The murine lymphocyte receptor for IgE. II. Characterization of the multivalent nature of the B lymphocyte receptor for IgE

Biotinylation of antibodies

Using the characteristic of a high-affinity complex between avidin and biotin, biotinylated antibodies have wide applications in various immunochemical assays, especially where signal amplification is required. A method is described here for the biotinylation of immunoglobulins. The procedure utilizes water-soluble succinimidyl ester of biotin that reacts with primary amines of the lysine residues or the amino terminus on the antibody to form amide bonds. The method is simple and specific and results in stable conjugates retaining full immunologic activity.

Fibroblast growth factor receptors 1 and 2 interact differently with heparin/heparan sulfate. Implications for dynamic assembly of a ternary signaling complex

Heparan sulfate (HS) regulates the kinetics of fibroblast growth factor 2 (FGF2)-stimulated intracellular signaling and differentially activates cell proliferation of cells expressing different FGF receptors (FGFRs). Evidence suggests that HS interacts with both FGFs and FGFRs to form active ternary signaling complexes. Here we compare the interactions of two FGFRs with HS. We show that the ectodomains of FGFR1 IIIc and FGFR2 IIIc exhibit specific interactions with different characteristics for both heparin and porcine mucosal HS. These glycans are both known to activate FGF signaling via these receptors. FGFR2 interacts with a higher apparent affinity than FGFR1 despite both involving 6-O-, 2-O-, and N-sulfates. FGFR1 and FGFR2 bind heparin with mean association rate constants of 1.9 x 10(5) and 2.1 x 10(6) m(-1)s(-1), respectively, and dissociation rate constants of 1.2 x 10(-2) and 2.7 x 10(-2) s(-1), respectively. These produced calculated affinities of 63 and 13 nm, respectively. Hence, FGFR1 and FGFR2 bind to heparin chains with markedly different kinetics and affinities. We propose a mechanistic model where the kinetic parameters of the HS/FGFR interaction are a key element regulating the formation of ternary complexes and the resulting FGF signaling outcomes.

Identification of the heparan sulfate binding sites in the cellular prion protein

Data from cell culture and animal models of prion disease support the separate involvement of both heparan sulfate proteoglycans and copper (II) ions in prion (PrP) metabolism. Though direct interactions between prion protein and heparin have been recorded, little is known of the structural features implicit in this interaction or of the involvement of copper (II) ions. Using biosensor and enzyme-linked immunosorbent assay methodology we report direct heparin and heparan sulfate-binding activity in recombinant cellular prion protein (PrP(c)). We also demonstrate that the interaction of recombinant PrP(c) with heparin is weakened in the presence of Cu(II) ions and is particularly sensitive to competition with dextran sulfate. Competitive inhibition experiments with chemically modified heparins also indicate that 2-O-sulfate groups (but not 6-O-sulfate groups) are essential for heparin recognition. We have also identified three regions of the prion protein capable of independent binding to heparin and heparan sulfate: residues 23-52, 53-93, and 110-128. Interestingly, the interaction of an octapeptide-spanning peptide motif amino acids 53-93 with heparin is enhanced by Cu(II) ions. Significantly, a peptide of this sequence is able to inhibit the binding of full-length prion molecule to heparin, suggesting a direct role in heparin recognition within the intact protein. The collective data suggest a complex interaction between prion protein and heparin/heparan sulfate and has implications for the cellular and pathological functions of prion proteins.

Do human bile salt stimulated lipase and colipase-dependent pancreatic lipase share a common heparin-containing receptor?

Bile salt stimulated lipase (BSSL), a lipolytic enzyme secreted with pancreatic juice and with human milk, is in concert with colipase-dependent pancreatic lipase, important for the intestinal digestion of dietary lipids. BSSL may also facilitate uptake of free cholesterol from the intestinal lumen, while colipase-dependent lipase has a similar role for fatty acids. According to this theory, the two lipases bind to the intestinal mucosa via a common heparin-involving receptor. In the present study, binding of the two lipases to heparin was explored in vitro using purified human lipases and heparin molecules varying in both chain length and charge density. Native, but not denatured, BSSL bound avidly to heparin and several of the heparin variants. In contrast, at physiologic salt concentration, colipase-dependent lipase did not bind to heparin. Thus, our data do not support the view that the two lipases share a common intestinal heparin-like receptor. Hence, it seems unlikely that such binding could be of physiologic relevance for colipase-dependent lipase, although for BSSL the data are supportive.

