Transplantation immunobiology

INTRODUCTION — The mammalian immune system is an extraordinarily complex system that has developed in response to evolutionary stressors provided by coexistence with micro-organisms over millions of years. The system can be divided into two components:

●Natural immunity, which refers to the nonspecific immune response

●Adaptive immunity, which refers to the response to a specific antigen

In organ transplantation, the principal target of the immune response to the graft are the major histocompatibility complex (MHC) molecules expressed on the surface of donor cells (allo-MHC); this feature is a form of adaptive immunity.

The immunobiology of solid transplantation will be reviewed here. The immunobiology of bone marrow or stem transplantation, which primarily involves graft-versus-host (GVH) and graft-versus-tumor effects, is presented separately:

●(See "Pathogenesis of graft-versus-host disease (GVHD)".)

●(See "Biology of the graft-versus-tumor effect following hematopoietic cell transplantation".)

To adequately understand the immune response to transplanted tissue, it is helpful to also review the general immune response; there will be an emphasis upon those elements involved in the response to donor antigens. More detailed discussions concerning various aspects of the immune system are discussed separately:

●(See "The adaptive humoral immune response".)

●(See "The adaptive cellular immune response: T cells and cytokines".)

NATURAL AND ADAPTIVE IMMUNITY — Natural or innate immunity refers to the older, nonspecific immune system that involves the recruitment and involvement of macrophages, neutrophils, natural killer cells, cytokines, certain cellular receptors, and complement components [1]. Physical damage and infective agents recruit a variety of inflammatory processes that do not involve recognition of specific antigens. However, they induce a robust inflammatory response due to the induction, presence, and/or absence of specific features, such as the absence of complement inhibitory receptors on most foreign organisms [2]. A detailed review of the innate immune system is provided elsewhere. (See "An overview of the innate immune system".)

By comparison, adaptive immunity involves recognition of specific antigen and confers both specificity and a memory effect by T and B lymphocytes:

●T cells recognize antigen in the form of peptide bound to major histocompatibility complex (MHC) proteins [3]. (See "The adaptive cellular immune response: T cells and cytokines".)

●B cells have immunoglobulin receptors that can recognize the antigenic portions of intact molecules. (See "The adaptive humoral immune response" and "Overview of therapeutic monoclonal antibodies".)

The natural and adaptive immune systems are closely interrelated. Antigen-specific T cell activation leads to the production and secretion of cytokines and chemokines, which recruit components of the "natural immune" system as well as the specific mechanisms of alloantibody production and CD8-positive cell-mediated cytotoxicity. In addition, local tissue production of complement components appears essential for full T cell activation [4].

An example of adaptive immunity involving B cells and its manipulation to permit transplantation involves ABO antigens. Previously, a classic contraindication to transplantation was the presence of ABO incompatibility between donor and possible recipient, a setting in which the presence of preformed donor-specific antibodies rapidly destroy an allograft. The principal method of circumventing this difficulty involves desensitization protocols, in which plasmapheresis, (possibly) splenectomy, and aggressive immunosuppressive regimens result in the removal of these antibodies. (See "Kidney transplantation in adults: HLA-incompatible transplantation".)

However, transplantation of such incompatible organs to infants, in whom humoral immunity has not yet developed, may also result in tolerance. This was reported in infants who have received and accepted ABO-incompatible hearts for several years, with evidence suggesting tolerance due to the deletion of donor-specific B-cell clones [5].

The role of antibodies that target non-human leukocyte antigen (HLA) endothelial cell antigens has long been suspected in solid organ transplantation. One study explored the relationship between antiendothelial cell antibodies and rejection in 226 kidney allograft recipients [6]. Although preexisting antibodies were not associated with increased rejection or decreased recipient/graft survival, de novo antibodies were associated with an increased risk of early acute rejection. Acute rejections were generally more severe and associated with glomerulitis and peritubular capillary inflammation. More patients with de novo antiendothelial antibodies developed graft dysfunction. A test to evaluate the presence of these antibodies is being developed (XM-ONE) [6,7].

T cell activation — T cell recognition of antigen is the primary event that initiates the effector mechanisms of the immune response. This key step requires the following two discrete signals (figure 1):

●Signal 1 is provided by the interaction of the T cell receptor (TCR) with antigen presented as a peptide by the antigen-presenting cell (APC).

●Signal 2 is provided by a costimulatory receptor/ligand interaction on the T cell/APC cell surface [8].

Once activated, T cells undergo clonal expansion under the influence of mitogenic growth and differentiation factors, such as interleukin-2 (IL-2). These activated T cells then perform the following functions:

●Induce CD8-positive T cell-mediated cytotoxicity

●Provide help for B-cell antibody production

●Provide help for macrophages to induce delayed-type hypersensitivity (DTH) responses (algorithm 1)

The individual components that contribute to the immune response are outlined below.

