Human Golgi beta-galactoside alpha-2,6-sialyltransferase generates a group of sialylated B lymphocyte differentiation antigens

The incorrect use of CD75 as a synonym for ST6GAL1 has fostered the expansion of commercial "ST6GAL1" antibodies that do not recognize ST6GAL1

The ST6GAL1 Golgi sialyltransferase is upregulated in many human malignancies, however, detection of ST6GAL1 protein in cancer tissues has been hindered by the prior lack of antibodies. Recently, numerous commercial antibodies for ST6GAL1 have become available, however, many of these do not, in fact, recognize ST6GAL1. Decades ago, the CD75 cell-surface epitope was mistakenly suggested to be the same molecule as ST6GAL1. While this was rapidly disproven, the use of CD75 as a synonym for ST6GAL1 has persisted, particularly by companies selling "ST6GAL1" antibodies. CD75 is reportedly a sialylated epitope which appears to encompass a range of glycan structures and glycan carriers. In this study, we evaluated the LN1 and ZB55 monoclonal antibodies, which are advertised as ST6GAL1 antibodies but were initially developed as CD75-recognizing antibodies (neither was raised against ST6GAL1 as the immunogen). Importantly, the LN1 and ZB55 antibodies have been widely used by investigators, as well as the Human Protein Atlas database, to characterize ST6GAL1 expression. Herein, we used cell and mouse models with controlled expression of ST6GAL1 to compare LN1 and ZB55 with an extensively validated polyclonal antibody to ST6GAL1. We find that LN1 and ZB55 do not recognize ST6GAL1, and furthermore, these 2 antibodies recognize different targets. Additionally, we utilized the well-validated ST6GAL1 antibody to determine that ST6GAL1 is overexpressed in bladder cancer, a finding that contradicts prior studies which employed LN1 to suggest ST6GAL1 is downregulated in bladder cancer. Collectively, our studies underscore the need for careful validation of antibodies purported to recognize ST6GAL1.

Complement System: a Neglected Pathway in Immunotherapy

Approved for the treatment of autoimmune diseases, hematological malignancies, and solid cancers, several monoclonal antibodies (mAb) make use of complement in their mechanism of action. Such an assessment is based on comprehensive investigations that used mouse models, in vitro studies, and analyses from patients at initiation (basal level to highlight deficiencies) and after treatment initiation (mAb impact on complement), which have further provided key insights into the importance of the complement activation and/or complement deficiencies in mAb activity. Accordingly, new approaches can now be developed with the final objective of increasing the clinical efficacy of mAb. These improvements include (i) the concurrent administration of fresh frozen plasma during mAb therapy; (ii) mAb modifications such as immunoglobulin G subclass switching, Fc mutation, or IgG hexamerization to improve the fixation and activation of C1q; (iii) optimization of the target recognition to induce a higher complement-dependent cytotoxicity (CDC) and/or complement-dependant cellular cytotoxicity (CDCC); and (iv) the control of soluble and cellular complement inhibitors.

Resistance to complement activation, cell membrane hypersialylation and relapses in chronic lymphocytic leukemia patients treated with rituximab and chemotherapy

The anti-CD20-specific monoclonal antibody rituximab (RTX), in combination with chemotherapy, is commonly used for primary treatment in chronic lymphocytic leukemia (CLL). However, relapses remain important and activation of the complement pathway is one of the mechanisms by which RTX generates the destruction of B cells directly by complement-dependent cytotoxicity (CDC), or indirectly by antibody-dependent cellular phagocytosis. In this study, the RTX capacity to induce CDC was established in 69 untreated CLL patients, this cohort including 34 patients tested before the initiation of RTX-chemotherapy. In vitro CDC-resistance to RTX predicts lower response rates to RTX-chemotherapy and shorter treatment free survival. Furthermore, the predictive value of CDC-resistance was independent from the clinical, cytogenetic and FcγR3A V158F polymorphism status. In contrast, CLL cell resistance to CDC predominates in IGHV unmutated patients and was related to an important α2-6 sialyl transferase activity, which in turn increases cell surface α2-6 hypersialylation. Suspected factors associated with resistance to CDC (CD20, CD55, CD59, factor H, GM1, and sphingomyelin) were not differentially expressed or recruited between the two CLL groups. Altogether, results provide evidence that testing RTX capacity to induce CDC in vitro represents an independent predictive factor of therapeutic effects of RTX, and that α2-6 hypersialylation in CLL cells controls RTX response through the control of the complement pathway. At a time when CLL therapy is moving towards chemo-free treatments, further experiments are required to determine whether performing an initial in vitro assay to appreciate CLL CDC resistance might be useful to select patients.

