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The biologic functions of gangliosides G(M3) and G(D3) in the attachment of human melanoma cells to extracellular matrix proteins (type I and IV collagens, fibronectin, and laminin) were investigated by using the G(D3)-deficient mutant clone (SK-MEL-28-N1) and the parent cell line SK-MEL28. SK-MEL-28-N1 (N1) (high G(M3) expression: G(M3), 97.3%; G(D3), 0%) was selected by treating SK-MEL-28 (high G(D3) but low G(M3): G(M3), 6.5%, G(D3), 93.5%) with an anti-G(D3) monoclonal antibody (R24) and rabbit complement and subsequent subcloning of the surviving cells. The N1 clone showed significantly higher ability to adhere to type I and IV collagens and laminin than the parent clone SK-MEL-28. In the N1 clone, the expression of alpha2beta1 and alpha3beta1 integrin receptors was increased, whereas in SK-MEL-28, their expression was very low or undetectable. The treatment with monoclonal antibodies directed specifically to G(D3) expressed on SK-MEL-28 inhibited the cell attachment to type IV collagen (33% inhibition of control), fibronectin (59%), and laminin (71%). These findings suggest that gangliosides G(M3) (by influencing integrin receptor levels) and G(D3) (by interacting directly with matrix proteins) might play some functional roles in attachment to extracellular matrix proteins and thereby enhance the metastatic potency of melanoma cells.

Understanding the distribution of tumor-associated antigens on cancers and normal tissues is essential for selection of targets for cancer immunotherapy. Seven carbohydrate antigens, potential targets for immunotherapy, were studied using a panel of well-characterized MAbs by immunohistochemistry on cryostat-cut tissue sections of 13 types of cancers and 18 normal tissues. GD2 and GD3 were present on most cancers of neuroectodermal origin and GD2 was also present on B cell lymphomas. 9-O-acetyl-GD3 was detected only on melanoma while fucosyl GM1 was detected only on small cell lung cancers (SCLC). Surprisingly, GM2 was strongly expressed on all tested tumors, including cancers of neuroectodermal origin and cancers of epithelial origin. Polysialic acid was primarily expressed on SCLC and neuroblastomas. Globo H was present on most cancers of epithelial origin. These antigens were also identified in normal tissues. Fucosyl GM1 was not expressed significantly on any of the normal tissues analyzed. GD3, GD2, GM2 and polysialic acid were detected in normal brain to varying degrees. GM2 and Globo H were expressed on the luminal surface of epithelia of a variety of organs. The unexpected expression of GM2 on a broad range of cancers and normal epithelial tissues was confirmed by loss after methanol fixation and by immune thin layer chromatography.

We have selected GD3-deficient human melanoma cell lines, in order to investigate the function of GD3 ganglioside. This was done by treating SK-MEL-28 cells with anti-GD3 antibody (R24) and rabbit complement and subsequent subcloning of the surviving cells, resulting in the derivation of two cell lines deficient in the cell surface expression of GD3. Neither cell line (designated SK-MEL-28-N1 and SK-MEL-28-N2) had detectable cell surface expression of GD3 as analyzed with monoclonal antibody R24, and no GD3 was detectable in either cell line by glycolipid isolation, thin-layer chromatography, or resorcinol-HC1 spray, but thin-layer chromatography immunostaining with monoclonal antibody R24 showed the presence of low amounts of GD3 in both N1 and N2 (1/40 of the amount in the parent cell line in N1 and 1/500 in N2). In SK-MEL-28-N1, the residual GD3 was shown by immunofluorescence assays on permeabilized cells to be present in discrete intracellular organelles, suggesting that these cells have a defect in the transport of GD3 as well as in its synthesis. Both SK-MEL-28-N1 and -N2 had an increase in detectable GM3 expression. The mutant cell lines had altered cell morphology in comparison to the parent cell line and both had slower growth rates in vitro and lower tumorgenicity in nu/nu mice. These results indicate that GD3 ganglioside plays an important role in proliferation and growth of melanoma cells.

