Entre asociaciones

As has been discussed earlier, T cells only recognize antigens that have been degraded and presented by class I and II MHC molecules. The molecules of the major histocompatibility complex play a fundamental role in the immune system, primarily allowing the distinction to be made between self and non-self.

The MHC, or its equivalent, is a set of genes found in all species of vertebrates first discovered by Peter Gorer in the 1930s when he was studying antibodies to red blood cell antigens in mouse models [59]. Subsequently, George Snell coined the term histocompatibility (H) antigen to describe antigens capable of provoking graft rejection and that differences in the H-2 antigen first described by Gorer provoked the strongest graft rejection response [15]. The human MHC was subsequently described by Jean Dausset in the 1950s [17].

The human MHC is located on the short arm of chromosome six and comprises of two sets of genes encoding the human leukocyte antigen (HLA) system which lie either side of a genetic region (often referred to as the MHC class III region) which includes a range of genes encoding proteins involved in immune regulation (figure 1.8).

Figure 1.8: Map of the human major histocompatibility complex on chromosome 6. Class I HLA (HLA-A,B,C) loci and class II HLA (HLA-DR, DP, DQ) are encoded alongside a range of genes encoding proteins involved in immune responses (e.g complement proteins, proteins involved in antigen processing and presentation).

The MHC class I region contains the genes required for HLA class I molecules. The HLA class I molecule consists of a heavy polypeptide alpha (α) chain of approximately 44kDa which is noncovalently linked to a 12kDa polypeptide called β2- microglobulin. The heavy chain is organized into three extracellular domains (α1, α2, and α3), that are anchored to the cell surface by a hydrophobic anchor region. A short hydrophilic sequence carries the C-terminus end of the molecule into the cytoplasm (figure 1.9).

The HLA class II molecule is also a transmembrane glycoprotein, but differs structurally from the class I molecule as it lacks β2-microglobulin and instead consists

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into membrane distal α1 and β1 domains above the membrane proximal α2 and β2 domains (figure 1.9).

Figure 1.9: Basic structure of the class I and class II HLA molecules. The antigen binding portion consists of α1 andα2 domains for class I and α1 and β1 domains for class II in association with antigenic peptide.

There is significant structural homology between class I and class II HLA molecules. This is particularly apparent in the membrane distal portion of both class I and class II HLA molecules, the region where peptide presentation and T cell recognition occurs. In the class I molecule the α1 andα2 domains combine to form two alpha helices which lie parallel to each other and above a β-pleated sheet ‘floor’ to produce a very distinct ‘groove’ into which an antigenic peptide is situated and held in position due to the presence of conserved ‘anchor’ residues in the peptide sequence. The class II molecule forms the groove in a similar way by combining the

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α1 and β1 domains. This structure in association with the antigenic peptide is often said to resemble ‘sausages on a barbeque’ (figure 1.10). The nature of the peptide binding groove differs slightly between class I and class II MHC. The peptide binding groove of the class I molecule is closed at either end and allows the binding of an antigenic peptide typically 8-10 amino acids in length. The class II binding groove is significantly more ‘open-ended’ allowing for a greater degree of peptide binding flexibility with 10- 13 amino acid peptides typically accommodated.

Figure 1.10: The peptide binding groove of the MHC class I and class II molecules. The antigenic peptide can be seen between the two parallel alpha helices producing a recognition complex seen by the T cell (circled). The class II peptide binding groove is open-ended and allows for a longer peptide to be accommodated.

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1.4.2 The Major Histocompatibility Complex (MHC): Function

The MHC molecule has a range of functions, the main one being to present peptides to T-cells, but they can also act as inhibitory ligands for NK cells via interaction with a range of NK cell surface receptors, most notably killer cell immunoglobulin-like receptors (KIRs) and the NKG2A receptor [60, 61].

