In vitro modelling of rat mucosal mast cell function in Trichinella spiralis infection

Intestinal infection with the parasitic nematode, Trichinella spiralis, provides a robust context for the study of mucosal mast cell function. In rats, mucosal mast cells are exposed to parasites during the earliest stage of infection, affording an opportunity for mast cells to contribute to an innate response to infection. During secondary infection, degranulation of rat mucosal mast cells coincides with expulsion of challenge larvae from the intestine. The goal of this study was to evaluate the rat bone marrow-derived mast cells (BMMC) and the rat basophilic leukaemia cell line (RBL-2H3) as models for mucosal mast cells, using parasite glycoproteins and antibody reagents that have been tested extensively in rats in vivo. We found that BMMC displayed a more robust mucosal phenotype. Although T. spiralis glycoproteins bound to mast cell surfaces in the absence of antibodies, they did not stimulate degranulation, nor did they inhibit degranulation triggered by immune complexes. Parasite glycoproteins complexed with specific monoclonal IgGs provoked release of rat mast cell protease II (RMCPII) and β-hexosaminidase from both cell types in a manner that replicated results observed previously in passively immunized rats. Our results document that RBL-2H3 cells and BMMC model rat mucosal mast cells in the contexts of innate and adaptive responses to T. spiralis.

Crosslinking a lipid raft component triggers liquid ordered-liquid disordered phase separation in model plasma membranes

The mechanisms by which a cell uses and adapts its functional membrane organization are poorly understood and are the subject of ongoing investigation and discussion. Here, we study one proposed mechanism: the crosslinking of membrane components. In immune cell signaling (and other membrane-associated processes), a small change in the clustering of specific membrane proteins can lead to large-scale reorganizations that involve numerous other membrane components. We have investigated the large-scale physical effect of crosslinking a minor membrane component, the ganglioside GM1, in simple lipid models of the plasma membrane containing sphingomyelin, cholesterol, and phosphatidylcholine. We observe that crosslinking GM1 can cause uniform membranes to phase-separate into large, coexistent liquid ordered and liquid disordered membrane domains. We also find that this lipid separation causes a dramatic redistribution of a transmembrane peptide, consistent with a raft model of membrane organization. These experiments demonstrate a mechanism that could contribute to the effects of crosslinking observed in cellular processes: Domains induced by clustering a small number of proteins or lipids might rapidly reorganize many other membrane proteins.

Fluorescence anisotropy measurements of lipid order in plasma membranes and lipid rafts from RBL-2H3 mast cells

Specialized plasma membrane domains known as lipid rafts participate in signal transduction and other cellular processes, and their liquid ordered (L(o)) phase appears to be important for their function. To quantify ordered lipids in biological membranes, we investigated steady-state fluorescence anisotropy of two lipid probes, 2-[3-(diphenylhexatrienyl)propanoyl]-1-hexadecanoyl-sn-glycero-3-phosphocholine (DPH-PC) and N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (NBD-PE). We show using model membranes with varying amounts of cholesterol that steady-state fluorescence anisotropy is a sensitive measure of cholesterol-dependent ordering. The results suggest that DPH-PC is a more sensitive probe than NBD-PE. In the presence of cholesterol, ordering also depends on the degree of saturation of the phospholipid acyl chains. Using DPH-PC, we find that the plasma membrane of RBL-2H3 mast cells is substantially ordered, roughly 40%, as determined by comparison with anisotropy values for model membranes entirely in a liquid ordered (L(o)) phase and in a liquid disordered (L(alpha)) phase. This result is consistent with the finding that approximately 30% of plasma membrane phospholipids are insoluble in 0.5% Triton X-100. Furthermore, detergent-resistant membranes isolated by sucrose gradient fractionation of Triton X-100 cell lysates are more ordered than plasma membrane vesicles, suggesting that they represent a more ordered subset of the plasma membrane. Treatment of plasma membrane vesicles with methyl-beta-cyclodextrin resulting in 75% cholesterol depletion leads to commensurate decreases in lipid order as measured by anisotropy of DPH-PC and NBD-PE. These results demonstrate that steady-state fluorescence anisotropy of DPH-PC is a useful way to measure the amount of lipid order in biological membranes.

Cross-correlation analysis of inner-leaflet-anchored green fluorescent protein co-redistributed with IgE receptors and outer leaflet lipid raft components

To investigate the structural basis for membrane interactions that occur between Lyn tyrosine kinase and IgE-Fc(epsilon)RI or other components of lipid rafts, we prepared a green fluorescent protein analog of Lyn (PM-EGFP) and used cross-correlation analysis to quantify co-redistributions of aggregates that occur after IgE-Fc(epsilon)RI is cross-linked on the cell surface. PM-EGFP, which contains minimally the palmitoylation and myristoylation sites on Lyn, was compared with another inner leaflet probe, EGFP-GG, which contains a prenylation site and a polybasic sequence similar to K-ras. Confocal fluorescence microscopy was used to examine co-redistributions of these inner leaflet components with IgE-Fc(epsilon)RI and outer leaflet raft components, ganglioside GD1b and glycosylphosphotidylinositol-linked Thy-1, under conditions where the latter were cross-linked externally to form large patches at the cell surface. The cross-correlation analysis was developed and characterized with simulations representing cell surface distributions, and parameters from the cross-correlation curves, rho(o) (peak height) and A (peak area), were shown to be reliable measures of the extent of co-redistributed aggregates and their size. Cross-correlation analysis was then applied to quantify co-redistributions of the fluorescently labeled inner and outer leaflet components on RBL-2H3 cells. As visually observed and parameterized in this manner, PM-EGFP was found to co-redistribute with lipid rafts significantly more than EGFP-GG or an endogenous prenylated protein, Cdc42. These quantitative results are consistent with previous analyses of Lyn co-redistributions and support the hypothesis that the functionally important interaction of Lyn with cross-linked IgE- Fc(epsilon)RI is due to their mutual co-association with lipid rafts.

Fc(epsilon)RI as a paradigm for a lipid raft-dependent receptor in hematopoietic cells

Lipid domains or rafts are currently embraced by immunologists as critical participants in receptor-mediated signaling events occurring at the plasma membrane. This view of membrane heterogeneity and its functional importance is supported by many years of different experimental approaches. We can now refine our investigations, moving beyond the simple models to ask more detailed questions about structural properties and mechanistic interactions. As highlighted for the IgE receptor (Fc(epsilon)RI), new information about initial engagement with src family kinases, cytoskeletal regulation, and coupling with downstream signaling is beginning to emerge.

Activated Cdc42/Rac reconstitutes Fcepsilon RI-mediated Ca2+ mobilization and degranulation in mutant RBL mast cells

Antigen stimulation of mast cells via FcepsilonRI, the high-affinity receptor for IgE, triggers a signaling cascade that requires Ca(2+) mobilization for exocytosis of secretory granules during an allergic response. This study investigates critical signaling components by using mutant RBL mast cells that are defective in antigen-stimulated phospholipase Cgamma (PLCgamma) activation, as well as other signaling activities downstream of stimulated tyrosine phosphorylation. We show that the expression of activated versions of the Cdc42 or Rac1 GTPase restores antigen-stimulated Ca(2+) mobilization necessary for degranulation in these mutant cells. Wild-type Cdc42 and Rac1, as well as activated Cdc42 containing effector domain mutations, all fail to restore antigen-stimulated signaling leading to exocytosis. Expression of oncogenic Dbl, a guanine nucleotide exchange factor for Cdc42 and Rac1, partially restores sustained Ca(2+) mobilization and degranulation, suggesting that activation of endogenous Cdc42 and/or Rac1 is impaired in the mutant cells. Overexpression of PLCgamma1 with either activated Cdc42 or Rac1 synergistically stimulates degranulation, consistent with a critical defect in PLCgamma activation in these cells. Thus, our results point to activation of Cdc42 and/or Rac1 playing an essential role in antigen stimulation of early events that culminate in mast cell degranulation.

Mutant RBL mast cells defective in Fc epsilon RI signaling and lipid raft biosynthesis are reconstituted by activated Rho-family GTPases

Characterization of defects in a variant subline of RBL mast cells has revealed a biochemical event proximal to IgE receptor (Fc epsilon RI)-stimulated tyrosine phosphorylation that is required for multiple functional responses. This cell line, designated B6A4C1, is deficient in both Fc epsilon RI-mediated degranulation and biosynthesis of several lipid raft components. Agents that bypass receptor-mediated Ca(2+) influx stimulate strong degranulation responses in these variant cells. Cross-linking of IgE-Fc epsilon RI on these cells stimulates robust tyrosine phosphorylation but fails to mobilize a sustained Ca(2+) response. Fc epsilon RI-mediated inositol phosphate production is not detectable in these cells, and failure of adenosine receptors to mobilize Ca(2+) suggests a general deficiency in stimulated phospholipase C activity. Antigen stimulation of phospholipases A(2) and D is also defective. Infection of B6A4C1 cells with vaccinia virus constructs expressing constitutively active Rho family members Cdc42 and Rac restores antigen-stimulated degranulation, and active Cdc42 (but not active Rac) restores ganglioside and GPI expression. The results support the hypothesis that activation of Cdc42 and/or Rac is critical for Fc epsilon RI-mediated signaling that leads to Ca(2+) mobilization and degranulation. Furthermore, they suggest that Cdc42 plays an important role in the biosynthesis and expression of certain components of lipid rafts.