MAJOR HISTOCOMPATIBILITY COMPLEX STRUCTURE AND FUNCTION — The major histocompatibility complex (MHC) is a region of highly polymorphic genes located on the short arm of human chromosome 6 in humans (figure 2) (see "Major histocompatibility complex (MHC) structure and function"). The protein products of the MHC are expressed on the surfaces of a variety of cells. The human system is called human leukocyte antigen (HLA) and is analogous to the H-2 and RT1 systems in mice and rats, respectively.

These cell surface proteins are the principal antigenic determinants of graft rejection. Thus, organs transplanted between MHC-identical individuals are readily accepted, whereas organs transplanted between MHC antigen-mismatched individuals are inevitably rejected in the absence of immunosuppressive agents.

The normal biological role of these proteins was elucidated during the 1970s. It was discovered that antigen-specific T cells do not recognize antigen in free or soluble form or as intact protein. Instead, T cells recognize portions of protein antigens that have been fragmented into peptides bound to MHC molecules. (See "The adaptive cellular immune response: T cells and cytokines".)

Thus, MHC molecules are a major component of the immune system as they provide the means for displaying antigenic peptides to T cells. T cells are exquisitely selected during development in the thymus to have moderate affinity with self-MHC molecules so that they may bind to antigenic peptides in the context of MHC. (See "Normal B and T lymphocyte development".)

The protein products of the MHC have been classified into two major groups (class I and II). The structures of antigen-presenting MHC class-I and II molecules are similar. They each contain a beta-pleated sheet that provides a platform supporting two alpha-helical regions, thus forming a groove in which rests a peptide antigen (figure 3). The beta-pleated regions are relatively conserved as there is a great deal of sequence homology between individuals with different MHC genes. By comparison, the alpha-helical regions are more variable or polymorphic. All the polymorphisms contribute to the variable shapes of the peptide-binding groove and determine the motifs for peptide selection.

Immunogens are defined as foreign proteins that induce an immune response, with antigens being the targets of that response. In general, immunogens are also antigens. Such proteins tend to have defined regions of immunogenic amino acid sequences, termed epitopes, that may incite an immune response, as well as sequences that do not incite an immune response. The ability of an MHC molecule to bind antigen is determined by the amino acid side chain structure of both the MHC molecule and the peptide that binds to it. The physiochemical structure of the peptide-binding groove determines the type of peptides that may be bound.

MHC polymorphism exists as a consequence of the vertebrate evolutionary response to microbial invasion and ensures the continuity of species in the presence of pandemic infection. Thus, a few individuals within a species will survive a pandemic (such as plague) because of the protective effect of MHC gene polymorphisms, thereby ensuring the survival of the species even if the majority of individuals succumb to the illness.

Major histocompatibility complex class I and II — MHC class I and II molecules differ in structure, expression, and the cellular compartment from which they obtain antigenic peptides to present to T cells (table 1).

Class I — Class I MHC molecules contain two separate polypeptide chains:

●MHC-encoded alpha chain, which is divided into three domains (a1, a2, and a3)

●Non-MHC-encoded beta chain, beta2 microglobulin

The portion of the MHC class-I molecule that interacts with peptide antigen consists of the a1 and a2 domains, which comprise approximately 180 amino acids starting at the amino terminus. Structurally, this region is comprised of a platform of beta-pleated sheet that supports two parallel strands of alpha-helix, which together form the peptide-binding groove.

Class-I MHC molecules present cytoplasm-derived peptides that are representative of the normal components of the same cell or of intracellular parasites, principally viruses. Such proteins are degraded in the proteasome within the cytoplasm; the resultant peptide residues are transferred to the endoplasmic reticulum by Transporters associated with Antigen Presentation (TAP transporters). The final MHC/antigen complex assembly then occurs within the endoplasmic reticulum before transportation to the cell surface.

Once on the cell surface, class-I MHC molecules present peptide antigens to CD8-positive T cells. The CD8-positive T cells subsequently induce cell lysis by either activating the cell's self-destruct cycle (inducing apoptosis) or actively killing the infected cell via the release of a number of cytotoxic proteins. As viral infection can occur in most nucleated mammalian cells, MHC class-I molecules are found on almost all cell types (except red blood cells), although the level of expression varies. Particularly high levels of class-I MHC expression are expressed on antigen-presenting cells (APC), including dendritic cells, macrophages, B lymphocytes, and vascular endothelial cells.

The nomenclature of MHC class-I proteins in different species is the following:

●Humans – HLA-A, HLA-B, and HLA-C

●Mouse – H-2 K, H-2 D, and H-2 L

●Rat – RT1.A and RT1.E

Class II — Class-II MHC molecules are alpha-beta heterodimers composed of noncovalently associated polypeptide chains. The two chains are similar in overall structure. Each chain contains a relatively conserved beta-pleated sheet region as well as a more polymorphic alpha-helical region.

The antigen-binding region of the class-II molecule is formed by an eight-stranded, pleated-sheet platform supporting two alpha-helices that make up the walls of the antigen-binding cleft, thereby resulting in an overall structure that is similar to that of class-I MHC. As with class-I molecules, genetic polymorphisms of class-II MHC molecules determine the chemical surface of the cleft and are the principal determinants of the specificity and affinity of peptide binding and T cell recognition.

