Following the initial encounter with nonself, the nonspecific innate immunity is triggered (◘ Fig. 27.2). It primarily consists of macrophages, natural killer (NK) cells, granulocytes, and the complement cascade [6]. Morphologically, NK cells strongly resemble B or T lymphocytes; however, they are not antigen-specific and do not express T-cell receptors or immunoglobulins. NK cells function by lysing target cells or antigens regardless of the presence or absence of any major MHC complexes. Thus, they are directly involved with the initial nonspecific inflammatory response against the donor allograft.

Cytokines , on the other hand, are secreted proteins that alter the immunologic response of nearby cells in an autocrine and paracrine fashion during both innate and humoral responses. The major cytokines in heart transplantation include interleukin-2 (IL-2), interleukin-6 (IL-6), interferon-γ, and tumor necrosis factor (TNF). IL-2 is a key growth factor that is required for the expansion of T lymphocytes during a T-lymphocyte-mediated response (◘ Fig. 27.2). Stimulated cells begin to produce IL-2 and respond to IL-2 via an IL-2 receptor in an autocrine manner. IL-6 primarily induces B lymphocytes to differentiate into plasma cells. TNF is pro-inflammatory and cytotoxic and is primarily secreted by granulocytes. Interferon-γ mainly affects the differentiation of T lymphocytes into T-helper CD4 T cells. It also stimulates NK cells and CD8 T lymphocytes.

A critical step in the immune response pathway is the aggregation and transport of leukocytes into areas targeted for immunologic attack. The adhesion molecules play a key role in maintaining the structural integrity and promoting adhesion of leukocytes to surrounding structures. They include integrins, selectins, and immunoglobulin superfamily adhesion molecules. Adhesion molecules mediate the initial interaction of T lymphocytes with antigens, their migration, and retention within the transplanted organ. Likewise, the activation and migration of T lymphocytes in the lymph nodes and spleen are also dependent on adhesion molecules.

Generally, integrins act as cellular adhesive agents as they anchor cells to the extracellular matrix. Cell-to-cell adhesion is the role of selectins and immunoglobulin superfamily molecules. Although initial graft reperfusion following implantation causes host leukocytes to enter the graft, it also allows leukocytes and other cells in the donor organ to be “flushed” into the host. These cells travel through the blood to the peripheral lymphoid organs, including the thymus (◘ Fig. 27.6), nodes, and spleen (◘ Fig. 27.3), which are primary sites of the initiation of specific T-lymphocyte-mediated response (◘ Fig. 27.4).

Another important arm of innate immunity is the complement system [7, 8]. As outlined in ◘ Fig. 27.7, the complement cascade can be activated through three different pathways: classical, lectin, and alternative. All three pathways of complement activation converge in the activation of complement component 3 (C3).

Activation of complement via the classical pathway is initiated by the first component of human complement (C1), which can bind to antibodies. When the classical pathway of complement is activated through an antibody-mediated process, the globular heads of a single C1 interact simultaneously with two or more Fc regions of IgM or IgG (◘ Fig. 27.8). Once C1 binds, its protease function is activated, and it cleaves the fourth component of complement (C4), yielding two fragments—C4a and C4b. C4 can also be activated by a mannose-binding lectin (MBL). After binding, the lectin can cleave C4 through two associated serine proteases (MASP-1, 2, and 3). C4b continues the complement cascade by binding to C2, which is then cleaved by C1 or MASP to generate C2b that diffuses away and by the remaining C2a that is associated with C4b. C4b and C2a together form the classical pathway C3 convertase (◘ Fig. 27.7). This complex is a protease that can cleave the central component of the complement system, C3.

C3 also can be directly activated by the alternative pathway of complement. Like C4, C3 contains an internal thioester group. In the alternative pathway, small numbers of C3 molecules sporadically undergo a spontaneous conformational change, and several of these C3 molecules covalently bind to nearby proteins. The microenvironment in which C3 binds determines whether this pathway continues through activation of factor B or whether factor H inactivates C3b. When factor B is bound to C3, it is a substrate for the serum protease factor D. Cleavage of factor B results in a soluble fragment Ba and a larger fragment Bb that remains complexed with C3. Properdin stabilizes the activity of the C3Bb complex, and this complex is the alternative pathway C3 convertase.

