Structural basis for altered soybean agglutinin lectin binding between a murine metastatic lymphoma and an adhesive low malignant variant

CD24 is a marker for human breast carcinoma

CD24 is a small, mucin-type glycosylphosphatidylinositol-linked cell surface molecule expressed by neutrophils, pre B lymphocytes and certain human tumor cell lines. CD24 has been identified as a ligand for P-selectin in both mouse and human cells. We previously reported that the P-selectin-CD24 binding pathway is important for the binding of the breast carcinoma cell line KS to platelets and the rolling of these cells on endothelial P-selectin. In the present study we have analyzed the expression of CD24 on human breast carcinoma cell lines and on fresh breast carcinoma specimens using the CD24-specific antibody ML-5. Our study clearly demonstrates that CD24 is abundantly expressed on cell lines and fresh tissues of breast carcinomas. We find a differential expression of CD24 in breast carcinomas (cytoplasmic pattern) versus benign breast lesions (apical pattern). Moreover, the intensity of CD24 expression increases with the histological grade of the tumor. Thus, CD24 expression might be a useful marker for human breast carcinoma and play a role in facilitating metastasis by the interaction between tumor cells and platelets or endothelial cells.

Metalloproteinase-mediated release of the ectodomain of L1 adhesion molecule

The L1 adhesion molecule is an approx. 200–220 kDa type I membrane glycoprotein belonging to the immunoglobulin (Ig) superfamily. L1 can bind in a homotypic fashion and was shown to support integrin-mediated binding via RGDs in the 6th Ig-like domain. In addition to its cell-surface expression, L1 can occur in the extracellular matrix (ECM). Here we demonstrate that L1 is constitutively released from the cell surface by membrane-proximal cleavage. L1 shed from B16F10 melanoma cells remains intact and can serve as substrate for integrin-mediated cell adhesion and migration. The release of L1 occurs in mouse and human cells and is blocked by the metalloproteinase inhibitor TAPI (Immunex compound 3). This compound has been shown previously to block release of L-selectin and TNF-alpha which is mediated by the membrane-bound metalloproteinase TNF-alpha converting enzyme (TACE). Using CHO cells that are low in TACE expression and do not release L-selectin we demonstrate that L1 release is distinct from L-selectin shedding. We propose that cell-surface release may be necessary for the conversion of L1 from a membrane into an ECM protein.

Asp-698 and Asp-811 of the integrin alpha4-subunit are critical for the formation of a functional heterodimer

The amino acid motif LDV is the principal binding site for alpha4 integrins in fibronectin, and homologous motifs are recognized in vascular cell adhesion molecule-1 and MAdCAM-1. Three conserved LDV motifs (LDV-1 to 3) occur in the ectodomain of the human and mouse alpha4-subunit, the functions of which are unknown. We demonstrate here that alpha4-transfected fibroblasts with mutation in LDV-1 (D489N) behaved like alpha4-wild type but that LDV-2 (D698N) and LDV-3 (D811N) mutants were impaired in binding and spreading on alpha4-specific substrates. On the RGD-containing fibronectin fragment FN-120 there was an inverse behavior; now the alpha4-wild type and the LDV-1 mutant could not adhere whereas the two other mutants could. The beta1 chain was critical for the differential integrin response. Biochemical analysis demonstrated that the LDV-2 and -3 mutations reduced the strength of the alpha4beta1 association, favored the formation of alpha5beta1, and prevented the expression of alpha4beta7 on the cell surface. Our results indicate that LDV-2 and LDV-3 are critical for the formation of a functional heterodimer. The presence of similar amino acid motifs in ligands and the alpha4-subunit suggest that metal coordination plays an important role in integrin-ligand binding as well as for heterodimer formation.

