Incorporation of exogenous glycosphingolipids in plasma membranes of cultured hamster cells and concurrent change of growth behavior

Phenotypic changes in 3T3 cells associated with the change of sphingolipid synthesis by a ceramide analog, 2-decanoylamino-3-morpholino-1-phenylpropanol (compound RV538)

A culture of BALB/c 3T3 cells grown in the presence of 40 microM of the ceramide analog compound RV538 (2-decanolyamino-3-morpholino-1-phenylpropanol) for several passages caused a substantial decrease in the level of all glycosphingolipids and an accumulation of ceramide and sphingomyelin. Associated with these chemical changes of sphingolipid composition and metabolism, the following phenotypic changes were observed: (i) loss of the cobblestone appearance at cell density saturation and development of fibroblastic appearance with partial overlapping of cells; (ii) reduction of cell growth rate; (iii) enhanced production of lactic acid; (iv) enhanced rate of glucose transport; and (v) higher incidence of large colony formation with infiltrating appearance in soft agar. Cell morphology changes, lactate production, and enhanced sugar uptake were reversed by co-culturing cells with gangliosides, particularly trisialogangliosides. Thus, these phenotypic changes mimicking those of oncogenically transformed cells are closely related to the blocked synthesis of glycolipids in these cells, whereas other changes may be caused by an accumulation of ceramide and sphingomyelin.

Blood group antigen staining pattern during experimental carcinogenesis in rat palate

During 4-nitroquinoline-1-oxide-induced carcinogenesis in the rat palate, animals were sacrificed at various intervals and stained for blood group antigens B and H (Type 2 chain) by an immunofluorescent method. In rats without signs of epithelial dysplasia, the staining pattern was identical with that in the normal control rats. In rats with definite or questionable (borderline cases) dysplasias, marked changes in blood group antigen staining pattern were seen. Thus, changes in cell-surface carbohydrates during malignant development in the rat palate seem to follow closely the histomorphological changes. As there is good evidence that carcinomas would eventually develop in all rats if they were not sacrificed, it seems that the blood group antigen staining pattern does not predict malignant development in the absence of histological suspicion.

Exogenous GM1 ganglioside caused G1-arrest of human diploid fibroblasts. Flow cytometric studies

Exogenous GM1 ganglioside (II3 NeuAc-Gg0se4-Cer) inhibited growth and DNA synthesis of human diploid fibroblasts, TIG-1 cells. We examined the effect of exogenous GM1 on their cell cycle traverse by flow cytometry. When the cells were partially synchronized by serum deprivation, addition of GM1 at the time of refeeding caused about 70% reduction of their reentry into S phase from the level observed in the control culture untreated with the ganglioside. However, the addition of GM1 6 h later caused only about 30% reduction of the reentry from the control level. These results suggest that the exogenous ganglioside blocks the cell cycle traverse in an early G1 period. This is consistent with the fact that GM1-treated cells showed a high level of histone H1(0) similar to that observed in G1-arrested cells in confluent culture.

The distribution of blood group antigens in experimentally produced carcinomas of rat palate

It has been shown previously that rat oral epithelia express antigens cross-reacting with antibodies against human blood group antigen B and its structural precursor, the H antigen (Type 2 chain). In the present study we investigated the expression of these antigens in malignant changes in the rat palate induced by a chemical carcinogen (4NQO). The H antigen, normally expressed on spinous cells in rats, was absent in malignant epithelium, whereas staining for the B antigen, normally expressed on basal cells, was variable. These changes are equivalent to those seen in human squamous cell carcinomas. The blood group antigen staining pattern in experimentally produced verrucous carcinomas showed an almost normal blood group antigen expression. This may have diagnostic significance. Localized areas of hyperplastic palatal epithelium with slight dysplasia revealed loss of H antigen and the presence of B antigen in suprabasal strata equivalent to the pattern seen in human premalignant epithelium. We conclude from these findings, that the rat model is well suited to study changes in cell surface carbohydrates during chemical carcinogenesis.

