Antigenic modulation induced by four monoclonal antibodies adsorbed on gold particles (specificity anti-CD4, anti-CD5, anti-CD7, and anti-150-kDa antigen): relationship between modulation and cytotoxic activity of immunotoxins

A deimmunized bispecific ligand-directed toxin that shows an impressive anti-pancreatic cancer effect in a systemic nude mouse orthotopic model

OBJECTIVE

The objective was to test a bispecific ligand-directed toxin (BLT), with reduced immunogenicity for enhanced efficacy in targeting orthotopic pancreatic cancer in vivo.

METHOD

A new BLT was created in which both human epidermal growth factor (EGF) and interleukin 4 cytokines were cloned onto the same single chain molecule with deimmunized pseudomonas exotoxin (dEGF4KDEL). Key amino acids dictating B-cell generation of neutralizing antitoxin antibodies were mutated. Bioassays were used to determine whether mutation reduced potency, and enzyme-linked immunosorbent assay studies were performed to determine whether antitoxin antibodies were reduced. A genetically altered luciferase MIA PaCa-2 xenograft model was used to image in real time and determine effects on systemic malignant human cancer. Bispecific ligand-directed toxins targeting B cells were used as specificity controls.

RESULTS

Deimmunized EGF4KDEL was significantly effective after systemic injection against established orthotopic MIA PaCa-2 pancreatic cancer and selectively prevented metastasis. Mutagenesis significantly reduced antitoxin levels in vivo with no apparent activity loss in vitro. The drug was effective against 3 human pancreatic cancer lines in vitro, MIA PaCa-2, SW1990, and S2VP10.

CONCLUSIONS

Despite the metastatic nature of the MIA PaCa-2 orthotopic tumor xenografted in nude mice, high percentages of tumors responded to extended dEGFKDEL treatment resulting in significant anticancer effects and disease-free survivors.

Cytotoxicity of human RNase-based immunotoxins requires cytosolic access and resistance to ribonuclease inhibition

Immunotoxins are targeted therapeutics designed to kill cancer cells. The targeting moiety of an immunotoxin selectively binds to a tumor cell and targets it for death via an attached toxin. Because the toxins are typically of plant or bacterial origin, their clinical use is limited by immunogenicity and nonspecific toxicity. To circumvent these problems, we have begun to engineer immunotoxins containing human pancreatic ribonuclease. Here we describe the generation of ribonuclease mutants designed to evade a ubiquitous cytosolic inhibitor that would otherwise block cytotoxicity. Two mutants retained catalytic activity and were relatively resistant to the inhibitor. To deliver them to human T leukemic cells, these ribonuclease variants were fused to a single chain Fv fragment specific for CD 7. The ribonuclease-sFv fusion proteins bound CD 7(+) T cells and were internalized yet were not cytotoxic. Transfection of the proteins directly into the cytosol reduced cell viability, suggesting that the failure of the immunotoxins to kill cells when added externally resulted from the inability of the ribonuclease moiety to access the cytosol efficiently. Our results indicate appropriate intracellular routing, as well as resistance to inhibition, is critical to the cytotoxicity of human ribonuclease-based immunotoxins.

Antibody microarray for correlating cell phenotype with surface marker

To correlate cell surface markers with the cell phenotype, an antibody microarray prepared by covalently immobilizing antibodies onto a cellulose membrane and subsequent immunocytochemical staining were employed. The direct binding assay of a lymphoblastic leukemia cell line on the microarray showed that the immobilized antibody served to capture cells expressing the specific antigen. The density of bound cells increased linearly with an increasing content of antigen-expressing cells in suspension. The method was further applied to the analysis of surface antigens expressed on neural stem cells. A binding assay was performed with neural cells obtained from the neurosphere culture of the rat fetal striatum on a microarray spotted with eight kinds of antibodies and four different proteins, followed by immunocytochemical staining of cells bound to the microarray using antibodies to the intracellular markers of immature (nestin and vimentin) and mature (beta-tubulin III and glial fibrillary acidic protein) neural cells. As a result, the phenotype of bound cells could be correlated to surface antigen expression, which illustrated the potential of the solid-phase cytometry developed here for the identification of surface markers.

