Genomic and functional changes induced by the activation of the peripheral cannabinoid receptor CB2 in the promyelocytic cells HL-60. Possible involvement of the CB2 receptor in cell differentiation

The function of the peripheral cannabinoid receptor (CB2), which is mainly expressed on hematopoietic cells, remains an enigma. In an attempt to decipher its role, we used Affymetrix DNA chips to investigate the gene expression profile of the promyelocytic cells HL-60 transfected with the CB2 receptor and activated with the cannabinoid agonist CP 55,940. Agonist exposure of these cells led to an activation of a mitogen-activated protein kinase cascade and a receptor desensitization, indicating a functional coupling of the transfected receptors. At the genomic level, activation of the CB2 receptors induced an up-regulation of nine genes involved in cytokine synthesis, regulation of transcription, and cell differentiation. A majority of them are under the control of the transcription factor NF-kappaB, whose nuclear translocation was demonstrated. Many features of the transcriptional events, reported here for the first time, appeared to be related to an activation of a cell differentiation program, suggesting that CB2 receptors could play a role in the initialization of cell maturation. Moreover, we showed that CB2-activated wild-type HL-60 cells developed properties usually found in host defense effector cells such as an enhanced release of chemotactic cytokines and an increased motility, characteristic of more mature cells of the granulocytic-monocytic lineage.

Stimulation of peripheral cannabinoid receptor CB2 induces MCP-1 and IL-8 gene expression in human promyelocytic cell line HL60

Using the recently developed methodology of nucleic acid microarrays spotted with specific cDNAs probes belonging to different gene families, we showed for the first time that nanomolar concentrations of the cannabinoid ligand CP-55940 upregulated the expression of two different members of the chemokine gene family: the alpha-chemokine interleukin-8 (IL-8) and the beta-chemokine monocyte chemotactic protein-1 (MCP-1), in the promyelocytic cell line HL60 transfected with peripheral cannabinoid receptors (CB2). These genomic modulations observed on large-scale cDNA arrays were first confirmed by Northern blot studies. Furthermore, ELISA evaluations in culture supernatants indicated that the cannabinoid-induced activation of these two chemokine genes was followed by enhanced expression and secretion of the corresponding proteins. These upregulations initially observed in transfected HL60 cells overexpressing CB2 receptors, also occurred in normal non-transfected HL60 cells. The enhancement of IL-8 and MCP-1 gene transcription and protein production was shown to be pertussis toxin sensitive attesting that this phenomenon was a Gi protein-coupled receptor-mediated process as expected for cannabinoid receptors. More specifically, the abolition of the cannabinoid-induced effect by the specific CB2 antagonist SR 144528 indicated a strict peripheral cannabinoid-mediated process. Altogether, our data highlight a possible new function of peripheral cannabinoid receptors in the modulation of immune and inflammatory responses.

Modulation and functional involvement of CB2 peripheral cannabinoid receptors during B-cell differentiation

Two subtypes of G-protein-coupled cannabinoid receptors have been identified to date: the CB1 central receptor subtype, which is mainly expressed in the brain, and the CB2 peripheral receptor subtype, which appears particularly abundant in the immune system. We investigated the expression of CB2 receptors in leukocytes using anti-CB2 receptor immunopurified polyclonal antibodies. We showed that peripheral blood and tonsillar B cells were the leukocyte subsets expressing the highest amount of CB2 receptor proteins. Dual-color confocal microscopy performed on tonsillar tissues showed a marked expression of CB2 receptors in mantle zones of secondary follicles, whereas germinal centers (GC) were weakly stained, suggesting a modulation of this receptor during the differentiation stages from virgin B lymphocytes to memory B cells. Indeed, we showed a clear downregulation of CB2 receptor expression during B-cell differentiation both at transcript and protein levels. The lowest expression was observed in GC proliferating centroblasts. Furthermore, we investigated the effect of the cannabinoid agonist CP55,940 on the CD40-mediated proliferation of both virgin and GC B-cell subsets. We found that CP55,940 enhanced the proliferation of both subsets and that this enhancement was blocked by the CB2 receptor antagonist SR 144528 but not by the CB1 receptor antagonist SR 141716. Finally, we observed that CB2 receptors were dramatically upregulated in both B-cell subsets during the first 24 hours of CD40-mediated activation. These data strongly support an involvement of CB2 receptors during B-cell differentiation.

