Dynamics of toxin and lectin receptors on a lymphoma cell line and its toxin-resistant variant using ferritin-conjugated, 125I-labeled ligand

The dynamics of the toxin Ricinus communis agglutinin II (RCAII or ricin) on cells of a murine lymphoma line (BW5147) and a toxin-resistant variant line (BW5147RicR.3) that is 200 times more resistant than the parent to direct RCAII cytotoxicity were examined using ferritin-conjugated, affinity purified, 125I-labeled RCAII (ferritin-125I-RCAII). Ferritin-125I-RCAII was indistinguishable from native RCAII in quantitative binding and cytotoxicity experiments. When RCAII-sensitive BW5147 and -resistant BW5147RicR.3 cells were labeled with ferritin-125I-RCAII at various toxin concentrations (1--10 microgram/ml), no differences in toxin binding were observed. These same cells were examined by electron microscopy. At low ferritin-125I-RCAII concentrations (1-3 microgram/ml RCAII) where only the parental BW5147 cells were significantly more sensitive to RCAII, toxin receptors were internalized by ferritin-125I-RCAII-induced endocytosis. In parallel experiments, ferritin-125I-RCAII that bound to the resistant BW5147RicR.3 cells remained relatively dispersed or clustered, and there was little evidence of transport into cells via endocytosis. At higher ferritin-125I-RCAII concentrations (greater than 7 microgram/ml RCAII) where both parental and resistant variant cells are sensitive to the cytotoxic effects of RCAII, more ferritin-conjugated toxin was bound, and subsequent endocytosis occurred to a similar degree in both cell types. Endocytosis of ferritin-conjugated concanavalin A was indistinguishable on RCAII-sensitive parental and resistant variant cells at all concentrations tested. The results suggest that a specific defect on the selected BW5147RicR.3 cells prevents RCAII entry into these cells a low toxin concentrations, rendering them more resistant to the cytotoxic effects of RCAII.     

A novel ganglioside expressed by mouse hematopoietic cell lines.

Mouse progenitor T cell-derived cell lines were established by fusion of cells of hematopoietic organs such as bone marrow and fetal liver with T lymphoma (BW5147) to determine their characteristic cell-surface components. The hybridomas with the phenotype of Thy-1+, CD3-, CD4-, CD8- and expression of T cell receptor gene mRNA (BM216 and FL339) were selected for progenitor T cell-derived cell lines, and their ganglioside compositions were studied. A ganglioside component with a mobility slightly faster than that of bovine brain GD1a on high-performance thin-layer chromatography was found in the cell extracts of these cell lines as one of the most abundant components and was absent in the extract of the parental cell line (BW5147). The structure of the ganglioside was determined to be: NeuAc alpha-Gal beta-Gal beta-Gal alpha-Gal beta-Glc beta-ceramide. Gangliosides with such a sequence have never been found before, suggesting the possibility that the ganglioside is expressed as a surface marker of the cells in hematopoietic organs committed to a specific cell lineage, presumably to T cell lineage. cells in hematopoietic organs committed to a specific     

Mouse IgA Fc receptor on CD3+ T cells. Molecular forms of IgA that bind to a 38-kDa Fc alpha R protein and development of an anti-Fc alpha R antisera

