Elimination of T cells from human peripheral blood and bone marrow using a cocktail of three anti-T cell immunotoxins

Synthesis of an Anti-CD7 Recombinant Immunotoxin Based on PE24 in CHO and E. coli Cell-Free Systems

Recombinant immunotoxins (RITs) are an effective class of agents for targeted therapy in cancer treatment. In this article, we demonstrate the straight-forward production and testing of an anti-CD7 RIT based on PE24 in a prokaryotic and a eukaryotic cell-free system. The prokaryotic cell-free system was derived from Escherichia coli BL21 StarTM (DE3) cells transformed with a plasmid encoding the chaperones groEL/groES. The eukaryotic cell-free system was prepared from Chinese hamster ovary (CHO) cells that leave intact endoplasmic reticulum-derived microsomes in the cell-free reaction mix from which the RIT was extracted. The investigated RIT was built by fusing an anti-CD7 single-chain variable fragment (scFv) with the toxin domain PE24, a shortened variant of Pseudomonas Exotoxin A. The RIT was produced in both cell-free systems and tested for antigen binding against CD7 and cell killing on CD7-positive Jurkat, HSB-2, and ALL-SIL cells. CD7-positive cells were effectively killed by the anti-CD7 scFv-PE24 RIT with an IC50 value of 15 pM to 40 pM for CHO and 42 pM to 156 pM for E. coli cell-free-produced RIT. CD7-negative Raji cells were unaffected by the RIT. Toxin and antibody domain alone did not show cytotoxic effects on either CD7-positive or CD7-negative cells. To our knowledge, this report describes the production of an active RIT in E. coli and CHO cell-free systems for the first time. We provide the proof-of-concept that cell-free protein synthesis allows for on-demand testing of antibody−toxin conjugate activity in a time-efficient workflow without cell lysis or purification required.

Combination fusion protein therapy of refractory brain tumors: demonstration of efficacy in cell culture

Primary brain tumors including anaplastic astrocytomas and glioblastoma multiforme are difficult to treat because of their locally invasive nature and relative resistance to chemotherapy and radiation therapy. Novel agents that can kill multi-drug resistant tumor cells and reach tumor cells at distant sites in the brain are needed. One such class of agents is fusion proteins which consist of brain-tumor-selective peptide ligands fused to peptide toxins. The ligand directs the protein to the glioma cell surface; the peptide toxin is then internalized into the cell, translocates to the cytosol and catalytically inactivates protein synthesis leading to cell death. Fusion proteins are toxic to multi-drug resistant brain tumor cells. Because of the large molecular weight of these molecules, a unique delivery system has been developed--convection-enhanced delivery (CED). The method creates a bulk flow which supplements diffusion and achieves drug concentrations in the brain parenchyma orders of magnitude greater than by systemic administration. Patients with recurrent glioma treated with individual fusion protein CED have obtained clinical remissions lasting years. However, toxicities to normal brain have been observed and relapses ultimately occurred. To address the clinical need of these patients and improve upon the therapeutic index observed to date with single fusion protein CED, we generated a novel fusion protein DAB389EGF and tested it in combination with another active fusion protein, IL13PE38QQR. We observed potent glioma cytotoxicity with each fusion protein and synergistic toxicity with the combination. Further, brain tumor cells showed heterogeneous expression of individual receptors suggesting that the combination--DAB389EGF and IL13PE38QQR may show improved efficacy and should undergo further preclinical development for therapy of patients with relapsed high-grade gliomas.

Autologous stem cell transplantation and purging in adult acute lymphoblastic leukaemia

The prognosis for adult acute lymphoblastic leukaemia (ALL) is poor. Only 20-30% of patients will be cured with conventional chemotherapy. Haematopoietic progenitor transplantation is thus an attractive option in these patients. Even if allogeneic transplantation allows a better control of the disease, autologous transplantation remains an important alternative for patients lacking a suitable donor or when allogeneic transplants imply excessive risk. Relapse is the main drawback of autologous transplants, but many strategies are being explored to overcome this problem. We focus here on transplant modality, the source of haematopoietic progenitors, and the best timing to apply the procedure. Also reviewed are the current situation and future strategies for improving results in this setting, such as ex vivo purging; immunotherapy and maintenance chemotherapy.