The endogenous cannabinoid anandamide is a lipid messenger activating cell growth via a cannabinoid receptor-independent pathway in hematopoietic cell lines

The effect of anandamide, an endogenous ligand for central (CB1) and peripheral (CB2) cannabinoid receptors, was investigated on the growth of the murine IL-6-dependent lymphoid cell line B9 and the murine IL-3-dependent myeloblastic cell line FDC-P1. In conditions of low serum level, anandamide potentiated the growth of both cytokine-dependent cell lines. Comparison with other fatty acid cannabinoid ligands such as (R)-methanandamide, a ligand with improved selectivity for the CB1 receptor, or palmitylethanolamide, an endogenous ligand for the CB2 receptor, showed a very similar effect, suggesting that cell growth enhancement by anandamide or its analogs could be mediated through either receptor subtype. However, several lines of evidence indicated that this growth-promoting effect was cannabinoid receptor-independent. First, the potent synthetic cannabinoid agonist CP 55940, which displays high affinity for both receptors, was inactive in this model. Second, SR 141716A and SR 144528, which are potent and specific antagonists of CB1 and CB2 receptors respectively, were unable, alone or in combination, to block the anandamide-induced effect. Third, inactivation of both receptors by pretreatment of cells with pertussis toxin did not affect the potentiation of cell growth by anandamide. These data demonstrated that neither CB1 nor CB2 receptors were involved in the anandamide-induced effect. Moreover, using CB2-transfected Chinese hamster ovary cells, we demonstrated that after complete blockade of the receptors by the specific antagonist SR 144528, anandamide was still able to strongly stimulate a mitogen-activated protein (MAP) kinase activity, clearly indicating that the endogenous cannabinoid can transduce a mitogenic signal in the absence of available receptors. Finally, arachidonic acid, a structurally related compound and an important lipid messenger without known affinity for cannabinoid receptors, was shown to trigger MAP kinase activity and cell growth enhancement similar to those observed with anandamide. These findings provide clear evidence for a functional role of anandamide in activating a signal transduction pathway leading to cell activation and proliferation via a non-cannabinoid receptor-mediated process.

SR 144528, the first potent and selective antagonist of the CB2 cannabinoid receptor

Based on both binding and functional data, this study introduces SR 144528 as the first, highly potent, selective and orally active antagonist for the CB2 receptor. This compound which displays subnanomolar affinity (Ki = 0.6 nM) for both the rat spleen and cloned human CB2 receptors has a 700-fold lower affinity (Ki = 400 nM) for both the rat brain and cloned human CB1 receptors. Furthermore it shows no affinity for any of the more than 70 receptors, ion channels or enzymes investigated (IC50 > 10 microM). In vitro, SR 144528 antagonizes the inhibitory effects of the cannabinoid receptor agonist CP 55,940 on forskolin-stimulated adenylyl cyclase activity in cell lines permanently expressing the h CB2 receptor (EC50 = 10 nM) but not in cells expressing the h CB1 (no effect at 10 microM). Furthermore, SR 144528 is able to selectively block the mitogen-activated protein kinase activity induced by CP 55,940 in cell lines expressing h CB2 (IC50 = 39 nM) whereas in cells expressing h CB1 an IC50 value of more than 1 microM is found. In addition, SR 144528 is shown to antagonize the stimulating effects of CP 55,940 on human tonsillar B-cell activation evoked by cross-linking of surface Igs (IC50 = 20 nM). In vivo, after oral administration SR 144528 totally displaced the ex vivo [3H]-CP 55,940 binding to mouse spleen membranes (ED50 = 0.35 mg/kg) with a long duration of action. In contrast, after the oral route it does not interact with the cannabinoid receptor expressed in the mouse brain (CB1). It is expected that SR 144528 will provide a powerful tool to investigate the in vivo functions of the cannabinoid system in the immune response.

