Anti-CD3 recombinant diphtheria immunotoxin therapy of cutaneous T cell lymphoma.

The recombinant CD3 immunotoxin, A-dmDT(390)-bisFv(UCHT1), composed of the catalytic and translocation domains of diphtheria toxin fused to two single chain Fv fragments of an anti-CD3epsilon monoclonal antibody was administered to five patients with cutaneous T cell lymphoma (CTCL) by eight 15 min intravenous infusions over four days. Side effects were fever, chills, nausea, hypoalbuminemia, transaminasemia and reactivation of EBV and CMV. Half-life of drug was 40 min. Anti-immunotoxin antibodies developed in all patients after two weeks. Two patients had partial remissions lasting 1 and 6+ months. The agent is undergoing further dose escalation and shows promising results in this disease.

A new yeast display vector permitting free scFv amino termini can augment ligand binding affinities.

Yeast surface display and sorting by flow cytometry are now widely used to direct the evolution of protein binding such as single-chain antibodies or scFvs. The available commercial yeast display vector pYD1 (Invitrogen) displays the protein of interest flanked on the N-terminus by Aga2, the disulfide of which binds the myristylated surface membrane protein Aga1. We have noted that two anti-CD3epsilon scFvs expressed as fusion proteins suffer a 30- to 100-fold loss of affinity when placed NH(2) terminal to either truncated toxins or human serum albumin. In the course of affinity maturing one of these scFv (FN18) using pYD1 we noted that the affinity towards the ectodomain of monkey CD3epsilongamma was too low to measure. Consequently we rebuilt pYD1 tethering the scFv off the NH(2) terminus of Aga2. This display vector, pYD5, now gave a positive signal displaying FN18 scFv with its ligand, monkey CD3epsilongamma. The apparent equilibrium association constant of the higher affinity scFv directed at human CD3epsilongamma increased approximately 3-fold when displayed on pYD5 compared with pYD1. These data show that for certain yeast-displayed scFvs a carboxy-tethered scFv can result in increased ligand-scFv equilibrium association constants and thereby extend the low range of affinity maturation measurements.

Chimeric fusion proteins--diphtheria toxin-based.

Most cancer patients receive chemotherapy drugs that target DNA or the cell division apparatus. Many of these patients develop multidrug-resistant tumor cells, thus, novel methods to overcome drug resistance are needed. One approach is to target tumor cell protein synthesis. Peptide toxins, which catalytically inactivate protein synthesis, have been re-engineered to selectively bind and intoxicate tumor cells. Diphtheria toxin (DT), a member of the class of peptide toxins, has been subjected to structural and genetic analysis and protein engineering for several decades. In this review, we will examine the structure, function, synthesis and pharmacology of anticancer DT conjugates.

Influence of relative binding affinity on efficacy in a panel of anti-CD3 scFv immunotoxins.

The in vitro cell killing potency of an immunotoxin reflects the aggregate of several independent biochemical properties. These include antigen binding affinity; internalization rate, intracellular processing and intrinsic toxin domain potency. This study examines the influence of antigen binding affinity on potency in various immunotoxin fusion proteins where target antigen binding is mediated by single chain antibody variable region fragments (scFv). Firstly, the relationship between affinity and potency was examined in a panel of four scFv immunotoxins generated from different anti-CD3 monoclonal antibodies fused to the 38 kDa fragment of Pseudomonas aeruginosa exotoxin A (PE38). Of these four scFv-PE38 immunotoxins, the one derived from the anti-CD3 monoclonal antibody UCHT1 has highest cell killing potency. Analysis of these four scFv-PE38 immunotoxins indicated a correlation between antigen binding affinity and immunotoxin potency in the cell killing assay with the exception of the scFvPE38 immunotoxin derived from the antibody BC3. However this scFv appeared to suffer a greater drop in affinity ( approximately 100x), relative to the parent Mab than did the other three scFvs used in this study (2-10x). Secondly, the scFv(UCHT1)-PE38 immunotoxin was then compared with a further panel of scFv(UCHT1)-derived immunotoxins including a divalent PE38 version and both monovalent and divalent Corynebacterium diphtheriae toxin (DT389) fusion proteins. When the scFv-UCHT1 domain was amino-terminally positioned relative to the toxin, as in the scFv(UCHT1)-PE38, an approximately 10-fold higher antigen-binding affinity was observed than with the C-terminal fusion, used in the DT389-scFv(UCHT1) molecule. Despite this lower antigen-binding activity, the DT389-scFv immunotoxin had a 60-fold higher potency in the T-cell-killing assay. Thirdly, a divalent form of the DT389-scFv construct, containing tandem scFv domains, had a 10-fold higher binding activity, which was exactly reflected in a 10-fold increase in potency. Therefore, when comparing immunotoxins in which scFvs from different antibodies are fused to the same toxin domain (DT or PE) a broad correlation appears to exist between binding affinity and immunotoxin potency. However, no correlation between affinity and potency appears to exist when different toxin domains are combined with the same scFv antibody domain.