A combination of anti-CD3 and anti-CD7 ricin A-immunotoxins for the in vivo treatment of acute graft versus host disease

This study evaluated the anti-graft versus host disease (GVHD) potential of a combination of immunotoxins (IT), consisting of a murine CD3 (SPV-T3a) and CD7 (WT1) monoclonal antibody both conjugated to deglycosylated ricin A. In vitro efficacy data demonstrated that these IT act synergistically, resulting in an approximately 99% elimination of activated T cells at 10−8 mol/L (about 1.8 μg/mL). Because most natural killer (NK) cells are CD7+, NK activity was inhibited as well. Apart from the killing mediated by ricin A, binding of SPV-T3a by itself impaired in vitro cytotoxic T-cell cytotoxicity. Flow cytometric analysis revealed that this was due to both modulation of the CD3/T-cell receptor complex and activation-induced cell death. These results warranted evaluation of the IT combination in patients with refractory acute GVHD in an ongoing pilot study. So far, 4 patients have been treated with 3 to 4 infusions of 2 or 4 mg/m2 IT combination, administered intravenously at 48-hour intervals. The T1/2 was 6.7 hours, and peak serum levels ranged from 258 to 3210 ng/mL. Drug-associated side effects were restricted to limited edema, fever, and a modest rise of creatine kinase levels. One patient developed low-titer antibodies against ricin A. Infusions were associated with an immediate drop of circulating T cells, followed by a more gradual but continuing elimination of T/NK cells. One patient mounted an extensive CD8 T-cell response directly after treatment, not accompanied with aggravating GVHD. Two patients showed nearly complete remission of GVHD, despite unresponsiveness to the extensive pretreatment. These findings justify further investigation of the IT combination for treatment of diseases mediated by T cells.

A combination of anti-CD3 and anti-CD7 ricin A-immunotoxins for the in vivo treatment of acute graft versus host disease

This study evaluated the anti-graft versus host disease (GVHD) potential of a combination of immunotoxins (IT), consisting of a murine CD3 (SPV-T3a) and CD7 (WT1) monoclonal antibody both conjugated to deglycosylated ricin A. In vitro efficacy data demonstrated that these IT act synergistically, resulting in an approximately 99% elimination of activated T cells at 10−8 mol/L (about 1.8 μg/mL). Because most natural killer (NK) cells are CD7+, NK activity was inhibited as well. Apart from the killing mediated by ricin A, binding of SPV-T3a by itself impaired in vitro cytotoxic T-cell cytotoxicity. Flow cytometric analysis revealed that this was due to both modulation of the CD3/T-cell receptor complex and activation-induced cell death. These results warranted evaluation of the IT combination in patients with refractory acute GVHD in an ongoing pilot study. So far, 4 patients have been treated with 3 to 4 infusions of 2 or 4 mg/m2 IT combination, administered intravenously at 48-hour intervals. The T1/2 was 6.7 hours, and peak serum levels ranged from 258 to 3210 ng/mL. Drug-associated side effects were restricted to limited edema, fever, and a modest rise of creatine kinase levels. One patient developed low-titer antibodies against ricin A. Infusions were associated with an immediate drop of circulating T cells, followed by a more gradual but continuing elimination of T/NK cells. One patient mounted an extensive CD8 T-cell response directly after treatment, not accompanied with aggravating GVHD. Two patients showed nearly complete remission of GVHD, despite unresponsiveness to the extensive pretreatment. These findings justify further investigation of the IT combination for treatment of diseases mediated by T cells.

Treatment of refractory Hodgkin's lymphoma patients with an anti-CD25 ricin A-chain immunotoxin

The anti-CD25 immunotoxin RFT5.dgA was constructed by coupling the monoclonal antibody RFT5 via a sterically hindered disulfide linker to deglycosylated ricin A-chain and was administered to patients with relapsed Hodgkin's lymphoma in four bolus infusions over 7 days (day 1, 3, 5 and 7). The maximum tolerated dose in these patients as defined in a previous phase I study was 15 mg/m2. Subsequently, further patients were enrolled at the maximum tolerated dose and a total of 18 patients were treated at this level. All patients had signs of progressive disease and were heavily pretreated. Side-effects in this trial were moderate and related to vascular leak syndrome. Five of 18 patients experienced NCI grade III toxicities including weakness, edema, dyspnea, and myalgia. Eleven of 16 (69%) patients receiving two or more cycles produced human anti-ricin antibodies and human anti-mouse antibodies (>/=1.0 microg/ml). Seventeen of 18 patients were evaluable for clinical response. These included two partial remissions. One patient demonstrated minor response and five patients stable diseases. We conclude that RFT5.dgA is of moderate clinical efficacy in this group of heavily pretreated refractory patients. Leukemia (2000) 14, 129-135.