Previous studies (Y. Nagata, S. Yamashiro, J. Yodoi, K. O. Lloyd, H. Shiku, and K. Furukawa (1992) J. Biol. Chem. 267, 12082-12089) had isolated a putative cDNA coding for beta 1,4 N-acetylgalactosaminyltransferase (GM2/GD2 synthase) and demonstrated the presence of GM2 and/or GD2 gangliosides in melanoma cell lines stably transfected with this gene. We have now measured the levels of glycosyltransferase activities and mRNA levels in five transfected cell lines in comparison with their parent cell lines (mouse melanoma B16 and human melanoma MeWo). Membrane preparations from the transfected cell lines demonstrated de novo synthesis of GM2 or GD2 in in vitro assays using GM3 or GD3, respectively, as ganglioside acceptors. The enzyme levels, however, varied considerably among the different transfectants, as did the mRNA levels for the beta 1,4 Gal-NAc-transferase. The effect of the transfected gene on levels of preexisting glycosyltransferases involved in ganglioside biosynthesis was also measured and the ganglioside composition of the cell lines was determined. The level of beta 1,4 GalNAc-transferase expressed in the different cell lines was found to dramatically influence the overall ganglioside composition of the cell. In the transfected cell line with the highest levels of the transferase, for example, biosynthesis was almost completely redirected away from the b pathway to the a pathway with the resulting expression of only GM2. These data from this family of related cell lines clearly demonstrate the primary roles that relative glycosyltransferase levels play in determining the ganglioside composition of cells.

The substrate specificity of beta 1,4-N-acetylgalactosaminyltransferase has been analyzed using a fusion enzyme which consisted of the catalytic domain of the enzyme and the IgG binding domain of protein A, and also by extracts from cDNA transfectants. Both enzyme sources were capable of producing not only GM2 and GD2, but also asialo-GM2, GalNAc-sialylparagloboside, and Gal-NAc-GD1a from appropriate acceptors, although the efficiencies were at most 1-3% of those of GM2/GD2. The biological significance of these low specificities was studied with transient and stable transfectant cells. From the results of transient expression of the cDNA, asialo-GM2 expression appeared to inversely correlate with GM2 synthase levels in those lines. Consequently, GM2 seemed to be preferentially synthesized when both GM3 and lactosylceramide are available, and asialo-GM2 is synthesized in the absence of GM3 synthesis. However, the results of double immunostaining of CHO transfectants with anti-GM2 and anti-asialo-GM2 antibodies indicated that another factor may be involved in asialo-GM2 synthesis. From the in vitro assay using mixed acceptors, it was concluded that the presence of certain levels of GM2 might enhance the asialo-GM2 synthesis. These results suggest that even acceptors showing low efficiencies in vitro might be used in certain cells depending on the availability of precursors, expression levels of other gangliosides, as well as the kinetic properties of the enzyme, and the compartmentation of the glycosylation machineries in the cells.

Melanomas and other cancers of neuroectodermal origin express multiple cell-surface gangliosides in patterns that vary significantly even within the same tumor type. Monoclonal antibodies (mAb) against four of these gangliosides (GM2, GD2, 9-O-acetyl-GD3 and GD3) were tested alone and in combination on 14 tumor cell lines (7 melanomas, 3 neuroblastomas, 3 sarcomas and 1 astrocytoma) using flow cytometry and complement-dependent cytotoxicity (CDC) assays. Increased tumor cell recognition and CDC resulting from the combination of three or four mAb were found in 14/14 tested cell lines, and this was most striking when each mAb was used at suboptimal concentration. At these concentrations, the average mean fluorescence intensity of the 14 cell lines with individual mAb was between 3.0 and 6.8 and increased to 10.8 and 18.8 with the three- and four-mAb mixtures. The average percentage CDC-specific release with individual mAb was 2.0%-8.3%, and 12.3% and 16.6% with the three- and four-mAb combinations. The number of cell lines showing significant mean fluorescence intensity and CDC increased from 2-8/14 with single mAb to 13-14/14 with the mixtures of three or four mAb. Our experimental results support the rationale for active immunization with a polyvalent ganglioside vaccine or passive therapy with a combination of mAb to different gangliosides in patients with tumors of neuroectodermal origin. In addition, our studies have demonstrated that 9-O-acetyl-GD3 is a surprisingly effective target for immune attack, although it is a minor constituent of these cells.