MHC class-II molecules bind peptides derived from extracellular (exogenous) proteins. Within the endocytoplasmic reticulum, the alpha and beta chains are associated, although the antigen-binding groove is initially filled and protected by an invariant chain. The invariant chain is degraded once proper folding has occurred, resulting in the release of a class-II associated invariant chain peptide (CLIP). Meanwhile, exogenous proteins that have been endocytosed and fragmented are transferred to the "compartment for peptide loading" (CPL), where they displace the CLIP from the MHC antigen-binding groove. The tripartite structure (MHC alpha and beta chain with peptide) is then brought to the cell surface, where it is presented to CD4-positive T cells.

Class-II MHC molecules are expressed constitutively only on the surface of interstitial dendritic cells, macrophages, and B cells. Epithelial cell and vascular endothelial cell MHC class-II expression is strongly upregulated after exposure to a variety of proinflammatory cytokines, which include interleukin-2 (IL-2) and interferon-gamma (IFN-g).

The nomenclature used to describe MHC class-II proteins in different species is as follows:

●Humans – HLA-DP, HLA-DQ, and HLA-DR

●Rat – RT1.H, RT1.B, and RT1.D

●Mouse – I-A and I-E

MINOR TRANSPLANTATION ANTIGENS — The immune response that develops in response to donor tissue is primarily directed against the major histocompatibility complex (MHC) proteins. In contrast, minor histocompatibility antigens were initially described in the mouse as non-MHC-encoded histocompatibility differences that led to allograft rejection. The tempo of rejection in mice that are mismatched for minor, but not major, antigens is slower compared with that observed with MHC incompatibility.

Minor antigens are derived from polymorphic cellular proteins bound to MHC class I of the recipient [9]. Such antigens, for example, may be encoded by the Y chromosome in males (H-Y) and may thus induce an alloimmune response if male tissue is transplanted into a female recipient [10]. Another possibility is that the peptide antigen may be autosomally derived and represent polymorphisms among autosomal proteins or enzymes [11].

These minor antigens are generally peptides recognized by T cells in the context of self-MHC and are most often recognized by CD8-positive T cells that are cytotoxic [9]. In a mouse model, slight modification of such a minor antigen resulted in the manufacture of regulatory T cells that aided tolerance induction, suggesting that chronic low-level stimulation of the T cell receptor (TCR) may promote tolerance [12]. Minor antigens do not induce alloantibody responses.

In bone marrow transplantation, minor antigens play an important role in graft-versus-host (GVH) disease in six-human leukocyte antigen (HLA)-matched individuals. In this setting, donor MHC that is identical to that of the recipient presents minor antigen in the context of "self-MHC," thereby enhancing the GVH response. (See "Pathogenesis of graft-versus-host disease (GVHD)".)

MECHANISMS OF ALLORECOGNITION — T cell recognition of alloantigen is the primary and central event that leads to the cascade of events that result in rejection of a transplanted organ. Individual T cells (or colonies of identical T cell clones) are monospecific as they recognize only a single peptide antigen presented in the context of major histocompatibility complex (MHC). Such clones are further defined on the basis of the identity of their TCR, their phenotype (CD4 or CD8 positive), and the pattern of cytokines produced. (See "The adaptive cellular immune response: T cells and cytokines".)

There are at least two distinct, but not necessarily mutually exclusive, pathways of allorecognition, the direct and indirect pathways (figure 4). Each leads to the generation of different sets of allospecific T cell clones with differential impact on early (direct) versus later (indirect) rejection.

Direct pathway — In the direct pathway, host T cells recognize intact allo-MHC molecules on the surface of the donor or stimulator cell. Since MHC molecules that lack bound peptide are unstable and are therefore unrecognizable by T cells, peptides derived from endogenous proteins that are bound into the groove of the donor MHC play a role in this mode of allorecognition [13].

Direct allorecognition by T cells of intact surface MHC molecules has not been demonstrated outside of alloimmunity, so it is this phenomenon that uniquely distinguishes alloimmunity from ordinary immunity to micro-organisms. This pathway is thought to be the dominant pathway involved in the early alloimmune response as the relative number of T cells that proliferate on contact with allogeneic or donor cells is extraordinarily high compared with the number of clones that target antigen presented by self-antigen-presenting cell (APC) [14].

Thus, direct allorecognition is of major importance in acute allorejection. The transplanted organ carries a variable number of passenger APCs in the form of interstitial dendritic cells. Such APCs have a high density of allo-MHC molecules and are capable of directly stimulating the recipient's T cells.

"Professional APCs" also provide the necessary costimulatory signals for full T cell activation. Parenchymal cells, which may express donor MHC antigens, are frequently deficient in costimulatory molecules and are less able to fully activate T cells ("nonprofessional APCs"). However, evidence in transgenic animals suggests that some nonprofessional APCs, particularly vascular endothelial cells, may be sufficient to activate T cells [15].

