Association of the receptor for immunoglobulin E with an endogenous polypeptide on rat basophilic leukemia cells

Mast Cell and Basophil Cell Lines: A Compendium

Mast cells and basophils play a crucial role during type I hypersensitivity reactions. However, despite efforts to elucidate their role in the pathogenesis of allergy and inflammation, our understanding of MC and basophil biology is still relatively scarce. The practical difficulty in obtaining a sufficient number of purified primary cells from biological samples has slowed down the process of reaching a full understanding of the physiological role of these functionally similar cell types. The establishment of several immortalized cell lines has been a useful tool to establish and perform sophisticated laboratory protocols that are impractical using primary cells. Continuous cell lines have been extensively used to investigate allergen/IgE-mediated cell activation, to elucidate the degranulation dynamics, to investigate structural and functional properties of the high-affinity receptor (FcεRI), and to test cell-stabilizing compounds. In this chapter, we review the most widely used and better-characterized MC and basophil cell lines, highlighting their advantages and drawbacks. It must be pointed out, however, that while cell lines represent a useful in vitro tool due to their easy manipulability and reduced culture costs, they often show aberrant characteristics which are not fully representative of primary cell physiology; results obtained with such cells therefore must be interpreted with due care.

From IgE to Omalizumab

IgE is the least abundant Ig isotype, yet it plays a critical role in allergic reactions and host protection from helminth infection. Although IgE was discovered 50 years ago, the ultimate evidence for its role in human allergic diseases was obtained by the efficacy of anti-IgE therapy in many clinical trials on asthma and other allergic diseases. Beginning from the discovery of IgE 50 y ago, followed by studies of IgE receptors and activation mechanisms, this review provides a historic perspective of allergy research that has led to the development of anti-IgE therapy and other strategies targeting IgE and its receptors. Current IgE studies toward future precision medicine are also reviewed.

Fc Receptors and Fc Receptor-Like Molecules within the Immunoreceptor Family

Receptors for the Fc portion of immunoglobulins (FcRs) account for most cell-mediated biological activities of antibodies. The majority of FcRs are encoded by a set of genes, clustered in the fcr locus, on chromosome 1 in humans and on chromosome 1 and 3 in mice. Eight (in humans) and six (in mice) new genes were found, intermixed with FcR genes in corresponding fcr loci, which encode FcR-like molecules (FcRLs). FcRs and FcRLs are genetically, phylogenetically, structurally, and functionally related. FcRs and FcRLs, however, markedly differ by their ligands, their tissue distribution, and, therefore, by the biological functions they control. A systematic comparison of their biological properties leads to the conclusion that FcRLs are not like FcRs. They altogether form a single family within the immunoreceptor family, whose members fulfill distinct but complementary roles in immunity by differentially controlling innate and adaptive responses.

Molecular versatility of antibodies

As immunology developed into a discrete discipline, the principal experimental efforts were directed towards uncovering the molecular basis of the specificity exhibited by antibodies and the mechanism by which antigens induced their production. Less attention was given to how antibodies carry out some of their effector functions, although this subject presents an interesting protein-chemical and evolutionary problem; that is, how does a family of proteins that can bind a virtually infinite variety of ligands, many of which the species producing that protein has never encountered, reproducibly initiate an appropriate response? The experimental data persuasively suggested that aggregation of the antibody was a necessary and likely sufficient initiating event, but this only begged the question: how does aggregation induce a response? I used the IgE:mast cell system as a paradigm to investigate this subject. Data from our own group and from many others led to a molecular model that appears to explain how a cell 'senses' that antigen has reacted with the IgE. The model is directly applicable to one of the fundamental questions cited above, i.e. the mechanism by which antigens induce the production of antibodies. Although the model is conceptually simple, incorporating the actual molecular events into a quantitatively accurate scheme represents an enormous challenge.

Eureka! And other pleasures

At first one is very pleased at being invited to write a Prefatory Chapter, but as the delivery deadline draws closer one begins to think, "Oh my God! What on earth can I say that all but family members and few close friends will not find a great bore?" One solution is to write a scientific essay, but I concluded that that was a cop-out. I decided that perhaps the best tack to follow was to try to convey to the reader the personal characteristics I bring to my science and to other aspects of my professional career. The writing of this chapter has certainly convinced me that my particular background influenced what problems I chose to work on and how I approached their solution, but I hope that my results have a more ecumenical significance. There's been much written recently about how one's cultural background affects one's science, but I think that thesis can also be exaggerated. Science is a method of inquiry that by using certain guidelines permits rational individuals to observe Nature in a way that their findings will agree and have permanence. We shouldn't be diffident about defending that claim of objectivity.

Characterization of the detergent insolubility of the T cell receptor for antigen

Aggregation of cell surface receptors plays an important role in signal transduction in many receptor systems. In the T cell receptor (TCR), as in many other cell surface receptors, this aggregation results in insolubility in certain nonionic detergents. We have characterized this insolubility for TCR, and we show it is not preexisting in HPB-ALL cells but increases with increasing TCR aggregation. It is not likely to be due to a direct interaction with cellular cytoskeletal elements, as it is not affected by inhibitors of actin or tubulin polymerization. It may be due to interaction with detergent-resistant membrane domains that have been found in various cell types and contain tyrosine kinases, the earliest known participants in TCR signal transduction. This aggregation-dependent insolubility occurs as rapidly as the anti-TCR antibody binds, so the kinetics are consistent with an involvement in signal transduction. It is not, however, dependent on signal transduction, as inhibitors of tyrosine kinases do not inhibit the insolubility. Insolubility is also enhanced by preaggregation of CD4, an important T cell surface molecule which also associates with the tyrosine kinase p56lck.

