Identification of Human Anti-HIV gp160 Monoclonal Antibodies That Make Effective Immunotoxins.

The envelope (Env) glycoprotein of HIV is the only intact viral protein expressed on the surface of both virions and infected cells. Env is the target of neutralizing antibodies (Abs) and has been the subject of intense study in efforts to produce HIV vaccines. Therapeutic anti-Env Abs can also exert antiviral effects via Fc-mediated effector mechanisms or as cytotoxic immunoconjugates, such as immunotoxins (ITs). In the course of screening monoclonal antibodies (MAbs) for their ability to deliver cytotoxic agents to infected or Env-transfected cells, we noted disparities in their functional activities. Different MAbs showed diverse functions that did not correlate with each other. For example, MAbs against the external loop region of gp41 made the most effective ITs against infected cells but did not neutralize virus and bound only moderately to the same cells that they killed so effectively when they were used in ITs. There were also differences in IT-mediated killing among transfected and infected cell lines that were unrelated to the binding of the MAb to the target cells. Our studies of a well-characterized antigen demonstrate that MAbs against different epitopes have different functional activities and that the binding of one MAb can influence the interaction of other MAbs that bind elsewhere on the antigen. These results have implications for the use of MAbs and ITs to kill HIV-infected cells and eradicate persistent reservoirs of HIV infection. IMPORTANCE: There is increased interest in using antibodies to treat and cure HIV infection. Antibodies can neutralize free virus and kill cells already carrying the virus. The virus envelope (Env) is the only HIV protein expressed on the surfaces of virions and infected cells. In this study, we examined a panel of human anti-Env antibodies for their ability to deliver cell-killing toxins to HIV-infected cells and to perform other antiviral functions. The ability of an antibody to make an effective immunotoxin could not be predicted from its other functional characteristics, such as its neutralizing activity. Anti-HIV immunotoxins could be used to eliminate virus reservoirs that persist despite effective antiretroviral therapy.

Design and In Vivo Characterization of Immunoconjugates Targeting HIV gp160.

The envelope (Env) glycoprotein of HIV is expressed on the surface of productively infected cells and can be used as a target for cytotoxic immunoconjugates (ICs), in which cell-killing moieties, including toxins, drugs, or radionuclides, are chemically or genetically linked to monoclonal antibodies (MAbs) or other targeting ligands. Such ICs could be used to eliminate persistent reservoirs of HIV infection. We have found that MAbs which bind to the external loop of gp41, e.g., MAb 7B2, make highly effective ICs, particularly when used in combination with soluble CD4. We evaluated the toxicity, immunogenicity, and efficacy of the ICs targeted with 7B2 in mice and in simian-human immunodeficiency virus-infected macaques. In the macaques, we tested immunotoxins (ITs), consisting of protein toxins bound to the targeting agent. ITs were well tolerated and initially efficacious but were ultimately limited by their immunogenicity. In an effort to decrease immunogenicity, we tested different toxic moieties, including recombinant toxins, cytotoxic drugs, and tubulin inhibitors. ICs containing deglycosylated ricin A chain prepared from ricin toxin extracted from castor beans were the most effective in killing HIV-infected cells. Having identified immunogenicity as a major concern, we show that conjugation of IT to polyethylene glycol limits immunogenicity. These studies demonstrate that cytotoxic ICs can target virus-infected cells in vivo but also highlight potential problems to be addressed. IMPORTANCE: It is not yet possible to cure HIV infection. Even after years of fully effective antiviral therapy, a persistent reservoir of virus-infected cells remains. Here we propose that a targeted conjugate consisting of an anti-HIV antibody bound to a toxic moiety could function to kill the HIV-infected cells that constitute this reservoir. We tested this approach in HIV-infected cells grown in the lab and in animal infections. Our studies demonstrated that these immunoconjugates are effective both in vitro and in test animals. In particular, ITs constructed with the deglycosylated A chain prepared from native ricin were the most effective in killing cells, but their utility was blunted because they provoked immune reactions that interfered with the therapeutic effects. We then demonstrated that coating of the ITs with polyethylene glycol minimized the immunogenicity, as has been demonstrated with other protein therapies.

A phase I study of a combination of anti-CD19 and anti-CD22 immunotoxins (Combotox) in adult patients with refractory B-lineage acute lymphoblastic leukaemia.

