Thymocyte Thy-1 Research Articles

Tissue-specific N-glycosylation, site-specific oligosaccharide patterns and lentil lectin recognition of rat Thy-1.

To examine the extent to which protein structure and tissue-type influence glycosylation, we have determined the oligosaccharide structures at each of the three glycosylation sites (Asn-23, 74 and 98) of the cell surface glycoprotein Thy-1 isolated from rat brain and thymus. The results show that there is tissue-specificity of glycosylation and that superimposed on this is a significant degree of site-specificity. On the basis of the site distribution of oligosaccharides, we find that no Thy-1 molecules are in common between the two tissues despite the amino acid sequences being identical. We suggest, therefore, that by controlling N-glycosylation a tissue creates an unique set of glycoforms (same polypeptide but with oligosaccharides that differ either in sequence or disposition). The structures at each of the three sites were also determined for the thymocyte Thy-1 that binds to lentil lectin (Thy-1 L+) and for that which does not (Thy-1 L-). Segregation of intact thymus Thy-1 into two distinct sets of glycoforms by lentil lectin was found to be due to the structures at site 74. Analysis of oligosaccharide structures at the 'passenger' sites (23 and 98) suggests that either Thy-1 L+ and Thy-1 L- molecules are made in different cell-types or that the biosynthesis of oligosaccharides at one site is influenced by the glycosylation at other sites.

Decay accelerating factor of complement is anchored to cells by a C-terminal glycolipid

Membrane-associated decay accelerating factor (DAF) of human erythrocytes (Ehu) was analyzed for a C-terminal glycolipid anchoring structure. Automated amino acid analysis of DAF following reductive radiomethylation revealed ethanolamine and glucosamine residues in proportions identical with those present in the Ehu acetylcholinesterase (AChE) anchor. Cleavage of radiomethylated 70-kilodalton (kDa) DAF with papain released the labeled ethanolamine and glucosamine and generated 61- and 55-kDa DAF products that retained all labeled Lys and labeled N-terminal Asp. Incubation of intact Ehu with phosphatidylinositol-specific phospholipase C (PI-PLC), which cleaves the anchors in trypanosome membrane form variant surface glycoproteins (mfVSGs) and murine thymocyte Thy-1 antigen, released 15% of the cell-associated DAF antigen. The released 67-kDa PI-PLC DAF derivative retained its ability to decay the classical C3 convertase C4b2a but was unable to membrane-incorporate and displayed physicochemical properties similar to urine DAF, a hydrophilic DAF form that can be isolated from urine. Nitrous acid deamination cleavage of Ehu DAF at glucosamine following labeling with the lipophilic photoreagent 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine ([125I]TID) released the [125I]TID label in a parallel fashion as from [125I]TID-labeled AChE. Biosynthetic labeling of HeLa cells with [3H]ethanolamine resulted in rapid 3H incorporation into both 48-kDa pro-DAF and 72-kDa mature epithelial cell DAF. Our findings indicate that DAF and AChE are anchored in Ehu by the same or a similar glycolipid structure and that, like VSGs, this structure is incorporated into DAF early in DAF biosynthesis prior to processing of pro-DAF in the Golgi.

Poly- N-acetyllactosamine alterations of Thy-1 glycoprotein in lymphocyte differentiation

Developmental changes in murine lymphocyte Thy-1 include both quantitative and qualitative alterations involving N-linked oligosaccharides. Comparison of immature with mature T-cells has shown that the oligosaccharides of Thy-1 are characterized by an increase in the number of sialic acid residues responsible for the acidic pI of peripheral T-cell Thy-1, and a decrease in those oligosaccharides responsible for M, heterogeneity of thymoctye Thy-1. The research reported here suggests that the basis of the large M, heterogeneity in Thy-1 of immature T-cells is the presence of repeating N-acetyllactosamine (R.'Ga.lβl,4Glc NAcβl,3R″) units in the oligosaccharide portion of the molecule. Lymphocytes were surface iodinated and 125I-Thy-1 was purified by immunoprecipitation and preparative nonequilibrium pH gradient electrophoresis. The minimal M r, of unglycosylated Thy-1 after endoglycosidase F digestion was 15,000–16,000. Digestion of Thy-1 with endo-β-galactosidase suggested that the complex type N-linked glycans in thymocytes, but not in lymph node T-cells, contained increased levels of polylactosamine. The presence of polylactosamine was confirmed by binding to a Datura stramonium lectin column which retarded and bound approx. 50% of thymocyte Thy-1 and only about 18% of lymph node T-cell Thy-1. Affinity chromatography using anti-i antibody immobilized on agarose beads indicated that the polylactosamine is probably present in a predominantly linear form. Since alterations of polylactosamine structures have been implicated in development and transformation in several systems, the present results suggest an important role for these glycans in immune-cell differentiation.