Heterogeneous glycosylation of immunoglobulin E constructs characterized by top-down high-resolution 2-D mass spectrometry

Posttranslational glycosylation is critical for biological function of many proteins, but its structural characterization is complicated by natural heterogeneity, multiple glycosylation sites, and different forms. Here, a top-down mass spectrometry (MS) characterization is applied to three constructs of the Fc segment of IgE: Fcepsilon(3-4) (52 kDa) and Fcepsilon(2-3-4)(2) (76 kDa) disulfide-bonded homodimers. Fourier transform MS of a reduced sample of Fcepsilon(2-3-4) gave molecular masses of 37 527, 37 689, 37 851, and 38 014 Da, directly characterizing multiple glycoforms (hexose = 162 Da) without chromatographic separation. Limited proteolysis of the nonreduced Fcepsilon(2-3-4)(2) protein yielded a peptide mixture with molecular weight values that agreed with those expected from the DNA sequence. The single glycosylation site in these constructs was identified, and quantities were determined of five glycoforms that agreed within +/-2% of the molecular ion values. The 2-D mass spectrum of two glycosylated peptides showed these to have high-mannose structures, -GlcNAc-(hex)(n)(), demonstrating that Fcepsilon(2-3-4) has a single such structure of n = 5-9. For a mutated sample of Fcepsilon(3-4), in addition to five glycoforms, MS showed a molecular discrepancy that could be assigned with proteolysis and 2-D mass spectra to the oxidation of two methionines and an additional residue difference.

Interactions between Fc(epsilon)RI and lipid raft components are regulated by the actin cytoskeleton

Previous studies showed that crosslinking of IgE-Fc(epsilon)RI complexes on RBL-2H3 mast cells causes their association with isolated detergent-resistant membranes, also known as lipid rafts, in a cholesterol-dependent process that precedes initiation of signaling by these receptors. To investigate these interactions on intact cells, we examined the co-redistribution of raft components with crosslinked IgE-Fc(epsilon)RI using confocal microscopy. After several hours of crosslinking at 4 degrees C, the glycosylphosphatidylinositol-linked protein Thy-1 and the Src-family tyrosine kinase Lyn co-redistribute with IgE-Fc(epsilon)RI in large patches at the plasma membrane. Under these conditions, F-actin also undergoes dramatic co-segregation with Fc(epsilon)RI and raft components but is dispersed following a brief warm-up to 37 degrees C. When crosslinking of IgE-Fc(epsilon)RI is initiated at higher temperatures, co-redistribution of raft components with patched Fc(epsilon)RI is not readily detected unless stimulated F-actin polymerization is inhibited by cytochalasin D. In parallel, cytochalasin D converts transient antigen-stimulated tyrosine phosphorylation to a more sustained response. Sucrose gradient analysis of lysed cells reveals that crosslinked IgE-Fc(epsilon)RI remains associated with lipid rafts throughout the time course of the transient phosphorylation response but undergoes a time-dependent shift to higher density that is prevented by cytochalasin D. Our results indicate that interactions between Lyn and crosslinked IgE-Fc(epsilon)RI are regulated by stimulated F-actin polymerization, and this is best explained by a segregation of anchored raft components from more mobile ones.

How does the plasma membrane participate in cellular signaling by receptors for immunoglobulin E?

Accumulating evidence strongly supports the view that the plasma membrane participates in transmembrane signaling by IgE-receptors (IgE-Fc epsilon RI) through the formation of lipid-based domains, also known as rafts. Ongoing biochemical and biophysical experiments investigate the composition, structure, and dynamics of the corresponding membrane components and how these are related to functional coupling between Fc epsilon RI and Lyn tyrosine kinase to initiate signaling in mast cells.

Electron spin resonance characterization of liquid ordered phase of detergent-resistant membranes from RBL-2H3 cells

The dynamic structure of detergent-resistant membranes (DRMs) isolated from RBL-2H3 cells was characterized using two different acyl chain spin-labeled phospholipids (5PC and 16PC), a headgroup labeled sphingomyelin (SM) analog (SD-Tempo) and a spin-labeled cholestane (CSL). It was shown, by comparison to dispersions of SM, dipalmitoylphosphatidylcholine (DPPC), and DPPC/cholesterol of molar ratio 1, that DRM contains a substantial amount of liquid ordered phase: 1) The rotational diffusion rates (R( perpendicular)) of 16PC in DRM between -5 degrees C and 45 degrees C are nearly the same as those in molar ratio DPPC/Chol = 1 dispersions, and they are substantially greater than R( perpendicular) in pure DPPC dispersions in the gel phase studied above 20 degrees C; 2) The order parameters (S) of 16PC in DRM at temperatures above 4 degrees C are comparable to those in DPPC/Chol = 1 dispersions, but are greater than those in DPPC dispersions in both the gel and liquid crystalline phases. 3) Similarly, R( perpendicular) for 5PC and CSL in DRM is greater than in pure SM dispersions in the gel phase, and S for these labels in DRM is greater than in the SM dispersions in both the gel and liquid crystalline phases. 4) R( perpendicular) of SD-Tempo in DRM is greater than in dispersions of SM in both gel and liquid phases, consistent with the liquid-like mobility in the acyl chain region in DRM. However, S of SD-Tempo in DRM is substantially less than that of this spin label in SM in gel and liquid crystalline phases (in absolute values), indicating that the headgroup region in DRMs is less ordered than in pure SM. These results support the hypothesis that plasma membranes contain DRM domains with a liquid ordered phase that may coexist with a liquid crystalline phase. There also appears to be a coexisting region in DRMs in which the chain labels 16PC and 5PC are found to cluster. We suggest that other biological membranes containing high concentrations of cholesterol also contain a liquid ordered phase.

Quantitative analysis of phospholipids in functionally important membrane domains from RBL-2H3 mast cells using tandem high-resolution mass spectrometry

We recently showed that ligand-mediated cross-linking of FcepsilonRI, the high-affinity receptor for immunoglobulin E, on RBL-2H3 mast cells results in its co-isolation with detergent-resistant membranes (DRM) and its consequent tyrosine phosphorylation by the co-localized tyrosine kinase Lyn that is a critical early event in signaling by this receptor [Field et al. (1997) J. Biol. Chem. 272, 4276-4280]. As part of efforts to determine the structural bases for these interactions, we examined the phospholipid composition of DRM vesicles isolated from RBL-2H3 cells under conditions that preserve FcepsilonRI association. We used positive and negative mode electrospray Fourier transform ion cyclotron resonance mass spectrometry to compare quantitatively the phospholipid composition of isolated DRM to that of total cell lipids and to a plasma membrane preparation. From these analyses, over 90 different phospholipid species were spectrally resolved and unambiguously identified; more than two-thirds of these were determined with a precision of +/-0.5% (absolute) or less. Quantitative characterization of lipid profiles shows that isolated DRM are substantially enriched in sphingomyelin and in glycerophospholipids with a higher degree of saturation as compared to total cellular lipids. Plasma membrane vesicles isolated from RBL-2H3 cells by chemically induced blebbing exhibit a degree of phospholipid saturation that is intermediate between DRM and total cellular lipids, and significant differences in the headgroup distribution between DRM and plasma membranes vesicles are observed. DRM from cells with cross-linked FcepsilonRI exhibit a larger ratio of polyunsaturated to saturated and monounsaturated phospholipids than those from unstimulated cells. Our results support and strengthen results from previous studies suggesting that DRM have a lipid composition that promotes liquid-ordered structure. Furthermore, they demonstrate the potential of mass spectrometry for examining the role of membrane structure in receptor signaling and other cellular processes.

Critical role for cholesterol in Lyn-mediated tyrosine phosphorylation of FcepsilonRI and their association with detergent-resistant membranes

Tyrosine phosphorylation of the high affinity immunoglobulin (Ig)E receptor (FcepsilonRI) by the Src family kinase Lyn is the first known biochemical step that occurs during activation of mast cells and basophils after cross-linking of FcepsilonRI by antigen. The hypothesis that specialized regions in the plasma membrane, enriched in sphingolipids and cholesterol, facilitate the coupling of Lyn and FcepsilonRI was tested by investigating functional and structural effects of cholesterol depletion on Lyn/FcepsilonRI interactions. We find that cholesterol depletion with methyl-beta-cyclodextrin substantially reduces stimulated tyrosine phosphorylation of FcepsilonRI and other proteins while enhancing more downstream events that lead to stimulated exocytosis. In parallel to its inhibition of tyrosine phosphorylation, cholesterol depletion disrupts the interactions of aggregated FcepsilonRI and Lyn on intact cells and also disrupts those interactions with detergent-resistant membranes that are isolated by sucrose gradient ultracentrifugation of lysed cells. Importantly, cholesterol repletion restores receptor phosphorylation together with the structural interactions. These results provide strong evidence that membrane structure, maintained by cholesterol, plays a critical role in the initiation of FcepsilonRI signaling.

