Carbohydrate-mediated clearance of antibody . antigen complexes from the circulation. The role of high mannose oligosaccharides in the hepatic uptake of IgM . antigen complexes

Mammalian sugar-binding receptors: known functions and unexplored roles

Mammalian glycan-binding receptors, sometimes known as lectins, interact with glycans, the oligosaccharide portions of endogenous mammalian glycoproteins and glycolipids as well as sugars on the surfaces of microbes. These receptors guide glycoproteins out of and back into cells, facilitate communication between cells through both adhesion and signaling, and allow the innate immune system to respond quickly to viral, fungal, bacterial, and parasitic pathogens. For many of the roughly 100 glycan-binding receptors that are known in humans, there are good descriptions of what types of glycans they bind and how selectivity for these ligands is achieved at the molecular level. In some cases, there is also comprehensive evidence for the roles that the receptors play at the cellular and organismal levels. In addition to highlighting these well-understood paradigms for glycan-binding receptors, this review will suggest where gaps remain in our understanding of the physiological functions that they can serve.

Transduction of binding affinity by B lymphocytes: a new dimension in immunological regulation

To date, immunologists have operated with two primary paradigms governing the antibody response: (1) that affinity maturation is primarily dependent upon antigen-driven selection of both the germline and somatically amended repertoires, and (2) that antibody effector function is isotypically determined. The teleost model now suggests that these classical paradigms should be broadened to incorporate the ability of the B cell to transduce the strength of antigen recognition (affinity) into structural modifications of its antibody product, which, in turn, modulates the antibody's effector function. Although this relationship, thus far, has only been examined and demonstrated in the teleost, we find a number of the individual elements of this structural/functional relationship have been reported for mammalian IgM, which prompts future investigations into its universality. In sum, these findings suggest a heretofore unrecognized feature of B lymphocyte affinity discrimination, which transduces the affinity of antigen recognition into functionally modified antibodies.

The strength of B cell interaction with antigen determines the degree of IgM polymerization

The induction of variable disulfide polymerization of IgM in the trout (Oncorhynchus mykiss) and its effect on its half-life were examined. An association between greater Ab affinity and increased disulfide polymerization was first indicated by the observation of this increased IgM disulfide polymerization during the process of affinity maturation. A direct association between Ab affinity and disulfide polymerization was then established by the fractionation of individual sera into high- and low-affinity subpopulations, which also resulted in the partitioning of high and low degrees of disulfide polymerization. The ability of high-affinity B cells to produce more highly polymerized Abs upon Ag induction was demonstrated by in vitro Ag-driven selection. Low Ag concentrations, which elicited only high-affinity Abs, also possessed the highest degree of polymerization, whereas higher concentrations of Ag elicited a broader array of Ab affinities, yielding a lower average affinity and degree of polymerization. Half-life studies revealed that the high-affinity, highly polymerized Abs possessed longer half-lives than the lower-affinity, lightly polymerized Abs. Finally, although the affinity for Ag is associated with elevated levels of polymerization, analysis of naive Ig revealed that the degree of polymerization alone, not affinity, appears sufficient to prolong Ig half-life.

A novel monoclonal antibody recognizing beta(1-3) glucans in intact cells of Candida and Cryptococcus

The cell walls of all medically important fungi contain a unique polyglucose compound, beta(1-3) glucan. In the present study, murine monoclonal antibodies were produced against linear and beta(1-6) branched beta(1-3) glucans, and their specificities were characterized for reactivity to other beta glucans, fungal cell wall fragments, and fungal cells. Their reactivity was also compared with that of rabbit polyclonal antibodies raised against the same immunogens. Two mouse monoclonal antibodies (AG and BG) recognized immunoreactive epitopes in beta(1-3)(1-6) glucan by ELISA. In an inhibition assay of the anti-beta(1-3)(1-6) activity of the monoclonals, the homologous antigen effectively inhibited the activity as expected, while beta(1-3) also inhibited the assay but to a much lesser extent. No inhibition was obtained by beta(1-3)(1-4) or beta(1-6), while a cell wall extract of Candida albicans (PPM) effectively inhibited both monoclonals. Cell wall fragments of C. albicans (CaCW) and Cryptococcus neoformans (CnCW) inhibited the anti-beta(1-3)(1-6) activity of AG, while BG was much less or not inhibited at all. Immunofluorescence confirmed the unique antibody specificity of AG by its recognition of a beta(1-3)(1-6)-associated epitope on the cell surfaces of C. albicans,C. krusei, C. glabrata, and nonencapsulated C. neoformans. The epitope for the AG antibody is suggested to be present in the branching point of beta(1-3)(1-6), or in the randomly coiled beta(1-3) polyglucan due to the presence of branches. Thus, monoclonal antibodies to beta(1-3)(1-6) glucans may have potential as tools in the laboratory diagnosis of invasive yeast infections.