Novel agents are needed for patients with refractory and relapsed acute lymphoblastic leukaemia (ALL). Combotox is a 1:1 mixture of two immunotoxins (ITs), prepared by coupling deglycosylated ricin A chain (dgRTA) to monoclonal antibodies directed against CD22 (RFB4-dgRTA) and CD19 (HD37-dgRTA). Pre-clinical data demonstrated that Combotox was effective in killing both pre-B-ALL cell lines and cells from patients with pre-B ALL. A clinical study of paediatric patients in which 3 of 17 patients with ALL experienced complete remission, supported the preclinical work and motivated this study. This study was a Phase I, dose-escalation trial using Combotox in adults with refractory or relapsed B-lineage-ALL. A cycle consisted of three doses, with one dose given every other day. Dose levels were 3, 5, 6, 7 and 8 mg/m(2) per dose. Seventeen patients, aged 19-72 years, were enrolled in this multi-institution study. The maximum tolerated dose was 7 mg/m(2) /dose (21 mg/m(2) /cycle) and vascular leak syndrome was the dose-limiting toxicity. Two patients developed reversible grade 3 elevations in liver function tests. One patient achieved partial remission and proceeded to allogeneic stem cell transplantation. All patients with peripheral blasts experienced decreased blast counts following the administration of Combotox. Thus, Combotox can be safely administered to adults with refractory leukaemia.

A phase 1 study of Combotox in pediatric patients with refractory B-lineage acute lymphoblastic leukemia.

BACKGROUND: Acute lymphoblastic leukemia (ALL) is the most common cancer in children. Combotox is a 1:1 mixture of RFB4-dgA and HD37-dgA which are immunotoxins that target the CD22 and CD19 antigens, respectively. Combotox has different toxicities and targets than chemotherapy and is, thus, a new candidate for the treatment of patients with relapsed ALL. Preclinical data have demonstrated which Combotox is effective in killing pre-B-ALL cell lines and cells from patients with pre-B ALL. METHODS: We designed and conducted a Phase 1 dose-escalation study using Combotox in children with refractory or relapsed B-lineage-ALL. Seventeen patients aged 1 to 16 years were enrolled in this multi-institution study. They were treated at 4-dose levels: 2 mg/m2, 4 mg/m2, 5 mg/m2, and 6 mg/m2. RESULTS: The maximum tolerated dose was 5 mg/m2 and graft versus host disease defined the maximum tolerated dose. Three patients experienced complete remission. Six additional patients experienced a decrease of >95% in their peripheral blood blast counts, and 1 patient experienced a decrease of 75%. CONCLUSIONS: Combotox can be safely administered to children with refractory leukemia. It has clinically important anticancer activity as a single agent. The recommended dose for future studies is 5 mg/m2/dose.

Partial reduction of human FOXP3+ CD4 T cells in vivo after CD25-directed recombinant immunotoxin administration.

The regulation of tolerance to self-proteins and the suppression of T-cell responses have in part been attributed to the activity of CD25+CD4+ T regulatory (Treg) cells. Further, Treg cells can inhibit the antitumor effectiveness of adoptive immunotherapy and active immunization approaches in preclinical models. In an effort to selectively eliminate Treg cells from human peripheral blood mononuclear cell to potentially bolster antitumor responses, we have evaluated the Treg-cell depleting capacity of the CD25-directed immunotoxin, RFT5-SMPT-dgA. In preclinical studies, incubation of human peripheral blood mononuclear cell with RFT5-SMPT-dgA mediated a partial reduction in the levels of CD25+, Foxp3-expressing CD4+ T cells in vitro. Administration of RFT5-SMPT-dgA to 6 patients with metastatic melanoma induced a transient but robust reduction in the number of CD25high CD4 T cells in vivo (a 97.5% mean reduction at nadir; from 69.4 +/- 12.4 cells/miroL to 1.7 +/- 0.3 cells/microL). The reduction in FOXP3+ CD4 T-cell number was less comprehensive (a 71.3% mean reduction at nadir; from 66.6 +/- 16.5 cells/microL to 14.2 +/- 3.9 cells/tL). This resulted in the selective persistence of a stable number of CD25(low/neg) FOXP3+ CD4+ T cells in vivo. No objective antitumor responses were seen in any patient. Our results indicate that the CD25-directed, RFT5-SMPT-dgA immunotoxin can mediate a transient, partial reduction in Treg-cell frequency and number in vitro and in vivo and suggest that comprehensive eradication of human Treg cells in vivo may require the ability to target and eliminate FOXP3+ CD4+ T cells expressing both high and low levels of CD25.

Reconstitution of FOXP3+ regulatory T cells (Tregs) after CD25-depleted allotransplantation in elderly patients and association with acute graft-versus-host disease.