Engagement of the high-affinity IgE receptor activates src protein-related tyrosine kinases

The high-affinity IgE receptor (Fc epsilon RI), which is expressed on the surface of mast cells and basophils, has a central role in immediate allergic responses. In the rat basophilic leukaemia cell line RBL-2H3, which is a model system for the analysis of Fc epsilon RI-mediated signal transduction, surface engagement of Fc epsilon RI induces histamine release and the tyrosine phosphorylation of several distinct proteins. Although the alpha, beta, and gamma subunits of Fc epsilon RI lack intrinsic tyrosine protein kinase (TPK) activity, a kinase that copurifies with Fc epsilon RI phosphorylates the beta and gamma subunits of the receptor on tyrosine residues. We report here that in RBL-2H3 cells, p56lyn and pp60c-src are activated after Fc epsilon RI crosslinking, and p56lyn coimmunoprecipitates with Fc epsilon RI. In the mouse mast-cell line PT-18, another cell type used to study FC epsilon RI-mediated signalling, tyrosine phosphorylation of proteins is also an immediate consequence of receptor crosslinking. Notably, the only detectable src protein-related TPK in PT-18 cells is p62c-yes, and it is this TPK that is activated on Fc epsilon RI engagement and coimmunoprecipitates with the receptor. Therefore, it seems that different src protein-related TPKs can associate with the same receptor and become activated after receptor engagement.

Antigen-dependent transition of IgE to a detergent-insoluble form is associated with reduced IgE receptor-dependent secretion from RBL-2H3 mast cells

In mast cells, basophils, and the RBL-2H3 tumor mast cell model, crosslinking cell surface IgE-receptor complexes by multivalent ligands activates a signal transduction pathway that leads to the secretion of histamine, serotonin, and other inflammatory mediators. Receptor crosslinking in RBL-2H3 cells also changes cell surface morphology and increases F-actin assembly. Previously, Robertson et al. demonstrated that crosslinked IgE-receptor complexes become associated with the Triton X-100-insoluble fraction (the "cytoskeleton") of RBL-2H3 cells and raised the possibility that receptor-cytoskeletal association may be a required step in the stimulation of secretion. The studies reported here confirm by flow cytometry that crosslinking cell surface IgE by antigen induces the association of the crosslinked complexes with the detergent-insoluble fraction. Dose-response studies, also reported here, indicate that the detergent insolubility of the complexes does not correlate with secretion. Thus, secretion increases with antigen concentration to a maximum beyond which more antigen causes less, not more, secretion. There is little residual detergent-insoluble IgE at the concentrations of antigen that promote optimal secretion, whereas the association of IgE with the detergent-insoluble fraction is maximal at the high antigen concentrations that result in reduced secretion. The addition of monovalent hapten to reduce the amount of crosslinking caused by high concentrations of antigen increases secretion and simultaneously reduces the association of IgE with the detergent-insoluble fraction. Dihydrocytochalasin B, an inhibitor of antigen-stimulated actin polymerization, also increases the rate and extent of secretion and simultaneously delays the association of crosslinked IgE-receptor complexes with the detergent-insoluble fraction. From these data, we propose that the association of crosslinked IgE receptors with the detergent-insoluble fraction of RBL-2H3 cells increases with increased receptor crosslinking, is enhanced by antigen-induced actin polymerization, and is more likely related to the termination than the stimulation of secretion. The ligand-induced conversion of receptors to a detergent-insoluble form is not restricted to mast cells but occurs in a variety of cell types. Its general function may be to limit the generation or transmission of transmembrane signals.

Early expression of high-affinity receptor for immunoglobulin E (Fc epsilon RI) during differentiation of mouse mast cells and human basophils

Immediate hypersensitivity is due to the release of mediators from mast cells and basophils after the crosslinking of Fc epsilon RI. The appearance of such receptors was examined during differentiation of human and mouse bone marrow cells cultured in the presence of IL-3. As already reported, mouse bone marrow yield cultures of greater than 95% mast cells by 3 wk, whereas human bone marrow develop into cultures comprising 25% basophils by 3 wk. Here we show that transcripts for Fc epsilon RI subunits and membrane-associated receptors are apparent by 1 wk in both human and murine IL-3-dependent bone marrow cells. These cells contain few, if any, granules. The expression of transcripts and the number of receptor-positive cells continue to increase over 3 wk of culture. In parallel, a progressively larger number of cells become increasingly granulated to finally resemble either basophils or mast cells. Mature peripheral human basophils also contain transcripts for Fc epsilon RI and, therefore, may have the potential to synthesize de novo receptors. The early appearance of Fc epsilon FI during cell differentiation may be important for these cells to respond to IgE-mediated stimuli before granulation. The physiologic role of Fc epsilon RI could be to mediate lymphokine production (IL-3, IL-4, IL-6, and granulocyte/macrophage colony-stimulating factor) without inducing cellular degranulation.

