Negative regulation of FcepsilonRI signaling by FcgammaRII costimulation in human blood basophils

Dominating IgE-binding epitope of Bet v 1, the major allergen of birch pollen, characterized by X-ray crystallography and site-directed mutagenesis

Specific allergy vaccination is an efficient treatment for allergic disease; however, the development of safer vaccines would enable a more general use of the treatment. Determination of molecular structures of allergens and allergen-Ab complexes facilitates epitope mapping and enables a rational approach to the engineering of allergen molecules with reduced IgE binding. In this study, we describe the identification and modification of a human IgE-binding epitope based on the crystal structure of Bet v 1 in complex with the BV16 Fab' fragment. The epitope occupies approximately 10% of the molecular surface area of Bet v 1 and is clearly conformational. A synthetic peptide representing a sequential motif in the epitope (11 of 16 residues) did not inhibit the binding of mAb BV16 to Bet v 1, illustrating limitations in the use of peptides for B cell epitope characterization. The single amino acid substitution, Glu(45)-Ser, was introduced in the epitope and completely abolished the binding of mAb BV16 to the Bet v 1 mutant within a concentration range 1000-fold higher than wild type. The mutant also showed up to 50% reduction in the binding of human polyclonal IgE, demonstrating that glutamic acid 45 is a critical amino acid also in a major human IgE-binding epitope. By solving the three-dimensional crystal structure of the Bet v 1 Glu(45)-Ser mutant, it was shown that the change in immunochemical activity is directly related to the Glu(45)-Ser substitution and not to long-range structural alterations or collapse of the Bet v 1 mutant tertiary structure.

FcepsilonRI-FcgammaRII coaggregation inhibits IL-16 production from human Langerhans-like dendritic cells

Langerhans-like dendritic cells (LLDC) express the high-affinity IgE receptor FcepsilonRI form that lacks the beta-chain, and may play an important role in allergic inflammation via production of IL-16. Secretion of mediators by human mast cells and basophils is mediated through FcepsilonRI and is decreased by coaggregating these receptors to the low-affinity IgG receptor, FcgammaRII. We used a recently described human Ig fusion protein (GE2), which is composed of key portions of the human gamma1 and the human epsilon heavy chains, to investigate its ability to inhibit IL-16 production from FcepsilonRI-positive Langerhans-like dendritic cells through coaggregation of FcgammaRII and FcepsilonRI. Unstimulated LLDC-derived from CD14-positive monocytes from atopic donors were shown to express FcgammaRII, an ITIM-containing receptor, but not FcepsilonRI or FcgammaRIII which are activating (ITAM) receptors. When passively sensitized with antigen-specific, human IgE and then challenged with antigen, LLDC were stimulated to produce IL-16. However, when FcepsilonRI and FcgammaRII were coaggregated with GE2, IL-16 production was significantly inhibited. Exposure of LLDCs to GE2 alone did not induce IL-16 production. Our results further extend our studies demonstrating the ability of GE2 to inhibit FcepsilonRI-mediated responses through coaggregation with FcgammaRIIB and at the same time show that human LDCC can be modulated in a fashion similar to mast cells and basophils.

Two regions of down-regulation in the IgE-mediated signaling pathway in human basophils

Previous studies demonstrated that after stimulation of human basophils with a polyclonal anti-IgE Ab, early signaling elements showed sustained phosphorylation, whereas later elements were transient, suggesting that a region of down-regulation involved inhibition of phosphatidylinositol (PI) 3 kinase or its products. However, the current studies show that under some conditions, syk phosphorylation is transient. Generally, stimulation with a variety of Ags makes this early form of down-regulation more apparent. An exploration of the conditions needed to induce early down-regulation indicates that both the nature of aggregation and the cell surface density of IgE play roles. It was also found that the previously described late form of down-regulation (PI3 kinase product transience) can occur in cells displaying early down-regulation (transient syk phosphorylation), but this phenomenon is revealed by testing for subsequent down-regulation of the response to non-cross-reacting stimuli, altering their ability to induce phosphorylation of Akt or extracellular signal-regulated kinase. In contrast, phosphorylation of syk kinase, in response to a non-cross-reacting stimulus, was relatively unaffected by prior stimulation. The magnitude of cross-desensitization of the Akt or extracellular signal-regulated kinase response was a function of the strength of the first stimulus. Mediator release showed a similar cross-desensitization effect. Therefore, stimulation induces two forms of down-regulation, one operating before or at the level of syk phosphorylation, possibly characterizing the process formerly known as specific desensitization, and one that operates in the region of PI3 kinase, accounting for the process formerly known as nonspecific desensitization, which is dependent on the strength of stimulus.