Interleukin 1 (IL-1) causes changes in lateral and rotational mobilities of IL-1 type I receptors

To investigate IL-1-dependent interactions of IL-1 type I (IL-1 RI) receptors on intact cells, lateral and rotational mobilities and detergent insolubility were investigated. Lateral mobility was measured by fluorescence photobleaching recovery, using a Cy3-modified, noncompetitive mAb specific for IL-1RI (M5) bound to wild-type IL-1 RI or mutant IL-1 RI with a truncated cytoplasmic tail. Addition of IL-1 causes significant reduction in the mobile fraction of wild-type IL-1 RI for two different transfected cell lines. For the mutant IL-1 RI, no significant decrease in response to IL-1 is observed, indicating that the missing cytoplasmic segment is involved in IL-1-dependent interactions of IL-1 RI that lead to reduced lateral mobility on the cell surface. The rotational mobility of IL-1 RI was assessed with phosphorescence anisotropy decay measurements using erythrosin-labeled M5. IL-1 decreases the rotational mobility of cell surface IL-1 RI on the microsecond time scale and also increases the initial anisotropy, indicating loss in segmental motion. Measurements of resistance to solubilization by Triton X-100 showed that IL-1 binding increases the fraction of IL-1 RI sedimenting with cytoskeletal residues. The IL-1 receptor antagonist protein (IL-1ra) causes partial effects in reducing rotational mobility and increasing detergent insolubility of M5-lableled IL-1 RI, indicating that this ligand causes structural changes in the presence of the dimerizing M5 mAb. These ligand-dependent physical interactions of IL-1 RI on the cell surface may be related to signal initiation by this receptor.

Membrane organization in immunoglobulin E receptor signaling

The structure and dynamics of the plasma membrane are proposed to be critical for the initial steps of signal transduction by the high-affinity immunoglobulin E receptor. Recent experimental advances indicate that interactions between the high-affinity immunoglobulin E receptor and the tyrosine kinase Lyn with cholesterol- and sphingolipid-rich regions within the plasma membrane are important for receptor function. This accumulating evidence points to spatio-temporal control of immunoglobulin E receptor signaling by the organization of the plasma membrane; an attractive hypothesis is that ligand-dependent receptor aggregation causes the segregation of Lyn-containing ordered regions of the plasma membrane from disordered regions.

Structural aspects of the association of FcepsilonRI with detergent-resistant membranes

We recently showed that aggregation of the high affinity IgE receptor on mast cells, FcepsilonRI, causes this immunoreceptor to associate rapidly with specialized regions of the plasma membrane, where it is phosphorylated by the tyrosine kinase Lyn. In this study, we further characterize the detergent sensitivity of this association on rat basophilic leukemia-2H3 mast cells, and we compare the capacity of structural variants of FcepsilonRI and other receptors to undergo this association. We show that this interaction is not mediated by the beta subunit of the receptor or the cytoplasmic tail of the gamma subunit, both of which are involved in signaling. Using chimeric receptor constructs, we found that the extracellular segment of the FcepsilonRI alpha subunit was not sufficient to mediate this association, implicating FcepsilonRI alpha and/or gamma transmembrane segments. To determine the specificity of this interaction, we compared the association of several other receptors. Interleukin-1 type I receptors on Chinese hamster ovary cells and alpha4 integrins on rat basophilic leukemia cells showed little or no association with isolated membrane domains, both before and after aggregation on the cells. In contrast, interleukin-2 receptor alpha (Tac) on Chinese hamster ovary cells exhibited aggregation-dependent membrane domain association similar to FcepsilonRI. These results provide insights into the structural basis and selectivity of lipid-mediated interactions between certain transmembrane receptors and detergent-resistant membranes.

Stimulated release of fluorescently labeled IgE fragments that efficiently accumulate in secretory granules after endocytosis in RBL-2H3 mast cells

Sensitization of RBL-2H3 mast cells with monomeric fluorescein-5-isothiocyanate (FITC)-labeled immunoglobulin E (IgE) results in slow but highly efficient accumulation of labeled IgE fragments in a pool of acidic peripheral vesicles that are visible by fluorescence microscopy after raising endosomal pH with ammonium chloride. Stimulation of cells containing these FITC-IgE fragments by aggregation of high affinity receptors for IgE (FcepsilonRI) or by Ca2+ ionophore and phorbol 12-myristate 13-acetate results in release of FITC fluorescence from the cells, which can be monitored continuously with a spectrofluorometer. The fluorescence release process corresponds to cellular degranulation: it is prevented under conditions that prevent stimulated beta-hexosaminidase release, and these two processes exhibit the same antigen dose-dependence and kinetics. Pulse-chase labeling reveals that aggregation of FITC-IgE bound to FcepsilonRI at the cell surface causes internalization and delivery to the regulated secretory vesicles with a high efficiency similar to monomeric IgE-FcepsilonRI, but more rapidly. Binding of Cy3-modified IgE to FcepsilonRI results in labeling of the same secretory vesicles as in FITC-IgE-sensitized cells, and these Cy3-labeled vesicles can be observed by fluorescence microscopy without neutralization of intracellular compartments. Simultaneous three-photon microscopy of serotonin fluorescence and two-photon microscopy of Cy3 fluorescence reveals that these Cy3-labeled vesicles coincide with serotonin-labeled secretory granules. After stimulation of the cells via aggregation of IgE-FcepsilonRI or addition of Ca2+ ionophore and phorbol 12-myristate 13-acetate, depletion of the Cy3 label from the intracellular vesicles is observed with confocal microscopy. These results provide strong evidence for the lysosomal nature of secretory granules in these cells. In addition, they provide the basis for a direct, real-time method for monitoring single cell degranulation.

Kinetics of multivalent antigen DNP-BSA binding to IgE-Fc epsilon RI in relationship to the stimulated tyrosine phosphorylation of Fc epsilon RI

Multivalent DNP-BSA is commonly used to cross-link anti-DNP IgE bound to Fc epsilon RI to stimulate cellular responses, although key features of the binding process are unknown. Fluorescence quenching can be used to study the kinetics of DNP-BSA binding to FITC-IgE. We observe that DNP-BSA binds more slowly to IgE than does an equimolar amount of a monovalent DNP ligand, suggesting that the average effective number of DNP groups per BSA is less than one. The binding data are well described by a transient hapten exposure model in which most of the DNP groups are unavailable for binding but have some probability of becoming exposed and available for binding during the time of the binding measurement. Additional experiments indicate that, for suboptimal to optimal concentrations of DNP-BSA, most of the FITC fluorescence quenching on the cell surface is due to cross-linking events. With these concentrations at 15 degrees C, the kinetics of FITC fluorescence quenching by DNP-BSA correlates with the kinetics of DNP-BSA-stimulated tyrosine phosphorylation of Fc epsilon RI. At 35 degrees C, the phosphorylation kinetics are biphasic during the time period in which cross-linking continues to increase. Our results establish a quantitative relationship between the time-course for cross-linking by multivalent Ag and Fc epsilon RI-mediated signaling, and they provide the means to predict the kinetics of cross-linking under a wide variety of conditions.

Evidence supporting a role for microfilaments in regulating the coupling between poorly dissociable IgE-Fc epsilonRI aggregates downstream signaling pathways

Aggregation of Fc epsilonRI, the high-affinity receptor for IgE, on RBL-2H3 mast cells caused by reversible ligands such as multivalent antigen causes cellular responses that can be halted by subsequent addition of excess monovalent ligand. In contrast, Ca2+ and degranulation responses elicited by effectively irreversible streptavidin cross-linking of biotinylated IgE-Fc epsilonRI are not stopped by addition of excess biotin after stimulation is initiated. These results support previous conclusions based on studies with covalent oligomers of IgE that stable cross-links can continue to deliver stimulatory signals for extended periods of time. Dissociation measured in the presence of monovalent hapten reveals two populations of IgE-Fc epsilonRI cross-linked by multivalent antigen that differ in functional effectiveness. Aggregates with readily dissociable cross-links are normally responsible for triggering essentially all of the degranulation response, whereas aggregates with poorly dissociable cross-links apparently do not trigger this response. Treatment of RBL-2H3 cells with cytochalasin D, an inhibitor of actin polymerization, enhances downstream signaling and enables the less readily dissociable aggregates to stimulate Ca2+ and degranulation responses. Under these conditions, cytochalasin D does not affect hapten-mediated dissociation of multivalent antigen, nor does it prevent hapten from reversing tyrosine phosphorylation of Syk. Cytochalasin D alone causes tyrosine phosphorylation of a protein at approximately 75 kDa, and it reduces hapten-induced reversal of antigen-stimulated tyrosine phosphorylation of several other proteins. Taken together, these results indicate that stimulated actin polymerization normally regulates the coupling of aggregated Fc epsilonRI to downstream signaling pathways, and they provide an explanation for seeming discrepancies between responses to stable and reversible cross-links.

