Coaggregation of FcεRI with FcγRIIB Inhibits Degranulation but Not Induction of Bcl-2 Family Members A1 and Bim in Mast Cells

The aggregation of high-affinity immunoglobulin E (IgE) receptors (FcεRI) on mast cells is a critical event in the initiation of an allergic reaction. Coengagement of FcεRI with immunoglobulin G (IgG) low-affinity receptor FcγRIIB/CD32 inhibits degranulation and the release of inflammatory mediators from mast cells and has therefore been proposed as a new therapeutic approach for the treatment of allergies. In this study, we investigated whether FcγRIIB, besides inhibiting degranulation, negatively regulates other signalling pathways downstream of FcεRI. For this, we determined the phosphorylation and/or expression of proteins involved in the regulation of mast-cell apoptosis. Coaggregation led to an attenuation of Akt phosphorylation but did not inhibit phosphorylation of transcription factor Foxo3a or its proapoptotic target, Bim. Similarly, FcεRI-dependent expression of the prosurvival gene A1 was not affected by coaggregation. Our data demonstrate that coengagement of FcεRI and FcγRIIB inhibits degranulation but not the signalling pathways regulating Bcl-2 family members Bim and A1.

Mast cells are critical effector cells mediating immunoglobulin E (IgE)-dependent allergic responses. Binding of an allergen to IgE, already bound to its high-affinity receptor Fc⑀RI on mast cells, leads to aggregation and subsequent activation. This initiates signalling events that typically result in degranulation, changes in gene expression, and the release of inflammatory mediators, contributing to acute and late-phase allergic responses. [1][2][3] Fc⑀RI consists of a tetrameric protein complex, the IgE-binding amplifying ␣ chain, a signalling ␤ chain, and two ␥ chains. 4 The ␤ and ␥ subunits of the Fc⑀RI each contain an immunoreceptor tyrosine-based activation motif (ITAM), which is phosphorylated upon Fc⑀RI aggregation and which is both necessary and sufficient for receptor-induced signal transduction. 5 Mast cells also express other Fc receptors, either constitutively or upon stimulation; among these, Fc␥RI (CD64), Fc␥RIIB (CD32), and Fc␥RIII (CD16) are receptors for immunoglobulin G (IgG). Fc␥RI (high-affinity IgG receptor) and Fc␥RIII (low-affinity IgG receptor) are activating receptors, both containing ITAM, that initiate signalling upon aggregation. 6,7 Fc␥RIIB is a low-affinity receptor containing an immunoreceptor tyrosine-based inhibitory motif (ITIM), 8 which negatively regulates the activating signal when coaggregated with activating receptors bearing an ITAM. 9 The coaggregation results in the recruitment of the inhibitory signalling molecule SHIP, leading to the abrogation of the ITAMinduced activation. 2,10,11 IgE-induced mast cell activation (ie, Fc⑀RI aggregation) is negatively regulated by coaggregation of Fc⑀RI with Fc␥RIIB. 9,12 The release of mediators and cytokines is inhibited in a process in which Fc⑀RI contributes to the ITIM-dependent inhibition of its own intracellular signalling. This is achieved by the Fc⑀RI-associated tyrosine kinase Lyn, which phosphorylates the Fc␥RIIB ITIM that recruits SHIP1, thus leading to Fc⑀RI signal abrogation. 11,13,14 The receptors interact with the Factin skeleton that enables Fc␥RIIB to recruit SHIP1, which is provided by filamin-1. Fc␥RIIB is believed to negatively regulate Fc⑀RI signalling in two ways: by facilitating the translocation of Fc⑀RI into the F-actin skeleton but also by concentrating SHIP1 at the site close to Fc⑀RI. 15 Investigations of the mechanism by which SHIP mediates its Fc␥RIIB inhibitory function have also suggested p62 dok as a possible mediator of Fc␥RIIB inhibition of Fc⑀RI signalling downstream of SHIP in mast cells. 16 Fc⑀RI-mediated degranulation and release of mediators are inhibited when Fc⑀RI is coaggregated with Fc␥RIIB. 12 In addition to elucidating the impact of coaggregation on mast-cell degranulation, this study has elucidated the effect on the activation of downstream signalling pathways involved in the regulation of mast-cell survival. The aggregation of Fc⑀RI induces rapid but transient phosphorylation of the signalling protein Akt and the forkhead transcription factor Foxo3a, known to regulate Bim expression at the transcriptional level. 17 Phosphorylated Akt phosphorylates and thereby inactivates Foxo3a, which in its unphosphorylated state is located in the nucleus and acts as a transcription factor for Bim. Bim is a proapoptotic protein of the Bcl-2 family, involved in the regulation of mast-cell apoptosis. 18,19 Another Bcl-2 family member of crucial importance for Fc⑀RI-mediated activationinduced mast-cell survival is A1. 20 Mast cells lacking A1 do not survive IgE receptor aggregation. 20 In this study, we investigated if Fc⑀RImediated activation/expression of Akt, Foxo3a, Bim, and A1 are inhibited when Fc⑀RI is coengaged with Fc␥RIIB. We report here that although mast-cell degranulation is inhibited and the phos-phorylation of Akt is attenuated by the coaggregation of Fc⑀RI with Fc␥RIIB, Foxo3a and Bim are still phosphorylated and up-regulated, respectively. We also demonstrate that the level of A1 messenger ribonucleic acid (mRNA) induced by Fc⑀RI is not significantly altered upon coaggregation with Fc␥RIIB. Altogether, this indicates that only certain signalling pathways are affected by the coaggregation of Fc⑀RI with Fc␥RIIB whereas others, closely related to cell survival, remain largely unaffected.

