Organic complexation of U(VI) in reducing soils at a natural analogue site: Implications for uranium transport

Understanding the long-term fate, stability, and bioavailability of uranium (U) in the environment is important for the management of nuclear legacy sites and radioactive wastes. Analysis of U behavior at natural analogue sites permits evaluation of U biogeochemistry under conditions more representative of long-term equilibrium. Here, we have used bulk geochemical and microbial community analysis of soils, coupled with X-ray absorption spectroscopy and μ-focus X-ray fluorescence mapping, to gain a mechanistic understanding of the fate of U transported into an organic-rich soil from a pitchblende vein at the UK Needle's Eye Natural Analogue site. U is highly enriched in the Needle's Eye soils (∼1600 mg kg^-1). We show that this enrichment is largely controlled by U(VI) complexation with soil organic matter and not U(VI) bioreduction. Instead, organic-associated U(VI) seems to remain stable under microbially-mediated Fe(III)-reducing conditions. U(IV) (as non-crystalline U(IV)) was only observed at greater depths at the site (>25 cm); the soil here was comparatively mineral-rich, organic-poor, and sulfate-reducing/methanogenic. Furthermore, nanocrystalline UO₂, an alternative product of U(VI) reduction in soils, was not observed at the site, and U did not appear to be associated with Fe-bearing minerals. Organic-rich soils appear to have the potential to impede U groundwater transport, irrespective of ambient redox conditions.

Multiple environmental factors influence 238U, 232Th and 226Ra bioaccumulation in arbuscular mycorrhizal-associated plants

Ecological consequences of low-dose radioactivity from natural sources or radioactive waste are important to understand but knowledge gaps still remain. In particular, the soil transfer and bioaccumulation of radionuclides into plant roots is poorly studied. Furthermore, better knowledge of arbuscular mycorrhizal (AM) fungi association may help understand the complexities of radionuclide bioaccumulation within the rhizosphere. Plant bioaccumulation of uranium, thorium and radium was demonstrated at two field sites, where plant tissue concentrations reached up to 46.93 μg g^{-1 238}U, 0.67 μg g^{-1 232}Th and 18.27 kBq kg^{-1 226}Ra. High root retention of uranium was consistent in all plant species studied. In contrast, most plants showed greater bioaccumulation of thorium and radium into above-ground tissues. The influence of specific soil parameters on root radionuclide bioaccumulation was examined. Total organic carbon significantly explained the variation in root uranium concentration, while other soil factors including copper concentration, magnesium concentration and pH significantly correlated with root concentrations of uranium, radium and thorium, respectively. All four orders of Glomeromycota were associated with root samples from both sites and all plant species studied showed varying association with AM fungi, ranging from zero to >60% root colonisation by fungal arbuscules. Previous laboratory studies using single plant-fungal species association had found a positive role of AM fungi in root uranium transfer, but no significant correlation between the amount of fungal infection and root uranium content in the field samples was found here. However, there was a significant negative correlation between AM fungal infection and radium accumulation. This study is the first to examine the role of AM fungi in radionuclide soil-plant transfer at a community level within the natural environment. We conclude that biotic factors alongside various abiotic factors influence the soil-plant transfer of radionuclides and future mechanistic studies are needed to explain these interactions in more detail.

Radioactivity and the environment: technical approaches to understand the role of arbuscular mycorrhizal plants in radionuclide bioaccumulation

Phytoaccumulation of radionuclides is of significant interest with regards to monitoring radionuclide build-up in food chains, developing methods for environmental bioremediation and for ecological management. There are many gaps in our understanding of the characteristics and mechanisms of plant radionuclide accumulation, including the importance of symbiotically-associated arbuscular mycorrhizal (AM) fungi. We first briefly review the evidence that demonstrates the ability of AM fungi to enhance the translocation of (238)U into plant root tissues, and how fungal association may prevent further mobilization into shoot tissues. We then focus on approaches that should further advance our knowledge of AM fungi-plant radionuclide accumulation. Current research has mostly used artificial cultivation methods and we consider how more ecologically-relevant analysis might be performed. The use of synchrotron-based X-ray fluorescence imaging and absorption spectroscopy techniques to understand the mechanisms of radionuclide transfer from soil to plant via AM fungi is evaluated. Without such further knowledge, the behavior and mobilization of radionuclides cannot be accurately modeled and the potential risks cannot be accurately predicted.