Gangliosides block the inhibition of macrophage Fc-dependent phagocytosis by lipopolysaccharide

The mechanism whereby bacterial lipopolysaccharide (LPS) exerts its biologic effects on mammalian cells is unknown. Plasma membrane gangliosides bind bacterial toxins and have been implicated in modulating the effects of a variety of immunoregulatory substances. We investigated the possibility that gangliosides can inhibit the effect of lipopolysaccharide on Fc-dependent phagocytosis by murine peritoneal macrophages. Protein-free lipopolysaccharide preparations significantly inhibited Fc-mediated phagocytosis (less than 71% of control) at concentrations of 100 ng/ml or greater after 90 min of incubation. The inhibitory effect of LPS (1 micrograms/ml) was blocked when macrophages were incubated with mono-, di-, or trisialogangliosides (25-50 micrograms/ml). Neither asialoganglioside nor sialic acid alone were capable of blocking the effect of LPS. When chromatographed separately on a Sepharose 4B column, LPS and trisialoganglioside had different elution profiles. LPS and trisialoganglioside coeluted, however, when premixed at 37 degrees C for 60 min and then applied to the column. Therefore, abrogation of the effect of LPS on Fc-dependent phagocytosis may occur as a consequence of direct interaction between LPS and gangliosides. These data suggest that gangliosides may modulate the response of macrophages to bacterial lipopolysaccharide.

Cytostatic effect of gangliosides present in the membrane of macrophages

Stimulated macrophages are known to inhibit the growth of certain tumor cells. Using mouse peritoneal exudates as a source of macrophages and the mastocytoma cell line P815 as the target, the inhibition was found to depend on direct contact between the macrophages and the growing cells. Cytostatic activities were detected in extracts of macrophages as well as in membranes of macrophages bound to substances of low molecular weight. Physical and biochemical characteristics of the cytostatic activity hint toward N-acetylneuraminic acid containing glycosphingolipids (gangliosides). The different macrophage gangliosides were separated by thin-layer chromatography. All types showed cytostatic activity, but the most effective gangliosides were identified as monosialoganglioside GM1 and disialoganglioside GD3.

Ganglioside GM3: an acidic membrane component that increases during macrophage-like cell differentiation can induce monocytic differentiation of human myeloid and monocytoid leukemic cell lines HL-60 and U937

When human myeloid and monocytoid leukemic cell lines HL-60 and U937, respectively, were treated with an exogenous sialoglycosphingolipid, ganglioside GM3, in serum-free medium, cell growth was markedly inhibited, and their morphological maturation along a monocytic lineage was observed. In addition to a significant increase in phagocytic and nonspecific esterase activities, marked increase of monocyte-specific surface antigens detectable with monoclonal antibodies such as OKM1 and OKM5 was observed in GM3-fed cells. Other sialoglycosphingolipids with the carbohydrate structure belonging to ganglio-series oligosaccharide, ganglioside GM1 and a brain ganglioside mixture, had no effect on the cell differentiation, showing instead stimulatory actions on the growth of these cell lines. We recently demonstrated that the ganglio-series ganglioside GM3 characteristically increased during macrophage-like cell differentiation of these cell lines. The present results indicate that ganglioside molecular species that specifically increase during monocytic cell differentiation of human myeloid and monocytoid leukemic cell lines may play, in turn, an important role in the differentiation-induction of these cell lines along a monocytic cell lineage.