Effect of substance P on cytokine production by human astrocytic cells and blood mononuclear cells: characterization of novel tachykinin receptor antagonists

Substance P (SP) has been reported to induce inflammatory cytokine production in human neuroglial cells and peripheral lymphoid cells as well. In order to evaluate the potency of novel non-peptide antagonists of the tachykinin receptors as inhibitors of SP-induced cytokines, we used the astrocytoma cell line U373MG and blood mononuclear cells as models of central and peripheral SP-target cells, respectively. In the first part of this study, we showed that SR 140333, an NK1 tachykinin receptor antagonist, was able to inhibit strongly the SP-induced production of interleukin (IL)-6 and IL-8 in the astrocytoma cell line. The antagonistic activity of SR 140333 toward SP-induced cytokine production was specific and could not be attributed to a general anti-cytokine effect, since cytokine release induced by another inflammatory protein such as IL-1beta was not blocked by this compound. In addition, NK2 and NK3 agonist neuropeptides were at least 1000-fold less effective than SP, while SR 48968 and SR 142801 which are selective NK2 and NK3 receptor antagonists, respectively, displayed a 2.5-3 orders of magnitude lower inhibitory potency than SR 140333. All these data indicated that SR 140333 blocked SP-induced cytokine production in U373MG astrocytic cells via a specific NK1 receptor-mediated process. Since SP has also been described to trigger peripheral blood mononuclear cells (PBMNC) or monocytes to release inflammatory cytokines, we attempted, in the second part of this study, to evaluate the potential antagonistic effect of our compounds on these cells. Experiments on human PBMNC from different donors were carried out to determine first their pattern of cytokine production upon SP stimulation. Surprisingly, we noticed that SP at concentrations ranging from 0.1 to 1000 nM was unable to stimulate the release of any inflammatory cytokine tested. This raises the question of the specificity of the reported in vitro effects of SP on cytokine production by human peripheral immune cells.

A sigma ligand, SR 31747A, potently modulates Staphylococcal enterotoxin B-induced cytokine production in mice

Sigma receptors originally described in distinct regions of the central nervous system are expressed on cells of the immune system. A sigma ligand, SR 31747A, was observed here to inhibit in vitro the Staphylococcal enterotoxin B (SEB)-driven lymphocyte proliferation. In mice, the drug confers a potent protection against the lethality induced by SEB, stimulates the SEB-induced serum release of interleukin (IL)-10 and inhibits at the same time the systemic release of IL-2, IL-4, granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-6 and tumour necrosis factor-alpha (TNF-alpha). The enhancement of IL-10 production by this compound is also effective in nude mice treated with SEB, indicating that IL-10 of T-cell origin is not involved in this process. The finding that a sigma ligand protects against the SEB-induced toxicity provides insights into the clinical use of this family of compounds, particularly in food poisoning and septic shock where Staphylococcal enterotoxins are involved. The observation that this compound stimulates IL-10 synthesis indicates that it could be a potent regulatory agent of chronic inflammatory diseases.

Echinomycin suppresses the pyrogenic effects of endotoxin and interleukin-1 beta in human endothelial cells and peripheral blood mononuclear cells

Endotoxin (LPS) and interleukin-1 beta (IL-1 beta) increased in a dose-dependent manner the expression of tissue factor, an ubiquitous membrane-anchored glycoprotein that initiates blood coagulation at the surface of human umbilical vein endothelial cells (HUVEC and human peripheral blood mononuclear cells (PBMC). Echinomycin, a cyclic octapeptide of microbial origin strongly inhibited LPS- and IL-1 beta-induced tissue factor expression in HUVEC and PBMC with IC50 values in the subnanomolar range at the same time it reduced LPS and IL-1 beta-induced expression of intercellular adhesion molecule-1 (ICAM-1, CD54) on HUVEC (IC50 = 0.4 +/- 0.1 and 0.3 +/- 0.2 nM respectively). Echinomycin also reduced LPS-induced secretion of IL-1 beta and IL-6 by human PBMC (IC50 = 10 +/- 2 and 3 +/- 0.5 nM respectively). These observations demonstrate that echinomycin protects endothelial cells and PBMC from the pyrogenic effect of LPS and IL-1 beta.