Altered patterns of tyrosine phosphorylation and Syk activation for sterically restricted cyclic dimers of IgE-Fc epsilonRI

Previous studies in our laboratory established that the symmetrical bivalent ligand, N,N'-bis-[[epsilon-(2,4-dinitrophenyl)amino]caproyl]-L-tyrosyl]-L-cystin e ((DCT)2-cys), stably cross-links anti-2,4-dinitrophenyl-immunoglobulin E (IgE) bound to high affinity receptors Fc epsilonRI on the surface of RBL-2H3 cells, forming mostly cyclic dimers containing two IgE-Fc epsilonRI and two (DCT)2-cys (Posner et al. (1995) J. Immunol. 155, 3601-3609). These cyclic dimers do not trigger Ca2+ or degranulation responses under a variety of conditions. However, we find that the linearly cross-linked IgE-Fc epsilonRI formed at higher concentrations of (DCT)2-cys do trigger degranulation in the presence of cytochalasin D, an inhibitor of actin polymerization. We further investigated stimulation by (DCT)2-cys of the earliest known events in the functional response, i.e., tyrosine phosphorylation of the beta and gamma subunits of Fc epsilonRI. At the higher (DCT)2-cys concentrations corresponding to linear dimers and maximal degranulation, tyrosine phosphorylation of both beta and gamma are observed. At lower (DCT)2-cys concentrations where cross-linking is maximal and cyclic dimers are overwhelmingly dominant, only gamma tyrosine phosphorylation is observed. Cytochalasin D does not affect these phosphorylation patterns, but instead appears to enhance coupling to downstream signaling events. Phosphorylation of Syk occurs at the higher (DCT)2-cys concentrations in parallel with beta phosphorylation but does not occur in its absence at the lower (DCT)2-cys concentrations. These results suggest that cyclic dimers of IgE-Fc epsilonRI are sterically restricted such that they stimulate tyrosine phosphorylation of gamma but not beta, and this is not sufficient for Syk binding and/or activation.

Compartmentalized activation of the high affinity immunoglobulin E receptor within membrane domains

The earliest known step in the activation of the high affinity IgE receptor, FcepsilonRI, is the tyrosine phosphorylation of its beta and gamma subunits by the Src family tyrosine kinase, Lyn. We report here that aggregation-dependent association of FcepsilonRI with specialized regions of the plasma membrane precedes its tyrosine phosphorylation and appears necessary for this event. Tyrosine phosphorylation of beta and gamma occurs in intact cells only for FcepsilonRI that associate with these detergent-resistant membrane domains, which are enriched in active Lyn. Furthermore, efficient in vitro tyrosine phosphorylation of FcepsilonRI subunits occurs only for those associated with isolated domains. This association and in vitro phosphorylation are highly sensitive to low concentrations of detergent, suggesting that lipid-mediated interactions with Lyn are important in FcepsilonRI activation. Participation of membrane domains accounts for previously unexplained aspects of FcepsilonRI-mediated signaling and may be relevant to signaling by other multichain immune receptors.

Fc epsilon RI-mediated association of 6-micron beads with RBL-2H3 mast cells results in exclusion of signaling proteins from the forming phagosome and abrogation of normal downstream signaling

Cells of the mucosal mast cell line, RBL-2H3, are normally stimulated to degranulate after aggregation of high affinity receptors for IgE (Fc epsilon RI) by soluble cross-linking ligands. This cellular degranulation process requires sustained elevation of cytoplasmic Ca2+. In this study, we investigated the response of RBL-2H3 cells to 6-micron beads coated with IgE-specific ligands. These ligand-coated beads cause only small, transient Ca2+ responses, even though the same ligands added in soluble form cause larger, more sustained Ca2+ responses. The ligand-coated 6-micron beads also fail to stimulate significant degranulation of RBL-2H3 cells, whereas much larger ligand-coated Sepharose beads stimulate ample degranulation. Confocal fluorescence microscopy shows that the 6-micron beads (but not the Sepharose beads) are phagocytosed by RBL-2H3 cells and that, beginning with the initial stages of bead engulfment, there is exclusion of many plasma membrane components from the 6-micron bead/cell interface, including p53/56lyn and several other markers for detergent-resistant membrane domains, as well as an integrin and unliganded IgE-Fc epsilon RI. The fluorescent lipid probe DiIC16 is a marker for the membrane domains that is excluded from the cell/bead interface, whereas a structural analogue, fast DiI, which differs from DiIC16 by the presence of unsaturated acyl chains, is not substantially excluded from the interface. None of these components are excluded from the interface of RBL-2H3 cells and the large Sepharose beads. Additional confocal microscopy analysis indicates that microfilaments are involved in the exclusion of plasma membrane components from the cell/bead interface. These results suggest that initiation of phagocytosis diverts normal signaling pathways in a cytoskeleton-driven membrane clearance process that alters the physiological response of the cells.

Antigen-mediated IGE receptor aggregation and signaling: a window on cell surface structure and dynamics

The high-affinity receptor for immunoglobulin E, Fc epsilon RI, serves as an archtype for multisubunit immunoreceptors that mediate cell activation in response to foreign antigens. Antigen-mediated aggregation of this receptor at the surface of mast cells and basophils initiates a biochemical cascade that uses nonreceptor tyrosine kinases as key participants in the earliest steps of this signal transduction process. Cross-linking of Fc epsilon RI with ligands of well-defined structure and valency has revealed detailed information about the fundamental requirements for functionally active receptor aggregates. Cross-linking-dependent changes in the interaction of these receptors with other cellular components have been characterized with biochemical and biophysical methods to develop a more complete view of signal initiation. Recent evidence suggests that this process involves the interaction of aggregated Fc epsilon RI with specialized plasma membrane domains that may localize important signaling molecules in the vicinity of aggregated receptors. Although these various studies were aimed toward understanding the operation of one cell surface receptor, they provide new insights into plasma membrane structure and dynamics that are generally relevant to the function of most nucleated mammalian cells.

Sustained T cell receptor-mediated Ca2+ responses rely on dynamic engagement of receptors

We have investigated the functional advantage of surface-attached ligands for TCR-mediated cell activation with flow cytometric measurements of cytoplasmic Ca2+ changes in T cells after aggregation of TCR by soluble and bead-attached mAb. Conjugation of HPB-ALL human leukemia cells with cell-sized beads coated with anti-TCR mAb causes a stronger, more sustained Ca2+ response than that produced by the soluble form of the same mAb. Addition of a large excess of the soluble mAb subsequent to stimulation with the beads causes a marked reduction in the response of the bead-conjugated cells, but only limited disruption of the conjugates. Free (nonconjugated) cells, sampled simultaneously in this mixture, respond to the soluble mAb with a transient Ca2+ increase that declines with the same kinetics as the bead-conjugated cells after addition of the soluble mAb. Fab fragments of the anti-TCR mAb cause a similar reduction in the response of the bead-conjugated cells, and they do not stimulate free cells. Following the Fab-mediated decline in cytoplasmic Ca2+ of conjugated cells to near-baseline concentrations, the addition of a second, noncompetitive, anti-TCR mab causes a Ca2+ response that is substantially reduced in magnitude compared with that for the free cells. The results indicate that soluble and surface-attached ligands cause TCR-specific desensitization of the Ca2+ response. Surface-attached ligands are more effective than soluble ligands in sustaining signaling in T cells at least in part because they facilitate steady association and/or reassociation of TCR into the bound state in the surface contact area.

Sustained T cell receptor-mediated Ca2+ responses rely on dynamic engagement of receptors

We have investigated the functional advantage of surface-attached ligands for TCR-mediated cell activation with flow cytometric measurements of cytoplasmic Ca2+ changes in T cells after aggregation of TCR by soluble and bead-attached mAb. Conjugation of HPB-ALL human leukemia cells with cell-sized beads coated with anti-TCR mAb causes a stronger, more sustained Ca2+ response than that produced by the soluble form of the same mAb. Addition of a large excess of the soluble mAb subsequent to stimulation with the beads causes a marked reduction in the response of the bead-conjugated cells, but only limited disruption of the conjugates. Free (nonconjugated) cells, sampled simultaneously in this mixture, respond to the soluble mAb with a transient Ca2+ increase that declines with the same kinetics as the bead-conjugated cells after addition of the soluble mAb. Fab fragments of the anti-TCR mAb cause a similar reduction in the response of the bead-conjugated cells, and they do not stimulate free cells. Following the Fab-mediated decline in cytoplasmic Ca2+ of conjugated cells to near-baseline concentrations, the addition of a second, noncompetitive, anti-TCR mab causes a Ca2+ response that is substantially reduced in magnitude compared with that for the free cells. The results indicate that soluble and surface-attached ligands cause TCR-specific desensitization of the Ca2+ response. Surface-attached ligands are more effective than soluble ligands in sustaining signaling in T cells at least in part because they facilitate steady association and/or reassociation of TCR into the bound state in the surface contact area.