Mast-Cell Activation
Mast _cells to be used for ribonuclease (RNAse) protection assay and ␣-hexosaminidase release assay were resuspended in RPMI-1640 medium supplemented with 0.2% bovine serum albumin, 2 mM of L-glutamine, and 100 µg/mL of penicillin/streptomycin. The cells were sensitized for 90 minutes at 37°C by the addition of 0.1 µg/mL of monoclonal anti-dinitrophenyl (anti-DNP) clone SPE-7 IgE mouse antibody (anti-DNP IgE). After washing, the cells were activated by the addition of either 45 µg/mL of RAM IgG (coaggregation of Fc⑀RI with Fc␥RIIB) or 30 µg/mL of RAM F(ab´) 2 (aggregation of Fc⑀RI) at 37°C for the time periods indicated. Mast cells to be used for Western blot analysis were resuspended in the previously mentioned medium. The cells were sensitized for 90 minutes at 37°C by the addition of 0.1 µg/mL of the same IgE as previously mentioned or 0.1 µg/mL of the same IgE together with 5 µg/mL of 2.4G2 rat Ab. After being washed, the cells were activated by the addition of 10 µg/mL of TNP7-F(ab´) 2 mouse anti-rabbit (MAR) at 37°C, causing either coaggregation of Fc␥RIIB with Fc⑀RI or aggregation of Fc⑀RI, for the time periods indicated. The conjugated antibody, TNP7-MAR F(ab´) 2 , functions as a multivalent antigen recognized by the Fc⑀RI-bound IgE but also recognizing bound 2.4G2 rat Ab. 13 Aggregation with 2.4G2 rat Ab together with TNP7-MAR F(ab´) 2 does not cause degranulation, which indicates that expression of Fc␥RIII (an activating low-affinity receptor for IgG) on C57 cells does not interfere with our system (data not shown). In experiments in which the phosphorylation pattern of Akt and Foxo3a as well as the total amount of these two proteins were measured, the mast cells were starved for approximately 24 hours at 37°C in RPMI-1640 medium supplemented with 0.5% FBS before sensitization and activation. For Bim expression experiments, the mast cells were resuspended in RPMI-1640 medium supplemented with 10% filtered FBS, 2 mM of L-glutamine, 100 µg/mL of penicillin/streptomycin, and 50 µM of 2-mercaptoethanol during both sensitization and activation, which lasted for 24 hours.

N-Acetyl-␤-D-Hexosaminidase Release Assay
For detection of the granular enzyme ␤-hexosaminidase, an enzymatic colorimetric assay was used. 23 After 30 minutes of activation, 60 µL of supernatant were transferred to a 96-well plate and mixed with an equal volume of substrate solution (7.5 mM of p-nitrophenyl-N-acetyl-␤-D-glucosaminide dissolved in 80 mM of citric acid, pH 4.5). The mixture was incubated on a rocker platform for 2 hours at 37°C. After incubation, 120 µL of glycine (0.2 M, pH 10.7) was added to each well, and the absorbance was measured with an Emax Precision Microplate Reader (Molecular Devices, Sunnyvale, CA).