Association of endogenous substrate with solubilized bovine brain sialidase

Nonidet P40 solubilized up to 90% of the sialidase, active towards added ganglioside substrate, that was associated with the total membrane fraction prepared from gray matter of bovine brains. Solubilized sialidase acted upon endogenous substrate (sialic acid containing compounds solubilized with the enzyme), hydrolyzing approximately 50% of the readily available sialosyl residues within 20 min. During a 2-hr reaction time 80% of the polysialylated gangliosides solubilized with the enzyme were acted upon. A 20-min lag was observed before sialidase acted upon added ganglioside substrate. The lag could be reduced to less than 2 min when the enzyme was allowed to act on endogenous substrate prior to exposure to exogenous substrate, suggesting that the solubilized enzyme acted preferentially on endogenous substrate. A protease inhibitor prevented much of the 86% loss of activity towards added substrate that was seen when the enzyme was stored at 4 degrees C for 6 days; activity towards endogenous substrate decreased only 34%.

The role of gangliosides in the interaction of a growth inhibitor with mouse LM cells

We have isolated and characterized glycopeptides, derived from mouse and bovine cerebral cortex cells, that inhibit protein synthesis and cell growth of normal but not transformed cells. The inhibitor binds to target cell surfaces, and gangliosides have previously been shown to influence cell sensitivity to the glycopeptides. Preincubation with 3.0 micrograms/ml ganglioside GM1 at 0 degrees C for 3 hr sensitized the mouse L-cell line to the inhibitor, as determined by protein synthesis assays. Preincubation of LM cells with ganglioside GM1 alone did not affect protein synthesis rates. In addition, the gangliosides GD1a and GM3 also sensitized the LM cells to the protein synthesis inhibitory effect of the glycopeptide inhibitor. Binding experiments were performed with 3T3 (sensitive) and LM (insensitive) cells to determine if sensitivity to the glycopeptide inhibitor was reflected in binding of the inhibitor to these cells. Binding of 125I-labeled inhibitor to 3T3 cells was maximal after 60 min at 0 degrees C and saturable at approximately 1 X 10(4) molecules/cell. Furthermore, binding of the inhibitor was dose-dependent, with half-maximal binding at 1.5-2.0 nM and saturation at 8.0-10.0 nM. Scatchard plot analysis indicated that the Kd was about 1 X 10(-9) M and that there are 1 X 10(4) receptors/cell. Binding of the inhibitor to LM cells was maximal after 30 min at 0 degrees C and saturation occurred at 5 X 10(3) molecules/cell. We then examined the possibility that gangliosides are the cellular receptor or co-receptor for the glycopeptide inhibitor. Binding of the inhibitor to ganglioside GM1 was first examined after the ganglioside had been preadsorbed to polystyrene tubes. These experiments indicated that the ganglioside did not bind the inhibitor. Ganglioside-containing liposomes from phosphatidylcholine or LM cell membrane components were also prepared; these artificial membranes did not bind appreciable amounts of the iodinated inhibitor. Competition experiments showed that the gangliosides GM1 and GD1a did not neutralize the protein synthesis inhibitory activity of the glycopeptides, indicating that gangliosides do not directly interact with the glycopeptide inhibitor. In addition, binding of the inhibitor to LM cells preincubated with ganglioside GM1 was studied. Although the binding of the inhibitor to LM cells was one-half that observed for 3T3 cells, incorporation of exogenous gangliosides into LM cells did not result in increased binding of the inhibitor.(ABSTRACT TRUNCATED AT 400 WORDS)

The differentiation of HL-60 cells in the synthetic medium induced by GM3 ganglioside

GM3 ganglioside, added exogenously to a promyelocytic leukemia cell line (HL-60 cells) in serum-free synthetic medium, induced differentiation into macrophage-like cells. Macrophagic morphology and function of differentiation-induced cells were determined by cell growth behavior, May-Grünwald-Giemsa staining, activities of nonspecific esterase, phagocytosis and nitroblue tetrazolium (NBT) reduction. GM3 ganglioside may play a role in triggering differentiation of HL-60 cells into macrophage-like cells.