Enhancement of endotoxin-induced interleukin-10 production by SR 31747A, a sigma ligand

SR 31747A is a new sigma ligand eliciting immunosuppressive and anti-inflammatory properties. Here, we show that SR 31747A greatly enhances lipopolysaccharide (LPS)-induced systemic release of interleukin (IL)-10, while it inhibits the secretion of tumor necrosis factor (TNF)-alpha and interferon (IFN)-gamma. In line with this finding, we also show by using quantitative reverse transcription-polymerase chain reaction analysis that SR 31747A increased LPS-induced IL-10 mRNA accumulation in spleen cells, whereas the level of both TNF-alpha and IFN-gamma mRNA was dramatically decreased. The enhancement of IL-10 production by SR 31747A treatment was also apparent in nude and severe-combined immunodeficient mice treated with LPS, clearly indicating that T and B cells were not involved. Finally, SR 31747A conferred protection against the lethal effect of LPS. The finding that SR 31747A strongly stimulates the synthesis of the natural anti-inflammatory cytokine IL-10, a property not observed with dexamethasone, provides new insights for the clinical use of this original compound, particularly in chronic inflammatory diseases where IL-10 is believed to be a pivotal regulatory component.

Cannabinoids enhance human B-cell growth at low nanomolar concentrations

This study examined the effect of cannabinoid ligands on human tonsillar B-cells activated either through cross-linking of surface immunoglobulins or ligation of the CD40 antigen. The two synthetic cannabinoids, CP55,940 and WIN55212-2, as well as delta 9-tetrahydrocannabinol (THC), the psychoactive component of marijuana, caused a dose-dependent increase of B-cell proliferation and displayed EC50 at low nanomolar concentrations. This cannabinoid-induced enhancing activity was inhibited by pertussis toxin which suggested a G-protein-coupled receptor process. In addition, the absence of antagonistic effect of SR141716A, a specific CB1 receptor antagonist, together with the demonstration that human B-cells displayed large amount of CB2 receptor mRNAs, led us to assume that the growth enhancing activity observed on B-cells at very low concentrations of cannabinoids could be mediated through the CB2 receptor.

In vivo inhibition of endotoxin-induced pro-inflammatory cytokines production by the sigma ligand SR 31747

In previous studies, the authors demonstrated that the new sigma ligand, cis-N-cyclohexyl-N-ethyl-3-(3-chloro-4-cyclohexyl-phenyl) propen-2-ylamine hydrochloride (SR 31747), elicited a suppressive effect on immune responses through the sigma receptor expressed on lymphocytes. Here the effect of SR 31747 on the proinflammatory cytokine production by endotoxin-activated macrophages is examined. In vivo, SR 31747 dramatically blocked lipopolysaccharide-induced production of interleukin (IL)-1, IL-6 and tumor necrosis factor-alpha in a dose-dependent manner (ED50, 2 mg/kg). Whereas SR 31747 suppression was not observed in vitro on lipopolysaccharide-induced IL-6 by macrophages, sera from SR 31747-treated animals displayed a strong inhibitory activity. It was shown that this effect could be completely reversed by the steroid receptor antagonist, mifepristone, which suggests that SR 31747 probably abrogated monokine production through an indirect mechanism that involves endogenous corticosteroids. This conclusion was supported by in vivo experiments that showed that 1) ablation of corticosteroids by use of mifepristone or adrenalectomy suppressed the effect of SR 31747 and 2) administration of SR 31747 induced an enhancement of the corticosterone level. It was also shown that this molecule improved the survival of animals with endotoxinic shock as a result of monokine inhibition. The combination of immunosuppression, previously described, along with anti-inflammatory properties makes SR 31747 a novel attractive molecule for therapeutic applications such as autoimmune diseases in which both immune and inflammatory disorders are involved.

Interleukin-13 stimulates interleukin-6 production by human keratinocytes. Similarity with interleukin-4

Interleukin-13 (IL-13) is a recently described human lymphokine which is produced by activated T-cells. Its effect on the production of IL-6 by normal keratinocytes and keratinocyte cell lines of human origin was studied and compared to that of IL-4. IL-13, similarly to IL-4, stimulated IL-6 expression by these cells in a dose- and time-dependent manner. Contamination with endotoxin was excluded by the use of polymyxin B and heat-inactivated cytokines. Further, we showed that IL-13, like IL-4, not only stimulated IL-6 production but also was able to induce overexpression of this cytokine in response to an inflammatory signal such as lipopolysaccharide (LPS). In a previous study, we demonstrated that IL-13, by inhibiting IL-6 and other cytokines produced by monocytes, exhibited an 'anti-inflammatory profile' comparable to that displayed by IL-4. In contrast, we show here that IL-13, by stimulating IL-6 production by keratinocytes, may favour the installation of an inflammatory process at a local level and, here again, it acted like IL-4. Therefore, according to the type of target cell these two 'TH2 type' cytokines induce similar opposing effects on IL-6 production and are likely to be important cytokines in the regulation of inflammation at both systemic and local levels.

