The Rejection of Neural Xenotransplants: A Role for Antibodies?

Discordant Neural Tissue Xenografts Survive Longer in Immunoglobulin Deficient Mice. Transplantation 1999; 68: 1153. Larsson LC, Czech KA, Widner H, and Korsgren O. Allotransplantation of embryonic neural tissue into the diseased central nervous system (CNS) has now passed from the experimental stage into clinical trials, at least with respect to Parkinson's disease and, more recently, Huntington's disease (1). These diseases of the CNS are characterized by the loss of specific populations of neurons, which are replaced with the implantation of that same neuronal population from the developing embryo. For example, in Parkinson's disease (PD), the central pathological event is the loss of the dopaminergic nigrostriatal neurons, which are replaced with the grafting of embryonic ventral mesencephalic (VM) tissue from 6-9-week old human fetuses. When this is done, assuming that appropriate care is taken with the age of tissue harvested and its preparation before implantation, then good clinical results can be obtained. This beneficial effect is seen not only clinically, in terms of improved motor performance, but also using functional imaging, such as positron emission tomography scans to look at dopaminergic activity in the graft, a measure which has been shown to correlate well with graft survival postmortem. Despite this success, a number of fundamental difficulties remain with the use of fetal human allografts, including ethical as well as practical problems. For example, a single PD patient will require between 8 and 12 fetuses for bilateral transplants to be most efficacious. As a consequence, alternative sources of cells have been sought, including embryonic neural tissue from other species, most notably pig. This animal has been chosen because (i) the size and development of its brain is similar to that seen in man; (ii) it is relatively easy to breed and produces large litters of embryos; (iii) it can be genetically modified to produce transgenic animals, and (iv) it has long been considered a possible source of peripheral whole organ transplants. Furthermore, in the case of neural tissue, there is some evidence that there may be a primary advantage of using xenotransplanted, rather than allografted, tissue in that it may have a greater potential for sending fibers into the host brain, possibly through the avoidance of species specific barriers to axonal growth. Despite all these attractions, the major problem with xenografted tissue is rejection, even with transplants into the relatively immunologically privileged CNS. This currently represents one of the major obstacles to the widespread use of this tissue in clinical programs of neural transplantation. Thus although some clinical transplants have taken place using porcine VM tissue in patients with PD, the results have been disappointing, with poor graft survival even in the presence of cyclosporin A immunotherapy (2). It is, therefore, pertinent that, in this issue of Transplantation, Larsson et al. have re-examined the role of humoral mediated processes in the rejection of neural xenografted tissue transplanted into the brain (3). The CNS has traditionally been thought of an immunologically privileged site for the following reasons: (i) it lacks a well developed lymphatic drainage; (ii) it possesses a blood brain barrier (BBB), which isolates the CNS from the circulation, and (iii) it contains little in the way of antigen presenting cells. However, it is now clear that all three attributes are in need of redefinition. For example, antigen placed into the CNS can drain into the deep cervical lymph nodes; activated lymphocytes can cross the intact BBB; and microglia, as well as possibly astrocytes, can act as antigen presenting cells. Therefore, it is not surprising that allografted and xenografted neural tissue can be rejected when transplanted into the CNS, especially given the fact that the BBB is disrupted during the implantation procedure. The implantation of allografted neural tissue into the CNS will induce an immunological response that is dependent on the extent to which the graft and host differ at MHC loci (4). The greater the disparity, the greater the rejection response, a response that is similar to that seen in the concordant xenograft situation. In these latter transplants, the tissue induces a T-cell response (both CD4 and CD8) that is apparent 5-10 days postimplantation, at the same time that there is increased MHC expression in the graft. This response then leads to rejection of the xenografted neural tissue over the ensuing 3-4 weeks in the majority of cases, although there are occasional reports of long-term survival of such tissue in the non-immunosuppressed host. However, the rejection of such grafts can be abrogated, although not always abolished, by the use of cyclosporin A as well as other immunosuppressive regimes, of which the most successful to date have been monoclonal antibody therapy to CD4 lymphocytes and the IL2R (5). Although these studies have clearly shown a role for T-cell directed processes in the rejection of neural xenografts, there has always been a suspicion that there may be a role for other immune mediators, as reported for peripherally xenotransplanted porcine islet cells (6). It is, therefore, significant that Larsson et al. have now demonstrated a possible role for antibodies in the rejection of neural xenotransplants (3). In this study, porcine embryonic VM tissue was transplanted into the striatum of normal or immunoglobulin knockout (IgKO) mice, and their survival was followed over time. This represents a concordant xenograft paradigm, assuming that mice (like rats) have low levels of circulating naturally occurring antipig antibodies. In the normal wild type mice, xenografted porcine VM tissue survived poorly, with the majority of the grafts being lost within 2 weeks of implantation. In contrast, similarly grafted tissue in the IgKO mice survived for longer periods of time, with the majority of the grafts being lost 4-6 weeks after implantation. Furthermore, the two different hosts seem to mount different T-cell responses, with the wild type animals showing an early CD4+ T-cell response and the IgKO mice, a later CD8+ T-cell infiltrate. This latter response was associated with more intense microglia staining, whereas non-specific immunoglobulin (Ig) binding was seen in and around the grafts of wild type hosts. Thus, this study suggests that Ig may be important in the early stages of neural xenograft rejection, in conjunction with a CD4+ T-cell response, while a later rejection event is the activation of CD8+ T-cells in association with, or as a result of, microglial activation in the graft site. This study, therefore, implicates humoral processes in the rejection of neural xenografts, which is in accordance with our recent demonstration of IgM and complement binding in pig to rat VM xenografts (Barker et al., manuscript in preparation; see Figure 1). This is, perhaps, surprising given the fact that this represents a concordant xenograft paradigm. It is also unclear from these studies, how the two arms of the immune process (humoral versus cellular) are activated and interact. Furthermore, it is not known how these processes initiate the actual loss of cells in the graft. For example, is there complement activated cell death in the graft, as our study might suggest? However, the study by Larsson et al. in this issue of Transplantation is important in highlighting our limited understanding of neural xenograft rejection and acts as a reminder to those considering clinical trials with embryonic porcine neural tissue.Figure 1: Ten-day-old xenografted porcine VM tissue transplanted into the 6-OHDA lesioned rat, showing (a) C3 binding and (b) IgM binding in and around the graft site. Scale bar: 1 mm.Acknowledgments. RAB is an MRC Clinician Scientist. The authors thank Richard Armstrong for his helpful comments. Roger A. Barker Department of Neurology; Addenbrooke's NHS Trust and MRC Cambridge Centre for Brain Repair Stephen B.