Isolation and characterization of IgE receptors from rat intestinal mucosal mast cells

High-(Fc epsilon RI) and low-(Fc epsilon RII) affinity IgE receptors were isolated from surface radioiodinated, Nonidet-P40-solubilized rat intestinal mucosal mast cells (IMMC) and compared with those on rat peritoneal mast cells (PMC) and rat basophilic leukemia (RBL) cells. Fc epsilon RII were isolated by affinity chromatography using IgE-Sepharose or by anti-Fc epsilon RII antisera and protein A-Sepharose. The surface-exposed, IgE-binding alpha subunits of Fc epsilon RI [Fc epsilon RI alpha] were isolated by affinity chromatography using IgE and anti-IgE-Sepharose. Fc epsilon RI alpha on IMMC had an apparent molecular mass of 59 kDa, somewhat larger than that of PMC (51 kDa), RBL-2H3 cells (51 kDa) or RBL-CA10.7 cells (46 kDa). Brief (45 s) incubation of IMMC or PMC in glycine-HCl, pH 3, prior to iodination removed much of the surface-bound IgE. This permitted more thorough labeling of the receptors, but had no affect on the estimate of receptor size. Surprisingly and in contrast to acid-treated PMC, upon anti-IgE-Sepharose isolation acid-treated IMMC yielded an intensely radioactive Fc epsilon RI alpha band in the absence of added IgE. Such a finding suggests that IMMC, more so than PMC, may have an intracellular store of IgE, as has been suggested by many others. IMMC also differed from PMC in the number of forms of Fc epsilon RII isolated; 50-kDa and 58-kDa forms of Fc epsilon RII were obtained from IMMC, whereas PMC yielded most often a single 56-kDa Fc epsilon RII band. These results were mimicked by the two RBL cell sublines: RBL-2H3 cells yielded two Fc epsilon RII (46 kDa and 55 kDa), but only one form of Fc epsilon RII (54-kDa) was obtained from RBL-CA10.7 cells. Thus, the two subtypes of rat mast cells, which have previously been shown to differ in mediator profile and responsiveness to secretagogues and antiallergic drugs, are also distinguished by differences in IgER profile.

Isolation and purification of an Fc epsilon receptor activated ion channel from the rat mast cell line RBL-2H3

Derivatives of the antiallergic drug cromolyn [disodium 5,5'-[(2-hydroxy-1,3-propanediyl)-bis(oxy)]bis [4-oxo-(4H-1-benzopyran)-2- carboxylate]], which can be conjugated covalently at the propane 2-position to macromolecules and to insoluble matrices, were synthesized. Conjugates of these derivatives with macromolecules were examined for their binding to cells of the rat basophilic leukemia line RBL-2H3, which is widely employed as a model for immunologically induced mast cell degranulation. Only those drug-protein conjugates in which the cromolyn analogue with an amino group at the propane 2-carbon instead of the hydroxyl was linked to the carrier by glutaraldehyde were found to exhibit specific and saturable binding to these cells. Analysis of the binding data for these conjugates yielded an apparent binding constant of 3.8 +/- 0.2 X 10(8) M-1 and an apparent number of binding sites for the probe of 4000-8000 per cell. The conjugates found to bind specifically to the cells were also immobilized on agarose matrices and employed in an affinity-based isolation of the membrane component responsible for the observed binding. A single labeled polypeptide was eluted from these columns, onto which either whole cell lysates or solubilized purified plasma membranes of surface-radioiodinated RBL-2H3 cells had been adsorbed. This membrane protein appears on autoradiograms of nonreducing SDS-PAGE as a single broad band of approximately 110,000 daltons (Da) apparent molecular mass. On autoradiograms of reducing gels, the only band detected has an apparent mass of approximately 50,000 Da and appears narrower. Elution of the columns with the drug and disulfide-reducing agents or with the latter alone resulted in significantly higher yields of the 50-kDa polypeptide. Both the intact and reduced proteins bind strongly to immobilized concanavalin A and less so to immobilized wheat germ agglutinin, suggesting that the isolated intact protein is probably a dimer of two glycosylated subunits of similar molecular mass. Treatment of the reduced protein with endoglycosidase F leads to a decrease in its apparent molecular mass by approximately 12 kDa, suggesting that the extent of glycosylation of this polypeptide is approximately 25%. As shown in the following paper, the intact protein constitutes a Ca2+ channel that is activated upon IgE-Fc epsilon receptor aggregation.

Cross-linking of IgE-receptor complexes by rigid bivalent antigens greater than 200 A in length triggers cellular degranulation

We have examined the effect of cross-linking IgE-receptor complexes with variable receptor-receptor distances on the transmembrane signaling that leads to degranulation of rat basophilic leukemia cells. Linear polymers of the biotin-binding protein avidin were generated with bis biotin-1,12-diamidododecane, and a dinitrophenyl-biotin conjugate was bound at each end of the polymers to form a series of rigid bivalent haptens of well-defined length. The polymers were fractionated by size with nondenaturing PAGE, electro-eluted, and tested for their ability to stimulate degranulation of rat basophilic leukemia cells sensitized with anti-DNP IgE. We found that hexamers of avidin (of length greater than or equal to 240 A) were as effective in triggering degranulation as dimers (of length approximately 80 A), while the monomeric avidin antigen (of length approximately 40 A) elicited a poorer degranulation response from the cells. The mechanism by which aggregation of cell surface receptors can initiate signal transduction is discussed in light of these results.