Previous studies have shown that splenic T cells from mice that bear IgA myelomas, as well as certain T cell lines, express receptors for the Fc of IgA, and are termed Fc alpha R. In this study, we have isolated and characterized two CD3+ T cell lines derived by fusion of murine Peyer's patch (PP) CD4+ T cells with the BW 5147 lymphoma cell line. These cell lines, designated PPT4-6 and PPT4-16, were shown to bind monomeric or dimeric IgA, whereas the fusion partner did not bind either form of IgA. However, polymeric IgA (m.w. 600,000) bound equally well to all three cell lines. Similar results were also obtained with two known Fc alpha R+ T cell lines, ThHA1 nos. 9 and 10. Immunoprecipitation studies with IgA on PPT4-16 and ThHA1 no. 9 have shown that IgA binds to a 38-kDa protein. A rabbit antiserum was prepared to a 38-kDa fraction of Fc alpha R+ T cell membranes, and heterophilic antibody was removed from the antiserum by adsorption with mouse thymocytes, BW 5147 and R1.1 lymphoma. The antiserum bound to both PPT4-16 and ThHA1 no. 9 as well as to other Fc alpha R+ T cells, but did not bind to thymocytes or to the T lymphomas R1.1 or BW 5147. The antiserum appeared specific for the Fc alpha R, because it failed to block binding of anti-CD3 (145 2C11) or other surface molecule-specific antibodies. Further, competitive inhibition studies with IgA and anti-Fc alpha R (38 kDa) showed that preincubation of Fc alpha R+ T cells with the anti-38-kDa protein completely eliminated IgA binding, whereas IgA partially blocked the binding of the anti-Fc alpha R antibodies to the cell membrane. Immunoisolation with the anti-Fc alpha R antibody of radioiodinated cell membrane proteins from Fc alpha R+ T cells, but not from Fc alpha R- cells, gave a distinct band at 38 kDa. To further test the specificity of this antiserum, we have isolated T cells from spleens of IgA-myeloma bearing mice, and tested the phenotype and IgA binding. A subset consisting of 15 to 20% of CD3+, CD8+ T cells was found that bound monomeric or dimeric IgA. Further, the anti-Fc alpha R antiserum also recognized this CD8+ T cell subset, and preincubation of the cells with antibody resulted in their failure to bind IgA. Our results indicate that the Fc alpha R on T cell lines derived from PP is a 38-kDa protein.(ABSTRACT TRUNCATED AT 400 WORDS)     

Mechanism of resistance to ricin toxin in selected mouse lymphoma cell lines

The role of impaired toxin uptake in conferring cellular resistance to the plant toxin RCA_II (ricin) has been examined using a murine BW5 147 lymphoma line and a toxin-resistant variant (BW5 147 Ric^R.3) selected by repeated exposure to RCA_II. The toxin-resistant variant is 250 times more resistant to RCA_II in long-term growth experiments and 1,000 times more resistant in short-term protein synthesis assays. Experiments with ferritin-conjugated ¹²⁵I-labeled RCA_II (ferritin-¹²⁵I-RCA_II) indicated that toxin binding to sensitive and resistant cells is similar at low toxin concentrations where maximum differential cytotoxicity occurs but that major difference exist with respect to toxin uptake. In sensitive cells toxin is internalized via endocytosis, and as seen previously in other systems subsequent rupture of some of the toxin-containing endocytotic vesicles releases toxin into the cytoplasm, where it inhibits protein synthesis. The process of toxin transfer to the cytoplasm is presumed to account for the one-hour lag before toxin-induced inhibition of protein synthesis can be detected. Endocytotic uptake of toxin is impaired in resistant BW5147Ric^R.3 cells, and they are unaffected by toxin concentrations that inhibit protein synthesis and kill sensitive parental cells. Killing of resistant cells at low toxin concentrations was accomplished by encapsulating RCA_II into lipid vesicles capable of fusing with the plasma membrane. Direct introduction of toxin into resistant cells using lipid vesicles as carriers produced rapid inhibition (< 15 min) of protein synthesis and eliminated the lag in toxin action seen in sensitive cells exposed to free toxin. These findings are discussed in relation to the mechanism of toxin action and proposals that toxin activity requires structural modification of the toxin molecule at the cell surface before transport into the cell.     

Activity of UDP-GlcNAc:alpha-mannoside beta(1,6)N-acetylglucosaminyltransferase (GnT V) in cultured cells using a synthetic trisaccharide acceptor

N-acetylglucosaminyltransferase V activity has been measured under saturating conditions in the extracts of seven cultured cell lines using as substrates, UDP-[3H]-GlcNAc and a synthetic 8-methoxylcarbonyloctyl trisaccharide. The unreacted sugar-nucleotide and its breakdown products were separated from the radiolabeled tetrasaccharide product by reverse-phase chromatography. Enzyme activity was present in six of the cell lines, which were derived originally from either human, mouse, or hamster tissues, with the highest activity in mouse lymphoma BW5147 cells. The PHAR 2.1 variant cell line, derived from the BW5147 line, expressed no detectable activity.     

