Transplant rejection 1

Transplant rejection

Transplant rejectionClassification and external resources

Micrograph showing lung transplant rejection. Lung biopsy. H&E stain.

ICD-10 T86 [1]

MedlinePlus 000815 [2]

MeSH D006084 [3]

Transplant rejection occurs when transplanted tissue is rejected by the recipient's immune system, which destroysthe transplanted tissue. Transplant rejection can be lessened by determining the molecular similitude between donorand recipient and by use of immunosuppressant drugs after transplant.

Pretransplant rejection preventionThe first successful organ transplant, performed in 1954 by Joseph Murray, involved identical twins, and so norejection was observed. Otherwise, the number of mismatched gene variants, namely alleles, encoding cell surfacemolecules called major histocompatibility complex (MHC), classes I and II, correlate with the rapidity and severityof transplant rejection. In humans MHC is also called human leukocyte antigen (HLA).Though cytotoxic-crossmatch assay can predict rejection mediated by cellular immunity, genetic-expression testsspecific to the organ type to be transplanted, for instance AlloMap Molecular Expression Testing, have a highnegative predictive value. Transplanting only ABO-compatible grafts, matching blood groups between donor andrecipient, helps prevent rejection mediated by humoral immunity.

ABO-incompatible transplantsBecause very young children (generally under 12 months, but often as old as 24 months[4]) do not have awell-developed immune system, it is possible for them to receive organs from otherwise incompatible donors. This isknown as ABO-incompatible (ABOi) transplantation. Graft survival and patient mortality is approximately the samebetween ABOi and ABO-compatible (ABOc) recipients. While focus has been on infant heart transplants, theprinciples generally apply to other forms of solid organ transplantation.The most important factors are that the recipient not have produced isohemagglutinins, and that they have low levels of T cell-independent antigens. UNOS regulations allow for ABOi transplantation in children under two years of age if isohemagglutinin titers are 1:4 or below,[5] and if there is no matching ABOc recipient. Studies have shown that the period under which a recipient may undergo ABOi transplantation may be prolonged by exposure to nonself A and B antigens. Furthermore, should the recipient (for example, type B-positive with a type AB-positive graft)

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require eventual retransplantation, the recipient may receive a new organ of either blood type.Limited success has been achieved in ABO-incompatible heart transplants in adults, though this requires that theadult recipients have low levels of anti-A or anti-B antibodies. Kidney transplantation is more successful, withsimilar long-term graft survival rates to ABOc transplants.

Immunologic mechanisms of rejectionRejection is an adaptive immune response via cellular immunity (mediated by killer T cells inducing apoptosis oftarget cells) as well as humoral immunity (mediated by activated B cells secreting antibody molecules), though theaction is joined by components of innate immune response (phagocytes and soluble immune proteins). Differenttypes of transplanted tissues tend to favor different balances of rejection mechanisms.

ImmunizationAn animal's exposure to the antigens of a different member of the same or similar species is allostimulation, and thetissue is allogenic. Transplanted organs are often acquired from a cadaver (usually a host who had succumbed totrauma), whose tissues had already sustained ischemia or inflammation.Dendritic cells (DCs), which are the primary antigen-presenting cells (APCs), of the donor tissue migrate to therecipient's peripheral lymphoid tissue (lymphoid follicles and lymph nodes), and present the donor's self peptides tothe recipient's lymphocytes (immune cells residing in lymphoid tissues). Lymphocytes include two classes that enactadaptive immunity, also called specific immunity. Lymphocytes of specific immunity T cellsincluding thesubclasses helper T cells and killer T cellsand B cells.The recipient's helper T cells coordinate specific immunity directed at the donor's self peptides or at the donor'sMajor histocompatibility complex molecules, or at both.

Immune memoryWhen memory helper T cells' CD4 receptors dock to MHC class II molecules, expressed on the surfaces of selectcells, the memory helper T cells' T cell receptors (TCRs) can recognize their target antigen being presented withinthe MHC class II. The memory helper T cell produces clones that, as effector cells, secrete immune signalingmolecules (cytokines) in approximately the cytokine balance that had prevailed at the memory helper T cell'spriming to memorize the antigen. As the priming event in this instance occurred amid inflammation, the immunememory is proinflammatory.

