The neglected of eosinophil biology, IL‐3 finds sustenance in the basophil

Short abstract

The understanding of the unique IL‐3 functions on eosinophils and basophils should provide further justification for examining the role of therapies targeting IL‐3 pathway.

Eosinophils and basophils are a vital component of allergic inflammation in asthma and other diseases. Both granulocytes express the 3 common βc‐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, Kämpfer 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 Kämpfer 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 Kämpfer et al. manuscript [1], which primarily focuses on basophils.

In agreement between Kämpfer 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, Kämpfer et al. [1] found that IL‐3 was unique among the common β‐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 Kämpfer 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 Kämpfer et al. study [1]; however, given our study [2], p90S6 kinase may be the distinguishing factor. Similarly to the Kämpfer 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 Kämpfer 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‐3‐activated 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 up‐regulation of granzyme B protein in basophils, as observed by Kämpfer 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 Kämpfer et al. study [1], the potential role of S6 phosphorylation on protein translation rate in basophils should have been discussed in their paper.

Finally, Kämpfer and coworkersˈ work [1] supports the premise that in basophils, the continuous intracellular IL‐3 signaling versus the ephemeral signaling induced by IL‐5 or GM‐CSF 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‐3Rα over time, whereas IL‐5Rα and GM‐CSFRα are quickly removed from the cell surface. On the contrary, in eosinophils, the basal level of IL‐3Rα 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‐5Rα, GM‐CSFRα is generally unaffected by the cytokine treatments [2, 4, 5]. The different dynamics of GM‐CSFRα in activated basophils versus eosinophils may explain why GM‐CSF can still trigger a weak, yet prolonged, signal in eosinophils, as opposed to in basophils [1]. Furthermore, the weakness of the prolonged GM‐CSF‐induced signal in eosinophils occurs despite continuously high surface levels of GM‐CSFRα, suggesting that receptor dynamics may not explain attenuation of GM‐CSF‐driven signals over time in eosinophils. For instance, GM‐CSF could trigger inhibitory signals via immunoreceptor tyrosine‐based inhibitory motif‐possessing receptors, although this supposition has not been explored by our group.

In conclusion, the publication by Kämpfer et al. [1] would have benefited from our recent publication that focused on the exclusive role of IL‐3 on eosinophils, as both studies showed prolonged IL‐3R and MAPK signalings. Furthermore, Kämpfer et al. [1] omitted discussion of the potential function of prolonged activation of the ERK/p90S6 kinase/S6 pathway in basophils. Because of the similarity between basophils and eosinophils, studies performed using either granulocyte will gain an understanding from each other to comprehend better the uniqueness of IL‐3 functions on these granulocytes and to provide further justification for examining the role of therapies targeting the IL‐3 pathway.