Effect of sialidase on the viability of erythrocytes in circulation

Sialic acid and platelet count regulation: Implications in immune thrombocytopenia

Platelets are blood components that survive in circulation for 7 to 10 days in humans. Thus, platelet production by bone marrow (BM) megakaryocytes (MKs), and their removal from the blood circulation is precisely orchestrated to maintain an average platelet count. Abnormalities in both processes can result in thrombocytopenia (low platelet count) or thrombocytosis (high platelet count), often associated with the risk of bleeding or overt thrombus formation, respectively. Platelet glycans, particularly sialic acids, are indicators of platelet count. Loss of platelet sialic acids leads to platelet clearance. A State-of-the-Art lecture titled "Platelet and Megakaryocyte Glycobiology" was presented at the ISTH virtual congress 2021 to discuss (i) the loss of O-glycan sialic acid on BM MKs, revealing the Thomsen-Friedenreich (TF) antigen as a new concept of thrombocytopenia; herein, impaired thrombopoiesis is attributed to activation of immune cells with a plasmacytoid dendritic cell signature; and (ii) upregulation of antibodies against the TF antigen in pediatric patients with immune thrombocytopenia (ITP), positing that glycan alterations such as MK asialylation can lead to immune cell responses. Here, we discuss our findings alongside new data presented at the 2020 and 2021 ISTH congresses on the role of sialic acids and glycans in regulating platelet count. Desialylation is a prominent feature in thrombocytopenia, notably in ITP presentation. We compare similarities between ITP mediated with shear-stress and with storage-related asialylation. We also discuss genes involved in sialic acid synthesis leading to thrombocytopenia. Increased awareness in gene-regulating MK and platelet glycans is a giant leap to understanding the underpinning mechanisms of ITP and other forms of thrombocytopenia.

The regulation roles of Ca2+ in erythropoiesis: What have we learned?

Calcium (Ca²⁺) is an important second messenger molecule in the body, regulating cell cycle and fate. There is growing evidence that intracellular Ca²⁺ levels play functional roles in the total physiological process of erythroid differentiation, including the proliferation and differentiation of erythroid progenitor cells, terminal enucleation, and mature red blood cell aging and clearance. Moreover, recent research on the pathology of erythroid disorders has made great progress in the past decades, indicating that calcium ion hemostasis is closely related to ineffective erythropoiesis and increased sensitivity to stress factors. In this review, we summarized what is known about the functional roles of intracellular Ca²⁺ in erythropoiesis and erythrocyte-related diseases, with an emphasis on the regulation of the intracellular Ca²⁺ homeostasis during erythroid differentiation. An understanding of the regulation roles of Ca²⁺ homeostasis in erythroid differentiation will facilitate further studies and eventually molecular identification of the pathways involved in the pathological process of erythroid disorders, providing new therapeutic opportunities in erythrocyte-related disease.

Use of mouse models to study the mechanisms and consequences of RBC clearance

Mice provide tractable animal models for studying the pathophysiology of various human disorders. This review discusses the use of mouse models for understanding red-blood-cell (RBC) clearance. These models provide important insights into the pathophysiology of various clinically relevant entities, such as autoimmune haemolytic anaemia, haemolytic transfusion reactions, other complications of RBC transfusions and immunomodulation by Rh immune globulin therapy. Mouse models of both antibody- and non-antibody-mediated RBC clearance are reviewed. Approaches for exploring unanswered questions in transfusion medicine using these models are also discussed.

Sialic acid in cardiovascular diseases

Sialic acid, the acylated derivatives of 9-carbon sugar neuraminic acid, present as terminal component of oligosaccharide chains of many glycoproteins and glycolipids, has been recognized to be involved in the regulation of a great variety of biological phenomena. Studies have shown that serum sialic acid predicts both coronary heart disease and stroke mortality and reflects the existence or activity of an atherosclerotic process. Most of the studies have shown an elevation in serum sialic acid concentration in coronary heart disease and a positive correlation between the raised serum sialic acid and the severity of the coronary lesions is observed. However, a few contradictory reports are also available. Racial differences in serum sialic acid have also been reported and correlated with international differences in the prevalence of atherosclerosis. Reduced sialic acid content of platelets, erythrocytes and lipoproteins may play important role in the pathogenesis of atherosclerosis. Elucidation of the mechanism of alternation in sialic acid concentration may throw more light on its potential clinical utility. Hence more studies are needed to designates sialic acid as a cardiovascular risk factor/marker.

Flow cytometric approach to the study of erythrophagocytosis: evidence for an alternative immunoglobulin-independent pathway in agammaglobulinemic mice

Neuraminidase treatment of red blood cells (RBCs) is believed to induce changes similar to RBC senescence, and leads to a rapid clearance of RBCs from the circulation in vivo. The objective of this study using immunodeficient SCID mice and the lipophilic fluorescent probe PKH-26 was to ascertain whether antibodies are required as the final signal allowing the phagocytosis of neuraminidase-treated murine RBCs. All of the methods we applied are based on flow cytometry analysis using fluorescent probes: fluoresceinyl isothiocyanate (FITC)-labeled lectins for membrane carbohydrate identification and PKH-26-labeled RBCs for in vitro phagocytosis and in vivo clearance studies. The results can be summarized as follows: (i) the rate of neuraminidase-induced desialylation of RBCs from normal and immunodeficient mice is identical as ascertained with FITC-labeled lectins (wheat germ agglutinin (WGA) and Ricinus communis agglutinin (RCA(120))); (ii) the rate of clearance of enzyme-treated RBCs from both types of mice is also similar, as is their localization in spleen, liver and lung; (iii) the rates of in vitro phagocytosis of untreated and neuraminidase-treated PKH-26-labeled RBCs from both species of mice are very similar in the presence of homologous sera. In the absence of serum or in the presence of heterologous sera, the rate of phagocytosis is markedly decreased but not totally abolished. These data suggest that neuraminidase-treated RBCs can be cleared via an alternative pathway that is antibody-independent. This pathway exists in immunocompetent mice but with a very low activity and is the only one active in immunodeficient mice. In accordance with results reported by Connor et al. [J. Biol. Chem. 269 (1994) 2399], it is possible that this antibody-independent mechanism is involved in the clearance of circulating senescent RBCs. Finally, the methods described here may also be of interest for the investigation of the mechanisms involved in the phagocytosis of apoptotic cells.

