Recent evidence for common signalling mechanisms among immunoreceptors that recognize foreign antigens

The beta subunit of the type I Fcepsilon receptor is a target for peptides inhibiting IgE-mediated secretory response of mast cells

Peptides originally derived from complement component C3a were earlier shown to inhibit the type I FcepsilonR (FcepsilonRI)-mediated degranulation of mucosal type mast cells. In the present study, we show that C3a7, a peptide with a natural sequence, and its modified derivative, C3a9, are powerful inhibitors of the above response of both serosal and mucosal type mastocytes. We demonstrate that these peptides inhibit FcepsilonRI-induced membrane proximal events, suppress phosphorylation of the FcepsilonRI beta subunit, the protein tyrosine kinase Lyn, as well as the transient rise in free cytosolic Ca2+ level. The late phase of cellular response was also inhibited, as demonstrated by the reduced TNF-alpha secretion. Experiments using two independent methods provided evidence that the interaction site of complement-derived peptides is the FcepsilonRI beta-chain. This was further supported by fluorescence confocal microscopic colocalization and resonance energy transfer measurements. Taken together, these results suggest the presence of distinct "activating" and "inhibitory" motifs in the C3a sequence. Response to both is in balance under physiologic conditions. Furthermore, present data predict that such inhibitory peptides may serve as potent agents for future therapeutic intervention.

Inhibition of IgE-mediated triggering of mast cells by complement-derived peptides interacting with the Fc epsilon RI

Mucosal type mast cells, in contrast to the serosal type ones, do not respond to cationic agents, or to the complement-derived peptides C3a and C5a. Earlier we have found that while C3a does not activate the rat mucosal type mast cells (line RBL-2H3), it strongly inhibits the IgE-mediated triggering of these cells, by interfering with the Fc epsilon RI-initiated signaling pathway. In the present study we further investigated the mechanism of this process. It is shown, that C3a interacts with the beta-chain of the Fc epsilon RI complex. Binding of the complement peptide to the cells apparently causes a decrease in the proximity of the IgE-binding Fc epsilon RI. Investigating certain sequences of C3a we found that the inhibition is caused by the C-terminal sequences of the complement-peptide, ranging from positions 56 to 77 and also by a shorter sequence, ranging from positions 56 to 64. The inhibitory effect of these peptides was observed both in the case of RBL-2H3 cells and mouse bone marrow derived mast cells.

Complement peptides and mast cell triggering

Mucosal type mast cells have been earlier shown to be unresponsive to the so called 'peptidergic' stimulus provided by cationic agents, such as anaphylatoxins, neuropeptides or polyamines. We studied the relationship between mast cells' secretory response to stimulation via their type I Fc epsilon receptors (Fc epsilonRI) and that provided by C5a and C3a fragments of the complement system, in the rat mucosal-type mast cell line RBL-2H3. Our results shown here reveal a novel function of C3a, its inhibitory capacity on IgE-mediated triggering of mucosal mast cells. This activity of C3a is most probably mediated by its interaction with the beta-chain of Fc epsilonRI. While connective tissue type mast cells are known to be activated by micromolar concentrations of the complement peptides C3a and C5a, the amount of C3a necessary for the inhibition of antigen-induced degranulation of mucosal cells in our assays is in the nanomolar range. Interestingly, the other anaphylatoxic peptide C5a, which is known to be much more effective in several biological assays, did not show any activity in the same test-system.

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

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

Novel functions of phosphatidylinositol 3-kinase in terminally differentiated cells

Importance of phosphatidylinositol 3-kinase (PI 3-kinase) in signalling pathways leading to growth stimulation has already been reviewed in this journal and others. Evidence has now been accumulating that PI 3-kinase is involved in transmission of activation signals in terminally differentiated cells, especially signals starting from receptors which have no intrinsic tyrosine kinase domain. The pioneer works showed the presence of PI 3-kinase activity and the accumulation of the reaction products of PI 3-kinase correlated with the cell responses. However, these studies were done in only limited cell responses such as respiratory burst in neutrophils and degranulation in platelets. Recent finding of a potent and selective inhibitor of PI 3-kinase, wortmannin, reported from three independent groups including us, gave a new and powerful tool not only to confirm the suggested functions but also to reveal new functions of PI 3-kinase such as histamine release from antigen-stimulated mast cells/basophils and glucose uptake in insulin-stimulated adipocytes. Nearly one hundred papers which describe the action of wortmannin on various cells have been reported during one year after the publication of the discovery of wortmannin as PI 3-kinase inhibitor, suggesting possible involvement of the enzyme in the diverse cell responses besides cell proliferation.

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

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

Signal transduction by Fc receptors: the Fc epsilon RI case

The family of proteins collectively known as Fc receptors (FcR) plays a variety of roles both in the initiation of the immune response and in its consequences. During the past five years the structure of these proteins and the genes that code for them have been largely elucidated. The most unexpected finding has been their extensive diversity. Considerable efforts are now being expended to define the molecular events initiated by these various FcR and these events are the focus of our review.

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

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