Selective depletion (SD) of host-reactive donor T cells from allogeneic stem-cell transplants (SCTs) using an anti-CD25 immunotoxin (IT) is a strategy to prevent acute graft-versus-host disease (aGvHD). There is concern that concurrent removal of regulatory T cells (T(regs)) with incomplete removal of alloactivated CD25(+) T cells could increase the risk of aGvHD. We therefore measured T(regs) in the blood of 16 patients receiving a T-cell-depleted allograft together with anti-CD25-IT-treated SD lymphocytes, in 13 of their HLA-identical donors, and in 10 SD products. T(regs) were characterized by intracellular staining for forkhead box protein 3 (FOXP3) and by quantitative reverse-transcription-polymerase chain reaction (qRT-PCR) for FOXP3 gene in CD4(+) cells. Patients received a median of 1.0 x 10(8)/kg SD T cells and a stem cell product containing a median of 0.25 x 10(4)/kg residual T cells. T(regs) reconstituted promptly after SCT and underwent further expansion. Of the CD4(+) T cells in SD products, 1.5% to 4.8% were CD25(-) T(regs). Acute GvHD (>or= grade II) was restricted to 5 patients whose donors had significantly (P = .019) fewer T(regs) compared with those without clinically significant aGvHD. These results suggest that rapid T(reg) reconstitution can occur following SD allografts, either from CD25(-) T(regs) escaping depletion, or from residual CD25(-) and CD25(+) T(regs) contained in the stem-cell product that expand after transplantation and may confer additional protection against GvHD.

Adoptive immunotherapy with allodepleted donor T-cells improves immune reconstitution after haploidentical stem cell transplantation.

Poor T lymphocyte reconstitution limits the use of haploidentical stem cell transplantation (SCT) because it results in a high mortality from viral infections. One approach to overcome this problem is to infuse donor T cells from which alloreactive lymphocytes have been selectively depleted, but the immunologic benefit of this approach is unknown. We have used an anti-CD25 immunotoxin to deplete alloreactive lymphocytes and have compared immune reconstitution after allodepleted donor T cells were infused at 2 dose levels into recipients of T-cell-depleted haploidentical SCT. Eight patients were treated at 10(4) cells/kg/dose, and 8 patients received 10(5) cells/kg/dose. Patients receiving 10(5) cells/kg/dose showed significantly improved T-cell recovery at 3, 4, and 5 months after SCT compared with those receiving 10(4) cells/kg/dose (P < .05). Accelerated T-cell recovery occurred as a result of expansion of the effector memory (CD45RA(-)CCR-7(-)) population (P < .05), suggesting that protective T-cell responses are likely to be long lived. T-cell-receptor signal joint excision circles (TRECs) were not detected in reconstituting T cells in dose-level 2 patients, indicating they are likely to be derived from the infused allodepleted cells. Spectratyping of the T cells at 4 months demonstrated a polyclonal Vbeta repertoire. Using tetramer and enzyme-linked immunospot (ELISPOT) assays, we have observed cytomegalovirus (CMV)- and Epstein-Barr virus (EBV)-specific responses in 4 of 6 evaluable patients at dose level 2 as early as 2 to 4 months after transplantation, whereas such responses were not observed until 6 to 12 months in dose-level 1 patients. The incidence of significant acute (2 of 16) and chronic graft-versus-host disease (GVHD; 2 of 15) was low. These data demonstrate that allodepleted donor T cells can be safely used to improve T-cell recovery after haploidentical SCT and may broaden the applicability of this approach.

Generation and characterization of a novel tetravalent anti-CD22 antibody with improved antitumor activity and pharmacokinetics.

The purpose of this study was to prepare a tetravalent anti-human CD22 recombinant antibody with improved antitumor activity and a half life longer than that of its divalent counterpart. We compared the ability of tetravalent vs. divalent antibody to associate/dissociate to/from CD22-positive Daudi cells, to interact with murine and human Fcgamma receptors (FcgammaR), to bind human complement component C1q, to inhibit the growth of tumor cells, to diffuse into various tissues, to be internalized by Daudi cells, to react with human neonatal Fc receptors (FcRn), and to persist in the circulation of normal mice. As compared to the murine or chimeric divalent antibodies, the chimeric tetravalent counterpart has a longer half life in mice. It also has an affinity for FcRns that is identical to that of human IgG. The tetravalent antibody has increased antitumor activity in vitro and completely conserved effector functions (binding to FcgammaR-positive cells and to C1q) in vitro. Despite its 33% higher molecular weight, it penetrates mouse tissues as well as its divalent antibody counterpart. Based on the improved in vitro performance and pharmacokinetics of the tetravalent antibody it will now be tested for its antitumor activity in vivo.