In vitro internalization, intracellular transport, and clearance of an anti-CD11a antibody (Raptiva) by human T-cells

Efalizumab (Raptiva) is a humanized CD11a-specific monoclonal antibody that was recently approved for the treatment of moderate to severe psoriasis. In psoriasis patients, the rate of efalizumab clearance from serum is related to T-cell surface expression of CD11a, suggesting a receptor-mediated clearance model for efalizumab (Bauer et al., 1999). However, limited experimental data are available to explain how the interaction with CD11a results in the systemic clearance of efalizumab. The following studies were designed to test the hypothesis that one mechanism of anti-CD11a antibody clearance is mediated in part by cellular internalization. This was tested in vitro using purified mouse and human T-cells as a model to study the cellular uptake and clearance of anti-CD11a antibodies. Data from these studies suggest that anti-CD11a antibodies are internalized by purified T-cells. Upon internalization, the antibodies appeared to be targeted to lysosomes and were cleared from within the cells in a time-dependent manner. CD11a-mediated internalization and lysosomal targeting of efalizumab may constitute one pathway by which this antibody is cleared in vivo.

Phagocytosis of immunobeads by CD8 positive lymphocytes during magnetic cell sorting

Lymphocytes of human blood were isolated by Ficoll gradients and separated into a CD8 positive fraction using the "Immunobead Technique". Electron microscopic examination approximately 45 min after mixing with the beads at room temperature revealed adhering beads in all cells of the CD8 positive fraction, whereas the negative fraction did not exhibit any adhering beads. About 2% of the lymphocytes of the positive fraction additionally showed bead incorporations. These cells exhibited a characteristic lymphocytic morphology and must be considered T lymphocytes. Formation of processes and ridges in the vicinity of the adhering beads points to a phagocytosis-like process. Since incorporations of beads can be demonstrated only in the CD8 positive fraction, this type of phagocytosis must be a specific; i. e. receptor-mediated phagocytotic process. The capability to perform phagocytosis by CD8 positive lymphocytes at room temperature should be considered during separation and when evaluating the behaviour of cells after magnetic separation, e. g. when attempting to remove the beads again. An exact experimental temperature (4 degrees C) should therefore be maintained.

Immunoelectron microscopic investigations of patching, capping, endocytotic and shedding processes in T and B lymphocytes

Lymphocytes were isolated from the blood of healthy juvenile test persons by the FICOLL method. Subsequently, CD4-, CD8-, and CD19-positive cells were obtained by the use of magnetic beads. The sandwich technique and gold labelling method (preembedding) served for the demonstration of receptors in the electron microscope. The gold-labelled receptors were primarily endocytosed via smooth-walled micropinocytotic processes, less frequently by coated pits/vesicles. The endocytosis cycle lasted only as far as the level of multivesiculated bodies. Lysosomes and structures of the Golgi apparatus were free from gold particles. It was surprising that after activation the capping phenomena were not associated with increased endocytotic activity. The inner surface of the membrane of endocytotic vesicles underneath the cap does not explain modulation or turnover of the receptor under these conditions, not even in view of a fast endocytotic cycle. Another possibility of a membrane turnover is the "shedding" process. We were indeed able to demonstrate gold labelling and surface coat-like material in the extracellular matrix.

Biological activity of soluble CD100. I. The extracellular region of CD100 is released from the surface of T lymphocytes by regulated proteolysis

CD100 is the first semaphorin described in lymphoid tissues, where it has been shown to be associated with a serine kinase activity. Semaphorins are molecules involved in axon pathfinding during nerve development and act as repellent guidance cues. In the nervous system semaphorins exist as either membrane-bound or secreted forms. We report here a spontaneous processing of membrane CD100, suggesting that it is also produced as a diffusable semaphorin from lymphoid cells. Monomeric and homodimeric forms of CD100 are expressed by T lymphocytes and CD100-transfected fibroblasts. We demonstrate that CD100 is released through a proteolytic process blocked by metalloprotease inhibitors. In T cells, only soluble CD100 dimers are produced, suggesting that CD100 dimerization is required for proteolysis. In agreement, we observe that increasing membrane dimers strongly favors shedding of the molecule. By expressing a CD100 molecule mutated at cysteine 674 into a COS cell system, we additionally demonstrate that this particular residue in the extracellular domain of the molecule is required for dimerization. Finally, we show that staurosporine, a serine kinase inhibitor, enhances the membrane cleavage of CD100. Together these results demonstrate that membrane CD100 is cleaved by a metalloprotease-dependent process, which is probably regulated by phosphorylation. Mainly, these findings shed light on a possible function for the semaphorin region of CD100 as a long range guidance cue in the immune system.