CD24, a Mucin-Type Glycoprotein, Is a Ligand for P-Selectin on Human Tumor Cells

P-selectin (CD62P) is a Ca2+-dependent endogenous lectin that can be expressed by vascular endothelium and platelets. The major ligand for P-selectin on leukocytes is P-selectin glycoprotein ligand-1 (PSGL-1). P-selectin can also bind to carcinoma cells, but the nature of the ligand(s) on these cells is unknown. Here we investigated the P-selectin binding to a breast and a small cell lung carcinoma cell line that are negative for PSGL-1. We report that CD24, a mucin-type glycosylphosphatidylinositol-linked cell surface molecule on human neutrophils, pre B lymphocytes, and many tumors can promote binding to P-selectin. Latex beads coated with purified CD24 from the two carcinoma cell lines but also neutrophils could bind specifically to P-selectin-IgG. The binding was dependent on divalent cations and was abolished by treatment with O-sialoglycoprotein endopeptidase but not endoglycosidase F or sialidase. The beads were stained with a monoclonal antibody (MoAb) to CD57 (HNK-1 carbohydrate epitope) but did not react with MoAbs against the sialylLex/a epitope. The carcinoma cells and CD24-beads derived from these cells could bind to activated platelets or P-selectin transfected Chinese hamster ovary cells (P-CHO) in a P-selectin–dependent manner and this binding was blocked by soluble CD24. Transfection of human adenocarcinoma cells with CD24 enhanced the P-selectin–dependent binding to activated platelets. Treatment of the carcinoma cells or the CD24 transfectant with phosphatidylinositol-specific phospholipase C reduced CD24 expression and P-selectin–IgG binding concomitantly. These results establish a role of CD24 as a novel ligand for P-selectin on tumor cells. The CD24/P-selectin binding pathway could be important in the dissimination of tumor cells by facilitating the interaction with platelets or endothelial cells.

L1 adhesion molecule on human lymphocytes and monocytes: expression and involvement in binding to alpha v beta 3 integrin

The L1 adhesion molecule is a member of the immunoglobulin (Ig) superfamily initially identified in the nervous system which contains six Ig-like domains. Besides the known L1-L1 homotypic interaction, L1 was recently shown to bind to very late antigen (VLA)-5 in the mouse and alpha v beta 3 in the human. The sixth Ig domain is critical for this function. We now demonstrate that human CD4+ peripheral blood T lymphocytes, monocytes and B lymphocytes, but not CD8+ T lymphocytes, express L1. When compared to the expression of CD31, another ligand for alpha v beta 3 on T lymphocytes, only a small proportion of cells were CD31+L1+ double positive. L1 was also detected on the surface of human monocytic and lymphoid tumor lines and was shown to have a molecular mass of approximately 220 kDa, similar to the molecule present on neuroblastoma cells. The function of the sixth Ig domain of human L1 as an integrin ligand was also investigated. Using an RGD-containing peptide derived from the sixth Ig domain as well as a fusion protein of the sixth Ig domain of L1 and the Fc portion of human IgG1 (6.L1-Fc), we demonstrated the binding of human MED-B1 (alpha v beta 3hi, alpha 5 beta 1lo) tumor cells and this binding was blocked by alpha v-specific mAb. In contrast, human Nalm-6 cells (alpha v beta 3lo, alpha 5 beta 1hi) did not bind to the 6.L1-Fc fusion protein. MED-B1 cells could also be stained with the 6.L1-Fc fusion protein. Our results suggest that human L1 binds predominantly to alpha v beta 3 and that its presence on leukocytes could be important for adhesion and migration.

