The ability of IL-33 to modify MC activation has recently been the topic of active investigation due to reports of elevated IL-33 levels in the lungs of asthmatics and affected tissues of patients with rheumatoid arthritis, Crohn’s disease, atopic dermatitis, and psoriasis; diseases that are associated with MC activation (5
). A role for IL-33 in MC-driven allergic reactions had also been suggested by the report that IL-33 alone induces anaphylaxis in a mouse model, and degranulation of MCs in culture (33
) (now retracted). However, as described by others in mouse BMMCs (8
), we observed that IL-33 itself failed to induce calcium mobilization and thus was unable to induce degranulation (). Furthermore, we observed no anaphylactic response in the mouse following direct challenge with IL-33 (). From these data, we conclude that, under acute conditions, IL-33 does not influence MC degranulation. Nevertheless, our data clearly demonstrate that prolonged exposure to IL-33 () markedly reduces FcεRI-mediated MC degranulation and that this was associated with MyD88-dependent ( and ) IL-33-induced attenuation of the FcεRI-induced calcium signal () and cytoskeletal rearrangement which are required for effective degranulation ().
The observed IL-33-dependent down-regulation of PLCγ1
and Hck likely accounted for, or at least contributed to, the impaired calcium signal and cytoskeletal rearrangement in the hypo-responsive phenotype. Of note, Hck-deficient BMMCs display impaired degranulation and associated actin reorganization (28
) and we have recently documented that a hypo-responsive mouse BMMC phenotype induced by prolonged exposure to SCF is also associated with down-regulation of Hck (24
). In contrast to the effects of IL-3, however, the SCF-induced hypo-responsive phenotype was not associated with similar defects in PLCγ1
expression and calcium signaling (24
). This difference may be related to the distinctly different signaling pathways through which IL-33 and SCF operate (4
is required for induction of the calcium signal (25
) and, as for Hck (28
), the associated actin reorganization (34
). Both of these signals are necessary formast cell degranulation. Thus, the IL-33-dependent down-regulation of Hck and PLCγ1
likely accounted for the impaired cytoskeletal rearrangement and marked attenuation of the FcεRI-mediated calcium signal in the hypo-responsive phenotype. In addition, as the antigen-induced PLCγ-dependent calcium signal interacts synergistically with weak calcium signals generated by SCF (23
) to further enhance antigen-mediated degranulation, this could explain why IL-33 also reduced the ability of SCF and PGE2
to enhance degranulation.
It is possible that IL-33 may act upon other essential signaling events even though we observed little change in the expression of other critical signaling proteins for mast cell activation. For example, antigen-induced LAT and LAT2 phosphorylation, but not protein levels, was substantially reduced in the IL-33-treated MCs. It has been proposed that the positive effects of Hck may in part be mediated through suppression of the inhibitory actions of Lyn (28
), and such actions would likely be diminished as a consequence of the decrease in Hck expression by IL-33. Certainly, the diminished levels of Hck in IL-33-treated cells would be expected to share similar phenotypic characteristics as the Hck−/−
We propose that the prolonged effects of IL-33 on MC activation through modification of the expression of critical signaling molecules can be viewed as phenotypic reprogramming. The extent to which other factors can modulate the HuMC activation phenotype in the manner described in this paper is currently unknown. Nevertheless, as noted above, we have recently shown that prolonged exposure to SCF also induces a hypo-responsive phenotype in the mouse (24
). Similarly, we observed that prolonged exposure to IL-1 which signals in a similar manner as IL-33 also down-regulates antigen-mediated degranulation in BMMCs (Fig. S3
). In contrast to the IL-33 receptor, ST2, which signals through a MyD88 and the interleukin-1 receptor accessory protein (IL-1RAcP) complex (4
), it has recently been shown that prolonged exposure to agonists of TLRs which require MyD88, but not IL-1RAcP, for signaling, enhance antigen-mediated degranulation (35
). This implies that although MyD88 is essential, signals conferred by IL-1AcP are also required for IL-33 induction of the hypo-responsive phenotype.
Overall, our results suggest that the secretory response of MCs to antigen in a pathophysiological setting may not only be finely regulated in an acute manner (36
), but that persistent increases in factors in the surrounding milieu might conceivably dampen pro-inflammatory responses of MCs following an innate immune reaction and assist resolution of the immune response. In the case of IL-33, the elevated level of IL-33 in asthmatic lungs rather than contributing to the disease may actually protect by tempering further MC-dependent inflammation. This, and other studies (37
), and our recent finding that BMMCs are rendered similarly quiescent by long-term exposure to a physiologic factor, SCF (24
), point perhaps to a broad array of conditioning factors to which MCs might be exposed in health and disease states. The possible implication is that, if the downregulation of signaling proteins by IL-33 or SCF is a mechanism for long-term prevention of inappropriate activation of MCs in a pathologic situation, then any disruption of such a process would render MCs hyperactive leading to MC-driven disease. Our observations may thus have implications for the treatment of allergic disorders if, for example, receptor ligands with more restricted actions than IL-33 or SCF were found to shift MCs to a quiescent state.