Clinical trials with an anti-CD25 ricin A-chain experimental and immunotoxin (RFT5-SMPT-dgA) in Hodgkin's lymphoma

Immunotoxins (ITs) consisting of a cell-binding component and a potent toxin were developed as a new class of biological anti-tumor agents to improve adjuvant therapy. Hodgkin's lymphoma (HL) has been demonstrated to be an excellent target for ITs because high concentrations of lymphocyte activation markers such as CD25 and CD30 are expressed on Hodgkin and Reed-Sternberg (H-RS). Several ITs against these antigens have shown potent antitumor effects against H-RS cells in vitro and in different HL animal models. On the basis of its superiority in preclinical models, the anti-CD25 IT RFT5-SMPT-dgA was subsequently evaluated in a phase I study in patients with refractory Hodgkin's lymphoma. The IT was constructed by linking the monoclonal antibody (Moab) RFT5 via a sterically hindered disulfide linker (SMPT) to deglycosylated ricin A-chain (dgA). All 15 patients enrolled in this trial were heavily pretreated with a mean of five different prior therapies. The IT was administered intravenously over four hours on days 1-3-5-7 for total doses per cycle of 5, 10, 15, or 20 mg/m2. Side effects were reversible and related to the vascular leak syndrome (VLS), i.e. decrease in serum albumin, edema, weight gain, hypotension, tachycardia, myalgia, and weakness. In all three patients receiving 20 mg/m2 NCI toxicity grade III was observed. Thus, 15 mg/m2 is the maximal tolerated dose (MTD) of RFT5-SMPT-dgA. 50% of the patients developed human anti-ricin A-chain antibodies (HARA) and/or human anti-mouse antibodies (HAMA). Clinical results included two partial remissions (PR), one minor response (MR), three stable disease (SD) and nine progressive disease (PD). In an extension of the phase I trial, five additional patients have been treated at the MTD.

Additive cytotoxicity of different monoclonal antibody-cobra venom factor conjugates for human neuroblastoma cells

Insufficient numbers of antigen molecules and heterogeneity of antigen expression on tumor cells are major factors limiting the immunotherapeutic potential of the few clinically useful monoclonal antibodies capable of mediating complement cytotoxicity and antibody-dependent cellular cytotoxicity. To overcome this limitation, we converted two non-cytotoxic monoclonal anti-neuroblastoma antibodies, designated 3E7 (IgG2b) and 8H9 (IgG1), and the non-cytotoxic F(ab')2 fragment of the cytotoxic monoclonal anti-GD2 antibody 3F8 (IgG3) into cytotoxic antibody conjugates by covalent attachment of cobra venom factor (CVF), a structural and functional homologue of the activated third component of complement. Competitive binding experiments confirmed the different specificities of the three antibodies. In the presence of human complement, all three antibody-CVF conjugates mediated selective complement-dependent lysis of human neuroblastoma cells. Consistent with the kinetics of the alternative pathway of complement, approximately seven hours incubation were required to reach maximum cytotoxicity of up to 25% for the 3E7-CVF conjugate, up to 60% for the 8H9-CVF conjugate, and up to 95% for the 3F8 F(ab')2-CVF conjugate. The different extent of maximal cytotoxic activity of the three conjugates was reflected by corresponding differences in the extent of binding of both unconjugated antibodies and the respective conjugates. Any combination of the three antibody-CVF conjugates caused an additive effect in complement-mediated lysis. Using a cocktail of all three conjugates, the extent of complement-mediated killing could be increased up to 100%. These data demonstrate that by coupling of CVF the relative large number of non-cytotoxic monoclonal anti-tumor antibodies of interesting specificity can be used to design cocktails of cytotoxic conjugates and, thereby, to overcome the problem of insufficient and heterogeneous antigen expression on tumor cells for immunotherapy.

