Physiochemical studies on achatininH, a novel sialic acid-binding lectin

Post-Glycosylation Modification of Sialic Acid and Its Role in Virus Pathogenesis

Sialic acids are a family of nine carbon keto-aldononulosonic acids presented at the terminal ends of glycans on cellular membranes. α-Linked sialoglycoconjugates often undergo post-glycosylation modifications, among which O-acetylation of N-acetyl neuraminic acid (Neu5Ac) is the most common in mammalian cells. Isoforms of sialic acid are critical determinants of virus pathogenesis. To date, the focus of viral receptor-mediated attachment has been on Neu5Ac. O-Acetylated Neu5Acs have been largely ignored as receptor determinants of virus pathogenesis, although it is ubiquitous across species. Significantly, the array of structures resulting from site-specific O-acetylation by sialic acid O-acetyltransferases (SOATs) provides a means to examine specificity of viral binding to host cells. Specifically, C₄ O-acetylated Neu5Ac can influence virus pathogenicity. However, the biological implications of only O-acetylated Neu5Ac at C_7-9 have been explored extensively. This review will highlight the biological significance, extraction methods, and synthetic modifications of C₄ O-acetylated Neu5Ac that may provide value in therapeutic developments and targets to prevent virus related diseases.

9-O-acetylated sialic acids differentiating normal haematopoietic precursors from leukemic stem cells with high aldehyde dehydrogenase activity in children with acute lymphoblastic leukaemia

Childhood acute lymphoblastic leukaemia (ALL) originates from mutations in haematopoietic progenitor cells (HPCs). For high-risk patients, treated with intensified post-remission chemotherapy, haematopoietic stem cell (HSC) transplantation is considered. Autologous HSC transplantation needs improvisation till date. Previous studies established enhanced disease-associated expression of 9-O-acetylated sialoglycoproteins (Neu5,9Ac2-GPs) on lymphoblasts of these patients at diagnosis, followed by its decrease with clinical remission and reappearance with relapse. Based on this differential expression of Neu5,9Ac2-GPs, identification of a normal HPC population was targeted from patients at diagnosis. This study identifies two distinct haematopoietic progenitor populations from bone marrow of diagnostic ALL patients, exploring the differential expression of Neu5,9Ac2-GPs with stem cell (CD34, CD90, CD117, CD133), haematopoietic (CD45), lineage-commitment (CD38) antigens and cytosolic aldehyde dehydrogenase (ALDH). Normal haematopoietic progenitor cells (ALDH(+)SSC(lo)CD45(hi)Neu5,9Ac2 -GPs(lo)CD34(+)CD38(-)CD90(+)CD117(+)CD133(+)) differentiated into morphologically different, lineage-specific colonies, being crucial for autologous HSC transplantation while leukemic stem cells (ALDH(+)SSC(lo)CD45(lo)Neu5,9Ac2 -GPs(hi)CD34(+)CD38(+)CD90(-)CD117(-)CD133(-)) lacking this ability can be potential targets for minimal residual disease detection and drug-targeted immunotherapy.

Critical stoichiometric ratio of CD4(+) CD25(+) FoxP3(+) regulatory T cells and CD4(+) CD25(-) responder T cells influence immunosuppression in patients with B-cell acute lymphoblastic leukaemia

Regulatory T (Treg) cells act to suppress activation of the immune system and thereby maintain immunological homeostasis and tolerance to self-antigens. The frequency and suppressing activity of Treg cells in general are high in different malignancies. We wanted to identify the role and regulation of CD4(+)  CD25(+)  FoxP3(+) Treg cells in B-cell acute lymphoblastic leukaemia (B-ALL). We have included patients at diagnosis (n = 54), patients in clinical remission (n = 32) and normal healthy individuals (n = 35). These diagnosed patients demonstrated a lower number of CD4(+)  CD25(+) cells co-expressing a higher level of FoxP3, interleukin-10, transforming growth factor-β and CD152/CTLA-4 than the normal population. Treg cells from patients showed a higher suppressive capability on CD4(+)  CD25(-) responder T (Tresp) cells than normal. The frequency and immunosuppressive potential of CD4(+)  CD25(+)  FoxP3(+) Treg cells became high with the progression of malignancy in B-ALL. Relative distribution of Tresp and Treg cells was only ~5 : 1 in B-ALL but ~35 : 1 in normal healthy individuals, further confirming the elevated immunosuppression in patients. A co-culture study at these definite ex vivo ratios, indicated that Treg cells from B-ALL patients exhibited higher immunosuppression than Treg cells from normal healthy individuals. After chemotherapy using the MCP841 protocol, the frequency of CD4(+)  CD25(+) cells was gradually enhanced with the reduction of FoxP3, interleukin-10 positivity corresponded with disease presentation, indicating reduced immunosuppression. Taken together, our study indicated that the CD4(+)  CD25(+)  FoxP3(+) Treg cells played an important role in immunosuppression, resulting in a positive disease-correlation in these patients. To the best of our knowledge, this is the first detailed report on the frequency, regulation and functionality of Treg cells in B-ALL.

A perspective on the emergence of sialic acids as potent determinants affecting leishmania biology

Leishmaniasis caused by Leishmania sp. has a wide range of manifestations from cutaneous to the deadly visceral form. They shuttle between the invertebrate and vertebrate hosts as promastigotes and amastigotes having adaptations for subverting host immune responses. Parasite-specific glycoconjugates have served as important determinants influencing parasite recognition, internalization, differentiation, multiplication, and virulence. Despite the steady progress in the field of parasite glycobiology, sialobiology has been a less traversed domain of research in leishmaniasis. The present paper focuses on identification, characterization, and differential distribution of sialoglycotope having the linkage-specific 9-O-acetylated sialic acid in promastigotes of different Leishmania sp. causing different clinical ramifications emphasizing possible role of these sialoglycotopes in infectivity, virulence, nitric oxide resistance, and host modulation in Leishmania spp. asserting them to be important molecules influencing parasite biology.