Cellular immunityAs a cell is indicated by the prefix cyto, a cytotoxic influence destroys the cell. Alloreactive killer T cells, also calledcytotoxic T lymphocytes (CTLs), have CD8 receptors that dock to the transplanted tissue's MHC class Imolecules,which display the donor's self peptides. (In the living donor, such presentation of self antigens helpedmaintain self tolerance.) Thereupon, the T cell receptors (TCRs) of the killer T cells recognize their matchingepitope, and trigger the target cell's programmed cell death by apoptosis.

Humoral immunityDeveloped through an earlier primary exposure that primed specific immunity to the nonself antigen, a transplantrecipient can have specific antibody crossreacting with the donor tissue upon the transplant event, a secondaryexposure. This is typical after earlier mismatching among A/B/O blood types during blood transfusion. At thissecondary exposure, these crossreactive antibody molecules interact with aspects of innate immunitysolubleimmune proteins called complement and innate immune cells called phagocyteswhich inflames and destroys thetransplanted tissue.

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Antibody

Secreted by an activated B cell, then called plasma cell, an antibody molecule is a soluble immunoglobulin (Ig)whose basic unit is shaped like the letter Y: the two arms are the Fab regions, while the single stalk is the Fc region.Each of the two tips of Fab region is the paratope, which binds a matching molecular sequence and its 3D shape(conformation), altogether called epitope, within the target antigen.

Opsonization

The IgG's Fc region also enables opsonization by a phagocyte, a process by which the Fc receptor on thephagocytesuch as neutrophils in blood and macrophages in tissuesbinds the antibody molecule's FC stalk, andthe phagocyte exhibits enhanced uptake of the antigen, attached to the antibody molecule's Fab region.

Complement cascade

When the paratope of Ig class gamma (IgG) binds its matching epitope, IgG's Fc region conformationally shifts andcan host a complement protein, initiating the complement cascade that terminates by punching a hole in a cellmembrane. With many holes so punched, fluid rushes into the cell and ruptures it.Cell debris can be recognized as damage associated molecular patterns (DAMPs) by pattern recognition receptors(PRRs), such as Toll-like receptors (TLRs), on membranes of phagocytes, which thereupon secrete proinflammatorycytokines, recruiting more phagocytes to traffic to the area by sensing the concentration gradient of the secretedcytokines (chemotaxis).

Tissue Mechanism

Blood Antibodies (isohaemagglutinins)

Kidney Antibodies, cell-mediated immunity (CMI)

Heart Antibodies, CMI

Skin CMI

Bonemarrow CMI

Cornea Usually accepted unless vascularised: CMI

Medical categories of rejection

Hyperacute rejectionInitiated by preexisting humoral immunity, hyperacute rejection manifests within minutes after transplant, and iftissue is left implanted brings systemic inflammatory response syndrome. Of high risk in kidney transplants is rapidclumping, namely agglutination, of red blood cells (RBCs or erythrocytes), as an antibody molecule binds multipletarget cells at once.While kidneys can routinely be obtained from human donors, most organs are in short supply leading toconsideration of xenotransplants from other species. Pigs are especially likely sources for xenotransplants, chosenfor the anatomical and physiological characteristics they share with humans. However, the sugargalactose-alpha-1,3-galactose (Gal) has been implicated as a major factor in hyperacute rejection inxenotransplantation. Unlike virtually all other mammals, humans and other primates do not make Gal, and in factrecognize it as an antigen. During transplantation, xenoreactive natural antibodies recognize Gal on the graftendothelium as an antigen, and the resulting complement-mediated immune response leads to a rejection of thetransplant.

Transplant rejection 4

Acute rejectionDeveloping with formation of cellular immunity, acute rejection occurs to some degree in all transplants, exceptbetween identical twins, unless immunosuppression is achieved (usually through drugs). Acute rejection begins asearly as one week after transplant, the risk being highest in the first three months, though it can occur months toyears later. Highly vascular tissues such as kidney or liver often host the earliest signsparticularly at endothelialcells lining blood vesselsthough it eventually occurs in roughly 10 to 30% of liver transplants, and 50 to 60% ofkidney transplants. A single episode of acute rejection can be recognized and promptly treated, usually preventingorgan failure, but recurrent episodes lead to chronic rejection. It is believed that the process of acute rejection ismediated by the cell mediated pathway, specifically by mononuclear macrophages and T-lymphocytes.