The generation of immunotoxins using chimeric anti-CD22 antibodies containing mutations which alter their serum half-life.

Murine and chimeric RFB4 (anti-human CD22) monoclonal antibodies (MAbs) with mutations in their Fc portions were conjugated to recombinant ricin toxin A chain to generate immunotoxins. The resulting immunotoxins (ITs) constructed with chimeric RFB4 MAbs were designed to have longer or shorter half-lives but similar binding and cytotoxic properties. These ITs can now be evaluated in vivo for improved therapeutic indices. The characteristics of these ITs are the subject of this report.

Selective depletion of alloreactive donor lymphocytes: a novel method to reduce the severity of graft-versus-host disease in older patients undergoing matched sibling donor stem cell transplantation.

We have selectively depleted host-reactive donor T cells from peripheral blood stem cell (PBSC) transplant allografts ex vivo using an anti-CD25 immunotoxin. We report a clinical trial to decrease graft-versus-host disease (GVHD) in elderly patients receiving selectively depleted PBSC transplants from HLA-identical sibling donors. Sixteen patients (median age, 65 years [range, 51-73 years]), with advanced hematologic malignancies underwent transplantation following reduced-intensity conditioning with fludarabine and either cyclophosphamide (n = 5), melphalan (n = 5), or busulfan (n = 6). Cyclosporine was used as sole GVHD prophylaxis. The allograft contained a median of 4.5 x 10(6) CD34 cells/kg (range, 3.4-7.3 x 10(6) CD34 cells/kg) and 1.0 x 10(8)/kg (range, 0.2-1.5 x 10(8)/kg) selectively depleted T cells. Fifteen patients achieved sustained engraftment. The helper T-lymphocyte precursor (HTLp) frequency assay demonstrated successful (mean, 5-fold) depletion of host-reactive donor T cells, with conservation of third-party response in 9 of 11 cases tested. Actuarial rates of acute GVHD were 46% +/- 13% for grades II to IV and 12% +/- 8% for grades III to IV. These results suggest that allodepletion of donor cells ex vivo is clinically feasible in older patients and may reduce the rate of severe acute GVHD. Further studies with selectively depleted transplants to evaluate graft-versus-leukemia (GVL) and survival are warranted.

Evaluation of a CD25-specific immunotoxin for prevention of graft-versus-host disease after unrelated marrow transplantation.

Donor T cells activated by recipient alloantigens cause graft-versus-host disease (GVHD) after hematopoietic cell transplantation. Activated T cells express CD25, among other components of the interleukin-2 receptor. We conducted a phase I/II study to determine whether administration of CD25-specific antibody conjugated to ricin toxin A could reduce the risk of grade III or IV GVHD after marrow transplantation from HLA-matched unrelated donors. All patients received methotrexate and cyclosporine after the transplantation. The immunotoxin was given to 36 patients for 4 consecutive days beginning approximately 36 hours after the marrow infusion was completed. Fourteen (40%) of the 35 patients who could be evaluated developed grade III or IV GVHD. In a contemporaneous population of 121 patients who received marrow from HLA-matched unrelated donors and were given methotrexate and cyclosporine without the immunotoxin, the incidence of grades III and IV GVHD was 24%. Cyclosporine blocked the induction of CD25 expression on alloactivated T cells in vitro but had no detectable effect on CD25 expression by T-regulatory cells. Taken together, these results are consistent with the hypothesis that cyclosporine protected alloactivated donor T cells from the effects of the immunotoxin, whereas the CD25+ T-regulatory cells remained susceptible, causing an unexpected exacerbation of acute GVHD.

Neonatal Fc receptors discriminates and monitors the pathway of native and modified immunoglobulin G in placental endothelial cells.

In the placenta, immunoglobulin G (IgG) is selectively transported from mother to fetus by a highly regulated transcellular mechanism aimed to achieve fetal humoral immunity. We questioned the role of neonatal Fc receptors (FcRn) in the traffic of IgG in human placental endothelial cells (HPEC). Cells were cultured in a double-chamber system and further exposed to IgG or Fc or to diethylpyrocarbonate-modified IgG or Fc in which the receptor recognition domain of the molecule (CH2-CH3) was altered. We provide evidence that the native IgG/Fc probes are transcytosed or recycled by HPEC, whereas the probes with the altered receptor recognition domain (which do not bind to FcRn) massively accumulate into the endocytic/lysosomal compartments. The results indicate that FcRn distinguishes between the intact and modified IgG and control their cellular traffic: native IgG is salvaged and released out of the cells, whereas modified IgG is retained and sorted to a degradative pathway. The data advance the understanding of the basic mechanism for IgG traffic in human endothelial cells, which may be exploited for the specific transport of antibodies in various immune disorders.