O-acetylated sialic acids: multifaceted role in childhood acute lymphoblastic leukaemia

Childhood acute lymphoblastic leukaemia (ALL), a malignant transformation of the lymphoblasts, is highly responsive to chemotherapy. However, due to certain inadequacy in detection of minimal residual disease (MRD), relapse is a common phenomenon. To address this question, the present review deals with the induction of an unique O-acetyl derivative of sialic acid on a few disease-associated glycoproteins and glycolipids at the onset of childhood ALL, a finding of our group in the last decade. This information has been successfully utilized for diagnosis and prognosis of the disease. Existing literature is included for comparison. Additionally, cell surface overexpression of 9-O-acetylated sialoglycoproteins and antibodies against them present in patients' sera aid the survival of the malignant lymphoblasts and suggest a multifaceted role played by these molecules. Taken together, monitoring these molecules helps not only in unravelling the biology of this paediatric malignancy but also in personalizing the treatment strategies for the betterment of the patient population.

9-O-acetylated sialoglycoproteins are important immunomodulators in Indian visceral leishmaniasis

Overexpression of disease-associated 9-O-acetylated sialoglycoproteins (9-O-AcSGPs) on peripheral blood mononuclear cells (PBMC) of visceral leishmaniasis (VL) patients (PBMC(VL)) compared to their levels of expression in healthy individuals has been demonstrated using a lectin, achatinin-H, with specificity toward 9-O-acetylated sialic acid derivatives alpha2-6 linkage with subterminal N-acetylgalactosamine (9-O-AcSAalpha2-6GalNAc). The decreased presence of disease-associated 9-O-AcSGPs on different immune cells of parasitologically cured individuals after successful treatment relative to the levels in patients with active VL prior to treatment was demonstrated. However, their contributory role as immunomodulatory determinants on PBMC(VL) remained unexplored. Accordingly, 9-O-AcSGPs on PBMC(VL) were sensitized with achatinin-H, leading to their enhanced proliferation compared to that observed with different known mitogens or parasite antigen. This lymphoproliferative response was characterized by evaluation of the TH1/TH2 response by intracellular staining and enzyme-linked immunosorbent assay for secreted cytokines, and the results were corroborated by their genetic expression. Sensitized PBMC(VL) evidenced a mixed TH1/TH2 cellular response with a predominance of the TH1 response, indicating the ability of 9-O-AcSGPs to modulate the host cell toward a favorable response. Interestingly, the humoral and cellular responses showed a good correlation. Further, high levels of anti-9-O-AcSGP antibodies with an order of distribution of immunoglobulin M (IgM) > IgG1 = IgG3 > IgG4 > IgG2 > IgE could be explained by a mixed TH1/TH2 response. A good correlation of enhanced 9-O-AcSGPs with both the cell-mediated (r = 0.98) and humoral (r = 0.99) response was observed. In summary, it may be concluded that sensitization of 9-O-AcSGPs on PBMC(VL) may provide a basis for the modulation of the host's immune response by their controlled expression, leading to a beneficial immune response and influencing the disease pathology.

Co-expression of 9-O-acetylated sialoglycoproteins and their binding proteins on lymphoblasts of childhood acute lymphoblastic leukemia: an anti-apoptotic role

Enhanced levels of 9-O-acetylated sialoglycoproteins (Neu5,9Ac2GPs) as disease-associated molecules was reported to act as signaling molecules for promoting survival of lymphoblasts in childhood acute lymphoblastic leukemia (ALL). Here, we searched for potential physiological ligands for Neu5,9Ac2GPs that could be involved in modulating the survival of lymphoblasts. Accordingly, we examined the presence of binding proteins for Neu5,9Ac2GPs on cell lines and primary cells of patients with B- and T-ALL, at presentation of the disease. Peripheral blood mononuclear cells from normal healthy donors and cells from myeloid leukemia patients were used for comparison. Neu5,9Ac2GPs-binding proteins (BPs) were specifically detected on the surface of both T- and B-ALL-lymphoblasts and ALL-cell lines along with the consistent presence of Neu5,9Ac2GPs. The Neu5,9Ac2GPs and BPs also co-localized on the cell surface and interacted specificallyin vitro. Apoptosis of lymphoblasts, induced by serum starvation, was reversed in the presence of purified Neu5,9Ac2GPs due to possible engagement of BPs, and the anti-apoptotic role of this interaction was established. This is the first report of the presence of potential physiological ligands for disease-associated molecules like Neu5,9Ac2GPs, the interaction of which is able to trigger an anti-apoptotic signal conferring a survival advantage to leukemic cells in childhood ALL.

Flow-cytometric monitoring of disease-associated expression of 9-O-acetylated sialoglycoproteins in combination with known CD antigens, as an index for MRD in children with acute lymphoblastic leukaemia: a two-year longitudinal follow-up study

Background

Over expression of 9-O-acetylated sialoglycoproteins (Neu5,9Ac₂-GPs, abbreviated as OAcSGP) has been demonstrated as a disease-associated antigen on the lymphoblasts of childhood acute lymphoblastic leukaemia (ALL). Achatinin-H, a lectin, has selective affinity towards terminal 9-O-acetylated sialic acids-α2-6-Nacetylated galactosamine. Exploring this affinity, enhanced expression of OAcSGP was observed, at the onset of disease, followed by its decrease with chemotherapy and reappearance with relapse. In spite of treatment, patients retain the diseased cells referred to as minimal residual disease (MRD) responsible for relapse. Our aim was to select a suitable template by using the differential expression of OAcSGP along with other known CD antigens to monitor MRD in peripheral blood (PB) and bone marrow (BM) of Indian patients with B- or T-ALL during treatment and correlate it with the disease status.

Methods

A two-year longitudinal follow-up study was done with 109 patients from the onset of the disease till the end of chemotherapy, treated under MCP841protocol. Paired samples of PB (n = 1667) and BM (n = 999) were monitored by flow cytometry. Three templates selected for this investigation were OAcSGP⁺CD10⁺CD19⁺ or OAcSGP⁺CD34⁺CD19⁺ for B-ALL and OAcSGP⁺CD7⁺CD3⁺ for T-ALL.