Studies with a monoclonal antibody to the beta subunit of the receptor with high affinity for immunoglobulin E

The receptor with high affinity for IgE consists of a tetrameric complex of polypeptides, one of which (alpha), contains the binding site for IgE. The function of the other chains--a single beta and two disulfide-linked gamma chains--is unknown. We report the cloning of a murine hybridoma that secretes an IgG1 antibody which specifically reacts with the beta subunit. Studies with this monoclonal antibody show that the subunit stoichiometry of the receptor is unaffected by the presence or absence of bound IgE. We also found that under certain conditions where the alpha beta gamma 2 complex dissociates, beta remains attached to the dimer of gamma chains, indicating that these chains contact each other in the native receptor. In rat basophilic leukemia cells--a neoplastic line of mucosal-type mast cells--all of the beta subunits expressed by the cells appeared to be associated with the high affinity receptor. However, in at least one cell line which has no high affinity receptors--a putative rat lymphoma line--beta or beta-like polypeptides were also expressed.

The interaction of IgG subclasses with solubilized Fc receptors of rat basophilic leukemia cells

Rat basophilic leukemia (RBL) cells carry two surface glycoprotein molecules named R (or alpha) and H which, when detergent solubilized, bind to rat IgE-Sepharose. The same two molecules also bind to rat IgG-Sepharose but with a lower affinity. R is a component of the high affinity Fc receptor for IgE. In the present study the inhibition of the binding of R and H to rat IgG-Sepharose by various homologous and heterologous immunoglobulins was used to assess their relative affinities for the two receptor molecules. Ranking the rat immunoglobulins in order of their affinities for the R receptor yielded: IgE much greater than IgG2a greater than IgG1 greater than IgG2b; and for H: IgE greater than IgG2b greater than IgG1 greater than IgG2a. Rat IgG2c inhibited the binding of both R and H but a precise ranking could not be assigned. Conclusive evidence has been obtained for the Fc specificity of these interactions. The affinities of the mouse IgG subclass/R interactions can be ranked: IgG1 greater than IgG2a greater than IgG2b; and for the H receptor: IgG1 greater than IgG2b greater than IgG2a. All of the mouse proteins and other heterologous IgGs, such as those of sheep, goat, equine and rabbit origin, interacted considerably more strongly with H than with R. No interaction with mouse IgG3 could be detected under the conditions tested.

An IgE-binding protein with a distinctive repetitive sequence and homology with an IgG receptor

Proteins that bind IgE play important roles in both the synthesis and function of IgE are therefore intimately involved in IgE-mediated human allergic disorders. This report describes the structure of an IgE-binding protein, as predicted from sequencing a cDNA cloned from rat basophilic leukemia cells. This protein contains two domains: the amino-terminal domain (140 amino acids) consists of a highly conserved repetitive amino acid sequence, Tyr-Pro-Gly-Pro/Gln-Ala/Thr-Pro/Ala-Pro-Gly-Ala, whereas the carboxyl-terminal domain (122 amino acids) shares significant sequence homology with a domain of lymphocyte/macrophage receptor for the Fc portion of IgG. Other proteins with this type of structure but with affinity for other immunoglobulin isotypes may exist and may represent a heretofore unidentified component of the immune system.

Glucocorticoid-regulated compartmentalization of cell surface-associated glycoproteins in rat hepatoma cells: evidence for an independent response that requires receptor function and de novo RNA synthesis

The role of glucocorticoid hormones in the compartmentalization of cell surface-associated mouse mammary tumor virus (MMTV) glycoproteins was examined in M1.54, a cloned line of MMTV-infected rat hepatoma tissue culture cells. The expression of cellular [2-3H]mannose-labeled and cell surface 125I-labeled MMTV glycoproteins was examined throughout a time course of exposure to dexamethasone, a synthetic glucocorticoid. Posttranslational localization of cell surface MMTV glycoproteins required 6 h of exposure to hormone and occurred approximately 4 h after their initial production in an intracellular fraction. This regulated localization to the cell surface correlated with glucocorticoid receptor occupancy and was inhibited by exposure to RU 38486, a powerful antagonist of glucocorticoid-mediated responses. Cell surface immunoprecipitation demonstrated that actinomycin D, an inhibitor of de novo RNA synthesis, prevented regulated expression of cell surface viral glycoproteins, suggesting that newly synthesized cellular components mediate this process. The localization of cell surface MMTV glycoproteins appeared normal in a transcriptional variant (CR1) that produces basal levels of MMTV RNA and glycoprotein precursors in the presence of dexamethasone. Thus, regulated compartmentalization of viral glycoproteins is not an obligate consequence of a critical precursor concentration. Taken together, our results suggest that posttranslational trafficking of cell surface-destined MMTV glycoproteins resulted from an independent glucocorticoid hormone response that required receptor function and de novo RNA synthesis.