The evaluation of recombinant, chimeric, tetravalent antihuman CD22 antibodies.

PURPOSE: The purpose of this study was to prepare chimeric antihuman CD22 tetravalent monoclonal antibodies (MAbs) with high functional affinity, long persistence in the circulation, increased antitumor activity, and conserved effector function in vitro. EXPERIMENTAL DESIGN: We investigated the association/dissociation rates of these tetravalent antibodies using CD22(+) Daudi lymphoma cells. We then tested their ability to interact with Fc receptors on a human cell line (U937), to mediate antibody-dependent cellular cytotoxicity with human natural killer cells, to bind human C1q, to inhibit the in vitro growth of CD22 Daudi cells, and to persist in the circulation. RESULTS: The rate of dissociation of the tetravalent MAbs versus the divalent antibody was considerably slower. These tetravalent MAbs inhibited the in vitro proliferation of CD22 Daudi cells at a concentration that was at least 100-fold lower than that of the divalent murine antibody. The tetravalent MAbs containing both the CH2 and CH3 domains and a chimeric recombinant divalent antibody bound similarly to Fc receptor, C1q, and mediate antibody-dependent cellular cytotoxicity equally well with human natural killer cells. The persistence in the circulation of chimeric tetravalent MAbs was considerably longer than that of chemical homodimers. CONCLUSIONS: The tetravalent anti-CD22 MAbs with intact Fc regions should make effective therapeutic agents for B-cell tumors.

The bidirectional transfer and fetal vascular pressure changes due to the presence of 125I-labeled inhibin A in the ex-vivo human placental model.

OBJECTIVE: The purpose of this study was to investigate the transport of inhibin A and to determine its effects on fetal vascular pressure at elevated levels in the human placenta using 125I-labeled synthetic glycoprotein. METHODS: Synthetic inhibin A was prepared and was shown to be consistent with the natural form by high-pressure liquid chromatography (HPLC) and molecular weight determination by gas-chromatography mass spectrometry. The standardized Na125I process yielded 125I-labeled inhibin A with a radioactivity of 10(6) cpm/microg. This compound was placed in the human placenta in maternal-fetal and fetal-maternal studies using antipyrine and 14C-labeled inulin as controls to determine the bidirectional transfer of the compound. RESULTS: Maternal-fetal and fetal-maternal clearance indices were 0.045 +/- 0.003 and 0, respectively. In eight placentas there was no evidence of vascular pressure changes due to the presence of up to 5000 pg of inhibin A. CONCLUSIONS: There is minimal maternal-fetal transfer and no detectable fetal-maternal transfer in normotensive and pregnancy-induced hypertensive placentas. In addition, there are no pressure changes in the fetal vascular system due to the clinically significant levels of inhibin A.

Generation of mutated variants of the human form of the MHC class I-related receptor, FcRn, with increased affinity for mouse immunoglobulin G.

Much data support the concept that the MHC class I-related receptor FcRn serves to regulate immunoglobulin G (IgG) concentrations in serum and other diverse body sites in both rodents and humans. Previous studies have indicated that the human ortholog of FcRn is endowed with unexpectedly high stringency in binding specificity for IgGs. In contrast to mouse FcRn, which binds promiscuously to IgGs across species, human FcRn does not bind to mouse IgG1 or IgG2a, and interacts weakly with mouse IgG2b. Here, we investigate the molecular basis for this high-level specificity. We have systematically mutated human FcRn residues to the corresponding mouse FcRn residues in the regions that encompass the FcRn-IgG interaction site. Notably, mutation of the poorly conserved residue Leu137 of human FcRn to glutamic acid (L137E) generates a human FcRn mutant that binds to mouse IgG1 and mouse IgG2a with equilibrium dissociation constants of 13.2 microM and 14.4 microM, respectively. From earlier high-resolution structural analyses of the rat FcRn-rat Fc complex, residue 137 of human FcRn is predicted to contact residue 436 of IgG, which can be either His436 (mouse IgG1, mouse IgG2a) or Tyr436 (human IgG1, mouse IgG2b). The simplest interpretation of our data for the L137E mutant is therefore that replacement of the Leu137-Tyr436 (human) by the Glu137-His436 (mouse) pair generates a receptor that can bind to mouse IgG1 and mouse IgG2a. The L137E mutation reduces the affinity of human FcRn for human IgG1 by about twofold, consistent with the introduction of a less favorable Glu137-Tyr436 interaction. However, the analysis of the effects of other mutations on the binding to different IgGs indicates that the contribution to binding of the interaction of FcRn residue 137 with IgG residue 436 can vary. This suggests the existence of distinct docking topologies that are accompanied by variations in contacts between these two residues for different FcRn-IgG pairs. Our observations are of direct relevance to understanding the molecular nature of the human FcRn-IgG interaction. In turn, understanding human FcRn function has significance for the optimization of the serum half-lives of therapeutic and prophylactic antibodies.

