Alloimmunity

Mechanisms of rejection

Unlimited graft acceptance is hindered by allogenic immune responses. 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. The mechanism of humoral rejection is relevant for hyperacute, accelerated and chronic rejection. Alloimmunity can be also regulated by neonatal B cells.

In addition to humoral immune responses, B-lymphocytes aid in T-cell activation by acting as antigen-presenting cells (APCs). This is activation by indirect allorecognition is crucial, as these T-cells are able to translocate the follicular border to retrieve a T-follicular helper cell (Tfh). This will then lead to an induced B-cellular response. They are important for lymphoid neogenesis (de novo formation of lymphoid tissues during transplant rejection) and forming in-graft tertiary lymphoid organs (TLOs). TLOs also encourage a local alloimmune response needed for recovery and long-term health outcomes.

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 Donor APCs or by anti-inflammatory cytokine IL-10 and TGF-β secretion. 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:

• 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.

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.

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 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.

Disparities in Maternal Alloimmunity/Alloimmunization Care

Although alloimmunization is not a common phenomenon, roughly 2% among prenatal women, lack of proper care has been shown to result in complications during pregnancy, mortality, and significant morbidity. Most often, pregnant women are susceptible to Red Blood Cell (RBC) alloimmunization. RBC alloimmunization is a result of exposure to non-self antigens, ranging from events like fetomaternal hemorrhage during pregnancy or blood and organ transfusions, among a plethora of different circumstances.

Historically, inaccessibility of proper care prior pregnancy and gaps in prevention strategies have affected certain groups of pregnant women more than others. There is evidence that, in the United States, insufficient care among racial and ethnic minorities are at higher risk of having severe alloimmunization that requires intrauterine transfusions (IUTs). For example, roughly 18.6% of the IUT cohort were non-English speakers compared to 4.6% of maternal alloimmunization cases. This demonstrates the need for research in alloimmunity, especially amongst prenatal women, as well as accounting for adequate prenatal care for women of racial/ethnic minorities in the United States. For example, ensuring pregnant women receive Rho(D) immune globulin (RhIG) during pregnancy is crucial to reduce the risk of maternal alloimmunization.

Another risk factor that has been identified is the presence of anti-D antibodies in pregnant women. Over half the women who suffer from maternal alloimmunization have anti-D antibodies which require anti-D prophylaxis and RhIG for treatment. Furthermore, the risk of alloimmunization was higher in women within the Rhesus D-negative blood group, histories of blood transfusion, and poor obstetric history. Other risk factors have been identified, such as an instance of fetomaternal hemorrhage (FMH; a relatively common occurrence in pregnancy), RBC and maternal genetics, as well as previous transfusions. Additionally, mothers who are expecting and have sickle-cell disorder or thalassemia are often more likely to come across complications with alloimmunity due to frequent transfusions. Transfusions expose these individuals, often of racial and ethnic minorities where these conditions are more prevalent, to many antigens from donors that results in alloimmunization. Identifying these risks early in pregnancies and screening more thoroughly prevents instances of hemolytic disease of the fetus and newborn (HDFN) as well as protect the mother's longterm health.

General and cited references

• Cellular and Molecular Immunology, 7th edition by Abul K. Abbas, Andrew H. Lichtman, Shiv Pillai. Saunders Copyright.