Results

Using each template the level of MRD detection reached 0.01% for a patient in clinical remission (CR). 81.65% of the patients were in CR during these two years while the remaining relapsed. Failure in early clearance of lymphoblasts, as indicated by higher MRD, implied an elevated risk of relapse. Soaring MRD during the chemotherapeutic regimen predicted clinical relapse, at least a month before medical manifestation. Irrespective of B- or T-lineage ALL, the MRD in PB and BM correlated well.

Conclusion

A range of MRD values can be predicted for the patients in CR, irrespective of their lineage, being 0.03 ± 0.01% (PB) and 0.05 ± 0.015% (BM). These patients may not be stated as normal with respect to the presence of MRD. Hence, MRD study beyond two-years follow-up is necessary to investigate further reduction in MRD, thereby ensuring their disease-free survival. Therefore, we suggest use of these templates for MRD detection, during and post-chemotherapy for proper patient management strategies, thereby helping in personalizing the treatment.

Identification of 9-O-acetylated sialoglycans on peripheral blood mononuclear cells in Indian Visceral Leishmaniasis

Although the existence of O -acetylated sialic acids is well known, it is only in recent years that steady refinement of analytical techniques have enabled detailed mapping of their structural diversity [1]. Fluorimetric analysis of peripheral blood mononuclear cells (PBMC) of patients with Visceral Leishmaniasis (VL) showed six fold increase in the percentage of surface 9-O-acetylated sialoglycoconjugates (9-O-AcSGs) as compared to normal human donors. Using Achatinin-H, a 9-O-acetyl sialic acid- binding lectin, an enhanced presence of 9-O-AcSGs in an alpha2 --> 6 linkage was demonstrated by flow cytometry; abolition of its binding by pretreatment with a recombinant 9-O-acetylesterase corroborated the presence of this glycotope. Western blotting of PBMC from VL patients indicated the presence of five O-acetylated sialoglycans corresponding to 144, 65, 56, 36 and 19 kDa as compared to 144 and 36 kDa in normal individuals. Taken together our data indicates that during active disease, there is an overexpression of 9AcSGs on the surface of PBMC of VL patients, thus opening up new research avenues wherein the expression of this biomarker could be exploited to monitor the clinical status of VL patients.

Detection of immune-complexed 9-O-acetylated sialoglycoconjugates in the sera of patients with pediatric acute lymphoblastic leukemia

Although childhood acute lymphoblastic leukemia (ALL) is highly responsive to chemotherapy, reliable techniques are needed to determine treatment outcome. Over expression of 9-O-acetylated sialoglycoconjugates (9-OAcSGs) on lymphoblasts and concomitant anti-9-OAcSGs was found to have a diagnostic and prognostic potential. However, the presence of circulatory immune-complexed antigens remains unknown. The present study was aimed to evaluate whether immune-complexed 9-OAcSGs can be harnessed for better disease management. Immune-complexed antigens were evaluated in ALL sera (n=262) by a Dot-blot using a 9-OAcSAalpha2-6GalNAc-specific lectin, Achatinin-H. Using three serum samples, the inter- and intra-assay imprecision was evaluated as 11-13% and 7-11%, respectively. The recovery of spiked 9-OAcSGs was 84.2-95.4%. The central 95% reference interval for immune-complexed 9-OAcSGs in normal human sera (NHS, n=144) was 2.9-3.4 mug/ml irrespective of sex and age. At disease presentation, the immune-complexed 9-OAcSGs were fivefold higher than NHS, decreased with remission induction and importantly, reappeared with clinical relapse. Sera from patients with other hematological disorders (n=86) showed negligible levels. The Dot-blot demonstrated the potential application of immune-complexed antigen as a disease-specific marker and its efficacy as a sensitive and specific method that could serve as an economical yet effective index for monitoring disease status.

Purification, characterization of O-acetylated sialoglycoconjugates-specific IgM, and development of an enzyme-linked immunosorbent assay for diagnosis and follow-up of indian visceral leishmaniasis patients

The surface expression of 9-O-acetylated sialic acid (9-OAcSA) is elevated on hematopoietic cells and erythrocytes of visceral leishmaniasis (VL) patients. In this study, we show that VL patients contain elevated levels of IgM antibodies directed against 9-O-acetylated sialoglycoconjugates (9-OAcSG). These antibodies were affinity purified with bovine submaxillary protein as the affinity matrix containing the terminal epitope, 9-OAcSAalpha2-6GalNAc. They also bound to 9-OAcSGs on hematopoietic cells of patients with VL and to epitopes in the cytosol of Leishmania donovani promastigotes. A novel enzyme-linked immunosorbent assay was employed that showed 4-fold higher anti-OAcSG titers in VL patients (n=38), mean +/- S.E.M. being 0.83 +/- 0.09 vs. 0.21 +/- 0.04 detected in normal donors (n=20) and patients with cross-reactive diseases such as malaria (n=4) or tuberculosis (n=4). Assay specificity and sensitivity was 100% and 92%, respectively, whereas positive and negative predictive values were 100% and 90%, respectively. Significantly, anti-OAcSG titers declined 30 days after completion of anti-leishmanial treatment, indicating that monitoring of anti-9-OAcSGs may be a valuable alternative toward increasing the efficiency of diagnosis and follow-up of VL.

