Alloimmunity

Alloimmunity is an immune response to foreign antigens (alloantigens). The body attacks especially transplanted tissue and even the fetus in some cases. Alloimmune response results in graft rejection which is manifested as deterioration or complete loss of its function. Alloimmunity is caused by the difference between products of highly polymorphic genes, primarily genes of MHC complex, of the donor and graft recipient. These products are recognized by T-lymphocytes and other mononuclear leukocytes which infiltrate the graft and damage it.

Types of the rejection

 * Acute rejection is caused by antigen-specific Th1 and cytotoxic T-lymphocytes. They recognize transplanted tissue because of expression of alloantigens. A transplant is rejected during first several days or weeks after transplantation.
 * Hyperacute and accelerated rejection is antibody-mediated immune response to the allograft. Recipient’s blood already contains circulating antibodies before the transplantation – either IgM or antibodies incurred by previous immunization (e.g. by repeated blood transfusion). In case of hyperacute rejection, antibodies activate complement; moreover the reaction can be enhanced by neutrophils. This type of rejection is very fast, the graft is rejected in a few minutes or hours after the transplantation. Accelerated rejection leads to phagocyte and NK cell activation (not of the complement) through their Fc receptors that bind Fc parts of antibodies. Graft rejection occurs within 3 to 5 days. This type of rejection is typical response to xenotransplantations.
 * Chronic rejection mechanism is not fully understood so far but it is known that it is associated with alloantibody and cytokine production. Endothelium of the blood vessels is being damaged, therefore the graft is not sufficiently supplied with blood and is replaced with fibrous tissue (fibrosis). It takes two months at least to reject the graft in this way.

Mechanisms of the rejection
CD4+ and CD8+ T-lymphocytes along with other mononuclear leukocytes (their exact function regarding the topic is not known) participate in the rejection. B-lymphocytes, NK cells and cytokines also play a role in it.
 * Cellular rejection – CD4+ and CD8+ T-lymphocytes, NK cells
 * Humoral rejection – B-lymphocytes
 * Cytokines

B-lymphocytes

Humoral (antibody-mediated) type of rejection is caused by recipient’s B-lymphocytes which produce alloantibodies against donor MHC class I and II molecules. These alloantibodies can activate the complement – this leads to target cell lysis. Alternatively, donor cells are coated with alloantibodies that initiate phagocytosis through Fc receptors of mononuclear leukocytes. Mechanism of humoral rejection is relevant for hyperacute, accelerated and chronic rejection. Alloimmunity can be also regulated by neonatal B cells.

Cytokines

Cytokine microenvironment where CD4+ T-lymphocytes recognize alloantigens significantly influences polarization of the immune response.
 * CD4+ T-lymphocytes differentiate into Th1 helper cells in the presence of IL-12 (which is usually secreted by mature dendritic cells). Th1 cells produce proinflammatory cytokine IFN-γ and destroy the allograft tissue.
 * If there is IL-4, CD4+ T-lymphocytes become Th2 cells secreting IL-4 and IL-5. Then allograft tolerance is mostly observed.
 * TGF-β induces expression of Foxp3 gene in the absence of proinflammatory cytokines and thus differentiation of CD4+ T-lymphocytes into regulatory T cells (Treg). Regulatory T cells produce anti-inflammatory cytokines IL-10 and TGF-β which ensures the allograft tolerance.
 * However, in the presence of IL-6 or IL-21 along with TGF-β, CD4+ T-lymphocytes acquire tissue-destructive Th17 phenotype and secrete IL-17.

NK cells

NK cells can also directly target the transplanted tissue. It depends on the balance of activating and inhibitory NK cell receptors and on their ligands expressed by the graft. Receptors of KIR (Killer-cell immunoglobulin-like receptor) family bind concrete MHC class I molecules. If the graft has these ligands on its surface, NK cell cannot be activated (KIR receptors provide inhibitory signal). So if these ligands are missing, there is no inhibitory signal and NK cell becomes activated. It recognizes target cells by “missing-self strategy” and induces their apoptosis by enzymes perforin and granzymes released from its cytotoxic granules. Alloreactive NK cells also secrete proinflammatory cytokines IFN-γ and TNF-α to increase expression of MHC molecules and costimulatory receptors on the surface of APCs (antigen-presenting cells). This promotes APC maturation which leads to amplification of T-cell alloreactivity by means of direct and also indirect pathway of alloantigen recognition (as described below). NK cells are able to kill Foxp3+ regulatory T-lymphocytes as well and shift the immune response from graft tolerance toward its rejection. Besides the ability of NK cells to influence APC maturation and T cell development, they can probably reduce or even prevent alloimmune response to transplanted tissue – either by killing the recipient’s APCs or by anti-inflammatory cytokine IL-10 and TGF-β secretion. However it is important to note that NK cell sub-populations differ in alloreactivity rate and in their immunomodulatory potential. Concerning immunosuppressive drugs, the effects on NK cells are milder in comparison to T cells.