Identification of a novel mechanism for the removal of glucose residues from high mannose-type oligosaccharides.

The role of glucosylated oligosaccharides in the biogenesis of the glycoprotein (G protein) of vesicular stomatitis virus was studied in PhaR2.7, a mouse lymphoma cell line deficient in glucosidase II activity. As expected, the great majority of cell-associated G protein remained glucosylated in PhaR2.7, and the G protein was rapidly deglucosylated in BW5147, the parental cell line. Despite these differences in glucosylation, the rates of G protein trimerization and transport to the cell surface were as rapid and efficient in the PhaR2.7 mutant as in BW5147. Surprisingly, greater than 73% of the oligosaccharides on G proteins recovered from released virions were complex-type units. The efficient processing of the G protein oligosaccharides coincided with the efficient removal of glucose residues from its oligosaccharides. After treatment with deoxynojirimycin, an inhibitor of endoplasmic reticulum (ER) glucosidases I and II, the total percentage of G protein-associated high mannose-type oligosaccharides increased more in the parental cells than in the mutant cells. Furthermore, when the G protein was retained in the ER of PhaR2.7 cells by depletion of the cellular ATP pools with carbonyl cyanide m-chlorophenylhydrazone, its oligosaccharides remained glucosylated. Under identical conditions, BW5147 cells removed the glucose residues from > 90% of the retained G protein's oligosaccharides. Thus, PhaR2.7 cells efficiently remove glucose residues from high mannose-type oligosaccharides of selected proteins using a deoxynojirimycin-insensitive enzyme located in a post-ER compartment. The existence of a second mechanism for the deglucosylation of N-linked oligosaccharides provides evidence for the important role of glucose removal in glycoprotein maturation.     

Variants of hamster fibroblasts resistant to Ricinus communis toxin (ricin).

1. Variant baby-hamster kidney (BHK) cell lines were isolated that grow in the presence of high concentrations of ricin, the toxic lectin of castor beans (Ricinus communis). The variant lines were independently derived from several cultures of normal BHK cells which had been exposed to the mutagen, methyl-N-nitro-N-nitrosoguanidine, before selection by ricin. 2. The cell lines maintain a high degree of resistance to ricin after growth in lectin-free medium for prolonged periods and therefore exhibit stable phenotypes that are different from normal BHK cells. 3. A preliminary classification of the phenotypes was made. Several cell lines bind normal amounts of 125I-labelled ricin, whereas other bind the lectin poorly. 4. A loss of surface receptors for two other lectins, R. communis RCA and Axinella polyploides, which have specificities similar to ricin, was also found in some but not all of the cell lines showing decreased surface concentrations of ricin receptors. 5. The binding to the ricin-resistant cells of lectins of different sugar specificity, namely Lens culinaris lectin and concanavalin A, was similar to, or higher than, to normal BHK cells. 6. Several of the ricin-resistant cell lines were shown to be cross-resistant to the weak cytotoxicity of Phaseolus vulgaris lectin. By contrast, some cell lines were more sensitive to concanavalin A than were normal BHK cells.     

Regulation of self-recognition by T-cell hybrids

Somatic cell hybrids were prepared between BW 5147, an AKR T lymphoma, and purified T cells from three sources: spleen cells exposed to sheep red blood cells, lymph node cells from mice sensitized to ovalbumin, and spleen cells of mice injected with azobenzenearsonate-IgG. Hybrid lines expressed constitutive markers of both parents which include H-2 antigens and the isoenzymes glucose phosphate isomerase and isocitrate dehydrogenase. Furthermore, they expressed both parental alleles of Thy 1, a differentiation antigen. Many of the hybrid lines formed rosettes with mouse erythrocytes. T-cell hybrids did not bind human or chicken red blood cells, though they did rosette with sheep erythrocytes to the same extent as with mouse red cells. We interpret the latter reaction as due to recognition of shared antigens by the murine T cells. This form of self-recognition is influenced by culture conditions and is expressed optimally by cells in late logarithmic phase of growth.     