The L1 adhesion molecule is a cellular ligand for VLA-5

The L1 adhesion molecule is a member of the immunoglobulin superfamily shared by neural and immune cells. In the nervous system L1 can mediate cell binding by a homophilic mechanism. To analyze its function on leukocytes we studied whether L1 could interact with integrins. Here we demonstrate that VLA-5, an RGD-specific fibronectin receptor on a wide variety of cell types, can bind to murine L1. Mouse ESb-MP cells expressing VLA-5 and L1 could be induced to aggregate in the presence of specific mAbs to CD24 (heat-stable antigen), a highly and heterogeneously glycosylated glycophosphatidylinositol-linked differentiation antigen of hematopoietic and neural cells. The aggregation was blocked by both mAbs to L1 and VLA-5, respectively. Aggregation was blocked also by a synthetic RGD-containing peptide derived from the Ig-domain VI of the L1 protein. ESb-MP subclones with low L1 expression could not aggregate. In heterotypic binding assays mouse bone marrow cells could adhere in an L1-dependent fashion to platelets that expressed VLA-5. Also purified L1 coated to polystyrene beads could bind to platelets. The binding of L1-beads was again inhibited by mAbs to L1 and VLA-5, by soluble L1 and the L1-RGD peptide in a dose-dependent manner. Thymocytes or human Nalm-6 tumor cells expressing VLA-5 could adhere to affinity-purified L1 and to the L1-derived RGD-containing peptide coated to glass slides. The adhesion was strongly enhanced in the presence of Mn(2+)-ions and blocked by mAbs to VLA-5. We also demonstrate a direct L1-VLA-5 protein interaction. Our results suggest a novel binding pathway, in which the VLA-5 integrin binds to L1 on adjacent cells. Given its rapid downregulation on lymphocytes after induction of cell proliferation, L1 may be important in integrin-mediated and activation-regulated cell-cell interactions.

The alpha 4 integrin chain is a ligand for alpha 4 beta 7 and alpha 4 beta 1

The heterodimeric alpha 4 integrins alpha 4 beta 7 lymphocyte Peyer's patch adhesion molecule ([LPAM]-1) and alpha 4 beta 1 (very late antigen-4) are cell surface adhesion molecules involved in lymphocyte trafficking and lymphocyte-cell and matrix interactions. Known cellular ligands include vascular cell adhesion molecule (VCAM)-1, which binds to alpha 4 beta 1 and alpha 4 beta 7, and the mucosal addressin cell adhesion molecule (MAdCAM)-1, which binds to alpha 4 beta 7. Here we show that the alpha 4 chain of these integrins can itself serve as a ligand. The alpha 4 chain, immunoaffinity purified and immobilized on glass slides, binds thymocytes and T lymphocytes. Binding exhibits divalent cation requirements and temperature sensitivity which are characteristic of integrin-mediated interactions, and is specifically inhibited by anti-alpha 4 integrin antibodies, which exert their effect at the cell surface. Cells expressing exclusively alpha 4 beta 7 (TK-1) or alpha 4 beta 1 (L1-2) both bound avidly, whereas alpha 4-negative cells did not. A soluble 34-kD alpha 4 chain fragment retained binding activity, and it inhibited lymphocyte adhesion to alpha 4 ligands. It has been shown that alpha 4 integrin binding to fibronectin involves an leucine-aspartic acid-valine (LDV) motif in the HepII/IIICS region of fibronectin (CS-1 peptide), and homologous sequences are important in binding to VCAM-1 and MAdCAM-1. Three conserved LDV motifs occur in the extracellular sequence of alpha 4. A synthetic LDV-containing alpha 4-derived oligopeptide supports alpha 4-integrin-dependent lymphocyte adhesion and blocks binding to the 34-kD alpha 4 chain fragment. Our results suggest that alpha 4 beta 7 and alpha 4 beta 1 integrins may be able to bind to the alpha 4 subunit on adjacent cells, providing a novel mechanism for alpha 4 integrin-mediated and activation-regulated lymphocyte interactions during immune responses.