Altered erythrocyte membrane characteristics during anemia in childhood acute lymphoblastic leukemia

Anemia is a prominent feature in children with acute lymphoblastic leukemia (ALL). To investigate the erythrocyte features during anemia in these patients, we studied the altered characters of these cells and oxidative stress imposed in their serum. This investigation reveals that erythrocytes from ALL patients show (1) increased membrane fluidity detected by fluorescence anisotropy studies, increased osmotic fragility detected by hemolysis of erythrocytes in different saline concentrations, and increased hydrophobicity as measured by binding with 8-anilino-1-naphthalenesulfonic acid, (2) enhanced (approximately threefold) glycosylation and sialylation, monitored by digoxigenin enzyme assay, and (3) expression of disease-specific 210, 105, 83, 54, and 28 kDa 9-O-acetyl sialoglycoconjugates (9-O-AcSGs) demonstrated by Western blot analysis and fluorescence-activated cell sorter (FACS) analysis studies using Achatinin-H with specificity towards 9-O-AcSAalpha2-6GalNAc as the analytical probe. (4) In addition, induced oxidative stress was observed in the sera of these children as indicated by increased nitric oxide (approximately fourfold) and thiobarbituric acid (TBA) reactive species (twofold) as detected by Griess reaction and TBA assay, respectively. For all the experiments, erythrocytes from normal individuals served as controls. Thus, the altered membrane characteristics together with their exposure to induced oxidative stress in serum are found to be a few features restricted to diseased erythrocytes. Taken together, our results are suggestive of their interplay in the contribution to the observed anemia in these patients, which may be exploited for better management of the disease.

Differential expression of 9-O-acetylated sialoglycoconjugates on leukemic blasts: a potential tool for long-term monitoring of children with acute lymphoblastic leukemia

Earlier studies have demonstrated overexpression of 9-O-acetylated sialoglycoconjugates (9-O-AcSGs) on lymphoblasts, concomitant with high titers of anti-9-O-AcSG antibodies in childhood acute lymphoblastic leukemia (ALL). Our aim was to evaluate the correlation between expression of different 9-O-AcSGs during chemotherapeutic treatment. Accordingly, expression of 9-O-AcSGs on lymphoblasts of ALL patients (n = 70) were longitudinally monitored for 6 years (1997-2002), using Achatinin-H, a 9-O-acetylated sialic acid (9-O-AcSA) binding lectin with preferential affinity for 9-O-AcSGs with terminal 9-O-AcSA alpha 2-->6GalNAc. Western blot analysis of patients (n = 30) showed that 3 ALL-specific 9-O-AcSGs (90, 120 and 135 kDa) were induced at presentation; all these bands disappeared after treatment in patients (n = 22) who had disease-free survival. The 90 kDa band persisted in 8 patients who subsequently relapsed with reexpression of the 120 kDa band. FACS analysis revealed that at presentation (n = 70) 90.1 +/- 5.0% cells expressed 9-O-AcSGs, which decreased progressively with chemotherapy, remained <5% during clinical remission and reappeared in relapse (80 +/- 10%, n = 18). Early clearance of 9-O-AcSG(+) cells, during 4-8 weeks of treatment showed a good correlation with low risk of relapse. Sensitivity of detection of 9-O-AcSG(+) cells was 0.1%. Numbers of both high- and low-affinity binding sites were maximum at presentation, decreased with treatment and increased again in clinical relapse. We propose that close monitoring of 90 and 120 kDa 9-O-AcSGs may serve as a reliable index for long-term management of childhood ALL and merits therapeutic consideration.

Identification of sialic acids on Leishmania donovani amastigotes

The presence of Neu5Ac on promastigotes of Leishmania donovani, the causative organism of Indian visceral leishmaniasis, has been reported recently. Here we report the occurrence of Neu5Ac as a major component on amastigotes, as well as Neu5Gc, Neu5,9Ac2 and Neu9Ac5Gc as indicated by fluorimetric high performance liquid chromatography and gas liquid chromatography/electron impact mass spectrometry. Furthermore, binding studies with Sambucus nigra agglutinin (SNA), Maackia amurensis agglutinin (MAA), and various Siglecs, showed the presence of both (alpha2-6) and (alpha2-3)linked sialic acids; their binding was reduced after sialidase pretreatment. Western blotting of amastigote membrane glycoproteins with SNA demonstrated the presence of two sialoglycoconjugates of Mr values of 164 000 and 150 000. Similarly, binding of MAA demonstrated the presence of five distinct sialoglycans corresponding to molecular masses of 188, 162, 136, 137 and 124 kDa. Achatinin-H, a lectin that preferentially identifies 9-Oacetylated sialic acid (alpha2-6)linked to GalNAc, demonstrated the occurrence of two 9-Oacetylated sialoglycans with Mr 158 000 and 150 000, and was corroborated by flow cytometry; this binding was abolished by recombinant 9-Oacetylesterase pretreatment. Our results indicate that Neu5Ac w(alpha2-6) and (alpha2-3)linked, as well as Neu5Gc and their 9-Oacetyl derivatives, constitute components of the amastigote cell surface of L. donovani.

Induction of glycosylation in human C-reactive protein under different pathological conditions

As an acute-phase protein, human C-reactive protein (CRP) is clinically important. CRPs were purified from several samples in six different pathological conditions, where their levels ranged from 22 to 342 microg/ml. Small, but significant, variations in electrophoretic mobilities on native PAGE suggested differences in molecular mass, charge and/or shape. Following separation by SDS/PAGE, they showed single subunits with some differences in their molecular masses ranging between 27 and 30.5 kDa, but for a particular disease, the mobility was the same for CRPs purified from multiple individuals or pooled sera. Isoelectric focusing (IEF) also indicated that the purified CRPs differed from each other. Glycosylation was demonstrated in these purified CRPs by Digoxigenin kits, neuraminidase treatment and binding with lectins. The presence of N-linked sugar moiety was confirmed by N-glycosidase F digestion. The presence of sialic acid, glucose, galactose and mannose has been demonstrated by gas liquid chromatography, mass spectroscopic and fluorimetric analysis. Matrix-assisted laser-desorption ionization analysis of the tryptic digests of three CRPs showed systematic absence of two peptide fragments, one at the N-terminus and the other near the C-terminus. Model-building suggested that the loss of these fragments exposed two potential glycosylation sites on a cleft floor keeping the protein-protein interactions in pentraxins and calcium-dependent phosphorylcholine-binding qualitatively unaffected. Thus we have convincingly demonstrated that human CRP is glycosylated in some pathological conditions.