The leukocyte semaphorin CD100 is expressed in most T-cell, but few B-cell, non-Hodgkin's lymphomas

The 150-kd transmembrane protein CD100 is the first semaphorin protein shown to be expressed in lymphoid tissue. CD100 is present in the interfollicular T cell zones and is also expressed by B cells in the germinal centers of secondary lymphoid follicles, but not in the mantle zones. The CD100 molecule was recently cloned, and CD100 transfectants were shown to induce homotypic aggregation of human B cells and improve their viability in vitro, suggesting that CD100 may play a role in lymphocyte aggregation and germinal center formation. We studied the expression of CD100 in 138 clinical cases representing a range of lymphoproliferative disorders, to determine whether this molecule is expressed in these neoplastic processes. In general, we found CD100 expression to be common in peripheral T-cell non-Hodgkin's lymphomas but rare in B-cell non-Hodgkin's lymphomas. CD100 expression was not detectable in low-grade B-cell non-Hodgkin's lymphomas, including cases of small lymphocytic lymphoma (18 cases), marginal zone lymphoma (10 cases), and mantle cell lymphoma (10 cases), as might be expected for these neoplasms that are not of follicular center cell origin. Surprisingly, we found that the vast majority of follicular lymphomas (37 of 40 cases) as well as diffuse large-cell lymphomas of B-cell type (35 cases) did not express CD100. The neoplastic cells in 3 of 11 cases of predominantly large-cell-type follicular lymphoma did express CD100. In contrast, all five cases of high-grade, small non-cleaved (Burkitt-like) B-cell lymphoma were immunoreactive for CD100 expression, as were 18 of 20 cases (90%) of malignant T cell neoplasms. Northern blot analysis of CD100 expression correlated with immunohistochemical findings. Absence of expression of CD100 by neoplastic follicular center B cells is a common feature in follicular lymphomas, but expression of CD100 by T cells is maintained in T-cell lymphoproliferative disorders.

Human CD100, a novel leukocyte semaphorin that promotes B-cell aggregation and differentiation

Herein we describe the molecular characterization of the human leukocyte activation antigen CD100 and identify it as the first semaphorin, to our knowledge, in the immune system. Semaphorins have recently been described as neuronal chemorepellants that direct pioneering neurons during nervous system development. In this study we demonstrate that CD100 induces B cells to aggregate and improves their viability in vitro. We show that CD100 modifies CD40-CD40L B-cell signaling by augmenting B-cell aggregation and survival and down-regulating CD23 expression. Thus, these results suggest that semaphorins as exemplified by CD100 also play a functional role in the immune system.

Different cytotoxic activity and intracellular fate of an anti-CD5-momordin immunotoxin in normal compared to tumour cells

We investigated the different sensitivity of peripheral blood mononuclear cells (PBMC) and human T cell leukaemias (Jurkat and CEM) to an anti-CD5-momordin immunotoxin. In a short-term assay, the immunotoxin displayed different cytotoxic activity on normal and tumour cells: for leukaemic cell lines an incubation time of 72 h was necessary for the immunotoxin to reach the IC50 of 41-53 pM, compared to the 1 h sufficient for 6 pM immunotoxin to inhibit 50% of PBMC protein synthesis. In a long-term clonogenic assay (15 days), the immunotoxin demonstrated a comparable efficacy of clonogenic cell killing for both cell types. We investigated the immunotoxin internalization pathway by a flow-cytometric method and our data seem to indicate that the molecules meet a different intracellular fate in the two cell populations. It may be assumed that the low cytotoxic activity of immunotoxins on tumour cells, detected in the short-term assay, is due to inefficient delivery to their cytoplasmatic target, while a longer exposure of the cells to the immunotoxin promotes adequate intracellular distribution.

Internalization and degradation of anti-CD4 monoclonal antibodies bound to human peripheral blood lymphocytes