Western Blot Analysis
The cells were lysed in SDS sample buffer (125 mM of tris-hydrochloric acid [pH 6.8], 4% w/v SDS, 20% glycerol, 0.02% w/v bromphenol blue, and 50 mM of dithiothreitol, added just before use) or in cell lysis buffer (1ϫ cell lysis buffer, 1 mM of phenylmethylsulfonyl fluoride [PMSF]) before being sonicated on ice. The phosphorylation and/or the total amount of proteins of interest were studied by Western blot with a NuPAGE Bis-Tris Western gel (Invitrogen, Carlsbad, CA). After electrophoresis, the proteins were electrically transferred to a nitrocellulose membrane (Hybond ECL, Amersham Biosciences). All was performed according to the manufacturers´ instructions. The membrane was incubated overnight at 4°C with the primary antibody and thereafter incubated with horseradish peroxidase-conjugated secondary antibody for 1 hour at room temperature. The proteins were visualized with an enhanced chemiluminescence system (LumiGLO) and exposure to a Hybond ECL film.

RNAse Protection Assay
TriPure isolation reagent was used for isolation of total cellular ribonucleic acid (RNA). An RNAse protection assay (RPA) was performed (according to RiboQuant System protocol) with an mAPO-2 multiprobe set (PharMingen, San Diego, CA). The gel was dried and exposed on Kodak film (Eastman Kodak Company, Rochester, NY) with intensifying screens at -70°C. Expression of RNA was detected with a phosphoimager device, and levels of expression were quantified with MacBAS 2.2 software (Fuji Photo Film Co., Ltd., Tokyo, Japan).

Statistical Analysis
We used an analysis of variance, followed by multiple comparison with the Wilcoxon matchedpairs test.

Coaggregation of Fc⑀RI with Fc␥RIIB Inhibits IgE-Dependent Mast-Cell Degranulation
To analyze the effect of Fc␥RIIB-mediated inhibition of mast-cell activation, we used murine C57 mast cells known to express the receptors Fc⑀RI and Fc␥RIIB. C57 cells were sensitized with murine IgE and challenged with polyclonal RAM F(ab´) 2 to aggregate Fc⑀RI or with RAM IgG to coaggregate Fc⑀RI and Fc␥RIIB. The RAM F(ab´) 2 induced activation of mast cells, leading to degranulation as measured by ␤-hexosaminidase release ( Figure 1A). When Fc⑀RI was coaggregated with Fc␥RIIB by the addition of RAM IgG, the release of ␤-hexosaminidase was inhibited (see Figure 1A). In addition to the activation system whereby RAM IgG or RAM F(ab´) 2 was added, we also used another system for Western blot analysis, one by which each receptor can be aggregated separately or coaggregated. C57 cells were sensitized with murine anti-DNP IgE and incubated with or without 2.4G2 rat Ab before challenge with TNP-MAR F(ab´) 2 , TNP-conjugated F(ab´) 2 fragments of mouse anti-rat IgG. The conjugated antibody, TNP7-MAR F(ab´) 2 , functions as a multivalent antigen recognized by the Fc⑀RI-bound IgE but also recognizing Fc␥RII-bound 2.4G2 rat Ab. 13 The addition of TNP7-MAR F(ab´) 2 will aggregate Fc⑀RI in cells sensitized with IgE, aggregate Fc␥RIIB in cells sensitized with 2.4G2 rat Ab, and (as a consequence) coaggregate Fc⑀RI and Fc␥RIIB in cells sensitized with both IgE and 2.4G2 rat Ab. Since aggregation using 2.4G2 rat Ab together with TNP7-MAR F(ab´) 2 does not cause degranulation, this indicates that expression of Fc␥RIII (an activating low-affinity receptor for IgG) on C57 cells does not interfere with our system (data not shown). Although not as sufficient as the other system for causing degranulation, this system induced the activation of mast cells, causing degranulation, and showed inhibition upon coaggregation of Fc⑀RI with Fc␥RIIB (see Figure 1B).