Overall, the corollary of this hypothesis is that, as donor-origin APCs are depleted over time after engraftment, the relative contribution provided by the direct pathway to the alloimmune response may dwindle.

Indirect pathway — In the indirect pathway, T cells recognize processed alloantigen presented as peptides by self-APCs (host-APCs) [16]. The basic premise for indirect allorecognition as a mechanism involved in allograft rejection is that donor MHC molecules are shed from the graft, taken up by recipient APCs, and then presented to T cells.

Despite the misleading name, this pathway corresponds to the normal system of antigen recognition whereby the host's immune system recognizes foreign or exogenously derived peptide as presented by self-MHC (see "The adaptive cellular immune response: T cells and cytokines"). Numerous experimental studies have indicated that, in general, the dominant epitopes are mostly confined to the hypervariable regions that encode the peptide-binding regions of the MHC molecule (see above). By comparison, the nonpolymorphic regions (structural regions) are relatively silent immunologically.

In the setting of organ transplantation, peptides corresponding to polymorphic regions of the MHC induce a strong alloimmune response, whereas nonpolymorphic peptides fail to induce such a response. Whether a given section of MHC is immunogenic depends upon its amino acid sequence and on the structure of the MHC molecules in which it is itself presented. There is increasing interest in this "indirect" pathway as peptide antigens are relatively simple structures that can be readily synthesized [17].

This novel experimental approach permits the investigation of the molecular mechanisms of allograft rejection, which has led to several important findings. Allopeptide-reactive T cells are present during both acute and chronic rejection [18,19]. Although primary immune responses are characterized by T cell-proliferative responses to a limited number of immunogenic MHC allopeptides, secondary responses such as those that occur in chronic or late acute rejection are associated with T cell-proliferative responses to a more variable repertoire [16]. This repertoire includes responses to peptides that were previously immunologically silent.

Such a change in the pattern of T cell responses has been termed epitope switching or spreading and can occur to peptides representing alternative regions within a given MHC hypervariable region (intramolecular spreading) or to peptides representing different MHC chains (intermolecular spreading) [20]. The precursor frequency of such MHC allopeptide reactive T cells is typically low, as evidenced by studies in humans with chronic kidney allograft rejection [18]. However, such a finding is not unexpected given the indolent nature of chronic rejection. Since it is possible to assay donor-specific MHC allopeptide-specific T cell activity, it may be possible to translate such information to clinical usefulness to predict rejection.

Studies have also provided a link between MHC allopeptide-primed T cells and the development of acute vascular-type rejection mediated in part by accelerated alloantibody production [21]. Such studies also suggest that chronic allograft vasculopathy, the sine qua non of experimental chronic rejection, may be mediated by T cells primed by the indirect pathway.

In addition to the data that cast light on the molecular mechanisms underlying T cell activation, MHC peptides may also be used therapeutically. Under some circumstances, animal studies reveal that administering donor-specific class-II MHC peptides to allograft recipients in the peritransplant period either orally or intrathymically can lead to transplantation tolerance [22,23].

Semi-direct antigen presentation — Intact donor antigen can be transferred between different T cell types, raising the possibility that direct T cell recognition of intact donor alloantigen on recipient APCs may also occur. The impact of this method of recognition remains unclear [24].

T CELL COSTIMULATION — The two-signal model of T cell activation implies that T cells require two separate signals to enter the cell cycle (figure 1) [25,26]:

●Signal 1 is antigen specific and is provided by the engagement of the T cell receptor (TCR) with peptide complexed with major histocompatibility complex (MHC) on the antigen-presenting cell (as above). (See "The adaptive cellular immune response: T cells and cytokines".)

●Signal 2 is provided by the interaction of one or more T cell surface receptors with their specific ligands on the antigen-presenting cell (APC) cell surface (costimulatory pathways).

CD28, B7, and CTLA4 — Of the numerous costimulatory pathways identified thus far, that provided by the interaction of CD28 on the T cell surface with its APC cell surface ligands, B7-1 or B7-2, has been most studied (figure 5) [8,27-32]. T cell anergy and apoptosis induced by TCR signaling alone is prevented by signaling through CD28 [33].

In addition to these ligands that transduce a costimulatory or activating signal, cytotoxic T-lymphocyte-associated antigen 4 (CTLA4), which also binds B7-1 and B7-2, provides an inhibitory signal [34,35]. Although CD28 is expressed on resting T cells, CTLA4 is expressed on the cell surface only after initial T cell activation.

Engagement of CTLA4 by binding with high affinity to its B7 counter-receptors appears to downregulate immune responses, suggesting that CTLA4 plays a critical feedback role in terminating T cell responses. The importance of CTLA4 can be illustrated by the following observations:

●Genetically engineered mice in which the genes for CTLA4 have been deleted develop massive lymphoproliferative disease, culminating in early death [36].

●The administration of blocking anti-CTLA4 monoclonal antibodies exacerbates autoimmune disease and prevents induction of T cell anergy [37].