Carbohydrate structure in tumor immunity

Researchers have endeavored to define surface alterations associated with neoplasia for at least 25 years. In comparisons of normal tissues with animal and human tumors, cultured cells before and after transformation with oncogenic agents, tumorigenic and nontumorigenic transformed cells, metastatic and nonmetastatic tumor cells, high- and low-metastatic variants, and tumor cells before and after induction of differentiation to a less malignant phenotype, a consistent finding has been some form of alteration in surface carbohydrate structures. These changes in glycolipids, glycoproteins and glycosaminoglycans are reviewed, and their structures are illustrated. Both nucleotide sugar biosynthesis and glycosyltransferase changes have been associated with these alterations. In some cases, alterations in transformed cells were related to growth, rather than transformation. In others, the altered glycoconjugates are truly tumor-associated. There is evidence that cell surface glycoconjugates may function in growth control. Altered carbohydrate structures could also serve as receptors for growth promoting factors and be directly responsible for altered growth control. Recent studies with monoclonal antibodies indicate that the vast majority of antibodies recognizing tumor-associated antigens are detecting altered carbohydrate structures. Mechanisms by which the immune system can recognize these carbohydrate structures are considered, and immune recognition of tumor-associated carbohydrate structural alterations is explored. A number of these hypotheses relating to alterations in glycosylation, growth control, and tumor immunity deserve further investigation.

Inhibition of interleukin-2-dependent cytotoxic T-lymphocyte growth by gangliosides

Previous work has shown that exogenous gangliosides suppress lectin-induced T-lymphocyte mitogenesis in mixed populations of immune cells. As one potential cellular site of this inhibition, the influence of gangliosides on interleukin-2-dependent T-cell proliferation was tested, using cultures of cytotoxic T cells (strain CT-6). Incubation of CT-6 cells with mixed bovine brain gangliosides resulted in a dose-dependent inhibition of cell proliferation. Cell viability was unaffected by gangliosides, and the inhibition was totally reversed when the gangliosides were removed. Individual purified gangliosides were tested and GM2 was most inhibitory (I50 = 15 microM). GD1a and GT1b were somewhat less potent, whereas GM1 and GM3 were only weakly inhibitory. Various nonpolar lipids, sulfatides, and sialic acid did not inhibit CT-6 cell growth. The results suggest that a primary mechanism whereby gangliosides inhibit lectin-induced lymphocyte mitogenesis is by inhibition of the interleukin-2-stimulated proliferation of T cells in these cultures.

Effects of cell density on the neutral glycolipid composition of cultured human brain and glioma cells

Density dependent chain elongation of neutral glycosphingolipids (NGSL) is associated with contact inhibition of mitosis in several normal cultured cell lines. Transformed non-neural cell lines which have impaired contact inhibition frequently lose this biochemical response. To determine if either of these phenomena occur in human neural cells we determined NGSL compositions of cultured glioblastoma multiforme and normal fetal brain cells. Fetal cells generally had more total NGSL than the tumor cells. As a percentage of total NGSL, both cell lines at higher cell densities had larger proportions of ceramide trihexoside and globoside, but smaller proportions of cerebroside. This decrease was mainly in non-hydroxy fatty acid cerebroside of glioma cells, but in hydroxy fatty acid cerebroside of normal fetal brain cells. These results demonstrate that although glioblastoma multiforme cells have markedly impaired growth control, they still preserve density dependent chain elongation of NGSL. A role for this phenomenon in normal cellular growth control has yet to be established.

Effect of exogenous gangliosides on human neural cell division

Human neural cells in exponential growth phase were transferred to a serum-free medium and maintained for 72 hr without any detectable loss in viability. The two normal fetal cell lines (CHI and CHII) showed a serum-dependent cell proliferation, but the glioblastoma multiforme cells (12-18) were able to continue proliferating in this totally synthetic medium. The incorporation of [3H]thymidine into the acid-precipitable fraction of both normal and neoplastic human neural cells was assayed in the presence and the absence of exogenous gangliosides by a convenient new method. In serum-free medium, gangliosides (50 microM) inhibited the thymidine incorporation into the normal fetal cells within 24 hr and, in serum containing medium, reduced their proliferation within 48 hr. No such effects were detectable in the glioma cells. The inhibition of thymidine incorporation in the normal cells was reversible upon removal of the gangliosides. These results indicate a role of gangliosides in the postmitotic phase of normal human neural cells resulting in the regulation of cell proliferation.