Treatment of patients with anti-CD4 mAbs induces both functional alterations of CD4+ cells and depletion of circulating CD4+ lymphocytes. Some of these effects depend on the amount of mAb molecules bound per CD4+ cell and on the properties of the Fc part of the mAb (isotype specificity). We have investigated the fate of anti-CD4 monoclonal antibodies (mAbs) after their interaction with CD4 protein on the surface of peripheral blood lymphocytes (PBL). We used seven anti-CD4 mAbs whose epitope specificity, equilibrium constant and kinetics of binding are reported. Lymphocytes were saturated with anti-CD4 mAbs either at +4 degrees C or 37 degrees C then washed and incubated in antibody-free medium. At different time intervals cells were processed for analysis. By indirect immunofluorescence, it was shown that the amount of surface-bound mAb decreased rapidly when cells were incubated at 37 degrees C, but not at 4 degrees C. With 125I-mAbs, we demonstrate that there was a rapid internalization of the molecules followed by the re-expression on the cell surface of a part of initially bound mAbs and by the release of partially degraded antibody in the cell supernatant. In the presence of sodium azide (10 mM) only a slow dissociation of intact antibody occurred, without internalization. The radioactive material eluted in the 100-200 kDa zone from supernatants was only partly adsorbed on protein A and hardly on CD4+ cells, indicating that alterations of the Fc region and loss of antigen binding activity, possibly by formation of CD4-anti-CD4 complexes, had occurred during the process of internalization and release into the extracellular medium. These data may be important to consider for adjusting the dosage of anti-CD4 mAbs to be administered.

Inhibition of human immunodeficiency virus infection by the lectin jacalin and by a derived peptide showing a sequence similarity with gp120

Jacalin is a plant lectin known to specifically induce the proliferation of CD4+ T lymphocytes in human. We demonstrate here that jacalin completely blocks human immunodeficiency virus type 1 (HIV-1) in vitro infection of lymphoid cells. Jacalin does not bind the viral envelope glycoprotein gp120. Besides other T cell surface molecules, it interacts with CD4, the high-affinity receptor to HIV. Binding of jacalin to CD4 does not prevent gp120-CD4 interaction and does not inhibit virus binding and syncytia formation. The anti-HIV effect of the native lectin can be reproduced by its separated alpha-subunits. More importantly, we have defined in the alpha-chain of jacalin a 14-amino acid sequence which shows high similarities with a peptide of the second conserved domain of gp120. A synthetic peptide corresponding to this similar stretch also exerts a potent anti-HIV effect. This peptide is not mitogenic for peripheral blood mononuclear cells and does not inhibit anti-CD3-induced lymphocyte proliferation. These results make jacalin alpha chain-derived peptide a potentially valuable therapeutic agent for acquired immunodeficiency syndrome.

Down-regulation of cell surface CD4 molecule expression induced by anti-CD4 antibodies in human T lymphocytes

Antigenic modulation was defined as the down-regulation of a cell surface antigen expression induced by exposure to specific antibody. We investigated the modulation of CD4 surface expression in human peripheral blood lymphocytes incubated in vitro with anti-CD4 monoclonal antibodies (mAbs). Modulation of surface CD4 was achieved at 37 degrees C, but not at 4 degrees C, with five different murine anti-CD4 mAbs of IgG1 and IgG2a subclasses, with different epitope specificities. Modulation was dose dependent with a maximum at nonsaturating mAb concentration. It was reversible upon culture in mAb-free medium. It was accelerated and amplified in the presence of monocytes or after cross-linking of anti-CD4 mAbs. It could be induced with solid phase anti-CD4 mAbs, but not with soluble F(ab')2 fragments. Its magnitude was identical on all CD4+ lymphocytes. It was associated with a moderate down-regulation of CD2 and CD3 but not of CD8 and HLA class I surface expression. Modulation was slightly augmented by addition of inhibitors of the endosome/lysosome pathway but not by protein synthesis inhibitors. The anti-CD4 mAb initially bound to cell surface was no longer detectable after 24 hr of culture. Most of surface CD4 proteins complexed with antibody were rapidly internalized and transiently replaced by CD4 from an intracytoplasmic pool and then no longer were expressed. CD4 mRNA was moderately decreased in cells incubated with anti-CD4 mAb while beta-actin and beta 2-microglobulin mRNAs remained at stable levels. It was concluded that down-regulation of CD4 surface expression induced by anti-CD4 mAb concerned only a part of CD4 molecules and was associated with a decreased synthesis. The delay required to achieve maximal modulation is likely to reflect exhaustion of the intracytoplasmic recycling pool of CD4 molecules.

