Integrity of glycoprotein complex sugars is required for homing but not for several other membrane-mediated functions

The effects of monensin on blood-borne arrest and glycosylation of BL/VL3 lymphoma cells

We have previously shown that inhibitors of N-glycan processing alter both the cell surface carbohydrates and the homing properties in lymphoid cells. We have now studied the effects of the ionophore monensin (MON) on these parameters. Arrest in the spleen of [111In]-labelled BL/VL3 murine T lymphoma cells, injected intravenously was clearly reduced if the cells had been cultured for 24 h in the presence of monensin (0.1-1.0 microgram ml-1). We have characterized glycopeptides from BL/VL3 murine T lymphoma cells. Following labelling with tritiated precursors (fucose, mannose, galactose, glucosamine), surface glycopeptides from BL/VL3 murine T lymphoma cells, were released by trypsin and separated by gel filtration on Bio-Gel P6 and by affinity chromatography on immobilized lectins. After treatment with MON, a class of high molecular mass glycopeptides was no longer found. There were less complex and more high mannose glycans, as a consequence of a reduction of terminal glycosylation (sialylation, fucosylation or incorporation of N-acetyl-glucosamine). Similar findings were obtained with immunoprecipitated Thy-1 antigen. However, as estimated by flow cytometry analysis, the cell surface expression of Thy-1 was not reduced in MON-treated cells. Taken together our results show that cell surface oligosaccharides are modified dramatically, but that at least, certain cell surface antigens are present in normal amounts. It is tempting to speculate that changes in glycosylation account for the abnormal homing properties of MON-treated cells.

Lymphocyte homing

The phenomenon of "homing" is discussed with respect to patterns of lymphocyte circulation and the molecules on the surface of both endothelium and lymphocytes that mediate this process. In addition, the data are analysed in the context of a model for lymphocyte homing.

An enzyme-linked lectin-binding assay on cells (CELLBA) for the comparison of lectin receptor expression on cell surfaces

Lectins can be used to specifically detect some cell surface glycans. Their expression on different cells or on cells of a given lineage throughout differentiation or following treatment with drugs can be compared using lectins labelled with radioactive, fluorescent or enzymatic probes. We describe a new method which, by analogy with CELISA (ELISA on cells), is called CELLBA (or ELLBA on cells) for cellular, enzyme-linked lectin-binding assay. It permits the comparison of the expression of specific glycans in a large number of different cell samples. As an example, it was able to detect alterations of cell surface glycan expression caused by inhibitors of N-linked oligosaccharide trimming.

Effects of radioprotective glycans on the arrest of blood-borne lymphoma cells

The mode of action of radioprotective glycans is not understood. In view of the known importance of cell migration in haematology, on the one hand, and of carbohydrates in homing processes, on the other, we have investigated the effect of several glycans on blood-borne arrest of lymphoma cells. Radioprotective glycans (but not heparin) modified the arrest of injected cells in the spleen. Altered recirculation and homing processes may play a role in the radioprotective properties of glycans.

Sialylation patterns of lymphocyte function-associated antigen 1 (LFA-1) differ between T and B lymphocytes

Lymphocyte function-associated antigen 1 (LFA-1) was immunoprecipitated from various types of surface-radioiodinated murine lymphocytes, and analyzed by two-dimensional polyacrylamide gel electrophoresis. LFA-1 alpha and beta chains from splenic B lymphocytes had the same apparent molecular weights as, but distinct isoelectrofocusing patterns from, their counterparts from thymocytes or splenic T lymphocytes. The splenic B lymphocytes lacked a basically charged population of alpha chain, while the thymocytes and the splenic T lymphocytes showed both the acidic and the basic portions. Furthermore, the beta chain of the former migrated more towards the acidic end than that of the latter. No difference was found between LFA-1 molecules of the same lineage of cells from several strains of mice whose H-2 haplotypes were different from one another. When murine lymphocyte lines were examined, LFA-1 with various isoelectrofocusing patterns were recognized. The charge difference again reflected the difference in lymphocyte lineage, but in a more exaggerated manner than that seen with cells from mice. The average acidity of both chains of LFA-1 decreased in the order of B cell lines, pre-B cell lines and T cell lines. The lineage-dependent charge difference of either chain disappeared after neuraminidase treatment of LFA-1, indicating that lymphocyte differentiation was accompanied by changes in LFA-1 sialylation.

Membrane carbohydrates of lymphoid cells: the receptor for interleukin 2

The contribution of sialic acids and of N-linked sugars to the biological activity of the receptor for IL 2 has been evaluated by treating activated cells with Neuraminidase or by growing them in the presence of inhibitors of N-linked glycosylation or processing. After treatment with Neuraminidase, Con A-activated spleen cells had not lost their ability to bind IL 2. The IL 2-absorbing capability was, however, strongly reduced after trypsinisation. 6 hours after Trypsin treatment, this property was again expressed. Proliferation of the IL 2-dependent CTLL cells was normal in the presence of Swainsonine but strongly impaired in the presence of Tunicamycin. Glycosylation of the IL 2 receptor may thus be required, but integrity of the sugars is not critical.

Effect of inhibitors of glycosylation and carbohydrate processing on invasion of malignant mouse MO4 cells in organ culture

Inhibitors of glycosylation and carbohydrate processing were used to investigate the role of carbohydrates exposed at the cell surface in invasion. Malignant mouse MO4 cells were confronted with embryonic chick heart in organ culture, an assay shown to be relevant for a number of aspects of invasion in vivo. Tunicamycin (1.0 microgram/ml), 2-deoxy-D-glucose (100 mM), beta-OH-norvaline (1.0 mM), and Monensin (0.1 microgram/ml) reversibly inhibited the invasion of MO4 cells. At these concentrations the drugs also inhibited the growth of MO4 cells. 1-Deoxynojirimycin (10mM), swainsonine (0.4 microgram/ml), and Marcellomycin (0.1 microgram/ml) permitted invasion. Marcellomycin also reversibly inhibited the growth of MO4 cells. These results show that drugs known to interfere with the glycosylation or processing of carbohydrate chains of glycoproteins in different ways have different effects on the invasion of MO4 cells in vitro.

Modification of blood-borne arrest properties of lymphoma cells by inhibitors of protein glycosylation suggests the existence of endogenous lectins

The requirement for intact carbohydrates of glycoproteins at the cell surface was investigated after treatment of lymphoma cells with compounds which interfere at different steps in N-linked glycosylation: swainsonine and 1-deoxynojirimycin act at different levels during the processing, so that complex oligosaccharides cannot be formed; 2-deoxyglucose, beta-hydroxynorvaline, and tunicamycin completely prevent the formation of N-linked (high-mannose as well as complex) oligosaccharides. The role of sialic acid was investigated by treating the cells with neuraminidase. These treatments resulted in altered patterns of surface-labelled glycoproteins after SDS-polyacrylamide gel electrophoresis. Blood-borne arrest of lymphoma cells in the spleen was sensitive to neuraminidase and to treatments interfering with the processing of complex N-linked oligosaccharides. It is suggested that carbohydrates are signals for cellular interactions involved in the recirculation and homing behaviour of lymphoid cells and probably interact with endogenous lectins at their site of homing.