Function and pathology of the sugar chains of human immunoglobulin G

Human immunoglobulin G (IgG) is unique among serum glycoproteins because it contains more than 30 different biantennary complex-type asparagine-linked oligosaccharides. This extremely high microheterogeneity is probably produced because human individuals have a series of B cell clones equipped with different sets of glycosyltransferases. Despite this complex composition, IgG samples purified from whole human sera have the same mole ratios of oligosaccharides, indicating that the ratio of B cell clones synthesizing IgGs with different sugar chains is constant in healthy individuals. We found that the glycosylation patterns of whole serum IgGs obtained from patients with rheumatoid arthritis (RA) are quite different from those of whole serum from healthy individuals. Structural studies of the oligosaccharides revealed that the sugar chains of the IgGs obtained from patients with RA are depleted of the beta-galactose residues. The sugar chains of transferrin from patients with RA are fully galactosylated. Therefore the galactose deletion from IgG is probably brought about by a decrease in galactosyltransferase activity in B cells rather than by degradation by galactosidase during circulation. Enzymic study revealed that human B cells contain various beta-galactosyltransferases which form the Gal(beta 1-4)GlcNAc groups in the sugar chains of different glycoproteins. Among these enzymes, abnormality in patients with RA was found only in the one that transfers beta-galactose residues specifically to degalactosylated IgG. This enzyme showed lower affinity toward UDP-Gal in B cells of patients with RA than that in healthy individuals.

Mannose-Binding Lectin: Clinical Implications for Infection, Transplantation, and Autoimmunity

Mannose-binding lectin (MBL) is a recognition molecule of the lectin pathway of complement and a key component of innate immunity. MBL variant alleles have been described in the coding region of the MBL gene, which are associated with low MBL serum concentration and impaired MBL structure and function. Both high and low serum levels of functional MBL have been associated with a variety of diseases and disease complications. Functioning as double-edged sword, low MBL serum levels have been shown to enhance the risk for infections. On the other hand, high MBL serum levels and high MBL activity have been associated with inflammatory diseases, transplant rejection, and diabetic nephropathy. Underscoring the Jekyll-and-Hyde character of MBL, both high and low serum MBL levels are associated with several aspects of autoimmune diseases. This review provides a general outline of the genetic and molecular characteristics of MBL and discusses MBL-disease association and its consequence in infection, transplantation, and autoimmunity.

The potential role of mannan-binding lectin in the clearance of self-components including immune complexes

Mannan-binding lectin (MBL) is a pattern recognition receptor in the innate immune system. It recognizes certain sugar residues arranged in a pattern that enables MBL to bind with sufficient strength. Such sugar patterns are common on the surface of many microorganisms, and MBL has therefore been considered to be an agent that can discriminate between self and nonself. There is, however, increasing evidence supporting that MBL, like many membrane-bound C-type lectin-like receptors, also helps to dispose of various outworn or abnormal body components. Most self-components are protected with sialic acid or galactose that disrupt the pattern of the sugars that MBL can bind, but MBL may be significantly involved in the elimination of self-components that have lost these protective terminal residues. The role of MBL in the clearance of invading pathogens has previously been thoroughly reviewed. Here, we review some findings that support the notion that MBL may contribute to noninflammatory removal of immune complexes and abnormal cells by the reticuloendothelial system. Defects in this clearance mechanism may cause an accumulation of potentially dangerous self-components, thereby increasing the likelihood of chronic inflammation and autoimmunity.

Increased sensitivity of mannanemia detection tests by joint detection of alpha- and beta-linked oligomannosides during experimental and human systemic candidiasis

An enzyme immunoassay (EIA)-the commercially available Platelia Candida antigen test-developed for the diagnosis of systemic candidiasis is based on the detection of alpha-linked oligomannose residues (alpha-Man) released from Candida cells into the serum. This test has good specificity but has to be repeated frequently because of the rapid clearance of detectable mannanemia. We have developed a second EIA based on detection of beta-linked oligomannoses (beta-Man), since beta-Man are linked to different Candida molecules and interact differently with the host immune system and endogenous lectins and should therefore present different kinetics of serum clearance. In a guinea pig model of Candida albicans systemic infection, the relative amounts of detectable alpha- and beta-Man differed considerably according to the virulence of the strain, the infecting dose, and the time after challenge that serum samples were drawn. Detection of alpha-Man was more sensitive per serum sample than that of beta-Man, and the sensitivity for the combination reached 90%. The same tests were applied to 90 sera from 26 patients selected retrospectively for having been infected with the most-pathogenic Candida species: C. albicans (19), C. tropicalis (4), and C. glabrata (3). A total of 22 patients had positive antigenemia, 4 had alpha-mannanemia, 4 had beta-mannanemia, and 14 showed the presence of both. For the patients showing the presence of both forms of mannanemia, the use of both tests enhanced the duration of the detection of mannanemia. Mannanemia was correlated with early clinical symptoms and isolation of Candida in culture, which occurred in 55% of the patients at an average of 4.7 days after the first positive mannanemia test result. A combination of the two tests had a cumulated specificity of 95%, and positive and negative predictive values were 79 and 97%, respectively. These findings provide evidence for different kinetics of beta- and alpha-Man circulation during experimental and human candidiasis and suggest the joint detection of both types of epitopes as a rational approach contributing to increases in the sensitivity and earliness of diagnosis.