Heat-stable antigen/CD24 on mouse T lymphocytes: evidence for a costimulatory function

Heat-stable antigen (HSA)/mouse CD24 (formerly termed Nectadrin) is a membrane glycoprotein with an unusual structure consisting of a small protein core and extensive glycosylation. It is expressed by hematopoietic cells but not by mature T lymphocytes. HSA on accessory cells is an important costimulatory molecule required for the clonal expansion of T lymphocytes. HSA is also involved in cell-cell adhesion events and the isolated antigen has been shown to possess self-binding properties. In the present study we have re-investigated the role of HSA in T cell proliferation. We find that following stimulation of T lymphocytes with concanavalin A or of CD4+ T lymphocytes with a combination of anti-CD3/CD28 monoclonal antibodies (mAb) the HSA antigen is transiently expressed. The expression correlated with the appearance of CD25 and a CD2 activation epitope at the cell surface. Induction of HSA was also seen in vivo on V beta 8+ T lymphocytes in BALB/c mice that were injected with Staphylococcal enterotoxin B. Biosynthetic labeling and analysis of mRNA by reverse transcriptase-polymerase chain reaction showed that HSA was synthesized by activated T lymphocytes. A combination of anti-CD3 and mAb 79 to HSA was incapable of inducing proliferation of purified CD4+ T lymphocytes. However, the antibody strongly enhanced the response obtained with a combination of anti-CD3/CD28 mAb. The augmenting effect of the HSA-specific mAb was dose dependent. Since HSA is bound to the membrane via a glycosyl-phosphatidylinositol (GPI) anchor and GPI-anchored molecules have been implicated in lymphocyte activation, it is conceivable that HSA is not only a costimulatory molecule on accessory cells but is also a signaling molecule in T lymphocytes. The possibility of a homotypic HSA/HSA interaction between T lymphocytes and accessory cells is discussed.

Expression of PNA-binding sites on specific glycoproteins by human melanoma cells is associated with a high metastatic potential

Lectin-binding patterns of seven human melanoma clones and variants selected from the same parental cell line and differing in their spontaneous metastatic potential in an animal model were compared by flow cytometry and Scatchard analysis. Human melanoma clones and variants with high and low metastatic potential could be distinguished by their peanut agglutinin (PNA)-binding patterns, but not by their wheat germ agglutinin (WGA)-, Ulex europaeus agglutinin I (UEA I)-, and soybean agglutinin (SBA)-binding patterns. Low metastatic clones and variants proved to be made up of single poorly peanut agglutinin-binding cell population (2.20-3.52 x 10(6) sites/cell, Ka = 2.48-2.75 x 10(6) M-1). By contrast, highly metastatic variants were found to be constituted by two cellular subpopulations, exhibiting respectively a moderate 2.62-3.72 x 10(6) sites/cell) and a high peanut agglutinin staining (17.68-18.76 x 10(6) sites/cell). One highly metastatic clone was found to be homogeneously constituted by a single population of cells strongly binding this lectin (18.86 x 10(6) sites/cell) with an association constant of 4.06 +/- 10(6) M-1. Using an EPICS V cytometer, these two subpopulations were sorted from a highly metastatic variant and tested for their metastatic abilities: cells with high PNA binding generated a higher frequency of metastases than did moderately PNA-binding cells. Following treatment with Vibrio cholerae neuraminidase, all cells from all variants and clones were brightly labeled by PNA, collecting in a single peak with similar fluorescence intensities. Electrophoresis of total cellular proteins and subsequent detection with labeled PNA om Western blots show two major PNA-reactive glycoproteins with apparent molecular weights of 140 and 110 kDa (MAGP1 and MAGP2), expressed only in highly metastatic cells, but which can be strongly labeled by PNA in slightly metastatic cells following a treatment with neuraminidase. These results provide evidence that the expression of terminal galactose (beta 1-3)N-acetyl galactosamine structure, positioned on MAGP1 and MAGP2 glycoproteins, is associated with the metastatic potential of human melanoma cells.

