell function by altering or inhibiting intracellular signaling, have played a significant clinical role as focused immunosuppressive agents in organ transplantation. [5] [11] Both MHC molecules and αβ-TCR are expressed on resting T cells; however, the IL-2 receptor (IL-2R) is expressed at only very low levels. When T-cell activation occurs, there is a decrease in the number of TCRs expressed on the T cell, accompanied by an increase in IL-2R expression. Activated T cells produce and secrete IL-2, thereby exerting an autocrine (acting on self) and paracrine (acting on cells nearby) response. Only T cells that have been activated by their specific antigen and express the high-affinity IL-2R can respond to IL-2. After IL-2R binds IL-2, T-cell proliferation begins. Once the antigenic stimulus is removed, the number of surface IL-2Rs starts to decrease, and the TCR complex is re-expressed on the cell surface. This inverse relationship between TCR and IL-2R suggests a negative feedback mechanism. This is an elegant system that is reactive only in the presence of an antigen and ceases to function as the antigen is removed. Molecular signaling via the TCR-CD3 complex and its relationship with IL-2 production and IL-2R expression have been characterized. Antigen binding initiates the activation of two signal transduction pathways through a conformational change in the TCR complex. The β chain of the complex is phosphorylated via a CD4- or CD8-associated tyrosine kinase–dependent pathway. The activated TCR complex is coupled via a G-binding protein to phospholipase C. Activation of phospholipase C results in the hydrolysis of phosphatidylinositol 4,5-biphosphate to produce diacylglycerol and inositol 1,4,5-triphosphate. These are the second messengers responsible for the mobilization of intracellular and extracellular Ca+2 that activates protein kinase C. The result of these changes is the transcription of early-activation genes (NFAT and c-fos) and the production of mRNA for IL-2 and its receptor ( Fig. 27-7 ).[1] Co-stimulatory Pathways Two signals are required for T-cell activation: an antigen-specific signal via TCR (signal 1) and a co-stimulatory signal (signal 2). The co- stimulatory pathways present on APC surface molecules provide the second signal for T-cell activation. If these co-stimulatory pathways are interrupted or blocked, such as with monoclonal antibodies directed at the receptors, the result of signal 1 alone is clonal anergy (specific Figure 27-7 Signal transduction pathways in T lymphocytes. Antigen recognition by nonresponsiveness). Co-stimulatory molecules on T cells induces early signaling events, which include tyrosine phosphorylation of the T-cell surface specifically interact with molecules of the T-cell receptor (TCR) complex and recruitment of adapter proteins to molecules on the APC surface. One of the most the site of T-cell antigen recognition. These early events lead to activation of several biochemical intermediates, which in turn activate transcription factors that stimulate well characterized important co-stimulatory the transcription of genes whose products mediate the responses of T cells. The pathway involves the T-cell surface molecule possible effects of co-stimulation on these signaling pathways are not shown. PLCg1 CD28. CD28 binds to B7 molecules found on refers to the g1 isoform of phosphatidylinositol-specific phospholipase C. AP-1, APCs (dendritic cells, monocytes, B cells). activator pro-tein-1; ERK, extracellular signal–regulated kinase; GDP, guanosine diphosphate; GTP, guanosine triphosphate; ITAM, immunoreceptor tyrosine activation Signaling through CD28 enhances the T-cell motif; JNK, Jun N-terminal kinase; NFAT, nuclear factor of activated T cells; NF-kB, nuclear factor kB; PKC, protein kinase C.
  8. 8. response to antigens ( Fig. 27-8 ). To balance the enhancing response, another T-cell surface molecule is present that inhibits T- cell activation, CD152 (CTLA-4). CD28 is constitutively expressed on all CD4+ T cells and on about 50% of CD8+ T cells. CD28 is up-regulated after the T cell receives signal 1. In contrast, CD152 is not expressed on any resting T cells but is induced after T-cell activation; its highest concentrations are reached 48 hours after stimulation. The postulated mechanism for this inhibitory function is through abrogation of the tyrosine kinase activity required for TCR signaling. [5] [12] The mechanism by which CD28 promotes T-cell activation has not been fully defined. Proposed mechanisms include CD28-mediated expression of IL-2 by the T cell. This expression is enhanced at the level of mRNA production and results in increased production. Another mechanism involving CD28 is protection of T cells from programmed cell death, or apoptosis. CD28 is associated with increased expression of Bcl-xL, a survival protein. Expression of this gene results in resistance to T-cell death by apoptosis.[1] Closely related to this CD28/CD152 pathway is the CD40/CD154 (also known as CD40 ligand) pathway. CD40 is a surface molecule constitutively expressed on B cells. After antigen recognition by B cells, there is up-regulation of CD80 (B7-1) and CD86 (B7-2); these molecules interact with T-cell CD28 and cause increased expression of CD154 by the activated T cell, Figure 27-8 The role of co-stimulation in T-cell activation. Resting antigen- which binds to the CD40 receptor on B cells. This presenting cells (APCs) that have not been exposed to microbes or adjuvants may present peptide antigens but do not express co-stimulators and are unable to activate CD40/CD154 interaction provides the stimulus naïve T cells. Naïve T cells that have recognized antigen without co-stimulation may for B cells to continue activation and become unresponsive to subsequent exposure to antigen, even if co-stimulators are proliferation.