within minutes to hours from the time of the transplant. it is mediated by the binding of xnas (xenoreactive natural antibodies) to the donor endothelium, causing activation of the human complement system, which results in endothelial damage, inflammation, thrombosis and necrosis of the transplant. xnas are first produced and begin circulating in the blood in neonates, after colonization of the bowel by bacteria with galactose moieties on their cell walls. most of these antibodies are the igm class, but also include igg, and iga. the epitope xnas target is an α-linked galactose moiety, galactose-alpha-1,3-galactose (also called the α-gal epitope), produced by the enzyme alpha-galactosyltransferase. most non-primates contain this enzyme thus, this epitope is present on the organ epithelium and is perceived as a foreign antigen by primates, which lack the galactosyl transferase enzyme. in pig to primate xenotransplantation, xnas recognize porcine glycoproteins of the integrin family. the binding of xnas initiate complement activation through the classical complement pathway. complement activation causes a cascade of events leading to: destruction of endothelial cells, platelet degranulation, inflammation, coagulation, fibrin deposition, and hemorrhage. the result is thrombosis and necrosis of the xenograft. overcoming hyperacute rejection since hyperacute rejection presents such a barrier to the success of xenografts, several strategies to overcome it are under investigation: interruption of the complement cascade the recipient's complement cascade can be inhibited through the use of cobra venom factor (which depletes c3), soluble complement receptor type 1, anti-c5 antibodies, or c1 inhibitor (c1-inh). disadvantages of this approach include the toxicity of cobra venom factor, and most importantly these treatments would deprive the individual of a functional complement system. transgenic organs (genetically engineered pigs) 1,3 galactosyl transferase gene knockouts – these pigs do not contain the gene that codes for the enzyme responsible for expression of the immunogeneic gal-α-1,3gal moiety (the α-gal epitope). increased expression of h-transferase (α 1,2 fucosyltransferase), an enzyme that competes with galactosyl transferase. experiments have shown this reduces α-gal expression by 70%. expression of human complement regulators (cd55, cd46, and cd59) to inhibit the complement cascade. plasmaphoresis, on humans to remove 1,3 galactosyltransferase, reduces the risk of activation of effector cells such as ctl (cd8 t cells), complement pathway activation and delayed type hypersensitivity (dth). acute vascular rejection also known as delayed xenoactive rejection, this type of rejection occurs in discordant xenografts within 2 to 3 days, if hyperacute rejection is prevented. the process is much more complex than hyperacute rejection and is currently not completely understood. acute vascular rejection requires de novo protein synthesis and is driven by interactions between the graft endothelial cells and host antibodies, macrophages, and platelets. the response is characterized by an inflammatory infiltrate of mostly macrophages and natural killer cells (with small numbers of t cells), intravascular thrombosis, and fibrinoid necrosis of vessel walls. binding of the previously mentioned xnas to the donor endothelium leads to the activation of host macrophages as well as the endothelium itself. the endothelium activation is considered type ii since gene induction and protein synthesis are involved. the binding of xnas ultimately leads to the development of a procoagulant state, the secretion of inflammatory cytokines and chemokines, as well as expression of leukocyte adhesion molecules such as e-selectin, intercellular adhesion molecule-1 (icam-1), and vascular cell adhesion molecule-1 (vcam-1). this response is further perpetuated as normally binding between regulatory proteins and their ligands aid in the control of coagulation and inflammatory responses. however, due to molecular incompatibilities between the molecules of the donor species and recipient (such as porcine major histocompatibility complex molecules and human natural killer cells), this may not occur. overcoming acute vascular rejection due to its complexity, the use of immunosuppressive drugs along with a wide array of approaches are necessary to prevent acute vascular rejection, and include administering a synthetic thrombin inhibitor to modulate thrombogenesis, depletion of anti-galactose antibodies (xnas) by techniques such as immunoadsorption, to prevent endothelial cell activation, and inhibiting activation of macrophages (stimulated by cd4+ t cells) and nk cells (stimulated by the release of il-2). thus, the role of mhc molecules and t cell responses in activation would have to be reassessed for each species combo. accommodation if hyperacute and acute vascular rejection are avoided accommodation is possible, which is the survival of the xenograft despite the presence of circulating xnas. the graft is given a break from humoral rejection when the complement cascade is interrupted, circulating antibodies are removed, or their function is changed, or there is a change in the expression of surface antigens on the graft. this allows the xenograft to up-regulate and express protective genes, which aid in resistance to injury, such as heme oxygenase-1 (an enzyme that catalyzes the degradation of heme). cellular rejection rejection of the xenograft in hyperacute and acute vascular rejection is due to the response of the humoral immune system, since the response is elicited by the xnas. cellular rejection is based on cellular immunity, and is mediated by natural killer cells that accumulate in and damage the xenograft and t-lymphocytes which are activated by mhc molecules through both direct and indirect xenorecognition. in direct xenorecognition, antigen presenting cells from the xenograft present peptides to recipient cd4+ t cells via xenogeneic mhc class ii molecules, resulting in the production of interleukin 2 (il-2). indirect xenorecognition involves the presentation of antigens from the xenograft by recipient antigen presenting cells to cd4+ t cells. antigens of phagocytosed graft cells can also be presented by the host's class i mhc molecules to cd8+ t cells. the strength of cellular rejection in xenografts remains uncertain, however, it is expected to be stronger than in allografts due to differences in peptides among different animals. this leads to more antigens potentially recognized as foreign, thus eliciting a greater indirect xenogenic response. overcoming cellular rejection a proposed strategy to avoid cellular rejection is to induce donor non-responsiveness using hematopoietic chimerism. donor stem cells are introduced into the bone marrow of the recipient, where they coexist with the recipient's stem cells. the bone marrow stem cells give rise to cells of all