Phenotypic and functional analysis of T-cell recovery after anti-CD3 immunotoxin treatment for tolerance induction in rhesus macaques.

T-cell reduction utilizing specific antibody has been widely used in human transplantation, and is a cornerstone of several tolerance induction strategies in nonhuman primates. We have established a population of long-term tolerant rhesus macaques induced with an anti-CD3epsilon immunotoxin (IT). This treatment effects transient, specific and profound ablation of T cells in blood and lymphoid tissues. In most instances the IT was used in combination with the NF-kappaB inhibitor, 15-Deoxyspergualin. This 2-week long protocol produces a "window of opportunity" for tolerization in which the animal exhibits an enduring quiescent state of unresponsiveness to the allograft, all accomplished without maintenance immunosuppressive drugs. During this induction period, the treated immune system bears some resemblance to that of the neonate, in that T cell numbers are abnormally low and antigen presentation by dendritic cells is precluded by an arrest in their NF-kappaB dependant maturation. In addition, IL-4 production is prominent during and after the tolerance induction interval. For this study we focused on measuring the monkey's ability to repopulate T cells with particular emphasis on the memory T-cell phenotype. Three "memory" phenotypes were utilized; CD3(+)CD45RO(+), CD3(+)CRTH2(+), and CD3(+)CD4(+)CD8(+). All three phenotypes exhibited different patterns of recovery, all of which included transient bursts in their numbers during repopulation. We also estimated thymic activity after T-cell ablation with the use of a newly-described RTE or recent thymic émigré phenotype (a naïve CD8(+)CD103(+) T cell). This marker revealed production of RTE cells including supranormal levels at approximately 6 months post-transplant, implicating thymic function in the repopulation of T-cells. Finally, we measured antibody responses to a panel of antigens (vaccines, environmental antigen, and foreign proteins) that indicated there was no apparent loss of immunologic function during or after the tolerance induction period. Results of studies of T-cell receptor repertoire expression suggest preservation of the pretreatment repertoire, which is consistent with rapid recovery of immune competence to the test antigens. Taken together, these results suggest that while aggressive, this tolerance induction protocol does not appear to incur a prolonged immunologically-compromised state, if at all.

Successful reversal of streptozotocin-induced diabetes with stable allogeneic islet function in a preclinical model of type 1 diabetes.

The recent focus on islet transplantation as primary therapy for type 1 diabetes has heightened interest in the reversal of type 1 diabetes in preclinical models using minimal immunosuppression. Here, we demonstrated in a preclinical rhesus model a consistent reversal of all measured glycemic patterns of streptozotocin-induced type 1 diabetes. The model used single-donor islet transplantation with induction of operational tolerance. The term "operational tolerance" is used to indicate durable survival of single-donor major histocompatibility complex (MHC)-mismatched islet allografts without maintenance immunosuppressive therapy and without rejection or loss of functional islet mass or insulin secretory reserve. In this operational tolerance model, all immunosuppression was discontinued after day 14 posttransplant, and recipients recovered with excellent health. The operational tolerance induction protocol combined peritransplant anti-CD3 immunotoxin to deplete T-cells and 15-deoxyspergualin to arrest proinflammatory cytokine production and maturation of dendritic cells. T-cell deficiency was specific but temporary, in that T-cell-dependent responses in long-term survivors recovered to normal, and there was no evidence of increased susceptibility to infection. Anti-donor mixed lymphocyte reaction responses were positive in the long-term survivors, but all showed clear evidence of systemic T-helper 2 deviation, suggesting that an immunoregulatory rather than a deletional process underlies this operational tolerance model. This study provides the first evidence that operational tolerance can protect MHC nonhuman primate islets from rejection as well as loss of functional islet mass. Such an approach has potential to optimize individual recipient recovery from diabetes as well as permitting more widespread islet transplantation with the limited supply of donor islets.

