ANALYSIS OF MECHANISM OF IMMUNOSUPPRESSIVE DRUGS IN RENAL HOMOTRANSPLANTATION

Naturally occurring cancers in pet dogs as pre-clinical models for cancer immunotherapy

Despite the significant progress in tumor prevention, early detection, diagnosis and treatment made over recent decades, cancer is still an enormous public health challenge all around the world, with the number of people affected increasing every year. A great deal of effort is therefore being devoted to the search for novel safe, effective and economically sustainable treatments for the growing population of neoplastic patients. One main obstacle to this process is the extremely low percentage of therapeutic approaches that, after successfully passing pre-clinical testing, actually demonstrate activity when finally tested in humans. This disappointing and expensive failure rate is partly due to the pre-clinical murine models used for in vivo testing, which cannot faithfully recapitulate the multifaceted nature and evolution of human malignancies. These features are better mirrored in natural disease models, i.e., companion animals affected by cancers. Herein, we discuss the relevance of spontaneous canine tumors for the evaluation of the safety and anti-tumor activity of novel therapeutic strategies before in-human trials, and present our experience in the development of a vaccine that targets chondroitin sulphate proteoglycan (CSPG)4 as an example of these comparative oncology studies.

Chimerism and tolerance in transplantation

Studies in experimental models (1953-1956) demonstrated that acquired donor-specific allotolerance in immunologically immature or irradiated animals is strongly associated with donor leukocyte chimerism. Bone marrow transplantation in immune-deficient or cytoablated human recipients was a logical extension (1968). In contrast, clinical (1959) and then experimental organ transplantation was systematically accomplished in the apparent absence of leukocyte chimerism. Consequently, it was assumed for many years that success with organ and bone marrow transplantation involved fundamentally different mechanisms. With the discovery in 1992 of small numbers of donor leukocytes in the tissues or blood of long-surviving organ recipients (microchimerism), we concluded that organ engraftment was a form of leukocyte chimerism-dependent partial tolerance. In this initially controversial paradigm, alloengraftment after both kinds of transplantation is the product of a double immune reaction in which responses, each to the other, of coexisting donor and recipient immune systems results in variable reciprocal clonal exhaustion, followed by peripheral clonal deletion. It was proposed with Rolf Zinkernagel that the individual alloresponses are the equivalent of the MHC-restricted T cell recognition of, and host response to, intracellular parasites and that the mechanisms of immune responsiveness, or nonresponsiveness, are governed by the migration and localization of the respective antigens. Elucidation of the mechanisms of nonresponsiveness (clonal exhaustion-deletion and immune ignorance) and their regulation removed much of the historical mystique of transplantation. The insight was then applied to improve the timing and dosage of immunosuppression of current human transplant recipients.

STEALTH in transplantation tolerance

Although contemporary immunosuppressive regimens are responsible for major improvements in allograft acceptance, there are indications that long-term survival may be compromised through drug toxicity and/or chronic immune deficiency. The ultimate goal for transplantation is tolerance, defined as durable, donor-specific allograft acceptance in the absence of long-term immunosuppression. This article reviews the nonhuman primate STEALTH model of tolerance recently developed by the transplant immunobiology group at University of Alabama at Birmingham. The STEALTH model was designed for future application to human transplantation and comprises a concise peritransplant treatment strategy of only 2 wk. Tolerance is induced by depletion of T cells, with concomitant inhibition of nuclear factor-kappaB/RelB-dependent proinflammatory signaling. This treatment has resulted in an unprecedented frequency of kidney allograft survival (62.5% at 3 yr), with some primate recipients remaining in good health more than 6 yr posttransplant, in the complete absence of chronic pharmacologic immunosuppression.

History of clinical transplantation

The emergence of transplantation has seen the development of increasingly potent immunosuppressive agents, progressively better methods of tissue and organ preservation, refinements in histocompatibility matching, and numerous innovations in surgical techniques. Such efforts in combination ultimately made it possible to successfully engraft all of the organs and bone marrow cells in humans. At a more fundamental level, however, the transplantation enterprise hinged on two seminal turning points. The first was the recognition by Billingham, Brent, and Medawar in 1953 that it was possible to induce chimerism-associated neonatal tolerance deliberately. This discovery escalated over the next 15 years to the first successful bone marrow transplantations in humans in 1968. The second turning point was the demonstration during the early 1960s that canine and human organ allografts could self-induce tolerance with the aid of immunosuppression. By the end of 1962, however, it had been incorrectly concluded that turning points one and two involved different immune mechanisms. The error was not corrected until well into the 1990s. In this historical account, the vast literature that sprang up during the intervening 30 years has been summarized. Although admirably documenting empiric progress in clinical transplantation, its failure to explain organ allograft acceptance predestined organ recipients to lifetime immunosuppression and precluded fundamental changes in the treatment policies. After it was discovered in 1992 that long-surviving organ transplant recipients had persistent microchimerism, it was possible to see the mechanistic commonality of organ and bone marrow transplantation. A clarifying central principle of immunology could then be synthesized with which to guide efforts to induce tolerance systematically to human tissues and perhaps ultimately to xenografts.