The essential roles of parenchymal tissues and passenger leukocytes in the tolerance induced by liver grafting in rats

Liver allografts in pigs and rodents are uniquely capable of inducing tolerance to themselves and to other grafts of donor tissues, instead of succumbing to the acute rejection that follows transplantation of other allogeneic tissues. We demonstrate here, using normal and chimaeric rat liver grafts, that both the allogeneic liver parenchyma and the intrahepatic leukocytes of donor type contribute to the establishment of long-term tolerance, each component being essential and complementary. The essential role of hepatic parenchyma may be related to its continuous release of soluble transplantation antigens that facilitate tolerogenesis. We suggest that clinical attempts at tolerance induction by the infusion of donor bone marrow-derived leukocytes may likewise be facilitated by the coadministration of soluble transplantation antigens of donor type.

Tolerance of porcine renal allografts induced by donor spleen cells and seven days' treatment with cyclosporine

Liver allografts in pigs and in rats elicit a substantial cellular immune response that can resolve spontaneously with the induction of donor-specific systemic tolerance. Self-limiting interactions between host and donor (graft)-derived leukocytes may be the basis for tolerogenesis. We have attempted to reproduce this effect of liver grafting in pigs by peroperative infusion of donor leukocytes into kidney graft recipients given an interrupted short course of CsA designed to promote donor leukocyte survival and interaction with host cells. This protocol can secure long-term kidney graft survival resistant to challenge by donor skin grafting. Donor skin is, however, rejected, but more slowly than third-party skin, indicating a degree of systemic specific unresponsiveness in these long-term kidney graft recipients.

Immunosuppression of canine renal allograft recipients by CD4 and CD8 monoclonal antibodies

A state of tolerance to MHC mismatched allografts can be generated in rodents by treatment with CD4 and CD8 monoclonal antibodies (mAb). In order to transpose this type of therapy to large animals and ultimately to the clinic, a suitable model is required. To this end we have generated a series of mAb to the canine CD4, CD8, and Thy-1 antigens and have tested their ability to prevent rejection of renal allografts. Donor-recipient pairs were selected from a colony of mongrel dogs in which untreated rejection of two haplotype-mismatched kidneys occurred by day 7 (defined as a serum creatinine > 300 mumol/l). Therapy with either the CD4 or the CD8 mAb, using no other immunosuppression, did not prolong graft survival. Depletion of T cells by a Thy-1 mAb prior to surgery only extended graft survival to day 9. However, treating with combinations of mAb up to day 10 (CD4 plus Thy-1; CD4 plus CD8; or CD4 plus CD8 plus Thy-1) prolonged renal allograft function up to 25 days. Combination of the triple mAb therapy with a sub-therapeutic immunosuppressive drug regimen (cyclosporin A plus azathioprine that alone gave a median survival of 15 days) favored survival to a median of 38 days. This protocol also inhibited the antiglobulin response that had curtailed the effects of mAb treatment, opening the way to more extended, and potentially tolerizing, mAb plus drug regimens.

CD4 and CD8 monoclonal antibody therapy: strategies to prolong renal allograft survival in the dog

The value of CD4 and CD8 monoclonal antibody therapy in tolerance induction has been demonstrated in rodent transplant models. In this paper the immunosuppressive potential of CD4 and CD8 monoclonal antibodies for dog renal allografts was evaluated as a preliminary to tolerogenic studies in this large animal model. Monoclonal antibodies were given for a maximum of 10 days after transplantation. Therapy was stopped prematurely following adverse reactions associated with the recipient developing an antibody response against the foreign (rat) therapeutic monoclonal antibody. Blood trough levels of CD4 and CD8 antibodies indicated that saturating doses were achieved. Although neither CD4 nor CD8 alone prolonged allograft survival (rejection by day 7), combination of CD4 and CD8 antibodies resulted in good graft function for a median of 14 days. The effect of removing circulating T lymphocytes was also assessed using a lytic Thy-1 monoclonal antibody. Alone Thy-1 had little effect but, when combined with CD4, the median allograft survival time was increased to 15.5 days. Reduction of the number of circulating T lymphocytes appears complementary to blockade of CD4 for immunosuppression, while blockade of CD4 combined with removal of CD8 also favours allograft survival.