Partial characterization of the oligosaccharides of mouse thymocyte Thy-1 glycoprotein.

Four glycopeptides (I, IIA, IIB, III) with different oligosaccharide structures were isolated from purified mouse thymocyte Thy-1 glycoprotein. The glycoprotein was digested with Pronase, and the glycopeptide fraction was isolated by gel filtration and acetylated with [3H]acetic anhydride. The different glycan structures were separated by affinity chromatography on concanavalin A-Sepharose 4B and lentil lectin-Sepharose 4B. Size determinations of intact and exoglycosidase- and endoglycosidase-digested glycopeptides were performed by gel filtration on Bio-Gel P-6, calibrated with glycopeptides of known structure. On the basis of these experiments and on the behaviour of the glycopeptides on the lectin columns, the following structures of the oligosaccharide chains were proposed: I, triantennary 'complex-type' with terminal fucose; IIA, biantennary 'complex-type' without fucose; IIB, biantennary 'complex-type' with fucose; III, a mixture of 'high-mannose' chains containing either five or six mannose residues (approx. 50% of each). Amino acid analysis of the glycopeptides showed that the predominant oligosaccharide at glycosylation-site Asn-23 was of 'high-mannose' type, whereas the other two sites (Asn-75 and Asn-99) were glycosylated with 'complex-type' chains. Both these sites were shown to be variably glycosylated. The major glycans linked to Asn-75 were of structures I and IIB, whereas all three 'complex-type' chains were represented at Asn-99. The results presented explain the previously reported carbohydrate heterogeneity of thymocyte Thy-1 glycoprotein.

Molecular weight and charge heterogeneity of Thy-1 glycoprotein in different populations of T-cells

Molecular weight and charge microheterogeneity of Thy-1 glycoprotein expressed on different T-cell populations were compared by two-dimensional gel electrophoresis and endoglycosidase treatment. Thy-1 was immunoprecipitated from detergent lysates of radioiodinated T-cells of spleens, lymph nodes, Peyer's patches, peripheral blood, IL-2-cultured thymocytes and peanut agglutinin (PNA) separated thymocytes. In general, thymocytes and IL-2-cultured thymocytes expressed the highest levels of Thy-1 with a large M r and charge variation. The Thy-1 of these cells was basic and contained low levels of sialic acid. On the other hand, peripheral T-lymphocytes exhibited a restricted M r and more limited charge heterogeneity with higher amounts of sialic acid. The Thy-1 glycoprotein of mature, low Thy-1 PNA − thymocytes had M r and charge heterogeneity identical to peripheral T-lymphocytes. The M r and charge variation of immature PNA + thymocytes was essentially identical to that of whole thymocytes. The molecular source of M r heterogeneity in thymocyte Thy-1 and restricted M r, in lymph node Thy-1 was studied by analysis with endoglycosidase-H (Endo-H) and Endo-D. The results of Endo-H digestion showed that most of the Thy-1 polypeptides from both thymocyte and lymph node contained one high-mannose type oligosaceharide which was relatively small and not heterogeneous with respect to M r. The complex-type oligosaccharides (Endo-D-sensitive) were larger and were responsible for the broad M r-heterogeneity found in thymocyte Thy-1. Both thymocyte and lymph node Thy-1 generally appear to contan three N-linked oligosaccharides (one high-mannose and two complex) and sequential hydrolysis with Endo-H and Endo-D resulted in an unglycosylated polypeptide with an M r of approx. 12,500.

Cellular mechanism of primary anti-Thy-1 antibody responses in vitro induced by uniquely immunogenic thymocyte antigens.