Science, specialization, and the American Surgical Association

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Variable chimerism, graft-versus-host disease, and tolerance after different kinds of cell and whole organ transplantation from Lewis to brown Norway rats

The bidirectional paradigm of tolerance involving reciprocal host vs. graft and graft vs. host reactions was examined after Lewis (LEW)-->Brown Norway (BN) transplantation of different whole organs (liver, intestine, heart, and kidney) or of 2.5 x 10(8) LEW leukocytes obtained from bone marrow, spleen, lymph nodes, and thymus. The experiments were performed without immunosuppression or under 14 daily doses of postoperative tacrolimus, which were continued in weekly doses to 100 days in a "continuous treatment" subgroup, and to 27 days in a short treatment group. Without immunosuppression, all organs and cell suspensions failed to engraft or were acutely rejected. GVHD (usually fatal) was always caused when either the long or short treatment was used for recipients of intestinal grafts and cell suspensions of spleen and lymph nodes. In contrast, both immunosuppressive protocols allowed engraftment of bone marrow cells, liver, heart, and kidney without clinical GVHD, whereas thymus cell suspensions and small doses of whole blood neither engrafted nor caused GVHD. At 100 days, now drug-free for 73 days, the liver, bone marrow, and heart recipients were tolerant in that they accepted all challenge LEW heart and/or liver grafts for 100 more days despite in vitro evidence of donor-specific reactivity (split tolerance). At 200 days, histopathologic studies of the challenge livers were normal no matter what the priming graft. However, the still-beating challenge hearts had a spectrum from normal to severe chronic rejection that defined the tolerogenicity of the original primary grafts: liver best-->bone marrow next-->heart least. Both the GVHD propensity and tolerogenicity in these experiments were closely associated with recipient tissue chimerism 30 and 100 days after the experiments began. The tissue chimerism was invariably multilineage, but the GVHD outcome was associated with T cell over-representation. These observations provide guidelines that should be considered in devising leukocyte augmentation protocols for human whole organ recipients. The results are discussed in relation to the historical tolerance studies of Billingham, Brent, and Medawar; Good; Monaco; and Calne.

Variable chimerism, graft-versus-host disease, and tolerance after different kinds of cell and whole organ transplantation from Lewis to brown Norway rats

The bidirectional paradigm of tolerance involving reciprocal host vs. graft and graft vs. host reactions was examined after Lewis (LEW)-->Brown Norway (BN) transplantation of different whole organs (liver, intestine, heart, and kidney) or of 2.5 x 10(8) LEW leukocytes obtained from bone marrow, spleen, lymph nodes, and thymus. The experiments were performed without immunosuppression or under 14 daily doses of postoperative tacrolimus, which were continued in weekly doses to 100 days in a "continuous treatment" subgroup, and to 27 days in a short treatment group. Without immunosuppression, all organs and cell suspensions failed to engraft or were acutely rejected. GVHD (usually fatal) was always caused when either the long or short treatment was used for recipients of intestinal grafts and cell suspensions of spleen and lymph nodes. In contrast, both immunosuppressive protocols allowed engraftment of bone marrow cells, liver, heart, and kidney without clinical GVHD, whereas thymus cell suspensions and small doses of whole blood neither engrafted nor caused GVHD. At 100 days, now drug-free for 73 days, the liver, bone marrow, and heart recipients were tolerant in that they accepted all challenge LEW heart and/or liver grafts for 100 more days despite in vitro evidence of donor-specific reactivity (split tolerance). At 200 days, histopathologic studies of the challenge livers were normal no matter what the priming graft. However, the still-beating challenge hearts had a spectrum from normal to severe chronic rejection that defined the tolerogenicity of the original primary grafts: liver best-->bone marrow next-->heart least. Both the GVHD propensity and tolerogenicity in these experiments were closely associated with recipient tissue chimerism 30 and 100 days after the experiments began. The tissue chimerism was invariably multilineage, but the GVHD outcome was associated with T cell over-representation. These observations provide guidelines that should be considered in devising leukocyte augmentation protocols for human whole organ recipients. The results are discussed in relation to the historical tolerance studies of Billingham, Brent, and Medawar; Good; Monaco; and Calne.