Acidic and neutral sialidase in the erythrocyte membrane of type 2 diabetic patients

The behavior of the 2 sialidase forms present in the erythrocyte membrane was investigated in 117 subjects with type 2 diabetes mellitus versus 95 healthy controls. A significant increase of the acidic form of sialidase, which is anchored to the membrane by a glycosylphosphatidylinositol bridge, was observed in erythrocyte resealed membranes. On the contrary, the neutral form of the enzyme, the only one capable of removing lipid- and protein-bound sialic acid from endogenous and exogenous sialoderivatives, was significantly reduced with a consequent increase of erythrocyte membrane total sialic acid content. Disease duration, therapy, glycemia, parameters of metabolic control, and presence of complications, except nephropathies, had no influence on the tested enzyme activities. Diabetic subjects showed a different erythrocyte age distribution, with an almost double proportion of young red cells and only one quarter of senescent ones compared with controls. In young erythrocytes, diabetic and control subjects had the same distribution of the 2 enzymes, while in senescent cells the acidic enzyme was increased 3.5-fold and the neutral form was reduced by half in the diabetic subjects. The increase of both acidic sialidase and total membrane-bound sialic acid, together with an overpresence of young red cells in diabetics, suggests that in this pathological condition there might be an altered aging process with a diminished expression of the neutral form of the enzyme and an increase of bound sialic acid. It has been suggested that the expression of the neutral enzyme requires some activation mechanism that is impaired in diabetes.

Different behavior of ghost-linked acidic and neutral sialidases during human erythrocyte ageing

Acidic and neutral sialidases (pH optimum 4.7 and 7.2, respectively) were assayed on human circulating erythrocytes during ageing. The assays were performed on intact erythrocytes and resealed erythrocyte ghost membranes. From young to senescent erythrocytes the acidic sialidase featured a 2.7-fold and 2.5-fold decrease in specific activity when measured on intact cells or resealed ghost membranes, whereas the neutral sialidase a 5-fold and 7-fold increase, respectively. The Ca2+-loading procedure was employed to mimic the vesiculation process occurring during erythrocyte ageing. Under these conditions the released vesicles displayed an elevated content of acidic sialidase, almost completely linked through a glycan phosphoinositide (GPI) anchor but no neutral sialidase activity, that was completely retained by remnant erythrocytes together with almost all the starting content of sialoglycoconjugates. The loss with vesiculation of acidic sialidase with a concomitant relative increase of neutral sialidase was more marked in young than senescent erythrocytes. The data presented suggest that during ageing erythrocytes loose acidic sialidase, and get enriched in the neutral enzyme, the vesiculation process, possibly involving GPI-anchors-rich membrane microdomains, being likely responsible for these changes. The enhanced neutral sialidase activity might account for the sialic acid loss occurring during erythrocyte ageing.

Cellular and molecular mechanisms of senescent erythrocyte phagocytosis by macrophages. A review

Human red blood cells (RBCs) have a life-span of 120 days in circulation, after which they are phagocytized by resident macrophages. Extensive studies have been undertaken by many investigators in order to elucidate the cellular and molecular mechanisms of the erythrophagocytosis. The critical questions addressed by physiologists, clinicians and biochemists are: 'which of the many traumatic blemishes that appear on the erythrocyte surface as it winds its way through the circulation is the primary signal for clearance of the effete RBC from the circulation?', or 'What is the critical signal that it, and it alone, will activate the resident macrophage to adhere to and engulf it?'. Numerous, and often conflicting, hypotheses have been proposed. Each investigator focusing on but one of the many modifications that afflict the cell surface of the ageing erythrocyte, viz changes in either or both the carbohydrate or peptidic moieties of glycoproteins; abolishment of the pre-existing asymmetry in the lipid bilayer with the exposure of phosphatidylserine residues; or alterations in spectrin, to mention but a few. Many of these investigators also have invoked an intermediary role for auto-immune antibodies that recognise the change(s) on the erythrocyte surface and thereby serve as opsonins as a prelude to the erythrophagocytosis. The objective of the present review is to evaluate the data in support of the various hypotheses, and to submit some of our own recent observations involving the use of flow cytometric procedures that: i) provide evidence that the cell surface sialic acid serves as a determinant of the life-span; ii) characterise the senescent erythrocyte population that is specifically captured and phagocytized by macrophages (utilising the rapid and sensitive procedure we developed for quantification of in vitro erythrophagocytosis); and finally iii) provide evidence for the existence of an alternative pathway that is independent of immunoglobulins.

Molecular mechanisms of erythrophagocytosis. Characterization of the senescent erythrocytes that are phagocytized by macrophages

We have recently developed a flow cytometric assay for the quantitation of erythrophagocytosis, using PKH 26-labeled erythrocytes as the target cells. Using this assay we have shown that there is extensive phagocytosis of desialylated erythrocytes. Furthermore, we have demonstrated that it is the densest population of erythrocytes obtained on a self-forming gradient of Percoll that shows the greatest susceptibility to phagocytosis. We designate this population of erythrocytes as fraction X; it is even denser than the fraction 5 found previously. This population of erythrocytes corresponds to zone X previously seen in the dot-plot of the flow cytometric analyses of human erythrocytes. Further scrutiny of this fraction indicates that a) it shows the greatest reactivity with annexin V, which is specific for the detection of phosphatidylserine (PS) exposed on the outer leaflet of the erythrocyte membrane, b) it is the most susceptible to erythrophagocytosis by resident murine peritoneal macrophages, and c) this erythrophagocytosis of PKH 26-labeled erythrocytes can be inhibited by annexin V and by liposomes containing PS. Scanning electron microscopy of fraction X shows two populations of erythrocytes: (A) spheroechinocytes with filipodes and (B) echinocytes without filipods. After a 2-h period of phagocytosis, the cells remaining in fraction X show a decrease in population A, commensurate with a decrease in reactivity with FITC-labeled annexin V from 65.5 to 24%.