Down-regulation of lymphocyte CD4 antigen expression by administration of anti-CD4 monoclonal antibody

Modulation of surface CD4 antigen expression was assessed by flow cytometry after calibration with 125I-labeled anti-CD4 monoclonal antibodies (mAbs). Three patients with severe psoriasis treated with BB14 (anti-CD4 mouse IgG1) and five patients with rheumatoid arthritis treated with BL4 (anti-CD4 mouse IgG2a) were analyzed for sequential changes in surface CD4 expression on CD4+ blood lymphocytes. Anti-CD4 mAb treatment induced a decrease of 50 to 80% of CD4 expression, with slow and partial recovery after cessation of mAb administration. CD4 modulation was related to mAb dosage and mAb concentration in plasma. It was achieved at nonsaturating concentration. In vitro incubation of blood mononuclear cells induced CD4 modulation of similar kinetics and magnitude, associated with decrease of 5-10% of CD3 expression. CD4 modulation required both an intact Fc part of the antibody and the presence of monocytes. The possible role of CD4 modulation should be considered along with other functional activities of anti-CD4 mAbs in analyzing the mechanisms of the clinical effects of these antibodies.

Effectiveness of combinations of bispecific antibodies for delivering saporin to human acute T-cell lymphoblastic leukaemia cell lines via CD7 and CD38 as cellular target molecules

We have investigated the effectiveness of three different F(ab' gamma)2 bispecific antibodies (BsAb) for delivering the ribosome inactivating protein (RIP) saporin via the CD7 or CD38 cell surface molecules to the human T-ALL cell lines HSB-2 and HPB-ALL. Inhibition of 3H-leucine uptake by target cells was used as the parameter of cellular cytotoxicity. Used singly against HSB-2 cells in the presence of varied concentrations of saporin, an anti-CD7 BsAb, (HB2 x DB7-18) and an anti-CD38 BsAb (OKT10 x RabSap), gave 435- and 286-fold increases in saporin toxicity, respectively. For HPB-ALL cells the anti-CD7 BsAb performed poorly giving only an eight-fold increase in toxicity whilst on the same cell line the anti-CD38 BsAb was highly potent giving an 80,000-fold increase in saporin toxicity. A combination of both BsAb used together against HSB-2 cells was ten times more effective, than the best single BsAb HB2 x DB7-18 used alone. Kinetic studies conducted with HSB-2 cells revealed that the BsAb combination also gave an increased rate of protein synthesis inactivation in comparison to either BsAb used alone. These investigations clearly demonstrate a synergistic action when both BsAb are used in combination to target saporin against CD7 and CD38 expressed on the surface of the HSB-2 cell line.

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.

Direct involvement of CD7 (gp40) in activation of TcR gamma/delta+ T cells

In this study we reported that on T cell receptor (TcR) gamma/delta+ cells from three cell lines Peer, MOLT-13 and ICRF-1, the T cell antigen CD7 (gp40) can be directly involved in the activation process. This is shown by a rapid increase in cytoplasmic free calcium after stimulation of these cells with an anti-CD7 monoclonal antibody (mAb). Activation through CD7 was further confirmed by measuring the production of interleukin 2 in ICRF-1 cells stimulated with anti-CD7 mAb. In addition induction of mRNA for tumor necrosis factor (TNF)-alpha and TNF-beta in Peer and for granulocyte-macrophage-colony-stimulating factor in MOLT-13 was observed in these anti-CD7-stimulated cells. The same anti-CD7 antibody was unable to activate TcR alpha/beta+ Jurkat cells or normal resting peripheral blood T lymphocytes. We further showed that normal resting TcR gamma/delta+ cells were likewise activated via the CD7 molecule. TcR gamma/delta+ cells obtained from a patient with acute lymphoblastic leukemia 3 months after autologous bone marrow transplantation were induced to proliferate, as measured by [3H]thymidine incorporation after stimulation with anti-CD7 mAb but not with anti-CD3 mAb. Interestingly TcR alpha/beta+ cells from the same donor tested in parallel were not stimulated by anti-CD7 but by anti-CD3 mAb. In essence these findings contribute to the idea that on TcR gamma/delta+ cell, the CD7 antigen could play an important role during T cell differentiation.

CD4 and CD7 molecules as targets for drug delivery from antibody bearing liposomes