Environmental polycyclic aromatic hydrocarbons, benzo(a) pyrene (BaP) and BaP-quinones, enhance IgE-mediated histamine release and IL-4 production in human basophils

Polycyclic aromatic hydrocarbons (PAHs) are major components of diesel exhaust particles found in pollutant respirable particles. There is growing evidence that these fossil fuel combustion products exacerbate allergic inflammation. Basophils contribute to allergic inflammation through the release of preformed and granule-derived mediators. To determine whether allergens and PAHs interact, we incubated human basophils with PAHs and measured the release of histamine and IL-4 with and without added antigen. None of the PAHs induced mediator release by itself and none affected total cellular histamine levels. However, several PAHs enhanced histamine release and IL-4 production in response to crosslinking the high-affinity IgE receptor, Fc epsilon RI. The enhancement seen with 1,6-BaP-quinone involved an increase in tyrosine phosphorylation in several different substrates, including the Fc epsilon RI-associated tyrosine kinase, Lyn, and elevated reactive oxygen species (ROS) levels detected by dichlorofluorescein fluorescence and flow cytometry. The PAH-induced enhancement of mediator release and ROS production could be inhibited with the antioxidant N-acetylcysteine. These data provide further evidence that environmental pollutants can influence allergic inflammation through enhanced Fc epsilon RI-coupled mediator release from human basophils.

4. IgE, mast cells, basophils, and eosinophils

IgE, mast cells, basophils, and eosinophils constitute essential elements in allergic inflammation. Allergen-specific IgE, synthesized in response to allergens in the environment and in susceptible individuals, becomes fixed to high-affinity receptors on cellular membranes, especially of mast cells and basophils. If these receptor-bound IgE molecules are aggregated on reexposure to specific allergen, these mast cells and basophils produce mediators that result in the allergic response. Principal among the cells drawn to sites of mediator release is the eosinophil.

Association of the Src homology 2 domain-containing inositol 5' phosphatase (SHIP) to releasability in human basophils

During the study of the biology of the Human recombinant Histamine Releasing Factor (HrHRF), we uncovered a hyperreleasable phenotype of basophils from HrHRF-responder donors. Basophils from these donors released histamne to HrHRF, IL-3 and D(2)O. While there has been a significant amount of work elucidating signal transduction events in human basophils, the reason for this hyperreleasable phenotype remained illusive. A clue to the releasability of these highly allergic, asthmatic HrHRF-responder donor basophils was demonstrated in studies using SHIP knockout mice. Bone marrow-derived mast cells from the SHIP knockout mice demonstrated hyperreleasability to stimuli through the IgE receptor and alteration of subsequent signal transduction events. We have demonstrated a highly significant negative correlation between the amount of SHIP protein per cell equivalent and maximum histamine release to HrHRF. These results provide a clue to the hyperreleasable phenotype and implicate SHIP as an additional regulator of secretion in human basophils.

Regulation of human basophil adhesion to endothelium under flow conditions: Different very late antigen 4 regulation on umbilical cord blood-derived and peripheral blood basophils

BACKGROUND

Although soluble mediators released by basophils in tissue sites contribute to the chronic injury that occurs in hypersensitivity diseases, only limited information is available about how circulating basophils are recruited to tissues. In particular, the interaction of basophils with endothelium under conditions that mimic physiologic flow has not been explored.

OBJECTIVE

We sought to identify adhesion molecules regulating the attachment of human basophils to IL-4-activated human umbilical vein endothelial cells (HUVECs) under flow conditions.

METHODS

A parallel-plate flow chamber and blocking mAbs were used to define the adhesion molecules involved in the interactions of peripheral blood basophils (PBBs) and cord blood-derived basophils (CBDBs) with IL-4-activated HUVECs and with Chinese hamster ovary (CHO) cell transfectants expressing specific adhesion molecules. A fluorescent ligand specific for very late antigen 4 (VLA-4) was used to directly examine the VLA-4 affinity state of basophils.

RESULTS

Flowing PBBs and CBDBs attached to activated HUVECs and to CHO cells expressing P- or E-selectin. However, only CBDBs attached to vascular cell adhesion molecule 1 (VCAM-1)-transfected CHO cells under flow conditions. The attachment of CBDBs to CHO cells was blocked by mAbs directed against E-selectin, P-selectin, and VCAM-1, whereas attachment of PBBs was blocked by E-selectin and P-selectin mAbs. Activating VLA-4 with Mn(2+) on PBBs resulted in adhesion to the VCAM-1-transfected CHO cells, indicating that VLA-4 activity on PBBs can be regulated, at least in part, through affinity changes. The Mn(2+)-induced upregulation of basophil VLA-4 affinity was demonstrated directly by using a fluorescent ligand for VLA-4 and flow cytometry.