Pharmacokinetics and immunologic consequences of exposing macaques to purified homologous butyrylcholinesterase

Exposure to organophosphorus compounds (OPs), in the form of nerve agents and pesticides poses an ever increasing military and civilian threat. In recent years, attention has focused on the use of exogenously administered cholinesterases as an effective prophylactic treatment for protection against OPs. Clearly, a critical prerequisite for any potential bioscavenger is a prolonged circulatory residence time, which is influenced by the size of protein, the microheterogeneity of carbohydrate structures, and the induction (if any) of anti-enzyme antibodies following repeated injections of the enzyme. Previously, it was demonstrated that multiple injections of equine butyrylcholinesterase (BChE) into rabbits, rats, or rhesus monkeys, resulted in a mean residence time spanning several days, and variable immune responses. The present study sought to assess the pharmacokinetics and immunological consequences of administration of purified macaque BChE into macaques of the same species at a dose similar to that required for preventing OP toxicity. An i.v. injection of 7,000 U of homologous enzyme in monkeys demonstrated much longer mean residence times in plasma (MRT = 225 +/- 19 h) compared to those reported for heterologous Hu BChE (33.7 +/- 2.9 h). A smaller second injection of 3,000 U given four weeks later, attained predicted peak plasma levels of enzyme activity, but surprisingly, the MRT in the four macaques showed wide variation and ranged from 54 to 357 h. No antibody response was detected in macaques following either injection of enzyme. These results bode well for the potential use of human BChE as a detoxifying drug in humans.

Glomerular deposition of immune complexes made with IgG2a monoclonal antibodies

The factors that determine whether immune complexes (IC) are cleared safely from the circulation or are deposited in vulnerable tissues such as glomeruli are not well defined. To better understand how IC are handled, the present study examined the fate in vivo of three model IC preparations with different immunochemical characteristics. Radiolabeled IC were constructed with murine IgG1, IgG2a, or IgG3 anti-DNP mAbs bound to DNP-BSA, designated IgG1 IC, IgG2a IC, and IgG3 IC, respectively. The IC were infused i.v. into BALB/c mice, and clearance and tissue localization of the three IC probes were compared. The results indicate that the major portion of each IC preparation was cleared from the circulation by the liver. However, compared with the other two probes, IgG2a IC were preferentially deposited in the kidney. Histologic examination revealed the presence of IgG2a IC in glomeruli. The enhanced renal uptake of IgG2a IC could not be attributed solely to such characteristics as IC size, Ag/Ab ratio, Ab charge, or affinity. However, the preferential renal deposition of IgG2a IC was abrogated by complement depletion. Thus, enhanced renal uptake in normal mice was complement dependent. These data suggest that interactions between IC and the complement system can influence the propensity of IC to deposit in tissues susceptible to IC-mediated injury.

Role of oligosaccharides in the pharmacokinetics of tissue-derived and genetically engineered cholinesterases

To understand the role of glycosylation in the circulation of cholinesterases, we compared the mean residence time of five tissue-derived and two recombinant cholinesterases (injected intravenously in mice) with their oligosaccharide profiles. Monosaccharide composition analysis revealed differences in the total carbohydrate, galactose, and sialic acid contents. The molar ratio of sialic acid to galactose residues on tetrameric human serum butyrylcholinesterase, recombinant human butyrylcholinesterase, and recombinant mouse acetylcholinesterase was found to be approximately 1.0. For Torpedo californica acetylcholinesterase, monomeric and tetrameric fetal bovine serum acetylcholinesterase, and equine serum butyrylcholinesterase, this ratio was approximately 0.5. However, the circulatory stability of cholinesterases could not be correlated with the sialic acid-to-galactose ratio. Fractionation of the total pool of oligosaccharides obtained after neuraminidase digestion revealed one major oligosaccharide for human serum butyrylcholinesterase and three or four major oligosaccharides in other cholinesterases. The glycans of tetrameric forms of plasma cholinesterases (human serum butyrylcholinesterase, fetal bovine serum acetylcholinesterase, and equine serum butyrylcholinesterase) clearly demonstrated a reduced heterogeneity and higher maturity compared with glycans of monomeric fetal bovine serum acetylcholinesterase, dimeric tissue-derived T. californica acetylcholinesterase, and recombinant cholinesterases. T. californica acetylcholinesterase, recombinant cholinesterases, and monomeric fetal bovine serum acetylcholinesterase showed a distinctive shorter mean residence time (44-304 min) compared with tetrameric forms of plasma cholinesterases (1902-3206 min). Differences in the pharmacokinetic parameters of cholinesterases seem to be due to the combined effect of the molecular weight and charge- and size-based heterogeneity in glycans.

Carbohydrate-mediated recognition of a circulating placental alkaline phosphatase-immunoglobulin M complex

We detected an abnormal alkaline phosphatase (AP) electrophoretically in the serum of a patient with rheumatoid arthritis, who had a macromolecular AP linked with immunoglobulin M (IgM) bearing a kappa light chain. The IgM isolated from the AP-IgM complex in the patient's serum reacted apparently with all of the AP isozymes tested, i.e. those originating in the liver, bone, intestine and placenta, but the alpha-mannosidase-treated IgM from the patient's serum bound to placental AP (PAP) alone. This suggests that untreated IgM recognizes multivalent epitopes of the AP and that the complex of AP with alpha-mannosidase-treated IgM is a specific antibody-antigen complex. In order to investigate further the multivalent binding capacity for the PAP-untreated IgM complex, we prepared a monoclonal antibody (MoAb) against PAP and identified it as an IgM with a kappa light chain. The binding affinities and their circulating half-lives of the synthetic complexes of PAP and respective MoAbs were examined with and without treatment with several glycosidases. The untreated MoAb bearing IgM had binding affinity for all of the AP isozymes tested, while alpha-mannosidase-treated IgM attached only to PAP, the same as the IgM isolated from the PAP-IgM complex in the patient's serum. The circulating clearance of the PAP-IgM complex in rabbits was faster than either component alone. In addition, the PAP-IgM complex treated with alpha-mannosidase was found to have the shortest half-life of all the complexes of PAP and Igs treated with the several glycosidases tested. These results suggest that the formation of the PAP-IgM complex as an enzyme-linked antibody and the clearance of the complex in vivo are dependent on the sugar moieties of the Igs.