We have examined two T lymphocyte cell surface molecules, CD4 and CD7, as targets for specific delivery of drugs from antibody-directed liposomes. The efficiency of uptake by peripheral lymphocytes, thymocytes, and two CEM sublines (CEM.MRS and CEM-T4) of anti-CD4 and anti-CD7 liposomes containing methotrexate was evaluated by the methotrexate-mediated inhibition of the incorporation of d-[3H]Urd into DNA. This was compared with similar liposomes targeted to MHC-encoded HLA class I molecules, which are known to be efficiently taken up by T cells. Despite the lower expression of CD7 molecules relative to HLA class I on most cell lines, CD7 was shown to be a good target for drug delivery. The results of an internalization study using radiolabeled Protein A showed that a higher proportion of CD7 molecules was internalized than HLA class I molecules. CD4-targeted liposomes, in contrast, were relatively ineffective for drug delivery for lymphoid cells, and only partially inhibited CEM-T4 cells. The lack of toxicity correlated with poor internalization of the target molecule on most cell lines. The drug effect of anti-CD4 liposomes was more pronounced on HeLa-T4, which is an epithelial cell line transfected with the CD4 gene. In contrast to lymphoid cells, these cells efficiently internalized CD4 molecules. PMA is known to down-regulate surface expression of CD4 molecules on various T cells. Internalization of CD4 was induced by PMA, but PMA failed to induce cytotoxicity of CD4-targeted liposomes for CEM.MRS. The internalized drug was probably degraded rapidly because internalized anti-CD4 antibody-bound Protein A was degraded very rapidly.

Immunotoxins

Immunotoxins (ITs) are chimeric molecules constructed by covalently conjugating monoclonal antibodies (MoAbs) to plant or bacterial toxins (e.g. ricin or pseudomonas exotoxin). The antibody moiety allows specific targeting of ITs to tumor-associated antigens, while the toxin moiety is responsible for cell killing by irreversible inactivation of protein synthesis. Since ITs must reach the cytosol to kill cells, the rates of endocytosis, the pathways of intracellular routing, and the rates of translocation to the cytoplasm are important determinants of the efficacy of an IT. Promising in vitro and in vivo IT results have been reported by many groups, and phase I clinical trials in cancer patients are currently underway.

Isolation and preliminary characterization of the major membrane boundaries of the endocytic pathway in lymphocytes

Plasma membrane, coated pits, endosomes, and lysosomes were isolated from a mouse T lymphoma cell line using a density shift protocol in which these compartments were selectively loaded with gold conjugates. The plasma membrane was prepared after selective labeling for 1 h at 2 degrees C with gold-ricin and gave a yield of 40% according to enzymatic and antigenic markers. Endosomes were obtained by loading the cells for 2 h at 22 degrees C with gold complexed to an antimouse transferrin receptor mAb. Coated pits were isolated using a similar procedure, but after an incubation at 10 degrees C, which allowed deep invagination of the pits but prevented internalization. The yield (calculated using the recovery of [125I]transferrin) was 32% for endosomes and 10% for coated pits. Finally lysosomes were prepared by loading the cells for 18 h at 37 degrees C with gold low density lipoproteins (LDLs) followed by a 3-h chase at 37 degrees C with LDL alone. The final lysosome yield (based on the recovery of lysosomal enzymes) was 16%. Studies of the protein composition of these cellular compartments on two-dimensional gels showed that while some major proteins are present throughout the pathway, specific proteins can be identified in each of the isolated fractions. The greatest change in the pattern of protein constituents seen along the pathway was between endosomal and lysosomal preparations.

Internalized ricin and the plasma membrane glycoprotein MAM-6 colocalize in the trans-Golgi network of T47D human breast carcinoma cells

We have analyzed the intracellular transport of endocytosed ricin in the human breast carcinoma cell line T47D. Cells were incubated with ricin (10 micrograms/ml) for 1 h at 37 degrees C. Marked reduction in the protein synthesis did not take place until the end of this period. To detect ricin immunocytochemically, a rabbit anti-ricin serum was used. Gel electrophoresis followed by immunoblotting revealed that the antiserum reacted specifically with ricin and detected both the ricin A-chain and the ricin B-chain. Immunofluorescence experiments showed endocytosed ricin in endosomal and lysosomal vacuoles throughout the cytoplasm, as well as in a typical perinuclear position corresponding to the Golgi region. Using the monoclonal mouse antibody 115D8 directed toward the high-molecular-weight membrane glycoprotein MAM-6 of human breast epithelial cells, we similarly obtained a swarms of small vesicles throughout the cytoplasm. To further analyze the apparent colocalization of ricin and MAM-6 in the perinuclear Golgi region, immunogold cytochemistry on ultracryosections was performed. MAM-6 was detected mainly in Golgi stacks and associated trans-Golgi network (TGN) profiles, in 0.1 to 0.2-micron secretory vesicles, and on the cell surface. Ricin was detected on the cell surface, in endosomes and lysosomes, and also in the TGN. Furthermore, by using immunogold double labeling, internalized ricin was found to colocalize with MAM-6 in the TGN.