Studies of the biosynthesis of the mannose 6-phosphate receptor in receptor-positive and -deficient cell lines

Phosphomannosyl residues present on lysosomal enzymes are specifically recognized by the mannose 6-phosphate receptor protein. This interaction results in the selective targeting of lysosomal enzymes to lysosomes. While this pathway is operative in many cell types, we have found four cultured cell lines that are deficient in the ability to bind lysosomal enzymes containing phosphomannosyl residues to their intracellular or surface membranes (Gabel, C., D. Goldberg, and S. Kornfeld, 1983, Proc. Natl. Acad. Sci. USA, 80:775-779). These cells appear to segregate lysosomal enzymes by an alternate intracellular pathway. To determine the basis for the lack of mannose 6-phosphate receptor activity in these cell lines, we studied the biosynthesis of the receptor in receptor-positive (BW5147) and receptor-deficient (P388D1 and MOPC 315) cells. The cells were labeled with [2-3H]mannose or [35S]methionine and the receptor was immunoprecipitated with an antireceptor antiserum. BW5147 cells synthesize a receptor protein whose size increases after translation/glycosylation. MOPC 315 cells produce an apparently normal receptor and degrade it rapidly. P388D1 cells fail to synthesize any detectable receptor. The receptor from BW5147 and MOPC 315 cells is a glycoprotein with both high mannose and complex asparagine-linked oligosaccharides. The complex-type units become fully sialylated and remain so during long periods of chase.     

Identification and characterization of a monoclonal antibody recognizing a galactose-binding domain of the toxin ricin

A monoclonal antibody raised against purified ricin B chain, 75/3B12, blocked ricin toxicity 30- to 100-fold in vitro. The 75/3B12 IgG and F(ab')2 blocked ricin binding to cell surface galactose-containing receptors. The 75/3B12 Fab bound ricin D with a Ka of 10(7) M-1, and this binding was blocked by asialofetuin, lactose, and N-acetylgalactosamine--molecules which interact with the ricin galactose-binding site--but not by fetuin, sucrose, or glucose. The 75/3B12 Fab contained no detectable carbohydrate and, according to several lines of evidence, did not bind ricin via Ig carbohydrate determinants. The monoclonal antibody appears to recognize a galactose-binding site on ricin D via the variable region of the antibody. The 75/3B12 Fab bound ricin E only 1/50 as well as ricin D and bound the Ricinus agglutinin only 1/80 as well as ricin D. The antibody specificity indicates that structural differences exist in the galactose-binding sites of the Ricinus communis lectins. Abrin and other lectins which bind galactose or N-acetylgalactosamine were not significantly bound by the monoclonal antibody. In vitro, the antibody blocked the nontarget toxicity of immunotoxins similarly to lactose. However, in vivo, unlike lactose, the 75/3B12 antibody protected mice from ricin toxicity.     

Endocytosis of MHC molecules by B cell-B lymphoma and B cell-T lymphoma hybrids

Different cells and different cell surface determinants of the same cells take up liposomes, bound to them via monoclonal antibodies, with variable efficiency. We have previously reported that T and B lymphocytes differ in the extent to which they take up liposomes bound to MHC class I molecules; T cells endocytose class I molecules rapidly, but B cells endocytose class I molecules much less efficiently, although their endocytosis of class II molecules is rapid. An approach toward understanding the molecular basis for this difference was made by evaluating the internalization patterns of somatic cell hybrids of B and T cells. Hybrid cells were constructed between LPS-stimulated purified B cell blasts from C57BL/6 mice (H-2b) and the HAT-sensitive AKR T lymphoma BW 5147 (H-2k). Hybrids between the BALB/c B lymphoma M12.4.1 (H-2d) and B cell LPS blasts from B10.BR (H-2k) mice were also evaluated. In all cases, for hybrid tumor cells, liposomes that were bound to class I molecules encoded by genes from the B cell donor were endocytosed as efficiently as liposomes bound to the class I molecules of the recipient lymphoid cell. T cell tumors efficiently internalized their own class I molecules and those donated by B cells; B cell tumors internalized liposomes that were bound to their own and the donor B cell class I molecules poorly. Thus, our results suggest that the internalization of an MHC molecule is not an intrinsic property of the molecule, but rather of the cell in which it is found.     