Abatacept (CTLA4-Ig) — Interest in the manipulation of the CD28:B7 pathway in transplantation has focused on the administration of CTLA4-Ig (abatacept), which is a recombinant fusion protein that contains the extracellular domain of soluble CTLA4 combined with an immunoglobulin G1 (IgG1) heavy chain. CTLA4Ig is a competitive inhibitor of CD28 binding, resulting in T cell anergy in vitro.

In some transplantation models, the systemic administration of CTLA4Ig effectively dampened the immune response and prevented experimental acute and chronic rejection, resulting in prolonged graft survival and tolerance:

●In a rat acute kidney allograft rejection model, systemic tolerance induced by the administration of CTLA4Ig was associated with selective inhibition of T helper cell type 1 (Th1; interleukin-2 [IL-2], interferon-gamma [IFN-g]) and sparing of T helper cell type 2 (Th2; interleukin-4 [IL-4], interleukin 10 [IL-10]) cytokines in the target organ (figure 6) [38].

●CTLA4Ig can prevent or interrupt the development of chronic rejection in rat models of cardiac and kidney transplantation [39]. These data indicate that T cell activation is a proximal event in the cascade that culminates in the arteriosclerosis of chronic rejection.

However, the beneficial effect of CTLA4Ig is abrogated when coadministered with cyclosporine [40]. Although acute rejection was prevented, chronic rejection still occurred with combination therapy.

This finding is of major potential importance with regard to human clinical transplantation since CTLA4Ig and other agents active in blocking T cell costimulation (eg, CD40L antibody, see below) are to undergo clinical trials.

To this end, a high-affinity variant of CTLA4-Ig, named LEA29Y (belatacept), with significant immunosuppressive characteristics has been developed [41,42].

CD40 and CD40L — There has been a great deal of interest in the role played by another costimulatory molecule, CD40, and its ligand, CD40L (CD154), in the alloimmune response [30,43]:

●CD40, a member of the tumor necrosis factor (TNF) receptor superfamily, is expressed on B cells and other APCs, including dendritic cells.

●CD40 ligand, CD40L (CD154), is expressed early on activated T cells.

●CD40L is responsible for the main form of X-linked hyperimmunoglobulin M (hyper-IgM) syndrome.

Binding of CD40L to CD40 is critical in providing cognate T cell help for B-cell Ig production and class switching; a defect in CD40L is responsible for the hyper-IgM syndrome [44] (see "Primary humoral immunodeficiencies: An overview"). In addition, CD40L-deficient T cells fail to undergo effective clonal expansion. CD40 ligand is expressed by human vascular endothelial cells, smooth muscle cells, and human macrophages in vitro and is coexpressed with its receptor CD40 in human atherosclerotic lesions in situ [45].

CD40 and CD40L are coexpressed in acutely rejecting murine cardiac allografts, although not in normal hearts or syngeneic grafts. In addition, CD40 and CD40L have been demonstrated in human cardiac allograft vessels during rejection. Such data provide the rationale for targeting this pathway to prevent rejection.

The majority of studies reveal some prolongation of allograft survival by the administration of monoclonal antibodies directed against CD40L [43]. Interestingly, costimulatory blockade (using the combination of CD28 and CD40L blockade) synergistically enhances allogeneic murine skin and cardiac allograft survival and prevents the development of allograft vasculopathy in a murine aortic transplantation model [46].

Novel costimulatory pathways — A number of additional T cell costimulatory pathways have also been discovered. These include molecules related to the CD28-B7 family, namely ICOS-B7RP-1 and PD-1-PD-L, and newer members of the TNF-TNF-receptor superfamily [47]. The exact role of these pathways in the transplant setting is unclear. Emerging evidence suggests that some of these pathways, particularly ICOS-B7RP-1, are important in activating effector T cells in transplant models [48].

T CELL HOMING — Naive T cells are generated and mature in the thymus. Such T cells wander within the blood and lymphocyte circulation; they preferentially leave via lymph nodes when they encounter antigen. Subsequently, T cell activation ensues, leading to a cascade of events orchestrated by both cell surface and soluble proteins that target and amplify the alloimmune response. Once activated, the T cells enter the cell cycle and express a variety of cell surface proteins that mediate adhesion to other cells and matrix proteins.

These changes result in an increase in the number of activated, antigen-specific T cells that are more adherent to accessory cells, particularly endothelial cells. In addition, antigen-nonspecific T cells may also be activated and recruited into a rejection lesion when the level of inflammation is intense.

Leukocyte migration — Leukocyte migration from the circulation to a site of inflammation involves four discrete, coordinated steps [49]. These steps include (figure 7):

●Rolling, which is selectin mediated

●Triggering, which is chemokine mediated

●Firm adhesion, which is integrin mediated

●Transmigration, which is platelet endothelial cell adhesion molecule (PECAM) and chemokine mediated

Rolling — The initial step of rolling is mediated by the selectins, a family of three closely related proteins found on leukocytes, endothelial cells, and platelets (L, E and P selectins, respectively) [50]. L-selectin is expressed constitutively on leukocytes. One of its ligands, GlyCAM, is expressed on high endothelial venules of lymph nodes.