Improved binding of a bivalent single-chain immunotoxin results in increased efficacy for in vivo T-cell depletion.

Anti-CD3 immunotoxins exhibit considerable promise for the induction of transplantation tolerance in pre-clinical large animal models. Recently an anti-human anti-CD3epsilon single-chain immunotoxin based on truncated diphtheria toxin has been described that can be expressed in CHO cells that have been mutated to diphtheria toxin resistance. After the two toxin glycosylation sites were removed, the bioactivity of the expressed immunotoxin was nearly equal to that of the chemically conjugated immunotoxin. This immunotoxin, A-dmDT390-sFv, contains diphtheria toxin to residue 390 at the N-terminus followed by VL and VH domains of antibody UCHT1 linked by a (G(4)S)(3) spacer (sFv). Surprisingly, we now report that this immunotoxin is severely compromised in its binding affinity toward CD3(+) cells as compared with the intact parental UCHT1 antibody, the UCHT1 Fab fragment or the engineered UCHT1 sFv domain alone. Binding was increased 7-fold by adding an additional identical sFv domain to the immunotoxin generating a divalent construct, A-dmDT390-bisFv (G(4)S). In vitro potency increased 10-fold over the chemically conjugated immunotoxin, UCHT1-CRM9 and the monovalent A-dmDT390-sFv. The in vivo potency of the genetically engineered immunotoxins was assayed in the transgenic heterozygote mouse, tgepsilon 600, in which the T-cells express human CD3epsilon as well as murine CD3epsilon. T-cell depletion in the spleen and lymph node observed with the divalent construct was increased 9- and 34-fold, respectively, compared with the monovalent construct. The additional sFv domain appears partially to compensate for steric hindrance of immunotoxin binding due to the large N-terminal toxin domain.

Durable donor-specific T and B cell tolerance in rhesus macaques induced with peritransplantation anti-CD3 immunotoxin and deoxyspergualin: absence of chronic allograft nephropathy.

Tolerance induction can prevent acute kidney allograft rejection without chronic immunosuppression. It is uncertain whether specific tolerance can prevent chronic allograft nephropathy (CAN), which involves both nonimmune and immune injury. This report provides evidence that immunologically tolerant macaques, induced with immunotoxin and deoxyspergualin, developed neither acute rejection nor CAN. Long survivors, bearing MHC-mismatched grafts without chronic immunosuppression for 0.8 to 3.4 years, exhibited general immune competence with donor-specific T and B cell tolerance and no functional or histological evidence of CAN. Stringent criteria for tolerance were satisfied by specific prolongation of donor skin grafts with rapid rejection of third-party skin, followed by indefinite acceptance of a second donor kidney graft and establishment of microchimerism. Primate tolerance with documented absence of CAN may give impetus to the clinical application of tolerance.

Mixed chimerism and tolerance without whole body irradiation in a large animal model.

Mixed hematopoietic chimerism may provide a treatment for patients with nonmalignant hematologic diseases, and may tolerize patients to organ allografts without requiring chronic immunosuppression. However, the toxicity of the usual conditioning regimens has limited the clinical applicability of this approach. These regimens generally include some level of whole body irradiation (WBI), which is thought to facilitate engraftment either by making room for donor hematopoietic stem cells or by providing sufficient host immunosuppression to enable donor cells to engraft. Here, we have established mixed chimerism across both minor and major histocompatibility barriers in swine, by using high doses of peripheral blood stem cells in the absence of WBI. After mixed chimerism was established, swine leukocyte antigen-matched (SLA-matched) donor skin grafts were tolerated and maintained for a prolonged period, whereas third-party SLA-matched skin was rejected promptly. Donor-matched kidney allografts were also accepted without additional immunosuppression. Because of its low toxicity, this approach has potential for a wide range of clinical applications. Our data may indicate that niches for engrafting stem cells are filled by mass action and that WBI, which serves to empty some of these niches, can be omitted if the donor inoculum is sufficiently large and if adequate host T-cell depletion is achieved before transplant.

Expression of an anti-CD3 single-chain immunotoxin with a truncated diphtheria toxin in a mutant CHO cell line.