Target antigens of malaria transmission blocking immunity exist as a stable membrane bound complex

Monoclonal antibodies (MoAbs) blocking transmission of malaria by blocking fertilization of female gametes by male gametes in the mosquito midgut immunoprecipitate three polypeptides (apparent non-reduced mol. wt of 230,000, 48,000, 45,000) from the surface of gametes and zygotes of Plasmodium gallinaceum and P. falciparum. Earlier studies have shown that the epitopes with which the MoAbs react are present only on the mol. wt 230,000 polypeptide in P. gallinaceum and the 48,000/45,000 mol. wt antigens in P. falciparum (Kumar 1985, Vermeulen et al. 1985). In gel permeation chromatography all three polypeptides were found to be co-eluted. Cross-linking reagents were used to cross-link proteins on the intact cells and in the soluble extracts. The cross-linked product immunoprecipitated by the MoAbs showed an approximate mol. wt of 290,000 to 300,000, suggesting a stoichiometry of 1:1 between the high and low mol. wt polypeptides. Evidence was also obtained for the existence of a subpopulation of the 48,000/45,000 mol. wt polypeptides which is not complexed with the 230,000 mol. wt polypeptide. These data indicate that the 230,000 polypeptide is physically associated with the 48,000 and 45,000 polypeptides; the high and low molecular weight polypeptides appear to exist in a stable membrane bound complex.

Glucocorticoid-regulated compartmentalization of cell surface-associated glycoproteins in rat hepatoma cells: evidence for an independent response that requires receptor function and de novo RNA synthesis

The role of glucocorticoid hormones in the compartmentalization of cell surface-associated mouse mammary tumor virus (MMTV) glycoproteins was examined in M1.54, a cloned line of MMTV-infected rat hepatoma tissue culture cells. The expression of cellular [2-3H]mannose-labeled and cell surface 125I-labeled MMTV glycoproteins was examined throughout a time course of exposure to dexamethasone, a synthetic glucocorticoid. Posttranslational localization of cell surface MMTV glycoproteins required 6 h of exposure to hormone and occurred approximately 4 h after their initial production in an intracellular fraction. This regulated localization to the cell surface correlated with glucocorticoid receptor occupancy and was inhibited by exposure to RU 38486, a powerful antagonist of glucocorticoid-mediated responses. Cell surface immunoprecipitation demonstrated that actinomycin D, an inhibitor of de novo RNA synthesis, prevented regulated expression of cell surface viral glycoproteins, suggesting that newly synthesized cellular components mediate this process. The localization of cell surface MMTV glycoproteins appeared normal in a transcriptional variant (CR1) that produces basal levels of MMTV RNA and glycoprotein precursors in the presence of dexamethasone. Thus, regulated compartmentalization of viral glycoproteins is not an obligate consequence of a critical precursor concentration. Taken together, our results suggest that posttranslational trafficking of cell surface-destined MMTV glycoproteins resulted from an independent glucocorticoid hormone response that required receptor function and de novo RNA synthesis.

Characterization of monoclonal antibodies produced by immunization with partially purified IgE receptor complexes

A series of monoclonal antibodies (mAb) were produced following the immunization of mice with partially purified IgE receptors from the rat basophilic leukemia cell line (RBL-2H3). Twelve hybridoma cell lines were selected that secreted monoclonal antibodies capable of binding RBL-2H3 plasma membranes. These antibodies were all of the IgG1, or IgG2a subclass. All 12 antibodies bound to either intact or glutaraldehyde-fixed RBL-2H3 cells. Only one monoclonal (mAb 2AC3) inhibited 125I-labeled IgE binding (IC50 = 65 micrograms/ml compared to 1.0 microgram/ml for unlabeled IgE). This same mAb weakly precipitated the alpha component of the receptor from 125I-surface-labeled cells and directly triggered histamine secretion when incubated with RBL-2H3 cells. Therefore, this hybridoma most likely represents a low affinity anti-receptor antibody. Among the other 11 monoclonals, two caused direct histamine secretion from RBL-2H3 cells. These same two, as well as four others, released greater than 10% of total cellular histamine when rabbit anti-mouse antibody was added to cross-link mAb bound to the cell surface. One monoclonal (mA 1AD3) did not trigger histamine secretion but did inhibit IgE-mediated histamine release when incubated with pre-sensitized RBL-2H3 cells. Except for mAb 2AC3, none of the other monoclonal antibodies that caused or inhibited histamine secretion immunoprecipitated receptor or other protein components from either 125I-surface-labeled or intrinsically-labeled cells. However, two monoclonal antibodies (mAb 1CC4 and 1CD1) immunoprecipitated 45,000 and 55,000 proteins from 125I-surface-labeled cells. One hybridoma (mAb 2AA2) that failed to immunoprecipitate surface-labeled proteins did precipitate a 20,000 band from intrinsically-labeled cells. This band increased slightly in apparent mol. wt after reduction and, therefore, was not the previously described gamma component of the receptor. Because several mAb were capable of modulating histamine secretion, it appeared that some of the present monoclonal antibodies bound to undefined membrane components that are crucial to the secretory process of rat basophilic leukemia cells.