Expression of L1 cell adhesion molecule is associated with lymphoma growth and metastasis

The cell adhesion molecule (CAM) L1 is involved in homotypic and heterotypic adhesion between neural cells. It has recently also been identified on leucocytes. We have investigated the expression of L1 on hematopoietic tumor cell lines and found that several tumors including the ESb-MP lymphoma are positive for L1. A potential role for L1 in spontaneous metastasis formation was examined using these cells. From wild-type (wt) L1high lymphoma cells we selected by a fluorescence-activated cell sorter (FACS) stable L1low expression variants. Syngeneic DBA/2 mice injected subcutaneously with L1low clones showed faster primary tumor growth, developed visceral metastases significantly faster and died earlier than animals carrying L1high wt cells. L1 high revertants from the L1low variants showed again a reduced metastatic capacity and a malignancy similar to the wt cells. Expression of L1 on the tumor variants and revertants correlated directly with their homotypic aggregation behaviour in vitro. L1 expression correlated negatively with metastatic capacity. These results suggest that L1 molecules may contribute to the overall malignant potential of the lymphoma cells, presumably by interfering with cell-cell interactions critical for tumor growth and dissemination.

Newly established human pancreatic carcinoma cell lines and their lectin binding properties

Two human pancreatic carcinoma cell lines, designated HuP-T3 and HuP-T4, were established from the ascites of pancreatic cancer patients with carcinomatous peritonitis. The cell lines were grown in monolayer cultures and had population doubling times of 38.6 and 37.1 h, respectively. HuP-T4 secreted large amounts of CEA and CA19-9 into the medium, and HuP-T3 produced a small amount of CEA, but no CA19-9. Both cell lines showed tumorigenicity in nude mice. Histologically, the HuP-T3-derived tumor was poorly differentiated adenocarcinoma, and the HuP-T4-derived tumor was well differentiated papillotubular adenocarcinoma. In lectin histochemistry at both light and electron microscopic levels, the most striking difference between the lectin binding properties of the two cell lines and those of control normal pancreatic ductal cells was that soybean agglutinin (SBA) bound to both cell lines, but not to controls. These newly established cell lines should be useful models that will contribute to clarifying the biological and biochemical characteristics of pancreatic cancer.

Nectadrin, the heat-stable antigen, is a cell adhesion molecule

Nectadrin, the cell surface glycoprotein recognized by the novel mAb 79, was found to be immunologically identical to the heat-stable antigen (HSA). It is a glycoprotein with a polypeptide core of only 30 amino acids and a very high carbohydrate content (Wenger, R. H., M. Ayane, R. Bose, G. Köhler, and P. J. Nielsen. 1991. Eur. J. Immunol. 21:1039-1046). Immunocytological studies using cultured splenic B-lymphocytes, neuroblastoma cells, and cerebellar cells indicated that nectadrin is preferentially expressed at sites of cell-cell contact. Purified nectadrin and monoclonal nectadrin antibody 79, but not other monoclonal nectadrin antibodies, inhibited the aggregation of B-lymphocytes by 70%, suggesting that nectadrin may act as a cell adhesion molecule. Nectadrin was purified from a mouse lymphoma cell line in two forms of 40-60 and 23-30 kD. The lower molecular weight form appears to be generated from the higher molecular weight form by degradative removal of saccharide residues characteristic of complex type oligosaccharide side chains. Latex beads coated with purified nectadrin aggregated and the rate of their aggregation depended on the molecular form of nectadrin, with the larger form being more potent than the smaller one in mediating bead aggregation. Nectadrin thus appears to be a self-binding cell adhesion molecule of a structurally novel type in that its extensive glycan structures may be implicated in mediating cell adhesion.