Presence in Human Erythrocyte Membranes of a Novel Form of Sialidase Acting Optimally at Neutral pH

The feature of intact human erythrocytes and erythrocyte white ghosts is a unique sialidase activity with acidic optimal pH (acidic sialidase). The treatment of white ghosts with mildly alkaline isotonic solutions at 37°C, like that used to produce resealed ghosts, is accompanied by the expression, together with the acidic sialidase, of a novel sialidase with a pH optimum of 7.2 (neutral sialidase) that remained masked in the inside-out vesicles prepared from white ghosts. Exhaustive treatment of resealed ghosts with Bacillus Thuringiensis phosphatidylinositol-specific phospholipase C causes an almost complete release of the acidic sialidase, with the neutral enzyme remaining totally unaffected. The treatment of resealed ghosts with 1.2% Triton X-100 resulted in the solubilization of only the neutral sialidase, whereas 3.6% octylglucoside also solubilized the acidic sialidase. The neutral enzyme affected not only the artificial substrate but also any sialoderivatives of a ganglioside, glycoprotein, and oligosaccharide nature; the acidic enzyme did not affect sialoglycoproteins. Erythrocyte endogenous gangliosides were hydrolyzed by both sialidases, whereas the endogenous sialoglycoproteins responded to only the neutral enzyme. It was definitely proved that the acidic sialidase is located on the outer erythrocyte membrane surface, so presumably the neutral enzyme has the same location. It could be that the newly discovered neutral sialidase has a physiologic role in the releasing of sialic acid from erythrocytes during the erythrocyte aging process, leading to eventual phagocytosis by macrophages.

Flow cytofluorimetric analysis of young and senescent human erythrocytes probed with lectins. Evidence that sialic acids control their life span

Comparing the properties of 'young' and senescent ('aged') O+ erythrocytes isolated by applying ultracentrifugation in a self-forming Percoll gradient, we demonstrate that the sialic acids of membrane glycoconjugates control the life span of erythrocytes and that the desialylation of glycans is responsible for the clearance of the aged erythrocytes. This capture is mediated by a beta-galactolectin present in the membrane of macrophages. The evidence supporting these conclusions is as follows: (1) Analysis by flow cytofluorimetry of the binding of fluorescein isothiocyanate labelled lectins specific for sialic acids shows that the aged erythrocytes bind less WGA, LPA, SNA and MAA than young erythrocytes. The binding of DSA and LCA is not modified. On the contrary, the number of binding sites of UEA-I specific for O antigen and of AAA decreases significantly. PNA and GNA do not bind to erythrocytes. (2) RCA120 as well as Erythrina cristagalli and Erythrina corallodendron lectins specific for terminal beta-galactose residues lead to unexpected and unexplained results with a decrease in the number of lectin binding sites associated with increasing desialylation. (3) The glycoconjugates from the old erythrocytes incorporate more sialic acid than the young cells. This observation results from the determination of the rate of transfer by alpha-2,6-sialyltransferase of fluorescent or radioactive N-acetylneuraminic acid, using as donors CMP-9-fluoresceinyl-NeuAc and CMP-[14C]-NeuAc, respectively. (4) Microscopy shows that the old erythrocytes are captured preferentially by the macrophages relative to the young ones. Fixation of erythrocytes by the macrophage membrane is inhibited by lactose, thus demonstrating the involvement of a terminal beta-galactose specific macrophage lectin. (5) Comparative study of the binding of WGA, LPA, SNA and MAA to the aged erythrocytes and to the in vitro enzymatically desialylated erythrocytes shows that the desialylation rate of aged cells is low but sufficient to lead to their capture by the macrophages.

The specificity of alternative complement pathway-mediated lysis of erythrocytes: a survey of complement and target cells from 25 species

Sera from 20 species of mammals were tested for their ability to lyse erythrocytes from 18 species of mammals and birds by the alternative complement pathway. Erythrocytes were not lysed by homologous complement, with one minor exception, but all erythrocytes tested were lysed by at least one complement source, and all sera tested except that of the horse lysed at least one type of erythrocyte. Control experiments indicated that lysis was via the alternative complement pathway and that antibodies were not involved. Complement from the various species could be ranked from most active to least active, and erythrocytes could be ranked from most susceptible to least susceptible. There was an inverse correlation between complement activity and erythrocyte susceptibility. The ranking of the orders of placental mammals, from strongest to weakest complement, was carnivore > artiodactyl (ruminants and swine) > primate = armadillo > rodent > rabbit > horse. Opossum serum had activity that placed it in the centre of this range. Ferret complement, the most potent tested, lysed all erythrocytes tested except for homologous erythrocytes, with APCH50 titres as high as 4000. Although the overall reactivity pattern was clear, there were several striking exceptions. For example, the only complement source which lysed ferret erythrocytes was sera of the mouse. The amount of sialic acid present on erythrocytes of 14 mammals was determined, and was, in general, directly correlated with resistance to alternative complement pathway lysis, although there were prominent exceptions to this correlation, involving erythrocytes of the horse, burro and human. All 20 types of complement were also tested for their ability to lyse antibody-coated human tumour cells, under conditions in which both the classical and alternative complement pathways were functional. The data obtained suggest that alternative pathway activation is, in some cases, a major factor determining the effectiveness of a particular complement source in the lysis of xenogeneic tumour cells.

Human erythrocyte sialidase is linked to the plasma membrane by a glycosylphosphatidylinositol anchor and partly located on the outer surface

Treatment of human erythrocyte ghosts with phosphatidylinositol-phospholipase C (PIPLC) from Bacillus cereus liberated the ghost-linked sialidase. Maximal release of sialidase (about 70% of total) was achieved by incubating ghosts at 37 degrees C for 60 min, at pH 6.0, with PIPLC (PIPLC total units/ghost protein ratio, 4.5 each time) added at the beginning of incubation and every 15 min (four subsequent additions). Liberated sialidase was fully resistant to at least four cycles of rapid freezing and thawing and to storage at 4 degrees C for at least 48 h. The liberated enzyme had an optimal activity at pH 4.2, degraded ganglioside GD1a better than methylumbelliferyl N-acetylneuraminic acid (about fourfold), and gave a Km value of 2.56 x 10(-4) M and an apparent Vmax of 2.22 mU per mg protein on GD1a. Treatment of intact erythrocytes with PIPLC (PIPLC total units/erythrocyte protein ratio, 8), under conditions where haemolysis was practically negligible, caused liberation of 10-12% of membrane linked sialidase, indicating that the enzyme is, at least in part, located on the outer surface of the erythrocyte membrane. It is concluded that the erythrocyte membrane sialidase is anchored by a glycosylphosphatidylinositol structure sensitive to PIPLC action, and is partly located on the outer surface.