CONCLUSIONS

The interaction of human CBDBs and PBBs with endothelium under flow conditions is mediated in part by both P- and E-selectin. VLA-4 additionally contributes to the adhesion of flowing CBDBs. However, the affinity of VLA-4 is too low to support the adhesion under flow conditions of unstimulated PBBs.

A novel human immunoglobulin Fc gamma Fc epsilon bifunctional fusion protein inhibits Fc epsilon RI-mediated degranulation

Human mast cells and basophils that express the high-affinity immunoglobulin E (IgE) receptor, Fc epsilon receptor 1 (Fc epsilon RI), have key roles in allergic diseases. Fc epsilon RI cross-linking stimulates the release of allergic mediators. Mast cells and basophils co-express Fc gamma RIIb, a low affinity receptor containing an immunoreceptor tyrosine-based inhibitory motif and whose co-aggregation with Fc epsilon RI can block Fc epsilon RI-mediated reactivity. Here we designed, expressed and tested the human basophil and mast-cell inhibitory function of a novel chimeric fusion protein, whose structure is gamma Hinge-CH gamma 2-CH gamma 3-15aa linker-CH epsilon 2-CH epsilon 3-CH epsilon 4. This Fc gamma Fc epsilon fusion protein was expressed as the predicted 140-kappa D dimer that reacted with anti-human epsilon- and gamma-chain specific antibodies. Fc gamma Fc epsilon bound to both human Fc epsilon RI and Fc gamma RII. It also showed dose- and time-dependent inhibition of antigen-driven IgE-mediated histamine release from fresh human basophils sensitized with IgE directed against NIP (4-hydroxy-3-iodo-5-nitrophenylacetyl). This was associated with altered Syk signaling. The fusion protein also showed increased inhibition of human anti-NP (4-hydroxy-3-nitrophenylacetyl) and anti-dansyl IgE-mediated passive cutaneous anaphylaxis in transgenic mice expressing human Fc epsilon RI alpha. Our results show that this chimeric protein is able to form complexes with both Fc epsilon RI and Fc gamma RII, and inhibit mast-cell and basophil function. This approach, using a Fc gamma Fc epsilon fusion protein to co-aggregate Fc epsilon RI with a receptor containing an immunoreceptor tyrosine-based inhibition motif, has therapeutic potential in IgE- and Fc epsilon RI-mediated diseases.

Localizing a control region in the pathway to leukotriene C(4) secretion following stimulation of human basophils with anti-IgE antibody

Mediator release from human basophils is a self-limited process, but down-regulation of the signaling cascades leading to secretion of leukotriene C(4) (LTC(4)) is controlled independently of the pathway leading to IL-4 secretion. In the current studies, we have explored the regulation of upstream signaling events leading to activation of extracellular signal-related kinases (ERKs; previously shown to be required for LTC(4) generation) in human basophils. IgE-, but not FMLP-mediated activation, induced sustained tyrosine phosphorylation of syk, of shc, and an association of shc to the Grb2/son of sevenless 2 complex. In contrast, IgE-mediated activation resulted in transient activation of p21(ras) and mitogen-activated protein/ERK kinase 1, which were kinetically associated with phosphorylation of ERKs. The canonical Shc/Grb2/son of sevenless pathway to activation of p21(ras) is therefore sustained, while p21(ras) activity is not. We have previously shown that phosphatidylinositol 3 kinase activity is required for p21(ras) activity and, in the current studies, we show that of the p85-sensitive forms of p110 possible, basophils express only p110 delta and that there are no changes in association between p21(ras) and p110 delta in stimulated basophils. We used the generation of phospho-Akt as a marker of the presence of phosphatidylinositol-3,4,5-trisphosphate and found that phospho-Akt is transient on a time scale consistent with p21(ras) activity. On the basis of information obtained in these and other studies, we localize down-regulation of IgE-mediated LTC(4) secretion to a region of the signaling cascade antecedent to p21(ras) activation, downstream of phosphatidylinositol 3 kinase activity and probably involving regulation of phosphatidylinositol-3,4,5-trisphosphate levels.