A Phase-I Study of an Anti-CD25 Ricin A-Chain Immunotoxin (RFT5-SMPT-dgA) in Patients With Refractory Hodgkin's Lymphoma

The anti-CD25 immunotoxin (IT), RFT5-SMPT-dgA, was used in a phase I dose escalation trial in patients with refractory Hodgkin's lymphoma. The IT was constructed by linking the monoclonal antibody RFT5 via a sterically hindered disulfide linker to deglycosylated ricin-A. All patients in this trial were heavily pretreated with a mean of 5 (range, 2 to 8) different prior therapies, including autologous bone marrow transplantation in 8 of 15. The mean age was 29 years (range, 19 to 34 years). Thirteen of 15 patients had advanced disease (stage IV) with massive tumor burdens and 6 of 15 had B symptoms. The IT was administered intravenously over 4 hours on days 1, 3, 5, and 7 for total doses per cycle of 5, 10, 15, or 20 mg/m2. Patients received one to four cycles of treatment. The peak serum concentration of intact IT varied from 0.2 to 9.7 μg/mL. The serum half life (T1/2 ) of the IT ranged from 4.0 to 10.5 hours (mean, 6.1 hours). Side effects were related to vascular leak syndrome (VLS), ie, decreases in serum albumin, edema, weight gain, hypotension, tachycardia, myalgia, and weakness. Two patients had a National Cancer Institute (NCI) grade 2 allergic reaction with generalized urticaria and mild bronchospasm. At 15 mg/m2, 1 patient experienced a grade 3 myalgia. All 3 patients receiving 20 mg/m2 experienced NCI grade 3 toxicities (edema, nausea, dyspnea or tachycardia) and 1 patient had NCI grade 4 myalgia. Thus, the maximal tolerated dose was 15 mg/m2. Seven of 15 patients made human antiricin antibodies (≥1.0 μg/mL) and 6 of 15 developed human antimouse antibodies (≥1.0 μg/mL). Clinical response included 2 partial remissions, 1 minor response, 3 stable diseases, and 9 progressive diseases. As has been predicted from the preclinical tests, these data seem to indicate clinical effecicacy of this new IT in heavily pretreated Hodgkin's patients, thus warranting further clinical investigation.

Development of new ricin A-chain immunotoxins with potent anti-tumor effects against human Hodgkin cells in vitro and disseminated Hodgkin tumors in SCID mice using high-affinity monoclonal antibodies directed against the CD30 antigen

The lymphocyte activation marker CD30 has been shown to be an excellent target for the immunotherapy of human Hodgkin's lymphoma. In order to develop new potent immunotoxins (ITs) against CD30, we chemically linked 6 recently described monoclonal antibodies (MAbs) via SMPT to deglycosylated ricin A-chain (dgA). Cross-blocking experiments demonstrated that these MAbs, termed Ki-2 to Ki-7, recognize 3 different clusters on the CD30 antigen: Ki-2, Ki-4, Ki-5 and Ki-7 recognize cluster A; Ki-6 recognizes cluster B; Ki-3 binds to cluster C. Staining of 29 sections of normal human organs revealed no major cross-reactivity of any MAbs tested. Binding to the CD30 antigen on L540Cy Hodgkin cells was assessed by flow cytometry, and demonstrated high affinities for Ki-2, Ki-3 and Ki-4. The concentration giving 50% of the mean fluorescence intensity (MFI50) was 0.58 micrograms/ml to 0.78 micrograms/l. MAbs Ki-5, Ki-6, and Ki-7 bound much more weakly. The staining intensity of the MAbs correlated with the cytotoxicity of the corresponding ITs. Ki-2.dgA, ki-3.dgA and Ki-4.dgA inhibited the protein synthesis of L540Cy cells by 50% at concentrations (IC50) of 3.5 x 10(-10)M to 4.0 x 10(-11)M. The most effective IT, Ki-4dgA, is 5-fold more potent than previously reported CD30 ricin A-chain ITs. Ki-4.dgA was subsequently used for the treatment of disseminated human Hodgkin's lymphoma in a SCID mouse model. The mean survival time (MST) of lymphoma-bearing SCID mice was extended from 42 days in untreated controls to more than 132 days when Ki-4.dgA was applied one day after tumor challenge. Ki-4.dgA is a new potent IT suitable for further evaluation against Hodgkin's lymphoma in man.