Effect of culture conditions on IgM antibody structure, pharmacokinetics and activity

Culture conditions affect the binding activity, charge heterogeneity, conformational stability, glycosylation, and pharmacokinetics of human monoclonal IgM HMAB-10058. The 10058 human/human/murine trioma was grown in serum-free airlift suspension culture, hollow fiber perfusion culture, or in nude mouse ascites. The ascites-produced antibody showed reduced conformational stability, greater charge and glycoform heterogeneity, and a lower average degree of sialylation than the in vitro culture-produced material. Mean residence time after IV injection in rats was approximately 80-fold greater for the ascites culture-produced material, but specific binding activity was less than 5% of that for the airlift-produced material. In vitro culture in serum-supplemented media (in a hollow fiber perfusion reactor or in shake-flasks) resulted in antibody with pharmacokinetics intermediate between the serum-free airlift and ascites-produced materials. Incubation of airlift-produced antibody in ascites fluid also resulted in material with intermediate pharmacokinetics. Conclusions regarding the effect of culture conditions on antibody product cannot be generalized, as in vitro-produced antibody derived from two related cell lines (HMAB-10233 and HMAB-10390) had long mean residence times similar to that of ascites-produced HMAB-10058.

Second Jenner international glycoimmunology meeting

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Galactose-containing epitopes on the surface of IgG model immune complexes are accessible for specific binding with the high molecular weight ligand, Ricinus agglutinin, in solution-light-scattering studies

Immunoglobulin G (IgG) molecules contain covalently linked carbohydrate chains with galactose residues in their branched "antennae". The ability of galactose-containing epitopes on the surface of IgG model immune complexes (IC) to interact with a high mol. wt ligand in solution has been elucidated. Different types of IgG model IC with pre-determined molecular mass were mixed with Ricinus Agglutinin (RCI), which is known to bind specifically to galactose-containing oligosaccharides. The relative light-scattering increases (delta I) in the reaction mixture were measured as a function of time. The galactose-associated epitopes of the IgG model IC were accessible for binding with RC1. The rate of the interaction between IgG model IC and RC1 was dependent on the molecular mass of the complexes; the larger the model IC molecular mass, the faster the rate of interaction. The binding of RC1 to IgG model IC was highly specific because it was completely abolished in the presence of lactose. The galactose-containing epitopes of monomeric IgG were also able to interact with RC1 but the kinetics of the interaction was much slower. We suggest than an increase in the density of the epitopes on the surface of the model IC, by close attachment of the IgG molecules, mainly determines the ability of galactose-containing epitopes to be recognized by RC1. The data presented support the importance of IgG glycans in recognition events of IgG by biologically active molecules.

Carbohydrates as recognition molecules in infection and immunity

Consideration of host-parasite interactions encompasses a wide range of phenomena from adhesion to epithelial surfaces to interactions with cells of the immune system. This review focuses on the role of carbohydrates as recognition molecules in these complex interactions. The abundant glycoproteins and glycolipids of cell surfaces of both prokaryotic and eukaryotic cells have the ability to exist in a variety of spatial configurations through alpha- and beta-linkages and the formation of branched structures. This ability carries with it the opportunity of acting as informational molecules greater than that possible for proteins or nucleic acids. The blood group substances are probably the best characterized of these carbohydrate containing molecules. Whilst at present a detailed understanding of the importance of these molecules in host-parasite interaction is lacking, the material covered in this discussion emphasizes the way in which carbohydrate based recognition has been shown to be involved and may provide the basis for further understanding.

Immune complexes in serum of rats during infection with Plasmodium berghei

Large amounts of immune complexes were present in the serum of infected rats early in infection when parasitemias were low. As the infection progressed and parasitemia increased and then decreased, the amounts of immune complexes in the serum also fell. This result suggests that increased efficiency of complex clearance was an important factor in determining the levels of immune complexes in the serum. In high performance liquid chromatography (HPLC), the complexes in the serum migrated as a peak with material of 350 kDa and greater in mass. They sedimented in a sucrose gradient as a band with a sedimentation coefficient of 22 s, which was calculated to yield a mass of approximately 1100 kDa. Immunoelectrophoresis and radial immunodiffusion showed that IgG was the major immunoglobulin in the complexes. As the IgG content of the complexes increased, the levels of complexes in the serum generally decreased. HPLC analysis of precipitated complexes suggested that they contained loosely bound albumin. Serum proteins were affected by the infection. A depletion of free immunoglobulin was observed during the initial period of immune complex formation.

One-step purification of murine IgM and human alpha 2-macroglobulin by affinity chromatography on immobilized snowdrop bulb lectin

A new mannose-specific plant lectin (GNA) isolated from the snowdrop bulb was immobilized on Sepharose 4B and employed for the purification of certain glycoproteins with high-mannose type glycan chains. Murine IgM bound tightly to this column and was eluted with 0.1 M methyl alpha-D-mannoside whereas bovine and murine IgG were not bound. When a murine hybridoma serum containing IgM monoclonal antibody was applied to this column, highly purified IgM antibody was obtained after elution with methyl alpha-D-mannoside. On the contrary, human IgM was not bound by this column despite reports that it contains high-mannose type glycan chains. alpha 2-Macroglobulin was the sole glycoprotein present in human serum which was bound by the immobilized snowdrop lectin column. It appears that only glycoproteins containing multiple Man(alpha 1,3)Man units are bound to the immobilized lectin.