[8] Co-stimulatory blockade with present, and this state of unresponsiveness is called anergy. Microbes, as well as anti-CD154 monoclonal antibodies is very cytokines produced during innate immune responses to microbes, induce the expression of co-stimulators, such as B7 molecules, on APCs. The B7 co-stimulators effective in inducing anergic and regulatory T are recognized by the CD28 receptor on naïve T cells, thereby providing signal 2, and cells. This further emphasizes the importance of in conjunction with antigen recognition (signal 1), T-cell responses are initiated. IL-2, the CD40/CD154 pathway in providing co- interleukin-2. (From Introduction to the immune system. In Abbas A, Lichtman AH: stimulation and up-regulating the effects of Basic Immunology: Functions and Disorders of the Immune System, Updated Edition 2006-2007, 2nd ed. Philadelphia, Elsevier, 2006.) CD28/B7 pathway. Manipulation of both of these important pathways is under investigation in clinical transplantation protocols in an attempt to induce antigen-specific tolerance. T-Cell Effector Functions In addition to acquiring the TCR complex during thymic maturation, T cells also acquire differentiation receptors called cluster of differentiation (CD) antigens. CD4 and CD8 are the best-known CD markers. Other frequently occurring CD markers can be found in Table 27-2 . The subpopulations of T cells have several different functional activities. T cells bearing the CD8 molecule interact with MHC class I–peptide complexes and can directly lyse a foreign or tumor cell on activation. These activated CD8+ T cells are the cytotoxic T lymphocytes (CTLs). In contrast, CD4+ T cells recognize antigen in the context of MHC class II molecules. CD4+ T cells become T helper (TH) cells after activation and primarily function through the secretion of distinct cytokines to induce either a cell-mediated response (TH1) or a humoral response (TH2).[1] Even the recognition of foreign cells is a complex process. The initial responding and proliferating T cells do not destroy foreign grafts; rather, they serve as helper T cells (CD4+ TH) that activate another group of T cells (CTLs), which in turn damage the graft ( Fig. 27-9 ). TH-cell proliferation is an important step in amplification of the immune response, and these actively dividing cells are particularly vulnerable to antimetabolites. The activity of CD4+ TH cells is thus one of the major targets of clinical immunosuppression with drugs or monoclonal antibodies.[2] TH cells have a central role in response to alloantigen. Once antigen has been processed and presented in the context of cell surface MHC class II molecules on an APC, the TH cell proliferates.
  9. 9. The two distinct TH populations (TH1 and TH2 subsets) differ in their pattern of cytokine synthesis (see Fig. 27-9 ).[9] In TH1 responses, the main cytokine is interferon-γ (IFN-γ). These cytokines in turn enhance macrophage activation and cell-mediated immunity. The TH1 response is balanced by the TH2 response. The TH2 response results in the production of IL-4 and IL-5. The effect of TH2 cells is to inhibit macrophage activation. An important feature of these CD4+ TH cells is the ability of one subset to regulate the activity of the other. Thus, IFN- γ directly inhibits the proliferation of TH2 cells, whereas IL-5 inhibits cytokine production by TH1 cells. This cross- regulation occurs at the level of the effector cells triggered by these subsets. IFN-γ inhibits IL-4–induced B-cell activation, whereas IL-4 suppresses IL-2–induced T- and B-cell proliferation. The current theory postulates that differentiation of naïve CD4+ T cells, down either pathway, is directly related to the neighboring cells and the cytokines that these neighboring cells produce.[9] Regulatory T cells (Treg) have received a great deal of attention recently and may hold significant promise for strategies to achieve antigen-specific tolerance in the clinic.[10] Treg are defined by their function: suppression of alloreactivity in vitro (a state reversed by exogenous IL-2) and down-regulation of the proliferation of other T-cell populations via IL-10. Initially, they are cell (antigen) contact dependent but, on maturation, become contact independent to amplify the response. The most important Treg subset is CD4+/CD25+.[5] CD4+/CD25+ Treg are characterized by the transcriptional factor Fox-3. In animal models, Treg have been shown to play a role in the maintenance of self-tolerance and prevention of graft- versus-host (GVH) disease after marrow transplantation. [10] In autoimmune type 1 diabetes, the Treg are dysfunctional, which results in a breakdown in self- tolerance.[11] Clinical protocols using Treg in transplant recipients are currently being developed. B Lymphocytes Similar to all other cells in the immune system, B cells are derived from pluripotent bone marrow stem cells. IL-7, Figure 27-9 Functions of the TH1 and TH2 subsets of CD4+ helper T produced by bone marrow stromal cells, is a growth factor lymphocytes. A, TH1 cells produce the cytokine interferon-γ (IFN-γ), which for pre–B cells. IL-4, IL-5, and IL-6 are cytokines that activates phagocytes to kill ingested microbes and stimulates the production stimulate the maturation and proliferation of mature of antibodies that promote the ingestion of microbes by phagocytes. B, TH2 primed B cells.[1] B cells are responsible for the humoral cells specific for microbial or nonmicrobial protein antigens produce the cytokines interleukin-4 (IL-4), which stimulates the production of IgE or antibody-mediated immune response against foreign antibody, and IL-5, which activates eosinophils. IgE participates in the antigen ( Fig. 27-10 ). Antibodies prevent infection by activation of mast cells by protein antigens and coats helminths for blocking the ability of microbes to enter the host cell. B destruction by eosinophils. C, The main differences between the TH1 and cells express immunoglobulin (antibody) on their cell TH2 subsets of helper T cells are summarized. Note that many helper T cells are not readily classified into these distinct and polarized subsets. The surface. These membrane-bound immunoglobulins are the chemokine receptors are called CCR or CXCR because they bind B-cell antigen receptors and allow specific antigen chemokines classified as CC or CXC according to whether key cysteines are recognition. Only one antigen-specific antibody is adjacent or separated by one amino acid. Different chemokine receptors produced by each mature B cell. Each antibody is control the migration of different types of cells. These, in combination with selectins, determine whether TH1 or TH2 cells dominate in different composed of two heavy chains and two light chains. Both inflammatory reactions in various tissues. APC, antigen-presenting cell; heavy and light chains have a constant region (Fc), as well GM-CSF, granulocyte-macrophage colony-stimulating factor; IFN-γ, as a variable, antigen-binding region (Fab). The antibody- interferon-γ; IL-12R, interleukin-12 receptor. (From Introduction to the binding site is composed of both the heavy- and light- immune system. In Abbas A, Lichtman AH: Basic Immunology: Functions and Disorders of the Immune System, Updated Edition 2006-2007, 2nd ed. chain variable regions.