Immunomodulation of dog islets using a cocktail of monoclonal antibodies

Islet allografts are particularly vulnerable to rejection, and current immunosuppressive agents are deleterious to their function. They are, however, highly suitable for 'immunomodulation', i.e., the removal or inactivation of passenger leukocytes to reduce their immunogenicity. For this purpose we have used 3 rat anti-dog monoclonal antibodies (Mabs) which are synergistic for leukocytolysis in the presence of autologous dog serum. Spleen cells or purified islets treated with these Mabs together with autologous serum were tested in mixed leukocyte and islet co-culture assays. The stimulatory properties of the Mab-pretreated splenocytes or islets were markedly reduced; moreover, the Mab cytolytic activity was shown to be confined to the leukocyte target cells and did not affect islet secretory function upon glucose stimulation. We conclude that this method of modifying the immunogenicity of dog islets could lead to successful islet grafting in vivo, allowing the reduction of conventional immunosuppression. Successful in vivo studies in this model, which are currently in progress, could have implications for clinical islet transplantation.

TNF staining of graft biopsy in renal transplantation

Tumour Necrosis Factor (TNF) is a cytokine which may be found in patients' plasma and urine in association with acute rejection in renal transplantation. TNF is produced mainly by macrophage/monocytes and activated lymphocytes and its release in acute rejection may damage the nephron leading to renal dysfunction. However localization of TNF in renal grafts has not yet been demonstrated. We investigated TNF localization in renal graft tissue and the association with acute rejection compared with non-immunological events (cyclosporine toxicity and acute tubular necrosis) in graft biopsy.

Soluble HLA antigens in the circulation of liver graft recipients

Human liver allografts deliver soluble class I (HLA-A and -B) transplantation antigens into the recipients' circulation. These molecules are detectable in recipient serum shortly after transplantation and they persist at high concentration for as long as the liver graft functions. Levels of graft-derived antigens in the recipient serum and self antigens in donor serum are comparable. Kinetic studies of these soluble antigens in donor and recipient show that donor antigens are continuously exported by the transplanted liver, while in the recipient, self antigens are derived from liver and other sources. At least two molecular forms of soluble HLA-A and -B antigens are present in sera from donors, recipients, and normal individuals. One form with m.w. approximately 50 kd seems to be a soluble monomer associated with beta 2-microglobulin, while the other forms of higher m.w. may be aggregates and/or complexes. Monitoring of these antigens in transplant recipients may be a useful indicator of graft pathology and function.

Long-term survival of kidney allografts in dogs after withdrawal of immunosuppression with ciclosporin and azathioprine

We have examined the immunosuppressive effects of combined azathioprine (Aza) and ciclosporin (CS) in two groups of mongrel dogs receiving kidney allografts. In group 1, Aza and CS were given together daily after transplantation and in group 2 twice the dose of each drug was given separately on alternate days. Doses were halved in each group at successive 84-day intervals and all immunosuppression was stopped on day 336. Thus the same total amounts of Aza and CS were given to all recipients in both groups. Up to day-60 the incidence of rejection in each group was similar, thereafter recipients in group 1 were more susceptible to fatal infection and marrow hypoplasia. This accounted for the difference in long-term survival between the two groups (1/14 in group 1, 5/12 in group 2 at day 420). Subsequently, two long-term survivors in group 2 died, 1 on day 452 from chronic rejection and the other on day 529 from gastroenteritis with a histologically normal allograft kidney. An in vitro analysis of the alloreactive repertoire of two healthy recipients in group 2, bearing each other's kidneys for more than 2.5 years and more than 500 days without immunosuppression, showed a profound donor-specific defect which could account for their operationally tolerant state.

Plasma exchange technique in the unheparinized, unanaesthetized rat

An inexpensive and convenient plasma exchange technique has been developed in the rat, which avoids heparin and membrane plasmapheresis technology. The use of a reliable venous vascular access on unanaesthetized, briefly restrained animals is described.