Significance of β-Galactoside α2,6 Sialyltranferase 1 in Cancers

Altered glycosylation is a common feature of cancer cells. It takes a variety of forms, which includes loss of expression or excessive expression of some structures, the accumulation of precursors, the appearance of novel structures, etc. Notably, these changes in glycan structure do not occur as a random consequence of disorder biology. Only a limited subset of oligosaccharides is found frequently enriched on the tumor cell surface and implicated in different tumor phenotypes. Among these, altered sialylation has long been associated with metastatic cell behaviors such as invasion and enhanced cell survival and accumulating evidence points to the alteration occurring in the sialic acid linkage to other sugars, which normally exists in three main configurations: α2,3, α2,6, and α2,8, catalyzed by a group of sialyltransferases. The aberrant expression of all three configurations has been described in cancer progression. However, the increased α2,6 sialylation catalyzed by β-galactoside α2,6 sialyltranferase 1 (ST6Gal I) is frequently observed in many types of the cancers. In this review, we describe the findings on the role of ST6Gal I in cancer progression, and highlight in particular the knowledge of how ST6Gal I-mediated α2,6 sialylated glycans or sialylated carrier proteins regulate cell signaling to promote the malignant phenotype of human carcinoma.

Image-based assessment of growth and signaling changes in cancer cells mediated by direct cell-cell contact

Background

Many important biological processes are controlled through cell-cell interactions, including the colonization of metastatic tumor cells and the control of differentiation of stem cells within their niche. Despite the crucial importance of the cellular environment in regulating cellular signaling, in vitro methods for the study of such interactions are difficult and/or indirect.

Methodology/Principal Findings

We report on the development of an image-based method for distinguishing two cell types grown in coculture. Furthermore, cells of one type that are in direct contact with cells of a second type (adjacent cells) can be analyzed separately from cells that are not within a single well. Changes are evaluated using population statistics, which are useful in detecting subtle changes across two populations. We have used this system to characterize changes in the LNCaP prostate carcinoma cell line when grown in contact with human vascular endothelial cells (HUVECs). We find that the expression and phosphorylation of WWOX is reduced in LNCaP cells when grown in direct contact with HUVECs. Reduced WWOX signaling has been associated with reduced activation or expression of JNK and p73. We find that p73 levels are also reduced in LNCaP cells grown in contact with HUVECs, but we did not observe such a change in JNK levels.

Conclusions/Significance

We find that the method described is statistically robust and can be adapted to a wide variety of studies where cell function or signaling are affected by heterotypic cell-cell contact. Ironically, a potential challenge to the method is its high level of sensitivity is capable of classifying events as statistically significant (due to the high number cells evaluated individually), when the biological effect may be less clear. The methodology would be best used in conjunction with additional methods to evaluate the biological role of potentially subtle differences between populations. However, many important events, such as the establishment of a metastatic tumor, occur through rare but important changes, and methods such as we describe here can be used to identify and characterize the contribution of the environment to these changes.