[1] The ability of antibody to Philadelphia, Elsevier, 2006.) neutralize microbes is entirely a function of the antigen- binding region.
  10. 10. In humans there are nine different immunoglobulin subclasses: IgM, IgD, IgG1, IgG2, IgG3, IgG4, IgA1, IgA2, and IgE. Resting naïve B cells express IgD and IgM on their cell surface. On antigen stimulation and with the help of CD4+ T cells, B cells undergo isotype switching. Distinct immune effector functions are assigned to each isotype. IgM and IgG antibodies provide a pivotal role in the endogenous or intravascular immune response. IgA is secreted into the lumen of the gastrointestinal and respiratory tracts and is responsible for mucosal immunity. The first isotype produced in response to a foreign antigen is IgM, which is very efficient at binding complement to facilitate phagocytosis or cell lysis. B cells undergo isotype switching with the maturation of the immune response against a specific antigen. This results in a decrease in IgM titer with a concomitant rise in IgG titer (see Fig. Figure 27-10 Phases of humoral immune responses. Naïve B lymphocytes 27-10 ).[1] A primed B cell may undergo further recognize antigens, and under the influence of helper T cells and other stimuli (not shown), the B cells are activated to proliferate, thereby giving rise to mutation within the variable regions that leads to clonal expansion, and to differentiate into antibody-secreting effector cells. increased affinity of antibody, termed somatic Some of the activated B cells undergo heavy-chain class switching and affinity hypermutation. maturation, and some become long-lived memory cells. (From Introduction to the immune system. In Abbas A, Lichtman AH: Basic Immunology: Functions and Disorders of the Immune System, Updated Edition 2006-2007, 2nd ed. Monocytes Philadelphia, Elsevier, 2006.) Mononuclear phagocytes, which also have an integral role in the immune response, are derived from bone marrow. This cell type initially emerges as a monocyte while in peripheral blood. Monocytes are recruited to the site of tissue inflammation, where they mature to become macrophages or histiocytes. The main function of monocytes and macrophages is phagocytosis of foreign antigen. After phagocytosis, they process antigen, present the antigen to lymphocytes, and produce various cytokines (see Table 27-1 ) that regulate the immune response.[12] Dendritic Cells The most potent APCs are dendritic cells, which are distributed ubiquitously throughout the lymphoid and nonlymphoid tissues of the body. Different types of dendritic cells serve distinct functions in inducing and regulating T-cell as well as B-cell immune responses ( Fig. 27-11 ). Immature dendritic cells are located along the gut mucosa, skin, and other sites of antigen entry. On contact with antigen, dendritic cells are activated to mature, and expression of both MHC class II and co-stimulatory molecules (e.g., CD40, CD80, and CD86) is increased. As dendritic cells mature, they migrate to peripheral lymphoid tissue, where they can activate T cells to respond to antigen. Dendritic cells provide signals that initiate clonal expansion of T cells, as well as provide signals to promote naïve T cells to either a TH1 or TH2 response.[13] A number of subsets of dendritic cells have been described. For example, myeloid dendritic cells (DC1) are more immunogenic, whereas plasmacytoid dendritic cells (pDC) are more tolerogenic. Antigen presentation by immature dendritic cells leads to TCR signaling without co-stimulation and induces T-cell anergy. Recent data in animals suggest that pDC, under specific conditions, can be potently tolerogenic in vivo and may therefore in the future provide cell-based therapies to induce tolerance to transplanted grafts. Natural Killer Cells NK cells are a critical component of innate immunity. NK cells express cell receptors that are distinct from the TCR complex. Functionally, these cells are defined by their ability to lyse target cells without requiring priming. NK cells lyse cell targets that lack expression of self-MHC class I. NK cells produce the cytokine IFN-γ, which in turn activates macrophages to kill host cells infected by intracellular microbes. NK cells also play an important role in immune defenses, especially after hematopoietic stem cell and organ transplantation. In addition, they contribute to the defense against virus-infected cells, graft rejection, and neoplasia and participate in the regulation of hematopoiesis through cytokine production and cell-to-cell interaction. NK cells also mediate rejection in xenotransplantation.[14] MAJOR HISTOCOMPATIBILITY LOCUS: TRANSPLANTATION ANTIGENS The MHC is a region of highly conserved polymorphic genes. The products of these genes are expressed on the cell surface of a wide array of cell types. MHC genes play a pivotal role in the immune response. The MHC is so important because antigen- specific T lymphocytes do not recognize antigens in the free form or in soluble form, only as small peptides, products of protein