The classical and alternative pathways converge at C3. Either convertase can cleave C3 into two fragments, C3a and C3b. The smaller C3a fragment can bind to C3a receptors (C3aR) on nearby tissue mast cells, basophils, or eosinophils. The larger fragment, C3b, is deposited together with components of either the classical or alternative C3 convertase pathway forming a complex, the C5 convertase, which is capable of cleaving C5. The small fragment, C5a, is quantitatively much more potent as a chemotactic agent than C3a and affects a wider range of cells, including neutrophils, monocytes, basophils, and eosinophils. The larger fragment, C5b, initiates assembly of the terminal components of complement (C5b-C9) into a pore-forming structure, the membrane-attack complex (MAC). The MAC forms a hole in the membrane of the foreign allograft cells that disrupts membrane integrity and causes cell lysis. Hyperacute rejection is one of the manifestations of the speed and potency of complement activation by preformed antibodies directed against the transplanted allograft [9].

The specific adaptive response is primarily mediated by T lymphocytes, B lymphocytes, and APCs [10]. The cell surface proteins primarily responsible for the immune response in heart transplantation are called MHC or human leukocyte antigens (HLA ). T lymphocytes only recognize antigenic peptides that are contained within the MHC-binding domain on the surface of APCs. The MHCs are highly polymorphic, which establishes the basis for graft rejection. The function of MHC molecules is closely related to their three-dimensional structure, which is composed of two ɑ-helices on top of a β-pleated sheet.

The proteins of the MHC complex are subdivided into class I, class II, and class III. Class I and class II directly relate to transplantation but differ in the composition of the polypeptide chains that constitute them and in their distribution on different cells and tissues.

Class I molecules are expressed on all nucleated cells, with subtypes A, B, and C. The HLA–A gene includes more than 50 alleles, the HLA–B gene includes more than 75 alleles, and the HLA–C gene includes more than 30 alleles. These genes are composed of two chains—a larger, highly polymorphic, heavy ɑ-chain encoded on chromosome 6 and a smaller, non-polymorphic β-chain termed β-2 microglobulin encoded on chromosome 15. Antigens originating from inside the cell are processed via the endogenous pathways; they are complexed with MHC class I molecules and presented to T lymphocytes (◘ Fig. 27.2).

Class II molecules are expressed on a limited subset of cells, such as macrophages, dendritic cells, and B lymphocytes. The MHC complex classes that are relevant to transplantation are HLA-DR, HLA-DP, and HLA-DQ. These molecules are composed of an ɑ-chain and a β-chain. Unlike class I molecules, which interact primarily with CD8 T lymphocytes, class II molecules interact primarily with CD4 T lymphocytes. Antigens originating from outside the cells are processed via the exogenous pathway and presented with the MHC class II molecules, which is the principal mechanism for processing and presenting the alloantigens following heart transplantation [11]. ◘ Figure 27.9 demonstrates the three-dimensional structure of the class I and class II molecules.

T lymphocytes represent the cornerstone of the immune system that mediates the rejection response to the alloantigens that are expressed in the transplanted heart. The CD4 helper T lymphocytes function primarily to detect foreign antigens. Once activated, the CD4 helper T lymphocytes facilitate other cell types, such as CD8 cytolytic T lymphocytes, B lymphocytes, and neutrophils, to participate in the immune response (◘ Fig. 27.1). The CD8 cytolytic T lymphocytes function primarily as effector cells, which kill targeted cells expressing foreign antigens. During their development, the precursor T lymphocytes originate from the bone marrow (◘ Fig. 27.10) and then migrate to the thymus (◘ Fig. 27.6) where they first appear in the subcapsular cortex. Subsequently, they begin to express CD3, CD4, and CD8 molecules resulting in distinct T lymphocytes, and they acquire specific T-cell receptors needed to detect presented foreign antigens. During the selection process, the cells migrate from the thymic cortex to the medulla and then to the periphery (◘ Fig. 27.6).