Production of a Chimeric Form of CD23 That Is Oligomeric and Blocks IgE Binding to the FcεRI

The low affinity receptor for IgE (FcεRII/CD23) has previously been shown to interact with IgE with a dual affinity. Three chimeric constructs were created containing the lectin domain (amino acids 172–188) or the “neck” and lectin domain (amino acids 157–188) attached to subunits of oligomeric proteins. All chimeras were incapable of interacting with IgE with either a high or low affinity, indicating that the α-helical stalk of CD23 is important for orienting the lectin heads such that an interaction with IgE can occur. This concept received further support in that a chimeric CD23 composed of the human CD23 stalk and the mouse CD23 lectin head bound mouse IgE with a dual affinity, but could only bind rat IgE with a low affinity. Effort was next concentrated on a construct consisting of the entire extracellular (EC) region of CD23. A mutation to the first cleavage site of CD23 (C1M) resulted in a more stable molecule as determined by a decrease of soluble CD23 release. A soluble chimeric EC-C1M was prepared by attaching an isoleucine zipper to the amino terminus (lzEC-C1M). The interaction with IgE by lzEC-C1M was found to be superior to that seen with EC-CD23. The lzEC-C1M could inhibit binding of IgE to both CD23 and the high affinity receptor for IgE, FcεRI, providing further evidence for a strong interaction with IgE. FcεRI inhibition (∼70%) was seen at equimolar concentrations of lzEC-C1M, implying the effectiveness of this chimera and suggesting its potential therapeutic value.

Interaction of lipoproteins with heparan sulfate proteoglycans and with lipoprotein lipase. Studies by surface plasmon resonance technique

Interaction of different classes of lipoproteins with heparan sulfate, heparin, and lipoprotein lipase was studied by a surface plasmon resonance based technique on a BIAcore. The proteoglycans were covalently attached to sensor chips as previously described [Lookene, A., Chevreuil, O., Ostergaard, P., & Olivecrona, G. (1996) Biochemistry 35, 12155-12163]. Binding of all lipoproteins, except for beta-VLDL, to endothelial heparan sulfate was low. Binding of chylomicrons (from rat lymph) and of human VLDL was much increased by the presence of lipoprotein lipase. With human LDL, binding was low in the absence of lipase or at low lipase concentrations. For efficient binding, 2-4 lipase dimers per LDL particle were necessary, indicating cooperativity in the interaction. In contrast, HDL did not bind under any conditions. Heparin had higher binding capacity for lipoproteins than heparan sulfate. This was due to a higher number of binding sites on the heparin chains. Binding of LDL, VLDL, and chylomicrons to heparan sulfate-covered surfaces, both in the presence and in the absence of lipoprotein lipase, was characterized by high values for association rate constants (10(4)-10(5) M(-1) s(-1)) and low values for dissociation rate constants (10(-4)-10(-5) M(-1) s(-1)). In some experiments, rabbit beta-VLDL were directly immobilized to the sensor chips. Binding of lipoprotein lipase to these surfaces was characterized by a very high association rate constant (10(6) M(-1) s(-1)). The dissociation of triacylglycerol-rich lipoproteins was more rapid with catalytically active lipase than with active site-inhibited lipase. It was also markedly increased in the presence of free heparin, suggesting fast exchange kinetics at the surface. Based on that, we propose that lipoproteins are relatively mobile at heparan sulfate covered surfaces. Our study emphasizes the important role of lipoprotein lipase, or molecules with similar properties (apolipoprotein E, hepatic lipase), as mediators for binding of lipoproteins to proteoglycans. It also demonstrates the great potential for the use of biosensors for studies of lipoprotein interactions.

Interaction of lipoprotein lipase with heparin fragments and with heparan sulfate: stoichiometry, stabilization, and kinetics

The interaction of lipoprotein lipase (LPL) with heparan sulfate and with size-fractionated fragments of heparin was characterized by several approaches (stabilization, sedimentation, surface plasmon resonance, circular dichroism, fluorescence). The results show that heparin decasaccharides form a 1:1 complex with dimeric LPL and that decasaccharides are the shortest heparin fragments which can completely satisfy the heparin binding regions in dimeric LPL. Equimolar concentrations of octasaccharides also stabilized dimeric LPL, while shorter fragments (hexa- and tetrasaccharides) were less efficient. Binding of heparin did not induce major rearrangements in the conformation of LPL, supporting the view that the heparin binding region is preformed in the native structure. Interaction of LPL with heparan sulfate, as studied by surface plasmon resonance, was found to be a fast exchange process characterized by a high value for the association rate constant, 1.7 x 10(8) M-1 s-1, a relatively high dissociation rate constant, 0.05 s-1, and as a result a very low equilibrium dissociation constant equal to 0.3 nM at 0.15 M NaCl. The contribution of electrostatics was estimated to be 44% for the binding of LPL to heparan sulfate, 49% for the binding of LPL to unfractionated heparin, and 60% for the binding of LPL to affinity-purified heparin decasaccharides at 0.15 M NaCl. The number of ionic interactions between LPL and high-affinity decasaccharides was estimated to be 10. We propose an essential role of electrostatic steering in the association. Monomeric LPL had 6000-fold lower affinity for heparin than dimeric LPL had, expressed as a ratio of equilibrium dissociation constants. A model for binding of LPL to heparan sulfate-covered surfaces is proposed. Due to the fast rebinding, LPL is concentrated to the close proximity of the heparan sulfate surface. As the dissociation is also fast, the enzyme exchanges rapidly between specific binding sites on the immobilized heparan sulfate, without leaving the surface. This model may also apply to LPL at the endothelium of blood vessels.