  11. 11. digestion, that are bound to MHC molecules. There are two types of cell surface MHC molecules: class I and class II ( Fig. 27-12 ). Any lymphocyte is restricted to one of these two classes. Antigens associated with class I are recognized by CD8+ T cells; antigens associated with class II are recognized by CD4+ T cells.[1] Figure 27-11 Capture and presentation of protein antigens by dendritic cells. Immature dendritic cells in the epithelium (skin, in the example shown, where the dendritic cells are called Langerhans cells) capture microbial antigens and leave the epithelium. The dendritic cells migrate to draining lymph nodes after being attracted there by chemokines produced in the nodes. During their migration and probably in response Figure 27-12 Structure of class I major to the microbe, the dendritic cells mature, and in the lymph nodes, the histocompatibility complex (MHC) and class dendritic cells present antigens to naïve T lymphocytes. Dendritic cells at II MHC molecules. The schematic diagrams different stages of maturation may express different membrane proteins. and models of the crystal structures of class I Immature dendritic cells express surface receptors that capture microbial and class II MHC molecules illustrate the antigens, whereas mature dendritic cells express high levels of major domains of the molecules and the histocompatibility complex molecules and co-stimulators, which function fundamental similarities between them. Both to stimulate T cells. (From Introduction to the immune system. In Abbas types of MHC molecules contain peptide- A, Lichtman AH: Basic Immunology: Functions and Disorders of the binding clefts and invariant portions that bind Immune System, Updated Edition 2006-2007, 2nd ed. Philadelphia, CD8 (the a3 domain of class I) or CD4 (the Elsevier, 2006.) b2 domain of class II). b2m, b2- microglobulin. (From Introduction to the immune system. In Abbas A, Lichtman AH: Basic Immunology: Functions and Disorders of the Immune System, Updated Edition 2006-2007, 2nd ed. Philadelphia, Elsevier, 2006. Crystal structures courtesy of Dr. P. Bjorkman, California Institute of Technology, Pasadena.) Human Histocompatibility Complex The strongest antigens present in transplantation are the MHC molecules and the peptides that they hold. The MHC in humans is located on chromosome 6. The gene products of the MHC molecules in humans are called human leukocyte antigens (HLA). Class I molecules important to transplantation in humans are expressions of HLA-A, HLA-B, and HLA-C genes. HLA-E, HLA- F, and HLA-G are more conserved but may later demonstrate importance in transplantation. The class II molecules are expressions of HLA-DR, HLA-DQ, HLA-DP, and HLA-DM genes.[1] There are class III molecules, but they are not cell surface proteins involved in antigen recognition. Instead, class III molecules contain mainly soluble mediators of immune function and
  12. 12. include tumor necrosis factor-α (TNF-α) and TNF-β, complement components, heat shock protein, and nuclear transcription factor-β. Class I and class II molecules were previously considered antigens. They are, however, vital to T-cell and B-cell interactions. HLA class I molecules are present on all nucleated cells. In contrast, class II molecules are found almost exclusively on cells associated with the immune system (macrophages, dendritic cells, B cells, and activated T cells). Resting T cells do not express class II molecules. Both class I and class II MHC molecules are similar in their structures. These structures have been elucidated by x-ray crystallography. This important advance added much to the understanding of antigen recognition ( Fig. 27-13 ). MHC molecules are composed of four domains: a peptide-binding domain, an immunoglobulin (Ig)-like domain, a transmembrane domain, and a cytoplasmic domain. The Ig-like domain has limited polymorphism and contains the interaction region for CD8/class I and CD4/ class II molecules. The considerable homology between class I and class II molecules suggests a common evolutionary origin.[1] Class I MHC Class I molecules in humans are expressions of HLA-A, HLA-B, HLA-C genes, which are recognized by cytotoxic CD8+ T cells. The class I molecules are composed of a 44-kd transmembrane glycoprotein in a noncovalent complex with a nonpolymorphic 12-kd polypeptide called β2- microglobulin. The peptide-binding region of class I, composed of the first and second domains of a protein, forms a binding cleft. The a3 Ig-like domain, which is the domain closest to the membrane and interacts with CD8, demonstrates limited polymorphism and contains conserved interactions restricted to CD8+ T cells. Class I molecules are expressed on nearly all cells in adults; however, this expression can be increased by cytokines, which is important as an amplification mechanism. Interferons (IFN-α, IFN-β, IFN-γ) induce an increase in the expression of class I molecules by increasing levels of gene transcription.[1] Interestingly, the areas specific for antigen binding are not con-served, whereas the non– antigen-binding regions are conserved. Class II MHC Class II molecules are expressions of the HLA-DR, HLA-DQ, HLA-DP, and HLA-DM genes. Class II molecules contain two MHC-encoded polymorphic chains, one approximately 32 kd and the other approximately 30 kd. The peptide-binding region is composed of the α1 and β1 domains. The Ig-like domain is composed of the α2 and β2 segments. Similar to the Figure 27-13 Binding of peptides to major class I Ig-like domain, there is limited polymorphism, and interactions are histocompatibility complex (MHC) restricted to CD4+ T cells. Class II molecules are constitutively expressed molecules. A, These top views of the on professional APCs, including dendritic cells, B lymphocytes, and crystal structures of MHC molecules show macrophages.[1] Exposure of these APCs to antigen or inflammation causes how peptides (in yellow) lie on the floors increased expression of class II and is associated with activation and of the peptide-binding clefts and are maturation of the APC. Expression can be induced on endothelial cells with available for recognition by T cells. B, A cytokine stimulation and on other cells in certain disease states, such as bile side view of a cut-out of a peptide bound to duct epithelium in primary sclerosing cholangitis and beta islet cells in a class II MHC molecule shows how diabetes. anchor residues of the peptide hold it in the pockets in the cleft of the MHC molecule. A, (Courtesy of Dr. P. Bjorkman, California Institute of Technology, Pasadena.) B, (Adapted from Scott CA, Peterson PA, Teyton L, Wilson IA: Crystal structures of two I-Ad-peptide complexes reveal that high affinity can be achieved without large anchor residues. Immunity 8:319-329, 1998. © Cell Press; with Expression of MHC Molecules permission. From Introduction to the immune system. In Abbas A, Lichtman AH: MHC molecules are essential for recognizing interactions between cells. Basic Immunology: Functions and They are the primary determinant of whether T lymphocytes can interact Disorders of the Immune System, Updated Edition 2006-2007, 2nd ed. Philadelphia, Elsevier, 2006.)