Kupffer cells and their function

The intention of this review is to stress new information regarding the quite versatile functions of Kupffer cells. Although their main function is phagocytosis and defence of the liver against bacteria, endotoxaemia and viral infections, they also fulfil other important roles. They will phagocytose and partially degrade bacterial antigens before handing them on to the hepatocytes for excretion into the bile. They handle LDL lipoproteins, whilst the HDL proceed directly into the hepatocytes. They produce lymphokine mediators that direct protein synthesis by the hepatocytes. Also they normally produce prostaglandins that are cyto-protective for the hepatocytes. Conversely, if they are required to attack infected hepatocytes or cancer cells, then they switch to the production of leukotrienes. Thus they function as specialised macrophages, and it is not surprising that other "activated macrophages" have to be recruited into the liver to support them in inflammatory reactions.

Current ideas on the significance of protein glycosylation

Carbohydrate has been removed from a number of glycoproteins without major effect on the structure or enzyme activity of the protein. Thus carbohydrate has been suggested to underly a non-primary function for proteins, such as in relatively non-specific interactions with other carbohydrates or macromolecules, stabilization of protein conformation, or protection from proteolysis. This non-specific concept is consistent with both the general similarity in carbohydrate structure on very diverse glycoproteins and the frequent structural microheterogeneity of carbohydrate chains at given sites. The concept is supported in a general sense by the viability of cells whose glycosylation processes have been globally disrupted by mutation or pharmacological inhibitors. In contrast to the above observations, other studies have revealed the existence of specific, selective receptors for discrete oligosaccharide structures on glycoproteins which seem to be important for compartmentalization of the glycoprotein, or the positioning of cells on which the glycoprotein is concentrated. Sometimes multivalency in the carbohydrate-receptor interaction is crucial. There are additional possible roles for carbohydrate in the transduction of information upon binding to a receptor. The possibility of specific roles for carbohydrate is supported by the existence of numerous unique carbohydrate structures, many of which have been detected as glycoantigens by monoclonal antibodies, with unique distributions in developing and differentiated cells. This article attempts to summarize and rationalize the contradictory results. It appears that in general carbohydrate does in fact underlie only roles secondary to a protein's primary function. These secondary roles are simple non-specific ones of protection and stabilization, but often also satisfy the more sophisticated needs of spatial position control and compartmentalization in multicellular eukaryotic organisms. It is suggested that there are advantages, evolutionarily speaking, for the shared use of carbohydrate for non-specific roles and for specific roles primarily as luxury functions to be executed during the processes of cell differentiation and morphogenesis.

T cell activation by anti-T3 antibodies: comparison of IgG1 and IgG2b switch variants and direct evidence for accessory function of macrophage Fc receptors

The human T3 antigen is closely associated with the T cell receptor. Some anti-T3 antibodies cause T cell proliferation in the presence of monocytes which have Fc receptors (FcR) that bind particular antibody subclasses. Such an interaction is thought to determine whether or not an anti-T3 antibody is mitogenic. We examined the mitogenicity of an IgG1 antibody, UCHT1, and an IgG2b switch variant of identical specificity, UCHT1B. With autologous monocytes, 76% of individuals responded to UCHT1 and 9% to UCHT1B, falling into three patterns of responsiveness. Both antibodies in the absence of monocytes induced responsiveness to recombinant interleukin 2, even for UCHT1B nonresponder T cells. The proliferation induced by UCHT1B, however, was always less than that induced by UCHT1. These findings demonstrate the critical role played by the Fc region for mitogenesis, and suggest a possible role for the hinge region. We then obtained direct evidence that mitogenicity can be mediated exclusively via FcR. Mouse macrophages have distinct FcR: FcRI binds IgG2a but FcRII binds IgG1 and IgG2b and its function can be inhibited by the specific antibody 2.4G2. Because UCHT1 and UCHT1B were of the correct subclass to interact with FcRII we examined the accessory function of mouse peritoneal macrophages. Without exception, human T cells now responded to both antibodies. Proliferation was drastically inhibited by 2.4G2 but not by an irrelevant anti-macrophage antibody, F4/80, nor by an anti-human neutrophil FcR antibody, 3G8. Furthermore, 2.4G2 did not inhibit the accessory function of mouse macrophages for OKT3, an IgG2a antibody that presumably interacts with FcRI, and did not inhibit the function of human monocytes for UCHT1 and UCHT1B. Mouse B cells, in contrast to macrophages, have an FcR which binds all three subclasses, but which can be inhibited by 2.4G2. B cells, however, were not accessory cells for mitogenesis with UCHT1, UCHT1B or OKT3. These findings are discussed in relation to other requirements for T cell activation by anti-T3 antibodies.

Mannose-specific binding sites in synoviocytes of rat synovial membrane: an ultrastructural-cytochemical study

Mannose-specific binding sites in the two types of synoviocytes (A and S) from the synovial membrane of normal rat knee joints were studied by an ultrastructural-cytochemical method using horseradish peroxidase (HRP) as a ligand. The A cells showed mannose-specific binding sites for HRP which were localized in coated pits and coated vesicles (110 nm) as well as in smooth-walled vesicles, vacuoles, and tubules. No mannose-specific binding sites for HRP were detected in the S cells. Together with previous observations on absorption of intraarticularly injected HRP in type A and type S synoviocytes, the present results strongly support the interpretation that the A cells internalize HRP by receptor-mediated endocytosis and that the S cells internalize HRP by fluid-phase endocytosis only. The present results agree with the suggestion that mannose-specific binding sites consist of two pools, the one concerned with internalization of ligand and the other with intracellular targeting of internalized ligand.