The Fc segment of IgE influences the kinetics of dissociation of a symmetrical bivalent ligand from cyclic dimeric complexes

As part of a systematic effort to determine the features of immunoglobulin E-receptor (IgE-Fc epsilon RI) aggregation that are critical for cellular activation, we used fluorescence to examine the dissociation of a soluble bivalent ligand, N, N'-bis[[epsilon-[(2,4-dinitrophenyl)amino]caproyl]-L-tyrosyl]-L-cystine ((DCT)2-cys), from soluble bivalent IgE and its bivalent F(ab')2 and monovalent Fab' fragments. Cross-linking of Fab' fragments by (DCT)2-cys is limited to linear dimers, and we find that (DCT)2-cys dissociation from Fab' occurs with a single kinetic coefficient [(4.2 +/- 0.6) x 10-3 s-1] that corresponds to the lower of the two kinetic coefficients observed with the bivalent IgE [(4.7 +/- 0.7) x 10-2 s-1 and (4.4 +/- 0.3) x 10-3 s-1]. Similarly, the lower value is obtained for dissociation of (DCT)2-cys that is monovalently bound to IgE after incubation with a large excess of the ligand. (DCT)2-cys can bind to bivalent F(ab')2 fragments and form a variety of linear and cyclic aggregates, similarly to IgE, but, unlike IgE, we find that dissociation occurs with a single kinetic coefficient similar to that observed for Fab'. We find that IgE and its (Fab')2 fragments form highly stable cyclic dimer rings with two (DCT)2-cys. We demonstrate that the kinetic coefficients are independent of enhanced fluorescence quenching observed for bound sites in cyclic dimers. Together, the results show that the rate constant for breaking a linear cross-link formed by (DCT)2-cys is the same as that for dissociation of the monovalently bound (DCT)2-cys. Further, they show that opening of a bond in a dimer ring for the F(ab')2 fragment occurs with approximately the same dissociation rate constant as opening a bond in a linear cross-link. This rate constant is about three times smaller than that observed with IgE, suggesting that steric strain is caused by apposed Fc segments in cyclic IgE dimers. Such structural interference may affect the functional consequences of IgE-Fc epsilon RI aggregation on the cell surface.

Fluorescence resonance energy transfer reveals interleukin (IL)-1-dependent aggregation of IL-1 type I receptors that correlates with receptor activation

Fluorescence resonance energy transfer (FRET) was used to investigate whether interleukin-1 (IL-1) causes the aggregation of IL-1 type I receptors (IL-1 RI) at the cell surface. For these experiments, a noncompetitive anti-IL1 RI monoclonal antibody, M5, was labeled separately with a donor probe, fluorescein isothiocyanate, or with an acceptor carbocyanine probe, Cy3. Donor-labeled M5 and acceptor-labeled M5 were simultaneously bound to transfected mouse IL-1 RI on either C-127 mouse mammary carcinoma cells or on Chinese hamster ovary (CHO)-K1 cells, and the ratio of acceptor emission at 590 nm to donor emission at 525 nm (excitation at 488 and 514 nm) was monitored with flow cytometry as an indicator of FRET. Addition of a saturating concentration of human IL-1 alpha at 22 degrees C causes a time-dependent increase in FRET for both cell lines that indicates IL-1-dependent self-association of IL-1 RI. Binding of the IL-1 receptor antagonist at 22 degrees C causes little or no FRET for both cell lines, indicating a correlation between receptor aggregation and the ability of the ligand to stimulate a functional response. When donor-labeled and acceptor-labeled Fab fragments of M5 are used to monitor FRET, IL-1 alpha causes efficient energy transfer in the CHO-K1 cells at 22 degrees C, but not at 4 degrees C. In contrast, IL-1 alpha causes much less FRET at 22 degrees C in C-127 cells when the M5 Fab fragments are used instead of the intact bivalent M5. In a striking parallel, IL-1 alpha-dependent activation of prostaglandin E2 production depends on the bivalent M5 antibody in the C-127 cells, but is independent of this monoclonal antibody in the CHO-K1 cells. These results provide a strong correlation between the ability of IL-1 to cause the aggregation of IL-1 RI and the stimulation of a functional response.

Simultaneous cross-linking by two nontriggering bivalent ligands causes synergistic signaling of IgE Fc epsilon RI complexes

We have used two bivalent ligands that bind IgE to study the relationship between the aggregation of receptors with high affinity for IgE (Fc epsilon RI) and the responses (receptor immobilization, Ca2+ influx, and degranulation) of rat basophilic leukemia (RBL-2H3) cells. One of these is a symmetric bivalent ligand, N,N'-bis[[epsilon-[(2,4-dinitrophenyl)amino]caproyl]-L-tyrosyl]-L- cystine ((DCT)2-cys), which binds specifically to the combining sites of a mAb anti-DNP IgE and efficiently cross-links cell surface IgE, but does not trigger significant degranulation or increases in intracellular Ca2+. Several lines of evidence, including lateral mobility measurements, indicate that this ligand preferentially forms stable cyclic complexes containing two (DCT)2-cys and two IgE. The second ligand is a mAb anti-IgE, B1E3, which causes lateral mobility changes consistent with dimerized IgE-Fc epsilon RI and also does not trigger increases in intracellular Ca2+ or degranulation. The two ligands together trigger robust responses. In the presence of B1E3, (DCT)2-cys causes immobilization of IgE-Fc epsilon RI in a broad concentration range; in a more narrow concentration range, it is a potent stimulant of changes in both degranulation and Ca2+. We have compared the dose-response curves for cellular activation to simulated IgE aggregation curves, i.e., curves that predict the equilibrium IgE aggregate size distribution as a function of the (DCT)2-cys concentration. Our results indicate that maximal cellular activation occurs at a much higher (DCT)2-cys concentration than maximal IgE aggregation. When IgE aggregation is maximal, almost all aggregated IgE is in cyclic dimers. Thus, cyclic dimers appear to be functionally ineffective, even after they have been cross-linked by B1E3. Aggregated IgE-Fc epsilon RI that is effective in stimulating a cellular response may have particular structural or dynamic properties that allow critical interactions for initiating the signaling cascade.

Simultaneous cross-linking by two nontriggering bivalent ligands causes synergistic signaling of IgE Fc epsilon RI complexes

We have used two bivalent ligands that bind IgE to study the relationship between the aggregation of receptors with high affinity for IgE (Fc epsilon RI) and the responses (receptor immobilization, Ca2+ influx, and degranulation) of rat basophilic leukemia (RBL-2H3) cells. One of these is a symmetric bivalent ligand, N,N'-bis[[epsilon-[(2,4-dinitrophenyl)amino]caproyl]-L-tyrosyl]-L- cystine ((DCT)2-cys), which binds specifically to the combining sites of a mAb anti-DNP IgE and efficiently cross-links cell surface IgE, but does not trigger significant degranulation or increases in intracellular Ca2+. Several lines of evidence, including lateral mobility measurements, indicate that this ligand preferentially forms stable cyclic complexes containing two (DCT)2-cys and two IgE. The second ligand is a mAb anti-IgE, B1E3, which causes lateral mobility changes consistent with dimerized IgE-Fc epsilon RI and also does not trigger increases in intracellular Ca2+ or degranulation. The two ligands together trigger robust responses. In the presence of B1E3, (DCT)2-cys causes immobilization of IgE-Fc epsilon RI in a broad concentration range; in a more narrow concentration range, it is a potent stimulant of changes in both degranulation and Ca2+. We have compared the dose-response curves for cellular activation to simulated IgE aggregation curves, i.e., curves that predict the equilibrium IgE aggregate size distribution as a function of the (DCT)2-cys concentration. Our results indicate that maximal cellular activation occurs at a much higher (DCT)2-cys concentration than maximal IgE aggregation. When IgE aggregation is maximal, almost all aggregated IgE is in cyclic dimers. Thus, cyclic dimers appear to be functionally ineffective, even after they have been cross-linked by B1E3. Aggregated IgE-Fc epsilon RI that is effective in stimulating a cellular response may have particular structural or dynamic properties that allow critical interactions for initiating the signaling cascade.

Fc epsilon RI-mediated recruitment of p53/56lyn to detergent-resistant membrane domains accompanies cellular signaling

Detergent-resistant plasma membrane structures, such as caveolae, have been implicated in signalling, transport, and vesicle trafficking functions. Using sucrose gradient ultracentrifugation, we have isolated low-density, Triton X-100-insoluble membrane domains from RBL-2H3 mucosal mast cells that contain several markers common to caveolae, including a src-family tyrosine kinase, p53/56lyn. Aggregation of Fc epsilon RI, the high-affinity IgE receptor, causes a significant increase in the amount of p53/56lyn associated with these low-density membrane domains. Under our standard conditions for lysis, IgE-Fc epsilon RI fractionates with the majority of the solubilized proteins, whereas aggregated receptor complexes are found at a higher density in the gradient. Stimulated translocation of p53/56lyn is accompanied by increased tyrosine phosphorylation of several proteins in the low-density membrane domains as well as enhanced in vitro tyrosine kinase activity toward these proteins and an exogenous substrate. With a lower detergent-to-cell ratio during lysis, significant Fc epsilon RI remains associated with these membrane domains, consistent with the ability to coimmunoprecipitate tyrosine kinase activity with Fc epsilon RI under similar lysis conditions [Pribluda, V. S., Pribluda, C. & Metzger, H. (1994) Proc. Natl. Acad. Sci. USA 91, 11246-11250]. These results indicate that specialized membrane domains may be directly involved in the coupling of receptor aggregation to the activation of signaling events.