Preferential reactivity of autoantibodies in murine lupus NZB mice to neuraminidase-treated monosialogangliosides on B cells of mouse spleen

When analyzed by flow cytometry, reactivity of IgM autoantibodies in sera from NZB mice to spleen B cells, but not to T cells, from BALB/c mice was remarkably increased after treatment of the cells with Vibrio cholerae neuraminidase. By TLC immunostaining with the antibodies, neither neutral nor acidic glycosphingolipids from both BALB/c and NZB mouse spleens were found to be reactive, but after neuraminidase treatment of the TLC plate, prior to the immunostaining, three components became reactive. All of the reactive glycosphingolipids were found to carry a single sialic acid residue and were at a concentration less than 1.3% of the total lipid-bound sialic acids. Their mobilities on TLC plate were close to those of IV3 NeuAcnLc4Cer, IV3 NeuAcII3 NeuAcGg4Cer, and IV3 NeuAcII3 NeuAc2Gg4Cer. In addition, the monosialogangliosides, which became reactive with the autoantibodies after neuraminidase treatment, were found to be predominantly distributed on B cells from BALB/c mice spleen, but not on T cells by TLC immunostaining. These studies demonstrate that the majority of IgM autoantibodies to spleen lymphocytes in sera from NZB mice might react preferentially to terminal sugar residues of three new glycosphingolipids masked by a single sialic acid on B cells, but not on T cells.

Flow cytometric analysis of human erythrocytes: I. Probed with lectins and immunoglobulins

A recent review (Aminoff, 1988) summarized the evidence for and against our hypothesis for the role of glycophorin in the senescence and clearance of mammalian red blood cells (RBC) from circulation. This hypothesis postulates the loss of sialic acid from RBC surface in two forms: (a) as vesicles containing the sialoglycoprotein glycophorin, and (b) as free sialic acid residues from glycophorin molecules remaining on cell surface. In this report we demonstrate the applicability of flow cytometric procedures to explore, at the cellular level, time-dependent changes on RBC surface with change in cell size, and with in vivo age. The RBC are probed with fluorescein isothiocyanate (FITC) labelled lectins and goat anti-human-IgG and -IgM. The relative intensity of fluorescence is correlated to the change in RBC size as measured by forward lightscatter. Reactivity of RBC with FITC-labelled wheat germ agglutinin can be inhibited with either 0.2M N-acetylglucosamine or by removal of sialic acid residues with neuraminidase. The properties of the smallest RBC correspond to those of the oldest RBC in their: (a) decreased reactivity with FITC-labelled lectins that recognize sialic acid residues, wheat germ and Limax flavus agglutinins, and (b) increased reactivity with FITC-labelled goat anti-human-IgG and -IgM. These results are compatible with our glycophorin hypothesis. Moreover, they suggest that the initial loss of sialic acid as glycophorin containing vesicles is gradual, while the subsequent step involving the loss of sialic acid residues is rapid and exposes multiple disaccharide galactose beta(1-3)N-acetylgalacosaminyl residues. These unmasked disaccharide sites are recognized by autoimmune IgG, IgM, and lectin-like receptors on macrophages resulting in the clearance of senescent RBC from circulation.

Immunohistochemical analysis of pulmonary and pleural neoplasms with monoclonal antibodies B72.3 and CSLEX-1

Sequential paraffin sections of 222 epithelial lung tumors comprising all common histologic types, and 31 pleural mesotheliomas of all variants were immunostained with monoclonal antibodies (Mabs) B72.3 and CSLEX-1. Reactivity with Mabs B72.3 and CSLEX-1 respectively was noted in 7/57 and 4/57 squamous carcinomas, in 44/70 and 60/70 adenocarcinomas, 9/16 and 11/16 bronchioloalveolar carcinomas, 8/25 and 14/25 large cell undifferentiated carcinomas, 3/3 and 3/3 adenosquamous carcinomas, 0/11 and 0/11 carcinoids, 0/10 and 2/10 well differentiated neuroendocrine (NE) carcinomas, 4/13 and 5/13 intermediate cell NE carcinomas, 0/17 and 0/17 small cell NE carcinomas, and 0/31 and 1/31 mesotheliomas. In most instances, both Mabs stained the same tumors; however, reactivity with CSLEX-1 was more intense and extensive, and involved more cases. Therefore, regardless of conventional histologic type, staining with Mabs B72.3 and CSLEX-1 defines 4 subsets of lung tumors: one expressing both antigens, two expressing one but not the other, and one expressing neither. The possible biological and/or clinical significance of these subsets remains undetermined. When correlated with conventional histologic tumor types, our findings indicate: 1). both of these Mabs recognize most but not all adenocarcinomas and bronchioloalveolar carcinomas, and since CSLEX-1 stained more cases than B72.3, it may be argued that the former is a broader exocrine phenotype marker than the latter; 2). both of these Mabs select exocrine subsets of large cell undifferentiated carcinomas; 3). both of these Mabs stain exocrine cell subpopulations in well differentiated and intermediate cell NE carcinomas but not in carcinoids or small cell NE carcinomas, and 4). except for rare cases, neither B72.3 nor CSLEX-1 reacts with mesotheliomas regardless of variant.(ABSTRACT TRUNCATED AT 250 WORDS)

Sialic acid content in serum IgG from patients with myotonic dystrophy compared with healthy controls

One of the determinants of the lifetime of circulating glycoproteins is their content of the terminal carbohydrate sialic acid. In order to elucidate a possible mechanism behind reduced concentration of IgG in patients with myotonic dystrophy, serum IgG was isolated by affinity chromatography and its content of sialic acid determined. No difference between patients and healthy controls was found. The results provide further support to the idea that a host factor rather than an abnormality of the IgG molecule itself is responsible for the low concentration of serum IgG in this disease.

Effects of lead on sialic acid content and survival of rat erythrocytes

The anemia frequently observed in lead poisoning is thought to result from a shortening of erythrocyte survival in combination with inhibition of hemoglobin synthesis. However, the exact mechanism by which lead shortens erythrocyte survival remains unclear. In the present study, the effects of lead, injected intraperitoneally, on sialic acid content and survival of rat erythrocytes were investigated in order to study the relationship between them. As indices of lead exposure, hemoglobin (Hb) levels, hematocrits (Ht) and blood lead (blood Pb) levels in the injected rats were also examined. Exposure to lead significantly decreased the sialic acid content of the erythrocyte membrane. The decreases in sialic acid content were evident to some extent below a blood Pb level of 100 micrograms/100 ml and generally present at a level of 100 micrograms/100 ml and higher. In the rats exposed to lead a significant negative correlation was found between sialic acid content and the logarithm of blood Pb level. A shortening of erythrocyte survival was also observed in the rats exposed to lead.