T-lymphocytes

Alloantigen recognition

Alloantigen on APC surface can be recognized by recipient’s T-lymphocytes through two different pathways:
 * Direct allorecognition – occurs when donor’s APCs are presenting graft antigens. Recipient’s T-lymphocytes can identify either MHC molecules alone or complex MHC molecule-foreign peptide as alloantigens. Specific T-cell receptors (TCR) of CD8+ T-lymphocytes recognize these peptides when form the complex with MHC class I molecules and TCR of CD4+ T-lymphocytes recognize a complex with MHC class II molecules.
 * Indirect allorecognition – recipient’s APCs infiltrate transplanted tissue, then they process and present as any other foreign peptides donor’s MHC glycoproteins by MHC class II molecules. Mechanism of indirect allorecognition and therefore the involvement of CD4+ T-lymphocytes is the main cause of graft rejection. That is why the compatibility between donor and recipient MHC class II molecules is the most important factor concerning transplantation.

Activation of T-lymphocytes

T-lymphocytes are fully activated under two conditions: Alloimmune response can be enhanced by proinflammatory cytokines and by CD4+ T-lymphocytes that are responsible for APC maturation and IL-2 production. IL-2 is crucial for memory CD8+ T cell development. These cells may represent a serious problem after the transplantation. As the effect of being exposed to various infections in the past, antigen-specific T-lymphocytes have developed in patient’s body. Part of them is kept in organism as memory cells and these cells could be a reason for “cross-reactivity” – immune response against unrelated but similar graft alloantigens. This immune response is called secondary and is faster, more efficient and more robust.
 * T-lymphocytes must recognize complex MHC-alloantigen presented by APC through direct or indirect allorecognition pathway.
 * T-lymphocytes must receive costimulatory signal. There are costimulatory molecules on T-cell surface and APCs express their ligands (e.g. molecule CD28, which is on the surface of all naïve CD4+ and CD8+ T-lymphocytes, can bind ligands CD80 and CD86). Receptor-ligand engagement triggers T-cell signaling resulting in IL-2 production, clonal expansion and therefore development of effector and memory T-lymphocytes. In contrast, there are also such receptors on T-lymphocytes that cause inhibition of T-cell activation (for instance CD152/CTLA-4 receptor which binds CD80 and CD86 as well). If T-lymphocyte does not receive costimulatory signal, its activation fails and it becomes anergic.

Graft tolerance
Transplanted tissue is accepted by immunocompetent recipient if it is functional in the absence of immunosuppressive drugs and without histologic signs of rejection. Host can accept another graft from the same donor but reject graft from different donor. Graft acceptance depends on the balance of proinflammatory Th1, Th17 lymphocytes and anti-inflammatory regulatory T cells. This is influenced by cytokine microenvironment, as mentioned before, where CD4+ T-lymphocytes are activated and also by inflammation level (because pathogens invading organism activate the immune system to various degrees and causing proinflammatory cytokine secretion, therefore they support the rejection). Immunosuppressive drugs are used to suppress the immune response, but the effect is not specific. Therefore organism can be affected by the infection or cancer much more easily. The goal of the future therapies is to suppress the alloimmune response specifically to prevent these risks. The tolerance could be achieved by elimination of most or all alloreactive T cells and by influencing alloreactive effector-regulatory T-lymphocytes ratio in favor of regulatory cells which could inhibit alloreactive effector cells. Another method would be based on costimulatory signal blockade during alloreactive T-lymphocytes activation.

Literature

 * Cellular and Molecular Immunology, 7th edition by Abul K. Abbas, Andrew H. Lichtman, Shiv Pillai, Saunders Copyright
 * Základy imunologie by Václav Hořejší, Jiřina Bartůňková, Triton, 2009