Cocktails of ricin A-chain immunotoxins against different antigens on Hodgkin and Sternberg-Reed cells have superior anti-tumor effects against H-RS cells in vitro and solid Hodgkin tumors in mice

Three ricin A-chain immunotoxins (ITs) recognizing different antigens on Hodgkin-Reed/Sternberg (H-RS) cells were evaluated for their anti-tumor effects when used in combination as "cocktails". These ITs, BB10.dgA (CD25), HRS3.dgA (CD30), and IRac.dgA (70 kDa), strongly inhibited the growth of L540Cy H-RS cells in vitro. The protein synthesis of this cell line was reduced more efficiently by the combination of 2 of these ITs than by BB10.dgA, HRS3.dgA or IRac.dgA alone. A cocktail of all 3 ITs was most effective in vitro. This was at least in part due to the non-homogeneous distribution of CD25, CD30 or IRac on the L540Cy H-RS target cells and to the fact that subpopulations deficient in one antigen nevertheless expressed appreciable levels of the other target antigens. IT cocktails were also superior as anti-tumor agents in nude mice with solid L540Cy tumors. Ninety percent of mice treated with cocktails containing 2 or 3 ITs had continuous complete remissions (CCR), as compared with only 40% of mice treated with the same dose of a single IT. Analysis of 7 L540Cy sub-lines re-established ex vivo from mice that relapsed after having achieved complete remission (CR) after therapy with a single IT showed that the surviving tumor cells were antigen-deficient variants which were resistant to the original IT, but which could be killed by ITs directed against other target antigens. Thus, IT cocktails give superior results against human H-RS cells, both in vitro and in vivo.

Mutational analysis of the CD2/CD58 interaction: the binding site for CD58 lies on one face of the first domain of human CD2

The adhesion interaction between the immunoglobulin superfamily molecules CD2 and CD58 (lymphocyte function-associated antigen 3) plays an important role in T cell and natural killer cell interaction with various antigen-presenting and target cells. Determination of the solution structure of rat CD2 domain 1 has allowed a model of human CD2 domain 1 to be generated, and a series of mutants based on this model have been made. Residues of domain 1 of human CD2 predicted to be solvent exposed were substituted with the equivalent residues present in the rat CD2 molecule. The ability of these mutants to mediate rosetting with human and sheep erythrocytes was studied. Results show that the binding site of CD2 for both human and sheep CD58 maps to the beta sheet containing beta strands CC'C"F and G. Residues K34 and E36 in beta strand C, R48 and K49 in beta strand C', and K91 and N92 in the loop connecting beta strands F and G are shown to be critical in the interaction. The data support the proposition that the interaction between CD2 and CD58 involves the major beta sheet face of CD2.

A method for enriching myeloid (CFU-GM) and erythroid (BFU-E) progenitor cells from human cord blood by accessory cell depletion

Human cord blood provides a convenient alternative to bone marrow as a rich source of hemopoietic progenitor cells. This study reports a simple means for enriching a cord blood progenitor cell population by accessory cell depletion. Two methods of monocyte depletion were tested. A Cytodex 3 microcarrier system using collagen coated dextran beads was compared to the more commonly used method of plastic plate adhesion. The method of plastic plate adhesion gave a significantly higher cell recovery. T cell depletion using a recently characterized rat monoclonal antibody which fixes human complement was also investigated. A combined method of monocyte depletion by plate adhesion and T cell depletion resulted in the removal of > 96% of monocytes and > 98% of T cells. This led to a significant enrichment of myeloid (CFU-GM) and erythroid (BFU-E) colony growth. Such enriched progenitor cell populations provide a useful starting population for any study on hemopoiesis.