The MHC class I molecule primarily presents antigen that derives from intracellular pathogen such as viruses. As viruses can attack virtually any nucleated cell the immune system responds by incorporating MHC class I expression on virtually all nucleated cells also. The processed peptide is presented by the MHC class I molecule for recognition by the CD8 positive cytotoxic T-cell (Tc). The CD8 positive T-cell induces the death of the infected cell before the infected cell can multiply. Initially the naïve cytotoxic T-cells are activated when the T-cell receptor (TCR) recognizes and interacts with a peptide-MHC class I complex. The CD8 positive T-cell can induce apoptosis of the infected cell via the release of the granular components, perforins, granzymes, and granulysin. Upon recognition of target antigen, CD8 positive cytotoxic T-cells release perforins and granzymes onto the surface of the target cell. Perforin allows granzymes to enter the target cell and cause apoptosis [62].

Once the infected cells have been cleared the majority of the cytotoxic T-cells die and are cleared by phagocytosis. A small number remain to serve as memory T- cells. These memory T-cells can respond rapidly upon encountering the same antigen, differentiating into an effector cell population in a fraction of the time required following the primary response to antigen.

Class II MHC molecules are primarily expressed on professional antigen cells such as B lymphocytes, dendritic cells, and macrophages. Importantly, class II

expression can also be induced on capillary endothelial and epithelial cells by γ- interferon [63]. The function of the MHC class II molecule is to present antigen of extracellular origin to CD4 positive T-helper (Th) cells.

Antigen is internalized by endocytosis, processed, then presented by the MHC class II molecule. The TCR/CD3 complex of the T-helper cell binds to the peptide- MHC complex, this interaction is strengthened by the binding of the T-helper cell marker CD4 to a conserved region of the MHC class II molecule. This close interaction causes the intracellular kinases present on the TCR, CD3 and CD4 proteins to activate each other via phosphorylation. This in turn causes the activation of the major biochemical pathways within the T-helper cell, the so-called signal 1 of Th-cell activation. Following receipt of the initial activation signal the naïve T-helper cell then seeks a second confirmatory signal designed to ensure that T-cell activation is in response to foreign antigen and not due to auto-reactivity. This signal 2 involves an interaction between the CD28 molecule expressed on the T-helper cell and either the CD80 or CD86 protein expressed alongside the MHC class II complex on the antigen presenting cell [64].

Following the second signal the T-helper cell proliferates, mediated by a potent release of the growth factor interleukin-2 (IL-2). Activated T-helper cells also begin to express a fully functional form of the IL-2 receptor (IL-2R), so IL-2 acts in an autocrine manner to stimulate proliferation. The released IL-2 can act upon its cell of origin or an adjacent cells leading to a potent clonal expansion response. The activated T-helper cell can then further differentiate into Th-1 or Th-2 cells depending on its local cytokine environment. Th-1 cells are induced in response to interferon-gamma (IFN-γ), and Th-2 cells arise from interleukin-4 (IL-4) stimulation.

properties. Th-1 cells also induce B cells to produce opsonizing antibodies, resulting in a powerful B-cell mediated immune response. Both Th-1 and Th-2 cells can provide the required co-stimulatory signals to B-cells in order that they can start producing antibodies by encouraging differentiation of B cells into antibody secreting plasma cells (figure 1.4). Signals derived from both Th-1 and Th-2 subsets can also insitigate class switching of activated B-cells to produce antibodies of different isotype (discussed in more detail in section 1.5).

In the context of transplantation differences in the MHC between graft donor and recipient can be detected by host T-cells. The two main routes by which foreign MHC is detected are by direct and indirect allorecognition (figure 1.11). Direct allorecognition occurs when host T-cells recognize an intact graft MHC molecule as foreign and mounts an immune response directly. The alternative route, indirect allorecognition, occurs when graft MHC is internalized and presented as antigen peptides in the context of recipient MHC on antigen presenting cells. Direct allorecognition is thought to be more important in acute rejection, particularly in the presence of MHC disparity between donor and recipient. Indirect recognition is thought to contribute to rejection by activating macrophages leading to fibrosis, although alloantibody is generated by the indirect pathway only [65].

Figure 1.11: Direct and indirect allorecognition. Direct allorecognition: The whole donor MHC/peptide is recognized by the T cell. In contrast, for indirect allorecognition foreign MHC sequences are presented as peptides in the context of self-MHC.