Peripheral benzodiazepine receptor modulation with phagocyte differentiation

Peripheral benzodiazepine receptor (PBR) was found to be less expressed in the immature phagocytic HL-60 and U-937 cell lines than in the more mature monocytic THP-1 cell line. Cell differentiation by several agents induced a strong enhancement of PBR density on these three phagocytic cell lines but not on the lymphocytic CEM cell line. Detailed analysis of phorbol 12-myristate 13-acetate-treated THP-1 cells showed an increased PBR expression and the rise came along with an increase of CD11a and CD11b antigens and a secretion of macrophagic cytokines tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1 beta and IL-8. Quantitation of mRNA using polymerase chain reaction (PCR)-based technique showed that overexpression of PBR did not parallel mRNA expression, indicating a gene-independent regulation. These results suggest that PBR predominance on phagocytic cells could be related to maturation process.

Interleukin-13 is a new human lymphokine regulating inflammatory and immune responses

The discovery of new cytokines normally relies on a prior knowledge of at least one of their biological effects, which is used as a criterion either for the purification of the protein or for the isolation of the complementary DNA by expression cloning. However, the redundancy of cytokine activities complicates the discovery of novel cytokines in this way, and the pleiotropic nature of many cytokines means that the principal activities of a new cytokine may bear little relation to that used for its isolation. We have adopted an alternative approach which relies on differential screening of an organized subtracted cDNA library from activated peripheral blood mononuclear cells, using the inducibility of lymphokine messenger RNAs by anti-CD28 as a primary screening criterion. The ligation of the CD28 antigen on the T lymphocyte by a surface antigen, B7/BB-1, expressed on activated B lymphocytes and monocytes is a key step in the activation of T lymphocytes and the accumulation of lymphokine mRNAs. Here we report the discovery by molecular cloning of a new interleukin (interleukin-13 or IL-13) expressed in activated human T lymphocytes. Recombinant IL-13 protein inhibits inflammatory cytokine production induced by lipopolysaccharide in human peripheral blood monocytes. Moreover, it synergizes with IL-2 in regulating interferon-gamma synthesis in large granular lymphocytes. Recent mapping of the IL-13 gene shows that it is closely linked to the IL-4 gene on chromosome 5q 23-31 (ref. 4). Interleukin-13 may be critical in regulating inflammatory and immune responses.

Golgi vacuolization and immunotoxin enhancement by monensin and perhexiline depend on a serum protein. Implications for intracellular trafficking

Vacuole formation around the Golgi and immunotoxin enhancement induced by low doses of the ionophore monensin were inhibited by 50% human plasma (final concentration), whereas the lysosomal pH increase remained unaffected. Immunotoxin enhancement by the Ca2+ antagonist perhexiline was also inhibited by plasma. The inhibiting factor was present in different species and highly concentration-dependent. After purification on DEAE- and CM-Sepharose it showed a heterogeneous distribution between 45 and 50 kDa, in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, an extreme isoelectric point near 3.5, and binding to wheat germ agglutinin-Sepharose. Maximum inhibition was found in the lower molecular mass fraction of 45 kDa. The 50-kDa fraction, although showing immunological identity reactions, remained almost inactive. The simultaneous inhibition of morphological alterations and the enhancement of immunotoxin activity by the highly enriched protein provides a first direct link between both events. Apart from a role of this serum glycoprotein on in vivo inhibition of immunotoxin enhancement, its ability to maintain normal intracellular trafficking in the presence of blocking agents, such as monensin and perhexiline, suggests a more fundamental role in the regulation of these mechanisms.

Biologically active A-chain of the plant toxin ricin expressed from a synthetic gene in Escherichia coli

To assess the biological activity and pharmacokinetic properties of nonglycosylated ricin A-chain (RA), we have obtained the polypeptide following expression of a synthetic 842-bp RA gene in Escherichia coli. Expression of the gene was carried out using the phage T5 PN25 promoter fused to the E. coli lac operator. The RA polypeptide was synthesized in a completely soluble form and was purified in one step by immunoabsorption. It was shown to be as cytotoxic for a human cell line as both native RA and chemically deglycosylated native RA. Reconstituted whole ricin and an immunotoxin containing the recombinant RA were also biologically active. Immunotoxins made with recombinant and deglycosylated RA had similar clearance rates in vivo showing, after a short period of rapid elimination, stabilities far higher than that of an immunotoxin made with native RA. Our results show that the complete elimination of sugar side chains from the RA is not sufficient to entirely eradicate the rapid initial in vivo clearance of RA-based biologicals.