ADP-ribosylating immunotoxins are generally expressed in Escherichia coli and then refolded in vitro. Because the efficiency of the in vitro refolding process decreases with the number of protein domains and internal disulfide bonds, these immunotoxins have been generally limited to single-chain monovalent structures. We now show that using the hamster cell line CHO K1 RE1.22c (J. M. Moehring and T. J. Moehring, 1979, Somat. Cell Genet. 5, 453-468) that has been mutated to ADP-ribosylation insensitivity, a level of 4 microg/ml of a truncated anti-T cell immunotoxin, DT390-scFvUCHT1, can be secreted into the medium. This immunotoxin is glycosylated at the two potential N-linked glycosylation sites in the toxin moiety: positions 16-18 in the A chain and residues 235-237 in the B chain. The glycosylated immunotoxin is relatively nontoxic (IC(50) 4.8 x 10(-10) M). Removal of the N-linked oligosaccharides by N-glycosidase F treatment or mutations at the two N-linked glycosylation sites results in a highly active immunotoxin with an IC(50) of 4 x 10(-12) M toward CD3(+) Jurkat cells. This is a 12-fold increase in toxicity over the same immunotoxin harvested from E. coli periplasm without refolding. A single Asn(235) Ala mutation that removed the B chain glycosylation was nearly as toxic as the double mutant. This suggests that B chain glycosylation is the major cause for the loss of toxicity.

Long-term functional islet mass and metabolic function after xenoislet transplantation in primates.

BACKGROUND: Pancreatic islet transplantation (PIT) is an attractive alternative for patients with type I diabetes mellitus. PIT is not yet an effective clinical reality due in part to the high incidence of rejection and early loss of functional islet mass. In addition, current immunosuppressive drugs have toxic effects on islets and increase the risk of morbidity and mortality. In the present study, the effects of PIT on glycemic parameters were assessed in spontaneously diabetic primates. METHODS: Five insulinopenic nonhuman primates (three Macacca fascicularis, one Ceropithecus aethiops, and one Macacca mulatta) were studied. All required twice-daily treatment with 4-10 U of insulin. For immunosuppression, the animals received anti-CD3-immunotoxin (100 microg/kg(initially infused 2 hr before transplantation and again on day +1), cyclosporine (CsA) (20 mg/kg(i.v./2 hr before transplantation), cyclosporine microemulsion (Neoral) 60 mg/kg/b.i.d. on days +1 to +3 with dose adjusted by blood levels, and methylprednisolone (15 mg/kg day 0 to +3). Three recipients were given islets from a single donor (M mulatta). The islets were prepared by a semiautomated technique using Liberase. A mean of 13,136 islet equivalents/kg was infused into the portal vein. Two animals (M fascicularis and M mulatta) were used as a diabetic, nontransplanted control. Several metabolic parameters were evaluated. RESULTS: All monkeys that underwent transplantation experienced reversal of diabetes mellitus with normalization of all diabetic glycemic parameters. In the nontransplanted primates given the same immunosuppression but no PIT, diabetic metabolic parameters were unchanged after 9 months of follow-up. In contrast, all three PIT recipients established fasting and nonfasting euglycemia within 1-2 weeks, and none required exogenous insulin after day 10. Normal intravenous glucose tolerance tests were observed at day 15, and no significant differences in the glucose disappearance rate (Kg) were observed at days 15, 45, 190, and 365 days after transplantation. The acute insulin response to glucose indicated no significant reduction of functional islet mass. CONCLUSIONS: PIT in severely insulinopenic type I diabetes mellitus primates resulted in restoration of normal glycemic parameters and durable islet mass. Operational tolerance was achieved with only 4 days of drug administration, sparing the animals from chronic exposure to potentially diabetogenic immunosuppressive drugs. These results offer an exciting new potential for type I diabetes mellitus treatment.

Stable mixed chimerism and tolerance using a nonmyeloablative preparative regimen in a large-animal model.

Bone marrow transplantation (BMT) has considerable potential for the treatment of malignancies, hemoglobinopathies, and autoimmune diseases, as well as the induction of transplantation allograft tolerance. Toxicities associated with standard preparative regimens for bone marrow transplantation, however, make this approach unacceptable for all but the most severe of these clinical situations. Here, we demonstrate that stable mixed hematopoietic cell chimerism and donor-specific tolerance can be established in miniature swine, using a relatively mild, non-myeloablative preparative regimen. We conditioned recipient swine with whole-body and thymic irradiation, and we depleted their T-cells by CD3 immunotoxin-treatment. Infusion of either bone marrow cells or cytokine-mobilized peripheral blood stem cells from leukocyte antigen-matched animals resulted in stable mixed chimerism, as detected by flow cytometry in the peripheral blood, thymus, and bone marrow, without any clinical evidence of graft-versus-host disease (GvHD). Long-term acceptance of donor skin and consistent rejection of third-party skin indicated that the recipients had developed donor-specific tolerance.