Cell surface glycosylation diversity of embryonic thymic tissues

In the thymus, glycosylation status of many cell surface molecules changes during the thymocyte maturation and selection processes. In this study, we evaluated the glycosylation changes and possible relationships with programmed cell death in the thymic tissues from mouse embryos at the days 14 (E14), 15 (E15), 16 (E16), 17 (E17) and 18 (E18) of embryonic development. In order to determine glycosylation changes we used three different plant lectins: peanut agglutinin (PNA), Maackia amurensis leucoagglutinin (MAL or MAAI) and Sambucus nigra agglutinin (SNA), which recognize core disaccharide galactose (1-3) N-acetylgalactosamine [Galbeta(1-->3)GalNAc], sialic acid linked (2-->3) to galactose [SAalpha(2-->3)Gal] and sialic acid linked to galactose [SAalpha(2-->6)Gal] structures, respectively. Our lectin histochemistry and lectin blotting studies indicated that glycosylation pattern was modified in thymocytes at the embryonic developmental stages analyzed. The immature cortical thymocytes were labeled by PNA, whereas medullary thymocytes were positive for MAL and SNA binding. Many medullary thymocytes exhibited alpha(2-->6)-linked sialic acid on their surface and this increased throughout the gestational stages. In the lectin blotting studies, different protein bands of various molecular weights were identified in thymocytes. Two of them were putatively identified as CD43 and CD45 glycoproteins. In addition, TUNEL (deoxynucleotdyltransferase-mediated dUDP nick end labeling) indicated that only PNA-positive cortical thymocytes were deleted in all embryonic stages. These results indicate that the glycosylation pattern was modified in thymocytes at all embryonic developmental stages, and these modifications can affect the T cell deletion, probably via the galectin-1 molecule in the embryonic thymus.

Over-expression of anti-CD75 reactive proteins on distal and collecting renal tubular epithelial cells in calcium-oxalate stone-forming kidneys in Egypt

OBJECTIVE

To assess the nature, distribution and expression pattern of CD75, a neuraminidase-sensitive lymphocyte cell surface differentiation antigen, in calcium oxalate (CaOx) stone disease, as cell-surface sialic acid might be involved CaOx crystal binding, and lectin-binding assays suggest that sialic acid in the alpha2,6 position is upregulated in stone-forming kidneys.

MATERIALS AND METHODS

Human CaOx stone-forming and normal kidneys (13 each) and primary kidney epithelial cells (CAKI-1, three samples) were analysed. The protein pattern, distribution and expression of CD75 were analysed using Western blotting, immunohistology and semi-quantitative confocal laser scanning microscopy (cLSM). Production was investigated by alpha2,6-sialyltransferase specific reverse transcription-polymerase chain reaction.

RESULTS

Western blotting showed one strong band at approximately 43 kDa that reacted with anti-CD75 when renal epithelial and CAKI-1 tumour cell extracts were analysed. However, in renal tissue extracts of CaOx stone formers there were additional bands at 120 and 205 kDa. Image processing after cLSM showed that anti-CD75 reactivity was significantly greater on E-cadherin-positive distal and collecting tubular cells from CaOx stone-forming kidneys, at a mean (sd) intensity of 87 (7), than on those from normal kidneys, at 41 (5) (P = 0.005).

CONCLUSION

CD75 expression in human kidney was primarily on the luminal surface of distal tubules and collecting ducts. Whether increased epithelial CD75 expression in CaOx stone disease is a cause or result of the disease remains to be clarified.

Differential expression of beta-galactoside alpha2,6 sialyltransferase and sialoglycans in normal and cirrhotic liver and hepatocellular carcinoma

SUMMARY

Sialyltransferases sialylate plasma glycoproteins in hepatocytes and may (as hepatic key enzymes) constitute markers for liver diseases. We examined expression of the prevalent alpha2,6 sialyltransferase (ST6Gal I) and sialoglycans in normal liver, cirrhotic liver, and hepatocellular carcinoma (HCC) using a new ST6Gal I-specific mAb and recombinant fusion proteins of CD22 and sialoadhesin recognizing alpha2,6- or alpha2,3-sialylated glycans in immunohistology and flow cytometry. In normal and cirrhotic liver, ST6Gal I and sialoglycans were localized in the Golgi region of hepatocytes surrounding the bile canaliculi and along the bile canaliculi, respectively. Sialoglycans were additionally recognized in Kupffer cells, bile ducts, endothelial cells, and oval cells. Well-differentiated and moderately differentiated HCC showed Golgi and diffuse cytoplasmic staining of ST6Gal I and sialoglycans, whereas the cytoplasmic staining for ST6Gal I and sialoglycans was decreased or even absent in poorly differentiated HCC. Detection of sialoglycans by the recombinant fusion proteins in Western blots of cell lysates derived from cell lines revealed two major double bands of sialoglycoproteins at 65 and 120 kDa for hepatocytes, three major bands at 54, 49, and 44 kDa for colonic epithelial cells, and one band at 60 kDa for endothelial cells. Our results describe the expression patterns of ST6Gal I and sialoglycans in various liver tissues and demonstrate an altered expression of these structures between benign and malignant hepatocellular lesions.