Expression and function of the neural cell adhesion molecule L1 in mouse leukocytes

The neural cell adhesion molecule L1 is a cell surface glycoprotein of the immunoglobulin superfamily which mediates adhesion between neural cells. The possibility that similar cell-cell recognition mechanisms may be shared by the nervous and immune systems prompted us to study the expression and function of L1 in cells of the hematopoietic system. Immunofluorescence analysis using monoclonal L1 antibody revealed that the molecule is expressed in the bone marrow, spleen, and thymus of the mouse. This observation was confirmed by amplifying cDNA derived from these organs by the polymerase chain reaction with L1-specific oligonucleotide primers. Two-color fluorescence analysis indicated that bone marrow lymphoid and granulocyte precursor cells express low and high levels of L1, respectively. In the thymus L1 is primarily expressed by mature cells that have a strong expression of CD3 and in the spleen both B cells and T cells express L1. The possible function of L1 in lymphoid cells was studied using subcloned ESb-MP lymphoma cells having high or low densities of L1 on the cell surface as well as activated splenic B lymphoblasts. Parental and subcloned ESb-MP cells that strongly expressed L1 could form homotypic aggregates in the presence of low Ca2+ levels, whereas subcloned ESb-MP cells with a weak expression of L1 did not aggregate, suggesting that L1 mediates the Ca(2+)-independent aggregation of the parental ESb-MP cells. Furthermore, the aggregation of activated B lymphoblasts under physiological concentrations of Ca2+ and Mg2+ was inhibited by 30% in the presence of Fab fragments of polyclonal L1 antibodies, implying that L1 also mediates adhesion among normal lymphoid cells. A possible role of L1 on lymphocytes in stimulating the innervation of lymphoid organs is discussed.

The neural cell adhesion molecule N-CAM enhances L1-dependent cell-cell interactions

On neural cells, the cell adhesion molecule L1 is generally found coexpressed with N-CAM. The two molecules have been suggested, but not directly shown, to affect each other's function. To investigate the possible functional relationship between the two molecules, we have characterized the adhesive interactions between the purified molecules and between cultured cells expressing them. Latex beads were coated with purified L1 and found to aggregate slowly. N-CAM-coated beads did not aggregate, but did so after addition of heparin. Beads coated with both L1 and N-CAM aggregated better than L1-coated beads. Strongest aggregation was achieved when L1-coated beads were incubated together with beads carrying both L1 and N-CAM. In a binding assay, the complex of L1 and N-CAM bound strongly to immobilized L1, but not to the cell adhesion molecules J1 or myelin-associated glycoprotein. N-CAM alone did not bind to these glycoproteins. Cerebellar neurones adhered to and sent out processes on L1 immobilized on nitrocellulose. N-CAM was less effective as substrate. Neurones interacted most efficiently with the immobilized complex of L1 and N-CAM. They adhered to this complex even when its concentration was at least 10 times lower than the lowest concentration of L1 found to promote adhesion. The complex became adhesive for cells only when the two glycoproteins were preincubated together for approximately 30 min before their immobilization on nitrocellulose. The adhesive properties between cells that express L1 only or both L1 and N-CAM were also studied. ESb-MP cells, which are L1-positive, but N-CAM negative, aggregated slowly under low Ca2+. Their aggregation could be completely inhibited by antibodies to L1 and enhanced by addition of soluble N-CAM to the cells before aggregation. N2A cells, which are L1 and N-CAM positive aggregated well under low Ca2+. Their aggregation was partially inhibited by either L1 or N-CAM antibodies and almost completely by the combination of both antibodies. N2A and ESb-MP cells coaggregated rapidly and their interaction was similarly inhibited by L1 and N-CAM antibodies. These results indicate that L1 is involved in two types of binding mechanisms. In one type, L1 serves as its own receptor with slow binding kinetics. In the other, L1 is modulated in the presence of N-CAM on one cell (cis-binding) to form a more potent receptor complex for L1 on another cell (trans-binding).