  13. 13. with foreign antigens. For the most part, class I molecules contain peptides that originate inside the cell, whereas class II molecules hold peptides that were outside the cell, have been internalized, and were degraded in lysozymes. Importantly, in the regulation of cytotoxic effector cell function, neither class I nor class II molecules can be expressed on the cell surface without a bound peptide. Therefore, the peptide-binding groove is always occupied with either self or foreign peptides. The class I and class II genes can generally be expressed in one of several states in a particular cell. First, the genes can be constitutively expressed and further up-regulation can occur with the presence of cytokines. Second, the genes are not expressed but rather are induced by cytokines. Third, the genes are not expressed and not inducible. These states are of tremendous importance in clinical transplantation and in determining the antigenicity of the transplanted allograft. Expression of MHC molecules is important in T-cell–mediated rejection because of recognition of nonself. Antigen Presentation: Direct Versus Indirect Recognition In conventional antigen recognition, the foreign antigen is ingested by the host APC, digested into small peptides, and presented to T cells that recognize the antigen, as well as class I or class II of the APC. This process is termed indirect antigen presentation or indirect recognition. In addition, when a solid organ is transplanted, the professional (dendritic cells, macrophages) and nonprofessional (activated vascular endothelial cells) APCs of the donor present themselves. This process is termed direct recognition ( Fig. 27-14 ). In solid organ transplantation, both pathways play an important role. Recent studies in knockout mice show that induction of tolerance through mechanisms of co-stimulatory blockade may be selective for the indirect pathway because elimination of direct antigen presentation alone does not induce tolerance when combined with co- stimulatory blockade. Graft prolongation is relatively easily achieved by blockade of the CD28/B7 or CD154/CD40 co- stimulatory pathways (signal 2) in mice.[15] HLA Typing: Prevention and Rejection Organ transplantation in a recipient with a fully functional immune system may result in rejection. To minimize rejection, approaches that make the graft less antigenic to the host can be applied. The major strategy in achieving this objective is to minimize alloantigen differences between the donor and host. ABO compatibility is determined to avoid hyperacute rejection of renal allografts. Another determining factor is HLA typing or tissue typing. Potential donors and recipients are typed for HLA- A, HLA-B, and HLA-DR molecules. On close examination of graft survival, HLA matching is the best means of prolonging allograft survival. The larger the number of HLA-A, HLA-B, and HLA-DR alleles that are matched between both donor and recipient, the better the survival rate, particularly in the first year after transplantation.[1] Current immunosuppressive regimens negate much of the impact of matching, however. Humans have two different HLA-A, HLA-B, and HLA-DR alleles (one from each parent, six alleles in total). Large, single-center trials have shown significant survival benefit for only six of six antigen matches. Matching remains controversial in the transplant community. It may be that previously imprecise tissue typing led to the contradictory results of some studies. Historically, serologic testing with the microcytotoxicity technique was used for both crossmatching and antibody testing. A gradual transition to molecular typing has occurred because of its greater accuracy. Poor HLA class II resolution, limitations in cell viability, and broad cross-reactivity for different cross-reactive antigens limit the utility of serologic testing, especially for bone marrow transplantation. The serologic method uses an antigen-specific serum that binds to cells expressing that particular antigen. The functional method measures the reactivity of the lymphocytes of a potential recipient to a donor. When antigens are recognized as foreign, lymphocyte proliferation results.[1] Molecular techniques for performing HLA typing that use polymerase chain reaction (PCR) have been developed and are now commonly used. PCR permits more complete typing of class II loci (HLA-DR, HLA-DQ, and HLA-DP subsets), as well as precise typing of HLA-A and HLA-B. This Figure 27-14 Direct and indirect recognition of alloantigens. A, Direct alloantigen DNA typing has become the predominant method recognition occurs when T cells bind directly to intact allogeneic major because it better defines the crucial sequence of histocompatibility complex (MHC) molecules on professional antigen-presenting cells (APCs) in a graft. B, Indirect alloantigen recognition occurs when allogeneic amino acids around the peptide-binding groove. MHC molecules from graft cells are taken up and processed by recipient APCs and then peptide fragments of the allogeneic MHC molecules are presented by recipient (self) MHC molecules. Recipient APCs may also process and present graft proteins other than allogeneic MHC molecules. (From Introduction to the immune system. In Abbas A, Lichtman AH: Basic Immunology: Functions and Disorders of the Immune System, Updated Edition 2006-2007, 2nd ed. Philadelphia, Elsevier, 2006.)