Thy-1 antigens are the only cell membrane antigens known to be able to induce primary antibody responses in vitro. We have shown that antigens from the thymocytes of mice and rats were highly immunogenic in cultures of murine spleen cells for the induction of Thy-1.1-specific plaque-forming cell responses, whereas antigens from other tissues, including brains and bone marrow, were poorly immunogenic, if at all. The thymocyte-specific Thy-1 immunogenicity was carried by disrupted cell membranes, and the specific activity for inducing responses was closely linked to Thy-1. We then tried to determine the mechanism of anti-Thy-1 antibody responses in vitro that were induced by the uniquely immunogenic thymocyte antigens. The thymocyte Thy-1 antigens behaved as T cell-independent class 2 (TI-2) antigens: they induced responses in athymic nude mice but not in CBA/N mice with a B cell defect. The apparent TI-2 responses to thymocyte Thy-1 did, however, require Thy-1+ cells in the responder, similar to anti-DNP-Ficoll responses. The full development of the anti-Thy-1 responses required the participation of splenic adherent cells (SAC). Nevertheless, the mechanism of the SAC dependency of anti-Thy-1 responses did not involve antigen presentation to lymphocytes by antigen-pulsed SAC, which contrasted with the finding that the presentation of antigen by live SAC to lymphocytes was indispensable for responses to DNP-Ficoll. The poor Thy-1 responsiveness of SAC-depleted spleen cells was fully restored by the addition of soluble factors (IL 1-like molecules) released from SAC into the culture, which did not replace the SAC-requirement of responses to DNP-Ficoll. It was concluded from these results that Thy-1 or Thy-1-linked structures on thymocyte membranes have an intrinsic activity to directly signal either TI-2 B cells or immature T cells, or both, for activation in the presence of soluble factors released from adherent accessory cells. This conclusion is discussed in relation to a hypothetical view that the thymocyte Thy-1 would physiologically mediate cell-to-cell interactions among special subsets of lymphocytes under thymic influence.

Purification and characterization of the mouse thymocyte Thy-1 glycoprotein.

The thymocyte Thy-1 glycoprotein was purified from mouse strain NMRI (Thy 1.2) thymus. Crude membranes were prepared in Tween 20 and solubilized in deoxycholate. The glycoproteins were isolated by affinity chromatography on concanavalin A-Sepharose, and Thy-1 was further purified by two gel-filtration cycles on Sephacryl S-200. The concentration of Thy-1 in fractions obtained during purification was measured by a solid-phase radioimmunoassay with pure mouse brain Thy-1 as standard. Analysis of the purified preparation by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed at least five distinct bands in the apparent-Mr region 25000-30000, the polymorphism probably being due to carbohydrate heterogeneity. Amino-acid-analysis data were compatible with the previously published sequence of mouse brain Thy-1. Sugar content was determined at 31% (w/w), and the carbohydrate composition indicates the presence of 'complex-type' oligosaccharide chains. The mean Mr of mouse thymocyte Thy-1 was calculated to be 18 100.

Intact and subcellular form of thymocytes of rats are exceptionally immunogenic in mice for inducing anti-Thy-1 T cell-independent class 2 antibody responses

Results of the present study show that the primary anti-Thy-1.1 antibody response to rat antigen in Thy-1.2 mice is induced exclusively by thymocyte antigen. Thy-1 antigens of brain and bone marrow, which expressed much Thy-1 antigen, were poorly immunogenic if at all. Brain Thy-1 antigen considerably inhibited the immunogenicity of thymocyte Thy-1. Moreover, we found that subcellular form of thymocytes induce as high antibody responses as intact thymocytes do. The subcellular thymocyte Thy-1 antigen behaved as TI-2 antigen, inducing a good response in athymic nude mice but not in CBA/N mice with a B cell defect. The significance of these findings is discussed in relation to the possible physiological activity of Thy-1 or Thy-1-linked molecules on thymocytes specifically mediating lymphocyte differentiation.