Effects of subunit mutation on the rotational dynamics of Fc epsilon RI, the high affinity receptor for IgE, in transfected cells

Erythrosin-labeled immunoglobulin E (IgE) and time-resolved phosphorescence anisotropy were used to monitor the rotational dynamics of transfected wild-type (alpha beta gamma 2) and four mutant Fc epsilon RI receptors in the monomeric and dimeric state on P815 cells. Erythrosin-IgE bound to Fc epsilon RI on cells transfected with either beta or gamma subunits with truncated COOH-terminal cytoplasmic segments exhibit faster rotational motion than when bound to Fc epsilon RI on cells transfected with wild-type subunits. Deletion of the NH2-terminal cytoplasmic segment of the beta subunit or the COOH-terminal cytoplasmic segment of the alpha subunit does not cause any significant change in the anisotropy decay. Dimers of IgE-receptor complexes formed with anti-IgE monoclonal antibody B1E3 exhibit substantially slower anisotropy decays for all the receptor constructs used, including a receptor construct that only contains the ectodomain of the alpha subunit anchored to Chinese hamster ovary (CHO) cell membranes through a lipid tail. This loss of rotational motion of dimeric IgE-Fc epsilon RI complexes may be due to nonspecific entanglement or to specific interactions involving IgE or the extracellular portion of alpha. The results suggest that the beta and gamma subunits of the tetrameric alpha beta gamma 2 receptor participate in interactions with other membrane components even in the absence of receptor aggregation. The loss of such interactions may be related to the functional impairments previously determined for these mutants.

Alteration of lipid composition modulates Fc epsilon RI signaling in RBL-2H3 cells

We have used sonicated liposomes of phosphatidylcholine (PC), sphingomyelin (SM), or a mixture of cholesterol (chol) and PC to investigate the role of cellular lipid composition in Fc epsilon RI-mediated stimulation of RBL-2H3 cells. Overnight treatment with either PC or SM liposomes causes a substantial enhancement of antigen-stimulated degranulation and phospholipase A2 activity, whereas treatment with a PC/chol mixture results in partial inhibition of the antigen-stimulated response. The most consistent change in the cellular lipid composition that results from the PC and SM liposome treatments is an approximate 40% decrease in the chol/phospholipid (PL) ratio. The lipid treatments do not alter degranulation stimulated by AlF4- or by Ca2+ ionophore in the presence or absence of PMA, suggesting that lipid alteration affects a receptor-specific signaling process. The lipid treatments do not appear to alter antigen-stimulated tyrosine phosphorylation or Ca2+ mobilization. Possible involvement of protein kinase C (PKC) activation in the signal-enhancing effect of the PL treatments was investigated by using calphostin C and phorbol-12-myristol-13-acetate (PMA) to inhibit PKC activity and degranulation in RBL-2H3 cells. Both SM and PC treatment restore the antigen-mediated degranulation response that is inhibited by long-term treatment (> or = 16 h) with 100 nM PMA or short-term treatment (10 min) with 5 microM calphostin C. The results indicate that a decreased chol/PL ratio facilitates or enhances the receptor-mediated activation of a PKC-like pathway that plays an important role in Fc epsilon RI-stimulated degranulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Large-scale co-aggregation of fluorescent lipid probes with cell surface proteins

Large scale aggregation of fluorescein-labeled immunoglobulin E (IgE) receptor complexes on the surface of RBL cells results in the co-aggregation of a large fraction of the lipophilic fluorescent probe 3,3'-dihexadecylindocarbocyanine (diI) that labels the plasma membranes much more uniformly in the absence of receptor aggregation. Most of the diI molecules that are localized in patches of aggregated receptors have lost their lateral mobility as determined by fluorescence photobleaching recovery. The diI outside of patches is mobile, and its mobility is similar to that in control cells without receptor aggregates. It is unlikely that the co-aggregation of diI with IgE receptors is due to specific interactions between these components, as two other lipophilic probes of different structures are also observed to redistribute with aggregated IgE receptors, and aggregation of two other cell surface antigens also results in the coredistribution of diI at the RBL cell surface. Quantitative analysis of CCD images of labeled cells reveals some differences in the spatial distributions of co-aggregated diI and IgE receptors. The results indicate that cross-linking of specific cell surface antigens causes a substantial change in the organization of the plasma membrane by redistributing pre-existing membrane domains or causing their formation.

Disparate modulation of plasma membrane protein lateral mobility by various cell permeabilizing agents

The mobility of a cell surface protein on cells osmotically swollen by treatment with several different cell permeabilizing agents retains specific restraints despite detachment of the plasma membrane from the cortical cytoskeleton. Fluorescence photobleaching recovery experiments indicate that the lateral diffusion constants of immunoglobulin E (IgE)-receptor complexes on the surface of rat basophilic leukemia cells increase 2-5x following permeabilization with streptolysin O or digitonin, with little change in their mobile fractions. Swelling by hypo-osmotic treatment in water enhances lateral diffusion of IgE-receptor complexes and raises the mobile fractions to near 100%. In contrast, swelling by treatment with filipin arrests lateral diffusion, although rotational mobility remains unhindered. Lateral mobility of a fluorescent lipid analogue remains unchanged under these conditions. Crosslinking by anti-IgE antibodies redistributes the IgE-receptor complexes into large patches on untreated cells and on cells swollen by permeabilization with streptolysin O or digitonin, but not on cells swollen by treatment with filipin. The results indicate a diversity of effects of the various permeabilizing agents on the mobility of membrane proteins. In particular, treatment with filipin appears to reorganize the plasma membrane into a network of fluid domains on a scale smaller than the bleaching spot size used (approximately 1.5 microns).

Heterologous desensitization of the high affinity receptor for IgE (Fc epsilon R1) on RBL cells

To study the properties of Fc epsilon RI desensitization induced by aggregation of that receptor, RBL cells were sensitized with a mixture of two different IgE mAb to create two different populations of IgE-receptor complexes. Cross-linking of one receptor population containing anti-dinitrophenyl (DNP) IgE with a bivalent Ag, 1-DNP-amino-12-biotinamidododecane)2-avidin ((DNP)2-avidin), results in desensitization of a subsequent response, both of the same receptor population (homologous desensitization) and of the second receptor population containing anti-5-dimethylaminonaphthalene-1-sulfonyl (dansyl) IgE (heterologous desensitization). The extent of heterologous desensitization is dependent on several parameters, including the concentration of both the first and the second Ag and the densities of the respective IgE populations. Heterologous desensitization of the Ca2+ response is more sensitive to the concentration of the second stimulus (dansyl-BSA) than heterologous desensitization of the degranulation response. AlF4-, which activates GTP-binding proteins, can effectively replace (DNP)2-avidin as the initial stimulant and desensitizing agent. Other agents that mobilize intracellular Ca2+ including thrombin and a Ca2+ ionophore are less effective at replacing (DNP)2-avidin. Because prestimulation with Ag does not desensitize subsequent responses to AlF4- or Ca2+ ionophore, it appears that signal transduction via Fc epsilon RI is impaired at an early step. Addition of monovalent DNP hapten within approximately 10 min after cross-linking by (DNP)2-avidin completely prevents the desensitization of the subsequent Ca2+ or degranulation response to dansyl-BSA. After longer times of incubation with DNP Ag, the DNP hapten becomes increasingly less effective at preventing the desensitization of the dansyl-BSA response, even though ongoing signal transduction by the DNP Ag is halted. These results suggest a form of cellular memory for the desensitized state.

Heterologous desensitization of the high affinity receptor for IgE (Fc epsilon R1) on RBL cells

To study the properties of Fc epsilon RI desensitization induced by aggregation of that receptor, RBL cells were sensitized with a mixture of two different IgE mAb to create two different populations of IgE-receptor complexes. Cross-linking of one receptor population containing anti-dinitrophenyl (DNP) IgE with a bivalent Ag, 1-DNP-amino-12-biotinamidododecane)2-avidin ((DNP)2-avidin), results in desensitization of a subsequent response, both of the same receptor population (homologous desensitization) and of the second receptor population containing anti-5-dimethylaminonaphthalene-1-sulfonyl (dansyl) IgE (heterologous desensitization). The extent of heterologous desensitization is dependent on several parameters, including the concentration of both the first and the second Ag and the densities of the respective IgE populations. Heterologous desensitization of the Ca2+ response is more sensitive to the concentration of the second stimulus (dansyl-BSA) than heterologous desensitization of the degranulation response. AlF4-, which activates GTP-binding proteins, can effectively replace (DNP)2-avidin as the initial stimulant and desensitizing agent. Other agents that mobilize intracellular Ca2+ including thrombin and a Ca2+ ionophore are less effective at replacing (DNP)2-avidin. Because prestimulation with Ag does not desensitize subsequent responses to AlF4- or Ca2+ ionophore, it appears that signal transduction via Fc epsilon RI is impaired at an early step. Addition of monovalent DNP hapten within approximately 10 min after cross-linking by (DNP)2-avidin completely prevents the desensitization of the subsequent Ca2+ or degranulation response to dansyl-BSA. After longer times of incubation with DNP Ag, the DNP hapten becomes increasingly less effective at preventing the desensitization of the dansyl-BSA response, even though ongoing signal transduction by the DNP Ag is halted. These results suggest a form of cellular memory for the desensitized state.