Phosphorylation of Akt Is Attenuated by Coaggregation of Fc⑀RI with Fc␥RIIB
To assess the effects of coaggregating Fc⑀RI with Fc␥RIIB on signals transduced downstream of Fc⑀RI, the phosphorylation pattern of Akt protein was investigated. Akt is a signal-transducing protein downstream of PI3-kinase, involved in a variety of cellular functions such as survival and metabolism. 24,25 Via 3-phosphoinositide-dependent protein kinases, the PI3-kinase can activate Akt by phosphorylation at three different sites, two of which-threonine 308 (Thr 308) and serine 473 (Ser 473)-were investigated in this report. We compared the pattern of Akt phosphorylation at the Thr 308 and Ser 473 sites in cell lysates after Fc⑀RI aggregation or coaggregation of Fc⑀RI with Fc␥RIIB. Fc⑀RI aggregation induced rapid phosphorylation of Akt at Thr 308; the maximum phosphorylation stage was reached within 1 minute, and phosphorylation decreased at 5 minutes. The phosphorylation of Akt after Fc⑀RI aggregation at Ser 473 was achieved within 1 minute and remained at a comparable level for 10 minutes before decreasing at 30 minutes ( Figure  2). Considerable reductions of Akt phosphorylation at Thr 308 and Ser 473 were observed as early as 1 minute after coaggregation of Fc⑀RI with Fc␥RIIB (see Figure 2).

Coaggregation of Fc⑀RI with Fc␥RIIB Does Not Affect the Phosphorylation of Transcription Factor Foxo3a
Phosphorylated Akt phosphorylates and thereby inactivates the forkhead protein Foxo3a. 26 The phosphorylation of Foxo3a prevents its translocation into the nucleus, where it acts as a transcription factor for certain genes. We investigated the phosphorylation of Foxo3a at sites Ser 253 and Thr 32. Phosphorylation of Foxo3a at Ser 253 occurred within 1 minute but reached background phosphorylation level again after 30 minutes (Figure 3). However, after rapid phosphorylation at site Thr 32 within 1 minute after Fc⑀RI aggregation, phosphorylation remained constant until 30 minutes had elapsed (see Figure 3). In contrast to the effect on Akt phosphorylation, coengagement of Fc⑀RI with Fc␥RIIB did not affect either the levels of phosphorylation or the duration of the Fc⑀RIinduced Foxo3a phosphorylation (see Figure 3).

Fc⑀RI-Induced Expression of Bim Is Not Inhibited by Coaggregation with Fc␥RIIB
A key regulator of apoptosis is the Bcl-2 family of proteins, which consists of proapoptotic and antiapoptotic proteins. Bim, one of the proapoptotic members, is transcriptionally regulated by Foxo3a, 27 and we recently showed that Bim is involved in the regulation of mast-cell apoptosis. 18,19 Furthermore, Bim is induced upon aggregation of Fc⑀RI. 18 Therefore, we next investigated if coaggregation of Fc⑀RI with Fc␥RIIB would have an effect on Bim expression. After Fc⑀RI aggregation and coaggregation of Fc⑀RI and Fc␥RIIB, respectively, the two isoforms of Bim (Bim EL and Bim L ) were up-regulated to similar levels ( Figure 4). Bim EL consisted of two bands, owing to a shift in band motility; this shift of the Bim EL band is probably the result of phosphorylation. 19,28 The results herein demonstrate that Bim induced by Fc⑀RI aggregation is not affected by coaggregation with Fc␥RIIB (see Figure 4).

Coaggregation of Fc⑀RI with Fc␥RIIB Does Not Affect the Induction of A1
Apoptosis is regulated by members of the Bcl-2 family. A1, one of the antiapoptotic Bcl-2 family members, is described as being important for the survival of mast cells during allergic reactions. 20 To determine whether the coaggregation of Fc⑀RI with Fc␥RIIB affects the induced transcriptional regu- 92 Allergy, Asthma, and Clinical Immunology / Volume 2, Number 3, Fall 2006