Cytochemical characterization and localization of oligomannosidic oligosaccharide receptors on the normal human spermatozoa using fluorescent lectins: comparison of different fixation procedures

Similar locations of the Con A and wheat germ lectin receptors were obtained by using fluorescent lectins, in nonfixed spermatozoa and in spermatozoa fixed with formaldehyde and methanol, showing that in samples with the same previous treatment, worked out by the same operator, in which enough determinations have been performed to eliminate individual variations, the different procedures of fixation produced similar results. The locatizations obtained with fluorescent lectins confirm previous results, produced with the peroxidase technique, indicating that the lectins interact with oligomannosidic oligosaccharide receptors situated mainly in the equatorial segment of the acrosome and postnuclear cap. They also indicate the presence of similar receptors that were not detected previously on the neck and intermediate segment. The larger size of the lectin-peroxidase-diaminebenzidine reagent compared to that of the fluorescent lectins suggests that the new receptors are semicriptic and were not detected by steric effects in the first case, but were able to interact with lower volume, fluorescent probes. It is suggested that these oligomannosidic chains could be recognition signals for the elimination of incompetent sperm during their passage through the female reproductive track. Also these oligosaccharides and its possible metabolic variations could be involved in the interaction between the acrosome-reacted spermatozoa with the zone pellucida.

Modification of the carbohydrate in ricin with metaperiodate and cyanoborohydride mixtures: effect on binding, uptake and toxicity to parenchymal and non-parenchymal cells of rat liver

The carbohydrate in the toxic glycoprotein ricin was chemically modified by simultaneous treatment with sodium metaperiodate and sodium cyanoborohydride. This treatment causes oxidative cleavage of the sugar residues and reduction of the aldehyde groups which are formed to primary alcohols. The modification markedly decreased the rapid removal of ricin from the blood by hepatic non-parenchymal cells with only a relatively small increase in accumulation of the toxin by parenchymal cells. Binding, uptake and toxicity of the modified ricin in primary monolayer cultures of hepatic non-parenchymal cells were all decreased to a much greater extent than in parenchymal cells. The results indicate that native ricin binds to non-parenchymal cells by a dual recognition process which involves both interaction of cell receptors with the mannose-containing oligosaccharides of the toxin and binding of ricin to galactose-containing glycoproteins and glycolipids on the cells. However, uptake and toxicity of native ricin in non-parenchymal cells appears to result principally from entry of the toxin through the mannose recognition pathway. By contrast, uptake and toxicity of the expressed essentially through the galactose-recognition route.

Lack of evidence for liver lectins functioning as IgM or IgG-Fc-receptors

We have tested whether mannose- and galactose-specific lectins on liver cells are able to bind antibody-antigen complexes and thus function as Fc-receptors. Rat hepatocytes and liver sinusoidal cells were isolated by collagenase perfusion and differential centrifugation. Rat erythrocytes were coated with purified IgM or IgG from rabbits immunized with rat erythrocytes. Both IgM and IgG coated erythrocytes bound to liver macrophages but not to hepatocytes. The binding of IgM and IgG coated red blood cells to liver macrophages could not be blocked by potent inhibitors for mannose- and galactose-specific macrophage lectins such as mannan, D-mannose-bovine serum albumin, N-acetyl-D-galactosamine, D-galactose-bovine serum albumin, or asialofetuin. Although lectin activity is calcium dependent and trypsin sensitive neither condition blocked rosette formation between liver macrophages and opsonized erythrocytes. Thus mannose- and galactose-specific lectins are not involved in the sequestration of IgM- or IgG-antibody-erythrocyte complexes in the liver.

Carbohydrate structure in tumor immunity

Researchers have endeavored to define surface alterations associated with neoplasia for at least 25 years. In comparisons of normal tissues with animal and human tumors, cultured cells before and after transformation with oncogenic agents, tumorigenic and nontumorigenic transformed cells, metastatic and nonmetastatic tumor cells, high- and low-metastatic variants, and tumor cells before and after induction of differentiation to a less malignant phenotype, a consistent finding has been some form of alteration in surface carbohydrate structures. These changes in glycolipids, glycoproteins and glycosaminoglycans are reviewed, and their structures are illustrated. Both nucleotide sugar biosynthesis and glycosyltransferase changes have been associated with these alterations. In some cases, alterations in transformed cells were related to growth, rather than transformation. In others, the altered glycoconjugates are truly tumor-associated. There is evidence that cell surface glycoconjugates may function in growth control. Altered carbohydrate structures could also serve as receptors for growth promoting factors and be directly responsible for altered growth control. Recent studies with monoclonal antibodies indicate that the vast majority of antibodies recognizing tumor-associated antigens are detecting altered carbohydrate structures. Mechanisms by which the immune system can recognize these carbohydrate structures are considered, and immune recognition of tumor-associated carbohydrate structural alterations is explored. A number of these hypotheses relating to alterations in glycosylation, growth control, and tumor immunity deserve further investigation.