Fc epsilon RI and the T cell receptor for antigen activate similar signalling pathways in T cell-RBL cell hybrids

In order to investigate the functional similarities of the high affinity receptor for IgE (Fc epsilon RI) and the T cell receptor for antigen, we have developed a high efficiency polyethylene glycol-mediated fusion method to make somatic hybrids between cells from a mast cell line (RBL-2H3) and cells from T lymphoma cell lines (Jurkat and HPB-ALL). Using flow cytometry to select for the heterologously fused cells, we demonstrated that aggregation of the T cell receptor results in the efficient secretion of [3H]5-hydroxytryptamine from RBL cell-derived granules. In addition, both receptors mediate Ca2+ mobilization in the hybrid cells that is insensitive to inhibition by the protein kinase C activator phorbol-12-myristoyl-13-acetate (PMA). In contrast, Ca2+ mobilization caused by aggregation of Fc epsilon RI in the parent RBL cells is completely inhibited by PMA. The results indicate that these two different receptors for foreign antigen can substitute for each other to trigger responses in the hybrid cells that are unique to each cell type. The methodology employed has general utility for studying signal transduction mediated by mammalian cell surface receptors.

Dynamic conformations compared for IgE and IgG1 in solution and bound to receptors

Dynamic conformations of two distinct immunoglobulin (Ig) isotypes, murine IgE and human IgG1, were examined with fluorescence resonance energy transfer measurements. The IgE mutant epsilon/C gamma 3* and the IgG1 mutant gamma/C gamma 3* each bind [5-(dimethylamino)naphthalen-1-yl]sulfonyl (DNS) in two identical antigen binding sites at the amino (N)-terminal ends of the Ig in the Fab segments. Eosin-DNS bound in these Fab sites served as the acceptor probe in these studies. Both Ig have a carboxy (C)-terminal domain (C gamma 3*) which contains genetically introduced cysteine residues. Modification of these cysteine sulfhydryls with fluorescein maleimide provided donor probes near the C-terminal ends of the Ig in the Fc segment. Energy transfer between the C-terminal and N-terminal ends was compared for these two Ig in solution and when they were found to their respective high-affinity receptors on plasma membranes: IgE-Fc epsilon RI on RBL cell membranes and IgG1-Fc gamma RI on U937 cell membranes. Previous energy-transfer measurements with these probes yielded an average end-to-end distance of 71 A for IgE in solution and 69 A for IgE bound to Fc epsilon RI, indicating that in both situations IgE is bent such that the axes of the Fab segments and the axis of the Fc segment do not form a planar Y-shape [Zheng, Shopes, Holowka, & Baird (1991) Biochemistry 30, 9125]. In the current study we found the average end-to-end distance for IgG1 in solution is 75 A and greater than or equal to 85 A for IgG1 bound to Fc gamma RI, suggesting an average bend conformation for IgG1 as well. The contributions of segmental flexibility to the average distances were assessed directly by measuring the efficiency of energy transfer as a function of variations in donor quantum yield caused by a collisional quencher and using these data to extract a Gaussian distribution of end-to-end distances. The distribution average (rho) and half-width (hw) were determined to be as follows: rho = 75 A, hw = 24 A for IgE in solution; rho = 71 A, hw = 12 A for IgE bound to Fc epsilon RI; and rho = 100 A, hw = 88 A for IgG in solution.(ABSTRACT TRUNCATED AT 400 WORDS)

Aggregation of IgE-receptor complexes on rat basophilic leukemia cells does not change the intrinsic affinity but can alter the kinetics of the ligand-IgE interaction

The aggregation of IgE anchored to high-affinity Fc epsilon receptors on rat basophilic leukemia (RBL) cells by multivalent antigens initiates transmembrane signaling and ultimately cellular degranulation. Previous studies have shown that the rate of dissociation of bivalent and multivalent DNP ligands from RBL cells sensitized with anti-DNP IgE decreases with increasing ligand incubation times. One mechanism proposed for this effect is that when IgE molecules are aggregated, a conformational change occurs that results in an increase in the intrinsic affinity of IgE for antigen. This possibility was tested by measuring the equilibrium constant for the binding of monovalent DNP-lysine to anti-DNP IgE under two conditions, where the cell-bound IgE is dispersed and where it has been aggregated into visible patches on the cell surface using anti-IgE and a secondary antibody. No difference in the equilibrium constant in these two cases was observed. We also measured the rate of dissociation of a monovalent ligand from cell surface IgE under these two conditions. Whereas the affinity for monovalent ligand is not altered by IgE aggregation, we observe that the rate of ligand dissociation from IgE in clusters is slower than the rate of ligand dissociation from unaggregated IgE. These results are discussed in terms of recent theoretical developments concerning effects of receptor density on ligand binding to cell surfaces.

Rotational motion of monomeric and dimeric immunoglobulin E-receptor complexes

Erythrosin 5'-thiosemicarbazide labeled immunoglobulin E (IgE) was used to monitor the rotational dynamics of monomeric and dimeric Fc epsilon RI receptors for IgE on rat basophilic leukemia (RBL) basophilic leukemia (RBL) cells using time-resolved phosphorescence anisotropy. Receptors were studied both on living RBL cells and on membrane vesicles derived from RBL cell plasma membrane. The un-cross-linked IgE-receptor complexes on cells and vesicles exhibit rotational correlation times that are consistent with those expected for freely rotating monomers, but a small fraction of these complexes on cells may be rotationally immobile. A comparison of the initial phosphorescence anisotropy values for erythrosin-labeled IgE-receptor complexes on cells and vesicles reveals a fast component of rotational motion that is greater on the vesicles and may be due to a site of segmental flexibility in the receptor itself. Dimers of IgE-receptor complexes formed with anti-IgE monoclonal antibodies appear to be largely immobile on cells, but they are mobile on vesicles with a 2-fold larger rotational correlation time than the monomeric complexes. The results suggest that dimeric IgE-receptor complexes undergo interactions with other membrane components on intact cells that do not occur on the membrane vesicles. The possible significance of these interactions to receptor function is discussed.

Conformations of IgE bound to its receptor Fc epsilon RI and in solution

Previous resonance energy transfer studies suggested that murine immunoglobulin E (IgE) is bent near the junction of its Fc and Fab segments when bound to its high-affinity receptor (Fc epsilon RI) on RBL cells. To examine further the conformations of IgE, both bound to this receptor and in solution, a mutant recombinant IgE (epsilon/C gamma 3*) was prepared that has a cysteine replacing a serine near the C-terminal ends of the heavy chain. The introduced cysteine residues provide a means for specific modification of IgE, and the sulfhydryl groups were selectively labeled with fluorescein-5-maleimide (FM-epsilon/C gamma 3*). This IgE also binds a 5-(dimethylamino)naphthalene-1-sulfonyl (DNS) group in the antigen-binding sites. Resonance energy transfer experiments carried out on receptor-bound FM-epsilon/C gamma 3* yielded a distance of 53 A between fluorescein near the C-terminal end of the Fc segment and amphipathic acceptor probes at the membrane surface. The average distance between this C-terminal fluorescein and acceptor eosin-DNS in the antigen-binding sites at the N-terminal ends of the Fab segments was found to be 69 A. These results combine with those from previous structural studies to provide an unprecedented detailed description of the bent geometry of IgE bound to its receptor on the membrane. Energy transfer measured for FM-epsilon/C gamma 3* in solution between fluorescein near the C-terminal end of the Fc segment and eosin-DNS at the N-terminal ends of the Fab segments indicates that the average distance between these probes is about 71 A.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of increased K+ permeability associated with the stimulation of receptors for immunoglobulin E on rat basophilic leukemia cells

Aggregation of immunoglobulin E-receptor complexes on the surface of rat basophilic leukemia cells stimulates an increase in plasma membrane K+ permeability that is monitored as an increase in the rate of efflux of preloaded 86Rb+. A major component of this stimulated 86Rb+ efflux appears to be due to a Ca(2+)-activated K+ channel because it is inhibited by quinidine in parallel with the inhibition of degranulation and membrane potential repolarization, it is blocked by 0.1 mM La3+, and it is dependent on external Ca2+. Depolarization of the plasma membrane by carbonyl cyanide 3-chlorophenylhydrazone inhibits stimulated Ca2+ influx and prevents antigen-induced 86Rb+ efflux, and increased external Ca2+ partially restores 86Rb+ efflux under these conditions. In addition, potentiation of antigen-stimulated Ca2+ influx by pretreatment with cholera toxin increases the initial rate of stimulated 86Rb+ efflux. Another component of antigen-stimulated K+ efflux appears to be mediated by a guanine nucleotide-binding protein because pretreatment of rat basophilic leukemia cells with pertussis toxin decreases the initial rate of antigen-stimulated 86Rb+ efflux to 40% of that for the untreated cells. Stimulated 86Rb+ efflux is also observed when ionomycin is used to increase cytoplasmic Ca2+ and to trigger membrane depolarization. The efflux stimulated by ionomycin is inhibited by quinidine but not by pertussis toxin pretreatment; thus, it appears to occur through the Ca(2+)-activated K+ efflux pathway. It is proposed that these K+ efflux pathways serve to sustain the Ca2+ influx that is necessary for receptor-mediated triggering of cellular degranulation.