Glycosphingolipids of a green monkey kidney cell line (GMK AH-1). Evidence for a novel pentaglycosylceramide based on globotetraosylceramide

Total non-acid glycolipid fractions have been isolated from GMK AH-1 cells grown in fetal calf serum and in horse serum. For comparison, glycolipids were also prepared from green monkey (Cercopithecus aetiops) kidney and from fetal calf serum. The major glycolipids from GMK AH-1 cells grown in fetal calf serum were isolated by silicic acid column chromatography and preparative thin-layer chromatography. These fractions were characterized mainly by thin-layer chromatography, mass spectrometry and gas chromatography. The structures of the glycolipids isolated were proposed as: Glc1 leads to 1Cer, Gal1 leads to 1Cer, Gal1 leads to 4Glc1 leads to 1Cer, Gal1 leads to 4Gal1 leads to 4Glc1 leads to 1Cer, GalNAcl leads to 3Gal1 leads to 4Gal1 leads 4Glc1 leads to 1Cer. In addition, a novel pentaglycosylceramide with the probable structure Ga1 beta 1 leads to 3GalNAc beta 1 leads to Gal alpha 1 leads to 4Gal beta 1 leads to 4Glc beta 1 leads to 1Cer was also present. THe ceramides contained mainly dihydroxy 18:1 long-chain base in combination with non-hydroxy 16:0-24:0 fatty acids. Small amounts of trihydroxy 18:0 long-chain base and hydroxy 22:0-24:0 fatty acids were also present in the mono- and diglycosylceramide fractions. The glycolipid patterns of GMK AH-1 cells grown in fetal calf serum or horse serum were identical. The pentaglycosylceramide present in the cultured cells could not be detected with certainty in the kidney tissue. The uptake of this glycolipid from the culture medium is unlikely as it seems to be lacking in calf serum.

Studies on the cell association of exogenously added sialo-glycolipids

We studied the mode of uptake of exogenous sialoglycolipids with a permanent mouse cell line (Clone-1D) and compared the cell association of ganglioside II3 NeuAc-Gg4 Cer with that of a synthetic one-C18-hydrocarbon-chain ganglioside analogue (desphingo-G1-stearamide). In contrast to monolayer cultures, Clone-1D cells in suspension bind both sialo-glycolipids rapidly and independently of time and temperature (4 degrees/37 degrees C) of incubation. Three modes of cell association were distinguished: 1) a serum-and serum albumin-sensitive association; 2) a trypsin-sensitive association and 3) a serum- and trypsin-insensitive association. Inhibition of pinocytosis by deoxyglucose/sodium azide did not significantly influence cellular uptake or release of exogenous ganglioside. In monolayer culture, Clone 1D cells in M-phase associate more ganglioside than randomly growing cells. This may explain in part the inhomogeneous cellular distribution of ganglioside binding. In contrast, uptake of desphingo-G1 -stearamide by cells in mitosis equalled that bound by control cells.

Lymphocyte transformation induced by autologous splenocytes incorporated with the tetrasialoganglioside GQ1b

Incorporation of the tetrasialoganglioside GQ1b into membranes of mouse splenocytes rendered these cells effective triggers in an autologous mixed lymphocyte reaction. Whereas GQ1b-incorporated cells increased mitogenic responses of thymocytes by 10 to 13-fold over untreated cultures, the trisialoganglioside GT1b was only partially effective, and the di- and monosialogangliosides tested had no effect. The mixed lymphocyte reaction generated in the autologous system by highly glycosylated gangliosides is abolished by neuraminidase or mile formic acid treatment prior to their insertion into target cell membranes suggesting a role for the extra sialic acid residues in imparting additional antigenic specificity to the modified cells.

