The neglected of eosinophil biology, IL-3 finds sustenance in the basophil (original) (raw)

Abstract

Eosinophils and basophils are a vital component of allergic inflammation in asthma and other diseases. Both granulocytes express the 3 common bc-chain signaling cytokine receptors for IL-5, GM-CSF, and IL-3. Therefore, the identification of the specific signaling pathways induced by IL-3 could contribute to new therapeutic options that will dampen the functions of these inflammatory cells. Recently, Kӓmpfer et al. [1] published an important study in the Journal of Leukocyte Biology, demonstrating long-term IL-3R-mediated signaling in basophils and eosinophils. Also recently, our group has reported very similar long-term IL-3 effects on eosinophils [2], but our work was not mentioned by Kӓmpfer et al. [1]. Therefore, I found it important to discuss further the similarities and differences in IL-3 signaling observed in basophils and eosinophils. This Letter will help answer some of the questions or uncertainties raised by the Kӓmpfer et al. manuscript [1], which primarily focuses on basophils. In agreement between Kӓmpfer and colleagues’ work [1] and our study [2], IL-3-induced, prolonged intracellular events required continuous presence of IL-3 and IL-3R signaling. Furthermore, Kӓmpfer et al. [1] found that IL-3 was unique among the common b-chain signaling cytokines to prolong signaling for .48 h and to induce the production of key basophil-specific proteins. Phosphorylation of STAT5 was the most remarkably prolonged event in IL-3-activated basophils. To some extent, STAT3, protein kinase B, ERK, p38, and the ribosomal S6 protein were all continuously phosphorylated exclusively by IL-3 [1]. Interestingly, Pim1 (a phosphorylated STAT5-induced protein), retinaldehyde dehydrogenase 2, and granzyme B protein products were induced by IL-3 at 18 h, whereas IL-5 and GM-CSF had no effect on these proteins. Similarly with basophils, the prolonged phosphorylation of STAT5 and production of Pim1 were observed in IL-3-activated eosinophils, whereas little effect was seen following stimulation with GM-CSF or IL-5 [1]. In agreement with Kӓmpfer et al. [1], our group has recently reported that IL-3 was more potent than GM-CSF or IL-5 in maintaining intracellular signaling in eosinophils [2]. STAT5 phosphorylation was not analyzed in our study, but phosphorylation of both p90S6 kinase and ribosomal S6 proteins was found to be maintained for at least 2 d after addition of IL-3 [2]. Therefore, as for basophils, IL-3 is unique in its ability to prolong ribosomal S6 phosphorylation in eosinophils. The mechanism responsible for the prolonged S6 phosphorylation in IL-3-activated basophils was not pursued in the Kӓmpfer et al. study [1]; however, given our study [2], p90S6 kinase may be the distinguishing factor. Similarly to the Kӓmpfer and coworkers’ basophil study [1], we also examined ERK activation [2], which was required upstream of S6 phosphorylation. Although we did not analyze the continuous phosphorylation of ERK in activated eosinophils, as was shown in the Kӓmpfer et al. study [1], we did find that inhibition of ERK activation, 3 h after addition of IL-3, blocked long-term p90S6 kinase and S6 phosphorylations, indicating that ERK must be activated for at least 3 h in IL-3activated eosinophils. Furthermore, among the proteins solely induced by IL-3 in basophils, granzyme B is quickly and strongly up-regulated at the mRNA level [3]. This can account for the IL-3-induced upregulation of granzyme B protein in basophils, as observed by Kӓmpfer et al. [1]. In eosinophils, however, IL-3 is unique compared with IL-5 and GM-CSF to induce an increased translation rate of semaphorin-7A without changing its mRNA level. Whereas transcriptional and post-transcriptional analyses of the IL-3-induced basophil proteins were clearly beyond the scope of the Kӓmpfer et al. study [1], the potential role of S6 phosphorylation on protein translation rate in basophils should have been discussed in their paper. Finally, Kӓmpfer and coworkers’ work [1] supports the premise that in basophils, the continuous intracellular IL-3 signaling versus the ephemeral signaling induced by IL-5 or GMCSF is probably a result of the dynamic of their respective receptors. However, it is important to note the differences and commonalities between basophils and eosinophils regarding the regulation of these receptors. In basophils, IL-3, GM-CSF, or IL-5 maintain the high basal level of IL-3Ra over time, whereas IL5Ra and GM-CSFRa are quickly removed from the cell surface. On the contrary, in eosinophils, the basal level of IL-3Ra is low and is increased over time (after 4 h) by IL-3, GM-CSF, or IL-5. Interestingly, whereas these cytokines result in the loss of surface IL-5Ra, GM-CSFRa is generally unaffected by the cytokine treatments [2, 4, 5]. The different dynamics of GM-CSFRa in activated basophils versus eosinophils may explain why GM-CSF