Membrane and cytoskeletal changes associated with IgE-mediated serotonin release from rat basophilic leukemia cells

Binding of antigen to IgE-receptor complexes on the surface of RBL-2H3 rat basophilic leukemia cells is the first event leading to the release of cellular serotonin, histamine, and other mediators of allergic, asthmatic, and inflammatory responses. We have used dinitrophenol-conjugated bovine serum albumin (DNP-BSA) as well as the fluorescent antigen, DNP-B-phycoerythrin, and the electron-dense antigen, DNP-BSA-gold, to investigate dynamic membrane and cytoskeletal events associated with the release of [3H]serotonin from anti-DNP-IgE-primed RBL-2H3 cells. These multivalent antigens bind rapidly to cell surface IgE-receptor complexes. Their distribution is initially uniform, but within 2 min DNP-BSA-gold is found in coated pits and is subsequently internalized. Antigen internalization occurs in the presence and absence of extracellular Ca2+. The F-actin content of the detergent-extracted cell matrices analyzed by SDS PAGE decreases during the first 10-30 s of antigen binding and then increases by 1 min to almost double the control levels. A rapid and sustained increase is also observed when total F-actin is quantified by flow cytometry after binding of rhodamine-phalloidin. The antigen-stimulated increase in F-actin coincides with (and may cause) the transformation of the cell surface from a finely microvillous to a highly folded or plicated topography. Other early membrane responses include increased cell spreading and a 2-3-fold increase in the uptake of fluorescein-dextran by fluid pinocytosis. The surface and F-actin changes show the same dependence on DNP-protein concentration as stimulated [3H]serotonin release; and both the membrane responses and the release of mediators are terminated by the addition of the non-cross-linking monovalent ligand, DNP-lysine. These data indicate that the same antigen-stimulated transduction pathway controls both the membrane/cytoskeletal and secretory events. However, the membrane and actin responses to IgE-receptor cross-linking are independent of extracellular Ca2+ and are mimicked by phorbol myristate acetate, whereas ligand-dependent mediator release depends on extracellular Ca2+ and is mimicked by the Ca2+ ionophore A23187.

Coordinate synthesis and degradation of the alpha-, beta- and gamma-subunits of the receptor for immunoglobulin E

The surface receptor for immunoglobulin E (IgE) on rat basophilic leukemia cells and their normal counterparts has been postulated to consist of four polypeptide chains: a 45-kDa alpha-chain which binds IgE, a 33-kDa beta-component and two disulfide-linked, 9-10-kDa gamma-polypeptides. The instability of this complex in mild detergents makes it possible that, in vivo also, the structure may not be stable and that there is an independent assembly or exchange of the chains. We studied this question using surface-labeling and biosynthetic labeling techniques and found that the chains turn over coordinately and do not independently exchange. The results provide further support for the proposal that the alpha beta gamma 2 complex is the unit receptor for IgE.

Identification of an IgE-binding protein by molecular cloning

The synthesis and function of IgE are dependent on IgE-binding proteins, which include cell surface IgE receptors and IgE-binding lymphokines. To further our understanding of the IgE system, we have engaged in the molecular cloning of genes for some of these proteins. In studying the in vitro translation products of mRNA from rat basophilic leukemia (RBL) cells, we have identified a Mr 31,000 polypeptide that binds IgE and is also reactive with antibodies to proteins affinity-purified from RBL cells with IgE immunoadsorbent. For the molecular cloning, double-stranded cDNA was synthesized from sucrose gradient-fractionated RBL mRNA, inserted into plasmid pBR322, and used to transform Escherichia coli. By screening transformants with a hybridization-selection/in vitro translation procedure, we identified one clone containing cDNA that hybridized to mRNA coding for a Mr 31,000 IgE-binding protein. The DNA sequence of this cloned cDNA (571 base pairs) was determined and the amino acid sequence corresponding to part of the protein was deduced. In RNA blot analysis, the cDNA hybridized with a mRNA of 1100 nucleotides found in RBL cells but absent in cells not expressing IgE receptors. This cloned cDNA most likely codes for the Mr 31,000 IgE-binding protein identified in RBL cells, which appears to be related to the IgE-binding phenotype of the cells and which may have a significant role in the IgE-mediated activation of basophils and mast cells.

Affinity labeling of the Fc receptor on human monocytes using bifunctional cross-linking agents

To affinity label the Fc receptor on human monocytes, Fc fragments of monoclonal human IgG1 radiolabeled with iodine 125 were covalently bound to the surface of intact monocytes using a variety of bifunctional cross-linking agents including ethylene glycol bis(succinimidyl succinate), dithio-bis-(succinimidyl proprionate), maleimidobenzoyl N-hydroxysuccinimide, glutaraldehyde and dimethyl suberimidate. After cross-linking, cells were solubilized and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by radioautography. Each of these cross-linkers caused a portion of cell-bound Fc fragments to form a covalent complex with a monocyte membrane component. This complex migrated on electrophoresis with an apparent molecular weight of 120,000. Deducting the molecular weight of Fc fragments alone (53,000) the molecular weight of the second component of the complex therefore was about 67,000. A similar estimate of receptor size also was obtained after reduction with dithiothreitol. Complex formation was potently inhibited by unlabeled Fc fragments, IgG1 or IgG3, all of which would be expected to compete with Fc fragments for IgG Fc receptor on human monocytes, but was not inhibited by Fab fragments, IgG2 or IgG4, which do not bind avidly to this receptor. By quantitating the amount of complex formed in the presence of varying concentrations of labeled ligand, it could be demonstrated that complex formation was saturable, and that Fc fragments formed complexes with avidity comparable to that with which Fc fragments bound to receptors on intact monocytes. The findings establish the feasibility of using radiolabeled Fc fragments to affinity label the IgG Fc receptors on human leukocytes. Potential advantages of this approach to studying receptor structure are discussed.