Glycosylation reactions and tumor establishment: modulation by vitamin A

The ability of retinoids to prevent or alter the course of experimental tumorigenesis is well established. We have extended these observations to include effects on establishment of tumors and tumor metastases. A diet containing excess retinyl acetate fed to rats prior to injection of a metastatic line of transplantable hepatoma, prevented establishment of secondary tumor foci while 75% of the animals fed adequate retinyl acetate showed pulmonary metastases. Metastatic ability may be related to the ability to bind fibronectins, proteins that link cells to an underlying stroma. Findings suggest involvement of higher gangliosides in the attachment of cells to a fibronectin-collagen complex. Prior to metastasis, hepatoma lines become depleted in the putative fibronectin receptor gangliosides as an end result of a complex cascade of altered glycosyltransferase activities. After metastasis, fibronectin receptors are apparently restored in those secondary tumor foci that become established. Analyses suggest that excess vitamin A may prevent the reappearance of fibronectin receptor gangliosides so that secondary tumor foci do not establish.

Ganglioside biosynthesis in rat liver: characterization of cytidine-5'-monophospho-n-acetylneuraminic acid:hematoside (GM3) sialyltransferase

CMP-NAcNeu:GM3 ganglioside sialytransferase (GD3 synthase) was concentrated 80-100-fold, relative to total homogenates, in Golgi apparatus fractions from rat liver. Ultrasound treatment of Golgi apparatus in a low salt medium extracted 40-60% of the original protein but did not dissociate the transferase from membranes. The acivity was greatest in the presence of certain detergents, had a pH optimum of 6.2, was stimulated by mg2+ and diacylphospholipids and was inhibited by lysophospholipids. Apparent Km values for CMP-NAcNeu and GM3 were about 0.8 and 0.2 mM, respectively. On chromatographic separation, virtually all the reaction product migrated as GD3. GD3 synthase appeared to be a glycoprotein since the activity bound to concanavalin A-Sepharose and was eluted, with increased specific activity, by alpha-methyl mannoside.

Isolation of cholera toxin receptors from a mouse fibroblast and lymphoid cell line by immune precipitation

Cholera toxin receptors have been isolated from both a mouse fibroblast (Balbc/3T3) and mouse lymphoid cell line labeled by the galactose oxidase borotritiide technique. Tritiated receptor-toxin complexes solubilized in NP40 were isolated by addition of toxin antibody followed by a protein A-containing strain of Staphylococcus aureus. In both cell types by far the major species of toxin receptor isolated was ganglioside in nature, although galactoproteins were also present in the immune complexes. Whether the galactoproteins form part of a toxin-receptor complex or are artifacts of the isolation procedure is presently unclear. The relative specificity of cholera toxin for a carbohydrate sequence in a glycolipid suggests that the toxin might prove a useful tool in establishing the function and organization of glycolipids in membranes. For example, interaction of cholera toxin with the mouse lymphoid cell line was shown to result in patching and capping of bound toxin, raising the possibility that the glycolipid receptor interacts indirectly with cytoskeletal elements. Cholera toxin might also be used to select for mutant fibroblasts lacking the toxin receptor and therefore having an altered glycolipid profile. Such mutants might prove useful in establishing the relationship (if any) between modified glycolipid pattern and other aspects of the transformed phenotype. Attempts to isolate mutants, based on the expectation that growth of cells containing the toxin receptor would be inhibited by the increase in cAMP levels normally induced by cholera toxin, proved unsuccessful. Cholera toxin failed to inhibit significantly the growth of either Balbc or Swiss 3T3 mouse fibroblasts although it markedly elevated cAMP levels.