The effect of glycosidases on the survival of rat erythrocytes in circulation

Enzymatic removal of sialyl groups from mammalian erythrocytes resulted in their rapid sequestration from circulation subsequent to autologous transfusion. It has been demonstrated by many investigators that the terminal beta-D-galactosyl group, exposed on red blood cell by in vitro desialosylation, is recognized by an autoimmune anti-galactosyl IgG and/or by a lectin-like receptor on monocytes and macrophages. It is demonstrated herein that the disaccharide structure beta-D-Galp-(1----3)-D-GalpNAc (a) is masked in normal rat RBC, but exposed in asialo-RBC; (b) could be detected with fluorescently-labeled peanut agglutinin; (c) could be released from the asialo-RBC with an endo-N-acetyl-alpha-D-galactosaminidase; and (d) upon its removal by treatment with the endo-N-acetyl-alpha-D-galactosaminidase, enhances the survival of the asialo-RBC in circulation.

Antigenicity of rat erythrocyte glycophorins

The relationship between rat red blood cell (RBC) glycophorins and the antigens recognised by anti-rat RBC antibodies was examined. Initially, murine monoclonal antibodies specific for surface epitopes on whole rat RBCs were tested for their reactivity with RBC membranes on Western blots and two were found which reacted with blotted antigens. These antibodies recognised two bands corresponding to the major PAS-stainable bands of rat RBC membranes (i.e., the glycophorins) and a number of minor bands, thus demonstrating that the bands are antigenically related. This band-pattern was remarkably similar to that obtained with mouse anti-rat RBC serum. Digestion with neuraminidase altered the electrophoretic mobility of most of the bands, providing additional evidence that they are sialoglycoproteins, although sialic acid was shown not to contribute to their antigenicity. The glycophorin nature of the major antigens was verified by reelectrophoresis and blotting of bands excised from SDS gels, which showed that they were interconvertible monomeric and dimeric forms of the same polypeptide chain. It is suggested that rat RBC glycophorins are a related family of sialoglycoproteins with the high molecular weight members being formed by dimerization of five lower molecular weight polypeptide chains in various combinations.

Evidence that superoxide-anion, produced by PMA-activated human polymorphonuclear leukocytes, is the cytolytic agent for rabbit, but not for sheep red blood cells

The lysis of sheep and rabbit red blood cells (SRBC and RRBC, respectively) upon exposure to PMA-activated human polymorphonuclear leukocytes (PMN) was investigated. The lysis of these target cells, which was measured by the release of 51Cr, showed different kinetics and scavenger-sensitivity. The lysis of RRBC, which was already detectable within 45 min of incubation, was sensitive to superoxide dismutase (SOD), but was only poorly influenced by scavengers of hydroxyl radical formation, such as desferal or thiourea. In contrast, lysis of SRBC was first detectable after 90 to 135 min of incubation and sensitive to desferal and thiourea, but not to SOD. Finally, only RRBC were sensitive to the artificial superoxide-generating system hypoxanthine/xanthine oxidase. Taken together, these data point at a cytolytic activity of superoxide anion O2- towards RRBC. SRBC are relatively resistant to O2-, but are lysed by an H2O2- and hydroxyl radical-dependent process.

Ultrastructure of a transmembrane glycoprotein, glycophorin A

The major sialoglycoprotein of the human red cell membrane, glycophorin A, was isolated and examined by rotary shadowing and transmission electron microscopy. The glycophorin A molecule appeared as a cloud-like structure with a short, dense core within a large cloud. Mild acid hydrolysis in 0.05 M H2SO4, 80 degrees C for 1 hr reduced the size of the cloud significantly but left the dense core intact indicating that the original cloud represented the sialylated oligosaccharide chains of glycophorin A with the dense core being the polypeptide chain and its associated linkage proteins. Incubating glycophorin A with cationized ferritin (CF) revealed that the CF was bound only to the cloud, a finding that supports the view that the cloud is comprised of the sialylated oligosaccharide chains of the glycophorin A molecule. SDS-polyacrylamide gel electrophoresis revealed that our preparation of glycophorin A, as well as commercial preparations, consisted of monomers, dimers and oligomers of glycophorin A with trace amounts of the minor glycophorins and linkage proteins. Knowledge of the ultrastructure of this important integral protein will enable one to design studies to determine its functional role in the membrane.

Role of membrane sialic acid content in the adhesiveness of aged erythrocytes to human cultured endothelial cells

Following our previous observation that the oldest normal red blood cells were the most adherent to human cultured endothelial cells, we attempted to simulate this age-related adherence. Among all the membrane modifications experienced by erythrocytes during their life-span, loss of sialic acids has attracted considerable attention. Using two different preparations of neuraminidase, we performed a sialic acid depletion on the youngest erythrocytes to reach a sialic acid content similar to that observed in physiologically aged erythrocytes. These pretreated youngest cells displayed limited increase in the adhesiveness to endothelial cells, lower than that found with intact oldest cells. To obtain an adhesiveness of pretreated cells similar to that of naturally aged cells, it was necessary to exceed 80% of sialic acid depletion. At this extent of desialation, modifications of the electrophoretic pattern of glycophorins were observed as well as the appearance of peanut agglutinin reactivity which were never found in physiologically aged erythrocytes. Therefore, the sialic acid loss cannot be considered as being a single determinant factor of the naturally aged red cell adhesiveness.

A new photometric method for the quantitation of Fc receptors for murine IgG1 on human monocytes and cell lines

We have previously shown a polymorphism of human Fc receptors for mouse IgG1 using an EA rosette technique in which human erythrocytes sensitized with a murine IgG1 monoclonal antibody against glycophorin A acted as indicator cells. We now describe a method to quantitate this EA rosetting using the pseudoperoxidase activity present in erythrocytes. This photometric assay allows the sensitive quantitative determination of Fc receptor expression on human monocytes and cell lines. Not only the human Fc receptor for murine IgG1 can be studied in this way, but the method can also be applied to other Fc receptors. An important factor in this type of rosette assay appears to be the amount of negative charge present on the surface of the indicator erythrocytes. Using alcian blue as a probe, we found that this negative charge is higher on human erythrocytes than on sheep erythrocytes, which may contribute to a better signal-to-noise ratio. The method described facilitates the characterization of Fc receptors and permits the rapid screening of monoclonal anti-Fc receptor antibodies.