The internalization and recycling of glycosphingolipids (GLs), added exogenously to cells, has been shown in a number of systems. In addition, a portion of the internalized GLs becomes elongated by further glycosylation, presumably by passage through the Golgi apparatus, and is recycled to the plasma membrane. We have previously shown (K. Furukawa, I. Thampoe, H. Yamaguchi, and K. O. Lloyd J. Immunol. 142, 848, 1989) that NeuGc-LacCer (GM3), added exogenously to cultured human melanoma cells, is converted to NeuAc-NeuGc-LacCer (GD3) and appears at the cell surface where it can be recognized by a monoclonal antibody (32-27M) specifically recognizing this structure. The mechanism of this process has been investigated by analyzing the effect of monensin and Brefeldin A (BFA), two drugs known to affect vesicular transport and Golgi function, on the recycling of NeuGc-LacCer. Using two different serological assays, BFA was shown to specifically inhibit the recycling process. BFA probably achieves this effect by inhibiting the transfer of endocytosed GL from endosomes and the trans Golgi network to the Golgi stacks by a retrograde route and thus prevents its entry into the biosynthetic compartments. Monensin had a similar, but less clear-cut, effect on GM3 recycling. These results have important implications for understanding the modulation of cell surface glycolipids.

For the isolation of ganglioside GD3 synthase (EC 2.4.99.8) cDNA, we developed an expression cloning approach that used an anti-GD2 monoclonal antibody for selection. A host recipient cell line that we have named KF3027-Hyg5 was also utilized. This cell line expresses high levels of GM2 as well as GM3 but no GD3 or GD2 and was constructed from mouse B16 melanoma cells transfected with the polyoma large tumor antigen gene (KF3027) and the previously cloned beta-1,4-N-acetylgalactosaminyltransferase (EC 2.4.1.92) cDNA. Four rounds of transfection, monoclonal antibody 3F8 panning, and Hirt extraction resulted in the isolation of two cDNA clones, transfection of which directed the expression of GD3 in KF3027 and B16 melanoma cells and GD3 and GD2 in KF3027-Hyg5 cells. The cDNA contained a 1650-bp insert and a single open reading frame. The deduced amino acid predicted a type II membrane topology consisting of cytoplasmic (14 aa), transmembrane (18 aa), and catalytic (309 aa) domains. The sequence also predicted the presence of a sialyl motif similar to that found in the other sialyltransferases cloned so far. As expected, mRNA of this gene (2.6 kb) was strongly expressed in human melanoma lines.

Using beta 1,4-N-acetylgalactosaminyltransferase (EC 2.4.1.92) complementary DNA, the correlation of gene expression, enzyme activity, and expression of ganglioside antigens was analyzed in 20 human tumor cell lines. In many lines, GM2 and/or GD2 were the most complex structures examined. Northern blot analysis revealed 5.2- and 3.0-kilobase mRNAs in almost all cell lines expressing GD2 and/or GM2. Some melanoma lines, however, showed no bands although they expressed fairly high levels of GD2. These cell lines expressed very high levels of alpha 2,8-sialyltransferase and the resulting product, GD3. Semiquantitative RT-PCR demonstrated that even cell lines with no bands in Northern blot contained 0.4-2.5% of mRNA level in the highest expressing cell line. These results indicate that GD2 expression on individual cell lines is regulated not only by the expression level of the N-acetylgalactosaminyl transferase but also by the amount of its precursor structure (GD3) and alpha 2,8-sialyltransferase present in the cells. beta 1,4-N-acetylgalactosaminyltransferase activities and mRNA levels generally correlated quite closely. A few lines, however, showed lower enzyme activities than expected from their mRNA levels, indicating the possibility that the enzyme is being regulated by translational or posttranslational modification such as phosphorylation and glycosylation as well as by transcriptional regulation. Depending on their patterns of ganglioside synthesis and expression, the lines examined were classified into 6 groups which were characteristic of different tumor cell types.