Functional cooperation between the neural adhesion molecules L1 and N-CAM is carbohydrate dependent

The neural cell adhesion molecules L1 and N-CAM have been suggested to interact functionally by formation of a complex between the two molecules (Kadmon, G., A. Kowitz, P. Altevogt, and M. Schachner. 1990. J. Cell Biol. 110:193-208). To determine the molecular mechanisms underlying this functional cooperation, we have studied the contribution of carbohydrates to the association of the two molecules at the cell surface. Aggregation or adhesion between L1- and N-CAM-positive neuroblastoma N2A cells was reduced when the synthesis of complex and/or hybrid glycans was modified by castanospermine. Fab fragments of polyclonal antibodies to L1 inhibited aggregation and adhesion of castanospermine-treated cells almost completely, whereas untreated cells were inhibited by approximately 50%. Fab fragments of polyclonal antibodies to N-CAM did not interfere with the interaction between castanospermine-treated cells, whereas they inhibited aggregation or adhesion of untreated cells by approximately 50%. These findings indicate that cell interactions depending both on L1 and N-CAM ("assisted homophilic" binding) can be reduced to an L1-dominated interaction ("homophilic binding"). Treatment of cells with the carbohydrate synthesis inhibitor swainsonine did not modify cell aggregation in the absence or presence of antibodies compared with untreated cells, indicating that castanospermine-sensitive, but swainsonine-insensitive glycans are involved. To investigate whether the appropriate carbohydrate composition is required for an association of L1 and N-CAM in the surface membrane (cis-interaction) or between L1 on one side and L1 and N-CAM on the other side of interacting partner cells (trans-interaction), an L1-positive lymphoid tumor cell line was coaggregated with and adhered to neuroblastoma cells in the various combinations of castanospermine-treated and untreated cells. The results show that it is the cis-interaction between L1 and N-CAM that depends on the appropriate carbohydrate structures.

Identical forms of the CD2 antigen expressed by mouse T and B lymphocytes

A monoclonal antibody (12-15) reactive with the mouse CD2 was used to study the expression of the antigen in different lymphoid cell subsets. By two-color immunofluorescence using B or T cell-specific reagents and cell sorting in combination with biochemical analysis we provide evidence that the CD2 antigen is present on mouse B and T cells. The antigen is expressed by both subsets at similar density and appears to be biochemically indistinguishable.

Antibody 12-15 cross-reacts with mouse Fc gamma receptors and CD2: study of thymus expression, genetic polymorphism and biosynthesis of the CD2 protein

Previously we described a monoclonal antibody (mAb 12-15) that reacted with murine Fc receptor proteins (beta 1, beta 2 and alpha) and an undefined molecule of 37 kDa (beta 3) on certain types of cells. Here we present serological and biochemical evidence that the beta 3 chain is expressed on mouse thymocytes and that it is identical to the CD2 antigen. By immunofluorescence staining and cytofluorographic analysis greater than 95% of thymocytes were positive. Brightly staining cells coincided with cortisone-resistant thymocytes suggesting that mature thymocytes expressed higher levels of the antigen. Biosynthetic labeling of DBA/2 thymocytes with [35S]methionine showed that the size of the CD2 precursor molecule was 43 kDa which was processed to approximately 55-65 kDa in the mature molecule. mAb 12-15 was also reactive with the tunicamycin-treated form of the CD2 antigen suggesting that the cross-reactive epitope was of protein nature. Comparison of different mouse strains indicated that two molecular forms of CD2 exist. On BALB/c thymocytes, the relative mass of the native molecule was approximately 60-70 kDa (CD2.1) and slightly larger than in DBA/2 (CD2.2). Following endoglycosidase F treatment both proteins still showed a slight difference in electrophoretic mobility. Several inbred mouse strains were analyzed for expression of CD2 forms. When mAb 12-15 was used in cytotoxic T lymphocyte inhibition experiments using specific CTL and tumor target cells it was found that the antibody could specifically inhibit CTL-mediated lysis presumably by interfering with CD2 function.