Expression of functional alpha beta T cell receptor gene rearrangements in suppressor T cell hybridomas correlates with antigen binding, but not with suppressor cell function

We have examined the expression of TCR genes in 4-hydroxy-3-nitrophenyl-acetyl (NP)-specific Ts cell hybridomas. Each of three independently isolated hybridomas expressed in-frame TCR alpha-chain rearrangements derived from the original suppressor Ts cell. Different V alpha and J alpha gene segments were rearranged and expressed in each Ts cell line. The only TCR beta-chain expressed in these cells was derived from the BW5147 fusion partner. Expression of the BW5147 beta-chain was found to correlate with cell surface Ag binding, inasmuch as subclones derived from one of the original Ts lines expressed greatly reduced levels of beta-chain mRNA and no longer bound to NP-coupled RBC. Subclones that continued to express beta-chain mRNA did bind to NP-coupled RBC. This suggests that the Ag receptor on Ts hybridomas is a TCR-alpha beta dimer composed of a unique alpha-chain and the BW5147 beta-chain. Ag binding could be modulated by preincubation of Ts hybridoma cells with anti-TCR-alpha beta antibody, thereby supporting this conclusion. Suppressor factor activity was measured in the conditioned media of Ts subclones that differed by 250-fold in levels of beta-chain mRNA expression. No difference in suppressor factor activity was found; conditioned media from these subclones suppressed both plaque-forming cell responses and delayed-type hypersensitivity responses at approximately equivalent dilutions. Suppressor factor activity in the conditioned media of both a beta-chain negative subclone and a beta-chain positive subclone could be absorbed with an antibody that recognizes the TCR alpha-chain, but not with an antibody that recognizes the TCR beta-chain. We conclude that suppressor factor activity in the conditioned media of these Ts hybridomas is not derived from surface TCR-alpha beta receptors, although it does share TCR alpha-chain determinants.     

beta-Glucuronidase is transported slowly to lysosomes in BW5147 mouse lymphoma cells: evidence that the prelysosomal enzyme is not restricted to the endoplasmic reticulum

The post-translational processing of beta-glucuronidase in BW5147 mouse lymphoma cells is slow relative to other newly synthesized lysosomal enzymes. To characterize this slow maturation the acid hydrolase was immunoprecipitated from cells pulse-labeled with [2-3H]mannose. Radiolabeled beta-glucuronidase migrated as the precursor form of the enzyme for up to 4 h of chase, whereas another acid hydrolase, beta-galactosidase, was processed completely to its mature form within this same time period. Both beta-glucuronidase and beta-galactosidase obtained high levels of mannose 6-phosphate (Man 6-P) within 60 min of their biosynthesis. The Man 6-P content of beta-galactosidase declined rapidly during a subsequent chase while that of beta-glucuronidase remained high during the first 4 h of chase and then slowly declined. 3H-Labeled phosphorylated high mannose-type oligosaccharides isolated from beta-glucuronidase after 1 h of chase were composed primarily of species with one or two phosphodiester groups, but oligosaccharides with one and two phosphomonoesters became the predominant phosphorylated species with longer chase times. The phosphorylated oligosaccharides attached to other newly synthesized acid hydrolases, on the other hand, contained primarily phosphodiester species at all chase times. When BW5147 cells were pulsed with [3H]mannose and chased in the presence of monensin to disrupt transport, the number of phosphorylated oligosaccharides recovered from beta-glucuronidase was comparable to the quantity recovered from the enzyme produced by non-drug-treated cells. The number of phosphorylated units recovered from all other newly synthesized acid hydrolases, however, was greater in the presence of the ionophore than in its absence. Nondenaturing gel electrophoresis studies indicated that beta-glucuronidase existed in two forms at steady state within BW5147 cells and, as such, was similar to liver beta-glucuronidase in which a large percentage of the enzyme was present as a complex bound to egasyn. These data suggest that newly synthesized beta-glucuronidase produced by BW5147 cells complexes with an egasyn-like protein within the endoplasmic reticulum. This interaction retards the enzyme's migration through the secretory apparatus but does not prevent its access to Golgi-associated processing enzymes.     

