Medical Inspection – Health Blog

Infections have proven to be a substantial cause of morbid­ity and mortality after allotransplantation. This is owing in large part to the immunosuppressive treatment needed to prevent graft rejection. Interspecies transplantation may require greater levels of immune suppression than are cur­rently used for allotransplantation, thereby further in­creasing the risk of opportunistic infection. An additional risk of xenotransplantation is that of ‘xenosis’, the transfer of animal-derived infectious pathogens with xenotrans- plants (Figure 2). This is in contrast to the natural spread of infection between species, referred to as ‘zoon- osis’. While xenograft recipients are susceptible to in­fection by bacteria, fungi and parasites, much attention has focused on infection by viral pathogens (Table 2).

Several mechanisms of xenogeneic infection have been identified. A pathogen may be infectious for both the do­nor species and the human recipient (eg, Toxoplasma gon­dii). Some animal viruses that are similar to their human counterparts, such as primate cytomegalovirus (CMV), have been documented to produce clinical disease in human xenograft recipients. A major concern is retrovi- ruses such as simian immunodeficiency virus, which can be transmitted across species and produce a more virulent reaction in the new host. Some pig retroviruses have been shown to reactivate after radiation exposure and might similarly reactivate when exposed to immunosup- pressive medication. Indeed, many of the conditions associated with retroviral activation (eg, immune suppression, graft rejection, cytotoxic therapy) are present in the transplant recipient. Latent animal viruses pres­ent in the xenograft, such as porcine CMV, may be unable to infect human tissue but may later reactivate in the ani­mal organ, resulting in graft failure. Finally, concern has been raised regarding the possibility of crossover of an animal virus with a human virus, leading to a more viru­lent recombinant organism. Indeed, dual infections can lead to recombination, as has been observed with in vitro mixing of CMV isolates from transplant recipients. Concurrent inoculation of two avirulent herpes simplex viruses into mice has been demonstrated to produce lethal recombinations. Recognition of xenogeneic infections may be complicated by the presence of novel pathogens for which laboratory testing is not available, new clinical syn­dromes and altered behaviour of these pathogens in the im- munocompromised recipient.

Read More…

Xenografted organs that are not destroyed by HAR or DXR are subject to elicited cellular and/or humoral rejec­tion. In pig to primate transplants, Alexandre et al reversed decreases in graft function by administering immunosuppressive agents. Fryer et al expedited the rejection of guinea pig to rat cardiac xenografts by trans­ferring lymphocytes from presensitized animals. It is not clear how cellular immune responses to a xenograft might differ from responses to allografts, and consequently whether novel therapies for immune modulation might be required. In vitro experiments suggest that T cells stimu­lated by xenogeneic cells in mixed leukocyte cultures un­dergo less vigorous proliferation than those stimulated by allogeneic cells. This is thought to reflect a limited ability of T cells to recognize xenogeneic cells directly (ie, without the contribution of antigen-presenting cells). Conversely, the indirect T cell response to a xenotrans- plant might be expected to be stronger than that towards an allotransplant, given the wide array of foreign peptides it would give rise to. Potent immunosuppressants such as methotrexate, cyclosporine and rapamycin can and have been systemically administered to blunt host T cell responses to allografts, albeit with a significant risk of in­fection, malignancy and organ toxicity. A ‘tolerant’ state, in which the recipient’s immune system regards donor an­tigens as ‘self’ would obviate the need for chronic immuno- suppressive therapy. One approach to induce tolerance involves introducing donor-derived hematopoietic cells be­fore grafting the recipient with a solid organ from the same animal. The interaction of two coexisting donor and recipient cell populations is believed to result in mutual down-regulation of both recipient and donor immune sys­tems, a phenomenon known as ‘microchimerism’. This approach appears to have worked across rodent spe­cies but has not been successfully applied across broader species lines.