Chronic rejection

Micrograph showing a glomerulus with changescharacteristic of a transplant glomerulopathy.

Transplant glomerulopathy is considered a formof chronic antibody-mediated rejection. PAS

stain.

The term chronic rejection initially described long-term loss offunction in transplanted organs via fibrosis of the transplanted tissue'sblood vessels. This is now chronic allograft vasculopathy, however,leaving chronic rejection referring to rejection due to more patentaspects of immunity.

Chronic rejection explains long-term morbidity in most lung-transplantrecipients, the median survival roughly 4.7 years, about half the spanversus other major organ transplants.[6] In histopathology the conditionis bronchiolitis obliterans, which clinically presents as progressiveairflow obstruction, often involving dyspnea and coughing, and thepatient eventually succumbs to pulmonary insufficiency or secondaryacute infection.

Airflow obstruction not ascribable to other cause is labeled bronchiolitis obliterans syndrome (BOS), confirmed by apersistent dropthree or more weeksin forced expiratory volume (FEV1) by at least 20%. BOS is seen in over50% of lung-transplant recipients by 5 years, and in over 80% by ten years. First noted is infiltration bylymphocytes, followed by epithelial cell injury, then inflammatory lesions and recruitment of fibroblasts andmyofibroblasts, which proliferate and secrete proteins forming scar tissue. Generally thought unpredictable, BOSprogression varies widely: lung function may suddenly fall but stabilize for years, or rapidly progress to death withina few months. Risk factors include prior acute rejection episodes, gastroesophageal reflux disease, acute infections,particular age groups, HLA mis-matching, lymphocytic bronchiolitis, and graft dysfunction (e.g., airway ischemia).

Rejection due to non-adherenceOne principal reason for transplant rejection is non-adherence to prescribed immunosuppressant regimens. This isparticularly the case with adolescent recipients, with non-adherence rates near 50% in some instances.

Rejection detectionDiagnosis of acute rejection relies on clinical datapatient signs and symptomsbut also calls on laboratory datasuch as tissue biopsy. The laboratory pathologist generally seeks three main histological signs: (1) infiltrating Tcells, perhaps accompanied by infiltrating eosinophils, plasma cells, and neutrophils, particularly in telltale ratios, (2)structural compromise of tissue anatomy, varying by tissue type transplanted, and (3) injury to blood vessels. Tissuebiopsy is restricted, however, by sampling limitations and risks/complications of the invasive procedure. Cellularmagnetic resonance imaging (MRI) of immune cells radiolabeled in vivo might offer noninvasive testing.

Transplant rejection 5

Rejection treatmentHyperacute rejection manifests severely and within minutes, and so treatment is immediate: removal of the tissue.Chronic rejection is generally considered irreversible and poorly amenable to treatmentonly retransplantgenerally indicated if feasiblethough inhaled cyclosporine is being investigated to delay or prevent chronicrejection of lung transplants. Acute rejection is treated with one or multiple of a few strategies.

Immunosuppressive therapyA short course of high-dose corticosteroids can be applied, and repeated. Triple therapy adds a calcineurin inhibitorand an anti-proliferative agent. Where calcineurin inhibitors or steroids are contraindicated, mTOR inhibitors areused.Immunosuppressive drugs: Corticosteroids

Prednisolone Hydrocortisone

Calcineurin inhibitors Cyclosporin Tacrolimus

Anti-proliferatives Azathioprine Mycophenolic acid

mTOR inhibitors Sirolimus Everolimus

Antibody-based treatmentsAntibody specific to select immune components can be added to immunosuppressive therapy. The monoclonalanti-T cell antibody OKT3, once used to prevent rejection, and still occasionally used to treat severe acute rejection,has fallen into disfavor, as it commonly brings severe cytokine release syndrome and late post-transplantlymphoproliferative disorder. (OKT3 is available in the United Kingdom for named-patient use only.)Antibody drugs: Monoclonal anti-IL-2R receptor antibodies

Basiliximab Daclizumab

Polyclonal anti-T-cell antibodies Anti-thymocyte globulin (ATG) Anti-lymphocyte globulin (ALG)

Monoclonal anti-CD20 antibodies Rituximab

Transplant rejection 6

Blood transferCases refractory to immunosuppressive or antibody therapy are sometimes given blood transfusionsremovingantibody molecules specific to the transplanted tissue.