Structure and modulation of Fc and complement receptors

Recent experiments have revealed the structure of some phagocytosis-promoting receptors. The C3b receptor is a single-chain membrane glycoprotein of Mr 205,000, while the C3bi receptor is composed of two surface glycoprotein chains, of Mr 180,000 and 100,000. Fc receptors all appear to be single-chain glycoproteins of approximately Mr 50,000. Despite this structural similarity, Fc receptors display a broad range of heterogeneity with respect to ligand specificity. One type of Fc receptor (Fc gamma 2b/gamma 1R) appears to function as a ligand-dependent ion channel; the ion flux initiated by the ligation of this receptor may represent the proximal signal sent by this Fc receptor. The second signal sent by other Fc receptors and by the C3 receptors is uncharacterized, except for the observation that the second signal generated by C3 receptors is distinct from that of Fc gamma 2b/gamma 1R.

Two classes of mutant mammary tumor virus-infected HTC cell with defects in glucocorticoid-regulated gene expression

We have isolated mutant derivatives of M1.54 (a mammary tumor virus [MTV]-infected rat hepatoma [HTC] cell line containing multiple integrated proviruses) that fail to express hormone-inducible cell surface viral glycoproteins. In wild-type M1.54, the synthetic glucocorticoid dexamethasone selectively stimulates the rate of synthesis of MTV RNA. In addition, dexamethasone is essential for posttranslational maturation of three of the four cell surface viral glycoproteins processed from the MTV glycosylated precursor polyprotein; the fourth mature species is produced constitutively. Two mutant phenotypes are described; each contains glucocorticoid receptors that are indistinguishable from the wild-type receptor with respect to hormone affinity, intracellular concentration, nuclear translocation efficiency, DNA-cellulose chromatography, and sedimentation rate. In one class, represented by the mutant line CR1, dexamethasone fails to stimulate the low basal rate of MTV gene transcription; surprisingly, hormonal regulation of tyrosine aminotransferase activity is also defective in CR1, whereas several other cellular responses to dexamethasone are normal. In the second class of mutants, represented by CR4, dexamethasone stimulates synthesis of MTV transcripts indistinguishable from those produced in M1.54, but only the constitutive cell surface viral glycoprotein is expressed. Thus, these mutants define two distinct and novel aspects of glucocorticoid regulated gene expression in HTC cells: CR4 contains a defect in a hormone inducible protein maturation pathway that acts on specific viral (and presumably cellular) precursor polypeptides, whereas the lesion in CR1 appears to affect the expression of a subset of the gene products normally under glucocorticoid control in M1.54.

Selective phosphorylation of the IgE receptor in antigen-stimulated rat mast cells

Purified rat serosal mast cells were sensitized with mouse immunoglobulin E (IgE) anti-2,4-dinitrophenyl antibody, partially depleted of phosphate, labeled with [32P]orthophosphate, and stimulated with dinitrophenylated bovine serum albumin or control protein. After 15-120 seconds at 37 degrees C, the cells were extracted with nonionic detergent. IgE receptors were purified by repetitive affinity chromatography and were analyzed by NaDodSO4/polyacrylamide gel electrophoresis and radioautography. Antigenic stimulation of intact rat mast cells produced a rapid and marked increase in the phosphorylation of the surface-exposed alpha component of the IgE receptor. However, phosphorylation of the 33,000 Mr beta component of the IgE receptor was not altered significantly by antigen stimulation. This suggests that the selective increase in phosphorylation of the IgE receptor alpha component may be part of the physiologic mediator secretion process triggered by antigen.

Two classes of mutant mammary tumor virus-infected HTC cell with defects in glucocorticoid-regulated gene expression

We have isolated mutant derivatives of M1.54 (a mammary tumor virus [MTV]-infected rat hepatoma [HTC] cell line containing multiple integrated proviruses) that fail to express hormone-inducible cell surface viral glycoproteins. In wild-type M1.54, the synthetic glucocorticoid dexamethasone selectively stimulates the rate of synthesis of MTV RNA. In addition, dexamethasone is essential for posttranslational maturation of three of the four cell surface viral glycoproteins processed from the MTV glycosylated precursor polyprotein; the fourth mature species is produced constitutively. Two mutant phenotypes are described; each contains glucocorticoid receptors that are indistinguishable from the wild-type receptor with respect to hormone affinity, intracellular concentration, nuclear translocation efficiency, DNA-cellulose chromatography, and sedimentation rate. In one class, represented by the mutant line CR1, dexamethasone fails to stimulate the low basal rate of MTV gene transcription; surprisingly, hormonal regulation of tyrosine aminotransferase activity is also defective in CR1, whereas several other cellular responses to dexamethasone are normal. In the second class of mutants, represented by CR4, dexamethasone stimulates synthesis of MTV transcripts indistinguishable from those produced in M1.54, but only the constitutive cell surface viral glycoprotein is expressed. Thus, these mutants define two distinct and novel aspects of glucocorticoid regulated gene expression in HTC cells: CR4 contains a defect in a hormone inducible protein maturation pathway that acts on specific viral (and presumably cellular) precursor polypeptides, whereas the lesion in CR1 appears to affect the expression of a subset of the gene products normally under glucocorticoid control in M1.54.