Peritransplant tolerance induction in macaques: early events reflecting the unique synergy between immunotoxin and deoxyspergualin.

BACKGROUND: Day of transplant T cell depletion with anti-CD3 immunotoxin or F(Ab)2 immunotoxin induces stable tolerance to renal allografts in rhesus monkeys given 15-deoxyspergualin (DSG), a NF-kappaB inhibitor that suppresses proinflammatory cytokine (PC) production. Because PC and NF-kappaB are involved in dendritic cell (DC) maturation, we asked if impaired DC maturation and Th2-type cytokine deviation might be related to the synergistic effect of DSG in this novel model. METHODS: Immunosuppression was initiated 4 hr before transplanting a major histocompatibility complex mismatched renal allograft. Some groups received a supplemental 5-day course of cyclosporine A or DSG or a 15-day course of DSG. Peripheral lymph nodes were sequentially examined for presence of mature DC. In vitro effects of DSG on PC-induced maturation of DC were also examined. RESULTS: Allografts survived without rejection in 87% of recipients given immunotoxin or F(Ab)2 immunotoxin with DSG x 15 days, in 50% with DSG x 5 days, and 0% with cyclosporine A. The longest DSG survivors are >1000 days with normal graft function and tolerance validated, including acceptance of challenge second donor kidneys without treatment. DSG-treated recipients were unique in developing polarized Th2-type plasma cytokines. In DSG recipients, mature DC were significantly reduced in day +5 lymph node biopsies, with complete repopulation by 30 days. In vitro studies verified an inhibitory effect of DSG on DC maturation. CONCLUSIONS: The study suggests DSG arrests DC maturation. The unusual synergy of immunotoxin and DSG apparently involves coincidental reduction in lymph node T cell mass and mature DC, a transient circumstance favoring development of stable tolerance.

In vivo T cell depletion in miniature swine using the swine CD3 immunotoxin, pCD3-CRM9.

BACKGROUND: Partially inbred miniature swine developed in this laboratory provide a unique preclinical large animal model for studying transplant tolerance. The importance of in vivo T cell depletion for establishing stable mixed hematopoietic cell chimerism using a clinically relevant sublethal regimen has been well documented in murine studies (1). Until now, the lack of an effective in vivo T cell-depleting reagent in swine has limited the progress of studies involving hematopoietic cell transplants. METHODS: The swine CD3 immunotoxin, pCD3-CRM9, was prepared by conjugating our porcine-specific CD3 monoclonal antibody 898H2-6-15 to the diphtheria toxin derivative, CRM9. The resultant immunotoxin was administered i.v. to several miniature swine at doses ranging from 0.15-0.2 mg/kg either in a single dose or two doses 2 days apart. T-cell depletion was monitored in the peripheral blood, mesenteric lymph node, and thymus by flow cytometric analysis and histological examination. RESULTS: T cells were depleted to less than 1% of their pretreatment levels based on absolute numbers in the peripheral blood. Fluorescence activated cell sorter analysis and histological examination of serial lymph node biopsies confirmed depletion of the CD3+ T cells rather than down modulation or masking of the surface CD3 expression. Depletion of the CD3 bright medullary thymocytes could also be detected by flow cytometry and histological examination after immunotoxin treatment. CONCLUSIONS: Administration of the immunotoxin i.v. drastically depletes mature T cells from the peripheral blood, lymph node, and thymus compartments of the pig. This first description of an effective in vivo T-cell depleting reagent for the pig provides a valuable tool for studies of transplant tolerance in this large animal model. It also makes possible preclinical studies of T cell depletion with anti-CD3 immunotoxin in this large animal model.

Tolerability and side effects of anti-CD3-immunotoxin in preclinical testing in kidney and pancreatic islet transplant recipients.