The ganglioside composition of 20 human malignant melanomas and 5 normal tissues (muscle, spleen, kidney, liver and brain) was analyzed by high-performance thin-layer chromatography (HPTLC) and immune HPTLC using a panel of antiganglioside monoclonal antibodies, and quantified by photodensitometry. The most prominent gangliosides were GM3 and GD3, present in all 20 melanomas; however these were expressed in the 5 normal tissues as well. GD2, GM2, GT3 and 9-O-Ac-GD3 were each expressed in at least 17 of 20 melanomas, but distribution on the normal tissues examined was largely restricted to brain. The detection of several additional glycolipids was studied. GMI was highly expressed in normal brain tissue, but was not detected in any melanoma biopsies, and SGPG was detected in neither. Fuc-GMI was identified in 3 melanoma specimens and a base-sensitive ganglioside, not previously identified in melanoma, was detected in 4 of 20 melanomas with the anti-GD2 MAb 3F8. This compound is most likely O-acetylated GD2. GD3 lactones were identified in 16 of 20 melanoma biopsies, however the proportion that are naturally occurring rather than artifacts of extraction is unclear. The total expression of the more restricted gangliosides (GM2, GD2, GT3 and 9-O-Ac-GD3) in these 20 melanomas ranged between 2.4 and 102.5 micrograms/g, representing 8 x 10(6) to 3 x 10(8) ganglioside molecules per cell. This number of tumor-surface antigens provides the rationale for a polyvalent anti-melanoma vaccine containing GM2, GD2, GT3 and 9-O-Ac-GD3.

In order to elucidate some of the factors that determine the characteristic expression of gangliosides in malignant melanoma and neuroblastoma the levels of ganglioside synthases (glycosyltransferases) were determined in a panel of cell lines from those tumors that exhibited a wide range of ganglioside composition. Sialyltransferases (GM3, GD3, GD1a, and GT1b synthases), N-acetylgalactosaminyltransferases (GM2 and GD2 synthases), and galactosyltransferase (GM1 and GD1b synthases) were analyzed in crude membrane preparations from these cells. The results confirmed the importance of GM3 and GD3 synthases in determining the prominence of the a (GM3 to GT1a) or b (GD3 to GQ1b) biosynthetic pathways. The overall ganglioside composition in cells was found to be dependent on the relative levels of specific enzymes acting sequentially or in competing pathways. In general, the pattern and levels of transferases correlated with the actual ganglioside content of the cell line, although several important discrepancies were noted. For example, in cell lines containing high amounts of GD2 ganglioside, the level of the preceding enzyme in the pathway (GD3 synthase) was unexpectedly low. Thus, the high GD2:GD3 ratios characteristic of most neuroblastomas result from low levels of GD3 synthase as well as high levels of GD2 synthase. In other cell lines, GD3 synthase was completely absent, resulting in the synthesis of GM2, but not GD2, by N-acetylgalactosaminyltransferase I, as would be expected. It was concluded that different glycosyltransferases play key roles in determining glycolipid expression in different cell types.

The reactivity of a panel of antiganglioside monoclonal antibodies with a number of melanoma cell lines having different ganglioside composition profiles was studied. One cell line synthesized only GM3, one produced both GM3 and GD2, 2 had GM3 and GD3 as their major gangliosides, and 2 others synthesized approximately equal amounts of GM3, GM2, GD3, and GD2 gangliosides. Antibody reactivity with viable cells was analyzed by: (a) flow cytometry on suspension cells; and (b) mixed hemagglutination assays or immune adherence assays on monolayer cells in culture. GM3 was efficiently detected only in the cell line having GM3 as its sole ganglioside. In the other cell lines, GM3 was difficult to detect even in cells in which it made up a high proportion (up to 50%) of the total ganglioside content. GM2 was easily detectable only in JB-RH melanoma cells (which contain only GM3 and GM2). GD3 was the most reactive ganglioside in 2 cell lines and GD2 in 2 other lines. In general, the most complex ganglioside present in a cell was the one most accessible to antibody. The differential exposure at the cell surface of specific gangliosides may have implications for antibody-directed tumor detection and therapy and for cell-protein or cell-cell interactions that involve glycolipids.