Versatile biosynthetic engineering of sialic acid in living cells using synthetic sialic acid analogues

Sialic acids are critical components of many glycoconjugates involved in biologically important ligand-receptor interactions. Quantitative and structural variations of sialic acid residues can profoundly affect specific cell-cell, pathogen-cell, or drug-cell interactions, but manipulation of sialic acids in mammalian cells has been technically limited. We describe the finding of a previously unrecognized and efficient uptake and incorporation of sialic acid analogues in mammalian cells. We added 16 synthetic sialic acid analogues carrying distinct C-1, C-5, or C-9 substitutions individually to cell cultures of which 10 were readily taken up and incorporated. Uptake of C-5- and C-9-substituted sialic acids resulted in the structural modification of up to 95% of sialic acids on the cell surface. Functionally, binding of murine sialic acid-binding immunoglobulin-like lectin-2 (Siglec-2, CD22) to cells increased after N-glycolylneuraminic acid treatment, whereas 9-iodo-N-acetylneuraminic acid abolished binding. Furthermore, susceptibility to infection by the B-lymphotropic papovavirus via a sialylated receptor was markedly enhanced following pretreatment of host cells with selected sialic acid analogues including 9-iodo-N-acetylneuraminic acid. This novel experimental strategy allows for an efficient biosynthetic engineering of surface sialylation in living cells. It is versatile, extending the repertoire of modification sites at least to C-9 and enables detailed structure-function studies of sialic acid-dependent ligand-receptor interactions in their native context.

The sialyl-alpha2,6-lactosaminyl-structure: biosynthesis and functional role

Sialylation represents one of the most frequently occurring terminations of the oligosaccharide chains of glycoproteins and glycolipids. Sialic acid is commonly found alpha2,3- or alpha2,6-linked to galactose (Gal), alpha2,6-linked to N-acetylgalactosamine (GalNAc) or alpha2,8-linked to another sialic acid. The biosynthesis of the various linkages is mediated by the different members of the sialyltransferase family. The addition of sialic acid in alpha2,6-linkage to the galactose residue of lactosamine (type 2 chains) is catalyzed by beta-galactoside alpha2,6-sialyltransferase (ST6Gal.I). Although expressed by a single gene, this enzyme shows a complex pattern of regulation which allows its tissue- and stage-specific modulation. The cognate oligosaccharide structure, NeuAcalpha2,6Galbeta1,4GlcNAc, is widely distributed among tissues and is involved in biological processes such as the regulation of the immune response and the progression of colon cancer. This review summarizes the current knowledge on the biochemistry of ST6Gal.I and on the functional role of the sialyl-alpha2,6-lactosaminyl structure.

Red blood cell surface adhesion molecules: their possible roles in normal human physiology and disease

Human erythrocytes express a relatively large number of known adhesion receptors, despite the fact that red blood cells (RBCs) are generally considered to be nonadhesive for endothelial cell surfaces. Some of these adhesion receptors are expressed by many other tissues, while others have more limited tissue distribution. Some adhesion receptors, including CD36 and VLA-4, are only expressed by immature erythroid cells, while others are present on mature erythrocytes. The structure and function of these proteins is reviewed here. LW, CD36, CD58, and CD147 have been shown in other tissues to mediate cell-cell interaction. Other receptors, such as CD44, VLA-4, and B-CAM/LU, can mediate adhesion to components of extracellular matrix. In addition, their roles in normal erythropolesis, as well as in the pathophysiology of human disease, are summarized. The most convincing evidence for a pathophysiologic role for any of these receptors on erythrocytes comes from studies of cells from patients homozygous for hemoglobin S, as RBC adhesion is thought to contribute to vaso-occlusion. Thus, receptors such as B-CAM/LU may become targets for future therapy aimed at preventing or ameliorating this thrombotic process.