Cell migration and chimerism after whole-organ transplantation: the basis of graft acceptance

Improvements in the prevention or control of rejection of the kidney and liver have been largely interchangeable (1 , 2 ) and then applicable, with very little modification, to thoracic and other organs. However, the mechanism by which anti rejection treatment permits any of these grafts to be “accepted” has been an immunological enigma (3 , 4 ). We have proposed recently that the exchange of migratory leukocytes between the transplant and the recipient with consequent long-term cellular chimerism in both is the basis for acceptance of all whole-organ allografts and xenografts (5 ). Although such chimerism was demonstrated only a few months ago, the observations have increased our insight into transplantation immunology and have encouraged the development of alternative therapeutic strategies (6 ).

Cell migration and chimerism after whole-organ transplantation: the basis of graft acceptance

Improvements in the prevention or control of rejection of the kidney and liver have been largely interchangeable (1, 2) and then applicable, with very little modification, to thoracic and other organs. However, the mechanism by which anti rejection treatment permits any of these grafts to be “accepted” has been an immunological enigma (3, 4). We have proposed recently that the exchange of migratory leukocytes between the transplant and the recipient with consequent long-term cellular chimerism in both is the basis for acceptance of all whole-organ allografts and xenografts (5). Although such chimerism was demonstrated only a few months ago, the observations have increased our insight into transplantation immunology and have encouraged the development of alternative therapeutic strategies (6).

Fluorescence in situ hybridization for the Y-chromosome can be used to detect cells of recipient origin in allografted hearts following cardiac transplantation

The purpose of this study was to determine if fluorescence in situ hybridization for the Y-chromosome can be used to detect cells of recipient origin in allografted hearts following cardiac transplantation. Formalin-fixed, paraffin-embedded tissue sections of coronary arteries from two hearts surgically explanted from heart transplant recipients undergoing retransplantation because of accelerated arteriosclerosis were examined by fluorescence in situ hybridization for the presence of cells containing the Y-chromosome using a biotinylated Y-chromosome cocktail probe. In both cases, the recipients were male and the original donor hearts were obtained from female donors. Hybridization was detected in cells morphologically recognizable as infiltrating lymphocytes, macrophages, and mast cells, establishing that these cells in the donor hearts were of recipient origin. In contrast, hybridization was not detected in cardiac myocytes, in vascular smooth muscle cells, or in the majority (>95%) of endothelial cells, suggesting that these cells were of donor origin. Although hybridization was detected in rare flattened cells lining vascular lumina, these cells did not stain for factor VIII, suggesting that they were, in fact, flattened inflammatory cells and not endothelial cells. These results demonstrate that, when the recipient and donor are of the opposite sex, fluorescence in situ hybridization for the Y-chromosome can be used to detect graft chimerism in transplanted hearts.

Human organ transplantation: background and consequences

The story of the renal transplant program of the Peter Bent Brigham Hospital (now the Brigham and Women's Hospital) in Boston weaves together three distinct threads: the study of renal disease, the phenomenon of skin grafting in twins, and the development of surgical procedures ultimately leading to the use of chemical immunosuppression. The common leitmotiv is one of a single event or report proving to be decisive. Unanticipated consequences of successful human organ transplantation include the reorganization of clinical and nonclinical disciplines, national and international cooperation in organ preservation and distribution, tissue-typing as a marker for disease, redefinition of death in terms of brain function, better understanding of disease processes, and new health care quandaries that result from the scarcity of organ donors.

Induction of unresponsiveness to major transplantable organs in adult mammals: a recapitulation of ontogeny by irradiation and bone marrow replacement