GM2 and GD2 gangliosides are sialic acid-containing glycosphingolipids expressed in some normal tissues such as brain and in various tumors such as neuroblastomas, astrocytomas, and malignant melanomas. We used a eukaryotic cell transient expression system to isolate cDNA clones that determine GM2 expression. We developed a new cell line from murine melanoma line B16 by transfecting with the polyoma T antigen gene that was suitable for this purpose. Two cDNA clones, both of which have a continuous open reading frame of 1683 base pairs, were isolated. Although the cloned cDNAs had no primary sequence similarity to reported glycosyltransferases, the deduced amino acid sequence predicted a type II transmembrane protein with an overall structure similar to other glycosyltransferases. The cDNA clones, when stably transfected, determined the expression of GM2 in B16 cells and GM2 and GD2 in the human melanoma line MeWo. Northern blot analysis revealed two transcripts in all cells that expressed either GM2 or GD2 or both. These findings indicate that the cDNAs catalyze the transfer of GalNAc onto GM3 and GD3 by a beta 1,4 linkage, resulting in the synthesis of GM2 and GD2, respectively. Namely they suggest that these cDNAs derive from the UDP-GalNAc: GM3/GD3 beta 1,4 N-acetylgalactosaminyltransferase (EC 2.4.1.92) gene.

Human skin melanocytes and melanocytes cultured in vitro express GM3 ganglioside almost exclusively, whereas malignant melanomas express high levels of both GM3 and GD3. We now show that treatment of cultured melanocytes with tumor necrosis factor-alpha, particularly in the presence of tetradecanoylphorbol-13-acetate, results in a change in morphology from spindle-shaped to epithelioid and greatly enhanced expression of GD3 ganglioside. This effect is specific and no other ganglioside is affected, except that GM3 expression (which is already high) is also increased. In contrast, these agents did not enhance the already high expression of GD3 on melanoma cells. This result provides an example of the plasticity of glycolipid expression in mammalian cells and their susceptibility to the influence of biological agents.

GM2 ganglioside is a common cell surface constituent of human melanoma and other tumors of neuroectodermal origin, and vaccination with GM2 ganglioside results in high levels of anti-GM2 antibodies in patients with melanoma. Lymphocytes from a GM2-vaccinated patient (VS) were transformed by Epstein-Barr virus and tested for production of antibodies with reactivity for GM2-positive tumor cells. A high percentage of antibody-producing B cells was detected, but antibody reactivity was generally lost during culture expansion. Two cultures, however, remained stable for antibody productivity and one was used to develop a stable hybrid line with mouse myeloma. The monoclonal antibody (designated 3-207) derived from patient VS has dual specificity for GM2 and GD2, despite the fact that only GM2 antibody could be detected in the patient's serum. Monoclonal antibody 3-207 shows high-titered reactivity with a range of melanoma, astrocytoma, neuroblastoma, and leukemia cell lines, cells with prominent cell surface expression of GM2 and GD2. The cell surface reactivity of monoclonal antibody 3-207 was not abolished by treatment of target cells with neuraminidase, as the enzyme converted GD2 to GM2, which was still detected by monoclonal antibody 3-207.