INTRODUCTION: Anti-CD3-immunotoxin (alpha-CD3-IT) promotes allograft tolerance in nonhuman primates owing to efficient depletion of sessile and circulating T cells. Common side effects of vascular leak syndrome, hepatotoxicity, and nephrotoxicity have limited tolerability of other immunotoxins. We report on preclinical studies of alpha-CD3-IT-related side effects. METHODS: Normal rhesus monkeys received a kidney transplant and alpha-CD3-IT alone (on day -to +2) or in combination with brief peritransplant adjunctive immunosuppressive therapy. Some received donor CD34+ cells. Blood chemistries, complete blood count, weight, liver, and kidney biopsies were examined for immunotoxin-related changes. Five spontaneously diabetic primates also received alpha-CD3-IT, three of whom had a pancreas islet transplant. RESULTS: The main side effect of alpha-CD3-IT, vascular leak syndrome, was entirely prevented by adjunctive immunosuppressive therapy. Renal and liver function tests and biopsies revealed a lack of nephrotoxicity and hepatotoxicity. All had transient weight loss (14+/-5%). Without infusion of donor CD34+ cells, 97% had full weight recovery. Of those given donor CD34+ cells, 50% were euthanized for wasting. CONCLUSIONS: Side effects of alpha-CD3-IT are manageable and should not prevent therapeutic application.

Reversal of naturally occuring diabetes in primates by unmodified islet xenografts without chronic immunosuppression.

BACKGROUND: Isolated pancreatic islet transplantation (IPITx) is an attractive alternative for treatment of insulin-dependent diabetes mellitus (IDDM). However, IPITx has been difficult to implement clinically because islets frequently fail to function, have a high incidence of rejection, and are susceptible to autoimmune recurrence and damage by chronic immunosuppressive therapy. Tolerance induction may be a rational approach to resolve several of these limitations. Because anti-CD3 immunotoxin (IT) has been successful in promoting stable primate kidney transplant tolerance in our experience, we considered that tolerance induction with IT might be duplicated in IPITx. MATERIALS AND METHODS: Three monkeys with spontaneous IDDM (two Macaca fascicularis and one Ceropithecus aethiops) were treated with xenogeneic pancreatic islets (Macaca mulatta). Intrahepatic islet transplantation was performed at a mean of 13136+/-3860 islet equivalents/kg. Islet xenograft acceptance was accomplished by tolerance induction with two injections of IT given on day 0 at 2 hr before transplantation and on day +1, respectively. IT treatment was supplemented with cyclosporine and steroids administered on days 0 through 4. No additional immunosuppression was given thereafter. Two additional control macaques with spontaneous IDDM received the immunosuppressive protocol without islet infusion. RESULTS: All recipients were restored to stable euglycemia, off exogenous insulin, within 1-2 weeks after transplantation. Glucose tolerance, C-peptide, and glycosylated hemoglobin tests confirmed the restoration of normal glucose homeostasis after islet transplantation. All three islet recipients have remained euglycemic at 410, 255, and 100 days of follow-up despite recovery of peripheral T cells to normal levels. In contrast, none of the controls presented changes in the diabetic status 4 and 8 months after treatment. CONCLUSIONS: These results represent the first demonstration in nonhuman primates of stable, long-term acceptance of nonencapsulated xenogeneic islets off all immunosuppression, suggesting operational tolerance. The findings have potential implications for islet transplantation as well as improved and more cost-effective therapy for IDDM.

Primate renal transplants using immunotoxin.

BACKGROUND: T-lymphocyte depletion 7 days before transplantation with immunotoxin FN 18-CRM9 has resulted in tolerance to subsequent renal allografts. We tested the effect of giving immunotoxin on the day of the transplantation and evaluated its effect on rhesus monkey and allograft survival, on antibody production, and on T-cell recovery. METHODS: Major histocompatibility complex mismatched renal allografts were performed in rhesus monkeys. Immunotoxin was given starting on the day of transplantation, with and without prednisone and mycophenolate mofetil for 3 days. T-cell subsets and alloantibody levels were measured by flow cytometry. The ability of treated monkeys to develop antibody to tetanus, diphtheria, and xenoantibody was measured. Histology of renal transplants was read in a blinded manner. RESULTS: Immunotoxin started on the day of transplantation resulted in prolonged allograft survival in all treatment groups. Graft loss between days 50 and 135 was most often due to interstitial nephritis. Later graft loss was due to chronic rejection. Monkeys had intact antibody responses to alloantigen, tetanus, diphtheria, and xenoantibody. Their CD4 cells recovered gradually over 6 months. CONCLUSIONS: Immunotoxin reliably prolongs renal allograft survival when started on the day of transplantation, but interstitial nephritis and chronic rejection limit the development of long-term tolerance. T-cell-dependent B-cell responses remain intact after treatment.