  14. 14. Studies have been conducted to compare HLA-DR typing with the traditional serologic method versus PCR methods. Serologic typing will probably be abandoned over time. In clinical transplantation, crossmatching is performed with microcytotoxicity or flow cytometric techniques. Crossmatching differs from tissue typing. In crossmatching, serum from the recipient is tested for preformed antibodies against donor cells to exclude the possibility of hyperacute rejection. Despite excellent histocompatibility matching, hyperacute rejection can still occur if preformed antibodies are present.[1] Preservation time is more severely limited in heart, lung, and liver transplantation; therefore, in these organs, crossmatching is performed before organ recovery for recipients with known antibody titers only. Rejection Graft rejection requires the participation of various combinations of immunologically specific and nonspecific cells. Three types of graft rejection occur ( Fig. 27-15 ). Hyperacute rejection occurs within minutes to days after transplantation and is mediated primarily by preformed antibody. This type of rejection is prevented by screening the recipient for preformed antibodies, not by classic antirejection pharmaceuticals. Acute rejection is mediated primarily by T lymphocytes and first occurs between 1 and 3 weeks after solid organ transplantation without immunosuppression. Acute rejection episodes are most common in the first 3 to 6 months after transplantation but can occur at any time. Acute rejection can quickly destroy a graft if left untreated. The new immunosuppressive agents have made acute rejection increasingly less common. Chronic rejection occurs over a span of months to years and is the most common cause of graft loss after 1 year. From an immunologic standpoint, chronic rejection is mediated by both T- and B-cell responses.[1] Hyperacute rejection is mediated by preformed antibodies that bind to endothelium and subsequently activate complement. This rejection is characterized by rapid thrombotic occlusion of the vasculature of the transplanted allograft. The thrombotic response occurs within minutes to hours after host blood vessels are anastomosed to donor vessels. Hyperacute rejection is mediated predominantly by IgG antibodies directed toward foreign protein molecules, such as MHC molecules. These IgG antibodies are the result of previous exposure to alloantigens from blood transfusions, pregnancy, or previous transplantation.[1] There are two forms of acute rejection: acute vascular rejection and acute cellular rejection. Acute vascular rejection is the more severe form, with greater potential for long-term complications for the graft. In the setting of acute vascular rejection, the response is mediated by IgG antibodies that develop in response to the graft against the endothelial antigens and involves the activation of complement. T cells contribute to the acute vascular rejection episode by responding to the foreign antigen. This response leads to direct lysis of the endothelial cells or the production of cytokines that further recruit and activate inflammatory cells. The end result is endothelial necrosis. This process occurs within the first week of allograft transplantation in the absence of immunosuppression.[1] In the setting of acute cellular rejection, necrosis of parenchymal cells occurs as a result of infiltration of T cells and macrophages. Figure 27-15 Mechanisms of graft rejection. A, In hyperacute rejection, preformed antibodies react with alloantigens on the The exact mechanism that underlies this process has not been vascular endothelium of the graft, activate complement, and trigger fully delineated. The effector mechanism in macrophage- rapid intravascular thrombosis and necrosis of the vessel wall. B, mediated lysis is similar to a delayed-type hypersensitivity In acute cellular rejection, CD8+ T lymphocytes reactive with alloantigens on graft endothelial cells and parenchymal cells cause response. The T-cell effector mechanism is mediated by CTL- damage to these cell types. Inflammation of the endothelium is induced lysis. Much of the evidence emerging has implicated the sometimes called endothelialitis. Alloreactive antibodies may also alloreactive CD8+ CTL. The CD8+ CTL recognizes and lyses contribute to vascular injury. C, In chronic rejection with graft foreign cells. To support this mechanism, the cellular infiltrate arteriosclerosis, T cells reactive with graft alloantigens may produce cytokines that induce proliferation of endothelial cells and present in acute rejection is enriched for CD8+ CTL.[1] intimal smooth muscle cells, thereby leading to luminal occlusion. This type of rejection is probably a chronic delayed-type hypersensitivity (DTH) reaction to alloantigens in the vessel wall. APC, antigen-presenting cell. (From Introduction to the immune system. In Abbas A, Lichtman AH: Basic Immunology: Functions and Disorders of the Immune System, Updated Edition 2006-2007, 2nd ed. Philadelphia, Elsevier, 2006.)