The specificities of two human monoclonal antibodies (2-39M and 32-27M), produced by hybridomas derived from the lymphocytes of melanoma patients (Yamaguchi, H., Furukawa, K., Fortunato, S. R., Livingston, P. O., Lloyd, K. O., Oettgen, H. F., and Old, L. J. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 2416-2420) have been elucidated. Using a large panel of glycolipids, it has been shown that the two monoclonal antibodies (mAbs) identified a number of N-glycolylneuraminic acid (NeuGc)-containing gangliosides. mAb 2-39M reacted with (NeuGc)GM3, (NeuGc)sialylparagloboside, and (NeuGc)sialylhexaglycosylceramide; no reactivity was observed with gangliosides containing only N-acetylneuraminic acid (NeuAc) or with disialogangliosides. These reactive species have the NeuGc alpha 2----3Gal- sequence in common. mAb 32-27M reacted strongly with (NeuGc)2 GD3 and (NeuGc)2disialylparagloboside, and moderately with (NeuAc-NeuGc-)GD3 and (NeuAc-NeuGc-)disialylparagloboside. The reactive species have sialic acid alpha 2----8NeuGc alpha 2----3Gal- sequences in common. These two antibodies were used to demonstrate the species-related presence of different NeuGc-containing gangliosides in various animal erythrocytes by thin layer chromatography immunostaining. No reactivity of either mAb was observed with gangliosides isolated from fresh human colon cancer, melanoma specimens, or some normal tissues, including brain. On the other hand, it was shown that mAb 32-27M reacted with gangliosides isolated from human melanoma and astrocytoma cells grown in fetal bovine serum but not from those grown in synthetic medium. Within the sensitivities of the methods used, these data, and related chemical analyses, do not support the presence of NeuGc-containing gangliosides in human tumors.

Antisera reactive with the ganglioside GM2 were raised by immunizing C57BL/6 mice with the C57BL/6 melanoma JB-RH. Fusion with NS-1 was performed using splenic mononuclear cells from a mouse with high antibody titer. An immunoglobulin M monoclonal antibody (monoclonal antibody 5-3) was identified which was reactive with an antigen that was resistant to heat, trypsin, and Pronase. A panel of purified glycolipids was used to determine the specificity of monoclonal antibody 5-3. Reactivity was restricted to N-acetyl- and N-glycolyl-GM2. No reactivity was detected with asialo-GM2 or other gangliosides. Monoclonal antibody 5-3 was used to define the expression of GM2 on the cell surface of cultured human normal and malignant cells. Reactivity was seen with cell lines derived from 8 of 8 astrocytomas, 5 of 5 neuroblastomas, 7 of 9 sarcomas, 4 of 18 human melanomas, 2 of 4 murine melanomas, 4 of 37 epithelial cancers and with 0 of 6 skin fibroblast and 0 of 2 brain fibroblast lines. GM2, like GD2 and GD3, appears to be a differentiation antigen largely restricted to cells of neuroectodermal origin.

After the observation that human mAb 32-27M reacts only with melanoma and astrocytoma cells cultured in the presence of fetal bovine serum, a novel pathway for the uptake of exogenous gangliosides, their further biosynthesis, and expression at the cell surface as novel Ag has been elucidated. The addition of fetal bovine serum to melanoma and astrocytoma cells growing in synthetic medium (insulin-transferrin-selenium) resulted in reactivity with Ab32-27M. As antibody 32-27M detects N-glycolylneuraminic acid (NeuGc)-containing gangliosides, the effect of adding a number of different gangliosides to melanoma and astrocytoma cells cultured in the synthetic medium was studied. Only the addition of NeuGc-GM3 resulted in the development of Ab32-27M reactivity. The identity of the antigenic structures developed after addition of fetal bovine serum or NeuGc-GM3 was determined by analysis of the gangliosides from both samples. The major component detected in melanoma cell lines was shown to be N-acetylneuraminic acid-NeuGc-GD3. Another, slower moving component, present in some melanomas and in astrocytomas may be N-acetylneuraminic acid-NeuGc-GD2. The cell type specificity for these processes can be most readily explained by postulating that all cells can take up exogenous gangliosides but only melanoma and astrocytoma cells have sufficiently high levels of GM3 alpha 2----8-sialyltransferase for the conversion of added NeuGc-GM3 to disialogangliosides to be effective. These results demonstrate a novel pathway for exogenous glycolipid processing that can lead to novel Ag expression but may also play a role in normal glycolipid metabolism and function.