Isolation and characterization of two mouse L cell lines resistant to the toxic lectin ricin.

Two variant mouse L cell lines (termed CL 3 and CL 6) have been selected for resistant to ricin, a galactose-binding lectin with potent cytotoxic activity. The resistant lines exhibit a 50 to 70% decrease in ricin binding and a 300- to 500-fold increase in resistance to the toxic effects of ricin. Crude membrane preparations of CL 3 cells have increased sialic acid content (200% of control), while the galactose, mannose, and hexosamine content is within normal limits. Both the glycoproteins and glycolipids of CL 3 cells have increased sialic acid, with the GM3:lactosylceramide ratios for parent L and CL 3 cells being 0.29 and 1.5, respectively. In contrast, the membranes of CL 6 cells have a decrease in sialic acid, galactose, and hexosamine content with mannose being normal. Both cell lines have specific alterations in glycosyltransferase activities which can account for the observed membrane sugar changes. CL 3 cells have increased CMP-sialic acid:glycoprotein sialyltransferase and GM3 synthetase activities, while CL 6 cells have decrease UDP-GlcNAc:glycoproteinN-acetylglucosaminyltransferase and DPU-galactose:glycoprotein galactosyltransferase activities. The increased sialic acid content of CL 3 cells serves to mask ricin binding sites, since neuraminidase treatment of this cell line restores ricin binding to essentially normal levels. However, the fact that neuraminidase-treated CL 3 cells are still 45-fold resistant to ricin indicates that either a special class of productive ricin binding sites is not being exposed or that the cell line has a second mechanism for ricin resistance.     

Genes activated in the presence of an immunoglobulin enhancer or promoter are negatively regulated by a T-lymphoma cell line.

The tissue-specific expression of immunoglobulin genes can be partially explained by a requirement for activating factors found only in B lymphocytes and their derivatives. However, loss of immunoglobulin expression upon fusion of an immunoglobulin-producing myeloma cell with a T lymphoma cell (BW5147) or fibroblast (L cell) suggests that negatively acting factors also play a role in the tissue specificity of immunoglobulin genes. Expression of a cloned immunoglobulin heavy-chain gene introduced into myeloma cells was suppressed after fusion of the myeloma transformants with BW5147. The presence of either the immunoglobulin heavy-chain enhancer or promoter conferred suppression, under similar conditions, upon a heterologous gene that is normally expressed in both B and T lymphocytes. These immunoglobulin heavy-chain gene control regions, or gene modifications induced by them, are subject to negative control by T-lymphocyte-derived factors.     

Antigen-specific T-cell hybrids: I. Ovalbumin binding T-cell hybrid

Somatic cell hybrids were prepared between BW 5147 AKR T lymphoma cells and thymus-derived suppressor lymphocytes obtained from ovalbumin (OVA)-immunized, urea-denatured-ovalbumin (UD-OVA)-treated, suppressed BDF1 female mice. These T-cell hybrids express parental constitutive enzymes and differentiation antigens. Of the four hybrid lines developed, Hybrid 49A (Hyb 49A) has exhibited antigen-specific binding of OVA-coated syngeneic erythrocytes which is inhibited by preincubation in soluble OVA. This hybrid also interacts with self epitopes expressed on syngeneic erythrocytes. Antigen-specific rosette formation by Hyb 49A is not inhibited by preincubation in soluble keyhole limpet hemocyanin or normal mouse serum. This line and its agar-derived subclones have maintained their antigen-specific activity for 9 months and will provide extensive homogeneous quantities of T-cell products for molecular analysis.     