Purification of hamster melanoma tyrosinases and structural studies of their asparagine-linked sugar chains

In cultured melanotic melanoma, a marked decrease of pigmentation has been found to be induced by the addition of tunicamycin [Y. Mishima and G. Imokawa (1983) J. Invest. Dermatol. 81, 106-114]. Since it appears that this impaired pigmentation arises from the loss of asparagine-linked sugar chains serving as a signal for transport of tyrosinase from GERL (Golgi-associated endoplasmic reticulum of lysosomes) to premelanosomes, tyrosinases from the membrane fraction of Greene's hamster melanoma have been purified, and the structures of their sugar chains have been analyzed. Two kinds of tyrosinases were purified by Triton X-100 solubilization; DEAE-cellulose, Sephadex G-200, and DEAE-Sephadex column chromatography; and preparative polyacrylamide gel electrophoresis. The two tyrosinases were separated by polyacrylamide gel electrophoresis, and both corresponded to Mr 69,000. Their asparagine-linked sugar chains were released by hydrazinolysis and analyzed. The sugar chains of the two tyrosinases were identical except for the sialic acid contents. One mole of each tyrosinase contained 1 mol of high-mannose-type sugar chains and 3 mol of complex-type sugar chains. The former chain has Man3 approximately 5 X GlcNAc2 and the latter has Man3 X GlcNAc beta 1----4(+/- Fuc alpha 1----6)GlcNAc as their core structures. The complex-type sugar chains are composed of mono-, bi-, tri-, and tetraantennary sugar chains, with +/- Sia alpha 2----3Gal beta 1----4GlcNAc beta 1----as their outer chains.

Carcinoembryonic antigen from human malignant melanoma cells. I. Production and shedding characteristics

Two major findings are reported in the present studies: (a) Long-term cultivation, followed by cloning, of a human malignant melanoma HMMC-ShA cell line gave melanotic and amelanotic cell variants. During in vitro proliferation, the melanotic melanoma (HMMC-ShAE+) cells released carcinoembryonic antigen (CEA) and an inhibitor of phytohaemagglutinin-induced lymphocyte stimulation. Gel filtration patterns of CEA on Sephadex-G200 varied from one culture condition to another. Glycosylation-deficient CEA obtained from cells harvested from media supplemented with non-toxic levels of tunicamycin showed lower molecular weight and delayed filtration through Sephadex-G200. (b) Human melanotic melanoma (HMMC-ShAE+) differed from amelanotic melanoma (HMMC-ShA-) and glycosylation-deficient cells in the amount of CEA and in the suppression of lymphocyte response activity which they shed into the medium and as well as in oncogenicity in athymic nu/nu BALB/c mice.

Endocytosis by the mononuclear phagocyte system and autoimmune disease

The mononuclear phagocyte system, formerly called the reticuloendothelial system, is an important element in basic immunology, cell biology, and clinical disease. Secretory products participate in inflammation and immunoregulation. Endocytosis mediated by specific receptors for immunoglobulin and complement or by other opsonins is important in removal of damaged self or foreign particles. The ability to assess receptor-specific endocytosis has led to the recognition of Fc-receptor dysfunction in certain autoimmune diseases. This defect in membrane receptor function, whether inherited or acquired, may be important in the pathogenesis of these diseases.

The clearance of human fibrinogen fragments D1, D2, D3 and fibrin fragment D1 dimer in mice

The clearance of human fibrinogen fragments D1, D2, D3 and fibrin fragment D1 dimer were studied in the mouse model. Clearance of these fragments is a complex process involving clearance from blood into three other compartments. The overall clearance of fragment D1 and its dimer were essentially identical. Fragments D2 and D3 cleared at a progressively slower rate. Competition studies were performed between 125I-labeled fragment D1 and large molar excesses of unlabeled human fragments D1, D2, D3, D1 dimer, fragment E, fibrinogen, macroalbumin, mannan and asialoorosomucoid. Of these ligands only the fragment D variants competed for the clearance of 125I-labeled fragment D1. Cross-competition was observed when 125I-labeled fragment D1 dimer was cleared in the presence of a large molar excesses of fragment D1. Autopsies demonstrated that injected fragments D1, D2, D3 and D1 dimer cleared primarily in liver and kidneys. In some clearance studies, livers were perfused with tissue culture fluid, subjected to light microscopic autoradiography, and silver grain counts performed to localize cleared fragment D1. These experiments indicated that 80% of the liver uptake was in hepatocytes. However, when silver grain counts were normalized for the number of parenchymal and nonparenchymal cells, the distribution of silver grains was essentially identical (1.8 and 1.6 grains per cells, respectively). It is concluded that fragments D1, D2, D3 and D1 dimer are recognized by a similar clearance pathway. Since neither fibrinogen nor fragment E competed for the clearance of fragment D1, it is suggested that determinants present in the fragment D domain become exposed after plasmin attack on fibrinogen and are responsible for clearance.

The role of the liver in clearance of glycoproteins from the general circulation, with special reference to intestinal alkaline phosphatase

Glycoproteins represent a wide variety of macromolecules with important physiological functions. Characteristic variations in carbohydrate composition and plasma concentration of these proteins may occur during pathological conditions. Steady-state plasma concentrations are determined by release from normal or diseased tissues and simultaneous clearance from the general circulation. The liver occupies a central position in the production but also clearance and catabolism of such glycoproteins. A number of specialized receptor-mediated transport processes for different types of glycoproteins in this organ is reviewed. Membrane recognition is generally followed by absorptive endocytosis and vesicle transport to lysosomes, Golgi system and/or bile canaliculis. The charge of the protein, the nature of the terminal sugar residue or complex formation with other glycoproteins may determine the extent of uptake in the various cell types of the liver. By means of these transport processes the liver is able to remove potentially dangerous macromolecules such as denatured proteins, aggressive enzymes and immunocomplexes from the general circulation. Drugs can bind to some of these proteins or may interact with the hepatic transport or catabolism processes. Special attention is paid to the hepatic clearance of asialoglycoproteins with terminal galactose groups. Intestinal alkaline phosphatase is used as a model compound to characterize the pharmacokinetic profiles of hepatic uptake and biliary excretion in the rat in vivo and isolated perfused rat livers. Histochemical and electron-microscopic studies demonstrated a galactose-specific, receptor-mediated endocytotic process, mainly but not exclusively localized in centrolobular hepatocytes. Drug interactions with these processes will be the subject of further investigations.