Decreased sialic acid content of erythrocytes in patients with aplastic anaemia

The sialic acid content of erythrocytes from patients with aplastic anaemia was determined and compared with those in patients with several haematological disorders and healthy individuals. The sialic acid was released enzymatically with Vibrio cholerae sialidase and quantitated by the thiobarbituric acid method (Aminoff, 1961). The sialic acid content of normal erythrocytes was 538 +/- 31 nmol/ml of packed erythrocytes. That of erythrocytes from patients with aplastic anaemia was 480 +/- 35 nmol/ml of packed erythrocytes, which was significantly lower than normal (P less than 0.01). In contrast, erythrocytes from patients with myeloproliferative disorders showed significantly (P less than 0.05) higher sialic acid contents (564 +/- 45 nmol/ml of packed erythrocytes). These results suggest that some membrane changes occur in erythrocytes in patients with these disorders.

Quantitation of picomole levels of N-acetyl- and N-glycolylneuraminic acids by a HPLC-adaptation of the thiobarbituric acid assay

A simple HPLC adaptation of the periodate-TBA assay for free N-acetyl- and N-glycolylneuraminic acids greatly extends the sensitivity and increases the specificity of this standard colorimetric assay. The method, employing a C18 reverse-phase column eluted isocratically with a phosphoric acid-MeOH buffer, is linear over a range of 2 pmol to 20 nmol. Analyses can be performed directly on cell lysates and digests without prior purification of released sialic acids from contaminating salts and biological materials. Interference from 2-deoxysugars is completely eliminated as the chromophore from these compounds is completely resolved from that derived from sialic acids. The application of the technique to quantify cell-surface and total cellular TBA-reactive sialic acids on the surfaces of a variety of tumor cells is described. Additionally, the extent of desialylation of erythrocytes necessary to expose the T antigen is determined.

The anaemia of chronic renal failure: a potentially treatable catabolic phenomenon?

It is proposed that the anaemia of chronic renal failure and the apparent lack of erythropoietin response is a result of increased proteolytic and lysosomal enzyme action on both the red cell and erythropoietin itself. Removal of the sialic acid from the red cell membrane is known to cause sequestration and a shortened cell survival. Similarly removal of sialic acid from the carbohydrate coat of erythropoietin both increases clearance by the liver and renders it susceptible to cleavage into inactive fragments by proteolytic attack. If the hypothesis is correct therapeutic intervention using enzyme inhibitors may offer hope for the amelioration of the anaemia.

Adhesion and erythrophagocytosis of human senescent erythrocytes by autologous monocytes and their inhibition by beta-galactosyl derivatives

Senescent human erythrocytes (RBC) are able to adhere to and be phagocytized by autologous monocytes in vitro to a greater extent than are young RBC. This adhesion and erythrophagocytosis of senescent RBC is inhibited by D-galactose, N-acetyl-D-galactosamine, their corresponding derivatives of bovine serum albumin, and lactose. On the other hand, D-glucose, D-mannose, L-fucose, N-acetyl-D-glucosamine, and their corresponding derivatives of bovine serum albumin are noninhibiting. The glycopeptides released by tryptic digestion of senescent RBC and purified on immobilized peanut agglutinin are the most effective inhibitors of both RBC adhesion and phagocytosis by autologous monocytes obtained from peripheral blood.

Increased plasma glycosidase and protease activity in uraemia: possible role in the aetiology of the anaemia of chronic renal failure

We have measured plasma N-acetyl-beta-D-glucosaminidase (EC 3.2.1.30) and neuraminidase (EC 3.2.1.18) activities as markers of glycosidase activity and immunoreactive trypsin (EC 3.4.21.4) levels as a marker of proteolytic potential in the plasma of normal and uraemic subjects. The levels of all of these enzymes are significantly elevated in the plasma of uraemic subjects when compared to normal. We have postulated that the combined attack of glycosidases and proteases on erythropoietin will lead to fragmentation of this glycoprotein hormone with loss of activity. This may be a major contributory cause to the anaemia of chronic renal failure.

Study of the optimal reaction conditions for assay of the mouse alternative complement pathway

The optimal reaction conditions for hemolytic assay of alternative complement pathway activity in mouse serum were investigated. A microtiter system was used, in which a number of 7.5 X 10(6) rabbit erythrocytes per test well appeared to be optimal. Rabbit erythrocytes were superior as target cells over erythrocytes from a number of other animal species. The optimal conditions were as follows: an incubation temperature of 39 degrees C, an ionic strength of about 200 mM, and a magnesium concentration of 2.5 mM. Incubation during 60 min was not sufficient for an end-point titration. Addition of 1 mg of zymosan A per test well, however, enhanced and accelerated the hemolytic activity of mouse serum via the alternative pathway resulting in a maximum value after 45 min. This, most probably, proceeded by a mechanism involving the formation of a zymosan-C5-convertase and bystander lysis of the target cells. In contrast to the normal alternative pathway assay the zymosan-potentiated test did, most probably, not involve natural antibodies. Cobra venom factor was more efficient in enhancing the sensitivity of the assay for the mouse alternative complement pathway than zymosan. This makes this factor very useful for testing C-poor body fluids.

Membrane structure and function of malaria parasites and the infected erythrocyte

According to the World Health Organization the global estimate of malaria is over 200 million infections, the majority of which are caused by the most life-threatening species,Plasmodium falciparum(Report of the Steering Committees of the Scientific Working Groups on Malaria, World Health Organization, June 1983). The causative agent of the disease, the malarial parasite, requires two hosts: a blood-sucking mosquito and a blood-containing vertebrate. Commonly, infection of the vertebrate begins when an infected mosquito bites a suitable vertebrate and injects minute sporozoites into the bloodstream. Within 30 mm the introduced sporozoites leave the bloodstream and enter parenchymal cells of the liver (mammals) or endothelial cells (birds). In these sites the parasite undergoes asexual multiplication (= exo-erythrocytic schizogony) producing daughter progeny called merozoites. The exo-erythrocytic merozoites are released from the tissues into the circulation where they invade red blood cells. Within an erythrocyte the merozoite undergoes asexual multiplication (= erythrocytic schizogony) producing a substantial number of merozoites. The erythrocyte lyses, merozoites are released, and invasion of another erythrocyte may then take place. The synchronous rupture of the red cell and merozoite release is marked by the periodic fever–chill cycles so characteristic of the malarial infection. Some merozoites continue to reinvade other erythrocytes and multiply by asexual means, whereas others enter erythrocytes and differentiate into sexual stages, male or female gametocytes. When a suitable mosquito feeds on an infected vertebrate gametocytes are ingested and the sexual cycle of development is initiated. In the mosquito stomach the gametocytes transform into gametes, fertilization takes place, the resultant worm-like zygote penetrates the cells of the mosquito gut and comes to lie on the outer surface of the stomach. Here each zygote forms a cyst-like body, the oocyst, within which thousands of sporozoites are produced by asexual multiplication. When the swollen oocysts burst, sporozoites are freed and these make their way to the salivary gland. At the next blood feeding the mosquito injects the infective sporozoites and the life-cycle is completed.