IgE receptors

IgE receptors are implicated as important components of the immunological pathway in allergic and inflammatory diseases. Recent investigations have begun to unravel the structure, signal transduction and function of IgE receptors from different cell types in rodent and human systems. Studies of the mechanisms involved might provide opportunities for therapeutic intervention strategies in the treatment of allergic and hypersensitivity reactions.

Src homology 2 domain-containing inositol 5' phosphatase is negatively associated with histamine release to human recombinant histamine-releasing factor in human basophils

BACKGROUND

The human recombinant histamine-releasing factor (HrHRF) acts as a complete stimulus for histamine release and IL-4 secretion from a subpopulation of highly allergic donor basophils, termed IgE(+) basophils. Additionally, IgE(+) basophils release histamine to other secretogues, IL-3, and deuterium oxide. We hypothesized that IgE(+) basophils were hyperreleasable.

OBJECTIVE

Deficiencies in early signal transduction events associated with Fc(epsilon)RI lead to a nonreleasable phenotype, whereas the Src homology 2 domain--containing inositol 5' phosphatase (SHIP) knockout mice have hyperreleasable mast cells. The purpose of this study was to ascertain whether a difference in intracellular signaling molecules could explain the hyperreleasable phenotype of human IgE(+) basophils.

METHODS

Basophils were purified by means of double Percoll gradients and negative selection with magnetic beads. Cell lysates were Western blotted for the tyrosine kinases Lyn and Syk and the phosphatase SHIP. Additionally, histamine release to HrHRF was performed in addition to real-time RT-PCR to investigate mRNA for SHIP.

RESULTS

We show a striking negative correlation between the amount of SHIP protein per cell equivalent, but not Lyn or Syk, and maximum histamine release to HrHRF. This deficiency of SHIP was observed in basophils, but not lymphocytes or monocytes, of these IgE(+) donors. Additionally, levels of mRNA for SHIP did not differ between IgE(+) and IgE(-) donor basophils, which is consistent with a posttranscriptional mechanism of protein regulation. SHIP and phosphatidylinositol 3-kinase reciprocally regulate phosphatidylinositol (3,4,5) triphosphate levels. We also demonstrated that Ly294002, the phosphatidylinositol 3 kinase inhibitor, prevented HrHRF-induced histamine release in IgE(+) donor basophils.

CONCLUSION

Taken together, these data suggest that the hyperreleasability of IgE(+) donors is associated with low levels of SHIP and implicate SHIP as an additional regulator of secretion in human basophils.

Overcoming the signaling defect of Lyn-sequestering, signal-curtailing FcepsilonRI dimers: aggregated dimers can dissociate from Lyn and form signaling complexes with Syk

Clustering the tetrameric (alphabetagamma(2)) IgE receptor, FcepsilonRI, on basophils and mast cells activates the Src-family tyrosine kinase, Lyn, which phosphorylates FcepsilonRI beta and gamma subunit tyrosines, creating binding sites for the recruitment and activation of Syk. We reported previously that FcepsilonRI dimers formed by a particular anti-FcepsilonRI alpha mAb (H10) initiate signaling through Lyn activation and FcepsilonRI subunit phosphorylation, but cause only modest activation of Syk and little Ca(2+) mobilization and secretion. Curtailed signaling was linked to the formation of unusual, detergent-resistant complexes between Lyn and phosphorylated receptor subunits. Here, we show that H10-FcepsilonRI multimers, induced by adding F(ab')(2) of goat anti-mouse IgG to H10-treated cells, support strong Ca(2+) mobilization and secretion. Accompanying the recovery of signaling, H10-FcepsilonRI multimers do not form stable complexes with Lyn and do support the phosphorylation of Syk and phospholipase Cgamma2. Immunogold electron microscopy showed that H10-FcepsilonRI dimers colocalize preferentially with Lyn and are rarely within the osmiophilic "signaling domains" that accumulate FcepsilonRI and Syk in Ag-treated cells. In contrast, H10-FcepsilonRI multimers frequently colocalize with Syk within osmiophilic patches. In sucrose gradient centrifugation analyses of detergent-extracted cells, H10-treated cells show a more complete redistribution of FcepsilonRI beta from heavy (detergent-soluble) to light (Lyn-enriched, detergent-resistant) fractions than cells activated with FcepsilonRI multimers. We hypothesize that restraints imposed by the particular orientation of H10-FcepsilonRI dimers traps them in signal-initiating Lyn microdomains, and that converting the dimers to multimers permits receptors to dissociate from Lyn and redistribute to separate membrane domains that support Syk-dependent signal propagation.