Epstein-Barr virus-lymphoid cell interactions. III. Effect of concanavalin A and saccharides on Epstein-Barr virus penetration

To study some aspects of Epstein-Barr virus (EBV) penetration into target cells, the effect of concanavalin A (ConA) and various saccharides on virus infectivity and cell susceptibility to EBV infection was examined. ConA treatment of the target cells, EBV, or EBV-cell complexes was found to inhibit virus antigen expression. Several control experiments with alpha-d-methyl-mannoside elution of ConA, removal of nonfused EBV particles from the cell surface by trypsin treatment, and addition of ConA at different times postinfection were performed to define the site of ConA action on EBV infection. ConA appeared to have a dual action: (i) it inhibited EBV binding to virus receptors, and (ii) it blocked the penetration of receptor-bound virus into target cells at a trypsin-sensitive stage, thus indicating that ConA prevented the fusion of viral envelope with the target cell membrane. A high sucrose concentration (0.25 M), known to inhibit cell membrane movements, was also found to block EBV penetration at a trypsinsensitive stage, thus suggesting the implication of cell membrane movements and underlying activities (or both) in viral envelope fusion. Lower concentrations of various monosaccharides (0.12 M) did not influence EBV infection. Under conditions of ConA treatment that did not influence EBV infectivity and target cells susceptibility, ConA was able to mediate virus binding to EBV receptornegative cell lines, but no virus antigens were expressed in these cells. These observations reinforced the idea that the mere attachment of EBV to lymphoid cells is not sufficient to lead to infection. In light of the present and previously published data, we postulate the existence of a specific cellular mechanism that allows the penetration of EBV into the target (B) lymphocyte.

Clearance and killing of Candida albicans in the perfused mouse liver

Hepatic interactions of C. albicans with perfused mouse livers were characterized and compared in normal and glucan-treated mice. Normal livers, in the absence of serum, trapped greater than 90% and killed greater than 20% of the infused yeast. Phenylbutazone had no effect. Silica treatment abolished killing and decreased trapping suggesting that candidicidal activity of the liver is mediated by Kupffer cells. Immune serum, but not normal serum, enhanced trapping and killing in normal livers. Liver hypertrophy was evident in mice treated with glucan, but no enhanced candidicidal activity was observed in the absence of humoral factors. Specific immune serum and normal serum increased killing of C. albicans in glucan stimulated liver, suggesting a requirement for serum opsonin in facilitating glucan enhanced killing. Specific immune serum potentiated the greatest increase in killing. Glucan treatment in conjunction with immune serum increased killing to approximately 40%. D-mannose, but not D-glucose or D-mannitol impaired trapping of the yeast in livers of normal mice. Together, the data suggest that hepatic trapping of C. albicans involves phagocytic events as well as interactions of the yeast with surface receptors on sinusoidal cells and support the role for the liver in restricting hematogenous dissemination of C. albicans in the infected host.

A comparison of the accumulation of ricin by hepatic parenchymal and non-parenchymal cells and its inhibition of protein synthesis

Rat liver non-parenchymal cells in vivo were found to accumulate 125I-labelled ricin to a much greater extent than parenchymal cells. Similarly, in monolayer cell cultures, the rate of ricin uptake by non-parenchymal Kupffer cells was several times that by parenchymal cells. Evidence is provided also to suggest that ricin is primarily recognized by Kupffer cells via terminal mannose residues in the toxin, whereas ricin uptake by parenchymal cells was consistent with a role of the previously postulated galactosyl-containing cell receptors. Protein synthesis in Kupffer cells in vitro, although observed to occur at a lower rate than in parenchymal cells, was 100--1000-times more sensitive to inhibition by ricin. The selective damage known to be caused to liver sinusoids by ricin, therefore, may reflect both the relative efficiency with which the toxin is taken up by these cells and the extreme sensitivity of protein synthesis in the cells to inhibition by ricin.

Evidence for in vivo hepatic uptake of a galactose-terminated glycoprotein in fish (Salmo alpinus L.)

125I-labelled asialofetuin injected intravenously into chars (Salmo alpinus L.) was cleared from the blood with a half-time of approx. 60 min. The injected asialofetuin was found to be accumulating in the liver when different organs were dissected out at various time points after the injection. 125I-labelled fetuin was not concentrated in the liver, and the uptake of 125I-labelled asialofetuin was almost completely inhibited by simultaneously injecting an excess unlabelled asialofetuin, indicating a specific uptake mechanism for the galactose-terminated glycoprotein in char liver. Denatured human serum albumin was not taken up by the liver, but was recovered in the kidneys, suggesting that the liver in char is devoid of macrophages. It therefore seems reasonable to assume that the injected 125I-labelled asialofetuin is taken up by the parenchymal cells of the char liver.