Specific elimination of alloreactive T cells by an anti-interleukin-2 receptor B chain-specific immunotoxin

Graft-versus-host disease and graft rejection remain the two principal causes of morbidity and mortality after major-histocompatibility-complex-mismatched bone marrow transplantation. Human and animal models suggest that both CD4+ and CD8+ T cell subsets present in the donor inoculum are responsible for their initiation. Since the human mixed lymphocyte culture (MLC) and the HLA-restricted cytotoxicity may reflect cellular interactions occurring during GVHD and graft rejection, inhibitions of these responses may represent useful approaches for screening functional T cell depletion in experimental bone marrow transplantation studies. For this purpose, we have tested the possibility of removing the host-specific allogeneic T cells present in the marrow. After a two-day MLC, the specifically activated host alloreactive blood or bone marrow T cells were incubated with the ricin A-chain toxin conjugated with the antibody 33B3.1 directed against the human receptor of interleukin 2 (33B3.1-IT). A complete inhibition of a primary MLC and of cytotoxic activities was observed as well as a disappearance of IL-2R(+) (p55) T cells. This method had limited consequence upon the alloreactivity of blood or marrow T cells toward a third unrelated party. The limiting-dilution analysis of residual alloantigen-reactive T lymphocytes has shown that this depletion results in a twentyfold to fiftyfold reduction of antihost reactivity. The procedure was also shown not to inhibit the growth of marrow precursors for granulocytes and macrophages.

Donor bone marrow treatment with T101 Fab fragment-ricin A-chain immunotoxin prevents graft-versus-host disease

Thirty-eight patients with haematological malignancies were treated with bone marrow transplantation using histocompatible immunotoxin T cell-depleted marrow siblings. All patients received conventional postgraft immunosuppression (methotrexate and/or cyclosporin A). Donor bone marrow was treated ex vivo with T101 Fab fragment coupled to ricin A-chain (T101 Fab-RTA) at a concentration of 10(-8) M of A-chain in association with NH4Cl (2 x 10(-2) M) in pH adjusted (7.8) incubation medium. A median cytoreduction of 99.5% (91-99.5) was obtained. The median of follow-up was 300 days. Only three patients developed grade II acute graft-versus-host disease (GVHD) (actuarial rate of acute GVHD: 9.1%). No chronic GVHD occurred. All patients but one engrafted. Six out of the 37 patients developed a documented bone marrow rejection (actuarial rate of graft failure: 18%). Ten patients relapsed (actuarial rate of relapse: 36.9%). These findings demonstrate that treatment of donor marrow with T101 Fab-RTA in association with NH4Cl at critical pH value can achieve a high level of mature T cell depletion and greatly reduce the incidence of bone marrow rejection and relapse after T cell-depleted allogeneic bone marrow transplantation.

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

The present study concerns the antibody-induced antigenic modulation of CD4, CD5, CD7, and 150-kDa antigens present on cells of the CCRF-CEM human T line. The immunogold electron microscopy method was used, and it was found that the entry routes associated with the various antigen-antibody complexes were different. Thus, the anti-CD7 monoclonal antibody (MoAb) was frequently internalized via the coated structures of the cell membrane, whereas anti-CD5 MoAb was rarely internalized via those structures and anti-CD4 and anti-150-kDa antigens never used this route. The delay required for 50% internalization of the labeled MoAb-receptor complexes was 30 min. 1 h, 2 h, and 9 h for anti-CD7, anti-CD5, anti-CD4, and anti-150-kDa antigen MoAbs, respectively. A shedding of complexes from the cell surface was never observed. The internalized labeled MoAbs were sequentially transferred into endocytic vacuoles, then into fine anastomosed tubulovesicular structures, and then into lysosomes. However, the anti-150-kDa antigen MoAb proceeded directly from endocytic vacuoles to lysosomes. Among the four MoAbs studied, anti-CD7 MoAb was the most abundant in the endosomal compartment (up to 34% of internalized particles) before it proceeded to the lysosomes. The overall valency of the anti-CD7 MoAb-labeled beads (from 3.8 to 14 MoAb molecules per bead) did not modify the intracellular routing. These results suggested that the subcellular fate of MoAbs was an intrinsic property of each MoAb-antigen complex. More importantly, the comparison between the MoAb-induced modulation and the cytotoxic level of the immunotoxin built with the same MoAb suggested that receptor-mediated endocytosis via coated pits, along with an abundant occurrence of the antigen-MoAb complex within the endosomal complex, could correspond to the best set of conditions for the transfer of the toxin moiety of the immunotoxin to the cytosol.