Monoclonal antibody (MAb) R24 was previously shown to be directed toward ganglioside GD3 [Pukel, C. S., Lloyd, K. O., Travassos, L. R., Dippold, W. G., Oettgen, H. F., and Old, L. J. (1982) J. Exp. Med. 155, 1133-1147]. The structural specificity of the MAb has now been further characterized based on binding to structurally related glycolipids, including four GD3 derivatives with different N-acetylneuraminic acid (NeuAc) and N-glycolylneuraminic acid (NeuGc) substituents. Three assay systems (enzyme immunostaining on thin-layer chromatography, enzyme-linked immunosorbent assay, and immune adherence inhibition assay) were used. MAb R24 was found to react with (NeuAc-NeuAc-)GD3 and (NeuAc-NeuGc-)GD3 but not with (NeuGc-NeuAc-)GD3 or (NeuGc-NeuGc-)GD3. These results clearly indicate that the outer sialic acid (Sia) moiety of GD3 is crucial and must be a NeuAc residue, while the inner sialic acid is less involved in binding to the MAb and can be either NeuAc or NeuGc. The MAb was also found to cross-react weakly with two gangliosides, GT1a and GQ1b, but none of other gangliosides nor neutral glycolipids tested reacted. These findings suggest that the epitope detected by MAb R24 is the trisaccharide structure NeuAc alpha 2----8Sia alpha 2----3Gal-, which must be in a terminal position.

Human melanoma cells express high levels of GM3 and GD3 gangliosides whereas normal melanocytes have only low levels of GD3 but maintain their expression of GM3. In order to understand the basis for this difference, the levels of the sialyltransferase that converts GM3 to GD3 (CMP-N-acetylneuraminic acid:GM3 sialyltransferase or GD3 synthase, EC 2.4.99.8) were analyzed in melanoma and other cell lines. Enzyme levels were determined in vitro using membrane preparations and measuring the addition of [14C]-N-acetylneuraminic acid from CMP-[14C]-N-acetylneuraminic acid to GM3 in the presence of Triton CF-54. Sialyltransferase levels in 44 human cancer cell lines (including melanoma, neuroblastoma, astrocytoma, various carcinomas, and leukemias) and cultures of normal melanocytes and kidney epithelial cells were compared, and the products were identified by thin layer chromatography and fluorography. Melanoma cell lines exhibited the highest levels of incorporation and GD3 was found to be the major product. GM3 was also formed, apparently from endogenous lactosylceramide. Very low levels of GD3 synthase were found in normal melanocytes. Neuroblastoma and some astrocytoma cell lines also had significant levels of GD3 synthase. Some other cell lines incorporated high levels of radioactivity but the products did not correspond to GD3 and the major product was usually GM3. In general the levels of GD3 synthase correlated with the expression of GD3 in the various cell types. These results point to higher levels of GD3 synthase being directly responsible for the enhanced expression of GD3 in melanoma.

A number of gangliosides were isolated from cat and sheep erythrocytes for use in analyzing the specificity of a panel of human anti-heterophile monoclonal antibodies. The structures of these compounds were determined by a combination of different procedures, including sugar analysis, glycosidase treatment, periodate oxidation, TLC immunostaining, methylation analysis, and mass spectrometry. These methods identified the cat erythrocytes gangliosides (C1 and C2) as N-glycolylneuraminic acid (NeuGc)-containing hematosides; C1 was shown to be NeuGc alpha 2----8NeuGc alpha 2----3Gal beta I----4Glc-Cer [NeuGc)2GD3) and C2 to be NeuAc alpha 2----8NeuGc alpha 2----3Gal beta 1----4Glc-Cer [NeuAc-NeuGc-)GD3). The two sheep gangliosides (S1 and S2) were found to be novel glycolipids based on the paragloboside sequence; S1 was identified as NeuGc alpha 2----8NeuGc alpha 2----3Gal beta 1----4GlcNAc beta 1----3Gal beta 1----4Glc-Cer [NeuGc)2-disialylparagloboside) and S2 as NeuAc alpha 2----8NeuGc alpha 2----3Gal beta 1----4GlcNAc beta 1----3Gal beta 1----4Glc-Cer [NeuAc-NeuGc-)-disialylparagloboside). Structural analysis of these compounds was aided by the use of 252Cf fission fragment ionization time-of-flight mass spectrometry. This method provided easily interpretable spectra on methylated derivatives which were particularly useful in determining the sialic acid composition of the gangliosides and the sequence of their disialosyl side chains.