Figure 2
Reduction of immunoglobulin E (IgE)-dependent phosphorylation of Akt by Fc␥RIIB. C57 mast cells were activated as in Figure 1B for the indicated periods of time. Cell lysates were prepared, and the phosphorylation of Akt was analyzed by Western blot with the indicated antibodies. The result is representative of three independent experiments. Ser = serine; Thr = threonine; 2.4G2 Ab = anti-mouse CD16/CD32 (Fc␥III/II receptor) monoclonal antibody.  Figure 1B for the indicated periods of time. Cell lysates were prepared, and the phosphorylation of Foxo3a was analyzed by Western blot with the indicated antibodies. The result is representative of three independent experiments. Ser = serine; Thr = threonine; 2.4G2 Ab = anti-mouse CD16/CD32 (Fc␥III/II receptor) monoclonal antibody. lation of A1, an RPA was performed. A1 was absent in cells incubated only with IgE but was substantially up-regulated after Fc⑀RI aggregation, as well as in cells where Fc⑀RI had been coaggregated with Fc␥RIIB for 6 hours ( Figure 5). The A1 mRNA level in cells activated by Fc⑀RI aggregation had increased 12-fold, and coaggregation of Fc␥RIIB with Fc⑀RI led to a ninefold increase when the signal was compared to control cells incubated with IgE alone (see Figure 5). Thus, although A1 up-regulation is slightly reduced after the coaggregation of Fc⑀RI with Fc␥RIIB when compared to Fc⑀RI aggregation, the induction of A1 in cells after either coaggregation of Fc⑀RI with Fc␥RIIB or Fc⑀RI aggregation (as compared to resting cells) was consistent in several experiments.