Blocked ricin-conjugated T cell immunotoxins: effect of anti-CD6-blocked ricin on normal T cell function

The biological properties of an immunotoxin composed of an anti-CD6 monoclonal antibody conjugated to whole ricin, which had been modified so that the galactose-binding sites of the B chain were blocked ("blocked ricin"), were examined. Treatment of peripheral blood lymphocytes with anti-CD6-blocked ricin for a 24-h period prevented T cell proliferation induced by phytohemagglutinin in a dose-dependent manner with concentrations causing 50% inhibition (IC50) ranging from 5 pM to 30 pM. In contrast, treatment with either blocked ricin alone or with a control immunotoxin prepared with a B-cell-lineage-restricted monoclonal antibody gave IC50 values of approximately 2 nM. Although shortening the duration of the anti-CD6-blocked ricin treatment to as little as 3 h had little significant effect on the observed inhibition, T cell viability experiments demonstrated that the magnitude of immunotoxin-induced killing after a given time period is significantly higher when the target cells become activated. Thus, from the initial concentration of cells treated with anti-CD6-blocked ricin placed in culture, 40%-45% viable cells remained after 2 days yet only 3%-9% remained if phorbol ester and Ca2+ ionophore were added; activation of T cells after mock treatment using blocked ricin plus nonconjugated anti-CD6 demonstrated that this effect was not the result of activation alone. The toxicity of anti-CD6-blocked ricin was also measured by inhibition of PHA-induced clonogenic growth of normal T cells. Continuous treatment of the cells using anti-CD6-blocked ricin at 0.1 nM resulted in a surviving fraction of about 3.5 x 10(-3); when immunotoxin treatment was for 24 h or less, the surviving fraction was only about 10(-1). As an indication of the unique specificity of anti-CD6-blocked ricin, immunotoxin pretreatment of potential responder cells prevented the generation of allogeneic cytolytic T lymphocytes in mixed lymphocyte cultures yet had little effect on the generation of interleukin-2-induced lymphokine-activated killer cell activity. We conclude that anti-CD6-blocked ricin demonstrates a cellular specificity and potency that make it a highly promising anti-T cell reagent.

Neuronotoxic effects of monoclonal anti-Thy 1 antibody (OX7) coupled to the ribosome inactivating protein, saporin, as studied by suicide transport experiments in the rat

As a first attempt to develop suicide transport agents based upon antineuronal antibodies, we studied an immunotoxin directed against the Thy 1 antigen which is on rat neurons. The immunotoxin was composed of mouse monoclonal anti-Thy 1 antibody (OX7) and the ribosome-inactivating protein, saporin, and was prepared using the heterobifunctional cross linker, SPDP, which provides a disulfide linkage between the two protein components. This immunotoxin reliably and selectively destroyed ipsilateral vagal motor and sensory neurons after injection into the cervical vagus. Injection of the immunotoxin into the caudate nucleus produced destruction of the ipsilateral substantia nigra, pars compacta and intralaminar thalamic nuclei (parafascicular and central median). Anti-mouse IgG immunoperoxidase staining confirmed axonal transport of OX7 by vagal sensory and motor neurons and by caudate afferents and efferents. Systemic toxicity was not observed with OX7-saporin. The neuronotoxic effects of OX7-saporin were specific since injections of a similarly constructed immunotoxin of irrelevant specificity or a mixture of OX7 and saporin were without suicide transport activity. These results show the feasibility of using immunotoxins as suicide transport agents.

Anti-Thy-1 immunotoxin, OX7-saporin, destroys cerebellar Purkinje cells after intraventricular injection in rats

Thy-1 is an abundant surface glycoprotein of rat neurons. OX7 is a monoclonal antibody with high affinity for Thy-1. This study sought to determine if intraventricularly administered OX7 could serve as a carrier to deliver cytotoxin to neurons, thus destroying those neurons. Saporin (Sap), a ribosome-inactivating protein was disulfide-coupled to OX7 (OX7-Sap). OX7-Sap, OX7, saporin alone, pooled non-immune mouse IgG, and an irrelevant immunotoxin, RFT-1-Sap, were injected into the lateral ventricles of anesthetized adult rats. Animals were observed for 1-8 days. OX7-Sap-injected animals developed coarse head tremor and gait/truncal ataxia in a dose-dependent manner beginning 24 h or more after injection. All control animals remained healthy. After OX7 or OX7-Sap injection, immunoperoxidase staining for mouse IgG was most intense and specific in the molecular and Purkinje cell layers of the cerebellar cortex. Cresyl violet staining demonstrated destruction of the Purkinje cell layer in the OX7-Sap-treated animals but not in controls. These results indicate that intraventricular injections of OX7 can be used to deliver biologically active moieties to the Purkinje cells. This approach may prove useful in analysis of Purkinje cell function and as a model of cerebellar degeneration.