Generation of adhesive tumor variants: chromosomal changes, reduction in malignancy and increased expression of a distinct membrane glycoprotein

Tumor cell variants which grow adherent to a plastic surface could be isolated in a reproducible way from the high metastatic tumor cell line ESb which grows in a suspension culture. This occurred when starting selection from the uncloned parental line as well as from a freshly derived non-adhesive subclone. The variants showed changes in their karyotype. These were quantitative (tetraploidization) and qualitative (single chromosome aberrations involving the chromosomes 12 and 17 and a marker MX-7). Phenotypic cell surface changes were documented in vitro by immunofluorescence using a monoclonal antibody (mAb 12-15) directed against a distinct plasma membrane glycoprotein of 60-69kD (gp 60-69). The expression of gp 60-69 increased with time of selection for adherence to plastic surface. The adherent cells showed in all cases a greatly reduced overall malignancy as seen by a prolonged survival time of respective tumor bearing animals compared with the suspension growing parental cells.

Concanavalin A-induced impairment of fibroblast spreading on laminin but not on fibronectin

The effect of concanavalin A (Con A) on the adhesion of 8-day-old chick embryo fibroblasts (CEFs) to fibronectin (FN) and laminin (LM) was studied. Con A was shown to inhibit the spreading of CEF on a LM substrate. In contrast, no inhibition of CEF spreading on the FN substrate could be detected when the quantity of FN coated varied from 0.5 to 4 pmoles. The effect induced by Con A was specific, since it was abolished by 100 mM alpha-methylmannopyranoside. The inhibition of CEF spreading was only observed when the lectin was added during the 20 min following cell plating. In addition, the effect of Con A on CEF spreading on the LM substrate was shown to be dependent upon its presence at the cell surface, since under conditions which accelerate the uptake of the lectin, the effect on cell spreading is no longer detectable. Furthermore, the number of CEFs attached to LM was not modified by the lectin. The molecular weight of the isolated Con A binding sites revealed glycoproteins ranging from 30,000 to 72,000. On the other hand, these Con A binding sites did not interact with LM-Sepharose. Only a protein with a molecular weight of 68,000 which did not express affinity for Con A bound tightly to the LM-Sepharose. These data suggested that cell surface Con A binding sites do not interfere with the initial step of CEF adhesion to LM but play a key role during their spreading on this glycoprotein.

Murine Fc gamma receptor proteins: identification of a previously unrecognized molecule with a monoclonal antibody (12-15)

Previously we studied differential expression of cell surface molecules between the metastatic murine lymphoma ESb and an adhesion variant ESb-MP. Here we describe the specificity of a monoclonal antibody (12-15) that showed strong binding to the adhesion variant and weak reactivity against ESb cells. The antibody also reacted to lymphoid but not to macrophage-derived cell lines and immunoprecipitated a molecule of approx. 60-69 kDa from ESb-MP cells. N-terminal sequencing of the antigen revealed identity to the beta protein of mouse Fc gamma receptors. Using monoclonal antibodies against Fc gamma receptors (2.4G2 and K9.361) in immunofluorescence assays and cDNA probes specific for alpha, beta 1 and beta 2 Fc receptor transcripts in Northern blot experiments the differential expression of Fc receptors in ESb and ESb-MP cells was confirmed. Biochemical analysis of endoglycosidase F-treated precipitates revealed that antibody 12-15 reacted to products of all three transcripts with molecular masses for the protein core of 38.5 kDa (beta 1), 34 kDa (beta 2) and 31 kDa alpha). In addition, an unknown protein of 37 kDa (termed beta 3) was identified by antibody 12-15 which could also be detected in ESb cells and EL4 cells. Antibodies 2.4G2 and K9.361 did not react to the beta 3 chain but reacted to varying extents to the other Fc proteins in macrophage and lymphoid cells. Comparison by peptide mapping of the novel beta 3 chain to beta 1, beta 2 and alpha proteins revealed similar, but also distinct peptides. The tissue-specific reactivity of monoclonal antibody 12-15 is likely to be due to a carbohydrate epitope associated with all Fc gamma receptors in lymphoid but not macrophage cell lines.