Effects of an anti HLA-DR immunotoxin on leukaemia cells and hematopoietic progenitors

An anti HLA-class II immunotoxin has been prepared by coupling to ricin A-chain (RTA) and anti-DR-DP monoclonal antibody (2G5-MoAb). Protein synthesis inhibition assays showed that 2G5-RTA immunotoxin is: highly cytotoxic to B-cell lymphoid neoplastic cells, and variable so to ALL cells, while AML cell lines display a generally poor susceptibility. Toxicity of 2G5-RTA on normal hematopoietic cells (HPC) was found to be dose-dependent, and to increase significantly with the addition of NH4Cl, used as an activating agent. After 4 h of incubation with 2G5-RTA (10(-8) M), without NH4Cl, percentages of CFU-GM and CFU-GEMM (colony forming units-granulo-monocytes and -multipotent, respectively) progenitor cell recovery were in the order of 50% and 30% respectively. In the same treatment conditions, 2G5-RTA induced a 6 log kill on RAJI cells--measured by clonogenic assay. Finally, this study shows that anti-DR immunotoxins may represent an original, efficient, and relatively safe approach for bone marrow purging of DR positive malignant B-cell populations; while their clinical potential pertaining to ALL and AML remains uncertain.

Sensitivity of fresh leukemic cells to T101 ricin A-chain immunotoxin: a comparative study between Fab fragment and whole Ig conjugates

We compared the cell killing potency of a whole Ig ricin A-chain immunotoxin (T101 IgG-RTA) against its Fab fragment counterpart (T101 Fab-RTA) on both CEM cells and fresh malignant lymphoid cells. A dye exclusion assay (DEA), was used to evaluate the kinetics of leukaemia cell viability mediated in vitro by each immunotoxin (IT). This study found that in the absence of ammonium chloride (NH4Cl), used as an enhancer agent, T101 Fab-RTA was significantly more toxic to both CEM and fresh leukaemia cells than T101 IgG-RTA. In the presence of NH4Cl (10(-2) M), while no differences could be found between the two IT on CEM cells, T101 Fab-RTA was clearly superior to T101 IgG-RTA on fresh leukaemia cells. These results suggest that T101 Fab-RTA may offer an excellent alternative to T101 IgG-RTA for IT treatment of CD5 positive leukaemia patients.

Comparison of the cytotoxic potency of T101 Fab, F(ab')2 and whole IgG immunotoxins

The in vitro killing of the human CEM cell line was studied by using ricin A-chain immunotoxins constructed with either the whole IgG or the Fab and F(ab')2 fragments of the same T101 (anti-CD5) antibody. In the presence of ammonium chloride as an activator, the "whole" immunotoxin as well as the "fragment" immunotoxins did not show any significant difference in the cell killing efficacy. In contrast, without the activator, the efficacy of the T101 immunotoxin was greatly improved when fragments were used. Indeed, at a saturating dose, a cytoreduction of three orders of magnitude was obtained with the fragment immunotoxins vs less than one order of magnitude for the whole immunotoxin, as assessed in a clonogenic assay. This enhancing effect was related to better cell killing kinetics, because with a similar amount of A-chain molecules bound per cell, T101 fragment immunotoxins achieved a twofold faster protein synthesis inactivation rate than the corresponding whole IgG immunotoxin. No significant difference in activity was shown between monovalent (Fab) and divalent (F(ab')2) forms of fragment immunotoxins. The observation that T101 fragment immunotoxins were more potent than intact immunotoxins was extended to another fragment immunotoxin constructed with an antibody (F111.98) directed against a different epitope of the CD5 Ag. In another model (anti-CD22 1G11 antibody on Raji cells), the fragment immunotoxin did not show any superiority over the IgG immunotoxin which was by itself very potent, strongly suggesting an Ag-dependent phenomenon.