Read More…

If HAR is averted, xenografts are still rejected in days in­stead of minutes or hours by a process referred to as ‘d­elayed xenograft rejection’ (DXR), also termed ‘acute vascular xenograft rejection’. This process is charac­terized pathologically by infiltration of leukocytes (par­ticularly monocytes and natural killer [NK] cells), focal ischemia and diffuse microvascular coagulation. There is increasing evidence that DXR may be caused by the ongoing interaction of XRA with the graft. Develop­ment of DXR coincides with a marked increase in the syn­thesis of XRA after exposure to xenogeneic cells. Furthermore, removal and/or inhibition of XRA from xe­nograft recipients delays or prevents DXR. In con­trast to HAR, the role of the complement system in the pathophysiology of DXR is unclear. DXR typically occurs in xenograft recipients depleted of complement. Yet the pathologic lesions characteristic of DXR appear to re­flect complement-mediated changes in endothelial struc­ture and function. Hancock hypothesized that even the most potent inhibitors of complement, such as cobra venom factor, are incompletely effective. While such treatments may reduce complement to levels that do not result in HAR, there may still be sufficient complement activity in serum to contribute to the development of DXR. Other possible effector mechanisms for DXR in­clude the action of host platelets and leukocytes on the xe- nograft. Platelets activated by exposure to complement fragments express P-selectin, resulting in the release of po­tent chemotactic signals, such as RANTES and MCP-1, that attract leukocytes, particularly monocytes. The presence of both monocytes and NK cells has been docu­mented in organs with DXR. Treatment of xeno- graft recipients with antibodies that inhibit the function of inflammatory cells has been shown to prolong graft sur­vival. Some studies suggest that NK cells act by dis­rupting the integrity of the endothelial monolayer and by activating endothelial cells. Endothelial acti­vation in animal models of DXR has been associated with a shift to a procoagulant state with induction of tissue fac­tor, production of chemokines such as MCP-1, and induc­tion of leukocyte adhesion molecules such as intercellular adhesion molecule 1.

Read More…

The first immunological barrier to xenotransplantation is hyperacute rejection (HAR), characterized by comple­ment activation leading to widespread thrombosis and in­terstitial hemorrhage of the donor organ. This phenomenon is initiated by the binding of antibodies of the recipient to the vascular endothelium of the donor organ. These naturally occurring ‘xenoreactive’ antibodies (XRAs) recognize cells from discordant species, and are present in humans and other Old World primates.

XRAs react predominantly to the carbohydrate epitope Gal-alpha-1,3-Gal, which is expressed as a terminal modi­fication on glycoproteins and glycolipids of New World monkeys and all lower mammals. Synthesis of Gal- alpha-1,3-Gal is catalyzed by alpha-1,3- galactosyltransferase, an enzyme that is absent in humans, apes and baboons. Evidence that Gal-alpha- 1,3-Gal is a critical target in HAR of pig to primate xenografts came from the demonstration by Collins et al that transplantation of a New World monkey heart into a ba­boon results in HAR, and that virtually all the antibodies that bind to the rejected heart are specific for Gal- alpha-1,3-Gal. Sachs and Sablinski subsequently demonstrated that specific depletion of anti-Gal- alpha-1,3-Gal antibodies from primates prevents HAR of porcine xenografts.

Activation of complement is a crucial step in the devel­opment of HAR. Administration of complement inhibi­tors such as cobra venom factor, soluble complement receptor type 1 or gamma globulin markedly prolongs the survival of a discordant xenograft. HAR does not occur in recipients with inherited deficien­cies of complement. In porcine to primate xenografts, complement activation appears to occur predominantly via the classical pathway. Because complement activa­tion plays a pivotal role in the pathogenesis of HAR, the ability to regulate human complement may help overcome a major hurdle in the clinical application of xenotransplan- tation. One way to regulate complement activation may be through membrane-bound regulators of complement activ­ity that inhibit the complement cascade at various critical steps. Examples of such regulators are decay acceler­ating factor (DAF [CD55]), which promotes disso­ciation of convertase complexes; membrane cofactor protein (MCP [CD46]), which inhibits complement acti­vation at the C3 convertase step; and membrane inhibitor of reactive lysis (CD59), which prevents assembly of lytic membrane attack complexes. These glycoproteins help protect autologous (ie, host) cells against inadvertent complement-mediated injury targeted against xenogeneic cells. Complement regulatory proteins appear to function poorly against heterologous complement and thus may make xenografts particularly susceptible to complement- mediated injury. For example, human DAF inhibits human C3 convertases effectively but rabbit C3 converta- ses poorly. The importance of complement regulation is suggested by the observation by McCurry et al that organs from transgenic pigs expressing human CD55 and CD59 generally resist HAR after transplantation into ba­boons.

Read More…

Xenotransplantation: A potential solution to the critical organ donor shortage

Orthotopic liver transplantation is the treatment of choice for many patients with end-stage liver disease. Advances in immunosuppression and perioperative care have resulted in a dramatic improvement in survival of pa­tients undergoing liver transplantation. However, the need for donor organs far outweighs the available supply (Table 1). This need has prompted the search for al­ternatives to conventional allotransplantation. Xe- notransplantation, the transplantation of cells, tissues or organs between members of different species, has emerged as a potential solution to the shortage of human organs. The concept of transplanting animal organs into humans is not new. The first such attempt was made by a Russian physician in 1682, who reportedly repaired the skull of a wounded nobleman by using a bone from a dog. In 1964, kidneys from chimpanzees were transplanted into 13 patients. Most died within days, although one sur­vived for nine months on an immunosuppressive regimen of azathioprine, actinomycin C and steroids. More re­cently, two patients received liver transplants from ba­boons. One patient survived for more than two months, but ultimately both patients died due to infection.

Read More…

Read More…