Transcription of the beta-galactoside alpha2,6-sialyltransferase gene (SIAT1) in B-lymphocytes: cell type-specific expression correlates with presence of the divergent 5'-untranslated sequence

A single gene, SIAT1, encodes ST6Gal I, the sialyltransferase that mediates transfer of alpha2,6-linked sialic acids to Galbeta1, 4GlcNAc termini of N-linked glycoproteins. In vivo, multiple SIAT1 mRNA forms, differing only in the 5'-untranslated region, are expressed in a tissue-specific manner. This mRNA heterogeneity has been attributed, at least in part, to transcription from a number of physically distinct promoter regions. In mature B-lymphocytes, SIAT1 transcription initiates at P2, a regulatory region known to function only in B-lineage cells. Bacterial chloramphenicol acetyltransferase (CAT) under the control of the P2 region encompassing 415 bp 5'- and 125 bp 3' of the transcriptional initiation site is efficiently expressed in Louckes, a mature B-lymphoblastoid cell line. In contrast, CAT expression in Reh, a T-null/B-null precursor line, and in HepG2, a hepatoma line, are 14-fold and >25-fold less than in Louckes, respectively. The data is consistent with the presence of cis -acting regulatory elements residing both 5' and 3' of the P2 transcriptional initiation site. At least 370 bp of 5'-flanking sequence, coinciding with the inclusion of AP2 and NF-kappaB sites, is necessary for high level expression in Louckes. Exon sequences 3' of the transcription start site are also important for expression. A segment from(+)32 to(+)125 (position(+)1 is transcription start site) is capable of exerting promoter-like activity in Louckes, but not in Reh or HepG2. CAT expression by P2 is negligible in Reh cells. However, enhanced CAT activity is not accompanied by elevated mRNA levels. This observation is consistent with the relief of translational restraints imposed by the(+)32 to(+)125 region. Together, the data demonstrate that efficient and cell-specific transcription regulation in mature B lymphocytes is contained in a 495 bp P2 segment that is comprised of 370 bp of 5'-flanking region and 125 bp of transcribed region of Exon X.

Differential sialylation of cell surface glycoconjugates in a human B lymphoma cell line regulates susceptibility for CD95 (APO-1/Fas)-mediated apoptosis and for infection by a lymphotropic virus

Sialic acid, as a terminal saccharide residue on cell surface glycoconjugates, plays an important role in a variety of biological processes. In this study, we investigated subclones of the human B lymphoma cell line BJA-B for differences in the glycosylation of cell surface glycoconjugates, and studied the functional implications of such differences. With respect to the expression level of most of the tested B cell-associated antigens, as well as the presence of penultimate saccharide moieties on oligosaccharide chains, subclones were phenotypically indistinguishable. Marked differences among subclones, however, were found in the overall level of glycoconjugate sialylation, involving both alpha-2,6 and alpha-2,3-linked sialic acid residues. Accordingly, subclones were classified as highly- (group I) or hyposialylated (group II). The function of two sialic acid-dependent receptor-mediated processes is correlated with the sialylation status of BJA-B subclones. Susceptibility to and binding of the B lymphotropic papovavirus (LPV) was dependent on a high sialylation status of host cells, suggesting that differential sialylation in BJA-B cells can modulate LPV infection via its alpha-2,6-sialylated glycoprotein receptor. CD95-mediated apoptosis, induced by either the human CD95 ligand or a cytotoxic anti-CD95 monoclonal antibody, was drastically enhanced in hyposialylated group II cells. An increase in endogenous sialylation may be one antiapoptotic mechanism that converts tumor cells to a more malignant phenotype. To our knowledge, this is the first report demonstrating that differential sialylation in a clonal cell line may regulate the function of virus and signal-transducing receptors.

UDP-GlcNAc 2-epimerase: a regulator of cell surface sialylation

Modification of cell surface molecules with sialic acid is crucial for their function in many biological processes, including cell adhesion and signal transduction. Uridine diphosphate-N-acetylglucosamine 2-epimerase (UDP-GlcNAc 2-epimerase) is an enzyme that catalyzes an early, rate-limiting step in the sialic acid biosynthetic pathway. UDP-GlcNAc 2-epimerase was found to be a major determinant of cell surface sialylation in human hematopoietic cell lines and a critical regulator of the function of specific cell surface adhesion molecules.