Improvements to parallel plate flow chambers to reduce reagent and cellular requirements

BACKGROUND

The parallel plate flow chamber has become a mainstay for examination of leukocytes under physiologic flow conditions. Several design modifications have occurred over the years, yet a comparison of these different designs has not been performed. In addition, the reagent requirements of many designs prohibit the study of rare leukocyte populations and require large amounts of reagents.

RESULTS

In this study, we evaluate modifications to a newer parallel plate flow chamber design in comparison to the original parallel plate flow chamber described by Lawrence et al. We show that modifications in the chamber size, internal tubing diameters, injection valves, and a recirculation design may dramatically reduce the cellular and reagent requirements without altering measurements.

CONCLUSIONS

These modifications are simple and easily implemented so that study of rare leukocyte subsets using scarce or expensive reagents can occur.

Differences in functional consequences and signal transduction induced by IL-3, IL-5, and nerve growth factor in human basophils

Previous studies have indicated a redundancy in the effects of the cytokines, IL-3, IL-5, and nerve growth factor (NGF) on acute priming of human basophils. In the current study, we have examined the effects of these three cytokines on 18-h priming for leukotriene C4 generation, their ability to induce Fc(epsilon)RIbeta mRNA expression, or their ability to sustain basophil viability in culture. We also examine a variety of the signaling steps that accompany activation with these cytokines. In contrast with the ability of IL-3 to alter secretagogue-mediated cytosolic calcium responses following 18-h cultures, 18-h treatment with IL-5 or NGF did not affect C5a-induced leukotriene C4 generation or alter C5a-induced intracellular Ca2+ concentration elevations. IL-3 and IL-5, but not NGF, induced Fc(epsilon)RIbeta mRNA expression and all three improved basophil viability in culture with a ranking of IL-3 > IL-5 > or = NGF. All three cytokines acutely activated the extracellular signal-regulated kinase pathway and the signaling elements that preceded extracellular signal-regulated kinase and cytosolic phospholipase A2 phosphorylation, consistent with their redundant ability to acutely prime basophils. However, only IL-3 and IL-5 induced Janus kinase 2 and STAT5 phosphorylation. This pattern of signal element activation among the three cytokines most closely matched their ability to induce expression of Fc(epsilon)RIbeta mRNA. Induction of the sustained calcium signaling that follows overnight priming with IL-3 appeared to be related to the strength of the early signals activated by these cytokines but the relevant pathway required was not identified. None of the signaling patterns matched the ability of the cytokines to promote basophil survival.

Fc-signalling in the modulation of immune responses by passive antibody

Passive antibody can both suppress and augment immune responses. Until recently, there was virtual unanimity on the importance of the interaction of the Fc portion of modulating antibody with Fc-receptors (Fc-signalling), especially in experiments involving the suppression by antibody. Experiments reported in the last few years, that do not demonstrate the range of Fc-portion/Fc-receptor influences on the suppression of immune responses by passive antibody, have introduced new uncertainty into this field. The purpose of this paper is to review how the initial controversy on the influence of Fc-signalling in inhibition by passive antibody was resolved. Old and new approaches are suggested that may help in resolving the current uncertainty engendered by recent experimental results that were interpreted to mean that passive suppressive antibody does not utilize the inhibitory FcgammaRIIB receptor. An understanding of the factors that influence negative Fc-signalling is needed in order to optimize clinical therapies whose action depends on the suppressive property of antibody.

Activating and inhibitory signaling in mast cells: new opportunities for therapeutic intervention?

Immune responses are tightly controlled by the activities of both activating and inhibitory signals. At the cellular level, these signals are generated through engagement of membrane-associated receptors and coreceptors. The high-affinity IgE receptor FcepsilonRI is expressed on mast cells and basophils and, on cross-linking by multivalent antigen (allergen), stimulates the release of inflammatory mediators that induce acute allergic responses. Activation signals mediated by a variety of immune receptors (eg, B-cell receptor, T-cell receptor, and FcepsilonRI) are subject to negative regulation by a growing family of structurally and functionally related inhibitory receptors. Recent studies indicate that mast cells express multiple inhibitory receptors that may regulate FcepsilonRI-induced mast cell activation through similar mechanisms. The ability of inhibitory receptors to attenuate IgE-mediated allergic responses implicates them as potential targets for therapeutic intervention in the treatment of atopic disease. Indeed, coaggregation of activating and inhibitory receptors has been suggested as one possible mechanism to explain the beneficial effects of specific immunotherapy in the treatment of allergy. In this review we summarize the current knowledge of inhibitory receptors expressed in mast cells and the mechanisms through which they regulate mast cell function.