CD23/Fc epsilon RII and its soluble fragments can form oligomers on the cell surface and in solution

Human CD23 (also known as Fc epsilon RII) is a 45,000 MW glycoprotein with homology to C-type animal lectins. It is involved in B-cell differentiation and IgE regulation, and is naturally cleaved to give soluble products of 37,000, 33,000, 29,000, 25,000 and 16,000 MW. Previous work has suggested that the region between the transmembrane sequence and the extracellular lectin head is capable of forming an alpha-helical coiled coil, one of the main consequences of which would be formation of dimers or trimers. Here we present protein-protein cross-linking data showing that CD23 forms trimers on the cell surface and hexamers in solution, and we use several different fragments to determine the regions of the protein involved in this self-association. The region of the putative coiled coil is indeed responsible for trimerization, with additional interactions between the lectin heads resulting in the formation of hexamers observed in solution.

Limiting dilution analysis of CD45Rhi and CD45Rlo T cells: further evidence that CD45Rlo cells are memory cells

Murine CD4+ T cells can be separated into two distinct populations on the basis of their levels of expression of the CD45RB antigen (CD45Rhi and CD45Rlo). Murine CD45Rlo cells arise from CD45Rhi cells after antigenic exposure and provide antigen-specific help to B cells in a secondary immune response. In the present study, the ability of CD45Rhi and CD45Rlo cells to proliferate in response to either soluble antigen or allogeneic cells was examined by limiting dilution analysis. CD45Rhi cells were the major responding cells in unprimed animals; priming caused a large increase in the frequencies of responding CD45Rlo cells and this increase was evident 11 months later. These results further support the notion that CD4+ CD45Rlo cells are long-term memory cells.

A versatile and potentially general approach to the targeting of specific cell types by retroviruses: application to the infection of human cells by means of major histocompatibility complex class I and class II antigens by mouse ecotropic murine leukemia virus-derived viruses

A technique for delivering genes carried by recombinant retroviruses into specific cell types could have numerous applications in oncology, developmental biology, and gene therapy. As a first step toward this remote goal we designed a procedure allowing in vitro cell targeting by retroviruses. Biotinylated antibodies against the viral envelope protein on one side, and against specific cell membrane markers on the other side, were bridged by streptavidin and used to link the virus to the host. The method was successfully used to infect human cells with ecotropic murine retroviruses by means of major histocompatibility complex class I and class II antigens and appears easily adaptable to other cell membrane markers. Moreover, the sequential protocol we designed, although allowing infection of human cells, requires less stringent safety constraints than would handling of amphotropic virus stocks.

Metabolism of glutamate and ammonia in astrocyte: an immunocytochemical study

Alpha-ketoglutarate (alpha-KG) reductive amination activity in rat brain was found to be mostly absorbed with an antibody against liver glutamate dehydrogenase. With this and anti-glutamine synthetase antibodies, alpha-KG reductive amination activity was immunocytochemically shown to coexist with glutamine synthetase activity in astrocytes. The results suggest that astrocytes de novo synthesize glutamate from alpha-KG and ammonia, and metabolize it to glutamine.

IgE-binding factors: their possible role in the regulation of IgE synthesis

B cell-derived IgE-BFs (sCD23) are cleavage fragments of surface Fc epsilon R II. Their production is increased by IL4 and suppressed by IFN-gamma and IFN-alpha. IgE-BFs are likely to play a role in the regulation of human IgE synthesis as shown by the following two observations: i. MabER specifically blocks both the spontaneous IgE by synthesis by atopic B cells and the IL4-induced IgE synthesis by normal lymphocytes, ii. purified IgE-BFs enhance the IL4-induced and the spontaneous IgE synthesis. Soluble fragments of Fc epsilon R II also display BCGF-like activity although the exact structure of these fragments is not yet identified. The cDNA coding for Fc epsilon R II has been cloned and functionally expressed. The predicted amino acid sequence reveals no homology between human and rodent IgE-BFs indicating that they are unrelated molecules.

Quantitative and qualitative analysis of the Fc receptor for IgE (Fc epsilon RII) on human eosinophils

In order to characterize the Fc receptor for IgE (Fc epsilon RII) on human eosinophils, we have compared the binding of human IgE myeloma protein to that of a monoclonal antibody (mAb BB10) directed against a common antigenic determinant of the Fc epsilon RII present on eosinophils, platelets and macrophages. Scatchard analysis of the binding to human eosinophils of the BB10 mAb revealed a linear monophasic binding curve, with a binding affinity of 1.17 x 10(7) M-1 and a number of 10(5) binding sites per cell. Biochemical analysis of the human eosinophil Fc epsilon R, performed by immunosorbent chromatography with either BB10 mAb or IgE, showed under nonreducing conditions a major component of 200 kDa. Under reducing conditions, 3 peptide fragments were obtained, with molecular masses of 45-50, 23 and 15 kDa. Finally, comparative analysis suggested that the Fc epsilon RII of human eosinophils and of a human macrophage cell line (U937) are structurally related and differ from the high-affinity Fc epsilon RI present on basophilic granulocytes.