Differential expression of the hepatic transcript of beta-galactoside alpha2,6-sialyltransferase in human colon cancer cell lines

The activity of beta-galactoside alpha2,6-sialyltransferase (ST6Gal.1), the enzyme responsible for the addition of sialic acid in alpha2,6-linkage to N-acetyllactosaminic (Gal beta1,4GlcNAc) units of glycoconjugates, is increased in the vast majority of colon cancer specimens, and a positive correlation with an invasive phenotype has been suggested by several studies. In many tissues, ST6Gal.1 is regulated mainly at the transcriptional level through the use of different cell-specific promoters which generate transcripts differing in their 5'-untranslated regions. With the aim of understanding the molecular bases of the increased ST6Gal.1 expression in colon cancer, we investigated the expression of mRNA species in colon cancer cell lines and the relationship with enzyme activity and extent of alpha2,6-sialylation of cell glycoproteins. All cell lines examined express the form containing the 5'-untranslated exons Y and Z, typical of the "basal" expression of the gene, while others express also the liver transcript. This indicates that colon cancer cell lines can be grouped according to expression of the liver transcript of ST6Gal.1. The cell lines expressing only the Y+Z form display, in general, a lower activity:mRNA ratio, which might indicate reduced translational efficiency. The level of alpha2,6-sialylation of cell glycoproteins, as determined by reactivity with the Sambucus nigra lectin, is closely associated with the level of enzyme activity.

Characterization of the mitogenic response of murine CD5+ and conventional B lymphocytes to lipopolysaccharide

The nature of the response of conventional and CD5+ B cells to stimulation in vitro with optimal mitogenic concentrations of LPS was examined to elucidate the contributions of these B cell subsets in polyclonal B lymphocyte responses. Stimulation of murine splenic lymphocytes with LPS resulted in an increase in total biomass, peaking at 72 h of culture. The viability of the cultures remained high (> 90%) until 48 h of culture. A combination of trypan blue and 7-aminoactinomycin D (7AAD) exclusion in conjunction with PE-anti-CD5 and FITC-anti-B220 enabled more detailed analysis of the cultures. The total number of conventional B cells, viable and non-viable, increased until 48 h of culture and then decreased when stimulated with LPS, while CD5+ B cells increased over the culture period. The numbers of conventional B cells in the control cultures decreased, but the CD5+ B cell numbers remained stable. An examination of the modes of death of the B cell subsets using 7AAD showed that unstimulated conventional B cells were apoptotic rather than degenerate but, following stimulation with LPS, apoptotic and degenerate cells were found. Apoptotic and degenerate CD5+ B cells were found in both stimulated and unstimulated cultures, but the percentage of these apoptotic and degenerate cells was increased significantly only at 72 h and 96 h of culture in stimulated cultures compared with 24 h onwards in the control cultures. Morphological analysis and gel electrophoretic studies of extracted DNA reflected these findings. It was also found that the increase in the number and percentage of non-viable cells in the cultures was not equal to the decrease in the number and percentage of viable cells. Activation of B cells was examined using expression of B7-1 (CD80) as a marker. When stimulated with LPS a greater proportion of conventional B cells expressed B7-1 after 24 h of culture than in the control cultures; however, only at 72 h and 96 h of culture was the proportion of CD5+ B cells expressing B7-1 significantly higher than in the control cultures. These results show that conventional B cells are stimulated to proliferate and to become activated by LPS and that death is apoptotic rather than degenerate or necrotic. CD5+ B cells were also shown to be stimulated by LPS; they became activated and death was delayed. The data suggest that in addition to the proliferative role, LPS acts to delay death and to activate conventional and CD5+ B cells.

Masking and unmasking of the sialic acid-binding lectin activity of CD22 (Siglec-2) on B lymphocytes