A new heterophile antigen: man distinguished from higher apes and other mammals

Human cell membranes acquired an antigenic substance from the bovine serum in culture medium. The antigen was named the heterologous membrane antigen (HM Ag). Cross-reactive antigen was found in sera of all mammals tested, except for man. The distribution of the HM Ag serologically distinguished man from other mammals.

Glycolipids as indicators of tumorigenesis

Hyperplastic liver nodules and hepatocellular carcinomas were induced in rats by oral administration of the carcinogen N-2-fluorenylacetamide. Neoplastic tissue was compared with control, fetal, neonatal, and precancerous liver tissues. The development of the tumors was slow, such that temporal changes in the biochemical and morphologic development of carcinogenesis could be identified. Ganglioside sialic acid levels were elevated in all but the most poorly differentiated tumors. Experiments to monitor individual enzymes suggested that the alterations in glycolipid composition were a direct effect of alterations in biosynthetic activities. The pattern during tumorigenesis was the inverse of that during normal development. Also, ganglioside patterns showed a progressive simplification from hyperplastic nodules to well-differentiated hepatomas and through two grades of poorly differentiated hepatomas. An increase in the activity of the branchpoint enzyme of ganglioside biosynthesis preceded both a decrease in the branchpoint enzyme of the disialoganglioside pathway and a marked increase in the galactosyltranferase of GM1 formation. The results indicate that ganglioside deletions are the end result of a cascade of events in the tumorigenic transformation. The onset of ganglioside deletions but not of the cascade per se may correlate with the onset of malignancy. Glycolipid levels are elevated early in certain surrounding tissues especially in the blood. In rats bearing transplantable hepatomas, serum levels of lipid-bound sialic acid were elevated 2.5-fold. Similar results were obtained with sera of mice bearing transplantable mammary carcinomas and of cancer patients. These findings provide new emphasis for gangliosides in both cancer detection and as regulatory signals for growth and multiplication of cells.

Human tumor cells grown in fetal calf serum and human serum: influences on the tests for lymphocyte cytotoxicity, serum blocking and serum arming effects

Peripheral blood lymphoid cells (PBL) from cancer patients and normal donors were tested against three melanoma cell lines grown in either 10% fetal calf serum (FCS) or 2.5-5% human AB serum in order to determine if the heterologous membrane (HM) antigen or other FCS antigens acquired from the bovine serum supplement could influence lymphoid cell-mediated cytotoxicity in vitro. FCS-grown melanoma cells were more susceptible than the AB serum-grown subline to lymphocyte cytotoxic effects. Arming effects by autologous sera on normal donor lymphocytes and to a lesser extent on lymphocytes of cancer patients were more pronounced on the FCS-grown M12 melanoma cells. This effect was abrogated when the cells were grown in human AB serum for at least 8 weeks. The non-HM tumor-associated antigen remained at the same original low level. Blocking effects were more evident on the AB-grown M14 melanoma line. These data suggest that the FCS antigens on the cell surface may have been responsible for the augmented PBL cytotoxicity. The anti-FCS antibody present in normal and cancer patients' blood induced an antibody-dependent cellular cytotoxicity (ADCC). Elimination of arming activity against HM or other FCS antigens from AB-grown cells may have made the serum blocking factors more apparent. However, cytotoxicity against tumor cells by PBL from normal donors was still apparent even on the human serum-grown cells, suggesting that a different antigen-antibody system was also responsible for this "non-specific" activity.

Ligand-induced redistribution of lymphocyte membrane ganglioside GM1

Dynamic aspects of the binding of cholera toxin to lymphocyte membranes have been studied. We have shown that the receptor for this ligand--the GM1 ganglioside--can be laterally redistributed into aggregates and caps. Exogenous purified GM1 inserted into GM1-deficient human leukaemic cells can undergo a similar pattern of ligand-induced mobilisation. These observations may have important implications for both the general behaviour of cell surface glycolipids and the mode of action of cholera toxin on adenyl cyclase.