Comparison of the cytotoxic potency of T101 Fab, F(ab')2 and whole IgG immunotoxins

The in vitro killing of the human CEM cell line was studied by using ricin A-chain immunotoxins constructed with either the whole IgG or the Fab and F(ab')2 fragments of the same T101 (anti-CD5) antibody. In the presence of ammonium chloride as an activator, the "whole" immunotoxin as well as the "fragment" immunotoxins did not show any significant difference in the cell killing efficacy. In contrast, without the activator, the efficacy of the T101 immunotoxin was greatly improved when fragments were used. Indeed, at a saturating dose, a cytoreduction of three orders of magnitude was obtained with the fragment immunotoxins vs less than one order of magnitude for the whole immunotoxin, as assessed in a clonogenic assay. This enhancing effect was related to better cell killing kinetics, because with a similar amount of A-chain molecules bound per cell, T101 fragment immunotoxins achieved a twofold faster protein synthesis inactivation rate than the corresponding whole IgG immunotoxin. No significant difference in activity was shown between monovalent (Fab) and divalent (F(ab')2) forms of fragment immunotoxins. The observation that T101 fragment immunotoxins were more potent than intact immunotoxins was extended to another fragment immunotoxin constructed with an antibody (F111.98) directed against a different epitope of the CD5 Ag. In another model (anti-CD22 1G11 antibody on Raji cells), the fragment immunotoxin did not show any superiority over the IgG immunotoxin which was by itself very potent, strongly suggesting an Ag-dependent phenomenon.

T-lymphocyte killing by T101-ricin A-chain immunotoxin: pH-dependent potentiation with lysosomotropic amines

To maximize T-lymphocyte killing with anti-pan-T-lymphocyte immunotoxin (IT), prepared by linking ricin A-chain to monoclonal antibody (MoAb) T101 (T101-RTA IT), we have established the nature and the extent of parameters that influence the sensitivity of T lymphocytes to the IT. We showed that peripheral blood T lymphocytes, which are much less susceptible than malignant T cells to the T101-RTA IT, could become highly sensitive to the IT when used in conjunction with NH4Cl. However, enhancement of the IT by NH4Cl only occurred when the pH rose above neutrality. This pH-sensitive process of IT activation by NH4Cl, which led to an all-or-nothing effect within an extremely narrow pH window of 0.7 pH unit width, is due to the fact that NH3 is the effective enhancing component of NH4Cl. We also showed that F(ab')2 or Fab containing IT were much more effective than those produced using the whole IgG counterpart. From these data, we defined a procedure for an optimal and specific elimination of T lymphocytes in vitro by treating them with (Fab)T101-RTA at 10(-8) mol/L at pH 7.8 in the presence of NH4Cl for two hours. This peripheral blood cell processing elicited an abrogation of three logs of functional T-cell response. Under the same conditions, there was no reduction in the number of marrow hematopoietic precursor granulocyte-macrophage colony-forming units (CFU-GM).

Rationale for the selection of ricin A-chain anti-T immunotoxins for mature T cell depletion

A series of 25 different ricin A-chain immunotoxins (IT) were prepared with monoclonal antibodies reacting with several T cell antigens belonging to different clusters of differentiation (CD) to select the most appropriate immunotoxins (IT) for mature T cell depletion. Our screening procedure was performed in 2 steps. First, IT were evaluated using protein synthesis inhibition assay on clonogenic malignant cells in order to determine the most active IT for a given CD. Second, IT thus selected were evaluated for both mature T cell killing efficacy and tolerance on hematopoietic progenitor cells (HPC). This study showed that (1) different IT directed against the same CD antigen displayed a wide range of activity, suggesting the need for a large screening of IT to determine the most appropriate antibody for a given target antigen; (2) anti-CD3 and anti-CD5 ricin A-chain IT are the most active, displaying a cytoreduction of more than 2 logs on mature T cells; (3) the 3 different anti-CD2 ricin A-chain IT evaluated in this study induced poor mature T cell cytoreduction despite their relative efficacy on leukemic cells; (4) anti-CD8 ricin A-chain IT showed almost no efficacy on relevant target cells; (5) no toxicity on HPC was found for concentrations up to 10(-8) of A-chain whatever the IT used.