Marrow transplantBone marrow transplant can replace the transplant recipient's immune system with the donor's, and the recipientaccepts the new organ without rejection. The marrow's hematopoietic stem cellsthe reservoir of stem cellsreplenishing exhausted blood cells including white blood cells forming the immune systemmust be of theindividual who donated the organ or of an identical twin or a clone. There is a risk of graft versus host disease(GVHD), however, whereby mature lymphocytes entering with marrow recognize the new host tissues as foreignand destroy them.

References[1] http:/ / apps. who. int/ classifications/ icd10/ browse/ 2010/ en#/ T86[2] http:/ / www. nlm. nih. gov/ medlineplus/ ency/ article/ 000815. htm[3] http:/ / www. nlm. nih. gov/ cgi/ mesh/ 2014/ MB_cgi?field=uid& term=D006084[4] ABO Incompatible Heart Transplantation in Young Infants. (2009, July 30). ABO Incompatible Heart Transplantation in Young Infants.

American Society of Transplantation. Retrieved from http:/ / www. myast. org/ podcasts/abo-incompatible-heart-transplantation-young-infants

[5] United Network for Organ Sharing. (2013, January 31). OPTN Policy 3.7 - Allocation of Thoracic Organs. Retrieved from http:/ / optn.transplant. hrsa. gov/ policiesAndBylaws/ policies. asp

[6] http:/ / www. OPTN. org

External links The Immune Tolerance Network (http:/ / www. immunetolerance. org) Transplant rejection (http:/ / www. nlm. nih. gov/ medlineplus/ ency/ article/ 000815. htm) // Medline Plus

Encyclopedia, 6/14/2011

Article Sources and Contributors 7

Article Sources and ContributorsTransplant rejection Source: http://en.wikipedia.org/w/index.php?oldid=594906297 Contributors: 16@r, 73D884, A5b, Abanima, Adam Conover, Airosol1, Ajtoffice, Alteripse,Anthonyhcole, Anypodetos, Arcadian, Arjayay, Axl, Bobsagat, CDN99, CRLBalla, CanisRufus, Colonies Chris, David Kernow, Davidruben, Diptanshu.D, Eleassar777, Elenseel, Emersoni,Epbr123, Episcopo, Fraggle81, FreeBird, Ft1, Gaius Cornelius, GilCahana, Grafen, GrahamColm, Hairy Dude, Hydrargyrum, Ihealthbuzz, ImGz, Inclusivedisjunction, J-Star, JackWasey,Jfdwolff, Jhill1346, Johnasher, JosephBarillari, KC Panchal, Keepcalmandcarryon, KittySaturn, Kusername, Kyoko, Lab-oratory, Lbuckler, Lexor, LilHelpa, Lybbar12, Margaret52, Mike2vil,Millahnna, Mlaffs, MyOwnLittlWorld, N328KF, Nephron, Niclamus, Niels Olson, Obaid221, Opelio, Paiamshadi, Petri Krohn, Phronima, Pratyya Ghosh, Pseudomonas, RainbowCrane,Remember, Rholton, Rich Farmbrough, Saravask, Seans Potato Business, Sepguilherme, Sigma 7, Slalger, Spiritia, Sugarcolt, Techelf, Thinking of England, TimBentley, Timemutt, Tiresias BC,ToxicAllure, Undsoweiter, Wwallacee, Xoloki, Zaxxon de, 148 anonymous edits

Image Sources, Licenses and ContributorsFile:Lung transplant rejection - high mag.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Lung_transplant_rejection_-_high_mag.jpg License: Creative CommonsAttribution-Sharealike 3.0 Contributors: NephronImage:Transplant_glomerulopathy_-_very_high_mag.jpg Source: http://en.wikipedia.org/w/index.php?title=File:Transplant_glomerulopathy_-_very_high_mag.jpg License: CreativeCommons Attribution-Sharealike 3.0 Contributors: Nephron

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Transplant rejectionPretransplant rejection preventionABO-incompatible transplants

Immunologic mechanisms of rejectionImmunizationImmune memoryCellular immunityHumoral immunityAntibodyOpsonizationComplement cascade

Medical categories of rejectionHyperacute rejectionAcute rejectionChronic rejection

Rejection due to non-adherence Rejection detectionRejection treatmentImmunosuppressive therapyAntibody-based treatmentsBlood transferMarrow transplant

ReferencesExternal links

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