Discussion
Although coaggregation of Fc⑀RI with Fc␥RI is known to inhibit mast-cell degranulation, the effect of coaggregation on other signalling pathways in mast cells has not been investigated previously. In this study, we found that even though coaggregation of Fc⑀RI with Fc␥RIIB inhibits degranulation and decreases the phosphorylation of Akt, we observed no effect on Foxo3a phosphorylation or Bim expression (see Figures 2, 3, and 4). Results from RPAs showed that the mRNA of A1 (an antiapoptotic Bcl-2 family member) was up-regulated both when mast cells were activated through Fc⑀RI aggregation and when they were activated through coaggregation of Fc⑀RI with Fc␥RIIB (see Figure 5). Thus, Fc␥RIIB inhibits some but not all signalling pathways downstream of Fc⑀RI.
One pathway affected by Fc⑀RI aggregation is the PI3-K pathway, where PI3-K is phosphorylated and thereby activated. 5 Activated PI3-K can, via 3-phosphoinositide-dependent protein kinases or specific lipid products, phosphorylate the protein Akt. 29,30 Phosphorylation of Ser 473 and/or Thr 308 enables Akt to carry out its multifunctional activities, which are involved in a variety of cellular functions such as survival and metabolism. 24,25,31,32 Akt became rapidly phosphorylated at the two sites that were investigated after Fc⑀RI aggregation. The phosphorylation at Thr 308 was clearly diminished already after 5 minutes whereas the phosphorylation of Ser 473 remained for at least 20 minutes. This difference in phosphorylation between the two sites might reflect a strict regulation of phosphorylation of Akt.  Figure 1B for 24 hours. Cells sensitized only with 0.1 µg/mL of anti-dinitrophenyl immunoglobulin E (lgE) and incubated with or without 5 µg/mL of rabbit anti-mouse CD16/CD32 (Fc␥III/II receptor) monoclonal antibody (2.4G2 rat Ab) were used as controls. Cell lysates were prepared, and the induction of Bim was analyzed by Western blot with the indicated antibodies. The result is representative of three independent experiments. MAR = mouse anti-rat; TNP = trinitrophenyl. . After coaggregation of Fc⑀RI with Fc␥RIIB, the phosphorylation of Akt was attenuated when compared to Fc⑀RI aggregation. Akt is more heavily phosphorylated after Fc⑀RI aggregation, but the duration of the phosphorylation does not change after coaggregation of Fc⑀RI with Fc␥RIIB. These data are in line with data from earlier studies showing that coaggregation of Fc␥RIIB and the B-cell receptor (as well as coaggregation with the receptor for stem-cell factor Kit, present on mast cells) affects the PI3-K pathway and thereby inhibits the activation of Akt. 33,34 Members of the transcription factor forkhead family, such as Foxo3a, can be inactivated through phosphorylation by activated Akt. 26 We found that Fc⑀RI aggregation and Fc⑀RI coaggregation with Fc␥RII result in the same phosphorylation pattern of Foxo3a. This is an interesting observation because one might expect the phosphorylation of Foxo3a to decrease in response to less phosphorylated Akt being available. A possible explanation is that because the phosphorylation of Akt is not totally abrogated, there might still be enough to phosphorylate Foxo3a to the same extent. Another interesting feature is that phosphorylated Foxo3a is present in cells that are not activated by either Fc⑀RI aggregation or coaggregation of Fc␥RIIB with Fc⑀RI. This suggests a natural equilibrium between phosphorylated and unphosphorylated Foxo3a in the cells, which is shifted toward phosphorylation upon activation. Akt is a major effector protein, and although the phosphorylation of Foxo3a by Akt does not seem to be affected, a pathway (or pathways) other than the one investigated might be where the inhibition of Akt phosphorylation plays a more crucial role.
A protein known to be under the transcriptional control of the forkhead transcription factor Foxo3a is Bim. 27 We previously found Bim to be strongly increased upon Fc⑀RI aggregation. 18 After coaggregation of Fc␥RIIB with Fc⑀RI or after Fc⑀RI aggregation, the two isoforms of Bim (BimEL and Bim L ) were up-regulated in comparison to unactivated control cells. The results demonstrate that Fc⑀RI-induced Bim up-regulation is not affected upon coaggregation with Fc␥RIIB. Bim EL consisted of two bands, probably due to phosphorylation. We have previously seen that stem-cell factor (SCF) promotes the survival of mast cells through inactivation of Foxo3a, preventing the up-regulation of Bim and leading to increased phosphorylation of Bim. Those results show that inhibition of Foxo3a and (consequently) Bim provides an important mechanism by which SCF acts to prevent apoptosis in mast cells. 19 Antiapoptotic members of the Bcl-2 family are needed for cell survival. One of the murine prosurvival Bcl-2 family members is A1, which plays a prominent role in preventing apoptosis in a variety of cell systems. 35,36 Previously, we demonstrated that mRNA levels for A1 are increased after Fc⑀RI aggregation and that A1 is critical for the activation-induced survival of mast cells. 20 Similarly, the human homologue bfl-1 is up-regulated in human mast cells upon Fc⑀RI aggregation. 37 We examined the mRNA induction of the antiapoptotic A1 protein after coaggregation of Fc⑀RI with Fc␥RIIB; we found that A1 mRNA was up-regulated both when mast cells are activated through Fc⑀RI aggregation and when Fc⑀RI is coaggregated with Fc␥RIIB. Our finding that both antiapoptotic A1 and proapoptotic Bim proteins are up-regulated as a result of Fc⑀RI aggregation could be an explanation of why this activation results in cell death or survival in some experimental settings, since the fate of cells is likely to be influenced by the relative balance of these molecules.
The only treatment of allergic diseases that leads to long-lasting effects is allergen-specific immunotherapy. The immunologic mechanisms responsible for a successful treatment are still not fully defined. One hypothesis is that the antigenspecific IgG that increases in serum during treatment blocks antibodies, 38 leading to possible coaggregation of Fc⑀RI with Fc␥RIIB. The finding that allergic activity is inhibited by coaggregating Fc⑀RI with Fc␥RIIB by using a human Fc␥-Fc⑀ fusion protein highlights a new promising therapeutic approach to immunomodulation. 39 The fusion protein showed antiallergic effects both in vitro and in vivo and was shown to inhibit IgEmediated activation of blood basophils and cord blood-derived mast cells. 40 Furthermore, evidence for negative regulation of allergic responses by Fc␥RIIB has been demonstrated by the use of Fc␥RIIB-deficient mice. These mice produce more immunoglobulin than wild-type mice in response to immunization, 41 in which this increase is partly due to the increase in IgG1. The negative regulation of IgG production by Fc␥RIIB probably decreases the production of IgE. This would work in favour of reduced Fc⑀RI expression on the cells and less IgE being available for activation. 42,43 Fc␥RIIB-deficient mice also display more vascular permeability in the IgG-dependent passive cutaneous anaphylaxis reaction than do wildtype mice, indicating mast-cell activation of a greater extent than that seen in wild-type mice. 41 During IgE-and IgG-dependent passive systemic anaphylaxis, the Fc␥RIIB-deficient mice undergo increased hypothermia and death. 44 These findings indicate an important role for Fc␥RIIB on mast cells in down-regulating immediate hypersensitivity reactions as a result of anaphylactic mast-cell activation.
This report shows that although mast-cell degranulation is inhibited by coaggregation of Fc⑀RI with Fc␥RIIB, other downstream signalling proteins that are closely related to cell survival remain largely unaffected. Figure 6 presents a schematic overview of how these processes could be separated in the cell. Our previous finding that both proapoptotic and antiapoptotic proteins are up-regulated as a result of Fc⑀RI aggregation suggests that the fate of cells is likely to be based on the balance between these proteins. 17