A sandwich CD7-ELISA for detection of solubilized CD7 from normal and leukemic T cells

CD7 is a T differentiation antigen which is useful in the identification of precursor T cells as well as an important marker for the identification of leukemic T cells. It has proved to be useful as a target for immunotherapy by immunotoxins in several clinical settings. Most monoclonal antibodies to this antigen bind to the same or similar epitope. We have produced a monoclonal antibody, 69, which identifies a different epitope as that of the prototypic mAb to CD7, 3A1. Using mAB 69, we have devised a sandwich CD7-ELISA to detect solubilized CD7 antigen, with mAb 69 in the solid phase as the capturing mAb. This assay is sensitive and is able to detect antigen present on 2-5 x 10(4) activated T cells. This assay has been used to study the fate of CD7 on the membrane and in the cytosol of T cells during the process of mitogenesis. We have also utilized this assay to demonstrate the presence of free CD7 antigen in culture supernatant of activated T cells. This method will be useful to analyze antigen recovery from bulk cell cultures or from molecularly engineered microbial organisms. In addition, the sandwich CD7-ELISA may prove useful in monitoring the effects of immunotherapy in patients with leukemia.

Human T lymphocyte differentiation antigens as target for immunotoxins or complement-mediated cytotoxicity

Graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation (BMT) is initiated by immunocompetent T cells present in the graft. Selective elimination of distinct T-cell subsets or a sufficient, but not complete T-cell depletion, might abolish severe GVHD without graft rejection and loss of the anti-tumour potential. In this study we analysed the efficacy of different monoclonal antibodies (MoAb) WT32 (CD3), OKT4 (CD4), T101 (CD5), WT1 (CD7), and WT82 (CD8) with respect to their cytotoxicity to T cells either as immunotoxin (IT) or in combination with complement. The cytotoxic potential was assessed by protein synthesis inhibition and clonogenic assays. The ricin A conjugated MoAb exerted only a minor effect on blood or bone marrow T cells, although they were highly inhibitory to T-cell lines. However, in the presence of 20 mM ammonium chloride, IT directed against CD3, CD5, and CD7 were highly cytotoxic. IT directed against CD4 and CD8 were less effective, due to a low internalization. The complement-mediated cytotoxicity was efficient for all antigens used. The natural killer (NK) activity, as measured by cytotoxicity to K562, was hardly depressed by anti-CD3, anti-CD4, anti-CD5, and anti-CD8, but was eliminated by anti-CD7. All procedures used had only a minimal effect on haematopoietic progenitors as measured by CFU-GM and BFU-E assays. We concluded that, although the T-cell population can be eliminated with the combination of anti-CD3, anti-CD5, and anti-CD7 antibodies plus complement, IT with 20 mM NH4Cl appear to kill higher amounts of T cells. Selective elimination of CD4- and CD8-positive cells is effectively obtained by MoAb with complement.

Different susceptibilities of normal T cells and T cell lines to immunotoxins

In the context of ex vivo T cell elimination from bone marrow, the anti-T cell cytotoxic potential of immunotoxins (IT) prepared by conjugation of the monoclonal antibodies (MoAb) WT32 (CD3), T101 (CD5), and WT1 (CD7) to ricin A chain was evaluated. The cytotoxicity of IT was based on protein synthesis inhibition in human T cell lines: GH1, CEM, HPB-ALL, and Jurkat, and appeared closely related to the antigen density and internalization rate of and appeared closely related to the antigen density and internalization rate of the IT. Normal unstimulated T cells appeared to be rather insensitive to IT not due to a low antigen density or decreased internalization. The cytotoxicity of IT to T cells could be enhanced considerably by NH4Cl. Treatment of T cells with a cocktail of IT (10(-8) M) and 20 mM NH4Cl resulted in a 5000-fold cytoreduction as measured by clonogenic assays of limiting T cell dilutions, whereas the haematopoietic progenitor cells remained unaltered. Stimulation of T cells with phytohaemagglutinin (PHA) prior to incubation with IT considerably increased the sensitivity to IT treatment. Thus, normal T cells are less sensitive to anti-T cell IT than T cell lines and activated T cells. This suggests that a low protein synthesis is responsible for the resistance to IT. However, a high specific cytotoxicity of IT to normal T cells can be achieved in the presence of 20 mM ammonium chloride.