Development of an assay for quantification of linkage-specific O-acetylated sialoglycans on erythrocytes; its application in Indian visceral leishmaniasis

We have developed a noninvasive approach for the quantification of linkage-specific 9-O-acetylated sialoglycans on mammalian erythrocytes using a lectin, Achatinin-H, whose lectinogenic epitope has previously been defined as 9-O-acetylated sialoglycoconjugates (9-O-AcSGs) alpha 2-->6 linked to subterminal GalNAc. Titration and checkerboard analysis were performed to optimize the assay using rabbit, rat and human erythrocytes that contain differing amounts of this glycotope. Assay specificity was established by decreased binding of erythrocytes to immobilised Achatinin-H when pre-incubated with excess lectin. The intra-assay coefficient of variation (CV) for rat and human erythrocytes was 8.6-9.2% and 11.1-13.0%, respectively. The inter-assay CV for rat and human erythrocytes was 9.9-10.1% and 15.2-16.6%, respectively. In previous studies, we have identified an enhanced presence of cell surface 9-O-AcSGs on the erythrocytes of patients with visceral leishmaniasis (VL) [Am. J. Trop. Med. Hyg. 58 (1998) 551]. Our assay when evaluated on erythrocytes from VL patients (n=30) showed a fourfold increase in lectin binding as compared to endemic controls. The mean +/- S.E.M. of the A(405) nm value was 1.14 +/- 0.04 vs. 0.23 +/- 0.03, respectively (p<0.0001). Following effective chemotherapy, a significant reduction of this glycotope on the erythrocytes of VL patients indicates that this assay has both a diagnostic and prognostic potential. Taken together, we conclude that this antigen-based assay is a specific and reproducible method for monitoring the disease status of VL patients and could be used in retrospective and prospective trials.

O-acetyl sialic acid specific IgM in childhood acute lymphoblastic leukaemia

Initial studies have revealed an enhanced surface expression of O-acetylated sialoglycoconjugates (O-AcSGs) on lymphoblasts concomitant with high titres of IgG in childhood Acute Lymphoblastic Leukaemia (ALL) (Mandal C, Chatterjee M, Sinha D, Br J Haematol 110, 801-12, 2000). In our efforts to identify disease specific markers for ALL, we have affinity-purified IgM directed against O-AcSGs that reacts with three disease specific O-AcSGs present on membrane proteins derived from peripheral blood mononuclear cells (PBMC) of ALL patients. Antibody specificity towards O-AcSGs was confirmed by selective binding to erythrocytes bearing surface O-AcSGs, decreased binding with de-O-acetylated BSM and following pretreatment with O-acetyl esterase. Competitive inhibition ELISA demonstrated a higher avidity of IgM for O-AcSG than IgG. Flow cytometry demonstrated the diagnostic potential of purified O-AcSA IgM as binding was specific with ALL patients and minimal with other haematological disorders and normal individuals. It therefore may be adopted as a non-invasive approach for detection of childhood ALL. Taken together, the data indicates that carbohydrate epitopes having terminal O-AcSA alpha2 --> 6 GalNAc determinants induce disease specific IgG and IgM, potentially useful molecular markers for childhood ALL.

Characterization of a Galactose Specific Adhesin of Enteroaggregative Escherichia coli

A fimbrial adhesin was identified from an enteroaggregative Escherichia coli strain. The adhesin was purified to 740-fold by sequential chromatography on an affinity matrix and gel filtration column in the FPLC system. The homogeneity of the purified protein was established by analytical isoelectrofocussing (pI 7.25). The native adhesin appeared as a high-molecular-weight aggregative protein as revealed by gel filtration chromatography on Superose 12HR10/30 column. However, in sodium dodecyl sulfate-polyacrylamide gel electrophoresis the molecular weight of the adhesin was found to be 18 kDa and this was further confirmed by gel filtration chromatography on Superose 6HR 10/30 column presence of 6 M guanidine hydrochloride. The N-terminal 15-amino-acid sequence of the adhesin did not show homology with any of the previously reported fimbrial adhesins. The purified adhesin showed adhesion to human erythrocytes in the presence of Ca(2+) (5 mM). The optimum temperature and pH for the hemadhesion activity was found to be 25 degrees C and 6.5, respectively. The inhibition study clearly suggested that the binding site of the adhesin could recognize galactose as the specific sugar. The fluorescence of 4-methylumbelliferyl-alpha-D-galactopyranoside was quenched on binding to the adhesin and maximum reversal of fluorescence quenching was observed by competitive substitution titration with raffinose. The adhesin was found to contain one binding site per monomer for its specific sugar residue. The association constant and the free energy of binding were obtained as 3.98 x 10(5) M(-1) and -31.97 kJ/mol, respectively. The adherence of the bacteria to HEp-2 monolayer was inhibited in presence of galactose and this was further supported by a significant reduction in the bacterial adherence to the HEp-2 cells, pretreated with beta-D-galactosidase.

Role of linkage specific 9-O-acetylated sialoglycoconjugates in activation of the alternate complement pathway in mammalian erythrocytes

Substitution of the -OH group at C-9 of sialic acid by an O-acetyl ester has been suggested to modify various biological phenomena that are regulated by sialic acids. Amongst them, enhancement of erythrocyte lysis by 9-O-acetylated sialic acid determinants through modulation of the alternate pathway of complement has been extensively studied on murine erythrocytes [1]. A variable expression of linkage specific 9-O-acetylated sialoglycoconjugates as defined by the lectinogenic epitope of Achatinin-H namely 9-O-acetylated sialic acid alpha2-->6Gal NAc was identified on rabbit, guinea pig, hamster, rat, mouse and human erythrocytes. This differential expression of linkage specific 9-O-acetylated sialoglycoconjugates strongly correlated with the susceptibility of mammalian erythrocytes to lysis by the alternate pathway of complement. Additionally, low levels of antibodies directed against O-acetylated sialic acids in these mammalian species suggested that these constitutively present determinants have low immunogenicity. Taken together, our results indicate that complement mediated hemolysis depends not simply upon the extent of surface 9-O-acetylated sialic acids present but more importantly upon the specific linkage.