  15. 15. The mechanism of chronic rejection is less clearly defined and is an area of intense study. Chronic rejection appears as fibrosis and scarring in all organs currently transplanted, although the specific histopathologic lesions vary with the organ. Chronic rejection is manifested as accelerated atherosclerosis in heart recipients, as bronchiolitis obliterans in lung recipients, as so-called vanishing bile duct syndrome in liver recipients, and as fibrosis and glomerulopathy in kidney recipients. It is unlikely that chronic rejection is strictly an immunologic phenomenon—ischemia and inflammation, among other processes, also play a role. Risk factors for development of the lesions of chronic rejection include the following: 1. Previous acute rejection episodes, with increased severity and an increased number of episodes further increasing the risk for chronic rejection 2. Inadequate immunosuppression, including patient noncompliance 3. Initial delayed graft function 4. Donor issues such as age and hypertension 5. Organ recovery–related issues, including preservation and reperfusion injury 6. Recipient diabetes, hypertension, or post-transplant infections In essence, almost any injury to the organ in the donor or after transplantation can contribute to the development of chronic rejection.[16] Therefore, given the multifactorial basis of chronic rejection, the transplant is not completely protected with currently available immunosuppression. Episodes of acute rejection are a very significant risk factor, however, for the subsequent development of chronic rejection.[16] To the extent that immunosuppressive agents prevent acute rejection episodes, the drugs do clearly decrease chronic rejection. New immunosuppressive drugs are evaluated not only by their ability to prevent acute rejection episodes and their safety profiles but also by their ability to prevent chronic rejection and improve recipient quality of life. Improved side effect profiles may enhance recipient compliance with immunosuppressive regimens. The preceding, abbreviated description of the development of allograft immunity discloses many processes that may potentially be manipulated to suppress the immune response: 1. Destroying the immunocompetent cells that would otherwise react to donor antigen before transplantation 2. Minimizing histoincompatibility or altering the antigen to make it unrecognizable or even toxic to the reactive lymphocyte clones 3. Interfering with antigen processing and presentation by the recipient cells 4. Inhibiting antigen recognition by lymphocytes 5. Inhibiting production or release by macrophages or lymphocytes of the signal substances or cytokines involved in differentiating lymphocytes into cytotoxic or antibody-synthesizing cells 6. Suppressing clonal expansion of lymphocytes 7. Activating sufficient numbers of suppressor lymphocytes 8. Interfering with the binding of immunoglobulins to graft target antigens 9. Preventing tissue damage by the nonspecific cells and molecules that are activated by sensitized cells or antigen-antibody
  16. 16. complexes 10. Inducing donor-specific transplantation tolerance[5] Potential sites for regulation are discussed in detail later. CLINICAL IMMUNOSUPPRESSION Immunosuppressive agents are, for the most part, essential to graft survival. It is rare for a transplant recipient to become drug free, even over a prolonged period. Shortly after cardiac transplantation, recipients must orchestrate taking approximately 60 pills per day. Over time, fewer numbers of medications are required, but the medication regimen still takes its toll. It is estimated that it costs at least $15,000 per year to prevent graft rejection and treat the complications of nonspecific immunosuppression in transplant recipients. The relatively nonspecific mechanism of action with the currently available immunosuppressive agents is associated with an increased rate of infections (particularly viral infections) and malignancy. In addition, the individual agents themselves have specific toxicities. The overall risks associated with immunosuppression, the individual agents used in modern-day immunosuppression, and possible immunosuppressive drug regimens will each be discussed separately. As the effector mechanisms responsible for graft rejection have been increasingly well defined, strategies to develop immunosuppressive agents with increasingly specific actions have emerged. Although tolerance remains the unattained goal of research in transplantation, significant improvements in immunosuppressive medication regimens have occurred in the past few years as newer agents and newer protocols have been developed. Overall Risks Associated With Immunosuppression Risk for Infection Prevention of rejection in any recipient is possible, but prevention itself is achieved at a high cost in terms of increased risk for infections and malignancies as a result of increased immunosuppression. Immunosuppressive drugs do not specifically block alloreactivity, and a certain degree of increased susceptibility to opportunistic infection plagues all transplant recipients ( Fig. 27-16 ). This increased risk is caused not only by environmental pathogens but also by reactivation of previously controlled internal pathogens. An important example of the latter is CMV infection, which can result in pneumonia, hepatitis, pancreatitis, and gastrointestinal side effects in transplant recipients ( Fig. 27-17 ). CMV has been implicated in the lesions of heart transplant recipients with chronic rejection. The risk for reactivation is highest approximately 6 to 12 weeks after transplantation and again after periods of increased immunosuppression for rejection episodes.[17] Prophylaxis has been successfully used to prevent post-transplant infections. Transplant programs use various prophylactic regimens, depending on the Figure 27-16 Usual sequence of infections after organ organs transplanted. Many regimens include transplantation. Exceptions to the usual sequence of infections pneumococcal vaccine; hepatitis B vaccine; after transplantation suggest the presence of unusual trimethoprim-sulfamethoxazole for Pneumocystis epidemiologic exposure or excessive immunosuppression. CMV, pneumonia and urinary tract infections (pentamidine cytomegalovirus; EBV, Epstein-Barr virus; HSV, herpes simplex nebulizers may be substituted in those allergic to virus; PTLD, post-transplant lymphoproliferative disorder; RSV, sulfa); acyclovir, ganciclovir, or valganciclovir for respiratory syncytial virus; VZV, varicella-zoster virus. Zero CMV; and clotrimazole troche or nystatin for oral indicates the time of transplantation, solid lines indicate the most and esophageal fungal infections. Hyper-CMV common period for the onset of infection, and dotted lines immunoglobulins are also used to prevent Ebstein- indicate periods of continued risk at reduced levels. (Reprinted Barr virus (EBV)-derived lymphomas in some high- from Am J Med, Vol 70, Rubin RH, Wolfson JS, Cosimi AB, risk populations. Tolkoff-Rubin NE: Infection in the renal transplant patient, pp. 405-411, Copyright 1981, with permission from Excerpta Medica, Inc.)