CD22 is a B cell-restricted glycoprotein involved in signal transduction and modulation of cellular activation. It is also an I-type lectin (now designated Siglec-2), whose extracellular domain can specifically recognize alpha2-6-linked sialic acid (Sia) residues. This activity is postulated to mediate intercellular adhesion and/or to act as a coreceptor in antigen-induced B cell activation. However, studies with recombinant CD22 indicate that the lectin function can be inactivated by expression of alpha2-6-linked Sia residues on the same cell surface. To explore whether this masking phenomenon affects native CD22 on B cells, we first developed a probe to detect the lectin activity of recombinant CD22 expressed on Chinese hamster ovary cells (which have no endogenous alpha2-6-linked Sia residues). This probe is inactive against CD22-positive B lymphoma cells and Epstein-Barr virus-transformed lymphoblasts which express high levels of alpha2-6-linked Sia residues. Enzymatic desialylation unmasks the CD22 lectin activity, indicating that endogenous Sia residues block the CD22 lectin-binding site. Truncation of the side chains of cell surface Sia residues by mild periodate oxidation (known to abrogate Sia recognition by CD22) also had this unmasking effect, indicating that the effects of desialylation are not due to a loss of negative charge. Normal resting B cells from human peripheral blood gave similar findings. However, the lectin is partially unmasked during in vitro activation of these cells. Thus, the lectin activity of CD22 is restricted by endogenous sialylation in resting B cells and may be transiently unmasked during in vivo activation, perhaps to modulate intercellular or intracellular interactions at this critical stage in the humoral response.

Characterization of terminal sialic acid linkages on human thymocytes. Correlation between lectin-binding phenotype and sialyltransferase expression

T cell surface sialylation changes during maturation in the thymus. We have previously demonstrated increased expression of mRNA encoding the Gal beta 1, 3GalNAc alpha 2,3-sialyltransferase in mature medullary human thymocytes, compared with immature cortical thymocytes. For this enzyme, increased expression of transferase mRNA correlated with increased sialylation of O-glycans. We have now examined the pattern of expression in the human thymus of two additional sialyltransferases, the Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase (ST6N) and the Gal beta 1,3/4GlcNAc alpha 2,3-sialyltransferase (ST3N). The patterns of mRNA expression were compared with the pattern of binding of two sialic acid-specific plant lectins, Sambucus nigra agglutinin and Maackia amurensis agglutinin, which preferentially recognize alpha 2,6- and alpha 2,3-linked sialic acids, respectively, on N-glycans. By in situ hybridization, mRNA encoding ST3N was detected uniformly throughout the thymus. All thymocytes bound M. amurensis agglutinin, demonstrating a direct correlation between the level of ST3N mRNA expression and cell-surface glycosylation. In contrast, mRNA encoding ST6N was also expressed uniformly throughout the thymus; however, only mature (CD3hi) medullary thymocytes bound S. nigra agglutinin. On mature thymocytes, S. nigra agglutinin appeared to bind primarily to the cell-surface glycoprotein CD45; since only the mature thymocytes expressed the CD45RA isoform, while both mature and immature populations expressed the CD45R0 isoform, CD45RA may be a preferred substrate for ST6N. These results demonstrate that glycoprotein sialylation is tightly regulated during T cell development and that the developmentally regulated expression of specific oligosaccharide structures on the cell surface may be influenced by expression of both the relevant glycosyltransferase and specific acceptor substrates.

CD22-mediated cell adhesion to cytokine-activated human endothelial cells. Positive and negative regulation by alpha 2-6-sialylation of cellular glycoproteins

We previously showed that cultured human umbilical vein endothelial cells (HEC) exposed to the inflammatory cytokines tumor necrosis factor-alpha or interleukin-1 display increased activity of beta-galactoside alpha 2,6-sialyltransferase. This is associated with enhanced expression of ligands for the B cell receptor CD22 beta, which recognizes alpha 2-6-linked sialic acids (Hanasaki, K., Varki, A., Stamenkovic, I., and Bevilacqua, M. P. (1994) J. Biol. Chem. 269, 10637-10643). Here we report that increased expression of CD22 ligands is a feature of dermal microvascular endothelial cells as well, and is also observed in response to the cytokine interleukin-4. Tumor necrosis factor-alpha stimulation of HEC causes no change in the profile of endothelial glycoproteins recognized by CD22, but doubles the proportion of total cellular N-linked oligosaccharides capable of binding tightly to CD22. This modest change is sufficient to cause a marked increase in alpha 2-6-linked sialic acid-dependent binding of Chinese hamster ovary (CHO) cells expressing recombinant human CD22. In contrast, B lymphoma cell lines expressing higher levels of cell surface CD22 do not show such sialic acid-dependent binding to activated HEC. Since B lymphoma cells themselves also express high levels of alpha 2-6-linked sialic acids, their CD22 molecules might be rendered nonfunctional by endogenous ligands. In support of this, the lectin function of CD22 can be directly detected on transfected CHO cells, but not on B lymphoma cells. Furthermore, coexpression of beta-galactoside alpha 2,6-sialyltransferase with CD22 in the CHO cells abrogates sialic acid-dependent binding to cytokine-activated HEC. However, such co-transfected cells can bind to B lymphoma cells in a manner apparently less dependent upon alpha 2-6-linked sialic acid, suggesting CD22-mediated interactions that may not be directly dependent on its lectin function. Thus, CD22-mediated interactions between B cells and activated vascular endothelium may be positively regulated by induction of alpha 2-6-linked sialic acid-bearing endothelial cell ligands, but negatively regulated by such ligands on the B cells expressing CD22. Since expression of both CD22 and beta-galactoside alpha 2,6-sialyltransferase are regulated during B cell ontogeny, these findings could be of importance in B cell function and/or trafficking.