Immunological significance of metal induced conformational changes in the mitogenic achatininH binding to carbohydrate ligands

9-O-Acetyl neuraminic acid specific lectin (AchatininH) was isolated from the hemolymph of the land snail Achatina fulica by affinity chromatography on sheep submaxillary mucin (SSM) coupled cyanogen bromide activated Sepharose 4B. The molecular weight of the native protein was 2.42 kDa. UV-Vis absorption, fluorescence and circular dichroism spectroscopic studies on AchatininH revealed the importance of divalent metal ions (Ca2 +, Mg2+ and Mn2+) on lectin conformational change associated with activity of lectins. The binding of these cations changes lambdamax to shorter wavelength in the far UV region (blue shift) and longer wavelength in UV region (red shift), indicating substantial contribution of aromatic side chain in the far UV region on binding with metal ions. The results infer that divalent cations cause conformational changes in lectin which may be responsible for affinity with their carbohydrate moiety.

Identification and purification of cytolytic antibodies directed against O-acetylated sialic acid in childhood acute lymphoblastic leukemia

Sialic acids typically present as terminal sugars of oligo-saccharides are reported to be modified by O-acetylation at the C-9 position on lymphoblasts of childhood acute lymphoblastic leukemia (ALL) patients (Sinha et al., 1999a, Leukaemia, 13, 119-125). We now report high titers of IgG antibodies directed against O-acetylated derivatives of sialic acids (O-AcSA) in serum of ALL patients. These antibodies were purified using bovine submaxillary mucin (BSM) and the IgG distribution was confined to IgG(1)and IgG(2)subclasses; their binding was totally abolished with de-O-acetylation confirming their specificity towards O-AcSA determinants. Flow cytometry demonstrated binding of these antibody fractions to peripheral blood mononuclear cells (PBMC) of both T- and B-ALL patients having increased cell surface 9-O-AcSA determinants. Western blotting of membranes derived from PBMC of ALL patients confirmed binding of the antibody to O-acetylated sialoglycoconjugates corresponding to 144, 135, 120, 90, and 36 kDa whereas binding to PBMC from normal individuals corresponded to 144 and 36 kDa. Specificity of the antibody fraction towards 9-O-AcSA was substantiated by hemagglutination and hemagglutination-inhibition assays. The antibody purified from ALL serum selectively mediates complement dependent cytolysis of lymphoblasts expressing O-AcSAs and thereby possibly confers passive protection. The enhanced anti O-AcSA antibody levels allowed for development of a serodiagnostic assay (BSM-ELISA) specific for ALL. Minimal crossreactivity was observed with other hematological disorders like acute myeloid leukemia (n = 16), chronic myeloid leukemia (n = 6), chronic lymphocytic leukemia (n = 7) and non-Hodgkin's lymphoma (n = 3) as well as normal healthy individuals (n = 28). The BSM-ELISA therefore provides a simple, noninvasive alternative diagnostic approach for ALL and merits clinical consideration.

Lectin like properties and differential sugar binding characteristics of C-reactive proteins purified from sera of normal and pollutant induced Labeo rohita

Different forms of C-reactive proteins (CRPs) have been purified to electrophoretic homogeneity from the sera of Labeo rohita confined in freshwater (CRP(N)) and water polluted with nonlethal doses of cadmium (CRP(Cd)) or mercury (CRP(Hg)). CRP(N), CRP(Cd), and CRP(Hg) show remarkable differences in their electrophoretic mobility but exhibit strong immunological cross reactivity. All these CRPs exhibit variable agglutination properties with erythrocytes from diverse sources in presence of Ca+2, which could be inhibited by a variety of sugars showing specificity for galactose. Inhibition results show that the potency of galactose as an inhibitor increases about 4 fold in the process of transformation of CRP(N) to CRP(Cd) and CRP(Hg). In case of CRP(N), Gal beta(1 --> 1) Gal and oNO2 phenyl beta-Gal show highest inhibitory potency while oNO2-phenyl beta-Gal is the most potent inhibitor for CRP(Cd) and CRP(Hg) but the potency of Gal beta(1 --> 1) Gal reduced drastically. 6-phosphate D-Gal and stachyose are 20 times weaker inhibitors than D-Gal for induced CRP mediated agglutination, in contrast, these sugars are only 6 times weaker for CRP(N). Dissociation constants of the binding of CRP(N) with phosphoryl choline (PC) and galactose are about 9 mM and PC binding causes a change in the alpha and beta conformations of these CRPs.

A colorimetric assay to evaluate the chemotherapeutic response of children with acute lymphoblastic leukemia (ALL) employing achatininH: a 9-O-acetyl sialic acid binding lectin

Employing a 9-O-acetyl sialic acid binding lectin, Achatinin(H) (ATNH), we have reported a non-invasive, blood based lymphoproliferation assay which measures the maximal lymphoproliferative dose (MLD) of ATN(H) to assess the status of 9-O-acetylated sialoglycoconjugates (9-OAcSGs) in patients with Acute lymphoblastic leukemia (ALL) (Mandal C, Sinha D, Sharma V, Bhattacharya DK. O-acetyl sialic acid binding lectin, as a probe for detection of subtle changes on the cell surface induced during acute lymphoblastic leukemia [ALL] and its clinical application. Ind J Biochem Biophys 1997;34:82; Sinha D, Mandal C, Bhattacharya DK. Development of a simple blood based lymphoproliferation assay to assess the clinical status of patients with acute lymphoblastic leukemia. Leuk Res 1999;13:309-312; Sinha D, Mandal C, Bhattacharya DK. A novel method for prognostic evaluation of childhood acute lymphoblastic leukemia. Leukemia 1999;13[in press]). Although the expression of 9-OAcSGs clearly serves as an index of treatment outcome, the assay has limitations in that it requires radioisotopes, i.e. [3H]-TdR. Therefore a colorimetric assay was developed as an alternative approach. The pre-treatment MLD, as measured by the colorimetric assay, was 0.15 +/- 0.02 microg which progressively increased during consolidation therapy (1.40 +/- 0.39 microg), maintenance therapy (4.20 +/- 1.60 microg) and in followed-up cases (5.20 +/- 0.43 microg) but sharply declined following relapse (0.25 +/- 0.02 microg). The colorimetric assay also showed a good correlation with radiometric assay (r = + 0.93) and their mean coefficient of inter-assay precision were also comparable (15.53% versus 14.86%). We therefore propose that the colorimetric assay is a safe, non-radiometric, user-friendly alternative for assessing individual chemotherapeutic responses in childhood ALL.