  17. 17. Although outcomes have significantly improved, infections remain a major problem in transplantation despite prophylaxis.[17] Recent attention has focused on the potential role of BK virus in the development of renal allograft dysfunction. Studies have previously reported the role of the virus in ureteral stenosis. BK virus–associated nephropathy is diagnosed by the presence of viral inclusion bodies on biopsy, along with urine and plasma PCR testing. Sixty percent to 80% of the adult population is seropositive for BK virus, so determining the true role of the virus as a pathogen may be difficult. The nephropathy has reportedly improved with decreases in immunosuppression. Low doses of cidofovir and intravenous (IV) immunoglobulin[18] (IVIG) have been used to eradicate the virus. Risk for Malignancy Malignancy is also a complication of chronic immunosuppressive therapy.[19] The rate of malignancy is increased approximately 10-fold over controls.[19] Most post-transplant malignancies are easily treatable in situ carcinomas of the cervix or low-grade skin tumors. Virus-mediated tumors occur with greater frequency in transplant recipients, similar to those found in patients with acquired immunodeficiency syndrome. Human papillomavirus is associated with cancer of the cervix, hepatitis B and C virus with hepatoma, and human herpesvirus 8 with Kaposi's sarcoma. Lymphomas, particularly those associated with EBV, have an increased incidence in immunosuppressed transplant patients. Recipients treated repeatedly for acute rejection are at increased risk, as are young recipients of liver and small bowel transplants. The EBV-associated lymphomas are often referred to as post-transplant lymphoproliferative disorders (PTLDs) to better distinguish the differences in etiology and treatment from lymphomas in nonimmunocompromised populations. PTLD varies from asymptomatic to life threatening, and treatment varies from no treatment, to reduction or withdrawal of immunosuppression in non-lifesaving transplants, to treatment with antiviral agents, to traditional chemotherapy.[19] Rituximab (anti-CD20), a monoclonal antibody that depletes B cells, has been successfully used in the treatment of EBV-associated PTLD in solid organ transplant recipients.[20] Hyper-CMV immunoglobulin has also been used as prophylaxis in high-risk recipient groups. Risk for Cardiovascular Disease Cardiovascular disease remains a significant cause of morbidity and mortality in transplant recipients. After the first year, the most common causes of death in transplant recipients are (1) allograft loss from chronic rejection and (2) death of the patient with a functioning graft secondary to cardiovascular death, disease, or infection. Atherosclerotic disease in heart transplant recipients is multifactorial. It can be related to chronic rejection, CMV infection, or classic hyperlipidemia. Pancreas allograft recipients suffer from the increased cardiovascular risk factors associated with diabetes, and renal allograft recipients are at increased risk for cardiovascular events as a result of underlying diseases, including diabetes and hypertension with concomitant left ventricular hypertrophy. These pretransplant risk factors are amplified by post- transplant immunosuppression. Cyclosporine and corticosteroids, in particular, are associated with increased coronary artery disease. Adequate pretransplant assessment for coronary artery disease, including liberal use of coronary angiography, can help identify patients at risk. Post-transplant manipulation of immunosuppression in high-risk recipients needs to be undertaken. For instance, switching from cyclosporine to tacrolimus is considered, as well as avoidance or withdrawal of steroids in selected patients. 3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors may lower lipid levels in transplant recipients, in addition to protecting the graft. Exercise and smoking cessation are also emphasized.[21] Figure 27-17 Role of cytomegalovirus (CMV) infection in transplant recipients. Mediators of systemic inflammation link the activation of CMV Induction Agents infection to allograft injury and rejection, to infection with opportunistic pathogens, and to the development of cancer in organ transplant recipients. EBV, Epstein-Barr virus; PTLD, post-transplant lymphoproliferative disorder. (From Fishman JA, Rubin RH: Infection in organ transplant recipients. N Engl J Med 338:1741-1751, 1998. Copyright © 1998 Massachusetts Medical Society. All rights reserved.)

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