L-selectin binding of its ligand occurs rapidly, is of low affinity, and permits leukocyte rolling. This process slows leukocyte movement through the vasculature [51].

Activation — Leukocyte activation or triggering is then required before the cascade of events can proceed further. Such an activating signal is provided by chemokines (see 'Chemokines and chemokine receptors' below), which are produced by both leukocytes and endothelial cells.

Firm adhesion — Firm adhesion of leukocytes to the endothelium is mediated by cell surface integrins. Very late antigen-4 (VLA-4) ligation of endothelial vascular cell adhesion molecule-1 (VCAM-1) provides the principal interaction leading to adhesion, although several other integrins play important roles [50].

Transmigration — Transmigration, the process whereby leukocytes traverse the endothelial barrier and extracellular matrix, then occurs. This process is mediated predominantly by the PECAM-1:PECAM-1 homotypic interaction, usually in the direction of an increasing concentration gradient of chemokines [49]. (See "Leukocyte-endothelial adhesion in the pathogenesis of inflammation".)

Chemokines and chemokine receptors — Attraction of leukocytes to sites of tissue injury, infection, or allotransplantation is essential for the induction of the acute inflammatory response. This process is controlled by a group of low-molecular-weight (8 to 10 kd) "chemoattractant cytokines" or chemokines [52]. More than 40 chemokines have been identified and have been divided into families on the basis of the structural location of cysteine residues within the protein.

The majority of chemokine proteins are composed of a single amino acid chain, which contains four cysteine residues that link to form two intra-chain disulphide bonds. The largest families are the alpha and beta chemokines. The alpha-chemokine family is characterized by cysteine residues that are separated by a single amino acid (CXC), while the cysteine residues in beta chemokines are adjacent (CC). By comparison, lymphotoxin, a member of a third group, has only a single pair of cysteines. Chemokines are tethered to the cell membrane by highly sulphated anionic structures on glycosaminoglycans, particularly heparin and chondroitin-sulfates [53].

Chemokines provide the signals that convert the low-affinity selectin-mediated leukocyte rolling into the integrin-mediated leukocyte-endothelial adhesion that presages transmigration. The local production of chemokines also results in a chemoattractant gradient, thereby directing leukocyte trafficking through the tissues, blood, and lymph nodes. This process brings naive lymphocytes into contact with antigen and so enhances the effecter and memory functions of the immune response. (See "Leukocyte-endothelial adhesion in the pathogenesis of inflammation".)

The leukocyte response to chemokines requires cell surface expression of the appropriate chemokine receptor(s). Among the large number of known receptors, certain members of the CC family of chemokine receptors are predominantly expressed during an allograft rejection. These include chemokine receptor 1 (CCR1), CCR2, CXCR3, and CCR5 [54-56]. Data from animal models and clinical observations suggest that CCR5, a high-affinity receptor for the CC chemokines, macrophage inflammatory protein (MIP)-1alpha, MIP-1beta, and regulated on activation normal T cell expressed and secreted (RANTES), has a significant role in leukocyte trafficking in transplanted allografts [56-59].

CYTOKINES AND T HELPER CELLS — T helper cells have classically been divided into two distinct populations, type 1 (Th1) and type 2 (Th2) cells; they each produce their own repertoire of cytokines that mediate separate effector functions (figure 6) [60-62]. It should be noted there is marked redundancy in the system with a degree of overlap between T cell subsets and function. (See "The adaptive cellular immune response: T cells and cytokines", section on 'Cytokine profiles and functions of CD4+ T helper cell subsets'.)

Several other T helper subsets have been identified, including T helper cell type 17 (Th17) that secretes interleukin-17 (IL-17).

IL-17 induces cytokines and chemokines and recruits neutrophils. Th17 cells appear to be involved in the early response to pathogens, such as bacteria and fungi, and function in autoimmunity and tissue inflammation [63-65].

Type 1 helper T cells — Th1 cells produce interleukin-2 (IL-2) and interferon-gamma (IFN-g) and induce macrophage activation, leading to delayed-type hypersensitivity (DTH) responses. Acute allograft rejection is predominantly mediated by a Th1 immune response. Acutely rejecting allografts clearly express both mRNA transcripts as well as protein levels of IL-2 and IFN-g.

Type 2 helper T cells — Th2 cells produce interleukin-4 (IL-4), interleukin-5 (IL-5), interleukin-10 (IL-10), and interleukin-13 (IL-13), and provide help for B-cell function [66]. IL-4 is a growth factor for B cells and antibody production and also can directly inhibit T cell maturation into the Th1 pathway. Such responses have been implicated in clearing parasitic (predominantly helminthic) infections in mammals.

A switch from a Th1 to a Th2 cytokine expression is associated with allograft tolerance [67]; however, a causal relationship between a deviation in the alloimmune response towards Th2 cell function and tolerance induction has not been demonstrated.