Abnormal erythrocyte survival in patients with myotonic dystrophy

In four of six patients with myotonic dystrophy whom we studied 51Cr labeled erythrocytes were found to have a biphasic survival kinetic. After in initial significantly more rapid disappearance than in controls, about 80% of the labeled cells reappeared in circulation, and were thereafter eliminated at a significantly faster rate than normal. This type of biphasic survival pattern may partly be related to reduced sialic acid concentration of the erythrocyte membrane in patients with this disease.

Phosphorylation of glycophorin A in membranes of intact human erythrocytes

The qualitative and quantitative contribution of glycophorin A phosphorylation to the general and specific pattern of membrane protein phosphorylation in intact erythrocytes pre-incubated with 32Pi was examined. Intense 32P-labeled bands at 88,000 and 38,000 Mr were identified as phosphorylated glycophorin A dimer and monomer respectively on the basis of several criteria. Quantitatively, phosphorylated glycophorin A dimer accounted for about 70% of 32P in the band 3 region. This value is at least three times that previously reported. The results of ancillary experiments involving selective extraction of ghosts in acidified chloroform/methanol solvents and electrophoresis in the presence of detergents make it unlikely that the 32P associated with glycophorin A was due to bound polyphosphoinositides.

A re-examination of the effects of chymotrypsin and trypsin on the erythrocyte membrane surface topology

Silver/Coomassie blue staining of human erythrocyte membrane electrophoretograms permits simultaneous visualization and color differentiation of asialoproteins, sialoglycoproteins and lipids in the same gel. Using this technique evidence is provided that chymotrypsin cleaves glycophorin A as well as band 3. The chymotryptic fragmentation pattern of glycophorin A in situ intact cells was different from that generated by trypsin treatment. Chymotryptic cleavage of band 3 generated two Coomassie blue stained fragments at 62,000 and 38,000 Mr, whereas simultaneous cleavage of glycophorin A dimer and glycophorin A B heterodimer yielded yellow silver stained fragments at 68,000 and 47,000 Mr. Trypsin cleaved glycophorin A dimer (88,000 Mr) and monomer (38,000 Mr) to form membrane associated fragments of Mr = 40,000 and 18,000 respectively.

Use of 75Se-labeled methionine to study the sequestration of senescent red blood cells

Labeling red blood cells with Na251CrO4 enabled us to study certain aspects of red cell survival and sequestration from the circulation. As a random labeling procedure, however, the 51Cr method has certain limitations. Therefore, we developed a cohort labeling method using 75Se-methionine as a two-rat procedure. This gives us a clear pulse-labeled population of rat red cells to study the dynamics of sequestration. With this labeling procedure, it was possible to demonstrate that 1) there is an increase in the density of red cells with age, 2) a significant sequestration of red cells from the circulation is apparent at the end of 48 days and essentially is complete at the end of 60 days, 3) there is a corresponding uptake of senescent red cells in the spleen, which peaks at 55 days, and 4) the 60-day end point is sharper and is more definitive when the "specific activity" (cpm per red blood cell) of the labeled red cells in the spleen is compared to that of the red cells still in the circulation. Asialo red cells, obtained by removal of sialic acid with sialidase, frequently have been used as a model for the study of sequestration of senescent red cells. With the technique herein described, it was possible to show that while asialo red cells will inhibit the uptake of labeled asialo red cells, they have no effect on the sequestration of senescent red cells. Presumably, different sites and mechanisms of sequestration are involved.

Surface-associated sialic acid is an immunological adjuvant

The influence of neuraminidase on the immunogenicity of heterologous erythrocytes as determined by serum haemagglutination titres was investigated in mice. For this study sheep and rabbit erythrocytes were selected because of their high and low N-acetylneuraminic (sialic) acid content, respectively. Preincubation with neuraminidase resulted in a ten-fold reduction of the immunogenicity of sheep erythrocytes (ShE). By contrast, the immune response to rabbit erythrocytes appeared to be resistant to sialidase treatment. Addition of the extrinsic adjuvant dimethyldioctadecylammonium bromide largely restored the immunogenicity of neuraminidase-treated ShE, but did not change the response to control-treated ShE. The maximal antibody level induced by neuraminidase-treated ShE was lower than that provoked by control ShE. These results suggest that sialic acid is both an intrinsic immunological adjuvant and an antigenic determinant of ShE. The adjuvant effect of sialic acid does not depend on complement component C3 as judged by the response of cobra venom factor-pretreated animals. In genetically C5-deficient and in nude mice, however, sialic acid showed diminished and absent adjuvant activity, respectively.

Membrane-bound immunoglobulins and complement components on young and old red cells

In order to characterize changes in membrane-bound immunoglobulins and complement components, red cells (RBCs) were separated into young and old populations by simple centrifugation. Old RBCs had reduced mean corpuscular hemoglobin volume, increased mean corpuscular hemoglobin concentration, and reduced sialic acid. Using radioactive anti-antiglobulin techniques, old RBCs were shown to have more IgG, IgM, IgA, and C3d on their surfaces than did young RBCs; there was no increase on old RBCs of C3b, factor B, C4b, or C5. Similar results were observed with RBCs strongly coated with C3d in vivo from a patient with cold agglutinin disease. RBCs taken into ethylenediamine tetraacetate, washed thoroughly in saline, and then stored for prolonged periods in Alsever's solution or kept in autologous ethylenediamine tetraacetate plasma, at 4 degrees C, showed no increase in RBC-bound C3d with increased storage time. If, however, blood was taken into citrate-phosphate-dextrose and maintained at 4 degrees C in autologous plasma, a significant increase in RBC-bound C3d was observed in the mixed-cell population with prolonged storage time. Order donor blood units, taken into citrate-phosphate-dextrose and stored at 4 degrees C as packed red cells, showed higher levels of RBC-bound C3d in the mixed-cell population than did units stored for a shorter time. In no case did donor unit RBCs give a positive direct antiglobulin test on serologic testing with anti-C3d. The findings complement data already collected on membrane and cytoplasmic changes in aging RBCs and may contribute to an understanding of RBC senescence.

