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IL1-IL1RAP axis in AML
less relevant for AML stem cells. Previously, studies showed reduced clonogenicity in AML cell lines as well as reduced engraftment capacity upon knockdown or inhibi- tion of IL1RAP using antagonistic antibodies.28,35 We also found a reduction in CFC frequency upon knockdown of IL1RAP in THP1 cells or primary IL1RAP+ AML cells, but not in K562 or OCI-AML3 cells. It was also shown that IL1RAP-targeting antibodies resulted in reduced cell prolif- eration, however, we did not observe any difference in cell proliferation upon knockdown of IL1RAP.28,35 Possibly, a reduction in expression is dissimilar to an antibody-medi- ated block in signaling. Importantly, we did observe an IL1b–induced activation of canonical NFkB signaling in IL1RAP-expressing AML, but this did not result in NFkB- driven survival under stress conditions such as serum dep- rivation.
Intriguingly, the IL1b-induced inflammatory secretome of AML cells grown on MSC affected normal hematopoi- etic proliferation and HSPC clonogenicity, while AML cells were much less affected. This observation was specific for IL1RAP-expressing AML cells that were cultured on stro- mal cells as CM of AML cells and MSC alone did not affect normal hematopoiesis. We observed only a mild increase in DAPI percentage or Annexin V positivity in CB CD34+ cells, suggesting that an increase in apoptosis does not fully explain the loss of cell growth. Recently, Waclawiczek et al. showed that transcriptionally remodeled MSC, due to the presence of AML cells, resulted in suppression of HSPC but did not affect their viability.37 Along the same lines, Miraki- Moud et al. suggested that AML cells do not impair the sur- vival of normal HSC but do inhibit their differentiation, from which HSC can recover once removed from the leukemic environment.38 Single-cell sequencing studies of the mouse BM provided a very detailed description of the cellular heterogeneity within the BM niche, which was remodeled upon stress or MLL-AF9 leukemia engraftment.39,40 This remodeling affected function and maturation of BM stromal cells resulting in the loss of sig- naling molecules known to be essential for normal hematopoiesis.39 Similarly, studies showed BM remodeling via exosome secretion, TGF-b, Notch and inflammatory signals.41,42 Together, these findings suggest that leukemic cells can impact on normal hematopoiesis in multiple ways. Likely, there are more proteins secreted by AML and/or MSC that also influence normal hematopoietic pro- liferation, as Anakinra could not fully rescue the negative phenotype. For example, Carter and colleagues showed that IL1b can result in a Cox2-dependent secretion of prostaglandin E2 (PGE2) by MSC, which ultimately result- ed in b-catenin-mediated augmented chemotherapy resist-
ance in AML cells.43 Besides inflammatory factors, GATA2 was also upregulated ~2.6-fold in AML cells upon IL1b stimulation. p38-dependent GATA2 activation has been associated with poor overall survival and increased tran- scriptional activation of IL1b and CXCL2.44,45 We hypothe- size that this p38-dependent activation of GATA2 is part of a positive feedback loop in IL1RAP-expressing AML that results in an inflammatory niche.
The formation of an inflammatory niche possibly plays a role in the early stages of leukemia development as well. It has been shown that hematopoietic clones harboring a pre-leukemia mutation in Tet methylcytosine dioxyge- nase 2 (TET2) can outgrow non-mutated clones after inflammatory stress, which in turn might be aggravated by the fact that TET2 knockout mice show increased lev- els of inflammatory proteins including IL1b, IL-6 and chemokines including Cxcl1-3 and Pf4.46,47 Upon aging, the BM niche changes and becomes more senescent and as a result, via a senescent-associated secretory phenotype (SASP), more inflammatory.48-50 Although the data are lim- ited, it is enticing to consider that an inflammatory BM niche might accelerate clonal expansion and that the IL1- IL1RAP signaling axis plays an important role already in early stages of leukemic initiation.
Overall, our study contributes to the understanding of the role that plasma membrane receptors play in the leukemic BM niche. Such insights might aid further devel- opment of therapies aimed at specifically targeting factors that are essential for leukemogenesis. Inhibition of the IL1-IL1RAP signaling axis might be a good therapeutic approach to reduce inflammation in the BM niche and thereby promote normal hematopoietic recovery over AML proliferation after chemotherapy.
Disclosures
No conflicts of interest to disclose.
Contributions
BdB, SS and JJS conceived the study concept; BdB, SS, ABM and KA carried out experiments; GH and EV provided funding; BdB, SS and KA analyzed the data; BB, SS and JJS wrote the original draft; BdB, SS, GH, EV, ABM and JJS co-wrote, reviewed and edited the manuscript; JJS acquired funding and supervised the project.
Funding
This work was supported by a grant from the European Research Council (ERC-2011-StG 281474-huLSCtargeting) awarded to JJS. The UMCG/MPDI program is acknowledged for the scholarship awarded to SS.
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