Diagnostic and prognostic potential of antibodies against O-acetylated sialic acids in canine visceral leishmaniasis

Employing bovine submaxillary mucin (BSM) as the coating agent, an enzyme-linked immunosorbent assay (BSM-ELISA) was developed to detect antibodies directed against O-acetylated sialic acids (O-AcSA) in canine visceral leishmaniasis (CVL). Serum samples were collected from 50 dogs previously screened by a parasite-ELISA to detect anti-leishmanial antibodies and designated as seropositive (n = 30) and seronegative (n = 20). The BSM-ELISA detected anti-O-AcSA antibodies in 29 out of 30 seropositive dogs and was negative in 15 out of 20 seronegative dogs; the sensitivity and specificity of the assay being 96.6% and 75%, respectively. Seven dogs from an endemic area in central Israel were longitudinally monitored for 15 months clinically, serologically and cultured for parasite. The levels of antibodies directed against O-AcSA increased with the appearance of clinical symptoms and/or seropositivity, disappeared when the disease was self-limiting as also with chemotherapeutic response and reappeared with relapse. The BSM-ELISA, therefore, represents a valuable tool for assessment of disease progression.

Development of a simple, blood based lymphoproliferation assay to assess the clinical status of patients with acute lymphoblastic leukemia

Although childhood acute lymphoblastic leukemia (ALL) is highly responsive to chemotherapy, reliable techniques are needed to determine treatment outcome and predict relapse. Employing a 9-O-acetyl sialic acid binding lectin, ATN(H), we have identified two 9-O-acetylated sialogycoconjugates (9-OAcSGs) as novel biomarkers expressed selectively on leukemic blasts of ALL patients. Presently, we report a non-invasive, blood based lymphoproliferation assay, which employs the maximal lymphoproliferative dose of ATN(H) (MLD) to assess the status of 9-OAcSGs with progressive therapy. A low MLD (0.18 +/- 0.01 microg) in untreated patients reflects increased expression of 9-OAcSGs which decline following therapy (MLD = 2.10 +/- 0.60 microg), persist during maintenance therapy (MLD = 4.50 +/- 1.60 microg)/follow-up (MLD = 5.50 +/- 0.85 microg) and are re-induced with relapse (MLD = 0.25 +/- 0.01 microg). Since the assay detects lymphoblasts with a sensitivity of 10(-4), shows no cross-reactivity with other hematological disorders (n = 48) and has been tested in 212 patients, it meets clinical consideration.

Identification of antibodies directed against O-acetylated sialic acids in visceral leishmaniasis: its diagnostic and prognostic role

A significantly increased O-acetylated sialic acid (O-AcSA) binding fraction was purified from serum of visceral leishmaniasis (VL) patients by affinity chromatography on immobilized bovine submaxillary mucin (BSM) and found to be immunoglobulin in origin. The serodiagnostic and prognostic potential of BSM as a capture antigen was established by ELISA with no cross reactivity with coendemic diseases like malaria, tuberculosis, leprosy, chagas disease and cutaneous leishmaniasis; however, a strong cross reactivity was present with trypanosomiasis patients. In 56 clinically diagnosed VL patients, the BSM-ELISA was compared with diagnosis by microscopy using Giemsa stained tissue smears and direct ELISA using crude parasite antigen (parasite-ELISA); 49/56(87.5%) and 5/56(9.0%) were positive and negative respectively by all 3 methods. The BSM-ELISA failed to diagnose 2/56(3.5%) patients which were biopsy and parasite-ELISA positive. The prognostic potential of the BSM-ELISA in 18 longitudinally monitored VL patients before and after conventional antimonial treatment showed a significant decrease in anti O-AcSA titres in drug responsive patients whereas anti O-AcSA levels persisted in drug unresponsive patients. The IgG subclass distribution of antibodies directed against O-AcSA showed increased IgG2 levels in VL patients as compared to healthy controls. The BSM-based ELISA holds great promise as a serodiagnostic and prognostic assay for VL.

The specificity of the binding site of AchatininH, a sialic acid-binding lectin from Achatina fulica

A sialic acid-binding lectin, AchatininH (ATNH), having unique specificity towards 9-O-acetylneuraminic acid, has been purified and characterized. The specificity of this lectin for O-acetylsialic acids was studied in detail, using various sialic acid derivatives and sialoglycoproteins. The potent inhibition of hemagglutination by bovine submaxillary mucin (BSM), which contains 9(7,8)-O-acetylsialic acid and by free 9-O-acetylneuraminic acid confirms the preferential affinity towards this sugar. Further support for the role of O-acetylsialic acid was obtained by sialidase treatment of BSM. O-Deacetylation of the sialic acid residue abolished its inhibitory potency. Moreover, when the trihydroxypropyl side chain of the sialic acid molecule was modified by periodate-borohydride treatment, the truncated C7-sialic acid was unable to bind ATNH. This result suggests that the glycerol side chain of Neu5Ac, especially the C-8 and/or C-9 portion is an important determinant for ATNH. The hemagglutination-inhibition results using several mono-, di-, and tri-saccharides containing terminal sialic acid and various sialoglycoproteins reveals that ATNH preferentially binds the alpha-(2-->6)-linked sialic acid. Furthermore, beta-D-GlcNAc-(1-->3)-[alpha-NeuGc-(2-->6)]-GalNAc-ol was found to be the best ligand for ATNH.