Type 17 helper T cells — Th17 cells are a T cell subset that may contribute to allograft rejection and act as a barrier to the induction of transplant tolerance. Despite accumulating evidence, the precise impact of Th17 cells on transplant rejection and the induction of tolerance require further clarification [68]. Chronic rejection, both clinically and experimentally, has been associated with a number of fibrogenic cytokines, which include tissue growth factor (TGF)-beta and fibroblast growth factor (FGF)-1. In addition, such cytokines may be upregulated by the concomitant use of calcineurin inhibitors such as cyclosporine, thus exacerbating graft fibrosis [69].

Regulatory T cells — Regulatory T cells suppress the immune response and maintain tolerance. There are multiple types of regulatory T cells with variable expression of CD4, CD25, and Foxp3 [70].

MOLECULAR MECHANISMS OF T CELL ACTIVATION — Following T cell activation, four major biochemical events occur within the cytoplasm. These include the following:

●Hydrolysis of membrane-bound inositol phospholipid

●Increases in cytoplasmic calcium

●Tyrosine phosphorylation of a variety of proteins

●Increases in protein kinase C (PKC) activity

These pathways ultimately lead to activation of cytokine DNA promoter regions within the nucleus, permitting transcription of mRNA (figure 5).

Within minutes of T cell receptor (TCR) engagement, membrane-bound phosphatidylinositol 4,5-bisphosphate (PIP) is hydrolyzed to inositol triphosphate (IP3) and diacylglycerol (DAG). The increase in IP3 causes the endoplasmic reticulum to release stored calcium, leading to a marked and sustained elevation in intracellular calcium. This elevation in calcium promotes the formation of calcium:calmodulin complexes that activate a number of kinases including the phosphatase calcineurin.

Calcineurin dephosphorylates cytoplasmic nuclear factor of activated T cells (NFAT), permitting its translocation to the nucleus where it binds to the interleukin-2 (IL-2) promoter sequence and then stimulates transcription of IL-2 mRNA [71]. Calcineurin is the ultimate target of both cyclosporine and tacrolimus (FK506) [72].

Cyclosporine binds to an intracellular protein, cyclophilin, a member of the immunophilin family (figure 5). It is this complex that binds to and inhibits the phosphatase activity of calcineurin. Tacrolimus binds to the FK506 binding protein (FKBP), another immunophilin, which inhibits calcineurin in a similar manner. (See "Pharmacology of cyclosporine and tacrolimus".)

However, rapamycin has a different mode of action. Although this compound binds to FKBP, it works by blocking a p70/S6 kinase that is involved in transducing the IL-2 receptor signal (figure 5) [73]. (See "Kidney transplantation in adults: Maintenance immunosuppressive therapy".)

In parallel with the events described above, a number of other intracellular events also occur, including PKC activation by DAG and activation of nuclear factor kB (NFkB).

●DAG induces protein phosphorylation, which leads to activation of immediate genes in the nucleus.

●NFkB is another important nuclear transcription factor that is activated by several different signals that include tumor necrosis factor (TNF), interleukin-1 (IL-1), and lipopolysaccharide (LPS) [74]. In the resting cell, this molecule is found in a heterodimeric form in the cytoplasm bound to inhibitors of kB (IkBs). Signal-induced degradation of the IkBs frees the NFkB, permitting it to enter the nucleus, where it binds to its specific binding site. Transcription of a number of different genes including major histocompatibility complex (MHC) class I, immunoglobulin (Ig), and IL-2 then ensues. This pathway is disrupted by the administration of corticosteroid therapy [75].

The Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway, which is involved in transducing signals provided by interferons and many other cytokines, also appears to be highly important [76]. Initial experiments demonstrated that interferon-activated transcription factors undergo tyrosine phosphorylation, with tyrosine kinase being essential for interferon responses [77,78]. Subsequently, four Janus kinases were identified that play central roles in cytokine-signaling pathways. More than 30 cytokines use the JAK-STAT pathway, making it an attractive target for therapeutic interventions.

EFFECTOR MECHANISMS ASSOCIATED WITH ALLOGRAFT REJECTION — Alloantigen-dependent and alloantigen-independent factors contribute to the effecter mechanisms underlying allograft rejection [79-82]. A unifying hypothesis linking these apparently divergent mechanisms has been proposed (algorithm 1) [83].

Nonimmunologic "injury responses" (such as the response to ischemic damage) first induces a nonspecific inflammatory response; this leads to increased antigen presentation to T cells by upregulating the expression of adhesion molecules, class-II major histocompatibility complex (MHC), chemokines, and cytokines [83]. Nonspecific inflammation also promotes the shedding of intact, soluble human leukocyte antigen (HLA), which may prime the indirect allorecognition pathway. Once activated, CD4-positive T cells initiate macrophage-mediated delayed-type hypersensitivity (DTH) responses and provide help to B cells for alloantibody production.