Dissociation kinetics of bivalent ligand-immunoglobulin E aggregates in solution

We study the dissociation of preformed bivalent ligand-bivalent receptor aggregates in solution, where the ligand is a symmetric bivalent hapten with two identical 2,4-dinitrophenyl (DNP) groups and the receptor is a fluorescein-labeled monoclonal anti-DNP IgE. We promote dissociation in two ways: by the addition of high concentrations of a monovalent hapten that competes for IgE binding sites with the bivalent hapten and by the addition of high concentrations of unlabeled IgE that binds almost all ligand binding sites that dissociate from labeled IgE. We investigate both theoretically and experimentally the two types of dissociation and find them to be quite different. Theory predicts that their kinetics will depend differently on the fundamental rate constants that characterize binding and aggregation. Using monovalent ligand to promote dissociation, we find that the fraction of labeled IgE sites bound to bivalent ligand decays with a slow and fast component. The fast decay corresponds to the dissociation of a singly bound DNP hapten. The interpretation of the slow decay depends on the detailed way in which ligand-receptor aggregates break up. We show that one possible explanation of these data is that small stable rings form before the addition of monovalent ligand. Other possible explanations are also presented.

Bivalent ligand dissociation kinetics from receptor-bound immunoglobulin E: evidence for a time-dependent increase in ligand rebinding at the cell surface

The bivalent ligand N,N'-bis[[epsilon-[(2,4- dinitrophenyl)amino]caproyl]-L-tyrosyl]cystine [(DCT)2-Cys] binds and cross-links anti-dinitrophenyl (DNP) immunoglobulin E (IgE)-receptor complexes on the cell surface of rat basophilic leukemia cells. The rate of dissociation of this bound ligand was monitored by using a fluorescence method under two different conditions. In one case the monovalent ligand DCT was added in large excess to prevent the dissociating ligand from rebinding to unoccupied antibody combining sites. Under these conditions, dissociation of the bivalent ligand from IgE-sensitized cells proceeds to completion with kinetics that are well described by two rate constants that are independent of the time of preincubation of the bivalent ligand with the cells. In the second case, dissociation of (DCT)2-Cys from cell-bound anti-DNP IgE was monitored in the presence of a large excess of anti-DNP IgE in solution that acts as a sink to absorb the dissociated ligand. Under these conditions, the bivalent ligand becomes more resistant to dissociation as the preincubation time of the bivalent ligand with the cells is increased. An increasing fraction of the bound ligand does not dissociate on a measurable time scale in the presence of this sink. The results indicate that cell-associated IgE-receptor complexes undergo a time-dependent change that facilitates the reformation of the cross-linked state when one end of the ligand dissociates to break up the existing cross-link. The possible physical basis and functional implications of these results are discussed.

Recombinant human IgG1-murine IgE chimeric Ig. Construction, expression, and binding to human Fc gamma receptors

We have constructed a set of chimeric Ig by exchanging corresponding H chain C domains between human (hu) IgG1 and murine (m) IgE. We used this set of Ig to dissect the interaction of individual Ig domains with human Fc gamma receptors. Only one of the chimeras, epsilon/C gamma 2,3 (an mIgE with C epsilon 3 and C epsilon 4 replaced by C gamma 2 and C gamma 3 from huIgG1), binds tightly to the human Fc gamma RI on U937 cells. We found that epsilon/C gamma 2,3 has only twofold lower affinity for Fc gamma RI as compared to huIgG1. The gamma/C epsilon 4 (huIgG1 with C epsilon 4 replacing C gamma 3) binds weakly to Fc gamma RI. The other chimeric Ig, epsilon/C gamma 3, epsilon/C gamma 2, and gamma/C epsilon 3, as well as mIgE do not bind detectably to Fc gamma RI. From these data we conclude that the C gamma 2 domain is crucial for binding and contains the majority of the binding site for Fc gamma RI on IgG1. The C gamma 3 domain makes a smaller contribution to the binding, and the C gamma 1 domain and the hinge region have very little effect on the Fc gamma RI-IgG1 interaction. The chimeric epsilon/C gamma 2,3 and huIgG1 both mediate the formation of rosettes between K562 cells and antigen-sensitized E with similar concentration dependences. These results suggest similar ability to bind to Fc gamma RII. The other chimeric Ig do not cause rosettes in this assay system. Hence, both C gamma 2 and C gamma 3 seem to be required for binding to Fc gamma RII, but the C gamma 1-hinge region has no detectable effect.

Recombinant human IgG1-murine IgE chimeric Ig. Construction, expression, and binding to human Fc gamma receptors

We have constructed a set of chimeric Ig by exchanging corresponding H chain C domains between human (hu) IgG1 and murine (m) IgE. We used this set of Ig to dissect the interaction of individual Ig domains with human Fc gamma receptors. Only one of the chimeras, epsilon/C gamma 2,3 (an mIgE with C epsilon 3 and C epsilon 4 replaced by C gamma 2 and C gamma 3 from huIgG1), binds tightly to the human Fc gamma RI on U937 cells. We found that epsilon/C gamma 2,3 has only twofold lower affinity for Fc gamma RI as compared to huIgG1. The gamma/C epsilon 4 (huIgG1 with C epsilon 4 replacing C gamma 3) binds weakly to Fc gamma RI. The other chimeric Ig, epsilon/C gamma 3, epsilon/C gamma 2, and gamma/C epsilon 3, as well as mIgE do not bind detectably to Fc gamma RI. From these data we conclude that the C gamma 2 domain is crucial for binding and contains the majority of the binding site for Fc gamma RI on IgG1. The C gamma 3 domain makes a smaller contribution to the binding, and the C gamma 1 domain and the hinge region have very little effect on the Fc gamma RI-IgG1 interaction. The chimeric epsilon/C gamma 2,3 and huIgG1 both mediate the formation of rosettes between K562 cells and antigen-sensitized E with similar concentration dependences. These results suggest similar ability to bind to Fc gamma RII. The other chimeric Ig do not cause rosettes in this assay system. Hence, both C gamma 2 and C gamma 3 seem to be required for binding to Fc gamma RII, but the C gamma 1-hinge region has no detectable effect.

Mapping the site of interaction between murine IgE and its high affinity receptor with chimeric Ig

We have investigated the interaction of mouse (m) IgE with its Fc epsilon RI on rat basophilic leukemia cells using a set of chimeric Ig that were constructed by exchanging homologous H chain C domains between human (hu) IgG1 and mIgE. Binding affinities were examined with equilibrium and kinetic measurements, and we found that epsilon/C gamma 3 (mIgE with C epsilon 4 replaced by C gamma 3) was indistinguishable from mIgE. The huIgG1 and the other chimeric Ig, which did not contain both C epsilon 2 and C epsilon 3, did not bind detectably to rat basophilic leukemia cells (Ka less than 10(6) M-1). The ability of these chimeric Ig to stimulate a cellular response (degranulation) in the presence of multivalent Ag was also tested. The epsilon/C gamma 3 was indistinguishable from mIgE in eliciting a high level of degranulation, whereas the other chimeric Ig stimulated no response even when they were preaggregated to enhance their binding avidity. These results demonstrate that C epsilon 4 may be replaced by C gamma 3 without affecting the binding and cell activating properties of mIgE. The lack of binding by the other chimeric Ig indicates that both C epsilon 2 and C epsilon 3 are necessary for the binding interaction.

Mapping the site of interaction between murine IgE and its high affinity receptor with chimeric Ig

We have investigated the interaction of mouse (m) IgE with its Fc epsilon RI on rat basophilic leukemia cells using a set of chimeric Ig that were constructed by exchanging homologous H chain C domains between human (hu) IgG1 and mIgE. Binding affinities were examined with equilibrium and kinetic measurements, and we found that epsilon/C gamma 3 (mIgE with C epsilon 4 replaced by C gamma 3) was indistinguishable from mIgE. The huIgG1 and the other chimeric Ig, which did not contain both C epsilon 2 and C epsilon 3, did not bind detectably to rat basophilic leukemia cells (Ka less than 10(6) M-1). The ability of these chimeric Ig to stimulate a cellular response (degranulation) in the presence of multivalent Ag was also tested. The epsilon/C gamma 3 was indistinguishable from mIgE in eliciting a high level of degranulation, whereas the other chimeric Ig stimulated no response even when they were preaggregated to enhance their binding avidity. These results demonstrate that C epsilon 4 may be replaced by C gamma 3 without affecting the binding and cell activating properties of mIgE. The lack of binding by the other chimeric Ig indicates that both C epsilon 2 and C epsilon 3 are necessary for the binding interaction.