Transplantation of renal allografts obtained from prospectively selected genotypically DLA-identical donors into supralethally irradiated dogs reconstituted with their own stored bone marrow has produced a state of unresponsiveness to these kidneys in the recipients. Eleven of 18 kidneys transplanted at 12 hours after marrow replacement currently survive with normal function and maintain life in the recipients for 757, 800, 825, 978, 1062, 1092, 1136, 1282, 1373, 1380, and 1381 days, respectively. Similar results occurred in eight of 13 allografts transplanted at 28 hours after marrow replacement, which currently survive for 349, 363, 377, 407,436,470, 485, and 513 days, respectively, and in eight of 13 kidneys grafted at 36 hours after marrow replacement, which are surviving for 197, 247, 298, 324, 337, 396, 443, and 472 days, respectively. Achievement of optimal results is dependent on the specific timing and sequence of each procedure. Only four of 16 recipients of kidneys transplanted at the time of marrow replacement were unresponsive to their allografts. Similarly, only five of 19 recipients of kidneys placed in irradiated dogs at 40 hours before marrow replacement accepted such allografts. When kidney transplants were placed into the recipients 20 hours before removal of marrow, irradiation, and reconstitution with stored marrow, only three of 21 dogs became unresponsive to such ailografts. In five of 12 instances, the recipients were also unresponsive to skin allografts obtained from their respective kidney donors. Such skin grafts currently survive for 606, 673, 687, 701, and 708 days, respectively. The remaining seven skin grafts were rejected at 28, 39,42, 84, 90, 92, and 115 days, respectively. Second- and third-set skin grafts from the same kidney donor were rejected by six of these dogs at 19, 20, 21, 29, 29, and 30 days, and at 21, 22, 23, 24, 27, and 27 days, respectively. Rejection of these skin grafts had no detectable effect on the function and survival of kidney allografts from the same source. Seven of eight skin grafts obtained from other DLA-identical donors were rejected at 13,14,16,25,28,38, and 84 days, respectively; one allograft continues to survive for 708 days. Eleven DLA-incompatible skin allografts placed on the recipients at the same time were rejected within 11-20 days. Supralethal total body irradiation and bone marrow replacement can establish in the adult canine host a privileged phase of immunological reactivity during which exposure to alloantigens produces specific long-term unresponsiveness rather than sensitization. The use of stored autologous rather than allogeneic bone marrow for reconstitution of the irradiated recipient eliminates the hazards of GVH complication usually associated with this procedure. This consideration and the apparent capacity of the tolerant host to maintain a long-term state of unresponsiveness without any further immunosuppressive therapy point to the potential relevance of the results to human transplantation.

The effects of azathioprine and 6 MP on immunity

The thiopurines, azathioprine and 6 MP are potent inhibitors of both experimental and clinical immune responses. The primary pharmacological activities are mediated by competitive inhibition of enzymes concerned with de novo purine base synthesis; Immunosuppressive activities appear to result from cytotoxic activities directed against antigen-responsive lymphocytes; this inhibition is maximal when the treatment course coincides with the proliferative expansion phase of the response. By contrast, thiopurines are comparatively ineffective if used during an effector phase of an immune response. Furthermore, administration prior to antigenic challenge does not lead to immune inhibition; in fact, it may lead to augmentation of selected immune responses. Treatment with thiopurines does not result in acute lymphopenia; prolonged courses will cause a moderate decrease in circulating lymphocytes. The drug does not selectively deplete peripheral T or B cells but can acutely reduce K (killer) cells, which are effectors in ADCC responses. In addition, short-lived thymocytes and marrow lymphocytes are rapidly depleted by these anti-metabolites. Many in vitro functions of lymphocytes, from treated animals remain normal. Recent studies indicate that, in vitro, azathioprine is specifically able to bind murine T lymphocytes; this can be shown by their ability to inhibit their capacity to rosette with sheep erythrocytes. Azathioprine is also a potent inhibitor of mixed lymphocyte culture responses and can readily suppress the in vitro generation of cytotoxic T cells. These observations suggest that drugs exert preferential toxicities for murine T cells. B lymphocytes for mice appear to vary in their susceptibility for thiopurines. By contrast, the activity of human B cells can be readily suppressed with this drug whereas T helper function is comparatively resistant. In addition to immunosuppressive properties, thiopurines are capable of exerting anti-inflammatory activities, primarily by inhibiting the replication of hematopoietic precursors.

Effects of immunosuppression on the rejection of methylcholanthrene tumor isografts

The effect of immunosuppression with azathioprine, methylprednisolone, or both, on primary and secondary immune responses to a transplanted tumor was studied. The ability to reject a methylcholanthrene-induced squamous cell carcinoma of the prostate in Lewis rats was determined by rechallenge with tumor cells after amputation of the primary transplant. Oral azathioprine (20 mg/kg/day) increased the incidence of tumors in a primary challenge, but did not affect the ability of immunized rats to reject subsequent tumor isografts. Similar results were obtained with 2 mg/kg day intraperitoneal (IP) azathioprine, 2 mg/kg IP methylprednisolone, and the combination of 2 mg/kg/day IP methylprednisolone and 10 mg/kg/day IP azathioprine. It appears that immunosuppression by azathioprine affects the proliferation of sensitized cells during the immunological response to antigens of this specific tumor. Immunosuppression does not appear to alter substantially the processing of tumor-specific antigens or the cytotoxic effectiveness of the immune system. This data is reassuring to the concern of whether transplantation in patients previously cured of cancer is safe.