CD8-positive T cells that mediate cell-mediated cytotoxicity reactions kill either by delivering a "lethal hit" or alternatively by inducing apoptosis. After encountering a class-I MHC molecule that is presenting antigen, the T cell secretes perforin, an inducer of pore formation, and granzyme B, a serine protease that activates the interleukin-1-beta converting enzyme (ICE) protease pathway, that together induce cell death. This pathway is probably dominant during microbial infections. Alternatively, the T cell may utilize the FAS pathway, which induces "activation-induced cell death." The FAS pathway is of importance in limiting T cell proliferation in response to antigenic stimulation. Cell-mediated cytotoxicity has been shown to play an important role in acute, although not chronic, allograft rejection [84].

Detailed sequential immunohistologic as well as reverse transcriptase-polymerase chain reaction (RT-PCR) studies of chronically rejected allografts reveal gene upregulation of T cells and T cell-derived cytokines (including interleukin-2 [IL-2] and interferon-gamma [IFN-g]) early after transplantation. Other, somewhat later events include expression of the beta-chemokines regulated on activation normal T cell expressed and secreted (RANTES), interferon-inducible protein-10 (IP-10), and monocyte chemoattractant protein-1 (MCP-1), which precedes and promotes intense macrophage infiltration of the allograft [85,86]. The potent smooth muscle and mesangial cell mitogen interleukin-6 (IL-6), the inflammatory cytokine tumor necrosis factor (TNF)-alpha, inducible nitric oxide synthase, and growth factors, such as transforming growth factor (TGF)-beta, platelet-derived growth factor, and endothelin, are also expressed at approximately the same time [87,88].

Endothelial cells that are activated by T cell-derived cytokines and macrophages express class-II MHC, adhesion molecules, and costimulatory molecules, and they can present antigen and activate more T cells. Growth factors, including TGF-beta and endothelin, lead to smooth muscle cell proliferation, neointimal thickening, interstitial fibrosis, and, in the case of the kidney, glomerulosclerosis.

SUMMARY

●The immune response involved in organ transplantation is a form of adaptive immunity, or one in which the response is to a specific antigen. The principal targets of the immune are the major histocompatibility complex (MHC) molecules expressed on the surface of donor cells (allo-MHC). (See 'Introduction' above.)

●T cell recognition of antigen is the primary event that initiates the immune response. This key step requires the interaction of the T cell receptor (TCR) with antigen presented as a peptide by the antigen-presenting cell (APC) and a costimulatory receptor/ligand interaction on the T cell/APC cell surface. Activated T cells are directly cytotoxic and provide help for B-cell antibody production and macrophage-induced delayed-type hypersensitivity (DTH) responses. (See 'T cell activation' above.)

●Proteins encoded by the MHC are the principal antigenic determinants of graft rejection. Organs transplanted between MHC-identical individuals are readily accepted, whereas organs transplanted between MHC antigen-mismatched individuals are inevitably rejected in the absence of immunosuppressive agents. (See 'Major histocompatibility complex structure and function' above.)

●The antigen-presenting protein products of the MHC have been classified into class I and II groups that are characterized by structure, expression, and the cellular compartment from which they obtain antigenic peptides to present to T cells. Class-I MHC molecules include human leukocyte antigen (HLA)-A, HLA-B and HLA-C molecules and are found on all cell types except red blood cells. Class-I molecules present cytoplasm-derived peptide antigens to CD8-positive T cells, which induce cell lysis.

Class-II MHC molecules include HLA-DP, HLA-DQ, and HLA-DR molecules. Class-II MHC molecules are constitutively expressed on interstitial dendritic cells, macrophages, and B cells, but expression may be upregulated on epithelial cell and vascular endothelial cell after exposure to proinflammatory cytokines. Class-II molecules present peptides derived from extracellular proteins to CD4-positive T cells. (See 'Major histocompatibility complex class I and II' above.)

●The activation of costimulatory pathways is required for T cell entry into the cell cycle. Multiple costimulatory molecules have been identified including CD28 and CD40. (See 'Mechanisms of allorecognition' above and 'T cell costimulation' above.)

●Leukocyte migration from the circulation to a site of inflammation involves four steps including selectin-mediated rolling, chemokine-mediated triggering, integrin-mediated firm adhesion, and platelet endothelial cell adhesion molecule (PECAM)- and chemokine-mediated transmigration. Chemokine-regulated attraction of leukocytes to sites of tissue injury, infection, or allotransplantation is essential for the induction of the acute inflammatory response. (See 'Leukocyte migration' above.)

●T helper cells are divided into two distinct populations, type 1 (Th1) and type 2 (Th2) cells. Th1 cells produce interleukin-2 (IL-2) and interferon-gamma (IFN-g) and induce macrophage activation, leading to DTH responses. Acute allograft rejection is predominantly mediated by a Th1 immune response. (See 'Cytokines and T helper cells' above.)

●T cell activation results in intracellular signaling that activates cytokine DNA promoter regions permitting transcription of mRNA. (See 'Molecular mechanisms of T cell activation' above.)

●Alloantigen-dependent and alloantigen-independent factors contribute to the effecter mechanisms underlying allograft rejection. (See 'Effector mechanisms associated with allograft rejection' above.)