Enhancement of the recognition by cytotoxic T lymphocytes (CTL) of target membrane antigens after fusion with whole cells

Plasma membrane vesicles (R4-PM) prepared from mouse lymphoma cells (RDM4,H2k) were employed to investigate requirements for recognition of target cell membranes by allogeneic cytotoxic T lymphocytes (CTL). Using immunofluorescent staining and fluorescence microscopy, the R4-PM were tested for binding to CTL and were found to bind to these effector cells in a specific manner. However, this binding was very inefficient compared to the binding of whole RDM4 cells to CTL. The R4-PM were then attached to P388D1 cells (H-2d) in the presence of wheat germ agglutinin and polyethylene glycol (PEG), both under conditions which promote membrane fusion (40% PEG) and under conditions which do not (10% PEG). About 1 cell equivalent R4-PM becomes associated per P388D1 cell in both situations. In the cytotoxicity assays that were carried out, the P388D1 cells which had R4-PM attached under fusion conditions were lysed by CTL directed against H2k in a specific manner, while the P388D1 cells which had R4-PM attached under nonfusion conditions were not lysed above background levels by these CTL. These results suggest that recognition of target cells by allogeneic CTL such that lysis occurs requires more than presentation of the alloantigens as they are expressed in plasma membrane vesicles. However, fusion of these vesicles back into living cells apparently enhances the ability of the alloantigens to be recognized.

Gel filtration in 6 M guanidine hydrochloride of the alpha-subunit (and its fragments) of the receptor for immunoglobulin E

The mol. wts of the alpha-chain of the receptor for immunoglobulin E and several of its enzyme-cleaved fragments have been evaluated by gel filtration on Sepharose 6B in 6 M guanidine HCl. The mol. wt of alpha-chains treated with endoglycosidase was 30% less than that of untreated alpha-chains. alpha-Chains digested with papain eluted in a single peak with a mol. wt approximately one-half of that of undigested alpha-chains. The results support the proposal that papain cleaves alpha-chains into two fragments of similar size [Goetze et al. (1981) Biochemistry 20, 6341-6349].

Proteolysis of soluble IgE-receptor complexes: localization of sites on IgE which interact with the Fc receptor

Mouse and rat IgE and the respective soluble IgE-receptor complexes purified from rat basophilic leukemia cells were digested with trypsin. The end product in each case was F(ab')2-like. It contained the Ce2 regions, had intact antigen combining sites but had lost all ability to bind to the cell receptor for IgE. With mouse IgE the two principal sites of cleavage are likely to be the interdomain regions between Ce4:Ce3 and Ce3:C32 respectively. Cleavage at these sites occurs sequentially with the rate constant for the cleavage at the second site being approximately four-fold greater than that for the initial cleavage. When IgE is bound to the receptor the rates of cleavage are inhibited approximately three-fold. With rat IgE, the principal initial cleavage occurs within the intrachain disulfide loop in the Ce3 domain. Even when this disulfide bond in the digested protein is reduced, the product retains a substantial binding activity. A second cleavage occurs at a similar rate as the first and at a site analogous to that seen with mouse IgE, i.e. between the Ce3 and Ce2 domains. Notably, when bound to the receptor, the rate of cleavage at the first site is inhibited approximately three-fold but at the second site more than or equal to 40-fold. These results strongly implicate thd Ce3 domain as the principal site of interaction between rodent IgE and its receptor.

Further characterization of the beta-component of the receptor for immunoglobulin E

A 30,000 mol. wt component (beta) is associated in a 1:1 ratio with the 50,000 mol. wt glycoprotein (alpha) which binds immunoglobulin E (IgE) on mast cells and related tumor cells. We show that alpha and beta are associated in membrane preparations. This is consistent with previous results which showed labeling of beta with the hydrophobic probe 5-iodonaphthyl-1-azide (INA). The beta-polypeptide is susceptible to proteolytic cleavage during preparation of the membranes and when this occurs a 20,000 fragment can be labeled with INA and remains associated with alpha. No incorporation of carbohydrate precursors into beta was observed. Since beta is also not modified when cells are surface-labeled, it may not be exposed on the cell surface. Rigorous washing of IgE-receptor complexes with non-ionic detergent results in dissociation of beta from the alpha-IgE complex. The latter will then not reassociate with beta when exposed to crude detergent extracts of the tumor cells.

Unexpected findings from target analysis of immunoglobulin E and its receptor

The membrane receptor for immunoglobulin E (IgE) and its ligand, IgE, were irradiated with high-energy electrons. Loss of binding activity was measured for each, and the size of the functional targets was assessed. In both cases, the target size was substantially smaller than the covalent structure of the molecule. The direction of this discrepancy is unprecedented on the basis of experience with loss of enzymatic activity by irradiation; indeed, two enzymes which were present in the receptor preparations gave expected values when measured simultaneously. We suggest that in instances where a function such as ligand binding resides in a conformationally stable domain, radiation inactivation may be capable of revealing this.