The AKR thymoma BW5147 is able to produce lymphokines when stimulated with calcium ionophore and phorbol ester

We produced the T cell hybridoma D9C1.12.17 by fusing an IL-4-producing T cell clone D9.1Hi with the AKR thymoma BW5147. The resulting hybridoma produced IL-2 as well as IL-4 even though none of the parental cells produced IL-2 after stimulation with Con A. The production of IL-2 was confirmed at the mRNA level by using an S1 nuclease protection assay. Further analysis indicated that Con A-induced IL-2 production was a common phenomenon among T cell hybridomas derived from this fusion. Although BW5147 does not produce detectable lymphokines after Con A stimulation, this line was able to produce IL-2, granulocyte-macrophage colony stimulating factor, and small amounts of IL-3 and IFN-gamma when stimulated with calcium ionophore and phorbol ester. The latter agents are thought to mimic the activating signal(s) delivered through the Ag:MHC TCR. This observation indicates that BW5147 has the ability to produce lymphokines but may lack component(s) which couple the extracellular signal to lymphokine production, and suggests that in T cell hybridomas, part of the spectrum of lymphokines produced may be contributed by BW5147.     

Autoantibody in MRL/Mp-lpr/lpr mice. A monoclonal antibody specific for the abnormal T cells from mice bearing the lpr/lpr gene

We generated mAb from an unimmunized autoimmune MRL/Mp-lpr/lpr mouse. One of these mAb A108, reacted with cell surface Ag present on abnormal T cells from MRL/Mp-lpr/lpr, C3H-lpr/lpr, and C57BL/6-lpr/lpr mice. We failed to detect significant numbers of A108 bearing cells in the lymph nodes of MRL-Mp/+/+, normal C3H or normal C57BL/6 mice. Therefore, the expression of A108 correlates with the presence of the lpr/lpr gene. A108 binds to a variety of murine T cell tumor lines (e.g., EL4, BW5147, and YAC-1) and human T cell tumor lines (e.g., MOLT-3, Sup T1, and Jurkat). A108 does not bind to normal human PBL. Immunoprecipitation of surface iodinated EL-4 and BW5147 with A108 identified one major protein with a Mr of about 17.5-kDa. The significance of these findings with respect to the development of lymphoid proliferation and autoimmune disease in mice bearing the lpr/lpr gene will be discussed.     

Differential expression of protein kinase C isoenzymes related to high nitric oxide synthase activity in a T lymphoma cell line

Protein kinase C (PKC) is critical for T lymphocyte activation and proliferation, while nitric oxide synthase (NOS) may function both as an activator or inhibitor of T cell apoptosis. Both enzymatic activities were studied in T lymphoma cells in comparison to normal and activated T lymphocytes. Here we show a higher translocation of PKC in BW5147 lymphoma cells than in mitogen-stimulated T lymphocytes. Tumor cells overexpressed PKC zeta isoform, while high levels of the PKC beta isotype were found in mitogen-stimulated T lymphocytes. Moreover, tumoral T cells showed high NOS activity, almost undetectable in normal or stimulated T lymphocytes. PKC and NOS inhibitors or the intracellular delivery of an anti-PKC zeta antibody diminished both NO production and proliferation in tumor cells.These results suggest that atypical PKC zeta isoform expression and its association with NOS activity regulation would participate in the multistep process leading to BW5147 cell malignant transformation.     

Receptor-mediated interaction of ricin with the lipid bilayer of ganglioside GM1-liposomes

The interaction of ricin with ganglioside GM1 or glycoprotein containing liposomes was investigated. At neutral pH, ricin bound to galactose moieties on the surface of the liposomes to form ricin-liposomes complexes, but did not associate with their lipid bilayers. When these ricin-liposomes complexes were exposed to a pH below 5, ricin bound to GM1-liposomes became associated with the lipid bilayer, whereas ricin bound to glycoprotein-liposomes (containing human erythrocyte Band 3) was only rarely associated. Association of ricin with the lipid bilayer of GM1-liposomes did not occur in the presence of lactose, which inhibits the binding of ricin to ganglioside GM1. Using a hydrophobic probe, 8-amino-1-naphthalene sulfonic acid (ANS), it was revealed that an acidity below pH 5 resulted in exposure of hydrophobic regions on the ricin molecule. These results strongly suggest that association of ricin with the lipid bilayer of GM1-liposomes at acidic pH is mediated by the binding of ricin to ganglioside GM1 at neutral pH and occurs through interaction between the exposed hydrophobic region on the ricin molecule and the lipid bilayer of GM1-liposomes at low pH.