In the context of the immunologic response , the T-cell antigen receptor on the surface of T lymphocytes must first bind to the antigen presented by the MHC molecule. This binding then transmits a signal to the interior of the cell along the designated transduction pathways. This recognition process is very specific, despite the presence of many diverse antigens. This process recognizes antigens only in the presence of self-MHC molecules.

In the case of CD4 T lymphocytes, which initiate the cellular response to transplant antigens, the antigen must first be associated with an MHC complex on the cell surface through a process called antigen presentation . This presentation is performed by APCs (◘ Fig. 27.4). The antigen first must be internalized within the APC, where it is broken down into small polypeptides. The peptides are then physically associated with MHC molecules and exported to the cell surface as a complex. Cells that serve as APCs are dendritic cells, macrophages, and B lymphocytes. Dendritic cells are the most efficient in presentation and can be found scattered throughout the lymphoid and nonlymphoid tissues. After the recognition phase, the CD4 T lymphocytes progress through a series of activations that culminate in the induction of the rejection response. Specifically in cellular rejection, CD8 cytolytic T lymphocytes are jointly activated and programmed for nonself-destruction through exocytosis-mediated cytolysis and ligand-mediated triggering of apoptosis. Figure 27.11 shows the various transduction pathways involved (calcineurin and mTOR) that provide the basis for the immunosuppression agents being used.

Similar to the T lymphocytes, the B lymphocytes originate or are produced in the bone marrow (◘ Fig. 27.10). However, they act via immunoglobulins, and the end products of differentiation for the B lymphocytes are plasma cells and memory B cells. B lymphocytes depend on surface immunoglobulins for the binding of antigens. Plasma cells produce immunoglobulins (antibodies) (◘ Fig. 27.5), that consist of four polypeptide chains in two pairs (◘ Fig. 27.8). Each pair of chains is identical to the other, consisting of a heavy chain and a shorter light chain. ◘ Figure 27.8 depicts the basic configuration of an antibody molecule.

In order for the immune system to contend with the vastly differing antigens, the antigen-binding portion of the antibody must be extremely heterogeneous (◘ Fig. 27.8). The heavy-chain regions form the basis of the classification of immunoglobulins into isotypes: IgG, IgA, IgM, IgD, IgE, and IgG [12]. The initial production of IgM and, later, IgG are the main isotypes involved in transplant rejection. The term humoral (or antibody-mediated) rejection refers to the production of antibodies against the nonself allograft cells, primarily mediated by the activation of B lymphocytes. The main effector mechanisms described in the humoral responses are neutralization, opsonization, and complement activation.

The discovery and characterization of such immune responses and their biological targets, as well as emerging classes of immunosuppressive therapies, have contributed to enhanced preservation of the transplanted allografts . Alteration of these biological mechanisms using agents such as steroids, antimetabolites, calcineurin inhibitors, mTOR pathway inhibitors and others have revolutionized the field of transplantation.

Successful human transplants have a relatively long history that preceded the use of immunosuppression agents for postoperative survival and graft preservation. After decades of failed solid organ transplantation, in 1951, Medawar from the British National Institute for Medical Research suggested that immunosuppressive agents could be used to mitigate the effects of rejection [13]. Corticosteroid s had been traditionally used, and the more effective azathioprine (AZA) was identified in 1959, but it was not until the discovery of cyclosporine A (CsA) in 1970 that transplant surgery found a sufficiently powerful immunosuppressive (◘ Fig. 27.11).

In 1968, surgical pioneer Denton Cooley, MD, performed more than 15 transplants. Fourteen of these recipients did not survive more than 6 months [14, 15]. By 1984, two-thirds of all heart transplant recipients survived for 5 years or more. In 1981, the first successful heart–lung transplant took place at the Stanford University Hospital. The lead surgeon, Bruce Reitz, MD, credited the patient’s recovery to CsA [16]. Since then, many other drugs have been developed, such as the various calcineurin inhibitors, mammalian target of rapamycin (mTOR) inhibitors, and mycophenolate mofetil (◘ Figs. 27.11 and 27.12).

Prospectively, the emerging field of regenerative medicine promises to solve the problem of organ transplant rejection by regrowing organs in vitro using allogeneic stem cells. Until then, the reliance on current pharmaceutical agents and the development of new agents remain vital for the continuation of transplant medicine.