Renal transplantation: a twenty-five year experience

Boston has played a significant role in the development of renal transplantation. In Boston was performed the first successful isograft between identical twins (1954) the first successful allograft between fraternal twins (1959) and the first successful allograft from a cadaveric donor (1962). An immunosuppressive drug was also described in Boston by hematologists Schwartz and Dameschek (1959) and modified for renal transplantation in dogs (1961) and used for the first time in a human recipient in March 1962. By 1965 renal transplantation had become a clinical reality. Three hundred and ninety-eight of 589 recipients (68%) since 1950 are still alive, a remarkable figure considering that it includes all the earliest experimental transplants. One hundred and ninety-five of 295 (68%) with living-related donor transplants still have functioning allografts; 104/265 (39%) with cadaveric donor transplants have functioning grafts currently. Since 1968 transplants from living-related donors have an 80% one year survival whereas cadaveric donor transplants have approximately a 50% one year survival. Seventy-nine per cent of all one year survivors have had excellent psycho-social rehabilitation.

A study of allograft kidney rejection occurring simultaneously in whole organ and ear chamber grafts in the rabbit

When portions of adult renal tissue are allografted into the rabbit ear chamber, they usually survive for periods of up to several months (6). When a kidney from the same donor is grafted as a whole organ, the ear chamber grafts then reject with the whole organ in 7 days. During that time serial needle biopsies of the whole organ are compared with the in vivo appearance of the ear chamber grafts. This establishes that the changes occurring in the ear chamber grafts are monitoring the rejection process proceeding in the whole organ grafts. Dramatic vascular changes herald the earliest stages of unmodified rejection. A highly characteristic form of individual discrete platelet adhesion to both endothelium and adherent leukocytes is observed which is associated with the release reaction. At times as many as 20 such discrete platelets are clearly visible in profile in one high-power field. This demonstrates in vivo a mechanism whereby vascular and parenchymal damage may be produced by platelet contents, without previous aggregation or thrombus formation being necessary.

Origin of endothelium in human renal allografts

Sex chromatin counts performed on the endothelial cells of 40 human kidneys transplanted to recipients of the opposite sex showed that the donor endothelium had persisted except in three poorly functioning and severely damaged grafts. In these a high proportion of the endothelial cells in peritubular capillaries and veins were derived from the host. Endothelial repopulation of organ allografts probably occurs only after severe tissue injury, and it cannot explain the phenomenon of graft adaptation. Repopulated endothelium may be derived from circulating cells.

Transplantation of the liver in man

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Mixed lymphocyte reactions and tissue transplantation tolerance

The induction of tolerance of Lewis histocompatibility antigens in BN rats inoculated at birth with BNI Lewis F(1) hybrid bone marrow cells, as revealed by the prolonged survival of Lewis skin grafts, is accompanied by markedly decreased reactivity in the mixed lymphocyte interaction. Blood lymphocytes from animals inoculated with lymph node cell suspensions also display diminished proliferative reactivity to hybrid BN/Lewis cells in the interaction. However, these recipients are not tolerant of Lewis skin grafts. Blood lymphocytes from BN rats inoculated neonatally with Lewis thymocytes fail to display any level of unresponsiveness in vitro, and such animals are not tolerant of Lewis skin grafts. The results suggest that in rats skin and marrow cells have histocompatibility antigens that are absent or poorly expressed on lymph node cells and thymocytes.

Allografts in genetically defined rats: difference in survival between kidney and skin

Although skin allografts from inbred donors of the Fisher strain to inbred male Lewis recipients regularly show acute rejection within 12 days, orthotopic kidney allografts between untreated animals, in this same combination of strains, usually remain functionally intact for longer than 100 days. Since such renal allografts persist despite previous or concomitant rejection of skin allografts, neither acquired tolerance nor nonspecific immunosuppression can explain the surprisingly prolonged kidney survival. Many factors appear to be responsible for the disparate survival times observed. Tentatively, these factors are (i) antigenic differences between kidney and skin, (ii) intervention of immunological enhancement, and (iii) physiological differences in vulnerability between kidney and skin.