Changes in (Na+ + K+) ATPase activity and the composition of surface carbohydrates in erythrocyte membranes in alcoholics

(Na+ + K+)ATPase activity and sensitivity to the inhibitory effect of ethanol and noradrenaline in vitro as well as the concentrations of sialic acid, galactose, and hexosamine were determined in erythrocyte membranes in 20 healthy controls and in 20 alcoholic patients within 24 hr of withdrawal. Basal (Na+ + K+)ATPase activity, the sensitivity of the enzyme to inhibition by ethanol and noradrenaline added in vitro, and the concentrations of sialic acid and galactose were significantly reduced in the patients (p less than 0.0005). All of these abnormalities were significantly correlated to each other as well as to the estimated daily quantity of ethanol consumed. After enzymatic removal of terminal sialic acid and terminal and sialic acid-bound beta-galactose, the (Na+ + K+)ATPase differences between the patients and the controls were eliminated. The results showed that the previously reported reduced inhibition of (Na+ + K+)ATPase by ethanol in the presence of noradrenaline in brain membranes in chronically ethanol-fed animals is also found in erythrocyte membranes in alcoholic humans. Abnormal carbohydrate composition of glycolipids and/or glycoproteins of the membrane surface appeared to be related to the (Na+ + K+)ATPase changes, possibly due to interference either with K+ transport or surface electrostatics or, directly or indirectly, with the conformation of (Na+ + K+)ATPase.

Detection of erythrocyte membrane proteins, sialoglycoproteins, and lipids in the same polyacrylamide gel using a double-staining technique

A silver/Coomassie brilliant blue R-250 staining technique that permits a color-coded differentiation of erythrocyte membrane proteins, sialoglycoproteins, and lipids in a single one-dimensional NaDodSO4/polyacrylamide gel has been described. Gels stained first with silver stain and then with Coomassie blue (CB) showed the characteristic blue staining of all conventional CB-sensitive membrane polypeptides, whereas periodic acid-Schiff reagent-sensitive sialoglycoproteins and lipids stained yellow. Several yellow Ag-stained bands corresponding to major and minor sialoglycoproteins were detected at Mr X 10(-3) of 88, 72, 65, 41, 35, 31, 28, 24, and 20. Neuraminidase treatment of intact erythrocytes caused shifts in the electrophoretic mobilities of several yellow-stained bands without affecting the CB-stained polypeptide pattern. These observations afforded evidence that the yellow-staining bands were sialoglycoproteins and lipids. The double-staining technique was used in a topological analysis of the membrane surface of the erythrocyte using protease digestion and selective solubilization. Trypsin cleaved the yellow bands at Mr 88,000 and 41,000. Membrane-associated cleavage products were noted at Mr 58,000 and 38,000. Pronase treatment of intact cells gave membrane-associated cleavage products at Mr 38,000 (yellow) and two CB-stained bands at Mr 58,000 and 60,000. These results suggested that the double-staining technique may be applicable in compositional and topological analyses of other biological membranes.

Carbohydrate composition of erythrocyte membranes and glycosidase activities in serum in patients with myotonic dystrophy, limb-girdle dystrophy and congenital myotonia

A number of abnormalities in cell membrane function, including cells other than muscle cells, have been described in patients with inherited muscular diseases such as myotonic dystrophy and congenital myotonia. The basic molecular defects are, however, still unknown. The complex carbohydrates of membrane-bound glycoconjugates are of vital importance for the normal performance of the cell membrane. In this study the concentrations of the three major carbohydrates (sialic acid, galactose and hexosamines) of the erythrocyte membrane were therefore determined in patients with myotonic dystrophy, limb-girdle dystrophy and congenital myotonia. The activities of relevant glycosidases in serum were also assayed. In each of the three diseases pertinent changes of the carbohydrate pattern were found. In patients with myotonic dystrophy the sialic acid and in patients with limb-girdle dystrophy the hexosamine concentration was significantly reduced (P less than 0.0005). The sialic acid, galactose and hexosamine concentrations were all significantly increased in patients with congenital myotonia. No increase of the neuraminidase (sialidase) activity was found in sera from patients with myotonic dystrophy. In patients with limb-girdle dystrophy, the activities of serum hexosaminidases were normal. These results support the contention that certain inherited muscular diseases may represent generalized membrane disorders, and suggests that disturbances of membrane-bound glycoproteins and/or glycolipids might be of importance in the pathogenesis of some of these disorders.

Isolation and characterization of type III group B streptococcal mutants defective in biosynthesis of the type-specific antigen

Four classes of mutants of type III group B streptococcus were isolated by serial subculture of the wild-type strain in the presence of type III-specific rabbit antiserum. Class I mutants no longer synthesized sialic acid but still elaborated the core antigen. Class II mutants maintained the ability to synthesize sialic acid but could not attach it to the core antigen. Class III mutants did not produce the core antigen but still synthesized intracellular sialic acid. Class IV mutants synthesized the complete antigen; however, only approximately 4% of the antigen synthesized was found associated with the cell wall peptidoglycan (in the wild-type strain greater than 85% of the antigen synthesized is covalently attached to the cell wall peptidoglycan), whereas greater than 90% of the antigen was secreted into the growth medium. Production of other components (CAMP factor, group B antigen, beta-hemolysin, neuraminidase) by these mutants appeared similar to those of the wild-type strain. Mouse lethality studies of these strains indicated that all four classes have greater than 3 log10-higher 50% lethal dose values than that of the wild-type strain. To understand the basis for this variation, the invasive ability of the wild-type strain and the sialic acid-deficient mutant strain M-10 (class I) was examined. Mice received 10(5) CFU of each organism; they were then sacrificed at various times postinoculation, and viable group B streptococci from different organs were enumerated. Mice were able to clear M-10 more efficiently, with greater than 80% of M-10 cells being phagocytized by macrophages within 1 h, whereas the wild-type strain was able to evade phagocytic killing and disseminate to other tissues. These data, therefore, strongly indicate that the sialic acid moiety greatly enhances the virulence of the type III antigen. In addition, the level of cell-associated type-specific antigen appears to contribute significantly to the pathogenicity of the organism.