Selective depletion of donor alloreactive T cells without loss of antiviral or antileukemic responses.

Poor immune reconstitution after haploidentical stem cell transplantation results in a high mortality from viral infections and relapse. One approach to overcome this problem is to selectively deplete the graft of alloreactive cells using an immunotoxin directed against the activation marker CD25. However, the degree of depletion of alloreactive cells is variable following stimulation with recipient peripheral blood mononuclear cells (PBMCs), and this can result in graft versus host disease (GVHD). We have refined this approach using recipient Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines (LCLs) as stimulators to activate donor alloreactive T cells. Our studies demonstrate that allodepletion with an anti-CD25 immunotoxin following stimulation with HLA-mismatched host LCLs more consistently depleted in vitro alloreactivity than stimulation with host PBMCs, as assessed in primary mixed lymphocyte reactions (MLRs). Allodepletion using this approach specifically abrogates cytotoxic T-cell responses against host LCLs. In interferon-gamma (IFN-gamma) enzyme-linked immunospot (ELISPOT) assays, antiviral responses to adenovirus and cytomegalovirus (CMV) were preserved following allodepletion. Likewise, using HLA-A2-pp65 tetramers, we have shown that the frequency of CMV-specific T cells is unaffected by allodepletion. Moreover, the donor anti-EBV response is partially retained by recognition of EBV antigens through the nonshared haplotype. Finally, we studied whether allodepletion affects the response to candidate tumor antigens in myeloid malignancies. Using HLA-A2-PR1 tetramer analysis, we found that the frequency of T cells recognizing the PR1 epitope of proteinase 3 was not significantly different in allodepleted and unmanipulated PBMCs from patients with chronic myeloid leukemia (CML) undergoing transplantation. Based on these data, we have embarked on a phase 1 clinical trial of addback of allo-LCL-depleted donor T cells in the haplo-identical setting.

Genetic engineering of an immunotoxin to eliminate pulmonary vascular leak in mice.

Vascular leak syndrome is a major and often dose-limiting side effect of immunotoxins and cytokines. We postulated that this syndrome is initiated by damage to vascular endothelial cells. Our earlier studies identified a three-amino acid motif that is shared by toxins, ribosome-inactivating proteins, and interleukin-2, all of which cause this problem. We have now generated a panel of recombinant ricin A chains with mutations in this sequence or in amino acids flanking it in the three-dimensional structure. These have been evaluated alone and as immunotoxins for activity, ability to induce pulmonary vascular leak in mice, pharmacokinetics, and activity in tumor-xenografted mice. One mutant was comparable to the ricin A chain used before in all respects except that it did not cause vascular leak at the same dose and, when used as an immunotoxin, was more effective in xenografted SCID mice.

Evidence to support the cellular mechanism involved in serum IgG homeostasis in humans.

IgG is the most abundant serum antibody and is an essential component of the humoral immune response. It is known that the 'neonatal' Fc receptor (FcRn) plays a role in maintaining constant serum IgG levels by acting as a protective receptor which binds and salvages IgG from degradation. However, the cellular mechanism that is involved in serum IgG homeostasis is poorly understood. In the current study we address this issue by analyzing the intracellular fate in human endothelial cells of IgG molecules which bind with different affinities to FcRn. The studies show that IgG which do not bind to FcRn accumulate in the lysosomal pathway, providing a cellular explanation for short serum persistence of such antibodies. We have also investigated the saturability of the homeostatic system and find that it has limited capacity. Our observations have direct relevance to the understanding and treatment of IgG deficiency, and to the effective application of therapeutic antibodies.