hematopoietic lineages, through the process of hematopoiesis. lymphoid progenitor cells are created by this process and move to the thymus where negative selection eliminates t cells found to be reactive to self. the existence of donor stem cells in the recipient's bone marrow causes donor reactive t cells to be considered self and undergo apoptosis. chronic rejection chronic rejection is slow and progressive, and usually occurs in transplants that survive the initial rejection phases. scientists are still unclear how chronic rejection exactly works, research in this area is difficult since xenografts rarely survive past the initial acute rejection phases. nonetheless, it is known that xnas and the complement system are not primarily involved. fibrosis in the xenograft occurs as a result of immune reactions, cytokines (which stimulate fibroblasts), or healing (following cellular necrosis in acute rejection). perhaps the major cause of chronic rejection is arteriosclerosis. lymphocytes, which were previously activated by antigens in the vessel wall of the graft, activate macrophages to secrete smooth muscle growth factors. this results in a build up of smooth muscle cells on the vessel walls, causing the hardening and narrowing of vessels within the graft. chronic rejection leads to pathologic changes of the organ, and is why transplants must be replaced after so many years. it is also anticipated that chronic rejection will be more aggressive in xenotransplants as opposed to allotransplants. dysregulated coagulation successful efforts have been made to create knockout mice without α1,3gt; the resulting reduction in the highly immunogenic αgal epitope has resulted in the reduction of the occurrence of hyperacute rejection, but has not eliminated other barriers to xenotransplantation such as dysregulated coagulation, also known as coagulopathy. different organ xenotransplants result in different responses in clotting. for example, kidney transplants result in a higher degree of coagulopathy, or impaired clotting, than cardiac transplants, whereas liver xenografts result in severe thrombocytopenia, causing recipient death within a few days due to bleeding. an alternate clotting disorder, thrombosis, may be initiated by preexisting antibodies that affect the protein c anticoagulant system. due to this effect, porcine donors must be extensively screened before transplantation. studies have also shown that some porcine transplant cells are able to induce human tissue factor expression, thus stimulating platelet and monocyte aggregation around the xenotransplanted organ, causing severe clotting. additionally, spontaneous platelet accumulation may be caused by contact with pig von willebrand factor. just as the α1,3g epitope is a major problem in xenotransplantation, so too is dysregulated coagulation a cause of concern. transgenic pigs that can control for variable coagulant activity based on the specific organ transplanted would make xenotransplantation a more readily available solution for the 70,000 patients per year who do not receive a human donation of the organ or tissue they need. physiology extensive research is required to determine whether animal organs can replace the physiological functions of human organs. many issues include size – differences in organ size limit the range of potential recipients of xenotransplants; longevity – the lifespan of most pigs is roughly 15 years, currently it is unknown whether or not a xenograft may be able to last longer than that; hormone and protein differences – some proteins will be molecularly incompatible, which could cause malfunction of important regulatory processes. these differences also make the prospect of hepatic xenotransplantation less promising, since the liver plays an important role in the production of so many proteins; environment – for example, pig hearts work in a different anatomical site and under different hydrostatic pressure than in humans; temperature – the body temperature of pigs is 39 °c (2 °c above the average human body temperature). implications of this difference, if any, on the activity of important enzymes are currently unknown. xenozoonosis xenozoonosis, also known as zoonosis or xenosis, is the transmission of infectious agents between species via xenograft. animal to human infection is normally rare, but has occurred in the past. an example of such is the avian influenza, when an influenza a virus was passed from birds to humans. xenotransplantation may increase the chance of disease transmission for 3 reasons: (1) implantation breaches the physical barrier that normally helps to prevent disease transmission, (2) the recipient of the transplant will be severely immunosuppressed, and (3) human complement regulators (cd46, cd55, and cd59) expressed in transgenic pigs have been shown to serve as virus receptors, and may also help to protect viruses from attack by the complement system. examples of viruses carried by pigs include porcine herpesvirus, rotavirus, parvovirus, and circovirus. porcine herpesviruses and rotaviruses can be eliminated from the donor pool by screening, however others (such as parvovirus and circovirus) may contaminate food and footwear then re-infect the herd. thus, pigs to be used as organ donors must be housed under strict regulations and screened regularly for microbes and pathogens. unknown viruses, as well as those not harmful in the animal, may also pose risks. of particular concern are pervs (porcine endogenous retroviruses), vertically transmitted microbes that embed in swine genomes. the risks with xenosis are twofold, as not only could the individual become infected, but a novel infection could initiate an epidemic in the human population. because of this risk, the fda has suggested any recipients of xenotransplants shall be closely monitored for the remainder of their life, and quarantined if they show signs of xenosis. baboons and pigs carry myriad transmittable agents that are harmless in their natural host, but extremely toxic and deadly in humans. hiv is an example of a disease believed to have jumped from monkeys to humans. researchers also do not know if an outbreak of infectious diseases could occur and if they could contain the outbreak even though they have measures for control. another obstacle facing xenotransplants is that of the body's rejection of foreign objects by its immune system. these antigens (foreign objects) are often treated with powerful immunosuppressive drugs that could, in turn, make the patient vulnerable to other infections and actually aid the disease. this is the reason the organs would have to be altered to fit the patients' dna (histocompatibility). in 2005, the australian national health and medical research council (nhmrc) declared an eighteen-year moratorium on all animal-to-human transplantation, concluding that the risks of transmission of animal viruses to patients and the wider community