Orthotopic liver allografts in the rat. The influence of strain combination on the fate of the graft

This report describes the fate of orthotopic liver allografts performed in 22 donor/recipient strain combinations. Of these, 2 were major histocompatibility complex (MHC)-congenic, 18 were fully allogeneic, and 2 were non-RT1 incompatible combinations considered to differ only in minor transplantation antigens. The fate of fully allogeneic liver grafts was strictly dependent on the donor/recipient strain combination, and survival times fell into three nonoverlapping groups corresponding to acute rejection (8-21 days), delayed rejection (28-63 days), and prolonged survival (greater than 100 days). Serial levels of recipient serum enzymes also fell into groups corresponding to the fate of the graft. In 7 fully allogeneic donor/recipient combinations, liver grafts showed very prolonged survival. In five combinations they were rejected almost as quickly as kidney or heart grafts. In the two MHC congenic combinations, liver grafts survived for prolonged periods. In the non-RT1-incompatible combinations, unexpectedly, PVG livers were rejected by AUG recipients. MHC-incompatible liver grafts of a given strain survived for long periods or were rejected (in acute or delayed fashion) depending on the recipient strain. This behavior is unique to liver amongst commonly transplanted vascularized organs, and the fate of the graft clearly indicates strain-dependent levels of recipient responsiveness to defined transplantation antigens. Independent evidence supporting this conclusion is discussed. Apparently anomalous results of liver grafting in certain strain combinations are briefly considered.

Liver transplantation in the rat. Biochemical and histological evidence of complete tolerance induction in non-rejector strains

Orthotopic liver allografts in the rat survive indefinitely without immunosuppressive agents, despite incompatibility between donor and recipient for antigens of the major histocompatibility complex. This is strain-dependent. In the DA to PVG strain combination, liver grafts are never rejected. This is not due to failure of the recipient to mount an immune response against the donor tissue, because there is unequivocal histological evidence of a rejection response during the first few weeks after grafting. This response is moderate and disappears to leave a histologically normal liver, apart from mild bile duct proliferation. Liver function tests show evidence of damage during the phase of cellular infiltration, but these test results also return to normal levels within a few weeks. In the DA to BN strain combination liver grafts are rapidly rejected, and this process is accompanied by histological signs of a violent and progressive destructive cellular response with gross alterations in liver function test results that are progressive until the death of the recipients. F1 hybrid recipients between these two strains (BN X PVG)F1 show intermediate levels of both histological damage and elevated liver function values, but they do not reject their grafts. Recovery from the rejection episode appears to be complete, as judged histologically. However, biochemical values remain slightly elevated, indicating either that the original damage was so severe that it was inconsistent with complete functional recovery or that there is continuous damage that is not visible histologically.

Major transplantation antigens of pig kidney and liver. Comparisons between the whole organs and their parenchymal constituents

We have previously shown that cultured renal and hepatic parenchymal cells of the pig differ in the expression of major transplantation antigens. Antigens apparently analogous to those of the mouse I region were detectable on kidney and some peripheral blood lymphocytes but not on liver, platelets, or other peripheral blood lymphocytes. The present study confirms these findings for liver parenchymal cells and for a mixture of kidney tubules and glomeruli prepared from fresh, perfused whole organs. However, homogenates prepared at the same time from the perfused whole organs showed no antigenic disparity. Whole liver homogenate contains I region analogues that are barely detectable on the parenchymal cell membrane. Liver parencymal cells may possess such antigens in a form not patnetly expressed, or the antigens may belong to other types of cell resident in the liver which are not removed by extensive perfusion. Similar results were obtained with liver homogenates derived from an orthotopic transplant to a fully allogeneic recipient and resident for 37 days, suggesting that blood lymphocytes persisting in the normal liver after perfusion could not account for these findings.

Differences between pig tissues in the expression of major transplantation antigens: possible relevance for organ allografts

Evidence is presented that there are differences between pig liver and kidney parenchymal cells in the expression of major histocompatibility complex antigens. Analogues of I-region antigens of the mouse are detectable on kidney cells and some peripheral blood leukocytes (PBL) but not on liver, platelets, and other PBL. Such differences between liver and kidney may explain (a) the fate of these organs as allografts and (b) the donor-specific protection of kidney by liver. Other antigens peculiar to skin and kidney have been detected but require further characterization.