Biosynthetic modulation of sialic acid-dependent virus-receptor interactions of two primate polyoma viruses

Sialic acids are essential components of the cell surface receptors of many microorganisms including viruses. A synthetic, N-substituted D-mannosamine derivative has been shown to act as precursor for structurally altered sialic acid incorporated into glycoconjugates in vivo (Kayser, H., Zeitler, R., Kannicht, C., Grunow, D., Nuck, R., and Reutter, W. (1992) J. Biol. Chem. 267, 16934-16938). In this study we have analyzed the potential of three different sialic acid precursor analogues to modulate sialic acid-dependent virus receptor function on different cells. We show that treatment with these D-mannosamine derivatives can result in the structural modification of about 50% of total cellular sialic acid content. Treatment interfered drastically and specifically with sialic acid-dependent infection of two distinct primate polyoma viruses. Both inhibition (over 95%) and enhancement (up to 7-fold) of virus binding and infection were observed depending on the N-acyl substitution at the C-5 position of sialic acid. These effects were attributed to the synthesis of metabolically modified, sialylated virus receptors, carrying elongated N-acyl groups, with altered binding affinities for virus particles. Thus, the principle of biosynthetic modification of sialic acid by application of appropriate sialic acid precursors to tissue culture or in vivo offers new means to specifically influence sialic acid-dependent ligand-receptor interactions and could be a potent tool to further clarify the biological functions of sialic acid, in particular its N-acyl side chain.

Double immunofluorescent staining of alpha 2,6 sialyltransferase and beta 1,4 galactosyltransferase in monensin-treated cells: evidence for different Golgi compartments?

Beta 1,4 galactosyl- and alpha 2,6 sialyltransferase (gal-T EC 2.4.1.22 and sialyl-T EC 2.4.99.1) sequentially elongate and terminate complex N-glycan chains of glycoproteins. Both enzymes reside in trans Golgi cisternae; their ultrastructural relationship, however, is unknown. To delineate their respective Golgi compartment(s) we conducted a double label immunofluorescent study by conventional and confocal laser scanning microscopy in HepG2, HeLa, and other cells in presence of Golgi-disturbing agents. Polyclonal, peptide-specific antibodies to human sialyl-T expressed as a beta-galactosidase-sialyl-T fusion protein in E. coli were developed and applied together with mABs to human milk gal-T. In untreated HepG2 and HeLa cells Golgi morphology identified by immunofluorescent labeling of sialyl-T and gal-T, respectively, was nearly identical. Treatment of cells with brefeldin A (BFA) led to rapid and coordinated disappearance of immunostaining of both enzymes; after BFA washout, vesicular structures reappeared which first stained for gal-T followed by sialyl-T; in the reassembled Golgi apparatus sialyl-T and gal-T were co-localized again. In contrast, monensin treatment produced a reversible swelling and scattering of gal-T positive Golgi elements while sialyl-T positive structures showed little change. Treatment with nocodazole led to dispersal of Golgi elements in which gal-T and sialyl-T remained co-localized. Treatment with chloroquine affected Golgi structures less than monensin and led to condensation of gal-T positive and to slight enlargement of sialyl-T positive structures. Sequential recovery from BFA of gal-T and sialyl-T and their segregation by monensin suggest that these enzymes are targeted to different Golgi subcompartments.