The fine specificity analysis of two human monoclonal antibodies (AbFCM1 and AbHJM1) reacting with gangliosides is described and their specificities are compared with analogous mouse monoclonal antibodies (mAbs). These two antibodies were generated from lymphocytes of melanoma patients by Epstein-Barr virus transformation followed by fusion with mouse myeloma NS-1. Using a wide variety of gangliosides, including N-glycolylneuraminic acid (NeuGc)-containing compounds, the precise structures recognized by these two antibodies were elucidated by enzyme-linked immunosorbent assay and immunostaining of thin-layer chromatograms. AbFCM1 reacted with N-acetylneuraminic acid (NeuAc)-type GM3, GD1a, sialylparagloboside, and GT1b in decreasing order of intensity. This antibody also reacted with (NeuAc-NeuGc-)-GD3 and -disialylparagloboside, but did not react with NeuGc-type GM3, GM2, sialylparagloboside, (NeuGc)2-GD3 and -disialylparagloboside. The main epitope structures recognized by AbFCM1 are, therefore, NeuAc alpha 2----3Gal beta 1- and NeuAc alpha 2----8NeuGc alpha 2----Gal beta 1-. These results are similar to the specificity of mouse mAb M2590. AbHJM1 reacted with NeuAc-type GD3 and disialylparagloboside, GD2, GD1b, GM3, and GT1b, in decreasing order of intensity. Among NeuGc-type gangliosides, this antibody reacts with (NeuAc-NeuGc-)-GD3 and -disialylparagloboside, but did not react with gangliosides containing only NeuGc. Consequently the epitope structure recognized by AbHJM1 is probably (R)-(NeuAc alpha 2----8Sialic acid alpha 2----3)Gal beta 1-. Mouse anti-GD3 mAbR24, in contrast, showed strong reactivity only with GD3 and -disialylparagloboside among NeuAc-type gangliosides, but showed a similar pattern to AbHJM1 in its reactivity with NeuGc-containing gangliosides. Although these two human monoclonal antibodies are not highly restricted in their specificities, they reacted best with the major gangliosides, GM3 and GD3, present in the majority of human melanomas.

AH antigen, initially defined by an antibody present in a melanoma patient, is a cell surface antigen found on approximately 65% of melanoma cell lines. Absorption analysis indicates that AH is a differentiation antigen marking normal and malignant cells of neuroectodermal origin. The AH determinant has been found to be related to GD2 ganglioside.

Mouse monoclonal antibody AbR24 has a high degree of specificity for human melanoma cells when tested on viable cultured cells using the protein A mixed hemagglutinin serological assay. The antigen detected by this antibody has been isolated from melanoma cells and shown to be GD3 ganglioside by compositional and partial structural analysis and by comparison with authentic GD3 in thin layer chromatography (TLC). AbR24 reacts with authentic GD3, but not with any other ganglioside tested. Using TLC and reactivity with AbR24, a wide range of cells and tissues was examined for the presence of GD3. A new serological assay, termed glycolipid-mediated immune adherence, was devised for assaying the reactivity of AbR24 with gangliosides. Melanomas (cultured cells or tumor tissue) were shown to have GD3 and GM3 as major gangliosides. Other cells and tissues examined also contained GD3, but usually only in low amounts. Melanomas (and MOLT-4, a T cell line) were characterized by a simplified ganglioside profile with GD3 and GM3 as major components. The apparent discrepancy between the ubiquitous presence of GD3 and the serological specificity of AbR24 for melanoma cells can be explained in terms of localization and concentration of GD3 in different cells.