O-acetylated sialic acid as a distinct marker for differentiation between several leukemia erythrocytes

AchatininH (ATNH) is a lectin, isolated from the hemolymph of Achatina fulica snail, which has been shown to have narrow specificity towards 9-O-acetyl sialic acid. Usually ATNH does not agglutinate normal human erythrocytes, however, it is capable of agglutinating erythrocytes of patients suffering from acute lymphocytic and acute myelogenous leukemia. Determination of binding constants, numbers of binding sites and lectin overlay experiments using patients' erythrocytes ghost, have suggested that some alterations in erythrocyte cell surface sialoglycoproteins or more precisely appearance of some O-acetylated sialoglycoprotein as a result of pathological transformations has caused this change in the binding of ATNH.

A sialic acid-binding lectin from the mushroom Hericium erinaceum

A lectin was isolated from the mushroom Hericium erinaceum. This lectin is composed of two different subunits of 15 and 16 kDa and the molecular mass of the intact lectin was estimated to be 54 kDa by gel filtration. It exhibits specificity towards sialic acids, especially N-glycolylneuraminic acid.

Structural properties of the myelin-associated glycoprotein ectodomain

Myelin-associated glycoprotein (MAG) has been proposed to mediate adhesive interactions during myelin development. We have used the baculovirus expression system to produce a truncated form of this molecule [soluble extracellular domain of MAG (sMAG)] consisting of the complete extracellular ectodomain. Spectroscopic studies indicate a high beta-sheet content, consistent with the prediction of Ig-like structure. Hydrodynamic studies indicate an asymmetric monomer, with a Stokes radius of 4.1-4.6 nm, a sedimentation coefficient of 3.6S, and a frictional ratio of approximately 1.6. We postulate that the outer two Ig-like domains form a unit that folds back over the rest of the molecule. Fluorescence quenching studies indicate that sMAG interacts with divalent cations and may have a functional lectin domain.

Albumen gland of the snail Achatina fulica is the site for synthesis of AchatininH, a sialic acid binding lectin

A sialic acid binding lectin, AchatininH was purified from the hemolymph of Achatina fulica snail. To identify the site of synthesis of AchatininH, in vitro incubation studies in presence of labelled amino acid precursor were performed. Different organs from the snail were sliced and incubated in methionine-deficient Eagle's minimum essential medium containing [35S]-methionine at 25 degrees C for 5 h. After termination of incubation, tissues were homogenized, centrifuged and the de novo synthesized protein was immunoprecipitated with specific AchatininH antibody, followed by protein-A. The precipitated antigen-antibody complex was analysed by SDS-PAGE. Data obtained from native gel electrophoresis and SDS-PAGE radioautographic analysis indicates that AchatininH is synthesized in the albumen gland.

Kinetics of haloperidol binding to monoclonal antibodies as measured by direct fluorescence quenching

Monoclonal antibodies which bind small neurogenic ligands may mimic certain aspects of the stereospecific binding present in the natural biological receptor, and may prove useful in designing engineered protein receptors, provided their interactions are correctly understood. We report here the kinetic and equilibrium characteristics of ligand-antibody complexes of haloperidol, a dopaminergic antagonist, with three of its monoclonal antibodies as studied by fluorescence quenching techniques. These antibodies possessed moderate to high affinity constants, ranging from 10(5) to 10(9) M-1, and caused fluorescence quenching of a fluorescein-labeled haloperidol as well as quenching (40-60%) of the internal tryptophan fluorescence. The dissociation rates of the ligand from the complexes were measured at different conditions of temperature, pH and ionic strength. The results provide important information regarding the kinetic and thermodynamic parameters of the binding pockets of these antibodies.

The polyclonal activation of lymphocytes and T cell mitogenicity by a unique sialic-acid-binding lectin from the hemolymph of Achatina fulica snail

A unique sialic-acid-binding lectin, AchatininH, isolated from the hemolymph of Achatina fulica snail was found to be strongly mitogenic, as monitored by [3H]thymidine incorporation assay, to rat and human lymphocytes. However, the degree of mitogenic response varied with the type of cell population. As indicated, this lectin induced proliferation of purified T lymphocytes and rat thymocytes, whereas it was less mitogenic towards peripheral lymphocytes of pregnant rats and was not mitogenic towards B lymphocytes. Furthermore, the mitogenic response was inhibited by the sialic-acid-containing disaccharide, a strong inhibitor of this lectin. This suggests that lymphocyte cell surface molecules containing sialic acid residues are important for this interaction and may have a structure similar to that of AchatininH receptors. Although this lectin showed strong mitogenicity towards lymphocytes, it showed very weak leucoagglutination. Surprisingly, PHA-induced blastogenesis was inhibited by the same dose of AchatininH as caused mitogenic activity in resting lymphocyte culture. The degree of suppression was higher for the lymphocytes isolated from pregnant rat blood compared to the non-pregnant control. Since the 51Cr-uptake assay and the cell viability test results negate the potential cytotoxic activity of this protein, the immunosuppression induced in the presence of PHA by AchatininH may be the additive effect of two mitogens causing an increased cell-density-dependent arrest. AchatininH does not have complement-like activity, but induces potent polyclonal activation of B cells as measured by the reverse hemolytic plaque assay.

Sialic acid binding lectins

The literature contains several reviews on lectins in general, covering mainly those from plants and invertebrates. However, the sialic acid binding lectins have not been reviewed so far. Considering the importance of sialic acids in cell sociology, lectins which specifically recognize terminal sialic acid residues are potentially useful as analytical tools in studying the biological functions of sialoglycoconjugates. These lectins, along with monoclonal antibodies raised against sialoglycoconjugates, have been used in the detection, affinity purification, cytochemical localization and quantitation of such glycoconjugates. In this review the main emphasis has been placed on the occurrence, general purification procedures, macromolecular properties, sugar specificities and applications of these lectins.