Purification and Chemical Characterisation of Membrane Glycoproteins from Rat Thymocytes and Brain, Recognised by Monoclonal Antibody MRC OX 2

The MRC OX 2 monoclonal antibody recognises antigens present on rat thymocytes, brain, follicular dendritic cells in lymphoid organs, vascular endothelium, some smooth muscle and B-lymphocytes. The OX 2 antigens recognised by this antibody were purified from brain and thymus, by solubilisation with sodium deoxycholate, affinity chromatography with MRC OX 2 antibody and gel filtration. The purified brain and thymocyte OX 2 antigens were glycoproteins with apparent Mr 41000 and 47000 respectively as determined by polyacrylamide gel electrophoresis in sodium dodecyl sulphate. Rabbit antisera raised against the purified antigens were analysed by radioimmunoassay and immunoperoxidase-staining of tissue sections. The brain and thymocyte OX 2 antigens were antigenically very similar to those on the other tissues. This indicates that the unusual pattern of distribution was not the result of fortuitous cross-reaction of the MRC OX 2 antibody, as the rabbit sera would be expected to recognise more determinants on the antigen than that recognised by the monoclonal antibody. The amino acid compositions of brain and thymus OX 2 antigens were very similar but with no distinguishing features. Carbohydrate compositions showed that the OX 2 antigens were highly glycosylated, with brain OX 2 antigen containing 24% and thymocyte OX 2 antigen 33% by weight of carbohydrate. Both OX 2 antigens contained carbohydrate residues typical of structures N-linked to asparagine but lacked galactosamine, indicating the absence of O-linked structures. Thymocyte OX 2 contained higher levels of galactose and sialic acid but less fucose than brain OX 2. Similar differences had been observed for brain and thymocyte Thy-1 antigens and were also observed in pooled glycoproteins purified by lentil affinity chromatography from these tissues, reflecting overall differences in the patterns of glycosylation in the two tissues. The OX 2 antigens showed many similarities to Thy-1 antigens in their odd patterns of distribution, characteristic migration on polyacrylamide gels in sodium dodecyl sulphate, and carbohydrate compositions. It is possible that OX 2 antigens, like Thy-1 antigens, have homologies with immunoglobulin domains. A possible role for OX 2 antigens in cell interactions necessary for tissue organisation is discussed.

Carbohydrate complexity of the mouse thymocyte Thy-1 glycoprotein as demonstrated by lectin affinity and isoelectric focusing.

The Thy-1 glycoprotein of mouse thymocytes was analysed with regard to the properties of its carbohydrate part. Of the different lectins tested, partial binding of Thy-1 from membrane extracts was found to Lens culinaris agglutinin (approximately 50%) and wheat germ agglutinin (approximately 25%). In contrast, concanavalin A bound all Thy-1. The results indicate that there is considerable microheterogeneity in the oligosaccaride chains within the Thy-1 population. Thy-1 was purified from thymocyte membrane extracts by concanavalin A affinity chromatography and gel filtration. The purified preparation revealed, on analysis by isoelectric focusing, at least six charge variants (isoelectric points ranging from approximately 5-9). These variants were isolated, treated with neuraminidase and tested for lectin binding and isoelectric point. It was shown that the charge heterogeneity was due to different amounts of sialic acid; even the most basic form contained at least one sialic acid residue. Furthermore, it was concluded that the degree of sialylation was not correlated to the lectin-binding properties. Analysis of Thy-1 from surface 125I-labelled thymocytes showed that the purified preparation was representative in its complexity of the Thy-1 glycoprotein exposed on the surface membrane. The possible biological implications of the findings are discussed.

Purification of the Thy-1 molecule from rat brain

The present paper describes the characterization of the Thy-1 molecule from rat brain. The molecule was recognized by its antigens, which could be solubilized from brain membrane with deoxycholate. In the solubilized form the Thy-1 antigens were associated with a homogenous component with the following hydrodynamic properties: s20,w=2.2s,v=0.72ml/g and Stokes radius=3.0nm. The mol.wt. of the deoxycholateantigen complex was estimated to be 27000; these values are not significantly different from those obtained thymocyte Thy-1. Brain Thy-1 was further purified by affinity chromatography with lentil lectin coupled to Sepharose 4B, and more than 80% of the antigen was bound. The material eluted with methyl alpha-D-glucopyranoside was then filtered on a column of Sephadex G-200, and only one glycoprotein was found in the antigenically active fraction. On sodium dodecyl sulphate-polyacrylamide-gel electrophoresis the glycoprotein was very similar to the Thy-1 from thymocytes that binds to lentil lectin. Its apparent mol.wt. on 12.5% acrylamide gels was 24000, and it electrophoresed as a symmetrical band. Brain Thy-1 was antigenically indistinguishable from thymocyte Thy-1 when analysed with rabbit antisera raised against brain or thymocyte Thy-1.